Mercaptobenzoic acid-palladium(0) complexes as active catalysts for S-benzylation with benzylic alcohols via (η~3-benzyl)palladium(Ⅱ) cations in water

摘要

Mercaptobenzoic acid-palladium(0) complexes show high catalytic activity for S-benzylation with benzylic alcohols via the (η~3-benzyl)palladium(Ⅱ) cation in water. Notably, these palladium(0) complexes could play an important role in formation of active (η~3-benzyl)palladium(Ⅱ) cation complexes followed by S-benzylation. Hammett studies on the rate constants of S-benzylation by various substituted alcohols show good correlation between log[k_x/k_H) and the σ~+ value of the respective substituents. From the slope, negative p 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. The catalytic system can be performed using only 2.5 mol% Pd2(dba)_3 without the phosphine ligand or other additives.