Visible Light-Mediated Aza-Norrish–Yang Type Cyclization

In this paper, we report a visible light-mediated aza-Norrish–Yang type cyclization upon energy transfer catalysis for the rapid construction of thiazaspiro[3.4]­octanes or hexahydropyrrolo­[b]­isothiazoles from benzyloxy cyclosulfonimides or acetal cyclosulfonimides, along with broad substrate scope. In addition, most of the thiazaspiro[3.4]­octane derivatives are obtained with complete diastereoselectivities. However, for ortho-electron-withdrawing group substituted substrates, moderate diastereoselectivities were observed, probably due to the influence of the torquoelectronic effect. This photochemical process is initiated by the formation of a triplet state of cyclic imine, followed by 1,5-hydrogen atom abstraction (1,5-HAA) and cyclization steps. The plausible reaction mechanisms have been validated through a series of experimental procedures, including control experiments, kinetic studies, deuterium labeling experiments, electrochemical analysis, Stern–Volmer analysis, and density functional theory (DFT) calculations. Based on DFT calculations, we have elucidated the origins of the stereoselectivity and chemoselectivity of this protocol.