Model System for the Template Effect of Alkali and Alkaline-Earth Metal Ions on the Formation of Benzo-18-crown-6 in MeOH Solution

Article abstract of DOI:10.1021/ja00357a029

Alkali and alkaline-earth metal ions exert a remarkable catalytic effect (template effect) on the cyclizatiion of o-hydroxyphenyl 3,6,9,12-tetraoxa-14-bromotetradecyl ether (ArOH) to benzo-18-crown-6 (B18C6) in MeOH solution.This phenomenon is interpreted as resulting from two opposing factors, a rate-enhancing factor due to an increased proximity of chain ends in the cation-associated precursor ArO^- and a rate-retarding factor due to interaction of the nucleophilic end of ArO^- with the metal ion.Combination of previous equilibrium data with kinetic measurements from the present work provides a complete set of equilibrium constants for association of seven cations Na⁺, K⁺, Rb⁺, and Cs⁺ and Ca²⁺, Sr²⁺, and Ba²⁺ with the four ligands ArOH, ArO^-, B18C6, and T^<*>, the latter being the transition state for cyclization.Following our earlier suggestion, according to which the proximity effect is viewed as a sort of macrocyclic effect, i. e., the one in which the cyclic ligand is T^<*> and the acyclic ligand is ArO^-, we now present an extrathermodynamic treatment indicating that the magnitude of the above macrocyclic effect is well represented by the K_B18C6/K_ArOH ratio.This ratio provides an estimate for the upper limit of the template effect.The actually observed values are lower by a factor of ca. 4 because of reduction of nucleophilicity due to cation association.