IS HOLE TRANSFER INVOLVED IN METALLOPORPHYRIN-CATALYZED EPOXIDATION

The possibility of a hole-transfer mechanism for the epoxidation of alkenes catalyzed by metalloporphyrins (MP) has been investigated. In the first approach, the results of MP-catalyzed epoxidation of a series of substrates were compared to the corresponding results of epoxidation under conditions where cation radicals are demonstrably formed (using a triarylaminium salt catalyst). In sharp contrast to the MP-catalyzed epoxidations (using M = Mn), the hole-catalyzed epoxidations do not generate carbonyl compounds and alcohols as byproducts and are rigorously stereospecific. These results are of interest primarily in that they provide support for the assumption that cation radicals can be efficiently and stereospecifically epoxidized by appropriate oxygen-transfer agents. However, the differences in product composition and stereochemistry for MP- vs hole-catalyzed epoxidation cannot be construed mechanistically to rule out a cation radical mechanism for the former, especially because the oxygen-transfer agents are different in the two reaction systems. In a second, and more rigorous, approach, a careful search for transient cation radical intermediates in MP-catalyzed epoxidations (using M = Mn and Fe) was carried out using newly developed cation radical probe reactions. Cation radical intermediates were, in fact, not detected and, if involved, must be extremely short lived (<2 X 10(-12)s). The results of this work, taken as a whole, are reasonably construed to suggest that, even for the relatively easily ionizable alkene functionalities present in many of the probe substrates, a hole-transfer mechanism is probably not operative in MP-catalyzed epoxidations.