Intermolecular transposed Paternò–Büchi reactions enabled by triplet sensitization

Abstract

The canonical mechanism of the Paternò–Büchi cycloaddition involves the reaction of an excited-state carbonyl compound with a ground-state alkene to afford an oxetane. Despite a century of sustained interest in this important organic photoreaction, the mechanistically complementary reaction proceeding via an excited-state alkene and ground-state carbonyl compound was first reported less than a decade ago and remains relatively underdeveloped. We report herein a visible light induced photocatalytic approach towards transposed Paternò–Büchi cycloadditions. This strategy involves the use of a photosensitizer that is capable of engaging in selective triplet energy transfer to the alkene component in preference to its carbonyl reaction partner. These more general reaction conditions have enabled a thorough investigation of the synthetic opportunities afforded by the transposed Paternò–Büchi reaction.