OXYGEN-EXCHANGE BETWEEN SULFOXIDES AND SULFIDES .3. THE HCL-CATALYZED REDUCTION OF ARYL METHYL SULFOXIDES BY DIALKYL SULFIDES IN AQUEOUS METHANOL

Dialkyl sulphides reduce aryl methyl sulphoxides in aqueous methanol in the presence of ca. 4 mol dm-3 HCl. Ring substitution in phenyl methyl sulphoxides moderately affects their reactivity towards dibutyl sulphide, the overall effect resulting in a Hammett rho value of -1.6. This parameter is a measure of the effect of structural changes on a protonation pre-equilibrium, the formation of chlorosulphonium ion, and its subsequent partitioning between reduction and return to reactants by the action of water. Changing the alkyl moiety of the sulphides induces moderate reactivity changes suggesting a compensation of opposite polar and steric effects. Sulphoxides with electron-releasing groups display the highest selectivity towards dialkyl sulphides. When a large concentration of sulphide is used, both racemization of chiral sulphoxide and O-18 exchange with the aqueous solvent are suppressed and this suggests that the species ArR1SCl+ is a common intermediate for these processes and for the reduction reaction. These and other kinetic findings indicate that the reduction step involves the displacement at the ArR1SCl+ chlorine of a sulphide by a sulphide molecule rather than by chloride ion and rules out, for the racemization too, the hypothesis involving a free halogen intermediate.