Mechanically Interlocked Chiral Self-Templated [2]Catenanes from 2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) Ligands

Abstract

We report the efficient self-templated formation of optically active 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) derived homocircuit [2]catenane enantiomers. This represents the first example of the enantiopure formation of chiral btp homocircuit [2]catenanes from starting materials consisting of a classical chiral element; X-ray diffraction crystallography enabled the structural characterization of the [2]catenane. The self-assembly reaction was monitored closely in solution facilitating the characterization of the pseudo-rotaxane reaction intermediate prior to mechanically interlocking the pre-organised system via ring-closing metathesis.

Keywords: chiral [2]catenanes; chiroptical; interlocked; pre-organisation; self-templated.

Scheme 1

Synthesis of chiral btp ligands 1R/S. (i) CuSO₄ ⋅ 5H₂O, ImSO₂N₃⋅H₂SO₄ and K₂CO₃ in CH₃OH, rt, 15 h; (ii) Sodium ascorbate, K₂CO₃, DMF/ tBuOH/ H₂O (5 : 5 : 3), rt, 18 h; (iii) NaOH_(aq), MeOH, rt, 17 h (iv) 1) Et₃N, DMAP, CH₂Cl₂/ DMF (4 : 1), 0 °C, 30 mins 2) EDCI⋅HCl, 0 °C, 30 mins, rt, 66 h. (v) Hoveyda‐Grubbs 2^nd Gen. Catalyst, DCM (anhydrous), rt, 7 days.

Figure 1

¹H NMR spectra of chiral btp olefin precursor 2(R,R) (A) and chiral btp [2]catenane 1(R,R) (B) (400 MHz, DMSO‐d₆ ) with resonances labelled.

Figure 2

Photophysical spectra of ligands olefin precursor 2R/S and [2]catenane 1R/S measured in CH₃CN at room temperature (c=1×10⁻⁵ M): (A) UV‐vis absorption spectra comparison of 2R/S and 1R/S; (B) CD spectra of 1R/S.

Figure 3

(A) Perspective view of the molecular structure of [2]catenane 1. Selected heteroatom labelling shown. Disordered contributor, selected hydrogen atoms and solvent molecules omitted for clarity. (B) Spacefill representation of [2]catenane 1. (C) Extended structure of [2]catenane 1 illustrating hydrogen bonding interactions between adjacent moieties.

Figure 4

¹H NMR spectra of the dimerisation of A) btp olefin (2S,S) with B) macrocycle (3S,S). C–D) Initial broadening of the proton resonance signals followed by a change in signal chemical shifts, resulting in a spectrum closely resembling that of E) [2]catenane (1S,S), indicated threading of (2S,S) through macrocycle 3S i.e. pseudo‐rotaxane formation. F–H) Addition of RCM catalyst demonstrated a ¹H spectrum which closely resembled that of the [2]catenane. (enlarged version can be seen in the Supporting Information).