Crystals chirality

Chiral liquid crystals Chiral recognition Chiral separation Chiral separations Chiral shift reagents... 

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

In these systems, after the crystal chirality induced the chirality of asymmetric carbon in external organic compound, the subsequent asymmetric autocatalysis gives the greater amount of enantiomerically amplified product. These results clearly demonstrate that the crystal chirality of achiral organic compound is responsible for the enantioselective addition of /-Pr2Zn to pyrimidine-5-carbalde-hyde Ic. 

Keywords acridine, diphenylacetic acid, photodecarboxylation, two-component molecular crystal, chiral crystal, 7,8-dihydro-7-diphenylmethyl-acridine... 

Chiral organic crystals composed of achiral compounds such as hippuric acid act as the initial source of chirality of asymmetric autocatalysis to produce the highly enantiomerically pure product. In this reaction, chiral organic crystals are utilized as a chiral inducer, not as a reactant. Therefore, these results are the realization of the process in which the crystal chirality of achiral organic compounds induces asymmetry in another organic compound whose chirality was amplified to produce a large amount of enantiomerically pure organic compound, pyrimidyl alkanol in conjunction with asymmetric autocatalysis. 

The extraordinarily strong chiral properties of [nfhelicenes provide an impetus for the development of synthetic approaches to nonracemic [nfhelicenes for applications as organic materials. From this point of view, asymmetric syntheses of functionalized long [n]helicenes (n > 7), and also [n]helicene-like molecules and polymers with novel electronic structures and material properties, are important. The properties of helicenes related to materials are relatively unexplored, compared with the more synthetically accessible n-conjugated molecules and polymers. Notably, redox states of helicenes are practically unknown [33, 34]. Assembly of helicenes on surfaces, their uses as liquid crystals, chiral sensors, ligands or additives for asymmetric synthesis and helicene-biomolecule interactions are in the exploratory stages [35-43],... 

Fig. 1 Quartz minerals consisting of covalently linked silicon-oxide tetrahedra that are corner-connected and twisted in a helical fashion around three- and six-fold screw axes, rendering individual quartz crystals chiral...

Host-guest crystal Chiral host molecule Moderate-high enantiodifferentiation... 

Even if a molecule is achiral, chiral crystals can form by spontaneous chiral crystallization [26]. The big advantage in utilizing a crystal as a reactant is that absolute asymmetric synthesis can be achieved by solid-state photoreaction of such a chiral crystal. The initial chiral environment in the crystal lattice is retained during the reaction process, owing to the low mobility of molecules in the crystalline state, and leads to an optically active product. The process represents transformation from crystal chirality to molecular chirality. This kind of absolute asymmetric synthesis does not need any external asymmetric source in the entire synthetic procedure [9-14]. 

A hydrogen bond pair of acridine and diphenylacetic acid molecules is present in the crystal lattice. The two phenyl planes and the carboxyl plane of the diphenylacetic acid molecule in the molecular pair form a propellerlike conformation of three blades (Fig. 6). The helicity around the C-H bond of the methine group is counterclockwise (minus), which is termed the M-crystal. The crystal chirality is generated from the existence of molecular pairs of counterclockwise propellers alone in the unit cell. Conversely, only the molecular pairs with the opposite conformation are observed in the P- crystal. 

Dowd, M. K. (2003). Preparation of enantiomeric gossypol by crystallization. Chirality 15, 486-493. 

Some chiral ILs have been designed and synthesized. They have already been applied in different fields snch as asymmetric synthesis, stereoselective polymerization, chiral chromatography, liquid crystals, chiral resolution, and NMR shift reagents [20,106, 107]. Chiral solvents have been reported in asymmetric syntheses. In the BayUs-HUlman reaction of benzaldehyde and methyl acrylate in the presence of bases, chiral ILs demonstrate their ability in the transfer of chirality, even if the enantiomeric excesses (ee) are stiU moderate. The presence of an alcoholic functional group on the Af-alkyl-fV methylephedrinium is primordial and acts as a fixing point of the chiral IL on the reactants. It is assumed that the OH is connected... 

Chirality is the non-identity of two objects that are related by a mirror symmetry relationship and can be a property common to both molecules and crystals (7). Whereas molecular chirality results from the disymmetric, three-dimensional arrangement of the constituent atoms, crystal chirality can be considered to result from a similar spatial arrangement in which the objects to consider are the molecules in the crystal (7b). It is for this reason that resolyed molecular chirality is a sufficient but not necessary requirement for crystal chirality (7a). 

There are many natural minerals and salts that posses optical activity in their crystalline state owing to their chiral lattices, sueh as quartz, einnabar, 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 sourees of chirality and the origin of homochirality in our biosphere The optieal activities of clays have not revealed reliable evidence of chirality and therefore they do not play any positive role in our understanding of the origin or of the amplification of homochirality in nature. 

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