Phenazines and Their N-Oxides as Products of Cyclization of N-Aryl-2-nitrosoanilines – Disproof of the Reported Homolytic Cross-Coupling Process Leading to Benzo[c]cinnoline Oxides

The base-promoted homolytic cross-coupling reaction of nitroarenes and anilines involving single electron transfer and leading to benzo[c]cinnoline oxides reported by Sythana et al. (S. K. Sythana, S. Unni, Y. M. Kshirsagar, P. R. Bhagat, Eur. J. Org. Chem. 2014, 311) was shown to be incorrect. In fact, nucleophilic substitution of hydrogen with anilide anions takes place, which leads to the formation of intermediate N-aryl-2-nitrosoanilines followed by base-promoted cyclization to phenazines and/or their N-oxide derivatives. Three of the phenazine N-oxides, which were erroneously described as benzo[c]cinnoline oxides by Sythana et al., were obtained independently in the cyclization of N-aryl-2-nitrosoanilines possessing a leaving group at the ortho position in the N-aryl ring. A competitive substitution of hydrogen and a leaving group lead to the formation of 1-substituted phenazines and phenazine N-oxides, respectively, depending on the reaction conditions and the leaving group.