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Aldol Reactions and Aldol-like Reactions


The term enamine is composed of the components "en" for an alkene, or carbon-carbon double bond and "amine" for an amine or an amino group. The name enamine is used for α,β-unsaturated amines, that is, for vinyl amines. Analogous to keto-enol tautomerism, enamines are in equilibirum with the corresponding imines. The interconversion of enamines and imines is called imine-enamine tautomerism, where the equilibrium position is usually mainly located on the imine side.

Imine-enamine tautomerism.

However, tertiary enamines cannot convert into the corresponding imine, as there is an absence of the nitrogen-bonded hydrogen atom required for tautomerism. Enamines are good nucleophiles. Thus, they can be added to aldehydes and ketones in an aldol-like reaction under acidic conditions. Amines that are frequently used as amine components of enamines are, for instance, the heterocyclic amines pyrrolidine, piperidine, and morpholine. Due to the +M effect of the amino group, enamines are electron-rich, good nucleophiles. With suitable alkyl halides (methyl halides, primary and some secondary alkyl halides), enamines react in an SN2 reaction in which the halide is exchanged for the enamines β carbon. Carbonyl compounds are converted into enamines by being treated with the respective amine under acidic conditions. The β carbon may then nucleophilically attack an electrophile, such as an alkyl halide. Last but not least, the carbonyl group can be recovered through hydrolysis. Consequently, the α position of carbonyl compounds can, for instance, be alkylated by an alkyl halide or acylated by an acyl halide. (Note that the α carbon of a carbonyl compound is the β carbon of the corresponding enamine.)

Alkylation of enamines.

If asymmetric ketones are applied, two different enamines are yielded by enamine formation. These are in equilibrium via the imine through imine-enamine tautomerism. However, subsequent alkylation theoretically leads to two different monoalkylated products. Moreover, it is possible that additional alkylations may occur, as long as the imine's α carbon carries at least one hydrogen.

Exercise 1

Exercise 2

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