Oxidation using sodium tungstate catalyst

Oxidation of secondary amines into nitrones has been extensively studied and a variety of well-known efficient oxidants and catalysts which can be employed in this process are available. Catalytic oxidation by hydrogen peroxide at room temperature is carried out by using sodium tungstate (Fig. 2.1) (28-47). 

Quinazoline and its derivatives 1 are converted to the corresponding quinazoline 3-oxides 3 when treated with hydroxylamine. The parent quinazoline 3-oxide is also obtained by oxidation of quinazoline with hydrogen peroxide using sodium tungstate as catalyst vide... 

Oxidation. Maleic and fumaric acids are oxidized in aqueous solution by ozone [10028-15-6] (qv) (85). Products of the reaction include glyoxyhc acid [298-12-4], oxalic acid [144-62-7], and formic acid [64-18-6], Catalytic oxidation of aqueous maleic acid occurs with hydrogen peroxide [7722-84-1] in the presence of sodium tungstate(VI) [13472-45-2] (86) and sodium molybdate(VI) [7631-95-0] (87). Both catalyst systems avoid formation of tartaric acid [133-37-9] and produce i j -epoxysuccinic acid [16533-72-5] at pH values above 5. The reaction of maleic anhydride and hydrogen peroxide in an inert solvent (methylene chloride [75-09-2]) gives permaleic acid [4565-24-6], HOOC—CH=CH—CO H (88) which is useful in Baeyer-ViUiger reactions. Both maleate and fumarate [142-42-7] are hydroxylated to tartaric acid using an osmium tetroxide [20816-12-0]/io 2LX.e [15454-31 -6] catalyst system (89). 

Sodium tungstate has also been used as a catalyst in the oxidation of dimethyl sulphoxide to the sulphone36. The kinetics of this reaction have been studied in great detail and it has been shown that oxygen transfer to the sulphoxide takes place via two peroxytungstic acid species (HW05 and HWOg ). 

The use of hydrogen peroxide as an oxidant is generally desirable due to cost and ease of use. Most of the development objectives could be met by removing or replacing acetic acid from the process. One of the seminal reports of the oxidation of thioethers, with hydrogen peroxide was made by Freyermuth,1 in which, sodium tungstate is used as a catalyst in the process. 

A greener method has been developed using hydrogen peroxide as the oxidant, with catalytic amounts of sodium tungstate and a quaternary ammonium phase-transfer catalyst (Equation 4.7) [16]. Since the solvent and the by-product are water, the reaction is indeed much greener (E-factor = 0.49, atom economy 67%). 

Use of sodium tungstate as catalyst in 30% aqueous hydrogen peroxide oxidizes primary alcohols to the corresponding carboxylic acids under microwave irradiation. 

A tungstate-exchanged Mg-Al layered double hydroxide (LDH) was employed to catalyze oxidation of aliphatic tertiary amines to N-oxides by 30% aqueous H2O2 (Table 8.8) [117]. The reaction takes 1-3 h at room temperature. The use of dode-cylbenzenesulfonic acid sodium salt as an additive increased the rate of the oxidation by a factor of 2-3 except for N-methyl morpholine. An advantage with the LDH-W04 catalyst is that it can be reused. 

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