Chiral Separation of Underivatized Amino Acids by Reactive Extraction with Palladium−BINAP Complexes

Abstract

In answer to the need for a more economic technology for the separation of racemates, a novel system for reactive enantioselective liquid-liquid extraction (ELLE) is introduced. Palladium (S)-BINAP complexes are employed as hosts in the separation of underivatized amino acids. The system shows the highest selectivity for the ELLE of tryptophan with metal complexes as hosts reported to date and shows a good selectivity toward a range of natural and unnatural amino acids. Furthermore, the host can be prepared in situ from commerically available compounds. Bulk-membrane transport in the form of U-tube experiments demonstrates the enantioselective and catalytic nature of the transport. The dependency of the system on parameters such as pH, organic solvent, and host-substrate ratio has been established. (31)P NMR spectroscopy has been used to confirm the preferred enantiomer in the extraction experiments. The intrinsic selectivity was deduced by determination of the association constants of the palladium complex with the tryptophan enantiomers.