Metal cations, catalytic efficiency

Synthesis.- The basic strategy - condensation of the separate halves of the ring by nucleophilic displacement - remains the same, The role of ion pairing of aryloxide ions and alkali-metal cations was studied. It was found that such ion pairs facilitated cyclisation, except when lithium was the cation. Catalytic efficiency decreases in the same order as for free cations. 

A DNA aptamer with iV-glycosylase activity has been isolated by in vitro selection." The catalytic rate enhancement is of the order of 10, and the aptamer catalyses the AT-glycosidic cleavage of a specific dG residue. It is dependent on divalent metal cations, and has optimal activity at pH 5. DNA aptamers that catalyse the capping of DNA with AMP to the 5 -end of the DNA, thus creating a 5, 5 -pyrophosphate linkage, have been evolved." The aptamers require Cu " and have a catalytic efficiency of 10 M min using either ATP or dATP. 

Mechanistic studies of the diacetoxylation of alkenes using (diacetoxyiodo)benzene have demonstrated a protio-catalytic nature of this reaction [255]. Systematic studies into the catalytic activity in the presence of proton-trapping and metal-complexing additives indicate that strong acids act as catalysts in the reaction. When trifluoromethanesulfonic acid is used as catalyst, the selectivity and reaction rate of the conversion is similar or superior to most efficient metal-based catalysts, such as Pd(II) and Cu(II) metal cations. Based on a kinetic study as well as in situ mass spectrometry, a mechanistic cycle for the proton-catalyzed reaction was proposed in this work [255]. 

Subsequently, it was found that the catalytic efficiency of a quaternary salt is directly related to its solubility in the organic phase, other conditions being the same. This and other observations support the mechanism reported in Fig. 2, in which anion transfer does not necessarily require the partitioning of the quaternary cation, the charge parity in both phases being ensured by the quaternary and the metal cation, respectively. ... 

The efficiency of the oxidation of 2,3 4,6-di-O-isopropylidene-a-L-sorbofuranose to 2,3 4,6-di-0-isopropylidene-a-L-arafciMo-hexulopyranosonic acid (a stage in the manufacture of L-ascorbic acid) with sodium hypochlorite and nickel sulphate (as a catalyst) has been shown to depend on the characteristics of the catalyst, the temperature of the reaction, and the concentration of the reactants. The catalytic activities of a number of metal cations e.g. Cu ", Ag", Co, Fe ", and Ce " ") in the electrochemical oxidation of 2,3 4,6-di-0-isopropylidene-a-L-sorbofuranose have also been examined, but none were as effective as Ni cations. ... 

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