Iron Catalysed Synthesis 2-Arylbenzimidazoles and Benzothiazoles Under Solvent Free Conditions

The authors (K Bahrami et al, SYN LETT, 2009, (4), 569) comment that there are many methods available for the preparation of benzimidazoles and benzothiazoles but that they have identified a potentially low cost and convenient procedure.  The general reaction is outlined below wherein an aromatic aldehyde is reacted with a diamine or thioanaline in the presence of iron (III) nitrate and hydrogen peroxide to give directly the heterocyclic product, generally in yields in excess of 90% in less than 30 minutes.

The paper provides 16 examples of benzimidazoles and 12 examples of benzothiazoles and gives some comparison data for alternative procedures to illustrate the utility of the reported chemistry.  It is interesting to note that in the absence of the iron (III) nitrate there is no reaction, so clearly the oxidant is an iron species.

In conclusion, the procedure offers an interesting procedure which uses low cost materials and minimal solvent.  However, it should be noted that the reported general procedure involves charging the full inventory of materials and heating to 50ºC, so its use in development and at larger scales would need a least a careful thermal evaluation and most likely modification for safer operation and control.  Once the reaction is complete, the work-up is simple, involving dissolution in ethanol and addition to water to precipitate the product.