Under thermodynamic control, the ( Z )- and ( E )-forms of the enolates are in rapid equilibrium, and the product distribution is determined by the relative stabilities of the six-membered chair-shaped cyclic transition states that includes the metal counter-ion (Scheme 4). Transition sate that leads to the syn product has R in the less stable axial position, whereas in that leading to anti product both R and R' are in the more stable equatorial position. The latter is therefore of lower energy, leading to a major anti product.

Scheme 4

In contrast, under kinetic control, the ( Z ) and ( E ) enolates are formed rapidly and irreversibly, and their relative amounts determine the product distribution (Scheme 5). For examples, for ketone CH₃ CH₃ CO t -Bu, the ( Z )-enolate is normally formed to afford the syn diasteroisomer as a

Scheme 5

major product. But, the selectivity falls to 4:1 ( syn : anti ) when the size of the R is reduced from t -Bu to isopropyl. This is presumably because of the difference in steric repulsion of the methyl group with t-butyl and isopropyl groups.

However, there is a general technique to increase the degree of diastereoselectivity. The enolate can be converted into silyl enol ethers that can be separated by distillation. The separated silyl enol ethers can then be converted into pure ( Z )- or ( E )-enoate by treatment with fluoride ion (Scheme 6).

Scheme 6