Methyltrioxorhenium pyridine

Rudolph, J. Reddy, L. Chiang, J. P Sharpless, K. B., Highly Efficient Epoxidation of Olefins Using Aqueous H202 and Catalytic Methyltrioxorhenium / Pyridine Pyridine-Mediated Ligand Acceleration. /. Am. Chem. Soc. 1997, 119, 6189 Rouhi, M., New reaction uncouples epoxidation from acidity. Chem. Eng. News 1997, 75(27), 6. 

J. Rudolph, K. L. Reddy, J. P. Chiang, K. B. Sharpless, Highly efficient epoxidation of olefins using aqueous H2O2 and catalytic methyltrioxorhenium/pyridine pyridine-mediated ligand acceleration, J. Am. Chem. Soc. 119,6189-6190 (1997). 

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). 

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

Re has recently come to the forefront in liquid phase oxidation catalysis, mainly as a result of the discovery of the catalytic properties of the alkyl compound CH3Re03 [methyltrioxorhenium (MTO)]. MTO forms mono-and diperoxo adducts with H2O2 these species are capable of transferring an oxygen atom to almost any nucleophile, including olefins, allylic alcohols, sulfur compounds, amides, and halide ions (9). Moreover, MTO catalysis can be accelerated by coordination of N ligands such as pyridine (379-381). An additional effect of such bases is that they buffer the strong Lewis acidity of MTO in aqueous solutions and therefore protect epoxides, for example. 

In methyltrioxorhenium-catalysed reactions, residual pyridine, pyrazole or imidazole may deactivate the active catalyst and the ionic liquid should be purified accordingly.118 191 The nature of the ionic liquid may decide the success or failure of a given reaction and, for example, Ru-catalysed alcohol oxidations appear to be very sensitive to the type and quality of ionic liquid used. 

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)... 

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). 

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). 

E. Da Palma Carreiro, G. Young-En, A. J. Burke, Approaches towards catalytic asymmetric epoxidations with methyltrioxorhenium(VII) (MTO) Synthesis and evaluation of chiral non-racemic 2-substituted pyridines, J. Mol. Cat. A.. Chem. 235, 285-292 (2005). 

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