Photochemical oxidative dehydrogenation of saturated N-heterocycles by an iminoquinone

Abstract

In this manuscript, we report an organic cofactor-inspired oxidative dehydrogenation reaction under aerobic atmosphere on a series of saturated N-heterocycles. Prior methods comprising transition metal catalysts relied on acceptorless dehydrogenation under thermal conditions and usually required high temperatures. Our designed iminoquinone molecule could be reduced easily upon photoexcitation, resulting in iminosemiquinonate generation. This putative radical further governs hydrogen atom transfer reactions on substrate N-heterocycles to convert them into their unsaturated analogues. A series of N-heterocycles, including indole, quinoline, quinoxaline, benzimidazole, and pyrimidine, were synthesized by this organocatalytic process under mild reaction conditions. An array of mechanistic studies disclosed that the process is radical-promoted, which was further substantiated by radical intermediate isolation. The operative mode was also distinctly different from other cofactor-driven oxidative processes, where two-electron chemistry is prominent via addition–elimination or transamination mechanisms.