Analysis of the 13C 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.

Abstract

We examined and discuss the proton- and deuterium-decoupled carbon-13 1D spectrum of a molecule, chiral by virtue of the isotopic substitution, dissolved in a chiral oriented medium which simultaneously exhibits chiral discrimination, enantiomeric enrichment and isotope effect. Using the 1-deutero-(2',3',4',5',6'-pentadeuterophenyl)phenylmethanol orientationally ordered in a chiral nematic liquid crystal as illustrative example, we point out three important features. First, we demonstrate that the absolute assignment of the pro-R/pro-S character may be derived from the absolute configuration of the isotopically chiral analogue. Second, we report evidence that isotopic effect on (13)C chemical shift anisotropy is negligible in a weakly orienting solvent. Third, we definitely establish that the molecular orientation of prochiral C(s) symmetry molecules and their parent compounds that are chiral by virtue of the isotopic substitution is the same.