Studies of amine‐induced ring opening of some mesoionic xanthines

Abstract

AbstractThe ring‐opening reactions of seven mesoionic thiazolo[3,2‐a]pyrimidine‐5,7‐diones by a series of primary and secondary amines have been investigated. The rates of the ring fission of five N(8)‐substituted mesoionic xanthines with benzylamine were measured and found to follow second order kinetics. The Hammett relationship is followed with ϱ value of + 0.48 in p‐dioxane as solvent. The dependence of rates on temperatures has been studied for the N(8)‐ethyl derivative; the energy of activation (ΔE*) is 25.3 kcals mol−1, the enthalpy of activation (ΔH*) is 24.7 kcals mol−1 and the entropy of activation (ΔS*) is — 4.9 e.u. A slight increase in rate of reaction was observed when the solvent was changed from p‐dioxane to dimethyl sulfoxide. In p‐dioxane at constant mesoionic xanthine concentration, the rate constant for ring opening decreased with increasing benzylamine concentration. These results are consistent with a bimolecular nucleophilic mechanism proceeding by the rate‐determining formation of a charged tetrahedral transition state.