Methyltrioxorhenium epoxidation

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. 

Methyltrioxorhenium, supported on silica functionalized with polyether tethers, catalyzed the epoxidation of alkenes with 30% aq H2O2 in high selectivity compared to the ring opening products observed in homogeneous media in the absence of an organic solvent.46... 

Several general reviews describe the state of the art of peroxide epoxidation catalyzed by TM compounds at about a decade ago [2A. Later on, specialized reviews dealt with particular peroxides ofCr, Mo, andW [5], V [6], and with epoxidation reactions catalyzed by methyltrioxorhenium (MTO) [7] that involve Re peroxo complexes as species responsible for the oxygen transfer. 

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]. 

Methyltrioxorhenium-based oxidants, i.e. MT0/H202, MT0/H202/substituted pyri-dines and MTO/UHP , are active in the epoxidation of many double-bond typologies, including allylic alcohols. Regiochemical and stereochemical probes have been... 

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)... 

Hi. Epoxidation of allylic alcohols with special synthetic utility or academic interest. In 1999, Adam and coworkers reported on the methyltrioxorhenium (MTO) catalyzed... 

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. 

Finally, a mention should be made about the one peroxo system which will become more and more dominant the organometallic oxides of rhenium(VII). Such compounds have been found to be of outstanding catalytic activity for a number of oxygen transfer reactions with hydrogen peroxide.92 The best studied complex is methyltrioxorhenium(VII) (MTO) and its congeners. Figure 2.32 illustrates its synthesis. Epoxidation, aromatic oxidation and halide oxidation with these complexes have been studied with hydrogen peroxide and shown to be remarkably efficacious. 

Methyltrioxorhenium(VII) (MTO) absorbance spectroscopy, 130, 132 alkene epoxidation, catalysis with hydrogen peroxide... 

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)... 

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]. 

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). 

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). 

Adam, W., Mitchell, C. M., Saha-Moeller, C. R. Regio- and Diastereoselective Catalytic Epoxidation of Acyclic Ailylic Alcohols with Methyltrioxorhenium A Mechanistic Comparison with Metal (Peroxy and Peroxo Complexes) and Nonmetal (Peracids and Dioxirane) Oxidants. J. Org. Chem. 1999, 64, 3699-3707. 

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. 

Catalytic oxidation of 239 to the quinone 240 was also effected with H2O2 catalyzed by methyltrioxorhenium(VII) (McRcOb) (Scheme 60)", where a small amount of hydroxy-substituted quinone 280 was produced in addition to 240 (70%). In this reaction, MeRe03 is stepwise converted by H2O2 into the mono- and bis(peroxo)rhenium complex MeRe(02)20-H20 (281). This active oxidant then reacts with the phenol to give the epoxide 282, which is further converted to the two quinones (240 and 280). 

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). 

W. M. Adam, C. M. Mitchell, Methyltrioxorhenium(VII)-catalyzed epoxidation of alkenes with the urea/hydrogen peroxide adduct, Angew. Chem. Int. Ed. Engl. 35, 533— 535 (1996). 

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). 

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). 

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