Silicon-oxygen bond formation

Acyl silanes often, however, exhibit abnormal behaviour, for example involving rearrangements leading to silicon-oxygen bond formation, especially when treated with... 

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. 

Whether the formation of alkene 3 proceeds directly from alkoxide 4 or via a penta-coordinated silicon-species 6, is not rigorously known. In certain cases—e.g. for /3-hydroxydisilanes (R = SiMes) that were investigated by Hrudlik et al —the experimental findings suggest that formation of the carbon-carbon bond is synchronous to formation of the silicon-oxygen bond ... 

Less usual carbosilane dendrimers were synthesised by reaction of acetyl-protected hydroxyethyl glycosides with chlorosilanes. Introduction of the carbohydrate component proceeds via alcoholysis of the chlorosilane with formation of silicon-oxygen bonds a carbosilane core unit is formed with carbosiloxane side arms [82]. Reverse assembly of carbosilanes with a carbohydrate core is also feasible [54]. 

Obviously the formation of a strong silicon-oxygen bond is also assumed to be the driving force in these rearrangements. In the presence of isonitriles, no silyl shift could be induced in complexes 3 and 4. 

The enhanced thermal stability of the silicones has been postulated by Andrianov and Sokolov (26) to be due to (1) the high energy of the SiOSi bond, (2) the stability of organic radicals to oxidation at the silicon atom, (3) stable SiOSi bond formation within the polymer after partial degradation, and (4) the formation of a layer of silicon-oxygen bonds on the surface of the polymer during thermal oxidation. 

Trimethylsilyl)methyl anion 328 reacts with N20 to generate Me3SiO 329 and the driving force for reaction 128 is obviously the formation of the strong silicon-oxygen bond and the conversion of the highly basic carbanion to the more stable oxide ion 330. 

Scheme 4.5. Concept of carbanion pump to convert oxyanion into carbanion by the aid of ring opening of silacyclobutane ring by oxyanion through the formation of silicon-oxygen bond.

As frequently noted before, the formation of the silicon-oxygen bond occurs here also with virtually complete retention of configuration based on studies with (+ )-l-naphthylphenylmethylbenzoylsilane. Similar photochemical formation of oxacarbenes have recently been reported by several workers 65-68). The role of base and acid in leading to these apparently unrelated pathways is not yet understood. Qualitative rate studies indicate that conversion of the mixed acetal to the alkoxysilane and acetal under comparable acid conditions is much slower than their direct photochemical formation so that the mixed acetal does not appear to be an intermediate in the reactions containing acid. 

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).

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