Introduction

The functionalization of the magnetic nanoparticles (the F-MNPs) is crucial for chemical processes, and they have many applications in medical and biological research. Meanwhile, the amine-functionalized magnetite nanoparticles (the AF-MNPs) have received more attention because of their properties such as excellent magnetism, tunable sizes, good crystallinity, and facile dispersion in water [¹-¹³].

The AF-MNPs and their derivatives have been used for different fields in organic chemistry such as bio-conjugated to biological molecules (proteins, nucleic acids, and peptides) [¹], for cellulase immobilization to prevent wastage of cellulosic resources [2], to prepare cis-pinane [³], catalyst for Heck reaction [⁴], synthesis of xanthenes [⁵], selective separation of catecholamines in urine [⁶], hydrogenation and Heck reactions by Pd immobilized on the AF-MNPs [⁷], for attachment of polyacrylic acid [⁸], for the treatment of gastric, colon, and pancreatic cancers in near future [9], for the synthesis of carbamates [¹⁰], for degradation of the methyl orange and p-nitrophenol reduction in aqueous media [¹¹], enhanced photocytotoxicity [¹²], and removal of carmoisine dye from aqueous solutions [¹³].

On the other hand, pyrrolin-2-one skeleton has biological activity in some synthetic drugs and natural products, for example: Cotinine [¹⁴], Doxapram hydrochloride [¹⁵], Ethosuximide [¹⁶], Rolipram [¹⁷], Lactacystin [¹⁸], Salinosporamide [¹⁹], (−)-Azaspirene [²⁰] and Pesticides [²¹] (Scheme 1).

Scheme 1 The chemical structure of some natural products and drugs with pyrrolinone moiety. 

Some procedures have been reported for the synthesis of pyrrolin-2-ones [²²-²⁷]. However, a coupling of arylamines and aromatic aldehydes with acetylenedicarboxylates is an important synthetic method for the synthesis of this class of compounds. This method has been studied by several protocols such as using of nano-TiO₂ [²⁸], in water as a green solvent [²⁹], by p-toluensulfonic acid [³⁰], under ultrasound irradiation [³¹], or microwave irradiation [³²], by ionic liquids [³³], in the presence of nano-magnetic particles [³⁴], and AmberChrom as a polymer catalyst [³⁵]. For improving the obtained result as a part of the synthetic project we turned attention to the synthesis of a more stable and convenient catalyst for this procedure. Thus, we decided to link oxalic acid on magnetite nanoparticles by a covalent bond. Our survey showed using of amine-functionalized magnetite nanoparticles (the AF-MNPs) is a good approach for this subject. Thus, pyrrolin-2-ones were synthesized by a new methodology from aldehydes, anilines and dimethyl acetylenedicarboxylate by Fe₃O₄@NH₂@oxalic acid as a reusable nano-magnetic catalyst at room temperature (Scheme 2).

Scheme 2 The synthesis of 1,5-diaryl-3-hydroxy-4-methoxycarbonyl-3-pyrrolin-2-ones by Fe₃O₄@NH₂@oxalic acid as a catalyst.  

Experimental

Results and discussion

A literature survey showed that the preparation of the AF-MNPs by direct coupling of amino agents to magnetite nanoparticles has been carried out by different chemical procedures. The chemical conditions for these methods have been summarized in table 1 for more consideration.

In continuous of these procedures (table 1), Fe₃O₄@NH₂ was synthesized by a new protocol. The magnetite nanoparticles were prepared from FeCl₃•6H₂O. Also, 1, 2-ethylenediamine was used as an amino agent. As shown in table 2, some experiments were carried out for optimizing the best ratio of the reagents, temperature, and reaction time. All reactions were performed by a one-pot reaction of FeCl₃•6H₂O and 1, 2-ethylenediamine in the ratio (1:10) in ethylene glycol at different temperatures & times under an inert atmosphere.

The quality of the obtained AF-MNPs was determined by comparing FT-IR spectra as shown in Fig. 1(a-c). In Fig. 1(a), the band at 645 cm^-1 assigned to the impurity some amount of maghemite [^36b] when the reactions were carried out at 200 °C (table 2, entry 2-3). The same results were achieved at 150-190 °C (table 2, entry 1). Due to the absorbance bands at 3428 cm^-1 (NH stretching vibration) and 2934 and 2862 cm^-1 (asymmetric stretching vibration and symmetric stretching vibration of CH₂), 1, 2-ethylenediamine was coated better at 110 °C within 6 hours (Fig. 1(c)) (Table 2, entry 5), while the coating was reduced in fewer time of reaction (3 hours, Fig. 1(b)) (Table 2, entry 4).

Fig. 1 (a) FT-IR spectrum of synthesized of Fe₃O₄-CH₂CH₂-NH₂ at 200 °C within 6 hours (Table 2, entry 3). (b) FT-IR spectrum of synthesized of Fe₃O₄-CH₂CH₂-NH₂ at 110 °C within 3 hours (Table 2, entry 4). (c) FT-IR spectrum of synthesized of Fe₃O₄-CH₂CH₂-NH₂ at 110 °C within 6 hours (Table 2, entry 5). 

Thus, the results (Table 2, entry 5) showed that a one-pot reaction of FeCl₃•6H₂O and 1, 2-ethylenediamine in the ratio (1:10) at 110 °C within 6 hours in ethylene glycol gave the best quality and quantity of Fe₃O₄-CH₂CH₂-NH₂ (Fe₃O₄@NH₂) as shown in Scheme 1. The chemical structure of Fe₃O₄@NH₂ was characterized by FT-IR and VSM spectra.

A one-pot reaction of FeCl₃.6H₂O and 1, 2-ethylenediamine in the ratio (1:10) at 110 °C within 6 hours in ethylene glycol gave the best quality and quantity of Fe₃O₄-CH₂CH₂-NH₂ (Fe₃O₄@NH₂) as shown in Scheme 3.

Scheme 3 The synthesis of Fe₃O₄@NH₂ nanoparticles.  

Then, oxalic acid as an organic acid was linked to surface of Fe₃O₄@NH₂ nanoparticles. The best reaction conditions were obtained by performing some reactions in different conditions as shown in Table 3.

The results showed (Table 3, entry 6) that oxalic acid can be linked by using ultrasonic irradiation at 60 °C within 4 hours under the Ar atmosphere. Thus, the Fe₃O₄-CH₂CH₂NH-CO-CO₂H (Fe₃O₄@NH₂@oxalic acid) nanoparticles were successfully synthesized, as shown in Scheme 4. The chemical structure of the Fe₃O₄@NH₂@oxalic acid was characterized by FT-IR, XRD, SEM, VSM, and EDAX spectra.

Scheme 4 The synthesis of Fe₃O₄@NH₂@oxalic acid nanoparticles from Fe₃O₄@NH₂ nanoparticles under ultrasonic irradiations.  

The FT-IR spectrum of Fe₃O₄-CH₂CH₂-NH₂ (Fe₃O₄@NH₂) is shown in Fig. 2(a). The band at 3428 cm^-1 was assigned to NH stretching vibration which was combined with OH stretching vibration of the absorbed water on Fe₃O₄. The bands at 2934 cm^-1 and 2862 cm^-1 were assigned to asymmetric stretching vibration and symmetric stretching vibration of CH₂. The band at 1626 cm^-1 was assigned to OH bending vibration of the absorbed water on Fe₃O₄. The band at 1547 cm^-1 was assigned to NH bending vibration. The band at 1316 cm^-1 was assigned to C-C stretching of vibration. The band at 1023 cm^-1 was assigned to C-N stretching vibration and the band at 583 cm^-1 was assigned to the Fe-O bending vibration [34b].

Fig. 2 The FT-IR spectra of (a) Fe₃O₄-CH₂CH₂-NH₂ (Fe₃O₄@NH₂), (b) Fe₃O₄-CH₂CH₂NH-CO-CO₂H, (Fe₃O₄@NH₂@oxalic acid) (c) oxalic acid.  

The FT-IR spectrum of Fe₃O₄-CH₂CH₂NH-CO-CO₂H (Fe₃O₄@NH₂@oxalic acid) has been shown in Fig. 2(b). The band at 3394 cm^-1 was assigned to OH stretching vibration of the COOH group. The band at 1684 cm^-1 and 1673 cm^-1 were assigned to C=O bond stretching vibrations of the COOH and CO-NH groups, respectively. The band at 1397 cm^-1 was assigned to OH bending vibration of the COOH group. The band at 1267 cm^-1 was assigned to C-O stretching vibration. The band at 1037 cm^-1 was assigned to C-N stretching vibration. The band at 787 cm^-1 was assigned to OH bending vibration of the COOH group, and 567 cm^-1 was assigned to the Fe-O bending vibration.

A comparison of the FT-IR spectra of Fe₃O₄@NH₂ in Fig. 2(a) and Fe₃O₄@NH₂@oxalic acid in Fig. 2(b) shows that a band at 1626 cm^-1 in Fig. 1(a) does not appear in the FT-IR spectrum of Fe₃O₄@NH₂@oxalic acid Fig. 2(b). Because this band was replaced by a doublet band at 1684 cm^-1 (acidic C=O) and 1673 cm^-1 (amide C=O). This result that was indicated the successful attachment of oxalic acid to Fe₃O₄@NH₂ nanoparticles.

The VSM spectrum was used to evaluate the magnetic measurement of magnetite nanoparticles at room temperature as shown in Fig. 3(a). Due to the lack of magnetic remanence and coercivity, the room-temperature magnetization curve of the Fe₃O₄@NH₂ and Fe₃O₄@NH₂@oxalic acid nanoparticles suggests a paramagnetic state of the nanoparticles. The magnetization curves indicate the saturation magnetization of Fe₃O₄@NH₂@oxalic acid and Fe₃O₄@NH₂ nanoparticles, which were diminished to 10.50 emu/g from 12.90 emu/g. These results are significantly smaller than reported values for bulk Fe₃O₄ (92 emu/g and 100 emu/g) [⁵⁴]. In the Fe₃O₄@NH₂ and Fe₃O₄@NH₂@oxalic acid adsorbed water, amine agent, and oxalic acid reduce the measured magnetic strength because magnetic nanoparticles are diluted with diamagnetic physically.

The SEM images of nano-Fe₃O₄@NH₂@oxalic acid (Fig. 3(b)) show the presence of spheric nanoparticles with 81 nm of average diameter.

Fig 3 a) The room-temperature magnetization curve of the Fe₃O₄@NH₂ and Fe₃O₄@NH₂@oxalic acid nanoparticles. b) The SEM spectra of Fe₃O₄@NH₂@oxalic acid.  

The XRD patterns of the Fe₃O₄@NH₂@oxalic acid nanoparticles are presented in Fig. 4. The peak positions are indexed as (111), (220), (331), (400), (422), (511), (440), (620), (533) and (444) in the 2θ range of 20-80°. These results reveal the existence of a Fe₃O₄ core in the Fe₃O₄@NH₂@oxalic acid which is in good agreement with the literature [⁵⁴-⁵⁵]. Fig. 5 shows EDAX analysis of Fe₃O₄@NH₂@oxalic acid nanoparticles. The sample peaks are observed for carbon, oxygen, iron, and nitrogen. It reveals that carbon was from 1, 2-ethylenediamine and oxalic acid, oxygen from oxalic acid and Fe₃O₄, iron from Fe₃O₄, and nitrogen from 1, 2-ethylenediamine. These results shows that surface of Fe₃O₄ was coated more than 93 % by organic materials (1, 2-ethylenediamine, and oxalic acid).

Fig 4 The XRD patterns of Fe₃O₄@NH₂@oxalic acid nanoparticles.  

Fig 5 EDAX analysis of Fe₃O₄@NH₂@oxalic acid nanoparticles.  

The synthesized Fe₃O₄@NH₂@oxalic acid nanoparticles were used for the synthesis of pyrrolin-2-one derivatives after optimum reaction conditions as shown in Table 4. It was best to combine benzaldehyde (1 mmol), aniline (1 mmol), and 0.2 g Fe₃O₄@NH₂@oxalic acid nanoparticles as the catalyst in ethanol. The reaction was completed after 8 hours to give the corresponding pyrrolin-2-ones in 95 % yields as shown in Scheme 2.

For more proofing, the reaction of a variety of anilines and aromatic aldehydes was performed under optimized reaction conditions. All the reactions were completed within 6-10 hours in the excellent yields of pure products (1-8a) (90-95 %) as shown in scheme 2. All products were characterized by FT-IR, ¹H-NMR, ¹³C-NMR and C. H, N elemental analysis. In the ¹H-NMR spectrum, the chemical shifts of the CH group in the heterocyclic ring appear around 5.62-6.41 ppm, as a single signal. In the FT-IR spectrum, the C=O stretching frequency of the products appears around 1675-1693 cm^-1. The melting points of the products were measured and compared with the literature [³⁰-³⁵].

Due to the chemical structure of Fe₃O₄@NH₂@oxalic acid, CO₂H groups can provide an acidic surface with good polarity. All moieties in this reaction are polar. Thus, they can react and condensate more easily on Fe₃O₄@NH₂@oxalic acid surface by dipolar-dipolar interactions. Fe₃O₄@NH₂@oxalic acid nanoparticles activate the aldehyde functional group for reaction with aniline. Thus, an imine as an intermediate (step (I), Scheme 3) is produced. Then, the obtained H₂O molecules from this condensation reaction, react with DMAD and produce a 1,3-dipolar intermediate (step (II), Scheme 3) [³⁰], which would undergo the addition to the imine (step (III & IV), Scheme 3). Then, an intramolecular attack of the amino group on one of the esters results in the formation of the five-membered ring as pyrrolin-2-one skeleton (a, step (V), Scheme 3).

Scheme 3 The schematic and the tentatively proposed mechanism for the synthesis of pyrrolin-2-one derivatives by Fe₃O₄@NH₂@oxalic acid as catalyst.  

The reusability of Fe₃O₄@NH₂@oxalic acid catalyst was examined. The recycled catalyst can be used three more times. For example, the second and third rounds yields of the product (Scheme 2, 1a) were 92 % and 93 % respectively. For more consideration, the efficiency of Fe₃O₄@NH₂@oxalic acid nano-magnetic catalyst for the synthesis of 1,5-diphenyl-3-hydroxy-4-methoxycarbonyl-3-pyrrolin-2-one (Sheme 2, 1a) has been compared with other reported protocols as shown in Table 5. The comparison shows Fe₃O₄@NH₂@oxalic acid catalyst has a good potential for the synthesis of pyrrolin-2-one derivatives.

Conclusions

In this research, the Fe₃O₄@NH₂ nanoparticle was synthesized from FeCl₃•6H₂O and 1, 2-ethylenediamine. Then, Fe₃O₄@NH₂@oxalic acid as an organoacid-magnetic nanoparticle was synthesized by coupling of Fe₃O₄@NH₂ nanoparticle and oxalic acid under ultrasonic irradiations. Also, the nano-Fe₃O₄@NH₂@oxalic acid was used for preparation of a variety of pyrrolin-2-ones. The reactions were performed in ethanol in the excellent of yields at room temperature. The recovered and regenerated catalyst can be useful for reaction with loss of activity. Thus, easy work-up, mild reaction conditions, recyclability, recovery, non-toxicity, economically affordable, and high efficiency are valuable characteristics that make this new method noteworthy.

Acknowledgements

The authors gratefully acknowledge the financial support of this work by the Research Council of the Islamic Azad University, Mahabad Branch, Mahabad, Iran. DOI: