Silaethene, XIV [1] Bildung und Reaktivität von l,l-Di-terf-butoxy-2-neopentylsilaethen ('BuO)2Si=CH Np Silaethenes, XIV [1] Formation and Reactivity of l,l-Di-ter/-butoxy-2-neopentyl Silaethene ('BuO)2Si = CHNp

The reaction of ('BuO)2Si(Cl)Vi (3) with Li'Bu has been investigated in pentane or toluene (non-polar) and mixtures o f pentane or toluene with THF (polar), respectively, in order to gain information about the influence o f substituents and the polarity o f the solvent on the reaction mechanism. In accord with earlier results, using Me2Si(Cl)Vi (1) [8] or 'Bu2Si(Cl)Vi (2) [5] as precursors, the course o f reaction in non-polar solvents is dominated by 1,2-elimination of LiCl from the a-lithio intermediate R2Si(Cl)CH(Li)CH2H'Bu yielding l,l,3,3-tetra(terfbutoxy)-l,3-disilacyclobutane (4) via the corresponding silaethene derivative. On the other hand, the presence o f small amounts (5% by volume) o f THF in the solvent system leads to a considerable decrease in the yield o f 4 and favours secondary reactions o f the a-lithio deriva­ tive by solvate formation giving chain compounds like ('BuO)-,Si(X)CH2CH-/Bu (X = Cl, 'Bu) and ('B u0)2Si(CH = CH'Bu)Si(0'Bu)2CH2CH2'Bu (5b). M NDO calculations on the model compounds Me-,Si = CH2, Me-,Si = CH, • OH2 and (H O )2Si = CH2, (H O)2Si = CH, OH2, respec­ tively, show that for the gas phase molecules the formation o f the dimethyl derivatives from the corresponding a-lithio intermediate is less endothermic than that o f the dihydroxy ana­ logues. Therefore, the differences observed in the product pattern o f the reactions o f 1 and 3, respectively, especially in polar systems, can only be explained by a combination o f substituent and solvent effects on the kinetics.