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Published: Kem. Ind. 59 (3) (2010) 111–124
Paper reference number: KUI-18/2009
Paper type: Review
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The Free Tricoordinated Silyl Cation Problem

H. Čičak


As the importance and abundance of silicon in our environment is large, it has been thought that silicon might take the place of carbon in forming a host of similar compounds and silicon-based life. However, until today there is no experimental evidence for such a hypothesis and carbon is still unique among the elements in the vast number and variety of compounds it can form. Also, the corresponding derivatives of the two elements show considerable differences in their chemical properties. The essential debate concerning organosilicon chemistry relates to the existence of the free planar tricoordinated silyl cations in condensed phase (R3Si+), in analogy to carbocations (R3C+) which have been known and characterized as free species. Although silyl cations are thermodynamically more stable than their carbon analogs, they are very reactive due to their high inherent electrophilicity and the ability of hypervalent coordination. On the other hand, stabilization by inductive and hyperconjugative effects and larger steric effects of carbocations make them less sensitive to solvation or other environmental effects than silyl cations. Hence, observation of free silyl cations in the condensed phase proved extremely difficult and the actual problem is the question of the degree of the (remaining) silyl cation character. The first free silyl cation, trimesitylsilyl cation, and in analogy with it tridurylsilyl cation, were synthesized by Lambert et al. Free silyl cations based on analogy to aromatic ions (homocyclopropenylium and tropylium) have also been prepared. However, in these silyl cations the cationic character is reduced by internal π-conjugation. Čičak et al. prepared some silyl-cationic intermediates (Me3Si-CH≡CR)+ in solid state. With the help of quantum-mechanical calculations it was concluded that these adducts have much more silyl cation than carbocation character.

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silicon, organosilicon compounds, silyl cations, carbocations, β-silyl effect, degree of silyl cation character