Methyltrioxorhenium catalyts

Immediately upon addition of the urea-hydrogen peroxide adduct to the solution containing methyltrioxorhenium, a yellow color develops due to formation of the catalytically active rhenium peroxo complexes.3... 

The low solubility of oxygen in most ionic liquids limits its application in oxidation catalysis in these liquids. However, oxidation by H2O2 or organoperoxide is not subject to this limitation when the ionic liquids are properly chosen. An example of catalytic oxidation is the methyltrioxorhenium (MTO)-catalyzed epox-idation of alkenes with the urea-H202 adduct in [EMIMJBF4 (228). High conversions and yields were obtained. 

Oxidation of alcohols may be achieved also using the methyltrioxorhenium/H202 catalytic system Primary benzylic and secondary aliphatic alcohols afforded the... 

A -sulfinylacetamide 297 in greater than 90% yield when a catalytic amount of methyltrioxorhenium is employed. Futhermore, the hetero-Diels-Alder adduct is highly soluble in both chlorinated and ethereal solvents. A detailed investigation of the retro-Diels-Alder reaction of 298 by thermogravimetric analysis revealed an onset temperature of 120 °C and complete conversion of bicycle 298 to pentacene 296 at 160 °C, which are temperatures compatible with the polymer supports typically used in electronics applications. The electronic properties of these newly prepared OTFTs are similar to those prepared by traditional methods. Later improvements to this chemistry included the use of A -sulfinyl-/< r/-butylcarbamate 299 as the dienophile <2004JA12740>. The retro-Diels-Alder reaction of substrate 300 proceeds at much lower temperatures (130 °C, 5 min with FlTcatalyst 150 °C, Ih with no catalyst). 

Selective oxidation of N-1 of adenine derivatives is typically carried out with peracids <1998JOC3213>, but has also been achieved with hydrogen peroxide and catalytic methyltrioxorhenium (Scheme 10) <2000T10031>. The inclusion of pyridazine-2-carboxylic acid as a stabilizer for reactive rhenium peroxides led to increased yields. Caffeine did not react under these conditions. 

Similarly, Ono et al. [189] have reported using a composed catalytic system, namely MTO/UHP/Zn[EMIm]2 Br4/[BMfm]BF4 (UHP, urea hydrogen peroxide and MTO, methyltrioxorhenium). With the multistep method described above, a... 

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. 

Hydrogen peroxide in the presence of catalytic amounts of methyltrioxorhenium(VII), ReMeOs, is a convenient and efficient method for the a-hydroxylation of ketones. Particularly interesting is the HiOi/cetylpyridinium peroxotungstophosphate system which, under phase transfer conditions, provides a facile method for preparing aldehydes with one carbon atom less than the parent precursors. The ratio of the products changes with the experimental conditions. 

Thiiranes are rapidly desulfurized by reaction with triphenylphosphine and catalytic methyltrioxorhenium in the presence of H2S in acetonitrile- 3 at room temperature (Scheme 20) <1999CC1003>. H2S reacts with MeReOs to form MeRe02S, MeReOS2, and MeReSs,which are the reactive species in this reaction. 

The discovery of the catalytic activity of methyltrioxorhenium (MTO), by Herrmann and co-workers [60], stimulated interest in its application as an (ep)oxida-... 

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]. 

Up to now, there is only slight knowledge of mechanistic details, and no kinetic data are yet available. Both catalytic systems depend on high oxidation-state metal centers, indicating that (reversible) redox steps Mo Mo and Re " Re, respectively, are in operation to move the oxo ligand(s) of the catalyst in and out. The following proposal for the mechanism related to methyltrioxorhenium (VII) 5 may apply in principle for the molybdenyl catalyst 4, too. 

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). 

As a result, the majority of contributions to the present edition have had to be either updated or completely replaced by new articles. This applies to the sections mentioned above, but also to the rapidly growing area of enantioselective synthesis (Sections 3.3.1 and 3.2.6), the catalytic hydrogenation of sulfur- and nitrogen-containing compounds in raw oils (Section 3.2.13), the Pauson-Khand reaction (Section 3.3.7), and a number of industrially relevant topics covered under Applied Homogeneous Catalysis in Part 2. New aspects of organometallic catalysis have emerged from the chemistry of renewable resources (Section 3.3.9) and the chemistry around the multi-talented catalyst methyltrioxorhenium (Section 3.3.13). 

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). 

With the bisalkaloid ligands, potassium ferricyanide can be used as the stoichiometric oxidant [84, 91]. As with the parent achiral osmium oxidation, NMMO can also be used as the oxidant (see above) [92]. However, rather than using NMMO in stoichiometric amounts, this morpholine component can be used in catalytic amounts by the addition of the biomimetic flavin 4 to set up a triple catalytic system where hydrogen peroxide is the oxidant [93-95], Methyltrioxorhenium can be used in place of the flavin mimic [96], as can tungsten(VI) [97] and carbon dioxide [98]. 

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). 

E. Da Palma Carreiro, G. Young-En, A. J. Burke, Approaches towards catalytic asymmetric epoxidations with methyltrioxorhenium(VII) (MTO) Synthesis and evaluation of chiral non-racemic 2-substituted pyridines, J. Mol. Cat. A.. Chem. 235, 285-292 (2005). 

Even by 1989, the importance of rhenium compounds in oxidation catalysis was still minimal [1], This picture has changed dramatically since organorhenium(VII) oxides, especially the water-soluble methyltrioxorhenium (MTO, 1), have proven to be excellent catalyst precursors for a surprisingly broad variety of processes, most notably for a variety of oxidation reactions [2, 3]. This article summarizes the behavior of MTO and its peroxo derivatives in the presence of water and under catalytic condition in aqueous systems, particular attention being given to the most recent findings. 

Miscellaneous Reactions of Phosphines.- The basicities of a series of bidentate phosphines have been determined by a study of their enthalpies of protonation with trifluoromethanesulphonic acid in 1,2-dichloroethane. Ring-opening of sultones via nucleophilic attack by nitrogen occurs on treatment with tri-(2-pyridyl)phosphine, with the formation of the water-soluble phosphine systems (91). A novel aldehyde-olefination procedure is afforded by the reactions of aldehydes, diazomethanes, and tertiary phosphines in the presence of a catalytic amount of the powerful Lewis acid methyltrioxorhenium. Attempts to prepare carboxyphenylphosphines by the ring metallation of triphenylphosphine followed by... 

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