Allyl silanes molecular orbitals

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a k bond and a C-Si O bond, to the allylic product with a new K bond and a new o bond to the electrophile. The intermediate cation is mainly stabilized by O donation from the C-Si bond into the vacant p orbital but it has other a-donating groups (C—H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

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. 

The first enantioselective organocatalytic a-allylation of cyclic ketones has been accomphshed via singly occupied molecular orbital catalysis. " Geometrically constrained radical cations (i.e., 196), forged from the one-electron oxidation of transiently generated enamines, readily undergo aUylic alkylation with a variety of commercially available allyl silanes to give 197 (Scheme 25.91). 

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