Reaction of imidazole with toluene-4-sulfonate salts of substituted phenyl N-methylpyridinium-4-carboxylate esters: special base catalysis by imidazole

Abstract

The reaction of imidazole in aqueous solution with toluene-4-sulfonate salts of substituted phenyl N-methylpyridinium-4-carboxylate esters obeys the rate law: k_obs − k_background = k₂[Im] + k₃[Im]² where [Im] is the imidazole concentration present as free base. The parameters k₂ and k₃ fit Brønsted type free energy correlations against the pK_a of the leaving phenol with β_Lg values of −0.65 and −0.42 respectively. The imidazolysis is insensitive to catalysis by general bases and yet k₃ for the 3-cyanophenyl ester possesses a deuterium oxide solvent isotope effect of 4.43 consistent with rate limiting proton transfer. A special catalytic function is proposed for decomposition of the tetrahedral addition intermediate (T^±) viak₃ whereby the catalytic imidazole interacts electrophilically with the leaving phenolate ion and removes a proton from the nitrogen in the rate limiting step with subsequent non-rate limiting ArO–C bond fission. This is consistent with the change in effective charge on the leaving oxygen in the transition structure of k₃ which is more positive (−0.42) than that expected (−0.60) for the equilibrium formation of the zwitterion intermediate. The catalytic function at the leaving oxygen is likely to be an electrophilic role of the NH as a hydrogen bond donor. In the k₂ step the deuterium oxide solvent isotope effect of 1.51 for the 3-cyanophenyl ester and the β_Lg of −0.65 are consistent with rate limiting expulsion of the phenolate ion from the T^± intermediate. The absence of general base catalysis of imidazolysis rules out the established mechanism for aminolysis of esters where T^± is stabilised by a standard rate limiting proton transfer. The kinetically equivalent term for k₃ where T⁻ reacts with the imidazolium ion as an acid catalyst would require this step to be rate limiting and involve proton transfer not consistent with departure of the good aryl oxide leaving group.