Schiff-base complexes formations

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

Activation volumes for aquation of Schiff base complexes [Fe(C5H4NCH=NHR)3]2+ (R = Me, Et, nPr, nBu) are between +11 and +14 cm3 mol-1 (107), and thus within the range established earlier (108) for (substituted) tris-l,10-phenanthroline-iron(II) complexes, viz. +11 to +22 cm3 mol-1. These positive values are consistent with dissociative activation. Kinetic studies of the reaction of a CH2S(CH2)3SCH2 -linked bis(terpy) ligand (L6) with [Fe(terpy)2]2+ showed a very slow two-step process. The suggested mechanism consisted of slow loss of one terpy, rapid formation of [Fe(terpy)(L6)], and finally slow displacement of the second terpy as the partially-bonded L6 becomes hexadentate (109). 

Martin and associates [124, 125] have studied the dehalogenation of CHC13 in boiling methanol by Schiff-base complexes of some transition metals in the presence of TMEDA. The kinetics of chloride ion formation has been measured without characterizing the organic products. Nahar and Mukhedkar [126, 127] found that the reactivities of related Schiff-base complexes in the above reaction decreased in the order Pd>Pt>Ni>Cu>Zn. 

In two earlier studies (106, 107), the oxidation of two Schiff base complexes were studied at room temperature, but in these cases only activation parameters for the overall process could be obtained since it was not possible to detect the formation of an intermediate species which could be attributed to a peroxo species. Nevertheless, the kinetic measurements provided indirect evidence for the existence of this intermediate. In both studies negative values for the activation entropies and the activation volumes were obtained. The oxidation of [Cu2(H-BPB-H)(CH3CN)2](PF6)2 (H-BPB-H = l,3-bis[iV-(2-pyridylethyl)-formidoyl]benzene) is accompanied by an activation entropy of -53 11 J K-1 mol-1 and an activation volume of -9.5 0.5 cm3 mol-1. In... 

The intervention of a metal ion in the stoichiometry of a reaction has been illustrated several times previously. Reaction is forced to completion in ester hydrolysis since the carboxylate grouping forms a more stable complex than the ester moiety does. A similar driving force underlies the formation of macrocycles and the completion of transamination by formation of the metal-Schiff base complex. The latter is particularly relevant in dilute solution and at low pH. For example, the extent of aldimine formation between pyridoxal and alanine is undetectable at the physiological pH but occurs to the extent of = 10% in the presence of zinc... 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

A strategy to accelerate p-BL formation was therefore developed, consisting of the synthesis of unsymmetrical tridentate Ci Schiff base) complexes (Fig. 44) that might open a reaction channel for this particular sequence after chloride abstraction with Na[Co(CO)4]. 

A Co(II) Schiff-base complex converts 1- and 2-alkenes into methyl ketones and the corresponding secondary alcohols in the presence of oxygen or H2O2 in primary alcohol solvent.543 A radical oxidation with cobalt hydroperoxide through the formation and subsequent decomposition of alkyl hydroperoxide was suggested.543 An efficient conversion of alkenylarenes to ketones was achieved by the use of molecular oxygen and EtjSiH in the presence of a catalytic amount of Co(II) porphyrin in 2-propanol.544... 

Table 53 Formation Constants for some Silver(I) Schiff Base Complexes in Methanol at 20°C, I - 0.I392...

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

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... 

Schiff base and related complexes of uranium and thorium are widely described in recent literature and covered in a review [463]. Those of U(VI) have a practical use as catalytic organic oxidants [460] or as part of a polystyrene-supported chelating resin [464,465]. Among other Schiff base precursors, salicylaldehyde [466] and triethylenetetramine [464], 3-formylsalicylic acid and o-hydroxybenzylamine [465], or salicylaldehyde and l-amino-2-naphthol-4-sulfonic acid [467] were used. In the example of Schiff base complexes, kinetics of formation of U(VI) complexes and their pK values were studied [468]. 

Vanadyl and copper(n) ions catalyse the /J-elimination reaction of O-phospho-threonine in the presence of pyridoxal.429 Equilibrium spectroscopic studies of the threonine-metal ion-pyridoxal system have identified a metal-ion complex of the amino-acid-pyridoxal Schiff base. The catalytic effect of the metal is ascribed to its electron-with drawing effecCIt was suggested that the specific catalytic effect of Cu2 + and V02+ arises from their reluctance to co-ordinate the phosphate in an axial position. Other metal ions such as nickel can also form the Schiff base complex but probably stabilize the phosphothreonine system by chelate formation. 

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