Enamine
An enamine is an unsaturated compound derived by the condensation of an aldehyde or ketone with a secondary amine. Enamines are versatile intermediates.
The word "enamine" is derived from the affix en-, used as the suffix of alkene, and the root amine. This can be compared with enol, which is a functional group containing both alkene (en-) and alcohol (-ol). Enamines are considered to be nitrogen analogs of enols.
If one of the nitrogen substituents is a hydrogen atom, H, it is the tautomeric form of an imine. This usually will rearrange to the imine; however there are several exceptions (such as aniline). The enamine-imine tautomerism may be considered analogous to the keto-enol tautomerism. In both cases, a hydrogen atom switches its location between the heteroatom (oxygen or nitrogen) and the second carbon atom.
Enamines are both good nucleophiles and good bases. Their behavior as carbon-based nucleophiles is explained with reference to the following resonance structures.
Enamines are labile and therefore chemically useful moieties which can be easily produced from commercially available starting reagents. . A common route for enamine production is via an acid-catalyzed nucleophilic reaction of ketone ( Stork, 1963) or aldehyde ( Mannich/Davidsen 1936) species containing an α-hydrogen with secondary amines. Acid catalysis is not always required, if the pKa of the reacting amine is sufficiently high (for example, pyrrolidine- pKa 11.26). If the pKa of the reacting amine is low, however, then acid catalysis is required through both the addition and the dehydration steps ( common dehydrating agents include MgSO4 and Na2SO4). Primary amines are usually not used for enamine synthesis due to the preferential formation of the more thermodynamically stable imine species. Methyl ketone self-condensation is a side-reaction which can be avoided through the addition of TiCl4 into the reaction mixture ( to act as a water scavenger). An example of an aldehyde reacting with a secondary amine to form an enamine via a carbinolamine intermediate is shown below:
...
Wikipedia