Chirality low-molecular weight

Due to special symmetry conditions, the presence of chiral molecules in tilted smectic phases leads to the appearance of a spontaneous electric polarization within each smectic layer [1], [2], Magnitude and direction of the polarization are coupled to the magnitude and direction of the molecular tilt giving rise to imique properties of tilted as well as orthogonal smectic phases of chiral molecules in external electric fields. In this chapter, the basic properties of smectic-C and smectic- l phases of chiral low-molecular weight molecules are summarized, mainly from the viewpoint of physics. 

Many chiral low-molecular-weight compounds have been tested as CS in chromatography resulting in a considerable number of CSPs. One main advantage of these CSPs is their... 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Cyclic low molecular weight compounds. Chiral separations using chiral crown ethers immobilized on silica or porous polymer resins were first reported in the... 

A scandium complex, Cp ScH, also polymerizes ethylene, but does not polymerize propylene and isobutene [125]. On the other hand, a linked amidocyclo-pentadienyl complex [ Me2Si( / 5-C5 Me4)( /1 -NCMe3) Sc(H)(PMe3)] 2 slowly polymerizes propylene, 1-butene, and 1-pentene to yield atactic polymers with low molecular weight (Mn = 3000-7000) [126, 115]. A chiral, C2-symmetric ansa-metallocene complex of yttrium, [rac-Me2Si(C5H2SiMe3-2-Buf-4)2YH]2, polymerizes propylene, 1-butene, 1-pentene, and 1-hexene slowly over a period of several days at 25°C to afford isotactic polymers with modest molecular weight [114]. 

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

In brief, we can say that the study of macromolecular compounds has introduced a new dimension into organic stereochemistry. This is true not only in the spatial sense if one considers the shape of the macromolecule, but also in the time sense if one examines the process of polymerization and the transmission of stereoregularity and chirality within each macromolecule. Finally, the study of macromolecules has necessitated the introduction of concepts and methods (e.g., the statistical approach), which are usually not pertinent to the stereochemistry of low molecular weight compounds (4). 

With the third model, also, the single chains are chiral but with negligible optical activity. Even in this case we are in the cryptochirality domain, except possibly with low molecular weight compounds. 

Another result of great importance—the conformational asymmetric polymerization of triphenylmethyl methacrylate realized in Osaka (223, 364, 365)— has already been discussed in Sect. IV-C. The polymerization was carried out in the presence of the complex butyllithium-sparteine or butyllithium-6-ben-zylsparteine. The use of benzylsparteine as cocatalyst leads to a completely soluble low molecular weight polymer with optical activity [a]o around 340° its structure was ascertained by conversion into (optically inactive) isotactic poly(methyl methacrylate). To the best of my knowledge this is the first example of an asymmetric synthesis in which the chirality of the product derives finom hindered rotation around carbon-carbon single bonds. 

It is worth pointing out that, besides o-sorbitol 19 and D-mannitol 36, other low-molecular weight building blocks have been already obtained on the ton-scale from low cost or waste polymeric carbohydrates (starch, cellulose, hemicellulose, chitin) [80, 81]. Most of these compounds are densely functionalized enantiopure molecules that can be easily converted into high-value added products, including chiral ionic liquids. Therefore, further studies are required to develop other synthetic approaches to environmentally sustainable ionic liquids based on renewable raw materials. 

Values of the skeletal complexity metric S for these alkaloids are small, in the range 36-49. In contrast, S/H values are relatively high, in particular for strychnos alkaloids (0.88, Chart 6.1.3.A) this reflects a high molecular complexity that stems from the many cycles and chirality centers, in spite of the low molecular weights. 

The low thermal stability and the volatility of some of the low molecular weight stationary phases restricted their general use. Therefore, thermally stable and nonvolatile polymeric chiral stationary phases were developed by coupling the diamide phase, via the amino functionality, to a statistical copolymer of dimethylsiloxane and (2-carboxypropyl)methylsiloxane of appropriate viscosity131. The fluid polymeric phase, referred to as Chirasil-Val (Table 2), exhibits excellent properties for the enantiomer separation of a variety of compound classes over a broad temperature range141142. 

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. 

---