Chemical On/Off Switching of Mechanically Planar Chirality and Chiral Anion Recognition in a [2]Rotaxane Molecular Shuttle

Abstract

We exploit a reversible acid-base triggered molecular shuttling process to switch an appropriately designed rotaxane between prochiral and mechanically planar chiral forms. The mechanically planar enantiomers and their interconversion, arising from ring shuttling, have been characterized by NMR spectroscopy. We also show that the supramolecular interaction of the positively charged rotaxane with optically active anions causes an imbalance in the population of the two enantiomeric coconformations. This result represents an unprecedented example of chiral molecular recognition and can disclose innovative approaches to enantioselective sensing and catalysis.

Figure 1

Schematic representation of a C_∞v symmetric axle and a C_s symmetric ring (a); the two enantiomers of a mechanically planar (MP) chiral rotaxane (b); the two enantiomers of a coconformationally MP chiral rotaxane and their interconversion by ring shuttling through an achiral coconformation that features a mirror plane (c).

Scheme 1. Rotaxanes 1H³⁺ and 2H³⁺ (top), and Their Base-Triggered Switching to 1²⁺ and 2²⁺ (bottom)

The latter species can exist in two interconverting co-conformations, which constitute an enantiomeric pair for 1²⁺ (see ref (6) for the assignment of the absolute configurations) while they are the same molecule in the case of 2²⁺. The starting rotaxanes are regenerated upon addition of an acid.

Figure 2

(a) Variable temperature (VT) ¹H NMR spectra (500 MHz, CD₂Cl₂) of 2²⁺ in the region of the triazolium protons (H_Tr^u, H_Tr^c). (b) VT ¹H NMR spectra (500 MHz, CD₂Cl₂) of 1²⁺ in the regions of the triazolium protons (H_Tr^u, H_Tr^c; left) and of the methylene protons in the pyrenyl tether of the macrocycle, adjacent to the dioxybenzene unit (H_Py^a, H_Py^b; right). See Scheme 1 and SI for proton labeling.

Figure 3

Absorption and fluorescence (inset, λ_exc = 328 nm) spectra of the free macrocycle (black), 1H³⁺ (blue) and 1²⁺ (red). Air equilibrated CH₂Cl₂, 20 °C.

Figure 4

(a) Interconversion between two diastereomeric ion pairs composed of a coconformationally MP chiral rotaxane dication, such as 1²⁺, and a chiral monoanion. In the proposed structures, one anion is coordinated to the unencircled triazolium, while another is weakly paired with the encircled site. Simplified potential energy curves for the location of the ring along the axle are also shown. As the two ion pairs can have different stabilities [ΔΔG° ≠ 0], the ring distribution between the two identical stations can become unbalanced. (b) Partial ¹H NMR spectra (500 MHz) of the H_Tr^uresonance in 1²⁺ after the addition of 8 equiv of the tetrabutylammonium salt of (1S)-(+)-10-camphorsulfonate (CD₂Cl₂, 223 K; left) or Δ-TRISPHAT (toluene-d₈, 243 K; right). Black, red, and gray traces show respectively the experimental spectrum, the deconvoluted peaks, and the fitting residuals.