Template Effects. 6. The Effect of Alkali Metal Ions on the Formation of Benzo-3x-crown-x Ethers over a Wide Range of Ring Sizes

Article abstract of DOI:10.1021/ja00313a034

The rate formation of benzo-3x-crown-x ethers with x = 4,5,7,10 and 16 via intramolecular alkylation of o-^-OC6H4(OCH2CH2)_x-1Br in 99percent aqueous Me2SO was found to be markedly affected by added alkali metal bromides.Catalysis or inhibition was observed, depending on the cation-substrate pair.Combination of the present results with those previously reported for the formation of B18C6 offers a large variety of patterns.The magnitude of the observed effects ranges over four powers of ten.The dependence of the observed rates (k_obsd) on metal ion concentration was expessed in terms of independent contributions from free and cation-paired aryl oxide ions, whose relative weights are ruled by the rate constants k_i and k_ip, respectively, and by the ion-pairing association constants K_ArO-.A self-consistent analysis was used to derive numerical values of the above parameters.A definite contribution from an additional reaction path involving two metal ions was detected in the case of the K⁺-catalyzed formation of B30C10.The equilibrium constants K_C for associations between many cation-crown pairs were also determined under the same conditions.A comparative analysis of structure and metal ion effects on the extent of interaction of the alkali metal ions with the reactants, transition states, and reaction products shows that the cation interaction in the transition state is strongly reminiscent of the host-guest interactions found in the cation-B3XCx complexes.The catalytic efficiency of the alkali metal ioms (log k_ip/k_i) shows a definite tendency to parallel the strength of interaction with the reaction products (log K_C), thus indicating that a metal ion capable of binding strongly with a crown ether is also a good catalyst for the formation of the crown ether itself.