Chiral: axis planes

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. 

Fig. 1. Introduction of one or more than one stereogenic elements (center, axis, plane or helix) leads to different types of chiral dendrimers...

In a tour de force in 1956, Cahn, Ingold and Prelog introduced the terms chirality axis (descriptors aR/aS) and chirality plane (descriptors pRjpS) in order to deal with compounds such as allenes, biaryls and cyclophanes. Rules for assigning the chirality sense were devised ad hoc. In 1966 the helieity concept was introduced and it was recognized that its use allows the corresponding models to be treated in an alternative way. The specific proposals, as illustrated in Table 1, however, were only published in 19821. 

Fortunately, the original assignment rule for the chirality plane is identical to the helieity assignment defined above with descriptors aRjaS and PjM, respectively, corresponding. However, the rule for the chirality axis was based on an elongated tetrahedron as the stereogenic unit and the descriptors aR and P or aS and M are not equivalent ... 

Chirality (handedness, from Greek cheir = hand) is the term used for objects, including molecules, which are not superposable with their mirror images. Molecules which display chirality, such as (S)-(+)-lactic acid (/, Fig. 1) are called chiral. Chirality is often associated with a chiral center (formerly called an asymmetric atom ), such as the starred carbon atom in lactic acid (Fig. 1) but there are other elements that give rise to chirality the chiral axis as in allenes (see below) or the chiral plane, as in certain substituted paracyclophanes.1,2)... 

Tochtermann W, Ruckling D, Meints C, Kraus J, Bringmann G (2003) Bridged bioxepines and bi[10]paracyclophanes - synthesis and absolute configuration of a bi[10]paracyclophane with two chiral planes and one chiral axis. Tetrahedron 59 7791-7801... 

Scheme 2-3. Stereochemical features of the vancomycin structure. The AB-ring (chiral axis) and the C-O-D-O-E-rings (chiral planes) fixate the heptapeptide aglycon in a rigid comformation.

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

DMAP itself is achiral but a chiral version would make the achiral reactive intermediate 6 chiral. Since the alcohol 7 reacts with this species, there is the possibility that one of the alcohol enantiomers will react more quickly than the other and there will be a kinetic resolution. All that needs is for a chiral version of DMAP to be developed. Because DMAP has two planes of symmetry, this takes some doing. One way that was developed by Spivey et al. was to use an axis of chirality.6 One plane of symmetry is removed because a naphthyl ring is attached on one side and not the other while the chiral axis differentiates the back and front faces of the pyridine ring 8. For the esterification of 9 the s factor was 27 which means good levels of ee in both the starting material and the product should be possible. Indeed 97% ee of the starting material can be achieved at around 56% conversion. 

Chirality may exist in many molecules that do not possess a chiral center. Such compounds may possess a chiral plane or a chiral axis, and are said to be dissymetric with respect to either that plane or that axis. Certain optically active allenes, biaryls, alkylidenecyclohexanes, and spiranes provide examples of axially dissymmetric molecules (chiral axis), irons-Cycloalkenes exemplify planar dissymmetry in molecules. The configurations of these classes may be specified by the Cahn-Ingold-Prelog convention using the usual R and 5 descriptors. Special subrules, which we will not describe here, are applied to this purpose. The interested reader is referred to references 8 (see p. 43) and 9 for details. Scheme 2.1 presents some molecules that are optically active because of planar or axial dissymmetry, and for which the absolute configurations have been determined. 

Axial Chirality Chirality induced in a molecule containing four out-of-plane groups arranged in pairs that produce asymmetry about a chiral axis. In organometallic arene complexes, this can be caused by restricted rotation about a single bond (axis) of a substituent in the arene [20]. 

Cholesteric liquid crystals consist of chiral molecules and therefore do not have reflection symmetry. The symmetry group of cholesteric hquid crystals is >2 [1,3]- A cholesteric liquid crystal is invariant for the two-fold (180°) rotation around n, which rules out the possibility of spontaneous polarization perpendicular to n. It is also invariant for the two-fold rotation around an axis that is perpendicular to the n - (the hehcal axis) plane, which mles out the possibility of spontaneous polarization parallel to n. Therefore there is no ferroelectricity in the cholesteric phase. 

Chirality is a property of the entire molecule and there are molecules that are chiral but do not contain a chiral centre. For example, 1,3-dimethylallene is a chiral molecule since it has no plane of symmetry, and it can exist as a pair of mirror images (Figure 20.73). The molecule has a chiral axis rather than a chiral centre. The presence of two rigid 7t bonds prevents free rotation around the three central carbon atoms. 

Often symmetry operations cannot be used in a simple way to classify chiral forms because, e.g., the molecule consists of a number of conformations. Therefore, independent of the symmetry considerations, a chemical approach to describe chiral molecules has been introduced by the use of structural elements such as chiral centers, chiral axis, and chiral planes. Examples for a chiral center are the asymmetric carbon atom, i.e., a carbon atom with four different substituents or the asymmetric nitrogen atom where a free electron pair can be one of the four different substituents. A chiral axis exists with a biphenyl (Figure 3.2) and chiral planes are found with cyclo-phane structures [17]. Chiral elements were introduced originally to classify the absolute configuration of molecules within the R, S nomenclature [16]. In cases where the molecules are chiral as a whole, so-called inherent dissymmetric molecules, special names have often been introduced atropiso-mers, i.e., molecules with hindered rotation about a helical molecules [18], calixarenes, cyclophanes [17], dendrimers [19], and others [20]. 

Fig. 2 Examples of chiral molecules with and without asymmetric center, a The sp hybridized carbon bearing four different substituents is by far the most common asymmetric center, b The C=C=C allene arrangement forms a chiral axis. The l-chloro-3-bromoallene is chiral, c Atropoisomerism occurs when the free rotation around a o bond is hindered, d Steric hindrances create a chiral plane in hehcenes...

A molecule is chiral if it cannot be superimposed on its mirror image (or if it does not possess an alternating axis of symmetry) and would exhibit optical activity, i.e. lead to the rotation of the plane of polarization of polarized light. Lactic acid, which has the structure (2 mirror images) shown exhibits molecular chirality. In this the central carbon atom is said to be chiral but strictly it is the environment which is chiral. 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

We have seen in Section 4.1.4 that = n and that S2 = i, so we can immediately exclude from chirality any molecule having a plane of symmetry or a centre of inversion. The condition that a chiral molecule may not have a plane of symmetry or a centre of inversion is sufficient in nearly all cases to decide whether a molecule is chiral. We have to go to a rather unusual molecule, such as the tetrafluorospiropentane, shown in Figure 4.8, to find a case where there is no a or i element of symmetry but there is a higher-fold S element. In this molecule the two three-membered carbon rings are mutually perpendicular, and the pairs of fluorine atoms on each end of the molecule are trans to each other. There is an 54 axis, as shown in Figure 4.8, but no a or i element, and therefore the molecule is not chiral. 

Chira.lNema.tlc, If the molecules of a Hquid crystal are opticaHy active (chiral), then the nematic phase is not formed. Instead of the director being locaHy constant as is the case for nematics, the director rotates in heHcal fashion throughout the sample. This chiral nematic phase is shown in Figure 7, where it can be seen that within any plane perpendicular to the heHcal axis the order is nematic-like. In other words, as in a nematic there is only orientational order in chiral nematic Hquid crystals, and no positional order. Keep in mind, however, that there are no planes of any sort in a chiral nematic Hquid crystal, since the director rotates continuously about the heHcal axis. The pitch of the helix formed by the director, ie, the distance it takes for the... 

Although the ultimate criterion is, of course, nonsuperimposability on the mirror image (chirality), other tests may be used that are simpler to apply but not always accurate. One such test is the presence of a plane of symmetry A plane of symmetry (also called a mirror plane) is a plane passing through an object such that the part on one side of the plane is the exact reflection of the part on the other side (the plane acting as a mirror). Compounds possessing such a plane are always optically inactive, but there are a few cases known in which compounds lack a plane of symmetry and are nevertheless inactive. Such compounds possess a center of symmetry, such as in a-truxillic acid, or an alternating axis of symmetry as in 1. A... 

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