Hydrogen peroxide vanadium complexes

The colour sequence already described, for the reduction of van-adium(V) to vanadium(II) by zinc and acid, gives a very characteristic test for vanadium. Addition of a few drops of hydrogen peroxide to a vanadate V) gives a red colour (formation of a peroxo-complex) (cf. titanium, which gives an orange-yellow colour). 

Vanadium(V) does not oxidise hydrogen peroxide but forms peroxy complexes VO(02) and VO(02)J. Kinetic data are available. ... 

Reports have appeared claiming that triperoxo vanadates behave as nucleophilic oxidants. In particular, triperoxo vanadium complexes, A[V(02)3]3H20 (A=Na or K), are proposed as efficient oxidants of a,-unsaturated ketones to the corresponding epoxide, benzonitrile to benzamide and benzil to benzoic acid, reactions which are usually carried out with alkaline hydrogen peroxide. Subsequent studies concerning the oxidation of cyclobutanone to 4-hydroxybutanoic acid, carried out with the above-cited triperoxo vanadium compound, in alcohol/water mixtures, clearly indicated that such a complex does not act as nucleophilic oxidant, but only as a source of HOO anion. 

An inner-sphere hydrogen atom abstraction from the alcohol by a peroxo metal complex, thus forming a coordinated ketyl radical [(CH3)2—C —O—V(0)(00H)]" , has been proposed for the aerobic oxidation of alcohols catalyzed by peroxidic molybdenum and vanadium derivatives (Scheme 16). While in the case of Mo-catalyzed reaction the H2O2 produced is quantitatively converted to products (ketone and H2O), in the vanadium mediated process, hydrogen peroxide accumulates . In this latter case, the direct involvement of a vanadium monoperoxo species has been substantiated by ESI-MS data. 

Enantioselective epoxidation of allylic alcohols using hydrogen peroxide and chiral catalysts was first reported for molybdenum 7B) and vanadium 79) complexe. In 1980, Sharpless 80) reported a titanium system. Using a tartaric acid derivative as chiral auxiliary it achieves almost total stereoselection in this reaction. 

While hydrogen peroxide cannot be employed to epoxidize C,C double bonds, its combination with boron trifluoiide is effective. Accentuation of polar character (in this case an acceptor) by an external agent through complex formation achieves the activation. Similar activation [242] of hydroperoxides by vanadium and titanium cations is now well known. 

Vanadium oxyfluorides also react with hydrogen peroxide to yield complex compounds.2 These are not so well defined as in the case of the peroxyfluorides of niobium and tantalum. 

Bolm and Bienewald discovered in 1995 that some chiral vanadium (IV)-Schiff base complexes were efficient catalysts (1 mol %) for sulfoxidation [71a]. The catalyst 20 was prepared in situ by reacting VO(acac)2 with the Schiff base of a fJ-aminoalcohol (Scheme 6C.8). Reactions were conveniently performed in air at room temperature by slow addition of 1.1 mol equiv. of aqueous hydrogen peroxide (30%). Under these experimental conditions the reaction of methyl phenyl sulfide gave the corresponding sulfoxide in 94% yield and 70% ee. The best enantioselectivity was obtained in the formation of sulfoxide 21 (85% ee). Many structural analogues of catalyst 20 were screened for their efficacy, but none of... 

Hamstra, B.J., G.J. Colpas, and V.L. Pecoraro. 1998. Reactivity of dioxovanadium(V) complexes with hydrogen peroxide Implications for vanadium haloperoxidase. Inorg. Chem. 37 949-955. 

The most-well-known-cationic peroxovanadate is the monoperoxide, V0(02)(H20)31+, which is a red vanadate derivative often utilized in a test for the presence of vanadium. Figure 5.2 shows the pH dependence of product distribution for the major peroxovanadates under a fixed overall concentration ratio of 2 mmol/L vanadate to 4 mmol/L hydrogen peroxide. It is evident from this diagram that any significant proportion of the cationic complex occurs only below pH 3. The bisper-oxide is the dominant product throughout the pH range to at least pH 10. 

Dipeptides complex in a trident fashion to form oxoperoxodipeptido products. The rate of complexation by these ligands is quite slow. Even the vanadium-catalyzed disproportionation of hydrogen peroxide occurs quickly when compared to the rate of complexation by dipeptides. The formation of the dipeptide complex is interesting, in the sense that the ligand is tridentate, and complex formation involves loss of a... 

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. 

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. 

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

Vanadium complexes of the first class of ligands catalyze the oxidation of bromide by hydrogen peroxide. These ligands (Figure 4) in-... 

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