Reusable Porphyrinatoiron(III) Complex Supported on Activated Silica as an Efficient Heterogeneous Catalyst for a Facile, One‐Pot, Selective Synthesis of 2‐Arylbenzimidazole Derivatives in the Presence of Atmospheric Air as a “Green” Oxidant at Ambient Temperature

An efficient and highly selective synthesis of 2‐arylbenzimidazoles by condensation of a wide range of aryl aldehydes bearing electron‐donating or ‐withdrawing substituents and phenylenediamines in a single pot using a catalytic amount of ( meso ‐tetrakis( o ‐chlorophenyl)porphyrinato)iron(III) chloride (5 mol‐%) in excellent isolated yields is described. The reactions were performed in the presence of atmospheric air as a “green” oxidant at ambient temperature in ethanol without any additives. To benefit from the recovery and reuse of the catalyst, a new iron(III) porphyrin–silica heterogeneous catalyst was prepared by simple and successful impregnation of ( meso ‐tetrakis( o ‐chlorophenyl)porphyrinato)iron(III) chloride onto activated commercial chromatographic silica gel. The silica was modified by treatment with HCl solution followed by NaOH solution, giving rise to the transformation of –O– strained siloxane atoms and silanol–OH groups into terminal negatively charged silanol–O – groups. The heterogeneous catalyst was characterized by powdered X‐ray diffraction (XRD), scanning electronic microscopy (SEM), atomic force microscopy (AFM), thermogravimetry (TG) to analyse for nitrogen adsorption, and Raman and FTIR spectroscopy. Leaching experiments showed that the catalyst is most strongly anchored to the activated support after 10 cycles. T( o ‐Cl)PPFe III –SiO 2 was successfully used for all runs performed by the homogeneous catalyst and exhibited high isolated yields, selectivity, reusability, and the potential for large‐scale synthesis. Mechanistically, the high‐valent oxido‐iron(IV)–porphyrin could be the key intermediate in this efficient and highly selective synthesis of benzimidazole derivatives. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)