Methyltrioxorhenium catalysts

Oxidative cleavage with hydrogen peroxide as oxidant is more important in oxidation processes of natural products. The use of a three-fold excess of hydrogen peroxide without further additives, except for the catalyst methyltrioxorhenium (MTO), enables the oxidation of certain natural products drawn from styrene... 

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). 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

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. 

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]. 

Methyltrioxorhenium (MTO) is now well established as a catalyst in a number of oxidations employing hydrogen peroxide. Two groups have now reported,... 

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). 

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)... 

Methyltrioxorhenium 41 has been established as a good catalyst for the selective oxidation of sulfides to sulfoxides by H2O2319 320. Both mono 42 and diperoxo species 1 (equation 30) are reactive in this oxidation. High selectivity of sulfoxide over sulfone has been reported, except in the presence of large contents of water in the reaction mixture319. 

Ketones can be olefinated with ethyl diazoacetate in the presence of triphenylphos-phine, using methyltrioxorhenium as catalyst.205... 

The BaeyerVilliger oxidation of flavanones 407 using methyltrioxorhenium (MTO) catalyst affords 1,5-benzodiox-epin-2-ones 408 in excellent yields (Scheme 214) . 

In methyltrioxorhenium-catalysed reactions, residual pyridine, pyrazole or imidazole may deactivate the active catalyst and the ionic liquid should be purified accordingly.118 191 The nature of the ionic liquid may decide the success or failure of a given reaction and, for example, Ru-catalysed alcohol oxidations appear to be very sensitive to the type and quality of ionic liquid used. 

Methyltrioxorhenium (MTO) is an extensively used catalyst for olefin epoxidation1241 and a number of reviews are available on its application.125"281 The complex reacts with hydrogen peroxide to form the mono r 2-peroxo species mpMTO and the diperoxide dpMTO. Both complexes are capable of transferring an oxygen atom to olefins or other substrates, as shown in Scheme 5.1, but the diperoxide species is about five times more reactive than the monoperoxo complex.1171 Care has to be taken that halide impurities are absent from the solvent as these are oxidised much faster than olefins.1171... 

Methyltrioxorhenium has been found to be a universal catalyst for a number of [2-1-1] cycloaddition reactions, including nitrene, carbene, or oxo-atom addition to olefins <2001GC235>. Typically, to increase the chemical yield of the reaction, at least 5 equiv of an olefin is required. As with most nitrene transfer reactions, simple cyclic olefins such as cyclohexene produce a low chemical yield of aziridine. The authors assume that the intermediate of the reaction is a reactive rhenoxaziridine intermediate. 1,2-Dihydronaphthalene provides aziridine 28 in 43% chemical yield under these reaction conditions (Equation 11). 

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