Electrochemical Transition-Metal-Catalyzed C-H Bond Functionalization: Electricity as Clean Surrogates of Chemical Oxidants

C-H Bond Functionalization of N-Heteroarenes Mediated by Selectfluor

Herein, recent developments for Selectfluor-mediated C-H functionalization of N-heteroarenes are described. This type of C-H bond activation is an attractive and competitive alternative to traditional methodologies, allowing the functionalization of a variety of chemical functions. In addition, Selectfluor is a more sustainable and economically accessible oxidant compared with expensive/toxic metals or hazardous peroxides. For a practical understanding, the current review classified systematically the reported strategies in four subsections as follows: (1) carbon-carbon formation, (2) carbon-nitrogen bond formation, (3) carbon-chalcogen bond, and (4) carbon-halogen bond formation. Mechanistic aspects and reaction conditions are fully discussed to provide an understanding of the aspects that govern C-H functionalization in N-heteroarenes mediated by Selectfluor.

Selective Electrochemical Halogenation of Functionalized Quinolone

This work describes an effective C3-H halogenation of quinoline-4(1H)-ones under electrochemical conditions, in which potassium halides serve as both halogenating agents and electrolytes. The protocol provides expedient access to different halogenated quinoline-4(1H)-ones with unique regioselectivity, broad substrate scope, and gram-scale synthesis employing convenient, environmentally friendly electrolysis, in an undivided cell. Mechanism studies have shown that halogen radicals can promote the activation of N-H bonds in quinolones.

Transition Metal-Catalyzed C-H Functionalization Through Electrocatalysis

Electrochemically promoted transition metal-catalyzed C-H functionalization has emerged as a promising area of research over the last few decades. However, development in this field is still at an early stage compared to traditional functionalization reactions using chemical-based oxidizing agents. Recent reports have shown increased attention on electrochemically promoted metal-catalyzed C-H functionalization. From the standpoint of sustainability, environmental friendliness, and cost effectiveness, electrochemically promoted oxidation of a metal catalyst offers a mild, efficient, and atom-economical alternative to traditional chemical oxidants. This Review discusses advances in the field of transition metal-electrocatalyzed C-H functionalization over the past decade and describes how the unique features of electricity enable metal-catalyzed C-H functionalization in an economic and sustainable way.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Electrification of a Milstein-type catalyst for alcohol reformation

Novel energy and atom efficiency processes will be keys to develop the sustainable chemical industry of the future. Electrification could play an important role, by allowing to fine-tune energy input and using the ideal redox agent: the electron. Here we demonstrate that a commercially available Milstein ruthenium catalyst (1) can be used to promote the electrochemical oxidation of ethanol to ethyl acetate and acetate, thus demonstrating the four electron oxidation under preparative conditions. Cyclic voltammetry and DFT-calculations are used to devise a possible catalytic cycle based on a thermal chemical step generating the key hydride intermediate. Successful electrification of Milstein-type catalysts opens a pathway to use alcohols as a renewable feedstock for the generation of esters and other key building blocks in organic chemistry, thus contributing to increase energy efficiency in organic redox chemistry.

Cathode enabled high faradaic efficiency: reduction of imines to amines with H2O as a H-source

Benefiting from a high overpotential of the competitive hydrogen evolution reaction with a carbon paper cathode, the desired electrochemical reduction of imines was achieved with high faradaic efficiency by using H₂O as a H-source. With this sustainable atom-economic strategy, a series of potentially versatile amines were obtained in medium-to-high yields (49-86%).

The Synthesis of α-Keto Acetals from Terminal Alkynes and Alcohols via Synergistic Interaction of Organoselenium Catalysis and Electrochemical Oxidation

Herein, an unprecedented electrochemical approach for the synthesis of α-keto acetals has been established from readily available terminal alkynes and alcohols. By merging the electrochemical and organoselenium-catalyzed processes, the desired products are obtained at room temperature in the absence of basic or metallic additives, with carbonyl and acetal motifs incorporated simultaneously across the triple bonds in a single operation.

A Convergent Paired Electrolysis Strategy Enables Cross-Coupling of Methylarenes with Imines

In this report, we have developed a metal-free convergent paired electrolysis strategy for α-benzyl amine synthesis from readily available imines and methylarenes, taking advantage of both anodic oxidation and cathodic...

Directing Group Enables Electrochemical Selectively Meta-Bromination of Pyridines under Mild Conditions

Without the use of catalysts and oxidants, a facile and sustainable electrochemical bromination protocol was developed. By introducing the directing groups, the regioselectivity of pyridine derivatives could be controlled at the meta-position utilizing the inexpensive and safe bromine salts at room temperature. A variety of brominated pyridine derivatives were obtained in 28-95% yields, and the reaction could be readily performed at a gram scale. By combining the installation and removing the directing group, the concept of meta-bromination of pyridines could be verified.

Acetonitrile and benzonitrile as versatile amino sources in copper-catalyzed mild electrochemical C-H amidation reactions

A mild, efficient electrochemical approach to the site-selective direct C–H amidation of benzene and its derivatives with acetonitrile and benzonitrile has been developed. It has been shown that joint electrochemical oxidation of various arenes in the presence of a copper salt as a catalyst and nitriles leads to the formation of N-phenylacetamide from benzene and N-benzylacetamides from benzyl derivatives (up to 78% yield). A favorable feature of the process is mild conditions (room temperature, ambient pressure, no strong oxidants) that meet the criteria of green chemistry.

Different pathways of C–H transformation depending on the substrate nature and oxidation potential.

Transition metal-catalyzed organic reactions in undivided electrochemical cells

Transition metal-catalyzed organic electrochemistry is a rapidly growing research area owing in part to the ability of metal catalysts to alter the selectivity of a given transformation. This conversion mainly focuses on transition metal-catalyzed anodic oxidation and cathodic reduction and great progress has been achieved in both areas. Typically, only one of the half-cell reactions is involved in the organic reaction while a sacrificial reaction occurs at the counter electrode, which is inherently wasteful since one electrode is not being used productively. Recently, transition metal-catalyzed paired electrolysis that makes use of both anodic oxidation and cathodic reduction has attracted much attention. This perspective highlights the recent progress of each type of electrochemical reaction and relatively focuses on the transition metal-catalyzed paired electrolysis, showcasing that electrochemical reactions involving transition metal catalysis have advantages over conventional reactions in terms of controlling the reaction activity and selectivity and figuring out that transition metal-catalyzed paired electrolysis is an important direction of organic electrochemistry in the future and offers numerous opportunities for new and improved organic reaction methods.

Electrochemical Oxidative C(sp3)-H/N-H Coupling of Diarylmethanes with Sulfoximines or Benzophenone Imine

Herein, we report an efficient electrochemical method for the synthesis of N-alkylated sulfoximines by electrochemical oxidative C(sp³)-H/N-H coupling of sulfoximines and diarylmethanes. In addition, we used the same conditions for electrochemical dehydrogenative amination of diarylmethanes with benzophenone imine as an aminating agent. The reactions showed good functional group tolerance and afforded the corresponding products in moderate to good yields without the use of a stoichiometric oxidant, a metal catalyst, or an activating agent.

Palladium-Catalyzed Dehydrogenative C-2 Alkenylation of 5-Arylimidazoles and Related Azoles with Styrenes

The construction of carbon–carbon bonds by direct involvement of two unactivated carbon–hydrogen bonds, without any directing group, ensures a high atom economy of the entire process. Here, we describe a simple protocol for the Pd(II)/Cu(II)-promoted intermolecular cross-dehydrogenative coupling (CDC) of 5-arylimidazoles, benzimidazoles, benzoxazole and 4,5-diphenylimidazole at their C-2 position with functionalized styrenes. This specific CDC, known as the Fujiwara–Moritani reaction or oxidative Heck coupling, also allowed the C-4 alkenylation of the imidazole nucleus when both 2 and 5 positions were occupied.

Palladium-Catalyzed Aromatic C-H Functionalizations Utilizing Electrochemical Oxidations

Transition-metal-catalyzed electrochemical C-H functionalizations have been extensively studied as atom- and step-economical clean methods in organic synthesis. In this account, we described our efforts on the palladium-catalyzed electrochemical C-H functionalizations, including C-H halogenations of arylpyridines and benzamide derivatives using HCl/HBr and I₂ as a halogen source, a one-pot process giving teraryls via the palladium-catalyzed electrochemical C-H iodination and subsequent Suzuki-Miyaura coupling, and an iodine-mediated oxidative homo-coupling reaction of arylpyridines.

Facilitating Ir-Catalyzed C-H Alkynylation with Electrochemistry: Anodic Oxidation-Induced Reductive Elimination

An electrochemical approach in promoting directed C-H alkynylation with terminal alkyne via iridium catalysis is reported. This work employed anodic oxidation of Ir(III) intermediate (characterized by X-ray crystallography) to promote reductive elimination, giving the desired coupling products in good yields (up to 95%) without the addition of any other external oxidants. This transformation is suitable for various directing groups with H2 as the only by-product, which warrants a high atom economy and practical oxidative C-C bond formation under mild conditions.

Ruthenium-Catalyzed Electrochemical Synthesis of Indolines through Dehydrogenative [3 + 2] Annulation with H2 Evolution

A dehydrogenative [3 + 2] annulation reaction of aniline derivatives and alkenes has been developed via the ruthenium-electron catalytic systems for the synthesis of versatile indolines. Electricity is used as a sustainable oxidant to regenerate the active Ru(II) catalyst and promote H₂ evolution. This protocol is ecofriendly and easy to handle as it uses a simple undivided cell in mild conditions without the employment of metal oxidants.

3d metallaelectrocatalysis for resource economical syntheses

Resource economy constitutes one of the key challenges for researchers and practitioners in academia and industries, in terms of rising demand for sustainable and green synthetic methodology. To achieve ideal levels of resource economy in molecular syntheses, novel avenues are required, which include, but are not limited to the use of naturally abundant, renewable feedstocks, solvents, metal catalysts, energy, and redox reagents. In this context, electrosyntheses create the unique possibility to replace stoichiometric amounts of oxidizing or reducing reagents as well as electron transfer events by electric current. Particularly, the merger of Earth-abundant 3d metal catalysis and electrooxidation has recently been recognized as an increasingly viable strategy to forge challenging C-C and C-heteroatom bonds for complex organic molecules in a sustainable fashion under mild reaction conditions. In this review, we highlight the key developments in 3d metallaelectrocatalysis in the context of resource economy in molecular syntheses until February 2020.

Tunable Electrochemical C-N versus N-N Bond Formation of Nitrogen-Centered Radicals Enabled by Dehydrogenative Dearomatization: Biological Applications

Herein, an environmentally friendly electrochemical approach is reported that takes advantage of the captodative effect and delocalization effect to generate nitrogen-centered radicals (NCRs). By changing the reaction parameters of the electrode material and feedstock solubility, dearomatization enabled a selective dehydrogenative C-N versus N-N bond formation reaction. Hence, pyrido[1,2-a]benzimidazole and tetraarylhydrazine frameworks were prepared through a sustainable transition-metal- and exogenous oxidant-free strategy with broad generality. Bioactivity assays demonstrated that pyrido[1,2-a]benzimidazoles displayed antimicrobial activity and cytotoxicity against human cancer cells. Compound 21 exhibited good photochemical properties with a large Stokes shift (approximately 130 nm) and was successfully applied to subcellular imaging. A preliminary mechanism investigation and density functional theory (DFT) calculations revealed the possible reaction pathway.

Electrochemical Ruthenium-Catalyzed C-H Hydroxylation of Amine Derivatives in Aqueous Acid

The development of an electrochemically driven, ruthenium-catalyzed C-H hydroxylation reaction of amine-derived substrates bearing tertiary C-H bonds is described. The reaction is performed under constant current electrolysis in a divided cell to afford alcohol products in yields comparable to those of our previously reported process, which requires the use of stoichiometric H₅IO₆ for catalytic turnover. With aqueous acid as solvent, the cathodic electrode reaction simply involves the reduction of protons to evolve hydrogen gas. The optimized protocol offers a convenient, efficient, and atom-economical method for sp³-C-H bond oxidation.

C-H Oxygenation Reactions Enabled by Dual Catalysis with Electrogenerated Hypervalent Iodine Species and Ruthenium Complexes

The catalytic generation of hypervalent iodine(III) reagents by anodic electrooxidation was orchestrated towards an unprecedented electrocatalytic C-H oxygenation of weakly coordinating aromatic amides and ketones. Thus, catalytic quantities of iodoarenes in concert with catalytic amounts of ruthenium(II) complexes set the stage for versatile C-H activations with ample scope and high functional group tolerance. Detailed mechanistic studies by experiment and computation substantiate the role of the iodoarene as the electrochemically relevant species towards C-H oxygenations with electricity as a sustainable oxidant and molecular hydrogen as the sole by-product. para-Selective C-H oxygenations likewise proved viable in the absence of directing groups.

Regioselective/electro-oxidative intermolecular [3 + 2] annulation for the preparation of indolines

Compared with the reported intramolecular electro-oxidative cyclization of alkenyl amines or vinyl anilines for the preparation of pyrrolidines or indolines, the intermolecular version is less studied. Herein, this electrochemical intermolecular oxidative annulation of anilines and alkenes for the preparation of indolines proceeded under external oxidant-free conditions. The most noteworthy achievement of our work is the facile generation of indolines with quaternary centers at the 2-position. In addition, alkenes and anilines bearing various functional groups can be well tolerated. Remarkably, electrolyte-free conditions were used in an electrochemical flow cell, which shows the application potential of this method.

Electrochemically generated N-iodoaminium species as key intermediates for selective methyl sulphonylimination of tertiary amines

This study presents a straightforward protocol for approaching N-sulphonylamidines via an electricity-driven, iodine-mediated cross dehydrogenative condensation (CDC) between sulphonamides and tertiary amines.

Electrosynthesis of polycyclic quinazolinones and rutaecarpine from isatoic anhydrides and cyclic amines

A direct decarboxylative cyclization between readily available isatoic anhydrides and cyclic amines was established to construct polycyclic fused quinazolinones employing electrochemical methods. This procedure was performed in an undivided cell without the use of a transition-metal-catalyst and external oxidant. A broad scope of polycyclic fused quinazolinones were obtained in moderate to good yields. Additionally, rutaecarpine was also prepared through our method in one step in good yield.

Polycyclic quinazolinones and rutaecarpine were synthesized from isatoic anhydrides and cyclic amines through an electrochemical method without an external oxidant and transition-metal-catalyst.

Rhodium(iii)-catalyzed C–H annulation of 2-acetyl-1-arylhydrazines with sulfoxonium ylides: synthesis of 2-arylindoles

An efficient Rh(iii)-catalyzed synthesis of 2-arylindole derivatives via intermolecular C–H annulation of arylhydrazines with sulfoxonium ylides was accomplished. A variety of 2-acetyl-1-arylhydrazines with sulfoxonium ylides were converted into 2-arylindoles in satisfactory yields. Excellent selectivity and good functional group tolerance of this transformation were also observed.

Rh(iii)-catalyzed intermolecular C–H annulation of arylhydrazines with sulfoxonium ylides for synthesis of 2-arylindole derivatives was well established.

Electrochemical sulfonylation of alkenes with sulfonyl hydrazides: a metal- and oxidant-free protocol for the synthesis of (E)-vinyl sulfones in water

An electrochemical sulfonylation of alkenes with sulfonyl hydrazides for the synthesis of (E)-vinyl sulfones in water is reported.

Metal-free benzoylation of imidazoheterocycles by oxidative decarboxylation of arylglyoxylic acids

A simple and straightforward approach has been realized for the direct benzoylation of imidazoheterocycles by oxidative decarboxylation of arylglyoxylic acids in the presence of K₂S₂O₈ as an oxidant.

The synthesis of sulfonated 4H-3,1-benzoxazines via an electro-chemical radical cascade cyclization

We achieved sulfonated 4H-3,1-benzoxazines under ambient conditions without any metals and external chemical oxidants via electrochemical radical cascade cyclizations.

Electrochemical Arylation of Electron-Deficient Arenes through Reductive Activation

An electrochemical method has been developed to achieve arylation of electron-deficient arenes through reductive activation. Various electron-deficient arenes and aryldiazonium tetrafluoroborates are amenable to this transformation within the conditions of an undivided cell, providing the desired products in up to 92 % yield. Instead of preparing diazonium reagents, these reactions can begin from anilines, and they can be carried out in one pot. Electron paramagnetic resonance studies indicate that cathodic reduction of quinoxaline occurs using the transformation. Moreover, cyclic voltammetry indicates that both quinoxaline and aryl diazonium salt have relatively low reduction potentials, which suggests they can be activated through reduction during the reaction.

Electrochemical Umpolung of Bromide: Transition-Metal-Free Bromination of Indole C⁻H Bond

A facile and sustainable electrochemical umpolung of bromide ion protocol was developed under mild reaction conditions. Transition metal catalysts and exogenous chemical oxidants were obviated for the bromination of C⁻H bond. Notably, graphite rod, which is commercially available at supermarkets and is inexpensive, was employed as the electrode material. This operationally easy and environmentally friendly approach accomplished the synthesis of 3-bromoindole in excellent yield and regioselectivity.

The Mn-catalyzed paired electrochemical facile oxychlorination of styrenes via the oxygen reduction reaction

Reported herein is the electrochemical engendering of chlorine radicals by a manganese catalyst with a controllable pattern, and inexpensive MgCl₂ as the chlorine source.