Chiral discrimination

An X-ray structure of the complex formed between 3-cinnamoyl-l,3-oxazohdin-2-one and a chiral TADDOL-Ti(IV) complex (see Chapters 1 and 6 by Hayashi and Gothelf, respectively) has been characterized [16]. The structure of this complex has the chiral TADDOLate and cinnamoyloxazohdinone ligands coordinated to titanium in the equatorial plane and the two chloride ligands in the axial plane and is similar to A in Fig. 8.8. The chiral discrimination was proposed to be due to... 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

J. Breinholt, A. Varming, Poster presented at the 11 International Symposium on Chiral Discrimination, Chicago, July 1999. 

Cabrera, K., Lubda, D., Bohne, S., Schafer, M., Poster presentation 3" International Symposium on Chiral Discrimination, Tubingen, 1992... 

Nagamatsu S. and Makino S., poster presented during the International Symposium on Chiral Discrimination, Nagoya, 1997. 

Sakai K, Sakurai R, Nohira H (2007) New Resolution Technologies Controlled by Chiral Discrimination Mechanisms. 269 199-231... 

Ikeda H. and Murata K., 4th Symposium on Chiral Discrimination, September 1993. 

Although somewhat more stable than its hexaammine relative, the air-sensitive [Co(en)3]2+ is still substitutionally labile and racemizes rapidly in solution. Chiral discrimination in its (racemic) solutions has been observed in outer sphere electron transfer reactions with optically active oxidants including [Coin(EDTA)], 209,210 [Cr(ox)3]3-,211,212 Co111 oxalate, malonate, and acetylacetonate (acac) complexes.213... 

Chiral recognition of A-[Co(phen)3]3+ has been observed in a modified /3-cyclodextrin.772 Chiral discrimination has also been seen in photoinduced energy transfer from luminescent chiral lanthanoid complexes773 to [Co(phen)3]3+ and between photoexcited [Ru(bpy)3]2+ and [Co(phen)3]3+ co-adsorbed on smectite clays.774 The [Co(bpy)3]3+ ion has been incorporated into clays to generate ordered assemblies and also functional catalysts. When adsorbed onto hectorite, [Co(bpy)3]3+ catalyzes the reduction of nitrobenzene to aniline.775 The ability of [Co(phen)3]3+ to bind to DNA has been intensively studied, and discussion of this feature is deferred until Section 6.1.3.1.4. 

Kammerer, B., Kahlich, R., Ufer, M., Laufer, S., Gleiter, C. (2005). Achiral-chiralLC/LC-MS/ MS coupling for determination of chiral discrimination effects in phenprocoumon metabolism. Anal. Biochem. 339, 297-309. 

The implications for films cast from mixtures of enantiomers is that diagrams similar to those obtained for phase changes (i.e., melting point, etc.) versus composition for the bulk surfactant may be obtained if a film property is plotted as a function of composition. In the case of enantiomeric mixtures, these monolayer properties should be symmetric about the racemic mixture, and may help to determine whether the associations in the racemic film are homochiral, heterochiral, or ideal. Monolayers cast from non-enantiomeric chiral surfactant mixtures normally will not exhibit this feature. In addition, a systematic study of binary films cast from a mixture of chiral and achiral surfactants may help to determine the limits for chiral discrimination in monolayers doped with an achiral diluent. However, to our knowledge, there has never been any other systematic investigation of the thermodynamic, rheological and mixing properties of chiral monolayers than those reported below from this laboratory. 

When compressed to surface pressures greater than their stability limits (see Table 10), diastereomeric mixtures of /V-(a-methylbenzyl)stearamides with both stearoylalanine and stearoylserine methyl esters provided clear evidence of chiral discrimination. Force-area isotherms at 35°C for homochiral and heterochiral pairs of N-(a-methylbenzyl)stearamide and stearoylalanine methyl ester show differences in both their lift-off and touchdown (the area per molecule where the surface pressure returns to zero on the expansion arm of the isotherm) areas per molecule (Fig. 32). In addition, monolayers of the heterochiral pair could be compressed to lower areas per molecule than monolayers of the homochiral pair. 

In theory, the two diastereomeric complexes will have different association constants. The evaluation of any chiral discrimination will depend upon measurement of the different proportions of the diastereoisomers formed. For example, nmr experiments have been successful in determining the degree of complex formation by each enantiomer. Alternatively, an extraction procedure has been employed this involves the interaction... 

An understanding of the recognition of chirality at a molecular level has become of interest in many fields of chemistry and biology. In the past decade, many attempts to clarify the mechanism of chiral recognition on CSPs for liquid chromatography have been made by means of chromatography, NMR spectroscopy,199 202 X-ray analysis, and computational methods.203 - 206 The successful studies have been mostly carried out for the small-molecule CSPs, especially cyclodextrin-based CSPs and Pirkle-type (brush-type) CSPs. In contrast, only a few mechanistic studies on chiral discrimination at the molecular... 

In the kinetic resolution, the yield of desired optically active product cannot exceed 50% based on the racemic substrate, even if the chiral-discriminating ability of the chiral catalyst is extremely high. In order to obtain one diastereomer selectively, the conversion must be suppressed to less than 50%, while in order to obtain one enantiomer of the starting material selectively, a higher than 50% conversion is required. If the stereogenic center is labile in the racemic substrate, one can convert the substrate completely to gain almost 100% yield of the diastereomer formation by utilizing dynamic stereomutation. 

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