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 pi-complex (CCN model, Figure 1). 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.