Enamines will form when secondary amines react with aldehyde/ketone that have at least one alpha (α) proton.
Here are the overall reactions.
This reaction is catalyzed by acid and is often driven to completion by removing water. There is an optimal amount of acid required, too much or too little and the reaction will slow down.
The mechanism proceeds exactly like that for imine formation in which an analogous carbinolamine is formed. The carbinolamine hydroxyl group must be protonated, activating it to be eliminated as water. Once the water is expelled an alkylated iminium ion is formed. This is where enamine formation differs from imine formation. Deprotonation of the proton alpha to the iminium results in the enamine.
In enamines, the lone pair of electrons is conjugated with the adjacent π system. The lone pair is delocalized as shown by the following resonance structures.
Notice the negative charge in the resonance structure on the right. This carbon is nucleophilic and is analogous to enolates we will talk about soon.
Examples of Reactions
Cyclic secondary amine work nicely!
In this example, acetone forms an enamine and an imine.