In this enolate reaction wouldn't we get the ethyl group attaching from the back side due to the minimal 1,3 allylic strain shown in confomer B. I am having trouble seeing how this reaction gives compound C instead of its diastereomer.
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The structure of the starting material could be redrawn as follows, maintaining the same stereochemistry, keeping the methyl group in the plane of the paper and the phenyl group behind the plane of the paper.
Upon minimizing the energy of the starting material in Chem3D, it could be seen that the conformer orients itself in such a way that the phenyl group is nearer to the lithium cation in a perpendicular fashion, allowing feasible cation-pi interaction. Thus, it is the most favourable conformation of the enolate.
The A value of the methyl group is around 1.8 kcal/mol, whereas the phenyl group has an A value of 3.0 kcal/mol.List of A values The allylic strain provided by the phenyl group would be almost twice as much as the methyl group. The phenyl group would hinder the incoming ethyl iodide electrophile from a backside attack; thus, the given structure of the product is observed.
