Mechanism of Sulfone Formation in the Reaction of Sulfides and Singlet Oxygen:  Intermediacy of S-Hydroperoxysulfonium Ylide

H−D exchange was observed in the methyl group during the formation of sulfones in the reaction of dimethyl sulfide-d₆ or thioanisole-α,α,α-d₃ with singlet oxygen in aprotic solvents. No exchange was observed in the sulfoxides obtained. The proton in the sulfones was shown to come from adventitious water, since the oxidation of C₆H₅SCH₃ in the presence of D₂O lead to the formation of sulfones with monodeuteriation. The ¹⁶O₂−¹⁸O₂ tracer study demonstrated no oxygen scrambling in the sulfones. All the results indicate that the sulfones are formed intramolecularly via an intermediate possessing one activated proton exchangeable with trace water, a suggested structure for which is S-hydroperoxysulfonium ylides (RS⁺(OOH)CH₂^-). Kinetic isotope effects (k_H/k_D = 2−4) observed for methyl protons in the sulfone formation suggest that the rate-determining step is the intramolecular proton abstraction in the persulfoxides (RS⁺(OO^-)CH₃) generating S-hydroperoxysulfonium ylides. The conversion of the ylide intermediates to sulfones is shown to be facile on the basis of PM3 theoretical calculations.