Dynamic Chirality: Keen Selection in the Face of Stereochemical Diversity in Mechanically Bonded Compounds

• Published in: Chemistry : a European journal Vol. 9; no. 2; pp. 543 - 556
• Main Authors: Tseng, Hsian-Rong, Vignon, Scott A., Celestre, Paul C., Stoddart, J. Fraser, White, Andrew J. P., Williams, David J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 20.01.2003; WILEY‐VCH Verlag; Wiley
• Subjects: Chemistry; Chemistry, Multidisciplinary; chirality; NMR spectroscopy; Physical Sciences; Science & Technology; stereochemistry; structure elucidation; supramolecular chemistry; ORGANIC-CHEMISTRY; chirality; HYDROGEN-BOND; AROMATIC INTERACTIONS; MOLECULAR MECCANO; STACKING INTERACTIONS; SOLID-STATE; CATENANES; TETRATHIAFULVALENE UNIT; stereochemistry; structure elucidation; NMR spectroscopy; CHARGE-TRANSFER; supramolecular chemistry; PI-PI INTERACTIONS
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200390057

The template‐directed syntheses, employing bisparaphenylene‐[34]crown‐10 (BPP34C10), 1,5‐dinaphthoparaphenylene‐[36]crown‐10 (1/5NPPP36C10), and 1,5‐dinaphtho‐[38]crown‐10 (1/5DNP38C10) as templates, of three [2]catenanes, whereby one of the two bipyridinium units in cyclobis(paraquat‐p‐phenylene) is replaced by a bipicolinium unit, are described. The crude reaction mixtures comprising the [2]catenanes all contain slightly more of the homologous [3]catenanes, wherein a “dimeric” octacationic cyclophane has the crown ether macrocycles encircling the alternating bipyridinium units with the bipicolinium units completely unfettered. X‐ray crystallography, performed on all three [2]catenanes and two of the three [3]catenanes reveals co‐conformational and stereochemical preferences that are stark and pronounced. Both the [3]catenanes crystallize as mixtures of diastereoisomers on account of the axial chirality associated with the picolinium units in the solid state. Dynamic 1H NMR spectroscopy is employed to probe in solution the relative energy barriers for rotations by the phenylene and pyridinium rings in the tetracationic cyclophane component of the [2]catenanes. Where there are co‐conformational changes that are stereochemically “allowed”, crown ether circumrotation and rocking processes are also investigated for the relative rates of their occurrence. The outcome is one whereby the three [2]catenanes containing BPP34C10, 1/5NPPP36C10, and 1/5DNP38C10 exist as one major enantiomeric pair of diastereoisomers amongst two, four, and eight diastereoisomeric pairs of enantiomers, respectively. The diastereoisomerism is a consequence of the presence of axial chirality together with helical and/or planar chirality in the same interlocked molecule. These [2]catenanes constitute a rich reserve of new stereochemical types that might be tapped for their switching and mechanical properties. The dynamic stereochemistry played out by a troupe of donor/acceptor catenanes earns them the right of entry into that hall of fame we associate with a dynamic combinatorial library. On account of the rare collection of chiral elements, including axes, planes, and helices, they house, these exotic molecules exhibit diastereoisomerism, as well as enantiomerism, on a grand scale (see scheme). In the event, it is their natural instinct for survival in essentially one of the numerous possible habitats that make them such a fascinating bunch of molecules to know.