Ferric hydroperoxo

An increase in the fraction of the four-electron reduction pathway at more reducing potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The first is based on the kinetic competition between the release of H2O2 from the ferric-hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible) reduction to a ferrous-hydroperoxo species, which undergoes rapid 0-0 bond heterolysis (18.13b). Because H2O2 and particularly HO2 are more basic ligands... 

Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphyrins can be effective ORR catalysts, since large overpotentials are required to access intermediates in which 0-0 bond heterolysis is facile. The only strategy discovered so far to facilitate this 0-0 bond heterolysis in the ferric-hydroperoxo intermediate is to control both the distal and the proximal environments of Fe porphyrins. In those cases, the overpotential of ORR reduction appears to be controlled by the potential of the (por)Fe / couple (see Section 18.6). 

The low limit on the rate constant fehetero of 0-0 bond heterolysis in the putative ferric-hydroperoxo intermediate by analyzing the turnover frequency of H2O2 reduction at potentials 0.6-0.4 V (vs. NHE at pH 7). 

Figure 18.20 A plausible ORR catalytic cycle by biomimetic catalysts 2 (Fig. 18.17). Cu is ligated by three imidazoles (omitted for clarity) and potentially an exogenous ligand, whose nature is not known. All intermediates other than ferric-peroxo and ferric-hydroperoxo were prepared independently.

The kinetics and the mechanism of superoxide reduction by SORs have been studied by several researchers. It was suggested that SORs react with superoxide via an inner-sphere mechanism, binding superoxide at ferrous center to form a ferric hydroperoxo intermediate [46,48 50]. The rate constant for this reaction is equal to 108 109 1 mol-1 s-1 [46,49], This... 

Figure 20 (a) An ultrafast radical clock substrate capable of distinguishing between radical and carbocation intermediates (b) The proposed two-oxidant pathway of C-H bond oxidation by P450 enzymes in which Compound 1 follows a concerted mechanism with no intermediate while the ferric-hydroperoxo attacks substrates by insertion of HO+ leading to carbocation intermediates which rearrange to give the alcohol product... 

Figure 25 (a) The aromatase activity of P450 enzymes in which androstenedione is converted to estrone via oxidative deformylation (b) The concerted and radical mechanism of deformylation via breakdown of a peroxohemiacetal intermediate formed by nucleophilic attack of the ferric-hydroperoxo intermediate on the aldehyde (c) Abstraction of a alcohol hydrogen leads to the formation of a ketone rather than a ring... 

The reaction sequence at the heme active site starts with the binding of unactivated triplet dioxygen forming the so-called oxy-heme complexes. The iron center in 02-activating heme enz5maes is then thought to be converted into a peroxo anion species. It can be protonated to form a ferric hydroperoxo intermediate usually termed compormd 0 (183), which is a crucial reactive species in catalase and peroxidase enz5nne catalysis (Fig. 21). These hydroperoxo intermediates of hemoproteins are important... 

E. A. Duban, K. P. Bryliakov, E. P. Talsi, Characterization of low-spin ferric hydroperoxo complexes with N,N -dimethyl-N,N -bis(2-pyridylmethyl)-l,2-diaminoethane, Mendeleev Commun. (2005) 12. 

Protonation of the distal oxygen of this peroxo anion to generate the Fe- -OOH , ferric-hydroperoxo complex. 

Two additional intermediates, the ferric peroxy anion and ferric hydroperoxo complex, have been proposed to substitute for the ferryl as the actual oxidizing species in at least some P450 reactions. The role of the ferric peroxy anion in some reactions is supported by good evidence and is discussed in the section on carbon-carbon bond cleavage reactions (see Section 8), but the proposed role of the ferric hydroperoxide in electrophilic double bond and heteroatom oxidations is discussed here. 

The cxurent interest in the ferric hydroperoxo complex as a P450-oxidizing species derives largely from the work by Vaz et ai, who observed that mutation of the conserved threonine (Thr303) in CYP2E1 to an alanine decreased the allylic hydrox-ylation of cyclohexene, cts-2-butene, and tmns-2-butene, but increased the epoxidation of the same... 

Figure 6.2. Hypothetical ipso-substitution mechanism involving the ferric hydroperoxo complex and ferryl species as the potential oxidizing species. ...

The discussion of epoxidation has been framed in terms of a ferryl catalytic species. It has been proposed that the ferric hydroperoxo intermediate may contribute to olefin epoxidation, but as discussed in Section 2, the support for this postulate is contradictory. Although it appears that the ferric hydroperoxo intermediate can oxidize double bonds at a low rate, the data strongly suggests that the ferric hydroperoxide... 

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