Intramolecular reductive carbonylation of o-nitroaniline by carbon monoxide to 2(3H)-benzimidazolone

o-Nitroaniline reacts with carbon monoxide under pressure in the presence of solvents (e.g. dioxane, tetrahydrofuran, N,N-dimethylformamide) and a catalytic system (sulfur or a low molecular sulfur compound - a basic medium - and optionally a vanadium(V) compound) at 370-445 K to give 2(3H)-benzimidazolone by intramolecular reductive carbonylation. Similarly to intermolecular carbonylation of nitroaromatic compounds also in the title reaction the efficiency of the sulfur component decreases in the order: COS > H2S much greater than CS2 > S. The promotor action of NH4VO3 and V2O5 is, however, less pronounced. The rate of the carbonylation increases with temperature, with optimal selectivity to the 2(3H)-benzimidazolone (>82%) at 390-425 K. The basicity of o-nitroaniline was found to be insufficient. and inorganic or organic bases had to be added to effect the reaction. The above two-(or three-) component system catalyze also the intramolecular oxidative carbonylation of o-phenylenediamine to 2(3H)-benzimidazolone.