Methyltrioxorhenium

The oxidation of alkenes and allylic alcohols with the urea-EL202 adduct (UELP) as oxidant and methyltrioxorhenium (MTO) dissolved in [EMIM][BF4] as catalyst was described by Abu-Omar et al. [61]. Both MTO and UHP dissolved completely in the ionic liquid. Conversions were found to depend on the reactivity of the olefin and the solubility of the olefinic substrate in the reactive layer. In general, the reaction rates of the epoxidation reaction were found to be comparable to those obtained in classical solvents. 

The 14e compound MTO readily forms coordination complexes of the type MTO-L and MTO-L2 with anionic and uncharged Lewis bases [96], These yellow adducts are typically five- or six-coordinate complexes, and the Re-L system is highly labile. Apart from their fast hydrolysis in wet solvents, MTO-L adducts are much less thermally stable then MTO itself. The pyridine adduct of MTO, for instance, decomposes even at room temperature. In solution, methyltrioxorhenium displays high stability in acidic aqueous media, although its decomposition is strongly accelerated at increased hydroxide concentrations [97, 98], Thus, under basic aqueous conditions MTO decomposes as shown in Equation (4). 

METHYLTRIOXORHENIUM CATALYZED OXIDATION OF SECONDARY AMINES TO NITRONES TV-BENZYLIDENE-... 

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, supported on silica functionalized with polyether tethers, catalyzed the epoxidation of alkenes with 30% aq H2O2 in high selectivity compared to the ring opening products observed in homogeneous media in the absence of an organic solvent.46... 

Methyltrioxorhenium(VII) (MTO) forms mono- or bis-complexes with di-Schiff bases being derivatives of various salicylaldehydes and fra s-l,2-diaminocyclohexane [37] 90... 

Several general reviews describe the state of the art of peroxide epoxidation catalyzed by TM compounds at about a decade ago [2A. Later on, specialized reviews dealt with particular peroxides ofCr, Mo, andW [5], V [6], and with epoxidation reactions catalyzed by methyltrioxorhenium (MTO) [7] that involve Re peroxo complexes as species responsible for the oxygen transfer. 

An important improvement in the catalysis of olefin epoxidation arose with the discovery of methyltrioxorhenium (MTO) and its derivatives as efficient catalysts for olefin epoxidation by Herrmann and coworkers [16-18]. Since then a broad variety of substituted olefins has been successfully used as substrates [103] and the reaction mechanism was studied theoretically [67, 68, 80]. 

Figure 6. Methyltrioxorhenium (MTO) (3) and the corresponding mono- (4) and di(peroxo) (5) complexes. The coordination of the base ligand L is also indicated.

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

The low solubility of oxygen in most ionic liquids limits its application in oxidation catalysis in these liquids. However, oxidation by H2O2 or organoperoxide is not subject to this limitation when the ionic liquids are properly chosen. An example of catalytic oxidation is the methyltrioxorhenium (MTO)-catalyzed epox-idation of alkenes with the urea-H202 adduct in [EMIMJBF4 (228). High conversions and yields were obtained. 

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. 

Bisperoxo methyltrioxorhenium complex is one of the most powerful oxidants in Baeyer-Villiger reactions. In the same paper, oxidation of Adam probe was performed in order to provide evidence for the nucleophilic character of this complex. 

Methyltrioxorhenium-based oxidants, i.e. MT0/H202, MT0/H202/substituted pyri-dines and MTO/UHP , are active in the epoxidation of many double-bond typologies, including allylic alcohols. Regiochemical and stereochemical probes have been... 

Oxidation of alcohols may be achieved also using the methyltrioxorhenium/H202 catalytic system Primary benzylic and secondary aliphatic alcohols afforded the... 

MSD (a-methylstyrene dimer), 697 MTO see Methyltrioxorhenium Multicomplexation model, intrazeohte photooxygenation, 878... 

Pyridine, methyltrioxorhenium hgands, 460-1 Pyridine-2,6-dicarboxylate (dipic), transition metal peroxides, 1060, 1061 Pyridinium dichromate, alcohol oxidation, 787-8... 

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