Abstract
Catenanes—molecules comprising two interlocking rings held together like links in a chain—are topologically non-trivial: a catenane is a topological isomer of its separated rings, but the rings cannot be disconnected without bond scission. Catenanes can exist as topological enantiomers if both rings have directionality conferred by a defined atom sequence, but this has led to the assumption that the stereochemistry of chiral catenanes composed of oriented rings is inherently topological in nature. Here we show that this assumption is incorrect by synthesizing an example that contains the same fundamental stereogenic unit but whose stereochemistry is Euclidean. One ring in this chiral catenane is oriented by the geometry of an exocyclic double rather than determined by atom sequence within the ring. Isomerization of the exocyclic double bond results in racemization of the catenane, confirming that the stereochemistry is not topological in nature. Thus, we can unite the stereochemistry of catenanes with that of their topologically trivial cousins, the rotaxanes, enabling a more unified approach to their discussion.

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Data availability
Raw characterization data are available through the University of Southampton data repository (https://doi.org/10.5258/SOTON/D2492)40. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2207578 ((S,Smp,Eco-c)-4) and CCDC 2207579 (S34). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.
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Acknowledgements
S.M.G thanks the European Research Council (Consolidator Grant, agreement no. 724987) and the Royal Society for a Research Fellowship (RSWF\FT\180010). E.M.G.J. thanks the EPSRC and University of Southampton for a Doctoral Prize Fellowship.
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S.M.G. secured project funding. S.M.G. and E.M.G.J. conceived the study. F.R. carried out the initial synthesis of catenane 3 and characterized the intermediates leading to this structure. N.P. optimized the synthesis of catenane 3 and its conversion to catenane 5. N.P. and D.L.M. completed the synthesis of the final compounds and their characterization. N.P. performed the isomerization studies of catenane 5, including the synthesis and analysis of model compounds. N.P. obtained single crystals of catenane 4 and model rotaxane S34 for X-ray analysis, which was performed by G.J.T. N.P. led the preparation of the Supplementary Information, including the stereochemical analysis of all interlocked products. S.M.G. wrote the manuscript. All authors contributed to the reviewing and editing of the manuscript and Supplementary Information.
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Extended data
Extended Data Fig. 1 Annotated representations of catenane 3 for assigning the absolute stereochemistry of the molecule.
a. The structure of catenane (S,Rmp,Eco-c)-3 produced from the reaction of (Z)-1 and (S)-2. The triazole C atom in bold is higher priority for the purpose of defining the co-conformational geometry. b. The (S,Rmp,Zco-c)-3 co-conformational covalent geometric isomer of 3. The triazole C atom in bold is higher priority for the purpose of defining the co-conformational geometry in this structure. c. The components of 3 with atoms A and B labelled. The triazole C atom in bold is higher priority for the purpose of defining the orientation of the triazole containing macrocycle (this is a fixed property of the covalent structure, as opposed to the bold atoms in a. and b. which depend on the position of the bipyridine ring). d. Catenane 3 redrawn such that the A→B vector of the bipyridine macrocycle passes through the triazole macrocycle away from the observer, confirming that the stereochemistry of 3 produced from (S)-2 is Rmp.
Extended Data Fig. 2 Annotated representations of catenane 4 for assigning the absolute stereochemistry of the molecule.
a. The structure of catenane (S,Smp,Eco-c)-4 produced from (S)-2. b. The bipyridine macrocycle with atoms A and B labelled (A and B atoms of the triazole macrocycle as in Extended Data Fig. 1c). c. Catenane 4 with the A→B vector of the bipyridine ring passing through the triazole macrocycle away from the observer, confirming that the stereochemistry of 4 produced from (S)-2 is Smp.
Extended Data Fig. 3 Annotated representations of catenane S16 for assigning the absolute stereochemistry of the molecule.
a. The structure of catenane (Smp,Eco-c)-S16 produced from (S)-2. b. The bipyridine macrocycle of S16 with atoms A and B labelled (A and B atoms of the triazole macrocycle as in Extended Data Fig. 1c). c. Catenane S16 with the A→B vector of the bipyridine macrocycle passing through the triazole macrocycle away from the observer, confirming that the stereochemistry of S16 produced from (S)-2 is Smp.
Extended Data Fig. 4 Annotated representations of catenane 5 for assigning the absolute stereochemistry of the molecule.
a. The structure of catenane (Rmp,Eco-c)-5 produced from (S)-2 as depicted in the manuscript. b. The triazole-containing macrocycle with atoms A and B labelled (A and B atoms of the bipyridine macrocycle as in Extended Data Fig. 3b). c. Catenane 5 with the A→B vector of the bipyridine macrocycle passing through the triazole macrocycle away from the observer, confirming that the stereochemistry of 5 produced from (S)-2 is Rmp.
Supplementary information
Supplementary Information
Experimental procedures and analytical data for all compounds. Elaborated discussions of manuscript content.
Supplementary Data 1
Crystallographic data for catenane 4 (CCDC 2207578>).
Supplementary Data 2
Crystallographic data for rotaxane S34 (CCDC 2207579).
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Pairault, N., Rizzi, F., Lozano, D. et al. A catenane that is topologically achiral despite being composed of oriented rings. Nat. Chem. 15, 781–786 (2023). https://doi.org/10.1038/s41557-023-01194-1
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DOI: https://doi.org/10.1038/s41557-023-01194-1