E-Enolate Formation with LDA

The stereoselective formation of an E-enolate from an aldehyde and LDA (lithium diisopropylamide) demonstrates how transition state geometry controls product stereochemistry. The reaction proceeds through a highly organized cyclic transition state.

1. Li⁺ first coordinates to the carbonyl oxygen of propionaldehyde. This organizes the reactants and makes the α-proton more acidic.
2. The reactants arrange into a six-membered cyclic transition state, with the terminal methyl group preferentially adopting an equatorial position to minimize steric interactions.
3. The base abstracts the α-proton, forming the lithium-coordinated E-enolate product.