Chiral supramolecular crystal

Chiralsil-val, 6 96-97 Chiral smectic C liquid crystals, 15 106-107 Chiral stationary phases, 6 79-82 Chiral supramolecular clusters, 24 61 Chiral synthons, 11 5 Chiral titanium complexes, 25 98—99 Chirobiotic phases, for chiral separations, 6 90-91... 

Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to die di-Jt-methane and Norrish type II photorearrangements, J. Am. Chem. Soc., 108, 5648-5649. (b) Chen, J., Pokkuluri, P. R., Scheffer, J. R., and Trotter J. (1990) Absolute asymmetric induction differences in dual pathway photoreactions, Tetrahedron Lett., 31, 6803-6806. (c) Fu, T. Y., Liu, Z., Scheffer, J. R., and Trotter, J. (1993) Supramolecular photochemistry of crystalline host-guest assemblies absolute asymmetric photorearrangement of the host component, J. Am. Chem. Soc., 115, 12202-12203. (d) Leibovitch, M.,... 

De Feyter S, De Schryver FC (2003) Two-dimensional supramolecular self-assembly probed by scanning tunneling microscopy. Chem Soc Rev 32 393 Ernst K-H (2006) Supramolecular surface chirahty. Top Curr Chem 265 209 Perez-Garcia L, Amabrlino DB (2007) Spontaneous resolution, whence and wither from enantiomorphic solids to chiral liquid crystals, monolayers and macro- and supramolecular polymers and assemblies. Chem Soc Rev 36 941... 

Aniline, (V.iV-dimethylaniline and toluene form molecular complexes with (4a, b,13b)-13-methyl-16-oxo-17-norkaurane-18-carboxylic acid (isosteviol) (93) investigated by X-ray diffraction159. 93 forms individual 2 1 molecular complexes with aniline or N,N-dimethylaniline and toluene. The supramolecular crystal structure of these complexes is formed by chiral double helices involving isosteviol molecules. The helices are linked together by intermolecular hydrogen bonds between the carboxy and the carbonyl groups160. 

Hisaki, I., Tohnai, N. and Miyata, M. (2008) Supramolecular tilt chirality in crystals of steroids and alkaloids. Chirality, 20, 330-336. 

In this chapter we will review the recent advances of supramolecular photochirogenesis in various confined media, excluding micelles, chiral solvents, liquid crystals, metal complexes, polymer matrices, clays, and crystals. Micelles are a typical supramolecular assembly with an internal hydrophobic core which shows a unique boundary effect, e.g., enhanced radical recombination of geminate radical pairs produced by ketone photolysis [26], but essentially no asymmetric photoreaction has hitherto been reported in micelles. Photochemical asymmetric induction in chiral solvents [27,28] and chiral liquid crystals [29,30] have been known... 

Related to the process of crystallization is the growth of a polymer from a pool of available molecules in a racemate. There are many studies of the spectacular generation of intrinsically chiral supramolecular structures such as hehces, often from achiral materials seeded by a small amount of a chiral additive [6-9], but these are not preparative methods for the separation of enantiomers in any meaningful sense, even though the helices can themselves be stable, chiral structures that are separable by chromatography - these are not the enantiomers of primary interest... 

The reasons for this preference are still under debate and the process is not yet predictable. But spontaneous resolution can also afford chiral liquid crystals, monolayers, and supramolecular polymers and is likely controlled by subtle noncovalent interactions including crystal packing forces and crystallization kinetics. Obviously, it would be both fascinating and exhemely useful to be able to predict and control this process. 

An interesting area, which involves chiral liquid crystal properties, is that of temperature sensors, used, for example, in the diagnosis of skin cancer, as well as in peripheral blood circulation problems. Another application in the medical field is represented by nematic elastomer films or fibers of liquid crystalline polymers with mesogene in the side chain, that can be used in the manufacture of muscle prosthesis [28]. To optimize their performance for different applications, the current knowledge on the relationship between the structure and properties of liquid crystals should be extended. In addition to further development of liquid crystals and their applications, the liquid crystal theories represent a sound basis for other areas of interest. For example, liquid crystals can be used as model compounds for the study of molecular interactions and of their effects on self-organization in supramolecular chemistry. 

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

FIGURE 5.11 Supramolecular, helical architecture and definition of pitch length p of chiral nematic liquid crystals. 

Another case of major enantiomer separation occurs when helical tubuland diols (Section 3.2.1) are crystallised with small phenol molecules and intimately hydrogen bonded co-crystals are produced. A typical example is (11 ) ( p-chloro-phenol) [33], The major supramolecular synthon is H-0 H-0 H-0 hydrogen bonding with eclipsed stacks of the participating molecules surrounding a pseudo-threefold screw axis (Figure 13). This chiral motif involves molecules of p-chloroplienol and only one of the enantiomers of 11. 

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