Stereoselective construction of coconformational mechanically helical and topologically chiral [2]catenanes induced by point chirality

Abstract

Supported by chiral stationary phase high-performance liquid chromatography HPLC (CSP-HPLC), examples of chiral mechanically interlocked organic molecules, including knots, rotaxanes, and catenanes, have been reported. However, the exploration of stereoselective construction of chiral cationic complexes, particularly those induced by point chirality, has been notably limited due to the constraints posed by the type of chiral chromatographic columns and separation efficiency. To address this, we have developed a construction strategy for generating coconformational mechanically helical and topologically chiral [2]catenanes through the induction of point chirality. In this study, by adjusting the symmetry of the ligand, we have easily realized the efficient construction of high-yield crystalline coconformational mechanically helical and topologically chiral [2]catenanes. Moreover, within the enantiomerically pure chiral environment of molecular self-assembly driven by L-alanine and L-valine residues in bidentate ligands, the coconformational mechanically helical and topologically chiral [2]catenanes exist exclusively as a single enantiomer, thus eliminating the need for laborious CSP-HPLC separation from racemic mixtures. The generation of the opposite enantiomer can be realized by employing unsymmetrical ligands containing corresponding D-alanine and D-valine residues, as confirmed through single-crystal X-ray diffraction, elemental analysis, electrospray-ionization time-of-flight mass spectrometry, solution-state NMR spectroscopy, and circular dichroism spectroscopy.

Keywords: catenanes; coconformational mechanically helical chirality; half-sandwich rhodium; point chirality; topological chirality.

Fig. 1.

Construction strategy of coconformational mechanically helical and topologically chiral [2]catenanes.

Fig. 2.

Binuclear half-sandwich organometallic units Rh-B and semienclosed ligands L1–L4 are mentioned in this work.

Fig. 3.

Stereoselective construction of coconformational mechanically helical chiral [2]catenanes Rh-1R, Rh-1S, Rh-2R, and Rh-2S.

Fig. 4.

(A) Chemical structure of coconformational mechanically helical chiral [2]catenanes Rh-1R based on the initially reduced structure. (B) Single-crystal X-ray crystal structure of Rh-1R. (C) ¹H NMR spectrum of Rh-1R. (D) ESI–TOF/MS spectra of Rh-1R. (E) The C − H···O hydrogen bonding interactions present in Rh-1R. Color code: C, gray; H, apple green; N, blue; Rh, turquoise. (F) CD spectra of the topologically chiral [2]catenanes Rh-1R/S in methanol solution at room temperature.

Fig. 5.

(A) The coconformational mechanically helical chirality in [2]catenanes in which coconformational isomerism is sterically prohibited. (B) The topological chirality in [2]catenanes. Note: the direction of the arrow is determined by the atomic level, according to the CIP rule, the direction is that the highest-priority atom (blue ball) rotates to the sub-high-priority atom (red ball).

Fig. 6.

Stereoselective construction of topologically chiral [2]catenanes Rh-3R_mt, Rh-3S_mt, Rh-4R_mt, and Rh-4S_mt.

Fig. 7.

(A) Single-crystal X-ray crystal structure of the topologically chiral [2]catenanes Rh-3R_mt/S_mt. Counter anions and solvent molecules are omitted for clarity. Color code: C, gray; H, apple green; N, blue; Rh, turquoise. (B) Dimensions of Rh-3R_mt fitted by D (2.62 × 10⁻⁶ cm² s⁻¹) according to the prolate spheroidal model. (C) CD spectra of the topologically chiral [2]catenanes Rh-3R_mt/S_mt in methanol solution at room temperature.