Ej. Corey et Mc. Noe, A CRITICAL ANALYSIS OF THE MECHANISTIC BASIS OF ENANTIOSELECTIVITY INTHE BIS-CINCHONA ALKALOID CATALYZED DIHYDROXYLATION OF OLEFINS, Journal of the American Chemical Society, 118(45), 1996, pp. 11038-11053
This paper presents a critical analysis of two transition state models
for the bis-cinchona alkaloid catalyzed enantioselective dihydroxylat
ion of olefins using a broad range of experimental data. In one model
(Sharpless) the transition state resembles a metallaoxetane structure
formed by [2 + 2] cycloaddition of Os=O and C=C, and in the other the
transition state is a five-membered structure in which one axial and o
ne equatorial oxygen of cinchona bound OsO4 are becoming attached to t
he olefinic carbons by a [3 + 2] cycloaddition process from an Os-olef
in pi-complex (CCN model, Figure 1). Data on the enantioselectivity of
the asymmetric dihydroxylation of a wide variety of olefinic substrat
es and on the selectivity of a range of catalyst structures agree well
with expectations based on the CCN model, but not the Sharpless model
.