[2]Rotaxane with multiple functional groups

Abstract

High-yield syntheses of Cu(II)- and Ni(II)-templated [2]pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis(azide)bis(amide)-2,2′-bipyridine axle into a bis(amide)tris(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π–π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu(I)-catalyzed azide–alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]rotaxane. Furthermore, a variable-temperature ¹H NMR study in DMSO-d₆ of [2]rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle.