Oxidative activation rhenium compounds

Related alkylrhenium(VI) complexes can be transformed to the same active species [MeRe0(02)2 H20] in the presence of hydrogen peroxide and therefore can also serve as catalysts. A comparison of the catalytic activity of various rhenium compounds is given in Reference 345. Because of its great oxidation activity, MTO can be used at room temperature and below. Although MTO is more active in the absence of bases like... 

Finally, a mention should be made about the one peroxo system which will become more and more dominant the organometallic oxides of rhenium(VII). Such compounds have been found to be of outstanding catalytic activity for a number of oxygen transfer reactions with hydrogen peroxide.92 The best studied complex is methyltrioxorhenium(VII) (MTO) and its congeners. Figure 2.32 illustrates its synthesis. Epoxidation, aromatic oxidation and halide oxidation with these complexes have been studied with hydrogen peroxide and shown to be remarkably efficacious. 

In contrast with the common activity of oxo-rhenium compounds in oxidation catalysis (see above), the Re -dioxo-complex [Re02l(PPh3)2] catalyzes the reductive hydrosilylation of aldehydes and ketones to give silyl ethers (reaction... 

In a recent improvement to this approach, poly(4-vinylpyridine) and poly(4-vinylpyridine) N-oxides were used as the catalyst carrier [91]. The MTO catalyst obtained from 25% cross-linked poly(4-vinylpyridine) proved to efficiently catalyze the formation of even hydrolytically sensitive epoxides in the presence of aqueous hydrogen peroxide (30%). This catalyst could be recycled up to 5 times without any significant loss of activity. Attempts have been made to immobilize MTO with the use ofeither microencapsulation techniques, including sol-gel techniques, to form silica-bound rhenium compounds, or by the attachment of MTO to silica tethered with polyethers. These approaches have provided catalysts with good activity using aqueous hydrogen peroxide as the terminal oxidant [91-93]. In the latter case, high selectivity for epoxide formation was also obtained for very sensitive substrates (e.g., indene). 

The catalysts applied to alkene epoxidation in fluorinated alcohol solvents can be subdivided into those which are metal/chalconide-based and those which are purely organic in nature (Scheme 4.5). The former comprise arsanes/arsane oxides [27,28], arsonic acids [29, 30], seleninic acids/diselenides ]31-35], and rhenium compounds such as Re207 and MTO (methylrhenium trioxide) ]36,37]. As shown in Scheme 4.5, their catalytic activity is ascribed to the intermediate formation of, for example, perseleninic/perarsonic adds or bisperoxorhenium complexes. In other words, their catalytic effect is due to the equilibrium transformation of hydrogen peroxide to kmetically more active peroxidic spedes. 

In general, these rhenium compounds are more active for the oxidation of cyclic (four-, five-, and six-membered rings) than acyclic ketones, consistent with the common lower reactivity of the latter ketones. The Re(III) tris(pyrazol-l-yl)methane compound [ReCl3 HC(pz)3 ] (11) is the most active one for 2-methylcyclopentanone and cyclohexanone or pinacolone BV oxidations, whereas the most effective oxidations are observed for cyclobutanone with [ReCl2 N2C(0)Ph (Hpz)2(PPh3)] (26) (54% yield, 99% conversion, TON of 537) (Table 22.3) [9]. 

As noted previously, the rhenium-catalyzed reactions dealt with to this point can be carried out in vessels open to the atmosphere. This is advantageous because of the convenient working procedures it allows. On the other hand, it means that molecular oxygen is not available as the stoichiometric oxidizing reagent (53-55). Three rhenium(V) compounds, 24-26, have been prepared that do activate 02. 

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