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.