Silicon-oxygen bond formation compounds

While this accounts satisfactorily for the behavior of silicon compounds, and suggests again, as in the acylmetalloids, that silicon-oxygen bond formation is a powerful driving force governing the reactions of these compounds, the behavior of j8-ketones with germanium halides is not explained, and appears to require a four-center transition state. 

Scheme 8.95. The formation of a generic silyl enol ether (R = alkyl, alkaryl, aryl, etc.) from a generic carbonyl compound (aldehyde or ketone) with imidazole as base in methylene chloride solvent. It is argued that the silylation occurs preferentially on oxygen because of the strength of the silicon-oxygen bond. The silyl enol ether can be reconverted to the corresponding aldehyde or ketone by treatment with tetra- -butylammonimn fluoride ( -Bu4N" E).

Carbon halides resist hydrolysis because only s and p orbitals are available for bond formation. This ensures a maximum covalency of four and precludes the donation of electrons by the oxygen atom in a water molecule to a carbon atom. But silicon compounds do hydrolyse, since the unoccupied silicon 3d orbitals lie not far above the 3s and 3p. 

The facile generation of this species involves transfer of a methyl group from silicon to boron and O—Si bond formation in the coordination sphere of the titanium atom and is due to the fact that the Si— CH3 bond is comparatively reactive and easily accessible. Attack of the Lewis base THF at the most Lewis acidic center of this compound (the Ti atom) induces an intramolecular nucleophilic attack of the CH2SiMe3 group on the oxygen-bound Si atom, which... 

Because transition-metal anions can be prepared conveniently in tetrahydrofuran solution, this cyclic ether was often used in earlier attempts to prepare silicon-metal compounds. It is now generally realized, however, that tetrahydrofuran can frustrate these attempts in two ways. First, it promotes electrophilic attack by the silicon compound on oxygen atoms of coordinated carbonyl groups this leads to the formation of products with Si-0 bonds (54, 138, 262, 300, 306, 310, 336,337), e.g.,... 

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