Electrocatalytic Generation of Amidyl Radicals for Olefin Hydroamidation: Use of Solvent Effects to Enable Anilide Oxidation

Electrocatalytic Anaerobic Oxidation of Benzylic Amines Enabled by Ferrocene-Based Redox Mediators

The generation and functionalization of carbon- or nitrogen-centered radicals are of great interest for their potential synthetic utility. Here, we report the anaerobic electrocatalytic oxidation of two primary benzylic amines, benzylamine and 2-picolylamine, in the presence of a catalytic quantity of an electron deficient ferrocene derivative as a single-electron redox mediator. The use of the appropriate redox mediator prevented fouling of the electrode surface and significantly decreased the potential at which the catalytic oxidation reaction occurred. Simulation of the electrochemical results revealed an ErCi' catalytic process between the redox mediator and both substrates and significant difference in the electron transfer rate between the two substrates and electrochemically oxidized mediator. Through anaerobic controlled-potential electrolysis, we demonstrated a method with a Faradaic efficiency of 90% forming the desired coupled imine product of benzylamine oxidation while avoiding an excess of problematic overoxidation, hydrolysis, and other side reactions. Based on the electrochemical data along with the product analyses using IR and 1H and 13C NMR spectroscopies, the proposed mechanistic steps for the redox mediated electrocatalytic process were laid out.

Total synthesis of bicyclomahanimbine by Cu(ii)-promoted photoredox process

Since the isolation of carbazole alkaloids, the synthetic chemists have witnessed an upsurge in research of them due to their potential pharmacological properties. Our approach shows the total syntheses of five such biorelevant pyrano-[3,2a]-carbazole alkaloids, emphasizing biomimetic and innovative synthetic methodologies such as cascade reactions and strategic bond formations through sustainable electrochemical and photochemical conditions.

Electrochemical deoxygenative amination of stabilized alkyl radicals from activated alcohols

Alkylamine structures represent one of the most functional and widely used in organic synthesis and drug design. However, the general methods for the functionalization of the shielded and deshielded alkyl radicals remain elusive. Here, we report a general deoxygenative amination protocol using alcohol-derived carbazates and nitrobenzene under electrochemical conditions. A range of primary, secondary, and tertiary alkylamines are obtained. This practical procedure can be scaled up through electrochemical continuous flow technique.

Intramolecular Radical Amidooxygenation of Alkenes for the Construction of Pyrrolidinones

An intramolecular 1,2-amidooxygenation of unactivated alkenes for the construction of the pyrrolidinone scaffold containing a masked 5-hydroxymethyl functionality is reported. γ,δ-Unsaturated N-aryloxyamides react with sodium 2,2,6,6-tetramethylpiperidin-1-olate (TEMPONa) to afford alkoxyaminated pyrrolidinones. The cascade proceeds through reductive amidyl radical generation with TEMPONa, 5-exo cyclization, and TEMPO trapping. No transition metal is required to perform these transformations, and complex (fused, bridged) pyrrolidinones are accessible in moderate to good yields. The product alkoxyamines are readily further converted into ketones and alcohols through either oxidation or reduction.

Visible light mediated iron-catalyzed addition of oxamic acids to imines

Oxamic acids where shown to add to imines, providing a broad range of α-aminoacid amides in generally good yields. The process is efficient on pre-formed imines but may also be conducted using a 3-component strategy by simply mixing aldehydes, amines and oxamic acids in the presence of ferrocene, acting both as a photocatalyst under visible light and as a Lewis acid. The reaction proceeds through the addition onto the imine of a carbamoyl radical intermediate generated through a charge transfer from the carboxylate ligand to a Fe(iii) species (LMCT).

Electrochemical Ring-Opening of Cyclopropylamides with Alcohols toward the Synthesis of 1,3-Oxazines

An electrochemical method is presented to construct 1,3-oxazines by the oxidative ring-opening of cyclopropylamides with alcohols. This method avoids the use of external oxidants and thus shows good functional group tolerance. The substrate scope covers primary, secondary, and tertiary alcohols as well as (hetero)aryl amide-substituted cyclopropanes.

Ferrocene/air double-mediated FeTiO3-photocatalyzed semi-heterogeneous annulation of quinoxalin-2(1H)-ones in EtOH/H2O

With both ferrocene and air as the redox catalysts, for the first time, the low-cost natural ilmenite (FeTiO3) was successfully used for photocatalytic bond formations. Under the assistance of a traceless H-bond, and HCHO as the methylene reagent, a variety of imidazo[1,5-a]quinoxalinones were semi-heterogeneously photosynthesized in high yields with good functional group compatibility.

Photoelectrochemical oxidative C(sp3)-H borylation of unactivated hydrocarbons

Organoboron compounds are of high significance in organic synthesis due to the unique versatility of boryl substituents to access further modifications. The high demand for the incorporation of boryl moieties into molecular structures has witnessed significant progress, particularly in the C(sp3)-H borylation of hydrocarbons. Taking advantage of special characteristics of photo/electrochemistry, we herein describe the development of an oxidative C(sp3)-H borylation reaction under metal- and oxidant-free conditions, enabled by photoelectrochemical strategy. The reaction exhibits broad substrate scope (>57 examples), and includes the use of simple alkanes, halides, silanes, ketones, esters and nitriles as viable substrates. Notably, unconventional regioselectivity of C(sp3)-H borylation is achieved, with the coupling site of C(sp3)-H borylation selectively located in the distal methyl group. Our method is operationally simple and easily scalable, and offers a feasible approach for the one-step synthesis of high-value organoboron building blocks from simple hydrocarbons, which would provide ample opportunities for drug discovery.

Paraformaldehyde as C1 Synthon: Electrochemical Three-Component Synthesis of Tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones in Aqueous Ethanol

A green and practical method for the electrochemical synthesis of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones through the three-component reaction of quinoxalin-2(1H)-ones, N-arylglycines and paraformaldehyde was reported. In this strategy, EtOH played dual roles (eco-friendly solvent and waste-free pre-catalyst) and the in situ generated ethoxide promoted triple sequential deprotonations.

Electrocatalytic Access to Azetidines via Intramolecular Allylic Hydroamination: Scrutinizing Key Oxidation Steps through Electrochemical Kinetic Analysis

Azetidines are prominent structural scaffolds in bioactive molecules, medicinal chemistry, and ligand design for transition metals. However, state-of-the-art methods cannot be applied to intramolecular hydroamination of allylic amine derivatives despite their underlying potential as one of the most prevalent synthetic precursors to azetidines. Herein, we report an electrocatalytic method for intramolecular hydroamination of allylic sulfonamides to access azetidines for the first time. The merger of cobalt catalysis and electricity enables the regioselective generation of key carbocationic intermediates, which could directly undergo intramolecular C-N bond formation. The mechanistic investigations including electrochemical kinetic analysis suggest that either the catalyst regeneration by nucleophilic cyclization or the second electrochemical oxidation to access the carbocationic intermediate is involved in the rate-determining step (RDS) of our electrochemical protocol and highlight the ability of electrochemistry in providing ideal means to mediate catalyst oxidation.

Redox Mediators in Homogeneous Co-electrocatalysis

Homogeneous electrocatalysis has been well studied over the past several decades for the conversion of small molecules to useful products for green energy applications or as chemical feedstocks. However, in order for these catalyst systems to be used in industrial applications, their activity and stability must be improved. In naturally occurring enzymes, redox equivalents (electrons, often in a concerted manner with protons) are delivered to enzyme active sites by small molecules known as redox mediators (RMs). Inspired by this, co-electrocatalytic systems with homogeneous catalysts and RMs have been developed for the conversion of alcohols, nitrogen, unsaturated organic substrates, oxygen, and carbon dioxide. In these systems, the RMs have been shown to both increase the activity of the catalyst and shift selectivity to more desired products by altering catalytic cycles and/or avoiding high-energy intermediates. However, the area is currently underdeveloped and requires additional fundamental advancements in order to become a more general strategy. Here, we summarize the recent examples of homogeneous co-electrocatalysis and discuss possible future directions for the field.

Toward Improving the Selectivity of Organic Halide Electrocarboxylation with Mechanistically Informed Solvent Selection

The use of a liquid electrolyte is nearly ubiquitous in electrosynthetic systems and can have a significant impact on the selectivity and efficiency of electrochemical reactions. Solvent selection is thus a key step during optimization, yet this selection process usually involves trial-and-error. As a step toward more rational solvent selection, this work examines how the electrolyte solvent impacts the selectivity of electrocarboxylation of organic halides. For the carboxylation of a model alkyl bromide, hydrogenolysis is the primary side reaction. Isotope-labeling studies indicate the hydrogen atom in the hydrogenolysis product comes solely from the aprotic electrolyte solvent. Further mechanistic studies reveal that under synthetically relevant electrocarboxylation conditions, the hydrogenolysis product is formed via deprotonation of the solvent. Guided by these mechanistic findings, a simple computational descriptor based on the free energy to deprotonate a solvent molecule was shown to correlate strongly with carboxylation selectivity, overcoming limitations of traditional solvent descriptors such as pK_a. Through careful mechanistic analysis surrounding the role of the solvent, this work furthers the development of selective electrocarboxylation systems and more broadly highlights the benefits of such analysis to electrosynthetic reactions.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

Recent advances in electrooxidative radical transformations of alkynes

During the past few years, electrochemical oxidative reactions through radical intermediates have emerged as an environmentally-benign, powerful platform for the facile formation of C–E (E = C, N, S, Se, O and Hal) bonds through single-electron-transfer (SET) processes at the electrodes. Functionalized unsaturated molecules and unusual structural motifs can, for instance, be directly constructed under exceedingly mild reaction conditions through initial radical attack onto alkynes. This minireview highlights the recent advances in electrooxidation in radical reactions until June 2022, with a particular focus on radical additions onto alkynes.

Electro-/photocatalytic alkene-derived radical cation chemistry: recent advances in synthetic applications

Alkene-derived radical cations are versatile reactive intermediates and have been widely applied in the construction of complex functionalized molecules and cyclic systems for chemical synthesis. Therefore, the synthetic application of these alkene-derived radical cations represents a powerful and green tool that can be used to achieve the functionalization of alkenes partially because the necessity of stoichiometric external chemical oxidants and/or hazardous reaction conditions is eliminated. This review summarizes the recent advances in the synthetic applications of the electro-/photochemical alkene-derived radical cations, emphasizing the key single-electron oxidation steps of the alkenes, the scope and limitations of the substrates, and the related reaction mechanisms. Using electrocatalysis and/or photocatalysis, single electron transfer (SET) oxidation of the CC bonds in the alkenes occurs, generating the alkene-derived radical cations, which sequentially enables the functionalization of translocated radical cations to occur in two ways: the first involves direct reaction with a nucleophile/radical or two molecules of nucleophiles to realize hydrofunctionalization, difunctionalization and cyclization; and the second involves the transformation of the alkene-derived radical cations into carbon-centered radicals using a base followed by radical coupling or oxidative nucleophilic coupling.

Electrochemically driven regioselective C-H phosphorylation of group 8 metallocenes

Metallocenes are privileged backbones for synthesis and catalysis. However, the direct dehydrogenative C−H functionalization of unsymmetric metallocenes suffers from reactivity and selectivity issues. Herein, we report an electrochemically driven regioselective C−H phosphorylation of group 8 metallocenes. Mechanistic investigations indicate this dehydrogenative cross coupling occurs through an electrophilic radical substitution of the metallocene with a phosphoryl radical, facilitated by the metallocene itself. This work not only offers an efficient and divergent synthesis of phosphorylated metallocenes, but also provides a guide to interpret the reactivity and regioselectivity for the C−H functionalization of unsymmetric metallocenes.

Metallocene-based phosphines are compounds with potential use in catalysis. Here, the authors report the electrochemical regioselective functionalization of group 8 metallocenes with phosphine oxides; over 60 examples of phosphorylated (benzo)ferrocenes and ruthenocenes can be accessed via this method without the need for a preinstalled directing group.

Progress in the Electrochemical Reactions of Sulfonyl Compounds

Electrosynthesis has recently attracted more and more attention due to its great potential to replace chemical oxidants or reductants in molecule-electrode electron transfer. Sulfonyl compounds such as sulfonyl hydrazides, sulfinic acids (and their salts), sulfonyl halides have been discovered as practical precursors of several radicals. As electrochemical redox reactions can provide green and efficient pathways for the activation of sulfonyl compounds, studies for electrosynthesis have rapidly increased. Several types of radicals can be generated from anodic oxidation or cathodic reduction of sulfonyl compounds and can initiate fluoroalkylation, benzenesulfonylation, cyclization or rearrangement. In this Review, we summarize the electrosynthesis developments involving sulfonyl compounds mainly in the last decade.

Electrochemical Enantioselective Nucleophilic α-C(sp3)-H Alkenylation of 2-Acyl Imidazoles

Merging electrochemistry with asymmetric catalysis promises to provide an environmentally friendly and efficient strategy for the construction of nonracemic chiral molecules. However, in practice, significant challenges arise from the instability or incompatibility of the chiral catalysts under the electrochemical conditions at the interface of electrode and solution. Herein, we report a catalytic asymmetric indirect electrolysis employing the combination of a redox mediator and a chiral-at-rhodium Lewis acid, which achieves a previously elusive enantioselective nucleophilic α-C(sp³)-H alkenylation of ketones. Specifically, 2-acyl imidazoles react with potassium alkenyl trifluoroborates in high yields (up to 94%) and with exceptional enantioselectivities (27 examples with ≥99% ee) without the need for any additional stoichiometric oxidants (overall 40 examples). The new indirect electrosynthesis can be scaled to gram quantities and was applied to the straightforward synthesis of intermediates of the natural product cryptophycin A and a cathepsin K inhibitor.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Complementary Copper-Catalyzed and Electrochemical Aminosulfonylation of O-Homoallyl Benzimidates and N-Alkenyl Amidines with Sodium Sulfinates

A complementary copper-catalyzed and electrochemical aminosulfonylation of O-homoallyl benzimidates and N-alkenyl amidines with sodium sulfinates was developed. The terminal alkene substrate produced sulfone-containing 1,3-oxazines and tetrahydropyrimidines in the presence of Cu(OAc)₂, Ag₂CO₃, and DPP, and under similar reaction conditions, sulfonylated tetrahydro-1,3-oxazepines were prepared from 1-aryl-substituted O-homoallyl benzimidates in moderate to good yields. For certain electron-rich 1,1-diaryl-substituted alkene substrates, the corresponding tetrahydro-1,3-oxazepines could also be obtained in similar or even higher yields via a green electrochemical technique.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Mediated Inner-Sphere Electron Transfer Induces Homogeneous Reduction of CO2 via Through-Space Electronic Conjugation

The electrocatalytic reduction of CO₂ is an appealing method for converting renewable energy sources into value-added chemical feedstocks. We report a co-electrocatalytic system for the reduction of CO₂ to CO comprised of a molecular Cr complex and dibenzothiophene-5,5-dioxide (DBTD) as a redox mediator, which achieves high activity (TOF=1.51-2.84×10⁵  s^-1 ) and quantitative selectivity. Under aprotic or protic conditions, DBTD produces a co-electrocatalytic response with 1 by coordinating trans to the site of CO₂ binding and mediating electron transfer from the electrode with quantitative efficiency for CO. This assembly is reliant on through-space electronic conjugation between the π frameworks of DBTD and the bpy fragment of the catalyst ligand, with contributions from dispersive interactions and weak sulfone coordination.

Remote Selective Decarboxylative Difluoroarylmethylation of 8-Aminoquinolines under Transition Metal-Free Conditions

A facile transition metal-free decarboxylative C4 selective C-H difluoroarylmethylation of 8-aminoquinolines has been developed. This strategy proceeds under simple aqueous conditions and displays a broad substrate scope and excellent functional...

Rapid construction of γ-lactam containing 3,3-disubstituted oxindoles via a silver-catalyzed cascade radical bicyclization reaction

An efficient cascade bicyclization strategy for the construction of γ-lactam containing 3,3-disubstituted oxindole derivatives is described, which enables the sequential assembly of both unactivated and activated double bonds in one pot.

Silver-catalyzed cascade cyclization and functionalization of N-aryl-4-pentenamides: an efficient route to γ-lactam-substituted quinone derivatives

The synthesis of γ-lactam-substituted quinone derivatives through a Ag₂O-catalyzed cascade cyclization and functionalization of N-aryl-4-pentenamides has been developed. Related 2-oxazolidinone substituted quinone products can be also obtained with N-aryl allyl carbamates. The reactions proceed through an amidyl radical-initiated 5-exo-trig cyclization and followed radical addition to quinones. They provide an efficient route to various γ-lactam-substituted quinone derivatives with a wide range of substrate scope.

The synthesis of γ-lactam and related 2-oxazolidinone substituted quinone derivatives through a Ag₂O-catalyzed cascade cyclization and functionalization of N-ary-4-pentenamides and N-aryl allyl carbamates has been developed.

Anodic Oxidation of Aminotetrazoles: A Mild and Safe Route to Isocyanides

A new electrochemical method for the preparation of isocyanides from easily accessible aminotetrazole derivatives has been developed, which tolerates an unprecedented range of functional groups. The use of chemical, rather than electrochemical, oxidation to afford isocyanides was also demonstrated, which provides access to these compounds for those without electrosynthesis equipment. The practicality of scale-up using flow electrochemistry has been demonstrated, in addition to the possibility of using electrochemically generated isocyanides in further reactions.

Electrochemical Reductive Arylation of Nitroarenes with Arylboronic Acids

The synthesis of diarylamine is extremely important in organic chemistry. Herein, a novel electrochemical reductive arylation of nitroarenes with arylboronic acids was developed. A variety of diarylamines were synthesized without the need for transition-metal catalysts. The reaction could be scaled up efficiently in a flow cell and several derivatization reactions were carried out smoothly. Cyclic voltammetry experiments and mechanism studies showed that acetonitrile, formic acid, and triethyl phosphite all played a role in promoting this reductive arylation transformation.

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.

Time-Resolved EPR Revealed the Formation, Structure, and Reactivity of N-Centered Radicals in an Electrochemical C(sp3)-H Arylation Reaction

Electrochemical synthesis has been rapidly developed over the past few years, while a vast majority of the reactions proceed through a radical pathway. Understanding the properties of radical intermediates is crucial in the mechanistic study of electrochemical transformations and will be beneficial for developing new reactions. Nevertheless, it is rather difficult to determine the "live" radical intermediates due to their high reactivity. In this work, the formation and structure of sulfonamide N-centered radicals have been researched directly by using the time-resolved electron paramagnetic resonance (EPR) technique under electrochemical conditions. Supported by the EPR results, the reactivity of N-centered radicals as a mediator in the hydrogen atom transfer (HAT) approach has been discussed. Subsequently, these mechanistic study results have been successfully utilized in the discovery of an unactivated C(sp³)-H arylation reaction. The kinetic experiments have revealed the rate-determined step is the anodic oxidation of sulfonamides.

Electrochemical Tandem Olefination and Hydrogenation Reaction with Ammonia

An electrochemical Horner-Wadsworth-Emmons/hydrogenation tandem reaction was achieved using ammonia as electron and proton donors. The reaction could give two-carbon-elongated ester and nitrile from aldehyde or ketones directly. This reaction could proceed with a catalytic amount of base or even without a base. The ammonia provides both the electron and proton for this tandem reaction and enables the catalyst-free hydrogenation of an α,β-unsaturated HWE intermediate. More than 40 examples were reported, and functional groups, including heterocycles and hydroxyl, were tolerated.

Paired Electrolysis Enabled Ni-Catalyzed Unconventional Cascade Reductive Thiolation Using Sulfinates

Herein, we have reported a nickel-catalyzed cascade reductive thiolation of aryl halides with sulfinates driven by paired electrolysis. This protocol uses sulfinates as the sulfur source, and various thioethers could be synthesized under mild conditions. By mechanism exploration, we find that a cascade chemical step is allowed on the electrode interface and could alter the reaction pathway in paired electrolysis, whose findings could help the discovery of novel cascade reactions with unique reactivity.

Discovery of a tetraarylhydrazine catalyst for electrocatalytic synthesis of imidazo-fused N-heteroaromatic compounds

The development of electrocatalytic synthetic methods hinges on efficient molecular catalysts. Triarylamines are well-known redox catalysts because of the good stability of their corresponding amine radical cations. Herein we show that tris(4-(tert-butyl)phenyl)amine decomposes unexpectedly during electrolysis in MeOH/THF to afford a tetraarylhydrazine, 1,1,2,2-tetrakis(4-(tert-butyl)phenyl)hydrazine. In addition, we have applied this tetraarylhydrazine, which is either preprepared or formed in situ from tris(4-(tert-butyl)phenyl)amine, as an electrocatalyst for the synthesis of imidazopyridines and related N-heteroaromatic compounds through intramolecular [3 + 2] annulation. This metal-free electrocatalytic method provides straightforward access to the N-heteroaromatic compounds from readily available materials without the need for external chemical oxidants.

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.

Visible light bromide catalysis for oxazoline, pyrrolidine, and dihydrooxazine syntheses via Csp3-H functionalizations

A catalytic benzylic C_sp³-H functionalization protocol is described here. This visible light-mediated process is centered on the utilization of a bromide catalyst and oxidant to generate a nitrogen (N)-centered radical for a site-selective hydrogen atom transfer (HAT) process. This strategy enabled the unconventional syntheses of a number of N-heterocycles dependent on the amide identity. We also discovered a nucleophilicity-dependent kinetic resolution for stereochemical differentiation of C_sp³-H bonds that enabled the stereoselective synthesis of cis- and trans-oxazolines.

Recent advances in electrochemical oxidative cross-coupling with hydrogen evolution involving radicals

Oxidative cross-coupling has developed into a robust method for carbon-carbon (C-C), carbon-heteroatom (C-X), and heteroatom-heteroatom (X-Y) bond formation. Despite considerable advances in this field, the traditional oxidative cross-coupling reactions usually employ stoichiometric amounts of chemical oxidants to clean up surplus electrons from substrates to form new chemical bonds. Organic electrosynthesis is recognized as an environmentally benign and particularly powerful synthetic platform. Recent advancements have revealed that radical-involved electrochemical oxidative cross-coupling reactions can be achieved under exogenous-oxidant-free conditions. This tutorial review provides an overview of the most recent developments in electrochemical oxidative cross-coupling with hydrogen evolution involving radicals. Emphasis is mainly placed on synthetic and mechanistic aspects. We hope that this tutorial review can promote the development of radical chemistry, electrochemistry, and oxidative cross-coupling reactions.

Recent progress in the radical α-C(sp3)-H functionalization of ketones

The direct use structurally simple ketones as α-ketone radical sources for α-C(sp³)-H functionalization is a sustainable and powerful approach for constructing complex and multifunctional chemical scaffolds with diverse applications. The reactions of α-ketone radicals with alkenes, alkynes, enynes, imides, and imidazo[1,2-a]pyridines have broadened the structural diversity and complexity of ketones. Through chosen illustrative examples, we outline the recent progress in the development of methods that enable the radical α-C(sp³)-H functionalization of ketones, with an emphasis on radical initiation systems and possible mechanisms of the transformations. The application of these strategies is illustrated by the synthesis of several biologically active molecules and drug molecules. Further subdivision is based on substrate type and reaction type.

Structurally Diverse Synthesis of Five-, Six-, and Seven-Membered Benzosultams through Electrochemical Cyclization

We have developed a metal- and oxidant-free approach to structurally diverse synthesis of benzosultams from aryl sulfonamides through an electrochemical cyclization. Upon variation of the ortho substituent on aryl sulfonamides, five-, six-, and seven-membered benzosultams were efficiently assembled in an atom- and resource-economic manner. The generality of the process is demonstrated by the formation of five- to seven-membered cyclic products from 42 substrates bearing substituents with different electronic effects and steric hindrance.

Electrooxidative dearomatization of biaryls: synthesis of tri- and difluoromethylated spiro[5.5]trienones

Radical spirocyclization via dearomatization has emerged as an attractive strategy for the rapid synthesis of structurally diverse spiro molecules. We report the use of electrochemistry to perform an oxidative dearomatization of biaryls leading to tri- and difluoromethylated spiro[5.5]trienones in a user friendly undivided cell set-up and a constant current mode. The catalyst- and chemical oxidant-free dearomatization procedure features ample scope, and employs electricity as the green and sole oxidant.

Electrocatalytic Dehydrogenative Cyclization of 2-Vinylanilides for the Synthesis of Indoles

Indole is prevalent in bioactive compounds and natural products. The development of efficient and sustainable methods to access this privileged structural scaffold has been a long-standing interest of synthetic chemists. Herein, we report an electrocatalytic method for the synthesis of indoles through dehydrogenative cyclization of 2-vinylanilides. The reactions employ an organic redox catalyst and do not require any external chemical oxidant, providing speedy and efficient access to 3-substituted and 2,3-disubstituted indoles.

Tailored cobalt-salen complexes enable electrocatalytic intramolecular allylic C-H functionalizations

Oxidative allylic C–H functionalization is a powerful tool to streamline organic synthesis as it minimizes the need for functional group activation and generates alkenyl-substituted products amenable to further chemical modifications. The intramolecular variants can be used to construct functionalized ring structures but remain limited in scope and by their frequent requirement for noble metal catalysts and stoichiometric chemical oxidants. Here we report an oxidant-free, electrocatalytic approach to achieve intramolecular oxidative allylic C–H amination and alkylation by employing tailored cobalt-salen complexes as catalysts. These reactions proceed through a radical mechanism and display broad tolerance of functional groups and alkene substitution patterns, allowing efficient coupling of di-, tri- and even tetrasubstituted alkenes with N- and C-nucleophiles to furnish high-value heterocyclic and carbocyclic structures.

Oxidative allylic C–H functionalizations minimise the need for functional group activation and generate alkenyl-substituted products amenable to further chemical modifications. Here the authors report an oxidant-free, electrocatalytic approach to achieve intramolecular oxidative allylic C–H amination and alkylation by employing tailored cobalt-salen complexes as catalysts.

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.

Catalyst- and Reagent-Free Formal Aza-Wacker Cyclizations Enabled by Continuous-Flow Electrochemistry

The development of efficient and sustainable methods to access saturated N-heterocycles is of great importance because of the prevalence of these structures in natural products and bioactive compounds. Pd-catalyzed aza-Wacker type cyclization is a powerful method and provides access to N-heterocycles bearing an alkene moiety available for further synthetic manipulations from readily available materials. Herein we disclose a catalyst- and reagent-free formal aza-Wacker type cyclization reaction for the synthesis of functionalized saturated N-heterocycles. Key to the success is to conduct the reactions in a continuous-flow electrochemical reactor without adding supporting electrolyte or additives. The reactions are characterized by broad tolerance of di-, tri- and tetrasubstituted alkenes.