Cleavage of silicon–carbon bonds in tris(trimethylsilyl)methylsilicon compounds by trifluoroacetic acid. Rearrangements and anchimeric assistance

Abstract

The fairly high rate of reaction of the highly sterically hindered compound (Me₃Si)₃CSiMe₂Ph with CF₃CO₂H[to give (Me₃Si)₃CSiMe₂O₂CCF₃and PhH] is consistent with the view that the ratedetermining step involves the transfer of a proton from the acid to the ipso-carbon atom of the ring. The formation of the rearranged species (Me₃Si)₂C(SiMe₂F)(SiMe₂O₂CCF₃) in the reaction of (Me₃Si)₃-CSiPhMeF with CF₃CO₂H suggests that the leaving of benzene from the initial protonated species generates a methyl-bridged silicon cation. Treatment of(Me₃Si)₃CSiPhMel with AgO₂CCF₃–CF₃CO₂H gives the rearranged (Me₃Si)₂C(SiPhMe₂)(SiMe₂O₂CCF₃), which reacts with CF₃CO₂H under reflux to give (Me₃Si)₂ C(SiMe₂O₂CCF₃)₂. In a remarkable example of anchimeric assistance by a γ-OMe group the compound (Me₃Si)₃CSiMe₂OMe reacts readily with CF₃CO₂H at room temperature with evolution of methane, and formation finally of (Me₃Si)₂C(SiMe₂O₂CCF₃)₂ apparently via(Me₃Si)₂C-(SiMe₂OMe)(SiMe₂O₂CCF₃). Sulphuric acid reacts very vigorously with (Me₃Si)₃CSiMe₂OMe, and hydrolysis of the initial product gives the diol (Me₃Si)₃C(SiMe₂OH)₂. This diol can also be obtained by hydrolysis of (Me₃Si)₂C(SiMe₂O₂CCF₃)₂.