Electrochemical behaviour of 3-iminoindolenine derivatives in dimethylformamide: II. Reduction of 2-phenyl-3-aryliminoindolenine-1-oxide

Abstract

The electrochemical behaviour of substituted 2-phenyl-3-aryliminoindolenine 1-oxides (Ia-f) in DMF solution was studied by polarography, cyclic voltammetry, controlled-potential coulometry and u.v spectroscopy. In aprotic medium (DMF with KClO4 as supporting electrolyte) compounds (Ia-f) are reduced at a mercury dropping electrode in two one-electron steps. The potentials of the first reversible step have been correlated with the Hammett constants. In the presence of a protonating agent such as 3,4-dimethylphenol or water 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 found with the purely aprotic medium are gradually replaced by a single two-electron wave (A) at less negative potentials and a new wave (D) appears at more negative potentials. The electrochemical reduction product is 1-hydroxy-2-phenyl-3-arylaminoindole corresponding to the first wave (A) and 2-phenyl-3-arylaminoindole corresponding to the second wave (D). A multi-step scheme for the electrochemical reduction of (Ia-f) is suggested to explain most of the experimental results obtained.