Alkaline earth metals, kinetic

Most metals (other than the alkali and alkaline-earth metals) are corrosion resistant when cathodically polarized to the potentials of hydrogen evolution, so that this reaction can be realized at many of them. It has thus been the subject of innumerable studies, and became the fundamental model in the development of current kinetic concepts for electrochemical reactions. Many of the principles... 

Hering, J. G. and Morel, F. M. M. (1988). Kinetics of trace metal complexation role of alkaline-earth metals, Environ. Sci. Technol., 22, 1469-1478. 

In spite of the overwhelming importance of the channel mechanism for the transport of alkali and alkaline earth metal ions in biological systems, only carrier transport has been studied extensively by chemists. Studies on ion channel mimics of simple structures have long been limited to antibiotic families of gramicidin, amphotericin B, and others. Several pioneers have reported successful preparation of non-peptide artificial channels. However, their claims have been based on kinetic characteristics observed for the release of metal ions through liposomal membrane and lacked the very critical proofs of channel formation. Such a situation was... 

MeO N Me -catalyzcd raethanolysis of phenyl acetate is accelerated by the addition of alkaline-earth metal salts. The kinetics are consistent with a reaction scheme [Eq. (1)] involving preassociation of MeO ion with the metal cation (Km) and independent contributions to the overall rate from free (ko) and cation-paired (Icm) MeO [3,4]. 

Similar effects of the cation of the supporting electrolyte occur, to a greater or lesser extent, in the reductions of alkali and alkaline earth metal ions in basic aprotic solvents [26a]. In dimethylacetamide (DMA), the reductions of alkaline earth metal ions are electrochemically masked by Et4N+. In DMF and DMSO, the reversibility of the reductions of alkali and alkaline earth metal ions decreases with the decrease in the cationic size of the supporting electrolyte. This effect is apparent from the kinetic data in Table 8.3, which were obtained by Baranski and Fawcett [23 b] for the reductions of alkali metal ions in DMF. 

Kinetic processes have been shown by NMR and other spectral data to involve both intra- and inter-molecular exchange for 1 1 complexes of alkaline earth metal ions and (52a) (Scheme 7).190 The internal exchange between the two monocycles occurs at a faster rate than the intermolecular process, however. 

The effect of trace contaminants on the reaction has been investigated carefully. All uncondensed effiuent gases were recycled to the reactor, except for the amounts present in the streams taken off for analysis or flashed upon depressuring of the organic phase. Aqueous phase from the separator containing the water soluble by-products has been used as the water feed to the reactor. Hydrogen chloride containing chlorinated hydrocarbons and acetylene was used in all operations. In addition, certain possible impurities were tested for their effect on the kinetics and selectivity of the process. Paraffins, carbon monoxide, sulfide, carbon dioxide, alkali, and alkaline earth metals were found to be chemically inert. Olefins, diolefins and acetylenic compounds are chlorinated to the expected products. No deleterious effects of by-product recycle were observed even when some of the main by-products were added extraneously. 

Similarly, CAL B has been used in combination with a palladium/alkaline earth metal based racemization catalyst to effect a dynamic kinetic resolution on the benzylic amine 57e (Figure 14.29). The R) amide 58e was obtained in very good yield and excellent optical purity. Several other substrates also underwent the reaction [41, 42]. 

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