Electrochemical C-H Amidation of Heteroarenes with N-Alkyl Sulfonamides in Aqueous Medium

Aggregation-induced C-C bond formation on an electrode driven by the surface tension of water

Electrochemical organic synthesis is typically conducted in organic media. The solvent and related supporting electrolytes negatively affect the greenness of electrosynthesis. In this work, with 100% water used as the solvent, we realize aggregation-driven electrochemical radical cross coupling of unsaturated compounds driven by water tension. A key finding is that aggregation of the substrate at the electrode confined the radical intermediate and prevented side reactions, thus providing a way to regulate radical reactions in addition to their native properties. The reaction provides up to 90% yields with almost quantitative chemoselectivity. The pure water system readily yields the products via cold filtration, and the solvent is recycled repeatedly. In particular, the life span of the radical species generated in the reaction increase significantly because of the confined environment in the aggregation state. The greenness of this protocol is further enhanced with readily separation of product from media using cooling and filtration.

Electro-oxidative intermolecular CSP2-H amination of heteroarenes via proton-coupled electron transfer

A new electrochemical proton-coupled electron transfer method for the intermolecular CSP2-H amination of heteroarenes without oxidants, metal catalysts and external electrolytes has been developed. Various new N-containing heteroarenes were prepared in medium to high yields, and the indole-containing product could be converted into practical 2-oxindole by simple basic hydrolysis. Mechanistic investigation indicated that ester sulfonyl-substituted N-radicals could be formed by the combination of 2,6-lutidine and electrochemical oxidation, which is the key to achieve the desired chemoselectivity.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Recent advances in metal catalyst- and oxidant-free electrochemical C-H bond functionalization of nitrogen-containing heterocycles

The C-H functionalization of nitrogen-containing heterocycles has emerged as a powerful strategy for the construction of carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds. In order to achieve efficient and selective C-H functionalization, electrochemical synthesis has attracted increasing attention. Because electrochemical anodic oxidation is ideal for replacing chemical reagents in C-H functionalization reactions. This mini-review summarizes the current knowledge and recent advances since 2017 in the synthetic utility of electrochemical transformations for the C-H functionalization of nitrogen-containing heterocycles.

Electrooxidative tricyclic 6-7-6 fused-system domino assembly to allocolchicines by a removable radical strategy

Natural allocolchicine and analogues derived thereof a tricyclic 6-7-6-system have been found as key scaffold of various biologically relevant molecules. However, the direct preparation of the allocolchicine motif remains difficult to date. Herein, we report on an electrooxidative radical cyclization of biarylynones with various carbon- and heteroatom-centered radical precursors via a sequential radical addition/7-endo-trig/radical cyclization domino reaction. This approach provides a step-economical and strategically novel disconnection for the facile assembly of a wide range of carbocyclic 6-7-6 fused ring systems. Remarkably, the sulfonyl group on the products could be easily removed by photocatalysis at room temperature with high yields.

Uncatalyzed Synthesis of Unsymmetrical Indole-substituted Methane Derivatives via One-pot Multicomponent Reactions under Solvent-free Conditions

Abstract:

For the first time, one-pot multicomponent reactions containing indole, aromatic aldehydes, and dimedone have been achieved in the absence of a catalyst under solvent-free conditions. A class of unsymmetrical indole-substituted methane derivatives and symmetrical diindolylmethanes has been synthesized through this green organic synthetic method. Based on experimental results, a reasonable mechanism for this condensation is emphasized here.

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.

Electro-Oxidative C-N Bond Formation through Azolation of Indole Derivatives: An Access to 3-Substituent-2-(Azol-1-yl)indoles

A practical protocol to synthesize 3-substituent-2-(azol-1-yl)indole derivatives has been developed via an electrochemical oxidative cross coupling process under mild conditions. This electro-oxidative C-N bond formation strategy tolerates a range of functional groups and is amenable to gram scale synthesis. Moreover, this method was applied to the late-stage functionalization of bioactive molecules.

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.

Electrochemical C-H Halogenations of Enaminones and Electron-Rich Arenes with Sodium Halide (NaX) as Halogen Source for the Synthesis of 3-Halochromones and Haloarenes

Without employing an external oxidant, the simple synthesis of 3-halochromones and various halogenated electron-rich arenes has been realized with electrode oxidation by employing the simplest sodium halide (NaX, X = Cl, Br, I) as halogen source. This electrochemical method is advantageous for the simple and mild room temperature operation, environmental friendliness as well as broad substrate scope in both C-H bond donor and halogen source components.

Iridium-catalyzed regioselective C-H sulfonamidation of 1,2,4-thiadiazoles with sulfonyl azides in water

We have developed a regioselective C-N cross-coupling of 1,2,4-thiadiazoles with sulfonyl azides through iridium catalysis in water. This method tactically linked the 1,2,4-thiadiazoles and sulfonamides together, and the novel molecules increased the diversity of 1,2,4-thiadiazoles which may have potential applications.

Organic Electrosynthesis Towards Sustainability: Fundamentals and Greener Methodologies

The adoption of new measures that preserve our environment, on which our survival depends, is a necessity. Electro-organic processes are sustainable per se, by producing the activation of a substrate by electron transfer at normal pressure and room temperature. In the recent years, a highly crescent number of works on organic electrosynthesis are available. Novel strategies at the electrode are being developed enabling the construction of a great variety of complex organic molecules. However, the possibility of being scaled-up is mandatory in terms of sustainability. Thus, some electrochemical methodologies have demonstrated to report the best results in reducing pollution and saving energy. In this personal account, these methods have been compiled, being organized as follows: • Direct discharge electrosynthesis • Paired electrochemical reactions. and • Organic transformations utilizing electrocatalysis (in absence of heavy metals). Selected protocols are herein presented and discussed with representative recent examples. Final perspectives and reflections are also considered.

Electrochemical Synthesis of Benzimidazoles via Dehydrogenative Cyclization of Amidines

The development of efficient and sustainable methodologies for the synthesis of N-heterocycles is a constant focus of organic synthesis. Herein an electrochemical method is reported for the synthesis of benzimidazoles through dehydrogenative cyclization of easily available N-aryl amidines. The reactions were conducted under simple constant current conditions in an undivided cell without need for catalysts, chemical oxidants, or additives, and produced H₂ as the only theoretical byproduct.

Iron-catalyzed cross-dehydrogenative C–H amidation of benzofurans and benzothiophenes with anilines

An efficient iron-catalyzed radical cross-dehydrogenative aromatic C–H amidation provides a straightforward access to structurally diverse diarylamine derivatives incorporating benzofuran/benzothiophene motifs.

Electrochemically facilitated oxidative C–H amination enables access to fluorescent N9-fused tricyclic xanthines

An electrochemically enabled intramolecular C−H amination route for accessing a broad range of fluorescent N9-fused tricyclic xanthines with various substitution patterns under simple, green, and mild condition is developed.

Electrochemical C-H Amidation of Heteroarenes with N-Alkyl Sulfonamides in Aqueous Medium

The construction of C-N bonds by free radical reactions represents a powerful synthetic approach for direct C-H amidations of arenes or heteroarenes. Developing efficient and more environmentally friendly synthetic methods for C-H amidation reactions remains highly desirable. Herein, metal-free electrochemical oxidative dehydrogenative C-H amidations of heteroarenes with N-alkylsulfonamides have been accomplished. The catalyst- and chemical-oxidant-free C-H amidation features an ample scope and employs electricity as the green and sole oxidant. A variety of heteroarenes, including indoles, pyrroles, benzofuran and benzothiophene, thereby underwent this C(sp² )-H nitrogenation. Cyclic voltammetry studies and control experiments provided evidence for nitrogen-centered radicals being directly generated under metal-free electrocatalysis.