A simple, convenient, and environment-friendly method for synthesis of 1,2-disubstituted benzimidazoles in high yields under extremely mild conditions, has been described. The methodology described, involves condensation of o-phenylenediamine derivatives with aldehyde derivatives in presence of catalytic amount of iron(III) phosphate under solvent-free conditions. Although previous methods are available in literature for synthesis of such compounds, the highlight of this new methodology is the demonstration of efficiency of solvent-free conditions in such reactions. The authors have specifically mentioned the usefulness of the catalyst as “iron(III) phosphate is reported as a green, reusable, recyclable, environmentally safe, inexpensive and commercially available catalyst”.
The Reaction of Interest:
Applying the procedure illustrated for synthesis of 1,2-disubstituted benzimidazoles in scheme 1, to reaction of o-phenylenediamine (1 mmol) and benzaldehyde (2 mmol), the reaction conditions could be easily optimized.
- The reproducibility of the reactions is very good with yields more or less as described. The best part in this methodology that I have found is the simple synthesis of 1,2-disubstituted benzimidazoles without the by-products like 1-substituted benzimidazoles, when the benzaldehyde was taken in 2.5 moles. This however, is a slight drawback, but the selectivity of the method compensates for it.
- The use of catalyst for synthesis of 1,2-disubstituted benzimidazoles is indeed very well thought out. The solvent-less conditions for the reactions are a big plus. All of the reactions listed in Table 3 were easily reproduced with yields greater than 78% (up to 92% for entry 5) and within 60 minutes.
- The procedure for synthesis is not well written. I found that grindstone technique is better suited to this methodology. On mixing the aldehyde (2.5 mmol), and o-phenylenediamine (1 mmol) with 2 mol% FePO4 in a mortar-pestle, the reaction proceeds smoothly within an hour.
- The recrystallization from n-hexane-ethyl acetate was not straightforward in all cases and increased humidity caused production of gummy mass. The authors did not mention all steps for purification of the product. Simple layer separation technique with dichloromethane water resulted in pure products but loss of catalyst. So, dissolving the product in dichloromethane followed by filtration was the best method. The products obtained by drying dichloromethane could be recrystallized from ethanol.
- The methodology is very good and does not require much modification. Use of mortar-pestle for grinding and dichloromethane for separation of products is recommended.