Vanadium complexes hydroxamic acids

SCHEME 35.6. Asymmetric epoxidation of homoallylic alcohols catalyzed by vanadium-chiral-hydroxamic-acid complexes. 

Vanadium-catalyzed asymmetric epoxidation has recently been re-examined with a newly designed chiral hydroxamic acid (3).43-45 The hydroxamic acid (3) forms a 1 1 complex with vanadium ions and induces high enantioselectivity (Scheme 6). 

The development of transition metal mediated asymmetric epoxidation started from the dioxomolybdcnum-/V-cthylcphcdrinc complex,4 progressed to a peroxomolybdenum complex,5 then vanadium complexes substituted with various hydroxamic acid ligands,6 and the most successful procedure may now prove to be the tetroisopropoxyltitanium-tartrate-mediated asymmetric epoxidation of allylic alcohols. 

Hydroxamic acid exists in two tautomeric forms, (1) and (2), and such keto-enol tautomerism provides a number of sites for coordination and chelation. The keto form (1) predominates in acid media and the enol form (2) in alkaline media 25 this has been corroborated by the extraction of vanadium benzohydroxamic acid complexes in organic solvents.26... 

Comparison of V(IV,V) hydroxamic acid complexes showed the V(V) complex induced a stronger insulin-enhancing effect than the V(IV) complex, and both complexes were better than either of the salts, vanadyl sulfate or sodium vanadate, in relieving the symptoms of mice with STZ-induced diabetes. The distribution of vanadium in tissues was the same irrespective of the complex administered however, the tissue distribution of vanadium when the salts were administered was different from that seen after vanadium complex administration [146], These results suggest that the difference in the antidiabetic activity of the hydroxaminic acid V complexes is related to the different oxidation state, although there was no difference in the final tissue distribution of these complexes. 

Dutta, R. L. Metal complexes of hydroxamic acids. Colored complexes of iron, vanadium and molybdenum with isonicotinohydroxamic acids and their analtyical uses. J. Ind. Chem. Soc. 36, 285 (1959). 

Simple complexes of hydroxamic acids with vanadium(V) have been reported, including ones with a variety of nuclearities.249,250 A wide range of TV-substituted mono- and dihydroxamic acids undergo oxygen abstraction upon reaction with V111 and Vlv to form Vv hydroxamate complexes and the respective amides and diamides.251... 

Formation constants of 3d metal ions with A-m-tolyl-p-substituted benzohydroxamic acids and of rare earths with thenoylhydroxamic acid have been determined. Formation constants of proton and metal complexes of iV-phenyl-2-thenoyl- and A-p-tolyl-2-thenoyl-hydroxamic acids have also been determined. In addition, study has been made of the mixed ligand complexes involving nicotine- and isonicotino-hydroxamic acids. A method of extraction and spectrophotometric determination of vanadium with chlorophenylmethylbenzohydroxamic acid has also been published. It may be mentioned that hydroxamic acids (in particular, the A-phenylbenzohydroxamic acid) have been widely used as analytical reagents for metal ions. Solvent extraction of titanium by benzo- or salicyl-hydroxamic acid in the presence of trioctylamine in the form of coloured complexes has been reported. A-w-Tolyl-p-methoxybenzohydroxamic acid has been used for extraction and spectrophotometric determination of Mo and W from hydrochloric acid media containing thiocyanate. 

Vanadium(V) extracted from clam tissue was determined as its A -phenylbenzo-hydroxamic acid (PBHA) complex on a silica column (A = 430 nm) using a 97/3 chloroform/methanol (0.9 mM PBHA) mobile phase [789]. Absorption spectra are presented for the PBHA-V(V) complexes in chloroform/methanol mobile phases ranging from 5% to 30% methanol. Choice of mobile phase is important not only because of its effects on the spectral characteristics of the complex, but also because of its effects on complex stability. Linear concentration curves were obtained up to 200 ig/L, with detection limits reported as 8 pg/L. 

In these reactions the tested organic substance and an inorganic salt give a more or less stable strong color due to a complex salt. In Part 2 of this monograph a series of such reactions is described, such as, for example, the reaction of hydroxamic acids with ferric salts (p. 276), the reaction of phenols with ferric chloride (p. 188), the reaction of molybdenum with o-dihydro-xybenzenes (p. 191), of diacetyl dioxime with nickel salts (p. 227), of phenols with Millon s reagent (p. 196), alcohols with ceric ammonium nitrate (p. 170), alcohols with vanadium-hydroxy quinoline complex (p. 171), and the reaction of cis-enols of )5-dicarbonylic compounds and a-dicarbonyl compounds with ferric chloride, (p. 294.)... 

Three other metals, namely, copper, vanadium, and molybdenium, possess appreciable affinities for siderophores. It is possible that siderophores or siderophore-like molecules are involved in the microbial transport of these metals. Copper(II) forms stable complexes with hydroxamate siderophores (for instance, schizokinen, desferriox-amine B. and desfeirithiocin) and catechol siderophores (for instance, enterobactin). However, copper(II), unlike iron(III), also possesses a high affinity for amino acids and small peptides, and this property renders it unlikely that copper uptake is dependent on siderophore-medi-ated transport. 

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