Reactions of Enolates

You may use these summaries and problems but you may NOT download them for use at another site, nor may you charge for access to them. Copyright Linda M. Sweeting 1997

The following example reactions are organized by type, with references to text chapters. Links within these summaries (which may show up as boxes around reagents) will provide further information about the reagents and their other reactions. Return to Reaction Summary menu for other functional groups.

Except for acid-base reactions, products are shown after neutralization in aqueous solution. A short essay on the reactions of enolates and their preparation using alkoxides is found in the Reagents, Bases file.

Nucleophilic Substitution by Enolates and Enamines
Mechanism!
Alkylation of an enolate is an SN2 reaction
McMurry 22.5, Fessenden 17.4B, Schmid 17.14

Alkylation of an enamine, plus hydrolysis with water
McMurry 24.7, Fessenden 17.5, Schmid 17.14
+
Mechanism!
Note that there is a possibility that the ethoxide could react with the halide (SN2) to form an ether. However, the acid base equilibrium with the diketone is much faster. The equilibrium strongly prefers the more stable, less basic, enolate over the ethoxide, essentially consuming the ethoxide. Thus, the very stable enolates from 1,3-dicarbonyl compounds are able to do SN2 reactions without competition from the alkoxide catalyst.
Other reagents: weaker bases
Decarboxylation upon hydrolysis of 3-ketoesters
McMurry 22.8, Fessenden 17.2B, Schmid 24.7

Mechanism!
Bromination of the enolate (think of Br- as a leaving group)
McMurry 22.7, Fessenden 13.10, 17.6, Schmid 17.4

Aldol and Claisen Condensations (Nucleophilic Addition)

+ OH- + OH- + H2O
Mechanism!
Aldol condensation.
McMurry 22.7, Fessenden 17.6, Schmid 17.6
  2
Reverse aldol condensation.
McMurry 24.3, Fessenden 17.6, Schmid 17.10
+    2 CH3O-
Mechanism!
Claisen condensation; Claisen condensation is not reversed by alkoxide because the product is converted to its conjugate base. See carboxylic acids for decarboxylation.
McMurry 23.6,Fessenden 17.8A, Schmid 17.7, 24.3
+
Mechanism!
Mixed Claisen condensation. Remember that all condensation reactions occur at equilibrium. Thus the most stable product is formed. In this basic solution, that will be the weakest base, namely the conjugate base of the product shown. The additions ofother enolates occur, e.g. aldol condensations, but the equilibria eventually produce this most stable product.
McMurry 23.2, Fessenden 17.8B, Schmid 17.7, 24.3

Addition to Conjugated Unsaturated Carbonyl Compounds


Mechanism!
Michael reaction; other bases can be used for this addition, such as alkoxides, amines, alkyl lithium, dialkyl copper lithium, giving a wide variety of products. See the next reaction.
Lithium aluminum hydride and Grignard reagents add to the C=O.
McMurry 23.4, Fessenden 17.9, Schmid 24.12
+
Mechanism!
Michael condensation, used as the first and key step in the Robinson annulation.
Note that there is a possibility that the ethoxide could react with the conjugated ketone (SN2) to form an ether. However, the acid base equilibrium with the diketone is much faster. The equilibrium strongly prefers the more stable, less basic, enolate over the ethoxide, essentially consuming the ethoxide. Thus, the very stable enolates from 1,3-dicarbonyl compounds are able to do addition reactions without competition from the alkoxide catalyst.

McMurry 23.11, Fessenden 17.9, Schmid 24.7

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Last update February 1999