Sharpless-Katsuki asymmetric epoxidation Mechanism

In 1980, Katsuki and Sharpless described the first really efficient asymmetric epoxidation of allylic alcohols with very high enantioselectivities (ee 90-95%), employing a combination of Ti(OPr-/)4-diethyl tartrate (DET) as chiral catalyst and TBHP as oxidant Stoichiometric conditions were originally described for this system, however the addition of molecular sieves (which trap water traces) to the reaction allows the epoxidation to proceed under catalytic conditions. The stereochemical course of the reaction may be predicted by the empirical rule shown in equations 40 and 41. With (—)-DET, the oxidant approaches the allylic alcohol from the top side of the plane, whereas the bottom side is open for the (-l-)-DET based reagent, giving rise to the opposite optically active epoxide. Various aspects of this reaction including the mechanism, theoretical investigations and synthetic applications of the epoxy alcohol products have been reviewed and details may be found in the specific literature . 

The mechanism of the asymmetric epoxidation of allylic alcohols with the Sharpless-Katsuki catalyst is assumed to be very similar to the one described for the Halcon-ARCO process in Section 2.5. The key point is that the chiral tartrate creates an asymmetric environment about the titanium center (Figure 18). When the allylic alcohol and the t-butyl hydroperoxide bind through displacement of alkoxy groups from the metal, they are disposed in such a way as to direct oxygen transfer to a specific face of the C=C double bond. This point is crucial to maximize enantioselectivity. 

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