C2-Symmetric chiral diamine ligands for enantiomeric recognition of amino acid esters and mandelic acid by proton NMR titration method

Two novel C2-symmetric chiral diamines containing alpha -phenylethyl and alpha -(1-naphthyl)ethyl chiral subunits were prepared with quantitative yields. Enantiomeric recognition properties of these simple structured diamine ligands towards D- and L-amino acid esters and D- and L-mandelic acid were examined by the ^1H NMR titration method. These ligands exhibited strong complexation (with K_f up to 2481 M^{-1}) and good enantioselectivity (up to K_L/K_D = 4.08) towards the mandelic acid enantiomers. The results show that simple structured and easily accessible acyclic C2-symmetrical compounds can also be used for enantiomeric recognition of racemic amino acids and mandelic acid in addition to complex molecules such as crown ethers and other cyclic molecules.