Chirality due to restricted rotation,

•	Some compounds are chiral, yet have no stereogenic centres. Consider 2,2'-dihydroxy-4,4',6,6'-tetramethylbiphenyl, the mirror images (enantiomers) shown below are not superimposable and so the molecule is chiral:

 

•	The presence of the ortho substituents means that the central bond linking the two phenyl groups cannot rotate freely due to steric hindrance. This hindered rotation prevents the enantiomers from interconverting and therefore gives rise to chirality.

•	Examine this molecule for yourself (refcodes: NIYQUH and NIYRAO). The crystal structure of NIYQUH consists of a single enantiomer, whereas the crystal structure of NIYRAO has both enantiomers present in the unit cell (unit cell packing can be displayed using the free visualiser Mercury).

•	Another example of a molecule that is chiral by virtue of restricted rotation is 2,2-bis(diphenylphosphino)-1,1'-binaphthyl, known as BINAP (refcodes: PASRAC and HUZGUE). This is an important ligand used in asymmetric hydrogenation reactions.

 

•	In order for the enantiomers of BINAP to interconvert, the PPh2 group would have to force its way past either the other PPh2 group or past the hydrogen. Both pathways are too strained for racemization to occur: