Oxidation of Other Substrates

The dioxygen complexes M02(CNBu-r)2 (A/ = Ni, Pd) have been isolated in the pure state.Treating the nickel complexes with excess tert-huty isocyanide at 30°C results in the formation of Ni(CNBu-r)4 and r-BuNCO(19)  

however, triphenylphosphine is added to the nickel complex, this compound is preferentially oxidized to triphenylphosphine oxide (20)  

Ni02(CNBu-f)2 + 4PPh3 - Ni(CNBu-r)2(PPh3)2 + 20PPh2 

The reaction of methyl or cyclohexyl isocyanide with Ni02(CNBu-r)2 produces a mixture of tert-butyl and either methyl or cyclohexyl isocyanate, and it is considered important that the formed isocyanate complex be kinetically labile for achieving successful isocyanide oxidation. 

Although the primary method of choice for the metal-catalyzed oxidation of carbon monoxide to carbon dioxide involves nucleophilic attack of water or hydroxide on a coordinated carbonyl, there are an increasing number of cases where a transition metal phosphine complex will catalyze this conversion. 

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Up to now the mediator properties have been discussed focusing on MET. There is however an additional aspect of the entire mediator concept. Mediators are also molecules that facilitate enzymatic oxidation of other substrates, the oxidation of which by enzymes only is slow (236,237). They are oxidative mediators that speed up the oxidation of poor substrates. They are oxidized by enzymes first and then, using their oxidative potential, react with substrate molecules to form product(s). Equations (47) and (48) illustrate the mechanism of action of oxidative mediators in catalysis by HRP. 

Oxidation of Other Substrates by the TCA Cycle The TCA Cycle Activity Is Regulated at Metabolic Branchpoints... 

Oxidation of IAA (2.49) results in cation 2.50, which undergoes decarboxylation and results in the skatolyl radical (2.51). This compound reacts with molecular oxygen to form peroxyl radical 2.52. With IAA or another cellular reductor, the hydroperoxide 2.53 is formed. It is this compound that activates the peroxidase, and thus allows the oxidation of other substrates, such as coniferyl alcohol. Among the degradation products of 2.53, 3-methylene 2-oxindole (2.54) is the most abundant. 

Early work showed that the rate of the silver ion-catalysed oxidation of oxalate is much faster than the oxidations of other substrates under similar conditions King ). Allen showed that with solutions of very low copper concentration, but not de-aerated, the rate is only slightly faster compared with other substrates. However, Kalb and Allen found that oxygen is a powerful inhibitor of the silver ion-catalysed oxidation, and that in the absence of oxygen low concentrations of copper have no effect on the rate. They studied the silver ion-catalysed reaction in the absence of oxygen. With peroxodisulphate concentrations greater than 0.004 M the rate equation is... 

As we have mentioned already, similar, but more complicated cycles can be written for the oxidation of other substrates. For example, cycle 3 is drawn for methane. 

One of the main objectives of using ionic liquid as reaction medium is the possibility to recycle the catalyst. The catalytic solution could indeed be reused eight times for the oxidation of benzyl alcohol to benzaldehyde, with only a slight loss of activity. In addition, the recovered ionic liquid solvent could be recovered and reused for new oxidations of other substrates. 

These Pd-Ti systems were active in the oxidation of other substrates such as alkanes, alkenes and alcohols. Hexane was hydroxylated into 2- and 3-hexanols, which were further oxidized in part to the corresponding ketones. In this case the product turnover was sensitive to the concentration of HCl. The addition of MeOH was effective as in the case of oxidation by H, , over TS-1. Finally we note that shape selectivity was found in the oxidation of alkanes and alkenes similarly to what was observed for the oxidation where H2O2 was used as oxidant the rates for oxidation of cyclic alkanes and alkenes were much lower than those of linear alkanes and alkenes. 

In order to estimate the rate of glycine oxidation we compared the oxidation of glycine to oxidation of other substrates. The ratios of the glycine oxidation rate relative to the oxidation rates of malate, succinate and NADH in the presence of ADP, i.g. state III, are shown in Table I. 

The o-quinones formed from phenolics further enhance the intensity of browning by oxidation of other substrates, complexing with amino acids and protein, and polymerization. Non-enzymatic discoloration is believed to involve metal-polyphenol complexing as reported in the processed potato (Bate-Smith et al., 1958), cauliflower (Donath, 1962), and asparagus (DeEds and Couch, 1948), conversion of leucoanthocyanidins to pink anthocyanidins in the processed broad bean (Dikinson et al., 1957), green bean puree (Roseman et al., 1957), and canned Bartlett pear (Luh et al., 1960), and protein-polyphenol complexing in chilled or stored beer (Schuster and Raab, 1961). 

In addition, a number of the developed catalysts are also active, under different experimental conditions, in oxidations of other substrates, in particular some of the copper catalysts for alcohol aerobic (TEMPO-mediated) or peroxidative oxidations [21, 22]. The reactions may be microwave-assisted (details are given in Chapter 18). 

Eldjarn [121] and his colleagues [129] considered first the possibility that co-oxidation, i.e. step-wise degradation from the opposite end of the molecule, might be blocked in Refsum s disease. Their eeurly results did indeed suggest that there was such a defect they showed that in patients with Refsum s disease there was a reduction in the excretion of sebacic acid (aCio dicarboxylic acid) after a loading dose of tricaprin [121]. However, these preliminary results were of uncertain significance because their patients did not show defects in co-oxidation of other substrates [130]. Later they showed that in patients whose serum phytanic acid was reduced by dietary treatment, the tricaprin test became normal [131], and in fact there is no direct evidence that co-oxidation is of importance in phytanic acid metabolism. 

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