Rapid and efficient functionalized ionic liquid-catalyzed aldol condensation reactions associated with microwave irradiation

Abstract

Five quaternary ammonium ionic liquid (IL) and two tetrabutylphosphonium ILs were prepared and characterized. An environmentally benign and convenient functionalized ionic liquid catalytic system was thus explored in the aldol condensation reactions of aromatic aldehydes with acetone. The aldol reactions proceeded more efficiently through microwave-assisted heating than through conventional thermal heating. The yield of products obtained under microwave heating for 30 min was approximately 90%, and the ILs can be recovered and reused at least five times without apparent loss of activity. In addition, this catalytic system can be successfully extended to the Henry reactions.

Figure 1.

Structures of functionalized ILs.

Figure 2.

Comparison of the yields of product 4a under the conditions of microwave heating and conventional heating. Reaction conditions: 2.5 mmol benzaldehyde, 7.5 mmol acetone, 3.4 mmol [N₂₂₂₂][EtNHC₃SO₃] (1.0 g) and 50 wt % of H₂O (1.0 mL H₂O), 80 °C.

Figure 3.

Recycling of [N₂₂₂₂][EtNHC₃SO₃] aqueous solution in the model aldol reaction. Reaction conditions: 2.5 mmol benzaldehyde, 7.5 mmol acetone, 3.4 mmol (1.0 g) [N₂₂₂₂][EtNHC₃SO₃] and 50 wt % of H₂O (1.0 mL), 80 °C, 0.5 h.

Scheme 1.

Model aldol reaction of benzaldehyde and acetone.

Scheme 2.

The reaction processes of aldol and Henry reaction when 4-nitrobenzaldehyde was used as reactant.

Scheme 3.

Aldol and Henry reactions.