A study on a primitive artificial esterase model: reactivity of a calix[4]resorcinarene bearing carboxyl groups

The host molecule octacarboxymethyl calix[4]resorcinarene 1 catalyses the hydrolysis of substituted phenyl Nmethylpyridinium- 4-carboxylate esters 3a-f by complexation followed by intracomplex reaction via an anhydride intermediate. The reactivity in the presence of 1 is higher than that of the background at low pH; at high pH an inversion of reactivity occurs, the background becomes predominant since the host inhibits hydrolysis of the esters. The reactivity of esters 3a-f complexed with the host suffers little change in effective charge on the phenolic oxygen (0.15 units) in contrast with the changes observed in alkaline hydrolysis (0.28 units) and in the hydrolysis of the model monoaryl glutarate esters (-1.02 units). The less negative effective charge in the transition state for host 1 catalysis compared with that in the glutarate case is ascribed to stronger solvation by water molecules in the complex compared with that due to water molecules in the bulk solvent.