Polyoxometalates alcohols

The applications of polyoxometalates in catalytic dehalogenation of halocar-bons have been succinctly reviewed by Hill and coworkers [188]. This reaction involves the photocatalytic transformation of organic halides coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides) (Eq. (9)) [189, 190] ... 

A more recent approach to the epoxidation of allylic alcohols makes use of a vanadium polyoxometallate, together with a sterically demanding chiral tartrate (or TADDOL) -derived hydroperoxide, to give a highly chemoselective, regiose-lective and enantioselective outcome (Figure 11.2). ... 

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. 

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. 

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 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. 

Table 6 Ruthenium-substituted polyoxometalates as catalysts for the oxidation of alcohols...

For example, PEG-200 and PEG-400 (the number refers to the average molecular weight) were used as solvents for the aerobic oxidation of benzylic alcohols catalyzed by the polyoxometalate, H5PV2Mo10O40 [8]. Combination of the same polyoxometalate with Pd(II) was used to catalyze the Wacker oxidation of propyl-... 

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... 

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>. 

It has previously been reported that hydrotalcite catalyzes the aldol condensation of acetone (25). Polyoxometalates are known to dehydrate alcohols due to their acidic nature (IS ). In order to compare the relative basicity of polyoxometalate-pillared hydrotalcites to that of hydrotalcite itself, a variety of hydrotalcites were screened for 2-propanol conversion (Table II). This reaction is known to give propylene when the catalyst contains acidic sites (such as alumina) and acetone when the catalyst contains basic sites (such as magnesium oxide). 

Polyoxometallates of Mo and W in homogeneous solutions are known to photo-catalyze oxidation of alcohols [176, 177]. Such species can be intercalated into... 

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]. 

Besides hydrogen, other reductants for O2 in the liquid-phase epoxidation of propene include carbon monoxide, aldehydes, alcohols and other organic compounds. The reaction proceeds very efficiently with methanol as the reductant, in the presence of Pd and the Ti-Al-MCM-22 catalyst or Pd and peroxo-polyoxometalate catalysts the latter have been intercalated inside layered double hydroxides to make them heterogeneous [29e,h]. A propene conversion of 47%, with 91.5% selectivity for PO, was obtained at 80°C [29h]. 

D. Sloboda-Rozner, P. L. Alsters, R. Neumann, A water-soluble and "self-assembled" polyoxometalate as a recyclable catalyst for oxidation of alcohols in water with hydrogen peroxide, /. Am. Chem. Soc. 125 (2003) 5280. 

FIGURE 16.4 Effect of 1,2- and/or 1,3-allylic strain on the diastereoselectivity observed with sandwich polyoxometalate (POM)-catalyzed epoxidations of chiral allylic alcohols. 

W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-Moller, D. Seebach, A. K. Beck, R. Zhang, Chiral hydroperoxides as oxygen source in the catalytic stereoselective epoxidation of allylic alcohols by sandwich-type polyoxometalates Control of enantioselectivity through a metal-coordinated template, /. Org. Chem. 68 (2003) 8222. 

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