Question

Question: Treatment of α, β-unsaturated carbonyl compound X with base forms the diastereomer Y. Write a stepwise mechanism for this reaction. Explain why one stereogenic center changes configuration but the other does not.

Short answer

Answer

Stepwise mechanism for the reaction:

Step 1

Step 2

$\gamma$-hydrogen is required for abstraction which is not present for other stereogenic centers. Therefore, only one stereogenic center changes configuration.

Step-by-step solution

Step 1:  α,β- unsaturated compound

Organic compound containing two functional groups, the carbonyl group and carbon-carbon double bond are known as $\alpha ,\beta$-unsaturated compound

$\alpha ,\beta -$ unsaturated compound contains a diene skeleton

Step-wise mechanism

The stepwise mechanism for the provided reaction is shown as follows:

The base abstracts the proton from $\gamma$hydrogen of $\alpha ,\beta -$unsaturated compound X and gives an enolate.

Abstraction of proton

Here, the reformation of the C=O group then protonation of hydrogen at asymmetric center can result in inversion of configuration

Protonation at asymmetric center

$\gamma$$\gamma$-hydrogen is required for abstraction which is not present for other stereogenic center. Therefore, only one stereogenic center changes the configuration