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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

1999; American Chemical Society; Volume: 121; Issue: 16 Linguagem: Inglês

10.1021/ja984304l

1943-2984

Marco Fachini, Vittorio Lucchini, Giorgio Modena, Manuela Pasi, Lucia Pasquato,

Synthesis and Catalytic Reactions

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 CD2Cl2 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 SN2-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.