Highly enantioselective cyclopropanation of styrenes and diazoacetates catalyzed by 3-oxobutylideneaminatocobalt(II) complexes, part 2. Semiempirical analysis of diastereo and enantioselectivities

A semiempirical analysis using the PM3(tm) method was performed on the 3-oxobutylideneaminatocobalt(II) complexes-catalyzed cyclopropanation. Although the energy diagram calculated by the PM3(tm) method had a contradiction, where the product was less stable than the reactant, the structures and the energies of the reactant and the transition state obtained by the PM3(tm) method could be relatively reliable by comparing with the results based on the density functional theory. Analysis of the transition states by the PM3(tm) method indicated that the olefin approached parallel to the cobalt-carbene bond with bisecting an O₁-Co-O₂ angle. It was revealed that the transition state affording the (1S,2S)-arylcyclopropanecarboxylic acid ester corresponding to the (1S,2S)-cobalt complex was the most stable, because of the lowest steric repulsion among the aryl group of the diamine unit, the ester moiety, and the approaching olefin. It was clarified that the enantioselectivity occurred due to the repulsion between the aryl group of the diamine unit and the ester moiety and that the diastereoselectivity was controlled by the repulsion between the ester moiety and the approaching olefin.