Silicon cation, tetracoordinate

TETRACOORDINATE SILICON CATIONS AS INTERMEDIATES IN NUCLEOPHILE-ASSISTED SUBSTITUTION AT SILICON... 

The tetracoordinate silicon cation is a rather common species in solution. It may be generated by heterolytic cleavage of a bond from silicon to a reactive ligand, as a result of interaction of the silicon center with an uncharged nucleophile like amine, imine, phosphine, phosphine oxide, and amide. Since these nucleophiles are also known to be effective catalysts for many displacements at silicon including important silylation processes (86,89,235-238), the cations of tetracoordinate silicon have received attention as possible intermediates in these reactions according to Eq. (40) (78,235,239-243). 

The greatly increased steric repulsion between the axial and the bulky equatorial groups in trisilylated 1-methyltriazasilatrane (198) leads to a significant weakening of the transannular Si — N bond. The Si—N distance in 198 is the longest ever recorded in a triazasilatrane (2.775 A)410. It makes the atom sufficiently basic to react with CFsSOsMe, forming the cationic species 199 in which the silicon atom is tetracoordinated... 

The other justification of the extension of the scope of this article to tetracoordinate Si+ intermediates is the notable interest in the nucleophilic activation to substitution at the silicon atom, which is closely related to the tetrahedral Si+ intermediacy problem. Little attention has been paid so far to this aspect of organosilicon chemistry in other review articles. This discussion does not include, however, the vast area of tetrahedral Si+ intermediates in reactions catalyzed by protic acids thus, the protic cations, i.e., those having acidic hydrogen, are in principle excluded. 

B. Formation of Stable Salts Having a Tetracoordinate Silicon Atom in the Cation... 

Cations having a tetracoordinate silicon may appear by three general routes. Apart from the ionization of a silicon-reactive ligand bond, the cations can be formed by transformation of a group bound to silicon, in particular by the addition of positively charged ion. For example, the quaternization of trimethylsilylamine with methyl iodide leads to the same ionic complex as the reaction of trimethylsilyl iodide with trimethylamine [Eq. (46)] (254). 

The molecular structure of 1 shown in Fig. 4 demonstrates that the vinyl cation is linear around the dicoordinated C atom, which indicates sp hybridi2ation for the C atom. The C -C bond is unusually short (122.1 pm) and approaches the length of a regular C C triple bond. A quite remarkable feature of the molecular structure of 1 is the unusual length of the C -Si bonds (198.4 and 194.6 pm), around 10 pm longer than regular single bonds between sp -hybridized carbon and tetracoordinated silicon atoms. 

Another important feature of extracoordinate silicon compounds is the increase of the cationic character of the silicon atom in 21.2 and 21.3 compared to the tetracoordinate precursors 21.1,which becomes even more substantial when anionic nucleophiles, such as F , are used. This is compensated by making the surrounding groups (R and Cl in 21.2 and 21.3) more nucleophilic and leads to an increased Lewis acidity of the hypercoordinate silicon. 

Treatment of tetracoordinated silanes by the NCS anion afforded some new pentacoor-dinate anionic spirobicyclic siliconates [(C6H402)2SiNCS] M+ with various M+ counter cations , as well as some hexacoordinate dianions [(C6H402)2Si(NCS)2]. ... 

In cases in which the high Lewis acidity of the cationic silicon is decreased by an intramolecular Lewis basic group (LB), as in the tetracoordinated cation 9 (and likewise in the siliconium ion 10 Scheme 2), interactions between the positively charged silicon atom and the solvent and/or anion are of minor importance. The structures, spectroscopic properties, and reactivities of these sUyl cations are greatly determined by the electron-donating ability of the LB groups. As a consequence, the structural and spectroscopic features of silyl cations 21 and 22, which are stabilized by intramolecular electron donation from an aryl substituent, closely resemble those of benzenium ions 11 (Scheme 5) [33, 34]. The intramolecular stabilization operative in silylium ions 9 offers the intriguing possibility of... 

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