enolate

An enolate is the conjugate base of a carbonyl compound after deprotonation at the α-carbon. In solution, the negative charge is delocalized between the α-carbon and the carbonyl oxygen, giving two resonance forms: an oxyanion form and a carbanion form. Enolates are ambident nucleophiles and commonly react at carbon to form C–C bonds, though attack at oxygen is possible under some conditions.

Enolates are typically generated by deprotonation of aldehydes, ketones, and esters with strong bases such as LDA, NaH, or metal amide bases, often in ethers like THF or DME at low temperature. The metal cation (lithium, sodium, potassium) and the reaction conditions influence aggregation and reactivity. Enolates can be isolated as salts (e.g., lithium or sodium enolates) or converted into silyl enol ethers for protection or use in further transformations.

Reactivity and selectivity: Enolates participate in alkylation with primary or secondary alkyl halides, acylation, and various carbonyl condensations. They are central to aldol reactions, Claisen condensations, and Michael additions. Kinetic enolates (formed under bulky bases at low temperature) favor less substituted α-carbons, while thermodynamic enolates (formed at higher temperature with stronger bases) favor more substituted α-carbons.

Acidity: The α-C–H of carbonyl compounds is relatively acidic, with pKa values in the ~20–25 range for ketones and esters, enabling selective deprotonation under appropriate conditions.

Derivative and protection: Silyl enol ethers are protected enolates obtained by silylation; they serve as versatile electrophilic partners in reactions such as Mukaiyama-type aldol reactions.