A Perspective on Organic Electrochemistry

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.

Electrochemical Multicomponent Synthesis of Alkyl Alkenesulfonates using Styrenes, SO2 and Alcohols

A novel electrochemical approach to access alkyl alkenesulfonates via a multicomponent reaction was developed. The metal-free method features easy-to-use SO2 stock solution forming monoalkylsulfites from alcohols with an auxiliary base in-situ. These intermediates serve a dual role as starting materials and as supporting electrolyte enabling conductivity. Anodic oxidation of the substrate styrene, radical addition of these monoalkylsulfites and consecutive second oxidation and deprotonation preserve the double bond and form alkyl β-styrenesulfonates in a highly regio- and stereoselective fashion. The feasibility of this electrosynthetic method is demonstrated in 44 examples with yields up to 81 %, employing various styrenes and related substrates as well as a diverse set of alcohols. A gram-scale experiment underlines the applicability of this process, which uses inexpensive and readily available electrode materials.

Synthesis of Aromatic N-Oxides Using Electrochemically Generated Peroxodicarbonate

Electrochemically generated green platform oxidizers like peroxodicarbonate (PODIC) constitute a game-changing technology in terms of sustainable chemistry while serving as an alternative counterreaction in the electrochemical hydrogen evolution. Peroxodicarbonate avoids the storage and shipping of concentrated hydrogen peroxide solution. We herein disclose an efficient method for the N-oxidation of quinolines, pyridines, and complex tertiary amines. The use of phenoyloxy succinimide (POSI) is the decisive factor for obtaining N-oxides (28 examples) in isolated yields of up to 98%.

Electrochemical labelling of hydroxyindoles with chemoselectivity for site-specific protein bioconjugation

Electrochemistry has recently emerged as a powerful approach in small-molecule synthesis owing to its numerous attractive features, including precise control over the fundamental reaction parameters, mild reaction conditions and innate scalability. Even though these advantages also make it an attractive strategy for chemoselective modification of complex biomolecules such as proteins, such applications remain poorly developed. Here we report an electrochemically promoted coupling reaction between 5-hydroxytryptophan (5HTP) and simple aromatic amines-electrochemical labelling of hydroxyindoles with chemoselectivity (eCLIC)-that enables site-specific labelling of full-length proteins under mild conditions. Using genetic code expansion technology, the 5HTP residue can be incorporated into predefined sites of a recombinant protein expressed in either prokaryotic or eukaryotic hosts for subsequent eCLIC labelling. We used the eCLIC reaction to site-specifically label various recombinant proteins, including a full-length human antibody. Furthermore, we show that eCLIC is compatible with strain-promoted alkyne-azide and alkene-tetrazine click reactions, enabling site-specific modification of proteins at two different sites with distinct labels.

Cathodically Coupled Electrolysis to Access Biheteroaryls

An electrochemical approach to biheteroaryls through the coupling of diverse N-heteroarenes with heteroaryl phosphonium salts is reported. The reaction features pH and redox-neutral conditions and excellent regioselectivity, as well as exogenous air or moisture tolerance. Additionally, a one-pot, two-step protocol can be established to realize formal C-H/C-H coupling of heteroarenes, thereby greatly expanding the substrate availability. The utility of this method is demonstrated through late-stage functionalization, the total synthesis of nitraridine, and antifungal activity studies.

Reductive cyclodimerization of chalcones: exploring the "self-adaptability" of galvanostatic electrosynthesis

The "self-adaptability" of galvanostatic electrolysis was shown to assist a multistage unprecedented chemo- and diastereoselective electrochemically promoted cyclodimerization of chalcones. The process, all involving the reductive events, delivered densely functionalized cyclopentanes featuring five contiguous stereocenters (25 examples, yields of up to 95%, dr values up to >20 : 1). Dedicated and combined experimental as well as electrochemical investigation revealed the key role of a dynamic kinetic resolution of the aldol intermediate for the reaction mechanism.

Electroreductive hydroxy fluorosulfonylation of alkenes

An electroreductive strategy for radical hydroxyl fluorosulfonylation of alkenes with sulfuryl chlorofluoride and molecular oxygen from air is described. This mild protocol displays excellent functional group compatibility, broad scope, and good scalability, providing convenient access to diverse β-hydroxy sulfonyl fluorides. These β-hydroxy sulfonyl fluoride products can be further converted to valuable aliphatic sulfonyl fluorides, β-keto sulfonyl fluorides, and β-alkenyl sulfonyl fluorides. Further, some of these products showed excellent inhibitory activity against Botrytis cinerea or Bursaphelenchus xylophilus, which could be useful for potent agrochemical discovery. Preliminary mechanistic studies indicate that this transformation is achieved through rapid O2 interception by the alkyl radical and subsequent reduction of the peroxy radical, which outcompete other side reactions such as chlorine atom transfer, hydrogen atom transfer, and Russell fragmentation.

Magnetic field-enhanced redox chemistry on-the-fly for enantioselective synthesis

Chemistry on-the-fly is an interesting concept, extensively studied in recent years due to its potential use for recognition, quantification and conversion of chemical species in solution. In this context, chemistry on-the-fly for asymmetric synthesis is a promising field of investigation, since it can help to overcome mass transport limitations, present for example in conventional organic electrosynthesis. Herein, the synergy between a magnetic field-enhanced self-electrophoretic propulsion mechanism and enantioselective redox chemistry on-the-fly is proposed as an efficient method to boost stereoselective conversion. We employ Janus swimmers as redox-active elements, exhibiting a well-controlled clockwise or anticlockwise motion with a speed that can be increased by one order of magnitude in the presence of an external magnetic field. While moving, these bifunctional objects convert spontaneously on-the-fly a prochiral molecule into a specific enantiomer with high enantiomeric excess. The magnetic field-enhanced self-mixing of the swimmers, based on the formation of local magnetohydrodynamic vortices, leads to a significant improvement of the reaction yield and the conversion rate.

Cobaltaelectro-Catalyzed C-H Annulation with Allenes for Atropochiral and P-Stereogenic Compounds: Late-Stage Diversification and Continuous Flow Scale-Up

The 3d metallaelectro-catalyzed C-H activation has been identified as an increasingly viable strategy to access valuable organic molecules in a resource-economic fashion under exceedingly mild reaction conditions. However, the development of enantioselective 3d metallaelectro-catalyzed C-H activation is very challenging and in its infancy. Here, we disclose the merger of cobaltaelectro-catalyzed C-H activation with asymmetric catalysis for the highly enantioselective annulation of allenes. A broad range of C-N axially chiral and P-stereogenic compounds were thereby obtained in good yields of up to 98% with high enantioselectivities of up to >99% ee. The practicality of this approach was demonstrated by the diversification of complex bioactive compounds and drug molecules as well as decagram scale enantioselective electrocatalysis in continuous flow.

Nickel-Catalyzed Electroreductive Coupling of Alkylpyridinium Salts and Aryl Halides

An electrochemical, nickel-catalyzed reductive coupling of alkylpyridinium salts and aryl halides is reported. High-throughput experimentation (HTE) was employed for rapid reaction optimization and evaluation of a broad scope of pharmaceutically relevant structurally diverse aryl halides, including complex drug-like substrates. In addition, the transformation is compatible with both primary and secondary alkylpyridinium salts with distinct conditions. Mechanistic insights were critical to enhance the efficiency of coupling using secondary alkylpyridinium salts. Systematic comparisons of the electrochemical and non-electrochemical methods revealed the complementary scope and efficiency of the two approaches.

Unveiling catalyst-free electro-photochemical reactivity of aryl diazoesters and facile synthesis of oxazoles, imide-fused pyrroles and tetrahydro-epoxy-pyridines via carbene radical anions

Herein, we report a reagent-less (devoid of catalyst, supporting electrolyte, oxidant and reductant) electro-photochemical (EPC) reaction [electricity (50 μA) and blue LED (5 W)] of aryl diazoesters to generate radical anions which are subsequently reacted with acetonitrile or propionitrile and maleimides to generate diversely substituted oxazoles, diastereo-selective imide-fused pyrroles and tetrahydroepoxy-pyridines in good to excellent yield. Thorough mechanistic investigation including a 'biphasic e-cell' experiment supports the reaction mechanism involving a carbene radical anion. The tetrahydroepoxy-pyridines could be fluently converted to fused pyridines resembling vitamin B₆ derivatives. The source of the electric current in the EPC reaction could be a simple cell phone charger. The reaction was efficiently scaled up to the gram level. Crystal structure, 1D, 2D NMRs and HRMS data confirmed the product structures. This report demonstrates a unique generation of radical anions via electro-photochemistry and their direct applications in the synthesis of important heterocycles.

Migration from Photochemistry to Electrochemistry for [2 + 2] Cycloaddition Reaction

Cyclobutane scaffolds are incorporated in several valuable natural and bioactive products. However, non-photochemical ways to synthesize cyclobutanes have scarcely been investigated. Herein, based on the principles of the electrosynthesis technique, we introduce a novel electrochemical approach for attaining cyclobutanes by a simple [2 + 2] cycloaddition of two electron-deficient olefins in the absence of photocatalysts or metal catalysts. This electrochemical strategy provides a suitable condition for synthesizing tetrasubstituted cyclobutanes with a variety of functional groups in good to excellent efficiency, compatible with gram-scale synthesis. In contrast to previous challenging methods, this approach strongly focuses on the convenient accessibility of the reaction instruments and starting materials for preparing cyclobutanes. Readily accessible and inexpensive electrode materials are firm evidence to prove the simplicity of this reaction. In addition, mechanistic insight into the reaction is obtained by investigation of the CV spectra of the reactants. Also, the structure of a product is identified by X-ray crystallography.

Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

Practical and General Alcohol Deoxygenation Protocol

Herein, we describe a practical protocol for the removal of alcohol functional groups through reductive cleavage of their benzoate ester analogs. This transformation requires a strong single electron transfer (SET) reductant and a means to accelerate slow fragmentation following substrate reduction. To accomplish this, we developed a photocatalytic system that generates a potent reductant from formate salts alongside Brønsted or Lewis acids that promote fragmentation of the reduced intermediate. This deoxygenation procedure is effective across structurally and electronically diverse alcohols and enables a variety of difficult net transformations. This protocol requires no precautions to exclude air or moisture and remains efficient on multigram scale. Finally, the system can be adapted to a one-pot benzoylation-deoxygenation sequence to enable direct alcohol deletion. Mechanistic studies validate that the role of acidic additives is to promote the key C(sp3 )-O bond fragmentation step.

Direct Electrochemical Synthesis of 2,3-Disubstituted Quinoline N-oxides by Cathodic Reduction of Nitro Arenes

The use of electric current in synthetic organic chemistry offers a sustainable tool for the selective reductive synthesis of quinoline N-oxides starting from easily accessible nitro compounds. The reported method employs mild and reagent-free conditions, a simple undivided cell, and constant current electrolysis set-up which provides conversion with a high atom economy. The synthesis of 30 differently substituted quinoline N-oxides was successfully performed in up to 90 % yield. Using CV studies, the mechanism of the selective formation of the quinoline N-oxides was elucidated. The technical relevance of the described reaction could be shown in a 50-fold scale-up reaction.

Access to Saturated Oxygen Heterocycles and Lactones via Electrochemical Sulfonylative Oxycyclization of Alkenes with Sulfonyl Hydrazides

A facile electrochemical sulfonylative cycloetherification of linear unsaturated alcohols with sulfonyl hydrazides under mild conditions has been accomplished. This catalyst- and oxidant-free protocol proceeds via electro-oxidation, followed by radical addition, as well as an intramolecular oxygen nucleophilic process. This methodology is compatible with a broad substrate scope and good functional group compatibility, which provides a valuable and convenient synthetic tool for the synthesis of saturated five-, six-, seven-, and eight-membered ring oxygen heterocycles. Furthermore, sulfonylative cycloesterification of linear unsaturated acids toward the lactone products has also been established under this electrochemical system. In addition, control experiments indicated that the N-H bonds of the sulfonyl hydrazide molecule are non-essential.

Electrochemical Oxidative Cleavage of Alkynes to Carboxylic Acids

A sustainable method for converting terminal alkynes into their corresponding carboxylic acids is reported using synthetic electrolysis in an undivided cell at room temperature. This protocol, avoiding transition metal catalysis and stoichiometric chemical oxidants, tolerates a variety of aryl, heteroaryl, and alkyl akynes. Preliminary mechanistic studies demonstrate that sodium nitrite serves a triple role as the electrolyte, nitryl radical precursor, and a nitrosating reagent.

Cobalt-electrocatalytic C-H hydroxyalkylation of N-heteroarenes with trifluoromethyl ketones

Trifluoromethyl carbinols and N-heteroarenes are both prevalent in bioactive molecules. However, access to high-value pharmacophores combining these two functional groups still remains a challenge. Herein, we report an electro-chemical redox-neutral coupling for the synthesis of N-heteroaryl trifluoromethyl carbinols from readily available N-heteroarenes and trifluoromethyl ketones. The reaction starts with reversing the polarity of ketones to nucleophilic ketyl radicals through an electrocatalytic proton-coupled electron transfer (PCET), followed by radical addition to heteroarenes and rearomatization to afford tertiary alcohol products. Importantly, the merging of paired electrolysis and cobalt catalysis is crucial to this regioselective C-H hydroxyalkylation of heteroarenes, and thus avoids several known competing pathways including the spin-center shift (SCS) process. Collectively, this protocol provides straightforward access to heteroaryl trifluoromethyl carbinols, featuring ideal atom economy, excellent regioselectivity, and paired redox-neutral electrolysis.

Electrochemical-Induced Cascade Reaction of 2-Formyl Benzonitrile with Anilines: Synthesis of N-Aryl Isoindolinones

An electrochemical initiated tandem reaction of anilines with 2-formyl benzonitrile has been developed. Thus, unprecedented 3-N-aryl substituted isoindolinones have been conveniently achieved by constant current electrolysis in a divided cell using catalytic amount of electricity and supporting electrolyte and a Pt-cathode as working electrode. The origin of the electrochemical activation as well as the mechanism of the subsequent chemical cascade reactions have been investigated by DFT calculations.

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%).

Electrosynthetic C-F bond cleavage

Fluorinated organic compounds are common among pharmaceuticals, agrochemicals and materials. The significant strength of the C-F bond results in chemical inertness that, depending on the context, is beneficial, problematic or simply a formidable synthetic challenge. Electrosynthesis is a rapidly expanding methodology that can enable new reactivity and selectivity for cleavage and formation of chemical bonds. Here, a comprehensive overview of synthetically relevant electrochemically driven protocols for C-F bond activation and functionalization is presented, including photoelectrochemical strategies.

Electroreductive coupling of 2-acylbenzoates with α,β-unsaturated carbonyl compounds: density functional theory study on product selectivity

The electroreductive coupling of 2-acylbenzoates with acrylonitrile in the presence of TMSCl and successive treatment with 1 M HCl gave 2-cyanonaphthalen-1-ols or 3-(3-cyanoethyl)phthalides. On the other hand, the reaction of 2-acylbenzoates with methyl vinyl ketone under the same conditions produced 3-(3-oxobutyl)phthalides as the sole products. What determines the product selectivity was studied using DFT calculations.

Regio- and Stereoselective Electrochemical Alkylation of Morita-Baylis-Hillman Adducts

Electrosynthesis is effectively employed in a general regio- and stereoselective alkylation of Morita–Baylis–Hillman compounds. The exposition of N-acyloxyphthalimides (redox-active esters) to galvanostatic electroreductive conditions, following the sacrificial-anode strategy, is proved an efficient and practical method to access densely functionalized cinnamate and oxindole derivatives. High yields (up to 80%) and wide functional group tolerance characterized the methodology. A tentative mechanistic sketch is proposed based on dedicated control experiments.

Electrochemical Synthesis of β-Keto Sulfonyl Fluorides via Radical Fluorosulfonylation of Vinyl Triflates

Electrochemical synthesis of versatile β-keto sulfonyl fluorides is accomplished by radical fluorosulfonylation of vinyl triflates with FSO₂Cl as the fluorosulfonyl radical source. This electroreductive protocol uses inexpensive graphite felt as electrodes, thus avoiding the use of a sacrificial anode. Moreover, this protocol, featuring metal-free, mild conditions and easy scalability, allows expedient access to valuable β-keto sulfonyl fluorides from readily available precursors, as well as the cyclic ones that are otherwise inaccessible using prior methods.

Electroediting of Soft Polymer Backbones

Synthetic methods that edit soft polymer backbones are critical technologies for tailoring the structures and properties of macromolecules. Developing strategies that leverage underexplored reaction manifolds are vital for accessing new chemical (and functional) space in soft materials. Here, we report a mild electrochemical approach that enables both degradation and functionalization of synthetic polymers. We found that bulk electrolysis (under either homogeneous or heterogeneous conditions) promoted facile, chemoselective chain scission in a variety of olefin-containing materials. Polymer degradation could also be coupled with functionalization (e.g., azidation) to afford new species that could serve as macromonomers.

A Unified Synthetic Strategy to Introduce Heteroatoms via Electrochemical Functionalization of Alkyl Organoboron Reagents

Based on systematic electrochemical analysis, an integrated synthetic platform of C(sp³)-based organoboron compounds was established for the introduction of heteroatoms. The electrochemically mediated bond-forming strategy was shown to be highly effective for the functionalization of sp³-hybridized carbon atoms with significant steric hindrance. Moreover, virtually all the nonmetallic heteroatoms could be utilized as reaction partners using one unified protocol. The observed reactivity stems from the two consecutive single-electron oxidations of the substrate, which eventually generates an extremely reactive carbocation as the key intermediate. The detailed reaction profile could be elucidated through multifaceted electrochemical studies. Ultimately, a new dimension in the activation strategies for organoboron compounds was accomplished through the electrochemically driven reaction development.

Kinetics-Based Approach to Developing Electrocatalytic Variants of Slow Oxidations: Application to Hydride Abstraction-Initiated Cyclization Reactions

Electrochemical oxidant regeneration is challenging in reactions that have a slow redox step because the steady-state concentration of the reduced oxidant is low, causing difficulties in maintaining sufficient current or preventing potential spikes. This work shows that applying an understanding of the relationship between intermediate cation stability, oxidant strength, overpotential, and concentration on reaction kinetics delivers a method for electrochemical oxoammonium ion regeneration in hydride abstraction-initiated cyclization reactions, resulting in the development of an electrocatalytic variant of a process that has a high oxidation transition state free energy. This approach should be applicable to expanding the scope of electrocatalysis to include additional slow redox processes.

Fe(iii)-catalysed selective C–N bond cleavage of N-phenylamides by an electrochemical method

An Fe(iii)-catalysed transformation of secondary N-phenyl substituted amides to primary amides by an electrochemical method is developed. Regioselective aryl C–H oxygenation occurs during the reaction, promoting selective C(phenyl)-N bond cleavage to form primary amides in yields of up to 92%.

An Fe(iii)-catalysed transformation of secondary N-phenyl substituted amides to primary amides by an electrochemical method is developed.

Organocopper(ii) complexes: new catalysts for carbon–carbon bond formation via electrochemical atom transfer radical addition (eATRA)

Electrochemical generation of a novel organocopper(ii) complex offers a new way to carry out atom transfer radical addition to alkenes under mild conditions with high yields and low catalyst loadings.

Electrochemical Oxo-Fluorosulfonylation of Alkynes under Air: Facile Access to β-Keto Sulfonyl Fluorides

Radical fluorosulfonylation is emerging as an appealing approach for the synthesis of sulfonyl fluorides, which have widespread applications in many fields, in particular in the context of chemical biology and drug development. Here, we report the first investigation of FSO₂ radical generation under electrochemical conditions, and the establishment of a new and facile approach for the synthesis of β-keto sulfonyl fluorides via oxo-fluorosulfonylation of alkynes with sulfuryl chlorofluoride as the radical precursor and air as the oxidant. This electrochemical protocol is amenable to access two different products (β-keto sulfonyl fluorides or α-chloro-β-keto sulfonyl fluorides) with the same reactants. The β-keto sulfonyl fluoride products can be utilized as useful building blocks in the synthesis of various derivatives and heterocycles, including the first synthesis of an oxathiazole dioxide compound. Furthermore, some β-keto sulfonyl fluorides and derivatives exhibited notably potent activities against Bursaphelenchus xylophilus and Colletotrichum gloeosporioides.

"How Should I Think about Voltage? What Is Overpotential?": Establishing an Organic Chemistry Intuition for Electrochemistry

Redox reactions are ubiquitous in organic synthesis and intrinsic to organic electrosynthesis. The language and concepts used to describe reactions in these domains are sufficiently different to create barriers that hinder broader adoption and understanding of electrochemical methods. To bridge these gaps, this Synopsis compares chemical and electrochemical redox reactions, including concepts of free energy, voltage, kinetic barriers, and overpotential. This discussion is intended to increase the accessibility of electrochemistry for organic chemists lacking formal training in this area.

Electrochemical Trifluoromethoxylation of (Hetero)aromatics with a Trifluoromethyl Source and Oxygen

Trifluoromethoxylated aromatics (ArOCF₃ ) are valuable structural motifs in the area of drug discovery due to the enhancement of their desired physicochemical properties upon the introduction of the trifluoromethoxy group (CF₃ O). Although significant progress has been made recently in the introduction of CF₃ O group into aromatics, current methods either require the use of expensive trifluoromethoxylation reagents or require harsh reaction conditions. We present a conceptually new and operationally simple protocol for the direct C-H trifluoromethoxylation of (hetero)aromatics by the combination of the readily available trifluoromethylating reagent and oxygen under electrochemical reaction conditions. This reaction proceeds through the initial generation of CF₃ radical followed by conversion to CF₃ O radical, addition to (hetero)aromatics and rearomatization. The utility of this electrochemical trifluoromethoxylation is illustrated by the direct incorporation of CF₃ O group into a variety of (hetero)aromatics as well as bio-relevant molecules.

Organic Sonochemistry: A Chemist's Timely Perspective on Mechanisms and Reactivity

Sonochemistry, the use of sound waves, usually within the ultrasonic range (>20 kHz), to boost or alter chemical properties and reactivity constitutes a long-standing and sustainable technique that has, however, received less attention than other activation protocols despite affordable setups. Even if unnecessary to underline the impact of ultrasound-based strategies in a broad range of chemical and biological applications, there is considerable misunderstanding and pitfalls regarding the interpretation of cavitational effects and the actual role played by the acoustic field. In this Perspective, with an eye on mechanisms in particular, we discuss the potentiality of sonochemistry in synthetic organic chemistry through selected examples of past and recent developments. Such examples illustrate specific controlling effects and working rules. Looking back at the past while looking forward to advancing the field, some essentials of sonochemical activation will be distilled.

Evolution towards green radical generation in total synthesis

The use of radicals as intermediates in total synthesis has evolved since their initial use in the latter half of the twentieth century. Radical generation from metal hydride methodologies has shifted to "greener" techniques including catalytic metal-mediated systems, electrochemical and photoredox-mediated processes. This review will focus on these classical and contemporary methods for radical generation and their applications in recent total syntheses.