Electrochemical behaviour of 3-iminoindolenine derivatives in dimethylformamide: III. Reduction of 2-phenyl-3-(2′-pyridyl)- and 2-phenyl-3-(1′-oxide-4′-pyridyl)-iminoindolenine 1-oxide

Abstract

Summary The electrochemical behaviour of 2-phenyl-3-(2′-pyridyl)-(III) and 2-phenyl-3-(1′-oxide-4′-pyridyl)-iminoindolenine 1-oxide (IV) in DMF solution was studied by polarography, cyclic voltammetry, controlled-potential coulometry and u.v. spectroscopy. In aprotic medium (DMF with tetrabutylammonium perchlorate as supporting electrolyte) both compounds (III) and (IV) are reduced at a mercury dropping electrode in two one-electron steps. In the presence of a protonating agent such as water or 3,4-dimethylphenol, the first wave grows in height at the expense of the second until with sufficient protonating agent only one wave of two-electron height is observed. On addition of a stronger protonating agent such as benzoic, monochloroacetic or salicylic acid the two normal waves of aprotic medium are gradually replaced by a single two-electron wave (A) at less negative potentials. For compound (III) another wave (D) appears at more negative potentials than the wave (A). The electrochemical reduction product corresponding to wave (A) is 1-hydroxy-2-phenyl-3-(2′-pyridyl) aminoindole (V) for compound (III) and 1-hydroxy-2-phenyl-3-(1′-oxide-4′-pyridyl) aminoindole(VI) for compound (IV). Compound (V) and 2-phenyl-3-(2′-pyridyl)aminoindole are the reduction products corresponding to wave (D). A multi-step scheme for the electrochemical reduction of (III) and (IV) is suggested to explain most of the experimental results obtained.