CHEMICAL AND ELECTROCHEMICAL REDUCTION OF ORTHO-NITROANILIDES - A COMBINED CHEMICAL, POLAROGRAPHIC AND EPR STUDY

The acetyl and benzoyl o-nitroanilines 1a and 1b and the acetyl and benzoyl o-nitrodiphenylamines 1c and 1d have been reduced chemically (Bu(t)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 1d.- 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 1a-d were reduced with Fe/AcOH to the benzimidazoles 5a-d. The catalytic reduction of 1a gave the hydroxy-2-methylbenzimidazole 7a together with the azoxy derivative 6a. The macroscale electrolysis of 1c and 1d 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.