Redox cycling in the activation of peroxides by iron porphyrin and manganese complexes. ‘Catching’ catalytic active intermediates

Mechanistic studies on versatile metal-assisted hydrogen peroxide activation processes for biomedical and environmental incentives

“…However, in a study involving the application of H2O2, the homolytic mode of O-O bond scission seemed to prevail [71]. The short cut of the P450 catalytic cycle via the 'peroxide shunt' path has been applied by us and others in model studies involving the application of artificial porphyrins or cytochrome P450 itself, in order to reveal mechanistic aspects of the cytochrome P450 activity [20,26].…”

“…Motivation for the choice of H2O2 was to generate and examine the true compound 0 and not its oxidant-dependent mimics, since generation of compound I under catalytic turnover conditions with application of various exogenous oxidants is not always convincing. Indeed, the chemical nature of the exogenous oxidant can determine the reactivity pathway (homolytic vs heterolytic O-O bond cleavage), which may not reflect how the actual mechanism in the catalytic cycle of cytochrome P450 operates [20,26,106]. Application of different artificial oxidants such as m-chloroperoxybenzoic acid or cumene peroxide showed that the O-O bond of peroxy P450cam species could be cleaved heterolytically or homolytically to produce compounds I and II, respectively [52,93,107].…”

“…In many cases, a detailed understanding of the intimate reaction mechanism and the nature of the active sites are critical for the design of biomimetic catalysts with predictable reactivity. Mechanistic studies on model metal-assisted H2O2 activation systems for the formation of a more aggressive oxidant, which can be applied to biological, medical, environmental or industrial processes, are mostly focused on iron and manganese complexes [19,20,26,27]. In the following sections of this review, we discuss metal-assisted thermal activation processes of H2O2 in the context of their significant biomedical and environmental role.…”

“…Therefore, in numerous biomimetic studies H2O2 (or other artificial oxidants) is commonly used, which enables the direct production of models for compound 0 and in turn undergo homolytic (Scheme 2 path B) or heterolytic (Scheme 2 path A) O-O bond cleavage to form iron(IV) oxo species (compound II) or iron(IV) oxo porphyrin π-cation radical species (compound I), respectively [55][56][57][58][59][60][61]. All three of these species are active in the oxidation of various organic substrates [20,24,62].…”

“…In some cases, biological enzymatic systems have indicated the way to proceed in the development of efficient catalysts for the activation of peroxides. Although most enzymes use dioxygen as oxidant, as in the case of the cytochrome P450 enzyme family, the actual catalytic species involves the production of reactive oxygen species (ROS), such as superoxide and peroxides [20]. In other cases, inorganic chemists have designed very efficient catalysts for the oxidation of inert hazardous substrates.…”