Vanadium V systems

Figure 3.32 Cleavage of olefins with hydrogen peroxide/ vanadium ( V) systems.

Since epoxidation at the vinyl double bond is unproductive, it is desirable to direct reaction on the al-lene moiety. This can be accomplished by taking advantage of the hydroxy-directed epoxidation of allylic alcohols using the t-butyl hydroperoxide/vanadium(V) system.The directing effects of both allylic and homoallylic type hydroxy groups have been examined at both positions of the vinylallene unit. " At the 1-position (64), primary, secondary and tertiary allylic tdcohols are effective, while only primary homoallylic alcohols have bran examined (equation 35). Presumably the directing effect of the hydroxy groups favors formation of the intermediate allene oxide (65). A sample of the compounds prepared by this route is shown in Scheme 32. ... 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

The effect of inorganic additives upon ignition delay in anilinium nitrate-red finning nitric acid systems was examined. The insoluble compounds copper(I) chloride, potassium permanganate, sodium pentacyanonitrosylferrate and vanadium(V) oxide were moderately effective promoters, while the soluble ammonium or sodium metavanadates were very effective, producing vigorous ignition. 

The effects of various metal oxides and salts which promote ignition of amine-red fuming nitric acid systems were examined. Among soluble catalysts, copperQ oxide, ammonium metavanadate, sodium metavanadate, iron(III) chloride (and potassium hexacyanoferrate(II) with o-toluidine) are most effective. Of the insoluble materials, copper(II) oxide, iron(III) oxide, vanadium(V) oxide, potassium chromate, potassium dichromate, potassium hexacyanoferrate(III) and sodium pentacyanonitrosylferrate(II) were effective. 

As mentioned above, some chemistry of a few heavier elements is also of concern in the development of the geosphere and of living systems as we shall see later. A striking case is the chemistry of molybdenum (Mo) and tungsten (W), which we take here with vanadium (V). The first two elements are in the second and third series of transition metals and all three are found in higher combining ratios and with a greater preference for S rather than O, W less so than Mo (the... 

DeBoer E, Plat H, Wever R (1987) Algal vanadium(V)-bromoperoxidase. A halogenating enzyme retaining full activity in apolar solvent systems. In Laane C, Tramper J, Lilly MD (eds) Biocatalysis in organic media. Elsevier, Amsterdam, p 317... 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

By analogy between the oxo forms of vanadium(V) and iron(IV), the latter being the active species in oxidations by cytochrome P-450, the system constituted by vanadium oxide as the catalyst, and t-butylhydro-peroxide, as the oxidant, gives good results in the conversion of olefins to the corresponding epoxides. With the supported "clayniac" catalyst, in the presence of i-butyraldehyde as a sacrificial reducer, olefins are epoxidized in good yields by compressed air at room temperature, in a convenient procedure. 

Electrochemically characterized V(V) complexes which lack oxo ligands are somewhat rare. Many of those which are known are the result of preparative studies guided by CV results on V(IV) systems. For example, a series of imidovanadium(IV) complexes containing the tetradentate dianionic 5,7,12,14-tetramethyldibenzo[b, i] [1, 4, 8, 11] tetraazacyclotetradecinato moiety (TMTAA ) was reported [73], and diamagnetic vanadium(V) cations [(TMTAA)V = NR]+ (4) were prepared by oxidation of... 

The quasi living polymerization of ethene and norbornene has been reviewed, among other topics in living polymerization of alkenes (19). Specifically, arylimido-aryloxo-vanadium(V) complexes with methylaluminoxane or Et2AlCl as co-catalyst have been used as catalyst systems. The polymers exhibit a low polydispersity and a high molecular weight (20). 

Figure 1 Potential versus pH diagram for the vanadium-water system at 25 °C. The dashed lines indicate the domains of relative predominance of the dissolved forms of the metal, but the various dissolved forms for each oxidation state are not explicit. The solid lines correspond to saturated solutions with a total vanadium concentration of 0,51 gdm-3. The long dashed lines correspond to oxidation and reduction of water (for E° values of 1.23 and 0.00 V respectively) (adapted from E. Deitombe, N. Zoubov and M. Pourbaix, in Atlas d Equilibres Electrochimiques , ed. M. Pourbaix,...

An equimolar amount of vanadium(V) oxide (V205) reacts with diphenyl disulfide to yield PPS (>90%) even under anaerobic media. V(acac)3 (vanadi-um(III) species) is oxidized to VO(acac)2 by 02. The activated VO(acac)2 in the presence of acid seems to exhibit the properties of both vanadium(III) and vanadium(V) in the catalytic system. 

Elvingson, K., M. Fritzsche, D. Rehder, and L. Pettersson. 1994. Speciation in vanadium bioinorganic systems. 1. A potentiometric and 51V NMR study of aqueous equilibria in the H+-vanadate(V)-L-a-alanyl-L-histidine system. Angew. Chem., Int. Ed. Engl. 48 878-885. 

Bhattacharyya, S. and A.S. Tracey. 2001. Vanadium(V) complexes in enzyme systems Aqueous chemistry, inhibition and molecular modeling in inhibitor design. J. Inorg. Biochem. 85 9-13. 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

Toste s vanadium oxidation system currently has more limited scope, although it seems to be orthogonal to the Pd(II) system. Whereas the Pd-sparteine system was unable to effectively resolve a-hydroxycarbonyl compounds, these alcohols were resolved with high selectivity using vanadium(V) (Table 2) [11]. Benzylic and allylic a-hydroxycarbonyls performed well in the reactions (entries 1-3,6). Alkyl hydroxy esters were oxidized more slowly, but with high selectivity (entry 5). Activated alcohols not a to a carbonyl led to poor selectivity or low reactivity. 

Equilibrium496 and kinetic497 studies have also been reported for the vanadium(v)-hydroxylamine system. The complex V02NH30H is suggested to be the major... 

The V-BrPO-catalyzed oxidation of bromide by hydrogen peroxide is the first step in a series of reactions that results in brominated organic products or the halide-assisted disproportionation of hydrogen peroxide. As studies continue on both the enzymatic and model systems, we can make several comments. The ability to catalyze this reaction seems to be a fairly general property of vanadium(V) complexes with an available coordination site in the equatorial plane in which to bind peroxide. The oxidation rates of the model complexes, with respect to the enzyme, raise the question of what microscopic differences give rise to the high... 

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