Alkali and alkaline-earth metal ion catalysis in the reaction of aryl acetates with methoxide ion. Effect of a poly(oxyethylene) side arm

Article abstract of DOI:10.1021/ja00157a062

The effect of added alkali (Na, K) and alkaline-earth (Sr, Ba) metal bromides on the rate of acyl transfer from o-acetoxyphenyl 3,6,9,12-tetraoxatridecyl ether (2) to methoxide ion in methanol at 25 °C has been investigated spectrophotometrically. All of the added salts increased significantly the rate of reaction in the order Na⁺ < K⁺ 2+ 2+, a maximum catalytic effect of nearly 10² being observed with the latter metal ion. A proper analysis of rate data afforded equilibrium constants for associations of metal ions with reactants and transition state, showing that in all cases the transition state binds metal ions much more strongly than the reactants. An analogous set of rate measurements was carried out for comparison purposes on the corresponding reaction of phenyl acetate (1). The fact that catalytic effects are much greater with 2 than with 1 reveals that a substantial contribution to the stability of the transition state of the metal ion assisted path arises from interaction of the metal ion with the oxygen donors of the side arm.