Formation of enols and enolates

The alpha carbon of compounds containing the carbonyl group (aldehydes, ketones, esters, diketones, diesters, nitrates, nitriles, etc.), is the center of many CC bond formation reactions. Due to the acidity of the H a , they undergo a -deprotonation in the presence of a suitable base, with the consequent formation of a carbanion. The resulting negative charge on C a to C=O is resonance stabilized by the same carbonyl group.


The selection of the base, for the formation of enolates, is subject to the fact that the pKa of the conjugate acid of the base must be greater by at least three units than the pKa of the carbonyl compound that has acidic H 's .


pK a = 20

MeO-   pK a = 15

Unfavorable enolate formation


pK a = 10

tBuO- _   pK a = 19

Very favorable enolate formation

Formation of enolates:


¨        The kinetic enolate

It occurs because the substrate has H α , easily accessible for deprotonation through a typical base such as LDA (pKa   approx   30)

LDA (lithium diisopropylamide) is a strong, non-nucleophilic, sterically hindered base.


¨        Enolates of esters:

Esters are susceptible to a substitution reaction for the base,   LDA can be problematic, which is why the non-nucleophilic base (lithium isopropylcyclohexyl amide) is used with esters.



¨        Thermodynamic enolate:

A reversible deprotonation can lead to more stable enolates, which occurs when the more substituted C=C of the enol form is obtained.


Typical conditions to form thermodynamic enolates are: RO-M+ in ROH as protic solvent (pKa of ROH =   15 to 18).

Kinetic and thermodynamic enolates can be trapped, isolated, separated, and purified to obtain regiochemically pure enolates. This can be accomplished by the formation of enol and silylene ether acetates.

¨        Enol acetate:


¨        Silylene ether:


¨        Enolates from enones:

The enolate is generated by reduction from the dissolution of a metal in liquid ammonia.




¨   Enolates from conjugate addition (1,4) in a , b – unsaturated ketonic compounds


¨ Enolates from reduction of a -halocarbonyls


¨        Enolates from a , b unsaturated ketones


synthetic equivalents

The formation of the indicated synthetic equivalents of the enol takes place through simple reactions with a high yield:

*      Formation of the silyl enol ethers:

Silyl enol derivatives can also be easily prepared from esters and amides.


Silyl enol ethers are to be used preferably in the case of aldehydes, since these enolize with strong bases, inducing aldol condensation.

*      Enamine formation:

The regioselectivity can be controlled by the appropriate combination of electronic and steric effects.


*      Formation of enol ethers by Birch reduction:

The regioselectivity of the reduction is controlled by electronic effects.


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