Electrophilic substitution of unsaturated silanes

[3] Since then, a number of studies have firmly established the mechanism of electrophilic attack on vinyl- and allylsilanes[4][5] The electron-releasing strength of the carbon-silicon bond is large, and as a result, the position of silicon in the unsaturated silane controls the site of reaction and stereoselectivity.

Electrophilic additions to allyl- and vinylsilanes take advantage of this, and site selectivity generally reflects this property—electrophiles become bound to the carbon γ to the silyl group.

The electron-donating strength of the carbon-silicon bond is similar to that of an acetamide substituent and equal to roughly two alkyl groups.

(2)A model of the most likely reactive conformation of the allylsilane (see below) suggests that the new double bond that forms will predominantly possess the (E) configuration.

This factor does not affect the outcome unless nucleophilic attack liberates free anions or allylic transposition occurs.

[9] Allylsilanes react through a conformation in which the smallest substituent on the carbon attached to silicon is essentially eclipsing the double bond.

After addition of the electrophile to the top or bottom face of the double bond, the silyl moiety rotates to become parallel to the adjacent empty 2p orbital.

(6)Allyl- and vinylsilanes react with a variety of electrophiles under conditions of nucleophilic catalysis or Lewis acid promotion (often stoichiometric).

The primary advantage of using Lewis acids versus fluoride catalysis is site selectivity—fluoride activation tends to generate allyl anions, which can then react at either the α or γ position.

Vinylsilane additions are less common, as a variety of methods to access allylic alcohols are available (including epoxidation of allylsilanes; see below).

Intramolecular Sakurai reactions provide ene-6-ones[16] (9)Acetals and ketals are excellent substrates for additions of allyl- and vinylsilanes.

[17] (10)Iminium ions generated in situ in the presence of protic acid react with allylsilanes to give homoallylic amines.