Chelation reactions

Electroanalytical methods in the study of chelation reactions. M. Kopanica and J. Zyka, Chelates Anal. Chem., 1972, 3,151-209 (196). 

Hole, M. and Chem, C., Chelation reactions of significance to the food indnstry. Process Biochem., 15, 16, 24, 1980. 

Co (I I) complex formation is the essential part of copper wet analysis. The latter involves several chemical unit operations. In a concrete example, eight such operations were combined - two-phase formation, mixing, chelating reaction, solvent extraction, phase separation, three-phase formation, decomposition of co-existing metal chelates and removal of these chelates and reagents [28]. Accordingly, Co (I I) complex formation serves as a test reaction to perform multiple unit operations on one chip, i.e. as a chemical investigation to validate the Lab-on-a-Chip concept. 

Water-repellent coatings for the protection of glass, plastic or metal surfaces can be obtained on the basis of a stannosiloxane chelate (reaction product from tributylchlorostannane and methyl octadecylchlorosilane)68). 

Quill, L. L. (1956). Chelation reactions, page 27 in Rare Earths in Biochemical and Medical Research A Conference Sponsored by the Medical Division, Oak Ridge Institute of Nuclear Studies, October 1955, Report No. ORINS-12, Kyker, G. C. and Anderson, E. B., Eds. (Office of Technical Services, Washington). 

Methylmagnesium bromide (191) exerts a great influence on the stereoselectivity of the reactions between mesitonitrile oxide 10 and the Baylis-Hillman adducts 192. In the absence of a Grignard reagent, a mixture of isomers is formed in which compounds 194 are the main products. The presence of a Grignard reagent reverses the stereoselectivity (Scheme 9.59). When Fisera [107] performed these reactions under microwave irradiation, the reaction times decreased from days to less than 5 min without any loss of stereoselectivity for noncatalyzed cycloadditions, but with a small change in the stereoselectivity in the chelated reactions. 

Additional relevant details of the kinetics and mechanisms of iron chelation reactions may be found in other recent reviews (10,22,134,160,161). 

To address the hydrolysis sensitivity issue for materials with highly polar bonds, one of two strategies is typically employed (1) alcohol exchange or (2) chelation (reaction) of the starting alkoxide with a ligand that is less susceptible to attack by water. Chelation processes are discussed in detail in the next section. 

Because of these precursor modification reactions, the process chemistry of chelate processes is as complex, or more so, than that involved in sol-gel processes.78 However, it is typical for chelate processes that some control of process chemistry is sacrificed in return for more expedient solution preparation. For example, the hour-long (or longer) reflux processes that have been historically used in 2-methoxyethanol based sol-gel processing of ferroelectric films are not used. Rather, the entire solution preparation procedure is generally completed within one hour, with only the initial phase of the procedure being carried out under dry box and inert atmosphere conditions. Once the chelation reaction(s) has occurred, the hydrolysis sensitivity of the precursor solution is reduced to the point where the remaining process chemistry may be carried out under ambient conditions.46... 

One obvious area in which anchimeric effects might materialize is in chelation reactions this is the underlying reason that chelation is dominated by the first step (Sec. 4.4.1). We have seen it operate in the formation of Pt(II) chelates (Sec. 4.7.6). The accompanying values of AS are less negative than is usual for the associative substitution in Pt(II), consistent with the intramolecular mechanism. The rate constant for the ring-closure reaction (6.1)... 

In recent years, C-NMR spectroscopy has found extensive use in studies on carbohydrate polymers, in some series overshadowing the importance of H-NMR spectroscopy. Applications range through determination of primary structure, analysis of mixtures, monitoring of chemical and enzymic transformations or of chelation reactions, and studies on conformational change. Measurements of coupling are utilized in determining the... 

Treatment of chromium (III) acetylacetonate with acetic anhydride and boron trifluoride etherate yielded a complex mixture of acetylated chelates but very little starting material. Fractional crystallization and chromatographic purification of this mixture afforded the triacetylated chromium chelate (XVI), which was also prepared from pure triacetylmethane by a nonaqueous chelation reaction (8, 11). The enolic triacetylmethane was prepared by treating acetylacetone with ketene. The sharp contrast between the chemical properties of the coordinated and uncoordinated ligand is illustrated by the fact that chromium acetylacetonate does not react with ketene. 

Chelation (literally pliers or pincers) is the formation of a complex made of a chelating agent, a ligand, and a cation or a metallic atom that is set in the center of the complex. The atom or cation is connected by at least two connections called coordination bonds. Illustrations of important chelating reactions found in ocular corrosive damages the calcium and magnesium ions with oxalic acid or with the fluor ion of hydrofluoric acid (Fig. 3.13). 

It is pertinent to note the chelation reaction which was reported31 in 1981 for a diphenylphosphido selenide (Scheme 3). 

A somewhat arbitrary selection of areas is based on their perceived synthetic utility. P-Diketone and a-amino acid chelate reactions have received much attention over the years and the latter are still at the forefront of current activity. Finally, some reactions of simple ketones are affected by metal coordination and this subject will also be considered. 

Immobilization of enzymes. Enzymes consist of amino acids which contain reactive groups such as amino(lysine, c-terminus), thiol (cysteine), carbonyl (aspartate, glutamate and c-terminus), aromatic hydroxyl (tyrosine) and aliphatic hydroxyl (serine and threonine). Chemical, ionic or chelation reactions with such groups can enable us to attach the amino acids and hence proteins to insoluble, inert supports. Immobilization is one of the best ways of stabilizing enzymes. There is a vast literature on this subject and the reader is directed to Barker (9) and Goughian et al (10) for further reading on specific systems, techniques and applications of immobilization. 

Depending on the nature of the base polymer, chelation reactions can occur either in a homogeneous (soluble linear homo- and copolymers) or heterogeneous (cross-linked, grafted polymers and gels) phases. Consideration of the polymeric nature of the chelating ligand requires a more deliberate, as compared with the LMWM, selection of solvents, pH control and specific synthesis conditions. 

Aquation of the Chloro Monoadducts and Chelation Steps. The data are presented in Table 7. The chelation reactions were easily studied starting from the diaqua complex (Y = H20, Scheme 4) [99-102], Fewer experiments have been done starting from cis-[ PtC I (NH3)2 (H20) + [103],... 

The sense of asymmetric induction in the a-oxygenated aldehydes, which shows a strong kinetic preference for the formation of the syn diol, is consistent with the classic Gram model as in 27 (Scheme 32). Once the complexation occurs, the allyl group is transferred to the carbonyl carbon from the less hindered 7r-face opposite to that occupied by the R group. In 28, the chelation pathway is able to adopt a chair conformation which accommodates the favored formation of the syn-diol. For /3-chelate reactions, the factors which influence the product formation appear to be the same. When 29 forms, the intramolecular attack occurs syn to the hydroxyl group. This reaction trajectory leads preferentially to the anti-dio, provided that a chairlike transition states such as 30 is followed. 

Savory et al. [635] determined chromium in serum. The sample was wet ashed by treatment with sulphuric, nitric and perchloric acids. Finally, the pH was adjusted to 6.0 with a buffer and chelation was effected with a dilute solution of trifluoroacetylacetone at 70°C. The analysis was carried out on 5% of QF-1 on Chromosorb W with the use of a 63Ni ECD. Booth and Darby [636] applied a very similar procedure to soft tissues and serum. They performed the chelation reaction also at 70°C for 1 h. Recoveries of chromium in serum and liver homogenates were 94 and 88—104%, respectively. Ross and Shafik... 

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