Enamines: synthesis and reactivity

Introduction

Enamines are products very deriving from the addition of secondary amines to carbonyl compounds, very similar to imines. The name makes clear that enamines contain a double bond carbon-carbon ("en", from alkene) and the amino functional group ("amine"). THe imagine shows that the double bond is adjacent to the amine group, then in total  C=C−N, the same carbon is unsaturated and carries the amino function. Here's the general structures of enamines:

enamine

Enamine synthesis

Enamines synthesis mechanism is really similar to what we've seen for imine formation (that's not a surprise, considering that imines are linked by a tautomeric equilibrium to corresponding enamines) except for a step. Indeed, once we got to the iminium ion, in the case of imine a proton (H+) was lost by the nitrogen atom. In the case enamine instead, considering that we've started with a secondary amine that has already lost its nitrogen proton, the iminium ion can't be made stable that way; what happen instead is that the proton is lost (removed) from the  carbon adjacent to the nitrogen-carbon double bond (C=N+).

enamine-synthesis-mechanism

Also the synthesis of enamine contemplate the loss of water (H2O) considering that's a condensation reaction.

enamine-synthesis-overall-reaction

Back to the reagents : enamines hydrolysis 

Again, just like imines, enamines require a slightly acidic environment to encourage the loss of water (the hydroxyl group -OH is turned in a better leaving group. But enamines and imines synthesis are equilibrium reactions. This means that properly adjusting reagents, we can have the reaction backwards, in other words, if we don't keep water far from enamines, the hydrolysis is unavoidable.

The hydrolysis of an enamine is therefore (obcviously) the exact opposite of its synthesis. Treating an enamine with water, especially in a slightly acidic environment (to turn the amine in a better leaving group, ammonium) we can obtain the corresponding keton or aldehyde. The process is actually favored over the other,  because the products (carbonyl compound and amine) are more stable than the condensed molecule (enamine or imine)

enamine-hydrolysis-reaction-mechanism

1) Electrophilic addition of H+ to the double bond. The resulting carbocation is stabilized by the adjacent nitrogen N (electron ricb) and is attacked by H2O.

2) Intermolecular (or intramolecular) protons exchange makes the amine a good leaving group. At the same time, oxygen lone pair moves on the carbon forming a new π bond..

3) Loss of H+ (that indeed as just a catalytic role in this reaction) and the neutral species is obtained (ketone or aldehyde).

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