Competition between the donor and acceptor hydrogen bonds of the threads in the formation of [2] rotaxanes by clipping reaction

The synthesis of benzylic amide [2] rotaxanes using 1,2-bis(aminocarbonyl-(1'-tert-butyl-1H-pyrazole-[3'] 5'-yl))-ethanes as templates is reported. These templates are equipped with tert-butyl pyrazole-based stoppers and have donor (N-H) and acceptor (C=O) hydrogen bond groups. The synthesis of [2] rotaxanes has been shown to be highly dependent on the tert-butylpyrazole stoppers. While the thread with 10,30-disubstituted pyrazoles as stopper units was shown to be an excellent template for the synthesis of [2] rotaxanes, the thread with 1',5'-disubstituted pyrazoles as stopper units did not yield the expected [2] rotaxane. The structure of the synthesized [2] rotaxanes was characterized using NMR experiments and X-ray diffraction, with the latter showing that the macrocycle adopts a distorted chair conformation. An in-depth study of the isolated thread's X-ray structures and concentration-dependent NMR experiments seem to explain the dependence of the rotaxane formation on the substitution pattern of the thread's pyrazole stoppers.