117 - chelate formation

1 Hartwig, J. F. Transition metal catalyzed synthesis of arylamines and aryl ethers from aryl halides and triflates scope and mechanism. Angew. Chem. Int. Ed. 1998, 37, 2046-2067. 

2 Suzuki, A. Recent advances in the cross-coupling reactions of organoboron derivatives with organic electrophiles. J. Organomet. 

3 Prim, D. Campagne, J.-M. Joseph, D. Andrioletti, B. Palladium-catalyzed reactions of aryl halides with soft, non-organometallic nucleophiles. Tetrahedron 2002, 58, 2041-2075. 

4 Kotha, S. Lahiri, K. Kashinath, D. Recent applications of the Suzuki-Miyaura crosscoupling reaction in organic synthesis. Tetrahedron 2002, 58, 9633-9695. 

6 Herrmann, W. A. Ofele, K. von Preysing, D. Schneider, S. K. Phospha-palladacydes and N-heterocyclic carbene Pd complexes efficient catalysts for CC-coupling reactions. J. Organomet. Chem. 2003, 687, 229-248. 

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Deprotonation of enols of P-diketones, not considered unusual at moderate pH because of their acidity, is faciUtated at lower pH by chelate formation. Chelation can lead to the dissociation of a proton from as weak an acid as an aUphatic amino alcohol in aqueous alkaU. Coordination of the O atom of triethanolamine to Fe(III) is an example of this effect and results in the sequestration of iron in 1 to 18% sodium hydroxide solution (Fig. 7). Even more striking is the loss of a proton from the amino group of a gold chelate of ethylenediamine in aqueous solution (17). 

Chelate Formation. Citric acid complexes with many multivalent metal ions to form chelates (9,10). This important chemical property makes citric acid and citrates useful in controlling metal contamination that can affect the color, stabiUty, or appearance of a product or the efficiency of a process. 

Bases function in a more complex way than simply by acting as a halogen acceptor, for they cannot always be used interchangeably, and the product may depend on the base used (58,68,69). Diamines, such as ethylene-, propylene-, or hexamethylene diamines may function differently than monoamines. Diamines were thought to function through chelate formation (32). Loven and Speckamp (40) concluded that selectivity in reduction of a... 

The factors which influence the stability of metal ion complexes have been discussed in Section 2.23, but it is appropriate to emphasise here the significance of the chelate effect and to list the features of the ligand which affect chelate formation ... 

Excellent /(-methyl selectivity is observed in the zinc chloride mediated condensation with 0-silyl enol ethers of 2-pyridinylmethyl thiopropionates109. Supposedly, chelate formation of zinc(II) with the sulfur and the nitrogen atom of the pyridinylmethyl thioester is essential for the high /(-selectivity. The geometry of the ketene acetal also seems to have some influence. 

This result is in strong contrast with that obtained by Tanner et al. [49] for ligand 52 (ee of 2% for the addition of diethylzinc to benzaldehyde. Fig. 3). It was argued by Tanner that the M-trityl group would be too large to allow efficient chelate formation with diethylzinc [49], which is however clearly not the case... 

The titration curves of the dendrimers in the presence of calcium ions are also shown in Fig. 6. In the presence of calcium ions, the titration curve of the G1.5 dendrimer shifted downward, suggesting chelate formation. Both titration curves of the G3.5 dendrimers in the absence and presence of calcium ions are the same, especially in the high pH region. The protonation of the G4.5 dendrimer, however, was promoted in the presence of calcium ions. These results... 

There are available also several kits for the assay of calcium, in 10 or 20 microliter samples by chelate formation colorimetrically or fluorimetrically. (Pierce Chem. Co., Rockford, 111.). These are read either with the spectrophotometer or by spectrofluorometry. In our experience, while these systems can be used for approximate results, the plot of concentration versus reading curves are rather flat and only an approximation of the values can be obtained. This may be very important late at night or at times when the atomic absorption machine is down, but if the atomic absorption instrument is available it should be used in preference to these procedures. 

The complexation constant for copper(II) is particularly high and Wall Gill (1954) have suggested that chelate formation takes place with two carboxyl groups ... 

The strength of ion binding is enhanced when the arrangements of the functional groups permit chelate formation (Begala Strauss, 1972). Thus, magnesium is more firmly bound to poly(vinyl methyl ether-maleic acid) than to either poly(acrylic acid) or poly(ethylene maleic add). 

As reaction proceeds, the polymer chain (which is in random coil form) unwinds as the charge on it grows as a result of neutralization and ionization. This contributes to thickening of the cement paste. Cations released become bound to the polymer chain. Countercations can either be bound to a polyanionic chain by general electrostatic forces or be site-bound at specific centres. More than one type of site binding is possible. Complex formation and, if the ligand is bidentate, chelate formation enhance the effect. 

The last stage in the development of the EBA cement is represented by the polymer cements. Brauer Stansbury (1984b), taking advantage of the fact that the EBA-HV liquid does not inhibit vinyl polymerization, included methacrylates into the cement composition. The object was to produce a material that set after mixing, both by polymerization and by salt or chelate formation. 

Figure 4.102 Dynamic evolution of Co chelate formation in the reaction/extraction and decomposition/removal areas at various concentrations ...

Extraction rates of zinc (II) and nickel (II) with ethyldithizone, butyldithizone, or hexyldithizone in an organic phase (chloroform, CCI4, w-heptane, or benzene) showed a first-order dependence on the ligand and metal ion concentration and an inverse-first-order dependence on the proton concentration. The results were explained by chelate formation in the interfacial region [59]. The effects of stirring on the distribution equili-... 

Strongly influences positions of equilibria involving complex ions and metal-chelate formation. 

The patent describes the formation of complex metal chelates by treatment of the ketoester simultaneously with an alcohol and a metal to effect trans-esterification and chelate formation by distilling out the by-product ethanol [1], This process was being applied to produce the zinc chelate of 2-tris(bromomethyl)ethyl acetoacetate, and when 80% of the ethanol had been distilled out (and the internal temperature had increased considerably), a violent decomposition occurred [2], This presumably involved interaction of a bromine substituent with excess zinc to form a Grignard-type reagent, and subsequent exothermic reaction of this with one or more of the bromo or ester functions present. 

A few typical examples shall be discussed here to explain the chelate-formation ... 

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