Rate measurements of certain Vilsmeier–Haack reaction. Part 4. The reactivities of various arenecarboxamide–phosphoryl chloride complexes

Abstract

Measurement of the rates of the Vilsmeier–Haack reaction of several amide–POCl₃ complexes (2) on ethyl 2,4-dimethylpyrrole-3-carboxylate (3) in 1,2-dichloroethane was used to observe the effect of changing the aroyl group of the amide (1). The results for the 4- and 3-substituted benzamides are in accord with the Hammett relationship when σ⁺ constants are employed. The reaction constant, ρ, was found to remain essentially the same on using N-benzoylmorpholines instead of NN-dimethylbenzamides (1.09 and 1.08, respectively). The presence of excess of POCl₃ increased the reaction rate of methoxy- and nitro-benzamide complexes and to a lesser extent those of furan-2-and thiophene-2-but not pyrrole-2-carboxamides; an explanation is offered for this unexpected reactivity.