Enantioselective liquid-liquid extraction of amino acid enantiomers using (S)-MeO-BIPHEP-metal complexes as chiral extractants

(S)-MeO-BIPHEP-metal complexes were firstly used as chiral extractants to separate amino acid enantiomers. Six amino acid enantiomers were employed to evaluate the stereoselectivities of (S)-MeO-BIPHEP-metal complexes. The results revealed that (S)-MeO-BIPHEP-Cu complex exhibited good abilities to separate phenylalanine (Phe), phenylglycine (Pheg), tyrosine (Tyr) and 3-chloro-phenylglycine (Cpheg) enantiomers with operational enantioselectivities (α) were 2.56, 2.24, 4.22 and 1.81, respectively. While (S)-MeO-BIPHEP-Pd complex exhibited good ability to separate 4-nitro-phenylalanine (Nphe) with operational enantioselectivity (α) was 3.87. All of (S)-MeO-BIPHEP-metal complexes showed poor abilities to separate homophenylalanine (Hphe) with α below 1.5. The influences of extraction temperature, concentrations of (S)-MeO-BIPHEP-metal complexes and pH of aqueous phase on extractions were systematically investigated. On the basis of single-factor experiments, the extraction conditions of separating Phe, Pheg, Nphe, Tyr and Cpheg were optimized by response surface methodology. After optimization, the maximum performance factors (pf) for Phe, Pheg, Nphe, Tyr and Cpheg were 0.11865, 0.13487, 0.17706, 0.21882 and 0.13579, respectively.