Template effects. 3. The quantitative determination of the catalytic effects of alkali and alkaline earth cations in the formation of benzo-18-crown-6 in methanol solution

Cyclization of the conjugate base of o-hydroxyphenyl 3,6,9,12-tetraoxa-14-bromotetradecyl ether to benzo-18-crown-6 in methanol solution has been found to be strongly and specifically accelerated by added alkali and alkaline earth bromides. The observed accelerations range from 13.2 for Cs+ to a spectacular 540 for Sr2+. The reported data refer to conditions in which the rate is independent of cation concentration, thus providing for the first time a quantitative determination of the template effect of added cations in the formation of a macrocyclic polydentate ligand. The different catalytic abilities as observed for the different cations are tentatively discussed in terms of a combination between proximity of the reactive ends in the bifunctional precursor complexed around the metal ion, and chemical factors arising from the extent of interaction of the cation with the nucleophilic oxide ion. © 1981 American Chemical Society.