Peroxometal species

Pinene hydroperoxide (PHP) when compared with r-butyl hydroperoxide has been proposed as an excellent mechanistic probe in metal-catalysed oxidations. " If inter-molecular oxygen transfer from a peroxometal species to the substrate is rate limiting, the bulky PHP is unreactive, but for reaction of an oxometal species as the rate-limiting step, little or no difference is observed and only small differences in reactivity are observed when re-oxidation of the catalyst by ROOH to an active oxometal species is the rate-limiting step. 

When the oxygen donor is HgOg or ROjH, the active oxidant in these processes is an oxometal or a peroxometal species formed as shown below ... 

It is worthwhile to comment on the catalytic species. As opposed to oxometal species, which convert amines to imines, hydroperoxymetal complexes (MOOH) convert amines to nitrones. Thus the oxidation of amines is a convenient way of distinguishing the active species. The reactivity of oxometal versus peroxometal species is illustrated in Fig. 22. In practice, tungsten is the catalyst of choice to convert amines to nitrones [130]. 

Heterolytic oxygen transfer processes can be divided into two categories based on the nature of the active oxidant an oxometal or a peroxometal species (Fig. 4.10). Catalysis by early transition metals (Mo, W, Re, V, Ti, Zr) generally involves high-valent peroxometal complexes whereas later transition metals (Ru, Os), particularly first row elements (Cr, Mn, Fe) mediate oxygen transfer via oxometal species. Some elements, e.g. vanadium, can operate via either mechanism, depending on the substrate. Although the pathways outlined in Fig. 4.10... 

The systems described above all involve peroxometal species as the active oxidant. In contrast, ruthenium catalysts involve a ruthenium-oxo complex as the active oxidant [1]. Until recently, no Ru-catalysts were known that were able to activate H202 rather then to decompose it. However in 2005 Beller and co-workers recognized the potential of the Ru(terpyridine)(2,6-pyridinedicarboxylate) catalyst [63] for the epoxidation of olefins with H202 [64]. The result is a very efficient method for the epoxidation of a wide range of alkyl substituted or allylic alkenes using as little as 0.5 mol% Ru. In Fig. 4.26 details are given. Terminal... 

The active oxidant in these processes may be an oxometal or a peroxometal species (see Fig. 5). Some metals (e.g. vanadium) can, depending on the substrate, operate via either mechanism. 

One problem associated with the transfer of an oxygen atom from the peroxometal species to the double bond of an olefin is that the second oxygen atom remains bonded to the metal. In order to complete a catalytic cycle this oxometal species (M=0) must be reduced back to the original oxidation state (M). Read and coworkers achieved this by employing triphenylphosphine as a coreductant [21] ... 

Fig. 22 Reactivity of oxometal (Ru) versus peroxometal (W) species in the oxidation of amines...

Primary amines are dehydrogenated by high-valent oxometal species to give nitriles, or imines depending on the number of available a-hydrogens. Oxidation of the amine via peroxometal-intermediates (e.g. with MTO, Na2Mo04,... 

Similarly, for tertiary amines a distinction can be made between oxometal and peroxometal pathways. Cytochrome P450 monooxygenases catalyze the oxidative N-demethylation of amines in which the active oxidant is a high-valent oxoiron species. This reaction can be mimicked with some oxometal complexes (Ruv=0), while oxidation via peroxometal complexes results in oxidation of the N atom (Fig. 4.93 a and b) [261]. A combination of MTO/hydrogen peroxide can... 

For selective oxygen-transfer processes, as in, for example, epoxidation, Ru-0x0 species in lower oxidation states have been commonly applied. In general, catalytic systems for oxygen-transfer processes can be divided into two major categories, involving peroxometal and oxometal species as the active oxidant, respectively [1]. The peroxometal mechanism is generally observed with early transition elements whereby high-valent peroxometal complexes of, for example, Mo, and TF, are the active oxidants (Fig. 2, pathway a). Cataly-... 

Metal-catalyzed oxidations of alcohols with peroxide reagents can be conveniently divided into two categories involving peroxometal and oxometal species, respectively, as the active oxidant (Figure 5.6). In the peroxometal pathway the metal ion remains in the same oxidation state throughout the catalytic cycle, and no stoichiometric oxidation is observed in the absence of the peroxide. In contrast, oxometal pathways involve a two-electron change in the oxidation state of the metal ion, and a stoichiometric oxidation is observed, with the oxidized form of the catalyst, in the... 

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