Enolaatins

Enolates, sometimes written as enolate ions, are the conjugate bases formed when carbonyl compounds such as ketones, aldehydes, esters, or amides lose an alpha hydrogen. The negative charge is delocalized between the alpha carbon and the carbonyl oxygen, giving two main resonance forms: an oxygen-centered form (O-enolate) and a carbon-centered form (C-enolate). This resonance makes enolates ambident nucleophiles, capable of attacking electrophiles at either carbon or oxygen, depending on the substrate and conditions.

Enolates are typically generated by deprotonating the alpha position of a carbonyl compound with strong bases. Common bases include lithium diisopropylamide (LDA), sodium hydride, NaHMDS, and organolithium reagents, usually at low temperatures in polar aprotic solvents such as THF or DME. Other methods include metal enolate formation using zinc or magnesium reagents or converting a carbonyl compound into a silyl enol ether, which can later be converted back to the enolate under mild conditions. The choice of base, solvent, and temperature influences whether the kinetic or thermodynamic enolate predominates.

Reactivity and applications form the core of enolate chemistry. Enolates readily undergo alkylation at the alpha position with alkyl halides and related electrophiles, enabling carbon–carbon bond formation. They participate in aldol reactions, Claisen condensations, and Michael additions, and can be employed in cross-aldol and Stork enolate-type strategies to achieve stereoselective transformations. Silyl enol ethers serve as protected enolates and are versatile intermediates in Lewis acid–catalyzed reactions and rearrangements.

Enolate chemistry is central to modern organic synthesis, enabling selective access to α-substituted carbonyl compounds and complex molecular architectures.