Oxidation vanadium-peroxo complexes

One of the breakthroughs in the field was reported in 1983 by Mimoun and coworkers . On that occasion they reported the synthesis, characterization and radical reactivity of a class of vanadium peroxo complexes representative of which is the species V0(02)pic (HaOIa, 36. The oxidative ability of this complex has been tested with several aliphatic and aromatic hydrocarbons and the synthetic results obtained can be summarized as in Scheme 20. 

Conte, V., R Di Furia, and S. Moro. 1995. Studies directed toward the prediction of the oxidative reactivity of vanadium peroxo complexes in water. Correlations between the nature of the ligands and the 51V-NMR chemical shifts. J. Mol. Catal. A 104 159-169. 

It would be of considerable interest to see whether vanadium-peroxo complexes are also able to oxidize bromide and display kinetic behavior similar to that of the vanadium-containing bromoperoxidases. In this respect the complexes reported by Li et al. (80) may provide a useful contribution. Conversely, some attention should be paid to whether bromoperoxidases show specificity only toward bromide or iodide. These enzymes may perhaps be tuned to catalyze the oxidation and oxygenation of other nucleophiles. 

A C(carbonyl)-C bond of aldehydes can be cleaved under oxidative reaction conditions [31]. For example, when a mixture of aldehyde 55 and aniline was treated with Mn(0Ac)3-2H20 under an oxygen atmosphere, formanilide was obtained, presumably through fragmentation of the 1,2-dioxetane intermediate 56 (Scheme 7.22). Treatment of the aldehyde 57 with KCl and HjOj in the presence of a vanadium peroxo complex promoted a decarbonylative chlorination reaction to produce aryl chloride 58 (Scheme 7.23) [32]. 

Figure 20. Oxidation of aromatics by a vanadium peroxo complex.

The kinetics and stoicheiometry of the oxidation of oxovanadium(iv) by peroxide have been studied and continuous-flow e.s.r. techniques used to investigate the intermediate which appears to be a vanadium(v) complex with a paramagnetic ligand, formulated as either [OVOO,aq] or [V02(H02). aq]. Six- and seven-co-ordinate vanadium(v) has been identified by. Y-ray crystallography in the peroxo-complexes (NH4)[V0(02)2-Nh ]36o (NH4)4[0 V0(02)2 2] respcctively. and seven-co-ordination... 

However, oxidation with H2O2 in acetone resulted in a high diol selectivity with an equilibrium mixture of the cis- and trans-diols, illustrating the role of the residual acidity of the support The reaction is suggested to occur via heterolyhc cleavage of the vanadium peroxo species. Less than 0.5% leaching of the bipy complex was observed over 50 h of operation. 

Radical reactions133,158,159 of peroxo metal complexes have been observed in the oxidation of thioethers160, alkenes161, aromatics161-163 and alcohols164. Vanadium peroxo derivatives, in particular those containing picolinic acid derivatives as ligands, are usually... 

Studies of the oxidation of organic sulfides with amino acid-derived ligands in acetonitrile revealed very little difference between the mechanism of their oxidation and that of halides, except for one major exception. Despite the fact that acid conditions are still required for the catalytic cycle, hydroxide or an equivalent is not produced in the catalytic cycle, so no proton is consumed [48], As a consequence, there is no requirement for maintenance of acid levels during a catalyzed reaction. Peroxo complexes of vanadium are well known to be potent insulin-mimetic compounds [49,50], Their efficacy arises, at least in part, from an oxidative mechanism that enhances insulin receptor activity, and possibly the activity of other protein tyrosine kinases activity [51]. With peroxovanadates, this is an irreversible function. Apparently, there is no direct effect on the function of the kinase, but rather there is inhibition of protein tyrosine phosphatase activity. The phosphatase regulates kinase activity by dephosphorylating the kinase. Oxidation of an active site thiol in the phosphatase prevents this down-regulation of kinase activity. Presumably, this sulfide oxidation proceeds by the process outlined above. 

Clague, M.J. and A. Butler. 1995. On the mechanism of cw-dioxovanadium(V)-catalyzed oxidation of bromide by hydrogen peroxide Evidence for a reactive, binu-clear vanadium(V) peroxo complex. J. Am. Chem. Soc. 117 3475-3484. 

Recovery of vanadium with peroxygens involves both oxidation and com-plexation. In solution, conversion of lower oxidation states into vanadium(V) allows separation by solvent extraction (Figure 6.18).269 This chemistry can be used for vanadium by-products in uranium extractions. With hydrogen peroxide, vanadium(IV) is not oxidized in acidic solution, but rather in alkaline conditions, e.g. 60 °C at pH 9 (Figure 6.19).270 Use of excess hydrogen peroxide readily forms peroxo complexes and this is of value in selective dissolution of vanadium from secondary sources. 

Binuclear vanadium(V) peroxo complexes such as (NH4)5[V202(02)4P04]-H20 are also known.30 They have been implicated as the reactive species in the bromide oxidation by hydrogen peroxide,31 a process which is important in some biological systems (Section 17-B-8). Some stable peroxo compounds display insulin-mimetic activity both in vivo and in vitro.32... 

Chemical Methods. In general, vanadium is reduced to the +II oxidation state by zinc amalgam, the vanadium can then be determined volumetrically by oxidation with a standard oxidizing agent (e.g. KMn04) to the +V state. The formation of the reddish-brown peroxo-complex is used as the basis of a colorimetric technique for the determination of vanadium. Gravimetric techniques are not as useM for the determination of vanadium as the volumetric and colorimetric techniques. 

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