Related Pairs of Enantiomers

The separation methods described in Section IV for pairs of optically active diastereoisomers have also been applied to mixtures of diaste-reoisomeric pairs of enantiomers. By one or other of the following methods, it was possible to separate diastereoisomeric pairs of enantiomers completely or, at least, to obtain enriched fractions. In chromatography under achiral conditions, enantiomers are eluted at identical rates. Therefore, the.pairs of enantiomers (RR )l(SS ) and (RS )I(SR ) can be separated chromatographically in the same way as a pair of optically active diastereoisomers (RS1) and (SS ). By application of the methods based on solubility differences for many examples, fractions containing the less soluble pair of enantiomers could be separated from fractions containing the more soluble pair of enantiomers. However, with respect to the separation.by solubility differences, the situation with two diastereoisomeric pairs of enantiomers is more complicated than that of a pair of 

As an analytical method for determining the diastereoisomeric purity, thin-layer chromatography is occasionally used, but most frequently H-NMR spectroscopy is employed. Enantiomers give identical spectra. Thus, the H-NMR spectrum of a mixture of diastereoisomeric pairs of enantiomers contains only two sets of signals, one set for each pair of enantiomers this is analogous to the situation with a pair of optically active diastereoisomers described in Section VI. Hence, by H-NMR spectroscopy, the separation of diastereotopic pairs of enantiomers can be monitored, and the diastereoisomeric purity can be controlled (143-MS, 151, 171-179). 

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Stereochemical changes occurring at the metal have been investigated for the iron alkyl (VII). The reaction shown in Eq. (16) was carried out with each of the two diastereomerically related pairs of enantiomers (i.e., RR—SS and RS—SR) (7). Complete (> 95%) stereospecificity. 

Two stereocenters in a molecule result in as many as four stereoisomers—two diastereomerically related pairs of enantiomers. The maximum number of stereoisomers that a compound with n stereocenters can have is 2". This number is reduced when equivalently substituted stereocenters give rise to a plane of symmetry. A molecule containing stereocenters and a mirror plane is identical with its mirror image (achiral) and is called a meso compound. The presence of a mirror plane in any energetically accessible conformation of a molecule is sufficient to make it achiral. 

Figure 15. Curves Relating Log(k ) against 1/T for Two Pairs of Enantiomers...

The curves relate log(k ) against 1/T for two pairs of enantiomers on a chiral BBP column containing 1,6-dipentyl-buteryl cyclodextrin that was 20 m long, 250 pm... 

Molecules that are not identical to their mirror images are kinds of stereoisomers called enantiomers (Greek encmtio, meaning "opposite"). Enantiomers are related to each other as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H). For example, lactic acid (2-hydroxypropanoic acid) exists as a pair of enantiomers because there are four different groups (—H, -OH, - CH3, -C02H) bonded to the central carbon atom. The enantiomers are called (-i-)-lactic acid and (-)-lactic acid. Both are found in sour milk, but only the (+) enantiomer occurs in muscle tissue. 

A pyridine derivative related to dien with respect to the number and distribution of N-donor atoms, namely bis(2-pyridylmethyl)amine (bpma), also gives comparable complexes with Cd, e.g., [Cd(bpma)2](C104)2 with potentially three isomers (including a pair of enantiomers). As shown by the structure analysis (C2/c, Z = 4), a distorted octahedral fac-isomer with symmetry 2 —C2 has been isolated, necessarily with both enantiomers in the crystal lattice. No significant difference in the two kinds of Cd—N bonds (rav(Cd—N) 235.0 pm) is observed.189 Solid-state 13C NMR spectra of this complex and related Mn and Zn complexes have been discussed. 

Their isolation by flash chromatography on silica gel was comparatively easy. The CD spectra of related pairs of diastereomers whose addition pattern represent pairs of enantiomers, reveal pronounced Cotton effects and mirror image behavior. It is the chiral arrangement of the conjugated Jt-electron system within the fullerene core that predominantly determines the chiroptical properties. Adducts with a C2-... 

There are numerous chiral stationary phases available commercially, which is a reflection of how difficult chiral separations can be and there is no universal phase which will separate all types of enantiomeric pair. Perhaps the most versatile phases are the Pirkle phases, which are based on an amino acid linked to aminopropyl silica gel via its carboxyl group and via its amino group to (a-naphthyl)ethylamine in the process of the condensation a substituted urea is generated. There is a range of these type of phases. As can be seen in Figure 12.23, the interactions with phase are complex but are essentially related to the three points of contact model. Figure 12.24 shows the separation of the two pairs of enantiomers (RR, SS, and RS, S,R) present in labetalol (see Ch. 2 p. 36) on Chirex 3020. 

Construct the model of the image projected in the mirror. You now have two models. If one is the object, what is the other (3a) Do either have a plane of symmetry (3b) Are both chiral (3c) Now try to superimpose one model onto the other, that is, to place one model on top of the other in such a way that all five elements (i.e., the colored atoms) fall exactly one on top of the other. Can you superimpose one model onto the other (3d) Enantiomers are two molecules that are related to each other such that they are nonsuperimposable mirror images of each other. Are the two models you have a pair of enantiomers (3e) ... 

For tetrasubstituted compounds of the type CRjR2R3R4 (Fig. 9) (1) if the molecule is planar, then three isomers are possible. (2) If the molecule is p3uramidal, then six isomers are possible. Each of the forms in Fig. 9, top, drawn as a pyramid, is not superimposable on its mirror image. Thus, three pairs of enantiomers are possible (one of which is shown in Fig. 9, middle). (3) If the molecule is tetrahedral, two isomers are possible, related to one another as object to mirror image. In actuality, only two tetrasubsti-tuted isomers of methane are known (pair of enantiomers). This is strong evidence for the tetrahedral model for the carbon atom. Similar reasoning leads to the same conclusion for trisubstituted methanes. 

When molecules composed of the same constituents have the same structural formulas but differ only with respect to the spatial arrangement of certain atoms or groups of atoms, they are defined as stereoisomers. Stereoisomers can be optical isomers or geometrical isomers. Optical isomers are members of a set of stereoisomers, at least two of which are optically active or chiral geometrical isomers are members of a set of Stereoisomers that contains no optically active members. If the relationship between optical isomers is one of nonsuperimposable mirror images, the isomers are defined as enantiomers. Molecules having at least one pair of enantiomers are considered chiral. Optical isomers not related to each other as enantiomers are diastereomers. 

A molecule having at least one pair of enantiomers Stereoisomers that are related as nonsuperimposable mirror images... 

The relationship between a chiral object and its mirror-related object is called enantiomerism. A knowledge of the existence of enantiomers was one of the reasons that van t Hoff and Le Bel proposed, as described in Chapter 1, that the four valences of carbon are spatially directed to the corners of a regular tetrahedron.The only difference between a pair of enantiomers is that, if one can be described as a left-handed form, the other will be a right-handed form they have identical chemical formulae. The major physical property that allows one to distinguish between enantiomers is the direction in which they, or their solutions, rotate the plane of polarized light, that is, when they are studied in the chiral environment provided by the polarized light. It is important to note that a molecule is not necessarily chiral just because it contains an asymmetric center, or that it is necessarily achiral because it lacks such an asymmetric center. These are not the criteria for molecular chirality. The test for chirality in a molecule is the nonsuperimposability of the object on its mirror image. 

Cyclic compounds can exhibit enantiomerism as well as geometric isomerism. A cyclic compound with two dissimilar chiral carbon atoms has two possible enantiomeric pairs. The cis isomer can exist as a pair of enantiomers and the trans isomer does the same. The two cis enantiomers are diastereomers of the two trans enantiomers. A cyclic compound with two similar chiral carbon atoms has a meso compound, the cis geometric isomer, and a pair of enantiomers, the trans geometric isomer. Again, the cis and trans isomers are related as diastereomers. 

Optical isomerism is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.2. The simplest case of optical isomerism among fi -block complexes involves a metal ion surrounded by three didentate ligands, for example [Cr(acac)3] or [Co(en)3] (Figures 3.16b and 19.12). These are examples of tris-chelate complexes. Pairs of enantiomers such as A-and A-[Cr(acac)3] or A- and A-[Co(en)3]Cl3 differ only in their action on polarized hght. However, for ionic complexes such as [Co(en)3], there is the opportunity to form salts with a chiral counter-ion A. These salts now contain two different types of chirality the A- or A-chirality at the metal centre and the (-I-) or (—) chirality of the anion. Four combinations are possible of which the pair (A-(- -) and A-(—) is enantiomeric as is the pair A-(—) and A-(- -). However, with a given anion chirality, the pair of salts A-(—) and A-(—) are diastereomers (see Box 19.2) and may differ in the packing of the ions in the solid state, and separation by fractional crystallization is often possible. 

For the related problem of finding the nnmber of chiral hexachlorocycloalkanes, Dias [80] described a procedure based on Pdlya s theorem whereby this number is Z Cny 1 + ) — Z Dny 1 + ), leading to one pair of enantiomers for rings with n — 6 and 7 atoms, and four pairs for n = 8. 

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