DIFFERENTIAL METAL-ION STABILIZATION OF REACTANTS AND TRANSITION-STATES IN THE TRANSACYLATION OF CROWN-ETHER ARYL ACETATES

Transfer to methoxide ion of the acetyl group of a series of crown ether aryl acetates ranging from 2-AcO-15C4 to 2-AcO-27C8 is accelerated by alkali and alkaline earth metal ions. The magnitude of the rate enhancements is dependent on the substrate-cation combination, the divalent metals being in general much stronger catalysts than the monovalent metals. A maximum catalytic effect of 760 is observed in the Ba2+-catalyzed reaction of 2-AcO-18C5. The dependence of the observed second-order rate constants (k(obsd)) on metal ion concentration was quantitatively accounted for in terms of 1:1 associations of the metal ions with reactants and transition states. A definite contribution from an additional reaction path involving two metal ions was detected in the cases of the Na+- and K-catalyzed reactions of 2-AcO-24C7 and of the K+- and Rb+-catalyzed reactions of 2-AcO-27C8. A comparative analysis of structure and metal ion effects on the strength of binding of metal ions with reactants and transition states indicates that transition-state stabilization by metal ions occurs through cooperation of coordination to the oxygen donors of the bridge and electrostatic binding to the negative charge being transferred from methoxide ion to the carbonyl oxygen.