Epoxidation radical-type pathway

Scheme 11.18 Radical-type pathways to epoxidation of alkenes [133].

From these observations, one may conclude that the diol product arises directly from an iron-based oxidant, rather than from a carbon-based radical intermediate that captures OH or other oxygen-based radicals as in the mechanism proposed for the pentadentate ligand based catalyst. The formation of epoxide, however, is proposed to occur in a similar pathway to that described above, although the Fe—O—C—C-type intermediate then must have a shorter lifetime. Based on this interpretation, which admittedly is not the only possible one, one would conclude that no OH are present, and this is in agreement with the DFT-based suggestion that there is a direct formation of the iron(IV) catalyst firom its iron(II) precursor and H2O2, a proposal that is in agreement with the fact that no iron(III) intermediates are observed (see Scheme 22, Fig. 22). 

TT-cation radical complexes.In the reactions, oxoiron(IV) porphyrin rr-cation radicals of electron-rich porphyrins reacted fast with ROOH (i.e., catalase and peroxidase type of chemistry one-electron oxidation of ROOH) (Scheme 2, pathway A). On the other hand, oxoiron(IV) porphyrin rr-cation radicals of electron-deficient porphyrins reacted fast with olefins to yield epoxide products (i.e., cytochrome P450 type of chemistry oxygen atom transfer) (Scheme 2, pathway B). These results demonstrated that electron-deficient iron porphyrin complexes are better catalysts in hydrocarbon oxygenations by hydroperoxides, since these complexes can avoid the facile decomposition of oxoiron intermediates by ROOH (Scheme 2, pathway A). Indeed, highly electron-deficient iron(III) porphyrin complexes efficiently catalyze alkane hydroxylations by H2O2 in aprotic solvents. 

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