The mechanism of diazo-coupling to indoles and the effect of steric hindrance on the rate-limiting step
Journal of the Royal Society of Chemistry, Perkin Transactions 2
The first syntheses of 4-methyl-2-t-butyl-and 2-methyl-4.6-di-t-butyl-indole are described. Rates of diazocoupling at the 3-position of both these compounds together with those for 2-methyl-and 2-t-butyl-indole are reported for various para-substituted arenediazonium ions in mixed aqueous and some aprotic solvents a t 25". The kinetic behaviour of 2-methylindole is examined in detail and it is shown that 3-coupling may proceed through either the neutral compound or the 2-methylindolyl anion
... ylindolyl anion depending on the pH, with the anion being ca. 1 Os-fold more reactive. For neutral 2-methylindole coupling, rates increase with increasing reactivity of the diazonium ion giving p 3.3 for the Hammett (PO+) plot, and for coupling with p-toluenediazonium ion A H 3 = 38 kJ mol-I and ASt = 138 J K-I mol-l. Kinetic isotope effects for coupling to [3-2H,]-and [3-3H,]-2-methylindole and 3deuteriated 2-t-butyl-and 4-methyl-2-t-butyl-indole are not apparent in aqueous dioxan. Coupling to 2-methyl-4,6-di-t-butylindole is also shown to proceed via both the neutral substrate and its conjugate base, but these reactions show significant primary hydrogen isotope effects (kH/kD ca. 7). The results are interpreted in terms of a classical A-SE2 mechanism for both neutral indoles and indolyl anions in which direct electrophilic attack by the arene diazonium ion at the 3-position is normally rate limiting. The change in the rate limiting step observed for the A-SE2 mechanism with 2-methyl-4.6-di-t-butylindole is attributed to steric hindrance acting only on proton expulsion in a very reactant-like transition state and not on attack of the electrophilic diazonium ion. There is no evidence for coupling to indole nitrogen and triazene intermediates are therefore not significant in any of these reactions. ARENEDIAZONIUM ions are mildly electrophilic reagents capable of substituting activated aromatic compounds. The mechanism of these reactions is well established by the work of Zollinger,l who demonstrated an A-&2 mechanism (Scheme 1) for coupling to naphthols. With 1 H. Zollinger, Helv. Chim. Acta (a) 1956Acta (a) , 38, 1597Acta (a) , 1617Acta (a) , 1623 (b) 1958, 41, 2274. For a recent review see H. Zollinger, Angew. Clzern. Internat. Edn., 1972Edn., , 11, 874. 1116 (b) 1615, 1625. certain hindered hydroxy naphthalenesulphonic acids, the coupling reactions exhibited large primary deuterium isotope effects, which implied that proton transfer from the Wheland intermediate (step kb) could be almost entirely rate limiting. la We have reported isotopic rate ratios for the hydrogen B. C.