Compounds with tervalent chiral atoms.

•	Pyramidal nitrogen atoms might be expected to give rise to optical activity if they are connected to three different groups. This is because the unshared pair of electrons is analogous to a fourth group and necessarily different from the others:

 

•	In practice, chirality is rarely observed in such systems due to pyramidal inversion. This is the rapid movement of the unshared pair from one side of the XYZ plane to the other which thus interconverts the molecule into its enantiomer.

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However, inversion is less rapid for nitrogen atoms in a three membered ring, and for nitrogen atoms connected to an atom with an unshared electron pair. When both of these features are present in a molecule the barrier to inversion is sufficient to allow isolation of separate isomers. This can result in compounds which are optically active due to a chiral tervalent nitrogen atom.

•	An example of this is 1-chloro-2-methoxycarbonyl-2-methylcarbamoylaziridine, for which both the cis- and trans epimers have been isolated:

 

•	Examine each of these two stereoisomers in turn (refcodes: KIRCOD and KUBZOW).

•	You should be able to see that KIRCOD has a (1R,2R) configuration at the chiral N(l) and C(2) atoms, whereas the crystal structure of KUBZOW is made up of racemic pairs of discrete molecules with (1S,2R) and (1R,2S) configurations.