Transient Inverted Metastable Iron Hydroperoxides in Fenton Chemistry. A Nonenzymatic Model for Cytochrome P450 Hydroxylation

Quantum mechanical calculations (DFT) have provided a mechanism for the oxidative bond cleavage step in Fenton-like hydrocarbon hydroxylation. A transition structure for hydrocarbon oxidation by aqueous solvated cationic iron(III) hydroperoxides ((H2O)(n)(FOOH)-O-III) is presented that involves a novel rearrangement of the hydroperoxide group (FeO-OH -> FeO center dot center dot center dot HO) in concert with hydrogen abstraction by the incipient HO center dot radical with activation barriers ranging from 17 to 18 kcal/mol. In every hydroperoxide examined, the activation barrier for FeO-OH isomerization, in the absence of the hydrocarbon, is significantly greater than the overall concerted activation barrier for C-H bond cleavage in support of the concept of O-O bond isomerization in concert with hydrogen abstraction. The transition structure for the oxidation step in simple anionic iron(III) hydroperoxides has been shown to bear a remarkable resemblance to model porphyrin calculations on cytochrome P450 hydroxylation.