Vanadate by Hydrogen Peroxide and Hydroxylamines

The reactions of hydrogen peroxide with vanadate have been of interest for many years. Much of the early work was concerned with the function of peroxovanadates as oxygen transfer agents. Alkenes and similar compounds such as allyl alcohols can be hydroxylated or epoxidized. Even alkanes can be hydroxylated, whereas alcohols can be oxidized to aldehydes or ketones and thiols oxidized to sulphones or sulphoxides. Aromatic molecules, including benzene, can be hydroxylated. The rich chemistry associated with the peroxovanadates has, therefore, led to extensive studies of their reaction chemistry. To this end, x-ray diffraction studies have successfully provided details of a number of peroxovanadate structures. 

Many peroxovanadates have potent insulin-mimetic properties [1,2]. Apparently, this functionality derives from the ability of these compounds to rapidly oxidize the active site thiols found in the group of protein tyrosine phosphatases that are involved in regulating the insulin receptor function [3], The discovery of vanadium-dependent haloperoxidases in marine algae and terrestrial lichens provided an additional stimulus in research toward obtaining functional models of peroxidase activity, and there is great interest in duplicating the function of these enzymes (see Section 10.4.2). 

Because of the asymmetry of hydroxylamines and the fact that they are com-plexed by vanadate in a side-on fashion similar to hydrogen peroxide, they provide details of the chemistry not otherwise easily accessible. Further interest in these compounds arises from both their in vivo and in vitro insulin-mimetic properties. Animal studies have shown that the bis(., ., -dimcthyl)hydroxamidohydroxooxovan-adate is as effective as other insulin-mimetics such as bis(maltolato)oxovana-dium(IV). Unlike the peroxo complexes, the hydroxylamine complexes influence enzyme activity by a nonoxidative mechanism [4,5], 

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Coordination of vanadate by hydroxylamines is similar in many respects to coordination by hydrogen peroxide. Various hydroxylamine complexes are known, although only detailed studies of the aqueous chemistry have been carried out with hydroxylamine and its A -melhyl and A,A-dimcthyl derivatives. -substitution apparently has only a small influence on product formation. There is, however, scope for a wider range of isomeric forms with the A-methylated ligand compared to the unsubstituted and dimethylated ligands, and a number of isomers have been observed... 

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