You have just seen that silyl enol ethers are easy to make. But, if enolate ions have most of their negative charge on the oxygen atom, it ought to be possible to make ordinary, carbon-based ethers from them too. It is—but only under strange conditions. Normally, enols and enolate ions prefer to react with alkylating agents (such as alkyl halides) at carbon.

 If enolate ions are prepared with potassium bases in dipolar aprotic solvents (such as dimethyl sulfoxide, DMSO) that cannot solvate the oxygen anion, and are treated with dimethyl sulfate or trimethyl oxonium ion—powerful methylating agents that react best with charged atoms—some at least of the enol ether is formed. The Me3O+ ion is found in the stable (though reactive) compound trimethyl oxonium tetrafluoroborate, or Meerwein’s salt, Me₃O+BF₄ −. This compound and dimethyl sulfate, Me₂SO₄, are hard electrophiles with highly polarized C–O bonds which therefore react with the enolate at hard O rather than soft C.

The yield in this reaction is about 60–70% of enol ether, the rest being mainly C-alkylated product. A more reliable method for making an enol ether is the acid-catalysed decomposition of an acetal in the strict absence of water:

The reaction starts as though the acetal were being hydrolysed, but there is no water to continue the hydrolysis, so a proton is lost instead. In other words, with no suitable nucleophile for SN1 substitution, E1 elimination takes place.

These enol ethers are rather unstable, particularly towards acid-catalysed hydrolysis (as described in the next section) and are not as useful as the silyl enol ethers. We shall next look at the enol-like reactions of both groups of enol ethers.

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

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