Solvent‐Controlled, Catalyst‐Free Aerobic Photooxidation of Thioethers to Sulfoxides at 370 nm

Abstract We report a catalyst‐ and sensitizer‐free photooxidation of thioanisoles using a 370 nm UV‐LED under molecular oxygen. The reaction is solvent‐controlled, delivering up to quantitative conversion with 100% selectivity to sulfoxides in protic media (TFE, ethanol), whereas aprotic solvents promote partial overoxidation to sulfones. The protocol is operationally simple and scalable—including the gram‐scale synthesis of methyl phenyl sulfoxide—and tolerates complexity, enabling the selective oxidation of albendazole to ricobendazole. Mechanistic probes and solvent trends are consistent with a singlet‐oxygen ( 1 O 2 ) pathway, in which solvent–substrate hydrogen bonding stabilizes a persulfoxide intermediate and suppresses further oxidation, providing a simple handle to tune outcomes. Using O 2 as the sole oxidant (air is compatible with longer irradiation) and a narrow‐band 370 nm LED, this environmentally benign method offers a green, practical route to sulfoxides.