Ion Pairing and Reactivity of the Alkali-Metal and Alkaline-Earth-Metal Derivatives of Ethyl (5-Bromopentyl)acetoacetate

Article abstract of DOI:10.1021/jo00246a033

The effect added alkali-metal and alkaline-earth-metal salts on the rate of intramolecular alkylation of the title compound has been investigated in 99percent aqueous Me2SO.It was found that the rate is practically insensitive to Et4N⁺ and K⁺ ions and it is slightly decreased by Na⁺ ion, but it is strongly depressed by Li⁺, Ba²⁺, Sr²⁺, Ca²⁺, and Mg²⁺ ions, the observed effects ranging over nearly 5 powers of ten.Analysis of rate data was carried out by means of a self-consistent approach, as based on the classical Acree hypothesis of independent contributions of free ions and ion pairs to the overall rate.In most cases it was possible to measure the ion pairing association constants (K_ip) together with specific rates for reactions of the ion pairs (k_ip), even when the latter are much lower than the specific rate for reaction of the free ion (k_i).The results show that ion pairing decreases in the order Li > Na > K and Mg > Ca > Sr > Ba, which clearly suggests that association is dominated by coulombic interactions.The strong inhibition caused by ion pairing is due to a greater cation interaction with the anionic reactant than with the transition state, where a significant fraction of the available negative charge is concentrated in the C-C bond-forming region.