EDA-Driven Radical Relay Strategy for the Strain-Release Functionalization of Bicyclobutanes.

Herein, we unveil a catalyst-free, visible-light-driven radical sulfonylation of bicyclobutanes (BCBs) enabled by an in situ-generated electron donor-acceptor (EDA) complex, providing efficient access to lactam-spirocyclobutyl oxindole sulfones through a cascade of radical cyclization, SO2 incorporation, and strain-release ring opening of BCBs. Comprehensive mechanistic studies, including UV-vis, fluorescence quenching, Job's plot analysis, and EPR spectroscopy, support the formation of EDA complex and light-driven radical intermediacy. The protocol shows broad substrate compatibility and is further extended to an arylsulfonylative cyclization of bicyclobutanes engaging aryl diazonium salts under Ru-photoredox catalysis.