Pyridine, methyltrioxorhenium ligands

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). 

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. 

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). 

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