Enamines--Milder Enolate Equivalents

We saw in Chapter 17 that secondary amines condensed with aldehydes and ketones to create enamines because of the available proton transfer chemistry and the stability of the C=C bond. The utility of an enamine lies in the fact that it shares electronic properties with an enolate anion. Examination of the resonance forms reveals our expectation of anionic, electron-rich character at one of the enamine carbons:
Formation of an enamine (condensation of a ketone with a secondary amine) and resonance showing the carbanionic form

Formation of an enamine (condensation of a ketone with a secondary amine) and resonance showing the carbanionic form

We can see this further by comparing ESP maps of an enolate anion and this enamine:


Load the ESP maps Background: White Black Make the surfaces translucent Note the yellow area on the enolate carbon, compared to the orange area on the 6-member rinf of the enamine. This denotes the high electron density and nucleophilic character.

Because the carbon is now nucleophilic, it can be alkylated under milder conditions than required for generating the enolate. Acid hydrolysis restores the aldehyde/ketone functional group.
3-step process: condense amine with 2-methylpropanal; alkylate with 1-bromobutane; hydrolyze back to the aldehyde.