Oxometal 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. 

M(n+1)+. For oxygenation involving oxometal species, M(" 2)+o, the regeneration mode in the catalytic cycle is substrate oxidation via equation 6. Occasional side reactions with M-based or other redox active species can lead to the intermediate oxidation state of the catalyst, Equations 9-12 are routes to in the... 

The key step in this category involves the oxidation of a coordinated substrate by a metal ion or an oxometal species (see later). Examples include the palladium(II)-catalyzed oxidation of olefins (Wacker process) and the oxidative dehydrogenation of alcohols, where the key steps are reactions (5) and (6), respectively. 

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]. 

In a variation on this theme, which pertains mainly to gas phase oxidations, an oxometal species oxidizes the substrate and the reduced form is subsequently re-oxidized by dioxygen (Fig. 4.7). This is generally referred to as the Mars-van Krevelen mechanism [7]. 

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... 

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,... 

X- and c-values were determined for many metal ions, plus the proton, certain organometal ions and neutral or cationic oxometal species like and... 

There is one case where both Equations (24) and (25) are facile under mild conditions, where the high valent oxometal species have been isolated and fully characterized,322,323 and where their oxo transfer to substrates (alkenes) has been studied in depth. This is the case where Mred(P0M) = Cr111Xra+W11039 9-")- and OM(POM) = (OCrv)X" +W11039 9-"), X = PV or... 

Usually, the aquametal(III) is reducible to aquametal(II) and oxidizable to the corresponding oxometal(IV), hydrox-ometal(IV) and oxometal(V) derivatives depending on the character of the incorporated metals. Further oxidations to oxometal(VI) and-(VII) are possible in the cases of metals like rhenium [87]. The interest of hydroxometal and oxometal species as oxidation catalysts in synthetic applications must be stressed. 

There are two major problems associated with selective oxidation catalysis with soluble oxometal complexes which are the propensity of certain oxometal species (e.g. Ti O) towards oligomerization to inactive p-oxo complexes, and oxidative destruction of organic ligands. These problems can be circumvented by site isolation of discrete oxometal species in an inorganic matrix, whereby... 

Site-Isolation of Oxometal Species in Inorganic Matrices — Mineral... 

A second disadvantage encountered with these biomimetic model systems, and with many other oxidations involving oxometal species, is deactivation via the formation of unreactive dimeric (or oligomeric) p-oxo complexes, e.g. 

Formation of high-valent oxometal species from (salen)metal complexes and iodosylbenzene (PhIO) and the mechanism of the oxygen transfer from these species to alkyl aryl sulfides have been investigated in detail [22]. Fluorous salen ligands 13 and 14 were synthesized and the corresponding (salen)manganese(III) complexes Mn-13 and Mn-14 were evaluated in the oxidation of alkyl aryl sulfides with PhIO under homogeneous and FB conditions, respectively [23]. 

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] ... 

One approach to creating heterogeneous oxidation catalysts with novel activities and selectivities is to incorporate redox metals, by isomorphous substitution, into the lattice framework of zeolites and related molecular sieves. Site-isolation of redox metals in inorganic lattices prevents the dimerization or oligomerization of active oxometal species which is characteristic of many homogeneous oxometal complexes and leads to their deactivation in solution. We coined the term redox molecular sieves to describe such catalysts . The first and most well-known example is titanium silicalite (TS-1) which has been shown to catalyze a variety of systhetically useful oxidations with H202. ... 

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-... 

These deoxygenation reactions by metals correspond to alternative oxygen trans-fer pathways, where oxometal species are -formed. However in order that catalytic reduction may take place in the presence o-f CO, the oxometal species must be susequently reduced by carbon monoxides... 

The last two catalytic systems could correspond to oxygen transfer pathways, where oxometal species are formed <3.2.1.). 

According to Ullrich [25] and Groves et aJ. [36-39], iodosylbenzene can serve as the source of 0-atoms in the catalytic epoxidation of olefins (and oxygenation of alkanes) in the presence of metalloporphyrin complexes. The reason for the considerable success of this oxygen source is that PhIO and the iodobenzene (Phi) formed upon 0-atom transfer are both quite stable to oxidation. So the otherwise unstable oxometal species generated from a metalloporphyrin and PhIO for example remains unchanged for extended periods of time, permitting spectroscopic characterization and/or selective reactions with oxidizable substrates. The 0-atom of PhIO is, however, activated for... 

In addition to epoxidation, Tabushi s system also catalyzes aromatic and aliphatic hydroxylation, as well as dealkylation of tertiary amines [81], i.e. all of the important types of reactions that are catalyzed by cytochrome P-450. A practical disadvantage of this artificial P-450 catalytic system is that the active oxometal species is to a considerable extent (typically 80-90%) consumed for water production via interaction with the reducing agent Pt/H. ... 

Another kind of metalloporphyrin catalyzed olefin epoxidation utilizes aldehydes, which generate peroxy radicals and subsequently peroxy acids [95,98,258]. The latter are complexed by the metalloporphyrin and serve as sources of 0-atoms for epoxidation either as the peroxy acid complex or as the oxidized ir-cation radical (oxometal species) [96,97]. The catalysis of propene epoxidation by... 

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