Abstract
The reaction of (tBuO)2Si(Cl)Vi (3) with LitBu has been investigated in pentane or toluene (non-polar) and mixtures of pentane or toluene with THF (polar), respectively, in order to gain information about the influence of substituents and the polarity of the solvent on the reaction mechanism. In accord with earlier results, using Me2Si(Cl)Vi (1) [8] ortBu2Si(Cl)Vi (2) [5] as precursors, the course of reaction in non-polar solvents is dominated by 1,2-elimination of LiCl from the α-lithio intermediate R2Si(Cl)CH(Li)CH2HtBu yielding 1,1,3,3-tetra(tertbutoxy)-1,3-disilacyclobutane (4) via the corresponding silaethene derivative. On the other hand, the presence of small amounts (5% by volume) of THF in the solvent system leads to a considerable decrease in the yield of 4 and favours secondary reactions of the α-lithio derivative by solvate formation giving chain compounds like (tBuO)2Si(X)CH2CH2 tBu (X = Cl, tBu) and (tBuO)2Si(CH=CHtBu)Si(OtBu)2CH2CH2 tBu (5b). MNDO calculations on the model compounds Me2Si=CH2, Me2Si=CH2 · OH2 and (HO)2Si=CH2, (HO)2Si=CH2 · OH2, respectively, show that for the gas phase molecules the formation of the dimethyl derivatives from the corresponding α-lithio intermediate is less endothermic than that of the dihydroxy analogues. Therefore, the differences observed in the product pattern of the reactions of 1 and 3, respectively, especially in polar systems, can only be explained by a combination of substituent and solvent effects on the kinetics.
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