Electrochemical Decarboxylative Sulfonylation of Cinnamic Acids with Aromatic Sulfonylhydrazides to Vinyl Sulfones

Electrochemical Decarboxylation Coupling Reactions

Carboxylic acids are attractive synthetic feedstocks with stable, non-toxic, and inexpensive properties that can be easily obtained from natural sources or through synthesis. Carboxylic acids have long been considered environmentally friendly coupling agents in various organic transformations. In recent years, electrochemically mediated decarboxylation reactions of decarboxylic acids and their derivatives (NHPI) have emerged as effective new methods for constructing carbon-carbon or carbon-heterocarbon chemical bonds. Compared with transition metal and photochemistry-mediated catalytic reactions, which do not require the addition of oxidants and strong bases, electrochemically-mediated decarboxylative transformations are considered a sustainable strategy. In addition, various functional groups tolerate the electrochemical decarboxylation conversion strategy well. Here, we summarize the recent electrochemical decarboxylation reactions to better elucidate the advantages of electrochemical decarboxylation reactions.

Electrochemical Synthesis of Vinyl, Alkyl, and Allyl Sulfones from Sodium Sulfinates and Olefins

An electrochemical synthesis of vinyl, alkyl, and allyl sulfones using sodium sulfinates and olefins was reported. This method uses direct current to pass through an undivided cell equipped with graphite carbon electrodes, and a series of diverse sulfone compounds can be synthesized at room temperature in high yields.

Electrochemical synthesis of sulfinic and sulfonic esters from sulfonyl hydrazides

An electrochemical synthetic method for the synthesis of sulfinic esters and sulfonic esters from sulfonyl hydrazides was developed. Alkyl sulfinic esters were synthesized by treating sulfonyl hydrazides with trialkyl orthoformate in a DMF solvent at a constant current of 5 mA and then optimizing the reaction conditions. Conversely, alkyl sulfonic esters were exclusively obtained when the reaction was conducted in alkyl alcohol solvents at a constant current of 15 mA. The various substituted arylsulfonyl hydrazides afforded moderate to good yields of the desired sulfinic esters and sulfonic esters. Mechanistic investigations revealed that sulfonyl radicals were formed through electrochemical oxidation and that they react with alkyl radicals or alkoxy radicals to generate the respective ester products.

Photochemical Halogen-Bonding Promoted Synthesis of Vinyl Sulfones via Vinyl and Sulfonyl Radicals

A photochemical halogen-bonding-assisted synthesis of vinyl sulfones via radical-radical cross-coupling of vinyl bromines and sodium sulfinates is developed. This methodology offers a facile and efficient approach to various vinyl sulfones with excellent functional group tolerance under metal-, photocatalyst-, base-, and oxidant-free conditions. The reaction is also applicable for the late-stage functionalization of drug molecules and the hectogram scale. Moreover, instead of sodium sulfites being prepared, these reactions could also be conducted using sulfonyl chlorides in a one-pot method.

Electrochemical Synthesis of Dibenzothiophenes via Intramolecular C-S Cyclization with a Halogen Mediator

Electrochemical synthesis of dibenzothiophene derivatives was achieved. Several bis(biaryl) disulfides are efficiently converted to dibenzothiophenes by electrochemical oxidation. The use of Bu4NBr as a halogen mediator was essential, and wide varieties of dibenzothiophene derivatives were obtained in good yields.

Benzylic C(sp3)-H Bonds Play the Dual Role of Starting Material and Oxidation Inhibitor for Hydrazides in the Electrochemical Synthesis of Hydrazones

The electrooxidation of benzylic C(sp³)-H bonds to produce hydrazones as an alternate for conventional pathways has an enormous dignity. Under the aegis of electricity, instead of hazardous metal catalysts and external oxidants, we unveil an electrochemical process for electrooxidation of various benzylic C(sp³)-H bonds in aqueous media in all pH ranges that subsequently produce hydrazones with further reactions. This electrooxidative reaction strategy provides an acceptable condition for synthesizing hydrazones with various functional groups in good efficiency and amenable to gram-scale synthesis. The electrochemical oxidation condition proves an excellent level of compatibility with super cheap electrolyte NaCl for the oxidation of benzylic C(sp³)-H position despite the highly oxidizable hydrazide group remaining intact in the reaction.

The Application of Sulfonyl Hydrazides in Electrosynthesis: A Review of Recent Studies

Sulfonyl hydrazides are viewed as alternatives to sulfinic acids and their salts or sulfonyl halides, which are broadly used in organic synthesis or work as active pharmaceutical substances. Generally, sulfonyl hydrazides are considered good building blocks and show powerful value in a diverse range of reactions to construct C-S bonds or C-C bonds, and even C-N bonds as sulfur, carbon, or nitrogen sources, respectively. As a profound synthetic tool, the electrosynthesis method was recently used to achieve efficient and green applications of sulfonyl hydrazides. Interestingly, many unique and novel electrochemical syntheses using sulfonyl hydrazides as radical precursors have been developed, including cascade reactions, functionalization of heterocycles, as well as a continuous flow method combining with electrochemical synthesis since 2017. Accordingly, it is necessary to specifically summarize the recent developments of electrosynthesis with only sulfonyl hydrazides as radical precursors to more deeply understand and better design novel electrochemical synthesis reactions. Herein, electrosynthesis research using sulfonyl hydrazides as radical precursors since 2017 is reviewed in detail based on the chemical structures of products and reaction mechanisms.

Electrosynthesis of (hetero)aryl nitriles from α-imino-oxy acids via oxidative decarboxylation/N-O cleavage

A new method for the synthesis of (hetero)aryl nitriles via iminyl radicals has been developed through the electrochemical oxidative decarboxylation of α-imino-oxy acids. This protocol provides an efficient approach to nitriles with a broad range of functional-group tolerance under ambient conditions and can be applied for one-pot gram-scale synthesis.

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.

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.

Electrochemical Synthesis of Sulfonyl Fluorides from Sulfonyl Hydrazides

The synthesis of sulfonyl fluorides via the reaction of sulfonyl hydrazides and Et3N3HF under electrochemical conditions is reported. Various sulfonyl fluorides were obtained in good yields under a constant current...

Atom-Economical, Catalyst-Free Hydrosulfonation of Densely Functionalized Alkenes: Access to Oxindole Containing Sulfones

An atom-economical hydrosulfonation of densely functionalized alkenes under catalyst-free conditions is described. Alkenes possessing hydroxy-oxindole moiety underwent hydrosulfonation on treatment with arylsulfinic acids in green media to afford the resulting...

Iron-catalyzed regioselective synthesis of (E)-vinyl sulfones mediated by unprotected hydroxylamines

An Fe-catalyzed unprotected hydroxylamine mediated Heck-type coupling between sulfinic acids and alkenes furnished structurally important (E)-vinyl sulfones with moderate to good yields, high atom-economy and regioselectivity.

Expeditious and Efficient ortho-Selective Trifluoromethane-sulfonylation of Arylhydroxylamines

A metal- and oxidant-free, practical and efficient method for the synthesis of highly versatile and synthetically useful ortho-trifluoromethanesulfonylated anilines from arylhydroxylamines and trifluoromethanesulfinic chloride was developed. This rapid transformation proceeded smoothly with good yields and excellent ortho-selectivity in the absence of any metals or ligands. Mechanistically, the reaction comprised a noncanonical O-trifluoromethanesulfinylation of the arylhydroxylamine, and the subsequent [2,3]-sigmatropic rearrangement to afford ortho-trifluoromethanesulfonylated aniline derivatives. The practical application of this reaction was demonstrated by further conversion into a series of functional molecules under different reaction conditions.

Recent Progress and Emerging Technologies towards a Sustainable Synthesis of Sulfones

Sulfones play a pivotal role in modern organic chemistry. They are highly versatile building blocks and find various applications as drugs, agrochemicals, or functional materials. Therefore, sustainable access to this class of molecules is of great interest. Herein, the goal was to provide a summary on recent developments in the field of sustainable sulfone synthesis. Advances and existing limitations in traditional approaches towards sulfones were reviewed on selected examples. Furthermore, novel emerging technologies for a more sustainable sulfone synthesis and future directions were discussed.

Electrochemical fluorosulfonylation of styrenes

An environmentally friendly and efficient electrochemical fluorosulfonylation of styrenes has been developed. With the use of sulfonylhydrazides and triethylamine trihydrofluoride, a diverse array of β-fluorosulfones could be readily obtained. This reaction features mild conditions and a broad substrate scope, which could also be conveniently extended to a gram-scale preparation.

Iminyl-radicals by electrochemical decarboxylation of α-imino-oxy acids: construction of indole-fused polycyclics

Iminyl radicals are reactive intermediates that can be used for the construction of various valuable heterocycles. Herein, the electrochemical decarboxylation of α-imino-oxy acids for the generation of iminyl radicals has been accomplished under exogenous-oxidant- and metal-free conditions through the use of nBu₄NBr as a mediator. The resulting iminyl radicals undergo intramolecular cyclization smoothly with the adjacent (hetero)arenes to afford a series of indole-fused polycyclic compounds.

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.

Unconventional approaches for the introduction of sulfur-based functional groups

Organosulfur compounds have a pivotal role in the functionalities of many natural products, pharmaceuticals and organic materials. For these reasons, the search for new methodologies for the formation of carbon-sulfur bonds has been the object of intensive work for organic chemists. However, the proposed strategies suffer from various drawbacks, such as volatility, toxicity, and instability of the sulfur sources or the use of VOC solvents. In this review, we summarise the recent protocols which have the goal of obtaining sulfones, thioethers, thiazines, thiazepines and sulfonamides in an unconventional and/or sustainable way. The use of starting materials less invasive and toxic with respect to the traditional reagents, alternative solvents such as water, ionic liquids or deep eutectic solvents, the exploitation of ultrasound and electrochemistry, increasing the efficiency of the process, are reported. Moreover, representative reaction mechanisms are also discussed.

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.

Organic synthesis via Kolbe and related non-Kolbe electrolysis: an enabling electro-strategy

Herein, the electrolysis process, where the anodic oxidation of carboxylic acids leads to decarboxylation, has been discussed to synthesize organic molecules.

Visible light-mediated applications of methylene blue in organic synthesis

This review presents the manipulation of methylene blue in visible-light-assisted organic synthesis.

Progress and perspectives of electrochemical insights for C–H and N–H sulfonylation

A comprehensive electrosulfonylation study has been carried out via cathodic and anodic approaches for the production of organosulfone and sulfonamide derivatives.

An update on the use of sulfinate derivatives as versatile coupling partners in organic chemistry

The use of sulfinic acids and their salts continues to be extensively developed in organic chemistry. This is attributable to their dual capacity for acting as nucleophilic or electrophilic reagents, as well as their ease of preparation and stability on storage. This report highlights the research accomplished since 2015 on this topic, updating a previous review published by our team in 2014.

Visible-light-induced anti-Markovnikov hydrosulfonation of styrene derivatives

A visible-light-induced anti-Morkovnikov hydrosulfonation reaction of styrene derivatives with sodium sulfinates has been developed, featuring mild reaction conditions, good functional-group tolerance, good yields and high regioselectivity.

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

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

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

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

Base-controlled divergent synthesis of vinyl sulfones from (benzylsulfonyl)benzenes and paraformaldehyde

A tuneable metal-free protocol for the selective preparation of α-substituted vinyl sulfone and (E)-vinyl sulfone derivatives has been described. The base played an important role in the selectivity control of transformation.