The Cahn-Ingold-Prelog system for assigning absolute configurations to chiral centers [Angew. Chem. Int. Ed., 1966, 5, 385.] is widely taught in organic chemistry. Of it, Prelog has written:
"An ancient Chinese proverb has it that a picture can obviate a thousand words. It is not less true that a thousand pictures may be obviated by a right word in a right place, and a left word in a left place."
If you don't remember this system, click HERE for a review.
Chinese students past have told me that no such "ancient Chinese proverb" exists, but otherwise, the truth of the quotation is indubitable. The system is even more valuable because it can be extended to cover situations in which no chiral center is present.
For example, consider the tetrahedral arrangement of ligands around a chiral center (below, left). If we grasp opposite edges of the tetrahedron and stretch, the chiral center is extended into a chiral axis.
The structure remains dissymmetric, lacking a plane of symmetry, so long as (a) ¹ (b) and (c) ¹ (d). Structures with (a) and (b) the same as (c) and (d) remain chiral. Both structures have the same projection upon a perpendicular plane:
Molecules having this elongated tetrahedral geometry belong to point groups Cn and Dn, the chief representatives of which are allenes, biphenyls, and spiro compoounds.
To extend the CIP rules to these molecules, we need three additional rules:
Here is an example, a biphenyl and its enantiomer.
If we view the structures from the left, and project them onto a perpendicular plane, we obtain:
Note that the application of the near > far rule means that the CO2H on the front ring takes precedence over the NO2 on the rear ring. Thus, the left hand structure is S, and the right hand, R.
It may seem that this result could be related to our chosing to view the molecules from the left; not so. Here are the projections obtained if each is viewed from the right.
The assigned configurations are unchanged.
|R and S Biphenyls|
Allenes in which the substituents on each terminus are different (the same condition as for stereoisomerism in alkenes) are chiral; the long axis of the molecule is a chiral axis. Here we assign a configuration of S.
|R and S Diphenylallenes|
Spiro compounds are another group of molecules in which ligands remote from the spiro junction can be placed in a stretched tetrahedral arrangement:
Finally, a very unusual example: a biphenyl in which two identical bridged maintain a twisted structure.
In this case we arbitrarily choose either front substituent as #1. The rear group bonded to #1 then becomes #3. You should draw the mirror image structure and verify that this procedure will assign it the S configuration.