Electrochemical Oxidative Cross-Coupling with Hydrogen Evolution Reactions

Oxidative cross-coupling has proved to be one of the most straightforward strategies for forming carbon-carbon and carbon-heteroatom bonds from easily available precursors. Over the past two decades, tremendous efforts have been devoted in this field and significant advances have been achieved. However, in order to remove the surplus electrons from substrates for chemical bonds formation, stoichiometric oxidants are usually needed. Along with the development of modern sustainable chemistry, considerable efforts have been devoted to perform the oxidative cross-coupling reactions under external-oxidant-free conditions. Electrochemical synthesis is a powerful and environmentally benign approach, which can not only achieve the oxidative cross-couplings under external-oxidant-free conditions, but also release valuable hydrogen gas during the chemical bond formation. Recently, the electrochemical oxidative cross-coupling with hydrogen evolution reactions has been significantly explored. This Account presents our recent efforts toward the development of electrochemical oxidative cross-coupling with hydrogen evolution reactions. (1) We explored the oxidative cross-coupling of thiols/thiophenols with arenes, heteroarenes, and alkenes for C-S bond formation. (2) Using the strategy of electrochemical oxidative C-H/N-H cross-coupling with hydrogen evolution, we successfully realized the C-H amination of phenols, anilines, imidazopyridines, and even ethers. (3) Employing halide salts as the green halogenating reagents, we developed a clean C-H halogenation protocol under electrochemical oxidation conditions. To address the limitation that this reaction had to carry out in aqueous solvent, we also developed an alternative method that uses CBr₄, CHBr₃, CH₂Br₂, CCl₃Br, and CCl₄ as halogenating reagents and the mixture of acetonitrile and methanol as cosolvent. (4) We also developed an approach for constructing C-O bonds in a well-developed electrochemical oxidative cross-coupling with hydrogen evolution manner. (5) Under mild external-oxidant-free electrochemical conditions, we realized the C(sp²)-H and C(sp³)-H phosphonylation with modest to high yields. (6) We successfully achieved the S-H/S-H cross-coupling with hydrogen evolution under electrochemical oxidation conditions. By anodic oxidation instead of chemical oxidants, the overoxidation of thiols and thiophenols was well avoided. (7) The methods for constructing structurally diverse heterocyclic compounds were also developed via the electrochemical oxidative annulations. (8) We have also applied the electrochemical oxidative cross-coupling with hydrogen evolution strategy to the alkenes difunctionalization for constructing multiple bonds in one step, such as C-S/C-O bonds, C-S/C-N bonds, C-Se/C-O bonds, and C-Se/C-N bonds. We hope our studies will stimulate the research interest of chemists and pave the way for the discovery of more electrochemical oxidative cross-coupling with hydrogen evolution reactions.

Transition-metal- and oxidant-free directed anodic C-H sulfonylation of N,N-disubstituted anilines with sulfinates

A new, practical directed anodic C-H sulfonylation of N,N-disubstituted anilines with sodium sulfinates for producing o- or p-amino arylsulfones and diarylsulfones is described. Employing the anodic strategy, the reaction proceeds efficiently under mild (room temperature) and transition-metal- and chemical oxidant-free conditions, and enables the formation of C-S bonds via directed activation of ortho- or para-C-H bond to the amino group with broad substrate scope and excellent site selectivity.

Isocyanide Reactions Toward the Synthesis of 3-(Oxazol-5-yl)Quinoline-2-Carboxamides and 5-(2-Tosylquinolin-3-yl)Oxazole

A palladium-catalyzed three-component reaction between 5-(2-chloroquinolin-3-yl) oxazoles, isocyanides, and water to yield 3-(oxazol-5-yl)quinoline-2-carboxamides is described. Interestingly, sulfonylation occurred when the same reaction was performed with toluenesulfonylmethyl isocyanide (TosMIC) as an isocyanide source. The reaction with 5-(2-chloroquinolin-3-yl)oxazoles and TosMIC in the presence of Cs2CO3 in DMSO afforded 5-(2-Tosylquinolin-3-yl)oxazoles. In basic media, TosMIC probably decomposed to generate Ts- species, which were replaced with Cl-. Tandem oxazole formation with subsequent sulfonylation of 2-chloroquinoline-3-carbaldehydes to form directly 5-(2-tosylquinolin-3-yl)oxazoles was also investigated.

Exogenous-oxidant-free electrochemical oxidative C-H phosphonylation with hydrogen evolution

We herein report a versatile and environmentally friendly electrochemical oxidative C-H phosphonylation protocol. This protocol features a broad substrate scope; not only C(sp2)-H phosphonylation, but also C(sp3)-H phosphonylation is tolerated well under exogenous-oxidant-free and metal catalyst-free electrochemical oxidation conditions.

Electrochemical Semipinacol Rearrangements of Allylic Alcohols: Construction of All-Carbon Quaternary Stereocenters

The first examples of electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols were developed using cheap and stable RSO₂Na (R = CF₃, Ph) as reagents. Various β-trifluoromethyl and sulfonated ketones were obtained in moderate to excellent yields. This strategy provides a facile, direct, and complementary approach to construct all-carbon quaternary stereocenters. In addition, the reaction has the advantages of being chemical oxidant-free and metal-free and has safe and mild reaction conditions.

Electrochemical Oxidative Clean Halogenation Using HX/NaX with Hydrogen Evolution

Organic halides (R-X) are prevalent structural motifs in pharmaceutical molecules and key building blocks for the synthesis of fine chemicals. Although a number of routes are available in the literature for the synthesis of organic halides, these methods often require stoichiometric additives or oxidants, metal catalysts, leaving or directing groups, or toxic halogenating agents. In addition, the necessity of employing different, often tailor-made, catalytic systems for each type of substrate also limits the applicability of these methods. Herein, we report a clean halogenation by electrochemical oxidation with NaX/HX. A series of organic halides were prepared under metal catalyst- and exogenous-oxidant-free reaction conditions. It is worth noting that this reaction has a broad substrate scope; various heteroarenes, arenes, alkenes, alkynes, and even aliphatic hydrocarbons could be applied. Most importantly, the reaction could also be performed on a 200-mmol scale with the same efficiency (86%, 50.9 g pure product).

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Metal catalyst free and exogenous oxidant free

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Commercially available, nontoxic, and green halogenating agents

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Broad substrate scope

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200-mmol-scale synthesis

Exogenous-oxidant- and catalyst-free electrochemical deoxygenative C2 sulfonylation of quinoline N-oxides

By employing quinoline N-oxides as the starting materials, the electrochemical C–H sulfonylation of electron-deficient quinolines was indirectly achieved at room temperature.

A metal- and oxidizing-reagent-free anodic para-selective amination of anilines with phenothiazines

A highly para-selective amination of anilines with phenothiazines for producing various functionalized 10-aryl-10H-phenothiazines is reported.

Electrochemical oxidative C–H/N–H cross-coupling for C–N bond formation with hydrogen evolution

We herein report an electrochemical oxidative C–H/N–H cross-coupling reaction in an undivided cell.

Electrochemically induced oxidative S–O coupling: synthesis of sulfonates from sulfonyl hydrazides and N-hydroxyimides or N-hydroxybenzotriazoles

A variety of sulfonates were synthesized from sulfonyl hydrazides and N-hydroxy compounds via electrochemically induced oxidative S–O bond formation.