Electrophilic substitution in indoles. Part 14. Azo-coupling of indoles with p-nitrobenzenediazonium fluoroborate

Abstract

Indole and its 1- and 2-methyl derivatives undergo second-order azo-coupling reactions with p-nitrobenzenediazonium tetrafluoroborate to afford the corresponding indole-3-azo-(4′-nitrobenzenes). Kinetic studies with related 3-deuterioindoles showed that there is no isotope effect, thus confirming that the initial attack of the electrophile is the rate-determining step, as in the majority of electrophilic aromatic substitutions.

In contrast, 3-methylindole afforded 3-methylindole-2-azo-(4′-nitrobenzene) and the reaction not only showed a small deuterium isotope effect (ca. 2) but was also affected by base catalysis by water added to the acetonitrile. The difference in behaviour of the 3-methylindole from indole and its 1- and 2-methyl derivatives could be accounted for by a mechanism involving primary attack of the diazonium salt at the 3-position followed by rearrangement; further evidence for this mechanism was provided by the straight line plot obtained in comparisons of the rates of reactions for the four indoles with acidity functions (which reflect protonation at the 3-position).