Question

How do you form a Z-enolate over an E-enolate? Does it have to do with kinetics vs thermodynamics or just starics, or both? What is the reasoning behind this selectivity?

Answer 1

Both thermodynamic and steric factors determine the formation of Z-enolate.
On the one hand, use of weak, sterically unhindered base in amounts a little bit less than
stoichiometric and protonic solvent encourage reverse reaction of the enolate formation.
In these conditions thermodynamically more stable (Z)-enolate is formed.
On the other hand, when R1 (R1C(O)CH2R2) group is bulky (for example, tert-butyl group),
interaction between R1 and R2 becomes strong and (Z)-enolate is preferably formed.