4.10: Chirality at Nitrogen, Phosphorus, and Sulfur
Chirality is most prevalent in carbon-based tetrahedral compounds, but this important facet of molecular symmetry extends to sp3-hybridized nitrogen, phosphorus and sulfur centers, including trivalent molecules with lone pairs. Here, the lone pair behaves as a functional group in addition to the other three substituents to form an analogous tetrahedral center that can be chiral.
A consequence of chirality is the need for enantiomeric resolution. While this is theoretically possible for all chiral amines, it is, in practice, difficult to separate the enantiomers of most chiral amines. This is due to pyramidal or nitrogen inversion, where the enantiomers are readily convertible from one form to another at room temperature, as the barrier to interconversion is ~25 kJ/mol. To briefly summarize the mechanism of this conversion, when the enantiomer passes through the transition state for inversion, the central nitrogen atom is sp2 hybridized, with its unshared electron pair occupying a p orbital. Therefore, ammonium salts that have no lone pair do not exhibit this phenomenon, and such quaternary chiral salts can be resolved into individual (relatively stable) enantiomers. Further, sp3 phosphorus and sulfur compounds, despite their lone pair, possess a high barrier for interconversion. Hence, their enantiomeric resolution is feasible.
Recall that enantiomers are non-superposable and are, therefore, different compounds with distinct identities. The nomenclature of chiral nitrogen, phosphorus and sulfur centers is like that of chiral carbon centers. The process of naming their enantiomers follows the Cahn–Ingold–Prelog rules or (R-S system), which involves three steps. The three steps are the same as with carbon centers—namely, assignment of priorities to the substituent groups, the orientation of the lowest-priority substituent away from the observer, and determining whether the priority sequence of the other three groups at the chiral center is clockwise or counterclockwise. However, in chiral centers with a lone pair, the lone pair is always assigned the lowest priority, as compared to hydrogen in systems without a lone pair. Accordingly, the molecule is rotated such that the lone pair points away. As with carbon, the chiral center is the R configuration if the one-two-three sequence is clockwise and the S configuration if the sequence is counterclockwise.