Absolute configuration liquid crystals

Selenurane oxides are also one of the hypervalent selenium compounds. Recently, the enantiomers of chiral selenurane oxide 38 were isolated for the first time by enantioselective liquid chromatography of the racemate or by spontaneous resolution occurring during slow evaporation of its acetonitrile solution or slow crystallization from the same solvent.57 The absolute configurations of the enantiomers were determined by X-ray crystallographic analysis (Scheme 17). 

The induction of chirality in liquid crystals (LCs) has a long history [100-104]. The supramolecular induction can be used to assign absolute configurations [105-108], conformations of molecules [109,110] and the interplay between inter- and intra-molecular interactions [111], and models can be developed to justify the sense of the inductions that are observed. Twisting powers of dopants—the twist per mole—can be pushed to extraordinary values [112]. Given the history and vast body of work, we will focus here on the more contemporary aspects of work in this area. 

As a first step towards the measurement of single molecule effects, Schrader and Korte (1972) reported the measurement of the infrared rotatory dispersion of carvone in liquid crystalline solution. They used a modified commercial spectrometer. They observed a huge effect which is not the result of the carvone itself but of the liquid crystal in which a helical arrangement (cholesteric state) is induced by the chiral solute (Sec. 4.6.4). In this case the liquid crystal acts as a kind of molecular amplifier which allows the absolute configuration of tiny amounts of solutes to be determined reliably. At about the same time Dudley et al., (1972) measured the infrared circular dichroism of (-)-menthol in a liquid crystal. Their equipment consisted of a normal infrared spectrometer supplemented by a Fresnel rhomb made from sodium chloride. 

Investigations into the lattice-controlled - asymmetric ortho-Claisen rearrangement of prostereogenic substrate 1 in a cholesteric liquid crystal solvent at 200 °C show that asymmetric induction can be achieved to some extent. However, the optical purity and absolute configuration of the product was reported654. 

James, T.D., Harada, T., Shinkai, S. (1993). Determination of the absolute configuration of monosaccharides by a colour change in a chiral cholesteric liquid crystal system, J. Chem. Soc. Chem. Comm 857. 

Chiral orienting media provide a powerful means of enantiomeric discrimination by NMR. An overview of chiral media including liquid crystals that are used to assign the absolute configuration of particular classes of compounds using NMR spectroscopy is available. Orientational properties of one of the well-utilized liquid crystalline chiral orienting system, viz. 

Liquid crystals can be used as sensors for stereoelectronic effects of molecules or as detectors of chirality. Even a small enantiomeric excess of less than 1% can create a texture change in a nematic phase and a measurable pitch. Dopants of type C2a can be used to determine the absolute configuration of chiral centers and changes of aggregation or conformation of molecules can be displayed by pitch changes. More common is the use of lyotropic cholesteric phases as solvent for nmr studies [40]. 

There is widespread interest in the capability of VCD to determine absolute configurations of molecules because the method is free of the need to obtain a single crystal for X-ray diffraction measurements, the standard approach to the determination of absolute configuration of a chiral molecule. VCD measurements are typically carried out in solution or with neat liquids. Since many molecules are difficult to crystallize, VCD promises to be an effective way to determine stereo-specific structures in the absence of the availability of crystals. 

BaiUif V, Robins RJ, BiUault 1, Lesot P. Assignment of absolute configuration of natural abundance deuterium signals associated with (R)- and (5)-enantioisotopomers in fatty acid aligned in a chiral liquid crystal enantioselective synthesis and NMR analysis. J. Am. Chem. Soc. 2006 128 11180-11187. 

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