To investigate the surface functionalization g-C3N4 substrate characterized by Fourier transform infrared (FT-IR), energy-dispersive X-ray (EDX) spectroscopy indicates the type of elements present in the g-C3N4/Pr/ THAM nanocatalyst. X-ray diffraction (XRD) pattern is shown to investigate the crystal structure of the compound and the field emission scanning electron microscope (FESEM) analysis indicates the morphology of the synthesized catalyst surface. Finally, the heat resistance of the desired compound is characterized by Thermogravimetric analysis (TGA).
FT-IR spectroscopy
According to the spectrum presented in Figure 1, a broad peak has appeared in the range of 3000-3300 cm-1, which is related to the stretching vibration of NH bonds; the peak width can be assigned to NH groups involved in the hydrogen bond or the existence of the OH group. The stretching vibrational peak C =N is observed at 1602 cm-1. Peaks 1303 and 1082 cm-1 are attributed to the tensile vibration of C-N bonds formed between triazine and N-H groups, and the stretching vibration of C-N bonds in the ring in 1448 and 1379 cm-1 is easily visible. Also, the peak is 786 cm-1 due to the vibration of tri-s-triazine units. The presence of C-H stretching peaks (2800-3000 cm-1) confirms the synthesis of the desired composite.
EDS analysis
In this section, we will go to the EDS spectrum to confirm the synthesis of the g-C3N4 /THAM nanocatalyst compound. As shown in Figures 2, presence elements such as C, N, O confirms the synthesis of g-C3N4 / THAM nanocatalyst. Very small amounts of sodium, potassium and iodine are observed in the analysis, all of which are related to substances that added to the reaction to modify the surface carbon dioxide. Due to many elements and their disintegration, the names of the elements are not mentioned in the device's intermediate diagram; they are listed in the quantitative table shape.
XRD analysis
The structure of the g-C3N4/Pr/ THAM nanocatalyst was characterized using by XRD pattern. As observed in Figure 3, the broad and intense reflection peaks at 2θ= 27.4 were assigned to the reflection peak of graphitic carbon nitride with Card No. JCPDS 87-1526. The new couriers have corresponded to the surface modification and nanocatalyst synthesis.
FE-SEM analysis
FESEM analysis was used to study the morphology and particle size distribution of the synthesized nanocomposite. Figure 4(a-d) has demonstrated images of nano-sheets g-C3N4 and nano-sheets g- C3N4/ Pr/THAM nanocatalyst by FE-SEM studies. As expected, FE-SEM images of graphitic carbon nitride (a,b) are smooth and are almost neatly linked and placed on each other due to being graphitic. In nano-sheets g-C3N4/ Pr/THAM nanocatalyst images (c,d), g-C3N4 plates are not smooth and have particles on them that exhibited the synthesis of the nanocomposite. Also, carbon nitride nano-sheets have become irregular.
TGA analysis
These observations also agree with the thermogravimetric analysis (TGA). Figure 5 shows the thermal stability of synthesized graphitic carbon nitride/ tris(hydroxymethyl)aminomethane composite which was carried out by TGA. The resulting catalyst as evidenced by TGA has displays high-temperature resistance up to approximately 400 °C. The mass-loss curve presents an inflection point at 400 °C. From 400°C, the diagram exhibits a gentle slope which is probably due to the decomposition of the composite before entering a nearly sharp slope being attributed to the thermal decomposition of carbon nitride.
Catalyst application of the g-C3N4/Pr/ THAM nanocatalyst in MCRs
The catalytic activity of the g-C3N4/Pr/ THAM nanocatalyst was studied in two MCRs for the synthesis of 1,4-dihydropyridine and pyranopyrazole derivatives to obtain the highest performance. To achieve the best result, different experimental conditions such as temperature, solvent, and amount of catalyst must be examined. To optimize the reaction conditions and performance evaluation of the g-C3N4/Pr/THAM nanocatalyst in Hantzsch reaction for the synthesis of 1,4-dihydropyridine derivatives, a one-pot four components reaction of chlorobenzaldehyde (1 mmol), ethyl acetoacetate (1 mmol), dimedone (1 mmol), and ammonium acetate (1 mmol) were investigated as a model reaction. Also, for the synthesis of pyranopyrazole derivatives a four-component reaction ethyl acetoacetate (1 mmol), hydrazine hydrate (1 mmol),4- chlorobenzaldehyde (1 mmol), and malononitrile (1 mmol) in ethanol was considered as the model reaction. Both model reactions showed almost the same behavior. One of the crucial factors in multicomponent reactions is solvent selection due to the different yields of each catalyst in different solvents. therefore, we have examined different solvents (protic and aprotic) to observe the effect. First, optimization experiments were conducted in ethanol, methanol, acetonitrile, DCM, DMF, DMSO, THF, toluene, water, ethanol/water (3:1), methanol/water (3:1), and solvent-free conditions as green conditions in both model reaction and the best efficiency were obtained in ethanol (Table 1, entries 1). Among these solvents, just MeOH, DCM, and ACN gave the intended product for 92, 60, and 66% yield, respectively (Table 1, Entries 2-4). Protic solvents have shown great progress in model reactions in comparison to aprotic solvents. In aprotic solvents, the intended product has not been seen in TLC even after 1h. Thus, we have desired to use protic solvents more than aprotic solvents. Unfortunately, model reactions showed no intended product in TLC in water as solvent (Table 1, entry 9). The insolubility of organic compounds in water could be the reason for this event. Therefore, we have used the combination of water and ethanol (Table 1, entries 10-11). The results showed that the existence of water would decrease the yields. We also tried model reactions in solvent-free conditions. In these conditions, the intended product has not been seen in TLC even after 1h. After concluding the results, ethanol was the best solvent among the others and was selected as the constant solvent for further studies.
The subsequent model reaction was performed without catalyst and in the presence of ethanol, the yield of this MCRs was not considerable (Table 1, entries 14). Another critical factor in multicomponent reactions is the amount of catalyst in the reaction. By adding the g-C3N4/Pr/ THAM nanocatalyst to the model reaction in the presence of a solvent, the efficiency increased dramatically and the yield was reached about 91%. Thus, we have examined the different amounts of the catalyst to find the optimum amount, and the results have shown in Table 1. Hence, we have added 0.01 g of catalyst to the reaction mixture. The TLC monitoring showed that the intended product is produced (Table 1, Entry 15). For optimizing catalyst amount, we have increased the amount by 30 mg. As a result, it showed that the amount of catalyst upper 0.02 g does not affect the yield of the intended product. (Table 1, Entry 18). Accordingly, it was observed that 0.02g nanocatalyst is sufficient to conduct this MCRs.
Table 1. Optimizing the reaction conditions in the synthesis of 1,4-dihydropyridinea and pyranopyrazoleb derivatives
Entry
|
Solvent
|
Catalyst amount (g)
|
Time (min)
|
Yield(a) (%)
|
Yield(b) (%)
|
1
|
EtOH
|
0.02
|
15
|
90
|
91
|
2
|
MeOH
|
0.02
|
15
|
87
|
85
|
3
|
MeCN
|
0.02
|
15
|
68
|
63
|
4
|
DCM
|
0.02
|
15
|
53
|
50
|
5
|
DMF
|
0.02
|
60
|
_
|
_
|
6
|
DMSO
|
0.02
|
60
|
_
|
_
|
7
|
THF
|
0.02
|
60
|
_
|
_
|
8
|
Toluene
|
0.02
|
60
|
_
|
_
|
9
|
H2O
|
0.02
|
60
|
_
|
_
|
10
|
EtOH/H2O (3:1)
|
0.02
|
15
|
75
|
73
|
11
|
EtOH/H2O (3:1)
|
0.02
|
15
|
68
|
64
|
12
|
_
|
0.02
|
60
|
_
|
_
|
13
|
_
|
0.02
|
60
|
_
|
_
|
14
|
EtOH
|
_
|
60
|
_
|
_
|
15
|
EtOH
|
0.01
|
15
|
53
|
50
|
16
|
EtOH
|
0.015
|
15
|
73
|
71
|
17
|
EtOH
|
0.02
|
15
|
90
|
91
|
18
|
EtOH
|
0.03
|
15
|
66
|
64
|
(a) Reaction conditions: chlorobenzaldehyde (1 mmol), ethyl acetoacetate (1 mmol), dimedone (1 mmol), and ammonium acetate (1 mmol), catalyst (0.01-0.03 g), the yields relate to the isolated product.
(b) Reaction of ethyl acetoacetate (1 mmol), hydrazine hydrate (1 mmol), 4- chlorobenzaldehyde (1 mmol) and malononitrile (1 mmol), catalyst (0.01-0.03 g), the yields relate to the isolated product.
After the optimization process, the generality of the procedure was evaluated by the reaction of different aromatic aldehydes with different types of electron-donating and electron-withdrawing groups. As observed in Table 2 and 3, all desired product was obtained with high yields in short reaction times. After optimizing the reaction conditions, we investigated the limitation and generality of the presented procedure using different aromatic aldehydes (Table 2 and 3).
Table 2. The synthesis of 1,4-dihydropyridine derivatives in optimized condition using g-C3N4/Pr/ THAM nanocatalyst.
|
Entry
|
R
|
Products
|
Time (min)
|
Mp (ºC)
|
Yielda (%)
|
Observed
|
Literature
|
1
|
H
|
5a
|
15
|
201-203
|
203-20444
|
90
|
2
|
4-
Cl
|
5b
|
15
|
245-246
|
245-24645
|
96
|
3
|
3-
NO2
|
5c
|
30
|
182-183
|
180-18346
|
89
|
4
|
4-
OH
|
5d
|
25
|
231-233
|
231-23346
|
87
|
5
|
2-Cl
|
5e
|
15
|
200-202
|
202-20447
|
95
|
6
|
4-Me
|
5f
|
25
|
259-261
|
259-26248
|
86
|
7
|
4-NO2
|
5g
|
20
|
241-243
|
241-24349
|
90
|
8
|
4-OMe
|
5h
|
25
|
256-258
|
258-26050
|
87
|
9
|
2,4-Cl
|
5i
|
35
|
241-243
|
240-24251
|
94
|
10
|
3-OH
|
5j
|
30
|
232-233
|
230-23252
|
89
|
a The yields relate to the isolated product.
Table 3. The synthesis of pyranopyrazole derivatives in optimized condition using the g-C3N4/Pr/ THAM nanocatalyst.
|
Entry
|
R
|
Products
|
Time (min)
|
MP (oC)
|
Yielda (%)
|
Observed
|
Literature
|
1
|
H
|
10a
|
20
|
244-246
|
243-24553
|
91
|
2
|
4-
Cl
|
10b
|
20
|
231-233
|
230-23243
|
95
|
3
|
4-
NO2
|
10c
|
25
|
250-252
|
250-25154
|
88
|
4
|
2,4-
Cl
|
10d
|
40
|
234-236
|
234-23655
|
92
|
5
|
4-
OH
|
10e
|
30
|
223-224
|
223-22455
|
93
|
6
|
4-
Me
|
10f
|
30
|
176-177
|
177-17855
|
91
|
7
|
3-
NO2
|
10g
|
35
|
213-215
|
213-21655
|
93
|
8
|
2-
Cl
|
10h
|
20
|
255-257
|
245-24655
|
93
|
9
|
4-
MeO
|
10i
|
30
|
176-177
|
177-17855
|
87
|
10
|
4-
Br
|
10j
|
35
|
180-182
|
179-18156
|
90
|
a The yields relate to the isolated product.
Suggested mechanism for synthesis of pyranopyrazole and 1,4-dihydropyridine derivatives
The characteristics of g-C3N4/Pr/THAM nanocomposite as a multifunctional catalyst was utilized for the synthesis pyrano[2,3-c]pyrazol and 1,4-dihydropyridine derivatives.
The Hantzsch reaction widely used for direct synthesis of 1,4-dihydropyridine (DHPs) derivatives. Based on previous studies 57,58, we propose the following mechanism for the synthesis of 1,4-dihydropyridine derivatives. We know that the surface of our g-C3N4/Pr/THAM catalyst plays an important role for the synthesis of desired derivatives. 1,4-dihydropyridine derivatives are synthesized by two slightly different methods. In the first method which presented in Scheme 3, the critical intermediate (I) formed through Knoevenagel condensation of benzaldehyde and dimedone in the presence of g-C3N4/Pr/THAM. On the other hand, the nucleophilic attack of the nitrogen of the ammonium acetate and the condensation of ethyl acetoacetate and ammonium acetate generated the intermediate (II). The next step involves Michael addition of (II) to (I), ring closing, and water elimination produced 1,4-dihydropyridine derivatives. But there is another mechanism for this reaction, In the second method, the reaction between dimedone and ammonium acetate in the presence of catalyst would obtain intermediate (III). The reaction between ethyl acetoacetate and aldehyde would obtain intermediate (IV). Eventually, by the reaction of two intermediates, the corresponding product would obtain and the g-C3N4/Pr/ THAM nanocatalyst was returned to reaction cycle. Also, the essential role of the catalyst is shown.
To form the pyranopyrazole derivatives, the g-C3N4/Pr/THAM nanocomposite as a bifunctional catalyst was utilized. According to reported articles 43,57 ,we have proposed a plausible reaction mechanism for the preparation of pyranopyrazole derivatives which is outlined in Scheme 4. First, the carbonyl groups were subjected to the nucleophilic attack of hydrazine hydrate with two nucleophilic sites. At this stage, by removing water and ethanol molecules respectively and also tautomerization intermediate (I) is formed. On the other hand, intermediate (II) was produced via Knoevenagel condensation reaction and with loss of H2O between catalyst-activated aromatic aldehyde and malononitrile. In the second stage, two intermediate pyrazolone ring and 2-phenylidenemalononitrile catalyzed with g-C3N4/Pr/ THAM compound and produced intermediate (III) and (IV). These compounds are formed by the reactions Michael addition, 6-exo-dig cyclization respectively. In the last step, by proton transfer and tautomerization of molecule (IV), the desired pyranopyrazole derivatives were obtained. Then g-C3N4/Pr/THAM nanocatalyst was returned to the reaction cycle to reuse.
Reusability
Reusability is one of the most important factors in any catalyst system, which highlights them as an efficient system due to time savings and economic benefits. The reusability of this synthesized catalyst was investigated in two MCRs. Initially, the heterogeneous catalyst was separated from the reaction mixture with filter paper. Then, it was washed several times with water and ethanol and dried in an oven at 80 ° C. It was observed that the catalyst can be reused at least five times without significantly reducing its activity (Figure 6).