SolutionsEnolates of α,β-unsaturated ketones
Exercise 1
3-Methylcyclohex-2-enone is treated with LDA at −78 °C and then with CH3I. Which product is obtained? And with t-BuOK in t-BuOH followed by CH3I?
With LDA (kinetic control) the H on the α′ carbon (C6) is removed, giving 3,6-dimethylcyclohex-2-enone, which keeps the enone. With t-BuOK (equilibrium) the conjugated dienolate forms by loss of the γ H, and methylation at α (C2) gives 2,3-dimethylcyclohex-3-enone, with the double bond deconjugated.
Exercise 2
Why does protonation (or alkylation) of the conjugated dienolate occur at the α carbon and not at γ, even though the γ product, the conjugated enone, is more stable?
It is a matter of kinetic control: in the dienolate, the charge density at the α carbon (close to the oxygen) is higher than at γ, and the transition state for attack at α is lower in energy. The product formed faster is not the most stable one; the β,γ-enone only converts into the conjugated one if a catalyst allows equilibration.