Nucleophilic Reactions at the Sulfur of Thiiranium and Thiirenium Ions. New Insight in the Electrophilic Additions to Alkenes and Alkynes. Evidence for an Episulfurane Intermediate

The thiiranium hexachloroantimonates 1a, 3, and 5a − c and the thiirenium hexachloroantimonates 6a − c and 7a with exocyclic S -R substituent (R = Me, Et, i- Pr) react at sulfur with dialkyl disulfides R‘ ‘SSR‘ ‘ (R‘ ‘ = Me, Et and R‘ ‘ ≠ R) in CD 2 Cl 2 at 25 °C to give S -R‘ ‘ substituted ions. The reaction rates are affected by the steric hindrance of the substituents at sulfur and at ring carbons. t -2, t -3-Di- tert -butyl- r -1-methylthiiranium hexachloroantimonate ( 2 ) does not react, and the t -2- tert -butyl- c -3-phenyl- r -1-methylthiiranium ( 5a ) reacts about 100 times faster than the c -2, t -3-di- tert -butyl- r -1-methylthiiranium ion ( 1a ). The analysis of the kinetic data suggests that the sulfonium sulfur undergoes attack by the disulfide in the ring plane from a direction that is parallel to the C−C ring bond. This is also the direction which ensures the maximum overlap with the LUMO of thiiranium or thiirenium ions (determined at the RHF/3-21G*//RHF/3-21G* level). The combined consideration of the approach modality and of the maximum orbital overlap suggests that the nucleophilic substitution at sulfonium sulfur is not an S N 2-like reaction but occurs via an intermediate with episulfurane-like structure. The principle of microscopic reversibility will dictate that this is also the first intermediate in the electrophilic sulfenylation of unsaturated C−C bonds.