Electrochemical Oxidative Cyclization: Synthesis of Polysubstituted Pyrrole from Enamines

Electrochemical Organoselenium Catalysis for the Selective Activation of Alkynes: Easy Access to Carbonyl-pyrroles/oxazoles from N-Propargyl Enamines/Amides

Intramolecular electro-oxidative addition of enamines or amides to nonactivated alkynes was attained to access carbonyl-pyrroles or -oxazoles from N-propargyl derivatives. Organoselenium was employed as the electrocatalyst, which played a crucial role as a π-Lewis acid and selectively activated the alkyne for the successful nucleophilic addition. The synthetic strategy permits a wide range of substrate scope up to 93% yield. Several mechanistic experiments, including the isolation of a selenium-incorporated intermediate adduct, enlighten the electrocatalytic pathway.

Modular Synthesis of Tetrasubstituted Pyrroles through a Four-Component Cyclization Strategy Using Ammonium Salt as the Nitrogen Source

A four-component synthesis of tetrasubstituted pyrroles was developed under metal-free conditions. The pyrrole ring was formed in one pot through [2 + 1 + 1 + 1] condensation using ammonium salt as the nitrogen source. In this strategy, 1,4-naphthoquinones and maleimides were used as the versatile C2 fragments to provide substituted benzo[f]isoindole-4,9-diones and pyrrolo[3,4-c]pyrrole-1,3-diones, respectively. This work is highlighted by using ammonium salt as the nitrogen source, readily available starting materials and multibond formation (two C-C and two C-N bonds) in a single operation.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

An electrochemical tandem Michael addition, azidation and intramolecular cyclization strategy for the synthesis of imidazole derivatives

An electrochemical-oxidation-induced intramolecular annulation used for the synthesis of imidazole was developed under undivided electrolytic conditions. In an undivided cell, amines, alkynes and azides could smoothly participate in the transformation to furnish a variety of substituted imidazoles through the tandem Michael addition/azide/cycloamine reaction. The reaction could be easily handled and avoided the use of both transition metal catalysts and peroxide reagents, which is in line with the concept of green chemistry.

Regioselective Synthesis of N2-Aryl 1,2,3-Triazoles via Electro-oxidative Coupling of Enamines and Aryldiazonium Salts

An efficient synthetic route for the construction of N²-aryl 1,2,3-triazoles is reported via sequential C-N bond formation and electro-oxidative N-N coupling under metal-free conditions. Readily accessible 2-aminoacrylates and aryldiazonium salts were used as starting materials, and the developed protocol displays excellent functional group tolerance, allowing an extensive range of substrate scope up to 91% isolated yield. Various mechanistic studies, along with the isolation of an intermediate adduct, refer to successive ionic and radical reaction sequences.

Trifluoroethanol as a Unique Additive for the Chemoselective Electrooxidation of Enamines to Access Unsymmetrically Substituted NH-Pyrroles

An electrochemical method for the synthesis of unsymmetrically substituted NH-pyrroles is described. The synthetic strategy comprises a challenging heterocoupling between two structurally diverse enamines via sequential chemoselective oxidation, addition, and cyclization processes. A series of aryl- and alkyl-substituted enamines were effectively cross-coupled from an equimolar mixture to synthesize various unsymmetrical pyrrole derivatives up to 84 % yield. The desired cross-coupling was achieved by tuning the oxidation potential of the enamines by utilizing a "magic effect" of the additive trifluoroethanol (TFE). Additionally, extensive computational studies reveal the unique role of TFE in promoting the heterocoupling process by regulating the activation energies of the reaction steps through H-bonding and C-H⋅⋅⋅π interactions. Importantly, the developed electrochemical protocol was found to be equally efficient for the homocoupling of enamines to form symmetric pyrroles up to 92 % yield.

Electrochemical synthesis of 1,2,4,5-tetrasubstituted imidazoles from enamines and benzylamines

An electrochemical method for synthesizing 1,2,4,5-tetrasubstituted imidazoles was developed under undivided electrolytic conditions. This synthesis was specifically realized based on electrochemical C(sp3)-H amination via enamines and amines. Readily available starting materials were used, avoiding the use of both transition metals and oxidants. The practicability of the method lies in its broad substrate adaptability and in its ability to provide a simple green pathway for synthesizing GABAA receptor analogs.