Recent Development of Aryl Diazonium Chemistry for the Derivatization of Aromatic Compounds

Fanyang Mo,* Di Qiu, Lei Zhang, and Jianbo Wang*

Cite This: https://doi.org/10.1021/acs.chemrev.0c01030 Read Online

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ABSTRACT: Aryl diazonium salts are versatile building blocks in organic synthesis. In light of the ever-increasing importance of aryl diazonium salts spanning most disciplines of the chemical sciences, we review the recent development of aryl diazonium chemistry over the past seven years (2013-2020). Special emphasis is put on various new transformations involving the generation of radical intermediates via thermal, photochemical, and electrochemical means. Recent advances in the development of transition metal-catalyzed reactions using aryl diazonium salts are also reviewed. Together, these newly developed transformations significantly expand the synthetic chemist’s repertoire of aromatic carbon-carbon and carbon-heteroatom bond forming methods using aryl diazonium precursors, providing powerful tools for the synthesis and modification of complex molecular scaffolds.

${\mathrm{{CF}}}_{3},{\mathrm{{SCF}}}_{3}$ halogen…

Introduction B
Aryl C-C Bond Formations D

2.1. Aryl C-C Bond Formations Involving Aryl Radical Intermediates D

2.1.1. Radical Addition to Double Bonds D

2.1.2. Difunctionalization of Alkenes E

2.1.3. Radical Addition to Aromatic Compounds 1

2.1.4. Radical Addition to Alkynes P

2.1.5. Radical Addition to CO Q

2.1.6. Cascade Radical Cyclization R

2.1.7. C-CN, $\mathrm{C} - {\mathrm{{CF}}}_{3}$ , and $\mathrm{C} - {\mathrm{{CF}}}_{2}\mathrm{X}$ Bond Formations V

2.1.8. Miscellaneous Y

2.2. Transition-Metal-Catalyzed Coupling Reactions of Aryl Diazonium Salts Y

2.2.1. Heck-type Reactions with Alkenes Z

2.2.2. C-H Bond Functionalizations of Arenes AB

2.2.3. Suzuki-Miyaura, Hiyama, Stille-type Coupling Reactions, and Others AE

2.2.4. Sonogashira-type Coupling Reactions with Alkynes AH

2.2.5. Gold-Catalyzed Coupling Reactions and Cyclizations AH

Aryl Diazonium Salts in Aryl C-Heteroatom Bond Formations AO

3.1. C-Halogen Bond Formations AO

3.2. C-O Bond Formations AP

3.3. C-N Bond Formations AQ

3.4. C-B Bond Formations AQ

3.5. C-P Bond Formations AS

3.6. C-Sn Bond Formations AT

3.7. C-Se, Te Bond Formations AU

3.8. C-S Bond Formations AV

3.8.1. Synthesis of Aryl Sulfides and Sulfox-ides AV

3.8.2. Synthesis of Aryl Sulfones through the Insertion of ${\mathrm{{SO}}}_{2}$ AY

3.8.3. $\mathrm{C} - {\mathrm{{SCF}}}_{3}$ and $\mathrm{C} - {\mathrm{{SCF}}}_{2}\mathrm{H}$ Bond Formations BG

3.9. Reactions with Nitriles and Cascade Reactions BJ

Reactions with the Retention of the Nitrogen Group BL

4.1. Nucleophilic Additions with Aryl Diazonium Cation BL

4.2. Radical-type Reactions with Aryl Diazonium Salts BP

Aryl Diazonium Salts in Flow Chemistry BR
Aryl Diazonium Salts in the Modification of Functional Materials and Molecules BT
Conclusions BW

Author Information BW

Corresponding Authors BW

Authors BW

Notes BW

Biographies BW

Received: September 23, 2020

Figure 1. Chronological development of diazonium salts chemistry.

Scheme 1. Summary of the Transformations and Applications of Aryl Diazonium Salts

synthesis and interdisciplinary science

Acknowledgments References

BX BX

1. INTRODUCTION

Conveniently synthesized from the corresponding anilines, aryl diazonium salts are widely used building blocks in modern organic synthesis. ${}^{1 - 3}$ Since the initial report by Griefs in ${1858}^{4}$ a series of prominent named reactions associated with aryl diazonium salts have been discovered and developed over the past centuries. Most notable examples include the Sandmeyer reaction (1884), ${}^{5,6}$ Pschorr reaction (1896), ${}^{7}$ Gomberg-Bachmann reaction (1924), ${}^{8}$ Balz-Schiemann reaction (1927), ${}^{9}$ and Meerwein arylation (1939). ${}^{10}$ In 1977, Doyle developed a diazotization method that could be conducted in the organic phase, allowing for the generation of diazonium salts from water-incompatible substrates. ${}^{{11},{12}}$ In the same year, Kikukawa and Matsuda pioneered transition-metal-catalyzed cross-coupling of aryl diazonium salts. ${}^{{13},{14}}$ Their work paved the way for further advancing cross-coupling chemistry using aryl diazonium salts as electrophilic coupling partners. However, since then, the field of aryl diazonium chemistry has been dormant for a long time.

The past decade has witnessed a renaissance of aryl diazonium chemistry. ${}^{{15} - {19}}$ In particular, a range of new functional group conversions based on the diazonium moiety have been developed. These processes encompass the formation of aromatic $\mathrm{C} - \mathrm{B},\mathrm{C} - \mathrm{P},\mathrm{C} - \mathrm{{Sn}}$ , and $\mathrm{C} - {\mathrm{{CF}}}_{3}$ bonds from aryl diazonium salts. Using these methods, a range of functionalized aromatic compounds can now be conveniently accessed from readily available arylamines. In recent years, the merger of aryl diazonium chemistry and visible-light photo-redox catalysis afforded a series of new tools for aromatic C-C and $\mathrm{C} - \mathrm{Y}\left( {\mathrm{Y} = \mathrm{B},\mathrm{P},\mathrm{S},\mathrm{{Sn}},\mathrm{{Se}}}\right.$ , etc.) bond formations. Aryl diazonium salts have also found new applications in the synthesis of nitrogen-containing heterocyclic compounds. Furthermore, the use of diazonium chemistry provided new strategies for the synthesis of medicinally active agents ${}^{{20},{21}}$ and the modification of bulk materials, such as electrode surfaces and graphenes. ${}^{{22},{23}}$ Biomolecules including peptides and proteins can now also be rapidly modified by aryl diazonium reagents (Figure 1). ${}^{24}$ Moreover, starting from abundantly available arylamines, transformations via the intermediacy of aryl diazonium salts enabled large-scale applications in the syntheses of functional molecules, such as haloarenes, azo dyes, and nanomaterials. ${}^{{25} - {31}}$

Figure 2. (a) Number of publications on reactions with aryl diazonium salts (2010-2020); (b) pie chart of numbers of publications on various types of reactions of aryl diazonium salts (2010-2020).

The reactions of aryl diazonium salts can be categorized into four classes, including transformations via aryl radical, transformations via aryl cation, transition-metal-catalyzed processes, and transformations with retention of the dinitrogen group (Scheme 1). The first three reaction classes are based on the intrinsic electrophilicity of diazonium salts with ${\mathrm{N}}_{2}$ as an excellent leaving group. In the last reaction type, in the presence of aromatic nucleophiles such as phenols and anilines, the arene diazonium ion can serve as a nitrogen electrophile, giving rise to products with dinitrogen retention (Scheme 1). The broad utility of diazonium chemistry originates from the wide availability of their aniline precursors and the diverse diazotization conditions for their preparation.

While it is regarded as classic chemistry included in every basic organic chemistry textbook, aryldiazonium still remains an active topic of research over the past decade (Figure 2). C-

H arylation using aryl diazonium reagents has been the most extensively studied transformation with respect to the number of publications (72 papers). This is followed by other functional group conversions and nitrogen retaining processes. All of these transformations continue to attract attention from both academia and industry.

Since our previous review in 2013, an array of exciting research has emerged in this area, which warrants a new review article to provide an update and guidance to this field. In this article, we provide a comprehensive review on recently developed diazonium chemistry. The review will primarily focus on recent studies published after 2013.

2. ARYL C-C BOND FORMATIONS

2.1. Aryl C-C Bond Formations Involving Aryl Radical Intermediates

As a commonly used class of aryl radical precursor, aryl diazonium salts have enjoyed numerous applications in the construction of aryl $\mathrm{C} - \mathrm{C}$ bonds. In recent years, alternative routes for aryl radical generation, particularly the employment of the visible-light photoredox system, electrolysis, and organic promoter-initiated radical reactions, made aryl diazonium chemistry an attractive area in organic synthesis. Various arylated molecules can be synthesized through selective aryl C-C bond formations based on aryl radical intermediates. In this section, recent advances will be classified according to different reaction pathways, including radical addition to double bonds to form arylated alkenes, difunctionalization of alkenes, radical addition to aromatic compounds to form biaryls, radical addition to alkynes, radical addition to CO, cascade radical cyclization to form a variety of cyclic compounds, and aryl $\mathrm{C} - \mathrm{{CN}},\mathrm{C} - {\mathrm{{CF}}}_{3}$ , and $\mathrm{C} - {\mathrm{{CF}}}_{2}\mathrm{X}$ bond formations (Scheme 2).

Scheme 2. C-C Bond Formations Based on Aryl Radical Intermediates

2.1.1. Radical Addition to Double Bonds. Meerwein arylation reactions have been well-established as a powerful method for the synthesis of arylated olefins. ${}^{10}$ It could be carried out on alkenes bearing electron-withdrawing groups such as coumarins, cinnamic acids, and acrylic acids. The substrate scope of this process was later expanded to electron-rich alkenes. Classical Meerwein arylations require a catalytic or stoichiometric amount of redox-active transition-metal salts, such as copper(I) salt, and have relatively limited substrate scope. In recent years, a series of improvements have been reported for Meerwein arylations, including the expansion of substrate scope in traditional Cu-catalyzed arylations, the employment of metal-free systems, and electrochemical systems, particularly, photocatalytic systems (Scheme 3). ${}^{32}$

Scheme 3. Different Routes for Selective Radical Addition to Double Bonds

In traditional Cu-mediated Meerwein arylations, the substrate scope is usually limited in vinyl esters and other electron-deficient alkenes. This classical approach was expanded using $N$ -substituted maleimides 1 as the substrates catalyzed by a substoichiometric amount of $\mathrm{{CuCl}}$ , affording 3- arylmaleimides 3 with high yields (Scheme 4). ${}^{33}$ Particularly, this methodology provided access to two classes of products by changing the transition-metal reagents. In the presence of ${\mathrm{{TiCl}}}_{3}$ , this reaction generated 3-arylsuccinimides 2 instead.

Scheme 4. Synthesis of 3-Arylsuccinimides and 3- Arylmaleimides

Given the cost of transition metal catalysis, a transition-metal-free direct $\mathrm{C} - 3$ arylation of quinoxalin-2(1H)-ones 4 has also been explored in the presence of ${\mathrm{{MeSO}}}_{3}\mathrm{H}$ (Scheme 5). ${}^{34}$ Using arylamines as the substrates, this work represents a metal-free protocol for Meerwein arylations at room temperature. The plausible mechanism involves the generation of aryl radical through the single-electron reduction of aryl diazonium mesylate. The aryl radical then adds to protonated quinoxalin- $2\left( {1H}\right)$ -one, followed by single-electron transfer (SET) and deprotonation to deliver the desired product 5.

The organic promoter-mediated protocol was illustrated by Kanai and co-workers, who developed metal-free C(3)-H selective arylation of coumarins 6 catalyzed by 5,10,15,20- tetrakis(4-diethylaminophenyl)porphyrin 8 (Scheme 6). ${}^{35}$ This metal-free Meerwein arylation provided 3-arylcoumarins 7 in moderate yields. This reaction is initiated by one-electron reduction of aryl diazonium salts with porphyrin, affording aryl radical with extrusion of dinitrogen gas. The addition of the aryl radical to coumarin then furnishes a benzyl radical intermediate 9, which is oxidized by either the porphyrin radical cation or the aryl diazonium substrate.

Scheme 5. Direct C-3 Arylation of Quinoxalin-2(1H)-ones with Arylamines

Scheme 6. Porphyrin-Promoted Meerwein C(3)-H Arylation of Coumarins

Another improvement in this area is the combination of photoredox catalysis. Mainly, eosin Y is an effective organic photocatalyst for the redox reactions involving aryl diazonium salts. ${}^{{36},{37}}$ In 2016, the visible-light-induced coupling of aryl diazonium salts with nitroalkenes 10 was developed using eosin Y as the photosensitizer, affording stilbene derivatives 11 (Scheme 7). ${}^{38}$ This protocol uses green LEDs as the light source. A radical mechanism was proposed for this reaction, with nitroalkenes 10 serving as the radical acceptor. After the radical addition, the nitro group is then cleaved via homolytic fragmentation.

Scheme 7. Visible-Light-Induced Cross-Coupling of Aryl Diazonium Salts with Nitroalkenes

On the other hand, the past decade has witnessed a resurgence of organic electrochemistry. In 2019, the electrochemical Minisci-type selective arylation of quinoxalinones 4 and aryl diazonium salts was realized, which represents an alternative route to Cu-catalyzed Meerwein arylation or visible-light-induced protocol (Scheme 8). ${}^{39}$ The success of this electrochemical arylation approach to quinoxalin-2(1H)-ones 4 has demonstrated the feasibility of performing radical-type $\mathrm{C} - \mathrm{H}$ arylation of aryl diazonium salts under paired electrolysis conditions. On the basis of control experiments and CV analysis, a reaction mechanism is proposed which involves the initial reduction of aryl diazonium salts at the cathode’s surface to generate corresponding phenyl diazo radicals 12. After the loss of dinitrogen, the resulting aryl radical adds to quinoxalin- 2 (1H)-one to generate $N$ -centered radical intermediate 13. Subsequently, this intermediate is oxidized at the anode to afford the corresponding product. An alternative mechanism involves the homogeneous electron transfer of 13 with the aryl diazonium species. This gives rise to the product 5 and regenerates 12, thus initiating a radical chain process.

Scheme 8. Electrochemical Cross-Coupling of Quinoxalin- 2(1H)-one with Aryl diazonium Salts

2.1.2. Difunctionalization of Alkenes. The difunction-alization of alkenes can be achieved through the selective radical addition to double bonds upon the employment of transition-metal catalysis, organic promoters, photoredox or electrolysis conditions (Scheme 9). The reaction is initiated by the decomposition of aryl diazonium salts through an SET process. The resultant aryl radical is subsequently trapped by alkenes to afford an alkyl radical intermediate 14, which undergoes single-electron oxidation to afford a carbenium ion 15. Finally, an intra- or intermolecular nucleophilic reaction occurs to afford the difunctionalization product. This difunctionalization strategy allows the synthesis of various functional molecules, including oxyarylation, aminoarylation, fluoroarylation, arylthio(seleno)cyanation products, and a variety of cyclic compounds. This section will sum up the representative advances according to the type of pathways to generate aryl radicals.

Scheme 9. General Routes for Difunctionalization of Alkenes

2.1.2.1. Transition-Metal Catalysis. Traditional radical addition to alkenes from aryl diazonium salts needs the participation of copper salts or other transition metals. A practical application of this strategy was disclosed through a Cu(II)-catalyzed bromoarylation reaction using 2-chloroacry-lonitrile 16 with in situ formed aryl diazonium salts, accessing the preparation of 2-bromo-2-chloro-3-arylpropanenitriles 17 with moderate to good yields in acetone/water (Scheme 10, up). ${}^{40}$ Another similar protocol involved the Fe(II)-catalyzed oxyarylation of electron-rich olefins 18, including styrenes, vinyl pyrrolidinone, and vinyl oxazolidinone with aryl diazonium tetrafluoroborates, furnishing corresponding carbo-hydroxylated and carbomethoxylated products 19, including functionalized dihydroisocoumarin and dihydrobenzofuran derivatives (Scheme 10, below). ${}^{41}$ These reactions involve single-electron transfer with the $\mathrm{{Cu}}\left( \mathrm{{II}}\right) /\mathrm{{Cu}}\left( \mathrm{I}\right)$ or $\mathrm{{Fe}}\left( \mathrm{{III}}\right) /$ Fe(II) redox pair, respectively.

Scheme 10. Cu- and Fe-Catalyzed Difunctionalization of Alkenes

In recent years, incorporating a trifluoromethylthio group into an organic molecule has received extensive attention. A recent representative advance of Cu-mediated difunctionaliza-tion is the aryltrifluoromethylthiolation of styrenes 20 by the reaction with aryl diazonium salts and ${\mathrm{{CuSCF}}}_{3}$ under room temperature (Scheme 11). ${}^{42}$ An oxidative radical intermolecular aryltrifluoromethylthiolation of styrenes with aryl diazonium salts and ${\mathrm{{CuSCF}}}_{3}$ has been established. Mechanistically, dimeric complex $\mathrm{{Cu}}\left( \mathrm{I}\right) - {\mathrm{{SCF}}}_{3}$ was in situ oxidized by the aryl diazonium salt to produce the corresponding $\mathrm{{Cu}}\left( \mathrm{{II}}\right) - {\mathrm{{SCF}}}_{3}$ species and aryl radicals. The selective addition of the aryl radical to styrenes resulted in the formation of benzyl radical 22. Two reaction pathways with $\mathrm{{Cu}}$ (II)- ${\mathrm{{SCF}}}_{3}$ were proposed. In the first mechanism, this benzyl radical can be trapped by $\mathrm{{Cu}}\left( \mathrm{{II}}\right) - {\mathrm{{SCF}}}_{3}$ to generate alkyl-Cu(III) species 23, followed by reductive elimination to afford the trifluoromethylthioarylation product 21. In the latter case, the benzyl radical is oxidized by Cu(II) to generate a carbocation 24, which then reacts with the ${\mathrm{{CF}}}_{3}\mathrm{\;S}$ anion to obtain the final product 21.

Scheme 11. Intermolecular Trifluoromethylthioarylation of Styrenes with Aryl Diazonium Salts and CuSCF ${}_{3}$

Serving as an extension of a copper-catalyzed protocol, a silver-catalyzed intermolecular and intramolecular Meerwein fluoroarylation of styrenes with aryl diazonium salts was also disclosed in 2015 (Scheme 12). ${}^{43}$ This fluoroarylation reaction is promoted by AgOTf at room temperature, and Selectfluor is used as the fluorine source. The intramolecular procedure can be applied to synthesize fluorinated heterocycles, including dihydrobenzofurans 25 and indolines 26.

2.1.2.2. Transition-Metal-free Approaches. Apart from the employment of transition-metal catalysis, another approach involves the transition-metal-free difunctionalization, in which the radical procedure was initiated under thermal or base-initiated conditions. In the former case, Heinrich developed a transition-metal-free hydroxyarylation of styrenes 20 with aryl diazonium tetrafluoroborates (Scheme 13). ${}^{44}$ It was proposed that the diazonium salt itself act as an oxidant to propagate the radical chain via a radical-polar crossover mechanism. This approach allows the synthesis of tertiary benzylic alcohols 27 or esters under mild conditions.

An electron-rich complex-induced carboalkoxylation of styrenes 20 with aryl diazonium salts was realized at room

Scheme 12. Ag-Catalyzed Inter- and Intramolecular Meerwein Fluoroarylation of Styrenes

Scheme 13. Thermally Induced Carbohydroxylation of Styrenes with Aryl Diazonium Salts

temperature (Scheme 14). ${}^{45}$ This Meerwein addition proceeded in the presence of a phenalenyl catalyst 28 (PLY) at room temperature, allowing the difunctionalization of styrene using alcohol as the alkoxy donor. On the basis of the experimental results and detailed DFT calculations, the authors proposed a mechanism in which a [PLY-K] complex serves as an electron carrier to catalyze the decomposition of the diazonium salt and the formation of the product.

Scheme 14. Light-free Phenalenyl-Catalyzed Carboalkoxylation of Styrenes

Using arylamines as the aryl radical precursors, the arylesterification of $\gamma$ -alkenoic acids 29 was developed under room temperature (Scheme 15). ${}^{46}$ Using salicylic acid and water facilitated this process, affording various $\gamma ,\gamma$ -disubstituted butyrolactones 30. Salicylic acid was proposed to play an important role in the generation of the aryl radical through N-O homolytic cleavage of in situ formed aryl diazobenzoate 31.

Scheme 15. Salicylic Acid Promoted Arylation-Lactonization Sequence of $\gamma$ -Alkenoic Acids with Anilines

2.1.2.3. Photoredox Approaches. The photoredox protocol provides a complementary route to generate an aryl radical under mild conditions. The aryl radical intermediate is subsequently intercepted by alkenes selectively to deliver difunctionalized products or carbonyl compounds. In 2015, a visible-light-induced oxygenative Meerwein arylation of 32 was reported, affording a straightforward approach to $\alpha$ -aryl esters 33 in moderate to good yields (Scheme 16). ${}^{47}$ By intercepting the incipient carbon-centered radical with dioxygen, this radical arylation process furnished $\alpha$ -aryl esters 33 as the final products upon the loss of the cyanide.

In 2018, using various porphyrins as the SET photocatalysts, the photoinduced arylation of enol acetates 34 with aryl diazonium salts was developed both in batch and continuous-flow conditions, affording $\alpha$ -arylated aldehydes and ketones 35 in excellent yields (Scheme 17). ${}^{48}$ This process is easily scaled-up under continuous-flow conditions. Mechanistically, upon blue light irradiation, the porphyrin catalyst is excited from the steady state (PS) to the singlet state ( ${}^{1}$ PS*). Intersystem crossing provided the triplet state $\left( {{}^{3}\mathrm{{PS}} * }\right)$ . The triplet species can reduce the diazonium salts to generate an aryl radical, which is trapped by enol acetates 34 to yield the final products.

Scheme 16. Visible-Light-Induced Meerwein Cascade Reaction for the Preparation of $\alpha$ -Aryl Esters

Scheme 17. Porphyrin-Catalyzed Arylations of Enol-Acetates in Batch and Continuous-Flow Conditions

Aryl diazonium salts can serve as effective aryl radical precursors upon visible-light irradiation to enable olefin difunctionalization. For example, the oxyarylation and amino-arylation of styrenes 20 with diazonium salts under photoredox catalytic conditions were realized (Scheme 18), ${}^{49}$ allowing for the addition of an aryl group and a heteroatom across the styrene double bond. This reaction involves the initial reduction of the aryl diazonium salts with photoexcited Ir(III)* to provide an aryl radical. The aryl radical then adds to the styrene $\mathrm{C} = \mathrm{C}$ double bond, forming benzylic radical 36 and then is oxidized by Ir(IV) to benzylic cation 37. Further reaction with the nucleophile then affords three-component coupling products.

When the reaction was performed in DMF, intermolecular formyloxyarylation of alkenes was accomplished in good yields. ${}^{50}$ Here, the nascent benzylic cation is intercepted by DMF to generate the iminium ion intermediate 38. Hydrolysis of the iminium ion 38 yields the formyloxyarylation product 39.

Similarly, using alcoholic solvents, an analogous visible-light-induced alkoxyarylation of olefins with aryl diazonium salts could be realized using a tris(bipyridine)ruthenium catalyst. ${}^{51}$ This simple and versatile method allows the preparation of various alkoxyarylation products under mild reaction conditions, with a low catalyst loading and a short reaction time.

The substrate scope of this visible-light photocatalytic arylation was expanded to olefinic carboxylic acids 40, affording $\gamma ,\gamma$ -disubstituted butyrolactones 41 as the products (Scheme 19). ${}^{52}$ This process is initiated by aryl diazonium salts oxidative quenching of the excited state of the photocatalyst. Regioselective addition of the aryl radical to the alkene moiety generates a tertiary carbon radical intermediate 42. The oxidation of this species by a $\mathrm{{Ru}}$ (III) catalyst affords tertiary carbocation 43, which then undergoes intramolecular nucleophilic addition with the carboxylic acid to form $\gamma ,\gamma$ - disubstituted butyrolactones 41.

Scheme 18. Photocatalytic Oxyarylation and Aminoarylation of Styrenes with Iodonium or Diazonium Salts

Scheme 19. Photocatalytic Arylation-Lactonization Sequence of Olefinic Carboxylic Acids with Aryl Diazonium Salts

In 2017, the photoinduced difunctionalization reactions were applied by König and Fagnoni to the preparation of isochromanones using 2-carboxy-substituted aryl diazonium salts. ${}^{53}$ In the same year, the photocatalyzed reactions of 2- (alkoxycarbonyl)benzenediazonium tetrafluoroborates with various alkenes were disclosed by Basso, affording isochroma-nones in good yields under batch and flow conditions. ${}^{54}$

In 2020, the domino reaction of ortho-hydroxyarylenami-nones 44 with in situ photogenerated aryl radicals was realized, enabling the assembly of 3-aryl-substituted chromenes 45 with moderate to high yields (Scheme 20). ${}^{55}$

Scheme 20. Visible Light-Mediated Arylation of ortho-Hydroxyarylenaminones to Synthesize Isoflavones

Besides carbooxygenation of olefins, other types of alkene difunctionalization have also been established. The photoredox intermolecular aminoarylation of alkenes under visible-light irradiation was realized, leading to amides as well as 3-aryl-3,4- dihydro isoquinoline 46 (Scheme 21). ${}^{56}$ The proposed mechanism involves the initial formation of the aryl radical by a single-electron transfer from the excited state of the photocatalyst Ru(II)* by aryl diazonium salts. The addition of the aryl radical to an alkene affords the corresponding radical intermediate 47, which is subsequently oxidized to give a carbocation 48. Finally, 48 is intercepted by a nitrile, followed by hydrolysis to obtain the aminoarylation product.

Scheme 21. Photoredox Intermolecular Aminoarylation of Alkenes

In 2019, Maity reported a photoredox-catalyzed, three-component intermolecular arylthiocyanation/arylselenocyana-tion of alkenes with diazonium salts (Scheme 22). ${}^{57}$ Using ammonium thiocyanate and potassium selenocyanate as the thiocyanate/selenocyanate source, chemoselective difunction-alization of alkenes was done under visible-light irradiation, affording aryl-substituted alkylthiocyanates and alkylselenocya-nates with moderate to high yields. On the basis of the Stern-Volmer quenching experiment, an SET process between the photoexcited Ru(II)* and the aryl diazonium salt leads to the aryl radical. Subsequent radical addition and radical-polar crossover provide the carbenium ion, which reacts with ${\mathrm{{SCN}}}^{ - }$ to give the product.

Scheme 22. Photoredox-Catalyzed Intermolecular Radical Arylthiocyanation/Arylselenocyanation of Alkenes

2.1.3. Radical Addition to Aromatic Compounds. The aryl radical addition to aromatic compounds with aryl diazonium salts, which was first discovered by Gomberg and Bachmann, ${}^{8}$ is a highly useful method for synthesizing unsymmetrical biaryls. Several outstanding reviews have discussed biaryl synthesis with arene diazonium salts over the past six years. ${}^{{58} - {63}}$ This radical-type C-H bond arylation has attracted interest from the synthetic community. ${}^{{64},{65}}$ Recent advances in this area mainly focus on developing transition-metal-catalyzed, organic initiator-mediated, photoinduced, and electrolysis processes (Scheme 23). The transition-metal-catalyzed $\mathrm{C} - \mathrm{H}$ bond functionalization and dual photoredox/ transition-metal-catalyzed approaches will be summarized in the following section.

2.1.3.1. Transition-Metal Catalysis. The addition of transition-metal can facilitate the SET process of aryl diazonium ions to generate aryl radicals. This strategy’s main advances are the extension of substrate scope and the efficiency of the reactions. Employing AgONO for the diazotization of anilines under acid-free conditions, $\mathrm{{Cu}}{\left( \mathrm{{OAc}}\right) }_{2}$ -catalyzed direct $\mathrm{C} - \mathrm{H}$ arylation of $N$ -methylpyrroles, furans, and thiophenes was reported in 2014 (Scheme 24). ${}^{66}$ This one-pot diazotization/arylation protocol provides a new approach for the $\mathrm{C} - \mathrm{H}$ arylation of electron-rich heteroarenes, affording a variety of heterobiaryl products in moderate to good yields.

A broader range of Cu-promoted Gomberg-Bachmann reactions was reported (Scheme 25). A one-pot diazotization/

Scheme 23. Different Methods for Arenes C-H Bond Arylations via Aryl Radicals

Beyond the Gomberg-Bachmann reaction

Scheme 24. Cu-Catalyzed C-H Arylation of Heteroarenes by Anilines with AgONO as the Diazotization Agent

Scheme 25. Cu-Catalyzed Arylations of Anilines

$\mathrm{{Cu}}{\left( \mathrm{{OAc}}\right) }_{2}\left( {{10}\mathrm{{mol}}\% }\right)$

$t$ -BuONO (1.2 equiv.)

${\mathrm{{Ar} - {NH}}}_{2}$

${\mathrm{{MeSO}}}_{3}\mathrm{H}$ (1 equiv.)

${\mathrm{{CaCO}}}_{3}$ (1 equiv.)

acetone ${\mathrm{H}}_{2}\mathrm{O},{25}^{ \circ }\mathrm{C}$

R = Boc, Ts 12 examples

28-83% yields

$\mathrm{{Cu}}{\left( \mathrm{{OAc}}\right) }_{2}\left( {{20}\mathrm{\;{mol}}\% }\right)$

$t$ -BuONO (3 equiv.)

$\mathrm{{Ar}} - {\mathrm{{NH}}}_{2}$

${\mathrm{{Cs}}}_{2}{\mathrm{{CO}}}_{3}$ (1 equiv.)

DMSO, ${50}^{ \circ }\mathrm{C}$

$X = O, S$ 26 examples

50-82% yields

Cu-catalyzed regioselective C2-arylation of $N$ -Boc pyrroles with anilines was disclosed. ${}^{67}$ These reactions were carried out at an ambient temperature in degassed aqueous acetone using $\mathrm{{Cu}}{\left( \mathrm{{OAc}}\right) }_{2}$ and $t$ -BuONO/ ${\mathrm{{MeSO}}}_{3}\mathrm{H}$ as the diazotizing agents and ${\mathrm{{CaCO}}}_{3}$ as the base, leading to $\mathrm{C}2$ -arylated pyrroles in high yields. A similar arylation protocol was realized to afford 2-aryl-substituted benzoxazoles and benzothiazoles. ${}^{68}$

Aryl diazonium salts were also used for the regioselective modification of BODIPY dyes 49 to fine-tune their emissive properties (Scheme 26). ${}^{69}$ This radical $\mathrm{C} - \mathrm{H}$ arylation used ferrocene as the promoter to generate aryl radical from aryl diazonium salts and allowed for the synthesis of brightly fluorescent $\left( {\Phi > {0.85}}\right)$ arylated boron dipyrrins. The role of ferrocene in aryl radical formation was confirmed by cyclic voltammetry. ${}^{70}$ Additionally, direct and sequential arylations of pyrrole with aryl diazonium salt can also be initiated by the SET process catalyzed by cobaltocene at room temperature. ${}^{71}$

Scheme 26. ${\mathrm{{Cp}}}_{2}\mathrm{{Fe}}$ -Catalyzed Arylation of BODIPYs

Gomberg-Bachmann arylation of pyridine usually leads to a mixture of regioisomers. Aiming to achieve the regioselective arylation of pyridine, a regioselective radical arylation of 3- hydroxypyridine 50 was realized using a stoichiometric amount titanium(III) complex. This process takes advantage of the C-3 hydroxy directing effect and the protonated pyridine nitrogen to access C-2 arylated products (Scheme 27). ${}^{72}$ Using aryl diazonium chlorides, a range of 3-hydroxy-2-phenylpyridines 51 were obtained in moderate to good yields. This methodology also enables a rapid assembly of tricyclic benzopyr-anopyridinones 52.

Scheme 27. Ti(III)-Mediated Radical Arylation of 3- Hydroxypyridines

2.1.3.2. Organocatalysis Approaches. Using organic initiators instead of transition-metal species for radical arylations using diazonium salts is desirable from a green chemistry perspective. These organic initiators should be redox-active molecules to serve as efficient electron donors and acceptors. In 2014, Martin and Carrillo reported a metal-free Gomberg-Bachmann reaction using ascorbic acid 53 as the radical initiator (Scheme 28). ${}^{73}$ This research represents the first of organic reductant for such radical processes. Additionally, in situ generations of aryl diazonium salts from anilines can be achieved in the presence of 53. Good yields of biaryl products are obtained for strongly electron-deficient diazonium salts and electron-rich (hetero)arenes. In 2016, the same group disclosed a radical $\mathrm{C} - \mathrm{H}$ arylation of (hetero)arenes with aryl diazonium tetrafluoroborates catalyzed by gallic acid in water-acetone at room temperature. ${}^{74}$ Biowastes rich in gallic acid, such as grape pomace, can also promote this arylation process.

Scheme 28. Ascorbic Acid Initiated Direct C-H Arylation of (Hetero)arenes with Anilines

In 2018, the same group reported heteroarenes’ radical arylation with anilines induced by aqueous sodium carbonate at room temperature. ${}^{75}$ This method was applied to the modification of naphthylimide or bisthienylcyclopentenes. The key step for generating aryl radicals involves the thermal fragmentation of a diazo anhydride, which is formed from aryl diazonium salts under basic aqueous conditions.

After Martin and Carrillo’s research, several in situ diazotization/radical arylations using various electron transfer agents have been developed. ${}^{76}$ In the presence of hydrogen peroxide, a C-H arylation of heteroarenes with in situ generated aryl diazonium compounds was reported (Scheme 29). ${}^{77}$ DFT calculations support a mechanism involving a covalent adduct of tert-butoxide and aryl diazonium, allowing for inner sphere electron transfer to occur. Besides, the graphene oxide has also been explored as a catalyst and/or initiator in the arylation of heteroarenes. ${}^{78}$ The localized radicals on reduced graphene oxide play a crucial role in generating an aryl radical.

Moreover, direct $\mathrm{C} - \mathrm{H}$ arylation of heterocyclic $N$ -oxides was developed (Scheme 30). ${}^{{79} - {81}}$ In 2016, Horan reported an additive-free method for the $\mathrm{C} - \mathrm{H}$ arylation of heterocyclic $N$ - oxides (Scheme 30a). ${}^{79}$ This chemistry delivered 2-phenyl-quinoline- $N$ -oxide, a key intermediate en route to the NK-3 receptor antagonist SB222200 (54), in 70% yield. The C2- arylation of quinoline- $N$ -oxides can be efficiently carried out with aryl diazonium salts in the presence of a base without exogenous reductants (Scheme 30b). ${}^{80}$

Scheme 29. Hydrogen Peroxide Promoted C-H Arylation of Heteroarenes

Scheme 30. C-H Arylation of Heterocyclic N-Oxides through in situ Diazotization of Anilines

Hydrazines were also explored as a new family of low molecular-weight catalysts to promote $\mathrm{C} - \mathrm{H}$ arylation using aryl diazonium salts (Scheme 31). ${}^{82}$ A range of substrates, including electron-rich heterocycles, vinyl acetates, and quinones, could be effectively arylated under these conditions.

Mandal and co-workers explored C-H arylation of heteroarenes using PLY 28 under photofree conditions (Scheme 32). ${}^{{83} - {85}}$ The mechanism is like reactions using this class of promoters.

In 2018, the same group developed a Lewis acid-redox dual catalytic system using the phenalenyl derivative with one-electron redox characteristics (Scheme 33). ${}^{84}$ The proposed mechanism starts with the single-electron reduction of phenalenyl cation 55 using an organic electron donor tetrakis(dimethylamino)ethylene (TDAE), leading to the phenalenyl radical 56. This radical then reacts with the aryl diazonium salt to generate an aryl radical and the cationic species 57. The authors also disclosed a protocol for the $\mathrm{C} - \mathrm{H}$ arylation of arenes and heteroarenes using a phenalenyl-supported iron(III) catalyst and potassium as the reductant. ${}^{85}$

Redox-active polyanilines have also been explored as a radical promoter (Scheme 34). ${}^{{86},{87}}$ Amaya and Hirao employed polyaniline 58 for the Gomberg-Bachmann reaction from aryl diazonium salts ${}^{86}$ or anilines, ${}^{87}$ giving the cross-coupling product in moderate to good yields. Only 1 mol % (based on aniline tetramer) of the polyaniline 58 was required for this arylation reaction. Polyaniline is sufficiently reducing to convert various arene diazonium salts to aryl radicals.

Scheme 31. Hydrazine-Catalyzed Reductive C-C Bond Formation

Scheme 32. Phenalenyl-Based Radicals in the $\mathrm{C} - \mathrm{H}$

Arylation of Heteroarenes

In 2019, 1,3,5-triphenylpyrazolines 59 was explored as an organic catalyst for the $\mathrm{C} - \mathrm{H}$ arylation of heteroarenes using aryl diazonium salts (Scheme 35). ${}^{88}$

Apart from the protocols using aryl diazonium ions as aryl radical precursors, the radical arylations of heteroarenes with in situ formed aryldiazotates was developed in strongly alkaline aqueous solution conditions. ${}^{89}$ Recently, the diazene species have been explored as the aryl radical precursors in Gomberg-Bachmann arylation of arenes, in which the Hantzsch ester 60 transfers a hydride to the aryl diazonium substrate. ${\mathrm{O}}_{2}$ then induces fragmentation of the diazene intermediate 61 to produce an aryl radical for subsequent arylation (Scheme 36). ${}^{90}$

Scheme 33. Phenalenyl-Based Lewis Acid-Redox Dual Catalytic System

Scheme 34. Polyaniline-Induced C-H Arylation of Arenes

Scheme 35. 1,3,5-Triphenylpyrazoline Catalyzed C-H Arylations of Heteroarenes

Scheme 36. Hantzsch Ester-Mediated Generation of Aryl Radicals

2.1.3.3. Visible-Light-Induced Arylations. In the previous introduction, the radical arylation of arenes with diazonium salts should be facilitated by the addition of transition-metals or redox-active organic initiators, including ascorbic and gallic acid, ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ , graphene, hydrazines, PLY, and polyanilines. Furthermore, the photoredox catalysis strategy represents a complementary and mild approach to synthesize (hetero)- biaryls. A pioneering work using a photoredox catalyst was communicated by König for the arylation of heteroarenes with aryl diazonium salts by using eosin Y as an organic photoredox catalyst under green light irradiation at ${530}\mathrm{\;{nm}}$ (Scheme 37). ${}^{91}$

Scheme 37. Eosin Y-Catalyzed Visible-Light-Induced Arylations of Heteroarenes

This photoredox pathway is highly dependent on the nature of the light source, solvent, and $\mathrm{{pH}}$ of the medium. ${}^{92}$ This photoredox protocol is mechanistically initiated by the oxidative quenching process of the excited state of the photocatalyst (eosin Y* or Ru(II)*) with an aryl diazonium salt to generate an aryl radical. This aryl radical undergoes subsequent addition to arenes, followed by single-electron oxidation and final deprotonation to obtain biaryls. The hypervalent photocatalyst can facilitate the single-electron oxidation of the resulting radical adduct. This process is independent of the aryl diazonium ion’s redox potential, which plays an important role in the radical-chain pathway. Thus, both electron-rich and -deficient aryl diazonium salts can smoothly react in the presence of photosensitizers.

Since this report by König, several photoredox catalysts have been reported to promote the photocatalytic Gomberg-Bachmann reaction. Recently, organic photoredox catalysts have demonstrated powerful applications in the visible-light-induced transformations of aryl diazonium salts. ${}^{{93} - {98}}$ Notably, eosin Y-catalyzed visible-light-induced intermolecular arylation of anilines, ${}^{99}$ as well as intramolecular arylation of aryl diazonium tetrafluoroborates, ${}^{100}$ was developed (Scheme 38).

Scheme 38. Photoredox Eosin Y-Catalyzed Arylation Reactions of Aryl diazonium Salts

Besides eosin Y, more organic dyes, including iodo-Bodipys 62, ${}^{101}$ electron-deficient porphyrins 63, ${}^{102}$ Rhodamine B, ${}^{103}$ and 9,10-dihydro-10-methylacridine, ${}^{104}$ have been utilized as photosensitizers to promote the visible-light-induced arylations of diazonium salts (Scheme 39).

Moreover, the ruthenium(II) complex has emerged as efficient photosensitizers for catalyzing $\mathrm{C} - \mathrm{H}$ arylations of aryl diazonium salts. ${}^{{105} - {109}}$ In 2014, the direct arylation of $N$ - heteroarenes with aryl diazonium salts was successfully implemented in water, using $\mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\mathrm{{Cl}}}_{2} \cdot 6{\mathrm{H}}_{2}\mathrm{O}$ as a photosensitizer and a commercial household light bulb as a light source (Scheme 40). ${}^{105}$ This water-phase arylation reaction provides a variety of arylated heterocycle scaffolds including xanthene, thiazole, pyrazine, pyridazine, and pyridine with good regioselectivity.

Isoquinoline alkaloids are attractive natural products due to their diverse chemical structures and bioactivities. The direct $\mathrm{C} - \mathrm{H}$ arylation of isoquinoline with aryl diazonium salts using TFA as an additive and $\mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\mathrm{{Cl}}}_{2} \cdot 6{\mathrm{H}}_{2}\mathrm{O}$ as the photoredox catalyst provides efficient access to substituted isoquinoline derivatives (Scheme 41). ${}^{106}$ With this visible-light-induced synthetic methodology, the concise total syntheses of two isoquinoline alkaloids menisporphine 64 and daurioxoisopor-phine C 65 have been accomplished.

In 2015, Maestri and Malacria utilized Ru(II)-catalyzed photoredox C-H arylation of unactivated arenes with aryl diazonium salts under visible-light irradiation (Scheme 42). ${}^{107}$ High yields of biaryls are obtained only when a large excess of arenes (40 equivalence) is used during the reaction. In 2016, Natarajan reported photoredox intramolecular cyclization of in situ formed aryl diazonium salts from the reactions of $N$ -(2- aminoaryl)benzoimines 66 and tert-butyl nitrite (Scheme 42). ${}^{108}$ Utilizing $\mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\mathrm{{Cl}}}_{2}$ as a photoredox catalyst and a blue LED as an irradiation source, this mild protocol affords 6- aryl phenanthridines 67 with good yields.

Scheme 39. Organic Photosensitizer-Promoted Photoredox C-H Arylation of Heteroarenes

Recently, from the substrates of arylamines as safe aryl radical precursors, the selective C-H arylation of pyridines was realized upon visible-light irradiation, allowing the synthesis of 2-aryl pyridines and 2-aryl bipyridines with moderate to good yields (Scheme 43). ${}^{109}$

Apart from utilizing homogeneous photosensitizers, a series of metal oxides have demonstrated promise as efficient heterogeneous catalysts for visible-light-induced radical-type arylation reactions, opening up the green protocol of a cost-effective, sustainable operation through catalyst recycling. In 2015, a photoredox direct arylation of electron-rich hetero-arenes had been disclosed with an easily accessible and recyclable, heterogeneous ${\mathrm{{TiO}}}_{2}$ catalyst (Scheme 44). ${}^{110}$ Mechanistically, ${\mathrm{{TiO}}}_{2}$ -diazoether 68 was detected as the key reactive intermediates, which, upon visible-light irradiation, promoted one electron from the valence band to the metal oxide’s conduction band and, in turn, reduced the diazoether to the aryl radical species. The aryl radical then reacts with heteroarenes via a radical chain pathway. The radical intermediate is oxidized by the hole at the valence band to furnish a cation species, losing a proton to give the final product.

Scheme 40. Ru(II)-Catalyzed Direct Arylation of $N$ - Heteroarenes with Aryl Diazonium Salts in Water

Other heterogeneous catalytic systems, including a three-dimensional porous ${\mathrm{{Fe}}}_{3}{\mathrm{O}}_{4}@{\mathrm{{Cu}}}_{2 - x}\mathrm{\;S} - {\mathrm{{MoS}}}_{2}$ framework, ${}^{111} \; {\mathrm{{Bi}}}_{2}{\mathrm{O}}_{3},{}^{112}$ and $2\mathrm{D}{\mathrm{{MoS}}}_{2}{}^{113}$ were found to exhibit catalytic reactivity in photoredox arylation of heteroarenes with aryl diazonium salts (Scheme 45). ${}^{111}$ The terminal sulfur groups, or the single-electron in the conductance band, can facilitate the electron transfer to the diazonium salt to generate an aryl radical.

Furthermore, visible light-promoted, catalyst-free Gomberg-Bachmann arylations have also been developed. ${}^{{114} - {116}}$ In situ formation of photoactive electron donor-acceptor (EDA) complexes from aryl diazonium salts and arenes eliminates the need for a photocatalyst. In 2017, Heinrich achieved the blue LED-irradiated radical arylation of diverse arenes and heteroarenes of aryl diazonium salts in the absence of any photocatalyst and other additives (Scheme 46). ${}^{114}$ In most cases, the substrate scope in this process is limited in electron-deficient aryl diazonium salts with electron-rich arenes due to the effective formation of a visible light-absorbing charge-transfer complex.

Scheme 42. Ru(II)-Catalyzed Photoredox C-H Intermolecular Arylation of Unactivated Arenes and Intramolecular Cyclization of N-(2-Aminoaryl)benzoimines

$\mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\mathrm{X}}_{2}\left( {{0.5}\mathrm{\;{mol}}\% }\right)$

$\mathrm{{Ar}} - {\mathrm{N}}_{2}{\mathrm{{BF}}}_{4}$ + Ar

MeCN, 1-96 h, 25 ${}^{ \circ }\mathrm{C}$

(40 equiv.)

blue LED

34 examples

$X = {\mathrm{{SbF}}}_{6},\mathrm{{Cl}}$ 32-92% yields

Me F

${\mathrm{{NO}}}_{2}$ F

80% 91% 71%

o/m/p = 1.8/1.3/1

$- {\mathrm{{NO}}}_{2}$

OMe

72% 34% 41%

o/m/p = 3.3/1/1.5

NH2

$t$ -BuONO (1.3 equiv.)

Ru(bpy) ${}_{3}{\mathrm{{Cl}}}_{2}$ (1 mol%)

DMSO, Ar, 6 h, rt

5 W blue LED

66 67, 13 examples

85-94% yields

Scheme 43. Photoredox Synthesis of 2-Arylpyridines and 2-

Arylbipyridines

Ru(bpy) ${\left( {\mathrm{{PF}}}_{6}\right) }_{2}\left( {2\mathrm{\;{mol}}\% }\right)$

$t$ -BuONO (1.2 equiv.)

${\mathrm{{CF}}}_{3}{\mathrm{{CO}}}_{2}\mathrm{H}$ (6.5 equiv.)

$\mathrm{{Ar}} - {\mathrm{{NH}}}_{2}$ R II R II

MeCN, Ar, 3 h, rt

24 W blue LED

(4 equiv.) C2 or C4

21 examples

34-84% yields

Ir[df(CF ${}_{3}$ )ppy] ${}_{2}$ (dtbpy)) ${\mathrm{{PF}}}_{6}$

(1 mol%)

$t$ -BuONO (1.2 equiv.)

${\mathrm{{CF}}}_{3}{\mathrm{{CO}}}_{2}\mathrm{H}$ (6.5 equiv.)

$\mathrm{{Ar}} - {\mathrm{{NH}}}_{2}$

MeCN, Ar, 12 h, rt

24 W blue LED

(4 equiv.) 4 examples

45-61% yields

To further expand the substrate scope of this photosensitizer-free photoredox chemistry, electron-rich heterocycles were investigated for their ability to form photoactive EDA complexes. As a result, pyridine is amenable to the visible light-promoted photocatalyst-free arylation of (halo)arenes ${}^{115}$ or

Scheme 44. Heterogeneous ${\mathrm{{TiO}}}_{2}$ -Catalyzed C-H Arylation of Heteroarenes

Scheme 46. Visible-Light-Induced Catalyst-Free Radical ${2H}$ -indazoles ${}^{116}$ (Scheme 47). In this catalyst-free transformation, the key intermediate is an electron donor-acceptor complex between aryl diazonium salts and pyridine, which subsequently generates an aryl radical through the photo-excitation SET process.

Scheme 47. Visible Light-Promoted Gomberg-Bachmann Reactions to Form Biaryls and C3-Arylated-2H-Indazoles

2.1.3.4. Electrochemical Approach. From the perspective of generating aryl radicals, the electrolytic process is considered as a complementary method for the Gomberg-Bachmann reaction. In 2018, Amaya et al. utilized the electrochemical method to the Gomberg-Bachmann reaction of aryl diazonium salts with heteroarenes (furans, pyrroles, thio-phenes, and mesitylene) in a simple undivided cell (Scheme 48). ${}^{117}$ However, the substrate scope of aromatics is limited to electron-rich arenes. A radical chain pathway and cathodic reduction of aryl diazonium salts followed by an anodic oxidation pathway are both proposed.

Scheme 48. Electrolytic Arylation of Aryl Diazonium Salts and Heteroarenes

mechanism

In 2019, the electrochemical method was applied to achieve the radical-type arylation of electron-deficient aromatics with aryl diazonium salts through reductive activation (Scheme 49). ${}^{118}$ Various electron-deficient arenes, including quinox-alinone, pyrazine, naphthyridine, pyridazine, quinoline, and isoquinoline derivatives, are amenable to this transformation within an undivided cell. Mechanistically, EPR results, control experiments, CV, and DFT calculations indicate that quinoxaline reduction serves as the vital step to achieve this electrolytic transformation. The in situ formed aryl radical reacts with the quinoxaline radical or radical anion 69 through radical-radical coupling to deliver 2-aryl-1,2-dihydroquinoxaline 70, followed by twice anode oxidation and deprotonation to provide the desired product.

Scheme 49. Electrochemical Arylation of Electron-Deficient Arenes

2.1.3.5. Mechanoredox Approach. Serving as a complement protocol to the previous transition-metal catalysis, organic molecule-initiated, photoredox, and electrochemical transformations, another novel oxidative quenching process involves the mechanoredox system upon ball milling, which promotes one electron from the valence band to the conduction band of ${\mathrm{{BaTiO}}}_{3}$ (Scheme 50). ${}^{119}$ The resulting aryl radical species successfully induce sequential arylation or borylation reaction. The practical applications of this strategy have been demonstrated by the gram-scale synthesis of 2- arylated heteroarenes and direct functionalization of PAH coronene 71.

Scheme 50. Mechanoredox Arylation by Using Ball Milling and Piezoelectric Materials

2.1.4. Radical Addition to Alkynes. Upon decomposing aryl diazonium salts, the in situ formed aryl radical can be intercepted by alkynes to generate an alkene radical, followed by sequential transformations to afford arylated products. Using gallic acid as the radical initiator, a photoredox- and metal-free aerobic difunctionalization of terminal alkynes 72 with aryl diazonium salts was disclosed (Scheme 51). ${}^{120}$ This mild protocol allows the synthesis of unprotected $\alpha$ -hydroxy

Scheme 51. Gallic Acid-Promoted Aerobic Coupling Reaction of Terminal Alkynes with Aryl diazonium Salts

ketones 73 with moderate yields. The plausible mechanism involves the initial generation of an aryl radical, which reacts with the alkyne to produce an alkenyl radical species 74, followed by the reaction with dioxygen and water to afford the desired $\alpha$ -hydroxy ketones.

In 2019, Cheng reported visible-light-induced decarbox-ylative alkynylation of aryl diazonium salts with alkynyl carboxylic acids 75 (Scheme 52). ${}^{121}$ Using eosin Y as the photosensitizer and hypervalent iodine(III) reagent as the additive, this protocol enables the synthesis of diversely functionalized aryl alkynes under mild and transition-metal-free conditions upon visible-light irradiation. This protocol merged photoredox catalysis with decarboxylative alkynylation.

Scheme 52. Decarboxylative Alkynylation of Aryl diazonium Salts with Alkynyl Carboxylic Acids

2.1.5. Radical Addition to CO. In recent decades, the radical alkoxycarbonylation reaction involving the carbon-centered radical addition to $\mathrm{{CO}}$ has provided an alternative route to synthesize esters. However, most of these alkoxycarbonylations suffered from the employment of transition-metals, ${}^{122}$ or upon the UV light irradiation by Xe lamp. ${}^{123}$ The first transition-metal- and UV-light-free radical alkoxycarbonylation was disclosed using aryl halides as the substrates. ${}^{124}$ This thermal alkoxycarbonylation proceeded through a base-initiated radical-chain mechanism. It is still highly desirable to develop a mild and efficient radical alkoxycarbonylation process.

In 2015, Lu and Xiao reported the first radical alkoxycarboxylation of aryl diazonium salts using CO gas through a blue LED irradiation and metal-free conditions (Scheme 53). ${}^{125}$ As a synthetic method, this protocol is featured by a low loading of an organic dye as the photosensitizer, wide substrate scope, and mild reaction conditions. Mechanistically, oxidative quenching of fluorescein’s excited state with aryl diazonium ion occurs to generate the aryl radical. The aryl radical is trapped by CO to form a benzoyl radical 76, which is subsequently oxidized by dye radical cation, resulting in the generation of highly electrophilic acylium ion 77. Trapping of acylium ions by alcohol affords the alkoxycarboxylation product.

Scheme 53. Fluorescein-Catalyzed Radical Alkoxycarbonylation of Aryl Diazonium Salts via Visible Light Photoredox Catalysis

Almost at the same time, Wangelin and co-workers reported a similar visible light-driven radical alkoxycarboxylation using CO gas catalyzed by eosin Y, affording alkyl benzoates under metal-, base-, and ligand-free conditions (Scheme 54). ${}^{126}$ Compared with Xiao’s work, this protocol is amenable to synthesize tert-butyl esters with good yields, and the pressure of the CO atmosphere is relative low. The reaction mechanism is similar to the previous work.

In 2017, the same group reported a metal- and photo-free radical aromatic carbonylations mediated by weak bases at room temperature (Scheme 55). ${}^{127}$ This reaction is initiated by SET of an in situ formed diazoformate adduct to form an aryl radical, followed by radical trapping with carbon monoxide and alcohols.

Scheme 54. Eosin Y-Catalyzed Radical Alkoxycarbonylation of Aryl Diazonium Salts

Scheme 55. Weak Base-Mediated Radical Alkoxycarbonylation of Aryl Diazonium Salts

2.1.6. Cascade Radical Cyclization. Cyclization of aryl diazonium salts with functionalized alkenes or alkynes has provided a facile and expeditious route to assembly of five- or six-membered rings, particularly for heterocyclic scaffold (Figure 3). This route usually proceeds through a transition-metal-catalyzed condition or radical cyclization pathway. The reaction pathway involves the photoinduced or organic promoted-initiated formation of aryl radicals, followed by selective radical addition to unsaturated bonds and a sequential cyclization process. This content will focus on the recent representative research in this area, summarized according to the scaffold of carbon or heterocyclic rings.

First, the facile assembly of an oxindoles scaffold has been realized through the intermolecular aryl radical addition and cyclization cascade. ${}^{{128} - {130}}$ As for constructing of a five-membered ring scaffold, a visible light-mediated radical arylation reaction of $N$ -arylacrylamides 78 with aryl diazonium salts was realized, affording a variety of 3,3-disubstituted oxindoles 79 under mild conditions (Scheme 56). ${}^{128}$ This photoredox protocol represents a novel approach for assembly heterocycles upon visible-light irradiation. The plausible mechanism includes the initial oxidative quenching process of excited state Ru(II)* to generate an aryl radical, which adds to the $N$ -arylacrylamides 78 affording an alkyl radical 80. Intramolecular cyclization, single-electron oxidation by Ru- (III), and final dehydrogenation occur in sequence, delivering oxindoles 79 derivatives and regenerating the photocatalyst.

Scheme 56. Synthesis of Oxindoles by Radical Reactions of Aryl Diazonium Salts with N-Arylacrylamides

Figure 3. Diverse cyclic scaffold derived from radical and catalytic cyclization cascade reactions.

Another similar assembly of the oxindole scaffold was realized by a mild radical $\mathrm{C} - \mathrm{H}$ cyclization reaction of anilines with $N$ -arylacrylamides 78, affording the pharmaceutically important 3-benzyl-3-alkyloxindole scaffold 79 with moderate to good yields (Scheme 57). ${}^{129}$ In the presence of BPO as the radical initiator and $t$ -BuONO as the diazotization reagent, this protocol proceeds under metal- and photoredox-free conditions. The same group also reported a Cu(I)-catalyzed Meerwein carboarylation process to form 3-benzyl-3-alkylox-indoles. ${}^{130}$

Scheme 57. Radical C-H Cyclization Reaction of Anilines with $N$ -Arylacrylamides

In 2018, with a photocatalyzed cascade intermolecular addition/cyclization cascade strategy, Tang and co-workers developed a rapid synthetic route to azaspirocyclic cyclo-hexadienones 82 from $N$ -benzylacrylamides 81 with aryl diazonium salts under mild conditions (Scheme 58). ${}^{131}$ The reaction pathway includes the Ru-catalyzed photoredox generation of the aryl radical, followed by radical addition/ intramolecular cyclization/single-electron oxidation by Ru(III) to form a carbenium ion 83. This cation undergoes C-F bond cleavage applying ${\mathrm{H}}_{2}\mathrm{O}$ as an external oxygen source and final base-promoted deprotonation to deliver azaspirocyclic cyclo-hexadienones 82 scaffolds.

Scheme 58. Photocatalyzed Cascade Cyclization to Afford Azaspirocyclic Cyclohexadienones

Apart from the intermolecular cyclization process, the Studer group developed the intramolecular radical carboiodination, carbotelluration, and carboaminoxylation of in situ generated aryl diazonium salts from arylamines (Scheme 59). ${}^{132}$ The reaction pathway includes the formation of aryl radicals, which undergo selective 5-exo or 6-exo cyclization. The cyclized alkyl radicals 84 proceed through iodine abstraction in a chain reaction to obtain carboiodinated products with good yields, or trapped by PhTeTePh or TEMPO to provide the corresponding carbotelluration and carboaminoxylation products, respectively.

Scheme 59. Radical Carboiodination, Carbotelluration, and Carboaminoxylation of Arylamines

In addition to radical addition with an alkene moiety, a radical 5-exo cyclization of 2-allyloxyphenyldiazonium salts 85 with alkynes has also been developed, achieving the diastereoselective synthesis of highly functionalized ortho-spirocyclohexadienones 86 with moderate to good yields (Scheme 60). ${}^{133}$ This formal $\left\lbrack {2 + 2 + 1}\right\rbrack$ cyclization process provides a complementary route to afford all-carbon spiro-five-membered rings.

Another formal $\left\lbrack {2 + 2 + 1}\right\rbrack$ cycloaddition from anilines was developed through a radical deaminative ipso-cyclization of 4- methoxyanilines with $N$ -tethered 1,7-enynes 87, affording spirocyclohexadienone-containing cyclopenta[c]quinolin-4- ones 88 with two all-carbon quaternary stereocenters in one step (Scheme 61). ${}^{134}$ This radical cascade reaction proceeds through radical type 6-exo-dig/5-exo-trig ipso-cyclization/ single-electron oxidation to assemble cyclopenta[c]quinolin- 4-ones under oxidant-free thermal conditions.

The benzo-fused five-membered heterocycles represent a ubiquitous scaffold in natural products. Particularly, benzo[b]- heteroles containing heteroatoms other than nitrogen and oxygen have received considerable attention for their potential applications in materials science. ${}^{135}$ In 2016, the synthesis of functionalized benzothiophenes 89 or benzoselenophene 90 was disclosed through the intermolecular radical cascade reaction of $o$ -methylthio- or $o$ -methylselanyl-arylamines (Scheme 62). ${}^{136}$ The proposed mechanism involves the initial diazotization and generation of an aryl radical, which adds to the alkyne moiety to afford the vinyl radical intermediate. Subsequently, intramolecular homolytic substitution at the sulfur atom or selenium atom occurs, delivering final benzo[b]heterole derivatives 89/90.

Scheme 60. Selective Radical Cyclization of 2- Allyloxyphenyldiazonium Ions with Alkynes

Scheme 61. Synthesis of Spirocyclohexadienone-Containing Cyclopenta[c]quinolin-4-ones

Scheme 62. Synthesis of Benzothiophenes or Benzoselenophene through Radical Cascade Reactions

In addition, the photoredox system has emerged as an efficient route to access the synthesis of benzo-fused five-membered heterocycles, especially for indoles. A visible light-mediated cyclization of o-azidoarylalkynes 91 with aryl diazonium salts was catalyzed by eosin Y in the presence of 1,4-cyclohexadiene (1,4-CHD), enabling a transition-metal-free route for the synthesis of unsymmetrical 2,3-diarylated indoles 92 (Scheme 63). ${}^{137}$

Scheme 63. Photoredox Cyclization to Afford Indoles

Moreover, the combination of a ring-opening rearrangement with radical arylation presents a novel route to synthesize cyclic ketones. According to this strategy, Kim and co-workers reported a visible-light-induced photoredox catalytic arylation/ ring expansion cascade reaction of 1-(1-arylvinyl)cyclobutanols 93 with aryl diazonium salts, giving rise to functionalized cyclic ketones under mild conditions (Scheme 64). ${}^{138}$ The proposed reaction pathway includes the photoinduced formation of an aryl radical and sequential selective addition to 1-(1-arylvinyl)- cyclobutanols to afford alkyl radical 94. This alkyl radical 94 is oxidized by Ru(III) or aryl diazonium ions through a radical-chain pathway to obtain a benzylic carbenium ion 95, followed by 1,2-migration to deliver ring expansion products.

On the other hand, this cyclization cascade strategy has also been utilized in the concise construction of six-membered rings. In 2014, the Studer group developed a base-promoted homolytic aromatic substitution of ortho-amino-biphenyls and alkynes 72, assembling phenanthrene scaffold 96 under transition-metal- and photofree thermal conditions (Scheme 65). ${}^{139}$ Mechanistically, the biaryl diazonium salts are generated in situ from biarylamines using isoamyl nitrite as the diazotization reagent. After single-electron reduction using TBAI as the chain initiator, the corresponding biaryl radical forms, followed by addition to an alkyne and subsequent base promoted homolytic aromatic substitution, affording phenan-threnes 96 in moderate to good yields.

Scheme 64. Photoredox Arylative/Ring Expansion of 1-(1- Arylvinyl)cyclobutanols

Scheme 65. Cyclization of ortho-Amino-Biphenyls and Alkynes

Cho and co-workers utilized a visible light-promoted photocatalytic strategy to realize cyclization to achieve the synthesis of polyheteroaromatic compounds (Scheme 66). ${}^{{140},{141}}$ With 2-heteroaryl amines as the substrates, this photoredox protocol allows the synthesis of polycyclic heteroaromatics, and the gram-scale synthesis of 9-phenyl-phenanthrene. This process is initiated by $t$ -BuONO-mediated diazotization, followed by photoredox SET oxidation to generate the aryl radical. Subsequent selective addition/ intramolecular cyclization/oxidation by Ir(IV)/deprotonation occurs, affording polyheteroaromatic products along with the regeneration of Ir(III).

Scheme 66. Photoredox Synthesis of Polyheteroaromatics

Additionally, this radical [4 + 2] benzannulation procedure from ortho-amino-biphenyls and alkynes enables the expeditious synthesis of various benzo-fused ring scaffolds, affording phenanthrenes ${97},{}^{142}$ naphthalenes ${98},{}^{143}$ and tetrazolo[1,5- a]quinoline ${99}^{144}$ under light-free thermal conditions or at room temperature (Scheme 67).

Scheme 67. Radical [4 + 2] Benzannulation from ortho-Amino-Biphenyls and Alkynes

Apart from the selective cyclization with the substrates of alkynes, a transition-metal- and light-free arylative cyclization of 2-isocyanobiphenyls 100 with arylamines was reported, accessing the synthesis of 6-aryl phenanthridine derivatives 101 in a one-pot manner (Scheme 68). ${}^{145}$ This reaction pathway is initiated by the addition of thermal-initiated aryl radical to isocyanobiphenyls 100 to form an imidoyl radical 102. Subsequently, the intramolecular homolytic aromatic substitution of 102 occurs on the pending ring, affording a cyclohexadienyl radical 103. Next, either the SET oxidation to afford cyclohexadienyl cation followed by deprotonative aromatization, or the deprotonation of the cyclohexadienyl radical first followed by SET, can both direct the formation of phenanthridine products 101.

Studer and co-workers realized the cascade radical cyclization of 1,6-enynes with aryl diazonium salts by electron catalysis, affording polycyclic structures in one step (Scheme 69). In 2016, they realized the facile construction of biologically important polycyclic $\gamma$ -lactams and lactones through this radical cascade strategy under transition-metal-free thermal conditions. ${}^{146}$ The reaction mechanism involves initial LiI-promoted SET of the aryl diazonium ion to form an aryl radical. Chemoselective radical addition to the alkene moiety of 1,6-enyne provides tertiary alkyl radical 104, which undergoes a subsequent 5-exo-dig cyclization to deliver vinyl radical 105. Next, intramolecular cyclization/deprotonation/ SET oxidation occurs to afford a polycyclic $\gamma$ -lactam product. The same group also reported a cascade radical cyclization of 1,6-enynes using TBAI as a chain initiator, affording a broad range of polycyclic compounds in moderate yields. ${}^{147}$

Scheme 68. Arylative Cyclization of 2-Isocyanobiphenyls to Afford 6-Aryl Phenanthridines

Scheme 69. Cascade Radical Cyclization of 1,6-Enynes with Aryl diazonium Salts

2.1.7. $\mathrm{C} - \mathrm{{CN}},\mathrm{C} - {\mathrm{{CF}}}_{3}$ , and $\mathrm{C} - {\mathrm{{CF}}}_{2}\mathrm{X}$ Bond Formations. 2.1.7.1. C-CN Bond Formations. Traditional Sandmeyer-type cyanation of aryl diazonium salts has been widely used in constructing the aryl $\mathrm{C} - \mathrm{{CN}}$ bond, which usually needs the participation of stoichiometric copper salts. The selective advances emphasize expanding substrates scope and investigating new efficient catalytic systems, including the radical-type cyanations and several transition-metal-catalyzed coupling reactions for constructing C-CN bonds.

Recently, Cu-mediated Sandmeyer-type, as well as Pd-catalyzed cyanation of aryl diazonium salts, has been applied as an efficient route to access benzonitrile derivatives. ${}^{148}$ As shown in Scheme 70, a ${\mathrm{{Cu}}}_{2}\mathrm{O}$ -mediated cyanation of aryl diazonium salts was realized by using trimethylsilyl cyanide 106 as the cyano source. ${}^{149}$ This mild protocol affords aromatic nitriles with good yields in the presence of a substoichiometric amount of nontoxic, inexpensive ${\mathrm{{Cu}}}_{2}\mathrm{O}$ . In 2016, Dughera disclosed a copper-free cyanation of aryl diazonium o-benzenedisulfonimides 107 using tetrabutyl ammonium cyanide 108 as a safe cyanide source, which represents a rare case of the transition-metal-free Sandmeyer cyanation reaction. ${}^{150}$ This reaction features good functional group tolerance and the ability to recover o-benzenedisulfoni-mide for preparing other salts. The o-benzenedisulfonimide anion is assumed to facilitate the SET process.

Scheme 70. Cu-Mediated and Cu-free Cyanation of Aryl Diazonium Salts

Additionally, another cyanation strategy involves the Pd-catalyzed oxidative cross-coupling reactions of aryl diazonium salts with various cyano sources (Scheme 71). A common and nontoxic organic solvent acetonitrile ${}^{151}$ and NCTS ( $N$ -cyano- $N$ -phenyl- $p$ -toluenesulfonamide ${)}^{152}$ have been developed in the cyanation in the presence of silver(I) salts under ambient air.

Scheme 71. Pd-Catalzyed Cyanation of Aryl Diazonium Salts

In 2019, Chia reported copper-free Sandmeyer-type cyanation of anilines using tert-butyl isocyanide under mild conditions. ${}^{153}$ This Cu- and solvent-free protocol affords aryl nitriles in moderate to good yields (10 examples, 52-81% yields), which reveals that the tert-butyl isocyanide can serve as a potential cyanide source for the cyanation reaction.

2.1.7.2. $\mathrm{C} - {\mathrm{{CF}}}_{3}$ and $\mathrm{C} - {\mathrm{{CF}}}_{2}\mathrm{X}$ Bond Formations. The introduction of trifluoromethyl groups and other fluorinated substituents into organic molecules can substantially alter their properties, such as metabolic stability, lipophilicity, permeability. Because of these fluorinated groups’ importance, chemists have been focusing on the selective aryl $\mathrm{C} - {\mathrm{{CF}}}_{3}$ and $\mathrm{C} - {\mathrm{{CF}}}_{2}\mathrm{X}$ bond formations. In 2013, Fu reported an elemental copper-mediated Sandmeyer-type trifluoromethyla-tion of arylamines (Scheme 72). ${}^{154}$ In their method,

Scheme 72. Cu-Mediated Sandmeyer-Type Trifluoromethylation of Anilines with Umemoto’s Reagent

Umemoto’s reagent 109 was employed as the ${\mathrm{{CF}}}_{3}$ source, and iso-amyl nitrite $\left( {i\text{ -AmONO }}\right)$ was used to effect the in situ diazotization reaction. This trifluoromethylation reaction proceeds smoothly under mild conditions and exhibits good tolerance of many synthetically relevant functional groups. A radical pathway is supported by a series of mechanistic studies, including the clock trapping reaction, ESR test, and a ${\mathrm{{CuCF}}}_{3}$ species 110 suggested by ${}^{19}\mathrm{\;F}$ NMR spectrum. The practical application of this protocol is demonstrated by the preparation of leflunomide and formal $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{H}$ bond trifluoromethyla-tion through a nitration-reduction-trifluoromethylation tandem process.

Meanwhile, Wang and co-workers realized a Ag-mediated trifluoromethylation of in situ formed aryl diazonium salts, converting an aromatic amino group into a ${\mathrm{{CF}}}_{3}$ group (Scheme 73). ${}^{155}$ Through the simple addition of preformed ${\mathrm{{AgCF}}}_{3}$ 111 into a diazonium chloride solution in EtCN under $- {78}^{ \circ }\mathrm{C}$ , this trifluoromethylation protocol turns out to be compatible with a wide range of substrate scopes, including both electron-rich and electron-deficient anilines, as well as heteroaromatic amine derivatives. Gram-scale experiments demonstrate the potential usefulness of this method. Mechanistically, an oxidative addition-reductive elimination pathway involving the hypervalent silver species 112 has been proposed.

Almost at the same time, Goossen and co-workers developed a copper-mediated trifluoromethylation of stable diazonium tetrafluoroborate salts (Scheme 74). ${}^{156}$ In their work, an in situ generated ${\mathrm{{CuCF}}}_{3}$ species is contrived to react with aryl diazonium salt to deliver a trifluoromethylated product at room temperature. Thus, the Ruppert-Prakash reagent 113 (TMSCF ${}_{3}$ ) is used as the ${\mathrm{{CF}}}_{3}$ source and is premixed with $\mathrm{{Cu}}\left( \mathrm{I}\right)$ thiocyanate and cesium carbonate prior to the diazonium salt. The reaction proceeded smoothly with good functional group tolerance and generally afforded moderate to high yields of trifluoromethylated products. This trifluoromethylation reaction is proposed to proceed via a radical process, which is in line with classic Sandmeyer halogenation reactions.

Scheme 73. Silver-Mediated Trifluoromethylation of Aryl Diazonium Salts

Scheme 74. Cu-Mediated Sandmeyer Reaction of Aryl Diazonium Salts with ${\mathrm{{TMSCF}}}_{3}$

In 2014, the one-pot in situ diazotization and copper-mediated trifluoromethylation of (hetero)aromatic amines was realized using the Ruppert-Prakash reagent 113, affording various benzotrifluorides in good yields (Scheme 75). ${}^{157}$

In 2014, Grushin and co-workers developed a Sandmeyer-type trifluoromethylation of aryl diazonium salts with fluoroform-derived ${\mathrm{{CuCF}}}_{3}$ 110 in aqueous media (Scheme 76). ${}^{158}$ Unlike the other reactions mentioned above, the ${\mathrm{{CF}}}_{3}$ moiety in this protocol originates from an inexpensive, nonflammable gas of fluoroform. Subsequently, ${\mathrm{{CuCF}}}_{3}$ 110 can be easily generated as a solution by treating CuCl with $t$ -BuOK and ${\mathrm{{CHF}}}_{3}$ successively. This radical-type trifluoromethylation features a broad substrate scope, including a couple of heterocyclic examples, as well as some complex molecules. A similar protocol trifluoromethylation using anilines as the substrates was realized using Langlois’ reagent 114, affording trifluoromethylated arenes with moderate to good yields in the presence of TBHP as the suitable oxidant. ${}^{159}$

Fluorine-18 is a major radionuclide for labeling such radiotracers in positron emission tomography (PET), serving as an important imaging modality for biomedical research and

Scheme 75. Copper-Mediated Trifluoromethylation of in situ Generated Aryl Diazonium Salts

Scheme 76. Cu-Mediated Trifluoromethylation from Fluoroform and Langlois’ Reagent

Grushin drug development. In 2019, Pike and co-workers reported a gas phase route to access $\left\lbrack {{}^{18}\mathrm{\;F}}\right\rbrack$ fluoroform, which can be used in labeling aryl organic compounds with trifluoromethyl groups. ${}^{160}$ In the presence of base and CuBr, $\left\lbrack {{}^{18}\mathrm{\;F}}\right\rbrack$ fluoroform is converted rapidly into $\left\lbrack {{}^{18}\mathrm{\;F}}\right\rbrack {\mathrm{{CuCF}}}_{3}$ for reaction with aryl diazonium salts, affording $\left\lbrack {{}^{18}\mathrm{\;F}}\right\rbrack$ trifuoromethyl arenes with moderate to good yields (five examples, 45-96% yields).

In 2015, a Cu-mediated catalyst-dependent transformation of aryl diazonium salts with Langlois’ reagent 114 was disclosed, affording trifluoromethanesulfonylated or trifluor-omethylated arenes with excellent chemoselectivity (Scheme 77). ${}^{161}$ The ${\mathrm{{Cu}}}_{2}\mathrm{O}$ -catalyzed reaction in DMSO affords aryl trifluoromethanesulfones with moderate to good yields. Alternatively, in the presence of $\mathrm{{Cu}}{\left( \mathrm{{MeCN}}\right) }_{4}{\mathrm{{BF}}}_{4}$ and Tpy as the ligand, this reaction gives rise to trifluoromethylated arenes in moderate yields. In the latter case, TBHP is used as an oxidant with Langlois’ reagent to generate a trifluoromethyl radical, which ligates with $\mathrm{{Cu}}\left( \mathrm{I}\right)$ to afford an active $\mathrm{{Cu}}\left( \mathrm{{II}}\right) - {\mathrm{{CF}}}_{3}$ intermediate.

Apart from the trifluoromethyl group, more fluorinated functionality has been introduced into arenes through the reaction with aryl diazonium salts. In 2014, Goossen accomplished a Sandmeyer-type difluoromethylation of (hetero)aryl diazonium salts by using an in situ generated difluoromethyl-copper complex 115 (Scheme 78). ${}^{162}$ This

Scheme 77. Cu-Promoted Trifluoromethanesulfonylation and Trifluoromethylation of Aryl Diazonium Tetrafluoroborates with ${\mathrm{{NaSO}}}_{2}{\mathrm{{CF}}}_{3}$

mild protocol allows the synthesis of difluoromethyl (hetero)- arenes from readily available aromatic amines.

Scheme 78. Sandmeyer Difluoromethylation of Aryl Diazonium Salts

In addition to the trifluoromethylation and difluoromethy-lation of aryl diazonium salts, a copper-mediated perfluor-oalkylation of diazonium salts was reported by using perfluoroalkyl iodide 116-derived stable ${\mathrm{R}}_{\mathrm{F}}\mathrm{{Cu}}\left( \mathrm{{MeCN}}\right)$ complex as the perfluoroalkylating agents (Scheme 79). ${}^{163}$ This radical protocol provides a general and regiospecific synthetic route to perfluoroalkylated arenes with moderate yields. In 2016, Wu and Cao disclosed a silver and copper comediated isoperfluoropropylation of in situ generated aryl diazonium salts by using hexafluoropropylene gas 117 (HFP) as the perfluoroalkyl source (Scheme 79). ${}^{164}$

Scheme 79. Synthetic Route to Perfluoroalkylated Arenes from Aryl Diazonium Salts

The previous difluoromethylation reactions require excess transition metal salts. As for the copper-free difluoromethyla-tion procedure, Lu and Cai disclosed a visible-light-induced photoredox difluoromethylation of aryl diazonium salts with $\alpha$ - aryl- $\beta ,\beta$ -difluoroenol silyl ether 118 (Scheme 80). ${}^{165}$ In the presence of a tiny amount $\left( {{0.05}\mathrm{\;{mol}}\% }\right)$ of photosensitizer and a short light exposure, this radical chain protocol is initiated by the selective radical addition of an aryl radical to $\alpha$ -aryl- $\beta ,\beta$ - difluoroenol silyl ether 118, followed by single-electron oxidation process from another aryl diazonium salt to afford a variety of $\alpha$ -aryl- $\alpha ,\alpha$ -difluoroketones 119 with moderate to good yields.

Scheme 80. Visible-Light-Induced Photoredox Difluoromethylation of Aryl Diazonium Salts

Furthermore, other fluorinated functionalities can be selectively introduced on aromatic rings through the Cu-mediated Sandmeyer-type reaction using TMSCF ${}_{2}\mathrm{{PO}}{\left( \mathrm{{OEt}}\right) }_{2}$ 120 as the fluorinated reagent ${}^{166}$ or the pentafluoroethylation reaction ${}^{167}$ with an active pentafluoroethylation reagent " ${\mathrm{{CuC}}}_{2}{\mathrm{\;F}}_{5}$ " 121, which is pregenerated from CuSCN, tetrafluoroethylene, and CsF (Scheme 81).

Scheme 81. Synthesis of Aryl Difluoromethylphosphonates and Pentafluoroethyl Arenes through Sandmeyer-Type Reaction

2.1.8. Miscellaneous. The aryl radical through photoinduced diazonium decomposition can undergo an intramolecular HAT process to obtain an alkyl radical 122, which allows remote $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ bond functionality (Scheme 82). ${}^{168}$

Scheme 82. Visible Light-Promoted Remote C-H Functionalization of o-Diazoniaphenyl Alkyl Sulfones

Subsequent interception by various nucleophiles provides the remote $\mathrm{C} - \mathrm{O},\mathrm{C} - \mathrm{N}$ , and $\mathrm{C} - \mathrm{C}$ bond formation products. Moreover, aryl diazonium ions have also been explored as radical initiates to allow the regioselective cleavage of aziridines, ${}^{169}$ as well as the preparation of spirobi[indene] derivatives. ${}^{170}$

Upon visible-light irradiation, a selective HAT process with acetone using aryl diazonium ions as radical precursors was realized under continuous-flow conditions, affording an acetonyl radical which was subsequently utilized to forge versatile C-C bonds (Scheme 83). ${}^{171}$

Scheme 83. Acetonyl Radical-Induced Synthesis of Various Heterocycles

2.2. Transition-Metal-Catalyzed Coupling Reactions of Aryl Diazonium Salts

Serving as the surrogates of aryl halides, aryl diazonium salts have been utilized in transition-metal-catalyzed cross-coupling reactions for $\mathrm{C} - \mathrm{C}$ bond formations (Scheme 84). In most cases, both the palladium and gold complex have been explored as catalysts in coupling reactions to synthesize versatile functional molecules.

Scheme 84. Transition-Metal-Catalyzed Coupling Reactions of Aryl Diazonium Salts

For the Pd-catalyzed coupling reactions of aryl diazonium salts, the reaction is initiated by the oxidative addition to form an aryl-Pd(II) species. The insertion reaction, electrophilic $\mathrm{C} - \mathrm{H}$ bond functionalization, or transmetalation occurs, followed by reduction elimination to afford the desired coupling product along with the regeneration of $\mathrm{{Pd}}\left( 0\right)$ species. An innovative strategy in this area is the combination of the dual palladium/photoredox catalytic system, allowing selective coupling reactions under sustainable and environmentally benign conditions. The recent improvements will be summarized according to the types of coupling reactions.

A type of related reaction is the gold-catalyzed coupling reactions of aryl diazonium salts upon or without the visible-light irradiation. Mechanistically, these reactions are initiated by two-step single-electron oxidations of $\mathrm{{Au}}\left( \mathrm{I}\right)$ by aryl diazonium ions to generate an aryl-Au(III) intermediate. The photoredox catalytic system can facilitate this SET process. With the sequential transformations of the active aryl-Au(III) species with alkenes, alkynes, or other coupling partners, versatile coupling products including difunctionaliza-tion of alkenes, C-H bond functionalization of arenes/alkynes, and (hetero)cyclic compounds are obtained.

2.2.1. Heck-type Reactions with Alkenes. Since the pioneering work by Kikukawa and Matsuda in 1977, ${}^{{13},{14}}$ aryldiazonium salts have been established as a type of electrophilic reaction partner in transition-metal-catalyzed cross-coupling reactions. Thus, aryldiazonium salts could be considered as aryl halide surrogates in cross-coupling reactions for C-C bond and C-heteroatom bond formations. In recent years, efforts were dedicated to the development of a new catalytic system, mechanistic study, ${}^{172}$ and synthetic application of Heck-Matsuda type chemistry. ${}^{{173} - {176}}$

A modified Heck-Matsuda reaction was developed to allow for anilines to be applied as the starting materials in the presence of methanesulfonic acid (Scheme 85). ${}^{177}$ Under these conditions, the aniline starting material underwent in situ diazotization for Pd-catalyzed coupling. A range of anilines were excellent substrates for this process.

The one-pot tandem process that generates Heck-Matsuda coupled products from anilines via stable aryl diazonium tosylate salts 124 was reported (Scheme 86). ${}^{178}$ The reaction pathway includes the rapid formation of aryl diazonium salts using a polymer-supported nitrite reagent 123 with $p$ -TsOH, followed by a base-free Heck-Matsuda reaction with acrylates or styrenes. Moreover, this procedure allows the direct synthesis of 3,4-dihydroquinolin-2-ones by using 2-nitroani-lines as the substrates.

Scheme 85. Heck-Matsuda Arylation of Olefins through in Situ Diazotization Approach

Scheme 86. Tandem Heck-Matsuda Reactions of Anilines and One-Pot Synthesis of 3,4-Dihydroquinolin-2-ones

The heterogeneous Pd-catalyzed Heck-Matsuda reaction has been developed using palladium nanoparticles supported on sulfonic acid functionalized silica as trifunctional heterogeneous catalysts for Heck-Matsuda and Suzuki coupling reactions (Scheme 87). ${}^{179}$ The arylation of acrylates and boronic acids with in situ generated aryl diazonium salts proceeds under mild conditions using this heterogeneous catalytic system. These new catalysts are trifunctional. They act as a proton donor for the diazotization step and provide a counterion for the diazonium salt. Most importantly, they are also the Pd catalyst for cross-coupling. Additionally, synthetic calcium-phyllosilicates can also serve as bifunctional bases in the Matsuda-Heck reaction using methanol as the solvent. ${}^{180}$ This heterogeneous catalytic protocol provides a complementary route to synthesize acid-sensitive products.

Scheme 87. Palladium Nanoparticles as Trifunctional Heterogeneous Catalysts for Heck-Matsuda and Suzuki Reactions

In 2015, an in-water Heck-Matsuda reaction with water-soluble aryl diazonium tosylates was developed. ${}^{181}$ A range of alkenes underwent arylation in good yields. Electron-deficient alkenes afforded high yields of product, while moderate yields were observed for electron-rich alkenes. The reaction is significantly accelerated using microwave irradiation.

The Heck-Matsuda reaction has been utilized in synthesizing various functional molecules (Scheme 88). Coupling of ethenesulfonyl fluoride 125 and aryl diazonium tetrafluor-oborates under mild reaction conditions furnished $\beta$ - arylethenesulfonyl fluorides (126), which are versatile bis-electrophiles for sulfur(VI) fluoride exchange click chemistry. ${}^{182}$

Scheme 88. Heck-Matsuda Reaction for the Synthesis of $\beta$ - Arylethenesulfonyl Fluorides and 2-Deoxy Aryl C-Glycosides

The stereocontrolled synthesis of aryl-C-glycosides has also been reported. Kandasamy disclosed a Pd-catalyzed Heck-Matsuda reaction of glycals 127 and aryl diazonium salts ${}^{183}$ or anilines ${}^{184}$ in the presence of $t$ -butyl nitrite, achieving a one-pot stereospecific synthesis of 2,3-deoxy-3-keto- $\alpha$ -aryl-C-glycosides and 2-deoxy aryl C-glycosides under room temperature.

The Pd-catalyzed Heck-Matsuda cascade reaction has been developed to synthesize cyclic compounds. In 2017, Li and Song reported a Pd-catalyzed dearomative $\left\lbrack \begin{array}{l} 2 + 2 + 1 \end{array}\right\rbrack$ carboannulation of 1,7-enynes 128 and aryl diazonium salts, affording spirocyclohexadienone-fused cyclopenta $\left\lbrack c\right\rbrack$ quinolin- 4(5H)-ones 129 (Scheme 89). ${}^{185}$ This reaction is initiated by the selective insertion of Ar-Pd(II) species, followed by 6-exo-dig insertion to alkyne, ipso-cyclization, and a water-mediated substitution sequence to achieve the expeditious assembly of the polycyclic scaffold.

Furthermore, enantioselective Heck-Matsuda arylations have been explored in recent years. In 2012, using a redox-relay strategy, Sigman developed an enantioselective Heck arylation of acyclic alkenyl alcohols with aryl diazonium salts by using low Lewis basicity ${\mathrm{{CF}}}_{3}$ -substituted pyridine oxazoline as the chiral ligand, delivering a series of $\beta$ -, $\gamma$ -, or $\delta$ -aryl carbonyl products (Scheme 90). ${}^{186}$ The carbopalladation step was found to be highly regioselective. Sequential carbopalla-dation/ $\beta$ -hydride elimination eventually provided the carbonyl compound as the final product.

Scheme 89. Pd-Catalyzed Dearomative $\left\lbrack {2 + 2 + 1}\right\rbrack$ Carboannulation of 1,7-Enynes with Aryl Diazonium Salts and Water

Scheme 90. Enantioselective Heck Arylations of Racemic Alcohols

In 2018, a Heck-Matsuda desymmetrization of meso (1R,2S)-cyclohex-4-ene-1,2-diol 130 was successfully realized (Scheme 91). ${}^{187}$ This method provides trisubstituted allylic alcohols with excellent diastereo- and enantiocontrol.

In 2017, an enantioselective Heck-Matsuda reaction of five-, six-, and seven-membered ring alkenes with aryl diazonium salts was established (Scheme 92). ${}^{188}$ Under phase-transfer conditions, in the presence of a Pd catalyst and chiral phosphoric acid catalyst 131, the resulting arylated carbocycles were synthesized with good enantiocontrol.

Scheme 91. Enantioselective Heck-Matsuda Desymmetrization of cis-Cyclohex-4-ene-1,2-diol

Scheme 92. Enantioselective Heck-Matsuda Arylations of Cyclopentene, Cyclohexene, and Cycloheptene Derivatives

This Pd-catalyzed asymmetric Heck-Matsuda reaction has merged with cascade cyclization through the double Heck reaction to assemble cyclic compounds (Scheme 93). ${}^{189}$ Functionalized dihydrobenzofurans bearing all carbon quaternary centers 132 were obtained in high enantioselectivity. This catalytic protocol found application in the synthesis of enantiopure tricyclic dihydrobenzofuran scaffold 133.

Scheme 93. Pd-Catalyzed Enantioselective Double Heck Reaction

In addition to Heck-type C-C bond formations, arylbora-tion of alkenes with aryl diazonium salts was also reported. ${}^{{190} - {192}}$ In 2015, an enantioselective Pd-catalyzed arylboration of $\alpha$ -olefins, using aryl diazonium salts, and bis(pinacolato)diboron 134 was disclosed, allowing the direct preparation of enantioenriched benzylic boronates 135 (Scheme 94). ${}^{190}$ Mechanistically, upon interaction with the lipophilic chiral phosphoric acid anion, the insoluble aryl diazonium salt undergoes phase transfer to provide a soluble chiral ion pair 136. Oxidative addition of $\mathrm{{Pd}}\left( 0\right)$ into the aryl diazonium substrate followed by carbopalladation with the alkene furnishes an enantioenriched alkylpalladium complex 137. Transmetalation of 137 with the diboron reagent and subsequent reductive elimination forms the arylboration product 135 and regenerates the $\mathrm{{Pd}}\left( 0\right)$ catalyst.

Scheme 94. Pd-Catalyzed Three-Component Reaction of $\alpha$ - Olefins, Aryl Diazonium Salts, and ${\mathrm{B}}_{2}{\text{ pin }}_{2}$

In 2016, a similar Pd-catalyzed 1,2-arylboration of vinyl-arenes was developed, affording arylated benzylic boronates with high regioselectivity (Scheme 95). ${}^{191}$ The proposed mechanism is analogous to those involving terminal olefins.

2.2.2. C-H Bond Functionalizations of Arenes. Transition metal-catalyzed arenes $\mathrm{C} - \mathrm{H}$ bond functionalization with aryl diazonium salts represents an alternative route to the radical type arylation reactions (Scheme 96). The main feature of this protocol lies in the participation of transition metals, such as palladium or iridium complex, instead of radical initiators. The excellent regioselectivity is usually controlled by the directing group on substrates. Additionally, the substrate scope of arenes has been expanded to inert aromatics, in addition to electron-rich heterocycles that are usually superior in Gomberg-Bachmann arylations.

Mechanistically, one reaction pathway includes the direct oxidative addition of aryl diazonium ions to the transition metal center to form a hypervalent carbometalated species, followed by reduction elimination to form biaryls. A Pd-

Scheme 95. Pd-Catalyzed Regioselective Arylboration of Vinylarenes

Scheme 96. Arenes C-H Bond Functionalizations with Aryl Diazonium Salts

catalyzed arylation of indoles was also reported, producing 2- arylindoles with ${0.5} - 1\mathrm{\;{mol}}\% \mathrm{{Pd}}{\left( \mathrm{{OAc}}\right) }_{2}$ under air (Scheme 97). ${}^{193}$ This arylation procedure could also be extended to benzofurans and benzothiophenes in MeOH, delivering C-2 and C-3 arylated products in good yields. DFT calculations support a Heck-Matsuda type mechanism, which proceeds through electrophilic carbopalladation and sequential anti- $\beta$ - deprotonation rearomatization.

Scheme 97. Pd-Catalyzed Base-Free C-H Arylation of Heteroarenes

Tryptophan is a rare proteinogenic amino acid that has gained much current attention as a handle for site-selective protein modification and conjugation. In 2017, Fairlamb and co-workers described a regioselective Pd-mediated C-H arylation for tryptophans 139 usingaryl diazonium salts. This process affords C2-arylated tryptophan derivatives 140 in good to excellent yields (Scheme 98). ${}^{194}$ Tosylic acid was found to have a rate-accelerating effect for this transformation.

Scheme 98. Pd(OAc)2-Catalyzed C-H Arylation of Tryptophans with Aryl Diazonium Salts

Moreover, a series of transition-metal-catalyzed, directing group-assisted C-H arylation using aryl diazonium salts have been reported. ${}^{{195} - {198}}$ Chang described an Ir(III)-catalyzed ortho- $\mathrm{C} - \mathrm{H}$ arylation of benzamides with aryl diazonium salts under oxidant-free conditions (Scheme 99). ${}^{195}$ Enamides and quinoline N-oxides could also be arylated under these conditions. The authors proposed that the turnover-limiting step is likely the formation of an aryliridium(V) intermediate derived from the aryl diazonium substrates.

Scheme 99. Cp*Ir(III)-Catalyzed C-H Arylation of Arenes and Alkenes with Aryl Diazonium Salts

In the same year, Gooßen described an oxidant-free Ir-catalyzed ortho-C-H arylation protocol for benzoic acids 141 by using aryl diazonium salts with a catalytic amount of ${\mathrm{{Ag}}}_{2}{\mathrm{{CO}}}_{3}$ and stoichiometric ${\mathrm{{Li}}}_{2}{\mathrm{{CO}}}_{3}$ as additives (Scheme 100). ${}^{196}$ After various directing groups were evaluated, only free carboxylic acids (as in benzoic acid) and secondary amides were effective as directing groups in this ortho-C-H arylation regime (versus 2-pyridyl, 2-pyridyl ether, acetamide, 8- aminoquinoline amide, ester, 2-oxazolyl), allowing the synthesis of diaryl carboxylates 142. On the basis of radical scavenger experiments, a single-electron oxidation of aryl radical to Ir(III) species is hypothesized.

Scheme 100. Iridium-Catalyzed ortho-Arylation of Benzoic Acids with Arene diazonium Salts

In addition to iridium catalysts, palladium complexes have also been explored as efficient catalysts for promoting directed $\mathrm{C} - \mathrm{H}$ arylation of $N$ -methoxybenzamides 143 with in situ generated aryl diazonium salts as arylating agents, affording phenanthridones 144 via formation of C-C and C-N bonds in a one-pot fashion (Scheme 101). ${}^{197}$ The reaction pathway involves the initial ortho-C-H arylation of $N$ -methoxybenza-mide 143 with aryl diazonium salts to generate the biaryl bond, followed by a second $\mathrm{C} - \mathrm{H}$ activation and subsequent intramolecular amidation reaction to form the $\mathrm{C} - \mathrm{N}$ bond, which builds up the phenanthridinone scaffold.

In 2018, Polley and co-workers reported a Pd-catalyzed C-H arylation of aniline carbamates with aryl diazonium salts for the synthesis of various of carbazole alkaloids under mild conditions in the absence of external oxidants, acids, or bases. ${}^{198}$

Furthermore, the combined systems consisting of a photo-redox and a transition-metal catalyst have been developed, which expands the utility of transition metal-catalyzed $\mathrm{C} - \mathrm{H}$ functionalization through a photoinduced SET process. In 2011, Sanford and co-workers first described a Pd-catalyzed directed ortho-C-H activation and arylation reaction with aryl diazonium salts under visible-light photoredox catalysis to obtain 2-arylpyridine derivatives (Scheme 102). ${}^{199}$ Mechanistically, this dual catalytic system is initiated by the oxidative quenching of the excited state of photosensitizer to afford an aryl radical, which subsequently oxidizes the cyclic Pd(II) species to a Pd(III) intermediate. This Pd(III) species undergoes single-electron oxidation by a hypervalent photosensitizer to form Pd(IV) intermediate 145, followed by final reductive elimination to forge the biaryl $\mathrm{C} - \mathrm{C}$ bond.

Scheme 101. Palladium-Catalyzed Deaminative Phenanthridinone Synthesis from Anilines via C-H Bond Activation

Scheme 102. Mechanism for the Pd/Photoredox-Catalyzed C-H Arylation

Subsequent to Sanford’s pioneering research, a series of dual photoredox/Pd-catalyzed C-H functionalization systems have been established (Scheme 103). ${}^{200}$ Employing the Pd/AcrH ${}_{2}$ dual catalytic system, visible-light-induced ortho-C-H aryla-tions of acetanilides, benzanilides, and $N$ -methylbenzamides were realized, which represents a new example for the combination of a transition metal catalyst with an organic photoredox catalyst. ${}^{201}$ Balaraman reported a Pd/Ru photo-redox catalytic system in dimethyl carbonate (DMC) as a green solvent for ortho-C-H arylation of pivalanilides with aryl diazonium salts. ${}^{202}$ The same group also disclosed a dual catalytic approach for $\mathrm{C} - \mathrm{H}$ functionalization with aryl diazonium salts using the Pd/eosin Y catalytic system. ${}^{203}$

Scheme 103. Visible Light-Promoted Pd/Photoredox-Catalyzed C-H Arylations of Arenes

This dual Pd/Ru cocatalyzed system has found applications in modifying various heterocycles, such as the selective arylation of 6-arylpurine nucleosides 146 employing purine as a directing group, ${}^{204}$ as well as $\mathrm{C} - \mathrm{H}$ arylation of benzodiazepines 147 (Scheme 104). ${}^{205}$

Apart from the $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{H}$ arylation reactions, the synergistic Pd/Ru-catalyzed photoredox system was explored for the arylation of unactivated $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ bonds with aryl diazonium salts by using 5-chloro-8-aminoquinoline 148 as the auxiliary group (Scheme 105). ${}^{206}$ This $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ arylation reaction represents an extension for the dual catalytic $\mathrm{C} - \mathrm{H}$ functionalization process. A plausible mechanism involves a one-electron oxidative addition, enabling a Pd(II)-Pd(III)- Pd(IV)-Pd(II) catalytic cycle.

Scheme 104. Visible Light-Promoted Pd/Ru-Catalyzed C-H Arylations of 6-Arylpurine and Benzodiazepines

Scheme 105. Auxiliary-Directed Visible Light-Promoted Pd/Ru C $\left( {\mathrm{{sp}}}^{3}\right) -$ H Arylations

2.2.3. Suzuki-Miyaura, Hiyama, Stille-type Coupling Reactions, and Others. Not only can aryl diazonium salts be applied in Heck-type reactions, these substrates have also been explored as electrophiles in transition metal-catalyzed cross-coupling reactions with nucleophilic organometallic compounds through Suzuki-Miyaura, Hiyama, Stille, and Kumu-da-type coupling reactions (Scheme 106). Palladium and nickel complexes have been utilized as catalysts for these coupling reactions. A gold-catalyzed coupling reaction will be summarized in a separate section below.

Scheme 106. Transition-Metal-Catalyzed Coupling Reactions of Aryl Diazonium Salts with Organometallic Compounds

Pd-catalyzed cross-coupling reactions of aryl diazonium salts are dominated by reactions with arylboronic acids. Recent advances mainly include sustainable, cost-effective catalytic procedures, ${}^{207}$ variations in the aryl organometallic partners, and one-pot procedures using anilines as substrates. The Suzuki-Miyaura coupling reactions of aryl diazonium tetrafluoroborates can be applied to the safe production of ${500}\mathrm{\;g}$ of 2-cyano-4-formylbiphenyl 149, an essential intermediate for manufacturing angiotensin II inhibitors (Scheme 107). ${}^{208}$

A series of heterogeneous catalytic systems have been explored for Suzuki coupling reactions of aryl diazonium salts. Utilizing palladium supported on activated carbon $\left( {\mathrm{{Pd}}/\mathrm{C}}\right)$ in an aqueous hydrotropic solution, a ligand-free protocol for the Suzuki-Miyaura reaction and base-free Heck-Matsuda reaction was reported under mild conditions. ${}^{209}$ In addition, the stabilizing effect of biopolymers and biomolecules has been exploited for preparing palladium composite catalysts. Both palladium nanoparticles immobilized on a titanium dioxide cellulose composite (PdNPs@TiO ${}_{2}$ -Cell) ${}^{210}$ and Pd@poly-dopamine-coated nano- ${\mathrm{{Fe}}}_{3}{\mathrm{O}}_{4}{}^{211}$ have been synthesized and utilized in heterogeneous catalytic Suzuki cross-coupling reactions.

Scheme 107. Hectogram-Scale Synthesis of 2-Cyano-4- Formylbiphenyl

From a green chemistry perspective, new Suzuki-Miyaura coupling procedures utilizing water as a solvent are attractive. Bräse et al. disclosed an environmentally benign Pd-catalyzed Suzuki-Miyaura cross-coupling reaction of aryl diazonium tosylates in neat water (Scheme 108). ${}^{212}$

Scheme 108. Suzuki-Miyaura Cross-Coupling of Aryl Diazonium Tosylates in Neat Water

A “cationic” Suzuki-Miyaura cross-coupling reaction was reported using a ${\mathrm{{PAd}}}_{3}$ -Pd catalyst in the absence of any base or metal mediator, allowing for the efficient coupling reaction of base-sensitive polyfluoroaryl and heteroaryl boronic acids (Scheme 109). ${}^{213}$ The low coordinate cationic palladium complexes 150 are stabilized by electron-rich bulky phosphine ligands such as ${\mathrm{{PAd}}}_{3}$ .

To expand the substrate scope of organoboron coupling partners, an ultrafast Pd-catalyzed coupling reaction of potassium vinyltrifluoroborate 151 was developed, facilitating the synthesis of styrenes that are prone to polymerization and the preparation of stilbenes within minutes through a Suzuki-Heck coupling sequence (Scheme 110). ${}^{214}$ The scope of the organoboron moiety can also be expanded to diarylborinic acids. ${}^{215}$

The advantage of easy availability of aniline starting materials may be counteracted by the lack of efficiency and flexibility in preparing and isolating aryl diazonium tetra-fluoroborates. In 2011, Wang and co-workers reported a Pd-catalyzed Suzuki cross-coupling reaction of in situ generated aryl diazonium salts with arylboronic acids. ${}^{216}$ This base-free, one-pot diazotization/cross-coupling protocol enables the synthesis of biaryls directly from anilines. Afterward, a base-, acid-, and ligand-free Pd-catalyzed Suzuki-Miyaura cross-coupling reaction of anilines was reported (Scheme 111). ${}^{217}$ This reaction proceeds through the formation of an aryl-Pd alkoxo complex 152, confirmed by ESI-MS as a trimeric complex, which promotes the transmetalation step with aryl

Scheme 109. “Cationic” Suzuki-Miyaura Coupling Reactions

Scheme 110. Ultrafast Suzuki-Miyaura and Heck Reactions for the Synthesis of Styrenes and Stilbenes

Pd(OAc) ${}_{2}$ (1 mol%)

$\mathrm{{Ar}} - {\mathrm{N}}_{2}\mathrm{{OTs}}$ BF3K

$\mathrm{{MeCN}}/{\mathrm{H}}_{2}\mathrm{O}$ , rt

151 within ${450}\mathrm{\;s}$ 13 examples

52-97% yields

Scheme 111. Pd-Catalyzed Suzuki-Miyaura Cross-Coupling Reactions of Anilines boronic acids without any external base. Another Suzuki-Miyaura cross-coupling of anilines has been reported using silver nitrite (AgONO) as an alternative nitrosating reagent. ${}^{218}$

Schmidt has developed a deacetylation/diazotization followed by Pd-catalyzed cross-coupling sequence of acetani-lides for selective C-C bond formations (Scheme 112). ${}^{{219} - {221}}$

Scheme 112. Deacetylation-Diazotization-Coupling Sequence for Pd-Catalyzed C-C Bond Formations

The reaction pathway includes the Lewis acid-mediated decomposition of the acetanilide to obtain anilines, followed by diazotization and sequential Pd-catalyzed coupling reaction with aryltrifluoroborates, furnishing unsymmetrical biaryls and stilbenes in moderate to good yields.

Moreover, a multicomponent Pd-catalyzed coupling reaction of aryl diazonium salts and arylboronic acids with ynamides 153 was disclosed, giving rise to functionalized triaryl-substituted enamides 154 (Scheme 113). ${}^{222}$ Mechanistically, this coupling reaction is initiated by the Pd-catalyzed coupling of 153 with aryl diazonium salts to afford a keteniminium species 155, followed by 1,3-aryl migration to generate a vinylic palladium(II) complex, and sequential trapping of the arylboronic acid.

Scheme 113. Pd-Catalyzed Coupling of Ynamides, Aryl Diazonium Salts, and Arylboronic Acids

Nickel complexes have also been explored as cheaper and efficient catalysts in cross-coupling reactions of aryl diazonium salts with arylboronic acids. In glycerol as a reaction medium, this base-free protocol is amenable to synthesize functionalized biaryls in good to excellent yields (15 examples, 71-90% yields). ${}^{223}$

Apart from the Suzuki-Miyaura coupling reactions, the Negishi cross-coupling of aryl diazonium $o$ -benzenedisulfoni-mides 107 was disclosed (Scheme 114). ${}^{224}$ In 2019, visible light-facilitated Ni-catalyzed one-pot Negishi coupling reactions was developed using diazonium salts with in situ generated ${\mathrm{{Ar}}}_{2}\mathrm{{Zn}}$ from Grignard reagents and ${\mathrm{{ZnBr}}}_{2}{}^{.225}$

Scheme 114. Negishi Cross-Coupling of Aryl Diazonium Salts

Pd-catalyzed cross-coupling reactions of aryl diazonium salts with organometallic partners other than arylboronic acids have also been established, including an aqueous phase Hiyama cross-coupling, ${}^{226}$ an $\mathrm{{Al}}{\left( \mathrm{{OH}}\right) }_{3}$ -supported Pd nanoparticles catalyzed Stille cross-coupling, ${}^{227}$ homocoupling of aryl diazonium salts in ionic liquids, ${}^{228}$ and cross-coupling with organoindium or organobismuth reagents ${}^{229}$ (Scheme 115).

In 2018, Shang reported a Pd-catalyzed desulfinative and denitrogenative cross-coupling reaction of aryl sulfonyl hydrazides with aryl diazonium salts in the presence of TBAI, which afforded biaryl products under mild conditions in good yields (37 examples, 66-95% yields). ${}^{230}$ A plausible pathway includes the initial formation of iodobenzene by in situ Sandmeyer-type iodination of the aryl diazonium salt with TBAI, followed by Pd-catalyzed desulfinative coupling to deliver the desired biaryls.

Scheme 115. Pd-Catalyzed Cross-Coupling Reaction of Aryl Diazonium Salts with Other Organometallic Partners

Another type of Pd-catalyzed coupling reaction involves the oxidative decarboxylative coupling of aryl $\alpha$ -keto carboxylic acid 156 with aryl diazonium fluoroborates, which represents a complementary synthetic route to the carbonylation reactions (Scheme 116). ${}^{231}$ Both radical trapping experiments and EPR spectroscopy results indicate that this protocol proceeds through a single-electron oxidation pathway involving the formation of an acyl radical.

Scheme 116. Pd-Catalyzed Decarboxylative Coupling of $\alpha$ - Oxocarboxylic Acid with Aryl Diazonium Salts

In 2014, a cobalt complex was explored as efficient catalysis in the cross-coupling reaction of aryl diazonium salts with vinyl diazoacetates 157, affording regiospecific synthesis of $N$ - arylpyrazoles with good yields (Scheme 117). ${}^{232}$ Particularly, the nitrogen moiety in the pyrazole scaffold is from the substrates of vinyl diazoacetates 157. The reaction mechanism is initiated by oxidative addition to form a cationic aryl-cobalt complex, followed by $N$ -coordination to form diazo-cobalt species 158. This complex undergoes 1,5-electrocyclization (1,5-EC)/tautomerization and loss of hydrogen. Finally, the reductive elimination of $N$ -cobalt pyrazole 159 furnishes the desired product and regenerates the active cobalt species. This reaction not only features many advantages such as cheap catalysts, mild conditions, and excellent regioselectivity but also explores the catalytic coupling between diazonium salts and diazo compounds.

Scheme 117. Co-Catalyzed Cross-Coupling Reactions of Aryl diazonium Salts with Vinyl Diazoacetates

2.2.4. Sonogashira-type Coupling Reactions with Alkynes. Transition metal-catalyzed Sonogashira-type cross-coupling reactions of aryl diazonium salts have opened an alternative avenue toward the synthesis of arylated alkynes (Scheme 118). Under copper- and phosphane-free conditions, a Pd-catalyzed Sonogashira coupling reaction of aryl diazonium o-benzenedisulfonimides 107 with terminal alkynes 72 was developed. ${}^{233}$ The $o$ -benzenedisulfonimide can be recovered in greater than 80% yields. In 2015, a ${\mathrm{{Pd}}}_{0} - {\mathrm{{MnFe}}}_{2}{\mathrm{O}}_{4}$ -catalyzed decarboxylative Sonogashira reaction of aryl diazonium salts with propiolic acid derivatives was reported. ${}^{234}$

Scheme 118. Pd-Catalyzed Ligand-free Sonogashira Coupling Reaction

2.2.5. Gold-Catalyzed Coupling Reactions and Cycli-zations. Taking advantage of aryl diazonium ions’ oxidative property, gold-catalyzed cross-coupling reactions of aryl diazonium salts have emerged as a novel strategy for constructing C-C bonds under redox-neutral conditions. Unlike traditional Pd-catalyzed coupling reactions of aryl diazonium salts, the oxidation process in gold catalysis includes two potential pathways (Scheme 119). Supported by DFT calculations, one pathway consists of single-electron oxidation (path a) of gold(I) complex by the in situ formed aryl radical under organic-initiated or photoredox conditions, giving rise to an Ar-Au(II) complex 160, followed by second single-electron oxidation to afford the Ar-Au(III) intermediate 161. ${}^{235}$ The other plausible pathway (path b) is initiated by the nucleophilic addition of gold(I) complex to aryl diazonium ions to afford an ${\mathrm{{ArN}}}_{2} - \mathrm{{Au}}$ (III) intermediate 162, which undergoes dinitrogen elimination upon visible-light irradiation or thermal condition to deliver the $\mathrm{{Ar}} - \mathrm{{Au}}$ (III) intermediate 161. Proceeded through this key intermediate, various coupling products were obtained through sequential transformations, including difunctionalization, Sonogashira-type reaction, biaryls synthesis, the Meyer-Schuster rearrangement reaction, cascade cyclization, and C-heteroatom bond formations (Scheme 119). In this section, advances in C-C bond formation will be introduced according to reaction types. Gold-catalyzed C-heteroatom bond formations will be the focus of the next section.

Scheme 119. General Routes for Gold-Catalyzed Coupling Reactions

2.2.5.1. Difunctionalization. In 2013, combining gold and photoredox catalysis, ${}^{{236},{237}}$ Glorius established an oxy- and aminoarylation of alkenes with aryl diazonium salts at room temperature (Scheme 120). ${}^{238}$ The use of this novel dual gold/ photoredox catalytic system allowed for the difunctionalization of alkenes under mild conditions without requiring external oxidants. Mechanistically, the cationic Au(I) catalyst 163 first reacts with the alkene substrate to afford the alkyl gold(I) intermediate 164. Aryl radicals generated from the aryl diazonium salt in the photoredox cycle then react with alkyl gold(I) species to afford the Au(II) intermediate 165. This Au(II) species is then oxidized by the Ru(III) catalyst, generating an Au(III) species 166, which then undergoes rapid reductive elimination to deliver the product.

With this dual gold and photoredox catalytic system, a wide range of three-component intermolecular oxyarylation reactions of simple alkenes with aryl diazonium salts were also developed (Scheme 121). ${}^{239}$ Inexpensive organic dyes could be employed as the photocatalyst, affording $\alpha$ -arylated ether products 167 in good yields under mild conditions.

In 2014, Toste and co-workers reported a ring expansion-oxidative arylation reaction of alkenyl or allenyl cycloalkanols 168 with aryl diazonium salts, affording functionalized cyclic ketones 169 in high yields (Scheme 122). ${}^{240}$ In 2015, Shin reported an arylative cyclization process from tert-butyl allenoates 170 with aryl diazonium salts, employing in situ generated vinyl gold complexes to deliver 3-arylated butenolides 171 (Scheme 122). ${}^{241}$

Shi developed nonphotocatalytic conditions for gold-catalyzed alkene difunctionalization using diazonium salts (Scheme 123). ${}^{242}$ In the presence of $\mathrm{{Au}}\left( \mathrm{I}\right)$ catalyst and ${\mathrm{{NaHCO}}}_{3}$ , various alkenes, alkynes, and allenes were converted to oxy- and aminoarylation products in good yields. Compared to the photoactivation method, this base-initiated approach also offered good efficiency, fast reaction rates, and broad substrate scope.

This gold-catalyzed visible-light-induced reaction also allows the difunctionalization of alkynes. In 2016, Hashmi reported a novel visible light-mediated gold-catalyzed intermolecular 1,2- difunctionalization of alkynes with aryl diazonium salts in methanol under photosensitizer- and oxidant-free conditions, providing an array of $\alpha$ -aryl ketones 172 in moderate to good yields (Scheme 124). ${}^{243}$ A proposed reaction pathway is initiated by light-induced single-electron oxidation of aryl diazonium salt to Au(I) center, giving rise to an Ar-Au(III) species 161. Subsequently, the addition of methanol to the $\pi$ - activated alkyne forms the vinyl gold species 173, followed by reductive elimination to generate arylated enol ether 174, which undergoes hydrolysis to afford $\alpha$ -aryl ketones.

Scheme 120. Dual Gold/Photoredox-Catalyzed Intramolecular Oxy- and Aminoarylation of Alkenes

Scheme 121. Dual Gold/Photoredox-Catalyzed Intermolecular Oxyarylation of Alkenes

Scheme 122. Arylative Ring Expansion of Alkenyl Cycloalkanols and Arylative Cyclization of Allenoates

In 2020, Echavarren developed a visible-light-induced gold-catalyzed arylative alkoxycyclization of 1,6-enynes with aryl diazonium salts in the presence of alcohols (Scheme 125). ${}^{244}\mathrm{\;A}$ plausible reaction pathway is initiated by the photoredox-assisted formation of an Ar-Au(III) complex, followed by 5- exo-dig cyclization to form a gold carbenoid species 175, which undergoes nucleophilic attack by alcohol to generate a vinyl gold(III) intermediate 176. Finally, reductive elimination affords the cyclization product along with the regeneration of the catalytically active Au(I) species.

The gold-catalyzed alkyne difunctionalization cascade demonstrates applications in synthesizing various cyclic compounds at room temperature, especially for benzo-fused scaffolds. In 2016, Du and Zhu reported a visible-light-mediated dual gold/photoredox catalyzed aminoarylation reaction of alkynes 177, affording multisubstituted indoles with good functional group tolerance in moderate to excellent yields (Scheme 126). ${}^{245}$ This cyclization proceeds through the photoredox SET oxidation to form Ar-Au(III) species, followed by 5-endo-dig intramolecular nucleophilic cyclization and reductive elimination to form indole products and regenerate the gold(I) complex.

Scheme 124. Visible-Light-Mediated Gold-Catalyzed 1,2- Difunctionalization of Alkynes

Scheme 125. Photoredox-Assisted Gold-Catalyzed Arylative Alkoxycyclization of 1,6-Enynes

Scheme 126. Visible-Light-Driven Aminoarylation of Alkynes

This visible-light-promoted dual gold/photoredox alkyne difunctionalization/cyclization strategy enables the selective coupling of heteroatom $\left( {\mathrm{N},\mathrm{S},\mathrm{{Se}},\mathrm{O}}\right)$ -tethered alkynes with aryl diazonium salts to rapidly access various heterocycles, including benzothiophenes 178, benzoselenophenes 179, 3H-indoles 180, vicinal diaryl-substituted ${2H}$ -benzo[e][1,2]- thiazine 1,1-dioxides 181 (benzosultams), ${1H}$ -isochromen-1- ones 182 (isocoumarins), 4H-chromen-4-ones 183 (flavones), and ${4H}$ -thiochromen-4-ones 184 (thioflavones) (Scheme 127). ${}^{246}$

Scheme 127. Selective Arylative Heterocyclization of Heteroatom (N, S, Se, O) Tethered Alkynes with Aryl Diazonium Salts

In addition, this dual gold/photoredox catalysis also opens a new avenue toward the synthesis of benzofurans (Scheme 128). ${}^{247}$ In 2016, Ollivier and Fensterbank developed an arylative cyclization of o-alkynylphenols 185 with aryl diazonium salts, affording 2,3-diarylated benzofuran derivatives 186 in the absence of base and/or additives upon visible-light irradiation. ${}^{248}$ Another visible-light-induced domino $\mathrm{C}\left( \mathrm{{sp}}\right)$ arylation/oxyarylation cascade reaction of 2-[(trimethylslyl)- ethynyl]phenol 185’ with aryl diazonium salts provides access to 2,3-diarylbenzofurans 186 via double arylation. ${}^{249}$

A photoredox-free gold(III)-promoted arylative cyclization of alkynoic acids with aryl diazonium salts was developed, furnishing seven/eight- and five/six-membered ring enol lactones under thermally induced conditions (Scheme 129). ${}^{250}$ A plausible mechanism involves the aryl diazonium salt oxidizing Au(I) to aryl gold(III) complex 161. Because of its Lewis acidity, aryl gold(III) complex 161 coordinates with the alkyne moiety to facilitate the carboxylic acid’s nucleophilic

Scheme 128. Dual Gold/Photoredox Catalyzed Synthesis of Benzofurans attack. Finally, the reductive elimination of 187/187’ furnishes the cyclic enol lactones.

Scheme 129. Gold-Promoted Arylative Cyclization of Alkynoic Acids with Aryl Diazonium Salts

2.2.5.2. C-H Functionalization. In 2017, a dual Au-/Ru-catalyzed photoredox arylation of arenes with aryl diazonium salts was established under mild conditions (Scheme 130). ${}^{251}$ Compared with the Pd/Ru photoredox system, this Au-catalyzed C-H functionalization enables the straightforward arylation of simple arenes without any directing group. Similar to Sanford’s mechanism, a single-electron reduction of the aryl diazonium salt from the photoredox catalyst generates the aryl radical, which follows an Au(I)-Au(II)-Au(III)-Au(I) catalytic cycle.

In 2020, Hashmi reported a photosensitizer-free photochemical synthesis of organogold(III) pincer complexes 188 from DMSAuCl and diazonium salts in the presence of a base (Scheme 131). ${}^{252}$ This method not only gives access to these pincer gold complexes in moderate to excellent yields but also provides evidence for the photoredox oxidation of gold(I)

Scheme 130. Dual Au/Ru Catalyzed Photoredox C-H Arylation of Arenes

Scheme 131. Synthesis of Pincer Au ${}^{\mathrm{{III}}}$ Complex by Oxidative Addition/C-H Activation Cascade species by aryl diazonium salts to form a gold(III) intermediate.

Aside from the photoredox system, ascorbic acid 53 has been employed as an organic initiator to accelerate the oxidative addition of in situ generated aryl diazonium chlorides onto Au(I) (Scheme 132). ${}^{253}$ The resulting aryl-Au(III) species 161 has been used to prepare 3-arylindoles with good regioselectivity from 1-methylindole. On the basis of EPR analysis, cyclic voltammetry experiments, and DFT calculations, the mechanism constitutes two steps of single-electron oxidation via a radical pathway.

Scheme 132. Ascorbic Acid as an Aryl Radical Initiator in Au-Mediated Arylation of Indoles with Aryl Diazonium Salts

2.2.5.3. Coupling Reactions with Ar-M Species. The dual gold/photoredox visible-light-promoted catalytic system was recently demonstrated as a reliable tool for oxidative addition of aryl diazonium salts to Au(I) complex to form the corresponding aryl-Au(III) species. ${}^{{254} - {257}}$ Utilizing this dual catalytic strategy, Suzuki-type coupling reactions of aryl diazonium salts and arylboronic acids have been developed. Catalysts include ${\mathrm{{PPh}}}_{3}{\mathrm{{AuNTf}}}_{2}{}^{255}$ or ${\mathrm{{Ph}}}_{3}\mathrm{{PAuCl}}$ in combination with $\mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\left( {\mathrm{{PF}}}_{6}\right) }_{2}$ or 9-mesityl-10-acridinium salt as the photosensitizer ${}^{256}$ (Scheme 133). Asymmetric synthesis of atropoisomeric biaryls using chiral (P, N) ligands was investigated, even though only up to 26% ee was obtained. ${}^{257}$ Mechanistic studies reveal that this dual catalytic pathway is highly catalyst-dependent: for the cationic PPh ${}_{3}{\mathrm{{AuNTf}}}_{2}$ catalyst, transmetalation of the arylboronic acid with $\mathrm{{Au}}\left( \mathrm{I}\right)$ occurs before oxidation of Au (I) to Au (III), while for the neutral ${\mathrm{{PPh}}}_{3}\mathrm{{AuCl}}$ catalyst, transmetalation occurs after oxidation of Au (I) to Au (III).

Scheme 133. Dual Gold/Photoredox System in Cross-Coupling Reaction of Aryl Diazonium Salts with Arylboronic Acids

Merging gold/photoredox catalysis under visible-light irradiation, Patil realized the Hiyama-type cross-coupling reactions of aryl diazonium salts with allylsilanes ${}^{258}$ and arylsilanes ${}^{259}$ (Scheme 134). This reaction is thought to proceed through a photoredox-promoted generation of an electrophilic Ar-Au(III) intermediate 161, which undergoes transmetalation with allyltrimethylsilane or arylsilane to form allylarenes or biaryls, respectively. In the latter case, the addition of copper salts as catalysts has been crucial for the success of this transformation, likely due to the acceleration of the transmetalation process.

Moreover, gold complexes have emerged as efficient catalysts in cross-coupling reactions of aryl diazonium salts in the absence of photoredox catalysts. Mechanistic investigations indicate that aryl diazonium salts can effectively serve as electrophiles for oxidative addition to $\mathrm{{Au}}\left( \mathrm{I}\right)$ to generate Au(III) species, either under mild photochemical conditions ${}^{254}$ or under thermal conditions. ${}^{260}$ Hashmi established a photosensitizer-free visible-light-driven gold-catalyzed Suzuki coupling reaction of aryl diazonium salts with arylboronic acids (Scheme 135). ${}^{261}$ The same group has extended this method to pinacol esters, MIDA boronates, and potassium trifluor-oboronates as the efficient coupling partners. ${}^{262}$ Mechanistically, these photosensitizer-free approaches proceed through path b, which is initiated directly by the nucleophilic addition of gold(I) complex to the terminal nitrogen of aryl diazonium ions, resulting in the formation of an Ar-Au(III) intermediate.

Scheme 134. Gold/Photoredox Catalytic Coupling Reactions of Allyl or Arylsilanes

Scheme 135. Photosensitizer-Free Visible-Light-Driven Gold-Catalyzed Suzuki and Hiyama Coupling Reaction

Organosilicon compounds have been explored as coupling partners in gold-catalyzed coupling reactions with aryl diazonium salts. ${}^{262}$ In 2018, Xie and Hashmi reported a photochemical gold-catalyzed chemoselective Hiyama arylation of B, Si-bifunctionalized aryl reagents 189 with aryl diazonium salts (Scheme 136). ${}^{263}$ A wide array of diversely functionalized ${\mathrm{{sp}}}^{2}$ - and ${\mathrm{{sp}}}^{3}$ -hybridized biarylboronates was obtained, which are versatile building blocks for the assembly of various multiaryls via one-pot iterative orthogonal Pd-catalyzed C-C or Cu-catalyzed C-heteroatom bond formations.

Furthermore, a photoredox-free gold-catalyzed Suzuki-type cross-coupling reaction of aryl diazonium salts with arylboronic acids was achieved without any external oxidants, affording substituted biaryls in moderate to good yields (Scheme intermediate 161. A similar approach using aryl diazonium o-benzenedisulfonimides as the electrophilic partners was also developed. ${}^{265}$

Scheme 136. Light-Induced Gold-Catalyzed Chemoselective Hiyama Arylation

137). ${}^{264}$ The bipyridine ligand plays an important role in facilitating dinitrogen extrusion to afford the key Ar-Au(III)

Scheme 137. Photoredox-Free Gold-Catalyzed Cross-Coupling with Aryl Diazonium Salts with Arylboronic Acids

Aside from the gold-catalyzed Suzuki-Miyaura coupling reaction, a photoredox-free gold-catalyzed Stille cross-coupling reaction of aryl diazonium salts with organostannanes was also reported, affording biaryls, vinyl arenes, and arylacetylenes in moderate yields (Scheme 138). ${}^{266}$

2.2.5.4. Coupling Reactions with Alkynes. Besides palladium catalysts, gold complexes have also been utilized to promote $\mathrm{C}\left( \mathrm{{sp}}\right) - \mathrm{C}\left( {\mathrm{{sp}}}^{2}\right)$ coupling reaction of aryl diazonium salts with alkynes (Scheme 139). Mechanistically, the key Ar-Au(III) intermediate can be generated under the light-free, base-mediated condition at room temperature, ${}^{264}$ light-free thermal conditions, ${}^{260}$ or using a photosensitizer-free visible light-enabled approach. ${}^{262}$

Scheme 138. Light-Free Gold-Catalyzed Stille Cross-Coupling Reactions

Scheme 139. Gold-Promoted C(sp)-C(sp ${}^{2}$ ) Coupling Reaction of Aryl Diazonium Salts

In 2016, Toste ${}^{267}$ and Glorius ${}^{268}$ explored a dual gold/Ru catalytic system to achieve the $\mathrm{C}\left( \mathrm{{sp}}\right) - \mathrm{C}\left( {\mathrm{{sp}}}^{2}\right)$ cross-coupling reaction of aryl diazonium tetrafluoroborates with alkynyl-trimethylsilanes 190 or terminal alkynes 72, respectively (Scheme 140). The reaction proceeds via visible light-mediated single-electron oxidative addition of aryl diazonium salts, resulting in the formation of alkynyl-Au(III) species, followed by reductive elimination.

Scheme 140. Dual Gold/Photoredox Catalytic C(sp)-C(sp ${}^{2}$ ) Cross-Coupling Reactions

2.2.5.5. Meyer-Schuster Rearrangement. Utilizing a dual gold/photoredox visible-light-promoted catalytic system, several groups disclosed the tandem Meyer-Schuster/arylation of propargyl alcohols with aryl diazonium salts, affording $\alpha$ - arylated enones in one pot (Scheme 141). In 2016, Shin ${}^{269}$ and Glorius ${}^{270}$ reported tandem Meyer-Schuster/cross-coupling of alkynols 191 with aryl diazonium tetrafluoroborates under blue or green LED irradiation, respectively. In this process, aryl diazonium salts play triple roles as oxidants, coupling partners, and catalytic activators to dissociate propargyl alcohols. In 2016, Alcaide and Almendros also developed the arylative coupling of alkynols 191 or alkynyl hydroperoxides, affording aryl-substituted $\alpha ,\beta$ -unsaturated ketones and 3-acylidene-2- oxindole derivatives. ${}^{271}$

Scheme 141. Dual Gold/Photoredox Catalysis for Meyer-Schuster/Arylation Reaction

The mechanism of this dual catalytic Meyer-Schuster/ arylation reaction has been proposed as shown in Scheme 142. The Au(I) species first undergoes single-electron oxidation with an aryl radical generated upon oxidative quenching of the visible-light-activated photocatalyst by the aryl diazonium salt. Subsequent single-electron oxidation of the resulting Ar-Au(II) complex 160 either by the oxidized photocatalyst Ru(III) or in a radical chain reaction with another equivalent of aryl diazonium cation will lead to the Ar-Au(III) species 161. With the activation of Au(III) species in MeOH solvent, Meyer-Schuster rearrangement occurs through an allenyl ether intermediate. Finally, reductive elimination from the subsequent $\sigma$ -complex 192 selectively affords the coupling product accompanied by the regeneration of $\mathrm{{Au}}\left( \mathrm{I}\right)$ species.

Scheme 142. Proposed Mechanism of Tandem Meyer-Schuster/Arylation Reaction

In 2017, Alcaide and co-workers employed a dual gold/ photoredox catalytic system to realize a domino $\mathrm{C}\left( \mathrm{{sp}}\right) -$ arylation/oxyarylation of TMS-terminated alkynols 193, affording tri- and tetrasubstituted $\alpha ,\beta$ -unsaturated ketones (Scheme 143). ${}^{249}$ This domino bis-arylative sequence proceeds

Scheme 143. Visible-Light-Induced Gold-Catalyzed Domino C(sp) Arylation/Oxyarylation of TMS-Terminated Alkynols with Aryl Diazonium Salts

via the initial dual catalytic $\mathrm{C}\left( \mathrm{{sp}}\right) - \mathrm{C}\left( {\mathrm{{sp}}}^{2}\right)$ coupling reaction of TMS-terminated alkynols 193 to form monoarylated intermediates 194, followed by Meyer-Schuster/arylation to produce the desired diarylated $\alpha ,\beta$ -unsaturated ketones.

3. ARYL DIAZONIUM SALTS IN ARYL C-HETEROATOM BOND FORMATIONS

Sandmeyer-type transformations of aryl diazonium salts have opened a superior avenue toward the selective construction of aryl $\mathrm{C} - \mathrm{C}$ and $\mathrm{C} - \mathrm{X}$ bonds. In the classical $\mathrm{C} - \mathrm{X}$ bond formations of aryl diazonium ions, copper salts are required for aryl radical generation. The scope of heteroatom is usually limited to halogen, nitrogen, oxygen, and sulfur atoms. Recent endeavors in the field have mainly focused on the expansion of the scope of the reaction, more types of carbon-heteroatom bond formations, and the combination with photoredox or electrochemical synthetic methods. In this review, the most recent selected advances are classified according to the types of C-X bonds.

3.1. C-Halogen Bond Formations

Traditional Sandmeyer halogenations of aryl diazonium salts have been widely used in constructing aryl $\mathrm{C} - \mathrm{X}\left( {\mathrm{X} = \mathrm{{Cl}},\mathrm{{Br}}}\right)$ bonds via denitrogenative substitution reactions. However, these halogenations need the participation of a stoichiometric amount of copper halides or harsh hydrohalic acid as the halogen sources and low reaction temperature for the diazotization process. In view of the importance of aryl halides in organic synthesis, the development of novel halogenation of aryl diazonium salts has garnered significant attention from the synthetic community. Recent developments are focused on expanding substrate scope and exploring new efficient catalytic systems for aryl $\mathrm{C} - \mathrm{X}$ bond formations, including the employment of halogen sources and alternative routes for generating aryl radicals.

For example, a diazotization/iodination process based on the lamination of Pickering emulsions has been established, which represents a complementary route for constructing an aryl C-I bond. ${}^{272}$ Additionally, aiming to avoid copper halides or HX as the halogen source, a metal- and acid-free halogenation of arylamines using N-halosuccinimide and sodium nitrite serves as a complementary approach to the Sandmeyer reaction (Scheme 144). ${}^{273}$ The direct hydro-deamination of arylamines using ${\mathrm{{BF}}}_{3} \cdot {\mathrm{{Et}}}_{2}\mathrm{O}$ in DMF has also been examined.

Scheme 144. Direct Halogenation and Hydrodeamination of Arylamines in DMF

Aside from copper salts, the gold(I) complex ${\mathrm{{Ph}}}_{3}\mathrm{{PAuCl}}$ has been explored as an efficient catalyst in C-heteroatom coupling reactions with different nucleophiles, achieving aryl $\mathrm{C} - \mathrm{{Br}},\mathrm{C} - \mathrm{S}$ , and $\mathrm{C} - \mathrm{P}$ bond formations under photoredox-free thermal conditions (Scheme 145). ${}^{274}$ On the basis of mechanistic investigations, the heteroatom nucleophiles are thought to facilitate the oxidation of $\mathrm{{Au}}\left( \mathrm{I}\right)$ to form $\mathrm{{Ar}} - \mathrm{{Au}}\left( \mathrm{{III}}\right)$ complex 195, which undergoes final reductive elimination to forge C-Nu bonds.

Scheme 145. Gold-Catalyzed Aryl C-Br, C-S, and C-P Bond Formations

In 2018, Mo and co-workers developed a general electrochemical protocol for the Sandmeyer halogenation process (Scheme 146). ${}^{275}$ With the employment of a simple and inexpensive halogen source, this electrochemical protocol enables the efficient synthesis of a variety of aryl halides in good yields. A practical application of this method has been demonstrated by the one-pot halogenation process using anilines as the starting material and the multigram scale synthesis of aryl halides using inexpensive graphite as the electrode. On the basis of the mechanistic studies, including radical clock and radical scavenger experiments, cyclic voltammetry analysis, and in situ electron paramagnetic resonance (EPR) analysis, a single-electron reduction mechanism for the halogenation of diazonium salts upon electrolysis has been proposed.

Scheme 146. Electrochemical Sandmeyer Reaction of Aryl Diazonium Salts

mechanism

One of the oldest and most commonly used C-F bond-forming reactions is the Balz-Schiemann reaction. The preformed aryl diazonium tetrafluoroborates undergo thermal decomposition at a high temperature to afford aryl fluorides. Aiming to forge aryl $\mathrm{C} - \mathrm{F}$ bond under mild conditions, $\mathrm{{Hu}}$ developed a hypervalent iodine(III) reagent/Lewis acid catalytic system to synthesize fluoroarenes at relatively low temperatures (Scheme 147). ${}^{276}\mathrm{{Hu}}$ and co-workers suggested that the aryl diazonium is partially reduced by the hypervalent iodine species, which enhances the leaving propensity of the diazo moiety of the aryl diazonium salt to give an aryl cation intermediate 196. This species 196 undergoes sequential intramolecular nucleophilic fluorination to obtain the desired aryl fluoride.

3.2. C-O Bond Formations

A standard method for converting aryl diazonium salts to the corresponding phenols is the thermal decomposition of diazonium salts in a highly acidic aqueous solution. ${}^{2}$ In 2017, Zarei reported the hydrolysis of diazonium salts immobilized on the surface of silica sulfuric acid. ${}^{277}$ Employing diazonium salt nanocomposites, this protocol features operational simplicity, availability of reactants, short reaction times and easy workup, affording phenol derivatives in moderate to good yields (11 examples, 20-83% yields).

Scheme 147. Hypervalent Iodine(III)-Catalyzed Balz-Schiemann Reaction

In combination with photoinduced transformations, a visible-light-induced photoredox intramolecular C-O bond formation of 2-(2’-aminophenyl)-phenol derivatives was developed, affording naturally occurring dibenzofuran scaffold 197 (Scheme 148). ${}^{278}$ In this protocol, intramolecular $\mathrm{C} - \mathrm{O}$ bond formation through in situ formed diazonium intermediate from 2-(2’-aminoaryl)phenol is facilitated by visible-light photocatalysis utilizing T(p-F)PPT 198 as the organic photosensitizer to initiate the generation of oxy radical intermediate 199.

Scheme 148. Visible-Light-Promoted Synthesis of Dibenzofuran Derivatives

mechanism

Furthermore, trifluoromethyl ethers are increasingly being investigated for pharmaceutical, agrochemical, and materials science applications due to their strongly electron-withdrawing and highly lipophilic properties. Therefore, the development of new methods for the synthesis of trifluoromethyl ethers has received much attention. In 2019, a dual photoredox/Cu-catalytic trifluoromethoxylation of aryl diazonium salts was developed upon visible-light irradiation using trifluoromethyl arylsulfonate as the trifluoromethoxylation reagent (Scheme 149). ${}^{279}$ This new strategy combined a photoredox catalytic system with Cu-promoted selective trifluoromethoxylation, enabling the synthesis of trifluoromethoxylated arenes in good yields, as well as the late-stage trifluoromethoxylation of complex small molecules. Mechanistic studies indicate that $\mathrm{{Cs}}\left\lbrack {\mathrm{{Cu}}{\left( {\mathrm{{OCF}}}_{3}\right) }_{2}}\right\rbrack$ intermediate 200 might be generated during the reaction, followed by single-electron oxidation by Ru(III) and aryl radical to form a key $\mathrm{{Ar}} - \mathrm{{Cu}}$ (III) $- {\mathrm{{OCF}}}_{3}$ adduct 201. Finally, reductive elimination from this hypervalent Cu(III) intermediate 201 yields the desired product and regenerates the $\mathrm{{Cu}}\left( \mathrm{I}\right)$ species.

Scheme 149. Visible Light Photoredox Cu-Promoted Trifluoromethoxylation of Aryl Diazonium Tetrafluoroborates

In 2019, a silver-mediated trifluoromethoxylation of (hetero)aryl diazonium tetrafluoroborates under light-free conditions was published (Scheme 150). ${}^{280}$ This light-free protocol uses ${\mathrm{{AgOCF}}}_{3}$ as the trifluoromethoxylation reagent. The reaction is initiated by an SET process to generate an aryl radical and Ag(II) species 202, followed by recombination and $\mathrm{C} - {\mathrm{{OCF}}}_{3}$ bond formation.

3.3. C-N Bond Formations

Aryl diazonium salts can serve as partners in transition-metal-catalyzed coupling reactions to form aryl $\mathrm{C} - \mathrm{N}$ bonds, such as the traditional introduction of azido group onto aromatic rings. ${}^{281}$ This section summarizes recent advances in $\mathrm{C} - \mathrm{N}$ bond formations through coupling reactions via nitrogen extrusion. The cascade reactions with nitriles or the synthesis of nitrogen-containing heterocycles with the retention of diazo moiety will be discussed in the following section.

Scheme 150. Ag-Mediated Trifluoromethoxylation of Aryl Diazonium Salts

Using arylamine substrates, an iron-mediated C-N reaction of the in situ formed aryl diazonium salts with dialkyl azodicarboxylate was reported, affording N-aryl hydrazide derivatives 203 in good yields (Scheme 151). ${}^{282}$ This reaction is initiated by single-electron reduction of $\mathrm{{Fe}}\left( \mathrm{{II}}\right)$ to generate an aryl radical, followed by radical addition to DIAD to deliver hydrazides.

Scheme 151. Fe-Mediated Reaction of Aryl Diazonium Salts with Azodicarboxylate

Aside from hydrazide formation, a Cu-catalyzed Ullmann-type arylation of pyrazoles with in situ generated aryl diazonium salts from anilines was carried out under ligand-free conditions (Scheme 152). ${}^{{283},{284}}$ For other nitrogen heterocycles, the addition of TBAI and base significantly improves the yields of this C-N bond formation protocol. Theoretical studies toward the copper-catalyzed arylation indicate that this coupling reaction proceeds through a $\mathrm{{Cu}}\left( \mathrm{I}\right) /\mathrm{{Cu}}\left( \mathrm{{III}}\right)$ catalytic cycle. ${}^{285}$ In addition, oxidative addition of aryl diazonium salts is found to be the rate-determining step.

3.4. C-B Bond Formations

Methods for the preparation of arylboronic acid derivatives have attracted much attention from organic chemists. In recent years, the selective construction of aryl $\mathrm{C} - \mathrm{B}$ bonds via the homolytic cleavage of the boron-boron bond opens up a new approach for the synthesis of arylboronic acid derivatives. ${}^{{286},{287}}$ In particular, Sandmeyer-type borylation of arylamines and aryl diazonium salts for the synthesis of (hetero)arylboronic acids and boronates has been well studied in the past decade. Several modified practical routes to arylboronic compounds have been established, which can be applied in both academic settings and industrial processes.

Scheme 152. Cu-Catalyzed Arylation of Nitrogen Heterocycles from Anilines

In 2010, Mo, Wang and co-workers reported a metal-free aryl $\mathrm{C} - \mathrm{B}$ bond forming reaction that directly converts arylamines into arylboronates with ${\mathrm{B}}_{2}{\mathrm{{pin}}}_{2}$ (134) in the presence of $t$ -BuONO (Scheme 153). ${}^{288}$ The addition of a catalytic amount of BPO as a radical initiator improves the yields at room temperature. The reaction displays a broad substrate scope with most meta- and para-substituted anilines. However, ortho-substituted arylamines with an electron-deficient group and meta-substituted arylamines with an electron-donating group give lower yields or only trace amounts of product. In addition, heteroaromatic amines are incompatible with this chemistry. In 2013, this borylation method was further expanded by increasing the reaction temperature to ${80}^{ \circ }\mathrm{C}{.}^{289}$ The modified protocol works well for heterocyclic arylamines.

Scheme 153. Sandmeyer-type Borylation of Arylamines for the Synthesis of Arylboronates

In 2014, Pucheault and co-workers disclosed a series of metallocene reagents capable of promoting borylation of anilines to form boronates at room temperature (Scheme 154). ${}^{290}$ This protocol uses diisopropylaminoborane (204, ${\mathrm{H}}_{2}{\mathrm{{BNi}}}^{ - }{\mathrm{{Pr}}}_{2}$ ) as the borylation agent and is presumed to proceed through a radical chain mechanism. Another zinc-catalyzed borylation of aryl diazonium salts and aryltriazenes was developed to synthesize arylboronate in the presence of 5% $\mathrm{{Zn}}{\left( {\mathrm{{ClO}}}_{4}\right) }_{2}$ . ${}^{291}$

Scheme 154. Borylation Using Group IV Metallocene and Zinc Catalysts

Other borylation reagents have also been employed to forge aryl $\mathrm{C} - \mathrm{B}$ bonds. Aside from ${\mathrm{B}}_{2}{\mathrm{{pin}}}_{2}$ (134), diboronic acid 205 has been explored as a new borylation reagent to facilitate the aqueous phase borylation of a wide range of diazonium salts, leading to the facile synthesis of (hetero)arylboronic acids (Scheme 155). ${}^{292}$ Furthermore, this one-pot fast borylation method was combined with other reactions such as esterification, Suzuki-Miyaura coupling, hydrogenolysis, and Petasis borono-Mannich reaction for rapid access to molecular diversity. ${}^{293}$

Scheme 155. Aqueous Phase Borylation of Aryl Diazonium Salts and Arylamines with Diboronic Acid

In addition to the synthesis of arylboronic acids and pinacol boronates, a metal-free transformation from arylamines with a nonsymmetrical diboron reagent B(pin)-B(dan) 206 to access aryl naphthalene-1,8-diamino boronamides 207 was reported (Scheme 156). ${}^{294}$ This Sandmeyer-type borylation reaction represents an important supplement to the preparation of aryl-B(dan) compounds.

To develop Sandmeyer-type borylation under aqueous conditions, Xue and co-workers reported aqueous phase diazotization conditions to convert aryl amines to arylboro-nates. ${}^{295}$ The borylation with ${\mathrm{B}}_{2}{\left( \mathrm{{OH}}\right) }_{4}$ as the boron source

Scheme 156. Transformation from Arylamines to Aryl Naphthalene-1,8-Diamino Boronamides

was also achieved, affording arylboronic acid products (Scheme 157).

Scheme 157. Methanol-Promoted Borylation of Arylamines to Access Arylboronates and Boronic Acids

Another aqueous phase borylation of aryl diazonium tetrafluoroborates with ${\mathrm{B}}_{2}{\text{ pin }}_{2}$ (134) was shown to proceed in acetone-water solution at room temperature (Scheme 158). ${}^{296}$

Merging photoredox catalysis with borylation, Yan and coworkers first disclosed an organic dye (eosin Y)-catalyzed visible-light-promoted reduction of stable aryl diazonium tetrafluoroborate salts to generate free aryl radicals that are borylated with ${\mathrm{B}}_{2}{\text{ pin }}_{2}$ . ${}^{297}$ The one-pot diazotization/photoinduced borylation of anilines with $t$ -BuONO and ${\mathrm{B}}_{2}{\text{ pin }}_{2}$ (134) occurs at room temperature upon visible-light irradiation (Scheme 159). ${}^{298}$ In 2020, Wu and co-workers reported the borylation of aryl diazonium salts in water using easily prepared, highly emissive, and crystalline carbon dots as efficient photoredox catalysts, affording arylboronates in moderate to good yields (22 examples, up to 78% yield). ${}^{299}$ These photoredox borylation protocols feature good functional group tolerance, nontoxic catalysts, and green solvents.

Scheme 158. Catalyst-Free Borylation of Aryl Diazonium Tetrafluoroborates in Acetone-Water Solution

Scheme 159. Visible-Light-Induced Photoredox Borylation of Anilines

As illustrated before, Kubota and Ito developed a novel mechanoredox system through agitation of piezoelectric materials via ball milling to achieve the borylation of aryl diazonium salts (Scheme 160). ${}^{119}$ This mechanoredox borylation can serve as a complementary option to the existing transition-metal-catalyzed or photoredox transformations.

Scheme 160. Mechanoredox Borylation by Using Ball Milling and Piezoelectric Materials

3.5. C-P Bond Formations

The cascade coupling reaction involving Cu-mediated Sandmeyer-type C-P bond formation of aryl diazonium salts with ${\mathrm{{PCl}}}_{3}$ followed by hydrolysis can afford arylphosphonic acid derivatives. ${}^{300}$ However, this protocol usually forms a mixture with the desired product in moderate yields. ${}^{301}$ In 2015, Toste utilized a dual gold/photoredox system in the cross-coupling to form C-P bonds, allowing the P-arylation of aryl diazonium salts with H-phosphonates (Scheme 161). ${}^{302}$

Scheme 161. Gold/Photoredox Catalyzed Aryl C-P Bond Formations

This mild protocol proceeds at room temperature in the absence of base or oxidants, affording arylphosphonates in moderate to good yields. The proposed mechanism starts with photoredox single-electron oxidation to form an electrophilic aryl gold(III) intermediate 161, followed by coupling with the H-phosphonate nucleophile to generate an aryl-Au-P(O)R ${}_{2}$ species 208. Finally, reductive elimination forms the desired arylphosphonates 209 along with the regeneration of $\mathrm{{Au}}\left( \mathrm{I}\right)$ species.

In 2016, our group reported metal-free Sandmeyer-type phosphorylation of arylamines with triarylphosphites as the phosphorus source (Scheme 162). ${}^{303}$ This one-pot phosphorylation of anilines is conducted under mild conditions without excluding air or moisture, affording functionalized arylphosph-onates 210 in moderate to good yields. This reaction is proposed to proceed through a radical pathway, initiated by the SET of diazonium salt with triaryl phosphite to give a diazonium radical anion 211 and a phosphite radical cation 212. Then, phosphite radical cation 212 reacts with water to generate a phosphono radical 213 and a phenol byproduct. Finally, the reaction between diazonium radical anion 211 and phosphono radical 213 gives the desired phosphorylation product.

Scheme 162. Metal-Free Sandmeyer-Type Aromatic C-P Bond Formations

3.6. C-Sn Bond Formations

The versatile applications of organotin reagents to forge $\mathrm{C} - \mathrm{C}$ bonds through Stille coupling have made organotin compounds synthetically valuable building blocks. However, existing methods for constructing aryl C-Sn bonds usually suffer from harsh conditions and limited scope. In 2013, Wang and co-workers reported a Sandmeyer-type stannylation reaction of arylamines with hexamethyldistannane 214 as the tin source (Scheme 163). ${}^{304}$ Using one equivalent of acid to promote the diazotization process, this mild stannylation reaction affords a wide range of aryl trimethylstannanes 215 in moderate to good yields. The synthetic application of this protocol has been successfully demonstrated by a gram-scale synthesis and the tandem Pd-catalyzed Stille cross-coupling reaction to obtain biaryl products in good yields. Mechanistically, this stannylation involves the diazotization of the amino group with $t$ -BuONO to form a diazonium salt, followed by SET from ${\left( {\mathrm{{SnMe}}}_{3}\right) }_{2}$ (214) to the diazonium salt, generating diazonium radical 216 and radical cation species 217. The decomposition of diazonium radical anion 217 forms an aryl radical, while the radical cation 217 reacts with tert-butoxide anion 218 to generate a trimethylstannyl radical 219. The reaction of the aryl radical with ${\left( {\mathrm{{SnMe}}}_{3}\right) }_{2}\left( \mathbf{{214}}\right)$ or the alternative direct combination of the aryl radical with trimethylstannyl radical 219 can both afford the stannylation product.

Scheme 163. Sandmeyer-type Stannylation Reaction of Arylamines

The same group further developed a consecutive Sand-meyer-type borylation and stannylation from nitroaniline derivatives, leading to the synthesis of a series of functionalized trimethylstannyl arylboronates 220 (Scheme 164). ${}^{305}$ Taking advantage of the difference in the reactivity of boronates and stannanes in Pd-catalyzed cross-coupling reactions, a series of $m$ - and $p$ -terphenyl derivatives can be synthesized in moderate yields through chemoselective consecutive Stille coupling and Suzuki-Miyaura coupling reactions.

Scheme 164. Synthesis of Trimethylstannyl Arylboronates from Nitro-Substituted Anilines

3.7. C-Se, Te Bond Formations

Sandmeyer-type transformation of aryl diazonium salts can also be used for the preparation of unsymmetrical diaryl selenides (Scheme 165). In 2012, Ranu and co-workers disclosed a microwave-assisted reaction of aryl diazonium fluoroborates with diaryl dichalcogenides 221 in the presence of zinc dust, affording various unsymmetrical diaryl chalcogenides in high yields. ${}^{306}$ In 2014, Alves and co-workers described a catalyst-free reaction of aryl diazonium tetrafluoroborates with in situ generated arylselenols from diaryl diselenides and hypophosphorous acid $\left( {{\mathrm{H}}_{3}{\mathrm{{PO}}}_{2}}\right) \cdot {}^{307}$ This direct nucleophilic aromatic substitution reaction enables the synthesis of diaryl selenides in moderate to good yields.

Scheme 165. Synthesis of Diaryl Selenides from Aryl Diazonium Tetrafluoroborates with Diaryl Diselenides

Ranu

Recently, the selective trifluoromethylselenolation of aryl diazonium salts has attracted substantial attention from chemists. The Sandmeyer protocol for synthesizing trifluor-omethyl selenoethers was achieved by a copper-catalyzed trifluoromethylselenolation of aryl diazonium salts using ${\mathrm{{Me}}}_{4}{\mathrm{{NSeCF}}}_{3}$ (222) as the trifluoromethylselenolation reagent (Scheme 166). ${}^{308}$ This Cu-catalyzed protocol allows the efficient synthesis of trifluoromethyl selenoethers in high yields under mild conditions.

On the basis of the on-water synthetic strategy with ${\mathrm{{Na}}}_{2}\mathrm{{Te}}$ , Tanini and Capperuccia reported a tellurenylation method through Sandmeyer-type reaction (Scheme 167). ${}^{309}$ A range of substituted anilines are converted into the corresponding valuable diaryl tellurides upon treatment with ${\mathrm{{Na}}}_{2}\mathrm{{Te}}$ through a copper-free on-water Sandmeyer-type reaction.

Moreover, visible-light catalysis has opened up convenient routes for direct late-stage $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right)$ -chalcogen- ${\mathrm{{CF}}}_{3}$ bond formations. ${}^{310}$ In 2018, Billard and Tlili reported the first visible-light-promoted photoredox trifluoromethylselenolation of aryl diazonium salts (Scheme 168). ${}^{311}$ Using an organic photocatalyst and shelf-stable reagent trifluoromethyl toluene-selenosulfonate ${\mathrm{{TsSeCF}}}_{3}$ , this metal-free protocol allows the synthesis of trifluoromethylselenolated arenes as well as their higher homologues in high yields at room temperature. On the basis of EPR spectroscopy, luminescence investigation, and cyclic voltammetry, this reaction is believed to proceed through the initial reduction of the diazonium salt by the exited eosin Y through SET. The resulting aryl radical reacts with the ${\mathrm{{CF}}}_{3}{\mathrm{{SeSeCF}}}_{3}$ dimer obtained through the photoinduced homolysis of ${\mathrm{{TsSeCF}}}_{3}$ , furnishing a trivalent selenium radical 223. This key trivalent selenium radical intermediate is then oxidized by the aryl diazonium salt to furnish the electrophilic species 224, which then reacts with DMSO to form the desired product.

Scheme 166. Sandmeyer-type Trifluoromethylselenolation of (Hetero)Aromatic Amines Catalyzed by Copper

Scheme 167. Sandmeyer-type Synthesis of Diaryl Tellurides

Scheme 168. Visible-Light-Mediated Metal-Free Synthesis of Trifluoromethylselenolated Arenes

In 2020, Lu and Zhao reported a visible-light photocatalytic difluoromethylselenolation of in situ generated aryl diazonium salts using a shelf-stable Se-(difluoromethyl) 4-methylbenze-nesulfonoselenoate 225 (Scheme 169). ${}^{312}$ Mechanistic investigations, including ${}^{19}\mathrm{\;F}$ NMR spectroscopy and radical trapping experiments, indicate this difluoromethylselenolation reaction is initiated by SET of photoexcited RB with an aryl diazonium salt to afford an aryl radical, which reacts with 1,2- bis(difluoromethyl)diselane generated by the visible-light-induced homolysis of ${\mathrm{{TsSeCF}}}_{2}\mathrm{H}$ 225 to deliver the corresponding aryl difluoromethylselenol ether and difluor-omethylselenol radical. The sulfone radical 226 subsequently undergoes SET with the radical anion of the photocatalyst, leading to the regeneration of the photocatalyst and the formation of sulfite cation 227, which is trapped by water to afford TsOH.

Scheme 169. Photoredox Metal-free Difluoromethylselenolation of Arylamines

3.8. C-S Bond Formations

Sandmeyer-type transformations of aryl diazonium salts have been widely used in aryl $\mathrm{C} - \mathrm{S}$ bond formations. Mechanistically, this C-S bond forging process may proceed through the following pathways (Scheme 170). One (path a) is initiated by SET of an aryl diazonium ion to generate an aryl radical, which is intercepted by sulfur sources to construct a C-S bond. The Stadler-Ziegler reaction involves this radical-type protocol using transition metals or other initiators to facilitate the generation of aryl radicals. Another route (path b) is the transition metal-catalyzed coupling process to form Ar-M species, followed by trapping a nucleophilic sulfur moiety to give the corresponding $\mathrm{{Ar}} - \mathrm{S}$ coupling product. The third one (path c) constitutes a direct nucleophilic substitution, in which the aryl diazonium acts as the precursor of carbenium ions. Recent advances mainly focus on developing new catalytic systems, exploring substrate scope, and synthesizing various functionalized S-containing aromatic molecules. This section will classify these reactions according to the types of products, including sulfides, sulfoxides, sulfones, tri- or difluoromethyl thioether, and other compounds.

Scheme 170. General Routes for Aryl C-S Bond Formations Using Aryl Diazonium Salts

3.8.1. Synthesis of Aryl Sulfides and Sulfoxides. Although SRN1 reaction of aromatic thiols with aryl diazonium salts (Stadler-Ziegler synthesis) has been widely employed for the synthesis of unsymmetrical diaryl sulfides, various modifications have also been made to achieve improved yields and milder reaction conditions in the recent research. For practical applications, the synthesis of aryl sulfides through aryl diazonium transformations has been utilized in the site-selective modification of amino acids in proteins, an active area of research in chemical biology. In 2018, Sengupta and Chandrasekaran reported a mild chemo-selective method for $S$ -arylation of cysteine using aryl diazonium salts in MeOH (Scheme 171). ${}^{313}$ This base-promoted metal-free protocol can serve as a tool for modifying cysteine derivatives through a radical chain mechanism.

Scheme 171. Metal-free S-Arylation of Cysteine by Using Aryl Diazonium Salts

Moreover, further C(aryl)-S coupling reactions have been explored to access aryl sulfides (Scheme 172). In 2016, a weak base-mediated radical chalcogenation of aryl diazonium salts was developed, affording various functionalized thioethers, selenides, and tellurides. ${}^{314}$ The reaction pathway involves the homolysis of the initially formed diazoacetate to generate an aryl radical, which is then intercepted by a chalcogenide. The divergent synthesis of symmetrical and unsymmetrical diaryl sulfides resulted from the coupling reaction of aryl diazonium salts with ${\mathrm{{Na}}}_{2}\mathrm{\;S}$ or disulfides under microwave- or iron-/base-mediated conditions, ${}^{315}$ or Cu-catalyzed C(aryl)-S coupling reactions of aryl dithiocarbamates 228 and aryl diazonium salts. ${}^{316}$

Scheme 172. C(aryl)-S Coupling Reactions for the Synthesis of Aryl Sulfides

The organocatalytic photoredox system has been explored for the generation of aryl radicals and the synthesis of aryl sulfides from aryl diazonium salts with disulfides 229 (Scheme 173). ${}^{317}$ In the presence of $2\mathrm{\;{mol}}\%$ eosin $\mathrm{Y}$ as the photosensitizer, this mild protocol provides aryl sulfides in good yields. The critical steps in this photocatalytic thiolation include the photoinduced generation of aryl radicals, followed by a nucleophilic attack of disulfide 229 to give trivalent sulfur radical 230. Subsequent single-electron oxidation of trivalent sulfur radical by the eosin Y radical cation furnishes an electrophilic species 231, which undergoes substitution in the presence of DMSO.

Scheme 173. Visible Light-Mediated Synthesis of Aryl Sulfides

Employing the photoredox catalytic system, a visible-light-mediated one-pot Stadler-Ziegler reaction of arylamines with thiols affords aryl sulfides in good yields under mild conditions (Scheme 174). ${}^{318}$ A scalable continuous-flow protocol has also been developed. Microreactors’ use leads to improved irradiation of the reaction medium, resulting in significant acceleration compared to its batch counterpart. On the basis of the control experiment result, this reaction is likely initiated by the oxidative quenching of photoexcited Ru(II) species with photolabile diazosulfides 232 to generate aryl radicals.

Scheme 174. Photoredox One-Pot Stadler-Ziegler Synthesis of Aryl Sulfides

In 2015, Jiang reported a visible-light-induced thioether formation through photoredox C-S coupling reaction of alkyl/ aryl thiosulfates 233 and aryl diazonium salts at room temperature (Scheme 175). ${}^{319}$ This protocol allows the late-stage sulfuration of pharmaceutical derivatives. Electron paramagnetic study reveals a thiosulfate radical species 234 as the key intermediate. The cercosporin has been found to initiate photocatalyzed C-S coupling of thiols and aryl diazonium salts, allowing the selective synthesis of aryl sulfides under mild conditions (Scheme 175). ${}^{320}$

Scheme 175. Visible-Light-Induced Thioether Formation from Thiosulfates or Thiols

Apart from the synthesis of aryl sulfides, the methylsulfox-idation of aryl diazonium salts has also been explored to obtain aryl sulfoxides (Scheme 176). In 2016, Rastogi developed a visible-light-induced methylsulfoxidation of (het)aryl diazonium salts with DMSO. ${}^{321}$ This photoredox protocol affords aryl methyl sulfoxides in moderate to excellent yields under mild conditions. The reaction pathway is initiated by SET between aryl diazonium salt with Ru(II)* species to generate an aryl radical and a Ru(III) species. The aryl radical then reacts with DMSO to form an O-centered radical 235, followed by sequential single-electron oxidation and methyl group transfer to deliver aryl sulfoxides. In 2018, Wu and Zhang reported a methylsulfoxidation of tetrazole-amines with neat DMSO or methyl-sulfinyl derivatives in the presence of $t$ - BuONO, which produces antifungal methylsulfinyl-1H-tetrazole derivatives 236 in moderate to excellent yields. ${}^{322}$

Scheme 176. Methylsulfoxidation of Aryl Diazonium Salts and Arylamines

Taking advantage of the dual photoredox/transition-metal-catalyzed system, Lee realized a visible-light-driven Ag-catalyzed C-S coupling reaction of aryl thiols and aryl diazonium salts, enabling the selective synthesis of diaryl sulfoxides and diaryl sulfones under open-flask condition (Scheme 177). ${}^{323}$ These reactions show strong solvent dependence. For example, the reactions in DMSO give exclusive aryl sulfoxides 239 as the product. While in DMF, aryl sulfoxides can be further oxidized to the final oxidation product, aryl sulfones 240. Also, pyridine benefits to the formation of sulfoxides 239. Mechanistically, the intermediate product aryl sulfides 238 are obtained through the radical combination of thiyl radicals 237 with aryl radicals under

Scheme 177. Silver-Catalyzed Synthesis of Diaryl Sulfoxides and Diaryl Sulfones

mechanism

oxidative conditions. Sequential selective oxidation of 238 results in the formation of aryl sulfoxides 239 or sulfones 240.

The development of a greener metal-free approach for the synthesis of sulfones using organic photoredox catalysts is substantially appealing. In 2019, a visible-light-induced cross-coupling reaction of aryl diazonium and sulfinate salts using eosin Y as an organic photoredox catalyst was reported (Scheme 178). ${}^{324}$ This transition metal- and external oxidant/ reductant-free protocol affords unsymmetrical diaryl/alkyl aryl sulfones in moderate to excellent yields. The key step for generating desired sulfones is the photoinduced radical combination of the aryl radical with sulfonyl radical 241.

Scheme 178. Eosin Y-Catalyzed Cross-Coupling Reaction of Aryl Diazonium and Sulfinate Salts

In addition to the photoredox protocol, Xu and Tang reported a Cu-catalyzed three-component coupling reaction of aryl diazonium salts, elemental sulfur, and ${\mathrm{R}}_{2}\mathrm{P}\left( \mathrm{O}\right) \mathrm{H}$ compounds (Scheme 179). ${}^{325}$ This phosphorothiolation reaction allows the direct formation of P-S and C-S bonds in one operation, affording $S$ -aryl phosphorothioates 242 in high yields. The chemistry is assumed to proceed through the

Scheme 179. Copper-Catalyzed Phosphorothiolation of Aryl Diazonium Salts

reaction of ${\mathrm{R}}_{2}\mathrm{P}\left( \mathrm{O}\right) \mathrm{H}$ with elemental sulfur to generate $S$ - hydrogen phosphorothioates 243, followed by Cu-catalyzed C-S bond formation of aryl diazonium salts, leading to the formation of $S$ -aryl phosphorothioates 242 via a radical pathway.

The $O$ -benzotriazolyl sulfonimidates are served as the key intermediates for synthesizing sulfonimidamides, which are sulfonamide monoaza analogues and regarded as bioisosteres of the parent compounds. To access the synthesis of these biochemical active molecules, Bolm reported the three-component reaction of aryl diazonium tetrafluoroborates, N-tritylsulfinylamine, and N-hydroxybenzotriazole hydrate in the presence of the tertiary amine, affording O-benzotriazolyl sulfonimidates 246 with good yields by using inexpensive and environmentally benign dimethyl carbonate as the solvent (Scheme 180). ${}^{326}$ Reaction mechanism includes the double coordination of the tertiary amine and sulfinylamine with the nitrogen of the diazonium salt. Upon SET from the tertiary amine to give $N$ -radical cation, the aryl radical is formed, followed by the addition to the sulfur reagent to give a sulfoximiminoyl radical 244. A combination of sulfoximiminopl radical 244 and hydroxybenzotriazole radical (oxygen-centered) 245 gives rise to the sulfonimidate product 246.

Scheme 180. Three-Component Synthesis of Benzotriazolyl Sulfonimidates

3.8.2. Synthesis of Aryl Sulfones through the Insertion of ${\mathrm{{SO}}}_{2}$ . In the recent years, aromatic sulfonylation reactions of aryl diazonium salts, particularly through the insertion of ${\mathrm{{SO}}}_{2}$ by using DABCO- ${\left( {\mathrm{{SO}}}_{2}\right) }_{2}$ (DABSO) or inorganic sulfites as gaseous sulfur dioxide surrogate, have opened a novel and superior route to construct $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - {\mathrm{{SO}}}_{2}$ bonds (Scheme 181). ${}^{{327},{328}}$ This radical-type sulfonylation reaction allows the synthesis of aryl sulfones, sulfonamides, sulfonyl halides, thiosulfonates, sulfonic esters, and a variety of cyclic compounds containing sulfonyl group. In this section, systematic recent advances explored by Wu, Loh, Tu, Jiang, and other groups will be summarized.

Scheme 181. General Features of Sulfonylation Reactions of Aryl Diazonium Salts through the Insertion of ${\mathrm{{SO}}}_{2}$

3.8.2.1. Trapped by Nucleophiles. In 2014, Li and Wu reported a novel aminosulfonylation of aryl diazonium tetrafluoroborates, DABSO and hydrazines, affording aryl $N$ - aminosulfonamides with good to excellent yields at room temperature (Scheme 182). ${}^{329}$ The reaction mechanism involves the addition of an in situ generated aryl radical to ${\mathrm{{SO}}}_{2}$ , giving rise to an aryl sulfonyl radical 247, which was intercepted by hydrazinium radical 248 to obtain aryl $N$ - aminosulfonamides. In the same year, the coupling of (hetero)aromatic amines with DABSO and hydrazines was disclosed, leading to aryl $N$ -aminosulfonamides through a tandem diazotization/aminosulfonylation reaction. ${}^{330}$

Scheme 182. Aminosulfonylation of Aryl Diazonium Salts or Anilines with DABSO and Hydrazines

mechanism

DABSO $\mathrm{{Ar}} - {\mathrm{N}}_{2} \; - {\mathrm{N}}_{2}$ R

Ar・ + ${\mathrm{{SO}}}_{2}$

HN-N

247 248

$\mathrm{{Ar}} - {\mathrm{{NH}}}_{2}$ t-BuONO (1.8 equiv)

(1.5 equiv)

${\mathrm{{BF}}}_{3} \cdot {\mathrm{{OEt}}}_{2}$ (1.8 equiv)

${\mathrm{H}}_{2}\mathrm{\;N} - \mathrm{N}$

DABSO

MeCN, ${30}^{ \circ }\mathrm{C}$

(1.0 equiv) 22 examples

23-95% yields

A three-component reaction of 2-(allyloxy)anilines or diazonium tetrafluoroborates, DABSO, and hydrazines were realized to afford 1-(2,3-dihydrobenzofuran-3-yl)-methanesul-fonohydrazides (Scheme 183). ${}^{331}$ This radical procedure includes the in situ formed aryl radical, followed by intramolecular 5-exo-cyclization and insertion of sulfur dioxide.

Scheme 183. Synthesis of 1-(2,3-Dihydrobenzofuran-3- yl)methanesulfonohydrazides

Besides, the in situ formed aryl sulfonyl radical can be trapped by hydroxylamines or amines, affording aryl O-aminosulfonates and sulfonamides, respectively (Scheme 184). ${}^{332}$

Scheme 184. Synthesis of O-Aminosulfonates and Sulfonamides

The copper complexes have been utilized to facilitate the coupling reaction of aryl diazonium salts, DABSO with various nucleophiles, allowing the aminosulfonylation to access sulfonamides, ${}^{333}$ and oxysulfonylation to synthesize aryl sulfonic esters, ${}^{334}$ respectively (Scheme 185). These Cu-catalyzed multicomponent reactions proceed through the oxidation of aryl sulfonyl radical 247 with copper species, followed by reductive elimination to afford the desired products.

In addition, the trapping process of aryl sulfonyl radical by thiols was disclosed by Yang and co-workers, allowing an acid-promoted synthesis of unsymmetrical aryl thiosulfonates (Scheme 186). ${}^{335}$ In the same year, Volla and co-workers developed a visible-light-induced ${\mathrm{{SO}}}_{2}$ insertion strategy toward the synthesis of thiosulfonates 249 from the same substrates under room temperature. ${}^{336}$ Both of these coupling transformations proceed through the radical-radial cross-coupling reaction of sulfenyl radical and the aryl sulfonyl radical under thermal or photoredox conditions.

The potassium/sodium metabisulfite has been recently explored as the inorganic sulfur dioxide surrogate, particularly for the ${\mathrm{{SO}}}_{2}$ insertion reactions with aryl diazonium salts. ${}^{337}$ To explore the ${\mathrm{{ArSO}}}_{2} - \mathrm{S}$ bond construction process, Wu and Ye reported a photoredox three-component reaction of aryl diazonium salts, sodium metabisulfite, and thiourea 250, giving rise to $S$ -aryl thiosulfonates 249 with moderate yields (Scheme 187). ${}^{338}$ Proposed mechanism includes the reaction of aryl diazonium tetrafluoroborate with thiourea 250, leading to salt intermediate 251, which reacts with ${\mathrm{{Na}}}_{2}{\mathrm{\;S}}_{2}{\mathrm{O}}_{5}$ to form thiophenolate anions ${252},{\mathrm{{SO}}}_{2}$ and urea. The sulfenyl radical 253 generated from the thiophenolate anion 252 assisted by the photocatalyst reacts with the photoinduced formed aryl sulfonyl radical 242 to produce the $S$ -aryl thiosulfonate 249. Recently, a TFA-promoted multicomponent reaction of aryl diazonium with sodium metabisulphite and thiols was reported, affording thiosulfonates in good yields under transition-metal-free conditions. ${}^{339}$

Scheme 185. Cu-Catalyzed Coupling of Aryl Diazonium Tetrafluoroborates, DABSO, and N-Chloroamines or Alcohols

Moreover, the synthesis of vinyl sulfones through ${\mathrm{{SO}}}_{2}$ insertion has been established (Scheme 188). In 2015, Feng and co-workers reported an iodide-catalyzed radical alkenyla-tion of diazonium tetrafluoroborates, terminal alkenes, and DABSO in the presence of TBHP. ${}^{340}$ Another synthetic route toward the synthesis of vinyl sulfones involves a Cu(II)- catalyzed three-component substrate-dependent reaction of aryl diazonium salts, DABSO with alkenes, affording $\left( E\right)$ - alkenyl sulfones or allylic sulfones via the insertion of sulfur dioxide. ${}^{341}$ This reaction is assumed to be initiated by the conversion of $\mathrm{{Cu}}\left( \mathrm{I}\right)$ to $\mathrm{{Cu}}\left( \mathrm{{II}}\right)$ in the presence of ${\mathrm{{SO}}}_{2}$ , which can promote the formation of an aryl radical. Next, the insertion of ${\mathrm{{SO}}}_{2}$ affords the aryl sulfonyl radical 247, which is added to alkenes to generate an alkyl radical 254. Subsequently, oxidation by Cu(II) and final deprotonation occurs to deliver the vinyl sulfone product.

Scheme 186. Synthesis of Thiosulfonates through Insertion of Sulfur Dioxide

Scheme 187. Photoredox Reaction of Aryl Diazonium Salts, Sodium Metabisulfite, and Thiourea

Scheme 188. Three-Component Synthesis of Vinyl Sulfones of Aryl Diazonium Salts, DABSO with Alkenes

The radical tandem procedure also allows the synthesis of $\beta$ - keto sulfones through the Cu-catalyzed multicomponent reaction of aryl diazonium salts, DABSO, 3-aryl propiolic acids, and water, ${}^{342}$ as well as the additive-free reaction with silyl enol ether ${}^{343}$ (Scheme 189). In the previous case, the key steps in this transformation involve sulfonylation of aryl radicals with ${\mathrm{{SO}}}_{2}$ , selective addition of 247 to 3-aryl propiolic acid, and decarboxylation. The latter reaction involves the trapping process of aryl sulfonyl radical 247 with silyl enol ether, followed by single-electron oxidation by tertiary amine cation radical and desilylation to provide the desired $\beta$ -keto sulfones.

Another multicomponent cascade reaction approach by exploiting the differential reactivity pattern of substrates under open-atmosphere and metal-free conditions has been established (Scheme 190). ${}^{344}$ This cascade protocol uses aryl diazonium salts and alkynes/alkenes as the coupling partners at air atmosphere in the presence of DABSO or sodium metabisulfite as the source of ${\mathrm{{SO}}}_{2}$ .

A four-component reaction of aryl diazonium salts, alkenes, and hydroxylamines was disclosed by using DABSO or potassium metabisulfite as the source of sulfur dioxide (Scheme 191). ${}^{345}$ This alkene vicinal difunctionalization strategy enables the synthesis of arylated $\beta$ -hydroxysulfones with good yields under room temperature.

The combination of $\mathrm{C} - \mathrm{H}$ functionalization strategy with ${\mathrm{{SO}}}_{2}$ insertion has been achieved by an Ir-catalyzed three-component reaction of enamides, aryl diazonium tetrafluor-oborates, and DABSO (Scheme 192). ${}^{346}$ This direct C-H aryl sulfonylation protocol allows the synthesis of a diverse array of $\beta$ -amidovinyl sulfones in moderate to excellent yields and high stereoselectivities under light-free conditions. In the same year, a $\mathrm{{Cu}}{\left( \mathrm{{OTf}}\right) }_{2}$ -mediated aryl sulfonylation protocol was disclosed to afford $\beta$ -amidovinyl sulfones with good yields (32 examples, 26-95% yields). ${}^{347}$

Recently, the synthesis of aryl sulfonyl fluorides 257 has been achieved by Cu-catalyzed fluorosulfonylation of aryl diazonium salts in the presence of ${\mathrm{{KHF}}}_{2},{}^{348}$ or Cu-free Sandmeyer-type protocol using sodium metabisulfite and Selectfluor as the sulfur dioxide and fluorine sources ${}^{349}$

Scheme 189. Synthesis of $\beta$ -Keto Sulfones by Cu-Catalyzed Multicomponent Reaction

(Scheme 193). The previous reaction pathway involves the ${\mathrm{{SO}}}_{2}$ insertion of the aryl radical to form the aryl sulfonyl radical 247, followed by either chlorine abstraction and sequential Cl-F exchange, or direct combination with fluorine anion, which depend on the electronic character of the arene ring.

3.8.2.2. C-H Functionalization Combined with ${\mathrm{{SO}}}_{2}$ Insertion. Moreover, the Wu and co-workers have merged direct $\mathrm{C} - \mathrm{H}$ functionalization strategy and insertion reaction of ${\mathrm{{SO}}}_{2}$ , giving rise to diverse sulfonated arenes with high efficiency and good selectivity. They realized the Pd-catalyzed directing group-assisted $\mathrm{C} - \mathrm{H}$ bond sulfonylation of indoles ${}^{350}$ and $N$ -(quinolin-8-yl)benzamides ${}^{351}$ through ${\mathrm{{SO}}}_{2}$ insertion (Scheme 194). These two protocols provide good examples for the combination of radical chemistry and Pd-catalyzed $\mathrm{C} - \mathrm{H}$ activation with the insertion of sulfur dioxide. The reaction mechanism involves the single-electron oxidation of aryl sulfonyl radical 247 with Pd(II)-chelated complex and sequential oxidation by tertiary amine radical cation, followed by reductive elimination of Pd(IV) species. tion was disclosed through a three-component reaction of 8- aminoquinoline amides 261, DABSO, and aryl diazonium tetrafluoroborates. ${}^{352}$ This catalytic protocol is triggered by a Cu-chelated complex via the copper catalyst’s coordination with the substrate and in situ generated aryl sulfonyl radical 247, thus providing 5-sulfonyl-8-aminoquinoline amides 262 in moderate to good yields with excellent para-selectivity. The three-component reaction of naphthols 263, DABSO, and aryl diazonium tetrafluoroborates was reported by using iron(III) chloride as the radical initiator. ${}^{353}$

Scheme 191. Vicinal Difunctionalization of Alkenes through Insertion of Sulfur Dioxide

Scheme 192. Ir-Catalyzed C-H Aryl Sulfonylation of Enamides

Scheme 194. Pd-Catalyzed Direct C-H Functionalization of Indoles and N-(Quinolin-8-yl)benzamides

Other transition metal complexes have been explored as catalysts in the sulfonylative $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{H}$ bond functionalization process (Scheme 195). A Cu-catalyzed C-H bond sulfonyla-

Scheme 195. Cu- and Fe-Catalyzed C-H Bond Sulfonylation Reactions

Besides, the transition-metal-free C-H bond sulfonylation reactions of aryl diazonium salts with the ${\mathrm{{SO}}}_{2}$ insertion have also been explored by Wu and co-workers, allowing the selective $\mathrm{C} - \mathrm{H}$ bond sulfonylations of anilines ${}^{354}$ and quinoline $N$ -oxides ${}^{355}$ (Scheme 196).

Scheme 196. Transition-Metal-free C-H Bond Sulfonylation Reactions

In 2018, Wu and co-workers achieved the benzylic $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) -$ H bond sulfonylation at the para position of the hydroxyl group selectively with the insertion of ${\mathrm{{SO}}}_{2}$ through a visible-light-induced three-component reaction of aryl diazonium tetrafluoroborates, 4-methylphenols and sodium metabisulfite (Scheme 197). ${}^{356}$ Mechanistically, an aryl radical from the aryl diazonium salt is generated in the presence of an excited state of Ir(III)* under visible-light irradiation, which then reacts with sulfur dioxide providing aryl sulfonyl radical intermediate 247. The photocatalyst also promotes the oxidation of phenol in the presence of base via SET, leading to phenoxy radical 265. This phenoxy radical undergoes an intermolecular HAT with phenol to produce benzylic radical intermediate 266. Finally, aryl sulfonyl radical 247 couples with benzylic radical 266, giving rise to the corresponding product 267.

Scheme 197. Ir-Catalyzed Benzylic C-H Bond Sulfonylation

3.8.2.3. Cyclizations. The combination of radical cyclization with ${\mathrm{{SO}}}_{2}$ insertion has been widely established to access the synthesis of various (hetero)cyclic sulfones derivatives. At first, this method has been applied to synthesize $S$ -containing heterocycles. In 2015, Wu and co-workers reported a Cu-catalyzed cyclization reaction of 2-alkynyl aryl diazonium salts with DASBO, affording benzo[b]thiophenes 1,1-dioxides 268 (Scheme 198). ${}^{357}$ The aryl sulfonyl radical was initially intercepted by morpholin-4-amine, followed by intramolecular 5-endo-dig cyclization.

Scheme 198. Cu-Catalyzed Cyclization/SO2 Insertion Leading to Benzo $\left\lbrack b\right\rbrack$ thiophenes 1,1-Dioxides

Aiming to synthesize other $S$ -containing rings, a radical-triggered three-component bicyclization cascade reaction of 2- alkynyl aryl diazonium tetrafluoroborates with DABSO and internal alkynes was disclosed, affording a variety of richly decorated indeno[1,2-c]thiochromene 5,5-dioxides 269 with moderate to good yields and high levels of regioselectivity (Scheme 199). ${}^{358}$

The radical cyclization process with the insertion of ${\mathrm{{SO}}}_{2}$ shows practical applications in synthesizing sulfonated six-membered rings (Scheme 200). The catalyst-free synthetic approach for 3-sulfonated coumarins was realized through a three-component reaction of aryl diazonium tetrafluoroborates, DABSO and aryl propiolates 272, ${}^{359}$ or using (allyloxy)anilines as the substrates. ${}^{360}$ This tandem reaction proceeds through the in situ formation of aryl sulfonyl radicals 247, radical addition to aryl propiolates 272, spirocyclization, single-electron oxidation, 1,2-ester migration and final aromatization to get desired 3-sulfonated coumarin derivatives 273.

The assembly of sulfonated tetrahydropyridine scaffold 275 was realized through the three-component reaction of 1,6- enynes, DABSO, and aryl diazonium salts under catalyst- and additive-free conditions (Scheme 201). ${}^{361}$ The reaction mechanism includes the formation of aryl sulfonyl radical 247, which undergoes radical addition and intramolecular 6- exo cyclization to deliver an alkyl radical. Finally, this radical is trapped by an additional aryl diazonium cation, followed by an SET process to deliver the desired product.

Scheme 199. Radical-Induced Bicyclization to Afford Polycyclic Sulfones

Scheme 200. Synthesis of Sulfonated Coumarin Derivatives through Insertion of ${\mathrm{{SO}}}_{2}$

This cyclization strategy also allows the synthesis of sulfonated phenanthridines 278 through aryl sulfonyl radical interception with vinyl azides 277 (Scheme 202). ${}^{362}$ This

Scheme 201. Synthesis of Tetrahydropyridine Derivatives through the Reaction of 1,6-Enynes, DABSO, and Aryl Diazonium Salts reaction proceeds via the addition of in situ generated aryl sulfonyl radical 247 to vinyl azides along with the extrusion of ${\mathrm{N}}_{2}$ , followed by intramolecular cyclization, single-electron oxidation, and deprotonation to obtain 6-(sulfonylmethyl)- phenanthridine scaffold.

This radical cyclization process with the insertion of ${\mathrm{{SO}}}_{2}$ has also been utilized in synthesizing sulfonated five-membered rings, including 3-sulfonated-2,3-dihydrobenzo $\left\lbrack d\right\rbrack$ isothiazole 1,1-dioxides ${280},{}^{363}$ sulfonated ${9H}$ -pyrrolo $\left\lbrack {1,2 - a}\right\rbrack$ indoles 282,364 and azaspiro[4,5]-decanes 285 and azaspiro[4,5]- trienones ${286}^{365}$ under transition-metal- and photoredox-free conditions (Scheme 203).

In addition, Cu-catalyzed combination strategy of sulfony-lation with ${\mathrm{{SO}}}_{2}$ insertion enables the synthesis of sulfonated five-membered heterocyclic compounds (Scheme 204). In 2017, Cheng reported a Cu(II)-catalyzed three-component reaction of aryl diazonium salts, DABSO with $N$ -(aryl sulfonyl)acrylamides 287, affording sulfonated oxindoles 288 with moderate yields. ${}^{366}$ Mechanistically, this catalytic transformation is triggered by the in situ formation of aryl sulfonyl radicals. Afterward, the sequential radical addition to a double bond, radical cyclization, and desulfonylative 1,4-aryl migration occur to provide the final product by merely forming four new bonds. Additionally, 2,3-allenoic acids have been explored as substrates in a Cu(II)-catalyzed three-component reaction of aryl diazonium tetrafluoroborates by using DABSO or sodium metabisulfite as the ${\mathrm{{SO}}}_{2}$ surrogate, giving rise to 4-sulfonylated furan-2(5H)-ones 290 in good yields. ${}^{367}$

Scheme 203. Synthesis of Sulfonated Benzosultams, 9H-Pyrrolo[1,2-a]indoles and Spirocyclic Compounds

Scheme 204. Cu-Catalyzed Synthesis of Sulfonated Oxindoles and Furan-2(5H)-ones through Insertion of ${\mathrm{{SO}}}_{2}$

Cheng

The cyclization strategy merged with the insertion of ${\mathrm{{SO}}}_{2}$ allows the synthesis of sulfonated seven-membered rings, including the Cu-catalyzed three-component reaction to afford sulfonated benzo-fused seven-membered nitrogen heterocycles 292, ${}^{368}$ and catalyst-free protocol for synthesizing sulfonated [3,1]-benzothiazepines 294 (Scheme 205). ${}^{369}$ These two protocols represent the efficient and attractive strategy for preparing seven-membered heterocyclic compounds containing a sulfonyl group through a radical-type cyclization pathway.

Although significant achievements have been made on DABSO, this ${\mathrm{{SO}}}_{2}$ surrogate has rarely been explored in enantioselective synthesis. In 2018, Han and co-workers

Scheme 205. Synthesis of Sulfonated Seven-Membered Rings through Insertion of ${\mathrm{{SO}}}_{2}$

reported the first example for the enantioselective synthesis of chiral sulfonyl lactones 296 with the insertion of ${\mathrm{{SO}}}_{2}$ through Cu-catalyzed three-component reaction of aryl diazonium salts, DABSO and unsaturated carboxylic acids 295 (Scheme 206). ${}^{370}$ This multiple-step asymmetric radical reaction affords the corresponding sulfonyl lactones 296 bearing quaternary stereogenic center with up to 95% yields and 88% ee. The reaction mechanism is proposed to involve the initial reduction of aryl diazonium tetrafluoroborate by $\mathrm{{Cu}}\left( \mathrm{I}\right)$ via SET to generate aryl radical and Cu(II) complex. The resulting Cu(II) complex coordinates with unsaturated carboxylic acid 295 in the presence of 2,6-ditert-butylpyridine and chiral ligand to provide a Cu(II)-unsaturated carboxylic acid complex 297, in which the double bond is attacked by aryl sulfonyl radical, leading to an alkyl radical intermediate 298. Subsequent single-electron oxidation followed by reductive elimination of Cu(III) intermediate 299 gives rise to the final chiral product 296 and the regeneration of $\mathrm{{Cu}}\left( \mathrm{I}\right)$ species.

Scheme 206. Cu-Catalyzed Asymmetric Radical Sulfonylation for the Synthesis of Chiral Sulfonyl Lactones

Recently, the photoinduced sulfonylation reactions through the insertion of ${\mathrm{{SO}}}_{2}$ have been explored as an avenue to synthesize sulfonated cyclic compounds. In 2018, Wu and coworkers realized a visible-light-induced photoredox three-component reaction of aryl diazonium tetrafluoroborates, DABSO with 2-vinyl benzoic acids 300, leading to sulfonated isobenzofuran-1(3H)-ones 301 with good yields under mild conditions (Scheme 207). ${}^{371}$ Mechanistically, the photocatalyst promotes the single-electron oxidation from radical 302 to cation 303 during the reaction process. Then, intramolecular nucleophilic attack of carboxylic acid to the cation 303 affords the corresponding sulfonated isobenzofuran-1(3H)-one 301.

Scheme 207. Photoredox Synthesis of Sulfonated Isobenzofuran-1(3H)-ones

mechanism

Subsequently, the same group studied the combination of photoredox transformation with ${\mathrm{{SO}}}_{2}$ insertion to realize the synthesis of various sulfonated functional molecules, including sulfonated 2,2-disubstituted tetrahydrofurans 304/tetrahydro-pyran and 1,3-dihydroisobenzofuran scaffold, ${}^{372}$ sulfonated 1- isoindolinones 305, ${}^{373}$ and $\beta$ -sulfonyl amides 306 (Scheme 208). ${}^{374}$

In 2018, Wu and co-workers reported a three-component reaction of aryl diazonium salts, DABSO and N-(2-vinyl phenyl)amides 307, leading to 4-((aryl sulfonyl)methyl)-4H-benzo[d][1,3]oxazines 308 by merging photoredox catalysis and radical-type insertion of ${\mathrm{{SO}}}_{2}$ (Scheme 209). ${}^{375}$ With the combination of the insertion of ${\mathrm{{SO}}}_{2}$ through semipinacol rearrangement, a visible-light-induced photoredox-catalyzed sulfonylation of alkenyl cyclobutanols 309 was reported. ${}^{376}$ This protocol proceeds through an aryl sulfonyl radical-induced 1,2-carbon migration process, affording $\beta$ -sulfonated ketones 310 bearing $\alpha$ -carbon quaternary centers with high yields.

As for the synthesis of other heterocycles, the construction of 2-sulfonyl-substituted 9H-pyrrolo[1,2-a]indole derivatives 312 has been achieved by a visible-light-catalyzed cascade sulfonylation/cyclization of $N$ -propargyl indoles 311 with aryl diazonium salts and DABSO with good yields (Scheme 210). ${}^{377}$ The proposed mechanism includes the addition of aryl sulfonyl radical to a triple bond, intramolecular cyclization, single-electron oxidation, isomerization, and deprotonation process.

Scheme 208. Photoredox Synthesis of Sulfonated 2,2- Disubstituted Tetrahydrofurans and 1-Isoindolinones and $\beta$ -Sulfonyl Amides

Scheme 209. Photoredox Synthesis of Sulfonated Benzo $\left\lbrack d\right\rbrack \left\lbrack {1,3}\right\rbrack$ oxazines and $\beta$ -Sulfonated Ketones

Scheme 210. Photoredox Synthesis of 2-Sulfonyl-substituted 9H-Pyrrolo[1,2-a]indoles

Furthermore, Jiang and co-workers utilized the ${\mathrm{{SO}}}_{2}$ insertion strategy in constructing polycyclic compounds through the multicomponent reaction of aryl diazonium tetrafluoroborates with functionalized alkynes. In 2017, Jiang and co-workers merged $\left\lbrack \begin{array}{ll} 2 + & 2 \end{array}\right\rbrack$ cycloaddition and 1,4-addition sequence induced by $S$ -centered radicals through the three-component reaction of aryl diazonium salts, DABSO and benzene-linked allene-ynes 313 (Scheme 211). ${}^{378}$ This metal-free cascade cyclization approach allows the synthesis of a wide range of cyclobuta[a]naphthalen-4-ols 314 with excellent diastereose-lectivity and good yields. Mechanistically, this reaction is proposed to proceed by [2 + 2] cycloaddition, insertion of ${\mathrm{{SO}}}_{2}$ , radical 1,4-addition, diazotization, single-electron reduction, and tautomerization.

Scheme 211. Synthesis of Cyclobuta[a]naphthalen-4-ols through Insertion of ${\mathrm{{SO}}}_{2}$

The same group utilized this radical-triggered multicomponent reaction to access the synthesis of sulfonated 1- indenones 316, ${}^{379}$ and sulfone-containing 1,3-dimethylene-substituted $\left( {Z, Z}\right)$ -isobenzofurans 319, ${}^{380}$ or sulfonated $\left( Z\right)$ - 1,3-dihydroisobenzofurans ${320}^{381}$ (Scheme 212).

Another synthetic route toward 1,3-dimethylene-substituted isobenzofuran derivatives 322 involves a Pd-catalyzed cyclization/radical addition cascade reaction of $\beta$ -alkynyl ketones 321, aryl diazonium salts, and DABSO (Scheme 213). ${}^{382}$ This reaction is proposed to proceed through a sequence of a Heck-type reaction, oxo-cyclization, sulfonyl radical addition, single-electron oxidation, and deprotonation process.

3.8.3. $\mathrm{C} - {\mathrm{{SCF}}}_{3}$ and $\mathrm{C} - {\mathrm{{SCF}}}_{2}\mathrm{H}$ Bond Formations. Functionalized trifluoromethyl aryl sulfides constitute highly attractive building blocks for their exceptional lipophilicity and chemical properties. For such reasons, selective trifluorome-thylthiolation, difluoromethylthiolation, and pentafluoroe-thylthiolation have attracted significant attention in recent years. The main topic is developing suitable and mild reagents for these transformations, including transition-metal- ${\mathrm{{SCF}}}_{2}\mathrm{X}(\mathrm{X}$ = F, H) complexes and organic ${\mathrm{{SCF}}}_{2}\mathrm{X}$ -containing molecules. The recent approaches include the Cu-mediated or Ag-mediated process, and photoredox protocol to forge $\mathrm{C} - {\mathrm{{SCF}}}_{3}$ and $\mathrm{C} - {\mathrm{{SCF}}}_{2}\mathrm{H}$ bonds.

Scheme 212. Synthesis of Sulfonated 1-Indenones and $\left( {Z, Z}\right)$ -Isobenzofurans

In 2014, Goossen and co-workers reported a Cu-mediated Sandmeyer trifluoromethylthiolation of aryl diazonium salts using Prakash reagent 113 as the trifluoromethylating reagent (Scheme 214). ${}^{383}$ In the presence of sodium thiocyanate and base, this regiospecific trifluoromethylthiolation protocol affords aryl trifuoromethyl thioethers 323 with moderate to excellent yields. Mechanistically, this reaction proceeds through an SET process to afford an aryl radical, followed by the reaction with 324 to form aryl thiocyanate, which undergoes nucleophilic substitution with trifluoromethyl anion. In the same year, Goossen also achieved the one-pot copper-mediated Sandmeyer trifluoromethylthiolation from (hetero)aromatic amines. ${}^{157}$

The development of new trifluoromethylating reagent has attracted great attention in the selective trifluoromethylth-iolation of aromatic compounds. In 2016, by using ${\mathrm{{Me}}}_{4}{\mathrm{{NSCF}}}_{3}$ 327 as the trifluoromethylating reagent, Goossen continuously reported a Sandmeyer-type trifluoromethylthiolation of aryl-amines or aryl diazonium salts (Scheme 215). ${}^{308}$

Scheme 213. Pd-catalyzed Oxo-Cyclization/SO ${}_{2}$ Insertion Cascade of $\beta$ -Alkynyl Ketones

Scheme 214. Sandmeyer Trifluoromethylthiolation of Aryl Diazonium Salts

Moreover, the photoredox trifluoromethylthiolation of aryl diazonium ions has been established in the absence of copper salts. ${}^{310}$ Wangelin and co-workers first developed the diverse synthetic trifluoromethylthiolation protocol of aryl diazonium salts with the commercial disulfide ${\mathrm{{CF}}}_{3}{\mathrm{{SSCF}}}_{3}{328}$ (Scheme 216). ${}^{384}$ They found that the photoredox procedure is superior to the base-mediated process.

The limitation of the previous trifluoromethylthiolation protocol is the employment of highly volatile liquid ${\mathrm{{CF}}}_{3}{\mathrm{{SSCF}}}_{3}$ 328. In 2018, Zhao reported a visible-light-induced photoredox trifluoromethylthiolation of aryl diazonium salts using $S$ - trifluoromethyl 4-methoxylbenzenesulfonothioate 329 as the

Scheme 215. Sandmeyer-type Trifluoromethylthiolation with ${\mathrm{{Me}}}_{4}{\mathrm{{NSCF}}}_{3}$

Scheme 216. Weak Base-Mediated and Photoredox Trifluoromethylthiolation with ${\left( {\mathrm{F}}_{3}\mathrm{{CS}}\right) }_{2}$

NaOAc (1 equiv.)

$\mathrm{{Ar}} - {\mathrm{N}}_{2}{\mathrm{{BF}}}_{4}$ + ${\mathrm{F}}_{3}{\mathrm{{CS} - {SCF}}}_{3} \; \mathrm{{Ar}} = {\mathrm{{SCF}}}_{3}$

DMSO, ${24}\mathrm{\;h},{20}^{ \circ }\mathrm{C}$

328 323, 6 examples

(1 equiv.) 31-76% yields

$\mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\mathrm{{Cl}}}_{2}\left( {{0.5}\mathrm{\;{mol}}\% }\right)$

$\mathrm{{Ar}} - {\mathrm{N}}_{2}{\mathrm{{BF}}}_{4} \; {\mathrm{F}}_{3}{\mathrm{{CS} - {SCF}}}_{3} \; \mathrm{{Ar}} = {\mathrm{{SCF}}}_{3}$

DMSO, $1\mathrm{\;h},{20}^{ \circ }\mathrm{C}$

328 3.8 W blue LED 323, 8 examples

(1 equiv.) 33-70% yields

trifluoromethylthiolating reagent (Scheme 217). ${}^{385}$ In the presence of Ru(II) complex, this mild protocol affords aryl trifluoromethyl thioethers with moderate to good yields. Additionally, one-pot conditions from anilines as the substrates using Rose Bengal as photocatalyst have been established, giving rise to the desired compounds with similar reaction outcomes.

Scheme 217. Visible Light Photocatalytic Trifluoromethylthiolation with $S$ -Trifluoromethyl 4- Methoxylbenzenesulfonothioate

In 2019, Billard and Tlili disclosed a visible-light-induced trifluoromethylthiolation of aryl diazonium salts using shelf-stable TsSCFs 330 as the trifluoromethylthiolating reagent in the presence of ruthenium-based photosensitizer under mild reaction conditions (Scheme 218). ${}^{386}$ In the same year, Tlili and Gampe have utilized this photocatalytic and copper-catalyzed method to rapidly access the S and Se homologues of two ${\mathrm{{OCF}}}_{3}$ -containing pharmaceuticals riluzole and pretoma-nid. ${}^{387}$

Scheme 218. Visible-Light-Mediated Trifluoromethylthiolation with ${\mathrm{{TsSCF}}}_{3}$

Except for the trifluoromethylthiolation of aryl diazonium salts, Goossen reported a Cu-mediated difluoromethylthiola-tion of aryl diazonium salts, affording pharmaceutical and agrochemical important aryl difluoromethyl thioethers 331 with high yields (Scheme 219). ${}^{388}$ This mild drifluorome-thylthiolation reaction proceeds through the in situ generations of organothiocyanates 325 as the key intermediates, followed by the reaction with copper- ${\mathrm{{CF}}}_{2}\mathrm{H}$ complex generated in situ from copper thiocyanate and TMSCF ${}_{2}\mathrm{H}$ to deliver difluor-omethylthiolation products.

Scheme 219. Cu-Mediated Synthesis of Aryl Difluoromethyl Thioethers Using TMSCF ${}_{2}\mathrm{H}$

In the same year, copper-promoted difluoromethylthiolation of (hetero)aryl diazonium salts was realized by using stable NHC-ligated difluoromethylthiolated silver complex [(SIPr)- Ag(SCF ${}_{2}$ H) 332 (Scheme 220). ${}^{389}$ In the presence of one equivalent of $\mathrm{{Cu}}\left( \mathrm{I}\right) /$ bpy, this difluoromethylthiolation reaction affords various difluoromethylthiolated arenes and heteroar-enes in good to excellent yields under mild reaction conditions. This reaction is proposed to proceed through an SET process of aryl diazonium salts with in situ formed $\mathrm{{Cu}}\left( \mathrm{I}\right) {\mathrm{{SCF}}}_{2}\mathrm{H}{333}$ to afford an aryl radical, followed by the sequential SET process with $\mathrm{{Cu}}\left( \mathrm{{II}}\right) {\mathrm{{SCF}}}_{2}\mathrm{H}$ species 334 to deliver the desired difluoromethylthiolated product along with the formation of Cu(I) species.

A Sandmeyer-type pentafluoroethylthiolation of aryl diazonium salts was also realized with ${\mathrm{{Me}}}_{4}{\mathrm{{NSC}}}_{2}{\mathrm{\;F}}_{5}{336}$ in the presence of catalytic amounts of elemental copper (Scheme 221). ${}^{390}$ In 2018, Wang and co-workers reported a visible-

Scheme 220. Sandmeyer-Type Difluoromethylthiolation of Aryl Diazonium Salts with Difluoromethylthiolated Silver Complex

Scheme 221. Synthesis of Pentafluoroethyl Thioethers via Sandmeyer Reaction

mechanism

$$ {\mathrm{{Ar}}}^{ + }{\mathrm{N}}_{2} + {\mathrm{{Cu}}}^{\prime }{\mathrm{{SC}}}_{2}{\mathrm{\;F}}_{5}\;\xrightarrow[{-{\mathrm{N}}_{2}}]{\mathrm{{SET}}}{\mathrm{{Ar}}}^{ * } + {\mathrm{{Cu}}}^{\prime \prime }{\mathrm{{SC}}}_{2}{\mathrm{\;F}}_{5}\; \rightarrow \text{ 335 } $$

light-induced difluoromethylthiolation of aryl diazonium salts by using shelf-stable, easily prepared ${\mathrm{{PhSO}}}_{2}{\mathrm{{SCF}}}_{2}\mathrm{H}$ as the difluoromethylthiolating reagent. ${}^{391}$ Under the irradiation with a 12 W white CFL and catalyzed by $5\mathrm{\;{mol}}\% \mathrm{{Ru}}{\left( \mathrm{{bpy}}\right) }_{3}{\left( {\mathrm{{PF}}}_{6}\right) }_{2}$ , this photoredox protocol affords a variety of difluoromethyl thioethers with moderate to good yields (28 examples, 17- 85% yields). In this reaction, sodium ascorbate was added to promote the SET process.

3.9. Reactions with Nitriles and Cascade Reactions

Aryl diazonium salts’ reaction with nitriles has opened a novel synthetic route to $N$ -containing heterocycles through the $N$ - aryl nitrilium intermediates (Scheme 222). During this process,

Scheme 222. General Pathway for the Reaction of Aryl Diazonium Salts with Nitriles

the aryl diazonium salt serves as the precursor of aryl cation, which undergoes nucleophilic attack by nitriles to generate $N$ - aryl nitrilium ions 337, followed by sequential cyclization or cascade reactions. The recent catalytic approaches are mainly focused on expanding the scope toward the facile assembly of various $N$ -containing heterocycles.

In 2017, Shen and Yu reported a three-component reaction of aryl diazonium salts, nitriles, and alkynes, providing multiply substituted quinolines under catalyst- and additive-free conditions (Scheme 223). ${}^{392}$ The author has proposed that the formation of nitrilium cation 337 initiates this cascade annulation through nucleophilic attack of nitriles to diazonium salts. This key nitrilium cation undergoes a concerted Diels-Alder reaction, followed by deprotonation to obtain quinoline 338, or proceeds through stepwise alkyne nucleophilic addition/electrophilic cyclization alternatively.

In addition to the cyclization process with alkynes, a three-component assembly reaction of aryl diazonium salts, nitriles,

Scheme 223. Synthesis of Quinolines through Three-Component Reaction of Aryl Diazonium Salts, Nitriles, and Alkynes approach proceeds via the generation of $N$ -aryl nitrilium intermediates 337 from aryl diazonium salts and nitriles, followed by the reaction with styrenes 20 to obtain 3,4- dihydroquinolinium salts 339 as the key intermediate. In the presence of ${\mathrm{{NaBH}}}_{4}$ or DDQ, functionalized tetrahydroquino-lines 340 or quinolines 341 are obtained, respectively. Moreover, the synthesis of $N$ -arylbenzimidazoles was achieved by a cyclization/oxidation cascade reaction of aryl diazonium salts, nitriles, and anilines. ${}^{394}$ This reaction proceeds through the in situ generations of $N$ -arylnitrilium salts, followed by the formation of $N,{N}^{\prime }$ -diarylamidines, and final oxidative cyclization by $\mathrm{{PhI}}{\left( \mathrm{{OAc}}\right) }_{2}$ to obtain $N$ -aryl benzimidazoles.

and styrenes 20 was represented under transition-metal-free conditions (Scheme 224). ${}^{393}$ This divergent synthetic

Scheme 224. One-Pot Synthesis of (Tetrahydro)quinolines

mechanism

Moreover, the annulation strategy of aryl diazonium salts with nitriles was explored to access a variety of heterocyclic compounds. In 2017, Liu and co-workers reported a direct [2 +2 + 2] annulation of aryl diazonium salts with two equivalent nitriles, affording multisubstituted quinazolines 342 in moderate yields under metal-free conditions (Scheme 225). ${}^{395}$ Reaction of the aryl diazonium salt with a nitrile provides the initial formation of a reactive nitrilium ion, which is attacked by another molecule of nitrile followed by electrophilic cyclization to deliver the desired product. In 2018, the same group also reported the transition-metal-free synthetic route for $N$ -arylketimines 343 through intermolecular arylation of $N$ -arylnitrilium ions by electron-rich arenes. ${}^{396}$

Scheme 225. Synthesis of Quinazolines and Ketimines

In 2017, the transition-metal-free synthesis of multi-substituted quinazolin-4(3H)-imines 345 had been achieved through the three-component reaction of aryl diazonium salts, nitriles, and 2-cyanoanilines 344 (Scheme 226). ${}^{397}$ This annulation reaction is initiated by the in situ formation of reactive $N$ -arylnitrilium intermediate 337, followed by nucleophilic amination/tandem cyclization with $o$ -cyanoani-lines 344 to deliver quinazolin-4(3H)-imines 345. This cyclization strategy has been utilized in the synthesis of quinazolino[3,4-a]-quinazolin-13-ones 346 through the annulation reaction of o-(methoxycarbonyl)benzene diazonium salts, nitriles, and 2-cyanoanilines 344. ${}^{398}$

Scheme 226. Synthesis of Quinazolin-4(3H)-Imines and Quinazolinoquinazolinones through Annulation Cascade

Furthermore, Liu has trapped the $N$ -arylnitrilium ion by 2- aminoarylketones 347 to furnish other heterocycles. The synthesis of $N$ -arylquinazolinium salts ${348}^{399}$ and 2-amino-3- cyanoquinolines ${349}^{400}$ was realized through the trapping process of $N$ -arylnitrilium ion 337 by 2-aminoarylketones 347, followed by sequential cyclizations (Scheme 227).

Scheme 227. Synthesis of N-Arylquinazolinium Salts and 2- Arylamino-3-Cyanoquinolines

In 2019, Sharma and co-workers reported a catalyst-free three-component reaction of aryl diazonium salts, quinoline $N$ - oxides in acetonitrile under microwave irradiation, affording various 2-anilinoquinolines in moderate yields (Scheme 228). ${}^{401}$ This reaction pathway includes the formal [3 + 2] cycloaddition of quinoline $N$ -oxides with in situ formed $N$ - arylnitrilium ion, followed by proton abstraction/rearomatiza-tion/hydrolysis sequence to deliver the desired product. However, using 2-substituted quinoline as the substrates, this protocol affords 3-hydroxyquinoline as the major product through a 1,3-O-transfer from $N$ -oxide to the C3 position of quinolines.

Scheme 228. Synthesis of 2-Anilinoquinolines and 3- Hydroxyquinolines

4. REACTIONS WITH THE RETENTION OF THE NITROGEN GROUP

Aryl diazonium salts have been applied not only in $\mathrm{C} - \mathrm{C}/\mathrm{C} - \mathrm{X}$ bond formation through the extrusion of ${\mathrm{N}}_{2}$ ; these substrates can also be subjected to $N$ -containing coupling reactions, affording aryl hydrazines, azo compounds, hydrazones, various $N$ -heterocycles, and other compounds. In this section, we will briefly introduce these reactions (Figure 4). Mechanistically, these $N$ -containing coupling reactions can be summarized according to the nucleophilic or radical-type reactions. The previous routes involve the direct addition of aryl diazonium ions by carbonic nucleophiles to obtain azo, hydrazone, and diazo-containing heterocycles. The latter pathway consists of the interception of diazonium ions by carbon-centered radicals.

Figure 4. Reactions with retention of the nitrogen group.

4.1. Nucleophilic Additions with Aryl Diazonium Cation

The aryl diazonium cation can be intercepted by nucleophiles to access the synthesis of various liner or cyclic compounds with the $N$ -containing moiety. The aryl diazonium ions can also be attacked by carbonic nucleophiles, including metal complexes, CH-acids, aromatic compounds, enolate species, diazo compounds, isocyanides, nitrile ylides, alkynes, enamines/enaminones, and others. Notably, the nucleophilic reactions with aryl diazonium salts represent a general method to synthesize azo compounds. In 2015, Shen reported a preparation and characterization of the thermally stable, well-defined NHC-ligated difluoromethylated silver complexes [(SIPr)Ag(CF ${}_{2}$ H) ${}^{3}$ SO. A variety of aryl diazonium salts were treated with $\left\lbrack {\left( \mathrm{{SIPr}}\right) \mathrm{{Ag}}\left( {{\mathrm{{CF}}}_{2}\mathrm{H}}\right) }\right\rbrack$ 350 in acetonitrile at room temperature, and the corresponding difluoromethylated azo derivatives, which are air- and moisture-insensitive compounds, were afforded in high yields (Scheme 229). ${}^{402}$

Scheme 229. Difluoromethylation of Aryl Diazonium Salts with $\left\lbrack {\left( \mathbf{{SIPr}}\right) \mathbf{{Ag}}\left( {{\mathbf{{CF}}}_{2}\mathbf{H}}\right) }\right\rbrack$

As for the heterocyclic diazonium analogues, preparing a series of stable, previously inaccessible 1,2,5-oxadiazolyl diazonium salts 351 were represented by using ${\mathrm{{NOBF}}}_{4}$ in trifluoroacetic acid. ${}^{403}$ Selective azo coupling reactions of obtained diazonium salts with a series of structurally diverse CH-acids, including arenes, dimedone, Meldrum’s acid, and 2- nitropropane, have also been successfully performed (Scheme 230).

Scheme 230. Azo Coupling Reactions of 1,2,5-Oxadiazolyl Diazonium Salts

As a complementary route to azo compounds under metal-free conditions, aryl diazonium salts can also undergo nucleophilic attack by imidazo-heterocycles, ${}^{404}$ as well as by 2,3-dihydropyrroles, ${}^{405}$ affording azo imidazo[1,2-a]pyridines and 4-arylazo-2-pyrrolines, respectively.

Enolate species can trap the aryl diazonium salts to obtain hydrazone derivatives. In 2018, Zhu and Jiang developed a base-mediated intermolecular $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ amination reaction of trifluoromethyl ketones 352 and aryl diazonium salts, giving rise to trifluoroacetylated hydrazones 353 in high yields with excellent $E/Z$ selectivity (Scheme 231). ${}^{406}$ This simple protocol features metal-, catalyst-, directing group-free, gram-scalable, and a wide range of substrate scope. This reaction is proposed to be imitated by the enolization of the trifluoromethyl ketone 352 in the presence of a base. A subsequent interception by enolate anion occurs, followed by 1,3-HAT to obtain 353.

Scheme 231. Synthesis of Trifluoroacetylated Hydrazones of Trifluoromethyl Ketones

In 2018, the synthesis of $\alpha$ -arylhydrazo- $\beta$ -keto sulfones 354 was realized by a Cu-catalyzed, four-component reaction of aryl diazonium salts, alkenes, and sodium sulfinates under aerobic conditions (Scheme 232). ${}^{407}$ This oxysulfonylation/ diazenylation cascade protocol allows the facile formation of $\mathrm{C} - \mathrm{O},\mathrm{C} - \mathrm{S}$ , and $\mathrm{C} - \mathrm{N}$ bonds in a one-pot manner. The reaction is proposed to be initiated by the single-electron oxidation of sulfonyl anion by oxygen in the presence of $\mathrm{{Cu}}$ (II) catalyst to afford sulfonyl radical 355. After several radical-type transformations, the key step is the nucleophilic addition of enol Cu (II) intermediate 356 at the terminal nitrogen atom, followed by the final tautomerization to sulfones 354.

Scheme 232. Copper(II)-Catalyzed Synthesis of $\alpha$ - Arylhydrazo- $\beta$ -keto Sulfones

In the same year, a Co-catalyzed, three-component cycloaddition reaction of aryl diazonium salts, diazoesters 357 and 1,3-dicarbonyl compounds was represented (Scheme 233). ${}^{408}$ The reaction pathway initiates at nucleophilic addition of the diazoester 357 to the terminal nitrogen of the aryl diazonium salt. After extrusion of ${\mathrm{N}}_{2},1,3$ -HAT and elimination of ${\mathrm{{HBF}}}_{4}$ generate nitrilimine 358, which undergoes [2,3]-cycloaddition with 1,3-dicarbonyl compounds to form fully substituted pyrazoles and eliminate water at room temperature.

Ma and co-workers demonstrated the synthesis of tetrazoles through a cycloaddition cascade reaction. ${}^{{409} - {412}}$ In 2015, they found that the aryl diazonium salts can be trapped by

Scheme 233. Synthesis of Pyrazoles of Aryl diazonium Salts, Diazoesters and 1,3-Dicarbonyls

${\mathrm{{CF}}}_{3}{\mathrm{{CHN}}}_{2}$ 359 catalyzed by silver salts, giving rise to a diverse array of 2-aryl-5-trifluoromethyltetrazoles with moderate to excellent yields (Scheme 234). ${}^{409}$ The one-pot diazotization/ cycloaddition sequence from commercially available aniline derivatives has also been achieved.

Scheme 234. Silver-Catalyzed [3 + 2] Cycloaddition of Aryl Diazonium Salts with ${\mathrm{{CF}}}_{3}{\mathrm{{CHN}}}_{2}$

In 2019, they explored ${\mathrm{{PhSO}}}_{2}{\mathrm{{CF}}}_{2}{\mathrm{{CHN}}}_{2}{\mathbf{{360}}}_{\text{ and }}$ the Seyferth-Gilbert reagent 361 as the substrates in a silver-catalyzed formal [3 + 2] cycloaddition strategy under mild conditions, enabling the regioselective synthesis of a variety of difluoromethylated tetrazoles, ${}^{410}$ as well as a broad set of phosphonylated tetrazoles (Scheme 235). ${}^{411}$ These two cycloaddition protocols exhibit a feasible one-pot operation from commercially available aniline derivatives. The trapping process by diazoacetates or $\alpha$ -diazo acetophenones was also disclosed, allowing the regioselective construction of ketone-derived or carboxylic tetrazoles (35 examples, 46-92% yields). ${}^{412}$

Scheme 235. Nucleophilic Addition of Aryl Diazonium Salts with ${\mathrm{{PhSO}}}_{2}{\mathrm{{CF}}}_{2}{\mathrm{{CHN}}}_{2}$ and Seyferth-Gilbert Reagent

Moreover, the cycloaddition of aryl diazonium salts has been applied in the facile synthesis of functionalized 1,2,4-triazoles. In this strategy, diazonium salts serve as the two N atom resource to target 1,2,4-triazoles skeletons and trap by isocyanides selectively. In 2018, the construction of 1,2,4- triazole scaffold had been achieved by a catalyst-dependent regioselective $\left\lbrack {3 + 2}\right\rbrack$ cycloaddition of isocyanides with aryl diazonium salts (Scheme 236). ${}^{413}$ This reaction selectively affords 1,3-disubstituted 1,2,4-triazoles 362 with high yields under Ag(I) catalysis, whereas 1,5-disubstituted 1,2,4-triazoles 363 with moderate to good yields by Cu(II) catalysis.

In 2018, Bao and Wan reported a copper-catalyzed, three-component reaction of aryl diazonium salts, diazo compounds, and nitriles to obtain fully substituted 1,2,4-triazoles (Scheme 237). ${}^{414}$ This catalytic reaction proceeds through selective [3 + 2] cycloaddition of in situ formed nitrile ylides 364 with diazonium ions.

Another synthetic route toward 1,3,5-trisubstituted 1,2,4- triazoles involves the metal-free cycloaddition/decarboxylation cascade reaction of aryl diazonium salts utilizing azlactones 365 as the cycloadditive partners (Scheme 238). ${}^{415}$ The selectivity in the formation of 1,2,4-triazoles depends on the structure of aryl diazonium salts.

Moreover, aryl diazonium salts have also been explored as the dinitrogen resource trapping by alkyne, achieving the one-pot assembly of 4-amidocinnolines 366 in the absence of transition-metal with high yields (Scheme 239). ${}^{416}$ Mechanistically, this reaction includes the sequence of diazotization with $t$ -BuONO, intramolecular nucleophilic addition of the alkyne to the diazonium ion, followed by nitrile addition to the intermediary vinyl cation 367 and hydrolysis.

Moreover, the copper-mediated annulation cascade reactions have opened a novel route to access functionalized $N$ -aryl 1,2,3-triazoles in a facile manner. In this process, the key step includes the nucleophilic attack of in situ formed Cu-coordinated enamines with aryl diazonium cations. The vinyl azides have been utilized as enamine precursors in the presence of copper to participate in cascade cyclization with aryl diazonium salts, affording densely functionalized ${N}^{2}$ -substituted 1,2,3-triazoles with moderate to good yields. ${}^{417}$ In 2020, Cheung and Ma developed a Cu-mediated annulation reaction of alkyl 3-aminoacrylates with aryl diazonium salts, affording

Scheme 236. [3 + 2] Cycloaddition of Isocyanides with Aryl Diazonium Salts

Scheme 237. Diazonium Ions Trapped by Nitrile Ylides

mechanism

diverse alkyl ${N}^{2}$ -aryl1,2,3-triazole-carboxylates and their analogues under mild conditions. ${}^{418}$ These two reactions are proposed to proceed through an ionic mechanism, which includes a Cu-based cyclized species 368/369 as the key intermediate, followed by a reductive elimination to furnish the ${N}^{2}$ -aryl triazole product (Scheme 240).

The copper-catalyzed annulation cascade reactions of aryl diazonium salts with oxime acetates ${}^{419}$ and arylamines ${}^{420}$ were developed, affording $N$ -2-aryl-1,2,3-triazoles and $N$ -2-aryl-

Scheme 238. Synthesis of 1,2,4-Triazoles from Azlactones

Scheme 239. ${\mathrm{{BF}}}_{3}$ -Mediated Cascade Reaction of 2- Alkynylanilines with Nitriles naphthotriazoles, respectively (Scheme 241). In the previous case, the proposed mechanism involves a nucleophilic attack of copper(II) enamide complex at the terminal $\mathrm{N}$ atom of aryl diazonium salts, followed by the isomerization to obtain cyclic hydrazonyl iminyl copper(II) as the key intermediate 370.

A divergent copper-catalyzed trapping process of ketene $N, S$ -acetals (alkylthio-substituted enaminones) with aryl diazonium salts was disclosed under an oxygen atmosphere at room temperature, allowing the regioselective generation of functionally diverse ${N}^{2}$ -substituted 1,2,3-triazoles 371 and 2,3- dihydro-1,2,4-triazines 372 (Scheme 242). ${}^{421}$ The selective formation of five- and six-membered $N$ -heterocyclic products depends on the amount of oxidant and base. The mechanistic studies have revealed that the reaction proceeds via the generation of six-membered imino-Cu(II) hydrazone complex 373, or seven-membered benzyl-Cu(II) hydrazone complex 374, respectively.

Scheme 240. Copper(I)-Mediated Synthesis of ${N}^{2}$ -Aryl 1,2,3-Triazoles from Vinyl Azides and Alkyl 3- Aminoacrylate

Scheme 241. Copper-Catalyzed Coupling of Oxime Acetates or Arylamines with Aryl diazonium Salts

Additionally, both the homogeneous or heterogeneous copper-catalyzed [4 + 2] annulation reaction of $\beta$ -enaminones with aryl diazonium salts were developed, affording densely functionalized 3H-1,2,4-triazines 375 with good yields (Scheme 243). ${}^{422}$

As for the case of heteroatom nucleophiles, Slootweg and coworkers reported the facile synthesis of tunable aryl azophosphonium salts through the reaction of para-substituted aryl diazonium salts with tritert-butylphosphine, leading to a range of colored compounds (16 examples, 55-96% yields). ${}^{423}$ Additionally, the one-pot reaction from anilines through diazotization and treatment with a trialkyl phosphite was reported, affording phosphonohydrazines in moderate yields (6 examples, 27-84% yields). ${}^{424}$ These two reactions utilized phosphine compounds as the nucleophiles.

Scheme 242. Copper(II)-Catalyzed C-H Nitrogenation/ Annulation Cascade of Ketene N, S-Acetals with Aryl Diazonium Salts

Scheme 243. Copper-Catalyzed [4 + 2] Annulation of $\beta$ - Enaminones and Aryl Diazonium Salts

Nitrogen nucleophiles can also attack the aryl diazonium salts. In 2016, a three-step synthetic sequence toward benzo[e]imidazo[1,2-c][1,2,3]triazine derivatives was disclosed. ${}^{425}$ The reaction pathway includes sequential imidazole formation, reduction of nitro group accompanied by diazonium formation, and intramolecular $N - N$ bond formation, delivering benzo[e]imidazo[1,2-c][1,2,3]triazines with good yields (8 examples, 79-89% yields).

The construction of functionalized tetrazoles can be achieved through this trapping process with nitrogen nucleophiles. In 2015, Liu and co-workers reported a one-pot sequential reaction of aryl diazonium salts with amidines under basic conditions, followed by the treatment of ${\mathrm{I}}_{2}/\mathrm{{KI}}$ at room temperature (Scheme 244). ${}^{426}$ This transition-metal-free protocol affords a series of 2,5-disubstituted tetrazoles in moderate to high yields. The key step involves the nucleophilic addition of amidines with diazonium cation to obtain iminotriazene 376, followed by ${\mathrm{I}}_{2}$ -mediated cyclization and deprotonation sequence.

The nucleophilic trapping process also allows the synthesis of sulfonated hydrazines through a transition-metal- and oxidant-free cross-coupling reactions of various aryl diazonium tetrafluoroborates with rongalite 377, affording N-amino-sulfonamide 378 with moderate to good yields under room arylsulfinate anion, which is generated through the radical combination of the aryl radical with sulfoxylate anion radical.

Scheme 244. Synthesis of 2,5-Substituted Tetrazoles from Aryl Diazonium Salts and Amidines

temperature (Scheme 245). ${}^{427}$ The essential step involves the nucleophilic addition of aryl diazonium ion by in situ formed

Scheme 245. Synthesis of N-Aminosulfonamides through Radical Coupling Reactions

4.2. Radical-type Reactions with Aryl Diazonium Salts

Another route in this section involves the radical-type interception of aryl diazonium ions, leading to the formation of diazonium radical cation, followed by a sequential SET process or radical-chain pathway to afford final products (Scheme 246). The radical chain may be initiated by copper complex, sulfinates, thermal, or photoredox conditions.

In 2015, Heinrich and co-workers developed a transition-metal-free carboamination reaction of inert alkenes under thermal conditions, affording the desired azo compounds with moderate to good yields (Scheme 247). ${}^{428}$ This base-induced radical carboamination protocol proceeds via the formation of an alkyl radical by the addition of aryl radical with an alkene. The critical step is the trapping process of aryl diazonium ion by this alkyl radical, followed by SET to get a difunctionaliza-tion product. The radical alkene functionalization by using frozen diazonium chlorides was also disclosed, enabling the azo formation with moderate yields. ${}^{429}$

Scheme 246. General Routes for Radical-type Interception

Scheme 247. Carboamination for the Synthesis of Azo Compounds

Besides, the copper complex has been utilized to promote the tandem ring-opening/cyclization cascade of cyclopropanols 379 with aryl diazonium salts at room temperature, achieving the regioselective synthesis of structurally diverse $N$ -aryl pyrazoles with good yields (Scheme 248). ${}^{430}$ The reaction pathway includes the SET and ring-opening process, leading to the formation of the $\beta$ -keto alkyl radical 380, trapped by aryl diazonium salt to afford a $N$ -radical cation 381. Subsequent single-electron reduction, 1,3-HAT, intramolecular cyclization, and water elimination occur successively, affording desired $N$ - aryl pyrazole.

Scheme 248. Synthesis of N-Arylpyrazoles through Cu-Mediated Tandem Ring-Opening/Cyclization Reactions

mechanism

Cu(OAc) ${}_{2}$ Cu(OAc) ${}_{2}$ - CuOAc

CuI

380 381

-Ar

$- {\mathrm{H}}_{2}\mathrm{O}$

( OH

The sulfinates have also been employed to initiate the radical-type interception reaction of aryl diazonium salts. In 2014, Antonchick and co-workers established a novel multicomponent cascade reaction of alkenes, diazonium salts, and sodium triflinate 114, representing a straightforward and regioselective synthetic route to access indoles, pyrazoles, and pyridazinones with moderate to good yields (Scheme 249). ${}^{431}$

Scheme 249. Cascade Multicomponent Synthesis of Indoles, Pyrazoles, and Pyridazinones

This divergent and selective protocol for synthesizing trifluoromethylated heterocycles is based on the radical trifluoromethylation of alkenes, followed by the coupling reaction with aryl diazonium salts and SET to afford aryl hydrazones 382 as the key intermediates. These hydrazones 382 proceed through acid-promoted [3,3]- $\sigma$ rearrangement/ cyclization/ammonia elimination to provide indoles, or undergo intramolecular condensation by elimination of water to afford pyrazoles or dihydropyridazinones.

In 2019, Wu and co-workers reported the selective assembly of fully substituted pyrazoles through rongalite-mediated three-component reaction of aryl diazonium salts and $\alpha ,\beta$ - unsaturated carbonyls under transition-metal-free condition at room temperature (Scheme 250). ${}^{432}$ During this radical annulation protocol, aryl diazonium salts have served as both the precursor of the aryl and aryl hydrazine units. This reaction is initiated by the SET to afford an aryl radical, which undergoes radical addition with benzylideneacetone to provide a benzylic radical 383. Subsequently, the aryl diazonium cation is intercepted by 383, followed by SET process and reduction to obtain a hydrazine intermediate 384. Finally, intramolecular annulation and aromatization occur to generate the desired pyrazole product.

Scheme 250. Synthesis of Pyrazoles via Rongalite-Mediated Three-Component Radical Annulation Reaction

This radical-type interception can be implemented upon visible-light irradiation. With the employment of visible-light-induced photoredox strategy, an azotrifluoromethylation of alkenes with aryl diazonium salts and sodium triflinate 114 was realized, affording corresponding trifluoromethylated azo compounds 385 with good yields (Scheme 251). ${}^{433}$ The reductive quenching process of $\mathrm{{Ru}}$ (III) results in the formation of ${\mathrm{{CF}}}_{3}$ radical 386, which adds to the alkene to obtain an alkyl radical 387. The key step involves the radical interception of the aryl diazonium ion with 387.

In 2020, Correia and Hammond developed a visible-light-induced redox-neutral coupling reaction of aryl diazonium salts with dihydropyridines (DHP), affording a wide range of alkylaryldiazenes 389 without the addition of additional reductant or oxidant (Scheme 252). ${}^{434}$ This reaction is initiated by the photoinduced formation of aryl radical, which undergoes HAT with DHP to obtain an alkyl radical. Trapping of diazonium ions with an alkyl radical occurs to form radical cation 390, followed by the SET process to deliver alkylaryldiazenes.

In 2020, Rafique and Braga reported a visible-light-promoted photoredox azo coupling reaction of imidazoheteroarenes and anilines under room temperate in the presence of eosin ${\mathrm{Y}}_{ - }{\mathrm{{Na}}}_{2}$ (Scheme 253). ${}^{435}$ Mechanistically, both the ionic and radical pathways may be simultaneously operating in this transformation.

Scheme 251. Photoredox Azotrifluoromethylation of Alkenes with Aryl diazonium Salts and Sodium Triflinate

Scheme 252. Photoredox-Catalyzed Synthesis of Alkylaryldiazenes

5. ARYL DIAZONIUM SALTS IN FLOW CHEMISTRY

The transformations of aryl diazonium salts are potentially explosive, both for a large-scale procedure in lab synthetic research and in industrial production. The utilization of continuous-flow protocol partially enhances the safe operation of aryl diazonium salts. With the employment of a continuous-flow setup, the aryl diazonium salts can be generated in situ with no need to separate. Additionally, the generation and transformation of aryl diazonium ions are thoroughly controlled under small and automated flow conditions, avoiding the accumulation of a high concentration of aryl diazonium salts. This has made the combination of flow protocol with aryl diazonium chemistry an attractive area.

In 2015, Felpin completed an elegant review, which includes the recent reactions involving the use of diazonium salts in flow conditions. ${}^{23}$ This review has covered the continuous-flow generations and transformations of aryl diazonium salts during 2012 to 2015. ${}^{{436} - {445}}$ Thus, this content will mainly focus on the recent reactions of aryl diazonium salts in the continuous-flow reactor after the year 2015. Recent advances are focused on the parameters optimization in a continuous-flow reactor, ${}^{{446} - {449}}$ particularly for synthesizing azo and hydrazone compounds ${}^{450}$ (Scheme 254), and the exploration of specific transformations of aryl diazonium salts.

Scheme 253. Visible-Light-Induced Azo Coupling of Imidazoheteroarenes and Anilines with Aryl Diazonium Salts

Scheme 254. Flow in Situ Formation of Hydrazine and Azo Derivatives

In 2016, Buchwald developed a continuous-flow Balz-Schiemann protocol by using ${\mathrm{{LiBF}}}_{4}$ as an effective additive that eliminates the need to isolate the aryl diazonium salts (Scheme 255). ${}^{451}$ This new flow to the stirred-batch reactor process allows the efficient synthesis of aryl fluorides with high yields through the safer generation and dediazotization of aryl diazonium salts. The utility of this system has been successfully identified by the scale-up Balz-Schiemann reaction without compromising the yield.

In 2019, Lévesque reported the Balz-Schiemann reaction of 2-cyano-5-aminopyridine to obtain 2-cyano-5-fluoropyridine using an ionic liquid (1-butyl-3-methylimidazolium tetrafluor-oborate, BMIMBF ${}_{4}$ ) as a solvent (Scheme 256). ${}^{452}$ Compared with Buchwald’s flow-batch method, this protocol allows the C-F bond formation under fully continuous-flow conditions.

Scheme 255. Synthesis of Aryl Fluorides in Continuous-Flow through Balz-Schiemann Reaction

In addition to C-F bond formations via the Balz-Schiemann reaction, the continuous-flow procedure has also been applied to constructing other carbon-heteroatom bonds from diazonium salts. In 2017, Su developed a fully continuous-flow diazotization-hydrolysis protocol to access the synthesis of $p$ -cresol 391 up to 91% total yield and productivity of ${388}\mathrm{\;g}/\mathrm{h}$ (Scheme 257). ${}^{453}$ This process is initiated from the diazotization of $p$ -toluidine to form diazonium intermediate, followed by urea quenching process and final hydrolysis to give the product $p$ -cresol. Besides, a continuous-flow deaminohydrogenation reaction was also realized to synthesize 1,3-difluorobenzene. ${}^{454}$

In 2020, the C-S bond forging reaction was disclosed upon the continuous-flow protocol, in which the first step diazotization was realized on graphene oxide. The thiols were added in succession to intercept aryl diazonium ions, giving rise to desired thioethers in very high yields (Scheme 258).

Moreover, the C-C bond construction of aryl diazonium salts was also realized through the continuous-flow Sonoga-shira and Suzuki-Miyaura cross-coupling reactions using anilines under continuous-flow conditions (Scheme 259). ${}^{456}$ The arylamines undergo initial diazotization and sequential iodination under the flow setup, followed by Pd-catalyzed coupling reaction in the third flow reactor.

With the combination of photoredox strategy, the radical-type arylation reaction of arenes with anilines was realized under continuous-flow conditions, enabling overall reaction times of 45 min (Scheme 260). ${}^{457}$ This UV-light-promoted flow protocol involves the formation and photoinduced decomposition of diazo anhydride intermediate 392 to afford an aryl radical, followed by the Gomberg-Bachmann reaction with a (hetero)arene to form the desired bi(hetero)aryls.

Apart from the flow process upon UV-light irradiation, Rueping and Rehm developed a continuous-flow $\mathrm{C} - \mathrm{H}$ arylation of heteroarenes with aryl diazonium salts irradiated by blue light using ${\mathrm{{TiO}}}_{2}$ as an immobilized photocatalyst in a microstructured falling film reactor (Scheme 261). ${}^{458}$ This method represents a novel combination of a visible-light-induced reaction with a continuous-flow process.

Merging with the transition-metal- and visible-light-induced photoredox catalysis and continuous-flow reactions, Acker-mann developed a manganese-catalyzed C-H arylation of (hetero)arenes employing continuous visible-light flow conditions (Scheme 262). ${}^{459}$ This visible-light-induced flow protocol allows the synthesis of biaryls with high yields, as well as the efficient, versatile transformations at room temperature.

Scheme 256. Continuous-Flow Balz-Schiemann Reaction Enabled by Ionic Liquids

Scheme 257. Fully Continuous-Flow Process for the Synthesis of $p$ -Cresol

Scheme 258. Graphene Oxide-Catalyzed Two-Step Continuous-Flow Synthesis of Unsymmetrical Thioethers

In 2018, the de Oliveira group developed a photoinduced arylation of enol acetates 34 with aryl diazonium salts using various porphyrins as the SET photocatalysts, affording $\alpha$ - arylated aldehyde and ketone building blocks with high efficiency (Scheme 263). ${}^{48}$ The success of this continuous-photoflow strategy has demonstrated the practical application in $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{C}\left( {\mathrm{{sp}}}^{3}\right)$ bond formation.

Except for a traditional flow protocol and the merging of the photoredox/flow process, a mechanochemical synthetic route was developed using a jacketed screw reactor, allowing the synthesis of the azo compounds with $\beta$ -napththol under continuous-flow conditions. ${}^{460}$

6. ARYL DIAZONIUM SALTS IN THE MODIFICATION OF FUNCTIONAL MATERIALS AND MOLECULES

Apart from the effective utilization of aryl diazonium salts in organic synthetic chemistry, recent applications of diazonium compounds have furnished new strategies for the modification of materials and biomolecules, including electrode surfaces, graphene, carbon nanotubes, as well as peptides and proteins. In most cases, aryl diazonium salts are utilized as synthon of the substituted (hetero)aryl group to modify materials’ surface through covalent functionalization. This transformation usually proceeds through traditional radical-type arylation to forge a $\mathrm{C} - \mathrm{C}$ bond, or constructing a $\mathrm{C} - \mathrm{S}$ bond by the reaction with amino acid, ${\mathrm{{MoS}}}_{2}$ , and forging a $\mathrm{C} - \mathrm{{Au}}$ bond.

Scheme 259. Continuous-Flow Sonogashira and Suzuki-Miyaura Coupling Reactions of Anilines

Several recent reviews and papers have surveyed the advances in the development and application of diazonium compounds for the surface modification of diverse materials application of diazonium compounds for the surface modification of various materials, ${}^{461}$ such as gra-phene, ${}^{{22},{462} - {464}}$ electrode surfaces, ${}^{{465} - {469}}$ carbon nano-tubes, ${}^{{470} - {477}}$ nanostructured mesoporous ITO films, ${}^{478}$ glycosides, ${}^{479}$ and graphene oxide. ${}^{{480},{481}}$ Mainly, a range of aryl functional groups can be employed to functionalize the basal plane of ${\mathrm{{MoS}}}_{2}$ through diazonium-based chemistry. ${}^{{482} - {484}}$ Several elegant reviews have recently focused on modifying functional materials and bioactive molecules by using aryl diazonium salts. In this section, we will just give a brief instruction toward the recent advances in this area.

Scheme 260. Light-Induced C-H Arylation of Arenes by in Situ Generated Diazo Anhydrides

Scheme 261. Continuous-Flow Light-Mediated C-H Arylation of Heteroarenes Using ${\mathrm{{TiO}}}_{2}$

Scheme 262. Continuous-Flow Visible-Light-Mediated Approach for Mn-Catalyzed C-H Arylation of Heteroarenes

Scheme 263. Continuous-Photoflow Setup for $\mathrm{C} - \mathrm{H}$ Arylations

For the wide-established graphene modifications, both the preprepared and the in situ formed aryl diazonium salts are utilized to access the aryl group functionality. In 2020, Assali synthesized an array of graphene materials covalently functionalized with hydroxyl-, amine-, or carboxyl containing groups through the direct diazonium functionalization of few-layer graphene (FLG) (Scheme 264). ${}^{485}$ Here, the arylamines are initially diazotized using isoamyl nitrite, followed by radical addition to graphene to afford the antibacterial materials.

The diazonium functionalization using 2-aminobenzene-1,4- disulfonic acid as the substrate was reported under aqueous phase conditions, allowing the surface modification of carbon fibers (CFs) (Scheme 265). ${}^{486}$

Using the in situ diazotization/modification strategy, the selective diazonium functionalization of graphite nanosheets (GNs) and Si nanoparticles (Si NPs) was developed, affording a useful electrochemical Si-Ar-GNs composite (Scheme 266). ${}^{487}$ This protocol features of forging of $\mathrm{C} - \mathrm{C}$ and $\mathrm{C} - \mathrm{{Si}}$ bonds in a two-step process.

Apart from the $\mathrm{C} - \mathrm{C}$ and $\mathrm{C} - \mathrm{{Si}}$ bond formations, $\mathrm{C} - \mathrm{P}$ bond formations on the surface of black phosphorus were realized by covalent aryl diazonium functionalization, enabling the material’s hole-carrier mobility to be tuned, affording an optimized layered two-dimensional semiconductor. ${}^{488}$

The aryl diazonium functionalization also allows the Au surface graft through the generation aryl diazonium gold(III) species. ${}^{489}$ A combination of photoredox protocol with surface modification was realized to the afford nanopores within polynitrophenylene films on Au(111) surfaces (Scheme 267). ${}^{490}$ Using 4-nitrobenzenediazonium tetrafluoroborate as the substrate, the corresponding aryl radical 393 is generated upon the visible-light irradiation, followed by C-Au bond formation. The sequential photoredox arylation reaction could lead to the formation of a polynitrophenylene scaffold.

In addition to the photoredox strategy, the electrochemical protocol of diazotized N-doped carbon nanodots (N-CD) was developed to generate aryl radicals, allowing the electrografting onto the surface of carbon electrodes (Scheme 268). ${}^{491}$ The

Scheme 264. Functionalization of the Few-Layer Graphene electrochemical method allows nanofilter film preparation on the Au electrode by combining the aryl diazonium salt reduction strategy and boronate affinity. ${}^{492}$ This research demonstrates the diazonium modification in the enhancement of the signal-to-noise ratio in electrochemical biosensors.

Scheme 265. CFs Surface Modifications

Moreover, the aryl diazonium functionalization has demonstrated chemical biology applications through the selective covalent functionalization of amino acids, peptides, and proteins. Several outstanding reviews have elegantly covered and emphasized these recent advances. ${}^{{24},{493} - {498}}$ Thus, we will only briefly introduce this content to highlight these recent advances, stimulating interest toward broader applications of aryl diazonium salts as aryl transfer agents in bioconjugation reactions.

The aryl diazonium salts can serve as both aryl and azo precursors to graft on the biochemical molecules. For the previous case, the aryl group functionality of the phenolic side chain of tyrosine was realized through the selective $\mathrm{C} - \mathrm{C}$ bond

Scheme 266. Illustration of Si-Ar-GNs Preparation

Scheme 267. Photocatalysis with Particle Lithography to Generate Polynitrophenylene Nanostructures formation on aromatic rings, affording 3-aryltyrosine derivatives. ${}^{499}$ The 3-monoaryl tyrosine hydroxamic acids possessing KDM4A demethylase activities were also synthesized through the diazonium functionalization. ${}^{500}$

Scheme 268. Diazotized N-CD Electrografting on Carbon Electrodes

For the latter case, the site-selective azo coupling between tyrosine and aryl diazonium salts was realized. ${}^{501}$ Besides, the modification of tyrosine and tyrosine-containing peptides was disclosed using ${}^{64}\mathrm{{Cu}}$ - and ${}^{68}\mathrm{{Ga}}$ -labeled 1,4,7-triazacyclono-nane-1,4,7-triacetic acid-diazonium salts 394 as building blocks (Scheme 269). ${}^{502}$ These bioconjugation reactions on proteins have successfully been achieved through the selective azo coupling reaction on the aromatic rings.

Scheme 269. Aryl Diazonium Salts for Chemo-selective Bioconjugation of Tyrosine Residues

7. CONCLUSIONS

While aryl diazonium salts have been known for more than 160 years, as shown in this review, the chemistry of these fascinating compounds still remains as an exciting and attractive research topic in synthetic organic chemistry in recent years. Owing to their versatile reactivity, aryl diazonium salts are pervasive and outstanding substrates in various transformations, which have found numerous new applications in organic synthesis, functional material modifications, biological chemical research, and other interdisciplinary fields. In particular, over the past decade, the merger of aryl diazonium chemistry with photocatalysis, transition-metal catalysis, continuous-flow technique, electrochemistry, and mechanor-edox catalysis leads to the generation of aryl radicals and other reactive species under mild conditions, thus opening new possibilities for further utilization of aryldiazonium salts. Furthermore, several new aryl $\mathrm{C} - \mathrm{C}$ bonds and $\mathrm{C} - \mathrm{Y}(\mathrm{Y} = \mathrm{B}$ , S, P, Sn, and Se) bond forming reactions have emerged in the past years, which significantly increase the synthetic utilities of aryldiazonium salts.

Although aryl halides (or pseudohalides), aryl boronic acids, and aryl hypervalent iodine compounds are the common choices for functional group conversion or coupling reactions, aryl diazonium salts have their own distinguishing features and advantages. First, aryl diazonium salts are synthesized from the corresponding aryl amines, which are widely available and cheap industrial feedstocks. Second, since the conditions of diazotization are orthogonal to many other reactions, the reactions involving diazonium salts have excellent compatibility. Third, amino groups are easily protected and deprotected; thus, the conversion of amino groups through diazonium salts may be advantageous for late-stage modification in the synthesis of drug molecules or natural products.

Despite the myriad recent advances, several limitations need to be addressed in the future, in order to further improve the practicality and versatility of these methods. For most of the reactions, the aryldiazonium salts are used as the reactants. Because of the stability problem and safety concerns related to aryldiazonium salts, ${}^{503}$ the wide applications of these transformations in organic synthesis are rather hampered. The in situ generation of aryldiazonium salts and flow technic may circumvent these problems to some extent; however, the diazotization process may not be compatible with many reactions. Recently, some aryldiazonium salts surrogates, such as aryl triazenes, ${}^{{504} - {506}}$ arylazo sulfones, ${}^{{507} - {512}}$ have been developed and applied in the arylation reactions. These compounds can be employed to introduce aryl groups (or sulfonyl groups) selectively under Lewis acid or visible-light-driven conditions. However, up to now, these nonexplosive precursors are still far from replacing the synthetic value of aryldiazonium salts in organic synthesis.

As for the generation of aryldiazonium salts from arylamines, although some progress has been made, the choice of reagents and the reaction conditions for the diazotization of arylamines are still limited. Further development along this line would be desirable. Moreover, the scope of transition-metal-catalyzed transformations with aryldiazonium salts is still narrow. Future exploration in this arena would lead to the development of new methods for aryl $\mathrm{C} - \mathrm{C}$ bond forming with arylamines as the factual cross-coupling partners.

AUTHOR INFORMATION

Corresponding Authors

Fanyang Mo - Department of Energy and Resources Engineering, College of Engineering and School of Materials Science and Engineering, Peking University, Beijing 100871, China; © orcid.org/0000-0002-4140-3020;

Jianbo Wang - Beijing National Laboratory of Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China; © orcid.org/0000-0002-0092-0937;

Authors

Di Qiu - Tianjin Key Laboratory of Structure and Performance for Functional Molecules, College of Chemistry, Tianjin Normal University, Tianjin 300387, China; © orcid.org/0000-0001-9706-6302

Lei Zhang - Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, China; (Corcid.org/0000-0001-9031-4318

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.chemrev.0c01030

Notes

The authors declare no competing financial interest.

Biographies

Fanyang Mo obtained his undergraduate and master’s degrees in applied chemistry from the Beijing Institute of Technology under the supervision of Professor Zhiming Zhou in 2004 and 2006, respectively. He then moved to Peking University and pursued his Ph.D. studies under the supervision of Professor Jianbo Wang. After receiving his Ph.D. in 2010, he went to the USA and worked with Professor Qinghai Zhang at The Scripps Research Institute and Professor Guangbin Dong at The University of Texas at Austin from 2010 to 2015. He joined Peking University as an Assistant Professor in April 2015. The current research interests of his group include resources orientated organic synthesis, modification of semiconductor oxides for energy and resource utilization, and molecular electronics.

Di Qiu was born in Tianjin, China. He received his B.S. degree from Peking University in 2010. He was a graduate student in Prof. Jianbo Wang’s laboratory from 2010 to 2015. He obtained his Ph.D. degree in 2015. His research projects are focused on the development of basic organic functional group transformations, in particular, the gold-catalyzed C-H bond functionalization and the transformation of aromatic $\mathrm{C} - \mathrm{N}$ bonds based on diazonium salts. Dr. Qiu has published 30 papers in renowned international journals. Since 2015, he has been an Assistant Professor at the College of Chemistry, Tianjin Normal University. His research interests lie in aromatic CSn and C-P bond formation and functional group transformations in aromatic compounds.

Lei Zhang is a Ph.D. candidate under the supervision of Prof. Fanyang Mo at Peking University. He obtained his B. Eng. degree from Beijing Institute of Technology in 2016, and his bachelor thesis was finished under the joint guidance of Prof. Jianbo Wang and Prof. Fanyang Mo. In the same year, he moved to Peking University and started his Ph.D. study. His research project focuses on green organic synthesis by using organic electrochemistry and investigation of reaction mechanism enabled by single-molecule platform.

Jianbo Wang received his B.Eng. from Nanjing University of Science and Technology in 1983 and his Ph.D. from Hokkaido University (under the supervision of Prof. Hiroshi Suginome) in 1990. He was a postdoctoral associate at the University of Geneva from 1990 to 1993 (with Prof. Charles W. Jefford) and at University of Wisconsin-Madison from 1993 to 1995 (with Profs. Howard E. Zimmerman and Leonard A. Fahien). He began his independent academic career at Peking University in 1995 as an associate professor and was promoted to full professor in 1999. His research interests focus on the catalytic metal carbene transformations.

ACKNOWLEDGMENTS

This project is supported by the Natural Science Foundation of China (Grant Nos. 21772003, 21933001, 22071004, 91956104), Beijing National Laboratory of Molecular Sciences (BNLMS), and Laboratory for Synthetic Chemistry and Chemical Biology of Health@InnoHK of ITC, HKSAR. This work was partially funded by the BHP-PKU CCUS project supported by BHP Billiton. We thank Prof. Yang Yang (University of California-Santa Barbara) for helpful discussions.

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Recent Development of Aryl Diazonium Chemistry for the Derivatization of Aromatic Compounds

CONTENTS

1. INTRODUCTION

2. ARYL C-C BOND FORMATIONS

2.1. Aryl C-C Bond Formations Involving Aryl Radical Intermediates

2.2. Transition-Metal-Catalyzed Coupling Reactions of Aryl Diazonium Salts

3. ARYL DIAZONIUM SALTS IN ARYL C-HETEROATOM BOND FORMATIONS

3.1. C-Halogen Bond Formations

3.2. C-O Bond Formations

3.3. C-N Bond Formations

3.4. C-B Bond Formations

3.5. C-P Bond Formations

3.6. C-Sn Bond Formations

3.7. C-Se, Te Bond Formations

3.8. C-S Bond Formations

3.9. Reactions with Nitriles and Cascade Reactions

4. REACTIONS WITH THE RETENTION OF THE NITROGEN GROUP

4.1. Nucleophilic Additions with Aryl Diazonium Cation

4.2. Radical-type Reactions with Aryl Diazonium Salts

5. ARYL DIAZONIUM SALTS IN FLOW CHEMISTRY

6. ARYL DIAZONIUM SALTS IN THE MODIFICATION OF FUNCTIONAL MATERIALS AND MOLECULES

7. CONCLUSIONS

AUTHOR INFORMATION

Corresponding Authors

Authors

Notes

Biographies

ACKNOWLEDGMENTS

REFERENCES