DOI: 10.1039/C7RA08253C (Paper) RSC Adv., 2017, 7, 46644-46650
Nano-Fe3O4@SiO2-supported boron sulfonic acid as a novel magnetically heterogeneous catalyst for the synthesis of pyrano coumarins†
Received 26th July 2017 , Accepted 20th September 2017
First published on 3rd October 2017
In this study, a novel magnetically heterogeneous catalyst based on the immobilization of boron sulfonic acid onto Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2–BSA) is reported. Fe3O4@SiO2–BSA was characterized via FT-IR, X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and vibrating sample magnetometer (VSM) analysis. The performance ability of this catalyst including acid sites was evaluated for the synthesis of pyrano coumarins under solvent-free conditions with high yields. Thermal stability of the catalyst and its easy separation by a magnetic field make this catalyst a good heterogeneous system and a useful alternative to other heterogeneous catalysts.
IntroductionSupported heterogeneous catalysts as environmentally friendly materials play a pivotal role in modern science and technology, in particular, in the organic synthesis area.1–3 Because of the many advantages of catalyst immobilization on solid supports, including easy handling, low solubility, increasing selectivity of the reactions, and non-toxicity, it is a technique that is widely used.4 Nanostructure supports are also advantageous because they exhibit higher activity and selectivity than their corresponding bulk materials.5–8 As the diameter of the particle decreases to the nanometer scale, the external surface area becomes available for chemical transformations.9 Recent studies show that magnetite (Fe3O4) nanoparticles have unique properties such as simple isolation from the reaction mixture using an external magnetic field, high surface area, thermal stability, low cost, their potential to immobilize functional groups, and excellent recyclability, which make them useful supports to prepare reusable heterogenous catalysts.10–12 A protective shell of silica as a coating can be formed on the surface of Fe3O4 nanoparticles to prevent them from oxidizing in an air atmosphere, provide simple surface functionalization, and enable aggregation between particles.13,14
Boron sulfonic acid, as a solid acid catalyst that was first introduced by Kiasat et al., and silica boron sulfonic acid have many advantages such as simplicity in preparation, availability of precursor, economically benign, non-toxicity, and high activity/selectivity with excellent yields in various chemical processes.15
Synthesis of hybrid heterocycles that contain biologically active skeletons is an interesting subject in organic synthesis.16,17 The chromene ring system is used regularly as a scaffold in medicinal and agricultural chemistry.18,19 Pyrano coumarins as a fused dihydropyran with a chromene nucleus received great attention due to their wide range of applications in various fields of chemistry.20,21 They have exhibited various pharmacological activities such as anti-HIV, antitumor, anticancer, antibacterial, and anti-inflammatory properties.22,23 Moreover, they can also be employed as cosmetics and pigments and utilized as potential biodegradable agrochemicals.24,25 The unique properties and broad applications of pyrano coumarins have promoted extensive studies for the synthesis of these useful compounds.
Because one-pot multi-component reactions (MCRs) play an important role in combinatorial chemistry, this field remained one of the most interesting areas of research in recent years. During multi-component reactions, target compounds are formed by joining at least three functional groups through covalent bonds.26 These reactions represent a very useful tool for the synthesis of complex molecules with potential biological properties because of their effective atom economy, convergent nature, and brief and straightforward experimental procedures.27
Herein, in continuation of our studies in the field of heterogeneous catalysts28–31 and according to importance of pyrano coumarins, we wish to disclose, for the first time, the preparation and characterization of novel immobilized boron sulfonic acid onto Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2–BSA) as well as the examination of their catalytic application in the synthesis of pyrano coumarin derivatives.
ExperimentalAll chemicals used in this research were purchased from Fluka and Merck chemical companies. The obtained products were identified by comparison of their spectral data and physical properties with previously reported data. The monitoring of the reaction progress and determination of purity of the compounds were accomplished using TLC performed with silica gel SIL G/UV254 plates. Melting points were determined by an electrothermal KSB1N apparatus and are uncorrected. The NMR spectra of 1H in DMSO were recorded on a Bruker Avance Ultra Shield 400 MHz spectrometer, and 13C NMR spectra were recorded at 100 MHz using TMS as an internal standard. Infrared (IR) spectra were obtained with a JASCO Fourier transform-infrared (FT-IR)/680 spectrometer using KBr pellets. X-ray powder diffraction (XRD) patterns were recorded using a Bruker AXS (D8 Advance) X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The measurement was made in 2θ ranging from 10° to 80° at the speed of 0.05° min−1. Energy dispersive spectroscopy (EDS) was performed using a TESCAN Vega model instrument. The morphology of the particles was observed by scanning electron microscopy (SEM) under an acceleration voltage of 26 kV. The magnetic measurement was carried out in a vibrating sample magnetometer (VSM; Kashan University, Kashan, Iran) at room temperature.
Preparation of Fe3O4To synthesize Fe3O4 magnetic nanoparticles through a chemical coprecipitation method, a solution of FeCl3·6H2O (2.7 g, 10 mmol) and FeCl2·4H2O (1 g, 5 mmol) in 45 mL double distilled water was mechanically stirred under an argon atmosphere at 80 °C for 30 min. In the next step, a sodium hydroxide solution (5 mL, 10 M) was gradually added dropwise. After continuous stirring at 80 °C for 1 h under an argon atmosphere, the black precipitate of Fe3O4 magnetic nanoparticles was decanted using an external magnetic field. The product was washed with double distilled water until pH 9 was obtained and then dried at 60 °C under a vacuum.32
Procedure for the synthesis of Fe3O4-silica-coated nanoparticlesAccording to the current method in the literature, a suspension containing Fe3O4 magnetic nanoparticles (1 g) was sufficiently dispersed in a mixture of ethanol (80 mL), distilled water (20 mL), concentrated ammonia aqueous solution (3 mL, 28%), and tetraethyl orthosilicate (0.5 mL). The reaction mixture was heated under reflux for 12 h. The Fe3O4-silica-coated (Fe3O4@SiO2) was separated by a magnet, washed several times with ethanol, and dried at 60 °C in air.33
Procedure for Fe3O4@SiO2–OB(OH)2A saturated solution of boric acid was added to a slurry containing Fe3O4@SiO2 nanoparticles (8 g) in dry toluene (45 mL). The mixture was refluxed for 24 h. The resultant suspension was collected using an external magnet and washed several times with distilled water and then methanol. It was dried at 80 °C to obtain the brown solid named nano Fe3O4@SiO2–OB(OH)2.34
Procedure for the preparation of Fe3O4@SiO2–OB(OSO3H)2In the final stage, a 100 mL suction flask was equipped with a dropping funnel containing chlorosulfonic acid (7.64 g, 0.066 mol), dry chloroform (40 mL), one argon inlet, and a gas outlet tube for conducting HCl gas over an adsorbing solution (10% NaOH). Then, Fe3O4@SiO2–OB(OH)2 (7.5 g) was charged into the flask. Chlorosulfonic acid was added drop-wise over a period of 60 min at 0 °C. HCl gas immediately evolved from the reaction vessel. When the addition was completed, the mixture was sonicated for 1 h. The functionalized magnetic nanoparticles were collected by a magnet. The supernatant was decanted, and the nanoparticles were washed with dry chloroform (3 × 5 mL) and then dried in high vacuum overnight.
General procedure for the synthesis of pyrano coumarins 4 and 6Fe3O4@SiO2–BSA (0.005 g) was added to a mixture of malononitrile/ethyl cyanoacetate, aryl aldehyde, and 5,7-dihydroxy-4-substituted coumarin/4-hydroxycoumarin at 80 °C under solvent-free conditions. The reaction progress was monitored by TLC (n-hexane/EtOAc, 3:2). After completion of the reaction, boiling EtOAc (10 mL) was added, and the catalyst was separated by filtration. To further purify the product, the obtained powder was recrystallized from EtOH.
Compound 4e. FT-IR (KBr): νmax 3477, 3423, 3315, 1702, 1681 cm−1. 1H NMR (DMSO-d6, 400 MHz) δ = 10.88 (s, 1H), 7.61 (s, 2H), 7.25–7.05 (m, 5H), 6.39 (s, 1H), 6.04 (s, 1H), 3.50 (s, 2H), 3.44 (s, 1H), 2.66 (s, 3H), 1.02–1.11 (m, 3H) ppm. 13C NMR (DMSO-d6, 100 MHz) δ = 168.47, 168.18, 159.97, 159.92, 159.45, 158.12, 158.12, 158.05, 154.23, 153.41, 147.54, 143.85, 143.69, 132.54, 131.78, 111.30, 110.01, 101.57, 98.09, 56.06, 24.11, 18.56, 14.27 ppm. Anal. calcd for C22H18ClNO6: C, 61.76; H, 4.24; N, 3.27. Found: C, 61.70; H, 4.26; N, 3.31. MS (m/z): 427 [M]+.
Compound 4h. FT-IR (KBr): νmax 3413, 3311, 1687, 1625 cm−1. 1H NMR (DMSO-d6, 400 MHz) δ = 10.81 (s, 1H), 7.58 (s, 2H), 7.22–7.84 (m, 7H), 6.36 (s, 1H), 6.11 (s, 1H), 5.87 (s, 1H), 2.74 (m, 5H), 1.06 (t, J = 5.22 Hz, 3H) ppm. 13C NMR (DMSO-d6, 100 MHz) δ = 168.41, 160.07, 160.02, 159.44, 157.86, 153.89, 153.39, 147.61, 144.95, 132.86, 130.67, 127.87, 126.31, 126.23, 125.61, 125.16, 125.08, 124.93, 112.66, 111.32, 101.88, 98.08, 78.46, 50.23, 24.04, 18.52 ppm. Anal. calcd for C26H21NO6: C, 70.42; H, 4.77; N, 3.16. Found: C, 70.38; H, 4.80; N, 3.21.
Compound 4r. FT-IR (KBr): νmax 3402, 1728, 1663 cm−1. 1H NMR (DMSO-d6, 400 MHz) δ = 11.10 (s, 1H), 8.18 (d, J = 8.8 Hz, 2H), 7.44 (d, J = 8.4 Hz, 2H), 7.17 (s, 2H), 6.51 (s, 1H), 6.12 (s, 1H), 4.82 (s, 1H), 2.63 (s, 3H) ppm. 13C NMR (DMSO-d6, 100 MHz) δ = 160.02, 159.76, 157.97, 155.17, 153.76, 153.10, 147.99, 146.72, 128.92, 124.29, 120.17, 112.12, 107.55, 102.46, 99.11, 56.64, 36.81, 24.49 ppm.
Compound 6c. FT-IR (KBr): νmax 3380, 3311, 3189, 1714, 1675 cm−1. 1H NMR (DMSO-d6, 400 MHz) δ = 7.92 (d, J = 8 Hz, 1H), 7.82 (s, 1H), 7.80 (d, J = 1.6 Hz, 1H), 7.76 (d, J = 1.6 Hz, 1H), 7.74 (s, 1H), 7.72 (d, J = 1.6 Hz, 1H), 7.54 (s, 1H), 7.52 (s, 1H), 7.49 (t, J = 3.6 Hz, 2H), 7.47 (s, 1H) ppm. 13C NMR (DMSO-d6, 100 MHz) δ = 160.0, 158.5, 154.4, 152.7, 149.2, 133.5, 132.9, 129.3, 125.1, 123.0, 119.4, 119.2, 117.0, 113.4, 110.4, 103.3, 57.3, 37.5 ppm. Anal. calcd for C20H11N3O3: C, 70.38; H, 3.25; N, 12.31. Found: C, 70.42; H, 3.20; N, 12.25.
Compound 6e. FT-IR (KBr): νmax 3391, 3180, 1712, 1674, 1608 cm−1. 1H NMR (DMSO-d6, 400 MHz) δ = 7.90 (dd, J = 8.0 Hz, 1H), 7.73–7.69 (m, 1H), 7.44–7.38 (m, 7H), 7.18 (d, J = 8.8 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H), 5.06 (s, 2H), 4.40 (s, 1H) ppm. 13C NMR (DMSO-d6, 100 MHz) δ = 159.51, 157.87, 157.45, 153.09, 152.06, 137.05, 135.60, 132.83, 128.76, 128.40, 127.79, 127.64, 124.63, 122.41, 119.30, 116.53, 114.61, 112.97, 104.19, 69.17, 58.09, 36.13 ppm.
Compound 6i. FT-IR (KBr): νmax 3461, 3295, 3162, 1716, 1673, 1631 cm−1. 1H NMR (DMSO-d6, 400 MHz) δ = 8.86 (s, 1H), 7.90 (d, J = 8 Hz, 1H), 7.74 (t, J = 8 Hz, 2H), 7.46–7.52 (m, 2H), 7.34 (s, 1H), 6.81 (d, J = 2 Hz, 1H), 6.72 (d, J = 2.1 Hz, 1H), 6.64 (d, J = 8.4 Hz, 1H), 4.35 (s, 1H), 3.94–4.01 (m, 2H), 1.32 (t, J = 6.8 Hz, 3H) ppm. 13C NMR (DMSO-d6, 100 MHz) δ = 160.0, 158.3, 153.4, 152.5, 146.5, 134.7, 133.2, 15.0, 122.9, 120.3, 119.8, 117.0, 116.0, 114.0, 113.5, 104.8, 64.4, 58.8, 36.9, 15.1 ppm. Anal. calcd for C21H16N2O5: C, 67.02; H, 4.28; N, 7.44. Found: C, 67.12; H, 4.20; N, 7.51.
Results and discussionThe new magnetic nano-catalyst Fe3O4@SiO2–BSA was prepared following the protocol shown in Scheme 1. Firstly, in order to prepare the magnetic Fe3O4 nanoparticles, the chemical co-precipitation of Fe2+ and Fe3+ ions in NaOH solution was performed.32 Subsequently, the silica-coated magnetic nanoparticles (Fe3O4@SiO2) were easily achieved with the known Stober method.32 Then, Fe3O4@SiO2–OB(OH)2 was synthesized from the reaction of Fe3O4@SiO2 with boric acid in dry toluene under reflux.34 Finally, the surface of Fe3O4@SiO2–OB(OH)2 was functionalized with chlorosulfonic acid to obtain Fe3O4@SiO2–OB(OSO3H)2. Chemical analysis of prepared Fe3O4@SiO2–OB(OSO3H)2 was performed using FT-IR, EDX, XRD, and SEM; magnetic measurements were performed using VSM.
|Scheme 1 Synthesis of Fe3O4@SiO2–BSA.|
The FT-IR spectra of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@OB(OH)2, and Fe3O4@SiO2–BSA were compared with each other (Fig. 1). The appearance of peaks around 570 cm−1, 796 cm−1, 1097 cm−1, 478 cm−1, and 3413 cm−1 in all of these four spectra are relevant to Fe–O stretching, Si–O–Si symmetric and asymmetric stretching, bending vibration, and OH vibration, respectively, which was confirmed to preserve the nano-structure of Fe3O4 and Fe3O4@SiO2.35 The peak in the region of approximately 1400 cm−1 of spectra c and d can be related to B–O.36 The OSO asymmetric and symmetric stretching vibrations near 1200–1250 cm−1 and 1010–1100 cm−1, and the S–O stretching vibration of –SO3H at 650 cm−1 and 3180 to 3419 cm−1 were seen in d spectra for the sulfonic group of the catalytic surface.37
|Fig. 1 The FT-IR of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2@OB(OH)2, and (d) Fe3O4@SiO2–BSA.|
To investigate the quantity of crystalline phases of the new catalyst, its XRD-diffraction pattern was obtained, as shown in Fig. 2. The control of six characteristic peaks at 30.3909, 35.7981, 43.4418, 53.9044, 57.4051, and 63.0226 that correspond to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) crystal planes is very important because the preservation of the cubic spinel structure could be strictly confirmed (JCPD 00-001-1111 standard). Additionally, there was a broad peak from 2θ = 19 to 28 that was due to the SiO2 shells of the coated Fe3O4.38 The other existing groups in the catalyst did not exhibit any changes in the crystal structure. The particle size of the prepared catalyst can be calculated using the Debye–Scherrer equation:
|D = 0.94λ/βcosθ|
|Fig. 2 XRD patterns of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–BSA.|
Energy-dispersive X-ray spectroscopy (EDS) is one of the best approaches to determine catalyst purity. Fig. 3 shows the EDS spectra indicating the elemental composition of the catalyst. It contains no impurity elements other than Fe, O, Si, S, and B. It must be noted that failure of the boron peak is observed because of the overlap between peaks of boron and other elements.36
|Fig. 3 (a) Energy-dispersive X-ray spectroscopy (EDS) spectra of Fe3O4@SiO2–BSA. (b) EDS data for Fe3O4@SiO2–BSA.|
The SEM image of Fe3O4@SiO2–BSA in Fig. 4 shows that the average diameter range was 58–86 nm, indicating good harmony in comparision with the calculated result from the Debye–Scherrer equation. Also, the uniform core–shell morphology was consistent with the spherical shape of the Fe3O4 nanoparticles. We commonly used a vibrating sample magnetometer to evaluate the magnetic measurement of our catalyst. Fig. 5 shows the magnetic behavior at room temperature for the catalyst. The below magnetization curves indicate the saturation magnetization of Fe3O4@SiO2 nanoparticles and Fe3O4@SiO2–BSA, which were diminished to 24.8 emu g−1 from 56.2 emu g−1 for Fe3O4@SiO2.
|Fig. 4 SEM image of the catalyst.|
|Fig. 5 The vibrating sample magnetometer (VSM) magnetization curves of Fe3O4@SiO2–BSA in comparison with Fe3O4@SiO2.|
To show the catalytic activity of Fe3O4@SiO2–BSA, it was used as a catalyst in the synthesis of a range of known and novel pyrano coumarins 4 and 6via the three-component reactions of aryl aldehydes 1, active methylene compound 2 (malononitrile or ethyl cyanoacetate), and hydroxycoumarin (5,7-dihydroxy-4-substituted coumarins 3 or 4-hydroxycoumarin 5) under solvent-free conditions (Scheme 2).
|Scheme 2 Preparation of pyrano coumarins 4 and 6 in the presence of Fe3O4@SiO2–BSA as catalyst.|
To initiate the synthetic work, several 5,7-dihydroxy-4-substituted coumarins 3 were prepared in good yields according to previously published methods.39 Subsequently, in order to find the most appropriate reaction conditions, a three-component reaction of ethyl cyanoacetate, benzaldehyde, and 4-methyl-5,7-dihydroxycoumarin was screened as a model reaction. The desired product was not produced in the absence of a catalyst even after a long reaction time. Therefore, the model reaction was performed using Fe3O4@SiO2@(CH2)3OMoO3H at various conditions. On the basis of the results obtained, we found that this reaction was efficiently performed in the presence of 0.005 g of Fe3O4@SiO2–BSA at 80 °C under solvent-free conditions (Table 1).
Table 1Screening conditions for the model reaction
With optimal conditions established, we then examined the scope of the reaction for the construction of various substrates including malononitrile, various aromatic aldehydes, and diverse 5,7-dihydroxy-4-substituted coumarin derivatives; the results are summarized in Table 2. In general, the reaction proceeded smoothly to afford the desired products 4 in good to excellent yields.
Table 2Fe3O4@SiO2–BSA-catalyzed synthesis of pyrano coumarins 4
Encouraged by these results, we extended the catalytic activity of Fe3O4@SiO2–BSA to condensation reactions of aromatic aldehydes, malononitrile, and 4-hydroxycoumarin to afford pyrano coumarins 6 (Scheme 2). A series of product 6 with different substituents was prepared from different aromatic aldehydes bearing electron-withdrawing and electron-donating groups (Table 3).
Table 3Synthesis of pyrano coumarins 6 using Fe3O4@SiO2–BSA
The structures of the obtained products 4 and 6 were deduced from their elemental analysis, IR, 1H, and 13C NMR spectroscopy and they were compared with authentic samples.40–42 Although both electron-rich and electron-poor aldehydes afforded the desired products 4 and 6, aldehydes having electron-withdrawing groups in comparison with those having electron-donating ones performed this transformation in better yields. This may be explained according to more positive charges located on the carbonyl group of aldehydes in electron-poor cases, which makes it a more reactive electrophile center.
A plausible mechanism for the synthesis of pyrano coumarins is outlined in Scheme 3. Initially, intermediate 7 is formed via the Knoevenagel condensation of the aldehyde and active methylene compound. For the formation of pyrano coumarin 4, adduct 8 results from a Michael-type addition of C-8 of dihydroxycoumarin to compound 7. Subsequently, cyclization of intermediate 8 gives pyrano coumarin 4. 4-Hydroxycoumarin can also attack intermediate 7 to produce 9, which is then converted to pyrano coumarin 6 after an intramolecular cyclization.
|Scheme 3 Proposed mechanism for the synthesis of 4 and 6 in the presence of Fe3O4@SiO2–BSA.|
The recovery and reusability of catalyst are quite preferable because they are eco-friendly procedures. The recovered Fe3O4@SiO2–BSA from the model reaction for the synthesis of 4a was regenerated by washing with chloroform and drying at 120 °C for 1 hour. Using the recycled catalyst four consecutive times in the model reaction gave the product with a gradually decreasing reaction yield (Fig. 6).
|Fig. 6 Reusability study of Fe3O4@SiO2–BSA in the synthesis of 4a at 80 °C under solvent-free conditions.|
ConclusionsIn summary, for the first time, a new modified magnetic nanoparticle-bearing boron sulfonic acid, Fe3O4@SiO2–BSA, was prepared, characterized, and applied as a heterogeneous and efficient acid catalyst for the synthesis of a series of functionalized heterocyclic compounds containing the coumarin moiety via a one-pot three-component reaction. Our work presents a very simple reaction performed in the absence of hazardous organic solvents. Reusability of the catalyst, operational simplicity, and good chemical yields, combined with step- and atom-economic aspects, make this strategy highly attractive. It is worthwhile to note that the products are potentially valuable for further synthetic manipulation because of the presence of transformable functionalities.
Conflicts of interestThere are no conflicts to declare.
AcknowledgementsThe authors gratefully acknowledge partial support of this work by Yasouj University, Yasouj, Iran.
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|† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra08253c|
|This journal is © The Royal Society of Chemistry 2017|
|Entry||Catalyst (g)||Solvent||T (oC)||Yielda (%)|
|Entry||Ar||R1||R2||Mp (°C)||Yielda (%)|
|Entry||Ar||Mp (°C)||Yielda (%)|
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