A Critical Analysis of the Mechanistic Basis of Enantioselectivity in the Bis-Cinchona Alkaloid Catalyzed Dihydroxylation of Olefins

This paper presents a critical analysis of two
transition state models for the bis-cinchona alkaloid
catalyzed
enantioselective dihydroxylation 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 one equatorial oxygen of cinchona
bound OsO4 are becoming attached to the olefinic carbons by
a [3 + 2] cycloaddition process from an Os−olefin
π-complex (CCN model, Figure ). Data on the enantioselectivity
of the asymmetric dihydroxylation of a wide
variety of olefinic substrates and on the selectivity of a range of
catalyst structures agree well with expectations
based on the CCN model, but not the Sharpless model.