Anchimeric assistance by γ-substituents Z, Z=MeO, PhO, MeS or PhS, in reactions of the bromides (Me3Si)2(ZMe2Si)CSiMe2Br with AgBF4

The new organosilicon bromides (Me₃Si)₂(ZMe₂Si)CSiMe₂Br with Z=PhO or MeS have been prepared and new spectroscopic data obtained for the previously reported compounds with Z=H, F, Br, Me, Ph, MeO or PhS. Competitions between pairs of bromides for a deficiency of AgBF₄ in Et₂O, with the determination of the ratio of the fluoride products by ¹⁹F-NMR spectroscopy, have led to the following approximate relative reactivities of the bromides and so to the relative abilities of the γ-Z groups to provide anchimeric assistance to the leaving of Br^? in this reaction: Me, 1; Ph, 40; PhO, 3400; PhS, 5000; MeS, 7000; MeO, 54 000. In methanolysis in CH₂Cl₂, (Me₃Si)₂(MeOMe₂Si)CSiMe₂Cl has been found to be roughly 120 times as reactive as (Me₃Si)₂(PhOMe₂Si)CSiMe₂Cl. Combination of the results with previously available information suggests the following approximate order of ability of γ-groups Z to provide anchimeric assistance in reactions at the Si---X bonds in compounds (Me₃Si)₂(ZMe₂Si)CSiMe₂X: OCOMe>OMe>OCOCF₃>MeS>PhS, PhO>N₃, Cl>NCS>Ph>CH=CH₂>Me.