Direct substitution of benzylic alcohols with electron-deficient benzenethiols viaπ-benzylpalladium(ii) in water.

An efficient and environmentally benign method for the direct nucleophilic substitution of benzylic alcohols with electron-deficient benzenethiols in water is developed. π-Benzylpalladium(ii) cation complexes show high catalytic activity for S-benzylation, which provides a mechanistic alternative to the borrowing hydrogen methodology. Hammett studies on the rate constants of S-benzylation by various substituted alcohols show good correlation between log(kX/kH) and the σ value of the respective substituents. From the slope, negative ρ values are obtained, suggesting that there is a build-up of positive charge in the transition state. Water plays an important role in the catalytic system for sp(3) C-O bond activation and stabilization of the activated Pd(ii) cation species.