Asymmetric Epoxidation of Alkenes other than Allyl Alcohols

4 Asymmetric Epoxidation of Alkenes Other Than Allyl Alcohols 

It is clear that in the asymmetric epoxidation of allyl alcohols, coordination by the OH functionality to the titanium center plays a crucial role. Such a coordination has a favorable effect on the entropy of activation, that is, the rate constant (see Question 14 of Chapter 8). It also helps to orient only one of the two possible enantiofaces for a facile oxygen atom transfer. With alkenes that do not have any such functional groups, the titanium tartarate system gives poor enantioselectivities. The precatalyst that has been found to exhibit re- 

The proposed mechanism for epoxidation with 9.38B as the catalyst is shown in Fig. 9.8. The oxidation state of manganese in 9.38A and 9.38B is three. The oxygen donor NaOCl or PhIO oxidizes Mn3+ to Mn5+, and an oxo complex such as 9.39 is produced. Reaction of 9.39 with the alkene produces the chiral epoxide and regenerates 9.38B. While there is enough evidence for the basic mechanism and the involvement of a catalytic intermediate such as 9.39, there is some controversy about the details of the oxygen atom transfer from 9.39 to the alkene. 

Two alternative mechanisms have been proposed. A direct substrate attack at the oxo ligand with concerted or sequential C-O bond formation is possible. Alternatively, the substrate may attack at both the metal and oxo centers to generate an oxametallocyclic intermediate. These two alternatives are shown in Fig. 9.9. Finally, note that in CytP450-catalyzed hydroxylation and epoxidation, an iron porphyrin intermediate of the type of 9.39 is involved. 

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The importance of this reaction also lies in the fact that asymmetric epoxidation of alkenes other than allylic alcohols is possible with this catalytic system (see Section 9.3.4). The third reaction relates to catalysts developed by Unilever for improved detergent action in the presence of hydrogen peroxide. The important point to note is that catalytic intermediates with metal-oxo groups play a pivotal role in all these reactions. 

Asymmetric epoxidation of alkenes other than allylic alcohols 99MI22. 

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