The maximum molecular electrostatic potential (MEP) values of 4a and 4b (Figure ) are located over the NDI unit, in keeping with its established applications in anion-π catalysis. The MEP value over the bipyridine is negligible. In contrast, the maximum MEP values of 4aH + and 4bH + (Table , entries 1 and 2; Figure ) associated with the NDI unit and protonated pyridine units are similar and larger than those observed in the neutral state, suggesting that either the NDI or the bipyridine motif can stabilise enolate 9. The increase in MEP on protonation is consistent with ELUMO values obtained by cyclic voltammetry of 4a and 4b in the absence or presence of TsOH (ELUMO = 170 meV for 4a/b vs 4a/bH + ; ESI section S8). Considering the similar values of MEP calculated for the NDI and bipyridine π surfaces, both possible catalyst-substrate complexes were computed for 4aH + and 4bH + with 9 (Figure ). In both cases, binding of 9 to the NDI face of the catalyst was found to be slightly preferred, as a result of increased π-π interactions between the electron rich aromatic substituent of the thioester and the larger aromatic surface provided by the NDI. A small difference in complex energies was calculated for [4aH.9] and [4bH.9], which is consistent with the small difference in their selectivities observed in CDCl3 and reinforces the point that the different reaction rates observed between 4a and 4b are most likely due to the pKa difference between both catalysts.