Claisen condensation

The Claisen condensation is a carbon-carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base, resulting in a β-keto ester or a β-diketone. It is named after Rainer Ludwig Claisen, who first published his work on the reaction in 1881.

Requirements
At least one of the reagents must be enolizable (have an &alpha;-proton and be able to undergo deprotonation to form the enolate anion). There are a number of different combinations of enolizable and nonenolizable carbonyl compounds that form a few different types of Claisen condensations.

The base used must not interfere with the reaction by undergoing nucleophilic substitution or addition with a carbonyl carbon. For this reason, the conjugate sodium alkoxide base of the alcohol formed (e.g. sodium ethoxide if ethanol is formed) is often used, since the alkoxide is regenerated. In mixed Claisen condensations, a non-nucleophilic base such as lithium diisopropylamide, or LDA, may be used, since only one compound is enolizable. LDA cannot be used in the classic Claisen or Dieckmann condensations, since virtually all ester will be converted to ester enolate and condensation will not occur.

The alkoxy portion of the ester must be a good leaving group. Methyl and ethyl esters, which yield the methoxy and ethoxy leaving groups, respectively, are usually used.

Types
The classic Claisen condensation, where only one enolizable ester is used.

The mixed (or "crossed") Claisen condensation, where an enolizable ester or ketone and a nonenolizable ester are used.

The Dieckmann condensation, where a molecule with two ester groups reacts intramolecularly, forming a cyclic β-keto ester. In this case, the ring formed must not be strained, usually a 5- or 6-membered ring.

Mechanism


In the first step, the ester with the α-proton is deprotonated by the base, resulting in the enolate anion, made relatively stable by the delocalization of electrons. Then, the carbonyl carbon of the other ester undergoes nucleophilic attack by the α-carbon of the enolate. The alkoxy group leaves (resulting in regeneration of the alkoxide), and aqueous acid (e.g. sulfuric or phosphoric acid) is added to neutralize the base and any enolate still present. The newly-formed β-keto ester or β-diketone is then isolated.