Enantiomers liquid crystals

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

One may consider a series of physical states ranging from the crystalline, where molecular aggregation and orientation are large, to the dilute gaseous state, where there are no significant orientational limits. States of intermediate order are represented by micelles, liquid crystals, monolayers, ion pairs, and dipole-dipole complexes. In the crystalline state, the differences between pure enantiomers, racemic modifications, and diastereomeric complexes are clearly defined both structurally and energetically (32,33). At the other extreme, stereospecific interactions between diastereomerically related solvents and solutes, ion pairs, and other partially oriented systems are much less clearly resolved. 

The phase transitions of cholesteryl nonanoate have been studied with a new apparatus for thermal analytical microscopy. The enantiomer ratio of some chiral sulphoxides can be changed from racemic to a modest preference for one enantiomeric form by dissolution in a cholesteryl ester in its liquid-crystalline ( cholesteric ) state. 5,6-Epoxycholestan-3-yl p-nitrobenzoates exhibit liquid-crystal properties, but 5,6-diols and dibromides are inactive. ... 

Jeong HS, Tanaka S, Yoon DK, Choi S-W, Kim YH, Kawauchi S, Araoka F, Takezoe H, Jung H-T (2009) Spontaneous chirality induction and enantiomer separation in liquid crystals composed of achiral rod-shaped 4-arylbenzoate esters. J Am Chem Soc 131 15055-15060... 

Reactive crystallization/precipitation plays a role in a number of industrially relevant processes, such as liquid-phase oxidation of para-xylene to produce technical-grade terephthalic acid, the acidic hydrolysis of sodium salicylate to salicylic acid, and the absorption of ammonia in aqueous sulfuric acid to form ammonium sulfate (135). Reactive crystalhzation/precipitation is also widely applied in the pharmaceutical industry, to facilitate the resolution of the enantiomers (diastereomeric crystallization). Here, the racemate is reacted with a specific optically active material (resolving agent) to produce two diastereomeric derivatives (usually salts) that are easily separated by crystallization ... 

Two mechanisms have been proposed for the racemization of AA-binuclear species in a homogeneous solvent one takes into account inversion of the coordination sphere when a new metal-to-ligand bond is formed and the other includes conversion to its opposite enantiomer by the pseudorotation. The second mechanism seems to be more feasable in homogeneous systems such as hexane solutions or liquid crystals. In hexane, enantiomeric [Cr(acac)3] undergoes recemization by photoisomerization, giving a mixture of the A,A-, A,A- and A,A-isomers. Spectral assignments were followed by changes in CD and UV-Vis spectra. 

For molecules dissolved in a nematic thermotropic liquid-crystal, the direct coupling constants can be determined and from these the molecular geometry can be calculated. If the satellites are determined, not only the proton structure but the carbon skeleton of the molecule can be established. Oxirane has been measured in two laboratories. It proved possible to determine the orientation, the sign of the indirect coupling constant, and the geometry. Enantiomers can readily be determined by recording measurements in optically active liquid-crystals as solvents. ... 

Figure 4.5.1 The cholesteric borate amide forms liquid crystals, which react stereose-lectively with stereoisomers of monosaccharides, e.g., the enantiomers of glucose. Double diboration may be involved in the recognition process (compare with Figs. 3.2.1, 3.5.3, and 3.5.4) (from James, Harada, Shimkin, 1993).

Chiral solvation agents, chiral solvents and chiral liquid crystals. The differentiation of two enantiomers by NMR is possible by studying them in the presence of a chiral solvent or in chiral liquid crystals. The molecule-solvent or molecule-liquid crystal interaction is sufhciently energetic for the diastereomeric association to have a lifetime longer than the NMR timescale. 

Dicarboxy-2,2, 5,5 -tetraethyl-3,3 -bithienyl (70) was resolved into enantiomers (67AK115). The dextrorotatory form was obtained through fractional crystallization of the cinchonidine salt from ethanol. Some remarkable features were associated to the optically active forms. The melting behavior favors the existence of liquid crystals. 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

A chiral compound, dissolved in a nematic liquid crystal phase, transforms this phase into a chiral phase that is very often a chiral nematic - cholesteric -phase. Under the same condition of concentration and temperature two enantiomers induce helical structures with the same pitch but of opposite sign. The helical pitch p is for low concentrations of the dopant a linear function of mole fraction x. The molecular measure for the chiral induction is the helical twisting power (ITTP) ... 

There are interesting reports of liquid crystals forming domains of chiral conformations, sometimes induced by the small effect of irradiation by circularly polarized light (see below for more about this influence) but there is no preparative separation of enantiomers as, in most cases, the constituent molecules are either formally achiral or easily racemize upon a phase change [11-13]. 

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