Chiral and Achiral Pendant-Bound Poly(biphenylylacetylene)s Bearing Amide and/or Carbamate Groups: One-Handed Helix Formations and Chiral Recognition Abilities
Tomoyuki Ikai, Shogo Okuda, Motoki Aizawa, Eiji Yashima
Abstract
A series of cis-poly(biphenylylacetylene) (PBPA) derivatives bearing chiral and achiral pendant groups at the 4′-position of the biphenyl units through an amide (−NHCO−) or carbamate (−NHCOO−) linker were synthesized by polymerization of the corresponding biphenylylacetylene (BPA) monomers that can be readily prepared in one step from a novel amino-functionalized BPA. An excess one-handed helix induction in the PBPAs through covalent and noncovalent chiral interactions and their chiral recognition abilities when used as chiral stationary phases for high-performance liquid chromatography were investigated. PBPAs bearing optically pure l-amino acid residues showed unique two-state helical conformational changes between the extended and contracted helices regulated by the solvent-mediated on/off switching of the intramolecular hydrogen-bonding formations between the pendants or at each pendant. The chiral recognition abilities of the helical PBPAs were significantly influenced by the kinds of the pendant l-amino acid residues. The preferred-handed contracted helical PBPA carrying an l-leucine-derived pendant showed an excellent chiral resolving power toward various racemic compounds including axially and point chiral compounds and chiral metal complexes. The elution orders of some racemates were completely reversed when its helical conformation was changed to the extended helix. On the other hand, the trans-enriched nonhelical l-leucine-bound PBPA derived from its preferred-handed cis-helical PBPA and achiral pendant-bound cis-helical PBPAs induced by noncovalent chiral interactions and subsequent static memory of the helicity showed a poor and no chiral recognition, respectively.