Chiral Catenanes and Rotaxanes: Fundamentals and Emerging Applications

Abstract

Molecular chirality provides a key challenge in host-guest recognition and other related chemical applications such as asymmetric catalysis. For a molecule to act as an efficient enantioselective receptor, it requires multi-point interactions between host and chiral guest, which may be achieved by an appropriate chiral 3D scaffold. As a consequence of their interlocked structure, catenanes and rotaxanes may present such a 3D scaffold, and can be chiral by inclusion of a classical chiral element and/or as a consequence of the mechanical bond. This Minireview presents illustrative examples of chiral [2]catenanes and [2]rotaxanes, and discusses where these molecules have been used in chemical applications such as chiral host-guest recognition and asymmetric catalysis.

Keywords: catalysis; catenanes; chirality; host-guest recognition; rotaxanes.

Figure 1

Schematic representations of: a) [n+2]catenane, and b) [n+2]rotaxane structures.

Figure 2

Schematic representation of possible stereoisomers of a homocircuit [2]catenane, where each ring contains a single stereogenic centre (the numbers in italics are to illustrate CIP priority assignment).

Figure 3

Stoddart's catenane 1 ⁴⁺(PF₆ ⁻)₄ containing a tetra‐pyridinium macrocycle with two stereogenic centres.

Figure 4

Diastereoselective synthesis of Stoddart's catenane 5 ⁴⁺(PF₆ ⁻)₄ containing two rings, both possessing axial chirality.

Figure 5

Stoddart's catenane 6 ⁴⁺(PF₆ ⁻)₄ containing two rings, both possessing planar chirality.

Figure 6

Vögtle's glucose stoppered rotaxanes 7 ⁴⁺(PF₆ ⁻)₄ and 8, prepared by: a) donor–acceptor and b) hydrogen‐bond templating.

Figure 7

Leigh's amino acid containing rotaxanes 9 that demonstrate solvent, temperature and chiral substituent dependent ICD.

Figure 8

Leigh's bistable molecular shuttle 10 that demonstrates chiroptical switching.

Figure 9

Schematic representations of enantiomers of [2]catenanes and [2]rotaxanes arising as a consequence of the mechanical bond: a) [2]catenane consisting of two directional rings; b) [2]catenane consisting of facially unsymmetric rings; c) [2]rotaxane consisting of directional ring and axle components and d) [2]rotaxanes consisting of a ring trapped on one side of what would be a prochiral centre of the non‐interlocked axle component.

Figure 10

Enantiomers of Sauvage's first topologically chiral catenane 11. The stereochemical labels R and S are assigned thus: in the direction of an arrow pointing from the highest priority atom (labelled 1) to its highest priority neighbour (labelled 2), interlocked rings that are disposed in a clockwise manner are R, those in an anticlockwise manner are S.

Figure 11

Enantiomers of Charbonnière and Tranolsi's catenane 12. The numbers and arrows are to illustrate how stereochemical labels are determined.

Figure 12

Puddephatt's catenanes, where chirality arises from facially unsymmetrical rings. The stereochemical labels R _ma and S _ma are assigned as one would assign R and S to allenes.

Figure 13

Diastereomers of Marinetti's facially unsymmetric mechanically chiral catenane 14. The numbers and arrows are to illustrate how the R _ma and S _ma stereochemical labels are determined.

Figure 14

Enantiomers of Vögtle's mechanically planar chiral [2]rotaxane 15. The stereochemical labels R _mp and S _mp are assigned thus: View the rotaxane along its axle from the highest priority atom in the axle (A1) to that atom's highest priority neighbour (A2). If in the macrocycle the highest priority neighbour (M2) is disposed clockwise from the highest priority atom (M1), then the stereochemical label is R _mp; if it is disposed anticlockwise then the label is S _mp.

Figure 15

Takata's catalytic asymmetric synthesis of a mechanically planar chiral rotaxane (only one rotaxane enantiomer depicted, note the configuration of the major enantiomer was not established).

Figure 16

Goldup's preparation of enantiopure mechanically planar chiral rotaxanes. The labels (A1, etc) and arrows are to illustrate how the R _mp and S _mp stereochemical labels are determined.

Figure 17

Leigh's conversion of achiral rotaxane 27 into rotaxane 29 that possesses point mechanical chirality. The asterisks mark the stereocentre in rotaxane 29, which may be assigned according to standard CIP rules.

Figure 18

Structure of one enantiomer of Kameta and Hiratani's mechanically planar chiral rotaxane 30, showing the tentative proposed mode of binding of phenylalaninol as suggested by the authors.

Figure 19

Niemeyers's catenane (S,S)‐31 host prepared from enantiopure axially chiral binaphthyl‐phosphoric acid units: a) structure of catenane, and b) ratios of association constants for the binding of bis‐HCl salts of enantiopure diamine guests by the tetrabutylammonium salt of the catenane in [D₆]‐DMSO.

Figure 20

Beer's chiral halogen bonding rotaxanes 32–34⁺PF₆ ⁻ capable of enantioselective anion binding a) structures of rotaxanes and b) structures of chiral anions investigated.

Figure 21

Takata's chiral rotaxanes for asymmetric benzoin condensations: a) example of one of the chiral rotaxanes studied, and b) example of benzoin condensation reaction where rotaxane (R)‐35 ⁺Cl⁻ acted as a catalyst.

Figure 22

Leigh's chiral rotaxane for asymmetric transition metal catalysis: a) structure of rotaxane (R,R)‐38, and b) example of nickel‐catalysed reaction where rotaxane (R,R)‐38 induces enantioselective bond formation.

Figure 23

Leigh's point mechanically chiral rotaxane for asymmetric organocatalysis: a) structure of rotaxane (S)‐42, and b) example of enamine reaction where rotaxane (S)‐42 induces enantioselective bond formation.

Figure 24

Leigh's switchable rotaxane for controlled asymmetric Michael additions: a) structure of rotaxane (R)‐46 ⁺PF₆ ⁻, and b) example of Michael addition asymmetrically catalysed by deprotonated rotaxane (R)‐46 ⁺PF₆ ⁻.

Figure 25

Example of transfer hydrogenation reaction catalyzed by catenane (S,S)‐31 investigated by Niemeyer and co‐workers.