Point chirality

The structures of phases such as the chiral nematic, the blue phases and the twist grain boundary phases are known to result from the presence of chiral interactions between the constituent molecules [3]. It should be possible, therefore, to explore the properties of such phases with computer simulations by introducing chirality into the pair potential and this can be achieved in two quite different ways. In one a point chiral interaction is added to the Gay-Berne potential in essentially the same manner as electrostatic interactions have been included (see Sect. 7). In the other, quite different approach a chiral molecule is created by linking together two or more Gay-Berne particles as in the formation of biaxial molecules (see Sect. 10). Here we shall consider the phases formed by chiral Gay-Berne systems produced using both strategies. 

Here, ry is the separation between the molecules resolved along the helix axis and is the angle between an appropriate molecular axis in the two chiral molecules. For this system the C axis closest to the symmetry axes of the constituent Gay-Berne molecules is used. In the chiral nematic phase G2(r ) is periodic with a periodicity equal to half the pitch of the helix. For this system, like that with a point chiral centre, the pitch of the helix is approximately twice the dimensions of the simulation box. This clearly shows the influence of the periodic boundary conditions on the structure of the phase formed [74]. As we would expect simulations using the atropisomer with the opposite helicity simply reverses the sense of the helix. 

The condition of handedness or lack of superimposabil-ity of molecules differing only with respect to the stereochemical arrangement of identical substituents around a tetrahedral center. In 1893, Lord Kelvin succinctly defined chirality I call any geometrical figure, or any group of points, chiral, and say that it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself. ... 

Schipper has developed a theory for the induction of optical activity in achiral molecules, first by treating the van der Waals coupling phenomena [19], and then in a more abstract sense through the use of point chirality functions [20],... 

Point Chirality Factors that Introduce Asymmetry in the (R,R)-Tartaric Acid Adsorbed Motif... 

I., Topiol, S. Two contact-point chiral distinction Model CHFClBr dimers. Chirality, 2005,17, S159-S170. 

Most commonly, chiral molecules have point chirality, centering around a single atom, usually carbon, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism), and is exemplified by the a-carbon of amino acids. 

It is also possible for a molecule to be chiral without having actual point chirality (stereocenters). Commonly encountered examples include l,r-bi-2-naphthol (BINOL) and 1,3-dichloro-allene which have axial chirality, and (E)-cyclooctene which has planar chirality. 

Elegant work by Tius and co-workers has demonstrated that the transfer of asymmetry need not be from an sp hybridized carbon atom. Instead, they have reported examples of the controlled Nazarov cyclization of allenyl vinyl ketones. In one such example, in situ formation of 67 resulted in efficient formation of cyclopentenone 68 with > 95% chirality transfer. The excellent axial to point chirality transfer is a result of the large ter/-butyl substituent forcing counterclockwise rotation (as viewed by the reader). 

In principle, there are two different motifs of helical stacks commonly employed for columnar materials that are shown in Figure 9. One motif would use the side chains as a means to introduce point chirality that could then be transferred to the cores stacking. This chirality transfer would bias one helical stack versus another. A second motif would start with a structure that has a chiral core and use this as a subunit to stack into a helical column. 

A particular case in this category is that of atropisomeric compounds (those where chirality originates from a barrier to rotation over a single bond). In a thermal reaction, racemization occurs unless the temperature is sufficiently low. The peculiar temperature independence of photochemical reactions allows to transform axis chirality into point chirality with no loss. 

Jesuraj JL, Sivaguni J (2010) Photochtanical type II reaction of atropchiral benzoylformamides to point chiral oxazolidin-d-raies. Axial chiral memray leading to enantiomeric resolution of photoproducts. Chem Commun 46 4791-4793... 

For such events, we consider that chirality is transferred. The chirality sensing that we describe below is based on the supramolecular transmission of point chirality to dynamic chirality. Most chiral substances, including naturally occurring ones show low activity levels in circular dichroism (CD) spectroscopy - due to lack of distinct chromophores. Less CD-active chirality becomes detectable as an enhanced readout if it is transferred to the dynamic chiralities of chromophoric molecules. 

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