Oxidation tetrachloroaurate

Very interesting transformations were reported in terminal alkynes RC=CH (R = alkyl, aryl, alkoxy, carboxylate, etc.). They react readily with nitric acid, in aqueous nitromethane (1 1) and in the presence of catalytic amounts of tetra-butylammonium tetrachloroaurate to give 3,5-disubstituted isoxazoles 15 in 35% to 50% isolable yield (92). The reaction might proceed via a nitrile oxide intermediate by attack of an electrophile (AuCh or H+) and of a nucleophile (N02 ) on the triple bond to form a vinyl nitrite, which is converted to a nitrile oxide by the action of gold(III) or of nitric acid (Scheme 1.8). 

The oxidation of glycolaldehyde by tetrachloroaurate was carried out in acetic acid-sodium acetate buffer and found to be first order in [Au(III)] and [glycolaldehyde]. H+ and Cr both retarded the reaction. A compatible mechanism was proposed, which involves a one-step, rate-determining, two-electron transfer and the involvement of three gold species, AuCH, AuClsCOHa), and AuClsCOH), the last being the most active. 

The polymer resulting from oxidation of 3,5-dimethyl aniline with palladium was also studied by transmission electron microscopy (Mallick et al. 2005). As it turned out, the polymer was formed in nanofibers. During oxidative polymerization, palladium ions were reduced and formed palladium metal. The generated metal was uniformly dispersed between the polymer nanofibers as nanoparticles of 2 mm size. So, Mallick et al. (2005) achieved a polymer- metal intimate composite material. This work should be juxtaposed to an observation by Newman and Blanchard (2006) that reaction between 4-aminophenol and hydrogen tetrachloroaurate leads to polyaniline (bearing hydroxyl groups) and metallic gold as nanoparticles. Such metal nanoparticles can well be of importance in the field of sensors, catalysis, and electronics with improved performance. 

Bond and coworkers [521] have described Au(III) electroreduction and Au(0) oxidation stripping processes in dilute aqua regia utilizing platinum, rhodium, iridium, gold, and glassy carbon electrodes. Sorption of tetrachloroaureate ions on carbon paste electrode modified with montmorillonite has been performed as a preconcentration step in the determination of gold in pharmaceutical preparation [522]. 

Many other cations of transition metals have been employed for the oxidation of sugars. For example, the oxidation of aldoses by hexachloroiridate(IV)183,184 and tetrachloroaurate(III)183,185 in hydrochloric acid led to the corresponding aldonic acids or aldonolactones. The observed... 

In order to prevent S-alkylation in the repetitive acid-cleavage steps required in a multistep peptide synthesis, the protection of methionine residues as sulfoxides was proposed by Iselin.f This approach has since been widely used for peptide synthesis in solution and on solid supports.Generally, oxidation of L-methionine yields a mixture of S- and R-sulf-oxides and, depending on the conditions used, even a mixture of the related sulfone. Thereby, at least by oxidation with hydrogen peroxide the R-sulfoxide is formed in a preferred manner (80%), whereas oxidation of L-methionine by tetrachloroauric(III) acid has been reported to produce stereospecifically the 5-sulfoxide diastereoisomer. Isomeric pure Met(O) derivatives are obtained by isolation of the isomers from the 5,/ -sulfoxide mixture, taking advantage of the differences in solubility of the picrate salts.0 ... 

The quantity of stabilizing substance may be increased to provide greater protection from atmospheric oxidation. For example, with the use of 30 to 40 ml. of potassium thiocyanate or tellurate(IV) (or a mixture of the two) it is possible to obtain the hydroxide in a form which may be filtered in air and dried for 17 hours on the filter funnel without oxidation. The stabilized hydroxide will return to its original unstable form by treatment with oxidation-accelerating substances such as tetrachloroauric(III) acid [hydrogen tetrachloroaurate(III)], copper(II) sulfate, or palla-... 

Sodium tetrachloroaurate cat yzes the photo-oxidation of hydrocarbons in solution. Typical of this is the irradiation with wavelengths >310 nm of aerated solutions of cyclohexane in acetonitrile or methylene chloride. The principal isolable products formed from this treatment are cyclohexanol and cyclohexanone . A mechanism that might involve the formation of a metal peroxo or metal oxo complex has been suggested. Such complexes are known to react with alkanes to yield hydroxy derivatives . The principal organic intermediate is the unstable cyclohexylhydroperoxide that readily decomposes to afford cyclohexanone and cyclohexanol. Further study has shown that the hydroperoxide accumulates during the oxidation . The influence of wavelength (A. 300, 365 or 436 nm) on the reaction has also been studied . Hexane and ethylbenzene can be similarly oxidized to yield analogous products . 

X. Zhang, V. Chechik, D. K. Smith, P. H. Walton, and A.-K. Duhme-Klair, Controlled synthesis of optically active polyanihne nanorods and nanostructured gold microspheres using tetrachloroaurate as an efficient oxidant of aniline. Macromolecules, 41, 3417-3421 (2008). 

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