Chelation mechanisms

However, the decarbonylation reaction can be suppressed by the use of specially tailored chelating groups. Intermolecular processes involving dienes and salicylaldehydes are now known, and are thought to proceed via a double chelation mechanism, akin to the Jun-type system. Rhodium-catalyzed reactions lead to hydroacylated products, under relatively mild conditions (Equation (134)).117... 

In a bidentate ligand system, three molecules of a dye containing either a terminal salicylic acid unit (as in 5.2) or an o-nitrosonaphthol residue are able to chelate simultaneously with a trivalent metal ion of CN6, such as chromium (III) or iron(III), to form a 1 3 metal-dye complex (as in 5.8). Historically, the most important bidentate ligand system was alizarin (5.1). It has been suggested that both hydroxy groups and the keto group in the peri position are all involved with the metal atom in the chelation mechanism. 

Fig. 5. Equilibrium binding and chelation mechanism for intestinal regulation and control of iron uptake with a mucosal directed, active transport system...

Therefore, and because of the Ca presence, a possible Ca catal)4ic role could be guessed. The chelating mechanisms reported [10,11] at polymeric pyrolysis can not be applied to coal combustion because the inert-reducing atmosphere at pyrolysis process has not relation to the oxidation conditions at the combustion atmospheres. However, other additional effect in the LCL runs could influence the PAH emission and distribution. This effect would be a possible catalytic role performed by Ca. 

The temperature effect could be explained by the chelation mechanism. - The Schiff s base composed of a-keto acid and optically active amine interacted with the catalyst to form a substrate-catalyst complex (14) at lower temperature at higher temperatures, the population of the unchelated structure (15) would increase as shown in Scheme 6. Asymmetric hydrogenation involving () )-a-phenylglycinate and ethyl pyruvate has also been studied. ... 

Fig. (7). Chelating mechanism proposed by Samia et al [167] for a lone 3-hydroxyl substitution on flavylium cation (B) besides on catechol group (A). M 2 Bivalent metals.

Another way to protect ascorbic acid from oxidation consists of the chelating mechanism. As has been discussed above, flavonoids with ortho-phenol structure have the ability to chelate bivalent metals, which may undergo the oxidation of ascorbic acid, and this mechanism of protection for anthocyanins is being proposed by Sarma et al [167], These former reports might explain what was proposed by Rusznyak and Szent-Gyorgyi more than six decades ago when they observed that the administration of lemon juice decreased the fragility and permeability of the capillary wall, where ascorbic acid alone had no protective effect, and led them to propose flavonoids as vitamin P. 

Humus by means of exchange reactions and chelation mechanisms is able to hold most of them except copper in an available form as they are either released from soil minerals or are added to the soil in plant and animal residues and in fertilizers. The ability of soil humus to hold microelements for a considerable time and release them as iweded to crops is undoubtedly one of the most important benefits derived from its presence in soils. 

Different mechanisms of alkaline catalysis have been suggested according to the alkali used. When caustic soda is used as the catalyst, the type of mechanism which seems the most likely is that which involves the formation of a chelate ring similar to that suggested by Caesar and Sachanen [7]. The chelating mechanism was thought to initially cause the formation of a sodium-formaldehyde complex or of a formaldehyde-sodium phenate... 

Chelate Mechanism for the Electrophilic Substitution of Methanal on Phenol in the Presence of Metal Ions (M)... 

Bhattacharyya MH, Peterson DP (1979) Action of DTPA on hepatic plutonium. III. Evidence for a direct chelation mechanism for DTPA-induced excretion of monomeric plutonium into rat bile. Radiat Res 80 108-115 Bluhm RE, Bobbit RG, Welch LW, Wood AJJ, Bonfiglio JF, Sarzen C, Heath AJ, Branch RA (1992) Elemental mercury vapour toxicity, treatment, and prognosis after acute, intensive exposure in chloralkali plant workers. I. History, neuropsychological findings and chelator effects. Hum Exp Toxicol 11 201-210 Buchet JP, Lauwerys RR (1989) Influence of 2,3 dimercaptopropane-1-sulfonate and dimercaptosuccinic acid on the mobilization of mercury from tissues of rats pretreated with mercuric chloride, phenylmercury acetate or mercury vapors. Toxicology 54 323-333... 

Sestili P, Guidarelli A, Dacha M, Cantoni O. Quercetin prevents DNA single strand breakage and cytotoxicity caused by tert-butyUiydroperoxide free radical scavenging versus iron chelating mechanism. Free Radic Biol Med 1998 25 196-200. 

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