ANCHIMERIC ASSISTANCE BY THE ACETOXY GROUP IN THE SOLVOLYSIS OF (ME3SI)2C(SIME2OCOME)(SIME2X), X=CL, NCS OR N3

Reaction of the acetoxy chloride (Me3Si)2C(SiMe2OCOMe)(SiMeCl), 1, at 35-degrees-C with 3:2 v/v MeOH-dioxane containing one equivalent of Et 3N gave exclusively the corresponding methoxide (Me3Si)2C(SiMe2OCOMe)( SiMe2OMe), 5, in a pseudo first-order process with a rate constant of 6.5 x 10(-3) s-1. [In the absence of the amine some of the initial pro duct underwent further reaction, catalysed by the generated HCI, to gi ve the dimethoxy compound (Me3Si)2C(SiMeOMe)2, 8.] Under similar condi tions the reaction of the methoxy chloride, (Me3Si)2C(SiMeOMe)-(SiMe2C l), to give 8, was 14 times as slow, and this is taken to indicate tha t the anchimeric assistance by the acetoxy group in 1 involves the car bonyl oxygen atom, to give the six-membered ring cation III. The much less reactive isothiocyanate (Me3Si)2(SiMe2OCOMe)(SiMe2NCS), 2, and az ide (Me3Si)2C(SiMe2OCOMe)(SiMe2N3), 3, react with MeOH to give solely 5 in the presence of Et3N but also some 8 in its absence; 3 is ca. 21 times as reactive at 50-degrees-C as the corresponding methoxy isothio cyanate 9, and 3 is > 100 times as reactive as the corresponding metho xy azide 10. The fluoride (Me3Si)2C(SiMe2OCOMe)(SiMe2F) was found to b e unreactive towards MeOH. The chloride 1 was, surprisingly, found to react much less readily with CF3CH2OH than with MeOH (possibly because hydrogen bonding between the acidic alcohol CF3CH2OH and the carbonyl oxygen inhibits anchimeric assistance through the latter) and the pro duct was not the expected (Me3Si)2C(SiMe2OCOMe)(SiMe2OCH2CF3) but inst ead the trifluoroethoxy hydroxide (Me2Si)2C(SiMe2OCH2CF3)(SiMe2OH), fo rmed possibly via an intermediate oxadisiletane, 13. In the presence o f 1 equiv. of Et3N some 1 was rapidly converted into the diacetate (Me 3Si)2C(SiMe2OCOMe)2 but the subsequent reaction was very slow; the cou rse of the reaction is obscure.