enolates

Enolates are organic anions formed by deprotonation at the alpha position of carbonyl compounds, including aldehydes, ketones, esters, and amides. They are the conjugate bases of enols and are stabilized by resonance between an oxyanion form and a carbon-centered anion on the alpha carbon. This resonance gives enolates high nucleophilicity at the alpha carbon and, in some cases, the possibility of O-alkylation or C-alkylation.

Enolates are typically generated by strong, non-nucleophilic bases such as lithium diisopropylamide (LDA), sodium hydride, or potassium tert-butoxide in suitable solvents (often THF or ether). Kinetic enolates form rapidly at low temperature with bulky bases, whereas thermodynamic enolates form under warmer conditions with weaker bases. Enolates derived from carbonyl compounds can be complexed with metal cations (lithium, sodium, potassium) to give metal enolates; silyl enol ethers are protected forms that can be used in subsequent reactions. Some enolates are ambident, capable of reacting at either the oxygen or the alpha carbon; the outcome is influenced by the electrophile, solvent, temperature, and the metal counterion. Hard electrophiles tend to favor O-alkylation, while soft electrophiles can promote C-alkylation.

Enolate chemistry is central to carbon–carbon bond formation. Key reactions include aldol condensations, Claisen condensations, and Michael additions. Enolates serve as nucleophiles for alpha-alkylation and acylation, and can be protonated or transformed into protected derivatives such as silyl enol ethers for further manipulation. The ability to tune enolate formation and reactivity makes them versatile tools in organic synthesis.