Trace metal catalysis

Plastourgou, M. and Hoffman, M.R. Transformation and fate of organic esters in layered-flow systems the role of trace metal catalysis. Environ. Sci Techno , 18(10) 756-764, 1984. 

Hoffmann, M. R. (1980).Trace metal catalysis in aquatic environments. Environ. Sci. Technol. 14,1061-1066. 

Plastourgou, M., and M. R. Hoffmann (1984), Transformation and Fate of Organic Esters in Layered-Flow Systems The Role of Trace Metal Catalysis, Environ. Set. Technol. 18, 756-764. 

Hoffman, M.R. (1 980)Trace metal catalysis in aquatic environments. Environmental Science and Technology 9, 1051-1055. 

The authors emphasize the point that they cannot exclude even in their conditions that trace metal catalysis may still be active. Indeed the addition of sequestering agents like EDTA (ethylenediamine tetra-acetic... 

The above outlined scheme leads to the conclusion that completely ionized thiols would give exclusively sulphinic and sulphonic acids nevertheless, the experimental results indicate formation of ca. 5% of disulphide in the oxidation of potassium benzenethiolate even with base in large excess. Since formation of disulphide would require the presence of undissociated thiol, other mechanisms must be operative. Again it is possible that the intervention of trace metal catalysis in the oxidation reaction has to be taken into account. Cullis, Hopton and Trimm reported that copper ions in concentrations as low as 10 M are still active as catalysts and indeed it is very hard to detect metal ions at such low concentrations and to exclude adventitious impurities of this order of magnitude. 

The decomposition to sulphate and oxygen is subject to trace metal-ion catalysis . Co(II) , Mo(VI) and Mn(II) are particularly effective, but the kinetics could not be resolved unequivocally in all cases. The rate expressions are... 

Mixtures react exothermally at 90°C, sometimes igniting. (Catalysis by trace metals in the carbon may well be involved). 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

The thiol-dependent release of nitric oxide may well involve metal catalysis from trace contamination of copper or iron in buffers. The disulfide radical may also reduce another nitrosothiol to produce more nitric oxide. 

Studies of oxygen isotope fractionation were undertaken to address the mechanisms of the oxidative and reductive phases of SOD catalysis.44 Experiments were conducted at pH 10 in either borate or carbonate buffer where the rate is only slightly diminished from that at physiological pH. Control experiments demonstrated that all of the 02 produced came from the enzymatic reaction rather than the spontaneous disproportionation that occurs in the presence of trace metal ions slowly as the pH is increased. The reaction proceeds rapidly to 100% completion giving equal concentrations of 02 and H202 therefore, a ratio of fractionation factors was determined from the distribution of the lsO isotope in the two products. The ratio of fractionation factors reflects the ratio of KIEs on the oxidative and reductive reactions, and is designated as (3 (Equation 9.9). The (3 was determined to be 1.0104 0.0012 under the conditions described above. 

In our next series of measurements we determined the effect of pH on the oxidation of H2S in buffered dilute solutions. The measurements were made at S5°C to speed up the acquisition of data. The results are shown in Figure 7. The results from pH = 2 to 8 are similar to the earlier measurements of Chen and Morris (41). Above a pH = 8 we find the rate to be independent of pH unlike the results of Chen and Morris (411 who find a complicated pH dependence. This could be related to trace metal impurities in the buffers used by Chen and Morris (41). Hoffmann and Lim (461 nave examined these trace metal effects and the base catalysis of the oxidation of H2S. 

The interaction of amine-modified silica with Cu2+ ion is the most documented.23,24,25 However, the retention of other transition metals as well as precious metals with dedicated modification layers has also been reported. The separation of Pd and Pt from base metals, Ir(III) and Rh(III) was effectuated by using silica-bound thioether sulfur and primary amine groups.26 A review on polymeric as well as modified silica supports for separation and preconcentration of trace metals is presented by Kantipuly.27 This metal immobilization also allows other applications such as metalion chromatography28 and heavy metal catalysis.29... 

Fluoride activation of Si-C bonds toward electrophiles has recently been exploited to synthesise alternating thiophene-perfluoroarene copolymers without using transition metal catalysis. This has the advantage of leading to products that are devoid of even traces of metal residues <2006JA2536>. Here the electrophiles are perfluoroarenes (ttF) the potential nucleophilic sites are the 2- and 5-positions of 3,4-dibutoxy-2,5-bis(trimethylsilyl)thiophene. The reaction is initiated with catalytic fluoride ion, which is regenerated with each C-C bond formed (Equation 106). 

The most important t5q)es of homogeneous catalysis in water are performed by acids, bases and trace metals. A wide variety of mechanisms have been outlined for acid/base catalysis and are presented in kinetics texts (e.g. Moore and Pearson, 1981 Laidler, 1965). A number of bases have been observed to catalyze the hydration of carbon dioxide (Moore and Pearson, 1981 Dennard and Williams, 1966). Examples are listed in Table 9.7 for OH and the base Co(NH3)gOH2. The most dramatic effect is the catalysis of HS-oxidation by cobalt-4,4, 4",4"-tetrasulfophthalocyanine (Co-TSP ). At concentrations of 0.1 nM Co-TSP the reaction rate was catalyzed from a mean life of roughly 50 h to about 5 min. The investigators attributed the reason for historically inconsistent experimentally determined reaction rates for the H2S-O2 system by different researchers partly to contamination by metals. Clearly, catalysis by metal concentrations that are present in less than nanomolar concentrations is likely to be effective in aquatic systems. We shall see that similar arguments apply to catalysis by surfaces and enzymes. 

Trace metal impurities in excipients can lead to oxidative catalysis resulting in drug substance degradation.14 The metals most commonly... 

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