Enols have an OH group and are alcohols of a sort. Normal alcohols form stable ethers that are difficult to convert back to the alcohol. Powerful reagents such as HI or BBr₃ are required and these reactions were discussed. The reaction with HI is an SN2 attack on the methyl group of the protonated ether and that is why a good nucleophile for saturated carbon, such as iodide or bromide, is needed for the reaction. Enol ethers, by contrast, are relatively unstable compounds that are hydrolysed back to the carbonyl compound simply with aqueous acid—dilute HCl or H₂SO₄, for example.

Why the big difference? The reason is that the enol ether can be protonated at carbon using the delocalization of the oxygen lone pair in the enol derivative to produce a reactive oxonium ion.

This oxonium ion could be attacked on the methyl group in the same way that the ordinary ether was attacked.

We wouldn’t really expect this reaction to happen much faster than the same reaction on an ordinary ether, so there must be another better and faster mechanism. That mechanism is attack on the π bond instead of attack on the σ bond.

In aqueous acid the nucleophile X− is just water and we find ourselves in the middle of the mechanism of hydrolysis of acetals, The oxonium ion is an intermediate common to both mechanisms.

A similar reaction occurs when enol ethers react with alcohols in acid solution and in the absence of water, but now we are starting in the middle of the acetal hydrolysis mechanism and going the other way, in the direction of the acetal. A useful example is the formation of THP (tetrahydropyranyl) derivatives of alcohols from the enol ether dihydropyran. You will see THP derivatives of alcohols being used as protecting groups.

Silyl enol ethers hydrolyse by a slightly different mechanism, although the fi rst step is the same—protonation at carbon using the lone pair on oxygen. We have already seen how easy it is to attack silicon with nucleophiles, especially those with oxygen or a halogen as the nucleophilic atom. This tips the balance towards attack by water at silicon for the next step.

The aldehyde is formed immediately. What happens to the other product illustrates again just how easy nucleophilic substitution at silicon can be. Two of these compounds combine together to give a disilyl ether, called a disiloxane.

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

Enol and enolate reactions at oxygen: preparation of enol ethers

Silyl 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