Second only to lithium enolates in usefulness are silyl enol ethers. Silicon is less electro positive than lithium, and silyl enol ethers are more stable, and less reactive, than lithium enolates. They are made by treating an enolate with a silicon electrophile. Silicon electrophiles invariably react with enolates at the oxygen atom firstly because they are hard and secondly because of the very strong Si–O single bond. The most com mon silicon electrophile is trimethylsilyl chloride (Me3SiCl), an intermediate made industrially in bulk and used to make the NMR standard tetramethyl silane (Me4Si). Silicon–oxygen bonds are so strong that silicon reacts with carbonyl compounds on oxygen even without a strong base to form the enolate: the reaction probably goes through the small amount of enol present in neutral solution and just needs a weak base (Et₃N) to remove the proton from the product. An alternative view is that the silicon reacts with oxygen first, and the base just converts the oxonium ion to the silyl enol ether. Both mechanisms are given below—either might be correct. This is one of the two best ways to make a stable enol derivative from virtually any enolizable carbonyl compound.

Silyl enol ethers can also be made from lithium enolates just by treating them with tri methyl silyl chloride.

Occasionally, it can be useful to run this reaction in reverse, generating the lithium enolate from the silyl enol ether. This can be done with methyllithium, which takes part in nucleophilic substitution at silicon to generate the lithium enolate plus tetramethylsilane.

We shall be returning to silyl enol ethers and lithium enolates later in the book, but for the moment you should view them simply as enol derivatives that are stable enough to be formed quantitatively from carbonyl compounds before being used in further reactions.

مواضيع ذات صلة

Nitration of benzene

Benzene and its reactions with electrophiles

enols and phenols

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers

Enol and enolate reactions at oxygen: preparation of enol ethers

Nitrosation of enols

The Robinson annelation

Intramolecular aldol reactions

How to control aldol reactions of aldehydes

How to control aldol reactions of esters

Specifi c enol equivalents from 1,3-dicarbonyl compounds