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The Use of a Fischer-Porter Apparatus for Chiral Homogeneous Catalytic Hydrogenation
Published in John R. Kosak, Thomas A. Johnson, Catalysis of Organic Reactions, 2020
Louis S. Seif, Daniel A. Dickman, Donald B. Konopacki, Bryan S. Macri
Enantioselective reactions employing catalytic processes are a challenging problem for the organic chemist. In the pharmaceutical industry, much time and effort are being spent to develop practical techniques for efficient chiral synthesis, and asymmetric hydrogenation shows much promise in this application. A small amount of an optically active catalyst has been shown in several cases to produce a large amount of an optically active product by multiplying rather than merely duplicating the chirality of the reactants [1]. The catalyst directs the delivery of the hydrogen molecule to the prochiral carbon atom. Rhodium complexes containing optically active phosphines are the usual catalysts in such hydrogenation reactions. They are chiral modifications of the standard rhodium catalyst originally invented by Wilkinson [2].
Enzyme Catalysis
Published in Harvey W. Blanch, Douglas S. Clark, Biochemical Engineering, 1997
Harvey W. Blanch, Douglas S. Clark
While enzymes are specific in function, the degree of specificity varies. Some may act on closely related substrates, and are said to exhibit group specificity; others are more exacting in their substrate requirements, and are said to be absolutely specific. The product formed from a particular enzyme and substrate is also unique. Enzymes are able to distinguish between stereochemical forms and only one isomer of a particular substrate may undergo reaction. Surprisingly, enzyme reactions may yield stereospecific products from substrates that possess no asymmetric carbon atoms, as long as one carbon is prochiral. This chirality is a result of at least three-point interaction between substrate and enzyme in the active site of the enzyme. In Figure 1.1, sites 1, 2 and 3 are binding sites on the enzyme. When two of the R groups on the substrate are identical, the molecule has a prochiral center and a chiral center can result from the enzymatic reaction, as the substrate can only "fit" into the active site in one configuration if the site has binding selectivity for three of the R-group substituents. If the substrate has four different R groups, then chirality can be preserved in the reaction as a result of the multipoint attachment.
Enantioselective Catalysis with Transition Metal Compounds
Published in Dale W. Blackburn, Catalysis of Organic Reactions, 2020
Outstanding success in enantioselective catalysis with transition metal compounds has been achieved in the hydrogenation of dehydroamino acids to give optically active amino acids. An example is the conversion of (Z)-α-acetamidocinnamic acid into N–acetylphenylalanine, shown in Scheme 1, which is frequently used as a standard reaction in asymmetric hydrogenation. The enantioselective catalysts control the formation of the natural L and the unnatural D isomer by differentiating the addition of the hydrogen atom to the prochiral carbon atom according to the arrows in Scheme 1.
Exploring the behaviour of the twist-bend nematic phase using NMR with a variety of spin probes
Published in Liquid Crystals, 2020
Alberta Ferrarini, Cristina Greco, Geoffrey R. Luckhurst, Bakir A. Timimi, Herbert Zimmermann
The final probe to be studied is the nematogen 5CB-d2 with its specifically deuteriated pentyl chain. In the nematic phase there is a single quadrupolar doublet with a splitting of 40.6 kHz shown in Figure 6(c). But on entering the NTB phase at 94°C we encountered another quadrupolar doublet had been added to the spectrum in the N phase, see Figure 6(c). This striking behaviour is in keeping with both the prochiral nature of the two deuterons and the chirality of the twist-bend nematic phase. The two peaks have, as they should, essentially equal intensities unlike those first observed in the NMR spectrum for the NTB phase of CB7CB-d4 [2,9]. The magnitudes of the two quadrupolar splittings are 49.4 kHz and 38.7 kHz giving a mean splitting of 44.1 kHz; these are akin to the corresponding splittings for CB7CB-d4 [2]. We shall return to this comparison shortly, together with that for the difference in the two quadrupolar splittings, of 10.7 kHz, which is of special interest because of its relation to the phase chirality [2,9].
Setting things straight in ‘The twist-bend nematic: a case of mistaken identity’
Published in Liquid Crystals, 2020
Ivan Dozov, Geoffrey R. Luckhurst
A large number of experiments have given additional information concerning the ‘NX’ phase in general and, in particular, for the most studied CB7CB liquid crystal dimer. These experiments show clearly that its symmetry and structure are identical with the predicted symmetry and structure of the NTB phase. First, several different experimental techniques have confirmed that the ‘NX’ phase is chiral. The first proof is due to the NMR experiment of Cestari et al. [9] and subsequently many other NMR studies have confirmed this result later. The chirality of the NTB phase can be demonstrated using NMR spectroscopy when the mesogenic molecule contains two prochiral deuterons. These are normally placed in one or more methylene groups in the alkyl chain of the spacer linking the cyanobiphenyl groups of the dimer. In the nematic phase these deuterons are equivalent but in the NTB phase because of its chirality the two deuterons lose their equivalence as revealed by the NMR spectrum. In addition, from such experiments it is also possible to determine the temperature dependence of the phase chirality in the NTB phase.
Mirror symmetry breaking in liquids and liquid crystals
Published in Liquid Crystals, 2018
Because enantiomeric conformations and configurations have identical scalar properties, and thus identical energies, they can only be distinguished under chiral conditions, as the combinations of like and unlike enantiomers provide different energies (diastereomerism). If interaction between like enantiomers is preferred, then chirality discrimination leads to the segregation of the enantiomers into distinct domains (Figure 1(a)). This local breaking of mirror symmetry by spontaneous resolution of racemic mixtures of permanently chiral molecules into a conglomerate of crystals with opposite handedness has been well-known since the groundbreaking work of Pasteur which marked the birth of stereochemistry [9]. In the meanwhile chiral segregation of permanently chiral molecules (Figure 1(a)) and chirality synchronisation of transiently chiral molecules or supramolecular assemblies (Figure 1(b)) during phase transitions to solid-state crystals or self-assembled crystalline fibres and gels has developed to a prosperous field which is extensively reviewed [10–12]. Chiral (in most cases helical) aggregates can in principle, be formed by achiral, prochiral, transiently chiral or permanently chiral molecules, i.e. chirality observed on a higher level of self-assembly does not require chirality at a lower level, but any lower level chirality affects the higher level by the diastereomeric relations.