Polyoxometalates alkenes

MIL-101 Supported Polyoxometalates Alkene and alcohol Oxidation and epoxidation 47... 

Neumann and Fish have studied the novel polyoxometalate salt 25, which features 12 fluorous ammonium cations [12]. This material was insoluble in EtOAc (and toluene) at room temperature, but dissolved at 80 °C to give an effective catalyst system for the oxidation of alkenes and alcohols by 30% aqueous H2O2. CooHng precipitated the catalyst, which was reused. Additional examples of thermomorphic fluorous catalysts have been briefly described in meeting abstracts [60,61] and will Hkely soon appear in the peer-reviewed literature. 

SCHEME 85. Epoxidation of alkenes and alkenols with a perfluorinated quaternary ammonium salt of a polyoxometalate catalyst... 

Goals and five limitations in conjunction with the development of selective catalytic homogeneous oxidation systems are evaluated. Systems are presented that address several of the problems or goals. One involves oxidation of alkenes by hypochlorite catalyzed by oxidatively resistant d-electron-transition-metal-substituted (TMSP) complexes. A second involves oxidation of alkenes by H2O2 catalyzed by specific TMSP complexes, and a third addresses functionalization of redox active polyoxometalate complexes with organic groups. 

We report here three studies that address three separate but significant issues in the emerging area of selective catalytic oxidation by TMSP-type complexes. The first study establishes for the first time that some TMSP complexes are compatible with basic oxidants and basic conditions. The second study reports the first oxidation, in this case selective alkene epoxidation, by the economically and environmentally desirable oxidant, aqueous hydrogen peroxide, catalyzed by TMSP complexes. The third study demonstrates that redox active polyoxometalates can be derivatized with alcohols in a manner that should prove useful for fabricating future generations of more sophisticated and selective TMSP catalysts. 

Studies in this field are just beginning, and the number of publications hardly exceeds a dozen. The most interesting results were obtained by the research groups of Yamada [160-162], Neumann [163,164] and Kozhevnikov [165, 166], Using various type catalysts (Ru porphyrene complexes, polyoxometalates, supported metals), the authors conducted selective oxidations of various types. These include epoxidation of alkenes, oxidation of alcohols, oxidation of alkylaromatics, oxidation and aromatiza-tion of dihydroanthracenes, and some other reactions. The experiments were typically conducted at 373—423 K under 1.0 MPa pressure of nitrous oxide. 

Scheme 13.1 Reaction of a hydrogen terminated surface with an alkene to generate surface mono-layers (a) monolayer formation by reaction of silicon with H2C=CH(CH2)90Ph(C6H40Me)2, (b, c) monolayer derivatisation to give a surface layer of tethered magnetic TiW5 polyoxometalate clusters.4...

Ben-Daniel, R Weiner, L. and Neumann, R. (2002) Activation of nitrous oxide and selective epoxidation of alkenes catalyzed by the manganese-substituted polyoxometalate, [Mnnl2ZnW... 

Terminal alkenes can be hydrogenated selectively in the presence of PdCI2 [63] or RhCl(PPhj)3 [64] and heteropoly compounds. The catalytic system is also highly active for the production of urethane or isocyanate compounds by the reductive carbonylation of nitrobenzene. It is considered that polyoxometalate coordinating with Pd2+ in the reduced form is the active species, since easily reducible heteropolyanions are more active [63]. 

Tatsumi, T., Yamamoto, K., Tajima, H. and Tominaga, FI. (1992). Shape selective epoxidation of alkenes catalysed by polyoxometalate-intercalated hydrotalcite. Chem. Lett., 815. 

A polyoxometalate is also at the heart of an enantioselective epoxidation of allylic alcohols using a C-2 symmetric chiral hydroperoxide 39 derived from l,l,4,4-tetraphenyl-2,3-0-isopropylidene-D-threitol (TADDOL). Thus, in the presence of the oxovanadium(IV) sandwich-type POM [ZnW(V0)2(ZnW9034)2]12- and stoichiometric amounts of hydroperoxide 39, the dienol 40 is converted to the (2R) epoxide 41 in 89% yield and 83% ee. The proposed catalytic cycle invokes a vanadium(V) template derived from the POM, substrate, and hydroperoxide, a hypothesis supported by the lack of enantioselectivity with unfunctionalized alkenes. The catalytic turnover is remarkably high at about 40,000 TON <03OL725>. 

Organic substrates (alkanes alkenes, alcohols) are also photooxidized by trans-dioxo Ru and Os complexes [93]. The interest in these catalysts may lie in the transformation of cyclohexane to cyclohexanone and cyclohexanol in reasonable yields. The presence of alcohol, ester, and ketone functional groups is tolerated in the catalytic functionalization [94] with polyoxometallates and Pt as co-catalyst [95]. 

Zhang, X. Hill, C. L. Alkene Epoxidation by p-Cyano-lV-lV-dimethylaniline IV-oxide Catalyzed by d-Electron-transition-metal-substituted Polyoxometalates. In Catalysis of Organic Reactions. Malz, R. E. J., Ed. Marcel Dekker New York, 1996 pp 445-450. 

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