In case of biphenyl derivatives, if both aromatic ring systems are asymmetrically substituted, the compounds are chiral. As the chirality of these structures originates not from an asymmetrically substituted atom center, but from an asymmetric axis around which rotation is hindered, these enantiomers are also called atropisomers. In the biphenyls, the ortho-substituents must be large enough to prevent rotation around the central single bond. Since only the hindered rotation about the central C-C single bond leads to the stereoisomerism of these compounds. Therefore, biphenyl- and binaphthyl-derivatives are conformational stereoisomers (not configurational stereoisomers).
Planar chirality may arise if an appropriately substituted planar group of atoms or ring system is bridged by a linker-chain extending into the space above or below the plane. Some common examples are the planar chirality of cyclophanes or alkenes as shown in Figure 4.
Figure 4
Helices are also chiral as they can exist in enantiomeric left- or right-handed forms. Typical examples for helical structures are provided by the helicenes (benzologues of phenanthrene). With four or more rings, steric hindrance at both ends of this molecule prevents the formation of planar conformations, and helicenes rather adopt non-planar, but helical and enantiomeric structures with C2 symmetry (Figure 5).
Figure 5
The stereochemistry of a substrate may have profound effect on the rate of a reaction or the composition of the products of a particular reaction. With respect to the composition of products obtained by a reaction two terms are important.