Visible light–mediated aza Paternò–Büchi reaction of acyclic oximes and alkenes to azetidines

The aza Paternò–Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò–Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light–mediated aza Paternò–Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis of epi- penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (Δ G ǂ ) values and ultimately promotes reactivity.