Recognizing Chiral Molecules

The work just discussed deals with the situation in which the chromophore per se is chiral. This is one of two main classes of chiral molecules recognized by specialists in the field180,181) it is commonly called the case of the inherently chiral chromophore . The second class of chiral molecules are those which have an achiral chromophore in a chiral environment . 

Lord Kelvin lla> recognized that the term asymmetry does not reflect the essential features, and he introduced the concept of chiralty. He defined a geometrical object as chiral, if it is not superimposable onto its mirror image by rigid motions (rotation and translation). Chirality requires the absence of symmetry elements of the second kind (a- and Sn-operations) lu>>. In the gaseous or liquid state an optically active compound has always chiral molecules, but the reverse is not necessarily true. 

The comprehensive manner by which VOA intensities relate to the details of molecular stereochemistry can be appreciated by recognizing that the set of 3N-6 vibrational degrees of freedom is defined in the same space that specifies the parameters of molecular conformation. No other form of molecular spectroscopy is so closely related to molecular stereochemistry. It is literally trae that VOA spectra arise from stereospecific vibrational oscillations of a chiral molecule. A challenge facing VOA spectroscopy at the present is how to fully extract this stractural and conformational information from the spectra. 

Soon thereafter, chirality was recognized as a necessary and indispensible part of synthetic receptor molecule design and function. Predictably, not only has nature s chiral pool been called upon to supply inexpensive and readily available sources of chirality, but the ability of the chemist to resolve optically active precursors from racemic modifications prepared in the laboratory has been exploited ingeniously in a number of different directions. The various elements of chirality centers, axes, planes, as well as helices, have been incorporated into both axially symmetric and asymmetric receptors. 

It was soon recognized that in specific cases of asymmetric synthesis the relation between the ee of a chiral auxiliary and the ee of the product can deviate from linearity [17,18,72 - 74]. These so-called nonlinear effects (NLE) in asymmetric synthesis, in which the achievable eeprod becomes higher than the eeaux> represent chiral amplification while the opposite case represents chiral depletion. A variety of NLE have been found in asymmetric syntheses involving the interaction between organometallic compounds and chiral ligands to form enantioselective catalysts [74]. NLE reflect the complexity of the reaction mechanism involved and are usually caused by the association between chiral molecules during the course of the reaction. This leads to the formation of diastereoisomeric species (e.g., homochiral and heterochiral dimers) with possibly different relative quantities due to distinct kinetics of formation and thermodynamic stabilities, and also because of different catalytic activities. 

Enzymes are chiral molecules with specific catalytic activities. For example, when an acylated amino acid is treated with an enzyme like hog kidney acylase or car-boxypeptidase, the enzyme cleaves the acyl group from just the molecules having the natural (l) configuration. The enzyme does not recognize D-amino acids, so they are unaffected. The resulting mixture of acylated D-amino acid and deacylated L-amino acid is easily separated. Figure 24-5 shows how this selective enzymatic deacylation is accomplished. 

At the time they were synthesized, these molecules were not recognized as having a non-planar graph, and actually the first topologically chiral K5 molecule recognized as such, is the centered polyquinane derivative 53, independently synthesized by H. E. Simmons and J. E. Maggio [80] on the one hand, and by L. A. Paquette and M. Vazeux [81] on the other hand (Fig. 9). That it is topologi-... 

The substitution of chiral molecules onto the crystal surfaces has been investigated by Mein Lahav, Lia Addadi, Leslie Leiserowitz and co-workers.The chiral molecule is recognized by the molecules already oriented in the crystal lattice. This can be true for either an enantiomorphous crystal composed of molecules with only one chirality sense, or for a centrosymmetric racemic crystal in which a face that is composed only of one enantiomer interacts with a chiral agent. 

The presence or absence of a chiral center is thus no criterion of chirality. However, most of the chiral molecules that we shall take up do contain chiral centers, and it will be useful for us to look for such centers if we find a chiral center, then we should consider the possibility that the molecule is chiral, and hence can exist in enantiomeric forms. We shall later (Sec. 4.18) learn to recognize the kind of molecule that may be achiral in spite of the presence of chiral centers such molecules contain more than one chiral center. 

The same equilibria are attained if to a solution of racemate in an initially achiral medium (equal concentrations of d and I species) there is added another chiral species d or L. The equilibrium is then displaced in favour of one or other of the constituents of the racemic mixture. This process has recently been termed enantiomerization, although examples of optically labile systems in equilibria sensitive to the presence of other chiral molecules or ions have long been recognized. A typical example of what was earlier termed an asymmetric transformation of the first kind (no second-phase involved) is that of Read and McMath (1925) in which solutions in dry acetone of (—) or ( ) chlorobromomethanesulphonic acid d-l ) together with (—)-hydroxyhydrindamine (l+) showed a change of optical rotation interpreted in terms of an equilibrium... 

The anthropomorphic notion that a chiral molecule can somehow recognize or discriminate the chirality sense of another chiral molecule is a convenience that is used commonly, realizing that it is the observer that does the recognizing, not the molecules [64]. 

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