An unconventional photochemical amide synthesis from excited-state nitroarenes

Abstract

Given the widespread significance of amide functional groups in organic synthesis and pharmaceutical investigations, an efficient and practical synthetic protocol that eliminates the utilization of any (photo)catalyst and stoichiometric metallic reductants is highly desirable. We herein disclose a direct amide-synthesis protocol using only light and solvent as inevitable elements, enabled by a photoexcited-nitroarene hydrogen atom transfer system. These processes unveil the unprecedented bifunctional nature of the photoexcited nitroarene, which serves both as an intramolecular C(sp³)–H bond activator and oxygen atom source. Mechanistic studies support that the transformation occurs via the direct photoexcitation of nitroarene to triplet biradical species, followed by a specific radical cyclization event. This leads to the formation of benzoxazole N-oxide and benzoxazole intermediates, which fragment to give the carbonyl products. A combination of radical trapping experiments and HRMS analysis revealed the identities of the key radical species and benzoxazole intermediates, respectively.