Silylium ions anion-stabilized

Scheme 4 The anion-stabilized silylium ion 17 and conventional silyl Lewis acids 18-20...

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... 

For the abovementioned reasons this review will concentrate on the silylium ions and related silicon cations with silicon in the oxidation state (+1V). In particular in view of silylium-like reactivity of species such as the solvent- or anion-stabilized silylium ions of the types 11-13,17 and intramolecular stabilized species, as, for example, 21, 22 will be also included in this overview. Emphasis is given here to structural and Si NMR spectroscopic properties of the silylium ions, synthetic aspects and the use of silylium ions in organic synthesis and catalysis will be also addressed. 

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