Photocatalyzed Ring Expansion of α‐Ketosulfonylaziridines: Ready Access to δ‐Sultams

Abstract Herein, a novel photocatalytic ring expansion reaction of aziridines to δ‐sultams through intramolecular radical cyclization is reported. In particular, versatile N ‐sulfonyl‐protected α‐ketoaziridines were envisioned to undergo a ring opening–expansion through a photoredox‐mediated radical–polar crossover (RPC) process to provide six‐membered sultams bearing an unprecedented 2,3‐disubstitution pattern. The generality of the reaction was explored with a wide variety of substrates, showing a good functional group tolerance. Moreover, the robustness of the method allows a 10‐fold upscaling, providing similar yield or diastereomeric preference for the trans ‐disubstituted products. Mechanistic investigations supported the postulated radical–polar crossover pathway, involving aziridine ring opening by photocatalytic single‐electron reduction via a carbonyl radical anion, followed by intramolecular cyclization with the aryl unit of the N ‐sulfonyl group and re‐aromatization. Thus, this strategy opens new possibilities for the construction of highly decorated heterocycles by photoredox‐mediated RPC ring expansion reactions.