Visible light-driven [3 + 3] annulation reaction of 2H-azirines with Huisgen zwitterions and synthesis of 1,2,4-triazines

Abstract

A visible light-driven [3 + 3] annulation reaction of 2H-azirines with in situ generated Huisgen zwitterions from azodicarboxylates and phosphines is herein described. Under very mild conditions and irradiation of blue LED light, disubstituted 2H-azirines readily undergo a formal [3 + 3] annulation reaction with in situ generated Huisgen zwitterions, affording polysubstituted dihydro-1,2,4-triazines in 51–97% yields with a broad substrate scope. Mechanistic studies unveil that this annulation reaction proceeds through a cascade sequence of nucleophilic addition, light-driven 1,3-ester migration and aza-Wittig-like cyclization. The light-driven ester migration step is believed to be a photochemical process of an intramolecular electron donor–acceptor (EDA) complex.