Enantiomeric chiral structures

DPIBF, the resulting 119 was optically active. This suggests that the progenitor of 119 has the allene structure 117 rather than other conceivable constitutions [80, 81]. Even early quantum-chemical calculations on 117 showed a strongly bent, chiral structure, although the enantiomerization barrier was not correctly estimated [19]. 

The ambiguity involved in assigning the absolute configuration of a chiral molecule in a chiral crystal is presented in Scheme 1. Scheme la depicts a chiral molecule of, say, configuration S, with individual atomic coordinates — x — y - z, (i = 1,. . . , n, for n atoms) in a crystal axial system a,b,c. Scheme 1 b represents the enantiomeric crystal structure containing a molecule of configuration... 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

A most interesting extension of this type of reaction was performed by Addadi and Lahav (175). Their aim was to obtain chiral polymers by performing die reaction in a crystal of chiral structure. They employed monomers 103. The initial experiments were with a chiral resolved 103 where R1 is (R)- or ( -sec-butyl and R2 is C2H3. This material indeed crystallizes in the required structure, and yields photodimers and polymers with the expected stereochemistry, and with quantitative diastereomeric yield. It was possible to establish that the asymmetric induction was due essentially only to the chirality of the crystal structure and not to direct influences of the sec-butyl. Subsequently they were able, using sophisticated crystal engineering, to obtain chiral crystals from nonchiral 103, and from them dimers and polymers with high, probably quantitative enantiomeric yields. This may be described as an absolute asymmetric polymerization. 

The culmination of the studies on asymmetric photodimerization reactions in the solid state was the successful elaboration of chemical systems that are achiral but crystallize in chiral structures, and that yield, on irradiation, dimers, trimers, and higher oligomers in quantitative enantiomeric yield (175,258). 

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

Several 1,4-disubstituted phenylenediacrylates 13 such as Me, Et, and Pr esters crystallized into chiral structures, and they photodimerized to either (SSSS)-cyclobutanes 14 or (/ / /M)-cyclobutancs ent-14 with medium to quantitative enantiomeric yields. [15] In addition to these dimers, the corresponding trimers and oligomers were also produced with high enantiomeric yields. 

At the time of van t Hoff s and Le Bel s work, there were only a few optically active compounds whose structures had been determined. All of these compounds had a carbon bonded to four different groups, a carbon that we today call a chirality center, van t Hoff and Le Bel pointed out that a tetrahedral arrangement of four different groups around a carbon produced a structure that is not superimposable on its mirror image, a chiral structure. Thus, their postulate of a tetrahedral carbon explained the existence of enantiomeric compounds. 

Since there have been no previous studies of spin-polarized electron induced reaction asymmetries in adsorbed chiral molecules, the exact manner by which the enhancement occurs is unclear. If the orbital occupied during DEA is sufficiently diffuse so as to sample the regions of the molecule responsible for the chiral structure [92] then enantiomeric specific dissociation will result. On the other hand, it has been theorized that two enantiomers will be ionized at different rates by longitudinally spin-polarized electrons [126]. If there are sufficient numbers of higher energy spin-polarized secondary electrons and the final state reached following ionization is dissociative, then this could lead to chiral enhancement. 

The pattern of disorder and the relationship between racemic and enantiomeric crystal structures for phthalamide (11) is similar to that of the phthalate (10). The A -ray diffraction data were measured at —170 °C. The racemate crystallizes in space group PT (Z = 4) and the enantiomer in FI (Z = 4) (Table 2). Thus there are two independent molecules in the racemate and four in the chiral crystal. The s-butyl moeities in the racemate adopt the trans conformation and exhibit configurational disorder of different measures (44 56 and 28 72) at the two independent sites, as shown in Figure 11. The chiral crystal structure is shown in Figure 12. 

The answers are shown in Figure 4.20. Structures (1) and (2) are enantiomeric pairs. Structures (1) and (3) and structures (2) and (3) are pairs of diastereoisomers (or diastereomers), while structure (3) is a meso compound. A meso compound is optically inactive since it possesses a plane of symmetry and is superimpos-able on its mirror image. It does, however, contain two chiral carbon atoms. This reminds us that not all compounds that contain chiral centres are optically active. 

Fig. 7.22 (A) Chemical structures of the quinine and quinidine tert-butylcarbamate-based CHIRALPAK QN-AX and CHIRALPAK QD-AX anion exchange-type CSPs. These CSPs show pseudo-enantiomeric chiral recognition pro-...

The most significant handicaps associated with directionally specific locants are the inability to distinguish enantiomeric pairs of compounds directly from the notation and the arbitrary and often complicated hierarchical rules to determine what ligands are to be associated with which locants. Another complexity of locant notations that is sometimes overlooked is the need for additional symbols or other notation to indicate that there is less information meant than is expressed in the name with locants. Often the geometric configuration of a compound is known but not the absolute configuration. Thus the X in the third name is necessary in the locant notation because it is not possible to draw and number the cis chiral structure ambiguously. Similarly, vac is the recommended term to indicate a mixture of enantiomers. 

An Interesting example of the role of the exclmer as Intermediate comes from the system l-(2,6-dlchlorophenyl)-4-phenylbuta-dlene. Irradiation of a mixed crystal of this compound together with the corresponding 4-thlenyl derivative gives dimers by a (2n + 2tr) photocycloaddltlon reaction. The products are homo-dlmers of the phenyl and of the thienyl derivatives, and hetero-dimers. The two heterodimers formed are enantiomeric and, because the crystal Is of chiral structure, they are formed In different yields (54). Thus Irradiation of a single crystal has been found (55) to lead to a 70% enantiomeric excess. 

The chiral structural isomer sopromidine (265) was prepared from L-histidine (262) Scheme 5.59.). The amino acid was esteiified and reduced to the chiral alcohol (263) which was converted to the alkyl chloride and hydrogenated to the amine (264), enantiomerically pure (i )(-)-a-methylhis-tamine. Reaction with the isothiourea (256) then gave (265) [317-320]. 

Further evidence for the existence of a helical conformation for meta-linked CPEs comes from circular dichroism (CD) studies of j-PPE-C02 (Figure 14.10). The basis for this experiment lies in the fact that a helix is an inherently chiral structure. Because the helix is chiral, one would expect that if an enantiomeric excess (ee) of the right- or left-handed forms (the P and M forms) exists in solution, then the CD spectrum would feature a bisignate signal due to exciton coupling of the backbone chromophores which are in a chiral environment [35]. However, when a polymer folds into a helical structure, because the polymer itself is achiral, the ensemble of helically folded polymers exists as a... 

A particularly interesting D3 structure is that reported for an isomer of the fuUerene C78, a chiral structure composed only of carbon atoms Diederich, F. Whetten, R. L. Thilgen, C. Ettl, R. Chao, I. Alvarez, M. M. Science 1991, 254, 1768. The C76 fuUerene was isolated in enantiomeric form by kinetic resolution Hawkins, J. M. Meyer, A. Sciemx 1993, 260, 1918. 

The term meso is commonly used to designate an achiral structure that is a diastereomer of one or more chiral structures. A meso compound contains chiral substructures but is not itself chiral because of overall molecular symmetry. Such structures often— but not always— have a plane of symmetry in at least one conformation, as is illustrated for 73 and 74. These structures are said to be internally compensated An example of a meso structure that does not have a plane of symmetry is the l,4-dichloro-2,5-difluorocyclohexane 75, which is achiral because it has a center of symmetry. Structure 75 is a diastereomer of the enantiomeric pair 76 and 77. 

---