N-Phosphinyl Imines Enable Intermolecular Aza Paternò–Büchi Reactions with Unactivated Alkenes

Intermolecular aza Paternò–Büchi (aPB) reactions provide a direct and convergent approach to azetidines through the photocycloaddition of imines and alkenes. However, reactions involving unactivated alkenes remain challenging, particularly with acyclic imines, whose excited states are susceptible to rapid nonradiative relaxation and competing photochemical pathways. Here, we report N-phosphinyl imines as readily accessible and stable photoreactive partners for visiblelight-mediated intermolecular aPB reactions with unactivated alkenes. Triplet sensitization using thioxanthone as an inexpensive organic photocatalyst enables productive [2+2]-cycloaddition across a range of alkenes, establishing a new reagent platform for intermolecular aPB chemistry to expand the accessible reactivity of acyclic imines with unactivated alkenes.