Synthesis of Stable and Soluble One-Handed Helical Homopoly(substituted acetylene)s without the Coexistence of Any Other Chiral Moieties via Two-Step Polymer Reactions in Membrane State: Molecular Design of the Starting Monomer

Yunosuke Abe,Masahiro Teraguchi,Yu Zang,Hongge Jia,Takashi Kaneko,Toshiki Aoki,Lijia Liu,Takeshi Namikoshi,Shingo Hadano,Yuriko Kakihana

doi:10.3390/molecules17010433

Yunosuke Abe, Masahiro Teraguchi + Show 8 more

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https://doi.org/10.3390/molecules17010433

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Abstract

A soluble and stable one-handed helical poly(substituted phenylacetylene) without the coexistence of any other chiral moieties was successfully synthesized by asymmetric-induced polymerization of a chiral monomer followed by two-step polymer reactions in membrane state: (1) removing the chiral groups (desubstitution); and (2) introduction of achiral long alkyl groups at the same position as the desubstitution to enhance the solubility of the resulting one-handed helical polymer (resubstitution). The starting chiral monomer should have four characteristic substituents: (i) a chiral group bonded to an easily hydrolyzed spacer group; (ii) two hydroxyl groups; (iii) a long rigid hydrophobic spacer between the chiral group and the polymerizing group; (iv) a long achiral group near the chiral group. As spacer group a carbonate ester was selected. The two hydroxyl groups formed intramolecular hydrogen bonds stabilizing a one-handed helical structure in solution before and after the two-step polymer reactions in membrane state. The rigid long hydrophobic spacer, a phenylethynylphenyl group, enhanced the solubility of the starting polymer, and realized effective chiral induction from the chiral side groups to the main chain in the asymmetric-induced polymerization. The long alkyl group near the chiral group avoided shrinkage of the membrane and kept the reactivity of resubstitution in membrane state after removing the chiral groups. The g value (g = ([θ]/3,300)/ε) for the CD signal assigned to the main chain in the obtained final polymer was almost the same as that of the starting polymer in spite of the absence of any other chiral moieties. Moreover, since the one-handed helical structure was maintained by the intramolecular hydrogen bonds in a solution, direct observation of the one-handed helicity of the final homopolymer has been realized in CD for the solution for the first time.

Highlights

Conjugated polymers like polyacetylenes have aroused interest because of their unique properties such as optical resolution and enantioselective recognition [1–3]
We found the efficiency in the asymmetric induction to the main chain in this two-step method (AIP-reaction in membrane state (RIM)) was better than that of helix-sense-selective polymerization (HSSP)
In spite of the long distance of the spacer (23 Å), effective chiral induction from the chiral group to the main chain was observed in Asymmetric-Induced Polymerization (AIP) owing to rigidity of the spacer [31,33]

Summary

Introduction

Conjugated polymers like polyacetylenes have aroused interest because of their unique properties such as optical resolution and enantioselective recognition [1–3]. We have successfully synthesized soluble chiral copoly(substituted phenylacetylene)s whose chiral structures are only in one-handed helical conformations by asymmetric-induced copolymerization of a chiral monomer with an achiral comonomer having a hydrophobic group(AIP). Effective chiral induction to the backbone from the chiral groups by the rigidity; (4) To enhance reactivity for resubstitution by enhancing the reaction space This method consisted of AIP of 1 and two-step polymer reactions (desubstitution and resubstitution) in membrane state (RIM): (1) removing the chiral groups while maintaining the one-handed helical backbone (desubstitution); and (2) introduction of achiral long alkyl groups at the same position to enhance solubility of the resulting polymer while maintaining the one-handed helical backbone (resubstitution). KOH/CH3OH/THF in membrane state (RIM); c. lauroyl chloride in membrane state (RIM)

Results and Discussion

General

Conclusions