Enantioseparation on helical poly(diphenylacetylene)s bearing optically-active pendants: Effects of differences in higher-order structures of kinetically-trapped and thermodynamically-stable states on chiral recognition ability

Abstract

An optically-active poly(diphenylacetylene) (PDPA) bearing carboxy pendant groups with left-handed helicity memory (M-h-poly-1), synthesized using the noncovalent helicity-induction-and-memory strategy, was converted into a PDPA bearing optically-active pendant groups through an amide bonding (M-hKT-poly-2S), while maintaining the left-handed helicity memory, by reaction with (S)-1-phenylethylamine ((S)-2) using a condensing reagent at room temperature. Its chiral recognition ability was investigated as a chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC). M-hKT-poly-2S exhibited significantly different chiral recognition ability towards racemates compared to the previously reported corresponding helical PDPA bearing the same optically-active pendant groups (M-hTS-poly-2S) (prepared by the reaction of an optically-inactive PDPA bearing carboxy pendants with (S)-2, followed by thermal annealing, to induce a left-handed helical structure in the polymer main chain). Although the main chains of both M-hKT-poly-2S and M-hTS-poly-2S formed almost completely left-handed helical structures, their higher-order structures varied slightly, as confirmed by various spectroscopic methods (UV-Vis, circular dichroism (CD), IR, and vibrational CD). M-hKT-TS-poly-2S, the PDPA formed on the thermal annealing of M-hKT-poly-2S, exhibited the same higher-order structure and chiral discrimination ability as M-hTS-poly-2S. Therefore, slight differences in the higher-order structures of the kinetically-trapped metastable state (M-hKT-poly-2S) and the thermodynamically-stable state (M-hTS-poly-2S), due to differences in synthetic procedures, significantly impact their chiral recognition abilities as CSPs, even with identical primary structures and helix-sense of the polymer main chain.