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.