Ephedrinium-Based Protic Chiral Ionic Liquids for Enantiomeric Recognition

We report the synthesis of a series of novel structurally related protic chiral ionic liquids (PCILs) derived from ephedrines. Enantiopure norephedrine, ephedrine, and methylephedrine were neutralized by use of fluorinated acids, bis(trifluoromethanesulfonyl) imide, and bis(pentafluoroethanesulfonyl) imide to afford six PCILs with protonated primary, secondary, and tertiary amines. The goal of this study is to investigate the influence of structure on both chiral recognition abilities and physicochemical properties of these closely related PCILs. The newly synthesized PCILs were characterized by use of nuclear magnetic resonance (NMR), thermal gravimetric analysis, differential scanning calorimetry, circular dichroism (CD), mass spectrometry, and elemental analysis. The PCILs were thermally stable up to 220 degrees C and had glass transition temperatures between -60 and -30 degrees C. Both enantiomers of the PCILs retained chirality throughout the synthesis as demonstrated by use of CD measurements. More interestingly, these ephedrinium PCILs displayed strong chiral recognition capabilities as evidenced by peak splitting of the chemical shift of the trifluoro group of potassium Mosher's salt by use of F-19-NMR. In addition, these PCILs demonstrated enantiomeric recognition capabilities toward a range of structurally diverse analytes using steady-state fluorescence spectroscopy. Chirality 23: 54-62, 2011. (C) 2010 Wiley-Liss, Inc.