Chirality element

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

In an asymmetric reaction, substrate and reagent combine to form diaster-eomeric transition states. One of the two reactants must have a chiral element to induce asymmetry at the reaction site. Most often, asymmetry is created upon conversion of trigonal carbons to tetrahedral ones at the site of the functionality. Such asymmetry at carbon is currently a major area of interest for the synthetic organic chemists. 

The representation of stereochemical features by a polycentric configuration contains the monocentric configurations and essential information on the conformations. Use of the polycentric skeleton concept is particularly advantageous for cyclic and polycyclic systems. Sometimes, the asymmetric C-atoms are regarded as central elements of chirality1 4>, and their enumeration is used in the comparison of chiral systems when the question of which has more chiral elements is important. 

In the enumeration of chirality elements of flexible molecules all arrangements are taken into account which are permitted by the given constraints under the observation conditions. Here, one must always assume a rigid skeletal model and freely rotating ligandsF That arrangement for which the lowest number of chirality elements is found equal zero determines the number of chirality elements for the whole ensemble. 

According to the classical concept of van t Hoff one would need two operations for the conversion of 18 into its mirror image, namely the inversion of both asymmetric C-atoms. These two examples demonstrate the advantages of the present procedure for enumerating the chirality elements of molecules. 

Molecules exist whose chirality elements can be enumerated according to the outlined procedure, but for which one finds a lower number of chirality elements by a different procedure. For example, the dissection of 20 and 21 into achiral skeletons and sets of achiral ligands (H, H, H, a, b, c) leads to three chirality elements by regarding the achiral... 

Hereby, it is also possible to enumerate the chirality elements of a variety of chiral molecules which cannot be treated according to Eq. (1) because they cannot be dissected into an achiral skeleton and a set of achiral ligands. 

The latter procedure results in the enumeration of chirality elements for 22 in analogy to 20 and 27, namely two. 

In such cases the enumeration of chirality elements is generally achieved stepwise. 

In one step, the chirality elements are counted with reference to the skeleton, and in another, the local chirality elements of the separated chiral ligands are enumerated. The resulting sum is the total number of chirality elements. This enumeration corresponds to the number of independent asymmetric C-atoms as assumed by van t Hoff. Furthermore, it yields the number of chirality elements in systems with chiral units of a different nature, e.g. some belonging to Ruch s class B. This simply... 

Chirality element enumeration is essential for the classification of stereoselective reactions 27>. For instance, in order to distinguish an asymmetrically induced synthesis from other reactions whose stereoselectivity is also due to a chiral reference system, one must compare the number of chirality elements in the starting materials and the products. 

The prochirality concept is not necessarily an expression of a precursor-product relationship because there exist stereoselective reactions at pro-chiral elements that do not generate elements of chirality. An illustration of this is the reversible enzymatic dehydration of citric to cu-aconitic acid. In this process two prochiral centers of citric acid disappear and we obtain an achiral line of stereoisomerism that physically coincides with a prochiral plane of prostereoisomerism. 

To start the polymerisation the X atoms in Fig 10.13 are replaced by a growing alkyl chain and a co-ordinating propene molecule. Co-ordination of propene introduces a second chiral element and several diastereomers can be envisaged. The step-by-step regulation of the stereochemistry by the site can be most clearly depicted by drawing the molecule as shown in Figure 10.14, a... 

In this section, we shall examine the various approaches by which crown compounds that have their chiral elements associated in some way with fused ring systems can be constructed. A selection of the wide and growing range of saturated chiral diols—many of them derived finom readily available carbohydrates—which have been incorporated, as relatively inexpensive sources of chirality, into crown ether derivatives are displayed in Figure IS. It may be noted that the saturated chiral diols rely for their chirality on centers of the classical type (C abcd)—not so the chiral dihydroxy compounds associated with the unsaturated systems listed in Figure 16. These examples reveal that axes and planes of chirality join with less conventional chiral centers (C aaaa) in being sources of chirality in optically active crown ethers. 

Note 2 The optical activity originates from the presence of chiral elements in a polymer such as chiral centers or chiral axes due to long-range conformational order in a polymer (helicity) (see [2], p. 182 for helicity). 

The modification of thiourea catalyst 93 through incorporation of the (S,S)-diaminocyclohexane backbone as an additional chirality element and a Schiff base imidazoyl-moiety led to the bifunctional catalyst 94 that, in contrast to 93 in the Strecker reaction (Scheme 6.99), exhibited enantioinduction (83-87% ee) in the nitro-Michael addition of acetone to trons-P-nitrostyrenes. The desired adducts were isolated in moderate yields (46-62%) as depicted in Scheme 6.100) [259]. 

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