SPECIFIC TRANSITION-STATE STABILIZATION BY METAL-IONS IN REACTIONS OF FUNCTIONALIZED CROWN ETHERS

The reactivity of groups lying in close proximity of a crown-ether moiety where a metal ion is hosted, has been investigated in model systems. We have found that methyl transfer and acyl transfer reactions are greatly accelerated by alkali and alkaline-earth metal ions. Rate enhancements in the order of 10^2-10^3 are commonly observed, but exceptionally high values in the order of 10^5-10^7 are also found in some cases, which correspond to transition state stabilizations of some 7-10 kcal/mol.