Methyltrioxorhenium -urea-hydrogen peroxide

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... 

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... 

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). 

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). 

An example of using deuterated reactants for detailed kinetic studies of transition metal catalyzed reactions in ionic hquids vras contributed by Abu-Omar and coworkers. They studied the Rh-catalyzed epoxidation of olefins at ambient temperatures using [Dglstyrene and [Dio]-cylcohexene [71]. They also applied H-NMR experiments of [D3]-diperoxorhenium, formed in situ by reaction of [D3]-methyltrioxorhenium and urea hydrogen peroxide (UHP) to determine rate constants in single turnover experiments. 

Since its discovery in 1991, methyltrioxorhenium (MTO, 80) has attracted much interest as one of the most versatile catalysts for oxidation.When it is associated with a stoichiometric amount of H2O2, the system can efficiently transform alkene to epoxide, although formation of undesired diol can occur. Alternatively, water-free conditions, using urea hydrogen peroxide (UHP), allow the formation of the desired epoxide without byproducts. A maj or drawback of the MTO/UHP system is its insolubility in organic solvents, leading to a kinetically slow heterogeneous system. 

Yokoyama et al. [75] further examined the combination of an epoxidation catalyst system of MTO/UHP/[BM[m]BF4 (MTO, methyltrioxorhenium UHP, urea hydrogen peroxide) [76]... 

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