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 oxetanes. Despite a century of sustained interest in this important organic photoreaction, the mechanistically copmlementary 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 selectively engaging in 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.