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

Abstract

The acetyl and benzoyl o-nitroanilines 1a and 1b and the acetyl and benzoyl o-nitrodiphenylamines 1c and 1d have been reduced chemically (Bu^tOK/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.