An efficient and environmentally benign method for synthesis of 1,2-disubstituted and 2 substituted benzimidazoles has been described. Although previous methods are available in literature for synthesis of such compounds, the ease of operation, solvent-free conditions, short reaction times and metal-free catalysis are the highlights of this new methodology.
The Reaction of Interest:
Applying the procedure described for synthesis of 1,2-disubstituted benzimidazoles, decent yields of about 70 to 90% were obtained for aromatic aldehydes having electron releasing and groups like –OH, –OMe, and –NMe2 under solvent-free grindstone conditions.
The use of aldehydes bearing electron withdrawing substituents like nitro group and halogens was found suitable for synthesis of 2-arylbenzimidazoles in yields ranging in mid 70s while for bulky substituents, it dropped to mid 60s. This is very much in line with the mechanistic rationale of PIL catalysed reactions as illustrated in scheme 3.
- The reactions are reproducible with yields more or less as described. Using 2-methoxy benzaldehyde, the yield of 1,2-disubstituted benzimidazole was 76%. This particular reaction can be done by simple grinding without any catalyst and with better yield of around 92% in methanol.
- The use of catalyst for synthesis of 1,2-disubstituted benzimidazoles seems a bit over-emphasized as all these reactions can be carried out without a catalyst.
- The recrystallization from ethanol-water was not straightforward in all cases and increased humidity caused production of gummy mass. The recrystallizations were best when carried out in dry atmospheric conditions.
- As the described procedure anyway involves recrystallization from ethanol-water, the use of PIL catalyst is redundant, especially for 1,2-disubstituted benzimidazoles. Grinding can be done in small volume of methanol/ ethanol without catalyst.
- About 0.5 mol% of Cu(NO3)2 as catalyst can also be used as an alternative catalyst. Although it contains metal, it is a better alternative as the overall energy consumption for preparing PILs can be much higher.