Planar Chirality

How to Manipulate JSmol Structures

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Index:

Introduction:

Planar chirality, like axial chirality, employs the descriptors P (plus) and M (minus). Chiral molecules having atoms a, b and x lying ina plane and y-z in a plane with atom z orthogonal to the plane fall into this category (Fig. 1). Implicit in this description is that z is restricted from lying in the plane. When b>a, the chirality is P (Rp, b>a>y>z).; when a>b, the assignment is M (Sp; a>b>y>z). Viewing down the y-x bond and b>a, P is set at 3 o'clock. When a>b, M is at 9 o'clock. To show that M = Sp, point your left (Sinister) thumb from y to x. Your fingers will point to atom a, the M-configuration. Conversely, your right (Rectus) hand will point to atom b, the P-configuration. [Note: JSmol is not able to assign planar configurations.]

 Fig. 1

 



Fig. 2

(P)-Cyclooctene

2a

 

(E)-Cyclooctene

The trans isomer of cyclooctene is the smallest ring in which an (E)-double bond can exist. The racemate has been resolved into its two enantiomers, Figs. 2 & 3, because there is a large barrier for the double bond to pass through the ring. Both enantiomers have a two-fold axis of symmetry.

In JSmol Fig. 2, click on "show carbon #". Orient the structure with C3 above C2 and view down the the C2-C1 axis. Notice that C5 is at "3-o'clock", which is the P-configuration. One may also orient the structure with the double bond in the vertical axis with C5 above C1. Now, C3 is at "3-o'clock". The configuration is still "P".

The two-fold axis of symmetry of (E)-cyclooctene allows the configuration to be determined on either side of the double bond. In 2a, when you point you're right thumb down the y-x bond, your fingers point from b → a → y. The P-configuration is equivalent to Rp.

structure 3a is (M)-cyclooctene. Now you're pointing your left thumb down the y-x bond, your fingers point from b → a → y. This configuration is also Sp.

For axial chirality M:P::R:S. The descriptors are in alphabetical order. Planar chirality has M:P::S:R which is not in alphabetical order.

 

Fig. 3

(M)-Cyclooctene

3a

(P)-12-Chloro-1,4(1,4)-dibenzenacyclohexaphane


Fig. 4

4a

The aromatic rings in enantiomeric cyclophanes 4a and 5a are incapable of racemizing by rotating one, or both, aromatic rings through the larger ring. The red bonds delineate the chirality elements as P and M, respectively. Align the JSmol structures with C16 above C15 in the foreground and view down the C15-C9 bond. In Fig. 4, C10, which bears the chlorine, is on the right which is the P-configuration. Fig. 5, using the same alignment, has C8 on the left or the M-configuration.

 (M)-12-Chloro-1,4(1,4)-dibenzenacyclohexaphane

Fig. 5


5a