Silver -catalysed oxidation

In cone, solutions the silver catalysed oxidation of ammonia to nitrogen may be very violent. 

Silver forms monovalent ion in solution, which is colourless. Silver(II) compounds are unstable, but play an important role in silver-catalysed oxidation-reduction processes. Silver nitrate is readily soluble in water, silver acetate, nitrite and sulphate are less soluble, while all the other silver compounds are practically insoluble. Silver complexes are however soluble. Silver halides are sensitive to light these characteristics are widely utilized in photography. 

The ketenes will then react readily with any nucleophiles present in the system, e.g. H20 below. The reaction can be brought about by photolysis, thermolysis, or by treatment with silver oxide. In the first two cases an actual carbene intermediate (55) is probably formed as shown above, in the silver catalysed reaction loss of nitrogen and migration of R may be more or less simultaneous. In the case where R is chiral, e.g. C4H9C MePh, it has been shown to migrate with retention of its configuration (cf. p. 117). 

Several different oxidants have been used in this work. The trend has been to stronger oxidants, in the hope that more complete oxidation would result. Among those used have been potassium peroxide [69], dichromate in sulfuric acid [70-73], silver-catalysed potassium dichromate [74,75], potassium per-... 

The ligand was then used to form a variety of transition metal carbene complexes [207] (see Figure 3.72). Interestingly, more than one method for the formation of transition metal carbene complexes was successfully employed presence of an inorganic base (IC COj) to deprotonate the imidazolium salt and the silver(I) oxide method with subsequent carbene transfer to rhodium(I), iridium(I) and copperfi), respectively. The silver(I) and copper(I) carbene complexes were used for the cyclopropanation of styrene and indene with 1,1-ethanediol diacetate (EDA) giving very poor conversion with silver (< 5%) and qnantitative yields with copper. The diastereomeric ratio (endolexo) was more favonrable with silver than with copper giving almost a pnre diastereomer for the silver catalysed reaction of indene. 

FIGURE 5. Proposed reaction mechanisms for flavin-catalysed oxidation of amines. A, the carbanion mechanism initially proposed for TMADH (Rohlfs and Hille, 1994). B, the amminium cation radical mechanism, as originally proposed for monoamine oxidase (Silver-man, 1995) although only the pathway passing through a transient covalent intermediate is shown, several alternative pathways for breakdown of the initial flavin semiquinone/... 

Many of the reactions of peroxodisulphate are catalysed by silver ions, and although the reactions then involve higher oxidation states of silver as oxidants, it is convenient to consider them along with the uncatalysed oxidations. Silver ions also catalyse the decomposition of peroxodisulphate in a second-order reaction, viz. 

In a study of the silver ion-catalysed oxidation of 2-propanol in 50 % acetic acid, Venkatasubramanian and Sabesan found first-order kinetics with respect to peroxodisulphate and zero-order with respect to the alcohol. They propose a chain mechanism involving either silver(II) or silver(III) ions. 

The mechanism is considered to be the same as that proposed by Ball et al., i.e. reactions (20)-(23). For the silver ion-catalysed oxidation, Subbaraman and Santappa found the rate equation to be... 

However, Venkatasubramanian and Sabesan report that the rate is zero order with respect to the substrate, as in most silver ion-catalysed oxidations by peroxo-disulphate. 

The silver ion-catalysed oxidation of cyclohexanol obeys the rate equation (Subbaraman and Santappa )... 

Subbaraman and Santappa found that the uncatalysed oxidation of terf-butyl alcohol is very slow, but the silver ion-catalysed oxidation proceeds at a readily measurable rate which is independent of the alcohol concentration, viz. 

Menghani and Bakore studied the silver ion-catalysed oxidation of pinacol. Acetone is the main product, and the stoichiometry is presumably... 

Srivastava and Ghosh report that the kinetics are first-order with respect to peroxodisulphate and zero-order with respect to formic acid, but Kappana reports first-order kinetics with respect to each reactant. The effect of trace amounts of metal ions and of oxygen on the rate is uncertain, and discussion of the mechanism is of doubtful significance at present. However, the reported observations definitely indicate a chain mechanism. Thus Srivastava and Ghosh found an induction period in the oxidation, and report that halide ions inhibit the reaction (inhibition by halide ions is a feature of reactions involving hydroxyl radicals). In a study of the silver ion-catalysed oxidation, Gupta and Nigam found that the reaction is approximately first-order with respect to both peroxodisulphate and the catalyst, and zero-order with respect to the substrate. 

Early work showed that the rate of the silver ion-catalysed oxidation of oxalate is much faster than the oxidations of other substrates under similar conditions King ). Allen showed that with solutions of very low copper concentration, but not de-aerated, the rate is only slightly faster compared with other substrates. However, Kalb and Allen found that oxygen is a powerful inhibitor of the silver ion-catalysed oxidation, and that in the absence of oxygen low concentrations of copper have no effect on the rate. They studied the silver ion-catalysed reaction in the absence of oxygen. With peroxodisulphate concentrations greater than 0.004 M the rate equation is... 

Sengar and Guptaclaim that the silver ion-catalysed oxidation is subject to inhibition by oxalate, and they report other features not noticed by Kalb and Allen. Sengar and Gupta do not mention any control of the amounts of copper and oxygen in their solutions, and possibly their results were affected by the presence of these substances. 

The oxidation of acetate by peroxodisulphate is much slower than that of formate. Glasstone and Hickling showed that the products, which include carbon dioxide, methane, ethane, and ethylene, are similar to those produced by the anodic oxidation of acetate ions (Kolbe electrolysis), and they inferred that the same organic radicals are formed as intermediates. Similar results are reported by Eberson et al. for the oxidations of ethyl terf.-butyl-malonate, tert.-butyl-cyanoacetate, and ferl.-butyl-malonamate ions. The oxidations of these ions and of acetate by peroxodisulphate are first order with respect to peroxodisulphate and zero order with respect to the substrate. Mechanisms involving hydroxyl radicals are excluded because the replacement of peroxodisulphate by Fenton s reagent leads to different products, so Eberson et al. infer that the initial attack on the substrate is by sulphate radical-ions. Sengar and Pandey report that the rate of the silver ion-catalysed oxidation of acetate is independent of the peroxodisulphate concentration. 

The oxidations of lactic, malic, and tartaric acids have been studied. In each case carbon dioxide is produced, and in addition lactic acid gives acetaldehyde, and malic acid gives malonic acid. The kinetics are not well defined, and most of the studies show changes of order during a run. In general, the observations are in accord with chain mechanisms. Kumar and Saxena found that the oxidation of lactic acid is first-order with respect to peroxodisulphate and zero-order with respect to the acid. Bakore and Joshi studied the silver ion-catalysed oxidation, and found approximately first-order kinetics with respect to both... 

Agrawal and Mushran studied the kinetics of the silver ion-catalysed oxidation of acetamide. The stoichiometry is uncertain, but acetic acid and nitrogen are the main products. The rate is approximately first-order with respect to both peroxodisulphate and silver ions, and is almost independent of the substrate concentration. No definite conclusions regarding the mechanism can be drawn, but the kinetics suggest a chain process. Agrawal et report similar results for the oxidation of formamide. 

In contrast to silver-catalysed cumene oxidation, the evidence concerning the mechanism of copper-catalysed reactions favours radical initiation via surface hydroperoxide decomposition. Gorokhovatsky has shown that the rate of ethyl benzene oxidation responds to changes in the amount of copper(ii) oxide catalyst used, in a manner which is characteristic of this mechanism. Allara and Roberts have studied the oxidation of hexadecane over copper catalysts treated in various ways to produce different surface oxide species, Depending on the catalyst surface area and surface oxide species present, a certain critical hydroperoxide concentration was necessary in order to produce a catalytic reaction. At lower hydroperoxide levels, the reaction was inhibited by the oxidized copper surface. XPS surface analysis of the copper catalysts showed a... 

R. B. Grant, R. M. Lambert, A single crystal study of the silver-catalysed selective oxidation and total oxidation of ethylene, /. Catal. 92 (1985) 364. 

Turbidity (AgCl) caused by traces of chloride present during silver-catalysed persulphate oxidation is prevented by the addition of a little mercury(II) sulphate [a stable complex, HgCL, is formed in the presence of Hg(ll)]. The mechanism and the conditions of Mn(ll) oxidation by persulphate in the presence of Ag(I) ions have been studied [11]. 

In contrast to the oxidative generation of radicals described above, redactions of alkyl iodides nsing tris(trimethylsilyl)silane also produces alkyl radicals under conditions suitable for Minisci-type substitution. Carboxylic acids (a-keto acids) are also useful precursors for alkyC° and/or acyC radicals via silver-catalysed peroxide oxidation, or from their l-hydroxypyridine-2-thione derivatives, the latter in non-aqueous conditions. 

The most significant application of the C5-unit 73 (Scheme 7) is in the BASF process for the production of vitamin A [22]. Industrial syntheses of 73 [23] proceed via but-l-ene-3,4-diol diacetate (74) by acetylation and copper-catalysed rearrangement of 75. A new route is emerging via the vinyloxirane 76, which has recently become accessible via silver-catalysed gas-phase oxidation [24]. The diacetate 74 is formed as a byproduct in the oxidative acetoxylation of butadiene (14), which is performed on an industrial scale to produce butane-1,4-diol (77) [25]. 

Induced electron transfer has been investigated in the action of one-electron oxidants on pyridinemethanolpentammine-cobalt(iii) complexes. The direct [and silver(i)- and cobalt(ii)-catalysed] oxidations of pentammine-(pyridine-4-methanol)cobalt(iii) (G) and the corresponding pyridine-3-methanol complex by cerium(iv) yield aldehydic complexes of cobalt(iii)... 

Metal ions are known to catalyse oxidations involving peroxodisulphate, especially silver(i), and the catalysis of the oxidation of [Ru(bipy)3] + has been described in the presence of this ion. The chelated complex is inert to hydrolysis and oxidised only slowly in the absence of metal ions. The mechanism in sulphuric acid (Scheme 11) involves the formation of a weak 1 1 complex between Ag+ and SaOg , with subsequent rapid reactions of... 

Metal-catalysed Epoxidation. Many of the reports of metal-catalysed formation of epoxides are associated with the development of more economic and efficient industrial routes to simple epoxides. A review has appeared on the manufacturing processes for production of ethylene oxide by direct gas-phase oxidation of olefins. Further reports on this process include the rather unusual use of potassium chloride and quartz catalysts. The more usual silver-catalysed epoxidation of ethylene has been studied in detail both from a kinetic and from a stereochemical point of view. The latter... 

Univalent silver catalyses the oxidation of water by peroxodiphosphate. The experimental rate law ([P208 ]> 5 x 10 mol 1- ),... 

The role of substituents on the silver(i)-catalysed oxidation of phenols by peroxy-disulphate correlates better with attack at the phenol oxygen rather than at the 3-carbon atom. Rate acceleration by electron-releasing groups is observed. 

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