Polyoxometalates ketones

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. 

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]. 

Despite the utilization of polyoxometallates as Bnansted acid catalysts in organic synthesis [13], the most important application is the palladium-catalyzed Wacker oxidation of ethylene to acetaldehyde in aqueous phase. Under standard conditions (PdCl2, CuCl2, 02, HC1), chlorine ions are corrosive and produce chlorinated by-products (mainly from CuCl2) these conditions are not suitable for the oxidation of higher olefins, such as 1-butene to methyl ethyl ketone. For this reason... 

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]. 

The aldol reaction of cyclic ketones and acetone with aromatic aldehydes were carried out in combination with triflic acid in water at 25°C [250]. Other chiral primary-tertiary diamine catalyst such as compound 167 (20 mol%) was used in combination with solid polyoxometalate acid support (6.67% mol) in the aldol reaction between dihydroxyacetone (149a) and aromatic aldehydes in NMP as solvent at 25°C to afford mainly iyn-aldol products in good yields (59-97%) and high diastereo- and enantioselectivities (78-99% de, 84-99% ee). The combination of catalyst 167 with triflic acid was used in the reaction of acyclic ketones and a-hydroxyketones 8 with aromatic aldehydes also with good results [251]. Simple chiral diamine 168 (10 mol%) in the presence of Iriflic acid (20 mol%) was applied as catalyst in the reaction between acetone and cyclohexanone with aromatic aldehydes in water at 25°C, giving aldol adducts 4 in low yields (15-58%) and moderate diastereo- and enantioselectivities (50-98% de, 45-93% ee) [252]. 

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... 

A series of 1-aryl alkanols were investigated using polyoxometalates such as Wio032" as a photocatalyst (Scheme 6.4). A clean oxidation reaction of the side chain of the aryl alkanol led to the formation of the aryl ketone. This process supports a hydrogen transfer mechanism as the rate determining step. The efii-ciency of the reaction was mainly determined in the presence of oxygen [27]. 

Fig. 3. Processes involved in the polyoxometalate-catalyzed photoredox transformations of alkanes. Equations (10) - (12) are photochemical in nature the rest are thermal in nature but occur as a result of the photochemical processes. Alkene production is quite general while A/-alkylacetamide and methyl ketone production are only seen with particular alkane substrates.

The selectivity of the different polyoxometalate catalysts has also been used to carry out different reactions with ketones. Thus, whereas the photolysis of cw-2-decalone in the presence of WioOfe as catalyst leads to the formation of tran5-2-decalone, the photolysis of cw-2-decalone in the presence of a-P2Wi8062 or aPWi20 gives two alkenyl ketones ... 

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