Photoinduced sulfonylation cyclization to medium-sized benzo[b]azocine and mechanistic insight

Owing to inherent unfavorable enthalpy and entropy factors, methods for constructing medium-sized N-heterocycles, especially via radical cascade cyclizations, have rarely been reported. In this work, designed dienes were used to obtain medium-sized benzo[b]azocines via a novel visible-light-induced 8-endo sulfonyl cyclization. Reactions were performed at room temperature under photocatalyst-free conditions. Large-scale synthesis, and derivatization via epoxidation and N-Ts deprotection, showed the potential utility of this method. Radical-inhibition and light on/off experiments, and apparent quantum efficiency calculations, were performed to decipher the radical pathway. Density functional theory calculations enabled rationalization of the rate-determining step and chemoselectivity of this transformation.