HETEROGENEOUS ENANTIOSELECTIVE HYDROGENATION Theory and Practice
Heterogeneous Enantioselective Hydrogenation: Theory and Practice reviews the development of enantioselective hydrogenation reaction catalysts. The book looks at the first relatively ineffective catalysts right through to modern highly effective enantiose
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Abstract Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessful, but the search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results from the finding that all studied locations of quartz crystals contain equal amounts of d- and l-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed.
1.1. Introduction There are many natural minerals and salts that posses optical activity in their crystalline state owing to their chiral lattices, such as quartz, cinnabar, mica, chlorates, bromates, and iodates. Crystal chirality of other minerals, like aluminosilicates, such as zeolites, were not investigated, but these minerals are considered by many investigators as possible sources of chirality and the origin of homochirality in our biosphere 1,2,3. The optical activities of clays have not revealed reliable evidence of chirality 4 and therefore they do not play any positive role in our understanding of the origin or of the amplification of homochirality in nature. On the other hand, if aluminosilicates are used as supports for chiral metal complex catalysts, they might reveal a contribution to asymmetric action as was found in the case of the hydrogenation catalyst [CoSalen] complex supported on hectorite 5. Even though clays have no chirality associated with their crystal structure, Julg and other authors 6,7,8,9,10, claim asymmetric effects. As stated by Julg 10: “...owing to the weak interactions, the two enantiomeric forms of kaolinite exhibit different energies. Experiments on synthesis and the
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Heterogeneous Enantioselective Hydrogenation
condensation of L-amino acids is a supplemental argument for the role played by kaolinite on the appearance of the first proteins and their Lhomochirality...”.
1.2. Attempts of asymmetric adsorption on clays The role of clays in chemical evolution was suggested by Bernal 1 in 1951 based on preferential adsorption of optical isomers of amino acids by optically active crystals of quartz and possibly also by clays. Cairns-Smith 2 suggested that the surfaces of minerals, probably clays, played important roles in biogenesis. Field and Spencer 11 showed that in the presence of alumina the formation of glycine from water, CO2, and NH3; fructose from water and CO; and other sugars from HCHO and hydrogen, can be accomplished. Gabel and Ponnamperuma 12 may have confirmed these data in studies of the condensation of HCHO on alumina. According to Bernal’s suggestion,1 such processes provide the mechanism for the conc
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