Migration of the methoxy group in an organosilicon cation. Reactions of (bromodiphenylsilyl)(methoxydimethylsiyl)bis(trimethylsilyl) methane with silver borofluoride and silver chloride. Crystal structure of (chlordiphenylsilyl)(methoxydimethylsilyl)bis(trimethylisilyl) methane

Abstract

The reaction of the bromide (Me₃Si)₂C(SiMe₂OMe)(SiPh₂Br)(1) with AgBF₄ in Et₂O is found to give a 4:1 mixture of the rearranged fluoride (Me₃Si)₂C(SiMe₂F)(SiPh₂OMe) and the unrearranged (Me₃Si)₂C(SiMe₂OMe)(SiPh₂F). The 1,3 Si–Si migration of the OMe group is interpreted in terms of rate-determining formation of an MeO-bridged cation formed by abstraction of Br^– by Ag⁺ with anchimeric assistance by the OMe group. Surprisingly the reaction of (1) with AgCl, although anchimerically assisted, gives only the unrearranged chloride (Me₃Si)₂C(SiMe₂OMe)(SiPh₂Cl)(2), possibly because the reaction takes place on the surface of the insoluble AgCl. The crystal structure of (2) shows that the MeO group is well placed to carry out a backside nucleophilic attack on the silicon of the SiPh₂Ci group to provide anchimerlic assistance to the departure of Cl^–.