Question

What is the major product of the Stork enamine synthesis, that is, the reaction of cyclopentanone with pyrrolidine, heat, `CH_3CH_2Br`, `H^+`, and `H_2O`?

Answer 1

The major product is 2-ethylcyclopentanone.

Explanation:

The reaction is often called the Stork enamine synthesis.

Step 1. Formation of an enamine.

Enamine formation

Step 2. `"S"_text(N)2` alkylation

The double-bonded carbon in the enamine has a partial negative charge, so it can act as a nucleophile and displace the `"Br"` from ethyl bromide and form an iminium salt.

Alkylation

Step 3. Hydrolysis of the iminium salt

The iminium salt is hydrolyzed to regenerate the ketone and the amine.

Hydrolysis

Note: The reaction is done this way because direct alkylation of the cyclopentanone enolate gives multiple alkylation, but the enamine stops at one alkylation.