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.