Highly Efficient Polyacrylonitrile Fiber Catalysts Functionalized by

Research Article pubs.acs.org/journal/ascecg

Highly Efficient Polyacrylonitrile Fiber Catalysts Functionalized by Aminopyridines for the Synthesis of 3‑Substituted 2‑Aminothiophenes in Water Pengyu Li, Jianguo Du, Yujia Xie, Minli Tao,* and Wen-Qin Zhang* Department of Chemistry, School of Science, Tianjin University, Tianjin, 300072, Peoples’ Republic of China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, Peoples’ Republic of China S Supporting Information *

ABSTRACT: Four aminopyridine (AP) functionalized polyacrylonitrile fibers (PANAPFs) were developed. UV−vis, FTIR, EA, and SEM were used to demonstrate the successful preparation of the fiber catalysts. Among the prepared catalysts, the PANp‑AP‑3F (with a C3 chain linker) and PANp‑AP‑6F (with a C6 chain linker) exhibited the best catalytic activity to efficiently catalyze the Gewald reaction of ethyl cyanoacetate and 2,5-dihydroxy-1,4-dithiane in water with yields up to 92% under a low catalyst dosage of 1 mol %. The influences of the position and length of the linker, i.e., the carbon chain between the fiber backbone and the aminopyridine moiety, on the catalytic activities were investigated in detail. In addition, the selected catalyst PANp‑AP‑3F was applied to the Gewald reaction of a series of active nitriles and 2,5-dihydroxy-1,4-dithiane and reused without further treatments. A possible mechanism and a microenvironment promoting process were conceived to explain the excellent catalytic properties of this catalytic system. Furthermore, the PANp‑AP‑3F performed well in a scaled-up experiment indicating its potential application in industry. KEYWORDS: Polyacrylonitrile fiber, Fiber catalyst, Aminopyridine, Microenvironment, Gewald reaction, 3-Substituted 2-aminothiophene, Water

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INTRODUCTION 2-Aminothiophene derivatives, a series of promising compounds, were widely used in the designs of varieties of pharmaceuticals,1−6 dyes,7−12 conducting polymers,12−15 biodiagnostic reagents,16−18 and other related compounds.19−22 Typically, 2-aminothiophenes were prepared via the Gewald reaction,23,24 which is a multicomponent condensation of a ketone with an α-active acetonitrile and elemental sulfur under the catalysis of a base such as morpholine, diethylamine, etc. Recently, many improvements have been achieved on the typical Gewald reaction.12 Taking the reaction components into consideration, α-sulfanylaldehydes or α-sulfanylketones were successfully used to react with active nitriles affording significant results. In addition, different catalysts, such as 1,4diazabicyclo[2.2.2]octane (DABCO),25 ionic liquids (ILs),26−28 29 L-proline, imidazole,30 triethylamine,12 zinc oxide,31,32 and other catalysts33,34 have been recommended to catalyze the Gewald reaction. Moreover, microwave,28,35 electrochemical methods,36 and ultrasonic irradiation technologies37 have also been used to accelerate the Gewald reaction. Despite their potential utility, many of those methods have limitations, such as the use of expensive reagents or hazardous solvents, high catalyst loading, long reaction time, and unsatisfactory yields, which restrict the further practical application of the Gewald © XXXX American Chemical Society

reaction. Therefore, it is still of great necessity to explore simpler, more efficient, and environmentally friendly methods for the synthesis of 3-substituted 2-aminothiophenes. 4-Dimethylaminopyridine (DMAP), a high-performance catalyst, has been broadly applied in organic synthesis, for example, esterification reactions,38−41 Baylis−Hillman reactions,42−44 multicomponent reactions,45,46 etc. However, homogeneous catalysis has its intrinsic disadvantage of difficult separation of the catalyst, which will cause resource consumption and environmental pollution. Immobilizing catalysts on solid supports is an appropriate strategy to solve these problems. In recent years, from the perspective of green chemistry, immobilized catalysts have been well studied.47−49 A series of materials such as silicas,50−52 polymers,53−55 metal oxides,56,57 or zeolites58,59 have been used as supports. Although some progress has been achieved, there are still certain questions to be resolved, such as high cost, low activity, and the complicated preparation process of the catalyst. Hence, economical, Received: October 2, 2015 Revised: December 8, 2015

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DOI: 10.1021/acssuschemeng.5b01216 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

influenced by the linker length between the fiber backbone and the aminopyridine (AP) moiety. The longer the linker length of the AP is, the easier it is to be immobilized on the PANF. Additionally, the position of the linker on the pyridine moiety also influences the grafting degree of PANAPFs. For example, the weight gain of PANo‑AP‑3F is slightly greater than that of PANp‑AP‑3F under the same reaction conditions and carbon linker length (Table 1, entry 2 and 4). The UV−vis spectra of four AP derivatives are shown in Figure S1. Amines 1−3 have almost the same spectra: except one peak at 212 nm and another one around 280 nm. According to the structural similarity of them, the two peaks could be attributed to the π → π* transition of the pyridine ring conjugated system.63,64 When the N atom of the linker end locates at the ortho-position of the pyridine ring, i.e., in the case of amine 4, the two π → π* transition peaks show red shifts to 246 and 312 nm, respectively. After immobilizing the AP derivatives into the PANF, all characteristic UV peaks of PANAPFs show red shifts from 280 to 304 nm for amines 1−3 and from 312 to 339 nm for amine 4 (Figure S2b−e). These results indicate that the amines are successfully immobilized into the PANF. Moreover, the red shifts imply that the immobilized amines and the −CN groups on the PANF polymer chains form a special microenvironment in which the ion separated resonance structure (Scheme 2N) of the aminopyridine moiety is stabilized by the polar −CN groups, which further decrease the π → π* transition energy and increase the nucleophilicity of the AP moiety.

efficient, eco-friendly, and easily prepared immobilized catalysts are still expected to be developed. Polyacrylonitrile fiber (PANF), a common material in our daily life, has excellent mechanical strength and contains an abundance of cyano groups which can be easily transformed into carboxyl, amide, amidoxime, etc.60,61 Therefore, PANF is a suitable starting material to prepare heterogeneous catalysts. In our previous work, we successfully prepared an aminopyridine functionalized polyacrylonitrile fiber (PANp‑AP‑3F) catalyst and used it to efficiently catalyze a three-component condensation reaction of aldehyde, malononitrile, and α-naphthol in water to afford the corresponding substituted 2-amino-2-chromenes.62 Encouraged by this, we subsequently investigated the catalytic activity of PANAPFs. Herein, PANp‑AP‑3F and three other PANAPFs (PANp‑AP‑2F, PANp‑AP‑6F, PANo‑AP‑3F) were prepared, and their activities were examined via the Gewald reaction of 2,5-dihydroxy-1,4-dithiane and active nitriles.

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RESULTS AND DISCUSSION PANp‑AP‑2F, PANp‑AP‑3F, and PANp‑AP‑6F were prepared by adding PANF into a mixture of water and amines 1, 2, and 3, respectively (Scheme 1). In order to make clear the steric effect Scheme 1. Preparation of PANAPFs

Scheme 2. Resonance Structures of the Aminopyridine Moiety

on the pyridine ring, PANo‑AP‑3F was also prepared from amine 4 (with C3 chain at the ortho position of the pyridine ring). The extents of functionalization determined by weight gain and acid exchange capacity are summarized in Table 1. It shows that the

Samples of the fiber catalysts were pulverized by cutting and prepared into KBr pellets. The FTIR spectra are shown in Figure 1. Compared with the spectrum of PANF, the spectra of

Table 1. Synthesis of PANAPFs entry

catalyst

1

PANp‑AP‑2F

2 3 4

PANp‑AP‑3F PANp‑AP‑6F PANo‑AP‑3F

V(amine): V(water)

time (h)

weight gaina (%)

1:1 2:1 3:1 1:1 1:1 1:1

18 18 18 18 15 18

4.0 6.7 10.8 16.4 27.0 21.6

catalyst loading (mmol/g) 0.23b 0.38b 0.58b 0.78b 0.96b 0.99b

0.21c 0.35c 0.55c 0.73c 0.94c 0.98c

Weight gain = [(W2 − W1)/W1] × 100%, where W1 and W2 are the weights of PANF and PANAPFs. bCalculated by weight gain. c Determined by titration. a

catalyst loading obtained by titration matched up well to the weight gain. With V(amine)/V(water) = 3:1, the PANp‑AP‑2F with a low weight gain of 10.8% (0.48 mmol/g) was obtained after being refluxed for 18 h (Table 1, entry 1). However, with V(amine)/V(water) = 1:1, the weight gain of PANp‑AP‑3F was 16.4% (0.73 mmol/g) for the same reaction time (Table 1, entry 2), and that of PANp‑AP‑6F was 27.0% (0.94 mmol/g) for a shorter reaction time of 15 h (Table 1, entry 3), which indicates that the extent of modification of PANAPFs is prominently

Figure 1. FTIR spectra of (a) PANF, (b) PANp‑AP‑2F, (c) PANp‑AP‑3F, (d) PANp‑AP‑6F, (e) PANo‑AP‑3F. B


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Table 2. Catalytic Activities of Different Catalystsa

the fiber catalysts show two new peaks at 1552 cm−1 and 807− 818 cm−1. They are assigned to the stretching vibration65,66 and the C−H out-of-plane bending vibration of the pyridine ring, respectively. The IR results also demonstrate the successful grating of the amines into the PANF (Figure 1b−e). In Figure S3, the FTIR spectrum of PANp‑AP‑3F1 (PANp‑AP‑3F was reused one time) is similar to that of PANp‑AP‑3F (Figure S3f and c), which proves that the AP moieties are still immobilized on the fiber after one run. However, in the spectrum of PANp‑AP‑3F4 (PANp‑AP‑3F was reused four times), the peak at 1552 cm−1 decreases slightly, which may be due to the minor loss of AP moieties from PANp‑AP‑3F4 (Figure S3g). The elemental analysis data of PANF, PAN p‑AP‑3 F, PANp‑AP‑3F1, and PANp‑AP‑3F4 are listed in Table S1. Compared to PANF, amine 2 has a lower carbon content (67.00%) and nitrogen content (23.44%) and higher hydrogen content (9.56%). After the modification, the carbon and nitrogen contents of PANp‑AP‑3F both decrease, and the hydrogen content increases as expected (Table S1, entry 2), which demonstrates the successful immobilization of amine 2. After being reused for the first cycle, a trace amount of intermediates and product of the Gewald reaction may be absorbed in the fiber catalyst, which causes the sulfur content (0.21%) of PANp‑AP‑3F1 to increase while the nitrogen content decreases slightly compared with that of PANp‑AP‑3F (Table S1, entry 3). Due to the continual absorption of sulfur-containing intermediates and products, the sulfur content of PANp‑AP‑3F4 subsequently increases to 1% (Table S1, entry 4) as expected. SEM images of PANF, PANp‑AP‑3F, PANp‑AP‑3F1, and PANp‑AP‑3F4 are depicted in Figure S4. The surface of PANF is very smooth (Figure S4a). After being functionalized, the surface of the fiber catalyst becomes slightly rough, and the diameter increases remarkably (Figure S4b), which is further evidence for the successful grafting of amine 2. Very few changes occur on the surface of PANp‑AP‑3F1 after being used to catalyze the Gewald reaction one time (Figure S4c). Although the fiber catalyst still keeps its original integrality after four times, the surface of PANp‑AP‑3F4 becomes much coarser. It may be caused by the absorption of intermediates and product which will occupy the “active-site” of the catalyst and reduce the activity of the fiber catalyst. (Figure S4d). Catalytic activities of different fiber catalysts for the Gewald reaction of 2,5-dihydroxy-1,4-dithiane and ethyl cyanoacetate in water are shown in Table 2. When the reaction was carried out at 80 °C for 40 min without catalyst or in the presence of PANF, only trace product was detected (Table 2, entries 1 and 2). In the case of DMAP as the catalyst, the yield was 79%. While in the presence of the fiber catalysts, PANAPFs (PANp‑AP‑2F, PANp‑AP‑3F, and PANp‑AP‑6F) high yields (81%, 92%, and 92%, respectively, Table 2, entries 4−6) were achieved within 5 min; i.e., the PANAPFs exhibited much higher catalytic activities than DMAP. This may be because, on one hand, less water soluble reactants are pushed into the microenvironment of PANAPFs and aggregated together, which makes the aminopyridine moieties much better at attacking the reactants; on the other hand, in the microenvironment of PANAPFs, the aminopyridine moieties that are induced by the polar −CN group prefer the ion separated resonance structure (M), which is more nucleophilic than the electron neutral resonance structure (N; Scheme 2). In addition, the yields catalyzed by PANp‑AP‑2F, PANp‑AP‑3F, and PANp‑AP‑6F are much higher than that of other fiber catalysts under the same conditions (Table 2, entries 4−6, 8, and 9).

entry

catalyst

temperature (°C)

yield (%)b

1 2 3 4

blank PANF DMAP PANp‑AP‑2F

5

PANp‑AP‑3F

6

PANp‑AP‑6F

7 8 9

PANo‑AP‑3F PANPF-3d P-PANFe

80 80 80 60 80 60 80 60 80 80 80 80

tracec tracec 79 41 81 61 92 73 92 70 69 72

a

Reaction conditions: 2,5-dihydroxy-1,4-dithiane (A; 2.5 mmol), ethyl cyanoacetate (B1; 5.0 mmol), and catalyst (1 mol % for B1, based on aminopyridine group) were heated to 60 or 80 °C in water (20 mL) for 5 min. bIsolated yield by column chromatography. cReaction time is 40 min. dPANPF-3 was prepared by grafting N,N-dimethyl-1,3propanediamine onto PANF. eP-PANF was prepared by grafting Nmethyl-N′-(3-aminopropyl)piperazine onto PANF.

Since the nucleophilicity of the AP moieties in PANAPFs is high enough so that it can attack the cyano group of intermediate E quickly to form the intermediate F (Scheme 3). After that, the intermediate F will generate the product easily after an intramolecular ring-closure reaction. However, in the case of tertiary amine functionalized fibers as catalysts (Table 2, entries 8 and 9), they act primarily as Brønsted bases, rather than nucleophiles. The catalytic activity of PANo‑AP‑3F is the poorest among the four PANAPFs (Table 2, entry 7). This suggests that the position of the linker nitrogen at the pyridine ring is also crucial for the catalytic activity of PANAPFs, since the ortho substitution increases the steric effect and subsequently decreases the nucleophilicity of the pyridine ring. When the reaction temperature was reduced to 60 °C, the yield in the presence of PANp‑AP‑3F decreases to 61%, which is lower than that with PANp‑AP‑6F (73%) (Table 2, entries 5 and 6). It turned out that the PANp‑AP‑6F has the highest catalytic activity. This is due to that the AP moiety of PANp‑AP‑6F has the longest linker carbon chain, which makes the “catalytic active-sites” more flexible to access the reactants and promote the reaction more smoothly. However, considering that the preparation of material N,N′-dimethyl-1,6-hexanediamine, which is used to synthesize amine 3, is very tedious, time-consuming, and environmentally unfriendly (poisonous reagents such as benzene and methyl sulfate were used), while the N,N′dimethyl-1,3-propanediamine used to synthesize amine 2 is an inexpensive commercial material and PANp‑AP‑3F has the same catalytic activity as PANp‑AP‑6F at 80 °C, the catalyst PANp‑AP‑3F was thus chosen for further research. A possible mechanism of the Gewald reaction catalyzed by PANp‑AP‑3F is depicted in Scheme 3. Initially, PANp‑AP‑3F acts as a Brønsted base and removes an α-hydrogen from the active nitrile to generate a carbanion B−. Then, the B− attacks the carbonyl carbon of mercaptoacetaldehyde to form a tetrahedral intermediate which captures a proton from the pyridinium moiety of PANp‑AP‑3F to produce an intermediate D. After an C


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ACS Sustainable Chemistry & Engineering Scheme 3. A Possible Mechanism for the Gewald Reaction Catalyzed by PANp‑AP‑3F

Table 3. Optimization of the Gewald Reactiona

intramolecular dehydration of D, an important intermediate E was formed. Afterward, the pyridine group in PANp‑AP‑3F functions as a nucleophile to attack the cyano group of E to generate the corresponding intermediate F, which performs as a crucial process in the Gewald reaction. Then, after an intramolecular nucleophilic addition of the sulfur anion to the CNH double bond, the target product C is generated exclusively in the amino form and released from the fiber catalyst. The process of the Gewald reaction catalyzed by PANp‑AP‑3F is vividly illustrated in Figure S5, which is a partial enlargement of the fiber catalysts in the reaction system. In this system, a special microenvironment is built with polymeric segments and aminopyridine groups beneath the surface of the fiber catalyst, in which reactants can be easily “dissolved.” In this microenvironment, active catalytic units of PANp‑AP‑3F become more nucleophilic and accessible to the accumulated reactants, which make the Gewald reaction proceed efficiently and exclusively. After the reaction is completed, the product is released from the surface of PANp‑AP‑3F, and the fiber catalyst is recovered and can be reused for the next cycle. To optimize the reaction conditions, initially, 2,5-dihydroxy1,4-dithiane (2.5 mmol), ethyl cyanoacetate (5 mmol), and PANp‑AP‑3F (5 mol %) were conducted in water at 80 °C for 5 min, and a yield of 91% was obtained (Table 3, entry 1). When the dosage of catalyst was reduced to 3 mol %, 2 mol %, 1 mol %, and 0.5 mol %, yields of 93%, 92%, 92%, and 76% were obtained, respectively (Table 3, entries 2−5). Therefore, 1 mol % was chosen as the most suitable catalyst dosage for further studies. Reaction time is another research item, when the reaction time was prolonged from 5 to 10 min, the yield kept unchanged (Table 3, entry 6). However, when the reaction time was shortened to 3 min, the reactant could not be consumed completely, and the yield was reduced to 73% (Table 3, entry 7). Additionally, reaction temperature has also

entry

solvent

catalyst loading (mol %)

T (°C)

time (min)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O CH3OH C2H5OH THF EtOAc CH2Cl2 toluene cyclohexane

5 3 2 1 0.5 1 1 1 1 1 1 1 1 1 1 1 1

80 80 80 80 80 80 80 100 70 60 65 78 66 77 40 80 80

5 5 5 5 5 10 3 5 5 5 5 5 5 5 5 5 5

91 93 92 92 76 92 73 91 78 61 53 17 10 NR NR NR NR

a

Reaction conditions: 2,5-dihydroxy-1,4-dithiane (A; 2.5 mmol), ethyl cyanoacetate (B1; 5.0 mmol), and the fiber catalyst (1 mol % for B1, based on aminopyridine group) were heated at an appropriate temperature in corresponding solvent (20 mL) for the given time. b Isolated yield by column chromatography.

been investigated, and the yield was unchanged if the temperature was raised to 100 °C (Table 3, entry 8). However, when the temperature was lowered to 70 and 60 °C, the reaction was slowed down, and the yields were decreased D


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ACS Sustainable Chemistry & Engineering Table 4. Reaction of Active Nitriles with 2,5-Dihydroxy-1,4-dithianea

Reaction conditions: 2,5-dihydroxy-1,4-dithiane (A; 2.5 mmol), active nitrile (B; 5.0 mmol), and the fiber catalyst (1 mol % for B1−14, 2 mol % for B15, based on aminopyridine group) were heated at 80 °C in water (20 mL) for the given time. bIsolate yield.

a

even failed to happen (Table 3, entry 14−17). In a word, 1 mol % of catalyst dosage, reacting for 5 min, at 80 °C in water was selected as the optimal reaction conditions. PANp‑AP‑3F was then applied to the Gewald reaction of different active nitriles with 2,5-dihydroxy-1,4-dithiane under the optimal conditions mentioned above. As can be seen from Table 4, three common active nitriles, B1−B3, can react smoothly with high yields (Table 4, entries 1−3). Furthermore, three active nitriles with an aliphatic chain or aliphatic ring can also proceed efficiently to afford high yields from 90% to 93% when the reaction time was prolonged to 10 min (Table 4,

significantly to 78% and 61%, respectively (Table 3, entries 9 and 10). Moreover, different solvents were used to study the influence of solvents on the Gewald reaction in the presence of PANp‑AP‑3F. As shown in Table 3 (entries 3, 11−17), the polarity of solvent greatly affects the reaction yields which decreased with a decline of the solvent polarities. In water, the Gewald reaction gives the highest yield of 92% (Table 3, entry 4). In methanol, the yield reduces to 53% (Table 3, entry 11), and in THF it drops further to just 10% (Table 3, entry 13). For other solvents such as EtOAc, CH2Cl2, toluene, and cyclohexane, whose polarities are lower than THF, the reaction E


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ACS Sustainable Chemistry & Engineering Table 5. Comparison of Different Catalytic Systems on the Gewald Reactiona

a

entry

catalytic

catalyst dosage (mol %)

solvent

T&Time

yield (%)b

1 2 3 4 5 6

NaOH Et2NH Et3N (microwave) morpholine P-PANF PANp‑AP‑3F

25 wt % 113

H2O EtOH MeOH MeOH EtOH H2O

10 °C, 2 h < 5 °C, 10 min 30−35 °C, 1 h 50 °C, 2 min 50 °C, 2h reflux, 2h 80 °C, 5 min

64 57 81 69 91 92

45.8 8.0 1.0

reusability (run)

ref

10 4

67 68 69 70 71 this work

Selected 2,5-dihydroxy-1,4-dithiane and malononitrile as the comparison objects. bYields under their optimized experimental conditions.

(92%). A possible mechanism has been proposed, in which the nucleophilicity of AP plays an important role in the Gewald reaction, and a microenvironment promotion process has been speculated to explain the high catalytic activity of PANp‑AP‑3F. A series of active nitriles proceed smoothly under the catalysis of PANp-AP-3F to afford moderate to excellent yields from 76% to 93%. After the completion of the reaction, the catalyst can be easily separated from the reaction system and conveniently reused at least four times. Moreover, the PANp‑AP‑3F performs well in a scaled-up experiment with a yield of 87%. It turns out that the catalyst has many advantages, such as highly efficient activity, broad substrate scope, an environmentally friendly solvent, and good recyclability. It is believed that more, new applications of PANAPFs will be developed in the future.

entries 4−6). However, when conjugated aromatic rings were introduced into the active nitriles, the reaction slowed down and the yields decreased more or less (Table 4, entries 7−14). It suggested that the low solubility and big size of the substrate restricted their entering inside the microenvironment, and the conjugation effect of the aromatic ring decreased the activity of the substrate. From entries 8−10, it can be speculated that an aromatic ring with electron-withdrawing group like −Cl will enhance the acidity of the α-hydrogen and thus promotes the reaction. Conversely, an electron-donating group like −OCH3 will weaken the acidity of the active nitrile and obtunds the reaction. Moreover, substrates with a sulfonyl group or two active methylene groups have been tested, and they all gave moderate yields of 76% and 77%, respectively. The excellent results suggest that the PANp‑AP‑3F is a highly active catalyst for the Gewald reaction in water. The recyclability of the fiber catalyst PANp‑AP‑3F was tested by catalyzing the Gewald reaction of 2,5-dihydroxy-1,4-dithiane (2.5 mmol) and ethyl cyanoacetate (5 mmol) in water (20 mL) at 80 °C. After the completion of each reaction, the fiber catalyst was taken out and washed with ethyl acetate to remove the adsorbed product. Then the recycled catalyst was dried and reused directly in the next cycle without any additional treatment. The yields of the first four cycles are 92%, 88%, 84%, and 77%, which indicates that the PANp‑AP‑3F can be reused at least four times. Under the optimized conditions, the Gewald reaction was scaled up with 2,5-dihydroxy-1,4-dithiane (25 mmol) and ethyl cyanoacetate (50 mmol). The yield of ethyl 2-aminothiophene3-carboxylate was 87% (7.5 g), which indicates that PANp‑AP‑3F is a highly efficient catalyst, and it may be used in industrialscale productions. Compared with different catalysts under their optimized conditions (Table 5), the PANAPF catalysts has overwhelming advantages such as, easy to be prepared and low catalyst dosage, short reaction time, high activity, green solvent, potential for scaled up production and good reusability.

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EXPERIMENTAL SECTION

PANAPFs were prepared according to our previous work62 with minor improvements, and the detailed procedures are as below (Scheme 1). PANp‑AP‑2F. Dried PANF (1 g) was put into a mixture of water (10 mL) and N,N′-dimethyl-N-(4-pyridyl)-1,2-ethanediamine (30 mL) in a 100 mL three-neck flask and heated to reflux for 18 h. Next, the modified PANF was filtered out and washed with deionized water at 70−80 °C until the pH of the washed water was 7. Then the modified fiber was dried overnight under a vacuum at 60 °C to give the PANp‑AP‑2F catalyst. The weight gain of PANp‑AP‑2F was 10.8%. PANp‑AP‑3F. PANp‑AP‑3F with a weight gain of 16.4% was synthesized as per PANp‑AP‑2F from PANF (1 g) with the mixture of water (20 mL) and N,N′-dimethyl-N-(4-pyridyl)-1,3-propanediamine (20 mL). PANp‑AP‑6F. PANp‑AP‑6F with a weight gain of 27.0% was synthesized as per PANp‑AP‑2F from PANF (1 g) with a mixture of water (20 mL) and N,N′-dimethyl-N-(4-pyridyl)-1,6-hexanediamine (20 mL) under refluxing 15 h. PANo‑AP‑3F. PANo‑AP‑3F with a weight gain of 21.6% was synthesized as per PANp‑AP‑2F from PANF (1 g) with a mixture of water (20 mL) and N,N′-dimethyl-N-(2-pyridyl)-1,3-propanediamine (20 mL) under refluxing 18 h. General Procedure for Gewald Reaction. A mixture of active nitrile (5.0 mmol), 2,5-dihydroxy-1,4-dithiane (2.5 mmol), and H2O (20 mL) was stirred at room temperature for 10 min in a three-necked flask (100 mL) to get a turbid solution. Then the fiber catalyst (0.01 mmol, 1 mol % aminopyridine group compared to active nitrile) was added, and the mixture was stirred at 80 °C until trace or no starting substrate was detected (monitored by TLC; in the case of ethyl cyanoacetate, the reaction mixture became a clear liquid, and when the temperature reached 80 °C, it became turbid again). The fiber catalyst was filtered out and washed with EtOAc (3 × 15 mL). The filtrate was extracted with ethyl acetate (4 × 30 mL). The combined organic phase was concentrated, and the crude product was purified by column chromatography (petroleum ether/ethyl acetate = 20:1).

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CONCLUSIONS In this paper, four aminopyridines with different carbon chain lengths and positions functionalized polyacrylonitrile fiber (PANAPF) catalysts were prepared to highly and efficiently catalyze the Gewald reaction of 2,5-dihydroxy-1,4-dithiane and active nitriles in water to afford the corresponding 3-substituted 2-aminothiophenes. The results demonstrate that the aminopyridine attached by a longer linker chain at the para position of the pyridine ring has higher catalytic activity. The PANp‑AP‑3F functionalized by aminopyridine with a C3 alkyl chain was selected as the most suitable catalyst for the Gewald reaction due to its easy preparation and excellent catalytic activity. It can complete the Gewald reaction for 5 to 30 min in good to high yield. Among all the tested solvents, water gives the best result F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01216. General experimental information; the acid exchange capacities of the fiber catalysts; the synthesis of amines 1−4; the synthesis of the activated nitriles; a UV−vis spectra figure of amines 1−4 and a UV−vis spectra figure of different fiber catalysts; a group of SEM photographs and a FTIR spectra figure of selected fiber catalyst in different stages; a simulative diagram of Gewald reaction; a table of elemental analysis data of fiber catalysts in different stages; and NMR data and spectra for selected compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Natural Science Foundation of China (No. 21572156 and No. 21306133).

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REFERENCES

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