Theoretical studies of the reactions of the sulphur–sulphur bond. Part 3. R3S3 ions as intermediates in thiol–disulphide exchange reactions

Abstract

Ab initio calculations have been performed on model systems selected to clarify the reaction mechanisms and intermediate states in thiol–disulphide exchange reactions. Evidence is presented for formation of a weak R₃S₃^– complex as intermediate in the exchange reaction R¹SSR²+ R³S^–→ R¹SSR³+ R²S^–. The complex is linear and not a triangular ring as previously speculated and the excess charge is distributed solely on the peripheral sulphur atoms. The stability is almost insensitive to different R groups (H, CH₃). The complete reaction proceeds to liberate the weakest nucleophile which is also the most stable anion. However, the stability and nucleophilicity of the anions did not correlate with the charge densities on the anion as is often taken for granted. Evidence is also presented for a corresponding R₃S₃⁺ complex as an intermediate in reactions like R¹SSR¹+ R²SCl → R¹SSR²+ R¹SCl. The complex is neither linear nor ring shaped but bent as for a regular trivalent sulphur atom; it has an almost neutral SSS region, and is likely to be quite stable.