These results are surprising for several reasons. Firstly, it is striking that 4b displays selectivities equivalent or higher than those previously reported for simple mono-NDI-based catalysts (Table ). Indeed, the reported example that is most comparable in selectivity to 4b (12 / 10 = 4.4, THF, 20 °C, entry 7) includes a sulfone-substituted NDI unit, which is proposed to enhance the πacidity of the system and so selectivity for 12. 9e Secondly, it is not immediately obvious why rotaxanes 4a and 4c are inactive in THF, whereas 4b performs reasonably well. Given that 4a and 4c produce 12 with good selectivity in CDCl3, albeit with lower activity and slightly reduced selectivity compared with 4b, we considered that the differences observed may relate primarily to differences in their Brønsted basicity. In keeping with this, when a mixture of rotaxanes 4 was titrated with MsOH in CDCl3 and the changes observed by 1 H NMR, selective protonation of 4b was observed (Figure ).