Methyltrioxorhenium olefin epoxidation

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]. 

P. Gisdakis, W. Antonczak, S. Kostlmeier, W. A. Herrmann, N. Rosch, Olefin epoxidation by methyltrioxorhenium A density functional study on energetics and mechanisms, Angew. Chem. Int. Ed. Engl. 37 (1998) 2211. 

W. A. Herrmann, R. M. Kratzer, F. E. Kuhn, J. J. Haider, R. W. Fischer, Multiple bonds between transition metals and main-group elements. Part 168. Methyltrioxorhenium/Lewis base catalysts in olefin epoxidation, J. Organomet. Chem. 549 (1997) 319. 

An example of monitoring non-steady state kinetics in ionic liquids using UV-Vis spectroscopy was published by Abu-Omar and coworkers [71]. They monitored the methyltrioxorhenium(MTO)olefin epoxidation in [EMIM][BF4] by observing the spectral changes of the Re complexes in the ionic liquid. The decreasing absorbance at 360 nm was attributed to the reactions of both the diperoxorhetiium and the monoperoxorhenium complex with the olefinic substrate. 

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 TB ( + )-l adduct of methyltrioxorhenium [(+ )-Re03CH3], characterized by its crystal structural and spectroscopic data, was reported by Herrmann et al. The catalytic properties of this complex were tested in the epoxidation of olefins and the oxidation of sulfides. However, no enantioselective reactions of the pro-chiral olefins and sulfides were observed (97JOM(538)203). 

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. 

MTO [methyltrioxorhenium(VII), cf. Chapter 3.3.13] can be used as a catalyst for the epoxidation of olefins with urea hydroperoxide in [EMIMJBF4 [19]. The activity is reported to be comparable with the reaction in organic solvents but side reactions are suppressed. The use of an ionic liquid as a co-solvent in CH2CI2 for the enantioselective Mn-salen complex-catalyzed epoxidation of olefins with Na(OCl) was reported to result in enhanced reaction rates at no loss of enantioselectivity [20]. Cr-salen complexes can further be used for the asymmetric kinetic resolution of epoxides by ring-opening with azide [21]. 

The simple organorhenium(VII) compound methyltrioxorhenium (Structure 1 in Scheme 1) - called MTO - has developed a plethora of applications in catalytic processes [1], This rapid development occurred in the decade of 1990-2000. The epoxidation of olefins (cf. Section 2.4.3) became attractive to industrial applications. There is sound evidence that MTO represents the most efficient catalyst for this process, being active even for highly dilute solutions of hydrogen peroxide. The latter oxidant is not decomposed by MTO, as opposed to many other metal complexes (cf. Section 3.3.13.1). 

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. 

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). 

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). 

W. A. Herrmann, R. M. Kratzer, H. Ding, W. R. Thiel, H. Gras, Methyltrioxorhenium/ pyrazole-a highly efficient catalyst in the epoxidation of olefins, J. Organometal. Chem. 555,293-295 (1998). 

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. 

Methyltrioxorhenium (MTO) has perhaps been the most successful homogeneous catalyst for the epoxidation of simple olefins with hydrogen peroxide, however, attempts to modify the catalyst to achieve enantioselective epoxidation or anchoring to a solid support have met with only moderate success, probably because... 

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