Enamines

Enamines are unsaturated amines derived from aldehydes or ketones by condensation with a secondary amine. They contain a carbon–carbon double bond adjacent to an amine substituent, commonly written as R2N-CH=CRR'. The nitrogen is attached to the α-carbon of the original carbonyl, and the C=C bond lies between the α- and β-carbons.

Enamines are typically formed by reacting a carbonyl compound with a secondary amine (for example diethylamine or pyrrolidine) with loss of water. Removal of water and often the use of a dehydrating agent or molecular sieves shifts the equilibrium toward the enamine. The reaction is reversible; under acidic conditions hydrolysis re-forms the carbonyl compound and the amine.

Enamines behave as nucleophiles at the α-carbon. They are used to form C–C bonds via alkylation, acylation, or other electrophilic additions, a strategy exemplified by the Stork enamine synthesis for α-alkylation of ketones. They can also participate in aldol-type reactions and related transformations. After reaction, hydrolysis removes the enamine framework, regenerating the carbonyl compound.

Compared to imines, enamines are the saturated enamine form and can be viewed as enolate equivalents in many reactions. Cyclic enamines derived from secondary amines (for example, pyrrolidine-derived enamines) are common and often more stable. Enamines are valuable intermediates in organic synthesis, enabling direct formation of α-substituted carbonyl compounds without using metal enolates.