The silyl enol ether can be prepared from its parent carbonyl compound by forming a small equilibrium concentration of enolate ion with weak base such as a tertiary amine and trap ping the enolate with the very efficient oxygen electrophile Me₃SiCl. The silyl enol ether is stable enough to be isolated but is usually used immediately without storing. You should look upon silyl enol ethers as rather reactive alkenes that combine with things like protons or bromine, but do not react with aldehydes and ketones without catalysis: they are much less reactive than lithium enolates. As with alkylation , a Lewis acid catalyst is needed to get the aldol reaction to work, and a Ti(IV) compound such as TiCl₄ is popular.

The immediate product is actually the silyl ether of the aldol product but this is hydrolysed during work-up and the aldol is formed in good yield. The Lewis acid presumably bonds to the carbonyl oxygen atom of the electrophile. Now the aldol reaction can occur: the positive charge on the titanium-complexed carbonyl oxygen atom makes the aldehyde reactive enough to be attacked even by the not very nucleophilic silyl enol ether. Chloride ion removes the silyl group and the titanium alkoxide captures it again. This last step should not surprise you as any alkoxide (MeOLi, for example) will react with Me₃SiCl to form a silyl ether.

This mechanism looks complicated, and it is. It is, in fact, not clear that the details of what we have written here are right: the titanium may well coordinate to both oxygens throughout the reaction, and some of the steps that we have represented separately probably happen simultaneously. However, all reasonable mechanisms will agree on two important points, which you must understand:

• Lewis acid is needed to get silyl enol ethers to react.

• The key step is an aldol reaction of the silyl enol ether with the Lewis-acid complexed electrophile. The use of silyl enol ethers can be illustrated in a synthesis of manicone, a conjugated enone that ants use to leave a trail to a food source. It can be made by an aldol reaction between pentan-3-one (as the enol component) and 2-methylbutanal (as the electrophile). Both partners are enolizable so we shall need to form a specifi c enol equivalent from the ketone. The silyl enol ether works well. The aldol product will be a mixture of diastereoisomers but it eliminates to give a single compound.

The silyl enol ether is not isolated but is treated immediately with the aldehyde to give an excellent yield of the aldol. Dehydration in acid solution with toluenesulfonic acid (TsOH) gives the enone. You can see by the high yield in the aldol reaction that there is no significant self-condensation of either partner in the aldol reaction.

مواضيع ذات صلة

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

Conjugated Wittig reagents as specific enol equivalents

Lithium enolates in aldol reactions

The Mannich reaction

Base-promoted halogenation

Halogenation

Racemization

Thermodynamically stable enols: 1,3-dicarbonyl compounds