Metal-chelate-catalyzed additions

Fats and Oils. The oxidation of fats and oils in food products can be prevented by the addition of citric acid to chelate the trace metals that catalyze the oxidation. Citric acid is also used in the bleaching clays and the degumming process during oil refining to remove chlorophyll and phosphohpids (59—63). 

Metal cations can catalyze reactions of certain dienophiles. For example, Cu2+ strongly catalyzes addition reactions of 2-pyridyl styryl ketones, presumably through a chelate involving the carbonyl oxygen and pyridine nitrogen.31... 

Proteins modified with 2-iminothiolane are subject to disulfide formation upon sulfhydryl oxidation. This can cause unwanted conjugation, potentially precipitating the protein. The addition of a metal-chelating agent such as EDTA (0.01-0.1M) will prevent metal-catalyzed oxidation and maintain sulfhydryl stability. In the presence of some serum proteins (i.e., BSA) a 0.1M concentration of EDTA may be necessary to prevent metal-catalyzed oxidation, presumably due to the high contamination of iron from hemolyzed blood. 

Metal catalysts, 10 46-47 Metal-catalyzed addition, polymers prepared by, 15 179-180 Metal chelation, 9 424 Metal chloride salts, 13 817-818 Metal chlorides, decomposition by acids, 13 822-824 Metal cleaning... 

Utilize a metal chelating additive to help minimize the effect of copper-catalyzed degradation. 

An important factor in the success of these reactions involves chelation-assistance by a heteroatom. Thus, the coordination of the heteroatom to the metal, brings the metal closer to the C-H bond and stabilizes the thermally unstable C-M-H species formed by the oxidative addition of a C-H bond to a low-valent transition metal complex. In addition, the use of the chelation-assistance leads to a high regioselectivity, which is an essential factor in organic synthesis. For reactions, a number of transition metal complexes - including ruthenium, rhodium, and iridium - are used as a catalyst, and ruthenium-catalyzed reactions will be described in this chapter [5]. 

The rhodium-catalyzed addition of ethylene to 1,3-butadiene to yield 1,4-hexadiene (5a, 151) proceeds via a similar mechanism (151) with the exception that, upon formation of the alkylrhodium(III) species, the hexadiene synthesis proceeds without further change in the oxidation state of the metal. In these reactions with butadiene the coordinated alkyl groups are either chelate or 7r-allyl structures which appear to stabilize Rh(III) (151). The addition of propylene to butadiene and isoprene to produce [Pg.297]

Metal chelation may enhance or inhibit the Fenton reaction, depending on the metal and the chelator in question. Chelation of iron (II) by EDTA enhances the formation of hydroxyl radical, while deferoxamine, another chelator, reduces its formation. This is significant because peptides or proteins can chelate metals in the body, thus influencing the resulting degree of damage. The formation of hydroxyl radicals by nickel (II) and cobalt (II) is enhanced by this type of chelation. In addition to the Fenton and Haber-Weiss reactions, metals can also catalyze the formation of the hydroxyl radical via reaction with hypochlorite (HOCl), which is prodnced by neutrophils. ... 

It is well known that certain metals can catalyze oxidation reactions. Trace metals can be present in a polymer article from many sources including other additives, residual catalyst, or contamination from the processing equipment, which is metal. A class of antioxidants is metal deactivators. Their purpose is to chelate or passivate the metal rendering it ineffective as an oxidation catalyst. Irganox MD 1024 is an example of a metal deactivator. 

Another method employed is the treatment of aqueous solutions of aminophenols with activated carbon (81,82). During this procedure, sodium sulfite, sodium dithionite, or disodium ethylenediaminotetraacetate (82) is added to increase the quaUty and stabiUty of the products and to chelate heavy-metal ions that would catalyze oxidation. Addition of sodium dithionite, hydrazine (82), or sodium hydrosulfite (83) also is recommended during precipitation or crystallization of aminophenols. 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

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