TOWARD AN UNDERSTANDING OF THE HIGH ENANTIOSELECTIVITY IN THE OSMIUM-CATALYZED ASYMMETRIC DIHYDROXYLATION .2. A QUALITATIVE MOLECULAR MECHANICS APPROACH

A molecular mechanics (MM2*) model for the osmium-catalyzed asymmetric dihydroxylation (AD) based on an osmaoxetane intermediate is presented. The high enantioselectivities in the reaction can be rationalized in terms of stabilizing, attractive interactions between the largest oxetane substituent and the O9 substituent on the ligand in addition to repulsive interactions between certain hydrogen atoms in the ligand and the oxetane. The model has been developed to identify the key factors responsible for the observed enantiofacial selectivities as well as to qualitatively explain the selectivity trends observed for the various olefin classes (i.e. mono-, 1,1-di-, trans-1,2-di-, cis-1,2-di-, and trisubstituted olefins). Stabilizing attractive interactions are considered to be responsible for the much higher enantioselectivities and rate constants observed with the phthalazine ligands in comparison to the first generation ligands, especially with aromatic olefins as substrates.