Substituent effects on azo coupling of indoles

The kinetics of the azo coupling of eight para-substituted benzenediazonium tetrafluoroborates 2a–h with indole and its 1-, 2-,and 3-methyl derivatives 1a–d, respectively, were studied in acetonitrile at 25 °C under pseudo-first-order conditions. The relation k1(obs) = k2[diazonium salt] was found applicable in all cases. The logarithms of the rate constants k2 for each reaction series were correlated by the Hammett equation. A plot of the values of the reaction constant, ρ, obtained against the acidity constants pKa of 1a–d gave a straight line: ρ = 2.97 – 0.15 pKa. These results indicate that the azo-coupling reactions of indoles 1a–d follow one general mechanism involving rate-limiting initial electrophilic attack at the 3-position for all four compounds, contrary to the previous conclusion of Jackson and Lynch that the final deprotonation step is rate limiting.