Vinyl silanes react with electrophiles in a similarly regioselective process in which the silicon is replaced by the electrophile at the ipso carbon atom. The stereochemistry of the vinyl silane is important because this exchange usually occurs with retention of geometry as well.

This is a curious and interesting reaction that deserves explanation. Addition of the electrophile next to silicon leads to the more stable cation β to silicon. In the vinyl silane the C–Si bond is orthogonal to the p orbitals of the π bond, but as the electrophile attacks the π bond, say from underneath, the Me3Si group starts to move upwards. As it rotates, the angle between the C–Si bond and the remaining p orbital decreases from 90°. As the angle decreases, the interaction between the C–Si bond and the empty p orbital of the cation increases. There is every reason for the rotation to continue in the same direction and no reason for it to reverse. The diagram shows that, in the resulting cation, the electrophile is in the position formerly occupied by the Me3Si group, trans to Ph. Loss of the group now gives retention of stereochemistry.

The intermediate cation has only a single bond and so rotation might be expected to lead to a mixture of geometrical isomers of the product but this is not observed. The bonding interaction between the C–Si bond and the empty p orbital means that rotation is restricted. This stabilization weakens the C–Si bond and the silyl group is quickly removed before any further rotation can occur. The stabilization is effective only if the C–Si bond is correctly aligned with the vacant orbital, which means it must be in the same plane—rather like a π bond. Here is the result for both E and Z isomers of the vinyl silane.

It is unusual for silicon to be required in the final product of a synthetic sequence and the stereospecific removal of silicon from vinyl silanes makes them useful reagents that can be regarded as rather stable vinylic organometallic reagents that will react with powerful electrophiles, preserving the double bond location and geometry. Protodesilylation, as the process of replacing silicon with a proton is known, is one such important reaction. The halogens are also useful electrophiles while organic halides, particularly acid chlorides, in the presence of Lewis acids, form vinyl halides and unsaturated ketones of defined geometry.

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مواضيع ذات صلة

Allyl silanes as nucleophiles

Aryl silanes undergo ipso substitution with electrophiles

Silicon stabilizes a positive charge on the β carbon

Alkynyl silanes are used for protection and activation

The Peterson elimination

Nucleophilic substitution at silicon

Silicon has an affinity for electronegative atoms

Silicon and carbon compared

Sulfoxonium ylids

Sulfonium ylids

Sulfonium salts as electrophiles

Sulfonium salts