Organosilicon pentacoordinate silicon

To explain the great interest in silatranes in recent decades one should bear in mind two points. First, for a long time silatranes remained the only accessible class of neutral derivatives of pentacoordinated silicon. Second, the specific physiological action of many silatranes refuted the traditional opinion that organosilicon compounds do not differ much... 

Compound 4a has already been described R. Tacke, O. Dannappel, M. MOhleisen, Syntheses, Structures, and Properties of Molecular k Si-Silicates Containing Bidentate 1,2-Diolato(2 ) Ligands Derived from a-Hydroxycarbocyclic Acids, Acetohydroximic Acid, and Oxalic Acid New Results in the Chemistry of Pentacoordinate Silicon , in Organosilicon Chemistry II From Molecules to Materials (Eds. N. Auner, J. Weis), VCH, Weinheim, 1996, pp. 427-446. 

Pentacoordinate Silicon , in Organosilicon Chemistry II From Molecules to Materials (Eds. N. Auner, J. Weis), VCH, Weinheim, 1996, pp. 427 46. 

ABSTRACT. Pentacoordinate organosilicon species behave as Lewis acid as well as nucleophile. This unique character provides several interesting applications in selective reactions useful for organic synthesis. Accordingly, new reduction of carbonyl compounds with pentacoordinate hydridosilicates, new regio- and enantioselective allylation of aldehydes with pentacoordinate allylsilanes, and the new cleavage reaction of C-Si bonds are discussed. In these reactions, it is demonstrated that unique characters of pentacoordinate silicon species as a Lewis acid play an important role. 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

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. 

An intramolecular coordination of a Lewis base atom affording pentacoordi-nate silicon similar to the situation in silatranes has been established for cyclo-bis(benzamidodimethyl)silane 192 (318) and, more recently, for a new class of organosilicon compounds (aroyloxymethyl)trifluorosilanes 193 (319) (Figure 26). The pentacoordinate character of silicon in compounds 192 and 193 has been confirmed by X-ray diffraction. In the latter case, the Si <- 0 bond length is 1.94 A which is much less than the sum of van der Waals radii (3.4 A) for silicon and oxygen. Similarly, the product of the reaction of bis-(trimethylsilyl)-... 

Pentacoordinate organosilicon species formed by coordination of a neutral two-electron ligand to tetracoordinate organosilanes constitute an interesting class of compounds. These systems can serve as models for the study of nucleophilic substitution at tetracoordinate silicon. 

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