Macromolecular helicity induction and static helicity memory of poly(biphenylylacetylene)s bearing aromatic pendant groups and their use as chiral stationary phases for high‐performance liquid chromatography

Abstract

Two novel poly(biphenylylacetylene)s (PBPAs) bearing achiral alkylphenyl groups at the 4′‐position of the biphenyl pendant through ester linkers with different sequences were synthesized by the rhodium‐catalyzed polymerization of the corresponding monomers. The influence of the alkylphenyl pendants and the ester sequences on the macromolecular helicity induction and subsequent static helicity memory was investigated. In addition, the chiral recognition ability as chiral stationary phases for high‐performance liquid chromatography of the helicity‐memorized PBPAs was also examined. Both polymers formed almost perfect right‐ and left‐handed helical conformations through noncovalent chiral interactions with enantiomeric alcohols, and their induced macromolecular helicities were completely retained (“memorized”) after removal of the helix inducer. A PBPA bearing a 4‐n‐butylphenoxycarbonyl pendant group with a static helicity memory showed a remarkably high chiral recognition ability toward a wide variety of chiral aromatics, including simple point chiral compounds, axially chiral biaryls, a chiral spiro compound, helicenes, and planar chiral cyclophanes, particularly under the reversed‐phase conditions.

Chiral and Achiral Pendant-Bound Poly(biphenylylacetylene)s Bearing Amide and/or Carbamate Groups: One-Handed Helix Formations and Chiral Recognition Abilities

“…The synthesis of novel helical polymers with a one-handed helical conformation has been one of the attractive research topics [1][2][3][4][5][6][7][8][9][10][11][12][13] because of the potential for their practical applications to highperformance chiral materials as chiral stationary phases (CSPs) for chromatographic enantioseparation, 14-17 asymmetric catalysts, 18-21 and enantioselective sensing devices. [22][23][24] We previously reported a series of dynamically-racemic helical poly(biphenylylacetylene)s (PBPAs) bearing methoxymethoxy (MOM) groups at the 2,2'-positions along with various substituents at the 4'-position of the biphenyl pendants, 25 such as achiral 26,27 or chiral 28 aliphatic (e.g., poly-A-poly-D in Figure 1a), aromatic, 29,30 and oligo(ethylene glycol) groups, 31 through ether (-O-), 26,27,29 ester (-COO-and -OCO-), 27,28,30,31 and carbamate (-OCONH-) 27 linkers. The PBPAs are inherently optically-inactive, but excess one-handed helical and twisted conformations are induced in the polymer backbones and biphenyl pendants in a synchronized manner, respectively, through noncovalent interactions with chiral inducers, such as (R)-and (S)-1phenylethanol ((R)-and (S)-PEA).…”

“…The PBPAs are inherently optically-inactive, but excess one-handed helical and twisted conformations are induced in the polymer backbones and biphenyl pendants in a synchronized manner, respectively, through noncovalent interactions with chiral inducers, such as (R)-and (S)-1phenylethanol ((R)-and (S)-PEA). [25][26][27][28][29][30][31] The main-chain helicity and biphenyl axial chirality induced in the PBPAs can be completely retained ("memorized") after removal of the chiral inducers. [25][26][27][28][29][30][31] The functional substituents introduced at the 4'-position of the biphenyl units significantly affected the helicity induction kinetics, helical sense preference, and stability of the static helicity memory.…”

“…[25][26][27][28][29][30][31] The main-chain helicity and biphenyl axial chirality induced in the PBPAs can be completely retained ("memorized") after removal of the chiral inducers. [25][26][27][28][29][30][31] The functional substituents introduced at the 4'-position of the biphenyl units significantly affected the helicity induction kinetics, helical sense preference, and stability of the static helicity memory. 25,27,30 Taking advantage of this exclusively unique static helicity memory observed in the PBPAs, we have developed the first switchable CSPs, 26,27 which enable the elution orders of the enantiomers to be reversibly switched during high-performance liquid chromatography (HPLC) enantioseparation as well as highly-sensitive chirality sensors, 32,33 switchable asymmetric catalysts, 29,34 and circularly polarized luminescent materials.…”

Catalytic One-Handed Helix Induction and Subsequent Static Memory of Poly(biphenylylacetylene)s Assisted by a Small Amount of Carboxy Groups Introduced at the Pendants

“…Either a right ( P )- or left ( M )-handed main-chain helicity as well as the axial chirality of the biphenyl units can be induced in response to the chirality of optically active guests, such as ( R )- and ( S )-1-phenylethanol (( R )- and ( S )-PEA). 40 − 44 Both the macromolecular helicity and the axial chirality induced in the PBPAs are retained (“memorized”) after complete removal of the chiral inducers, resulting in the one-handed helical PBPAs with a static helicity memory. 40 − 44 Based on this “helicity induction and its static helicity memory” approach, we have succeeded in developing a series of unique helicity-memorized PBPA-based chiral materials, 43 such as switchable chiral stationary phases 40 , 45 and asymmetric organocatalysts, 46 , 47 capable of switching the elution orders of enantiomers in HPLC and the product chirality in asymmetric reactions, respectively.…”

“… 40 − 44 Both the macromolecular helicity and the axial chirality induced in the PBPAs are retained (“memorized”) after complete removal of the chiral inducers, resulting in the one-handed helical PBPAs with a static helicity memory. 40 − 44 Based on this “helicity induction and its static helicity memory” approach, we have succeeded in developing a series of unique helicity-memorized PBPA-based chiral materials, 43 such as switchable chiral stationary phases 40 , 45 and asymmetric organocatalysts, 46 , 47 capable of switching the elution orders of enantiomers in HPLC and the product chirality in asymmetric reactions, respectively. However, a large excess amount of homochiral alcohols, such as ( R )- or ( S )-PEA ([PEA]/[polymer] > 1000), was necessary for the one-handed helix induction and subsequent static memory of the helicity in PBPAs, probably due to the relatively weak noncovalent chiral interactions between the MOM groups of PBPAs (e.g., poly- 1a and poly- 1b ) and ( R )- or ( S )-PEA.…”