Mechanism of the oxidation of sulphides with sodium periodate

Abstract

The kinetics of the oxidation of RSAr, R₂S, and (CH₂)_nS sulphides with NaIO₄yielding sulphoxides were investigated in ethanol–water solutions, and the rate equation v=k₂[sulphide][IO₄^–] was found to be valid. The observed substituent (ρ– 1.40 for YC₆H₄SMe; ρ*–1.06 and –0.60 for RSPh and R₂S, respectively) and solvent effects (m 0.722 for MeSPh) are explained by an electrophilic attack of periodate oxygen on sulphide, leading to a polar transition state. The slight steric requirements of the reaction (δ 0.27 and 0.105 for RSPh and R₂S, respectively) and the lack of solvent isotope effect exclude the participation of water in an S_N2-type nucleophilic displacement on a sulphonium centre, which might be considered to have been formed from sulphide and periodate in a fast pre-equilibrium. A moderate anchimeric assistance was observed for the conversion of XC₆H₄SMe sulphides with o-X =CO₂Me, CO₂H, and CO₂^–(k₂°/k₂^pca. 1). From the results it is concluded that oxidation proceeds by a one-step electrophilic oxygen transfer from IO₄^– to sulphide through a polar product-like transition state that is stabilized by a polar neighbouring group or by the increasing polarity of the medium.