Measurement of the optical purity of fluorinated compounds using proton decoupled 19F NMR spectroscopy in a chiral liquid crystal solvent

The use of trifluoroacetic anhydride as an achiral derivatizing agent for enantiomeric excess measurement through chiral liquid crystal NMR is described. Examples involving amines, amino acids and alcohols are provided. It is shown that the proton decoupled fluorine NMR spectra of the -CO-CF3 group allow good enantiomeric discrimination based on both the chemical shifts and the dipolar splitting. The spectral separation is due to the: differential ordering effect of enantiomers in the oriented chiral solvent. The results are then compared to the discriminating power of the -CO-CD3 group that has been previously described using deuterium NMR in the same anisotropic medium. (C) 1997 Elsevier Science S.A.