Methyltrioxorhenium -hydrogen peroxide

D Accolti, L. Fiorentino, M. Fusco, C. Crupi, P Curci, R. Selective oxidation of acetylenic 1,4-diols with dioxiranes in comparison with the methyltrioxorhenium-hydrogen peroxide oxidant. Tetrahedron Lett. 2004, 45, 8575. 

A 250-mL, two-necked, round-bottomed flask equipped with a magnetic stirbar, thermometer, and a reflux condenser fitted with a rubber septum and balloon of argon is charged with a solution of methyltrioxorhenium (MTO) (0.013 g, 0.05 mmol, 0.1% mol equiv) in 100 mL of methanol (Note 1). Urea hydrogen peroxide (UHP) (14.3 g, 152 mmol) is added (Notes 1, 2, 3, 4), the flask is cooled in an ice bath, and dibenzylamine (9.7 mL, 50.7 mmol) is then added dropwise via syringe over 10 min (Notes 1, 5). After completion of the addition, the ice bath is removed and the mixture is stirred at room temperature (Note 6). A white precipitate forms after approximately 5 min (Note 7) and the yellow color disappears within 20 min (Note 8). Another four portions of MTO (0.1% mol equiv, 0.013 g each) are added at 30-min intervals (2.5 hr total reaction time). After each addition, the reaction mixture develops a yellow color, which then disappears only after the last addition does the mixture remain pale yellow (Note 9). The reaction flask is cooled in an ice bath and solid sodium thiosulfate pentahydrate (12.6 g, 50.7 mmol) is added in portions over 20 min in order to destroy excess hydrogen peroxide (Note 10). The cooled solution is stirred for 1 hr further, at which point a KI paper assay indicates that the excess oxidant has been completely consumed. The solution is decanted into a 500-mL flask to remove small amounts of undissolved thiosulfate. The solid is washed with 50 mL of MeOH and the methanol extract is added to the reaction solution which is then concentrated under reduced pressure by rotary evaporation. Dichloromethane (250 mL) is added to the residue and the urea is removed by filtration through cotton and celite. Concentration of the filtrate affords 10.3 g (97%) of the nitrone as a yellow solid (Note 11). 

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... 

Oxidation of thiophene with Fenton-like reagents produces 2-hydroxythiophene of which the 2(570 One isomer is the most stable (Eq. 1) <96JCR(S)242>. In contrast, methyltrioxorhenium (Vn) catalyzed hydrogen peroxide oxidation of thiophene and its derivatives forms first the sulfoxide and ultimately the sulfone derivatives <96107211>. Anodic oxidation of aminated dibenzothiophene produces stable radical cation salts <96BSF597>. Reduction of dihalothiophene at carbon cathodes produces the first example of an electrochemical halogen dance reaction (Eq. 2) <96JOC8074>. 

Methyltrioxorhenium (MTO) is now well established as a catalyst in a number of oxidations employing hydrogen peroxide. Two groups have now reported,... 

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. 

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. 

Similarly, Ono et al. [189] have reported using a composed catalytic system, namely MTO/UHP/Zn[EMIm]2 Br4/[BMfm]BF4 (UHP, urea hydrogen peroxide and MTO, methyltrioxorhenium). With the multistep method described above, a... 

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. 

Methyltrioxorhenium(VII) (MTO) absorbance spectroscopy, 130, 132 alkene epoxidation, catalysis with hydrogen peroxide... 

Methyltrioxorhenium (MTO) is an extensively used catalyst for olefin epoxidation1241 and a number of reviews are available on its application.125"281 The complex reacts with hydrogen peroxide to form the mono r 2-peroxo species mpMTO and the diperoxide dpMTO. Both complexes are capable of transferring an oxygen atom to olefins or other substrates, as shown in Scheme 5.1, but the diperoxide species is about five times more reactive than the monoperoxo complex.1171 Care has to be taken that halide impurities are absent from the solvent as these are oxidised much faster than olefins.1171... 

Hydrogen peroxide in the presence of catalytic amounts of methyltrioxorhenium(VII), ReMeOs, is a convenient and efficient method for the a-hydroxylation of ketones. Particularly interesting is the HiOi/cetylpyridinium peroxotungstophosphate system which, under phase transfer conditions, provides a facile method for preparing aldehydes with one carbon atom less than the parent precursors. The ratio of the products changes with the experimental conditions. 

Methyltrioxorhenium (MTO) catalyses direct epoxidation by hydrogen peroxide. The reaction is carried out in pyridine, avoiding acidic conditions detrimental to high epoxide yield and uses less concentrated hydrogen peroxide (30%) than other methods (58). This method epoxidized soybean and metathesized (see Section 7.4)... 

Unsaturated fatty compounds are the preferred educts in industrial epoxidation. Numerous methods are available to transform then to the corresponding epoxides. Epoxidation with molecular oxygen [3], dioxiranes [4], hydrogen peroxide with methyltrioxorhenium as catalyst [5, 6], the Halcon process [7], or enzymatic reactions [8] are the most important industrial processes (cf. Section 2.4.3). 

Oxidative cleavage with hydrogen peroxide as oxidant is more important in oxidation processes of natural products. The use of a three-fold excess of hydrogen peroxide without further additives, except for the catalyst methyltrioxorhenium (MTO), enables the oxidation of certain natural products drawn from styrene... 

The simple organorhenium(VII) compound methyltrioxorhenium (Structure 1 in Scheme 1) - called MTO - has developed a plethora of applications in catalytic processes [1], This rapid development occurred in the decade of 1990-2000. The epoxidation of olefins (cf. Section 2.4.3) became attractive to industrial applications. There is sound evidence that MTO represents the most efficient catalyst for this process, being active even for highly dilute solutions of hydrogen peroxide. The latter oxidant is not decomposed by MTO, as opposed to many other metal complexes (cf. Section 3.3.13.1). 

Stankovic, S., Espenson, J. H. Facile Oxidation of Silyl Enol Ethers with Hydrogen Peroxide Catalyzed by Methyltrioxorhenium. J. Org. Chem. 1998, 63, 4129 130. 

With the bisalkaloid ligands, potassium ferricyanide can be used as the stoichiometric oxidant [84, 91]. As with the parent achiral osmium oxidation, NMMO can also be used as the oxidant (see above) [92]. However, rather than using NMMO in stoichiometric amounts, this morpholine component can be used in catalytic amounts by the addition of the biomimetic flavin 4 to set up a triple catalytic system where hydrogen peroxide is the oxidant [93-95], Methyltrioxorhenium can be used in place of the flavin mimic [96], as can tungsten(VI) [97] and carbon dioxide [98]. 

Some efforts were made in order to obtain good enantioselectivities in the epoxidation of simple olefins using methyltrioxorhenium (MTO), urea hydrogen peroxide (UHP) and six different chiral non racemic 2-substituted pyridine ligands, some of which are novel UHP was chosen as the hydrogen peroxide source in order to avoid unfavourable competition from water for vacant sites on the metal. However, poor enantioselectivity was reached (3-12% ee). 

W. M. Adam, C. M. Mitchell, Methyltrioxorhenium(VII)-catalyzed epoxidation of alkenes with the urea/hydrogen peroxide adduct, Angew. Chem. Int. Ed. Engl. 35, 533— 535 (1996). 

H. Adolfsson, C. Coperet, J. P. Chiang, A. K. Yudin, Efficient epoxidation of alkenes with aqueous hydrogen peroxide catalyzed by methyltrioxorhenium and 3-cyano-pyridine, J. Org. Chem. 65, 8651-8658 (2000). 

O. A. Bouh, J. H. Espenson, Epoxidation reactions with urea-hydrogen peroxide catalyzed by methyltrioxorhenium(VII) on niobia. J. Mol. Cat. A Chem. 200, 43 7 (2003). 

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