Rate measurements of certain Vilsmeier–Haack reactions. Part 1. The benzoylation of a trisubstituted pyrrole

Abstract

Rate measurements on the reaction between the NN-dimethylbenzamide–POCl₃ complex (1) and ethyl 2,4-di-methylpyrrole-3-carboxylate (2) in 1,2-dichloroethane show that the reaction follows second-order kinetics, first order in each of (1) and (2), for reactions taken to at least 90% of completion. The rate constant remained un-changed for reactions in which an excess of POCl₃ was present, or where the concentration of the pyrrole (2) was varied, but became inconsistent when the concentration of the complex (1) was varied. The Arrhenius activation energy was determined and the low value obtained is consistent with a very early transition state. The presence of traces of water was found to have a marked effect on both the rate of the reaction and the yield obtained.