Fluorosilicates hexacoordinate

Hexacoordinate fluorosilicates were obtained from neutral pentacoordinate complexes by addition of K[18-crown-6]F191,192. In this way compounds 168-173 were prepared from their pentacoordinate precursors. These compounds, in addition to being fluorosilicates, contain in each chelate ring a dative Si—N bond. 

The 29 Si NMR (Table 20) supports a hexacoordinate structure in solution which is similar to that in the solid state. (529Si for the hexacoordinate fluorosilicates is ca 50 ppm shifted upheld relative to the pentacoordinate precursors. The multiplicity of the 29 Si resonances indicates the nonequivalence of fluorine ligands at low temperatures (between 25 and — 60 °C). These become equivalent as the temperature is raised, such that a symmetrical 29Si multiplet is observed. 

Berry pseudorotation as the mechanism of intramolecular exchange of ligands in pentacoordinate fluorosilicate anions represents a possible pathway. However, no definitive evidence has been presented for systems with mono-dentate ligands, and a pathway involving an intermediate hexacoordinated species can constitute an alternative mechanism. 

The first studies of fluorosilicates were reported by Miiller and by Kumada and coworkers , and later reviewed by Voronkov , GeTmboTdt, Corriu and their coworkers. More recently some interesting hexacoordinate fluorosilicates were reported by Corriu and coworkers and by Tacke and Miihleisen . ... 

Dimethyl(phenyl)silane reduces aldehyde and ketone carbonyls with the aid of fluoride ion or acid. a-Acylpropionamides, 1-aminoethyl ketones, and 1-alkoxyethyl ketones are readily converted into the corresponding -hydroxy amides, o -amino alcohols, and a-alkoxy alcohols, respectively. The stereoselectivity is complementary and generally high erythro (or syn) isomers are obtained with trifluoroacetic acid (TFA), whereas threo (or anti) isomers are obtained with fluoride ion activator (eq 1). The erythro selectivity in the acid-promoted carbonyl reduction is ascribed to a proton-bridged Cram s cyclic transition state. On the other hand, the threo selectivity in the fluoride-mediated reduction is explained in terms of the Felkin-Anh t) e model, wherein a penta-or hexacoordinated fluorosilicate is involved. No epimerization at the chiral center is observed during the reaction. 

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