PENTACOORDINATE DIHYDRIDOSILICATES - SYNTHESIS, STRUCTURE, AND ASPECTS OF THEIR REACTIVITY

The reactions of H- with trialkoxysilanes provide a unique and facile entry to anionic pentacoordinate dihydridosilicates [H2Si(OR)3]-, isolated as stable crystalline products in the case of bulky R groups (R = iota-Pr, s-Bu, c-C6H11). Their dynamic behavior (R = i-Pr and s-Bu) has been investigated by Si-29 and variable-temperature H-1 NMR. Intramolecular exchange of H atoms between axial and equatorial sites is indicated and was found to be significantly dependent on the nature of the solvent. Compared to HSi(OR)3, K[H2Si(OR)3] are versatile reagents: (1) the silicon atom is an electrophilic center and this leads to nucleophilic displacements of the alkoxy groups with Grignard reagents and organolithiums; (2) they are able to reduce very easily carbonyl derivatives to alcohols without any catalyst; (3) they react with primary halides through an ionic mechanism leading to the alkane; (4) the reaction with benzylic halides is more complex and affords the dimer as the major product, the formation of which can be explained by a SET process; (5) the ability to donate one electron is confirmed in the reaction with Cp(CO)2FeI and by direct ESR observation of stable radical intermediates.