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Polyphosphoric acid supported on Ni0.5Zn0.5Fe2O4 nanoparticles as a magnetically-recoverable green catalyst for the synthesis of pyranopyrazoles
⁎Corresponding author. Tel.: +98 7616670242. f.moeinpour@gmail.com (Farid Moeinpour)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Polyphosphoric acid supported on silica coated Ni0.5Zn0.5Fe2O4 nanoparticles was found to be magnetically separable, highly efficient, eco-friendly, green and recyclable heterogeneous catalyst. This new catalyst at first was fully characterized by TEM, SEM, FTIR and XRD techniques and then catalytic activity of this catalyst was investigated in the synthesis of 5-cyano-1,4-dihydropyrano[2,3-c]pyrazoles. Also the Ni0.5Zn0.5Fe2O4 magnetic nanoparticle-supported polyphosphoric acid could be reused at least six times without significant loss of activity. It could be recovered easily by applying an external magnet.
Keywords
Polyphosphoric acid
Catalysis
Pyranopyrazoles
Magnetic nanoparticles
1 Introduction
Green chemistry aims to eliminate pollution by preventing it from happening in the first place and by using resources for chemical products that are renewable. The other key component of green chemistry is to close the cycle in chemical production so that there is little to no waste product. In designing a reaction according to green chemistry principles, chemists pay close attention to whatever is known about the possible hazards a chemical presents to health or the environment before using that chemical in a reaction or creating it as a product. In other words, they treat the hazard posed by a substance as a property that must be considered along with other chemical and physical properties and select substances that minimize harm. Thus green chemistry processes are benign by design.
Multicomponent reactions (MCRs) have emerged as efficient and powerful tools in modern synthetic organic chemistry because the synthesis of complex organic molecules from simple and readily available substrates can be achieved in a very fast and efficient manner without the isolation of any intermediate (Strubing et al., 2005; Moeinpour and Khojastehnezhad, 2012; Khojastehnezhad et al., 2011; Moeinpour et al., 2011). In this type of reactions three or more components are reacted to form ideally one product, which contains the essential parts of all the initial reactants. MCRs contribute to the requirements of an environmentally friendly process by reducing the number of synthetic steps, energy consumption and waste production. Therefore, developing new MCRs and improving known MCRs are popular areas of research in current organic chemistry.
Because of the high surface area:volume ratios, nanoparticles can effectively improve the loading and the catalytic efficiency of immobilized catalysts. Therefore the use of nanostructured materials as support for different types of catalyst immobilizations remains an attractive field to researchers (Lu et al., 2007; Gao, 2005). Magnetic nanoparticles have obtained considerable interest in recent years (Schmid, 2004). The magnetic nature of these particles allows for easy recovery and recycling of the catalysts by an external field, which may optimize operational cost and improve product purity. Recently, magnetic nanoparticles such as Fe (Kim et al., 2006), FePt (Hong et al., 2005), Fe2O3 (Mornet et al., 2002), Fe3O4 (Fan et al., 2009), ZnFe2O4 (Shah et al., 2010), MnFe2O4 (Fan et al., 2009), NiFe2O4 (Chaudhuri et al., 2009), CoFe2O4 (Fan et al., 2009; Kim et al., 2008), etc., have been studied mostly for biomedical applications. As Fe-based metallic nanoparticles are chemically not stable (Nunez et al., 2003) and iron oxides, in particular spinel ferrites with a general formula (AB2O4) have attracted a great consideration. Magnetic properties of ferrites are closely related to the essential property of the element located at A position in ferrites. So that, to obtain the required magnetic and chemical performances the combination of transition elements of Zn, Mn, Ni, Co etc., is generally a fundamental approach. Ni–Zn ferrites are one of the most versatile magnetic materials as they have high saturation magnetization, high Curie temperature, chemical stability and relatively high permeability (Goldman, 1990). Due to the sensitivity of the magnetic nanoparticles and strong surface affinity toward silica, these nanoparticles can be directly coated with amorphous silica. There are several advantages to use silica shells in place of organic stabilizers in biomedicine application. The more important advantages of application of silica in the protection of magnetic nanoparticles are: physical blocking of the surface without any affect redox reactions at the core surface; the optical transparency of silica shell that leads to modulate the position and intensity of colloidal metal surface plasmon absorption bands; the prevention of coagulation during chemical reactions (Chaneiac et al., 1995).
Aoyama and Takido have introduced a heterogeneous version of polyphosphoric acid entitled silica-supported polyphosphoric acid (PPA–SiO2) that has emerged as a powerful catalyst for various organic reactions; including conversion of carbonyl compounds into oxathioacetals and dithioacetals, synthesis of 2-substituted-1,2,3,4-tetrahydro-4-quinazolinones, N-[α-(β-Hydroxy-α-naphthyl)(benzyl)]O-alkyl carbamate derivatives, amidoalkyl naphthols, polyhydroquinoline derivatives, 2H-indazolo[2,1-b]phthalazine-trione derivatives, 3,4-dihydropyrimidin-2(1H)-ones and thiones and 1,8-dioxodecahydroacridines (Aoyama et al., 2004, Shaterian and Oveisi, 2009; Shaterian et al., 2009a, 2008, 2009b Zeinali-Dastmalbaf et al., 2011). In spite of recoverability of the silica supported polyphosphoric acid, the slow recycling of catalysts by filtration and the unavoidable loss of some solid catalyst in the separation process, especially with air sensitive materials, are some of the drawbacks of the traditional heterogeneous PPA–SiO2. Therefore, there is more interest to introduce a novel version of PPA immobilized onto magnetic silica for more efficient catalyst recovery, especially viewed from green chemistry. Due to the reasonable needs to clean and green recovery of the heterogeneous catalyst, especially acid catalyst, we decided to design a novel version of silica-supported polyphosphoric acid to facilitate efficient recycling of this catalyst. For this purpose, we prepared the PPA-functionalized silica-coated magnetic nanoparticles [Ni0.5Zn0.5Fe2O4 @SiO2–PPA] and after characterization, synthesis of pyranopyrazoles for instance is selected to investigate its catalytic activity and recyclability.
Pyranopyrazoles are an important class of heterocyclic compounds. They find applications such as pharmaceutical ingredients and biodegradable agrochemicals (Bonsignore et al., 1993). The first reported pyranopyrazole was synthesized from the reaction between 3-methyl-1-phenylpyrazolin-5-one and tetracyanoethylene (Junek and Aigner, 1973). Different 6-amino-5-cyano-4-aryl-4H-pyrazolo[3,4-b]pyrans were synthesized by the reaction of arylidiene malononitrile with 3-methyl-pyrazol-ine-5-ones or by the condensation of 4-arylidienepyrazoline-5-one with malononitrile (Wamhoff et al., 1993). Vasuki and co-workers reported an efficient four-component reaction protocol for the synthesis of pyranopyrazole derivatives in the presence of a catalytic quantity of bases such as piperidine, pyrrolidine, morpholine and triethylamine at ambient temperature (Gnanasambandam Vasuki and Kumaravel, 2008).
We developed approaches for the synthesis of biologically interesting products via multicomponent reactions (Moeinpour and Khojastehnezhad, 2012, 2011; Moeinpour et al., 2011), here we report the new efficient green synthesis of pyranopyrazoles using Ni0.5Zn0.5Fe2O4 @SiO2–PPA as the catalyst (Scheme 1). To the best of our knowledge, this is the first report on the synthesis, characterization and catalytic performance of a Ni0.5Zn0.5Fe2O4 @SiO2–PPA catalyst.
2 Experimental section
2.1 Materials and techniques
All chemicals were available commercially and used without additional purification. Melting points were recorded on an electrothermal type 9100 melting point apparatus. The IR spectra were obtained using a 4300 Shimadzu spectrophotometer as KBr disks. The 1H NMR (500 MHz) spectra were recorded with a Bruker DRX500 spectrometer. TEM and SEM images were obtained by a TEM microscope (Philips CM 120 kV, The Netherlands) and SEM microscope (SEM, JSM-6700F), respectively. The crystal structure of the products was characterized by X-ray diffraction (XRD) patterns recorded by using a Rigaku RAD-IIA diffractometer with CuKα radiation (1.5418 Å) generated at 40 kV and 30 mA.
2.2 Preparation of the catalyst
The Ni0.5Zn0.5Fe2O4 @SiO2–PPA was prepared according to the reported procedure by R. Massart with minor modifications (Zins et al., 1999). The solution of metallic salts (FeCl3 160 mL, 1 M), NiCl2 (40 mL, 1 M) and ZnCl2 (40 mL, 1 M)] was poured as quickly as possible into the boiling alkaline solution [NaOH (1000 mL, 1 M)] under vigorous stirring. Then the solution was cooled and continuously stirred for 90 min. The resulting precipitate was then purified by a four times repeated centrifugation (4000–6000 rpm, 20 min) and decantation.
Coating of a layer of silica on the surface of the Ni0.5Zn0.5Fe2O4 nanoparticles was achieved by premixing (ultrasonic) a dispersion of the nanoparticles (8% w/w, 25 mL) obtained with ethanol for 2 h at 60 °C. A concentrated ammonia solution was added and the resulting mixture was stirred at 60 °C for 40 min. Then, TEOS (tetraethylorthosilicate) (1.0 mL) was added to the reaction mixture and continuously stirred at 60 °C for 24 h. The silica coated nanoparticles were collected by an external magnet, followed by three times with methanol and drying in a vacuum for 48 h.
PPA (polyphophoric acid) (2.2 g) was charged in the round-bottom flask, and CHCl3 (100 mL) was added. After the mixture was stirred at 50 °C for 1 h, Ni0.5Zn0.5Fe2O4 @SiO2 (5.0 g) was added to the solution, and the mixture was stirred for another 4 h. The CHCl3 was removed with a rotary evaporator and the resulting solid was washed with cold absolute ethanol, and dried in vacuo at 70 °C for 2 h. The amount of H+ in the Ni0.5Zn0.5Fe2O4 @SiO2–PPA determined by acid–base titration was 0.50 mmol/g.
2.3 General procedure for the synthesis of 5-cyano-1,4-dihydropyrano[2,3-c]pyrazoles 5a–5i by Ni0.5Zn0.5Fe2O4 @SiO2–PPA
Typically, to a mixture of hydrazine hydrate 1 (2 mmol), ethyl 3-oxopropanoate 2 (2 mmol), aromatic aldehyde 3 (2 mmol) and malonitrile 4 (2 mmol) was added the catalyst (0.03 g) at room temperature in water. The reaction mixture was vigorously stirred for the period of time denoted in Table 1. During the procedure, the reaction was monitored by TLC. Upon completion, Ni0.5Zn0.5Fe2O4 @SiO2–PPA could be placed on the side wall of the reaction vessel with the aid of an external magnet, and water was removed from the mixture to leave a residue (including the product and the catalyst). Then, the product was dissolved in ethanol and the catalyst was easily separated from the product with the aid of an external magnet onto the reaction vessel, followed by decantation of the product solution. Then, the solution was concentrated, dried at room temperature and recrystallized from ethanol. All products were identified by comparing their spectral data with those of authentic samples (Peng et al., 2006; Kanagaraj and Pitchumani, 2010).
| Entry | Solvent | Time (min.) | Yield (%)b |
|---|---|---|---|
| 1 | H2O | 12 | 91 |
| 2 | CH3CH2OH | 12 | 75 |
| 3 | CH3OH | 12 | 72 |
| 4 | CH3CN | 12 | 65 |
| 5 | ClCH2CH2Cl | 12 | Trace |
The 1H NMR data of some representative 5-cyano-1,4-dihydropyrano[2,3-c]pyrazoles are as follows:
2.3.1 6-Amino-3-methyl-4-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (5a)
δH (500 MHz, DMSO-d6): 11.98 (s, 1H, NH), 7.45–7.16 (m, 5H, ArH), 6.98 (s, 2H, NH2), 4.74 (s, 1H, 4-H), 1.91 (s, 3H, CH3) ppm.
2.3.2 6-Amino-3-methyl-4-(4-nitrophenyl)-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (5e)
δH (500 MHz, DMSO-d6): 11.65 (s, 1H, NH), 8.14 (d, J = 8.1 Hz, 2H), 7.49 (d, J = 8.1 Hz, 2H), 6.94 (s, br, 2H), 4.71 (s, 1H), 1.98 (s, 3H) ppm.
3 Results and discussion
The one-pot synthesis of pyranopyrazoles was achieved by the four-component condensation of hydrazine hydrate 1, ethyl 3-oxopropanoate 2, aromatic aldehydes 3 and malononitrile 4 in the presence of Ni0.5Zn0.5Fe2O4 @SiO2–PPA as a heterogeneous catalyst (Scheme 1). Ni0.5Zn0.5Fe2O4 @SiO2–PPA nanocrystallites were prepared according to the reported procedure by R. Massart with minor modifications: fine particles are precipitated in an alkaline solution (Zins et al., 1999). Ni0.5Zn0.5Fe2O4 @SiO2–PPA nanocrystallites were characterized by FT-IR (Fig. 1), TEM (Fig. 2), SEM (Fig. 3) and XRD (Fig. 4). FT-IR spectra of Ni0.5Zn0.5Fe2O4, Ni0.5Zn0.5Fe2O4 @SiO2, PPA and Ni0.5Zn0.5Fe2O4 @SiO2 -PPA are compared in Fig. 1. In the FT-IR spectrum of Ni0.5Zn0.5Fe2O4 @SiO2–PPA (Fig. 1(d)), most of the bands of Ni0.5Zn0.5Fe2O4 (Fig. 1(a)), Ni0.5Zn0.5Fe2O4 @SiO2 (Fig. 1(b)) and PPA(Fig. 1(c)) with a slight shift in some of them, are observable, which shows PPA has been adsorbed well on the SiO2 surface. To confirm Ni0.5Zn0.5Fe2O4 formation in the synthesized MNPs, the XRD pattern of the sample was studied. The XRD pattern (Fig. 4) indicates that these NPs have the spinel structure, with all the major peaks matching the standard pattern of bulk Ni0.5Zn0.5Fe2O4 (JCPDS 08-0234). The FT-IR spectrum of Ni0.5Zn0.5Fe2O4 exhibits strong bands in the low-frequency region (1000–500 cm−1) due to iron oxide skeleton, which is in agreement with the magnetite spectrum. The characteristic bands of Si–O–Fe and Si–O were observed at 798 and 1087 cm−1, respectively. The peaks at 1440–1650 cm−1 also showed the existence of Fe–O. The TEM and SEM photographs of the sample are illustrated in Figs. 2 and 3, respectively. Both the SEM and TEM images demonstrate that the prepared magnetic nanoparticles are spherical, narrowly distributed, and well dispersed, with average size less than 70 nm in diameter.
At first, the synthesis of compound 5a was selected as a model reaction to optimize the reaction conditions. The reaction was carried out by mixing hydrazine hydrate (2 mmol), ethyl 3-oxopropanoate (2 mmol), benzaldehyde (2 mmol) and malononitrile (2 mmol) at room temperature under neat conditions. We found that the best amount of catalyst was 0.03 g to obtain the corresponding pyranopyrazole in 91% yield in 12 min. Additionally, we performed the above reaction at the same conditions in the presence of 0.03 g of Ni0.5Zn0.5Fe2O4 @SiO2. Also we found that in this condition and without the catalyst after 120 min, the corresponding pyranopyrazole was obtained in 10% yield.
Furthermore, the reaction was carried out in different solvents. As shown in Table 1, the yield of the reaction in water was greater and the reaction time was generally shorter than the other solvents. The best result was obtained in water for 12 min. Subsequently, therefore, all reactions were carried out at room temperature in the presence of 0.03 g Ni0.5Zn0.5Fe2O4 @SiO2–PPA in water.
Using these optimized reaction conditions, the scope and efficiency of this approach was explored for the synthesis of a wide variety of 5-cyano-1,4-dihydropyrano[2,3-c]pyrazoles and the obtained results are summarized in Table 2. All reactions, delivered good product yields and accommodated a wide range of aromatic aldehydes bearing both electron-donating and electron-withdrawing substituents. In all cases, the obtained product was isolated by a very simple work-up.
| Entry | Ar | Productsb | Time(min) | Yields(%)c | Mp °C | |
|---|---|---|---|---|---|---|
| Found (Ref) | Reported | |||||
| 1 | C6H5 | 5a | 12 | 91 | 242–244 | 243–245 (Peng et al., 2006) |
| 2 | 3-BrC6H4 | 5b | 20 | 86 | 220–221 | 220–222 (Peng et al., 2006) |
| 3 | 3-ClC6H4 | 5c | 20 | 89 | 244–246 | 246–248 (Peng et al., 2006) |
| 4 | 4-CH3OC6H4 | 5d | 15 | 88 | 210–212 | 208–210 (Peng et al., 2006) |
| 5 | 4-NO2C6H4 | 5e | 12 | 93 | 249–252 | 249–252 (Peng et al., 2006) |
| 6 | 4-ClC6H4 | 5f | 10 | 94 | 233–235 | 233–235 (Peng et al., 2006) |
| 7 | 4-BrC6H4 | 5g | 18 | 91 | 180–182 | 180–183 (Kanagaraj et al., 2010) |
| 8 | 4-MeC6H4 | 5h | 20 | 92 | 174–176 | 175–176 (Kanagaraj et al., 2010) |
| 9 | 3-NO2C6H4 | 5i | 18 | 90 | 190–192 | 190–192 (Kanagaraj et al., 2010) |
A probable mechanism for condensation of hydrazine hydrate, ethyl 3-oxopropanoate, aromatic aldehyde and malonitrile at room temperature in the presence of Ni0.5Zn0.5Fe2O4 @SiO2–PPA (as a bronsted acid catalyst) for the synthesis of pyranopyrazoles is proposed as shown in Scheme 2.
From the view point of green chemistry, good recovery and reusability of the catalyst are highly preferable. For this purpose, the same model reaction was again studied under optimized conditions. After the completion of the reaction, the reaction mixture was then separated by an external magnet. Then the product was dissolved in ethanol and the catalyst was easily separated by an external magnet. The catalyst was washed with diethyl ether, dried at 60 °C under vacuum for 1 h, and reused for a similar reaction. As shown in Fig. 5, the catalyst could be reused at least six times without significant loss of its activity. To determine the percent leaching of the acid, the model reaction was carried out in the presence of Ni0.5Zn0.5Fe2O4 @SiO2–PPA. The catalyst was removed by an external magnet after washing with acetone at half of the reaction time. The filtrate solution was evaporated, and the reaction was continued using the resulting mixture. The reaction stopped without the solid heterogeneous catalyst (Table 1, entry 1). Thus, the absence of free PPA species in the reaction medium was confirmed. Therefore, these experiments are further evidence to the heterogeneous nature of the catalytic system. When the separated catalyst was transferred to the reaction medium, the conversion of benzaldehyde to 6-amino-3-methyl-4-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5 carbonitrile progressed and completed. To reconfirm that there were no active catalyst species in the solution, the PPA–SiO2 catalyst was subjected to vigorous stripping in acetone for 1 h. The catalyst was filtered, and the filtrate solution was evaporated. No residue and species of PPA were observed by IR spectroscopy (Fig. 1d). This fact confirmed that PPA bonded on the silica surface and that this bond is stable in the reaction medium. In addition, the weight of the recovered catalyst is the same as the amount of the fresh catalyst that was used the first time in the reaction. Furthermore, The XRD patterns of Ni0.5Zn0.5Fe2O4 @SiO2–PPA nanoparticles before use and after reuse six times were studied. As shown in Fig. 6, the XRD pattern of the used catalyst showed that the structure of catalyst particles remained almost the same after six-run reuse. In addition, the weight of the recovered catalyst is the same as the amount of the fresh catalyst that was used the first time in the reaction.
4 Conclusion
In conclusion, we report on a new simple and green catalytic method for the synthesis of 5-cyano-1,4-dihydropyrano[2,3-c]pyrazoles by one-pot condensation reaction of hydrazine hydrate, ethyl 3-oxopropanoate, aromatic aldehyde and malonitrile using Ni0.5Zn0.5Fe2O4 @SiO2–PPA as an efficient, reusable, and green heterogeneous catalyst in water. The catalyst could be recycled after a very simple work-up (with the aid of an external magnet), and reused at least six runs without appreciable reduction in its catalytic activity. High yields, short reaction times, easy work-up, and absence of any volatile and hazardous organic solvents are some advantages of this protocol.
Acknowledgment
The authors are thankful to the Islamic Azad University, Bandar Abbas Branch for financial support.
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