Generate the active oxidant within the confines of

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generate the active oxidant within the confines of the enzyme active site in the immediate proximity of the bound substrate. Without the constraints im- posed by the enzyme, however, iron porphyrins generally will either (1) cata- lyze the oxidation of the reducing agent by dioxygen, leaving the substrate un- touched, or (2) initiate free-radical autoxidation reactions (see Section II.C). A different approach was suggested by the observation of the peroxide shunt re- action (Reactions 5.68 and 5.69) using organic peroxides or single-oxygen-atom donors, and the earliest successful studies demonstrated that Fe(TPP)CI (TPP = tetraphenylporphyrin) would catalyze the epoxidation of olefins and the hydroxylation of aliphatic hydrocarbons by iodosylbenzene 8 1(Reactions 5.70 and 5.71). IPh OH 0" OIPh '"""'''. 6" IPh 0+ OIPh Fe(TPP)CI, 0 0 + IPh (5.70) (5.71) Reactions (5.70) and (5.71) were postulated to occur via an iron-bound ox- idant such as 3 in Reaction (5.67). This hypothesis was tested by studying the reaction of dioctyl Fe(PPIX)CI with iodosylbenzene, which resulted in 60 per- cent hydroxylation at positions 4 and 5 on the hydrocarbon tail (see 5.72), po- sitions for which there is no reason to expect increased reactivity except for the fact that those particular locations are predicted from molecular models to come closest to the iron center when the tail wraps around the porphyrin molecule. 82
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~4 ~ o 5,,----, O~F'{I"J_ Rooe V.OXYGENASES 289 (5.72) The nature of the species produced when single-oxygen-atom donors react with FeIII-porphyrin complexes has been deduced from studies of an unstable, bright-gr,een porphyrin complex produced by reaction of FeIlI(TMP)CI (TMP = 5,1O,15,20-tetramesitylporphyrin) with either iodosylbenzene or per- oxycarboxylic acids in solution at low temperatures. 81 ,83 Titrations of this green porphyrin complex using I - as a reducing agent demonstrated that this species is readily reduced by two electrons to give the ferric complex FeIlI(TMP) + , i.e., that the green complex is two equivalents more oxidized than Fe III. A logical conclusion would be that the green species is Fe V(p2-)(02-) +. How- ever, spectroscopic studies of this species have led to the conclusion that it is, in fact, an Fe lV oxo porphyrin-radical complex, FeIV(p' -)(0 2 -) +, and that this formulation is the best description of 3, the product formed from heterolytic cleavage of the hydroperoxy intermediate in Reaction (5.67).81,83 EXAFS stud- ies indicate that the green porphyrin complex contains iron bonded to an atom at an unusually short distance, i.e., 1.6 A, in addition to being bonded to the porphyrin nitrogens at 2 A. This short Fe-O distance is consistent with the formulation of the complex as a "ferryl" complex, i.e., Fe IV = O. In such a complex, the oxo ligand, 0 2 -, is bonded to the Fe IV center by a combination of a and 1T bonding, the latter because of overlap of the filled ligand p-orbitals with the partially filled d xz and d yz orbitals of the metal. Confirmation that the oxidation state of iron is indeed Fe IV comes from comparison of the Mossbauer parameters (Ope = 0.06 and flE Q = 1.62) with those of other known Fe IV -porphyrin complexes (see Figure 5.11).83
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