Electrochemical oxidation of methylindoles

Electrochemical oxidation of 1-methylindole ( I ), 1,3-dimethylindole ( II ) and 1,2-dimethylindole ( III ) on a platinum anode in methanol, containing ammonium bromide, was studied. A more profound oxidation as compared with benzofuran or benzothiophene derivatives is indicated by the obtained products: 1-methyl-2,2-bis(1-methylindol-3-yl)-3-indolinone ( IV ), 1,1'-dimethyl-Δ 3,3 -biindolin-2,2'-dione ( V ), 3,3-dibromo-1-methyl-2-indolinone ( VI ), 3,3,5-tribromo-1-methyl-2-indolinone ( VII ), 3-bromo-1-methyl-2,3-indoledione ( VIII ), 3-methoxy-1,3-dimethyl-2-indolinone ( IX ), 3-hydroxy-1,3-dimethyl-2-indolinone ( X ), 3,5-dibromo-1,3-dimethyl-2-indolinone ( XI ) and 3-bromo-1,2-dimethylindole ( XII ). The ratio of the products can be significantly influenced by the electrolysis conditions. The oxidation takes place in positions 2 and 3 of the indole molecule, the position 2 influencing decisively the oxidation path.