Abstract
Background: For the synthesis of tri and tetra-substituted alkenes, Knoevenagel condensations have previously been performed in solvents like dichloromethane, DMF, toluene, acetonitrile, DMSO etc. These conditions suffer from certain limitations like long reaction times are required in DMSO or harsh heating conditions are necessary using DMF as a solvent decreasing the yields to below 50%. Tedious work up is required after long reaction times or/and high temperatures, if the reaction is performed in toluene.
Methods: In a typical reaction, 1 mmol of aromatic aldehyde with 1.1 mmol of malanonitrile in the presence of bismuth nitrate (3 mmol%) was refluxed in water (10 ml) for 25-30 min. After completion of the reaction (TLC analysis), excess bismuth nitrate was filtered for next use and the filtrate was kept at room temperature over night for crystallization. Crystals were filtered, washed with water and dried. In a separate reaction the catalyst was re-used for the one of the same reactions and found satisfactory results.
Results: In order to generalize our newly developed methodology, we selected a variety of aldehydes to synthesize different trisubstituted alkenes and found that this method can also be applied on aromatic aldehydes. For comparison purposes, the reaction was carried out in ethanol in parallel to water and found that water is the best solvent for this reaction.
Conclusion: In summary, we have discovered a facile and rapid method for Knoevenagel condensations using bismuth nitrate as Lewis acid. The advantages of bismuth nitrate as a catalyst are: easy to remove from the reaction mixture by simple filtration, recyclable, requires water as solvent, is eco-friendly and afforded high yields (90-97%).