Selective access to constitutionally identical, orientationally isomeric calixarene-based rotaxanes by an active template approach†
Tris(phenylureido)calixarene is endowed with unique properties that make it a valuable macrocyclic component for the synthesis of mechanically interlocked molecules. Its three-dimensional and intrinsically nonsymmetric structure is kinetically selective toward two processes: (i) in apolar media, the threading of bipyridinium based axle-like components takes place exclusively from the upper rim; (ii) SN2 alkylation reactions of a pyridylpyridinium precursor engulfed in the cavity occur selectively at pyridylpyridinium nitrogen atom located at the macrocycle upper rim (active template synthesis). Here we exploit such properties to prepare two series of rotaxanes, each consisting of three sequence isomers that arise from the threading of two identical but nonsymmetric wheels on a symmetric thread differing only for the reciprocal orientation of the macrocycles. The features of the calixarene and the active template synthetic approach, together with a careful selection of the precursors, enabled us to selectively synthesise the rotaxane sequence isomers of each series with fast kinetics and high yields.