Diamagnetic lanthanide tris β-diketonate complexes with aryl-containing ligands as chiral NMR discriminating agents

Abstract

Diamagnetic lanthanum(III) and lutetium(III) tris β-diketonate complexes with the aryl-containing ligands 3-benzoyl-(+)-camphor and 3-(2-naphthoyl)-(+)-camphor are effective organic-soluble chiral NMR discriminating agents for oxygen- and nitrogen-containing compounds. Enantiomeric discrimination of sufficient magnitude to determine the enantiomeric purity is observed in the 1H NMR spectra of compounds with hydroxyl, carbonyl, oxazolidinone, amine, and sulfoxide groups. Diamagnetic lanthanide complexes with the aryl-containing β-diketonate ligands are almost always more effective than those with 3-trifluoroacetyl-(+)-camphor, 3-heptafluorobutyryl-(+)-camphor, and d,d-dicampholylmethane that have been previously reported. Many hydrogen atoms of the substrates are significantly shielded in the presence of the lanthanide chelates with the aryl-containing ligands, which likely enhances the extent of enantiomeric discrimination in the NMR spectra. No combination of metal and ligand is most effective for all substrates. Larger enantiomeric discrimination is usually observed in benzene-d6 or cyclohexane-d12 than in chloroform-d. Diamagnetic lanthanide tris β-diketonates with the aryl-containing ligands provide an alternative to paramagnetic chelates that often cause too much broadening in the 1H NMR spectrum.