Tungsten-catalyzed oxidation systems

The effect of electron transfer between tungsten oxide and titanium oxide is also important in photochromatic applications. In an excellent study of aqueous sols. He et al. analyze the electronic structure of the tungsten oxide-titanium oxide system, finding that titanium oxide can catalyze the generation of W+. This state is a colored state and can be generated from WO3 WH2O by the following reactions ... 

Tungsten-based catalytic systems for H2O2 oxidations of sulfides have attracted considerable interest, and some early reports include the use of H2WO4 [20]. More recently, various tungsten-catalyzed methods have been used [21-25]. 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

Transition Metai-Cataiyzed Epoxidation with Hydrogen Peroxide. There are few transition metal-catalyzed epoxidations of unsaturated fatty acids with hydrogen peroxide. By using a procediue developed by Ishii et al. (26), Bavaj (27) in oiu group epoxidized various fatty acid esters in a two-phase system with a combination of a tungsten heteropoly acid and a phase-transfer catalyst, using 30% H2O2 as the oxidant. (Table 1). 

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