Organophotoelectrocatalytic C(sp2)-H alkylation of heteroarenes with unactivated C(sp3)-H compounds

An organophotoelectrocatalytic method for the C(sp2)-H alkylation of heteroarenes with unactivated C(sp3)-H compounds through dehydrogenation cross-coupling has been developed. The C(sp2)-H alkylation combines organic catalysis, photochemistry and electrochemistry, avoiding the need for external metal-reagents, HAT-reagents, and oxidants. This protocol exhibits good substrate tolerance and functional group compatibility, providing a straightforward and powerful pathway to access a variety of alkylated heteroarenes under green conditions.

Visible-Light-Promoted Deoxygenative Alkylation of Quinoxalin-2(1H)-ones with Activated Alcohols

A one-pot strategy for deoxygenative alkylation of alcohols with quinoxalin-2(1H)-ones was developed by using xanthate salts as alcohol-activating groups for radical generation in the presence of tricyclohexylphosphine under visible-light-promoted conditions. The remarkable features of this reaction include a broad substrate scope, excellent functional group tolerance, mild conditions, and simple operation. Moreover, the synthetic utility of this reaction was validated by the success of two-step one-pot reactions, scale-up synthesis, and chemoselective radical monodeoxygenation of diols.

Exploring an Electrochemical Route for Water-Enhanced Oxygenation Reactions Utilizing Nickel Molecular Structures: A Case Study

Recently, Ni molecular catalysis has been extensively applied in oxygenation reactions. This work is underpinned by the characterization techniques and the discovered instability of the Ni-bipyridine/phenanthroline system, which results in Ni (hydr)oxide production under oxidative conditions. The practical applications of this mechanism by employing a prepared Ni (hydr)oxide-based electrode specifically in the oxygenation of sulfides, achieving noteworthy yields in contrast to noncatalyst control experiments, are explored. Thus, a Ni (hydr)oxide-based material is proposed as a candidate for the true catalyst for sulfide oxidation in the presence of the Ni-bipyridine/phenanthroline system. The findings of this study are expected to stimulate discussion and encourage new viewpoints within the chemical community regarding the potential applications and mechanisms of molecular catalysts in oxidation reactions.

Direct alkylation of quinoxalinones with electron-deficient alkenes enabled by a sequential paired electrolysis

The metal-free alkylation of N-heterocycles with alkenes has remained a synthetic challenge. We report here the successful implementation of metal-free alkylation of quinoxalinones with electron-deficient alkenes enabled by a sequential paired electrolysis. This protocol provides a mechanistically distinct approach to prepare a variety of C-3 alkylated quinoxalinones that are otherwise quite difficult to synthesize by other means.

para-Selective, Direct C(sp2)-H Alkylation of Electron-Deficient Arenes by the Electroreduction Process

Direct para-selective C(sp²)-H alkylation of electron-deficient arenes based on the electroreduction-enabled radical addition of alkyl bromides has been developed under mild conditions. In the absence of any metals and redox agents, the simple electrolysis system tolerates a variety of primary, secondary, and tertiary alkyl bromides and behaves as an important complement to the directed alkylation of the C(sp²)-H bond and the classic Friedel-Crafts alkylation. This electroreduction process provides a more straightforward, environmentally benign, and effective alkylation method for electron-deficient arenes.

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.

Acid-Promoted Direct C-H Carbamoylation at the C-3 Position of Quinoxalin-2(1H)-ones with Isocyanide in Water

Described herein is a concise and practical direct amidation at the C-3 position of quinoxalin-2(1H)-ones through an acid-promoted carbamoylation with isocyanide in water. In this conversion, environmentally friendly water and commercial inexpensive isocyanide were used as a solvent and carbamoylation reagent, respectively. This study not only provides a green and efficient strategy for the construction of 3-carbamoylquinoxalin-2(1H)-one derivatives that can be applied to the synthesis of druglike structures but also expands the application of isocyanide in organic chemistry.

Copper-catalyzed C-3 benzylation of quinoxalin-2(1H)-ones with benzylsulfonyl hydrazides

An unprecedented use of benzylsulfonyl hydrazides as benzylating agents has been demonstrated in the direct C-3 benzylation of quinoxalin-2(1H)-ones. A range of benzylsulfonyl hydrazides participated in the C-3 benzylation of quinoxalin-2(1H)-ones with CuCN as the catalyst and DTBP as the oxidant, delivering structurally diverse 3-benzylquinoxalin-2(1H)-ones in moderate to good yields.

General electrochemical Minisci alkylation of N-heteroarenes with alkyl halides

Herein, we report, a general, facile and environmentally friendly Minisci-type alkylation of N-heteroarenes under simple and straightforward electrochemical conditions using widely available alkyl halides as radical precursors. Primary, secondary and tertiary alkyl radicals have been shown to be efficiently generated and coupled with a large variety of N-heteroarenes. The method presents a very high functional group tolerance, including various heterocyclic-based natural products, which highlights the robustness of the methodology. This applicability has been further proved in the synthesis of various interesting biologically valuable building blocks. In addition, we have proposed a mechanism based on different proofs and pieces of electrochemical evidence.

Diastereoselective addition of redox active esters to azomethine imines by electrosynthesis

Thanks to metal- and catalyst-free electrochemical conditions in an undivided cell, a series of readily available redox-active N-(acyloxy)phthalimide esters led to an efficient and highly stereoselective addition (85 : 15 to 95 : 5 dr) of putative radical species to chiral (racemic and enantioenriched) C5-substituted azomethine imines to provide an array of 31 polyaminated hydrazine derivatives as a single diastereoisomer.

Electrochemical-Promoted Nickel-Catalyzed Reductive Allylation of Aryl Halides

Compared with conventional reductive coupling, reductive coupling under electrochemical conditions without external reductants is greener, milder, and more efficient and is of increasing interest to organic chemists. In this work, we report the sacrificial anode, nickel-catalyzed electrochemical allylation reaction of aryl and alkyl halides. The reaction can be applied to a range of allylation reagents such as trifluoroalkenes, oxalates, and acetates.

Recent synthetic methods involving carbon radicals generated by electrochemical catalysis

Driven by a resurgence of interest in electrode-driven synthetic methods, this paper covers recent activity in the field of mediated electrochemical and photoelectrochemical bond activation, inclusive of C-H, C-C, C-N, and other C-heteroatom bonds.

Recent Advances in C(sp3)-C(sp3) and C(sp3)-C(sp2) Bond Formation through Cathodic Reactions: Reductive and Convergent Paired Electrolyses

The formation of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds is one of the major research goals of synthetic chemists. Electrochemistry is commonly considered to be an appealing means to drive redox reactions in a safe and sustainable fashion and has been utilized for C-C bond-forming reactions. Compared to anodic oxidative methods, which have been extensively explored, cathodic processes are much less investigated, whereas it can pave the way to alternative retrosynthetic disconnections of target molecules and to the discovery of new transformations. This review provides an overview on the recent achievements in the construction of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds via cathodic reactions since 2017. It includes electrochemical reductions and convergent paired electrolyses.

Phenanthrenequinone (PQ) catalyzed cross-dehydrogenative coupling of alkanes with quinoxalin-2(1H)-ones and simple N-heteroarenes under visible light irradiation

A direct and convenient strategy to 3-alkylquinoxalin-2(1H)-ones and other alkyl N-heteroarenes via a photocatalyzed alkylation of quinoxalin-2(1H)-ones and other N-heterocycles with commercially available, low-cost alkanes under ambient conditions using phenanthrenequinone (PQ) as a photocatalyst was developed. This transformation has advantages of environment-friendly protocol, mild conditions, good functional-group tolerance, and high yields of products.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Metal-Free Alkylation of Quinoxalinones with Aryl Alkyl ketones

The first metal-free method for alkylation of quinoxalinones using cheap and stable aryl alkyl ketones as nucleophilic alkylation reagents is reported. This strategy greatly broadens the application channels of aryl...

C–H benzylation of quinoxalin-2(1H)-ones via visible-light riboflavin photocatalysis

An efficient visible light promoted riboflavin-catalyzed direct benzylation of substituted quinoxalin-2(1H)-ones for the synthesis of various C3-benzylated quinoxalin-2(1H)-one derivatives has been developed under mild conditions.

Photo-Induced Cross-Dehydrogenative Alkylation of Heteroarenes with Alkanes under Aerobic Conditions

We report a Minisci-type cross-dehydrogenative alkylation in an aerobic atmosphere using abundant and inexpensive cerium chloride as a photocatalyst and air as an oxidant. This photoreaction exhibits excellent tolerance to functional groups and is suitable for both heteroarene and alkane substrates under mild conditions, generating the corresponding products in moderate-to-good yields. Our method provides an alternative approach for the late-stage functionalization of valuable substrates.

Aminomethylation of Aryl Bromides by Nickel-Catalyzed Electrochemical Redox Neutral Cross Coupling

We develop an electrochemical nickel-catalyzed aminomethylation of aryl bromides under mild conditions. The convergent paired electrolysis makes full use of anode and cathode processes, free of a terminal oxidant, a sacrificial anode, a metal reductant, and a prefunctionalized radical precursor. In addition, this method exhibits wide functional group tolerance (63 examples), including some sensitive substituents and aromatic heterocycles. This redox neutral cross coupling provides a more environmentally friendly and synthetic practical protocol for forging C(sp²)-C(sp³) bonds.

Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

Organic electrosynthesis has been widely used as an environmentally conscious alternative to conventional methods for redox reactions because it utilizes electric current as a traceless redox agent instead of chemical redox agents. Indirect electrolysis employing a redox catalyst has received tremendous attention, since it provides various advantages compared to direct electrolysis. With indirect electrolysis, overpotential of electron transfer can be avoided, which is inherently milder, thus wide functional group tolerance can be achieved. Additionally, chemoselectivity, regioselectivity, and stereoselectivity can be tuned by the redox catalysts used in indirect electrolysis. Furthermore, electrode passivation can be avoided by preventing the formation of polymer films on the electrode surface. Common redox catalysts include N-oxyl radicals, hypervalent iodine species, halides, amines, benzoquinones (such as DDQ and tetrachlorobenzoquinone), and transition metals. In recent years, great progress has been made in the field of indirect organic electrosynthesis using transition metals as redox catalysts for reaction classes including C-H functionalization, radical cyclization, and cross-coupling of aryl halides-each owing to the diverse reactivity and accessible oxidation states of transition metals. Although various reviews of organic electrosynthesis are available, there is a lack of articles that focus on recent research progress in the area of indirect electrolysis using transition metals, which is the impetus for this review.

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

Electrochemical-Promoted Nickel-Catalyzed Oxidative Fluoroalkylation of Aryl Iodides

This work describes a general strategy for metal-catalyzed cross-coupling of fluoroalkyl radicals with aryl halides under electrochemical conditions. The contradiction between anodic oxidation of fluoroalkyl sulfinates and cathodic reduction of low-valent nickel catalysts can be well addressed by paired electrolysis, allowing for direct introduction of fluorinated functionalities into aromatic systems.

Electrochemical Deoxygenative Thiolation of Preactivated Alcohols and Ketones

This work describes an electrochemically promoted nickel-catalyzed deoxygenative thiolation of alcohols and ketones under mild conditions. Excellent substrate tolerance and good chemical yields can be achieved by graphene/nickel foam electrodes in an undivided cell. Further study to gain mechanistic insight into this electrochemical cross-coupling has been carried out.

Electrocatalysis as an enabling technology for organic synthesis

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. Electrochemistry's unique ability to generate highly reactive radical and radical ion intermediates in a controlled fashion under mild conditions has inspired the development of a number of new electrochemical methodologies for the preparation of valuable chemical motifs. Particularly, recent developments in electrosynthesis have featured an increased use of redox-active electrocatalysts to further enhance control over the selective formation and downstream reactivity of these reactive intermediates. Furthermore, electrocatalytic mediators enable synthetic transformations to proceed in a manner that is mechanistically distinct from purely chemical methods, allowing for the subversion of kinetic and thermodynamic obstacles encountered in conventional organic synthesis. This review highlights key innovations within the past decade in the area of synthetic electrocatalysis, with emphasis on the mechanisms and catalyst design principles underpinning these advancements. A host of oxidative and reductive electrocatalytic methodologies are discussed and are grouped according to the classification of the synthetic transformation and the nature of the electrocatalyst.

Electrochemical Nozaki-Hiyama-Kishi Coupling: Scope, Applications, and Mechanism

One of the most oft-employed methods for C-C bond formation involving the coupling of vinyl-halides with aldehydes catalyzed by Ni and Cr (Nozaki-Hiyama-Kishi, NHK) has been rendered more practical using an electroreductive manifold. Although early studies pointed to the feasibility of such a process, those precedents were never applied by others due to cumbersome setups and limited scope. Here we show that a carefully optimized electroreductive procedure can enable a more sustainable approach to NHK, even in an asymmetric fashion on highly complex medicinally relevant systems. The e-NHK can even enable non-canonical substrate classes, such as redox-active esters, to participate with low loadings of Cr when conventional chemical techniques fail. A combination of detailed kinetics, cyclic voltammetry, and in situ UV-vis spectroelectrochemistry of these processes illuminates the subtle features of this mechanistically intricate process.

Electrochemical Generation and Use in Organic Synthesis of C-, O-, and N-Centered Radicals

During the last decade several research groups have been developing electrochemical procedures to access highly functionalized organic molecules. Among the most exciting advances, the possibility of using free radical chemistry has attracted the attention of the most important synthetic groups. Nowadays, electrochemical strategies based on these species with a synthetic purpose are published continuously in scientific journals, increasing the alternatives for the synthetic organic chemistry laboratories. Free radicals can be obtained in organic electrochemical reactions; thus, this review reassembles the last decade's (2010-2020) efforts of the electrosynthetic community to generate and take advantage of the C-, O-, and N-centered radicals' reactivity. The electrochemical reactions that occur, as well as the proposed mechanism, are discussed, trying to give clear information about the used conditions and reactivity of these reactive intermediate species.

Reaching the Full Potential of Electroorganic Synthesis by Paired Electrolysis

Electroorganic synthesis has recently become a rapidly blossoming research area within the organic synthesis community. It should be noted that electrochemical reaction is always a balanced reaction system with two half-cell reactions-oxidation and reduction. Most electrochemical strategies, however, typically focus on one of the two sides for the desired transformations. Paired electrolysis has two desirable half reactions running simultaneously, thus maximizing the overall margin of atom and energy economy. Meanwhile, the spatial separation between oxidation and reduction is the essential feature of electrochemistry, offering unique opportunities for the development of redox-neutral reactions that would otherwise be challenging to accomplish in a conventional reaction flask setting. This review discusses the most recent illustrative examples of paired electrolysis with special emphasis on sequential and convergent processes.

Advances in Electrochemical Decarboxylative Transformation Reactions

Owing to their non-toxic, stable, inexpensive properties, carboxylic acids are considered as environmentally benign alternatives as coupling partners in various organic transformations. Electrochemical mediated decarboxylation of carboxylic acid has emerged as a new and efficient methodology for the construction of carbon-carbon or carbon-heteroatom bonds. Compared with transition-metal catalysis and photoredox catalysis, electro-organic decarboxylative transformations are considered as a green and sustainable protocol due to the absence of chemical oxidants and strong bases. Further, it exhibits good tolerance with various functional groups. In this Minireview, we summarize the recent advances and discoveries on the electrochemical decarboxylative transformations on C-C and C-heteroatoms bond formations.

Nickel-catalyzed electrochemical reductive relay cross-coupling of alkyl halides with alkyl carboxylic acids

A highly regioselective Ni-catalyzed electrochemical (undivided cell) reductive relay cross-coupling between alkyl carboxylic acids and alkyl bromides has been developed.

Metal-free electrochemical synthesis of α-ketoamides via decarboxylative coupling of α-keto acids with isocyanides and water

A mild and convenient electrochemical protocol has been developed for the preparation of α-ketoamides via a decarboxylative coupling reaction of α-keto acids with isocyanides and water.

Recent progress on electrochemical synthesis involving carboxylic acids

Carboxylic acids are not only essential sections of medicinal molecules, natural products and agrochemicals but also basic building blocks for organic synthesis. However, high temperature, expensive catalysts and excess oxidants are normally required for carboxylic acid group transformations. Therefore, more eco-friendly and efficient methods are urgently needed. Organic electrochemistry, as an environmentally friendly and sustainable synthetic method, can potentially avoid the above problems and is favored by more and more organic chemists. This review summarized the recent progress on the electrochemical synthesis of carboxylic acids to construct more complex compounds, emphasizing the development of electrosynthesis methodologies and mechanisms in order to attract more chemists to recognize the importance and applications of electrochemical synthesis.

Harnessing Radical Chemistry via Electrochemical Transition Metal Catalysis

The merger of transition metal catalysis and electroorganic synthesis has recently emerged as a versatile platform for the development of highly enabling radical reactions in a sustainable fashion. Electrochemistry provides access to highly reactive radical species under extremely mild reaction conditions from abundant native functionalities. Transition metal catalysts can be used as redox-active electrocatalysts to shuttle electrons, chiral information to organic substrates, and the reactive intermediates in the electrolytic systems. The combination of these strategies in this mechanistic paradigm thus makes the generation and utilization of radical species in a chemoselective manner and allows further application to more synthetically attractive enantioselective radical transformations. This perspective discusses key advances over the past few years in the field of electrochemical transition metal catalysis and demonstrates how the unique features of this strategy permit challenging or previously elusive transformations via radical pathways to be successfully achieved.

Evolution of High-Valent Nickela-Electrocatalyzed C-H Activation: From Cross(-Electrophile)-Couplings to Electrooxidative C-H Transformations

C-H activation has emerged as one of the most efficient tools for the formation of carbon-carbon and carbon-heteroatom bonds, avoiding the use of prefunctionalized materials. In spite of tremendous progress in the field, stoichiometric quantities of toxic and/or costly chemical redox reagents, such as silver(I) or copper(II) salts, are largely required for oxidative C-H activations. Recently, electrosynthesis has experienced a remarkable renaissance that enables the use of storable, safe and waste-free electric current as a redox equivalent. While major recent momentum was gained in electrocatalyzed C-H activations by 4d and 5d metals, user-friendly and inexpensive nickela-electrocatalysis has until recently proven elusive for oxidative C-H activations. Herein, the early developments of nickela-electrocatalyzed reductive cross-electrophile couplings as well as net-redox-neutral cross-couplings are first introduced. The focus of this Minireview is, however, the recent emergence of nickel-catalyzed electrooxidative C-H activations until April 2020.