TOWARD AN UNDERSTANDING OF THE HIGH ENANTIOSELECTIVITY IN THE OSMIUM-CATALYZED ASYMMETRIC DIHYDROXYLATION .3. NEW INSIGHTS INTO ISOMERIC FORMS OF THE PUTATIVE OSMAOXETANE INTERMEDIATE
Po. Norrby et al., TOWARD AN UNDERSTANDING OF THE HIGH ENANTIOSELECTIVITY IN THE OSMIUM-CATALYZED ASYMMETRIC DIHYDROXYLATION .3. NEW INSIGHTS INTO ISOMERIC FORMS OF THE PUTATIVE OSMAOXETANE INTERMEDIATE, Journal of the American Chemical Society, 118(1), 1996, pp. 35-42
A systematic study of the possible diastereomeric osmaoxetane intermed
iates in the asymmetric dihydroxylation reaction (AD) has been underta
ken. Several of these intermediates have been examined by DFT-calculat
ions using ruthenium as a model for osmium. Changes in the puckering o
f 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 b
etween the osmaoxetane and the ester/imino ester moiety of the ligand
is proposed to play an additional role in the transition state stabili
zation. The osmaoxetane intermediate explains all observed enantiosele
ctivities in the asymmetric dihydroxylation reaction.