Silicon-carbon compounds silyl cations

The differences in the chemical behavior of silicon, relative to carbon, is responsible for the elusiveness of SiR3 compounds in condensed phases. Just is it that what hampers the observation of silyl cations, although they are intrinsically thermodynamically more stable than carbenium ions ... 

The synthesis and the structural analysis of silicon unsaturated compounds impressively demonstrate that tricoordinate silicon species are able to exist under normal conditions. Since more than 50 years organosilicon research tries to answer the question if this might be true for silyl cations RsSi, in which silicon also is surrounded by three neighbors. Those compounds seem to meet the requirements because silicon is more electropositive than the carbon homologue and therefore should accommodate a positive charge more efficiently. And in fact, calculations confirm that [HsSi] is thermodynamically much more stable than [HsC]. ... 

Even if silicon chemistry is new to you, you should by now have a picture of stable compounds with C-Si bonds and selective reaction with fluoride. You are already familiar with silyl enol ethers as nucleophilic enolate equivalents and allyl silanes resemble these in many ways. The missing link is the i-silyl effect. A Si atom stabilises a cation in the p-posit ion by overlap of the populated and relatively high energy C-Si c-orbital with the empty p orbital of the cation. This overlap is already present in the preferred conformation 95a of the allyl silane 95 as an anti-bonding interaction 95b between the C-Si c-orbital and the n orbital of the double bond. The resultant molecular orbital (the new HOMO) 95c increases the nucleophilic reactivity of the carbon atom in the y-position. 

Silicon cation based Lewis acids have been reported in the Hterature for a long time and their apphcation in catalysis has been reviewed [25]. Most examples in the literature are based mainly on achiral species and will be presented in this section only briefly. A broad variety of reactions can be catalyzed with compounds like MesSiOTf, Me3SiNTf2, or Me3SiC104. These Lewis acids are compatible with many carbon nucleophiles such as silyl enol ethers, allyl organometaUic reagents, and cuprates. 

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