Metal-Free Active Template: A Straightforward Route to 4-Aminopyridinium-Based Rotaxane Molecular Shuttles

We report on the metal-free active template synthesis of [2]rotaxanes through nucleophilic aromatic substitution between an amine and halogenopyridinium derivatives in the presence of a crown ether as the reaction promoter. The influence of the leaving group and counter-ion of the pyridinium moiety was studied. Optimal conditions gave conversion of macrocycle to rotaxane up to 90% with rotaxane:axle ratio as great as 96:4. The ability of the 24-crown-8 and dibenzo-24-crown-8-containing [2]rotaxanes to behave as molecular shuttles was also considered. Mesomeric effects in the original 4-aminopyridinium moiety revealed two discrete, though conjugated, sites of interaction for crown ethers, which allowed for small-amplitude shuttling of the macrocycle. Unexpectedly, crown ether macrocycles mainly interacted around the amino site of the 4-aminopyridinium conjugated system, highlighting the more preponderant iminium mesomeric form. Sole deprotonation of the amino group led to a neutral ylide-kind dipolar [2]rotaxane, in which the crown ether freely shuttled along the entire axle. However, deprotonation-then-carbamoylation of the amino group triggered the small-amplitude shuttling of the macrocycle around the sole positively charged pyridinium site of the 4-aminopyridinium moiety.