Hydrogen peroxide—Rhenium oxide

Hydrogen peroxide-Rhenium(VII) oxide, H202-Re207. 

Rhenium is attacked by neither hydrochloric acid nor by cold sulfuric or hydrofluoric acid. However, oxidizing acids, such as nitric acid or hot sulfuric acid, vigorously react with the metal forming perrhenic acid, HRe04. The metal is oxidized by hydrogen peroxide in ammoniacal solution forming ammonium perrhenate, NH4Re04. 

Selective oxidation of N-1 of adenine derivatives is typically carried out with peracids <1998JOC3213>, but has also been achieved with hydrogen peroxide and catalytic methyltrioxorhenium (Scheme 10) <2000T10031>. The inclusion of pyridazine-2-carboxylic acid as a stabilizer for reactive rhenium peroxides led to increased yields. Caffeine did not react under these conditions. 

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... 

The oxidation chemistry of methylrhenium trioxide (MTO) has been reviewed.72 The oxidation of thiophenes by hydrogen peroxide has also been studied, using MTO as a catalyst.73 The latter reacts with H2O2 to generate 1 1 and 1 2 rhenium peroxides, which are able to transfer an oxygen atom to the sulfur of the substrate, to give first the sulfoxide and then the sulfone. Whilst electron-donating substituents accelerate the first oxidation, the reverse trend is observed for oxidation of the sulfoxide. 

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. 

Nonahydridorhenate is conveniently oxidized to perrhenate by slow addition of the solid to 5 % hydrogen peroxide. Sodium and rhenium may then be determined on the solution by standard methods. Hydrogen is determined by ordinary combustion techniques. 

Except in the presence of perchlorate, rhenium is determined gravimetrically by oxidation with hydrogen peroxide in a... 

Although high-valent rhenium-oxo complexes such as perrhenate, Rc207, and ReOjX (X = F, Cl, Br) are widely known, their use as oxidants is rather rare. The stoichiometric oxidation of alkenes to a mixture of ketone and epoxide by Rc207 has been reported in the patent literature. For example, 2-butene is transformed to 2,3-epoxybutane and 2-butanone by Rc207 at 100 °C. A catalytic oxidation was observed in the presence of excess hydrogen peroxide but precise data on this reaction are lacking. 

Alkylrhenium trioxide-catalyzed oxidations of hydroxy-substituted arenes (i.e. phenol or naphthol derivatives, discussed as intermediates on the way to the corresponding quinones [9]) by 85 % aqueous hydrogen peroxide (diluted in AcOH) affords the corresponding p-quinones in fair to high yields [10]. Control experiments without rhenium catalysts yielded very slow oxidations (less than 10 % conversion). Furthermore, under the conditions of the H202/CH3Re03/Ac0H oxidation, the quinones formed are quite stable thus hydroxy-substituted p-quinones are not derived from overoxidation of the p-quinones. 

The first major breakthrough came in 1991 when Herrmann introduced methyltrioxorhcnium (MTO) as a powerful catalyst for alkene epoxidation, using hydrogen peroxide as the terminal oxidant." This organometallic rhenium compound, now commercially available, was first detected by Beattie and Jones in 1979, produced in tiny amounts from the reaction of (CH3)4ReO with air. The high solubility of MTO in virtually any solvent from pentane to water makes this compound particularly attractive for catalytic applications. 

Z. Shu, J. H. Espenson, Kinetics and mechanism of oxidation of anilines by hydrogen peroxide as catalyzed by Methy I rhenium trioxide, J. Org. Chem. 60 (1995) 1326. 

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