Hydrogen polyoxometalates

Oxygenation of alkanes with hydrogen peroxide in homogeneous reactions can be obtained also with Keggin-type iron-substituted polyoxometalates. Evidence against radical reactions has been offered. 

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. 

The use of benzene as a solvent eliminates the chlorinated organic products in Table II. This point and the products in the table are consistent with radical abstraction (hydrogen atom transfer) from the substrate, cyclohexene, by the high valent (and likely oxometal) intermediate form of the TMSP complexes, followed by chlorine abstraction from the solvent by intermediate organic radicals. Separation and analysis of the two phases after the reaction reveals that the polyoxometalate is intact. 

Hydrogen peroxide is an inexpensive oxidant, but it requires a catalyst to effect oxidation of an alcohol to the ketone. Removal of the catalyst then becomes an issue. Ronny Neumann of the Weizmann Institute of Science reports (J. Am. Chem. Soc. 2004,126, 884) the development of a hybrid organic-tungsten polyoxometalate complex that is not soluble in organic solvents, but that nonetheless catalyzes the hydrogen peroxide oxidation of alcohols to ketones. The solid catalyst is removed by filtration after the completion of the reaction. The catalyst retained its activity after five recyles. 

Studies with unpromoted and Pt-promoted solid acids, specifically, H morde-nite,308 Pt Beta-zeolite,313 and acidic polyoxometalate cesium salts314,315 applied in the presence of hydrogen, showed that the isomerization of n-butane follows a monomolecular pathway. This consists of dehydrogenation on the metal site and isomerization on acidic sites. A new study using isotope labeling provided direct... 

Liquid-Phase Oxidations with Hydrogen Peroxide and Molecular Oxygen Catalyzed by Polyoxometalate-Based Compounds... 

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...

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]. 

An interesting example of the use of a recyclable, thermoresponsive catalyst in a micellar-type system was recently reported by Ikegami et al. [38]. A PNI-PAM-based copolymer containing pendant tetraalkylammonium cations and a polyoxometalate, PW1204o, as the counter anion was used as a catalyst for the oxidation of alcohols with hydrogen peroxide in water (Fig. 9.25). At room temperature the substrate and the aqueous hydrogen peroxide, containing the catalyst, formed distinct separate phases. When the mixture was heated to 90 °C a... 

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