Functional Reactions Facilitated by Carbon Electrodes

Abstract

This account highlights new strategies in electrochemical organic synthesis, focusing on Csp³–H/Csp²–H activation, cyclization, redox processes, and three-component reactions. Traditional organic synthetic methods encounter numerous challenges in these areas, including harsh reaction conditions, limited substrate scope, difficulties in selective control, and poor environmental sustainability. Electrochemical synthesis utilizes electrons as reagents to precisely control redox processes by accurate managing of the electrode potential, which provides a green and sustainable technique for organic synthesis. Our research team has been dedicated to the field of electrochemical organic synthesis for a long time, and has developed a series of innovative strategies that successfully enable the efficient synthesis of various high-value organic compounds. This account systematically summarizes our research findings in these areas, emphasizing design strategies, potential mechanisms, and prospective applications of the reactions. Our aim is to provide a valuable reference for researchers in the field of organic synthesis and to promote the further advancement of electrochemical organic synthesis technology.

1 Introduction

2 Activation of Csp³–H/Csp²–H Bonds

3 Electrochemical Cyclization

4 Electrochemical Redox Reactions

5 Outlook

Biographical Sketches

(from left to right) Xinyu Zhang graduated from the School of Chemistry and Chemical Engineering of Dezhou University with a B.S. degree in 2024. She is currently a master’s degree student in organic chemistry at Qingdao University of Science and Technology.

Ziyun Wang graduated from the School of Chemistry and Chemical Engineering of Liaocheng University with a B.S. degree in 2022. She is currently a master’s degree student in organic chemistry at Qingdao University of Science and Technology.

Jiaxiu Liu graduated from the School of Chemistry and Chemical Engineering of Qilu Normal University with a B.S. degree in 2022. He is currently a master’s degree student in organic chemistry at Qingdao University of Science and Technology.

Lirong Wen graduated from Beijing Normal University with a Master of Science degree in1992 and from Nanjing Tech University with a Doctor of Science degree in 2006. Her main research focuses on the synthetic methods and applications of biologically active organic nitrogen-, sulfur- and oxygen-containing heterocyclic compounds and drugs.

Ming Li graduated from Shaanxi Normal University with a Bachelor of Science degree in 1986, from Lanzhou University with a Master of Science degree in 1992, and from Nankai University with a Doctor of Science degree in 2004. He has been engaged in the design and synthesis of biologically active heterocyclic compounds and organic synthesis methodology for many years.

Weisi Guo received his B.S. and M.S. degrees from Qingdao University of Science & Technology. He obtained his Ph.D. in organic chemistry from Nankai University. In 2019, he joined Professor Jieping Zhu’s group as a postdoctoral fellow at EPFL, Switzerland. Currently, he is a professor at Qingdao University of Science & Technology, and focuses on developing new electrochemical methodologies for the synthesis of functionalized molecules.

Lin-Bao Zhang received his bachelor’s degree from Liaocheng University in 2010, and his master’s and Ph.D. (2016) degrees from Zhengzhou University under the supervision of Professor Maoping Song. He then joined Qingdao University of Science and Technology, where he is now an associate professor. His research interests are focused on organic synthetic chemistry.

Introduction

In the field of contemporary organic synthetic chemistry, the construction of chemical bonds is a crucial step in achieving diversity and complexity of molecules, which is of great significance for the advancement of drug discovery, materials science, and the total synthesis of natural products.[1]Among these, Csp³–H/Csp²–H activation, along with cyclization, oxidation, reduction and three-component reactions, serve as core reaction types that play important roles in the structural modification and functional regulation of organic molecules.[2] Traditional organic synthetic methods encounter numerous challenges in these bond-building reactions, including harsh reaction conditions, limited substrate scope, difficulties in selectivity control, and poor environmental sustainability.[3] For instance, certain C–N bond-forming reactions necessitate high temperatures and high pressures, and exhibit poor functional group compatibility with substrates.[4] Commonly employed methods for Csp³–H/Csp²–H activation may involve toxic reagents, which contradict the principles of green chemistry.[5] Additionally, cyclization reactions often struggle with accurately regulating selectivity, leading to complex and diverse arrays of products.[6] Furthermore, redox reactions frequently require excess amounts of oxidizing or reducing agents, resulting in low atom economy. Meanwhile, three-component reactions often suffer from low efficiency, resulting in poor yields of the target product. These limitations have driven chemists to continue to explore new synthetic methods and strategies to address the demands of modern chemical synthesis.[7]

As a very useful synthetic technology, electrochemical organic synthesis offers unique advantages and significant potential for the construction of chemical bonds.[8] This technique utilizes electrons as reagents and enables precise regulation of redox processes through accurate control of electrode potential, providing a green and sustainable approach to organic synthesis.[9] Electrochemical synthesis eliminates the need for large quantities of chemical oxidizing or reducing agents, thereby reducing waste generation and aligning with the modern trends of green chemistry.[10] Over the past few decades, the application of electrochemistry in organic synthesis has garnered extensive attention and has undergone in-depth research, particularly in Csp³–H/Csp²–H activation, as well as in cyclization, oxidation, reduction, and three-component reactions.[11]

Carbon electrodes play crucial roles in electrochemical synthesis due to their unique physical and chemical properties. Carbon materials, such as graphite and carbon felt, exhibit excellent electrical conductivity, which facilitates efficient electron transfer and promotes the progression of electrochemical reactions. This characteristic enables carbon electrodes to perform effectively in both oxidation and reduction reactions. For instance, in electrochemical desaturation and β-thiocyanation reactions, graphite rods used as working electrodes can significantly enhance the reaction efficiency and selectivity. Carbon electrodes exhibit good chemical stability in various solvents and under different reaction conditions, being resistant to corrosion and degradation. This stability allows them to be used for long periods in complex electrochemical environments. For example, in organic electrochemistry, carbon electrodes can remain stable in systems containing multiple organic solvents and electrolytes. The surfaces of carbon electrodes can be modified through physical or chemical methods to regulate their catalytic performance and reaction selectivity. For instance, surface functionalization or composite formation with other materials can further optimize the performance of carbon electrodes to meet the requirements of different electrochemical reactions. Carbon materials are widely available, relatively inexpensive, and do not introduce harmful metal impurities during use. Overall, this makes carbon electrodes an economical and environmentally friendly choice, being especially suitable for large-scale industrial applications.

In the field of electrochemical energy storage, carbon electrodes are essential components of supercapacitors and lithium-ion batteries, and their performance directly influences the energy density, cycle life, and safety of these energy storage devices. Additionally, carbon electrodes play a significant role in the realm of renewable energy; for instance, in perovskite solar cells, they can replace precious-metal electrodes, thereby reducing costs and enhancing device stability. Looking ahead, as materials science advances, carbon electrode materials are expected to evolve towards higher specific capacities, improved cycle stability, and faster charging performance. Furthermore, the development of environmentally friendly carbon electrodes is anticipated to garner increased attention. The carbon electrodes commonly used in laboratories primarily include carbon rods, carbon plates, carbon cloths, and graphite felts (Figure [1]).

Our group has long been dedicated to research in the field of electrochemical organic synthesis (Figure [2]). We have developed a series of innovative electrochemical synthesis strategies to address key challenges in Csp³–H/Csp²–H activation, as well as in cyclization, oxidation, reduction, and three-component reactions.[12] These challenges include low reactivity due to the high reduction potential of substrates, difficulties in controlling reaction selectivity, and the instability of intermediates.[13] By designing and optimizing the electrochemical reaction system, we have successfully achieved the efficient synthesis of a variety of high-value organic compounds, providing new ideas and methods for the advancement of organic synthetic chemistry.[14] In this account, we systematically present the results of our research in these areas, focusing on the design strategies of the reactions, the underlying mechanisms, and the potential applications. We aim to provide a valuable resource for researchers in this field and to promote advances in the development of electrochemical organic synthesis.

Activation of Csp³–H/Csp²–H Bonds

In modern chemical research, the activation and transformation of carbon–hydrogen (C–H) bonds have always been significant research topics in the field of organic synthesis. The direct functionalization of C–H bonds not only facilitates the efficient construction of molecular structures but also eliminates the cumbersome steps associated with functional group transformations in traditional synthetic methods. This advancement offers new strategies for green chemistry and sustainable synthesis. Among the various methods for C–H bond activation, electrochemical techniques have garnered considerable attention due to their environmentally friendly, efficient, atom-economical, and highly operable characteristics.

The electrochemical benzyl Csp³–H isothiocyanation method achieves highly selective and site-specific activation of Csp³–H bonds without the need for an external oxidant. Using 1-ethyl-4-methoxybenzene as the model substrate and reacting it with trimethylsilyl isothiocyanate (TMSNCS), the reaction conditions were optimized through systematic screening. Ultimately, in a mixed solvent of DCE/HFIP (3:1) at room temperature, with a constant current of 7 mA, and employing a graphite felt anode alongside a nickel cathode, the isothiocyanate product was successfully obtained (Scheme [1]).[15] For substrates containing both primary and secondary benzyl C–H bonds, the reaction selectively occurs at the more substituted carbon atom, yielding the corresponding α-tertiary isothiocyanate. Control experiments led to the proposal of a possible reaction mechanism. The substrate is initially oxidized to form a radical cation intermediate, which subsequently releases a proton to generate a benzyl radical. This radical undergoes further anodic oxidation to form a carbocation, which is then captured by the thiocyanate ion and isomerized in situ under electrolytic conditions to produce the isothiocyanate (Scheme [2]).

Similarly, selective thiocyanation of benzyl Csp³–H bonds is accomplished via an electrochemical reaction, which also leads to unique site selectivity. The results of experimental optimization indicate that when 1-ethyl-4-bromobenzene and trimethylsilyl isothiocyanate are used as substrates, the reaction in DCE as the solvent, utilizing a graphite felt anode, a platinum plate cathode, and nBu₄NClO₄ as the electrolyte at a constant current of 5 mA, yields the corresponding benzyl thiocyanate (Scheme [3]).[16] In substrate expansion experiments, ethylbenzenes with various halogen, electron-donating, or electron-withdrawing groups, as well as substrates with ortho- and meta-substituents, were smoothly converted into the corresponding thiocyanates. Mechanistic studies suggest that the reaction may proceed through a radical-polar crossover process, which differs from the mechanism described in the work of Liu and Chen.[15] The reaction first generates radical cation A through the anodic oxidation of the substrate, then releases a proton to form the benzyl radical intermediate, which is further oxidized at the anode to produce a carbon cation that is ultimately captured by the thiocyanate ion to generate the corresponding thiocyanate (Scheme [4]). This unique site selectivity suggests that the reaction may involve a nucleophilic addition process of the benzyl cation.

In addition, chemical selectivity and site-selective isothiocyanation of Csp³–H bonds in internal olefins has been achieved through electrochemical Csp³–H isothiocyanation via [3,3]-sigmatropic rearrangement (Scheme [5]).[17] Notably, the method demonstrates broad substrate compatibility for various internal alkenes and holds potential for post-synthetic isothiocyanation of bioactive molecules. Experimental and computational studies indicate that this reaction may proceed through an unexpected [3,3]-σ rearrangement process. The proposed mechanism involves the following steps. Initially, anodic oxidation of trimethylsilyl isothiocyanate (TMSNCS) in the electrochemical cell generates the SCN radical. Subsequent hydrogen atom transfer (HAT), in which the SCN radical selectively abstracts a hydrogen atom from the alkene, results in the formation of an aliphatic radical species. This aliphatic radical is subsequently captured by (SCN)₂ to yield an aliphatic isothiocyanate intermediate. Finally, a [3,3]-σ migration rearrangement process yields the isothiocyanate product (Scheme [6]).

In the electrochemical long-range Csp³–H thiocyanation reaction (Scheme [7]),[18] the remote thiocyanation of Csp³–H bonds is promoted by directly cleaving an N–H bond under electrolytic conditions to generate amide radicals with tetrabutylammonium acetate (TBAOAc) as the solvent, followed by a 1,5-hydrogen atom transfer process. This approach not only eliminates the need for metal catalysts and additional oxidants but also supports the radical-polarity crossing mechanism, as demonstrated by density functional theory (DFT) calculations and control experiments. Below is a brief summary of the reaction mechanism. First, the sulfonamide is deprotonated by the anion of hexafluoroisopropanol. The deprotonated compound is then anodized to produce an N radical. Subsequent 1,5-HAT of the N-radical produces the migrating C-radical, which is oxidized at the anode to form a carbocation. The carbocation is then captured by a thiocyanogen ion (SCN^–) to form the final thiocyanate product (Scheme [8]).

In an electrochemical regioselective Csp³–H etherification reaction, an electrochemical method is utilized for synthesizing functionalized 1-indenones. 1-Indone was selected as the model substrate in an unseparated cell, utilizing nBu₄NHSO₄ as the electrolyte and TFE as the solvent. The reaction was conducted at a constant current of 3 mA, employing graphite felt as the anode and nickel foam as the cathode, at room temperature for 4.5 hours to afford the target product (Scheme [9]).[19] By optimizing the reaction conditions, the researchers determined that the current intensity, the electrode material, and the electrolyte selection significantly influenced the reaction yield. Furthermore, the reaction was performed in an atmospheric environment without the need for inert gas protection. Mechanistic studies indicate that 1-indone is oxidized to an aryl radical cationic intermediate through a single-electron transfer (SET) process. Subsequently, this intermediate is further oxidized to a carbon-cationic intermediate via single-electron oxidation, after which an alcohol reacts with the intermediate to yield the target product (Scheme [10]).

In the electrochemical benzyl Csp³–H amination, the direct C–H/N–H cross-coupling between alkylenes and n-aminopyridine salts has been successfully realized. This method has good site selectivity, wide substrate compatibility, no redox reagent is required and the process is easy to scale-up (Scheme [11]).[20] The mechanism of this reaction involves initial anodic oxidation of the alkylaromatic substrate to generate a radical cation species. This species then releases a proton to form a benzyl radical. Further oxidation of the benzyl radical at the anode forms a benzyl cation, which is captured by the N-amino pyridinium salt to form a benzylamino pyridinium salt. Finally, through an electrochemical reduction strategy, the N–N bond in the benzylamino pyridinium salt is cleaved to generate the benzylamine product (Scheme [12]).

The efficient functionalization of BODIPY was achieved through an electrochemical C–H thiocyanation method. Using BODIPY and potassium thiocyanate (KSCN) as model substrates, reaction optimization was carried out in an undivided electrolytic cell. The role of potassium thiocyanate (KSCN) is to perform a nucleophilic attack on the cationic intermediate, resulting in the formation of the target product. Under the conditions of graphite felt as both the anode and cathode, with a mixed solvent of acetonitrile (MeCN) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and a constant current of 2 mA, the reaction was conducted for 2 hours at room temperature in a nitrogen atmosphere to afford the target product (Scheme [13]).[21] Further substrate expansion experiments demonstrated that this reaction exhibits good compatibility with various BODIPY substrates, including those with methyl, ethyl, trimethyl, and meta-2,6-chlorophenyl groups, as well as BODIPY derivatives containing halogen substituents (chlorine, bromine, iodine), all of which were smoothly converted into the corresponding thiocyanation products under the developed reaction conditions. Control experiments and cyclic voltammetry studies allowed a possible reaction mechanism to be proposed. BODIPY is first oxidized at the anode to form a cationic intermediate, which subsequently undergoes nucleophilic attack by KSCN to generate the target product, while hydrogen is reduced at the cathode (Scheme [14]).

In the study of the functionalization of BODIPY dyes, an environmentally friendly electrochemical method was developed to achieve the direct sulfonation of these dyes. By applying a constant current of 6.0 mA, the reaction was conducted under a nitrogen atmosphere at room temperature for 2.5 hours, utilizing graphite felt as both the anode and cathode, inexpensive NaCl as the electrolyte, and a mixed solvent of HFIP/MeNO₂/H₂O to successfully afford the target product (Scheme [15]).[16] Further experiments on substrate expansion demonstrated that BODIPY substrates with various substituents could be chemically sulfonated using p-toluenesulfonyl chloride to yield the corresponding monosulfonated BODIPYs. Additionally, this method also facilitated the synthesis of a disulfated BODIPY after optimizing the reaction conditions. The electrochemical sulfonation method is not only environmentally friendly and sustainable but also exhibits excellent substrate applicability and functional group tolerance, thus providing a practical alternative for the functionalization of BODIPY dyes, which are anticipated to demonstrate significant roles in the fields of bioimaging, fluorescent labeling, and solar cells. The mechanism for this process is shown in Scheme [16].

For the synthesis of 2,3-disubstituted quinoline derivatives, an electrochemical cascade cyclization approach has been established through the reaction of a benzoxazinone with aryl sulfonyl hydrazines under simple and mild conditions (Scheme [17]).[22] The reaction mechanism is briefly summarized as follows. Initially, the sulfonyl hydrazide is oxidized at the anode to form sulfonyl radicals, which subsequently participate in deprotonation, anodization, and release of nitrogen. Next, the sulfonyl free radicals react with benzene and benzene acetylene oxazine ketone (phenylethynylbenzoxazinone) substrates. The obtained intermediates undergo further oxidation at the anode, losing electrons to form carbocation intermediates. Finally, decarboxylation (loss of one hydrogen ion and one molecule of carbon dioxide) gives the target products. At the same time, a reduction reaction occurs at the cathode, and the resulting hydrogen ions gain electrons to form hydrogen gas (Scheme [18]).

Electrochemical Cyclization

For the synthesis of thiazoline-containing thiocyanates, the electrochemical regioselective cyclization of thiocyanide (SCN) with N-allyl thiamides can be achieved. In the absence of catalysts, additives, and oxidants, the reaction products are obtained with good yields and high regioselectivity, demonstrating excellent substrate compatibility (Scheme [19]).[23] At the anode, SCN⁻ ions are oxidized to form (SCN)₂, a critical step that initiates the subsequent cyclization reaction. The resulting (SCN)₂ reacts with the N-allyl thiamide to form a sulfide intermediate. During the intramolecular cyclization, the intermediate undergoes nucleophilic ring opening of the thiamide, followed by deprotonation, resulting in either a thiazoline or a thiazide. Simultaneously, a reduction reaction occurs at the cathode, where protons are reduced to hydrogen gas (Scheme [20]).

For the cyclization reaction between aromatics and alkynes, an electrochemically driven, Rh(III)-catalyzed regioselective cyclization was developed (Scheme [21]).[24] This method, which combines a rhodium catalyst with an electric current, eliminates the need for a large number of external oxidants, is conducted under mild green conditions, and exhibits a wide range of functional group compatibility for various substrates, including drugs and drug moieties. The mechanism of the reaction was studied by isotope-labeling experiments, kinetic isotope effect studies, cyclic voltammetry, and DFT calculations. Furthermore, its synthetic potential was validated through gram-scale experiments.

Isoquinolin-1(2H)-ones, which contain isooxazoline rings, were synthesized via an acyl radical reaction through intramolecular cyclization induced by electrochemical oxidation. The reaction mechanism was proposed based on control experiments and DFT calculations, suggesting that the reaction involves a radical process. In this process, acyl radicals are generated through anodic oxidation under electrochemical conditions, which is followed by cyclization to yield the target products (Scheme [22]).[25] The mechanism of this reaction involves an intramolecular cyclization process induced by electrochemical oxidation. Initially, ethanol is reduced at the cathode to form an ethoxy anion, which deprotonates the substrate to form an anion. The formed anion undergoes single-electron transfer through anodic oxidation, generating an N-centered radical. This radical participates in a cyclization reaction to form a vinyl radical, which undergoes a further cyclization to produce a delocalized radical. Finally, this radical undergoes rearomatization through electron oxidation to yield the target product (Scheme [23]).

In addition, an electrochemically facilitated [3+2] cycloaddition reaction has been developed for the synthesis of polycyclic aromatic hydrocarbons (PAHs). Specifically, in an undivided cell, we employed cost-effective graphite felt electrodes with Et₄NBF₄ as the electrolyte and a mixed solvent of hexafluoroisopropanol (HFIP) and tetrahydrofuran (THF) in a 4:1 ratio. The reaction was carried out at 60 °C under a constant current of 8 mA in an N₂ atmosphere for 2 hours, yielding the target product (Scheme [24]).[26] Mechanistic studies suggest that the reaction may involve radical intermediates. Cyclic voltammetry experiments revealed that the imidazo[1,2-a]pyridine first undergoes anodic oxidation with loss of an electron to form a radical cation. This radical cation then reacts with DABCO to generate an intermediate and DABCOH⁺. The intermediate subsequently reacts to form a radical, which undergoes intramolecular addition to create a new radical. This radical can directly generate an intermediate through anodic oxidation and ultimately undergoes deprotonation to yield the target product. This study not only provides a green and efficient method for synthesizing polycyclic heteroarenes but also offers new insights into the application of electrochemistry in the synthesis of complex molecules (Scheme [25]).

3-Thioalkylquinoline derivatives were synthesized under mild conditions. In these investigations, 4-phenylethynyl benzoxazinone and diphenyl disulfide served as model substrates. The synthesis of the target product was successfully achieved using a graphite felt electrode, a constant current of 16 mA, an N₂ atmosphere, and room temperature, with nEt₄NPF₆ as the electrolyte and MeNO₂ as the solvent (Scheme [26]).[27] Additionally, this reaction system demonstrated good tolerance for various functional groups, including methyl, halogen, methoxy, phenyl, and n-butyl on the benzene ring, as well as different substituents on the alkyne. Mechanistic studies suggest that the reaction may involve radical intermediates. Cyclic voltammetry experiments revealed that diphenyl disulfide first undergoes anodic oxidation with loss of an electron, forming an intermediate that subsequently generates phenylthio radicals and phenylthio cations. These radicals then react with the phenylethynyl benzoxazinone to form another intermediate, which is further oxidized at the anode to yield a carbocation intermediate. Finally, loss of a hydrogen ion and one molecule of carbon dioxide results in the formation of the target product (Scheme [27]).

Sulfur heterocyclic compounds were synthesized through electrochemical oxidation. Specifically, 3-oxo-3-phenyl-N-(p-methyl)propyl thiamine was selected as the model substrate, with 1,4-diazabicyclo[2.2.2]octane (DABCO) serving as the additive, nBu₄NBF₄ was utilized as the electrolyte, and MeCN was employed as the solvent, all at a constant current of 2.0 mA. Using graphite felt as both the anode and cathode, 2,5-dihydrothiophene compounds were obtained at room temperature in 2 hours (Scheme [28]).[28] The mechanistic study indicated that DABCO first generates the radical cation DABCO^+• through anodic oxidation, which then undergoes a hydrogen atom transfer process with the substrate to produce a radical intermediate. For the synthesis of dihydrothiophenes, the free-radical intermediate attacks the substrate to form a second intermediate, then a new third intermediate is formed through intramolecular cyclization, ultimately leading to the target product via a proton-transfer process (Scheme [29]). This study not only provides a green and efficient synthetic method for sulfur heterocycles but also offers a novel approach for the application of electrochemistry in the synthesis of complex molecules.

A novel and efficient method for synthesizing benzothiophene-1,1-dioxides under electrochemical conditions has been reported, involving the reaction of a sulfonyl hydrazide with internal alkynes (Scheme [30]).[29] This reaction proceeds through selective ipso-addition rather than ortho-attack, resulting in the formation of a quaternary spirocyclic intermediate, which is subsequently transformed into the target product via an S-migration process. During optimization of the reaction conditions, it was determined that the best results were achieved using a graphite felt electrode in an undivided electrolytic cell with constant current electrolysis, tetrabutylammonium hexafluorophosphate as the electrolyte, and a mixed solvent of HFIP/MeNO₂ (Scheme [31]). The yield and selectivity of the reaction were meticulously investigated by varying the current intensity, electrode material, electrolyte type, and solvent. Through DFT calculations, the researchers proposed a plausible reaction mechanism that includes the formation of sulfonyl radicals, addition to the alkyne, intramolecular cyclization, and S-migration steps. The computational results were consistent with the regioselectivity observed experimentally, providing theoretical support for understanding the intrinsic mechanism of the reaction. These electrochemical cyclization reactions offer a new, green, and efficient approach to organic synthesis and demonstrate significant potential in drug synthesis, materials science, and other fields.

Electrochemical Redox Reactions

Electrochemical oxidation plays a significant role in organic synthesis, offering a green and efficient method for the conversion of organic molecules. For the synthesis of flavones, the electrochemical α,β-dehydrogenation reaction is employed. This reaction utilizes nBu₄NHSO₄ as the electrolyte and is conducted in trifluoroethanol as the solvent within a system comprising a graphite felt anode and a nickel foam cathode. Without the need for metal catalysts or chemical oxidants, flavanones can be effectively transformed into a variety of flavones (Scheme [32]).[30] The mechanism of this reaction involves an electrochemically driven α,β-dehydrogenation process utilizing flavanones, azaflavanones, and thioflavanones as substrates. The substrate undergoes a hydrogen atom abstraction reaction with sulfate ions (SO₄ ^2–) at the anode, generating a carbanion intermediate. This intermediate loses an electron at the anode, forming a carbon radical intermediate, which is further oxidized at the anode, generating an α-carbocation intermediate. The α-carbocation intermediate reacts again with sulfate ions to form the final α,β-unsaturated ketone compound. Simultaneously, bisulfate ions (HSO₄ ^–) are reduced at the cathode, generating sulfate ions and releasing hydrogen gas. The interaction of the α-carbocation intermediate with the sulfate ions (SO₄ ²⁻) was crucial for forming the final α,β-unsaturated ketone compound through a hydrogen atom transfer (HAT) process (Scheme [33]). Alternatively, the reaction may proceed through a proton-coupled electron transfer pathway, which cannot be completely ruled out.

In the context of sulfide oxidation, a selective electrochemical oxidation method has been developed that utilizes NaCl as the electrolyte and redox medium, a mixed acetone/water solution as a green solvent, and a graphite felt electrode to successfully oxidize a sulfide to a sulfoxide at room temperature (Scheme [34]).[31] The mechanism of this reaction involves the electrochemical selective oxidation of sulfides to sulfoxides, with the specific steps as follows. The reaction takes place with sulfides as substrates in a mixed solvent (acetone/water). At the anode, the sulfides are oxidized to form sulfide radical cations through the action of chlorine radicals. The chlorine radicals then react with sulfide radical cations to form sulfides. At the cathode, OH is generated on the electrode surface through electrocatalytic dehydrogenation, which reacts with the sulfides to form further sulfides. Finally, the sulfoxide product is formed through a cathodic reduction process accompanied by the evolution of hydrogen. Overall, this reaction mechanism involves an electrochemical redox cycle, achieving the conversion of sulfides into sulfoxides through the generation and oxidation of chlorine radicals at the anode and the evolution of hydrogen at the cathode (Scheme [35]).

In electrochemical desaturation and β-thiocyanation reactions, the method achieves site-selective functionalization of a cycloamide without the need for an additional oxidant. Specifically, (1-phenylsulfonyl)piperidine was selected as the model substrate and reacted with trimethylsilyl isothiocyanate (TMSNCS). Electrolysis was conducted at a constant current of 7 mA in an unseparated cell, utilizing graphite rods as electrodes, nBu₄NPF₆ as the electrolyte, HCOOH as an additive, and TFE as the solvent. After reaction at room temperature for 5.5 hours, the target product was obtained (Scheme [36]).[32] Mechanistic studies indicate that the reaction may involve acrylamide intermediates. Specifically, the SCN anion generates thiocyanine (SCN)₂ through anodic oxidation, followed by the formation of SCN free radicals via homolysis. Concurrently, the substrate undergoes two successive single-electron oxidations and proton release to generate an acrylamide intermediate. Subsequently, the SCN radical adds to the acrylamide to form an intermediate, which is further oxidized to yield a carbocation intermediate. Finally, the target product is produced through a proton transfer process (Scheme [37]).

Electrochemical reduction reactions, as a sustainable chemical approach, play an increasingly significant role in organic synthesis. Recent years have witnessed a growing demand for sustainable chemical synthesis methods and electrochemical reduction reactions due to their high efficiency, selectivity, and eco-friendliness. Utilizing HFIP as a hydrogen donor, 1,4-diones were synthesized through electrochemical reduction without metal catalysts and external reducing agents (Scheme [38]).[33] In this process, 1,4-enediones are converted into 1,4-diones (Scheme [39]). This method exhibits excellent substrate compatibility and tolerates a variety of functional groups, including aromatic rings, heterocyclic rings, and alkyl groups. Furthermore, the reaction can be scaled up, providing an economical and green alternative for the synthesis of 1,4-diketones.[34] There is also an electrochemical method for the reduction of benzothiophene 1,1-dioxide (Scheme [40]). The reaction is carried out in the absence of a metal catalyst with HFIP as the hydrogen donor. By optimizing the reaction conditions, such as current density, solvent type, and electrolyte, the efficient reduction of benzothiophene 1,1-dioxide was successfully achieved (Scheme [41]). The method demonstrates good substrate compatibility and tolerates a wide range of functional groups, such as methyl, halogens, and aromatic groups. In addition, this reaction can also be performed on gram scale, further demonstrating its potential for practical applications.

In addition, a large-scale electrochemical reduction method for converting sulfur oxides into sulfides has been developed. This method, conducted under mild reaction conditions, utilizes substoichiometric AlCl₃ as a Lewis acid to activate the sulfur oxide, which is regenerated by combining an inexpensive aluminum anode with chloride ions. This deoxidation process demonstrates a broad range of substrate applicability, including acid-sensitive substrates and pharmaceutical compounds. Specifically, 1-bromo-4-(methylthio)benzene was selected as the model substrate in an unseparated battery, with an aluminum plate as the anode, a graphite felt electrode as the cathode, and a constant current of 10 mA at room temperature for 6 hours, to afford the corresponding sulfur ether (Scheme [42]).[35] By optimizing the reaction conditions, the researchers discovered that the amount of AlCl₃, the choice of electrode material, and the selection of electrolyte significantly influenced the reaction yield. Furthermore, the reaction can also be performed in an air atmosphere, but the yield is slightly decreased. A mechanistic study showed that AlCl₃ forms a complex with the sulfur oxide, which subsequently reduces the corresponding sulfide by breaking the S–O bond at the cathode (Scheme [43]).

In electrochemical deoxygenation reactions of alcohols, alkanes are formed through electrochemical reduction in the presence of AlCl₃. Specifically, propan-2-ol was selected as the model substrate, utilizing an unseparated cell with an aluminum plate as the anode, a tin plate as the cathode, and TBAClO₄ as the electrolyte, at a constant current of 15 mA in MeCN/EtOAc (3:1) at room temperature for 4 hours, resulting in the formation of propane (Scheme [44]).[36] By optimizing the reaction conditions, the researchers discovered that the amount of AlCl₃, the choice of electrode materials, and the electrolyte had significant effects on the reaction yield. Additionally, the reaction could be carried out in an air atmosphere, albeit the yield was significantly reduced. Mechanistic studies indicated that AlCl₃ forms a complex with the alcohol, which is subsequently reduced to a carbanion intermediate via a two-electron process at the cathode, ultimately yielding the target product through protonation (Scheme [45]).

In recent years, electrochemical three-component reactions have attracted extensive attention due to their high efficiency, high selectivity, and environmental friendliness. These reactions typically involve the cooperative reaction of three different reactants under electrochemical conditions to generate organic compounds with specific structures and functions.

The synthesis of 1,4-dicarbonyl Z-olefins via an electrochemical three-component coupling reactions has been reported (Scheme [46]).[37] This reaction employs thio-oxo ylides, alkynes, and water as reactants and proceeds without the need for metal catalysts or oxidants. Through electrochemical oxidation, thio-oxo ylides generate ylide radicals, which couple with alkynes to form alkyl radicals. These alkyl radicals are subsequently oxidized to alkyl cations, which then react with water to yield the target products (Scheme [47]). This method is notable for its simplicity, good functional group tolerance, and high Z-stereoselectivity, offering a novel approach for the synthesis of complex organic molecules. Additionally, an electrochemical oxidation for the direct dual-functionalization of alkynes to construct organic sulfates has been described (Scheme [48]).[38] This reaction uses sulfonyl hydrazides and alcohols as reactants and proceeds under electrochemical conditions. Through electrochemical oxidation, sulfonyl hydrazides generate sulfonyl radicals, which react with alkynes to form alkyl radicals. Subsequent oxidation of the alkyl radicals affords alkyl cations, which finally react with alcohols to produce organic sulfates (Scheme [49]). This method does not require metal catalysts or chemical oxidants and demonstrates good regioselectivity and stereoselectivity, providing a new route for the synthesis of biologically active organic molecules.

In addition, a novel electrochemical three-component method has been developed for the synthesis of sulfonyl sulfates (Scheme [50]).[39] The method directly difunctionalizes alkynes through electrochemical oxidation, eliminating the need for metal catalysts and chemical oxidants. Using a carbon rod as the anode and platinum as the cathode, sulfonyl alkenyl sulfates were successfully synthesized at room temperature (Scheme [51]). The reaction exhibits excellent substrate compatibility and tolerates a wide range of substituents, including methyl groups, halogens, and strong electron-withdrawing groups. Furthermore, this method demonstrated for the first time that the SO₄ ^2– ion could be used as a nucleophile in the preparation of organic sulfates, providing a new concept for electrochemical synthesis. The reaction mechanism was elucidated through control experiments and cyclic voltammetry. It is believed that the sulfonyl radical reacts with the alkyne to form an alkenyl radical, which is subsequently oxidized to generate an alkenyl cation that ultimately reacts with the sulfate ion to yield the target product. This study not only broadens the scope of electrochemical three-component reactions but also offers a new method for green chemical synthesis.

Outlook

In summary, our group has conducted a series of comprehensive studies on Csp³–H and Csp²–H activation, as well as cyclization, oxidation, reduction, and three-component reactions within the field of electrochemical organic synthesis, achieving significant progress. By developing innovative electrochemical strategies, we have successfully facilitated the efficient and environmentally friendly synthesis of various organic compounds. However, we also acknowledge that the field continues to face numerous challenges. For instance, despite some advancements in reaction selectivity, achieving high stereoselectivity and regioselectivity for certain complex substrates and reaction types remains a formidable challenge. Furthermore, for large-scale synthetic applications, electrochemical synthesis technology requires further optimization of the reaction conditions and equipment to enhance the efficiency and reproducibility of the reactions. Additionally, the development of novel electrode materials, electrolytes, and catalysts, along with a deeper understanding of the reaction mechanisms, are crucial areas for future research.

Conflict of Interest

The authors declare no conflict of interest.

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Corresponding Authors

Publikationsverlauf

Eingereicht: 06. März 2025

Angenommen nach Revision: 14. April 2025

Accepted Manuscript online:
14. April 2025

Artikel online veröffentlicht:
17. Juni 2025

© 2025. Thieme. All rights reserved

Georg Thieme Verlag KG
Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany

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