Chiral NMR Shift Reagents: Mixtures of Lanthanide Tris (β-diketonates) with Chiral Carboxylate Anions

Abstract

Chiral carboxylic acids such as N-(R)-1-(1-naphthyl)ethylaminocarbonyl-L-tert-leucine, N-(R)-1-(1-naphthyl)ethylaminocarbonyl-L-valine and N-(3,5-dinitrobenzoyl)-L-leucine are solubilized in chloroform by the addition of triethylamine. The resulting ion pairs are useful chiral resolving agents for sulfoxides, amines and alcohols. For certain substrates, particularly amines, the ion pairs were more effective chiral resolving agents than the corresponding organic-soluble ester derivatives. Addition of lanthanide tris(β-diketonate) complexes to mixtures of the ion pairs causes enhancements in the enantiomeric resolution in the spectra of certain substrates. Evidence indicates that the chiral carboxylate compound bonds directly to the lanthanide tris(β-diketonate) to form an anionic species. Shifts in the spectra of substrates and enhancements in enantiomeric resolution in the presence of the anionic species appear to be dominated by bonding of the substrate to the carboxylate moiety of the lanthanide complex. These new reagents are therefore complementary to mixtures of lanthanide tris(β-diketonate) complexes with the ester forms of the resolving agents. © 1997 John Wiley & Sons, Ltd.