Cascade Synthesis of Benzimidazole-Linked Pyrroles via Copper Catalyzed Oxidative Cyclization and Ketonization.

A three-component cascade reaction of 2-aminobenzimidazole, aldehyde, and terminal alkyne has been explored for an efficient synthesis of benzimidazole-linked tetrasubstituted pyrroles. The reaction sequence involves the formation of propargylamine, insertion of a terminal alkyne, and a ring opening reaction followed by an intramolecular carbonylative cyclization under aerobic conditions. It represents a novel strategy to the construction of C═N, C═C, C═O bonds and a new five-membered 2-ketopyrrole ring. In this process, the reaction conditions are crucial and an attempt to elucidate the novel reaction pathway is well supported by X-ray crystallography.