Hakimeh Ziyadi and
Akbar Heydari*
Chemistry Department, Tarbiat Modares University, P. O. Box 14155–4838, Tehran, Iran. E-mail: heydar_a@modares.ac.ir; Fax: +98-21-82883455; Tel: +98-21-82883444
First published on 15th October 2014
PVA/Fe(NO3)3 nanofiber mats are found to be a heterogeneous recyclable catalyst for the efficient synthesis of quinoline derivatives in the Friedländer condensation of 2-aminoarylketones with carbonyl compounds and β-keto esters. The novel catalytic system could be recovered and reused at least 5 times without any loss of catalytic activity.
Poly(vinyl alcohol) (PVA) is a water soluble and biodegradable synthetic polymer. The polymerization of vinyl acetate, followed by hydrolysis of the product, leads to the production of PVA. It is white (colourless), odourless and granular in its commercial form. The polyhydroxy structure of this polymer is shown below.
PVA is used in various industries, such as membrane, coatings, textile sizing, adhesive and medical devices, because of its biocompatibility, good chemical resistance and physical properties.22
This inexpensive and non-toxic polymer is a good candidate as a supporting matrix to immobilize Lewis acid catalysts. Transition metal species can be chelated with the numerous surface hydroxyl functional groups in PVA to prepare a heterogeneous solid support, which is insoluble in most organic solvents and can also be functionalized. However, the semi-crystalline structure, high melting point and low active sites of bulk PVA makes it difficult to fabricate it into a suitable solid matrix for catalytic applications.23 Electrospinning is a versatile method for the preparation of ultra-long, one-dimensional PVA nanofibers. This method uses an electromagnetic field with a voltage sufficient to overcome the surface tension of a polymer solution and to draw it into an ultrafine nanofiber. The electrospun nanofibers exhibit a number of unique features and properties such as 1D morphology, extraordinary length, high surface area, and hierarchically porous structure.24 When compared to nanoparticles, electrospun nanofibers can be easily prepared as membranes (supported or free-standing, and non-woven or woven) that can be conveniently handled and manipulated during an application.25 Few studies have been reported on the catalytic activity of PVA nanofibers. L. Shao used Pd/PVA nanofiber mats as a catalyst in a coupling reaction.26 However, to the best of our knowledge, no study has been conducted on the use of electrospun PVA nanofiber mats as a heterogeneous iron salt catalyst. We herein report the use of a PVA/Fe(NO3)3 nanofiber mat as an effective heterogeneous catalyst for quinoline synthesis via a Friedländer annulation. This catalyst showed good catalytic activity and high selectivity under mild conditions. Moreover, it can be easily separated using forceps or simple filtration and reused at least 5 times without any loss of catalytic activity.
The uniform and bead-free structure of the nanofiber mats could be achieved when the PVA weight concentration was at 6.0–8.0% in water.27 The PVA weight (8.0 wt%) was chosen for the preparation of the PVA solution and was mixed with ferric nitrate.28 Then, the electrospinning of the solution produced PVA/Fe(NO3)3 nanofiber mats (Scheme 1). The SEM images shown in Fig. 1a and b show the morphologies of the PVA/Fe(NO3)3 nanofibers at different magnifications. As this figure shows the nanofibers appear uniform in size, with smooth surfaces and no significant beading in the samples. The resultant nanofiber mats offer unique properties such as a high surface area to volume ratio, porous membrane structure, good interconnectivity of pores and potential to functionalize on a nanoscale. The obtained histograms (Fig. 1c) confirm the narrow diameter distribution for 50 observed nanofibers. The diameter of the PVA/Fe(NO3)3 nanofibers is between 80–200 nm. Fig. 2 provides the FTIR spectra of PVA, Fe(NO3)3·9H2O and PVA/Fe(NO3)3 nanofibers. The FTIR spectra of the PVA/Fe(NO3)3 nanofiber mats are similar to that of PVA with only a slight increase in the transmittance. Moreover, the peaks corresponding to the Fe(NO3)3 stretch were not observed indicating that the Fe(NO3)3 was well retained in the PVA matrix. X-ray diffraction analysis (Fig. 3) indicated that the nanofibers have an approximately amorphous structure and the crystallites of the PVA powder diminished during the solution preparation followed by electrospinning. In addition, there is no characteristic diffraction peak for the ferric nitrate, suggesting that the metal salt dispersed on the PVA nanofiber mats has an amorphous structure and was successfully chelated to PVA.29 Previously, groups of researchers have studied the binding energy of O 1s and Fe 2p during the formation of the PVA/Fe(III) complex in nanofibers. These results indicated the bonding of the hydroxyl groups in PVA to the metal ion through a Fe–O–C bond.30
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Fig. 1 (a and b) SEM image of electrospun PVA/Fe(NO3)3 fiber at different magnifications, (c) fiber diameter distribution of the electrospun PVA/Fe(NO3)3 fibers. |
The reaction is normally carried out in toluene, which is the normal solvent for the Friedlander reaction.31 It is well-known, that Fe(NO3)3 in its homogenous form is effective for the synthesis of quinoline (Table 1, entry 1). This reaction proceeded in a lower time and good yield because of the accessibility of the catalytic Fe3+center in solution, but the salt was soluble and homogenous in the solvent resulting in contamination of the desired product by salt and required a difficult work up without catalyst recycling. In the presence of the heterogeneous PVA/Fe(NO3)3 nanofibers, the reaction cleanly and simply proceeded, and the condensation product precipitated in excellent isolated yield (Table 1, entry 2). Our result showed that this catalyst was insoluble in toluene after 1 week at reflux. To optimize the reaction conditions and improve the yield, our attention was focused on the role of temperature. It was noticed that increasing the temperature to 80 °C considerably enhances the yield. By elevating the temperature up to 80 °C, the yield remained unchanged (Table 1, entries 3 and 4). In the next step, different amounts of catalyst were used for this transformation (Table 1, entry 5 and 6). Increasing the amount of catalyst increased the yield of quinoline. In the absence of the catalyst, the reaction takes more than 24 h for completion (Table 1, entry 7). The PVA nanofibers with no iron catalyst react to give a very low yield over a long time (Table 1, entry 8). Using the PVA nanofibers catalyst containing 3 mol% of Fe3+ is most effective in terms of both time and yield. Therefore, the optimum conditions for this reaction are as follows: PVA/Fe(NO3)3 nanofiber catalyst containing 3 mol% of Fe3+ at 80 °C.
Entry | Catalystb (mol%) | Temp. (°C) | Time (h) | Yieldc (%) |
---|---|---|---|---|
a Reaction conditions: 2-aminobenzophenone (2 mmol), ethyl acetoacetate (2 mmol), toluene (5 mL).b mol% of Fe3+ in catalyst.c Isolated yield.d Reaction completed after 18 h. | ||||
1 | Fe(NO3)3·9H2O (1) | 60 | 2 | 84 |
2 | PVA/Fe(NO3)3 nanofiber (1) | 60 | 10 | 70 |
3 | PVA/Fe(NO3)3 nanofiber (1) | 80 | 10 | 78 |
4 | PVA/Fe(NO3)3 nanofiber (1) | 100 | 5 | 80 |
5 | PVA/Fe(NO3)3 nanofiber (2) | 80 | 4 | 85 |
6 | PVA/Fe(NO3)3 nanofiber (3) | 80 | 4 | 96 |
7 | No catalyst | 80 | 24 | 0 |
8d | PVA nanofiber (0) | 80 | 12 | Trace |
We extended these reaction conditions to a variety of 2-aminoarylketones with carbonyl compounds. As shown in Table 2, this method is equally effective for substituted 2-aminoarylketones such as 2-aminobenzophenone or 2-amino-5-chlorobenzophenone and 2-aminoacetophenone. Various 1,3-diketones and β-ketoesters reacted with 2-aminoarylketones to obtain the corresponding quinolines. Under the same conditions cyclohexane as a simple cyclic ketone afforded the tricyclic quinoline; however, the reaction needed an excess amount of ketone and proceeded slower than the other substrates (Table 2, entry i).
Entry | Compound 1 | Compound 2 | Product 3 | Melting point (°C) | Time (h) | Yieldb (%) | TONc | TOFd (h−1) |
---|---|---|---|---|---|---|---|---|
a Reaction conditions: compound 1 (2 mmol), compound 2 (2 mmol), PVA/Fe(NO3)3 nanofibers (3 mol% Fe3+), toluene (5 mL), 80 °C.b Isolated yield.c Turnover number (total number of product moles per mole of the catalyst).d Turnover frequency (turnover number per hour).e 2 eq. of compound 2 was used. | ||||||||
a | ![]() |
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107 | 4 | 95 | 32 | 8 |
b | ![]() |
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100 | 4 | 96 | 32 | 8 |
c | ![]() |
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108 | 4 | 90 | 30 | 7.5 |
d | ![]() |
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108 | 6 | 80 | 27 | 4 |
e | ![]() |
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131 | 4 | 96 | 32 | 8 |
f | ![]() |
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100 | 4 | 98 | 33 | 8 |
g | ![]() |
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135 | 4 | 90 | 30 | 8 |
h | ![]() |
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156 | 6 | 75 | 25 | 4 |
ie | ![]() |
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163 | 8 | 70 | 23 | 3 |
j | ![]() |
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215 | 4 | 78 | 26 | 6.5 |
k | ![]() |
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100 | 5 | 85 | 28 | 6 |
l | ![]() |
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Oil | 5 | 85 | 28 | 6 |
m | ![]() |
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Oil | 5 | 78 | 26 | 5 |
n | ![]() |
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105 | 6 | 70 | 23 | 4 |
The reusability of the nanofibers was also examined. After completing the first run of model reactions, the catalyst was washed three times with CH2Cl2, and then was dried in a vacuum and reused in the subsequent reaction. The large structure of the nanofiber mat catalyst leads to its easy separation process using forceps. No significant loss of catalyst activity was observed even after 5 times of reuse (Fig. 4). A negligible decrease in the catalyst activity, rules out the deactivation of catalyst in the Friedländer reaction.
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Fig. 4 The reusability of the PVA/Fe(NO3)3 nanofibers in the reaction between 2-amino benzophenone and ethyl acetoacetate. |
Leaching of Fe(NO3)3 from the nanofibers to the solvent was analysed using ICP. We analysed the concentration of Fe3+ in the PVA/Fe(NO3)3 nanofibers catalyst before and after the reaction. The Fe3+ contents have been determined to be 0.055 and 0.052 mg for each mg of fresh and used catalyst, respectively. After the completion of the reaction (monitored by TLC), the catalyst and product were separated from the solvent. Having evaporated the solvent, water was added as the new solvent. The resultant aqueous solution was analysed using ICP. The content of Fe in the solution was 0.0002 mg for each mg of catalyst, indicating that the Fe leaching was less than 1%. These results show that the Fe leaching from the PVA/Fe(NO3)3 nanofiber mats is not significant during the reaction. SEM analysis of the nanofibers after the reaction (Fig. 5) showed that this catalyst is stable under the reaction conditions and was insoluble in toluene. Furthermore, the SEM image of the nanofibers after 5 runs showed no distinct change in morphology.
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Fig. 5 SEM image of the electrospun PVA/Fe(NO3)3 fibers (a) after reaction (b) after 5 times of reuse. |
The electrospinning process was carried out using a Electroris (FNM Ltd., Iran, http://www.fnm.ir) as an electrospinner device having a high voltage and a syringe pump controllable in a range of 1–35 kV and 0.1–100 mL h−1, respectively. This device can control the electrospinning parameters such as injection rate, drum rotation speed, working distance, needle scanning rate and temperature of the electrospinning media. Electroris has also been used for the industrial preparation of nanofibers. SEM images were observed using a SEM (Philips XL 30 and S-4160) equipped with gold coating. Powder XRD spectra were recorded at room temperature using a Philips X'pert 1710 diffractometer with Co Kα radiation (α = 1.54056 Å) in the Bragg–Brentano geometry (θ–2θ). FTIR spectra were obtained over the region of 400–4000 cm−1 using a NICOLET IR100 FTIR and spectroscopic grade KBr. NMR spectra were recorded in CDCl3 (Bruker, 250 MHz, Switzerland) and the proton chemical shifts reported in ppm relative to TMS as an internal reference.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07643e |
This journal is © The Royal Society of Chemistry 2014 |