Exactly the same sort of mechanism accounts for the reactions of aryl silanes with electrophiles under Friedel–Crafts conditions. Instead of the usual rules governing ortho, meta, and para substitution using the directing effects of the substituents, there is just one rule: the silyl group is replaced by the electrophile at the same atom on the ring—this is known as ipso substitution. Actually, this selectivity comes from the same principles as those used for ordinary aromatic substitution: the electrophile reacts to produce the most stable cation—in this case β to silicon. Cleavage of the weakened C–Si bond by any nucleophile leads directly to the ipso product.

There is an alternative site of attack meta to silicon that would lead to a cation β to Si. But this cation is not particularly stable because the vacant p orbital is orthogonal to the C–Si bond and so cannot interact with it as the C–Si bond is still in the plane of the ring. This illustrates that it is more important to understand the origin of the effect based on molecular orbitals rather than simply to remember the result.

This reactivity of aryl silanes is used to convert stable phenyldimethylsilyl compounds into more reactive compounds such as alcohols by a reaction such as that shown in the margin. Several reagents can be used, all of which induce ipso substitution of the phenyl silane. The reaction with bromine is typical. Bromobenzene is produced together with a silyl bromide that is activated towards subsequent oxidation. The mechanism of electrophilic desilylation is the same as that for electrophilic aromatic substitution except that the proton is replaced by the trimethylsilyl group. The silicon stabilizes the intermediate cation, and hence the transition state leading to it, to such an extent that the rate is many orders of magnitude faster. This is the fi rst step with bromine.

The rest of the reaction sequence involves displacement of Br ⁻ by HOO⁻, addition of hydrox ide, rearrangement, and hydrolysis.

●Trimethylsilyl and other silyl groups stabilize a positive charge on a β carbon and are lost very easily. They can be thought of as very reactive protons or ‘super protons’.

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مواضيع ذات صلة

Allyl silanes as nucleophiles

Vinyl silanes offer a regio- and stereoselective route to alkenes

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