MgFe₂O₄@Tris magnetic nanoparticles: an effective and powerful catalyst for one-pot synthesis of pyrazolopyranopyrimidine and tetrahydrodipyrazolopyridine derivatives

Abstract

Magnesium (Mg) as a metal has wide applications, but its use in chemical reactions is rarely reported. Currently, magnesium catalytic processes are being developed to synthesize basic chemical compounds. Therefore, an effective and recyclable nano-catalyst was synthesized using MgFe₂O₄@Tris in this study. The structure of MgFe₂O₄@Tris was characterized by various techniques including Fourier-transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM) techniques. Finally, the catalytic activity of this nano-catalyst was evaluated for the synthesis of pyrazolopyranopyrimidine and tetrahydrodipyrazolopyridine derivatives. Among the advantages of this catalyst are its high catalytic activity, high yields, use of environmentally friendly solvents, easy magnetic separation, and the possibility of reusing the catalyst.

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1. Introduction

Nanoparticles are an important class of nanometer-scale materials that exhibit unrivaled physicochemical attributes for widespread applications, such as in biomedicine,¹ biofuels,² sensors,³ batteries,⁴ and catalysts.⁵ The increase in the amount of scientific discussion on the topic of magnetic materials shows the increasing interest in this field of science.⁶ Spinel ferrites with the generic formula MFe₂O₄ (M = Mg, Mn, Ni, Co, or Zn) are important magnetic substances due to their high stability, excellent electrical and magnetic properties, catalytic properties and biocompatibility.⁷ Magnesium ferrite (MgFe₂O₄) is a magnetic nanomaterial that has good magnetic properties and electrical and thermal resistance.⁸ Recently, MgFe₂O₄ nanoparticles have attracted increasing amounts of interest because they have various applications in Li-ion batteries,⁹ gas sensors,¹⁰ catalysts,¹¹ and adsorbents.¹²

Multicomponent reactions (MCRs) are reactions between more than three compounds in one step and are important organic chemistry reactions for the synthesis of intricate molecules. They are widely used in all fields of organic synthesis.¹³ In the process of multicomponent reactions, magnetic nanocatalysts are the best option as heterogeneous catalysts due to their easy product separation technique, easy recovery, and environmentally friendly properties.

Heterocyclic compounds¹⁴ are one of the largest families of organic compounds. They are compounds with a cyclic structure that have at least one carbon atom and at least one heteroatom such as O, N, or S in their structure. Pyrazolopyranopyrimidines¹⁵ and tetrahydrodipyrazolopyridines¹⁶ are two important classes of nitrogen-containing heterocyclic compounds with diverse applications. Pyrazolopyranopyrimidines are a group of polycyclic-fused heterocyclic compounds, consisting of rings containing pyrimidine, pyran, and pyrazole. Pyrazolopyranopyrimidine derivatives are of great interest because of their broad pharmacological and biological properties and activities as antimicrobial,¹⁷ antituberculosis,¹⁸ anti-inflammatory,¹⁹ analgesic,²⁰ and anticancer agents.²¹ Tetrahydrodipyrazolopyridines are among the most important nitrogen-containing heterocyclic compounds, and they consist of two parts, pyrazole and 1,4-dihydropyridine. Due to anticancer,²² antiviral,²³ and anti-leishmanial²⁴ properties, they are of particular importance in organic chemistry. Nanocatalysis has found numerous applications in the synthesis of heterocyclic compounds due to the development of nanotechnology.²⁵ A majority of currently available and commercially approved drugs contain heterocycles containing nitrogen and oxygen. Due to the versatile applications of heterocycle synthesis catalyzed by metal nanoparticles, its significance should not be overlooked.²⁶

In this research, we report tris(hydroxymethyl) aminomethane-functionalized MgFe₂O₄ magnetic nanoparticles (MgFe₂O₄@Tris) as a new, efficient and recyclable catalyst for the synthesis of pyrazolopyranopyrimidine and tetrahydrodipyrazolopyridine derivatives. These heterocyclic compounds were prepared under mild conditions in a green solvent and at room temperature (Scheme 1).

Scheme 1 Synthesis of pyrazolopyranopyrimidine and tetrahydrodipyrazolopyridine compounds using MgFe₂O₄@Tris MNPs.

2. Experimental

2.1. General

All reactants used in this research were purchased from Merck, Fluka, or Sigma-Aldrich chemical companies. The melting points were defined using a Barnstead Electrothermal 9100 in capillary tubes. The infrared (IR) spectra of samples were recorded in KBr pellets using a VRTEX 70 spectrophotometer (Bruker, Germany). ¹³C and ¹H NMR spectra (in Hertz) were obtained using a Bruker DRX-250 AVANCE instrument in DMSO-d₆ as the solvent and TMS as the internal standard. Energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) were carried out and utilized a Czech TESCAN instrument. Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (PerkinElmer-STA6000, USA), and magnetic measurements of the nanocatalyst were obtained using a vibrating sample magnetometer (VSM; MDKB). X-ray diffraction (XRD) was carried out using a Holland Philips PW1730 and TEM of the magnetic nanoparticles (MNPs) was recorded with a Philips-EM 208S TEM.

2.2. Synthesis of MgFe₂O₄@Tris MNPs

2.2.1. General method for the synthesis of MgFe₂O₄ nanoparticles. A mixture of FeCl₃ (8 mmol = 1.3 g) and MgCl₂·6H₂O (4 mmol = 0.81 g) was dissolved in 40 mL of deionized water in a round-bottomed flask (250 mL). Then, it was stirred for 30 minutes at 500 rpm (revolutions per minute) by a mechanical stirrer. Then, 10 mL of sodium hydroxide solution (0.1 M) was added to the reaction mixture and heated for 2 h at 80 °C until brown precipitates were obtained. The product was separated by an external magnet and washed with deionized water, then washed twice with ethanol and dried in an oven at 80 °C for 24 h and calcined at 700 °C for 2 h.¹¹

2.2.2. General method for the synthesis of MgFe₂O₄@Tris nanoparticles. First, 1 g of MgFe₂O₄ produced in the previous step was dispersed in a mixture of 20 mL of H₂O and 30 mL of EtOH for 20 min. Then, 2.0 g of tris(hydroxymethyl)aminomethane was added and refluxed for 24 h. Finally, the obtained MgFe₂O₄@Tris MNPs were separated by an external magnet and washed many times with water and dried at 80 °C.²⁷

2.3. General method for the synthesis of pyrazolopyranopyrimidine derivatives

Initially, 1 mmol of hydrazine hydrate, 1 mmol of ethyl acetoacetate, and 2 mL of H₂O were added to the reaction vessel. Then, 1 mmol of aldehyde, 1 mmol of barbituric acid and 10 mg of MgFe₂O₄@Tris were added to the reaction mixture, and the resulting mixture was stirred at 300 rpm at room temperature. The progress of the reaction was monitored by thin-layer chromatography (TLC). After completion of the reaction, 10 mL of hot ethanol at a temperature of about 60 °C was added to the reaction mixture to dissolve the solid product. The catalyst was separated using an external magnet. The solvent was evaporated, and the crude product was obtained. The pure product was prepared through recrystallization from hot ethanol. The obtained pure products were characterized using melting points, FT-IR spectra, ¹H NMR spectra and ¹³C NMR spectra.

2.4. General method for the synthesis of tetrahydrodipyrazolopyridine derivatives

In a 5 mL round-bottomed flask, a mixture of ethyl acetoacetate (2 mmol), hydrazine hydrate (2 mmol), aldehyde (1 mmol), ammonium acetate (3 mmol), and MgFe₂O₄@Tris (15 mg) in water (2 mL) was stirred at 300 rpm at room temperature for an appropriate amount of time. The reaction progress was followed using thin-layer chromatography. After completion of the reaction, 10 mL of hot ethanol (at about 60 °C) was added to the reaction mixture to dissolve the solid product. Then, the catalyst was separated by a magnet. The solvent was evaporated, and the crude product was obtained. The pure product was prepared through recrystallization from hot ethanol. The obtained pure products were characterized using melting points, FT-IR spectra, ¹H NMR spectra and ¹³C NMR spectra.

3. Results and discussion

3.1. Synthesis and characterization of MgFe₂O₄@Tris

MgFe₂O₄@Tris nanoparticles were successfully prepared by the method outlined in Scheme 2. First, magnesium ferrite MNPs were synthesized using the co-precipitation technique.¹¹ Next, MgFe₂O₄@Tris was prepared by reacting MgFe₂O₄ with tris(hydroxymethyl)aminomethane in refluxed EtOH/H₂O under nitrogen for 24 h.²⁷ Next, the prepared catalyst was fully identified by various characterization methods such as FT-IR, VSM, EDX, EDX mapping, TEM, SEM, XRD, and TGA analysis.

Scheme 2 Stepwise synthesis of MgFe₂O₄@Tris MNPs.

3.1.1. FT-IR analysis. The FT-IR spectra of MgFe₂O₄ and MgFe₂O₄@Tris MNPs are displayed in Fig. 1. The FT-IR spectrum of MgFe₂O₄ displayed bands at 3412 and 1627 cm⁻¹ corresponding to the stretching and bending vibrations of the O–H bond. In addition, two peaks at 575 and 438 cm⁻¹ correspond to Mg–O stretches (Fig. 1A). Peaks at 2855, 2923, and 1383 were observed in the FT-IR spectra of MgFe₂O₄@Tris due to stretching and bending vibrations in C–H aliphatic groups. The absorption peaks at 3430 and 1631 cm⁻¹ are attributed to the stretching and bending vibrations of N–H groups. The vibrations at 1081 and 1150 cm⁻¹ are related to C–C and C–N bonds (Fig. 1B).

Fig. 1 FT-IR spectra of (A) MgFe₂O₄ and (B) MgFe₂O₄@Tris MNPs.

3.1.2. XRD analysis. To recognize the formation of the magnetite crystal phase in the nanocatalyst, the synthesized samples were analyzed by X-ray diffraction (XRD). The XRD analyses of MgFe₂O₄ (green) and MgFe₂O₄@Tris (red) nanoparticles are shown in Fig. 2a. The X-ray diffraction planes at (111), (220), (311), (400), (422), (511), (440), (533), (622), and (444) confirm the formation of a cubic spinel structure. The locations of all peaks in the XRD analysis of MgFe₂O₄ were matched to the standard XRD analysis of MgFe₂O₄.²⁸ These peaks showed that the structure of MgFe₂O₄ MNPs corresponds to the standard pattern with reference card numbers ICSD: 96-100-6064, ICSD: 96-901-3215 and ICSD: 96-901-5066 (Fig. 2b). The particle size determined from the XRD data was 59.09 nm, which was calculated using the Debye–Scherrer formula (D = Kλ/β cos θ).

Fig. 2 (a) XRD patterns of MgFe₂O₄ (green line) and MgFe₂O₄@Tris MNPs (red line). (b) Standard XRD patterns.

3.1.3. SEM analysis. The morphology and particle size of the nano-solid magnesium ferrite were analyzed using scanning electron microscopy (SEM). As shown in Fig. 3, the images show a spherical structure with an average particle diameter of 65–90 nm and a uniform distribution and size.

Fig. 3 SEM images of MgFe₂O₄@Tris MNPs at different magnifications.

3.1.4. TEM analysis. Transmission electron microscopy (TEM) analysis was used to examine the shape and size of the magnetic nanoparticles (Fig. 4). The images obtained confirm the formation of particles with a spherical morphology. Additionally, the particle size distribution histogram obtained from the TEM images revealed that the MgFe₂O₄@Tris nanoparticles have an average size of 46 nm.

Fig. 4 TEM images and histograms of MgFe₂O₄@Tris nanoparticles at different magnifications.

It is shown from SEM, TEM, and XRD analyses that the catalyst has a nanometer-sized structure.

3.1.5. EDX analysis. Energy dispersive X-ray spectroscopy (EDX) is a powerful analytical tool for characterizing and determining elements present in catalyst components and the purity of the nanoparticles. The EDX spectrum of MgFe₂O₄@Tris is shown in Fig. 5, which confirms the elements Mg, Fe, O, N, and C in the nano-catalyst structure. Additionally, there are no peaks related to any impurities. It can be concluded that the nano-catalyst has been successfully produced. EDX mapping spectra of MgFe₂O₄@Tris confirm the synthesis of the catalyst. The elements were shown to be uniformly distributed in the nano-catalyst (Fig. 6).

Fig. 5 EDX spectrum of MgFe₂O₄@Tris MNPs.

Fig. 6 EDX mapping spectra of MgFe₂O₄@Tris MNPs.

3.1.6. TGA analysis. The thermogravimetric analysis (TGA) diagram of the MgFe₂O₄@Tris nanoparticles is represented in Fig. 7. The TGA diagram of the MgFe₂O₄@Tris nanoparticles shows a small weight loss below 200 °C, corresponding to the evaporation of physically adsorbed solvents and OH groups on the nano-catalyst surface, and the next weight loss at temperatures above 200 °C is related to the removal of organic moieties on the MgFe₂O₄@Tris nanoparticles.

Fig. 7 TGA thermogram of MgFe₂O₄@Tris MNPs.

3.1.7. VSM analysis. The magnetization characteristics of the MgFe₂O₄ and MgFe₂O₄@Tris nanoparticles were analyzed using a vibrating sample magnetometer (VSM). The magnetization diagrams of MgFe₂O₄ (red line) and MgFe₂O₄@Tris (blue line) are shown in Fig. 8. The values of the saturation magnetization of the MgFe₂O₄ and MgFe₂O₄@Tris nanoparticles are 18 and 10 emu/g, respectively. The observed decrease in magnetic moment (MS) results from the addition of diamagnetic organic species to the surface of the MNPs, which confirms the presence of Tris on the surface of the MgFe₂O₄ nanoparticles. Meanwhile, the catalyst can be easily recycled from the reaction mixture using a simple magnetic field.

Fig. 8 VSM diagram of MgFe₂O₄ (red line) and MgFe₂O₄@Tris MNPs (blue line).

3.2. Catalytic studies

Following the successful synthesis and full characterization of MgFe₂O₄@Tris MNPs, we investigated the efficiency and activity of this nano-catalyst in the synthesis of pyrazolopyranopyrimidine and tetrahydrodipyrazolopyridine derivatives.

3.2.1. Pyrazolopyranopyrimidine. A four-component reaction between para-chlorobenzaldehyde, ethyl acetoacetate, hydrazine hydrate and barbituric acid was used as the sample reaction, and the effect of diverse conditions involving different temperatures, catalyst amounts, and solvents was examined. The results are summarised in Table 1. The best efficiency of product 2a is obtained when conducting the reaction in water at room temperature and in the presence of 10 mg of MgFe₂O₄@Tris MNPs (Table 1, entry 3). To verify the catalytic activity, we expanded the reaction to a range of aromatic aldehydes and barbituric acid under optimal reaction conditions (Scheme 3), and the results are reported in Table 2.

Table 1 Optimization of reaction conditions in the sample reaction^a

Entry	Amount of catalyst (mg)	Solvent	Temp (°C)	Time (min)	Yield^b (%)
a Reaction conditions: hydrazine hydrate (1 mmol), ethyl acetoacetate (1 mmol), para-chlorobenzaldehyde (1 mmol) and barbituric acid (1 mmol) in the presence of catalyst and solvent (2 mL).b Isolated yield.
1	30	H2O	rt	15	98
2	20	H2O	rt	10	98
3	10	H2O	rt	10	98
4	5	H2O	rt	10	90
5	10	H2O/EtOH	rt	10	97
6	10	EtOH	rt	20	85
7	10	DMF	rt	30	75
8	10	CH3CN	rt	50	65
9	10	DCM	rt	120	20
10	10	EtOAc	rt	60	75
11	10	n-Hexane	rt	120	Trace
12	10	H2O	40	10	98
13	10	H2O	60	10	98

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Scheme 3 Synthesis of pyrazolopyranopyrimidine derivatives catalysed by MgFe₂O₄@Tris MNPs.

Table 2 Synthesis of pyrazolopyranopyrimidine derivatives catalysed by MgFe₂O₄@Tris MNPs

Entry	Aldehyde	R1	X	Product	Time (min)	Yield^a (%)	M.p. (°C) (ref.)
a Isolated yield.
1	4-Cl C6H5	H	O	2a	10	98	218–220 (ref. 15)
2	C6H5	H	O	2b	7	98	215–218 (ref. 15)
3	4-OMe C6H5	H	O	2c	15	97	226–228 (ref. 15)
4	2-OH C6H5	H	O	2d	10	97	264–268 (ref. 17)
5	2-OMe C6H5	H	O	2e	15	92	228–230 (ref. 15)
6	3-OH C6H5	H	O	2f	10	94	278–280 (ref. 30)
7	3-NO2 C6H5	H	O	2g	20	80	266–268 (ref. 17)
8	4-Br C6H5	H	O	2h	20	95	210–212 (ref. 30)
9	4-Cl C6H5	H	S	2i	5	98	224–226 (ref. 35)
10	4-OMe C6H5	H	S	2j	10	92	224–225 (ref. 15)
11	C6H5	H	S	2k	5	94	219–220 (ref. 15)
12	2-OH C6H5	H	S	2l	5	95	267–270 (ref. 35)
13	2-OMe C6H5	H	S	2m	5	97	222–225 (ref. 15)
14	4-Cl C6H5	CH3	O	2n	10	98	200–202 (ref. 15)

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A proposed mechanism for the synthesis of pyrazolopyranopyrimidines using MgFe₂O₄@Tris MNPs as the catalyst is shown in Scheme 4.¹⁵

Scheme 4 Proposed mechanism for the synthesis of pyrazolopyranopyrimidines using MgFe₂O₄@Tris MNPs.

3.2.2. Tetrahydrodipyrazolopyridine. We used MgFe₂O₄@Tris MNPs to prepare tetrahydrodipyrazolopyridine derivatives (Scheme 5). To optimize the amount of catalyst, solvent, and reaction temperature, the reaction of ethyl acetoacetate, hydrazine hydrate, para-chlorobenzaldehyde, and ammonium acetate was selected as the model reaction (Table 3). The best efficiency of product 3a is achieved when the reaction is carried out in H₂O as the solvent at room temperature and in the presence of 15 mg of MgFe₂O₄@Tris MNPs (Table 3, entry 3). Subsequently, a vast range of tetrahydrodipyrazolopyridine derivatives was synthesized with success under optimal conditions with excellent yields and short reaction times, and the results of these reactions are presented in Table 4. The suggested mechanism for the production of tetrahydrodipyrazolopyridine using MgFe₂O₄@Tris MNPs as a catalyst is shown in Scheme 6.³⁸

Scheme 5 Synthesis of tetrahydrodipyrazolopyridine derivatives catalysed by MgFe₂O₄@Tris MNPs.

Table 3 Optimization of reaction conditions in the sample reaction^a

Entry	Amount of catalyst (mg)	Solvent	Temp (°C)	Time (min)	Yield^b (%)
a Reaction conditions: hydrazine hydrate (2 mmol), ethyl acetoacetate (2 mmol), para-chlorobenzaldehyde (1 mmol) and ammonium acetate (1.5 mmol) in the presence of catalyst and solvent (2 mL).b Isolated yield.
1	30	H2O	rt	10	98
2	20	H2O	rt	10	98
3	15	H2O	rt	10	98
4	10	H2O	rt	15	90
5	15	H2O/EtOH	rt	10	95
6	15	EtOH	rt	45	50
7	15	DMF	rt	30	95
8	15	CH3CN	rt	120	20
9	15	EtOAc	rt	120	Trace
10	15	n-Hexane	rt	120	Trace
11	15	H2O	40	10	98
12	15	H2O	60	10	98

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Table 4 Synthesis of tetrahydrodipyrazolopyridine derivatives catalysed by MgFe₂O₄@Tris MNPs

Entry	Aldehyde	Product	Time (min)	Yield^a (%)	M.p. (°C) (ref.)
a Isolated yield.
1	4-Cl C6H5	3a	10	98	252–254 (ref. 16)
2	C6H5	3b	5	98	241–243 (ref. 16)
3	4-OMe C6H5	3c	20	90	187–190 (ref. 16)
4	2-OMe C6H5	3d	15	93	180–182 (ref. 37)
5	2-NO2 C6H5	3e	20	88	189–190 (ref. 16)
6	3-NO2 C6H5	3f	20	85	284–286 (ref. 16)
7	4-Br C6H5	3g	25	91	169–170 (ref. 16)
8	4-OH C6H5	3h	20	92	269–270 (ref. 36)
9	2-Cl C6H5	3i	15	90	165–167 (ref. 16)
10	4-NO2 C6H5	3j	20	80	298–300 (ref. 16)
11	3-Br C6H5	3k	30	91	247–248 (ref. 16)
12	(CH3)2 N C6H5	3l	30	90	240–243 (ref. 16)
13	Terephthalaldehyde	3m	10	96	>300 (ref. 37)

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Scheme 6 Proposed mechanism for the synthesis of tetrahydrodipyrazolopyridine using MgFe₂O₄@Tris MNPs.

3.2.3. Comparison with other catalysts. To demonstrate the superiority and high efficiency of the nano-catalyst synthesized in this research, the results obtained for the synthesis of pyrazolopyranopyrimidines and tetrahydrodipyrazolopyridines using MgFe₂O₄@Tris MNPs were compared with the catalysts reported in previous studies. As shown in Table 5, this nano-catalyst has advantages such as high catalytic activity, low reaction time and excellent product yields.

Table 5 Comparison of the catalytic activity of MgFe₂O₄@Tris MNPs with other catalysts in the synthesis of products 2a (entries 1–5) and 3a (entries 6–10)

Entry	Catalyst	Conditions	Time (min)	Yield (%)	Ref.
1	[BNPs-Caff]HSO4	H2O, 50 °C	45	92	29
2	β-Cyclodextrin	H2O, 40 kHz, 50 °C	30	90	30
3	[MerDABCO-SO3H]Cl	H2O, 80 °C	15	94	17
4	H3[PMo7W5O40]·24H2O	Solvent free, 80 °C	40	94	31
5	MgFe2O4@Tris	H2O, rt	10	98	This work
6	Pseudopolymeric magnetic nanoparticles	EtOH, rt	60	63	16
7	HANCD@urease	H2O, 70 °C	70	95	32
8	Nano-CdZr4(PO4)6	EtOH, reflux	40	92	33
9	Nano-ovalbumin	H2O, 55 °C	35	94	34
10	MgFe2O4@Tris	H2O, rt	10	98	This work

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3.2.4. Catalyst recovery and reuse. One of the important features of the catalyst synthesized in this study is its magnetism. This feature simplifies catalyst recovery and separation. The possibility of reusing the MgFe₂O₄@Tris nano-catalyst in the synthesis of products 2a and 3a (as model reactions) was considered. The results are presented in Fig. 9. In summary, the catalytic activity was tested over 5 cycles, and similar activities were displayed, without any notable loss of the original catalytic activity. Additionally, the FT-IR spectra and SEM images of MgFe₂O₄@Tris before and after recovery (Fig. 10 and 11) exhibited good similarity due to the stability of the catalyst.

Fig. 9 Possibility of reusing MgFe₂O₄@Tris MNPs in the synthesis of products 2a and 3a.

Fig. 10 FT-IR spectrum of fresh MgFe₂O₄@Tris (a) and MgFe₂O₄@Tris after recovery (b).

Fig. 11 SEM images of MgFe₂O₄@Tris MNPs after recovery.

4. Conclusions

We introduced an innovative magnetic heterogeneous nano-catalyst, made from MgFe₂O₄ nanoparticles and tris(hydroxymethyl)aminomethane using a co-precipitation method. Various techniques were used for the full characterization of the synthesized nanocatalysts. The catalytic activity of this nanocatalyst for the synthesis of pyrazolopyranopyrimidine and tetrahydrodipyrazolopyridine derivatives was evaluated. A key feature of this protocol was a high product yield, and this catalyst can simply be separated from the reaction mixture using an external magnet and reused for multiple cycles without appreciable loss of its original catalytic activity. To summarize this study, this method can serve as a powerful strategy for the synthesis of important active molecules.

Author contributions

All authors contributed to data analysis, supervised the project, drafted and revised the paper and agreed to be responsible for all aspects of this work.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors are grateful to Ilam University for supporting this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra07934a

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