When rotaxane 13 was applied in the reaction of 7 with 11, extremely high levels of activity and selectivity were observed, even compared with rotaxane 4b. In THF, the reaction was complete after 16 h with a high selectivity for 12 (5.9) whereas In CDCl3 the reaction was complete in just 3 h with still higher chemoselectivity (18.6). Most strikingly, the activity of 13 remained high even at 7 ºC in CDCl3, with only 16 h required for complete conversion of 7 and a significant further enhancement in chemoselectivity (62.3), which is one of the highest values reported to date. Calculations suggest that the excellent selectivity observed in the case of catalyst 13 is due to the high polarizability of 13H + , which is almost double that of 4bH + , rather than the value of MEP, which is actually calculated to be lower for 13H + than 4bH + (Table , entry 6); in keeping with the high selectivity observed, Eint was calculated to be significantly higher for substrate-catalyst complex [13H + .9] (Figure ). Double protonation to give 13H2 2+ was also considered but seems unlikely under the reaction conditions; electrochemical measurements (ESI section S8) are consistent with mono-protonation taking place as, although ELUMO on addition of TsOH is large for 13 (230 meV), the value of ELUMO determined in the presence of acid is actually less negative than that of 4bH + (-3.77 vs -3.68 eV respectively). Interestingly, despite the higher calculated value of MEP of the doubly protonated structure, similar stabilities were found for complexes [13H + .9] and [13H2 2+ .9], which is consistent with their comparable calculated polarizabilities (Figure ). This suggests that, regardless of protonation state, polarizability is the key factor in determining the catalytic performance of rotaxane 13.