Thirdly, we were extremely surprised to observe that rotaxane 6 (in all solvents examined) and macrocycle 1 (in CDCl3) deliver 12 in much higher selectivity than a simple Brønsted base such as NEt3, albeit less selectively than rotaxanes 4. These results suggest that the protonated bipyridine unit may operate as a -acidic surface in this context, with the lower activity of 1 attributed to the lower basicity of the non-interlocked macrocycle whereas, conversely, the similar activity of 4b and 6 can be attributed to the similarity of their proton binding pocket. This proposal is consistent with previous observations of anion-π interactions involving protonated or metal coordinated pyridine units in the solid state and reports of alkylated pyridinium-based anion-π catalysts. 10a