Catalyst identification
The synthesis steps of magnetic CS-SA nanocomposite are illustrated in Scheme 2. After the synthesis of the magnetic-CS-SA, FT-IR, XRD, EDX, FESEM, TGA, VSM, etc. techniques were used to confirm its synthesis and structure. The FTIR technique was applied to show the main functional groups in the structure of the nanocomposite. FTIR spectra were taken from all nanocomposite synthesis steps (Fig. 2).
As FTIR spectrum of CS (Fig. 2a) shows, bands in 3414.4 cm− 1 and 3474.3 cm− 1 are related to N-H stretching, and O-H adsorption appears at 3550.5 cm− 1. The aliphatic C-H stretching vibration appeared at 2927.1 cm− 1. The bending frequencies of N-H and O-H are appeared at 1521.1 cm− 1 and 1616.9 cm− 1. The emerging frequencies at 1155.9 cm− 1 and 1083.7 cm− 1 belong to the C-N, C-O [26]. The CS-Pr-Br (Fig. 2b) shows almost identical spectrum to CS with a slight alteration due to appearance of C-Br adsorption peak at 621.6 cm− 1 [27]. As can be seen from the IR spectrum of CS-SA (Fig. 2c), all peaks seen in the FTIR spectrum of CS-Pr-Br are repeated here with a slight shift. It is worth noting that three new peaks are visible in the spectrum, the first at 755.9 cm− 1 (belonging to the S-O tensile frequency) [28], the second at 1617.6 cm− 1 belonging to the C = C aromatic and the third adsorption at 1388.2 cm− 1 relating to the S = O stretching frequency. In the IR spectrum of magnetic-CS-SA nanocomposite, Fe-O at 612.1 cm− 1 is emerged (Fig. 2d). The IR spectrum was taken again from the catalyst after the fifth thigh, based on the spectrum; the structure of the nanocatalyst is preserved.
The crystalline structure of CS (a) Fe3O4 (b) and magnetic-CS-SA (c) was investigated using XRD analysis (Fig. 3). In the XRD pattern of CS, only one peak is seen in 2Ѳ = 20.21 ο, which is related to amorphous structure of chitosan. As it is obvious, the peaks appearing in the 2Ѳ = 30.57 ° (220), 35.8 ° (311), 43.36 ° (400), 53.77 ° (422), 57.43 ° (511), 62.99 ° (440) and 74.67 ° confirm the cubic structure of Fe3O4 NPs (JCPDS card no. 01-075-0449). In the XRD pattern of final nanocomposite, all peaks related to iron nanoparticles appear with a small amount of shift, and the peak observed at 2Ѳ = 20.2 indicates the presence of CS in the structure [29, 30]. Also, the presence of chitosan did not change the crystalline phase of iron nanoparticles (Fig. 3).
FESEM analysis was also used to obtain the structure of nanocomposite. According to the FESEM images (Fig. 4a, 4b), nanoparticles have a spherical structure with a unique distribution. Energy dispersive X-ray analysis (EDS) was also used to confirm the presence of the main elements in the nanocomposite structure (Fig. 4c). According to the EDS pattern, iron (20.26%), carbon (44.20%), oxygen (32.35%), sulfur (1.71%), and nitrogen (1.69%) are present in the structure of magnetic-CS-SA. Another practical technique to confirm the synthesis of the nanocatalyst is elemental mapping. As Fig. 5 shows, all the key elements are well and evenly distributed in the nanocomposite texture.
The magnetic behavior of the magnetic-CS-SA and Fe3O4 nanoparticles was evaluated by VSM technique (Fig. 6). The Fe3O4 showed 52.85 emu g− 1and the magnetic-CS-SA 38.57 emu g− 1. According to the results, both samples showed superparamagnetic properties. The decrease in magnetic property of magnetic-CS-SA indicates the binding of iron nanoparticles to CS-SA and its successful synthesis.
Thermal stability evaluation of magnetic-CS-SA was evaluated using TGA analysis (Fig. 7). The weight loss observed before 200°C is consistent with the evaporation of organic solvents and adsorbed water. The next weight losing happens in 200–400°C, which is due to the separation and disintegration of organic groups such as 4-amino-3-hydroxy naphthalen-1-sulfonic acid and chitosan from the nanocomposite surface. Also, due to the presence of chitosan and organic groups, the nanocomposite is stable up to about 250°C, which can be easily used in organic reactions and at high temperatures [31]. Iron nanoparticles can be attached to chitosan through Fe+ 3 ions and also attached to chitosan through transverse bridging. The weight loss observed at 400 to 700°C is probably related to the detachment of the chitosan connected by the bridge [27].
Catalytic Studies
Catalytic performance of magnetic-CS-SA in the one-pot reaction between benzaldehyde (1 mmol), ammonium acetate (1 mmol) and ethyl acetoacetate (2 mmol) was evaluated as the selected reaction for the formation of 1,4-DHPs (Table 1). Also for the preparation of polyhydroquinolines, we used 1 mmol of daimedone and 1 mmol of ethyl acetoacetate instead of 2 mmol of ethyl acetoacetate (Table 2). First, both of these model reactions were carefully investigated with different solvents like water, ethanol, solvent-free conditions, tetrahydrofuran, methanol, and toluene. The results show that the highest yield was obtained when ethanol solvent was used. In the next step to optimize the amount of composite loading (0.02, 0.3, 0.035, 0.04 g), both model reactions were evaluated (Tables 1,2, entries 8–13). Based on the attained evidence, 0.035 g of catalyst has the best performance. Also, as Tables 1,2, entry13 showed, the amount of products did not improved as the catalyst increased. After obtaining the appropriate amount of catalyst (0.035 g), temperature was investigated as another effective parameter. With elevation of the reaction temperature to 50°C the yield increased by about 12–13%. (Tables 1,2, entries 10, 12). Based on the results, different aldehydes containing 3-NO2, 4-NO2, H, 4-CH3, 4-OCH3, 4-OH, 2-Cl, 4-Cl groups produced the desired products with excellent yields (85–96%) (Table 3). Also, the best time and the highest efficiency were obtained in the presence of benzaldehyde (entries 5 and 14). As Table 3 displays, the electronic effects have little influence on reaction rate and yield. Moreover, thiophene-2-carbaldehyde was also converted to its relative 1,4-DHP and polyhydroquinoline compounds with high yields in short times in the presence of the magnetic composite (Table 3, entries 9, 18).
The possible mechanism for the preparation of 1,4-DHPs and polyhydroquinolines has been summarized in Scheme 3. The SO3H groups as present on the surface of the catalyst play the role of the Brønsted acid and the Fe3O4 nanoparticles groups act as Lewis acid. First, the catalyst activates aldehyde and the 1,3-di-carbonyl compounds (step 1). Then, the Knoevenagel condensation reaction occurs between the aldehyde and dimedone or ethyl acetoacetate, which forms the intermediates (I) and (III). The reaction of ammonium acetate with ethyl acetoacetate (step 2) also creates the intermediate (II). Then, magnetic-CS-SA accelerates the Michael addition between intermediates (I) and (II) in order to create 5a-5i products. Also, to create products 6a-6i, intermediates (II) and (III) react together in the presence of nanocomposites (Scheme 3).
In order to evaluate the efficiency of the synthesized nanocomposite, its catalytic activity was compared with other catalysts. As can be seen from Table 4, magnetic-CS-SA performs better in a shorter time than most previously reported nanocatalysts under mild and safe reaction conditions.
Table 4
Comparison of magnetic-CS-SA with some other systems in the synthesis of 1,4-DHP and polyhidroquinolines
Entry
|
Catalyst
|
Solvent
|
T (°C)
|
Time (min)
|
Yield (%)
|
1
|
PEG1000-DAIL
|
Toluene
|
80
|
40
|
91 [37]
|
2
|
Fe3O4-TiO2-SO3H
|
EtOH
|
reflux
|
50
|
95 [38]
|
3
|
MgO nanoparticles
|
EtOH
|
reflux
|
110
|
85 [39]
|
4
|
AFGONsa
|
EtOH
|
r.t
|
180
|
89 [40]
|
5
|
Cell-Pr-NHSO3H
|
EtOH
|
reflux
|
42
|
90 [41]
|
6
|
MGCSb
|
EtOH
|
reflux
|
15
|
89 [32]
|
7
|
Alumina sulfuric acid (ASA)
|
MeOH
|
70
|
120
|
92 [42]
|
8
|
Fe3O4@FSM-16-SO3H
|
EtOH
|
78
|
25
|
86 [43]
|
9
|
Magnetic-CS-SA (This work)
|
EtOH
|
50
|
15
|
96
|
a: Amine functionalized graphene oxide nano sheet
b: Magnetic guanidinylated chitosan
|
Reusability of the magnetic-CS-SA
In the last decade, the issue of recycling heterogeneous nanocatalysts has been one of the most serious issues for researchers, because recyclable catalysts are not only economically viable but also environmentally friendly. For this reason, the recoverability of acidic magnetic chitosan nanocomposite in model reactions (Tables 2,3, entry 12) under optimized conditions was surveyed. After completion of the reaction and separation of the catalyst with a magnet, the nanocatalyst was rinsed with ethanol, dried and used for five consecutive runs. Despite five intermittent uses of the nanocomposite, no significant reduction in its catalytic activity was observed (Fig. 8). The percentage of Fe3O4 nanoparticles leached after 5th run was found to be 1.5% for synthesis of 5e and 1.8% for 6e based on the AAS.
Hot filtration study
In the hot filtration test, the model reactions for the synthesis of 1,4-dihydropyridine and polyhydroquinoline derivatives under optimal conditions (Tables 2,3, entry 12) in the presence of heterogeneous magnetic nanocomposite CS-SA were investigated. After half the reaction time (8 min), the catalyst was removed with the help of an external magnet and the reaction was continued without catalyst-with filtrate under similar conditions. It is noteworthy that after removal of the nanocomposite, the reaction did not show any improvement, so no remarkable leaching of acidic groups or Fe3O4 from the nanocomposite surface was occurred. Based on hot filtration experiments, the asymmetric and symmetric Hantzsch reactions are catalyzed by heterogeneous magnetic-CS-SA nanocomposite (Fig. 9).