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-1 H -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&z.dbd;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 1′,3′-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. The steric effect of the t -butyl group of 1′5′-thread prevents the formation of [2]rotaxane. On the other hand, the 1′3′-thread acts a template to obtain [2]rotaxanes.