Analysis of the13C NMR Spectra of Molecules, Chiral by Isotopic Substitution, Dissolved in a Chiral Oriented Environment: Towards the Absolute Assignment of the pro-R/pro-S Character of Enantiotopic Ligands in Prochiral Molecules
Résumé
We examined and discuss the proton- and deuterium-decoupled carbon-13 1D spectrum of a molecule, chiral by virtue of the isotopic substitu-tion, dissolved in a chiral oriented medium which simultaneously exhibits chiral discrimination, enantiomeric en-richment and isotope effect. Using the 1-deutero-(2’,3’,4’,5’,6’-pentadeutero-phenyl)phenylmethanol orientationally ordered in a chiral nematic liquid crystal as illustrative example, we point out three important features. First, we demonstrate that the absolute assign-ment of the pro-R/pro-S character may be derived from the absolute configu-ration of the isotopically chiral analogue. Second, we report evidence that isotopic effect on 13C chemical shift anisotropy is negligible in a weakly ori-enting solvent. Third, we definitely es-tablish that the molecular orientation of prochiral Cs symmetry molecules and their parent compounds that are chiral by virtue of the isotopic substitu-tion is the same.