Complexes Schiff base vanadium

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... 

In another type of mixed ligand oxo-vanadium Schiff base complexes chloro-oxo-bis[N(4-bromophe-nyl) salicylideneiminato] vanadium (V) chelate, VOL2CI, and methoxo-oxobis[N(4-bromophenyl)sali-cylideneimine vanadium (V) chelate, VOL2OCH3 Aiiwi and Salih [43] show that these complexes are active pho-... 

Oxo-vanadium derivatives include numerous examples of Schiff base complexes, the importance of which is increasing in that they can act as insulin-enhancing agents.3 For instance, the vanadyl(IV) derivatives of TV.TV -ethylenebisCsalicylideneimine), [VO(salen)], A TV -disalicylidene-o-phenylenediamine, [VO(saloph)], and iV,Ar -ethylenebis(acetylaceto-natoimine), [VO(acen)], are shown in Scheme 1. 

Scheme 10.2 Chiral self-dimerization of V-Schiff-base monomer complexes on Si02. Two vanadium monomers are dimerized through two sets of hydrogen bonds between Ph-OH and COO.

Novel vanadium(V)-alkyl peroxide complexes with the general formula VO(OOBu )(ROPh-salR ) (197 ROPhsalR = tridendate N-(2-oxidophenyl)salicylidenaminato Schiff base ligand) were found to epoxidize alkenes stoichiometrically with high selectivity.620... 

Mitchell and Valero (1982, 1983) studied VO-phthalocyanine (VO-PC) and a Schiff base complex, VO-salen, as model vanadium compounds. The VO-PC provides a metal coordination environment comprised of four nitrogens, similar to the porphyrin. The benzo rings at the /3-pyrrolic positions contribute to enhanced aromaticity in the metal ligand. 

Chiral Self-Dimerization of Vanadium Schiff-Base Complexes on Si02 and Their Catalytic Performances lor Asymmetric Oxidative Coupling of 2-Naphthol... 

The chemistry already described is reproduced by numerous ligands that have not specifically been addressed in the previous discussion. The V-salicylidenehydrazides (Scheme 4.18a) and related compounds provide a good example. The structure [76] of a typical complex, represented in Scheme 4.18b, is not very different from that proposed for the solution structures of dipeptide complexes (Scheme 4.17). Interestingly, other similar complexes, based on Schiff base-derived ligands, form dimeric [VO]2 core complexes (Scheme 4.1) via two long ( 2.4 A) VO bonds [2], The cyclic core is not necessary for dimer formation, and a dimer can form via a linear VOV bond [77], These complexes otherwise are not significantly different in their vanadium coordination from that depicted in Scheme 4.18b. 

Li, X., M.S. Lah, and V.L. Pecoraro. 1988. Vanadium complexes of the tridentate Schiff base ligand A,-salicydene-/V -(2-hydroxyethyl)ethylenediamine Acid-base and redox conversion between vanadium(IV) and vanadium(V) imino phenolates. Inorg. Chem. 27 4657 -664. 

Later, they developed a catalytic method using the technology developed by Bolm et al. based on the use of chiral Schiff base-vanadium complexes. After extensive work optimizing the catalytic system, they found that the use of 0.25 equiv of the... 

Many vanadium(V) complexes with Schiff bases also are known.23... 

Stabilizing the vanadium with auxiliary ligands makes it possible to form complexes with various functionalities including the alkoxy functionality. Such classes of ligands include pyrazolyl borates (15)102 and Schiff bases, and will be described in the appropriate multidentate ligand section. 

New complexes containing azo- and hydrazinato-derivatized Schiff base ligands298 375 continue to be used for the analytical determination of vanadium.376 The vanadium complex was characterized for the Schiff base derived from the condensation reaction of benzoyl acetone and Girard s reagent P.377 Azophenolates of mononuclear and mixed valence dinuclear species have been reported.378 Naphthanol- or quinolinol-derivatized ligands prefer to chelate iron over vanadium.379... 

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