Chemical and electrochemical reduction of ortho-nitroanilides. A combined chemical, polarographic and ESR study

The acetyl and benzoyl o-nitroanilines 1 a and 1 b and the acetyl and benzoyl o-nitrodiphenylamines 1 c and I d have been reduced chemically (Bu'OK/DMSO) and electrochemically inside the cavity of an EPR spectrometer. For all compounds the EPR signal of the radical anions could be recorded and interpreted. In DMSO the radical anions 1c'- and Id'- evolved to the phenazine radical anion within 1 h. The polarographic study showed that the four compounds are reduced in two different steps, the first being monoelectronic and reversible, as demonstrated by cyclic voltammetric experiments. Compounds l a 4 were reduced with Fe/AcOH to the benzimidazoles 5a-d. The catalytic reduction of 1 a gave the hydroxy-2-methylbenzimidazole 7a together with the azoxy derivative 6a. The macroscale electrolysis of l c and I d may be regarded as a convenient synthetic method of preparing benzimidazoles 5c and 5d, while the catalytic reduction may be considered the best route to benzimidazole N-oxides 4c and 4d.