Effect of pendant stereostructure on backbone conformation and enantioseparation ability of helical polyacetylene-based chiral stationary phases

Six proline-derived acetylene monomers bearing either two stereocenters (S-mR, S-mS, R-mS, Rac-mS and S-mRac) or one stereocenter (S-mBn) were obtained from commercially available N-(tert-butoxycarbonyl)-prolinal. Under the catalysis of Rh-diene complex, they were converted to the corresponding optically active helical polymers, S-pR, S-pS, R-pS, Rac-pS, S-pRac, and S-pBn. The correlations between configuration and position of stereocenters in pendants with the polymer conformation as well as chiral resolution performance were systematically explored by a combination of nuclear magnetic resonance (NMR), Raman, UV-Vis absorption, electronic/vibration circular dichroism spectroscopies, high-performance liquid chromatography (HPLC), and computational simulation. The configuration of the stereocenter adjacent to polymer mainchain determined the sense of helical conformation and the elution order of analytes, while that of the remote one affected the arrangement of pendants and the scope of analytes that could be discriminated. Among 18 aromatic analytes selected, S-pR could discriminate 10, while S-pS recognized 12. The racemization of adjacent or remote stereocenters greatly reduced the scope of analytes that could be resolved. Based on computer simulations, S-pS had larger recognition space than S-pR, favoring the steric fit with the racemates containing axial chirality. The strength and number of intermolecular hydrogen bondings between enantiomers and CSPs predominantly determined the chiral discrimination.