Ultra‐fast One‐Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts

Abstract

We report an ultra‐fast helix induction and subsequent static helicity memory in poly(biphenylylacetylene) (PBPA‐A) assisted by a catalytic amount of nonracemic ammonium salts comprised of non‐coordinating tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BArF−) as a counter anion. The remarkable acceleration of the helix‐induction rate in PBPA‐A accompanied by the significant amplification of the asymmetry relies on the two methoxymethoxy groups of the biphenyl pendants, which can gain access to enfold the chiral ammoniums in a crown‐ether manner in specific aromatic solvents, leading to ultra‐fast helicity induction, which is completed within 30 s. In aromatic solvents, helicity memory is lost rapidly, but is quite stable in long‐chain hydrocarbons. The best use of specific solvents for helicity induction and static helicity memory, respectively, provides a highly sensitive chirality sensing system toward a small amount of chiral amines and amino acids when complexed with BArF−.

Synthesis and applications of helical polymers with dynamic and static memories of helicity

“…12 C), leading to the strongest “diluted MR” effect. 100 – 102 ) As anticipated, in aromatic solvents, the static memory of the induced helicity in 24a was lost rapidly but was quite stable in long-chain hydrocarbons, such as n -dodecane (Fig. 12 E).…”

“…12 D). 100 ) The chiral ammonium salts of different counter anions, such as those of bis(trifluoromethanesulfonyl)imide (NTf 2 − ) and tetraphenylborate (BPh 4 − ), induced no CD in 24a in toluene. A catalytic amount of nonracemic 53 (60% ee, 0.1 equiv) could also induce an almost full ICD in 24a in toluene (Fig.…”

“…Hence, the best use of aromatic and aliphatic solvents for helicity induction and its static memory of helicity, respectively, enabled highly sensitive chirality detection toward nonracemic amines and amino acids when complexed with BArF − that relied on the MOM groups of the PBPAs capable of enfolding ammoniums salts in a crown-ether manner. 100 )…”

“…12D). 100) The chiral ammonium salts of different counter anions, such as those of bis(trifluoromethanesulfonyl)imide (NTf 2…”

Recent advances of optically active helical polymers as adsorbents and chiral stationary phases for chiral resolution

“…As mentioned above, once the achiral polymers are induced to have a preferred‐handed helix in the addition of chiral reagents via an acid‐base interaction, the conformation keep unchanged even after adding the inversed configuration of the chiral inducers to make the reagents racemic, called “chirality memory” effect [133−135]. In 2020, Maeda et al.…”