Host-guest interactions. The binding mode of 6-nitrobenzisoxazole-3-carboxylate to quaternary ammonium macrocycles

In order to investigate the mode of substrate binding and the factors involved in catalysis by artificial host compounds, the rate augmentation of the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (4) in the presence of the non-aggregating macrocyclic quaternary ammonium salts (1)-(3) was analysed. The kinetic results indicate that (1) and (3) consisting of 27-membered macrocycles form host-guest complexes with (4) whereas (2) having 21-membered rings does not. The macrotricycle (3) is the most effective catalyst, exhibiting a temperature-independent maximum rate enhancement of 110. The rate and binding constants and their temperature dependence for (3) leads to the conclusion that (4) penetrates with the nitroaromatic moiety first into the molecular cavity of (1) and (3). This mode of association is impossible with (2) for steric reasons. The cavity of (3), however, is large enough to accommodate two substrate molecules thus allowing the observation of the rare case of co-operative substrate binding to a low molecular weight enzyme model.