Heteroaromatic azo-activated substitutions. Part 3. Catalysis in the reaction of 4-(4-methoxyphenylazo)pyridinium methiodide with amines in dipolar aprotic solvents. Kinetic form and the mechanism of the uncatalysed reaction in nucleophilic aromatic substitution

Abstract

The kinetics of the reaction of the title compound with piperidine, n-butylamine, morpholine, and benzylamine have been studied in dimethyl sulphoxide and, for piperidine and n-butylamine only, in acetonitrile. The predominant reaction in all cases was nucleophilic displacement at the aryl carbon (S_NAr reaction), accompanied by an S_N2 attack at the alkyl carbon when secondary amines were the nucleophiles. The S_N2 rate constants were independent of amine concentration. The S_NAr reactions were all base-catalysed independent of whether the nucleophile was a primary or secondary amine, indicating that decomposition of the first formed intermediate was rate limiting in all cases. The activating influence of the azopyridinium function is discussed. The hitherto observed dichotomy in the reactions of the two amine types is considered. It is shown that in the absence of an ortho-nitro group in the substrate, as in the present case, both the catalysed and uncatalysed reactions occur by the specific base-general acid (SB–GA) mechanism.