Catalysis of the ethanolysis of aryl methyl phenyl phosphinate esters by alkali metal ions: transition state structures for uncatalyzed and metal ion-catalyzed reactions

Abstract

This paper reports on a spectrophotometric kinetic study of the effects of the alkali metal ions Li⁺ and K⁺ on the ethanolysis of the aryl methyl phenyl phosphinate esters 3a–f in anhydrous ethanol at 25 °C. Rate data obtained in the absence and presence of complexing agents afford the second-order rate constants for the reaction of free ethoxide (k_EtO⁻) and metal ion-ethoxide ion pairs (k_MOEt). The sequence k_EtO⁻ < k_MOEt is established for all the substrates, contrary to the generally observed reactivity order in nucleophilic substitution processes. The quantities δG_ip, δG_ts and ΔG_cat, which quantify the observed alkali metal ion effect in terms of transition state stabilization through chelation of the metal ion, give the order δG_ts > δG_ip for Li⁺ and K⁺. Hammett plots show significantly better correlation of rates with σ and σ^o substituent constants than with σ⁻, yielding moderately large ρ(ρ^o) values that are consistent with a stepwise mechanism in which formation of a pentacoordinate (phosphorane) intermediate is the rate-limiting step. The range of the values of the selectivity parameter, ρ_n (= ρ/ρ_eq), 1.3–1.6, obtained for the uncatalyzed and alkali metal ion catalyzed reactions indicates that there is no significant perturbation of the transition state (TS) structure upon chelation of the metal ions. This finding is relevant to the mechanism of enzymatic phosphoryl transfer involving metal ion co-factors. The present results enable one to compare structural effects for nucleophilic reactions of several series of organophosphorus substrates. It is shown that the order of reactivity of the substrates: 4-nitrophenyl dimethyl phosphinate (2) > 3a > 4-nitrophenyl diphenyl phosphinate (1) is determined mainly by the steric effects of the alkyl/aryl substituents around the central P atom in the TS of the reaction.