Time to recap and summarize the various kinds of enol and enolate that can form from car bonyl compounds. You have already seen that ketones and aldehydes enolize. With an unsymmetrical ketone, more than one enol or enolate ion is possible.

Aldehydes may enolize, but of course enolization is impossible in any carbonyl compound without hydrogen atoms adjacent to the carbonyl group.

All carboxylic acid derivatives can form enols of some kind. Those of esters are particularly important and either enols or enolates are easily made. It is obviously necessary to avoid water in the presence of acid or base, as esters hydrolyse under these conditions. One solution is to use the alkoxide belonging to the ester (MeO− with a methyl ester, EtO− with an ethyl ester, and so on) to make enolate ions.

Then, if the alkoxide does act as a nucleophile, there’s no harm done as the same ester is simply regenerated.

The carbonyl group is accepting electrons in both the enolization step and the nucleophilic attack. The same compounds that are the most electrophilic are also the most easily enolizable. This makes acyl chlorides very enolizable. To avoid nucleophilic attack, we cannot use chloride ion as base since chloride is not basic, so we must use a non-nucleophilic base such as a tertiary amine. The resulting enolate is not stable as it can eliminate chloride ion, a good leaving group, to form a ketene. This works particularly well in making dichloro ketene from dichloroacetyl chloride as the proton to be removed is very acidic.

Carboxylic acids do not form enolate anions easily as the base first removes the acidic OH proton. This also protects acids from attack by most nucleophiles.

In acid solution, there are no such problems and ‘ene-diols’ are formed.

Amides (unless they are tertiary) also have rather acidic protons, though not, of course, as acidic as those of carboxylic acids. Attempted enolate ion formation in base removes an N–H proton rather than a C–H proton. Amides are also the least reactive and the least enolizable of all acid derivatives, and their enols and enolates are rarely used in reactions.

It is not even necessary to have a carbonyl group to observe very similar reactions. Imines and enamines are related by the same kind of tautomeric equilibria.

With a primary amine (here PhNH2) a reasonably stable imine is formed, but with a second-ary amine (here a simple cyclic amine) the imine itself cannot be formed and the iminium salt is less stable than the enamine. Just as enamines are the nitrogen analogues of enols, aza-enolates are the nitrogen ana-logues of enolates. They are made by deprotonating enamines with strong base. Nitroalkanes are much more acidic and form enolate-like anions in quite weak base.

Nitriles (cyanides) also form anions and require strong base as the negative charge is de localized onto only a single nitrogen atom. The anion is a linear system like ketene, allene, or carbon dioxide.

● Requirement for enolization Any organic compound with an electron-withdrawing functional group, with at least one π bond joined to a saturated carbon atom having at least one hydrogen atom, may form an enol in neutral or acid solution. Many also form enolates in basic solution (exceptions being carboxylic acids, and primary and secondary amides).

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

Silyl enol ethers in aldol reactions

Lithium enolates in aldol reactions

The Mannich reaction

Base-promoted halogenation

Halogenation

Racemization