Triplet Energy Transfer‐Driven Intermolecular [2+2] Photocycloaddition of Acyclic Imines and Indoles: Facile Access to Azetidine‐Fused Indolines

Currently, successful examples of the aza-[2+2] photocycloaddition (aza-Paternò-Büchi reaction) of indoles primarily rely on intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò-Büchi reaction in indoles, it is essential to extend the reaction to acyclic imine. Here, an intermolecular [2+2] photocycloaddition of indoles with acyclic imines has been developed, enabling visible light-mediated aza-Paternò-Büchi reaction by triplet energy transfer. A range of azetidine-fused indolines are successfully provided with yields of up to 93% for 30 examples. Given the growing significance of nitrogen-containing heterocycles in pharmaceutical research and the challenge of their limited accessibility, this study afforded a novel synthetic methodology to construct previously inaccessible structural motifs with high molecular complexity by extending indole intermolecular [2+2] photocycloaddition with acyclic imines.