The commercially available '1994 Reagent of the Year',  Jacobsen's catalyst (1), converts achiral olefins to chiral epoxides with enantiomeric excesses regularly better than 90%  and sometimes exceeding 98%. To date, the origin of this dramatic selectivity has not been explained.
Jacobsen's Catalyst (1) is one member of a class of over 75 epoxidation catalysts  based on a chiral salen ligand complexed with Manganese(III) (the base salen ligand in Figure 2 is drawn in red).
Jacobsen's catalyst is a convenient and effective tool for stereoselective synthesis as demonstrated by its use in the synthesis of the key side chain in the antitumour drug Taxol. Jacobsen has recently reported a simple large-scale preparation of catalyst, making it readily available in kilogram quantities. The reaction requires only a few percent of catalylst to substrate while making use of a low cost oxidant - common bleach. These simple requirements result in less chemical waste, an increasingly important concern in the chemical industry.
The mechanism of the reaction has two related but distinct aspects: the mode of oxygen transfer and the method of chiral induction. Neither aspect is fully understood.
The oxygen transfer is known to occur by a two step catalytic cycle (Figure 3), similar to the accepted mechanism of metal porphyrin catalysed epoxidations. An oxidant transfers atomic oxygen to the MnIII catalyst in the first step. The oxygen presumably coordinates to the metal in a site normal to the porphyrin or salen plane, and the activated oxygen is then delivered to the alkene. This cycle, as opposed to assisted transfer by a complex aggregate, is supported by observations that several oxidants have been used with identical selectivity, that the reaction consumes stoichiometric quantities of an oxidant in the process of converting an alkene to an epoxide, and that related salen complexes show incorporation of oxygen from in situ labelled water, which does not occur via exchange with the oxidant or epoxide.
Figure 3 The epoxidation occurs by a two step catalytic cycle
Oxygen transfer from the catalyst to the olefin has been hypothesized to occur by several different mechanisms (Figure 4) including attack of an oxygen radical intermediate on the double bond (A), concerted oxygen delivery (B), and formation of a metallaoxetane intermediate (C).
Figure 4 Three possible modes of oxygen delivery
Although Norrby and coworkers have recently proposed a mechanism by which a metallaoxetane is a plausible intermediate, and Jacobsen has demonstrated that it is likely oxidation of different olefins occurs by different mechanisms, it is generally accepted that oxygen transfer to olefins with radical stabilizing groups occurs by an attack of an oxygen centred radical on the olefin. This mode of oxygen delivery is supported by the observation that cis olefins result in both cis and trans epoxides.
The second aspect of the epoxidation mechanism - the cause of stereoselectivity in the reaction - must occur in the second step of the catalytic cycle where oxygen is transferred from the oxo-manganese complex to the epoxide. Fundamental to this stereoselection is the relative orientation of the activated catalyst and the olefin in the first irreversible bond formation. In the following section, we will provide a novel hypothesis for the origin of stereoselection in the Jacobsen Epoxidation.