Question

I saw the following reaction on Modern methods of Organic Synthesis by Carruthers. The reaction is about the selective alkylation of trimethylsilyl enolates, and is shown below. However, I have some doubts regarding the formation of the enolate in the first step.

1. Why does deprotonation occur from the alpha carbon on the left, and not from the gamma carbon? Is it due to steric repulsion between lone pair of oxygen and negative charge on Nitrogen? But there two gamma carbons, and only one would be hindered?
2. The condition shown is the kinetic condition (LDA,THF, -78 °C). So the given enolate is the kinetic enolate. Is it classified as a kinetic enolate due to the lesser number of resonance structures (less stable alkene)?
(Normally, if the enolate is less substituted, it is termed as kinetic, and more substituted one is the thermodynamic enolate)
3. Should the carbanion stability of each resonance hybrid be accounted to determine the type of enolate?
Kindly provide answers to all questions precisely. Otherwise, please do not attempt. I really need clear answers. Thanks in advance.

Answer 1

The deprotonation occurs from the alpha carbon due to steric repulsion. The given enolate is classified as kinetic due to its stability. Carbocation stabilization is not relevant to the formation of the enolate.

Step-by-step explanation:

In the selective alkylation of trimethylsilyl enolates, the deprotonation occurs from the alpha carbon on the left due to steric repulsion between the lone pair of oxygen and the negative charge on the nitrogen. The gamma carbon is less hindered and therefore more accessible for deprotonation.

The given enolate is classified as a kinetic enolate because it has fewer resonance structures and is less stable compared to the thermodynamic enolate. The carbanion stability of each resonance hybrid can also be accounted for to determine the type of enolate.

Carbocations are stabilized by neighboring carbons, making tertiary carbons more prone to carbocation formation. This stabilization is not relevant to the formation of the enolate in this reaction.