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

ABSTRACT 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.