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.