Unraveling the Hidden Pathway of Catalyst‐Free Direct Photochemical Conversion of Sulfides to Sulfoxides: A Universal Pathway Under UVA Radiation

Selective conversion of sulfides to sulfoxides is an important class of chemical transformation having tremendous importance in medicinal chemistry. However, the primary process associated with the photoexcitation during the direct photochemical conversion of sulfides to sulfoxides is poorly understood and misrepresented in the literature. Herein, we discover a hidden pathway responsible for the efficient and selective conversion of sulfides to sulfoxides in the absence of any photocatalysts (PCs) or photosensitizers (PSs) under UVA illumination (λex = 370 nm). We show that this hidden pathway directly generates singlet oxygen (a1Δg,1O2) via solvent‐oxygen (X3Σg–,3O2) charge transfer (CT) excitation under ambient conditions without the need of any additional PCs or PSs. Our experimental and computational findings reveal that the 1O2 generated via CT excitation efficiently and selectively oxidizes sulfides to sulfoxides via the generation of persulfoxide intermediates with an excellent yields, fast kinetics, and broad substrate scope. In addition, we found that the presence of marginal amount of water favors faster kinetics and prevents overoxidation to sulfones due to the stabilization of the sulfoxide products via specific hydrogen‐bonding interactions. Finally, we demonstrate that the present hidden pathway is equally efficient and operational for other classes of photoredox reactions under UVA illumination; and thus, making this pathway a universal route for efficient and sustainable photoredox conversions under ambient conditions.