Amidolithium-mediated enolization: Does proton transfer occur via a dimer intermediate with bridging carbonyls?

Ab initio RHF/6-31G* and MP2-fc/6-31G* computations predict that a very shallow local minimum of the LiNH₂·OCHMe dimer exists in which the carbonyl oxygens adopt the bridging role, and further that the activation enthalpy of acetaldehyde enolization by LiNH₂ is significantly lower for such a dimeric intermediate than for an unsolvated monomeric intermediate. Solvation of the monomeric intermediate with dimethyl ether reduces the activation enthalpy of enolization, but not to the near-zero level found for the bridged carbonyl dimer. However, starting from the amide-bridged dimeric global minimum, the solvated monomeric local minimum from which the transition state evolves is more easily accessible than the carbonyl-bridged dimeric local minimum. Inclusion of electron correlation effects is imperative in the determination of the transition state barriers.