Toward an understanding of the high enantioselectivity in the osmium-catalyzed asymmetric dihydroxylation .3. New insights into isomeric forms of the putative osmaoxetane intermediate

A systematic study of the possible diastereomeric osmaoxetane intermediates in the asymmetric dihydroxylation reaction (AD) has been undertaken. Several of these intermediates have been examined by DFT-calculations using ruthenium as a model for osmium. Changes in the puckering of the metallaoxetane, together with interactions of the substrate with the ''binding pocket'' of the ligand, seem to be responsible for the selectivities observed for the different olefin substitution patterns. Two levels of enantioselection, formation of the ligated osmaoxetane and its rearrangement to the glycolate, add up to the usually observed high enantioselectivity. An electrostatic dipole-dipole interaction between the osmaoxetane and the ester/imino ester moiety of the ligand is proposed to play an additional role in the transition state stabilization. The osmaoxetane intermediate explains all observed enantioselectivities in the asymmetric dihydroxylation reaction.