Electrocatalytic Oxidative Transformation of Organic Acids for Carbon–Heteroatom and Sulfur–Heteroatom Bond Formation

CHEMISTRY & SUSTAINABILITY

CHIPMSUSCHIM

ENERGY & MATERIALS

Accepted Article

Title: Electro-catalytic oxidative transformation of organic acids for carbon-heteroatom and sulfur-heteroatom bond formation

Authors: Man Li, Junting Hong, Wei Xiao, Yang Yang, Di Qiu, and Fanyang Mo

This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article.

To be cited as: ChemSusChem 10.1002/cssc.201902657

Link to VoR: http://dx.doi.org/10.1002/cssc.201902657

Electrochemical transformation of organic acids for carbon- heteroatom and sulfur-heteroatom bond formation

Man Li, ${}^{\left\lbrack \mathrm{a}\right\rbrack }$ Junting Hong, ${}^{\left\lbrack \mathrm{a}\right\rbrack }$ Wei Xiao, ${}^{\left\lbrack \mathrm{a}\right\rbrack }$ Yang Yang, ${}^{\left\lbrack \mathrm{b}\right\rbrack }$ Di Qiu, ${}^{\left\lbrack \mathrm{c}\right\rbrack }$ and Fanyang Mo*[a, d]

Dedication ((optional))

REVIEW

Abstract: The electrolysis of organic acids has garnered increasing attention in recent years. In addition to the famous electrochemical decarboxylation known as Kolbe electrolysis, a number of other electrochemical processes have been recently established, allowing for the construction of carbon-heteroatom and sulfur-heteroatom bonds from organic acids. In this review, we survey recent advances in electrochemical C-X and S-X (X = N, O, S, Se) bond forming reactions from five classes of organic acids and their conjugate bases, namely carboxylic, thiocarboxylic, phosphonic, sulfinic and sulfonic acids.

1. Introduction

Organic electrochemistry represents one of the most powerful and sustainable methods in organic chemistry. ${}^{\left\lbrack 1\right\rbrack }$ Hazardous stoichiometric external oxidants or reductants generally used in conventional organic synthesis can be avoided or used in catalytic amount by organic electrosynthesis. ${}^{\left\lbrack 2\right\rbrack }$ Among the oxidative coupling reactions, external oxidant-free methods with hydrogen evolution are particularly attractive, which can be realized by employing organic electrochemistry.[3] What’s more, in certain cases some chemical regents such as strong bases or highly active oxidants can be electrogenerated in situ, which are both environmentally friendly and mechanistically interesting. ${}^{\left\lbrack 4\right\rbrack }$ Recent studies have demonstrated the usefulness of organic electrosynthesis in inert bond functionalization ${}^{\left\lbrack 5\right\rbrack }$ and difunctionalization of alkenes or alkynes. ${}^{\left\lbrack 6\right\rbrack }$

The electrodecarboxylation of aliphatic carboxylic acids to form dimeric products, known as the Kolbe reaction has found widespread applications in organic electrochemistry. ${}^{\left\lbrack 7\right\rbrack }$ Mechanistically, alkyl radicals are generated and coupled in classical Kolbe reaction. In recent years, the concept of Kolbe electrolysis has been largely expanded due to the rich chemistry of alkyl radicals. ${}^{\left\lbrack 8\right\rbrack }$ In addition, a number of non-decarboxylative electrochemical transformations of carboxylic acids have also been developed in recent years. Prior to the extrusion of carbon dioxide, the carboxylic acid group can participate in various C-O bond-forming reactions to provide esters and lactones as the product. The anodic oxidation of other organic acids such as sulfinic acids are very similar with that of carboxylic acids. Such sulfinic acids as ${\mathrm{{CF}}}_{3}{\mathrm{{SO}}}_{2}\mathrm{{Na}}$ were well known to release ${\mathrm{{SO}}}_{2}$ via anodic oxidation. It should be noted that generally C-S or S-X instead of C-O or O-X bond-forming reactions are observed in the electrolysis of thiocarboxylic acids and sulfinic acids. In this review, recent advances in C-O, C-S and S-X (X = N, O, S, Se) bond formation reactions involving organic acidic functional groups using electrochemical methods are summarized based on the types of the reacting organic acid substrates. The organic acids involved in this review include carboxylic acids, thiocarboxylic acids, phosphonic acids, sulfinic acids and sulfonic acids. Representative examples involving C-H functionalization and alkene/alkyne difunctionalization processes are highlighted. Electrochemical reactions involving merely expulsion of ${\mathrm{{CO}}}_{2}$ or ${\mathrm{{SO}}}_{2}$ from respective carboxylic acids or sulfinic acids are out of the reach of this review.

Man Li received his B.S. degree from Henan Normal University in 2010, and obtained his M.Sc. degree from Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS) in 2014. He has been a Research Assistant at LICP, CAS, Shanghai Institute of Organic Chemistry, CAS and Peking University. His research interests include chemical transformations of carbon dioxide and organic electrochemistry.

Junting Hong received her B.S. degrees from Beijing Institute of Technology in 2017. She is currently a Ph.D. candidate in Professor Fanyang Mo’s group in Peking University.

Wei Xiao is a third-year undergraduate student of Peking University. He is now studying in Professor Mo’s group.

Yang Yang received his Ph.D. degree from the Massachusetts Institute of Technology under the guidance of Prof. Stephen Buchwald. At MIT, his research focused on copper-catalyzed asymmetric hydrofunctionalization of simple olefins. In particular, he developed a copper-catalyzed

[a] M. Li, J. Hong, W. Xiao, Prof. Dr. F. Mo,

Department of Energy and Resources Engineering

College of Engineering

Peking University, Beijing, 100871, China

[b] Dr. Y. Yang

Division of Chemistry and Chemical Engineering

California Institute of Technology, Pasadena, California 91125,

United States

[c] Prof. Dr. D. Qiu,

Tianjin Key Laboratory of Structure and Performance for Functional

Molecules, MOE Key Laboratory of Inorganic-Organic Hybrid

Functional Materials Chemistry

College of Chemistry, Tianjin Normal University, Tianjin

300387, China

[d] Prof. Dr. F. Mo,

Jiangsu Donghai Silicon Industry S&T Innovation Center, Donghai

County, Jiangsu 222300, China

REVIEW

enantioselective hydroamination of unactivated internal olefins and a set of methods for the enantioselective addition of olefin-derived nucleophiles to carbonyls and imines. He is currently an NIH Postdoctoral Fellow in the laboratory of Prof. Frances Arnold at the California Institute of Technology, where he is using directed evolution to engineer cytochrome P450 enzymes for enantioselective $\mathrm{C} - \mathrm{H}$ amination.

Di Qiu was born in Tianjin, China, in 1987. He received his B.S. degree from Peking

University in 2010. He continued his

graduate study in Prof. Jianbo Wang’s

laboratory and obtained his Ph.D. in 2015. In

the same year, he joined in Tianjin Normal

University as an assistant professor. His

research interests lie in aromatic C-P bond

formation and functional group

transformations in aromatic compounds.

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 ${\mathrm{{CO}}}_{2}$ utilization in organic synthesis, organic electrochemistry and modification of semiconductor oxides for energy and resources utilization.

2. Ester C-O bond formation via electrolysis of carboxylic acids

2.1 Direct oxidative coupling of carboxylic acid with aryl $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{H}$ bond

Early works of this topic including examples involving transition-metal-catalyzed electrochemical transformations such as the oxygenation of $\mathrm{C} - \mathrm{H}$ bonds have been summarized by other reviews ${}^{\left\lbrack 5a,5b,9\right\rbrack }$ and are not included in this review. We will only focus on the recent advances in this part.

The oxidative cyclization of 2-arylbenzonic acid via aryl $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) -$ H bond functionalization is an ideal approach towards biaryl lactones. In 2018, several groups independently reported the anodic oxidative cyclization of 2-arylbenzonic acids under transition-metal-free conditions (Scheme 1). ${}^{\left\lbrack {10}\right\rbrack }$ Previously, this transformation has been achieved by many groups by means of metal catalysis (Pd, Cu, etc.) in combination with stoichiometric oxidant. The catalysis system is somewhat complicated. And the use of large amount of oxidant cause safety issue, excessive waste generation and poor atom economy. In this regard, electrochemistry offers a more superior way than traditional chemistry.

Scheme 1. Oxidative cyclization of 2-arylbenzonic acids.

Constant current electrolysis in an undivided cell at room temperature were employed in all these methods. In Zeng’s report, the utilization of inexpensive graphite electrodes can be quite attractive for large-scale electrosynthesis. ${}^{\left\lbrack {10}b\right\rbrack }$ This method had been applied to a ${40}\mathrm{\;g}$ scale synthesis. In Mo’s report, catalytic amount of NaOH was used and no further supporting electrolyte was required, which is distinct from others’ works.[10c] In Lei’s report, ${\mathrm{{Na}}}_{2}{\mathrm{{SO}}}_{4}$ aqueous solution was used as a cheap and green supporting electrolyte. ${}^{\left\lbrack {10}c\right\rbrack }$ Notably, they found that acids such as acetic acid were beneficial for the reaction. In Xu’s report, the electrolysis of 2-phenylbenzonic acid could be conducted on 100 g scale under an air atmosphere. ${}^{\left\lbrack {10}\mathrm{\;d}\right\rbrack }$ In Luo’s report, catalytic amount of DDQ and 2,6-lutidine was used as a redox mediator and base, respectively. ${}^{\left\lbrack {10}e\right\rbrack }$ Therefore, indirect electrolysis was used in Luo’s method and direct electrolysis was used by others’ works. It’s noteworthy that Zeng and Luo also reported the electrochemical lactonization of diaryl acrylic acids affording valuable coumarins. As for the reaction mechanism, the most popular proposed mechanism was the generation of carboxylate radical by anodic oxidation of carboxylate anion, which was followed by an intramolecular radical trapping cyclization. However, the prior anodic oxidation of the aromatic ring containing the reacting C-H bond cannot be ruled out especially for the electron-rich aromatic ring substituted with a methoxyl group as suggested by Lei and co-workers.

In 2018, Neumann and co-workers reported an electrocatalytic formylation reaction of benzene and its halogenated derivatives in the presence of ${\left\lbrack {\mathrm{{Co}}}^{\mathrm{{III}}}{\mathrm{W}}_{12}{\mathrm{O}}_{40}\right\rbrack }^{5 - }$ as a catalyst and lithium formate as an electrolyte(Scheme 2). ${}^{\left\lbrack {11}\right\rbrack }$ The aryl formates products are easily hydrolyzed by water affording the corresponding phenols and formic acid. Hydrogen was formed at the cathode. Therefore, the sum reaction is an indirect hydroxylation of benzene with water to yield phenol and ${\mathrm{H}}_{2}$ as products. It’s proposed that the formyloxy radical serves as the reactive species based on EPR study.

Scheme 2. Pathway for the electrochemical hydroxylation of benzene to phenol.

In 2019, Ackermann and co-workers reported the first electrochemical cobalt-catalyzed C(sp ${}^{2}$ )-H acyloxylation in biomass-derived y-valerolactone (GVL) as a renewable solvent (Scheme 3). ${}^{\left\lbrack {12}\right\rbrack }$ Electricity serves as sustainable oxidant with molecular hydrogen as the sole byproduct in the absence of stoichiometric amounts of metal oxidants. Both aromatic and aliphatic carboxylic acids were amenable substrates. A plausible catalytic cycle was slightly revised from original paper according to the related works afterwards. ${}^{\left\lbrack {13}\right\rbrack }$ The reaction was initiated by the oxidation of $\mathrm{{Co}}\left( \mathrm{I}\right)$ catalyst with carboxylic acid to form carboxylate Co(II), followed by coordination with directing group on the substrate. Further oxidation this Co(II) to Co(III) occurs with concomitant activation of C-H bond. At last, reductive elimination and proto-demetallation affords the final product and regenerates Co(I) catalyst (Scheme 3).

Scheme 3. Cobalt-catalyzed C-H acyloxylation.

2.2 Direct oxidative coupling of carboxylic acid with vinyl $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{H}$ bond

In 2018, one example of intramolecular oxidative cyclization of carboxylic acid with vinyl $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right)$

-H bond was reported by Luo and co-workers (Scheme 4). ${}^{\left\lbrack {11}\right\rbrack }$ Tetrabutylammonium bromide was a better redox mediator than DDQ in this case.

Scheme 4. Oxidative cyclization of carboxylic acid with vinyl $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{H}$ bond in the presence of tetrabutylammonium bromide.

2.3 Intramolecular coupling of carboxylic acid with $\mathrm{C}\left( {\mathrm{{sp}}}^{2}\right) - \mathrm{X}$ bond

In 2018, during the investigation of the intramolecular lactonization of 2’-halobiphenyl-2-carboxylic acids, Luo and coworkers found that C-X substitution products were obtained instead of C-H substitution products (Scheme 5). ${}^{\left\lbrack {11}\right\rbrack }$ Halogens including chloride, bromide, and fluoride, could be viable leaving groups. This kind of electrolysis-initiated aromatic radical substitution is relatively rare.

Scheme 5. Intramolecular lactonization of 2’-halobiphenyl-2-carboxylic acids.

2.4 Electrocatalytic dehydrogenative esterification of carboxylic acids with ketones

In light of the competing decarboxylation of aliphatic acids, it would be significant to construct ester C-O bonds from aliphatic acids suppressing this side reaction. In 2017, Xu and co-workers reported the intra- and intermolecular electrocatalytic dehydrogenative esterification of carboxylic acids with ketones by using an indirect electrolysis strategy (Scheme 6). ${}^{\left\lbrack {14}\right\rbrack }$

Scheme 6. Electrocatalytic dehydrogenative esterification of aliphatic carboxylic acids.

For intermolecular dehydrogenative esterification, both aliphatic and aromatic acids were well-tolerated. They proposed the following mechanism (Scheme 7). Cathodic reduction of the carboxylic acid forms the corresponding carboxylate anion. With $n - {\mathrm{{Bu}}}_{4}\mathrm{{NI}}$ as a mediator, the in situ-generated ${\mathrm{I}}_{2}$ could transform the ketone to $\alpha$ -iodinated ketone. The $\alpha$ -iodinated ketone could also be generated from acyl hypoiodite intermediate.The final nucleophilic substitution affords the ester product. The utility of this method is illustrated by the synthesis of a natural product cytosporanone A on a gram scale.

Scheme 7. Proposed mechanism for electrocatalytic dehydrogenative esterification of aliphatic carboxylic acids.

In 2019, Terent’ev and co-workers reported an electrochemically induced intermolecular cross-dehydrogenative C-O coupling of $\beta$ -diketones and $\beta$ -ketoesters with carboxylic acids by using KBr as supporting electrolyte (Scheme 8). ${}^{\left\lbrack {15}\right\rbrack }$ They proposed that brominated $\beta$ -diketones or $\beta$ -ketoesters serve as an important intermediate in this reaction, similar with that of Xu’s work.

Scheme 8. Intermolecular cross-dehydrogenative C-O coupling of $\beta$ -diketones and $\beta$ -ketoesters with carboxylic acids.

Recently, Wirth and co-workers reported the enantioselective electrochemical lactonization of diketo acid derivatives using chiral iodoarenes as redox mediators for the first time (Scheme 9). ${}^{\left\lbrack {16}\right\rbrack }$ CV studies show that chiral iodoarene has a lower oxidative potential than the substrate, indicating hypervalent iodine regent is first generated in situ in this electrochemical reaction, which then works as a homogeneous chiral organocatalyst to catalyze enantioselective lactonization reaction. Notably, this reaction could also be accomplished using an electrochemical flow microreactor with a lower supporting electrolyte concentration. In 2019, an efficient cross-coupling reaction of carbon dioxide with amines and aryl ketones was developed, providing a variety of O- $\beta$ -oxoalkyl carbamates with moderate to good yields. ${}^{\left\lbrack {17}\right\rbrack }$

Scheme 9. Enantioselective electrochemical lactonization of diketo acid derivatives.

2.5 Esterification of benzylic $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ bond

In 2018, Zeng and co-workers reported the external oxidant-free electrochemical lactonization of benzylic $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ bonds (Scheme 10). ${}^{\left\lbrack {10}b\right\rbrack }$ Lactones with a new quaternary carbon center could be forged in some cases. Notably, an example of the electrochemical lactonization of aliphatic carboxylic acid was given. A plausible mechanism was proposed by the authors. The initial electrogenerated carboxylate radical might trigger a remote 1,5-hydrogen-atom-transfer (HAT) resulting in the formation of a stabilized benzylic radical. The final cyclization leads to the product by losing an electron and a proton possibly via a cation intermediate. It should be mentioned that 2-methylbenzonic acid and 2-ethylbenzonic acid were not suitable for this protocol, which indicates that the stability of the benzylic radical intermediate is highly important for this reaction.

Scheme 10. Electrochemical lactonization of benzylic $\mathrm{C}\left( {\mathrm{{sp}}}^{3}\right) - \mathrm{H}$ bonds.

At the same year, Muñiz and co-workers also reported an example of the electrochemical lactonization of aliphatic carboxylic acid in higher yield (Scheme 11).[18]

Scheme 11. Electrochemical lactonization of aliphatic carboxylic acid.

In 2018, Neumann and co-workers reported the acetoxylation of alkylarenes catalyzed by ${\left\lbrack {\mathrm{{Co}}}^{\mathrm{{IV}}}{\mathrm{W}}_{12}{\mathrm{O}}_{40}\right\rbrack }^{4 - }$ in acetic acid. ${}^{\left\lbrack {11}\right\rbrack }$ Dehydrogenation or oxidation of alkylarenes were also observed in these reactions (Scheme 12).

Scheme 12. Electrochemical oxidation of alkylarenes catalyzed by ${\left\lbrack {\mathrm{{Co}}}^{IV}{\mathrm{\;W}}_{12}{\mathrm{O}}_{40}\right\rbrack }^{4 - }$ in acetic acid. FE = Faradaic efficiency for oxidation of the organic substrate.

2.6 Esterification of carboxylic acids via difunctionalization of alkenes

2.6.1 Lactone formation via intramolecular trapping of intermediate with carboxylic acids

In 2013, Moeller and co-workers reported the anodic coupling of carboxylic acids to electron-rich double bonds (Scheme 13). ${}^{\left\lbrack {19}\right\rbrack }$ Although aliphatic carboxylic acids were used as substrates, Kolbe decarboxylation does not appear to be a significant competing pathway. Electron-rich olefins including ketene dithioacetal, vinyl sulfide, enol ether and substituted styrene were all suitable for this reaction. Mechanistically, it’s likely that oxidation occurs at the olefin and carboxylate traps the resulting radical cation intermediate. After the second oxidation and nucleophilic attack by methanol or methoxide, the final products are furnished.

Proposed mechanism:

Scheme 13. Coupling of carboxylic acids to electron-rich double bonds.

Using the similar strategy of Moeller, in 2019, Xu and coworkers reported the lactone formation from carboxylic acid-containing styrene substrates (Scheme 14). ${}^{\left\lbrack {20}\right\rbrack }$ The endo-type cyclization deserves attention in Xu’s work.

Scheme 14. Electrochemical lactones formation.

In 2018, Kam and co-workers investigated the effect of ortho’- substituted side chains bearing nucleophilic groups such as ${\mathrm{{CO}}}_{2}\mathrm{H}$ on the reactivity of anodically generated 4-methoxy- and 3,4- dimethoxystilbene cation radicals (Scheme 15). ${}^{\left\lbrack {21}\right\rbrack }$ With COOH as the ortho’substituent, bis-δ-lactones were obtained via direct intramolecular cation-nucleophile reaction.

Scheme 15. Reactivity of anodically generated 4-methoxystilbene cation radicals.

Integration of electrochemical oxidative cyclization and chemical oxidation is an intriguing strategy in the difunctionalization of alkenes. Based on their previous work, in 2013, Yoshida and co-workers reported the integration of electrooxidative cyclization of alkenes bearing a nucleophilic moiety in the presence of DMSO and chemical oxidation of the resulting alkoxysulfonium ions. ${}^{\left\lbrack {22}\right\rbrack }$ The alkoxysulfonium ions can be converted to ketones by treatment with ${\mathrm{{Et}}}_{3}\mathrm{\;N}$ via Swern-Moffatt type oxidation. The reaction is useful for synthesis of lactones when the ${\mathrm{{CO}}}_{2}\mathrm{H}$ group serves as an intramolecular nucleophile (Scheme 16).

Scheme 16. Electrooxidative cyclization of alkenes

In 2017, Lam and co-workers reported an electrochemical synthesis of phthalides using anodically generated aroyloxy radicals in combination with aliphatic carboxylic acid as alkylating agent precursors (Scheme 17). ${}^{\left\lbrack {23}\right\rbrack }$ They proposed that aroyloxy radical and alkyl radical are key intermediates of this reaction. The electrogenerated alkyl radical from aliphatic carboxylic acid might oxidize the aromatic carboxylate forming the aroyloxy radical. The possibility of the direct oxidation of the olefin into a radical-cation followed by its capture through a nucleophilic attack from the carboxylate was not favored by a control experiment.

Scheme 17. Electrochemical synthesis of phthalides via anodic activation of aromatic carboxylic acids

In 2018, Han and co-workers reported the synthesis of thio-substituted lactones via electrochemical oxysulfuration of alkenes (Scheme 18). ${}^{\left\lbrack {24}\right\rbrack }$ Unsaturated carboxylic acids including aliphatic and aromatic acids were all suitable for this reaction. It’s likely that aryl sulfur radical generated from anodic oxidation of thiophenol triggers the addition reaction. The resulting radical intermediate undergoes further anodic oxidation to afford the carbocation intermediate, which is attacked by the nucleophile furnishing the final product.

Scheme 18. Synthesis of thio-substituted lactones via electrochemical oxysulfuration of alkenes

Using the similar strategy, in 2019, Xu and co-workers reported an electrochemical fluoromethylation triggered lactonization of alkenes under additional supporting electrolyte- and catalyst-free conditions (Scheme 19). ${}^{\left\lbrack {25}\right\rbrack }$ With readily prepared ${\mathrm{{CF}}}_{2}{\mathrm{{HSO}}}_{2}\mathrm{{Na}}$ or ${\mathrm{{CF}}}_{3}{\mathrm{{SO}}}_{2}\mathrm{{Na}}$ as the fluoromethylating reagents, the desired ${\mathrm{{CF}}}_{2}\mathrm{H}$ - or ${\mathrm{{CF}}}_{3}$ -containing lactones were obtained in an undivided cell under semi-aqueous conditions. Cyclic voltammetric (CV) experiments indicated that ${\mathrm{{CF}}}_{2}{\mathrm{{HSO}}}_{2}\mathrm{{Na}}$ and ${\mathrm{{CF}}}_{3}{\mathrm{{SO}}}_{2}\mathrm{{Na}}$ are easier to be electrochemically oxidized to generate fluoromethyl radicals than the alkene moiety. Thus, ${R}_{f}$ radical generated by anodic oxidation of ${\mathrm{R}}_{\mathrm{f}}{\mathrm{{SO}}}_{2}\mathrm{{Na}}$ adds to alkene, followed by further anodic oxidation of the resulting benzylic radical species to give benzylic carbocation. Intramolecular trapping by carboxylic acid furnishes the final product.

Scheme 19. Electrochemical fluoromethylation triggered lactonization of alkenes.

2.6.2 Ester formation via intermolecular trapping of intermediate with carboxylic acids

REVIEW In 2014, Kam and co-workers investigated the electrochemical oxidation of 4,4’-dimethoxystilbene under different conditions. When the reaction was carried out in 25% AcOH/MeCN/0.1 M ${\mathrm{{LiClO}}}_{4}$ in the presence of NaOAc (0.25 M), isomeric acetate derivatives were obtained in combined yields of 62% (Scheme 20). ${}^{\left\lbrack {26}\right\rbrack }$ The radical cation intermediate was proposed as the key species in this anodic oxidation.

Scheme 20. Electrochemical oxidation of 4,4’-dimethoxystilbene.

In 2015, DeVos and co-workers reported the divergent paired electrosynthesis of diacid and diol precursors through simultaneous cathodic carboxylation and anodic acetoxylation of conjugated dienes (Scheme 21). ${}^{\left\lbrack {27}\right\rbrack }$ As for the acetoxylation reaction, the choice of the tetraethylammonium salt seemed very important. When an acetate anion was used, no diacetate ester product was observed; when a trifluoroacetate salt was employed, a good yield of diacetoxylated compound product was obtained.

Scheme 21. Divergent paired electrosynthesis of diacid and diol precursors.

In 2018, Lei and co-workers reported the electrochemical oxidative oxysulfenylation (including acyloxysulfenylation) and aminosulfenylation of alkenes with hydrogen evolution. ${}^{\left\lbrack {28}\right\rbrack }$ Neither external chemical oxidants nor metal catalysts are required in these reactions. One example of acyloxysulfenylation of styrene was reported (Scheme 22). The proposed mechanism was similar with that of Han and co-workers. ${}^{\left\lbrack {24}\right\rbrack }$ Notably, the authors pointed out that although thiyl radical was the most likely species triggering the reaction, the possibility of arylbis(arylthio)sulfonium ion serves as the key intermediate could not be completely ruled out.

Scheme 22. Electrochemical oxidative oxysulfenylation of alkenes.

On the basis of this work, Lei and co-workers further developed the electrochemical oxytrifluoromethylation and aminotrifluoromethylation of alkenes in the presence of yttrium triflate using ${\mathrm{{CF}}}_{3}{\mathrm{{SO}}}_{2}\mathrm{{Na}}$ as trifluoromethylation reagent. ${}^{\left\lbrack {29}\right\rbrack }$ Formic acid and acetic acid could participate in the electrochemical oxytrifluoromethylation to afford the corresponding ${\mathrm{{CF}}}_{3}$ - containing esters (Scheme 23). It’s likely that ${\mathrm{{CF}}}_{3}$ radical generated from oxidation of ${\mathrm{{CF}}}_{3}{\mathrm{{SO}}}_{2}\mathrm{{Na}}$ at the anode triggers this reaction.

Scheme 23. Electrochemical oxytrifluoromethylation of alkenes.

In 2019, the Lei group reported the electrochemical aminoselenation and oxyselenation of styrenes without any acids or oxidants as additives. Formic acid, acetic acid, butyric acid, benzoic acid, and cyclopropyl carboxylic acid are all amenable nucleophiles in the oxyselenation of styrene (Scheme 24).[30] Two possible mechanisms involving seleno radical-initiated addition or cyclic selenium intermediate were proposed.

Scheme 24. Electrochemical oxyselenation of styrenes.

In 2019, Li and co-workers developed an intermolecular 1,2- bromoesterification of alkenes with carboxylic acids and $N$ - bromosuccinimide (NBS) under electrochemical oxidative conditions (Scheme 25). ${}^{\left\lbrack {31}\right\rbrack }$ They found that TEMPO promoted this reaction significantly. Inorganic bromine sources, such as $\mathrm{{LiBr}},{\mathrm{{NH}}}_{4}\mathrm{{Br}}$ , and KBr were not suitable for this reaction. A wide range of alkenes, including 1,1-disubstituted ethylene, 1,2- disubstituted ethylene, terminal styrenes, cycloolefins, and aliphatic alkene, were transformed to their corresponding products via C-Br and C-O bond formation. Various carboxylic acids, such as aromatic acids, cinnamic acid, aliphatic acids, and amino acids, were tolerated by the electrooxidative system. The cyclic bromonium ion intermediate was proposed as the key species in this transformation.

Scheme 25. Electrooxidative 1,2-bromoesterification of alkenes with acids and NBS.

2.7 Anodic allylic esterification of nonactivated alkenes

In 2018, Siewert and co-workers reported the anodic allylic esterification and amination of nonactivated alkenes catalyzed by diselenides (Scheme 26). ${}^{\left\lbrack {32}\right\rbrack }$ , A detailed mechanistic study revealed a plausible mechanism. The selenolactone is proposed as the key intermediate of this reaction. Both intramolecular and intermolecular anodic allylic esterification were presented in this work. The high regioselectivity of the intermolecular anodic allylic esterification is noteworthy. The required potential to drive the reaction can be adjusted easily, which is an important advantage of this electrochemical reaction compared with conventional methods.

Scheme 26. Anodic allylic esterification and amination of nonactivated alkenes.

2.8 Electrochemical reactions involving electrogenerated carboxylates

In 2015, Senboku and co-workers reported the electrochemical three-component coupling reaction of benzylic halides, carbon dioxide and N, N-dimethylformamide (DMF) by using paired electrolysis (Scheme 27). ${}^{\left\lbrack {33}\right\rbrack }$ In this reaction, carboxylate ion was generated in situ from the carboxylation of benzylic halides at the cathode. At the same time, N-acyliminium ion intermediate was generated from electrochemical oxidation of DMF at the anode. Coupling of carboxylate ion with $N$ -acyliminium ion gives the desired coupling product.

Scheme 27. Electrochemical three-component coupling reaction of benzylic halides, carbon dioxide and DMF.

3. C-S bond formation via electrolysis of thiocarboxylic acids

It is well established that the electrolysis of thiocarboxylic acid can generate the corresponding sulfur-centered radical. In 2017, Wirth and co-workers reported an example of electrochemical addition of thioacetic acid to phenyl acetylene using the electrochemical flow reactor without supporting electrolyte (Scheme 28). ${}^{\left\lbrack {34}\right\rbrack }$ The thioacetyl radical is stable and adds to phenyl acetylene smoothly to afford thioester. The authors pointed out that it is crucial to use hexafluoroisopropanol (HFIP) to stabilize the formed radical. Dimerization of thioacetic acid is observed with other solvents.

Scheme 28. Electrochemical addition of thioacetic acid to phenyl acetylene.

In 2016, Zeng and co-workers reported a paired electrosynthesis of 3-amino-2-thiocyanato- $\alpha ,\beta$ -unsaturated carbonyl derivatives from $\beta$ -dicarbonyl compounds and ammonium carbamodithioate mediated by bromide ions (Scheme

REVIEW 29). ${}^{\left\lbrack {35}\right\rbrack }$ In this reaction, bromide is oxidized anodically and the ammonium ion is reduced cathodically. The carbamodithioate anion here serves as the thiocyanato group source via expulsion of ${\mathrm{H}}_{2}\mathrm{\;S}$ .

Scheme 29. Electrosynthesis of 3-amino-2-thiocyanato- $\alpha ,\beta$ -unsaturated carbonyl derivatives.

4. C-O bond formation via electrolysis of phosphonic acids

Compared with the electrolysis of carboxylic acids, the electrolysis of phosphonic acids received less attention. In 2018, Mo and co-workers reported an anodic oxidation/cyclization of 2- (aryl)arylphosphonic acid monoesters for ethoxy dibenzooxaphosphorin oxide synthesis (Scheme 30).[36] Similar with their previous work on the anodic oxidation/cyclization of 2- arylbenzonic acids, catalytic amount of NaOH was used in this work. This electrochemical reaction features transition-metal-, external oxidant- and electrolyte-free. The reaction proceeds at room temperature with hydrogen as the only byproduct. A phosphonic Kolbe oxidation initiated mechanism was proposed. In addition to $\mathrm{C} - \mathrm{O}$ bond formations via electrochemical transformations of carboxylic acids, the electrochemical N-acylation of carboxylic acids has also been reported recently.[37]

Scheme 30. Anodic oxidation/cyclization of 2-(aryl)arylphosphonic acid monoesters.

5. C-S bond formation or S-X (X = N, O, S, Se) bond formation via electrolysis of sulfinic acids

5.1 C-S bond formation via electrolysis of sulfinic acids

5.1.1 Synthesis of vinyl sulfones from sodium sulfinates

In 2015, Yuan and co-workers reported an electrochemical method for the synthesis of vinyl sulfones from sodium sulfinates and olefins (Scheme 31). ${}^{\left\lbrack {38}\right\rbrack }$ With NaI as supporting electrolyte, the reaction can be performed under transition-metal- and external oxidant-free conditions. Apart from sodium phenylsulfinates, aliphatic sodium sulfinate like sodium methanesulfinate is also amenable.In a proposed mechanism, the in situ electrogenerated ${I}_{2}$ played an important role in this reaction. Additionally, a sulfonyl radical is proposed to be generated from the sulfonyl iodide intermediate. In 2018, in a report of a radical triggered migration reaction (vide infra, Scheme 38), ${}^{\left\lbrack {39}\right\rbrack }$ Han and co-workers reported an electrochemical synthesis of vinyl sulfone from 1,1-diphenylethylene and para-methylbenzenesulfinate without any mediators (Scheme 32).

Scheme 31. Electrochemical synthesis of vinyl sulfones from sodium sulfinates and olefins.

Scheme 32. Electrochemical synthesis of vinyl sulfone from 1,1- diphenylethylene and para-methylbenzenesulfinate.

REVIEW

In 2016, Wang, Zha and co-workers developed an electrochemical synthesis of (E)-vinyl sulfones from cinnamic acids and sodium sulfinates via decarboxylative sulfono functionalization (Scheme 33). ${}^{\left\lbrack {40}\right\rbrack }$ Both electronic effect and steric effect of substituents for cinnamic acids and sodium sulfinates were observed in this reaction. Aliphatic sodium sulfinates (e.g., sodium methanesulfinate and sodium trifluoromethanesulfinate) are not compatible substrates. When styrene was employed as the coupling partner instead of cinnamic acid under the standard conditions of this decarboxylative sulfono functionalization, only a trace amount of vinyl sulfone was obtained. A sulfonyl radical-triggered mechanism was proposed.

Scheme 33. Electrochemical synthesis of $\left( E\right)$ -vinyl sulfones from cinnamic acids and sodium sulfinates.

5.1.2 Synthesis of $\beta$ -keto sulfones from sulfinates and 1,3- dicarbonyl compounds

In 2015, Yuan and co-workers reported an efficient electrochemical synthesis of $\beta$ -ketosulfones from sodium sulfinates and 1,3-dicarbonyl compounds with ${\mathrm{{NH}}}_{4}\mathrm{I}$ as the supporting electrolyte (Scheme 34). ${}^{\left\lbrack {41}\right\rbrack }$ Notably, C-C bond cleavage occurs in this transformation. lodide salts are necessary for this reaction since no desired product was obtained with ${\mathrm{{NH}}}_{4}\mathrm{{OAc}}$ as the supporting electrolyte. The authors proposed that the in situ electrogenerated ${\mathrm{I}}_{2}$ plays an important role and the iodinated 1,3-dicarbonyl compound serves as the key intermediate in this reaction.

Scheme 34. Electrochemical synthesis of $\beta$ -ketosulfones from sodium sulfinates and 1,3-dicarbonyl compounds.

5.1.3 Difunctionalization of alkenes or alkynes via electrolysis of sulfinic acids

In 2016, Zeng and co-workers developed an efficient electrochemical cyclization method for synthesis of 3- sulfonyloxindoles via difunctionalization of acrylamide from electrolysis of a mixture of sodium sulfinates and acrylamide under transition-metal- and external chemical oxidant-free conditions (Scheme 35). ${}^{\left\lbrack {42}\right\rbrack }$ A catalytic amount of ${\mathrm{{NH}}}_{4}\mathrm{{Br}}$ is employed as a redox catalyst and no supporting electrolyte is required. Mechanistically, it is proposed that sulfonyl radical generated from sulfonyl bromide serves as a key intermediate in this reaction.

REVIEW

Scheme 35. Electrochemical cyclization method for the synthesis of 3- sulfonyloxindoles.

In 2017, Chang and co-workers reported an efficient organic electrosynthesis of tertiary $\beta$ -hydroxysulfones from functionalized $\alpha$ -methylstyrenes with substituted sodium sulfinates (Scheme 36). ${}^{\left\lbrack {43}\right\rbrack }$ With KI as supporting electrolyte, a mixture of acetonitrile and water as solvent, the desired product can be obtained in 85- 96% yields under constant potential electrolysis. Sulfonyl radical generated from sulfonyl iodide added to the double bond of $\alpha$ - methylstyrene furnishing a benzylic radical, followed by loss of one electron to form the stable tertiary carbocation. With the hydration of carbocation, the desired product was produced. Notably, the simple styrene is also amenable to this transformation.

Scheme 36. Electrosynthesis of tertiary $\beta$ -hydroxysulfones.

In 2019, Li and co-workers reported an electrochemical intermolecular 1,2-aminosulfonylation of alkenes with sulfinates and amines without the need of additive redox catalysts, metal catalysts and chemical oxidants (Scheme 37).[44] While arylalkenes were viable substrates in this reaction, aliphatic alkene had no reactivity. This protocol was applicable to both arylsulfinates and alkylsulfinates. A wide range of primary and secondary amines can undergo the aminosulfonylation reaction. A sulfonyl radical-involved mechanism was proposed. Amines serve as nucleophiles in this aminosulfonylation reaction.

Scheme 37. Electrochemical three-component 1,2-aminosulfonylation of alkenes.

In 2018, Han and co-workers reported an electrochemical 1,2- sulfonylation/alkynylation of alkenes via radical 1,4-alkynyl migration of alkynyl-substituted tertiary alcohols (Scheme 38). ${}^{\left\lbrack {39}\right\rbrack }$ Aryl- and alkyl-substituted sodium sulfinates were used as sulfonyl source in this reaction. Apart from 1,4-alkynyl migration, 1,4-alkenyl migration was also demonstrated. The reaction proceeds smoothly in an undivided cell without the use of any metal catalyst, additive and external chemical oxidant. This reaction represents the first electrochemical distal radical migration reaction. A possible mechanism was proposed. Anodic oxidation of sodium sulfinate gives the oxygen-centered radical, which resonates to the more stable sulfonyl radical. The sulfonyl radical adds to the terminal position of the alkene to give a radical, followed by a 1,4-alkynyl migration.

Scheme 38. Electrochemical 1,2-sulfonylation/alkynylation of alkenes via radical 1,4-alkynyl migration.

In the same year, Guo and co-workers reported an electrochemical sulfonylation/heteroarylation of alkenes with sulfinic acids via distal heteroaryl ipso-migration (Scheme 39). ${}^{\left\lbrack {45}\right\rbrack }$ Various sulfonated functionalized heteroarenes were prepared in an undivided cell, avoiding the use of any metal catalysts, additives and external chemical oxidants. A sulfonyl radical-initiated mechanism similar with that of Han and co-workers was proposed. ${}^{\left\lbrack {39}\right\rbrack }$

Scheme 39. Electrochemical sulfonylation/heteroarylation of alkenes with sulfinic acids.

In 2019, Zhang and co-workers reported the electrochemical sulfonylation/semipinacol rearrangement of allylic alcohols using sodium arylsulfinates as reagents (Scheme 40).[46] All-carbon quaternary stereocenters were constructed by utilizing this strategy. It’s possible that the sulfonyl radical initiates this difunctionalization of alkenes and the radical adduct might be oxidized to carbocation, which subsequently undergo a cationic 1,2-rearrangement process. Stoichiometric oxidants and transition-metal catalysts were not required in this protocol.

Scheme 40. Electrochemical sulfonylation/semipinacol rearrangement of allylic alcohols.

In 2017, Lei and co-workers developed an efficient electrochemical arylsulfonylation reaction of ynones with sulfinic acids under external chemical oxidant-free conditions (Scheme 41). ${}^{\left\lbrack {47}\right\rbrack }$ A series of sulfonatedindenones could be obtained with tetrabutylammonium iodide (TBAI) as redox catalyst. A sulfonyl radical-initiated tandem reaction mechanism was proposed. Notably, a similar arylsulfonylation product was obtained when the chalcone was used as the substrate to react with benzenesulfinic acid under the electrooxidation conditions.

Scheme 41. Electrochemical arylsulfonylation reaction of ynones with sulfinic acids.

5.1.4 Arylation of sulfinic acids via electrolysis of sulfinic acids

Electrochemical arylation of sulfinic acids is an important method for the synthesis of arylsulfones. Electrogeneration of phenoquinone and its analogues as electrophiles is a popular strategy for the arylation of sulfinic acids. ${}^{\left\lbrack {48}\right\rbrack }$ Besides, in 2017, Yu and co-workers reported an electrochemical α-sulfonylation of $1\mathrm{H}$ -indoles with sodium sulfinates by using a catalytic amount of TBAI. ${}^{\left\lbrack {49}\right\rbrack }$ As Baran and Waldvogel has summarized part of this field in their reviews, ${}^{\left\lbrack 1a,1b\right\rbrack }$ we will only focus on some representative advances in recent years and not reiterate the same contents here. Notably, for some cases not only C-S bonds but also S-N bonds are constructed in sequential reactions. We will discuss these cases in a following part (see 5.3).

In 2016, Nematollahi and co-workers reported the electrochemical synthesis of arylsulfonyl-4,4’-biphenol and bis-phenylsulfonyl-4,4’-biphenol derivatives via the electrochemical oxidation of 4,4’-biphenol in the presence of arylsulfinic acids as nucleophiles (Scheme 42). ${}^{\left\lbrack {48}\mathrm{a}\right\rbrack }$ Different products can be obtained depending on electrode potential.

Scheme 42. Electrochemical synthesis of arylsulfonyl-4,4’-biphenol and bis-phenylsulfonyl-4,4’-biphenol derivatives.

In 2017, Momeni and Nematollahi reported the convergent paired electrochemical synthesis of new aminonaphthol derivatives via oxidation (and reduction) of acid orange 7 in the presence of arylsulfinic acids as nucleophiles (Scheme 43). ${}^{\left\lbrack {48}b\right\rbrack }$ It’s proposed that the electrogenerated 1-iminonaphthalen-2(1H)-one participates in Michael addition reaction with arylsulfinic acids to form the 1-amino-3-(phenylsulfonyl)naphthalen-2-ol derivatives.

Scheme 43. Electrochemical synthesis of new aminonaphthol derivatives

In 2019, Waldvogel and co-workers developed a novel electrochemical strategy for the synthesis of arylsulfones by direct sulfonylation of phenols with sodium sulfinates (Scheme 44). ${}^{\left\lbrack {50}\right\rbrack }$ In this method, the sulfinates are used both as supporting electrolytes and coupling components. Therefore, additional reagent waste is prevented. HFIP (1,1,1,3,3,3- hexafluoroisopropanol) might play a role of stabilizing radical intermediate. By adding water to the solvent, decomposition of the generated sulfones can be diminished, since hydrogen is preferably generated at the cathode. The following mechanism was proposed. Initially, a phenoxyl radical is formed via electrochemical oxidation of the phenol component. The phenoxyl radical is trapped by a nucleophilic attack from the sulfinate anion, which forms a C-S bond. The final product is furnished after a further anodic oxidation. The mechanistic assumption is supported by oxidation potential determinations, revealing the lower oxidation potentials of the phenol components in comparation with sulfinates.

Scheme 44. Electrochemical synthesis of arylsulfones by direct sulfonylation of phenols with sodium sulfinates.

5.2 S-X (X = N, O, S, Se) bond formation via electrolysis of sulfinic acids

5.2.1 S-N bond formation via electrolysis of sulfinic acids

The electrochemical approach has provided a complementary method to access sulfonamides. ${}^{\left\lbrack {51}\right\rbrack }$ In 2016, Zeng and co-workers reported an efficient electrochemical synthesis of sulfonamides via oxidative amination of sodium sulfinates mediated by ${\mathrm{{NH}}}_{4}\mathrm{I}$ as a redox catalyst (Scheme 45). ${}^{\left\lbrack {52}\right\rbrack }$ A wide range of substrates, including aliphatic or aromatic secondary and primary amines, as well as aqueous ammonia, proved to be compatible with this method. External oxidants or corrosive molecular iodine were avoided in this transformation. Later, Terent’ev[53] and Yuan[54] independently reported similar protocols. Notably, in Yuan’s report, with ${\mathrm{H}}_{2}\mathrm{O}$ as the solvent and $\mathrm{{NaI}}$ as the supporting electrolyte, the electrolyte solution could be reused up to 10 times without an obvious reduction in the yields.

Scheme 45. Electrochemical oxidative amination of sodium sulfinates.

Apart from using halide anions as redox catalyst, another important strategy in this field is the electrogeneration of nitrogen-centered electrophiles in the presence of sulfinic acids as nucleophiles. ${}^{\left\lbrack {55}\right\rbrack }$ We will highlight several most important advances employing this strategy below.

In 2012, Nematollahi and co-workers reported the electrochemical synthesis of some sulfonamide derivatives via the electrooxidation of 4-propyl and 4-butyl derivatives of urazole in the presence of arylsulfinic acids as nucleophiles (Scheme 46). ${}^{\left\lbrack {55}\mathrm{a}\right\rbrack }$ It’s proposed that electrogenerated 4-alkyl-4H-1,2,4- triazole-3,5-diones are attacked by arylsulfinic acids.

Scheme 46. Electrochemical oxidation of 4-substituted urazoles in the presence of arylsulfinic acids.

Furthermore, in 2017, Varmaghani and co-workers reported the electrochemical synthesis of two different series of sulfonamides using 4-(4-nitrophenyl)urazole and arylsulfinic acids as substrates by controlling the potential during electrolysis (Scheme 47). ${}^{\left\lbrack {55}b\right\rbrack }$ One series of sulfonamides were obtained via direct electrochemical oxidation of 4-(4-nitrophenyl)urazole in the presence of arylsulfinic acids and another one were obtained via a paired electrosynthesis. In addition, they also investigated the electrochemical oxidation of 4-cyclohexylurazole in the presence of arylsulfinic acids.

Scheme 47. Electrochemical synthesis of diverse sulfonamide derivatives.

In 2015, Nematollahi and co-workers reported the synthesis of disulfonamide derivatives of N, N-dimethyl-1,4-benzenediamine by electrochemical oxidation of 4-nitroso-N, N-dimethylaniline in the presence of arylsulfinic acids as nucleophiles (Scheme 48). ${}^{\left\lbrack {55}\mathrm{c}\right\rbrack }$ Interestingly, $N$ -arylsulfonyl-3-arylsulfonyl derivatives were obtained by the chemical reaction of 4-nitroso-N, N-dimethylaniline with arylsulfinic acids.

Scheme 48. Electrochemical oxidation of 4-nitroso- $N, N$ -dimethylaniline in the presence of arylsulfinic acids.

In the same year, they also reported the synthesis of N-(4- (dimethylamino)phenyl)benzenesulfonamide via electrochemical reduction of 4-nitroso- $N, N$ -dimethylaniline with arylsulfinic acids as nucleophiles (Scheme 49). ${}^{\left\lbrack {55}\mathrm{\;d}\right\rbrack }$ This is the first report describing generation of Michael acceptor via electrochemical reduction. ${}^{\left\lbrack {55}e\right\rbrack }$

Scheme 49. Electrochemical reduction of 4-nitroso- $N, N$ -dimethylaniline with arylsulfinic acids.

In 2018, Nematollahi and co-workers reported a paired electrochemical method for the synthesis of sulfonamides, diarylsulfones and bis(arylsulfonyl)aminophenols using nitrobenzene derivatives and arylsulfinic acids as starting materials (Scheme 50). ${}^{\left\lbrack {55}f\right\rbrack }$ For the electrochemical oxidation of nitrobenzene, $p$ -nitroaniline, and $p$ -nitrophenol in the presence of arylsulfinic acids, the products were sulfonamides, diarylsulfones and bis(arylsulfonyl)aminophenols, respectively. Therefore, the product selectivity was greatly influenced by the nature of the functional group.

Scheme 50. Electrochemical synthesis of sulfonamides, diarylsulfones and bis(arylsulfonyl)aminophenols.

5.2.2 S-O bond formation via electrolysis of sulfinic acids

In 2019, Terent’ev and co-workers reported the electrochemical synthesis of sulfonates from sodium sulfinates and $N$ - hydroxysuccinimide (Scheme 51). ${}^{\left\lbrack {56}\right\rbrack }$ The yields were lower compared with sulfonylhydrazides as sulfonylation reagent. ${\mathrm{{NH}}}_{4}\mathrm{{Br}}$ serves as a supporting electrolyte in this reaction.

REVIEW

Scheme 51. Electrochemical synthesis of sulfonates from sodium sulfinates and $N$ -hydroxysuccinimide.

5.2.3 S-S/S-Se bond formation via electrolysis of sulfinic acids

In 2019, Sun and co-workers reported the electrochemical oxidative cross-coupling of arylsulfinic acids with thiophenols, providing unsymmetrical thiosulfonates in good to excellent yields (Scheme 52). ${}^{\left\lbrack {57}\right\rbrack }$ The reactions of arylsulfinic acids with disulfides or diselenides were also practical under similar conditions to afford thiosulfonates or selenosulfonates. External oxidants or redox catalysts were not required in these reactions.

Scheme 52. Electrochemical oxidative cross-coupling of arylsulfinic acids with thiophenols.

5.3 Sequential C-S and S-N bond formation via electrolysis of sulfinic acids

Electrochemical sequential C-S and S-N bond formation can be realized with the electrogeneration of electrophiles possessing both carbon- and nitrogen-centered electrophilic positions such as $p$ -methylquinoneimine in the presence of sulfinic acids as nucleophiles. ${}^{\left\lbrack {58}\right\rbrack }$

In 2017, Nematollahi and co-workers reported the synthesis of sulfonyl derivatives of $p$ -methylaminophenol by reaction of the electrogenerated $p$ -methylquinoneimine with sulfinic acids. Some mono and bis (or tris)sulfonyl- $p$ -(methylamino)phenol derivatives were obtained by changing the applied potential (Scheme 53). ${}^{\left\lbrack {58}\mathrm{a}\right\rbrack }$ This method doesn’t deal with organic solvent and strong acid/base. For the synthesis of mono or bissulfonyl-p- (methylamino)phenol derivatives, only C-S bond was forged. For the synthesis of trissulfonyl- $p$ -(methylamino)phenol derivative, both C-S and S-N bonds were constructed.

Scheme 53. Synthesis of sulfone derivatives of $p$ -methylaminophenol.

In 2018, Momeni and Nematollahi reported the electrochemical synthesis of new quinonesulfonimide derivatives by the oxidation of 4-aminophenyl ether in the presence of arylsulfinic acids in both electrochemical conventional batch and flow cells (Scheme 54). ${}^{\left\lbrack {58}b\right\rbrack }$ No use of a supporting electrolyte is an important advantage for electrochemical flow cell compared with the conventional batch cell.

Scheme 54. Electrochemical synthesis of quinone sulfonimide derivatives.

In 2019, Nematollahi and co-workers reported a tunable pair electrochemical synthesis of benzenesulfonamide derivatives by using reductive controlled potential electrolysis of dinitrobenzene in the presence of arylsulfinic acids (Scheme 55). ${}^{\left\lbrack {58}\mathrm{c}\right\rbrack }$ By adjusting

the potential, $N$ -hydroxy- $N$ -(4-nitrophenyl)benzenesulfonamide derivatives and N-(4-amino-3- (phenylsulfonyl)phenyl)benzenesulfonamide derivatives could be selectively obtained.

Scheme 55. Electrochemical synthesis of benzenesulfonamide derivatives.

6. C-O bond formation via electrolysis of sulfonates

One of the most important applications of electrolysis of sulfonates lies in the electrochemical glycosylation. Using triflate anions as nucleophiles in the presence of glycosylcation pools furnished a-glycosyltriflates selectively. The OTf group can be displaced by an alcohol to construct a $\beta$ -selective glycosyl linkage. As this field has been discussed by other reviews, ${}^{\left\lbrack 1a,{59}\right\rbrack }$ we will not reiterate in depth here.

In 2019, Waldvogel and co-workers developed the first electrochemical protocol for the direct synthesis of aryl mesylates by dehydrogenative coupling (Scheme 56). ${}^{\left\lbrack {60}\right\rbrack }$ In order to trap the anodically generated radical cations efficiently, tetrabutylammonium methanesulfonate $\left( {{\mathrm{{Bu}}}_{4}{\mathrm{{NCH}}}_{3}{\mathrm{{SO}}}_{3}}\right)$ in acetonitrile was used as a supporting electrolyte to ensure a sufficient concentration of the nucleophile methanesulfonate. The formation of regioisomeric products was observed in some cases. In the case of biphenyl, the corresponding dimesylated product could be isolated. In addition, anodic formation of aryl benzenesulfonates was also successfully performed.

Scheme 56. Anodic formation of aryl mesylates through dehydrogenative coupling reaction.

7. Conclusion

In conclusion, electrolysis of organic acids has gained increasing attention in recent years and impressive advances have been achieved. Despite the well-known decarboxylation possibility of carboxylates via anodic oxidation, a number of ester-forming reactions involving carboxylic acids have been reported. By employing electricity as the oxidant, many oxidative bond-forming reactions can be performed under external chemical oxidants-free conditions. Furthermore, transition-metal catalysts can also be obviated in many cases. Many challenging transformations including inert chemical bond functionalization and difunctionalization of alkenes/alkynes can be realized by using electrochemical methods. As for the design of the electrochemical reactions involving transformations of organic acids, it is not limited to anodic oxidation; cathodic reduction and paired electrolysis can also be considered. The most common roles of organic acids in these electrochemical reactions are radical precursors and nucleophiles. What’s more, the scalability of electrosynthesis has been demonstrated in many reactions. Notably, the selectivity of the electrochemical reactions can be finely tuned by several factors including the electrode potential, which represents a great advantage compared with other methods. Interestingly, the product selectivity of certain electrochemical reactions can be greatly influenced by the nature of the functional groups or different from conventional chemical reactions.

Several issues remain to be solved in the field of electrochemical functionalization of organic acidic groups. The substrate scope remains limited in some reactions. For some intramolecular reactions, it is challenging to develop the corresponding intermolecular version. There is still much room to develop in the field of inert chemical bond functionalization. For example, electrochemical reactions involving C-C bond cleavage are relatively rare. Furthermore, compared with the difunctionalization of alkenes, the difunctionalization of alkynes received less attention. As paired electrolysis can possess higher current efficiency than single anodic oxidation or cathodic reduction in theory, it is significant to develop practical paired electrolysis. It is expected that more kinds of mechanistically interesting protocols to be developed. Moreover, more detailed mechanistic studies should be undertaken to help our understanding of these electrochemical reactions. The enantioselective electrochemical transformations remain elusive. For indirect electrolysis, the loading of redox catalyst is large in some cases. As for the reaction solvents, green solvents such as water are favored for environmental reasons. As supporting electrolyte is required in most electrochemical reactions, it is important to reduce the amount of supporting electrolyte and develop effective recovery methods to save cost. At last, electrochemical flow reactions deserve more attention due to their unique advantages compared with batch reactions.

Acknowledgements ((optional))

This project is supported by National Natural Science Foundation of China (Grant 21772003). We also thank Peking University for a start-up fund. This work was also partially funded by the BHP-PKU CCUS project supported by BHP Billiton.

Keywords: Electrosynthesis • Organic acids • Carbon-heteroatom bond formation - Sulfur-heteroatom bond formation

Oxidative coupling

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Electrolysis of organic acids for carbon-heteroatom and sulfur-heteroatom bond formation Man Li, Junting Hong, Wei Xiao, Yang

Yang, Di Qiu, Fanyang Mo*

Organic acids C-X and S-X

bond formation

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Electrochemical transformation of

C-s organic acids for carbon-heteroatom

and sulfur-heteroatom bond N-s

0-s formation

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Author(s), Corresponding Author(s)*

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Title

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Chem. 2015, 39, 3852-3858; f) M. Sharafi-Kolkeshvandi, D. Nematollahi, F. Nikpour, M. Bayat,

E. Soltani, Electrochim. Acta 2016, 222, 845-855; g) M. Sharafi-Kolkeshvandi, D. Nematollahi,

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Synthesis 2017, 49, 1555-1560; k) D. Nematollahi, A. Namdar, S. Momeni, J. Electroanal.

Chem. 2018, 810, 161-170.

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[60] S. Möhle, S. Herold, N. D. Hillerson, S. R. Waldvogel, ChemElectroChem 2019, 6, 121-125.