Triplet dioxygen reactivity

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... 

With increasing knowledge and experience on the reactivity of singlet and triplet carbenes (see Sect. 8.1), and of singlet and triplet dioxygen (see Sect. 8.5) in the 1960 s and 1970 s, it seemed to be feasible to consider that a 02-molecule, as a highly electrophilic reagent, may attack the nucleophilic C (a)-atom of diazoalkanes... 

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

As noted earlier, there is a kinetic restriction on the reactivity of dioxygen, as the one-electron reduction to superoxide is unfavoured. This is demonstrated in the redox potentials shown in Figure 51. The triplet ground state of the molecule may also impose spin restrictions on its reactions. 

Dioxygen in its ground (triplet) state reacts as a rather stable diradical. One of its most important radical reactions is [4] in which radicals R are converted into peroxyl radicals R-OO which generally have quite different reactivities from those of the parent R species. This reaction is, in general, reversible, and stable radicals such as nitrox-ides fail to undergo reaction [1.1] to any measurable extent. 

Because the ground state of dioxygen is a triplet with two unpaired electrons with parallel spins, its reactivity with organic molecules in the singlet state is severely... 

Beyond the relevance in this particular study, the peroxy-like defects can be reaction intermediates in other zeolites-based oxidations in industrial applications. On the basis of present results, it can be argued that more effective oxidizing media can be obtained by modifying zeolites and mesoporous aluminosilicates in order to allow an easier formation of peroxy-like structures. In this respect, the presented data may suggest a possible activation mechanism of the inert triplet state of dioxygen in the cavities of nitrite sodalite to the more reactive singlet O2 [26]. 

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