Silyl-substituted pentacoordinated silicon

The rate and pathway of the methanolysis appear to depend strongly on the steric and electronic effects of the equatorial substituents. Because of the steric hindrance at the pentacoordinate silicon induced by N(Y/ substitution in N-silylated triazasilatranes, nucleophiles can attack their four-coordinate silicon atoms (equation 189)414. 

The system HSiC /DBU may be used as a SiClj synthon in the presence of electrophilic heterobutadienes. The resulting heterocycles are not only precursors for fluorine substituted imidazoles [13], but also may be transformed to amidinate complexes with silicon in a pentacoordinated state. Monophosphorous analogues to amidinates are synthons for silyl-substituted phosphaalkenes. 

Reverting to the simple phenylated substrate of Scheme 33, if the styryl moiety is orf/jo-substituted by a phenyldimethylsilyloxy group, the 5-exo-trig cych-zation is now followed by a retro-[ 1,4]-Brook rearrangement. Whatever the ( ) or (Z) configuration of the styryl group, the intermediate benzyllithium epimer-izes, and a pentacoordinated silicon allows transfer of the silicon moiety diaster-eoselectively to form a unique C-silylated adduct (er>98 2, dr>99 l) [59] (Scheme 35). 

Scheme 30 Diels-Alder reactions of 2-silyl-substituted 1,3-dienes with pentacoordinated silicon...

Summary 2-(Trimethylhydrazino)phenyl substituted organosilicon compounds 3, 4, and 5 were prepared. Their NMR spectra reflect the pentacoordination of the silicon center in solution at room temperature. Compound 5 is hexacoordinated in the solid state, whereas 4 shows pentacoordination. The phenyltrimethylhydrazino as well as the dimethylbenzylamino substituent proved to be highly suitable for the intramolecular stabilization of silyl cations. 

However, with aryl(trialkyl)silanes, sometimes for electronic reasons, the reaction does not take place. In order to reduce the electron density on the silicon atom, alkyl groups were replaced by fluorine. The fluorine effect could be explained as follows i) the van der Waals radius of fluorine is roughly comparable to that of hydrogen and hence the fluorine-substituted silyl group is not so bulky than the trimethylsilyl one ii) electronegativity of fluorine favours the formation of the reactive pentacoordinated silicate, enhances the Lewis acidity of the silicate and... 

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