Natural Abundance Deuterium NMR Spectroscopy in Polypeptide Liquid Crystals as a New and Incisive Means for the Enantiodifferentiation of Chiral Hydrocarbons

Abstract

Polymeric chiral liquid-crystalline solvents based on homopolypeptides are of interest with the view to discriminate between enantiomeric pairs of chiral hydrocarbons using proton-decoupled deuterium one- and two-dimensional NMR spectroscopy at natural abundance level. This method offers the major advantage that neither chemical modification nor isotopic labelling of the solutes to be studied is required. Chiral differentiation between optical isomers is observed through a difference in residual deuterium quadrupolar splittings. The spectroscopic separations and the S/N ratio from the spectra are usually large enough to measure the enantiomeric excess with an accuracy varying between 5 to 10 %. This analytical approach is successfully applied to a large collection of chiral, rigid or flexible unsaturated as well as saturated hydrocarbons, including cases of axial chirality, atropoisomerism, and moieties existing as a mixture of enantiomers interconverting by ring inversion. Using the results reported in literature, a systematic comparison with other analytical strategies (NMR, GC, HPLC, VCD) is made and discussed. Also, a tentative proposal to rationalise the various results in terms of chiral differentiation and enantioselective shape recognition is presented. We show that this original tool provides an attractive and incisive alternative to the existing analytical techniques for studying nonfunctionalised chiral materials.