Peroxometal mechanism

A borderline case is V(V) which has an intermediate oxidation potential [3] and can react via both mechanisms depending on the substrate used. It catalyzes epoxidations with TBHP via the peroxometal mechanism and alcohol oxidations via the oxometal pathway [7]. 

In this present investigation we used pinane hydroperoxide (PHP) as a mechanistic probe to distinguish between peroxometal and oxometal pathways. In the peroxometal pathway the bulky pinane group is present in the active oxidant while in the oxometal pathway it is not. Hence, in the case of the peroxometal mechanism one might expect more steric constraints and consequently a slower reaction using the bulky PHP compared to the much less bulky TBHP [8]. In the case of oxometal mechanisms the difference between PHP and TBHP should be much smaller as the alkyl group is not present in the active oxidant. 

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-... 

A number of soluble oxymetal salts were screened, particularly those of Mo, W, V, Ti and Zr, which are known to act via peroxometal and/or oxometal mechanisms (8). Unfortunately, no alternative to the existing KBr based protocol was found, so it was decided to, at least, improve the aqueous buffer system to significantly reduce the volume of the aqueous phase. 

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 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. 

The most well-known example is the catalytic epoxidation of olefins with alkyl hydroperoxides that is used for the commercial production of propylene oxide (see earlier). The reaction is catalyzed by high-valent compounds of early transition metals, e.g. Movl, WVI, Vv and TiIV, and involves a peroxometal type mechanism [6,7] as shown (reaction 12). Mo compounds are particularly effective homo-... 

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