Electrochemical difunctionalization of alkenes and alkynes for the synthesis of organochalcogens involving C–S/Se bond formation

Abstract

Organochalcogen compounds containing C–S/Se bond are motifs frequently found in various natural products, bioactive compounds, as well as functional materials. On the other hand, electrochemical organic synthesis has been regarded as a green and sustainable technology in which electrons serve as redox reagents. Given the wide presence of alkenes and alkynes as substrates, the electrochemical difunctionalization of alkenes and alkynes involving C–S/Se bond formation with high step-economy has recently attracted much attention from synthetic organic chemists. This review is devoted to highlighting the latest achievements in the development of the electrochemical difunctionalization of alkenes and alkynes for the synthesis of organochalcogens, including the oxychalcogenation, carbochalcogenation, aminochalcogenation, halochalcogenation, and dichalcogenation of alkenes and alkynes from 2018 to 2023.

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Jianchao Liu

Jianchao Liu received his Ph.D. from the South China University of Technology (SCUT) under the supervision of Prof. Biaolin Yin (2012–2017). In 2017, he joined the National Engineering Research Center for Carbohydrate Synthesis at Jiangxi Normal University. His current research interests mainly focus on the development and applications of new synthetic methods.

Jie-Ping Wan

Prof. Dr Jie-Ping Wan studied chemistry during 2000–2004 in the Department of Chemistry, Nanchang University. After obtaining a B.Sc. degree in 2004, he moved to the Department of Chemistry, Zhejiang University, in 2005 for postgraduate study under the guidance of Prof. Yuanjiang Pan. After receiving a Ph.D. degree there in 2010, he joined the College of Chemistry and Chemical Engineering, Jiangxi Normal University, as an assistant professor in the same year, and was promoted to full professor in 2017. He conducted postdoctoral research at RWTH Aachen University with Prof. Dieter Enders from Sep. 2011 to Aug. 2012. His current research interests are diversity-oriented synthesis, the discovery and application of platform synthons and sustainable catalysis and synthesis.

Yunyun Liu

Dr Yunyun Liu obtained her B.Sc. degree from Qufu Normal University in 2005. She then moved to Zhejiang University to continue her postgraduate study in the Department of Chemistry under the supervision of Prof. Weiliang Bao where she obtained her Ph.D. degree in 2010. Dr Liu joined the College of Chemistry and Chemical Engineering, Jiangxi Normal University, as assistant professor in 2010, and was promoted to associate professor in 2013. She is currently interested in the research of metal-catalyzed organic synthesis and sustainable catalysis.

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1. Introduction

Organochalcogen compounds, particularly molecules encompassing C–S/Se bonds, are ubiquitous structural frameworks present in a number of natural products,¹ and pharmacologically active substances.² Diverse biological activities, including anticancer,³ anti-Alzheimer's,⁴ antimicrobial,⁵ antiviral,⁶ anti-inflammatory,⁷ anti-diabetic⁸ and antidepressant,⁹ are present in those organochalcogen derivatives. Furthermore, they also play a significant role in organic synthesis,¹⁰ agrochemicals¹¹ and materials science.¹² Therefore, the development of effective synthetic methodologies for rapidly constructing various functionalized sulfur- and selenium-containing molecules and related scaffolds from simple and readily available starting materials has been of great interest to many organic chemists. Representative methods mainly involve transition-metal-catalyzed cross-coupling of a chalcogen source with partners,¹³ the addition of an organochalcogen to double or triple bonds under free-radical or metal-catalysed conditions¹⁴ and the direct electrophilic addition of electron-rich arenes with thiols or diselenides.¹⁵ Despite the significant utility of these transformations, some key issues still remain to be addressed, such as the use of noble metal catalysts and extra sacrificial oxidants. Therefore, it is highly desirable to achieve efficient chalcogenation under chemical oxidant- and metal-free conditions. In the past few years, environmentally friendly electrochemical synthesis has become a sustainable tool for the synthesis of functional molecules.¹⁶ This strategy features the advantage of using electrons as green reagents, so it does not need the use of stoichiometric chemical oxidants or transition metal catalysts, and can avoid the generation of reagent waste.¹⁷ In this context, several efficient protocols have been established via electrochemical reactions for the direct construction of C–S/Se bonds.¹⁸

On the other hand, unsaturated carbon–carbon bonds, such as in alkenes and alkynes, represent an appealing class of synthesis unit in organic synthesis because they are available in bulk quantities from petrochemical feedstocks and are easily prepared.¹⁹ The electrochemically enabled difunctionalization of unsaturated carbon–carbon bonds has emerged as a reliable and powerful tool for the rapid construction of complex molecules by the addition of two functional groups across the double bond with high step economy.²⁰ In recent years, the electrochemical difunctionalization of alkenes and alkynes via C–S/Se bond formation has attracted great attention. By this strategy, various structurally diverse organochalcogen compounds were prepared under environmentally friendly conditions with high efficiency.

Mechanistically, chalcogen precursors are firstly oxidized to chalcogen radicals or/and chalcogen cations under electrochemical conditions (Scheme 1). Two major reaction pathways have been proposed for the addition of chalcogen to alkenes or alkynes. The first is the radical pathway (path a), which is initiated by the addition of chalcogen radicals with an unsaturated carbon–carbon bond to form alkyl or alkenyl radicals. Then, the alkyl or alkenyl radicals undergo subsequently oxidation and nucleophilic attack to give the functionalized organochalcogens. Alternatively, the alkyl or alkenyl radicals can, through a radical addition of arenes, generate the products. The other reaction way is that the chalcogen cations are added to unsaturated carbon–carbon bonds to give cation intermediates, which are then attacked by nucleophiles to form the organochalcogen compounds (path b).

Scheme 1 General pathway of the electrochemical difunctionalization reaction via C–S/Se bond formation.

Considering the rapid growth of this area, it is urgent to summarize the recent advancements in this field to inspire future innovations. However, to the best of our knowledge, only the electrochemical seleno-functionalization and dichalcogenation of unsaturated bonds have been summarized recently by Liu's group^21a and Sun's group,^21b respectively. A comprehensive review on the chalcogenation (S/Se) reactions of unsaturated compounds has not been available so far. In continuation of our interest in the electrochemical synthesis and chalcogenation reactions,²² herein we highlight the recent developments of the electrochemical difunctionalization of alkenes and alkynes involving C–S/Se bond formation. The review mainly covers the relevant articles from 2018 to 2023, and is mainly organized according to different types of the reaction including oxychalcogenation, carbochalcogenation, aminochalcogenation, halochalcogenation, and dichalcogenation. It is noteworthy that the reaction scopes, mechanisms and potential applications are also discussed to gain more insights into the details of recent research progress.

2. Oxychalcogenation of alkenes and alkynes

2.1 C–S and C–O bond formation

β-Alkoxy sulfones are frequently found in a variety biologically active compounds and building blocks for various organic molecules.²³ In 2018, Lei and co-workers discovered an electrochemical oxidative alkoxysulfonylation of alkenes from sulfonyl hydrazines and alcohols (Scheme 2).²⁴ This approach provided facile access to β-alkoxy sulfones with a broad substrate scope, and good functional group tolerance. Benzenesulfonyl hydrazides bearing both electron-withdrawing and electron-donating groups, as well as heteroaryl sulfonyl hydrazides are suitable. Various styrenes with different substituents on the aromatic ring were competent substrates. However, no desired products were detected when aliphatic alkenes were used as the substrates. Additionally, primary alcohols including ethanol, n-butanol, 2-chloroethanol, and ethylene glycol give access to the corresponding β-alkoxy sulfones in moderate to good yields. In contrast, secondary and tertiary alcohols furnished the desired β-alkoxy sulfones in relatively low yields.

Scheme 2 Electrochemical oxidative alkoxysulfonylation of alkenes.

Detailed control experiments were performed to examine the mechanism. First, cyclic voltammetry (CV) experiments indicated that sulfonyl hydrazines are oxidized preferentially at the anode. Then, a radical clock experiment indicated that the reaction involves a sulfonyl radical addition process. On the basis of the above results, the proposed mechanism is depicted in Scheme 3. First, the deprotonation, electrochemical anodic oxidation, and subsequently N₂ liberation of sulfonyl hydrazide give the sulfonyl radical B. Then, the sulfonyl radical B undergoes insertion of the double bond of the alkene to provide the radical intermediate C, which undergoes further oxidation to produce a benzyl cation intermediate D. Finally, the desired alkoxysulfonylation product is obtained via the nucleophilic attack of alcohol and deprotonation. Meanwhile, alcohols and protons were reduced to produce H₂ and alkoxy anion on the cathode.

Scheme 3 Proposed mechanism for the reaction.

Oxidative difunctionalization of alkenes by using thiophenols/thiols and O/N nucleophiles to form the C–S and C–O/N bonds remains a challenge due to the fact that thiophenols/thiols are often easily over-oxidized to sulfoxides or sulfones under oxidation conditions.²⁵ In 2018, Lei et al. established an electrochemical oxidative oxysulfenylation of alkenes and alcohols with thiophenols/thiols as the thiolating agents (Scheme 4).²⁶ In the electrochemical reactions by altering the operating voltage and current, the oxidation and over-oxidation of thiophenols/thiols could be suppressed.

Scheme 4 Oxysulfenylation of alkenes with thiophenols/thiols.

A variety of thiophenols bearing electron-neutral and electron-withdrawing groups, as well as thiols such as benzyl mercaptan and cyclohexyl mercaptan, were competent reaction partners, delivering the desired products smoothly. Moreover, aliphatic alcohols containing halogens, alkoxy and hydroxyl groups, and sterically hindered tertiary alcohols were tolerated in this transformation. It is particularly noteworthy that the corresponding acyloxysulfenylation and hydroxysulfenylation products could be accessed by using acetic acid and water as nucleophiles, respectively.

The radical trapping experiment and radical clock experiment of 4-chlorothiophenol indicated that thiyl radicals might be involved in this transformation and disulfides could be converted to the corresponding thiyl radicals under the standard conditions. Further investigations suggested that the equilibrium of disulfides and thiyl radicals exists under electrochemical conditions, and thiyl radicals were the key intermediate in this reaction.

As show in Scheme 5, the plausible mechanism suggested that the thiyl radical A is firstly formed via the electrochemical anodic oxidation and subsequently deprotonation of thiols. Then, a radical addition of thiyl radical A to alkenes afforded the carbon-centered radical B, which could be further oxidized to give the benzyl cation intermediate C. Finally, C is attacked by the nucleophile (NuH), and then the deprotonation of intermediate D generates the desired products. However, the pathway in which the disulfide is oxidized to the arylbis(arylthio)sulfonium ion E, and intermediate E then undergoes nucleophilic attack by alkenes and the nucleophile to form the desired product, could not be completely ruled out.

Scheme 5 Proposed mechanism for the difunctionalization reaction.

In 2019, Han et al. successfully employed sodium sulfinate as an available, easy-to-handle and stable sulfonyl source to achieve the electrochemically enabled alkoxysulfonylation reaction of styrenes in an undivided cell at room temperature (Scheme 6).²⁷ This protocol provides a green access to β-alkoxy sulfones with a broad substrate scope. It worth mentioning that TsOH played a crucial role in this transformation. Moreover, the reactions of ethanol and propanol could proceed smoothly to afford the corresponding alkoxysulfonylation products while water was used as a co-solvent (Scheme 7).

Scheme 6 Alkoxysulfonylation reaction of styrenes.

Scheme 7 Alkoxysulfonylation reaction of ethanol and propanol.

A control experiment indicates that a radical pathway may be involved in the electrochemical transformation. The proposed mechanism is described in Scheme 8. Initially, sodium 4-methylbenzenesulfinate is oxidized at the graphite anode to form radical A, which was followed by tautomerization to sulfonyl radical B. Then, sulfonyl radical B adds to the double bond of α-methyl styrene to generate the alkyl radical C, which undergoes anodic oxidation to afford alkyl cation D. Then, a nucleophilic attack reaction of methanol and D produces intermediate E. Finally, deprotonation of E gives the target product.

Scheme 8 Proposed mechanism for the alkoxysulfonylation reaction.

Later, Sun et al. developed an electrochemical vicinal heterodifunctionalization of olefins using sulfinic acid as the sulfonyl source in an undivided cell with a carbon rod anode and platinum plate cathode at a constant current of 10 mA (Scheme 9).²⁸ Various β-hydroxysulfones could be obtained in the presence of water as nucleophile. Additionally, either external alcohol nucleophiles or internal carboxylic acid nucleophiles were well tolerated, giving the desired β-alkoxysulfones, and β-sulfonyl lactones under a modification of conditions. The authors suggested the reaction was initiated by an oxidation of sulfinic acid to form a sulfonyl radical, which undergoes further oxidation, and is then attacked by O-nucleophile to give the desire products (Scheme 10).

Scheme 9 Heterodifunctionalization of olefins with sulfinic acid.

Scheme 10 Proposed mechanism for the hetero-difunctionalization.

In 2020, Huang and co-workers reported an electrochemical radical cascade cyclization of styrenyl amides with sulfonylhydrazines for the synthesis of sulfonated 4H-3,1-benzoxazines (Scheme 11).²⁹ This reaction was conducted in a undivided cell equipped with a carbon rod anode and a platinum foil cathode by employing ⁿBu₄NBF₄ as the supporting electrolyte. Sulfonylhydrazines bearing electron-donating groups or electron-withdrawing groups at the aromatic ring were all amenable for constructing 4H-3,1-benzoxazines. Notably, the reaction of the sulfonylhydrazine with a hydroxy or an amino group on the benzene ring could not afford the desired product. Alkyl amides could be converted to the corresponding products in moderate to good yields. However, mono- or tri-substituted alkene was not suitable for this reaction.

Scheme 11 Cascade cyclization of styrenyl amides with sulfonylhydrazines.

Based on the results of control experiments, a possible reaction mechanism is shown in Scheme 12. Firstly, the sulfonyl radical B is generated from p-toluenesulfonyl hydrazide via electro-oxidation and deprotonation together with the release of N₂. Then, the sulfonyl radical B is added to the alkene to form the radical intermediate C, which could be oxidized at the anode to afford the corresponding carbon cation D. Finally, the desired sulfonated 4H-3,1-benzoxazine is generated through the nucleophilic attack and deprotonation of D.

Scheme 12 Proposed mechanism.

In 2021, He and co-workers explored a green and sustainable electrochemical oxidative difunctionalization of alkynes with sulfonyl hydrazides for the synthesis of β-keto sulfones (Scheme 13).³⁰ In general, new C–O bonds and C–S bonds were formed with H₂O as nucleophile. This process does not require any oxidizing agent, and is not only suitable for terminal alkynes, but also for internal alkynes. Moreover, in addition to aromatic sulfonyl hydrazides, it is also applicable to fatty sulfonyl hydrazides. Radical trapping experiments indicated that the reaction involves a radical pathway, in which the phenylsulfonyl radical is one of the key intermediates. Then, an isotope labeling experiment was conducted under standard reaction conditions using MeCN/H₂¹⁸O as the mixed solvent, and the ¹⁸O-product was detected by HRMS, suggesting that the carbonyl oxygen in the product comes from H₂O. Moreover, cyclic voltammetry experiments indicated that phenylsulfonyl hydrazide is preferentially oxidized during the reaction process.

Scheme 13 Electrochemical difunctionalization of alkynes with sulfonyl hydrazides.

The described mechanism involves the addition of sulfonyl radicals to alkynes, and the resulting alkenyl radicals are oxidized to alkenyl cations, which are then hydroxylated in the presence of H₂O, and provide β-keto sulfones via subsequent tautomerization (Scheme 14).

Scheme 14 Proposed mechanism for the difunctionalization of alkynes with sulfonyl hydrazides.

The electrochemical oxysulfenylation of alkenes with thiophenols/thiols has been disclosed by Lei and co-workers.²⁶ However, when acetic acid (AcOH) was used as a nucleophile, the sulfuration–acetoxylation of styrene was realized in low yield. In 2021, Zhao et al. described a PhB(OH)₂-promoted electrochemical sulfuration–formyloxylation reaction of styrenes with thiophenols/thiols by employing DMF as a formyloxylation reagent (Scheme 15).³¹ It is worth noting that the cyclic alkene was suitable, giving the expected cyclic β-formyloxy sulfide in high yield, while acyclic 1,1- and 1,2-disubstituted alkenes failed in this transformation. The sulfuration–formyloxylation product could be readily hydrolyzed to deliver β-hydroxy sulfide. Moreover, the product could be converted to (E)-vinyl sulfones via selectfluor-mediated oxidation–olefination (Scheme 16).

Scheme 15 PhB(OH)₂-promoted electrochemical sulfuration–formyloxylation reaction.

Scheme 16 Derivatization of the products.

Control experiments indicated that the transformation might involves a radical pathway, thiophenol was more easily oxidized, and two one-electron oxidation processes may be involved in this process. The mechanism is proposed in Scheme 17. Firstly, PhSH is converted to the thiyl radical A at the anode. Then, the thiyl radical A reacts with styrene to generate the benzyl radical intermediate C, which can be further oxidized at the anode to furnish the benzyl carbocation D. D is further converted to the iminium Evia the nucleophilic addition with DMF. Finally, the desired product can be afforded through the hydrolysis of iminium intermediate E. H₂O involved here may be in situ generated by the substitution reaction of PhB(OH)₂ or slowly seeped into the reaction system. The beneficial effect of adding PhB(OH)₂ could be to remove excess dimethylamine (or water) from the solvent. Alternatively, the sulfuration–formyloxylation product could also be furnished via a nucleophilic addition and elimination process between E and PhB(OH)₂.³²

Scheme 17 Proposed mechanism for the sulfuration–acetoxylation.

In 2021, Lei et al. reported a mild and efficient electrochemical oxidative radical cascade cyclization strategy to construct benzoxazines, oxazolines and iminoisobenzofurans from olefinic amides and thiophenols (Scheme 18).³³ The reaction was performed in a simple undivided cell equipped with a carbon anode and platinum cathode, employing ⁿBu₄NBF₄ as supporting electrolyte and acetonitrile as solvent under catalyst-free and oxidant-free conditions. The synthetic utility of this protocol was fully demonstrated by a large scale reaction and synthetic derivatization of the products.

Scheme 18 Electrochemical oxidative radical cascade cyclization of olefinic amides and thiophenols.

Control experiments results indicated that this reaction probably proceeded via a radical pathway (Scheme 19), and the sulfur radical might be involved in the transformation. A plausible mechanism for this electrochemical transformation is illustrated in Scheme 19. Firstly, the sulfur radical A is generated via single-electron-oxidation of thiophenol at the anode. The sulfur radical A undergoes a reversible dimerization to generate a disulfide, which obtained one electron at the cathode to give the disulfide radical anion B. Subsequently, this radical anion B cleaved to give the thiyl radical A and thiolate anion C. The thiolate anion C could be oxidized to the sulfur radical at the anode. Then, the sulfur radical A is added to the C=C double bond of olefin to generate the alkyl radical D. D undergoes a radical cyclization and oxidation to provide intermediate E. Finally, E undergoes deprotonation to afford the desired product. At the cathode, the proton was reduced to give hydrogen gas during the reaction.

Scheme 19 Proposed mechanism.

CF₃-substituted tertiary alcohols play a significant role in biological molecules and synthesis intermediates.³⁴ In 2022, Lei et al. realized the synthesis of α-CF₃–substituted tertiary alcohols by utilizing cheap and easily available sodium sulfinates as the precursor via a green electrochemical hydroxysulfonylation of α-CF₃ alkenes (Scheme 20).³⁵ Heteroaromatic rings and alkyl-substituted sodium sulfonates were well tolerated in this reaction, furnishing the desired products in good yields. The target reaction is only efficient for α-CF₃ alkenes with electron-rich or neutral groups, while alkenes with electron deficient and ortho-position substitutes cannot be tolerated in this electrolysis process.

Scheme 20 Synthesis of α-CF₃-substituted tertiary alcohol.

The preliminary mechanistic investigation indicated that this difunctionalization reaction involves a radical process via a sulfonyl radical. Gram-scale synthesis shows the significant potential application of this protocol. Moreover, H₂¹⁸O-labeled experiments indicated that the OH group of the target product was derived from water. A proposed mechanism is described in Scheme 21. Firstly, sodium sulfinate is oxidized to produce sulfonyl radical A. Subsequent radical addition to α-CF₃ alkenes gives the radical intermediate B, which would be further oxidized to furnish the cation intermediate C. Meanwhile, the cathodic reduction of H₂O was accompanied by the release of hydrogen and the formation of hydroxyl anions. The following hydroxylation was accomplished between the cation intermediate C and hydroxyl anion to form the α-CF₃-substituted tertiary alcohols. Nevertheless, the direct reaction of cation C with water may also be the pathway for the formation of the product.

Scheme 21 Proposed mechanism for the hydroxysulfonylation of α-CF₃ alkenes.

Sulfonate esters are privileged structures in medicinal chemistry and many natural products.³⁶ Meanwhile, metabisulfite salts have attracted continuous interest in synthetic chemistry, because they are stable, cheap, and abundant in nature.³⁷ In 2022, Han et al. reported an electrochemical difunctionalization of alkenes and alcohols with potassium metabisulfite as a sulfur dioxide source for the synthesis of β-alkoxy sulfonate esters (Scheme 22).³⁸ This process enables the formation of C–S, S–O, and C–O bonds in a single operation under mild reaction conditions. Alkene substrates with a hindered cyclohexyl or phenyl group at the α-position of the olefinic moiety of styrene were compatible with this electrochemical reaction, affording the desired sulfonate esters in good yields. Using ethanol, n-propanol, n-propanol, or n-butyl alcohol as the substrate and co-solvent were also successful, and the corresponding product were obtained in moderate yields.

Scheme 22 Synthesis of β-alkoxyl sulfonate esters.

Control experiments suggest that this reaction proceeds through the formation of a sulfur dioxide radical cation, and cyclic voltammetry experiments disclosed that the reaction preferably started from the initial oxidation of potassium metabisulfite.

A postulated reaction mechanism is presented in Scheme 23 for this reaction. Firstly, sulfur dioxide generated from potassium metabisulfite, which undergoes anodic oxidation to form the sulfur dioxide radical cation A. Then, the reaction of the sulfur dioxide radical cation A with a-methyl styrene affords intermediate B. Subsequently, the anodic oxidation of intermediate B occurs leading to the formation of intermediate C, which is nucleophilically attacked by methanol to give the intermediate D. Deprotonation of the intermediate D affords the cation intermediate E with the release of a proton, which is reduced at the cathode to hydrogen gas. Finally, the generated cation intermediate E reacts with another equivalent of methanol, followed by the second deprotonation to afford the corresponding product.

Scheme 23 Proposed mechanism for the synthesis of β-alkoxyl sulfonate esters.

In 2022, Chen et al. reported an electrochemical cascade sulfonylation and lactonization process of alkenes with sulfonyl hydrazines as an arylsulfonylation reagent (Scheme 24).³⁹ This electrochemical sulfonyl lactonization avoided the use of toxic metal catalysts or stoichiometric oxidants and was carried out under mild conditions. The target product sulfonyl lactones with broad and excellent substrate tolerance were achieved. A methyl group at the meta- and para-positions at the aromatic ring of 2-vinylbenzoic acids displayed superior yield compared with the ortho-substituted counterparts, indicating that the steric hindrance effect has an influence on this reaction. Apart from aromatic sulfonylhydrazines, aliphatic sulfonylhydrazines, such as ethanesulfono-hydrazide, participated well in this transformation, giving the product in good yield.

Scheme 24 Cascade sulfonylation and lactonization.

On the basis of the control experiments and CV studies, a postulated reaction mechanism is presented in Scheme 25. Firstly, sulfonyl hydrazide lost electrons through anodic oxidation and deprotonation to form the radical A, which is unstable and immediately releases N₂ to form the sulfonyl radical B. Subsequently, sulfonyl radical B attacked the double bond of the o-vinyl benzoic acid to generate the radical intermediate C, which is oxidized at the anode to afford the corresponding carbon cation D. Finally, the lone-pair electrons on the ortho-carboxyl oxygen attacked the carbon cation D and then deprotonation occurred to obtain the desired γ-sulfonylated lactones.

Scheme 25 Proposed mechanism for the cascade sulfonylation and lactonization.

In 2022, Liu and co-workers developed an electrochemical oxidative three-component cyclization of allylic alcohols, boronic acids, and disulfides (Scheme 26).⁴⁰ Cyclic sulfide boronic esters were successfully obtained via this electrochemical cascade esterification reaction by using KBr as the electrolyte and AcOH as an additive under a 10 mA constant current. Substrate scope investigation indicated that the reaction has a broad substrate scope and high functional group tolerance. Various boronic acids, allyl alcohols, and disulfides were suitable for this process, giving the desired cyclic sulfide boronic esters in good yields. Interestingly, the resulting cyclic boronic esters were easily converted to chalcogenated 1,3-diols, which could be further oxidized by H₂O₂ to give sulfone-containing 1,3-diols.

Scheme 26 Three-component cyclization of allylic alcohols, boronic acids, and disulfides.

In 2023, Lei et al. reported an electrochemical oxidative cyclization of ortho-vinyl aniline to access various SCN-containing benzoxazines (Scheme 27).⁴¹ This protocol featured a mild condition, an extra catalyst-free and oxidant-free system, and good tolerance for air. Various olefinic benzamide derivatives were compatible radical acceptors for achieving the desired transformation. Furthermore, a set of alkyl amides realized the desired transformation, forming target products in moderate yields.

Scheme 27 Synthesis of SCN-containing benzoxazines.

A plausible mechanism is proposed in Scheme 28. Firstly, the thiocyanate anion is oxidized to form thiocyanate radical A in the anode. Then, the thiocyanate radical A reacts with ortho-vinyl aniline to offer C-centered radical intermediate B, which transformed to carbon cation Cvia SET in the anode. Finally, the product is generated via a subsequent intramolecular nucleophilic attack and deprotonation of C. In the cathode, two protons were reduced to furnish hydrogen.

Scheme 28 Proposed mechanism for the oxidative cyclization.

Similarly, Huang et al. demonstrated an electrochemical oxythiocyanation of ortho-olefinic amides for the synthesis of thiocyanated benzoxazines (Scheme 29).⁴² Different from the method from Lei et al., this protocol provides not only various monothiocyanates, but also a series of dithiocyanates and trithiocyanates involving C(sp³)–H functionalization. More importantly, the synthetic utilities of this method were highlighted in the derivatization of thiocyanates.⁴³

Scheme 29 Oxythiocyanation of ortho-olefinic amides.

Recently, Zhang et al. accomplished a facile electrochemical sulfonylative cycloetherification of linear unsaturated alcohols with sulfonyl hydrazides (Scheme 30).⁴⁴ This protocol features a broad substrate scope and good functional group compatibility, which provides a convenient synthetic tool for the synthesis of saturated five-, six-, seven-, and eight-membered ring oxygen heterocycles. Furthermore, sulfonylative cycloesterification of linear unsaturated acids toward the lactone products has also been established under this electrochemical system. In addition, the utility of this protocol was also demonstrated by a gram-scale reaction and diversified synthetic transformations of the desired products.

Scheme 30 Sulfonylative cycloetherification of linear unsaturated alcohols with sulfonyl hydrazides.

The difunctionalization of internal alkenes equipped with nucleophilic groups to produce sulfonylated heterocycles is challenging due to the relatively low reactivity of internal alkenes and the difficulty in regioselectivity control. More recently, Chen et al. developed a simple and versatile electrochemical intramolecular oxysulfonylation of alkenes with sodium sulfonates (Scheme 31).⁴⁵ Sulfonylated O-heterocycles,⁴⁶ such as tetrahydrofurans, tetrahydropyrans, oxepanes, tetrahydropyrroles, and δ-valerolactones, were efficiently prepared with high diastereoselectivities from easily accessible unsaturated alcohols, and carboxylic acids without the need for additional metal or exogenous oxidant. Notably, the reaction yields were dramatically diminished in the absence of H₂O, probably due to its good capability of dissolving sodium p-tolylsulfinate.

Scheme 31 Oxysulfonylation of alkenes with sodium sulfonates.

2.2 C–Se and C–O bond formation

In 2019, Pan et al. explored an electrochemical difunctionalization of olefins with selenides for the synthesis of selenomethyl-substituted cyclic ethers or lactones (Scheme 32).⁴⁷ This transformation efficiently provided a range of selenomethyl-substituted cyclic ethers, particularly 9- and 11-membered, selenomethyl-substituted lactones (4–6 membered), and selenomethyl-substituted phthalides with high yields in a simple undivided cell containing acetonitrile as solvent, equipped with platinum as the cathode and RVC as the anode at room temperature, employing a catalytic amount of ammonium iodide as electrolyte.

Scheme 32 Synthesis of selenomethyl-substituted cyclic ethers or lactones.

The results of control experiments exclude the possibility of a radical mechanism and proved that the regeneration of iodide was from the reduction of iodine at the cathode. A plausible mechanism is proposed in Scheme 33. First, the electrochemical oxidation of I⁻ results in the formation of I⁺ by losing two electrons at the anode. The I⁺ then react with olefinic alcohols to provide the iodonium cation intermediate A, which then undergoes intramolecular cyclization and releases a proton to afford intermediate B. Finally, the target product and I₂ are obtained by a chemical selenation.⁴⁸ At the cathode, the I₂ and proton are reduced to the iodine anion and hydrogen (Scheme 33).

Scheme 33 Proposed mechanism for the reaction.

In 2019, De Sarkar et al. reported a tandem electro-oxidative selenation of amides and oximes to synthesize chalcogen-functionalized oxazolines and isoxazolines (Scheme 34).⁴⁹ This protocol enables the formation of C–Se without using any external oxidant under neutral reaction conditions. The synthetic practicability of the developed method was proved on a gram scale and with derivatization of the product. In addition, disulfides could also take part in the reaction, leading to the desired oxysulfenylation products.

Scheme 34 Chalcogenation of amides and oximes.

Control experiments indicated the possibility of both radical and ionic reaction pathways for the tandem reaction. The probable mechanistic paths are depicted in Scheme 35. In pathway I, oxidative activation of the diphenyl diselenide generates phenyl selenium cation A and phenyl selenium radical B. Furthermore, one-electron oxidation of B forms another cation A. Addition of A to the double bond can generate intermediate C, which upon nucleophilic cyclization by the amide oxygen produces oxazoline. In pathway II, reduction of diphenyl diselenide delivers radical B and phenyl selenium anion D. One electron oxidation of D forms radical B. On the other hand, oxidation of the amide produces delocalized radical E, followed up by radical cyclization and coupling with B to generate the product.

Scheme 35 Proposed mechanism.

β-Hydroxy(alkoxy) selenides containing two different functional groups (α-OH or β-OR and β-SeR) at the same single bond are attractive building blocks for the synthesis of bioactive molecules (Scheme 36).⁵⁰ In 2019, Chen and Shi et al. demonstrated an electrochemical iodide-catalyzed oxyselenation of styrene derivatives with dialkyl(aryl) diselenides and nucleophilic oxygen sources (water or alcohols) for the construction of β-hydroxy(alkoxy)selenides.⁵¹ Primary, secondary and tertiary alcohols could be also applied in this oxyselenation to give the corresponding ether selenides in excellent yields. It is worth mentioning that KI was not only used as electrolyte, but also as catalyst in this process. Radical trapping experiments indicated that no single-electron transfer was involved through the whole reaction process. Moreover, the cyclic voltammetry experiments indicated that the oxidation of iodide should occur first at the anode.

Scheme 36 Construction of β-hydroxy(alkoxy)selenides.

A possible mechanism is depicted in Scheme 37. First, I⁺ is generated from I⁻ by losing two electrons. Then the diphenyldiselenide undergoes electrophilic substitution by I⁺ to give PhSeI (A), and PhSe⁺ (B). B can also be converted from PhSeI (A). Then the coordination of electrophile PhSe⁺ with styrene forms seleniranium ion C, which reacts with ROH to give the products.

Scheme 37 Proposed mechanism.

In 2019, Lei, Zhang, and Tang et al. reported an electrochemical oxidative cyclization of olefinic carbonyls with diselenides toward C–Se and C–O bond formation (Scheme 38).⁵² A series of functionalized dihydrofurans⁵³ and oxazolines were synthesized in the absence of metal catalysts and external oxidants. Symmetric and unsymmetric olefinic carbonyls were suitable for this reaction, and the corresponding dihydrofurans were obtained in moderate to good yields. It is noteworthy that dimethyl diselenides and dibenzyl diselenides were also amenable for this approach and furnished the target products in acceptable yields. Moreover, N-allylamides, with fragile thiophene, furan and pyridine rings, as well as N-allyvinylamide and N-allycinnamide, also were well-tolerated to furnish the corresponding oxazolines with moderate yields. A gram scale reaction and the derivatization of product were carried out to demonstrate the potential application of this tandem cyclization reaction.

Scheme 38 Oxidative cyclization of olefinic carbonyls with diselenide.

On the basis of control experimental results, a proposed mechanism is outlined in Scheme 39. In path a, firstly, the anion radical intermediate B is formed from the reduction of diphenyl diselenide at the cathode. Then, the intermediate B is further decomposed to give phenylselenium radical C and phenyl selenium anion D. Thereafter, phenylselenium radical C is captured by the alkenyl of olefinic carbonyls A to form alkyl radical E, which is further oxidized at the anode. The fast ring closing followed by nucleophilic attack of the oxygen atom of the carbonyl, as well as deprotonation, furnishes the formation of desired product. In path b, reaction of the olefinic carbonyl A with the phenylselenium radical to form the alkyl radical F could not be completely ruled out.

Scheme 39 Proposed mechanism.

Meanwhile, Lei and Chen et al. accomplished an electrochemical oxyselenation of styrenes by employing ⁿBu₄NBF₄ as a supporting electrolyte and CH₃CN/HFIP as the co-solvent (Scheme 40).⁵⁴ It should be noted that the HFIP played an important role in the stabilization of radicals. Carboxylic acids, water, and alcohols, are all suitable in this reaction, affording the corresponding products in moderate to excellent yields.

Scheme 40 Oxyselenation of styrenes.

On the basis of mechanistic studies, a proposed mechanism is depicted in Scheme 41. First, seleno radical B and selenium anion C are formed via cathodic reduction. Then, radical addition of B to alkene provides radical D with good regioselectivity. Subsequently, further anodic oxidation of D gives E, which undergoes nucleophilic attack with a nucleophile and then deprotonation, leading to the formation of the product. Alternatively, the pathway in which diphenyl diselenide undergoes anodic oxidation followed by the formation of the cyclic selenium intermediate G, which reacts with NuH to form the final product, cannot be ruled out.

Scheme 41 Proposed mechanism.

Iso-coumarins are privileged scaffolds found in bioactive natural products and pharmaceuticals.⁵⁵ In 2020, Guo et al. reported an efficient continuous electrocatalytic strategy via the difunctionalization of alkynes to synthesise selenium-substituted iso-coumarin derivatives under oxidant-free and metal-free conditions (Scheme 42).^56a Substrates containing cycloalkane, a thiophene ring, and a naphthalene ring, as well as alkyl substituents attached to alkynes also proved to be well tolerated under optimized conditions, providing the corresponding products in good yields. In addition, except for diaryl diselenides, dialkyl diselenides are also suitable for this conversion, and the corresponding products can be obtained in excellent yields. Of note is that the diphenyl ditelluride was also proved to be well tolerated, providing the corresponding product in 48% yield. Recently, Mei, Ma, Gu and co-workers developed an electrochemical selenylative cyclization of alkynyl phosphonates with diselenides, which provides a green way for the synthesis of selenoether-substituted cyclic enol phosphonate.^56b

Scheme 42 Continuous electrocatalytic difunctionalization of alkyne.

Selenium-containing quinones constitute the key skeletons of bioactive natural products and drugs.⁵⁷ In 2020, Júnior, Ackermann and Jacob et al. reported an efficient electrochemical selenation/cyclization of naphthoquinones for the synthesis of selenium-containing multifunctional redox quinoidal compounds (Scheme 43).⁵⁸ This reaction is simple and versatile, and results in considerable yields. Some of the quinone-hybrid molecules produced with this electrochemical method also exhibit considerable biological activity including activity against cancer cell lines and pathogenic microorganisms. The authors suggested that the reaction mechanism involves a carbophilic reaction of the selenium dication with lapachol to give a cationic intermediate, followed by a nucleophilic cyclization to access the products (Scheme 44).

Scheme 43 Selenation/cyclization of naphthoquinones.

Scheme 44 Proposed mechanism.

Dihydrobenzofuran (DHB) plays a significant role in natural bioactive products and pharmaceuticals.⁵⁹ In 2020, Braga et al. reported an electrochemical intramolecular oxyselenylation of allyl-naphthols/phenols (Scheme 45).⁶⁰ The electrolysis reaction performed at a constant current of 5 mA using a platinum plate as both anode and cathode, and ⁿBu₄NClO₄ as a supporting electrolyte at room temperature in a simple undivided cell, afforded the desired selenyl-dihydrofurans in good to excellent yields. Of note, several of the synthesized products have been disclosed as acetylcholinesterase (AChE) inhibitors and exhibit potential in the treatment of Alzheimer's.

Scheme 45 Synthesis of selenyl-dihydrofurans.

In 2021, Liu and Cao et al. accomplished the electrochemical oxidative three-component cyclization of allylic alcohols, boronic acids, and diselenides (Scheme 46).⁴⁰ This protocol provided an efficient method to construct chalcogenated boronic esters with a broad substrate scope by using KBr as electrolyte and TFA as additive in an undivided cell equipped with a graphite anode and platinum plate cathode. Phenylboronic acids with single substituents of electron-donating or electron-withdrawing groups at the para- or ortho-position all converted to boronic esters in moderate to good yields. Moreover, double substituent phenylboronic acids could react smoothly, delivering oxyselenation products with high yields. Boronic acids with heteroaromatic rings, as well as bearing alkyl groups, were also transformed to the corresponding boronic esters. Based on control experiments, two plausible reaction mechanisms including cation and radical pathways for the electrochemical cyclization have been presented.

Scheme 46 Three-component cyclization of allylic alcohols, boronic acids, and diselenides.

Meanwhile, Kim and co-workers reported electrochemical oxidative radical selenylation and cyclization sequences of alkenoic acid derivatives with diselenides (Scheme 47).⁶¹ The various selenated-lactone derivatives⁶² could be synthesized in moderate to high yields in an undivided cell with LiClO₄ as the supporting electrolyte. Two mechanisms involving cation or radical pathways are also proposed in this work.

Scheme 47 Selenylation and cyclization sequences of alkenoic acids with diselenides.

In 2021, Fang and Hu et al. also accomplished an electrochemical cyclization strategy to prepare selenium-substituted benzoxazines from o-vinylaniline and diselenide (Scheme 48).⁶³ When an aliphatic, naphthalene or thiophene is substituted for the benzene of olefin amide, the desired products can still be obtained in moderate to good yields. Whether it is dialkyl diselenides or diaryl diselenides, the desired product can also be obtained in good yields. Control experiments indicated that the reaction proceeds via a radical pathway and diselenide is oxidized preferentially over o-vinylaniline at the anode.

Scheme 48 Electrochemical synthesis of selenium-substituted benzoxazines.

In 2021, an electrochemical oxidative tandem cyclization of unsaturated oximes with diselenides has been reported by Xu and Zhang et al. (Scheme 49).⁶⁴ In the presence of ⁿBu₄NBF₄ as electrolyte, and HFIP and CH₃CN as co-solvents at a constant current of 10 mA, this approach provided a facile way to access seleno isoxazolines containing a quaternary carbon center with good functional group tolerance. Radical control experiments indicated the existence of both radical and ionic reaction pathways under electrochemical conditions.

Scheme 49 Cyclization of unsaturated oximes with diselenides.

In the same year, Ruan et al. developed an electrochemical selenylation/cyclization of alkenes to deliver diverse functionalized benzheterocycles, including iminoisobenzofuran, and lactones (Scheme 50).⁶⁵ Diphenyl diselenides bearing various different substituent groups, as well as aliphatic diselenide derivatives, are all amenable to this reaction, giving the desired products in good yields. What is more, 2-isopropenylbenzoic acid was also suitable for this process.

Scheme 50 Electrochemical selenylation/cyclization of alkenes.

Later, De Sarkar and co-workers reported an electro-oxidative selenocyclization of easily accessible homo-propargyl alcohols to access polysubstituted selenofuran derivatives with a broad range of substrate scope and good functional group compatibility (Scheme 51).⁶⁶ The reaction employed LiClO₄ as the electrolyte, using MeCN and TFE as co-solvents in a simple undivided cell. Of note, the TFE can stabilize the reaction intermediates and improves the efficiency of cathodic proton reduction. However, terminal or alkyl-substituted alkynes failed to give the corresponding products, presumably due to the aryl group present in the alkyne moiety, which can stabilize the reaction intermediates.

Scheme 51 Selenocyclization of homo-propargyl alcohols.

Intramolecular migratory rearrangement has unique features in reactivity and selectivity,⁶⁷ while electrosynthesis exhibited the advantages of using electrons as green reagents. As a result, electrosynthesis with migratory rearrangements exhibit great potential in synthetic chemistry. In 2022, Zhu et al. reported a water-promoted migratory oxyselenation of N-acyl allylamine in an electrochemical system under metal-free and external-oxidant-free conditions (Scheme 52).⁶⁸ The reaction gave the β-acyloxy-γ-selenyl amines in up to 98% yield with controllable regio- and diastereoselectivities, and the regioselectivity of the reaction was controlled by both Markovnikov's rule and Baldwin's rule. With different N-acyl groups, both migration products and cyclization products were successfully obtained in good yields. Some derivatives of drug or natural products such as proline, cyclic sulfone, thioether, progesterone, and ibuprofen were well tolerated, which further showcases the utility of this protocol.

Scheme 52 Migratory oxyselenation of N-acyl allylamines.

Isotope labeling experiments indicated a water promoted acyl migration pathway. A plausible mechanism is proposed in Scheme 53. Firstly, the anodic oxidation of diphenyl diselenide generated electrophilic PhSe⁺, which then reacted with allylamine derivatives to give selenonium intermediate A, which was followed by 5-exo cyclization to provide intermediate B. Nucleophilic attack of water on intermediate B gives intermediate C. If the migration groups are aromatic or alkyl acyl groups, the migration products F are obtained via the elimination of the NTs moiety. If the migration group is Boc or Cbz, the elimination of the alkoxy group is preferred, which led to cyclization products G.

Scheme 53 Proposed mechanism for the migratory oxyselenation.

In 2022, an electrochemical oxidative intramolecular cyclization reaction of 2-alkynylphenol derivatives and diselenides under mild conditions has been realized by Braga and co-workers (Scheme 54).⁶⁹ This annulation protocol provided an efficient method to synthesize various selenylbenzo[b]furans in good to excellent yields with satisfactory functional group compatibility. Of note, the transformation is not limited to 2-alkynylphenols, and the methoxy-2-(phenylethynyl) benzene was also tolerated well and smoothly converted to the desired product in good yield under the same reaction conditions. Bis-selenylation products could be obtained when using 2-[(trimethylsilyl)ethynyl]phenol, 2-[(phenylselanyl)ethynyl]phenol, or 2-[(phenylthio)ethynyl]phenol as the substrates.

Scheme 54 Cyclization reaction of 2-alkynylphenols and diselenides.

Two plausible mechanisms are proposed as shown in Scheme 55. Pathway I: the intermediate cationic radical A is firstly formed via anodic oxidation. Meanwhile, the anodic oxidation of 2-alkynylphenol gives the radical species E, which after addition at the triple bond forms the intermediate F, which, followed by an addition of B, delivers the desired product. However, the way through the formation of a reversible seleniranium intermediate G, followed by nucleophilic attack to deliver the product, cannot ruled out (pathway II).

Scheme 55 Proposed mechanism for the cyclization reaction.

1,3-Dihydroisobenzofurans (phthalanes) widely exist in many natural products, biologically active molecules and pharmaceuticals.⁷⁰ In 2022, Liu et al. described an electrochemical radical selenylation with substituted o-divinylbenzenes and diselenides toward the formation of two C–Se bonds and two C–O bonds (Scheme 56),⁷¹ which enables the efficient construction of 1,3-diselenyl-dihydroisobenzofurans in the presence of LiBF₄ as the electrolyte under a constant current of 6 mA. Control experiments indicated that the reaction proceeds through a radical pathway. In addition, the H₂¹⁸O-labeled experiment indicated that the oxygen atom in the framework of 1,3-dihydroisobenzofuran was probably from H₂O.

Scheme 56 Construction of 1,3-diselenyl-dihydroisobenzofurans.

A plausible mechanism was proposed as shown in Scheme 57. Firstly, the intermediate radical cation A is formed via the oxidation of diphenyl diselenide at the anode. Then, the radical cation A decomposes into phenylselenium radical B and a phenyl selenium cation C. The phenylselenium radical B subsequently reacts with o-divinyl-benzene to provide radical D, which undergoes further anodic oxidation, nucleophilic attack and deprotonation leading to the formation of the intermediate F. Then, radical addition of seleno radical B to intermediate F provides G, which is followed by further anodic oxidation, nucleophilic attack and deprotonation to give the final products. Meanwhile, the phenyl selenium cation is reduced to regenerate diselenide, and the protons can be reduced to generate H₂ at the cathode.

Scheme 57 Proposed mechanism.

In 2022, Lei et al. reported an electrochemical oxidative radical cascade cyclization of olefinic amides and diselenides to access iminoisobenzofurans without transition-metal catalysts and external oxidants (Scheme 58).⁷² This protocol was conducted at a higher constant current of 30 mA in a simple undivided cell equipped with a carbon anode and platinum cathode. A broad range of olefinic amides and diselenides were suitable and gave the desired products in moderate to high yields. Control experiments indicated that a selenium radical intermediate might be involved in this transformation and diphenyl diselenide is more easily oxidized at the anode.

Scheme 58 Cascade cyclization of olefinic amides and diselenides.

In 2023, Liu, Li and Jin et al. developed a general electrochemical methodology for the oxyselenation of inert alkenes with carboxylic acids and diselenides (Scheme 59a).^73a Carboxylic acid, such as condensed, polyhalogenated, and aryl alkynyl carboxylic acid, reacted well with saccharin and diphenyl diselenide to generate the corresponding products. In addition, aliphatic carboxylic acids, as well as 5-, 7-, 8-membered cyclic alkenes and the linear aliphatic alkenes, were compatible. However, aliphatic alcohols such as methanol and ethanol failed in this transformation.

Scheme 59 Multicomponent oxyselenation reactions.

Very recently, Liang, Pan and co-workers developed an elegant electrochemically mediated multicomponent reaction of carbon dioxide, propargyl amides and diselenides for the preparation of selenium-containing oxazolidine-2,4-diones (Scheme 59b).^73b Meanwhile, this strategy is further extended to the reaction with ditellurides to produce tellurium-containing oxazolidinones (Scheme 59c).^73c Importantly, the in vitro anticancer activity of the novel tellurium-containing oxazolidinones has been tested and the result showed that the oxazolidinones can act against various types of human cancer cells.

3. Carbochalcogenation of alkenes and alkynes

3.1 C–S and C–C bond formation

In 2018, Guo et al. reported an electrooxidative sulfonylation/heteroarylation reaction of unactivated alkenes with sulfinic acids via ipso-migration, which would efficiently construct C–S and C–C bonds (Scheme 60).⁷⁴ The reaction was conducted at a constant current of 10 mA in an undivided cell equipped with graphite as both anode and cathode, employing ⁿBu₄NBF₄ as electrolyte at room temperature, and afforded the desired sulfonated functionalized heteroarenes.

Scheme 60 Sulfonylation/heteroarylation of unactivated alkenes with sulfinic acids.

Control experiments indicate that the migration favors five- and six-membered cyclic transition states, and the heteroaryl migration proceeded in an intramolecular manner. Radical trapping experiments indicated that the sulfinic acid may undergo a deprotonation process, resulting in sulfonyl radicals. A mechanism for this reaction is proposed in Scheme 61. First, the sulfonyl ion intermediate A is generated from a deprotonation process. Then, the intermediate A is oxidized to radical intermediate B and the resonance structure C through a single electron transfer (SET) at the anode. The sulfonyl radical C undergoes intermolecular radical addition to the double bond of alkenes to generate D, which undergoes an ipso-migration to give a ketyl radical Fvia a spiro N-radical E. Finally, F undergoes sequential single-electron oxidation and deprotonation, affording the corresponding products.

Scheme 61 Proposed mechanism for the sulfonylation/heteroarylation reaction.

In 2019, Lin et al. developed an enantioselective electrochemical cyanofunctionalization of vinylarenes by using serine-derived chiral bisoxazolines with ancillary coordination sites as optimal ligands (Scheme 62).⁷⁵ This electrochemical protocol could be extended to the cyanosulfinylation of vinylarenes by using sulfinic acids as nucleophiles, and delivering the desired product in high efficiency and selectivity. It should be noted that the pyridine was needed to balance the pH of the medium and ensure the facile generation of sulfinyl radical.

Scheme 62 Electrochemical cyanofunctionalization of vinylarenes.

Zhang et al. reported electrochemical sulfonylation/semipinacol rearrangements of allylic alcohols with aryl sulfinates in the same year (Scheme 63).⁷⁶ This strategy provides a facile approach to construct sulfonated ketones containing all-carbon quaternary stereogenic centers. Control experiments were conducted and showed that the reaction might go through a radical pathway.

Scheme 63 Electrochemical sulfonylation/semipinacol rearrangements of allylic alcohols.

In 2021, Lei and co-workers developed a practical methodology for the radical selenylation of alkenes and activated arenes through electrochemical Se–Se bond activation (Scheme 64).⁷⁷ Under undivided electrolysis conditions, a diversity of unsymmetric aryl–alkyl, and alkyl–alkyl selenoethers could be synthesized in moderate to high yields, which provides a novel way for organoselenium synthesis. Of note, Y(CF₃CO₂)₃ serves as the Lewis acid catalyst in this reaction to stabilize the reactive radical intermediates.

Scheme 64 Radical selenylation of alkenes and arenes.

Preliminary kinetic studies, density functional theory (DFT) calculation, and cyclic voltammetry experiments were conducted to provide insights into the reaction mechanism. A possible mechanism for electrochemical radical selenylation is shown in Scheme 65. Initially, the selenium radical and selenium anion are formed through cathode reduction. Then, the addition of the selenium radical to olefins resulted in radical A, which could be oxidized to the carbocation intermediate, subsequently combining with activated arenes, and deprotonation resulted in the formation of the product. Of note, the way that diphenyl diselenide undergoes anodization and then forms a phenylselemnium radical and phenyl selenium cation cannot ruled out.

Scheme 65 Proposed mechanism for the selenylation reaction.

Phenanthrenes represent one of the essential polycyclic aromatic hydrocarbon (PAH) motifs in materials science. In 2022, Zhang and Wen et al. reported an efficient electrochemical reaction of internal alkynes with sulfonyl hydrazides for the synthesis of sulfonated phenanthrenes (Scheme 66).⁷⁸ This protocol provided a green route to functionalized phenanthrenes without a metal catalyst. Notably, the synthesized phenanthrenes also exhibited intense fluorescence and control experiments demonstrated the existence of the sulfonyl radicals and alkenyl radical in the process.

Scheme 66 Synthesis of sulfonated phenanthrenes.

Functionalized 2H-chromene frameworks are crucial oxygenated heterocycles which have a widespread existence in natural products and biological activities molecules. In 2023, Satyanarayana and co-workers reported a facile approach for the synthesis of 3-sulfonated 2H-chromenes through the electrochemical oxidative annulation of inactivated propargyl aryl ethers with sulfonyl hydrazides (Scheme 67).⁷⁹ The process exhibited a broad scope and functional group tolerance to deliver 2H-chromenes in good yields by employing LiClO₄ as electrolyte at room temperature for 8 hours. However, aliphatic sulfonyl hydrazides failed to generate the expected products under the standard conditions.

Scheme 67 Synthesis of 3-sulfonated 2H-chromenes.

Control experiments indicated that the formation of the tosyl radical and the radical mechanism was involved. A plausible reaction mechanism for the electrochemical oxidative cyclization is proposed in Scheme 68. Firstly, tosyl radical B is generated via the oxidation of sulfonyl hydrazides and the subsequent release of nitrogen gas. Then, tosyl radical B attacks the triple bond of propargyl aryl ethers to give vinyl radical C, which undergoes an intramolecular cyclization to give D. D, through further oxidation at the anode and aromatization, would afford the desired sulfonated oxidative cyclized product.

Scheme 68 Proposed mechanism for the synthesis of 3-sulfonated 2H-chromenes.

3.2 C–Se and C–C bond formation

In 2019, an electrochemical synthesis of β-selenated cyclic ketones from alkenylcyclobutanols with diselenides via oxidative selenylation and semipinacol rearrangement sequences has been realized by Kim et al. (Scheme 69).⁸⁰ This process using ⁿBu₄NBF₄ as electrolyte, and CH₃CN as the solvent at a constant current density of 7 mA cm⁻², afforded the β-selenyl-substituted cyclopentanones in high yields. Notably, various alkyl-substituted vinyl cyclobutanols, as well as dibenzyl diselenides, were tolerated well and smoothly converted to the cross-coupling products. Interestingly, the products could deliver β-substituted cyclohexanones in the presence of tri-n-butyltin hydride and azobis (isobutyronitrile) (AIBN) through a one-carbon ring-expanded named Dowd–Beckwith-type rearrangement.

Scheme 69 Synthesis of β-selenated cyclic ketones.

A possible mechanism for the electrochemical cyclization is presented in Scheme 70. Firstly, seleno radical A and selenium cation B are generated via anodic oxidation. Then, seleno radical A undergoes radical addition with 1-(1-arylvinyl)cyclobutanol to give the radical intermediate C, which is oxidized to afford the cation D. Finally, the 1,2-carbon migration of cation D yields the desired product. As another possibility, the ionic pathway b could not be ruled out.

Scheme 70 Proposed mechanism for the electrochemical cyclization.

Introducing a selenium functional group into oxindoles presents an attractive research direction due to the fact that selenium compounds that contain heterocyclic units also have unique biological activities,⁸¹ as well as that oxindoles are privileged scaffolds found in natural products and pharmaceutical agents.⁸² In 2020, Pan et al. developed an electrochemical tandem cyclization of acrylamide with diselenides (Scheme 71).⁸³ This methodology has good functional group tolerance and leads to various seleno oxindoles in high yields under mild conditions. Notably, in vitro antitumor activity experiments were conducted, and the results indicated that some seleno oxindoles exhibited better antitumor activity than other oxindoles.

Scheme 71 Electrochemical tandem cyclization of acrylamide with diselenides.

In 2019, Guo et al. explored an electrochemical oxidative cyclization of alkynoates and alkynamides with diselenides for the synthesis of chalcogen-substituted coumarins and quinolinones (Scheme 72).⁸⁴ This protocol featured a broad substrate scope and under metal-/oxidant-free conditions. The reaction was carried out in an undivided cell equipped with a graphite anode and a platinum cathode, using acetonitrile and HFIP as a co-mixture, and employing ⁿBu₄NPF₆ as supporting electrolyte.

Scheme 72 Cyclization of alkynoates and alkynamides with diselenides.

Based on the results of control experiments and cyclic voltammetry experiments, a plausible mechanism is proposed in Scheme 73. Initially, the anodic oxidation of diphenyl diselenide generates the cationic radical intermediate A, which then divided into phenylselenium radical B and phenyl selenium cation C. The regioselectivity addition of radical B to the C–C triple bond of alkynoates provides vinyl radical D. Subsequently, vinyl radical D proceeds through an intramolecular cyclization to give E, which undergoes further anodic oxidation to give intermediate F. Finally, the 3-selenated coumarin product is produced via the deprotonation of intermediate F.

Scheme 73 Proposed mechanism.

In 2020, Guo and co-workers accomplished the electrochemical radical-initiated dearomative spirocyclization of alkynes and diselenides for the synthesis of spiro[4.5]trienones⁸⁵ with a broad scope and good functional group tolerance (Scheme 74).^86a This reaction was performed in an undivided cell equipped with a graphite anode and platinum cathode, using ⁿBu₄NPF₆ as electrolyte. Notable, N–H unsubstituted alkynamides also showed great tolerance to give the products in moderate yield. The reaction of alkynamide with ditelluride also proceeded smoothly, giving the desired tellurium substituted spiro[4.5]trienones in good yields. The scale-up reaction was successfully conducted with the assistance of an electrochemical continuous flow system. Later, Chen, Xu et al. also realized a similar electrochemical spirocyclization reaction of alkynamides with diphenyl selenium.^86b

Scheme 74 Electrochemical dearomative spirocyclization of alkynes and diselenides.

A possible reaction mechanism for the electrochemical transformation is proposed in Scheme 75. Firstly, diphenyl diselenide is oxidized to generate the cationic radical intermediate A, which then decomposed to give phenylselenium radical B and phenyl selenium cation C. Phenylselenium radical B undergoes radical addition of the C–C triple bond of alkynoates to produce vinyl radical D. Vinyl radical D tends to perform intramolecular spirocyclization to provide E due to the relative stability of the resonant free radical and the five-membered ring structure. Intermediate E then undergoes further oxidation at the anode to give oxygenium cation intermediate F. Finally, the product is generated through the demethylation and dearomatization sequence. Alternatively, the way that phenyl selenium cation C firstly reacts with alkynoates to form the intermediate G, which undergoes intramolecular nucleophilic attack of the electron-rich aromatic ring on the cyclic selenium cation to give intermediate F, cannot be ruled out.

Scheme 75 Proposed mechanism.

In 2021, Ruan et al. reported an electrochemical selenylation/cyclization of acrylamides with diselenides for the synthesis of selenium-containing oxindoles in a undivided cell equipped with a graphite plate anode and platinum plate cathode, employing nBu₄NPF₆ as electrolyte (Scheme 76).⁶⁵ In addition, this protocol was also extended to the transformation of N-arylcinnamamides to functionalized 3,4-dihydroquinolin-2(1H)-ones under a modified reaction conditions, which used a Pt plate as an anode and an Fe plate as a cathode under a slightly high temperature.

Scheme 76 Selenylation/cyclization of acrylamides with diselenides.

4. Aminochalcogenation of alkenes

In 2018, intramolecular electrochemical aminosulfenylation of alkenes has been investigated by Lei and co-workers (Scheme 77).²⁶ The primary aromatic or aliphatic amines, secondary aromatic amines, heteroarylamines, and amides all worked well under the standard conditions and furnished the corresponding products in moderate to good yields.

Scheme 77 Aminosulfenylation of alkenes.

In 2019, the same group reported an electrochemical aminoselenation of styrenes at room temperature without any acids, oxidants, or transition metals, which provides a green way for the synthesis of vicinal difunctionalized organoselenium compounds (Scheme 78).⁵³ Various amino sources such as benzotriazole, saccharin, pyrazole, and sulfonamide were amenable for this reaction, affording the corresponding β-amido selenides in good yields. Of note, secondary phenylalkyl amines can also act as a substrate to generate aminated products.

Scheme 78 Aminoselenation of styrenes.

In 2021, Sarkar et al. reported a green electro-oxidative tandem selenocyclization of thioallyl benzoimidazoles with diselenides (Scheme 79).^87a This protocol provided an efficient and convenient approach for producing diverse selenylated dihydro-benzoimidazothiazine derivatives via C–Se and C–N bond formation in an undivided cell, which exhibited excellent functional group tolerance and a broad substrate scope. This selenocyclization strategy could also be extended to construct benzoimidazo-[1,3]-thiazine through the annulation of thiopropergyl benzoimidazole. Very recently, Meng, Pan, Terent′ev and co-workers also explored a three-component electrochemical aminoselenation of 1,3-dienes with cyclic N-nucleophiles and diselenides to access selenium-containing allylazole skeletons.^87b

Scheme 79 Tandem selenocyclization of thioallyl benzoimidazoles with diselenides.

Imidates, derived from alcohol and nitrile, could serve as versatile amine sources in the alkene difunctionalization reaction.⁸⁸ In 2022, an electrochemical aminosulfonylation of N-alkenyl amidines with sodium sulfonates has been reported by Chen et al. (Scheme 80).⁸⁹ The electrolysis reaction, performed at a constant current of 4 mA using a graphite plate anode and platinum plate cathode, and ⁿBu₄NBF₆ as a supporting electrolyte at room temperature in a undivided cell, afforded the desired seven-membered tetrahydro-1,3-oxaze-pines in good to excellent yields. Of note, electron-rich 1,1-diaryl-substituted alkene substrates were competent reaction partners, delivering the tetrahydro-1,3-oxazepines smoothly.

Scheme 80 Electrochemical aminosulfonylation of N-alkenyl amidines with sodium sulfonates.

In 2023, Chen and co-workers reported a practical and efficient electrochemical intramolecular amino sulfonylation of internal alkenes with sodium sulfonate (Scheme 81).⁴⁵ Under undivided electrolytic cell conditions, a variety of sulfonylated N-heterocycles were efficiently prepared in high yields.

Scheme 81 Electrochemical amino sulfonylation of internal alkenes.

In 2023, Liu et al. reported a general and eco-friendly electrochemical aminoselenation of alkenes (Scheme 82).^73a This protocol features excellent chemoselectivity, ample substrate scope, and high functional group tolerance. Various alkenes including the challenging 1-aryl-1,3-dienes, and unactivated aliphatic alkenes were tolerated and gave the desired products in moderate to good yields.

Scheme 82 Aminoselenation of alkenes.

5. Halochalcogenation of alkenes

In 2021, Ye and co-workers reported an environmentally friendly and efficient electrochemical fluoro-sulfonylation of styrenes (Scheme 83).⁹⁰ A diverse array of β-fluorosulfones⁹¹ could be readily obtained from sulfonylhydrazides and triethylamine trihydrofluoride with a broad substrate scope under mild conditions. Notably, Et₃N-3HF acts as the fluorinating agent and electrolyte in this process due to its unique ionic properties.⁹² Besides arylsulfonyl hydrazides, heteroaryl hydrazides and aliphatic sulfonyl hydrazides were also applicable and gave the desired β-fluorosulfones in good yields. Unactivated olefins, such as cyclohexene or 1-octene, could not be tolerated under the standard conditions, probably due to the high reactivity of carbocations derived from unactivated olefins. The potential synthetic utility of this protocol was demonstrated, such as the facile defluorinative hydrogenation, dehydrofluorination, as well as the dehydrofluorination–lithiation–allylic substitution.

Scheme 83 Electrochemical fluoro-sulfonylation of styrenes.

The radical rearrangement experiment with (1-(2-phenylcyclopropyl)-vinyl)benzene resulted in the desired fluorosulfonylation and the corresponding ring-opened product was obtained, and the result of radical-trapping experiments all indicated that this electrochemical transformation may involves an arylsulfonyl radical. The mechanism of this electrochemical fluorosulfonylation of styrenes is proposed in Scheme 84. Firstly, the arylsulfonyl hydrazide undergoes anodic oxidations to form an arylsulfonyl radical A, along with the release of nitrogen. Then, radical A adds to the alkene to generate the benzyl radical B, which is further oxidized to intermediate C. Finally, the desired β-fluorosulfone is generated via the nucleophilic attack of C by Et₃N-3HF.

Scheme 84 Proposed mechanism.

In the same year, the Ye group also realized an electrochemical vicinal fluorosulfenylation and fluorosulfoxidation reactions of alkenes, thiophenol with triethylamine trihydrofluoride (Scheme 85).⁹³ Preliminary mechanistic investigations revealed that the fluorosulfenylation reaction proceeded through a radical-polar crossover mechanism involving a key episulfonium ion intermediate. Subsequent electrochemical oxidation of fluorosulfides to fluorosulfoxides was readily achieved under a higher applied potential with the adventitious H₂O in the reaction mixture.⁹⁴

Scheme 85 Vicinal fluorosulfenylation and fluorosulfoxidation reactions of alkenes.

In 2022, Wang and co-workers reported a selective electrochemical chlorosulfuration or chlorosulfoxidation of unactivated olefins and thiophenols (Scheme 86).⁹⁵ Evaluation of various chlorine sources revealed that HCl was optimal, and water in the HCl may have participated in the reaction. Interestingly, when the reaction time was increased to 22 h, the sulfoxide product was obtained in high yield. The similarity of the yields from the reaction under air and a reaction under O₂ indicated that sulfide was not oxidized by O₂. ¹⁸O-labeled experiments indicated that the oxygen atom in the product came from H₂O present in the concentrated HCl rather than from O₂.

Scheme 86 Chlorosulfuration or chlorosulfoxidation of olefins.

In 2022, Ye et al. explored an electrochemical β-chlorosulfoxidation of alkenes with readily available thiols and hydrochloride (Scheme 87).⁹⁶ This methodology has good functional group tolerance and leads to various β-chlorosulfoxides in good yields under mild conditions. It is worth mentioning that one equivalent of hydrochloric acid serves both as the chlorinating agent and as the redox mediator in the electrochemical sulfoxidation step.

Scheme 87 β-Chlorosulfoxidation of alkenes.

Ye and co-workers also reported an electrochemical fluoroselenation reaction of olefins with diaryl diselenides, and triethylamine trihydrofluoride in 2022 (Scheme 88).⁹⁷ This transformation was successfully applied for the construction of β-fluoroselenides by taking advantage of the ionic triethylamine trihydrofluoride both as a fluorinating agent and the electrolyte. This electrochemical protocol features mild conditions, high selectivity, good functional group tolerance and can be extended to the gram-scale synthesis of vicinal fluoroselenides.

Scheme 88 Electrochemical fluoroselenation reaction of olefins.

6. Dichalcogenation of alkenes and alkynes

In 2020, Wen and Guo et al. reported an electrochemical thiocyanothiocyclization of N-allylthioamides for preparing SCN-containing 2-thiazolines^98,99 with a broad substrate scope and good yields.¹⁰⁰ Isothiocyanato fused-thiazines were formed via a 6-endo-cyclization of cyclic thioamides (Scheme 89).Furthermore, the large-scale reactions and diversified transformations of the product demonstrate the practicality of this protocol.

Scheme 89 Electrochemical thiocyanothiocyclization of N-allylthioamides.

The results of radical-trapping experiments and radical-clock experiments excluded the formation of thiyl radical intermediates in this reaction. The cyclic voltammetry experiments indicated that NH₄SCN was easier to oxidize under electrochemical conditions. A possible mechanism for the tandem reaction is proposed in Scheme 90. Firstly, (SCN)₂ is generated from SCN anion via anode oxidation.¹⁰¹ Then, (SCN)₂ reacts with N-allylthioamide to give sulfonium intermediate A,¹⁰² which undergoes intramolecular regioselective nucleophilic ring-opening by thioamide and subsequently deprotonation to afford the thiazoline or thiazine. Meanwhile, the proton is reduced to hydrogen at the cathode.

Scheme 90 Proposed mechanism.

In 2021, Yang and Wang et al. reported an electrochemical sulfonylation/cyclization of 2-alkynylthioanisoles with sodium sulfonates to access sulfonated benzothiophenes (Scheme 91).¹⁰³ The reaction was conducted at a constant current of 2 mA using TBAB as the electrolyte in an undivided cell, in which RVC acted as the anode and a platinum plate acted as the cathode in the co-solvent of CH₃CN and H₂O. Notably, aliphatic sodium methanesulfinate was not amenable for this approach, probably because of the lack of stability of its sulfone radicals. Similarly, the aliphatic internal alkyne also could not provide the desired product.

Scheme 91 Sulfonylation/cyclization of 2-alkynylthioanisoles with sodium sulfonates.

Based on control experiments and cyclic voltammetric examination, a possible mechanism is illustrated in Scheme 92. Initially, TsNa is first oxidized at the anode to generate the Ts radical B. Then, the radical addition of B to the alkyne unit of 2-alkynylthioanisoles gives a vinyl radical C. The radical cation D is then formed via the intramolecular cyclization/oxidation of C. Subsequently, D undergoes further oxidation to give the intermediate cation E, which then produces the product and CH₃OH with hydroxyl anions through demethylation.

Scheme 92 Proposed mechanism.

In 2022, Huang and co-workers developed an electrochemical three-component thiocyanatosulfonylation of aryl acetylenes with sodium sulfinates and NH₄SCN (Scheme 93).¹⁰⁴ The reaction was carried out in a undivided cell equipped with a graphite anode and platinum cathode, employing LiClO₄ as an electrolyte with a constant potential current of 8 mA, and the thiocyanated vinylsulfones were obtained in moderate yields. The results of control experiments implied that the reaction involved a radical pathway. Moreover, cyclic voltammetry indicated that TsNa was easier to be oxidized at the anode.

Scheme 93 Thiocyanatosulfonylation of aryl acetylenes with sodium sulfinates and NH₄SCN.

A plausible mechanism was proposed as outlined in Scheme 94. Firstly, p-toluenesulfinate anion is oxidized preferentially to afford the sulfone radical at the anode. Then, vinyl sulfone radical A is generated via a radical addition of the sulfone radical to alkyne. Meanwhile, SCN radical is formed at the anode. Finally, the final product is generated through radical–radical coupling between radicals A and the SCN radical (pathway a). Besides, pathway b that involves the further oxidation of A, followed by the nucleophilic addition with SCN⁻, cannot be ruled out.

Scheme 94 Proposed mechanism for the reaction.

In 2020, Xu et al. reported an efficient electrochemical oxidative selenosulfonylation of alkynes to construct β-(seleno)vinyl sulfones (Scheme 95).¹⁰⁵ Interestingly, the alkyl alkynes are tolerated in this transformation, giving the desired product in slight lower yields. In addition, the alkyl sulfonyl hydrazides, as well as thiophene-containing sulfonyl hydrazides were also suitable for this transformation.

Scheme 95 Construction of β-(seleno)vinyl sulfones.

Meanwhile, Guo et al. described an electrochemical selenylation/cyclization of N-allylthioamides with diselenides. A series of selenium-containing 2-thiazolines were synthesized under mild conditions (Scheme 96).¹⁰⁶ The reaction exhibited a wide range of functional group tolerances, such as aryl substituted diselenides bearing electron-donating or withdrawing groups, and alkyl substituted diselenides also afforded the desired products in good yields. Notably, trifluoromethyl thioamides could also react with different substituted diselenides smoothly to efficiently produce trilfluoromethyl substituted thiazolines.

Scheme 96 Electrochemical selenylation/cyclization of N-allylthioamides with diselenides.

Recently, Dong et al. accomplished an electrochemical oxidation strategy to prepare β-selenylethyl dithiocarbamates from CS₂,¹⁰⁷ amines, alkenes, and diphenyl diselenides without oxidant or metal (Scheme 97).¹⁰⁸ Aliphatic alkenes, such as cyclohexene, cyclopentene, and allylbenzene, were well tolerated in in this transformation, giving the desired products in good yields. Cyclic secondary amines including pyrrolidine and piperidine are proved to be suitable substrates. However, primary amine was not compatible with the reaction.

Scheme 97 Preparation of β-selenylethyl dithiocarbamates.

7. Conclusions

Electrochemical difunctionalization of alkenes and alkynes via C–S/Se bond formation has emerged as a versatile and sustainable tool for the rapid synthesis of organochalcogens in the last decade. In this review, we have summarised the recent remarkable development of novel electrochemical methods for the construction of structurally diverse chalcogen-containing compounds via the oxychalcogenation, carbochalcogenation, aminochalcogenation, halochalcogenation, and dichalcogenation of alkenes or alkynes by the utilization of different chalcogen sources including sodium sulfinates, ammonium thiocyanate, sulfonylhydrazides, thiols, sulfinic acids, disulfide, ditelluride, and potassium metabisulfite. Despite the rapid development in this field, there are still a number of challenges to be addressed.

Firstly, there are few examples that do not use supporting electrolyte. It is highly desirable to develop low-cost reaction systems that reduce the use of electrolytes. Secondly, novel enantioselective electrochemical difunctionalization strategies should be established for the preparation of chiral organochalcogens. Thirdly, application of the developed reactions in the synthesis and modification of bioactive functional molecules should also be explored. In addition, the combination of electrochemical chalcogenation with the field of flow chemistry would be a fascinating research field in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22261026, 21807051), and Natural Science Foundation of Jiangxi Province (20202BABL203007).

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