Sustainable Self-Aldol Condensation of Unmodified Aldehyde: A Combination of Ionic Liquid and Heterogeneous Secondary-Amine Grafted on Silica

Abstract

Self-aldol condensation of an unmodified aldehyde has been effected by N-methyl-3-aminopropylated silica as a catalyst in ionic liquid, [bmim]PF₆, to give an α,β-unsaturated aldehyde (enal). Aldehydes having acid or base sensitive substituents provided desired enals in satisfactory yields. The reaction system has been efficiently recycled more than eight times.

aldehydes - aldol reactions - amine - catalyst - silica support - ionic liquid

References

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<A NAME="RU03703ST-19">19</A>

Preparation of NMAP-SiO ₂ : SiO₂ (JRC-SIO-8, 5.4 g, surface area = 290 m²/g) supplied from the Catalysis Society of Japan was evacuated at 150 °C prior to use. To the SiO₂ was added a solution of N-metlyl-3-aminopropyl(tri-ethoxy)silane (2.54 g, 10.8 mmol) in toluene (15 mL). The resulting mixture was heated at reflux for 12 h. The solid was filtered off, washed with toluene and acetone and dried at 100 °C for 6 h. Nitrogen loading was 0.75 mmol/g according to combustion analysis.

<A NAME="RU03703ST-20">20</A>

Typical Experimental Procedure: To a stirred solution of n-decanal 1 (188 µL, 1.0 mmol) in [bmim]PF₆ (1 mL) were added NMAP-SiO₂ pellets (264 mg, 0.75 mmol/g, 0.2 mmol) and the flask was heated at 80 °C for 5 h under nitrogen atmosphere. After being cooled to room temperature, Et₂O was added and stirring was continued for a few minutes. The upper ether layer was removed with a Pasteur pipette. This procedure was repeated until no product was observed in the [bmim]PF₆ layer by TLC monitoring (usually 20 times with ca. 5 mL of Et₂O). The combined Et₂O layer was evaporated in vacuo and the residue was purified by medium-pressure LC (eluent: EtOAc:n-hexane = 1:5) to give enal 2 (122 mg, 83%).
The ionic liquid involving silica pellets was heated at 70 °C for 1 h at 1.5 mmHg and used for subsequent reaction.