Enamine

An enamine is an organic compound featuring a carbon–carbon double bond adjacent to a nitrogen atom, typically written with the structural motif R2N–CH=CH–R. Enamines are formed from a ketone or aldehyde and a secondary amine, yielding the enamine as the condensation product of the carbonyl compound with the amine.

Preparation and scope: Enamines arise by condensation of a carbonyl compound (aldehyde or ketone) with a secondary amine under acid catalysis, with removal of water. The equilibrium favors enamine formation under dry conditions and often requires a drying agent or azeotropic water removal. Tertiary amines do not form enamines, and primary amines tend to give imines rather than enamines.

Reactivity and applications: Enamines are nucleophilic at the alpha carbon next to the nitrogen, acting as enolate equivalents in carbon–carbon bond formation. A principal use is the Stork enamine synthesis, wherein an enamine derived from a carbonyl compound reacts with electrophiles (alkyl, acyl, allyl, or other activated partners) and is then hydrolyzed to reveal the corresponding α-substituted carbonyl compound. This enables α-alkylation or α-acylation of ketones and aldehydes. Enamines also participate in catalytic, stereo- and enantioselective transformations when used as organocatalysts in enamine catalysis, enabling aldol-type and related reactions.

Overview and notes: Enamines are distinct from imines, representing a saturated carbon–nitrogen bond adjacently linked to a C=C system. They are typically prepared from secondary amines and carbonyl compounds in dry conditions; hydrolysis regenerates the carbonyl. Enamines are sensitive to moisture and heat, and their formation is reversible, governed by the relative stability of the starting carbonyl compound and the secondary amine.