Ring Expansion of Oximes via Photochemical Aza‐di‐π‐methane Rearrangement

Skeletal editing has emerged as a transformative strategy for manipulating cyclic architectures, with radical‐based approaches providing particularly mild and versatile routes. While significant progress has been made, molecular editing via diradical intermediates under visible‐light irradiation remains rare. Herein, we report an unprecedented visible‐light‐driven ring expansion of cyclic oximes through a photochemical aza‐di‐π‐methane rearrangement, mediated by energy transfer catalysis. This diradical‐mediated photochemical rearrangement provides a mild and efficient synthetic route, exhibiting broad substrate scope, excellent functional group tolerance, and versatile downstream reactivity of the rearranged scaffolds. The transformation can also be conducted under photochemical continuous‐flow conditions. Mechanistic studies support a stepwise sequence involving triplet diradical formation, a challenging 4‐exo‐trig cyclization, β‐scission elimination, and diradical recombination. This method expands the toolbox of radical‐based ring expansions, complementing classical approaches such as the Dowd–Beckwith reaction.