The products of alkylation reactions of indole are complex because they are subject to both kinetic and equilibrium
control. With methyl iodide, eg, initial attack is on the 3-position to give first 3-methylindole then 3,3-dimethylindolenine. This is followed by
equilibration between the 2- and 3-positions to form 2,3-dimethylindole which, in turn, is attacked to give 2,3,3-trimethylindolenine and, finally,
1,2,3,3-tetramethylindoleninium iodide. The latter compound loses HI, when treated with aqueous alkali, to form 1,3,3-trimethyl-2-methyleneindolenine,
a widely used intermediate for the manufacture of cationic dyestuff. |