Achiral systems

The uncatalyzed rates in the Triton X-100 micelle are much smaller than those of the CTAB micelle as expected in ionic micellar reactions. However, in the catalyzed reactions much larger rate enhancements occur in the former micelle than in the latter micelle, similarly as in achiral systems (Table 4, 5). In Table 10, ester 50 shows... 

We note that the bilayer smectic phase which may be formed in main-chain polymers with two odd numbered spacers of different length (Fig. 7), should also be polar even in an achiral system [68]. This bilayer structure belongs to the same polar symmetry group mm2 as the chevron structure depicted in Fig. 17b, and macroscopic polarization might exist in the tilt direction of molecules in the layer. From this point of view, the formation of two-dimensional structure of the type shown in Fig. 7, where the polarization directions in neighbouring areas have opposite signs, is a unique example of a two dimensional antiferroelectric structure. 

Because of the extreme sensitivity of monolayers to contamination and the resulting likelihood of erroneous results, every precaution must be taken to ensure that all materials and instruments are clean. A great bonus for the stereochemical studies reported here is that comparison of the properties of enantiomers provides a rigorous check for internal consistency and purity that is not available to studies of achiral systems. Details have been given elsewhere and will not be included here (Thompson, 1981). 

The difference in the n/ A properties of these mixed chiral/achiral systems was also observed in the films dynamic properties. Figure 25 gives the surface shear viscosities of the palmitic acid/SSME systems at surface pressures of 2.5 and 5.0 dyn cm -1 at 25°C. It is clear that stereo-dependence of film flow... 

An unexpected but possibly related phenomenon is the separation of enantiomers of nicotine in a totally achiral system [256]. The mechanism is unclear but may involve the formation of in situ diastereometric dimers, where a dimer formed from the same two enantiomers could possibly resolve from a racemic dimer. 

K. C. Cundy and P. A. Crook, Unexpected phenomenon in the high-performance liquid chromatographic analysis of racemic Relabelled nicotine, separation of enantiomers in a totally achiral system, J. Chromatogr., 281 11 (1983). 

The preference for the [3+2] mechanism does not provide in itself an explanation for the high enantioselectivity observed in these reactions. The theoretical studies undertaken to discern the mechanism were carried out at quantum mechanical level on a model achiral system. To consider the asym-... 

To use models to distinguish between chiral and achiral systems. 

Chiral catalysis is in its infancy. The results described in this review represent only crude pylons marking the entrance to what will probably prove to be an extraordinarily productive and useful arena for future research. There are a great many catalytically active achiral systems which can, in principle, be modified by the incorporation of chiral ligands to produce catalysts for asymmetric hydrogenation and other chiral reactions. Only a few chiral ligands have been synthesized there are almost limitless possibilities in this area for the synthetic chemist. 

Finally, we feel it is worthwhile to stress one more time the importance of the kinetic inertia in the (reversible) chiral transfer and memory processes of our porphyrin systems. Inertia provides evidence that the system is trapped in an energy minimum. In the above examples the minimum is local the real minimum is that reached from the achiral system whose formation involves the same enthalpic contribution of the chiral one but a more favourable entropic contribution. In particular, the network of electrostatic interactions ensures a quite deep local energy minimum (that is a high value of EA). 

If all the components of a sample loaded on an HPLC column are baseline-separated, any conventional detector will work, unless the object of the separation is to determine or confirm the stereochemical conformation of an enantiomer. In achiral systems, (solvent and/or stationary phase) enantiomers have identical retention times and are not separable. The problem has a solution if two detectors are used in series, e.g., CD and absorbance. Because the enantiomers elute together, the absorbance detector measures the sum of their concentrations, and the CD detector measures the difference AA. Solving the simultaneous equations gives the concentrations for both enantiomers. 

The adsorption of (7 ,/ )-tartaric acid on a Cu(llO) surface formed the basis of our initial studies and we present it to demonstrate the hierarchical expression of chirality at surfaces. The behaviour of this system reveals the explicit role played by the surface and the adsorption process in introducing additional asymmetry at an interface, and we use this to demonstrate how such surface events can lead to chiral expression in initially achiral systems such as succinic acid on Cu(llO). 

The optimised structure of Fig. 5.12d shows how local chirality is created from an achiral system, with the diagonal adsorption site and the distortion of the molecular... 

These shortcomings have been circumvented by using a heterocycle containing a double bond, as illustrated by the examples in Scheme 21. As was the case with the achiral systems (c/. Scheme 11), a mixture of regioisomers is obtained, but now the a-substitution product (10) predominates. The amount of y-substitution product (11) produced depends on the ring size, and may be as high as 30% of the product mixture for the dehydropiperidines.Nevertheless, the unwanted y-isomer (11) is destroyed chemose-lectively when the mixture is treated with hydrazine to remove the chiral auxiliary. Double bond reduction would then give the a-alkylated saturated heterocycle. 

A chirality classification of crystal structures that distinguishes between homochiral (type A), heterochiral (type B), and achiral (type C) lattice types has been provided by Zorkii, Razumaeva, and Belsky [11] and expounded by Mason [12], In the type A structure, the molecules occupy a homochiral system, or a system of equivalent lattice positions. Secondary symmetry elements (e.g., inversion centers, mirror or glide planes, or higher-order inversion axes) are precluded in type A lattices. In the racemic type B lattice, the molecules occupy heterochiral systems of equivalent positions, and opposite enantiomers are related by secondary lattice symmetry operations. In type C structures, the molecules occupy achiral systems of equivalent positions, and each molecule is located on an inversion center, on a mirror plane, or on a special position of a higher-order inversion axis. If there are two or more independent sets of equivalent positions in a crystal lattice, the type D lattice becomes feasible. This structure consists of one set of type B and another of type C, but it is rare. Of the 5,000 crystal structures studied, 28.4% belong to type A, 55.6% are of type B, 15.7% belong to type C, and only 0.3% are considered as type D. 

The most conunonly used descriptors cannot reflect enantiomerism, due to their determination in achiral systems. Predictions based on these descriptors must yield the same activity for both enantiomers of a chiral compound. But in natural (chiral) systems their impacts may be dramatically different, as, for example, the teratogenicity of thalidomide. Especially methods for calculating descriptors do not generally discriminate between enantiomers, not even between diastereomers, which leads to identical predictions for different compounds. The (optical) isomerism of chemical structures must, therefore, be deliberately considered in QSAR studies. 

The need to resolve the d- and L-enantiomers of amino acids has grown in recent years as it has been recognized that they differ in biological and physicochemical properties. Amino acid enantiomers cannot be resolved in achiral systems it is therefore necessary to have a second chiral center either in the chromatographic system or to create one by derivatization in the molecule to be separated (Figure 2). 

This will be referred to as the El-Ml mechanism and is the most important one for the majority of chiral systems. The importance of other terms, notably E1-E2 will be discussed later but for the moment it will be assumed that the El-Ml mechanism is sufficient. A pseudoscalar quantity is a rotational invariant but, unlike a true scalar, it has odd parity and, therefore, inverts its sign between enantiomeric systems and vanishes in achiral systems. This is what we expect of a quantity related to CD. 

The aspect of stereocontrol in achiral systems is particularly evident in the case of the regiospecific copolymerization of styrene (and of homologues thereofi ) with [(L L)Pd(S2)](X2) or [(L L)Pd(CH3)(S)](X) catalyst precursors (L L is 1,10-phenanthro-line or 2,2 -bipyridine) to poly(l-oxo-2-phenyl-l,3-propanediyl) with the prevailing ( 90%) formation of M-diads, independent of the anionic tigand. According to... 

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