Halide promoted organotin-mediated carbohydrate benzylation: mechanism and application

In the present study, the mechanistic origin of the promoted organotin-mediated carbohydrate benzylation by halides was explored by the comparison of the activation ability of halides on benzylation of methyl β-d-galactoside. It was demonstrated that the improvement of benzylation in terms of speed, yield and general convenience in the presence of halides was due to the formation of an activated oxide species by the coordination of halide anions to the tetracoordinate tin atoms. The halide, being able to form a stronger bond with tin, showed stronger activation ability. The catalytic amount of bromide should be preferentially chosen as an activation additive in the reaction with consideration of economy, convenience and moderate activation ability. The results were further applied to mono- and multi-benzylation of additional carbohydrate structures. A range of prototype carbohydrate structures were efficiently prepared with the guidance of the principle. In light of the previous studies and the present experimental results, the general rules for organotin-mediated benzylation of carbohydrates have been summarized.