Methyltrioxorhenium epoxidation catalyst

Unsaturated fatty compounds are the preferred educts in industrial epoxidation. Numerous methods are available to transform then to the corresponding epoxides. Epoxidation with molecular oxygen [3], dioxiranes [4], hydrogen peroxide with methyltrioxorhenium as catalyst [5, 6], the Halcon process [7], or enzymatic reactions [8] are the most important industrial processes (cf. Section 2.4.3). 

Epoxidation with Hydrogen Peroxide Methyltrioxorhenium as Catalyst... 

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]... 

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. 

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]. 

Allylic alcohols can also be epoxidized with methyltrioxorhenium (MTO). However, in contrast to the early transition metal catalysts, metal-alcoholate binding does not appear to be operative, but rather straightforward hydrogen bonding, as demonstrated by the epoxidation of geraniol (20)... 

Under the name Oxone an oxidation agent has been introduced, consisting of KHSO4-K2SO4-2KHSO5. Solid Oxone converts methylenic functions under anhydrous, biphasic conditions to carbonyl compounds under the catalytic influence of ligand-modified Mn porphyrins and phase-transfer catalysts (e. g., acetophenone is obtained from ethylbenzene). In the case of cyelohexane, e-caprolactone results as well as cyclohexanol and -one ([219 b, 241] cf. also Baeyer-Villiger oxidation). Biphasic oxidations with methyltrioxorhenium (e. g., to epoxides) are reviewed in Section 3.3.13 [244 i]. 

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). 

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). 

Key Words Lewis acid adducts, Radical oxidations, Epoxidation, Hydrogen peroxide, Bond dissociation energy, Catalyst durability, Methyltrioxorhenium, Cross-bridged cyclam, Mn(IV), Late transition metal. Propylene oxide. Titanium silicalite (TS-1) catalyst, Ethylanthrahydroquinone/H2 process, Polyoxometallates, Mn(IV) catalyst. Hydrogen abstraction. Rebound mechanism, Isotopic label, t-BuOOH, Peroxide adduct. 2008 Elsevier B.v. 

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. 

Molybdenum catalysts, Ruthenium porphyrins, Ruthenium(lll) complexes, Iron catalysts, Titanium catalysts. Sharpless epoxidation, Tungsten catalysts, Methyltrioxorhenium, Cobalt, Nickel, Platinum, Aerobic epoxidation, Lanthanum, Ytterbium, Calcium, BINOL-complexes. 2008 Elsevier B.v. 

A Pt salt, [(dppe)Pt(C6F5)(H20)]0Tf, is able to catalyze selective epoxidation of a monosubstituted (terminal) aUcene with H2O2, without affecting an internal double bond. As catalysts for epoxidation, methyltrioxorhenium is modified by converting into the more stable 2 °... 

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