Vanadium-based Schiff-base complex

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. 

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... 

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... 

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 redox reactivity of divanadium salen615 and other complexes555 have been investigated. Disproportionation of [ V(salen) 2(//-0)] was observed under electrochemical conditions.6 The reduced, [Vin(salen)]+ complex was found to be the essential species in the catalysis of the electroreduction of 02 by four electrons in CH2C12.616 While [ V(salcn) 2(/x-0)] was proposed to be the active species in the redox reaction, more recently [Vin(salen)]+ was identified as a reservoir from which the active species forms.616 Schiff base complexes encapsulated in zeolite Y are catalytically active in the oxidation of thioanisole with H202.14 Acid-promoted disproportionation of a Vlv phenolate under anaerobic conditions was proposed as a model reaction for the vanadium uptake in tunicates.139... 

Complexes of ligands with O-donors and two or three N-donors are known. The kinetics of the outer-sphere oxidation of cis-aq uaoxovanad i u m (IV) complexes of [2-(pyridylmethyl)imino]diacet-ate and its derivatives were determined.675 Complexes with Schiff bases have been used to mimic the structure and chemistry of vanadium bromoperoxidase.275 The ligation of an imidazole functionality in the ligand has been found to readily dissociate, and is important to the functional aspects of this complex.275 A variety of five-coordinate complexes with tridentate Schiff base complexes have been prepared, several of which have been found to form supramolecular polymeric structures through association between the V=0 groups in a V=0 V=0 V=0 pattern (140).627... 

Two diastereomers of a vanadium Schiff base complex with a chiral vanadium centre plus three chiral centres in the ligand periphery. ] The constituents of the Schiff base are o-hydroxynaphthaldehyde and S-alanine the additional monodentate ligands are i ec-butanol and i ec-butanolate. 

Other encapsulated complexes, e.g. we have recently shown that vanadium-SchifFs base complexes encapsulated in zeolite Y undergo facile leaching by reaction with TBHP [85],... 

Schiff s base complexes of vanadium were encapsulated in the supercages of zeolite NaY and their catalytic activities were tested in the epoxidation of several alkenes and allylic alcohols with /er/-butylhydroperoxide The complexes investigated were VO(HPS) (vanadyl-N-(2-hydroxyphenyl)salicylideneimine) and VO(salen) (vanadyl-N,N (bis)salicylidene-imine). Particular attention was devoted to the question of leaching of vanadium during reaction. 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

An unusual electronic effect of substituents in the asymmetric oxidation of para-substituted thioanisoles and benzyl phenyl sulfide to sulfoxides, catalysed by the vanadium-Schiff base complexes (34), prepared from the synthesized enantiomers of Schiff bases, was observed. 

The mechanism of olefin epoxidation with iodosylbenzene in the presence of Cr(III) Schiff base complexes has been studied.The same reaction is catalysed by V0(acac)2 probably yij free radicals.Trans-stilbene is epoxidised by NalOi, in the presence of RuClg and substituted phenanthroline ligands. Vanadium(V) supported on a functionalised polystyrene resin is a good catalyst for the epoxidation of allylic alcohols by ButQOH a similar Mo(VI) catalyst is more suitable for cyclohexene epoxidation. ... 

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