Helical molecules, chirality

Stnicture/chiroptics relationships of planar chiral and helical molecules, in particular of heteracyclophanes and heterohelicenes 98EJ01491. 

Binaphthyl molecules (chiral 2,2/-dihydroxy-l,T-binaphthyl) have been studied by Hicks et al., who reported the first experimental observation of CD-SHG on surfaces.4,29,43 In this section, we investigate the nonlinear optical properties of a chiral 1,1-binaphthyl-based helical polymer. 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

A review of the older literature on compounds with a stereogenic axis is available22, as are reviews on planar chiral molecular structures 23, on the stereochemistry of twisted double bond systems 24, on helical molecules in organic chemistry 25, and on the synthesis and stereochemistry of chiral organic molecules with high symmetry 26. 

It is of great significance to correlate the absolute configurations of helical molecules frozen in the chiral crystalline environment with the sign of specific rotations. An attempt to determine the absolute configuration of (+)-57a giving rise to a positive... 

Supramolecular architectures are highly sensitive to chiral perturbations in general, and in systems that form liquid crystals in particular. Small amounts of enantiopure guest molecule added to a nematic host can induce a transition to a cholesteric phase, and the helical organization in the mesoscopic system is very sensitive to the structure of the guest molecule. Chiral amplification was successfully achieved in such liquid crystals, using CPL as the chiral trigger for the phase transition [183]. 

Grimme S, Harren J, Sobanski A, Vogtle F (1998) Structure/chiroptics relationships of planar chiral and helical molecules. Eur J Org Chem 1491-1509... 

Helices are chiral objects often encountered in nature, for example helical shells. The absolute configuration of these objects is designated P (plus) and M (minus) for right- and left-handed helices, respectively. A number of chemical structures resort to helicity, the most famous example being the class of molecules known as helicenes. Thus, the compound shown in diagram XXXVIII is (/>)-( + )-hexahelicene [33]. 

M-), (P-) Stereochemical descriptors (M = minus, P = plus) introduced to describe the chirality of helical molecules. Extension of the CIP system to planar chirality gave an alternative description aR/aS for helical molecules such as helicenes, aR invariably = (M) and aS = (P), but for compounds showing planar chirality the reverse, with pR = (P) and pS = (M). Best avoided. See Section 7.3.2. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

Control of information in the components can maximize the thermodynamic stability, but it can do little to change the lability of the systems. This lability is indeed inherent to the self-assembly process, "in order to allow the exploration of the energy hypersurface of the system and the evolution towards the final architecture" [21]. However this lability means that the system will react rapidly to any change in the thermodynamic conditions. In the case of chiral systems such as the helicates this means that the racemate will be formed unless the ligands are themselves chiral and can induce a specific chirality [22]. We are faced therefore with the problem of a simple synthesis of helical molecules which can never be resolved into enantiomers. 

PDBSMA " " and its derivatives, 2PyDBSMA and SPyDBSMA, afford nearly complete isotactic polymers by radical polymerization regardless of the reaaion conditions. The high isotactic specificity implies that the polymer obtained is an equimolar mixture of completely right- and left-handed helical molecules, suggesting that introduction of a nonracemic chiral influence to the polymerization reaction could result in the production of a single-handed helical, optically active polymer with an almost complete isotactic stmcture. 

Cholesteric liquid crystals are similar to nematic phases except that the molecular orientation between one layer and the next shows a progressive helical order. This helical structure arises from the chiral properties of the constituent molecules. Chiral molecules differ from their mirror image and have a left- or right-hand sense and are called enantiomorphic. The director is not fixed in space and rotates throughout the sample as shown in Figure 3.3. 

Chiroptic Relations of Planar Chiral and Helical Molecules. 

Often symmetry operations cannot be used in a simple way to classify chiral forms because, e.g., the molecule consists of a number of conformations. Therefore, independent of the symmetry considerations, a chemical approach to describe chiral molecules has been introduced by the use of structural elements such as chiral centers, chiral axis, and chiral planes. Examples for a chiral center are the asymmetric carbon atom, i.e., a carbon atom with four different substituents or the asymmetric nitrogen atom where a free electron pair can be one of the four different substituents. A chiral axis exists with a biphenyl (Figure 3.2) and chiral planes are found with cyclo-phane structures [17]. Chiral elements were introduced originally to classify the absolute configuration of molecules within the R, S nomenclature [16]. In cases where the molecules are chiral as a whole, so-called inherent dissymmetric molecules, special names have often been introduced atropiso-mers, i.e., molecules with hindered rotation about a helical molecules [18], calixarenes, cyclophanes [17], dendrimers [19], and others [20]. 

There appear to be no reports of small molecule chiral chromonic mesogens, but it is easy to convert the nonchiral N phase into a cholesteric phase by adding a small water-soluble chiral solute such as an amino acid. Lee and Labes carried out an extensive study of the relationship between the pitch of the induced chiral phase and the concentration of various dopants [55]. They found that the twist varies linearly with concentration over large concentration ranges, and the helical twisting power is characteristic of the particular dopant. This approach offers a novel form of assay for small quantities of water-soluble chiral compounds. 

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