Hierarchical communication of chirality for aromatic oligoamide sequences

Abstract

The communication of chirality at a molecular and supramolecular level is the fundamental feature capable of transmitting and amplifying chirality information. Yet, the limitation of one-step communication mode in many artificial systems has precluded the ability of further processing the chirality information. Here, we report the chirality communication of aromatic oligoamide sequences within the interpenetrated helicate architecture in a hierarchical manner, specifically, the communication is manipulated by three sequential steps: (i) coordination, (ii) concentration, and (iii) ion stimulus. Such approach enables the information to be implemented progressively and reversibly to different levels. Furthermore, the chiral information on the side chains can be accumulated and transferred to the helical backbones of the sequences, resulting in that one of ten possible diastereoisomers of the interpenetrated helicate is finally selected. The circular dichroism experiments with a mixture of chiral and achiral ligands demonstrate a cooperative behavior of these communications, leading to amplification of chiral information.

Fig. 1. Synthesis of the monomeric and dimeric helicates.

The reaction of the corresponding ligand (2 equiv.) with Pd(BF₄)₂ (1 equiv.) led to the formation of the mixtures of the monomeric and dimeric helicates.

Fig. 2. ¹H NMR (600 MHz) spectra of the helicates and the corresponding Cl⁻ complexes in CD₃CN (500 μL).

a Excerpts of ¹H NMR spectra of mixture of 1 and 2 (ca. [L] = 1.8 mM) at equilibrium. b 1 formed from coordination of L and Pd²⁺ at low concentration (ca. [L] = 0.2 mM). c 2 formed from coordination of L and Pd²⁺ at high concentration (ca. [L] = 8.0 mM). d Excerpts of ¹H NMR spectra of [2Cl⊂2]. e Excerpts of ¹H NMR spectra of 3 S and 4 S (ca. [L] = 1.8 mM) at equilibrium. f, Excerpts of ¹H NMR spectra of [2Cl⊂4 S]. The Cl⁻ complexes (d, f) were formed by addition of NBu₄Cl 0.5 mmol to the correspongding helicate solutions (a, e), respectively. Amide and aromatic signals of the monomeric helicates and of the dimeric helicates are marked with filled and empty circles, respectively.

Fig. 3. Schematic representation of the chirality dynamics and hierarchical communication of the helicates.

a A mixture of sequences with different twisting conformations. b The monomeric helicates with three conformations (PM, MM, and PP) in fast exchange on the NMR timescale. c Four isomers (PPPP, MMMM, PMMP, and MPPM) of the dimeric helicates exist due to the communication of the ligands. d Two conformations (PPPP and MMMM) of the dimeric helicates complexed with two Cl⁻ anions.

Fig. 4. Crystal structures of [2Cl⊂2].

Top (left) and side (right) views of the crystal structures of [2Cl⊂2]. Only the PPPP enantiomer is shown. The structures belong to the centrosymmetric space groups and thus also contain the MMMM enantiomer. The solvent molecules, anions outside of cavity, and disorders have been removed for clarity.

Fig. 5. Chiral transmission and amplification from side chains to backbones.

a Circular dichroism spectra in CH₃CN of 3 R and 4 R mixture ([3 R] + 2[4 R] = 1 × 10⁻⁴ M, blue dashed line), 3 S and 4 S mixture ([3 S] + 2[4 S] = 1 × 10⁻⁴ M, red dashed line), [2Cl⊂4 R] (1 × 10⁻⁴ M, blue solid line), and [2Cl⊂4 S] (1 × 10⁻⁴ M, red solid line). b Main figure, circular dichroism spectra in CH₃CN for mixtures of [2Cl⊂2] and [2Cl⊂4 S] at various molar ratios at equilibria (the total concentration is kept constant at 1 × 10⁻⁴ M). Inset, a plot of the normalized intensities at 331 nm versus the percentages of [2Cl⊂4 S]. c Main figure, circular dichroism spectra in CH₃CN for mixtures of 2 and 4 S at various molar ratios at equilibria (the total concentration is kept constant at 1 × 10⁻⁴ M). Inset, a plot of the normalized intensities at 359 nm versus the percentages of 4 S.