Ethylene epoxidation by an iron(III)-hydroperoxo model of cytochrome P450 Fe(OOH)(C20N4H12)(SCH3) is investigated with the B3LYP density functional theory (DFT) method to look at whether or not the iron(III)-hydroperoxo species can participate in olefin epoxidation with the iron(IV)-oxo species (compound I). The answer is negative. There are two possible entrance channels in the epoxidation reaction, a proximal oxygen transfer mechanism and a distal oxygen transfer mechanism, both mechanisms being stepwise processes involving radical intermediates. In the proximal oxygen transfer mechanism, the homolytic cleavage of the Fe-O bond of the iron(III)-hydroperoxo species initially occurs with a change in the Fe atomic charge from +3 to +2. The proximal oxygen transfer mechanism involves an energetically less stable radical CH2CH2(OOH). In contrast, in the initial stages of the distal oxygen transfer mechanism, the O-O bond of the hydroperoxo ligand is homolytically cleaved, and the iron(IV)-oxo complex thus formed retains a weak interaction with the hydroxy group of the resultant radical intermediate involving CH2CH2 (OH). This mechanism is more realistic, but the activation energy of the transition state of the O-O bond cleavage is comparable to that of the corresponding transition state by hydrogen peroxide. The present result suggests that the mechanism and energetics of ethylene epoxidation remain unchanged by the involvement of the iron-porphyrin complex. The epoxidation of ethylene by a compound I model is also calculated to increase our understanding of olefin epoxidation by P450. The mechanism and energetics of olefin epoxidation by iron(III)-hydroperoxo species are less plausible than epoxidation by compound I.
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