High-Yield Epoxidations with Hydrogen Peroxide and tert-Butyl Hydroperoxide Catalyzed by Iron(III) Porphyrins: Heterolytic Cleavage of Hydroperoxides

The reactions of hydrogen peroxide or tert-butyl hydroperoxide with cyclooctene and norbornene, catalyzed by iron(III) tetrakis(pentafluorophenyl)porphyrin chloride and other electronegatively-substituted porphyrins, produce 60–100% epoxide yields. The epoxynorbornane yield has the same ratio of exo/endo isomers as is obtained using pentafluoroiodosylbenzene, an oxidant which produces the iron(IV) radical cation (oxene) intermediate. These results demonstrate heterolytic cleavage of these peroxides in the catalyzed reaction. We use the fast-reacting pentafluoroiodosylbenzene (or m-chloroperbenzoic acid) to produce the oxene in the presence of tert-butyl hydroperoxide. Under these conditions, the hydroperoxide does not react directly with the iron(III) porphyrin. The same loss of epoxide stereochemistry and reduction in yield were previously submitted as evidence for homolytic cleavage; this earlier explanation is invalid. Formerly, iron(III) tetraphenylporphyrin chloride and iron(III) tetramesitylporphyrin chloride failed to produce high epoxide yields or significant stereospecificity. These results are explained as follows: heminderived oxenes highly prefer hydroperoxide oxidation to epoxidation. This preference is much lower for electronegatively-substituted hemins, pointing the way to high epoxide yields. Previous evidence for homolysis of hydroperoxide bonds in these catalytic reactions is shown to accord with heterolysis; definitive evidence for heterolysis of hydrogen peroxide and hydroperoxides in protic solvents is presented.