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Aldol Reactions and Aldol-like Reactions

Enamines

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.

Fig.1
Imine-enamine tautomerism.
Fig.2
Enamines.

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.)

Fig.3
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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