Ab initio transition states for the reaction of α-hydroxyl lithium ester enolates with imine

The transition states for the reactions of α-hydroxyl ester lithium enolates with imine to form β-lactams were located using ab initio molecular orbital theory at the RHF/3-21G and RHF/6-31G** levels. MP2/6-31G* energy calculations indicated that the Z-isomer of the ester enolates is much more stable than the E-isomer (5.0–6.9 kcal/mol). This can be rationalized by the coordination of lithium with the α-hydroxyl oxygen. The reaction of the Z-enolate with imine proceeds through a boat-like transition state, which is energetically favored over an envelope-like transition state for the reaction of the E-enolate with imine (9.7 kcal/mol, MP2/6-31G*//RHF/3-21G). A solvent molecule (NH3) was incorporated into the boat-like transition state at the RHF/3-21G level. Neither the change of basis set nor inclusion of solvents had any significant effect on the transition state geometry.