$C_2$-symmetric chiral diamine ligands for enantiomeric recognition of amino acid esters and mandelic acid by proton NMR titration method

Abstract

Two novel $C_2$ -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 $C_2$-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.