Asymmetric molecule, enantiomorphic

Asymmetric carbon atom, description, 57 Asymmetric molecule, enantiomorphic forms, 57... 

Enantiomorphs are identical in most of their properties such as melting points, solubilities, and chemical reactivity. However, when another asymmetric molecule or polarized light is involved, they are markedly different. This behavior is especially pronounced in biological systems, because the enzymes are also asymmetric molecules, and frequently one enantiomorph is handled in biological systems quite differently from the other. n-Glucose is readily utilized by man, whereas its mirror image, L-glucose, is not utilizable. 

Consider a methane molecule CH, and suppose that some or all of its hydrogen atoms are replaced by some other monovalent atom. If the atoms attached to the carbon are all different, that is, the carbon atom is asymmetric, the resulting molecule is chiral and exists in two so-called enantiomorphic forms mirror images of each other. (For further information on chirality see the interesting expository paper [PreV76]). 

Let us now differentiate between structures which are asymmetric and dissymmetric. The word asymmetric conveys the idea that the molecule is completely devoid of the elements of symmetry. Dissymmetric on the other hand means not completely devoid of elements of symmetry but possessing so few elements of symmetry that on the whole it will posses two structures which will be the mirror images of each other. Therefore to avoid confusion the term asymmetric is used to cover examples which rotate the plane polarized light. The two forms of an optically active compound are called enantiometers or enantiomorphs or optical antipodes. They are also said to have enantiomeric relationship to each other. 

Since all the molecules are asymmetric and have no plane of symmetry, all are optically active. Further structures I and II are enantiomorphs and so are structures III and IV. But structures III and I or IV and I are although stereoisomers but are not enantiomorphs. Such pairs of steroisomers which possess chirality but are not the mirror images of each other are called diastereomers. Thus III and IV are diastereomers of 1. So diastereomers will always be formed when the compound contains two dissimilar asymmetric carbon atoms and will exist in four stereoisomeric forms. 

Crystallization and reactivity in two-dimensional (2D) and 3D crystals provide a simple route for mirror-symmetry breaking. Of particular importance are the processes of the self assembly of non-chiral molecules or a racemate that undergo fast racemization prior to crystallization, into a single crystal or small number of enantiomorphous crystals of the same handedness. Such spontaneous asymmetric transformation processes are particularly efficient in systems where the nucleation of the crystals is a slow event in comparison to the sequential step of crystal growth (Havinga, 1954 Penzien and Schmidt, 1969 Kirstein et al, 2000 Ribo et al 2001 Lauceri et al, 2002 De Feyter et al, 2001). The chiral crystals of quartz, which are composed from non-chiral Si02 molecules is an exemplary system that displays such phenomenon. 

The formation of an isotactic polymer requires that insertion always occur at the same prochiral face of the propylene molecule. Theoretically, both a chiral catalytic site (enantiomorphic site control) and the newly formed asymmetric center of the last monomeric unit in the growing polymer chain (chain end control) may... 

Chiral crystals, like any other asymmetric object, exist in two enantiomorphous equienergetic forms, but careful crystallization of the material can induce the entire ensemble of molecules to aggregate into one crystal, of one-handedness, presumably starting from a single nucleus (Figure 2). However, it is not uncommon to find both enantiomorphs present in a given batch of crystals from the same recrystallization. 

The coadsorption of chiral molecules into racemic layers is an efficient way to induce further asymmetrization towards single handedness. While in heterogeneous chiral catalysis the stationary ratio of modifier and reactant at the surface is assumed to be one, a small amount of chiral dopant can be sufficient for induction of homochirality on the entire surface SU on Cu( 110), for example, forms two enantiomorphous domains in its bisuccinate phase [27]. 

An ideal approach to achieving chiral induction in a constrained medium such as zeolite would be to make use of a chiral medium. No zeolite that can accommodate organic molecules, currently exists in a stable chiral form. Though zeolite beta and titanosilicate ETS-10 have unstable chiral polymorphs, no pure enantiomorphous forms have been isolated. Although many other zeolites can, theoretically, exist in chiral forms (e.g., ZSM-5 and ZSM-11) none has been isolated in such a state. In the absence of readily available chiral zeolites, one is left with the choice of creating an asymmetric environment within zeolites by the adsorption of chiral organic molecules. 

Replacement of the oxygen ring atom in 30 by sulfur affords a thia-analog with two different solid-state conformations (see 31,32 in Figs. 14 and 15). X-ray crystallography of the crystals showed spontaneous resolution had occurred in which the conglomerate of enantiomorphic chiral crystals contained two symmetry independent molecules of identical configuration in the asymmetric unit of each chiral crystal.1 This is similar to the case for the chiral crystals of 27, but now there are two different conformations instead of only one. One of the chiral crystals showed both... 

Indeed, the group of radicals R, R, R"", A when considered as material points differing among themselves form a structure which is enantiomorphous with its reflected image, and the residue, M, cannot re-establish the symmetry. In general then it may be stated that if a body is derived from the original type M A4 by the substitution of three different atoms or radicals for A, its molecules will be asymmetric, and it will have rotatory power. 

In order to distinguish enantiomorphic structures, molecular chirality has conventionally been expressed in terms of center-, axis-, and plane-chirality. In the case of compounds involving several stereogenic centers, however, these terms seem to be insufficient to express their whole molecular chirality or anisotropy, although their local chirality is well confirmed in the conventional manner. Indeed, the steroidal molecules (Figure 26.13a), which have asymmetric, amphiphilic, facial structures with multiple stereogenic carbon atoms, are saddled with such a structural complexity. In order to solve this problem, we introduced a simple but unique concept, three-axial chirality , as shown in Figure 26.13b. Such three-axial chirality is based on the orthorhombic three axes applied in a molecular structure and is expressed by... 

Two ° independent and different degradations have been carried out to establish the absolute stereochemistry, and in each case the final compound contained only the C(3a) asymmetric centre. One approach was to degrade the alkaloid to the oxindole (10), the enantiomorph of which was then synthesised starting from (R)-( — )-2-methyl-2-phenylbutyric acid. In the second approach the molecule was broken down to an amino-acid (11) which was characterised as its 2,4-dinitrophenyl derivative. Its enantiomer was synthesised from 3-ethyl-3-methoxycarbonyl-3-methylpropionic acid of known absolute configuration. 

Molecules having an asymmetric carbon atom are not the only ones to rotate the plane of polarization. A molecule having a helical (spiral) structure also exists as two enantiomorphs, one resembling a right-handed screw and the other a left-handed screw. These two enantiomorphs also rotate the plane of polarization of light in opposite directions. 

---