Peroxometal pathways

For the sake of completeness we also note that oxygen transfer processes can be mediated by organic catalysts which can be categorized on the same basis as metal catalysts. For example, ketones catalyze a variety of oxidations with mono-peroxysulfate (KHS05) [14]. The active oxidant is the corresponding dkmrane and, hence, the reaction can be construed as involving a peroxometal pathway. Similarly, TEMPO-catalyzed oxidations of alcohols with hypochlorite [15, 16] involve an oxoammonium salt as the active oxidant, i.e. an oxometal pathway. 

MTO-catalyzed epoxidations proceed via a peroxometal pathway involving a di-peroxorhenium(VII) complex as the active oxidant (see Fig. 4.25). Major disadvantages of MTO are its limited stability under basic H202 conditions [62] and its rather difficult and, hence, expensive synthesis. 

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... 

The general trend is that metals which react via an oxometal pathway show a very similar behaviour using these two hydroperoxides as oxidant, e.g. the selenium catalyzed allylic oxidation of olefins to the corresponding a, p-unsaturated alcohols. Reactions which involve a peroxometal pathway, e.g. the molybdenum catalyzed epoxidations of olefins, show a completely different behaviour using these two hydroperoxides, namely virtually no reaction is observed with the bulky PHP. We conclude that PHP is a suitable mechanistic probe for distinguishing between oxometal and peroxometal pathways in catalytic oxidation. 

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. 

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... 

Peroxometal pathways are typically observed with early transition metal ions with a d configuration, for example, Mo(VI), W(VI), Ti(IV), and Re(VII), which are relatively weak oxidants. Oxometal pathways are characteristic of late transition elements and first row transition elements, for example, Cr(VI), Mn(V), Os(VIII), Ru(VI), and Ru(VIII), which are strong oxidants in high oxidation states. Some metals can operate via both pathways depending, among other things, on the substrate for example, vanadium(V) operates via a peroxometal pathway in alkene epoxidations, but an oxometal pathway is involved in alcohol oxidations [31]. 

Figure 5.6 Oxometal versus peroxometal pathways in metal-catalyzed alcohol oxidations.

Methyltrioxorhenium (MTO) also catalyzes the oxidation of alcohols with H2O2 via a peroxometal pathway [137,138]. Primary benzyhc and secondary aliphatic alcohols afforded the corresponding aldehydes and ketones, respectively, albeit using two equivalents of H2O2. In the presence of bromide ion the rate was increased by a factor 1000 [137]. In this case the active oxidant could be hypobromite (HOBr), formed by MTO-catalyzed oxidation of bromide ion by H2O2. 

Keywords Alcohol oxidations, peroxometal pathway, oxometal pathway, hydridometal pathway, ruthenium catalyzed oxidations, palladium catalyzed oxidations, copper catalyzed oxidations, hydrogen peroxide, fcrt-butyl hydroperoxide, dioxygen... 

Molybdenum and vanadium compovmds have also been widely investigated as catalysts for the oxidation of alcohols with tert-butyl hydroperoxide (TBHP) as the oxidant. With the former a peroxometal pathway is involved while with the latter an oxovanadium(V) intermediate is the active oxidant. As with the H202-based systems described above, these systems exhibit a preference for the oxidation of secondary hydroxyl functionalities over primary ones. In contrast, zirconyl acetate, SO(OAc)2, catalyzes the selective oxidation of primary alcohol moieties with TBHP (Reaction 29) °". ... 

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