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

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.