Chiral compounds, spiranes

Axial Chirality. For a system with four groups arranged out of the plane in pairs about an axis, the system is asymmetric when the groups on each side of the axis are different. Such a system is referred to as an axial chiral system. This structure can be considered a variant of central chirality. Some axial chiral molecules are allenes, alkylidene cyclohexanes, spiranes, and biaryls (along with their respective isomorphs). For example, compound 7a (binaphthol), which belongs to the class of biaryl-type axial chiral compounds, is extensively used in asymmetric synthesis. Examples of axial chiral compounds are given in Figure 1-5. 

Among other types of compounds that contain the system illustrated in Figure 4.2 and that are similarly chiral if both sides are dissymmetric are spiranes (e.g., 21) and compounds with exocyclic double bonds (e.g., 22). 

The chirality observed in this kind of substituted allene is a consequence of dissymmetry resulting from restricted rotation about the double bonds, not because of a tetrahedral atom carrying four different groups. Restricted rotation occurs in many other kinds of compounds and a few examples are shown in Table 13-3, which includes trans-cycloalkenes (Section 12-7), cycloalkyli-denes, spiranes, and ort/zo-substituted biphenyl compounds. To have enantiomers, the structure must not have a plane or center of symmetry (Section 5-5). 

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. 

Another interesting group of compounds where chirality often occurs is spir-anes (Figure 2.17). It is sufficient to substitute any two hydrogen atoms in both spirane rings with any substituents (they can be identical) to make this molecule optically active. 

Compounds containing nitrogen atom that are capable of forming four bonds and having different substituents may be optically active. In such compounds, there is a certain analogy to a chiral tetrahedral carbon atom. This type of compounds includes ammonium salts, amine oxides, and spiranes where nitrogen atom is a central one (Figure 2.26). 

---