Z-Enolate Formation with LDA

The stereoselective formation of a Z-enolate from a ketone 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 the ketone. This organizes the reactants and makes the α-proton more acidic.
2. The reactants arrange into a six-membered cyclic transition state. The terminal methyl group adopts an axial position to avoid allylic strain with the bulky tert-butyl group.
3. The base abstracts the α-proton, forming the lithium-coordinated Z-enolate product.

This stereocontrolled deprotonation shows how substrate structure affects enolate geometry. While aldehydes favor E-enolates through minimizing diaxial interactions, ketones can favor Z-enolates when allylic strain becomes the dominant factor.