Aim and Objective:
In order to preserve the environment from harmful organic solvents, a synthesis
of coumarin derivatives was performed in deep eutectic solvents, which are considered as “green” due to their
characteristics.
Materials and Methods:
Choline chloride based deep eutectic solvents (DESs) were employed, both as
solvents and as catalysts, in the synthesis of coumarin derivatives via Knoevenagel condensation. In order to
find the best DES for coumarin synthesis, 20 DESs were tested for the reaction of salicylaldehyde and dimethyl
malonate at 80 °C.
Results:
Among the twenty tested deep eutectic solvents only five were adequate for this kind of synthesis. The
best DES for this reaction was found to be the one composed of choline chloride:urea (1:2). Most coumarin
compounds were obtained in good to excellent yield. Compounds 1g, 2g and 2p should be pointed out due to
their yields of 85, 88 and 98 %, respectively. 3-Acetylcoumarins 5a, 5c, 5d, 5e, 5f and 5g were synthesized
under ultrasound irradiation and were also obtained in excellent yields of 90, 95, 98, 93, 94 and 85 %,
respectively.
Conclusion:
Series of coumarin derivatives were successfully synthesized, either in choline chloide:urea DES
at 80 °C or in ultrasound-assisted reaction, from different salicylaldehydes and active methylene compounds.
These “green” methods were found to be very effective in Knoevenagel condensation, while DES was recycled
for several cycles without any significant influence on the product yield.
Syntheses of ZnO with Different Morphologies: Catalytic Activity toward Coumarin Synthesis via the Knoevenagel Condensation Reaction
