1,2-Diphenylethane-1,2-diamine: an effective NMR chiral solvating agent for chiral carboxylic acids

(1R,2R)-1,2-diphenylethane-1,2-diamine, 1, acts as an effective chiral solvating agent (CSA) in the ¹H NMR analysis of the enantiomeric purity of chiral carboxylic acids. In the 2:1 salt complexes with a range of acids including α-arylpropanoic, α-halo carboxylic acid and primary carboxylic acids, RCH₂CO₂H, the diastereotopic resonances in ¹H NMR were typically more than 0.05 ppm shift non-equivalent. The effect of temperature, stoichiometry, acid enantiomeric purity, concentration and solvent on the observed shift non-equivalence was studied. The structure of the CSA was varied systematically and the observed non-equivalence with 1, may be attributed to the anisotropy of the second aryl ring which is proximate to the substituents α to the carboxylic acid group.