d) Oxidation of amines (primary amines to hydroxylamines and nitro derivates and secondary amines to nitrones)
Sodium tungstate dihydrate catalyzes the oxidation of primary amines to the corresponding N-monoalkylhydroxylamines, important biologically active compounds, in the presence of UHP (urea-hydrogen peroxide adduct). This method can be applied for several primary amines, including chiral benzylic amines, α-1,2-hydroxylamines and α-amino esters, yields ranging from 65% to 80%.


 

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Poly-tungsten is lead alternative material in medical devices following regulation in the use of lead and other hazardous substances in electronic and electrical equipment. The material alternative, poly-tungsten thermoplastic, can be machined and fabricated in a variety of configurations.

Poly-tungsten thermoplastic-based material has the density of traditional metals without the safety issues associated with lead. Poly-tungsten is more durable than other lead substitutes and offers equal or better radiation-shielding capabilities. Because it is malleable, poly-tungsten offers increased design flexibility and the opportunity to create molded parts in lieu of multi-part assemblies.

Most expense associated with hazardous materials – from material handling and manufacturing to employee safety and disposal – is eliminated with poly-tungsten. In addition, the malleable material gives designers freedom that they haven't enjoyed in the past. As a result, poly-tungsten to be a cost-effective and attractive alternative to lead.

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The same approach can be used for primary alcohols. However, in the case of primary benzylic alcohols, the amount of hydrogen peroxide used affects the final product. Noyori and co-workers reported that when a small amount of hydrogen peroxide (< 1.5 molar amount) is used to oxidize primary benzylic alcohols, the corresponding aldehydes can be obtained; if a higher amount of hydrogen peroxide (2.5 - 5 molar amount) is added, the corresponding benzoic acids can be produced. The efficiency is highly dependent on the electronic propertiesof substituents.


 

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For many decades, lead (Pb) was the best option for protecting against radiation. It was cheap and plentiful, but it was toxic and highly-hazardous. Manufacturing with lead is currently banned in Europe, and is on track to be banned in the U.S. Poly tungsten shields are effectively and safely shield against radiation, that alternative for lead shields.

Proprietary combination of tungsten and polymers called poly tungsten has been developed. Poly tungsten shields against radiation even better than lead does —but far more safely. Poly tungsten has the same density as lead, meaning it can shield radiation at the same level as lead. It retains its shape during use, and can be used in many applications. Poly tungsten is safe and reliable.

Injection-molded poly tungsten parts have been successfully used in:
CT scanners
X-ray machines
Radiation therapy
Nuclear medicine equipment.

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c) Oxidation of alcohols
Secondary alcohols can be converted into the corresponding ketones (83-96% yield) under biphasic conditions (organic/aqueous) using 30% hydrogen peroxide in the presence of sodium tungstate dihydrate as catalyst and an appropriate PTC such as [CH3-(n-C8H17)3N]HSO4. Noyori and co-workers studied several cases with alkyl or aryl groups, namely 2-octanol or 1-phenylethanol, under solvent-free conditions or using toluene as solvent.


 

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Recently, a catalytic system based on Na2WO4.2H2O/CH3CO2H/H2O2 was developed as an effective method to oxidize natural rubber (with a large number of carbon-carbon double bonds in its structure) into the corresponding epoxy derivatives. This catalytic system possesses much higher epoxidation efficiency than the traditional CH3CO2H/H2O2 system. The proposed method also induces further oxidation of the epoxides into the corresponding ketones and aldehydes, resulting from C-C cleavage.


 

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b) Cleavage of double bonds (to carboxylic acids, aldehydes or ketones)

A convenient and efficient method for converting cyclohexene into adipic acid has been reported. This protocol represents a new and green viewpoint for the synthesis of adipic acid, since this approach replaces usual hazardous oxidizing agents, such as nitric acid, for an environmentally benign one, aqueous hydrogen peroxide. When a mixture of cyclohexene, hydrogen peroxide, sodium tungstate dihydrate and [CH3(n-C8H17)3N]HSO4 as PTC is kept at 75ºC to 90 ºC for 8 hours, adipic acid is obtained in 93% yield. Adipic acid is an importantchemical, whose production is necessary for the manufacture of nylon-6,6. This procedure is equally applicable to the oxidation of substituted cyclohexenes. Moreover, the oxidation cyclopentene produced crystalline glutaric acid in 90% yield.


 

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The epoxidation of 1,4-bis(allyloxy)butane in the presence of sodium tungstate dihydrate and orthophosphoric acid gives 1-allyloxy-4-glycidoloxybutane and 1,4-bis(glycidoloxy)butane under phase transfer conditions, using 30% hydrogen peroxide as oxidant. Methyltrioctylammonium chloride (Aliquat 336) was used as phase transfer catalyst (PTC). Sodium tungstate dihydrate can be used also as catalyst in the epoxidation of terpenes, such as limonene, geraniol, nerol or 3-carene, using a phosphate buffer solution of H3PO4/NaH2PO4.


 

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2. Applications
Sodium tungstate dihydrate can be used as catalyst or as catalyst precursor in several organic reactions, namely:

a) Epoxidation
The sodium tungstate catalyzed epoxidation of α,β-unsaturated acids by hydrogen peroxide in water has been developed by Payne in 1959, and revisited by Sharpless in 1985, also in water. According to Sheldon, the epoxidation of allyl alcohol to glycidol using hydrogen peroxide and sodium tungstate was applied industrially as an alternative to thetraditional route via epichlorohydrin. The epoxidation procedure developed by Venturello and co-workers, using a Na2WO4.2H2O/H3PO4/quaternary ammonium chloride/aqueous 8% hydrogen peroxide system presents some drawbacks, since requires an excess of olefinic substrates and uses 1,2-dichloroethane, a toxic and carcinogenic chlorinated hydrocarbon solvent. In contrast, Noyori and co-workers reported a clean, halide-free, biphasic epoxidation method with 30% hydrogen peroxide in the presence of sodium tungstate dihydrate, (aminomethyl)phosphonic acid, and a phase transfer catalyst (PTC) such as Q+HSO4- (Q+ =CH3(n-C8H17)3N+) to epoxidize olefins such as cyclooctene (98% yield) or 1-dodecene (87% yield). Functionalized olefins containing an ester, ether, alcohol, or α,β-enone linkage in the same molecule can also be epoxidized by this procedure.


 

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1.Introduction
Sodium tungstate dihydrate is a commercially available reagent which exists in the form of a crystalline white solid. Sodium tungstate dihydrate is usually used in the synthesis of inorganic compounds, namely in polyoxometalates synthesis. It can also be used in organic chemistry as a green catalyst or catalyst precursor, since sodium tungstate dihydrate is able to catalyze the oxidation of several substrates in the presence of hydrogen peroxide, generating only water as the by-product (Scheme 1). On the other hand, the conjugation of this reagent with other inorganic reagents used in polyoxometalates synthesis permits the formation of several complexes, mainly peroxocomplexes, which can catalyze some oxidation reactions by themselves.


 

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