Pd-Catalyzed Reductive Carbonylation-Ring Closure of Aryl Halides: A Direct Approach for Synthesis of Benzimidazoles

A novel protocol is reported for the facile one-pot benzimidazole derivatives formation from Pd-catalyzed reaction of aryl halides (X= I, Br, -Cl), Fe(CO) 5, and o-diaminobenzene or o-nitroaniline. Aryl halides are first converted to aryl aldehydes using Fe(CO)(5) which then undergo ring closure with o-diaminobenzene or o- nitroaniline in DMF at 110 degrees C under Ar. In the case of o- nitroaniline, the reduction of the nitro group takes place before the cyclization step in the presence of re-ducing agents such as Fe(CO) 5, released CO, Pd(0) and also P(III) in the reaction mixture. There is no need for employing P(III) ligand in case of aryl iodides while P(III) ligand is necessary for aryl bromides and chlorides to facilitate the reaction. In this reaction, Fe(CO) 5 is not only responsible as the CO source and reduction of -NO2 functionality in o-nitroaniline, but also produces iron oxides residue in the reaction which are responsible for the dehydrogenation step.