A Computational Study of the 1,4-Addition of Lithium Enolates to Conjugated Carbonyl Compounds

The MNDO potential energy surface for the reaction of acetaldehyde lithium enolate with acrolein was studied. Two water molecules were used to mimic ether solvent. Relative energies of stationary points were computed at the 3-21G//MNDO level. Four cyclic eight-membered transition structures 8-11 could be located, which allowed a qualitative rationalization of the observed stereoselectivity trends in the title reaction. The effects of enolate geometry and 3-enone substituent were evaluated at this level. Transition structure geometries were also optimized by ab initio molecular orbital calculations (3-21G) on the unsolvated structures 20-23. The ab initio optimized geometries were found to be very similar to the MNDO-optimized geometries. The relative energies of 20-23 were computed up to the MP2/6-31G*//3-21G level, which helped to explain the unrealistically high stereoselectivity predicted at the 3-21G//MNDO level.