Chapter 21: Ester Enolates

Summary

• Malonic esters are the esters of malonic acid (systematic name : propanedioic acid)

• The most common example of a malonic ester is diethyl malonate
• Diethyl malonate is:
• systematically known as diethyl propandioate
• a readily available, commercial starting material
• an example of an active methylene compound
• Diethyl malonate is useful because it:
• has a relatively acidic a-proton, pK_a = 13, so the enolate is readily formed with ethoxide, EtO^-
• can be readily substituted then hydrolysed and decarboxylated to give a substituted carboxylic acid.
• The sequence of reactions:
• Alkylation of an active methylene enolate
• Hydrolysis of the esters to a dicarboxylic acid
• Decarboxylation of a 1,3-dicarboxylic acid
• This process is the equivalent of taking R-X and substituting X with -CH₂COOH
• In effect, the acyl group of the ester has been used to activate the ester enolate towards alkylation and has then been removed.
• In principle, both of the a-H can be substituted with alkyl groups and these alkyl groups need not be the same (review)

• Acetoacetic ester synthesis
• Other reactions of enolates

OVERVIEW OF MALONIC ESTER SYNTHESIS
Step 1: First, an acid-base reaction. Ethoxide functions as a base and removes the acidic a-hydrogen giving the reactive enolate which is then alkylated.
Step 2: Acid or base catalysed hydrolysis of both esters  to give the parent dicarboxylic acid, a substituted malonic acid.
Step 3: Loss of CO2 = decarboxylation, readily occurs giving a substituted carboxylic acid.
Mechanisms for the individual steps can be found on the linked pages.

© Dr. Ian Hunt, Department of Chemistry