Racemates chiral discrimination

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... 

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. 

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. 

There are two possible approaches for the preparation of optically active products by chemical transformation of optically inactive starting materials kinetic resolution and asymmetric synthesis [44,87], For both types of reactions there is one principle in order to make an optically active compound we need another optically active compound. A kinetic resolution depends on the fact that two enantiomers of a racemate react at different rates with a chiral reagent or catalyst. Accordingly, an asymmetric synthesis involves the creation of an asymmetric center that occurs by chiral discrimination of equivalent groups in an achiral starting material. This can be done either by enan-tioselective (which involves the reaction of a prochiral molecule with a chiral substance) or diastereoselective (which involves the preferential formation of a single diastereomer by the creation of a new asymmetric center in a chiral molecule) synthesis. 

The influence of the counterion on the stability of crown-ether complexes in general was reviewed in detail in one of the preceding sections. There it was shown to be an important parameter. The nature of the counterion in diastereomeric complexes of chiral crown ethers with primary ammonium salts also influences the chiral recognition. First of all it greatly determines whether salt can be extracted into the organic phase where the chiral discrimination takes place. In a series of experiments (Kyba et al., 1978) it was shown that when S,S -6zs(dinaphthyl)-22-crown-6 [284] in chloroform was equilibrated with racemic er-phenylethylammonium salts the type of anion also influences the degree of enantiomeric differentiation (Table 70). The highest... 

The work reviewed here shows that chiral discrimination in mono-layers is probably quite general and can be modulated by variations in the temperature and pH of the aqueous subphase. This property alone renders the many monolayer studies of racemic materials inconclusive for comparison with naturaUy occurring optically active systems (or vice versa). 

Zeelen found the extent of chiral discrimination to be dependent on the type of monomolecular phase that was formed. Thus, racemic and optically active samples displayed identical force-area curves (Fig. 14) when both formed liquid-expanded films, but owed considerably different curves (Fig. 15) under conditions where both samples formed a more highly condensed monolayer. 

In her initial investigation, Lundquist studied the monolayer behavior of racemic and optically active forms of both tetracosan-2-ol and its acetate derivative on 0.0 lA aqueous HCl over a considerable range of temperature (77). In each case, it was possible to demonstrate chiral discrimination between pure enantiomers versus the racemic substance. Furthermore, the extent of enantiomer discrimination was significantly temperature dependent, being enhanced at lower temperatures and frequently disappearing at higher ones. Under favorable conditions of temperature, however, the appearance of the force-area curves could be very sensitive to the optical purity... 

Probably the most interesting and important example of diastere-omer discrimination known to us is part of the body of work conducted by Monica Lundquist, whose enormous contribution to the area of chiral discrimination in monolayers is discussed in Sect. V. In her third paper (79), she demonstrated that Fredga s method of quasi-racemates (33) can be appUed to the two-dimensional organization of chiral surfactants in monolayers. 

Racemic solid solution. The phase-composition diagram has a zero slope across the whole composition range thus, there is little difference in the affinity among molecules of either configuration (i.e., no chiral discrimination). 

Thus, chiral discrimination may be observed that differentiates the force-area curves of the enantiomers of some surfactants from their racemic modifications. Apparent phase changes in the monolayer can be related to parallel behavior in the crystalline state through X-ray diffraction and differential scanning calorimetry. Formation of racemic compounds and quasi-racemates can be observed in some cases. 

Expanding on the encapsulation of organometallic guests, half-sandwich complexes of the form CpRu(q -diene)(H2O) were encapsulated in 1 [32]. When the diene portion of the half sandwich complex is unsymmetrically substituted, the ruthenium atom becomes a chiral center. Addition of CpRu(2-ethylbutadiene) (H2O) (4) to 1 revealed the existence of two diastereomers. Encapsulation of these racemic ruthenium complexes in racemic 1 leads to diastereomeric pairs of enantiomeric host-guest complexes (A/R, A/S, A/R, A/S) (Figure 7.3). However, chiral discrimination was not observed with the diastereomeric ratio (d.r.) being 50 50. 

Schurig, V, and Schmidt, R. (2003) Extraordinary chiral discrimination in inclusion gas chromatography. Thermodynamics of enantioselectivity between a racemic perfluorodiether and a modified 7-cyclodextrin. J. Chromatogr. 1000, 311-324. 

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