A New Synthetic Strategy towards Molecules with Mechanical Bonds: Nonionic Template Synthesis of Amide-Linked Catenanes and Rotaxanes

After Sauvage et al. and Stoddart et al. had developed the now commonly used and well-established catenane and rotaxane synthesis utilizing phenanthroline/Cu+ and bipyridinium/crown units, a new way to synthesize mechanical bonds was discovered in 1992—the synthesis of amide-linked catenanes and rotaxanes around a neutral template. The formation of the template, which leads to the host–guest bond, does not proceed by covalent or ionic interaction, but by weak supramolecular interactions, such as hydrogen bonding, π-stacking, π-donor–π-acceptor interaction, and steric complementarity. The simple synthetic units (arene dicarboxylic acid dichlorides and diamines) can be varied in an astonishing number of ways, few steps are required to obtain the target molecules, and the yields of these syntheses approach those of ordinary organic syntheses. After the preparation of [2]catenanes, it took only a few years to prepare [2]-, [3]-, and bis[2]-rotaxanes of the amide type. The introduction of sulfonamide groups enabled alkylation of the sulfonamide nitrogen atom, and this made chemical reactions with catenanes and rotaxanes feasible. Intra- and intermolecular connections of catenanes and rotaxanes were then possible, and a molecule with the topology of a “pretzel” has recently been prepared. This allowed new insights into the molecular recognition processes of neutral guest and host substances, as well as further understanding of these processes in template synthesis, isomerism in catenanes, and the regioselectivity of the formation of catenane isomers. Competitive binding studies of guest molecules in the cavity of concave receptors with more than one binding site can be perfomed. These studies might result in higher synthetic yields of molecules with mechanical bonds and lead to further progress in the development of supramolecular nanostructures and molecular switches.