Post-Synthetic Macrocyclization of Rotaxane Building Blocks

Pretzelanes, [1]rotaxanes and molecular figures-of-eight - crossing the bridge from fundamentals to functional applications

There are myriad [2]catenanes and [2]rotaxanes that consist of two interlocked molecular components. On occasion, supramolecular chemists prepare interlocked molecules where there are covalent linkages between the interlocked molecular components. In this review, progress on pretzelanes ([1]catenanes), [1]rotaxanes and molecular figures-of-eight is surveyed. Particular attention is paid to the application of such molecules, especially where the interlocked structure and/or the covalent linkage(s) play a key functional role.

Rotaxane Formation from Borate Ion-Containing Crown Ether and Ammonium Ion: Enhancement of Their Association through Ion-Pairing

We synthesized [2]rotaxanes featuring a catechol borate ion-containing crown ether and secondary ammonium ions. These rotaxane components show both ion-ion interactions and hydrogen bonds. X-ray crystallography and NMR spectroscopy allowed elucidation of the rotaxane structure. Moreover, 1H NMR spectroscopy revealed the rotaxane synthesis can be thermodynamically controlled. The binding affinity between the borate-containing crown ether and ammonium ions is enhanced by ion-pairing.

Conjugated bis(enaminones) as effective templates for rotaxane assembly and their post-synthetic modifications

The development of efficient methods for the synthesis of mechanically interlocked compounds is currently considered a major challenge in supramolecular chemistry. Twofold vinylogous fumaramides, a class of conjugated bis(enaminones), successfully achieve the assembly of hydrogen-bonded amide-based rotaxanes, with a templating ability comparable to that of their parent fumaramide-based systems, showcasing full conversions and impressive yields up to 92%. Computational calculations offer a compelling explanation for the remarkable efficiency of these bis(enaminones) in driving the synthesis of unprecedented rotaxanes. The reactivity of these interlocked species was thoroughly investigated, revealing that a one-step double stopper-exchange process can be successfully performed while preserving the mechanical bond. This approach facilitates the formation of controllable rotaxanes, including a three-station molecular shuttle, whose assembly via a clipping methodology is highly unselective. The internal translational motion of this latter species has been successfully controlled in a reversible way by means of a cycloaddition/retrocycloaddition sequence.

Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.