Anchimeric assistance by and migration of the vinyl group in reactions of sterically hindered organosilicon compounds of the type (Me3Si)2C(SiMe2CHCH2)(SiR2X)

Abstract

The compound VsiSiMe₂I [Vsi =(Me₃Si)₂C(SiMe₂CHCH₂)](1) is very similar in reactivity to (Me₃Si)₃CSiMe₂I (2) in reactions with methanol and with alkali-metal salts in MeOH or MeCN, but (1) is much the more reactive towards electrophiles which induce rate-determining ionization of the Si–I bond, viz. CF₃CH₂OH (the factor f is > 500), CF₃CO₂H (f > 1 800), AgBF₄ in CH₂Cl₂(f ca. 150), or AgSCN in CH₂Cl₂(f > 500). The large differences are attributed to anchimeric assistance by the γ-vinyl group to the leaving I^– under the influence of the electrophile, leading to formation of a 1,3 vinyl-bridged cation. In accord with this, reactions of VsiSiEt₂I with AgBF₄ or AgO₂CCF₃ in CH₂Cl₂ give ca. 1 : 2 mixtures of unrearranged and rearranged products, VsiSiEt₂Y and (Me₃Si)₂C(SiEt₂CHCH₂)(SiMe₂Y)(Y = F or O₂CCF₃). Anchimeric assistance by the γ-vinyl group is much greater than that by a γ-Me group but much smaller than that by a γ-OMe group. The chloride VsiSiEt₂Cl reacts with AgO₃SCF₃ in CH₂Cl₂ by loss of the vinyl group (with anchimeric assistance by the γ-Cl atom), apparently to give a chlorine-bridged cation, and hence a mixture of the unrearranged (Me₃Si)₂C(SiEt₂Cl)(SiMe₂O₃SCF₃) and the rearranged (Me₃Si)₂C(SiEt₂O₃SCF₃)(SiMe₂Cl).