What is an Enamine?

Enamines (English: Enamines) are a type of unsaturated amine compounds containing double bonds and carbon atoms on the double bonds connected to nitrogen atoms ^[1] , which can be regarded as nitrogen-containing enols and are widely used in medicine and biology ^{[ 1]} , synthesis and other fields ^[2] . Enamines can be formed by dehydration condensation of an aldehyde or ketone with a secondary amine.

Enamines are made from ketones

Enamines are usually prepared by using a ketone with at least one -hydrogen and a secondary amine under the catalysis of an acid (such as p-toluenesulfonic acid), and adding a strong dehydrating agent (such as titanium tetrachloride) or an azeotrope to make The reaction went to completion. The reaction mechanism is shown in the following figure:

Commonly used secondary amines are tetrahydropyrrole, morpholine and hexahydropyridine, and their reactivity decreases from left to right. The reaction of asymmetric ketone with secondary amine mainly produces the enamine with the least substitution on the double bond carbon. This is because the methyl group connected to the double bond carbon will mutually repel the hydrogen on the tetrahydropyrrole ring, reducing the product. The stability.

Enamine synthesis is a reversible reaction. Under the action of dilute acid, enamine is hydrolyzed to ketone and secondary amine ^[3] .

Enamines are prepared from alkynamines

The reduction reaction of alkynamine can be used to prepare enamine with cis-trans selectivity. ^[4-6]

Reduced alkynamine