NMR discrimination in nonrigid prochiral solutes dissolved in chiral liquid crystals: symmetry considerations
Résumé
Enantiodiscrimination in the NMR spectra of flexible prochiral solutes dissolved in chiral liquid crystals (CLCs) is reviewed and
compared with the analog phenomenon in such rigid solutes. In rigid prochiral solutes, the discrimination is brought about by
the cancellation of improper symmetry elements upon dissolving in CLC within the frame of solute-solvent ordering mechanisms.
If this reduction in symmetry renders the ordering of enantiotopic sites dissimilar, spectral discrimination may be observed.
Symmetry considerations indicate that this is only possible for improper nonaxial groups lacking inversion symmetry. Nonrigid
prochiral solutes consist of rapidly (on the NMR timescale) interconverting enantiomers, in which the racemization is accompanied
by exchange of nonequivalent sites. These sites become, on the average, enantiotopically related, and in CLC, they exhibit
spectral discrimination. The mechanismof the effect and the symmetry selection rules are different for the two cases. Specifically,
the discrimination in flexible prochiral compounds results from the different ordering of the interchanging enantiomers in CLC.
Using Altman’s definition of average symmetry (Proc. R. Soc. A, 1967, 298, 184), selection rules for the phenomenon are derived.
It follows that chiral discrimination in nonrigid prochiral solutes is much more abundant and can occur in all symmetry types
except those possessing inversion. In particular, contrary to earlier thoughts, the effect can occur in compounds with axial
symmetry. Illustrative examples of such studies with particular emphasis on compounds with average axial symmetry of the type
D3h, C3v and C3h are reviewed in this contribution.