Catalyst Formation and Catalytic Applications

An important step in the imderstanding of the role of MTO in epoxidation and oxidation catalysis was the isolation and characterization of the reaction product of MTO with excess H2O2, i.e., a bisperoxo complex of stoichiometry (CH3)Re(02)20 H2O [18,19]. This reaction takes place in any organic solvent or water. The structures of(CH3)Re(02)20 H20and(CH3)Re(02)20 (0=P(N(CH3)2)3) (X-ray diffraction) were determined the structure of ligand-free complex (CH3)Re(02)20 is known from the gas phase [19, 20]. 

Experiments with the isolated bis(peroxo) complex (CH3)Re(02)20 H2O have shown that it is an active species in olefin epoxidation catalysis and several other catalytic reactions [19, 20]. In-situ experiments show that the reaction of MTO with 

The most important drawback of the MTO-catalyzed process is the concomitant formation of diok instead of the desired epoxides, especially in the case of more sensitive substrates [30]. It was quickly detected that the use of Lewis base adducts of MTO significantly decreases the formation of diols due to the reduced Lewis acidity of the catalyst system. However, while the selectivity increases, the conversion decreases [30-32]. It was found that biphasic systems (water phase/organic phase) and addition of a significant excess of pyridine as Lewis base not only hamper the formation of diols but also increase the reaction velocity in comparison to MTO as catalyst precursor [33, 34]. 

Furthermore, in the presence of olefins which are not readily transformed to their epoxides, 2,2 -bipyridine can be oxidized to its N-oxide by the MTO/H2O2 system [42]. Low to moderate stereo induction values (up to about 40% ee with conversions of aroimd 10% at -5 °C reaction temperature) can be achieved when prochiral olefins, e.g., ds-p-methylstyrene or a-pinene are epoxidized with chiral amine adducts of MTO [43]. 

MTO has also been successfully applied as an olefin epoxidation catalyst in ionic liquids ([44, 45] see also Chapter 5). 

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