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

Abstract

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 8C under Ar. In the case of o-nitroaniline, the reduction of the nitro group takes place before the cyclization step in the presence of reducing 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 -NO 2 functionality in o-nitroaniline, but also produces iron oxides residue in the reaction which are responsible for the dehydrogenation step.

Synthesis of Substituted Benzimidazoles and Benzoxazoles by Iron‐catalysed Reductive Carbonylation of Aryl Iodides

“…For instance, Fe(III)‐porphyrin has facilitated the utilization of precursors such as benzo‐1,2‐quinones, aryl aldehydes, and ammonium acetate in benzimidazole synthesis [8] . Likewise, palladium‐mediated reactions involving metal carbonyls, aryl halides, and o‐diaminobenzenes have enabled in situ aldehyde formation to produce the desired products [9] . A novel approach utilizing carbon monoxide, aryl halides, o‐diaminobenzenes, and manganese (Mn) metal catalyst has also been reported for benzimidazole synthesis [10] .…”

“…[8] Likewise, palladium-mediated reactions involving metal carbonyls, aryl halides, and o-diaminobenzenes have enabled in situ aldehyde formation to produce the desired products. [9] A novel approach utilizing carbon monoxide, aryl halides, o-diaminobenzenes, and manganese (Mn) metal catalyst has also been reported for benzimidazole synthesis. [10] In this method, aryl halides are transformed into aldehydes in situ, which then react with corresponding amino compounds to yield the desired products.…”