What's the mechanism of a Dieckmann condensation?

A Dieckmann condensation is an intramolecular process where an ester undergoes base-catalysed condensation to form a β-keto ester.

In more detail, the Dieckmann condensation is a specific type of condensation reaction, named after the German chemist Walter Dieckmann. It is essentially an intramolecular version of the Claisen condensation. The reaction involves a diester (a molecule with two ester groups) and a strong base. The base is typically an alkoxide ion, which is an oxygen atom bonded to an alkyl group and carrying a negative charge.

The mechanism of the Dieckmann condensation involves several steps. First, the base deprotonates one of the α-hydrogens of the diester. This forms a carbanion, which is a carbon atom carrying a negative charge. This carbanion is also known as an enolate ion, due to its resemblance to an enol (a compound with a hydroxyl group attached to a carbon-carbon double bond).

Next, the enolate ion attacks the carbonyl carbon of the other ester group in the molecule. This is a nucleophilic attack, as the enolate ion is a nucleophile (an electron-rich species) and the carbonyl carbon is an electrophile (an electron-poor species). This attack forms a five- or six-membered ring, depending on the structure of the original diester.

The oxygen atom of the ester group then picks up a proton from the solvent, forming a hydroxyl group. Finally, the hydroxyl group is deprotonated by the base, forming a double bond between the carbon atoms and releasing a molecule of water. The final product is a β-keto ester, which has a carbonyl group (C=O) and an ester group on the same carbon atom.

The Dieckmann condensation is a useful reaction in organic chemistry because it allows the formation of cyclic β-keto esters, which can be further manipulated to form a variety of other compounds. It is also a good example of how bases can be used to catalyse condensation reactions.