Unraveling the Hidden Pathway of Catalyst-Free Direct Photochemical Conversion of Sulfides to Sulfoxides: A Universal...

Selective conversion of sulfides to sulfoxides is an important class of chemical transformation that has enormous potential in medicinal chemistry. However, the primary process associated with 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 direct photochemical conversion of sulfides to sulfoxides in the absence of any catalysts under UVA illumination (λ_ex = 370 nm). We show that this hidden pathway directly generates singlet oxygen (a¹Δ_g,¹O₂) via solvent-oxygen (X³∑_g^-,³O₂) charge transfer (CT) excitation in neat solvents under ambient conditions. Our findings reveal efficient and selective oxidation of sulfides to sulfoxides in the presence of ¹O₂ through the generation of persulfoxide intermediates. In addition, we found that the presence of a 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.