Rhenium peroxides

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. 

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. 

Tertiary phosphines, triarylarsines, and triarylstibines are converted to their oxides, R3EO (E = P, As, Sb) by MTO/H2O2. Kinetic studies lead to the assumption that 2 and 3 have similar catalytic activities in all cases. The kinetic data support a mechanism involving nucleophilic attack of the substrate at the rhenium peroxides. 

Under standard MTO-catalyzed oxidation conditions with hydrogen peroxide, tertiary phosphines, triphenylarsine, and triphenyl-stibene were all efficiently oxidized. A detailed account of the rates of reactions of these oxidative processes has been reported which supports a mechanism that allows for nucleophilic attack of the substrate at the rhenium peroxide species. ... 

The procedure is experimentally simple, and the workup involves only the destruction of the traces of hydrogen peroxide with manganese dioxide and evaporation of the hexamethyldisiloxane. Pyridine additives serve to buffer the highly acidic rhenium species and to shut down the detrimental acid-catalyzed epoxideopening pathways. The scope of this transformation is best appreciated through the examples presented in Table 12.2 [28],... 

Immediately upon addition of the urea-hydrogen peroxide adduct to the solution containing methyltrioxorhenium, a yellow color develops due to formation of the catalytically active rhenium peroxo complexes.3... 

A highly explosive red/orange solid, obtained from rhenium trioxide and hydrogen peroxide reacting in hexamethylphosphoramide as solvent, was tentatively assigned this structure. 

Compounds and complexes of the early transition metals are oxophilic because the low d-electron count invites the stabilization of metal-oxo bonds by 7T-bond formation. To a substantial extent, their reactivity is typical of complexes of metals other than rhenium. That is particularly the case insofar as activation of hydrogen peroxide is concerned. Catalysis by d° metals - not only Revn, but also CrVI, WVI, MoVI, Vv, ZrIV and HfIV - has been noted. The parent forms of these compounds have at least one oxo group. Again the issue is the coordination of the oxygen donating substrate, HOOH, to the metal, usually by condensation ... 

The catalytic capabilities of rhenium compounds burst on the scene about one decade ago, featuring MeRe03 as a catalyst for reactions of hydrogen peroxide. It was quickly verified that peroxorhenium(VII) compounds were the active intermediates. With them, practical reactions and fundamental questions of mechanism could then be resolved. 

Methylbis( r-peroxo)rhenium oxide hydrate, 0497 Triethyltin hydroperoxide, 2583 Trimethylsilyl hydroperoxide, 1330 Triphenyltin hydroperoxide, 3758 See also alkyl trialkyllead peroxides... 

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. 

The oxidation of substituted pyridines to iV-oxides was reported by Sharpless and coworkers to proceed with yields between 78 and 99% (Scheme 154). A variety of substituents like electron donor as well as acceptor groups and alkenyl substituents are tolerated. In 1998, Sharpless and coworkers reported an alternative method for the preparation of pyridine-A-oxides in which the MTO/H2O2 catalyst could be replaced by cheaper inorganic rhenium derivatives (ReOs, Re207, HOReOs) in the presence of bis(trimethylsilyl) peroxide (equation 73). Yields of the prepared A-oxides after simple workup (filtration and bulb to bulb distillation) ranged from 70-98%. Molecular sieves slowed down the reaction while small amounts of water (0-15%) were essential for the reaction. Both electron-poor or electron-rich pyridines give high yields of their A-oxides and while para-... 

One of the most important peroxo complexes synthesized after 1983 is the rhenium species formed from methyltrioxorhenium (MTO) precursor. The synthesis of this complex is achieved in the way indicated in equation 2, by reacting hydrogen peroxide with MTO . The isolated peroxo complex 1 contains in the coordination sphere two /7 -peroxide bridges, a direct metal carbon bond and a molecule of water. The crystal structure of the peroxo rhenium derivative, however, was obtained by substitution of the water molecule with other ligands " more details on this aspect are enclosed in the structural characterization paragraph. 

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... 

Primary amines at a primary carbon can be dehydrogenated to nitriles. The reaction has been carried out with a variety of reagents, among others, IF5,"9 lead tetraacetate, 20 nickel peroxide,121 NaOCl in micelles,122 S g-NiSO, 2-1 and CuCl-02-pyridine.124 Several methods have been reported for the dehydrogenation of secondary amines to imines.125 Among them126 are treatment with(l) iodosylbenzene PhIO alone or in the presence of a ruthenium complex, 27 (2) Me2SO and oxalyl chloride, 2" and (3) f-BuOOH and a rhenium catalyst. 29... 

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