At least one of the reagents must be enolizable (have an α-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.
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 chain or ring.
MechanismSee the mechanism
In the first step of the mechanism, an α-proton is removed by a strong base, resulting in the formation an enolate anion, which is made relatively stable by the delocalization of electrons. Next, the carbonyl carbon of the (other) ester is nucleophilically attacked by the enolate anion. The alkoxy group is then eliminated (resulting in (re)generation of the alkoxide), and the alkoxide removes the newly-formed doubly α-proton to form a new, highly resonance-stabilized enolate anion. Aqueous acid (e.g. sulfuric acid or phosphoric acid) is added in the final step to neutralize the enolate and any base still present. The newly-formed β-keto ester or β-diketone is then isolated. Note that the reaction requires a stoichiometric amount of base as the removal of the doubly α-proton thermodynamically drives the otherwise endergonic reaction.
Stobbe condensationThe Stobbe condensation is a modification specific for the diethyl ester of succinic acid requiring less strong bases. An example is its reaction with benzophenone:
A reaction mechanism which explains the formation of both an ester group and a carboxylic acid group is centered around a lactone intermediate (5):