Document Type : Original Article
Department of Chemistry, Payame Noor University (PNU), P.O. Box, 19395-3697, Tehran, Iran
In the present study, an efficient and recyclable heterogeneous phase-transfer catalyst was prepared through the functionalization of magnetic nanoparticles with ethylenediamine and polyethylene glycol. After being characterized by various physico-chemical techniques, the bifunctional magnetic nanocomposite was used as a heterogeneous phase-transfer catalyst in the Knoevenagel condensation of aryl aldehydes with active methylene compounds under mild and green conditions. This procedure has several advantages such as high yield of products, short time of reaction, easy workup, mild reaction conditions, low amount of catalyst, and catalyst recoverability.
- Knoevenagel Condensation Reusable Catalyst Phase
- Transfer Catalyst Bifunctional Nanocomposite Aqueous Media
The Knoevenagel condensation, which is one of the greatest valuable and extensively working methods for carbon-carbon bond formation, has often been used for the production of biologically important heterocyclic compounds and fine chemicals [1, 2]. In the Knoevenagel condensation reaction, an aldehyde reacted with an active methylene compound, including dimedone, malononitrile, ethyl acetoacetate, etc. [3-5]. This reaction is generally catalyzed by homogeneous or heterogeneous catalysts containing weak bases such as Nano CeO2 , ethylenediamine , and alkaline earth metals , Nano CeO2/Al2O3 , LaCl3.7H2O , NbCl5 , KF–Al2O3 , and L-proline-Cu/TCT@NH2@Fe3O4  have been used as catalysts in this reaction.
Homogeneous catalysts show high efficiency in chemical reactions [14-17], but they commonly have low chemical and thermal stability. In addition, their recovery is mostly difficult and expensive, and causes additional waste . These issues can be solved using environmentally friendly heterogeneous catalysts instead of homogeneous ones .
The simplest way to achieve this goal is to support homogeneous catalysts on insoluble materials like silica, graphene, polymers, and magnetic nanoparticles (MNPs) [20-23]. Today, MNPs are widely used due to their special features including large surface area, biocompatibility, low toxicity, and retrievability [24-27]. Magnetic separation makes it much easier to recover the catalyst from the reaction mixture by magnet than by centrifugation and filtration .
In the present work, continuing our research on the preparation of nanomagnetic-supported organocatalysts [29-34], we synthesized a recoverable heterogeneous phase-transfer catalyst, and after characterization, its catalytic performance was investigated in Knoevenagel condensation. The results showed that this catalyst efficiently catalyzes the reaction between active methylene compounds and aryl aldehydes.
Materials and Methods
The materials were procured from Sigma-Aldrich and Merck companies. The magnetic property was determined at room temperature on VSM (Meghnatis Daghigh Kavir Co Iran). TEM measurements were carried out on a Zeiss EM10C microscope at 100 kV. SEM and EDS analyses were carried out using a Zeiss-Sigma VP instrument for the morphology and elemental analysis of the synthesized nanocatalyst. XRD analysis was carried out using PANalytical X'Pert Pro X-ray diffractometer. FT-IR spectra were recorded using a Shimadzu IRPrestige-21 spectrometer and samples were analyzed as a KBr disk.
Preparation of the catalyst
PEG-300 (3 mmol) and sodium hydride (3 mmol) were poured in 20 mL toluene at 0 ᵒC under an inert atmosphere. After stirring the mixture for one hour at 60 ᵒC, 1.0 g of Fe3O4@SiO2@(CH2)3-Cl  suspended in 50 mL toluene was added and stirred under reflux conditions for 12 h. The resulting MNPs were dried at 60 ᵒC after washing by ethanol and acetone. In the next step, 1.0 g of Fe3O4@SiO2-PEG was dispersed by ultrasonic irradiation in 100 mL toluene for 20 min. After adding 2 mL of (3-chloropropyl) triethoxysilane and refluxing the mixture for 12 h, the obtained Fe3O4@SiO2-PEG/Cl was washed with EtOH and dried at 60ºC. Finally, to the ultrasonicated suspension of Fe3O4@SiO2-PEG/Cl (1.0 g) in 100 mL acetonitrile, 5 mL ethylenediamine was added and the mixture was stirred under reflux conditions for 12 h. After cooling, the resultant MNPs were separated by a magnet and dried at 60ºC after washing by ethanol .
Knoevenagel condensation catalyzed by Fe3O4@SiO2-PEG/en
The Fe3O4@SiO2-PEG/en catalyst (0.01 g) was added to a mixture of an aldehyde (1 mmol) and an active methylene compound (1 mmol) in 3 mL water. The mixture was stirred at room temperature until TLC indicated the end of reaction. After completion of the reaction, the catalyst was easily removed with an external magnet. The residue suspension was filtered and recrystallized from EtOH to give a pure compound.
Results and Discussion
Scheme 1 represents the concise route for preparing the nanomagnetic-supported bifunctional polyethylene glycol/ethylenediamine phase-transfer catalyst, Fe3O4@SiO2-PEG/en.
As previously reported,34 various physico-chemical techniques such as FT-IR, SEM, EDS, TEM, XRD, and VSM were utilized to characterize the Fe3O4@SiO2-PEG/en nanocatalyst (Figures 1, 2, 3, 4 and 5).
The catalytic performance of the catalyst was surveyed in the reaction of active methylene compounds with aryl aldehydes. The most acceptable result was obtained when 0.01 g of the nanomagnetic PTC was used (refer to Table 1, entry 1 for more information about the catalyst amount optimization).
After optimization, the reaction of various aryl aldehydes (1) with active methylene compounds, including malononitrile (2), ethyl cyanoacetate (3), 1,3-cyclohexanedione (4), and dimedone (5) achieved product 6a-i (2-benzylidene malononitrile, 2-(2-Chlorobenzylidene) malononitrile, 2-(3-Chlorobenzylidene) malononitrile, 2-(4-Chlorobenzylidene) malononitrile, 2-(4-hydroxybenzylidene) malononitrile, 2-(3-nitrobenzylidene) malononitrile, 2-(4-metylebenzylidene) malononitrile, 2-(4-methoxyebenzylidene) malononitrile, 2-(2-(furan-2-yl) benzylidene) malononitrile)) product 7a-f ((E)-Ethyl 2-cyano-3-phenylacrylate, (E)-Ethyl 3-(3-chlorophenyl)-2-cyanoacrylate, (E)-Ethyl 3-(4-chlorophenyl)-2-cyanoacrylate, (E)-Ethyl 3-(4-hydroxyphenyl)-2-cyanoacrylate, (E)-Ethyl 3-(3-nitrophenyl)-2-cyanoacrylate, (E)-Ethyl 2-cyano-3-(2-(furan-2-yl) phenyl) acrylate) product 8a-i (2-benzylidenecyclohexane-1, 3-Dione, 2-(2-chlorobenzylidene) cyclohexane-1, 3-Dione, 2-(3-chlorobenzylidene) cyclohexane-1, 3-Dione, 2-(4-chlorobenzylidene) cyclohexane-1, 3-Dione, 2-(4-hydroxybenzylidene) cyclohexane-1, 3-Dione, 2-(3-nitrobenzylidene) cyclohexane-1, 3-Dione, 2-(4-methylbenzylidene) cyclohexane-1, 3-Dione, 2-(4-methoxybenzylidene) cyclohexane-1, 3-Dione, 2-((furan-2-yl) methylene) cyclohexane-1, 3-Dione) and product 9a-i (2-benzylidene-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(2-chlorobenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(3-chlorobenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(4-chlorobenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(4-hydroxybenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(3-nitrobenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(4-methylebenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-(4-methoxybenzylidene)-5, 5-dimethylenecyclohexane-1, 3-Dione, 2-((furan-2-yl) methylene)-5, 5-dimethylenecyclohexane-1, 3-Dione) was done under optimal conditions (Scheme 2 and Table 1). Aromatic aldehydes with electron-withdrawing and electron–donating groups have led to the related alkenes in high yield (Table 1).
The reusability of Fe3O4@SiO2-PEG/en in the Knoevenagel condensation between benzaldehyde and malononitrile in aqueous media at room temperature is depicted in Figure 6. After separation by a magnet, the catalyst was used for the next run. The catalyst can be reused six times with no considerable loss of catalytic activity.
In this work, to produce an efficient bifunctional phase-transfer catalyst, ethylenediamine, and polyethylene glycol were anchored on the MNPs surface. After characterization, the magnetic nanocomposite was utilized in Knoevenagel condensation at room temperature under aqueous media. The high yield of products, short reaction time, mild reaction conditions, and reusability of the catalyst have made the present method a green and effective alternate to the existing methods in the literature.
No potential conflict of interest was reported by the authors.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
All authors contributed to data analysis, drafting, and revising of the paper and agreed to be responsible for all the aspects of this work.
Fateme Hakimi: https://orcid.org/0000-0002-4580-4139
Elham Golrasan: https://orcid.org/0000-0003-0853-0033
HOW TO CITE THIS ARTICLE
Hakimi, A. Sharifi-Zarchi, E. Golrasan. Bifunctional Polyethylene Glycol/ethylenediamine Nanomagnetic Phase-Transfer Catalyst: Preparation, Characterization, and Application in Knoevenagel Condensation. Chem. Methodol., 2023, 7(6) 489-498