Polyoxometalate catalyzed reactions

Oxidation photocatalyzed by polyoxometalates [66k] has been applied to the fimctionalization of 1,8-cineole (structure IX-10) [661], widely distributed in the plant kingdom. The photooxygenation of IX-10 gave a mixture of ketones and alcohols which were transformed by the subsequent action of pyridinium chloro-chromate into 5- and 6-keto derivatives in the ratio IX-11 IX-12 = 2.5 1. A laser flash photolysis study of the mechanism has been carried out for the deca-tungstate anion catalyzed reaction [66m]. 

For reactive aldehydes, proline-derived 7V-sulfonylcarboxamides were investigated as catalysts by Ley and co-workers [45], Berkessel et al. [46], and Kokotos and co-workers [47]. Not only did amino acid derivatives promote the aldol reaction, but also chiral diamines in the presence of an acid were also found to be effective [48]. The yield and enantioselectivity were the same as for the proline-catalyzed reactions. In combination with polyoxometalate acid, a diamine could be used in 0.33 mol%, but for less reactive aldehydes the yields are still low [49]. 

It is important to note that the synthesis of many heterocycles is often carried out under acid-catalyzed reactions, so much effort has been put into the search for solid acid catalysts (Rosati et al., 2007 Dhakshinamoorthy et al., 2011 Sreekumar and Padmakumar, 1998 Kandarpa et al., 2011 Krishnakumar and Swaminathan, 2011 Huang et al., 2008). From this point of view, catalysis by heteropoly acids (HPAs) and related compounds is a field of increasing importance worldwide. To avoid the use of conventional acid catalysts (sulfuric, phosphoric, and hydrofluoric acids and boron trifluoride) and the related environmental pollution and corrosion problems (Vdzquez et al., 2002), insoluble solid acid catalysts such as HPAs can be used. HPAs are mixed oxides composed of a central ion or heteroatom, generally P, As, Si or Ge, bonded to an appropriate number of oxygen atoms and surrounded by a shell of octahedral MOg units. HPAs with Keggin structure and related polyoxometalates are quite common and are represented by the formula Hg [XM,204o], where X is the... 

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. 

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). 

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. 

The use of HPAs and multicomponenf polyoxometalates as catalysts in liquid-phase reactions was reviewed by Kozhevnikov. Moreover, an interesting minireview was published concerning the Friedel-Crafts acylation of arenes and the Fries rearrangement catalyzed by HPA-based solid acids. The results show that HPA-based solid acids, including bulk and supported heteropoly acids as well as heteropoly acid salts, are efficient and environmentally friendly catalysis for all reacfions analyzed. 

Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates... 

Further research into the reaction mechanism revealed that the reaction rate was correlated with the electron structure of the sulfoxide the more electropositive sulfoxides were the better oxygen donors. Excellent correlation of the reaction rates with the heterolytic benzylic carbon-hydrogen bond dissociation energies indicated a hydride abstraction mechanism in the rate-determining step to yield a carbocation intermediate. The formation of 9-phenylfluorene as by-product in the oxidation of triphenylmethane supports this suggestion. Further kinetic experiments and NMR showed the formation of a polyoxometalate-sulfoxide complex before the oxidation reaction, this complex being the active oxidant in these systems. Subsequently, in a similar reaction system, sulfoxides were used to facilitate the aerobic oxidation of alcohols [29]. In this manner, benzylic, allyUc, and aliphatic alcohols were all oxidized to aldehydes and ketones in a reaction catalyzed by Ke jn-type... 

Summarizing the information disclosed in the section above, one notes that polyoxometalates appear to be versatile oxidation catalysts capable of activating various mono-oxygen donors such as iodosobenzene, periodate, ozone, nitrous oxide, and sulfoxides. Some of these reactions are completely new from both a synthetic and mechanistic point of view. The various reaction pathways expressed are also rather unusual and point to the many options and reaction pathways available for oxidation catalyzed by polyoxometalates. 

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