Hypervalent silicon compounds

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

Hypervalent silicon compounds have generated much interest in recent years. This is evident from the numerous papers published every year on this topic, and from the extensive reviews which have appeared in recent years1-7. In this review we have attempted to cover only the most recent (five year) literature. Only for subjects for which we felt that the new work would not be fully appreciated without the previous reports did we include earlier citations, or for those subjects which in our judgment, were not sufficiently covered in earlier reviews. 

The most recent comprehensive review focused primarily on the reactivity aspects of hypervalent silicon complexes6, while the latest review covers only silicon-oxygen coordination7. The present chapter focuses on synthesis and structure, silicon-29 NMR spectroscopy, and on the nonrigidity of hypervalent silicon compounds and the resulting kinetic and stereochemical studies. 

A new direction in the chemistry of hypervalent silicon compounds has recently been developed by Tacke and coworkers the chemistry of zwitterionic organosilicates28-31. In these compounds the silicon is formally negative, with a cationic nitrogen attached to it by an alkyl chain. One of the groups of products of this general class is the pentacoordinate zwitterionic fluorosilicates31. 

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