Chapter 21: Ester Enolates

Decarboxylation of b-carbonyl esters

Summary

• Loss of carbon dioxide is called decarboxylation.
• Esters or carboxylic acids  with a carbonyl group at the 3- (or b-) position readily undergo thermal decarboxylation.
• Decarboxylation was first encountered in Chapter 19 for carboxylic acids (review).
• The reactive species is the carboxylic acid or the carboxylate anion.

• So the ester must first be converted to the acid by hydrolysis (review)
• The decarboxylation reaction then proceeds via a cyclic transition state giving an enol intermediate that tautomerises to the carbonyl.

DECARBOXYLATION OF A b-KETOACID
Step 1: Remember curly arrows flow.... Start at the protonation of the carbonyl, break the O-H bond and form the p bond, break the C-C and make the C=C. Note the concerted nature of this reaction and the cyclic transition state.
Step 2: Tautomerisation of the enol of the ketone leads to the more favourable ketone form (mechanism not shown here).