Anchimeric assistance by the acetoxy group in the solvolysis of (Me3Si)2C(SiMe2-OCOMe)(SiMe2X), X = Cl, NCS or N3

Abstract

Reaction of the acetoxy chloride (Me₃Si)₂C(SiMe₂OCOMe)(SiMe₂Cl), 1, at 35 °C with 3 : 2 v/v MeOH–dioxane containing one equivalent of Et₃N gave exclusively the corresponding methoxide (Me₃Si)₂C(SiMe₂OCOMe)(SiMe₂OMe), 5, in a pseudo first-order process with a rate constant of 6.5 × 10^–3 s^–1. [In the absence of the amine some of the initial product underwent further reaction, catalysed by the generated HCl, to give the dimethoxy compound (Me₃Si)₂C(SiMe₂OMe)₂,8.] Under similar conditions the reaction of the methoxy chloride, (Me₃Si)₂C(SiMe₂OMe)(SiMe₂Cl), to give 8, was 14 times as slow, and this is taken to indicate that the anchimeric assistance by the acetoxy group in 1 involves the carbonyl oxygen atom, to give the six-membered ring cation III. The much less reactive isothiocyanate (Me₃Si)₂(SiMe₂OCOMe)(SiMe₂NCS), 2, and azide (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 °C as the corresponding methoxy isothiocyanate 9, and 3 is > 100 times as reactive as the corresponding methoxy azide 10. The fluoride (Me₃Si)₂C(SiMe₂OCOMe)(SiMe₂F) was found to be 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 product was not the expected (Me₃Si)₂C(SiMe₂OCOMe)(SiMe₂OCH₂CF₃) but instead the trifluoroethoxy hydroxide (Me₃Si)₂C(SiMe₂OCH₂CF₃)(SiMe₂OH), formed possibly via an intermediate oxadisiletane, 13. In the presence of 1 equiv. of Et₃N some 1 was rapidly converted into the diacetate (Me₃Si)₂C(SiMe₂OCOMe)₂ but the subsequent reaction was very slow; the course of the reaction is obscure.