Anchimeric assistance by the methoxy group in solvolysis of the compounds (Me3Si)2C(SiMe2OMe)(SiMe2X)

Abstract

In a study of the solvolysis of the compounds (Me₃Si)₂C(SiMe₂OMe)(SiMe₂X)(1), it has been shown that the reaction of (1;X= Cl) with MeOH–dioxane is (a) little accelerated by NaOMe and (b) markedly slower than that with CF₃CH₂OH–dioxane; these results confirm that the much greater reactivity of (1; X= Cl) than of (Me₃Si)₃CSiMe₂Cl in such solvolyses arises from anchimeric assistance by the γ-OMe group to the rate-determining departure of Cl^– and formation of a cationic intermediate. In reactions with MeOH or MeOH–dioxane the order of reactivity of compounds (1) appears to be (X =) Br > NO₃ > O₃SC₆H₄Me-p > Cl, SCN > O₂CCF₃ > NCS > O₂CPh, O₂CMe > F, N₃, H. Autocatalysis was observed for X = Br, O₂CMe, and O₂CPh, and in these cases and for X = O₂CCF₃ catalysis by HX was demonstrated. Marked catalysis by HBr was observed in several cases, and the reactivity sequence in the catalysed reactions is (X =) Br N_3″ O₂CMe > O₂CCF_3″ the catalysis being especially marked for X = N₃. The chloride (1; X = Cl) was found to be ca. 13 times as reactive as (Me₃Si)₃CSiMe₂Cl towards KOCN or KSCN in refluxing MeCN (possibly because of smaller steric hindrance by OMe than Me in this direct bimolecular displacement) but the factor is very small compared with that (>10⁶) in methanolysis.