SUBSTITUENT EFFECTS ON AZO COUPLING OF INDOLES

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