Mechanism of the reaction of sulphides with N-chloroarenesulphonamides

Abstract

The kinetics of the reaction of R¹R²S (R¹= R²= alkyl or aryl) and ArSO₂NCI^–Na⁺ were investigated in buffered water–ethanol solution. Analytical methods have been elaborated to follow the reaction which affords R¹R²-SNSO₂Ar and R¹R²SO. From the reaction scheme proposed, kinetic equations are deduced and supported experimentally. In fast and slow (k_d′) equilibrium reactions, respectively, ArSO₂NCl^–Na⁺ in water is transformed into ArSO₂NHCl and ArSO₂NCl₂ which are reactive electrophiles and which, in their turn, react with R¹R²S, the first in a slow (k₁′) and the second in a relatively fast (k₂) step to form reactive intermediates. The rate constants k_d′, k₁′, and k₂ have been determined for different reactants and the observed substituent effects are correlated with the formation of an (R¹R²SCl⁺ArSO₂NQ^–) ion-pair intermediate (Q = H or Cl). Me₂S is converted by TsNHCl (k₁′k_d′), Ph₂S by TsNCl₂(k₁′k_d′), and MeSPh by both chlorinating agents (k₁′∼k_d′). For the reactions of MeSPh with TsNCl₂ and TsNHCl k₂/k₁′ is 3·5 × 10⁶. MeSPh is chlorinated by TsNCl₂ 1500 times faster than Ph₂S. For XC₆H₄SMe, electron-donating X groups increase the rate of reaction with both TsNHCl and TsNCl₂(ρ–4·25 and –3·56, respectively). For YC₆H₄SO₂NHCl, electron-withdrawing Y groups produce an increase in reactivity toward MeSPh (ρ+ 1·76). From the ion-pair intermediate R¹R²SNSO₂Ar and R¹R²SO are formed, probably via a sulphurane intermediate, in fast, competitive S_N reactions under different steric control. The formation of sulphoxides is more hindered by bulky R groups than that of sulphimides. Other product-controlling factors are also discussed.