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“Fiddler crab-type” imidazolium salt as remote substituents tuning organocatalyst for the cycloaddition of epoxides with carbon dioxide
Journal of CO2 Utilization 16 (2016) 391–398
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
“Fiddler crab-type” imidazolium salt as remote substituents tuning organocatalyst for the cycloaddition of epoxides with carbon dioxide Fei Chen, Dengtai Chen, Lei Shi, Ning Liu* , Bin Dai* School of Chemistry and Chemical Engineering, Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, North 4th Road, Shihezi, Xinjiang 832003, China
A R T I C L E I N F O
Article history: Received 8 August 2016 Received in revised form 28 August 2016 Accepted 6 October 2016 Available online xxx Keywords: Carbon dioxide fixation Cyclic carbonate Organocatalysis Imidazolium salt
A B S T R A C T
A series of unsymmetrical pyridine-bridged pincer-type imidazolium salts are synthesized and demonstrated as highly active catalyst. This catalyst is for the cycloaddition of carbon dioxide with a broad range of terminal epoxides without organic solvent, and co-catalyst. The performance of catalyst considerably depends on the remote structural feature bearing pyridine ring. In addition, bulky internal epoxide is efficiently converted to the desired product. The robustness of the catalytic system is shown by recycling the catalyst over seven consecutive times without significant loss of activity. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Carbon dioxide can serve as a renewable C1 building block for constructing C-C, C-O, C-N, and C-H bonds by reacting with epoxides [1], aziridines [2], amines [3], and terminal alkynes [4]. The 100% atom-economic conversion of CO2 and epoxides yielding cyclic carbonates is one of the most promising reaction routes for CO2 chemical utilization. Cyclic carbonates are considerably interesting and might be utilized as intermediates for preparation of fine chemicals and polymers. Some of the metal-based catalysts exhibit high catalytic activity in the cycloaddition reaction between CO2 and epoxides [5]. Furthermore, organocatalysts for the synthesis of cyclic carbonates could represent an attractive alternative [1e]. To date, various organocatalysts for the cycloaddition reaction between CO2 and epoxides, such as imidazolium salts [6], ammonium salts [7], phosphonium salts [8], and organic amines [9], have been developed. Among these organocatalysts, imidazolium salts are generally synthesized in simple one or two-step procedures by using abundant and inexpensive precursors. Accordingly, various imidazolium salts catalysts have been developed for the synthesis of cyclic carbonates from epoxides and CO2. The mechanistic studies of imidazolium salt-catalyzed the cycloaddition reaction proposed that the proton in the C2 position of the imidazolium ring activates the epoxide through hydrogen
bonding, which facilitates the nucleophilic attack for ring opening of the epoxide (Fig. 1, left) [10]. Research on the improvement of catalytic activity focused on tuning the acidity of the C2 proton by varying substituents R1 and/or R2 that directly bears an imidazolium ring (Fig. 1, left). Cokoja and co-workers [10] suggested that the acidity of imidazolium salts should be proper; significantly weak acidity leads to difficultly in activating the epoxides, whereas considerably strong acidity inhibits the insertion of CO2 because of strong hydrogen bonding contacts. However, the electronic influence of directly functionalized strategy on C2 proton is strong. Consequently, controlling the acidity of catalysts accurately is difficult. The pyridine ring is a typical electronic transfer structure. Thus, this ring is an alternative for the design of imidazolium salts when used as a bridge to tune the acidity of C2 proton by varying remote substituents bearing pyridine ring (Fig. 1, right). In our previous work, we developed a copper-catalyzed [11] and metal-free [12] selective C-N bond construction method, which was applied successfully to the synthesis of unsymmetrical pyridine-bridged pincer-type (fiddler crab-type) imidazolium salts. The resulting the “fiddler crab-type” imidazolium salts were investigated for their use as catalysts in the cycloaddition reaction of CO2 to epoxides. This catalyst design provided insight into the rational development of highly efficient catalysts. 2. Results and discussion
* Corresponding authors. E-mail addresses: [email protected] (N. Liu), [email protected] (B. Dai). http://dx.doi.org/10.1016/j.jcou.2016.10.005 2212-9820/ã 2016 Elsevier Ltd. All rights reserved.
In the present work, 18 “fiddler crab-type” imidazolium salts 1a–r bearing various N-heterocycle substituents through pyridine
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Fig. 1. Design of imidazolium salts.
ring as an electronic transfer bridge were initially prepared with good to excellent yield according to our developed method (Fig. 2) [11]. The molecular structure of imidazolium salts 1q was confirmed by X-ray crystallography (Fig. 2, 1q). The influence of structural features on catalytic behavior was investigated in detail using the cycloaddition of propylene oxide (PO) to CO2 as a model reaction, and the results are summarized in Table 1. They are efficient single-component, metal-free catalysts under solvent-free condition. The remote electronic effect of substituents bearing pyridine ring was initially studied under the same reaction conditions. The structure of substituents bearing pyridine ring, using benzimidazole salt as a catalytic active site, shows an obvious effect on the catalytic activity toward the synthesis of propylene carbonate (PC). The catalytic activities are in the following order: imidazolyl group > benzimidazolyl group > indazolyl group > pyrazolyl group (Table 1, entries 1–4; 5–8; 9–12; and 13–16). The preliminary experimental results first demonstrated that the remote substituents show evident effect on the catalytic performance of imidazolium salts catalyst for the cycloaddition of epoxides to CO2. The influence of alkyl chain lengths bearing cation of imidazole on the catalytic performance was evaluated. The catalytic activity of imidazolium salts with a long alkyl chain is stronger than those with a short alkyl chain (Table 1, entries 2, 6, 10, and 14; 1, 5, 9, and 13); this result is consistent with those reported by Cokoja group
[10b] who suggested that the steric effect of substituents bearing the cation of imidazolium salts is responsible for catalytic activity. The effect of the counter anions on catalytic performance was also investigated (Table 1). The activity order of anion is I > Br > Cl (Table 1, entries 2, 6, 10, and 14; 3, 7, 11, and 15; 4, 8, 12, and 16), which is consistent with results obtained in previous reports [6a,13]. The nucleophilic order of these anions is responsible for the results. Maginn and co-workers [14] suggested that the most significant effect on CO2 solubility in alkylimidazolium-based ionic liquids relies on the nature of the anion. These results indicated that the anion structure has a significant effect on CO2 solubility and capture. Catalysts 1b and 1q show nearly equal catalytic activity on the synthesis of PC (Table 1, entries 2, and 17). Catalyst 1b exhibits an obvious advantage over the synthetic procedure compared with catalyst 1q; therefore, 1b was chosen as the model catalyst for further investigation. The influence of reaction parameters on the reaction was optimized to achieve mild reaction conditions. As shown in Table 1, reaction rate for synthesis of PC was increased with prolonged reaction time. Thus, the optimal reaction time was 5 h (Table 1, entry 18). Pressure influenced the cycloaddition largely in terms of the reactivity when the CO2 pressure varied from 0.1 MPa to 1.0 MPa (Table 1, entries 18–20). The dependence of the PC yield on the catalyst amount was examined. The catalytic activity of 1b was sensitive to the amount of catalyst. A decrease in the molar ratio of 1b to PO, from 1 mol% to 0.1 mol% resulted in evident decrease of PC yield (Table 1, entries 2, 21, and 22). To explore the correlation between structural feature of our developed imidazolium salts and catalytic activity, the pH of two known benzimidazolium salts and four selected imidazolium salts, namely, 1b, 1j, 1m, and 1n, was determined in the mixture of dichloromethane and methanol. In the substitution of imidazolium salts bearing N-atom varying from methyl group to propyl group, the pH of benzimidazolium salts shows evident change from 3.59 to 4.67 (Table 2, entries 1, and 2). In contrast to the abovementioned results, the pH change range of our developed imidazolium salts 1m, and 1n was slow (Table 2, entries 5, and 6). Our developed catalysts are relatively accurate in controlling the acidity of C2 proton by remote substitution effect. As shown in Table 2 (entries 3, 4, and 5), the acidity of imidazolium salts is
Fig. 2. Imidazolium salts in this work.
F. Chen et al. / Journal of CO2 Utilization 16 (2016) 391–398 Table 1 Optimization of PC synthesisa .
entry
Catal. (mol%)
t (h)
Yieldb (%)
Selectivityb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19c 20d 21 22 23
1a (1.0) 1b (1.0) 1c (1.0) 1d (1.0) 1e (1.0) 1f (1.0) 1g (1.0) 1h (1.0) 1i (1.0) 1j (1.0) 1k (1.0) 1l (1.0) 1m (1.0) 1n (1.0) 1o (1.0) 1p (1.0) 1q (1.0) 1b (1.0) 1b (1.0) 1b (1.0) 1b (0.5) 1b (0.1) 1r (1.0)
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 5 5 5 5 5 3
69 88 43 31 57 79 72 45 44 54 46 12 45 63 41 33 90 98 67 36 78 38 42
97 99 93 91 97 96 95 92 96 95 96 97 93 96 95 95 98 99 99 98 98 95 98
a Conditions: unless otherwise specifically notified, all the reactions were carried out with PO (0.58 g, 10 mmol), catalyst (1.0 mol%), no solvent, 90 C, CO2 (1.0 MPa), 3 h. b Determined by GC using biphenyl as an internal standard. c CO2 (0.5 MPa). d CO2 (0.1 MPa).
Table 2 The pH of the imidazolium salts. Entry
1
2
Imidazolium salt
pH Product yield
3.59
4.67
3
4
5
6
1j
1b
1n
1m
4.02 54%
4.47 88%
4.80 63%
5.00 45%
Conditions: imidazolium salts (0.1 mmol) was added in CH2Cl2: MeOH = 10:1 (1 ml). The solution was stirred for 90 min at room temperature and then the pH value was determined.
crucial for its catalytic activity. Significantly weak acidity easily leads to difficultly in activating the epoxides, whereas considerably strong acidity inhibits the insertion of CO2 because of strong hydrogen bond contacts. Our developed method provides a tunable alternative to imidazolium salt design and synthesis. Various epoxides were subjected to cycloaddition in our developed catalytic system to survey the substrate scope. As shown in Table 3, both aliphatic and aromatic substrates were well tolerated to yield desired products 2a–g, and 2i with 78–99% yield under mild conditions. Internal epoxide, which is a challenging substrate because of its high hindrance effect, was also evaluated in the developed catalytic system. A satisfying product yield was obtained by prolonging the reaction time to 12 h (Table 3, 2h). Moreover, the current catalytic system is also effective for the
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reaction of two enantiopure terminal epoxides, thereby affording the corresponding cyclic carbonates (S)-2j, and (S)-2e with 99% ee’s. In addition to the catalyst activity, the recyclability and stability potential also play an important role for possible industrial application. Catalyst 1b was used to evaluate reusability under optimized conditions. In each cycle, the catalyst was separated from the volatile organic products and starting materials by distillation under reduced pressure, and reused for subsequent recycles. As shown in Fig. 3, no evident decrease was observed in PC yield after eight runs, thereby indicating high stability and reusability of the catalyst 1b. The recycled the catalyst 1b was characterized by the NMR analysis, as shown in Fig. S1 (for details, see Supporting information), and there was no difference in the 1H NMR spectra between the fresh and the last-used catalysts, which provided evidence for the good structural stability of the our developed catalyst. The mechanism of the cycloaddition of epoxides with carbon dioxide was investigated widely in previous works [1e,10a]. To determine which substrate, whether CO2 or epoxides, preferentially binds to our developed catalysts, control experiments were designed by conducting the reaction in the presence of CO2 but in the absence of epoxides. However, CO2 adducts were not obtained from catalyst 1b under CO2 pressure of 1 MPa. D’Elia et al. [15] demonstrated that CO2 adducts formation is not relevant to the achievement of catalytic activity. Cokoja and co-workers [10] investigated the cycloaddition reaction mechanism using FTIR spectroscopy technology and proposed that imidazolium salts first activate the C-O bond of epoxide through the hydrogen interactions between the C2 proton of the imidazolium salt and the oxygen atom of epoxide. The polarization enhancement of the C-O bond of epoxide increases the electrophilicity of the epoxide carbon, thereby facilitating ring opening of the epoxide. Kleij and co-workers [16] synthesized an iron(III) amine triphenolate complex which is active for the cycloaddition reaction of CO2 to epoxides. UV–vis spectroscopy analysis showed that the dimeric iron(III) complex reacts with epoxide to form the monomeric complex. He and co-workers [17] reported a Zn-salen catalysts with multiple active sites including Lewis acidic zinc center, phenolic hydroxyl group amd protonated tertiary ammonium. The synergistic effects of multiple hydrogen bonding donors renders the reaction to perform smoothly at atmospheric CO2 pressure. These results showed that catalysts first interact with epoxide instead of CO2 to trigger the reaction. To investigate the hydrogen bonding between PO and 1b, UV– vis titration studies were carried out. The UV–vis spectroscopy of neat catalyst 1b (Fig. 4, black curve) exhibited evident hyperchromic effect (l = 297, and 362 nm) upon addition of titrant of PO to dichloromethane solution of catalyst 1b (Fig. 4). This UV hyperchromic effect can be ascribed to the intermolecular interaction between PO and 1b. We infer that the electrondonating effect of oxygen atom bearing epoxide promotes the conjugated effect of catalysts, which is responsible for the hyperchromic phenomenon. To determine the hydrogen bond interaction between PO and catalysts, catalyst 1r bearing methyl group at the C2 position of the imidazolium ring was evaluated in the same reaction conditions (Table 1, entry 23). The result showed that the alkylation of the C2 position leads to a decrease of PC yield, which might be caused by the lack of hydrogen bonding interaction between PO and catalyst 1r; this result is consistent with previous reports [10,18]. On the basis of previous works [1e,10a,17] and our preliminary experimental studies, we proposed a mechanistic cycle for the “fiddler crab-type” imidazolium salts-catalyzed cycloaddition of CO2 to epoxide as depicted in Fig. 5; this cycle consists of four steps. Starting from the intermediate (1) between catalyst 1b and
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Table 3 Scope of substratesa.
crucial for catalyst activity. The pH determination experiments demonstrates the use of a series of unsymmetrical pyridinebridged pincer-type imidazolium salts as tunable alternatives to the previously reported imidazolium salts. We anticipate that the correlation between the remote electronic effect and catalyst activity provides insight into the design of efficient catalysts. Further efforts on developing highly active catalyst under mild condition for CO2 utilization are underway in our laboratory. 4. Experimental section
Fig. 3. Recycling of 1b in the cycloaddition of CO2 to PO. Reaction condition: PO (0.58 g, 10 mmol), 1b (43.1 mg, 1.0 mol%), CO2 (1 MPa), no solvent, 90 C, 5 h.
epoxide through hydrogen bonding interaction, the rate-determining step occurs where the epoxide ring is opened by nucleophilic attack of the iodide to form an oxyanion species (2) Subsequently, the negatively charged oxygen atom attacks the electrophilic carbon atom of CO2 and generates the open-chain carbonate. (3) The negatively charged oxygen atom attacks the carbon atom through an intramolecular nucleophilic substitution to release the desired product and catalytically active iodide. 3. Conclusion A series of unsymmetrical pyridine-bridged pincer-type imidazolium salts are synthesized and demonstrated as an efficient and recyclable catalyst for the cycloaddition of CO2 to epoxides under metal-free, solvent-free, and co-catalyst-free conditions. Investigation on the influence of catalyst structure on the catalytic performance shows the remote effect of substitution is evident. UV–vis spectroscopy studies suggest that the catalyst selectively reacts with the epoxides through the hydrogen bonding interaction, thereby showing that the proton acidity of C2 position is
All reactions were carried out in Schlenk tubes under an atmosphere of nitrogen. All solvents and reagents were purchased from commercial sources used without additional purification. NMR spectra were recorded on a Bruke Avance III HD 400 spectrometer using TMS as internal standard (400 MHz for 1H NMR and 100 MHz for 13C NMR). Mass spectroscopy data of the compounds were collected on a Bruker ultrafleXtreme mass spectrometer. GC analysis were carried on a Shimadzu GC-2014 equipped with a packed column (GDX-301, 2 m 4 mm) using a flame ionization detector. pH analysis were performed on a HANNA pH-211 meter. All products were isolated by short chromatography on a silica gel (300–400 mesh) column. 4.1. General procedure for the synthesis of imidazolium salts A mixture of 2,6-dibromopyridine (0.5 mmol), amines (1.0 mmol), Cu (0.1 mmol), tetramethylethane-1,2-diamine (0.2 mmol) and K2CO3 (1.5 mmol) in DMSO (2 ml) was stirred for 30 min at room temperature, and then heated to 90 C for 24 h under nitrogen atmosphere. The solvent was concentrated under vacuum and the product of 2-bromo-6-substituent-pyridine was isolated by short chromatography. Thereafter, 2-bromo-6-substituent-pyridine (0.5 mmol), amines (0.75 mmol), Cu (0.1 mmol), N, N-dimethylethylenediamine (0.2 mmol) and K2CO3 (1.5 mmol) in DMSO (2 ml) was allowed to react under nitrogen atmosphere. The solvent was removed under reduced pressure and the remaining solid was isolated by short chromatography. The resultant 2,6disubstituent-pyridine (0.5 mmol), and haloalkanes (1 ml) was heated to the desired temperature and the reaction stirred for 8 h under air. The reaction mixture was added to brine (15 ml) and extracted three times with dichloromethane (3 15 ml). The desired products were isolated by short chromatography.
F. Chen et al. / Journal of CO2 Utilization 16 (2016) 391–398
395
Fig. 4. UV–vis spectra of 1b (5 10 5 M) titrated with propylene oxide (0–100, 120, and 140 equiv.) in dichloromethane at room temperature. The arrows indicate the increase/decrease of the major absorption bands upon increase of the amount of titrant.
brine (15 ml) and extracted three times with dichloromethane (3 15 ml). The solvent was removed under reduced pressure and the desired products were isolated by short chromatography. 4.3. General procedure for the recycling experiments PO (0.58 g, 10 mmol) and the catalyst 1b (43.1 mg, 1.0 mol%) were put into a 25 ml stainless steel reactor equipped with a magnetic stirrer. The reactor was pressurized with CO2 to 1.0 MPa and heated to 90 C for 5 h. After the reaction completion, the reactor was cooled to room temperature, and the remaining CO2 was removed slowly. The catalyst was separated from the volatile organic products and starting materials by distillation under reduced pressure, and reused for subsequent recycles. The volatile organic phase containing product were analyzed by GC using biphenyl as internal standard. 4.4. Preparation of the crystalline
Fig. 5. Proposed mechanism of “fiddler crab-type” catalyst for the cycloaddition of CO2 to epoxide.
The crystalline was prepared through the layer-to-layer diffusion method. The imidazolium salt 1q was added into THF solution, and layered with n-hexane. After 3 days, a crystal suitable for single-crystal X-ray diffraction was obtained. Crystallographic data of the structures have been deposited at the Cambridge Crystallographic Database Centre, Supplementary publication no.: CCDC 1484922 for imidazolium salt 1q. 4.5. Characterization data for imidazolium salts
4.2. General procedure for the synthesis of cyclic carbonates In a typical reaction, 10 mmol epoxide and 1.0 mol% of the catalyst were put into a 25 ml stainless steel reactor equipped with a magnetic stirrer. The reactor was pressurized with CO2 to 1.0 MPa and then heated to 90 C for the required time. After reaction, the reactor was cooled to room temperature and excess CO2 was carefully vented off. The mixture was added biphenyl as internal standard for GC to determine conversion, selectivity and yield. For the product purified procedure, the reaction mixture was added to
4.5.1. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-methyl-1H-benzo[d] imidazol-3-ium iodide (1a) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (145 mg, 72%), mp = 217.8–218.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.12 (s, 1H), 9.19 (s, 1H), 8.63 (s, 1H), 8.47 (t, J = 8.0 Hz, 1H), 8.30 (d, J = 8.0 Hz, 1H), 8.18 (d, J = 8.0 Hz, 1H), 8.03–7.99 (m, 2H), 7.84 (d, J = 8.0 Hz, 1H), 7.49–7.39 (m, 2H), 4.03 (s, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.23, 145.93, 144.82, 144.34, 143.16, 136.43, 131.86, 125.44, 125.03, 124.07, 120.64, 119.81, 115.55, 114.14,
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111.96, 37.00, ppm; HRMS (ESI): m/z calcd for C16H14N5 [M]+ 276.1244, found 276.1244. 4.5.2. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium iodide (1b) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (144 mg, 67%), mp = 168.0–168.4 C; 1H NMR (400 MHz, DMSO-d6): d 10.19 (s, 1H), 9.21 (s, 1H), 8.67 (s, 1H), 8.48 (t, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.19–8.14 (m, 2H), 8.02 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.49–7.39 (m, 2H), 4.32 (t, J = 7.2 Hz, 2H), 1.96 (hex, J = 7.2 Hz, 2H), 0.96 (t, J = 7.6 Hz, 3H), ppm; 13 C NMR (100 MHz, DMSO-d6): d 149.12, 145.97, 144.83, 144.30, 143.18, 135.90, 131.84, 125.03, 124.19, 124.05, 120.63, 120.09, 115.48, 114.06, 112.12, 51.61, 23.17, 10.96, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1558. 4.5.3. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium bromide (1c) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (90 mg, 47%), mp = 190.2–190.6 C; 1H NMR (400 MHz, DMSO-d6): d 10.25 (s, 1H), 9.22 (s, 1H), 8.68 (s, 1H), 8.48 (t, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.20–8.15 (m, 2H), 8.05 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.49-7.39 (m, 2H), 4.33 (t, J = 7.2 Hz, 2H), 1.96 (hex, J = 7.2 Hz, 2H), 0.96 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.13, 145.98, 144.84, 144.29, 143.22, 135.93, 131.86, 125.02, 124.19, 124.04, 120.64, 120.09, 115.48, 114.07, 112.13, 51.58, 23.18, 10.95, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1561. 4.5.4. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium chloride (1d) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (49 mg, 32%), mp = 142.7–143.1 C; 1H NMR (400 MHz, DMSO-d6): d 10.46 (s, 1H), 9.24 (s, 1H), 8.72 (t, J = 2.0 Hz, 1H), 8.46 (t, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.19 (d, J = 8.0 Hz, 2H), 8.12 (d, J = 8.0 Hz, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.48–7.38 (m, 2H), 4.34 (t, J = 7.2 Hz, 2H), 1.96 (hex, J = 7.2 Hz, 2H), 0.95 (t, J = 7.6 Hz, 3H), ppm; 13 C NMR (100 MHz, DMSO-d6): d 149.09, 145.99, 144.82, 144.27, 143.23, 136.08, 131.82, 125.02, 124.18, 124.01, 120.61, 120.04, 115.43, 114.06, 112.17, 51.53, 23.18, 10.94, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1557.
4.5.7. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d] imidazol-3-ium bromide (1g) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (163 mg, 75%), mp = 277.2–277.8 C; 1H NMR (400 MHz, DMSO-d6): d 10.70 (s, 1H), 9.19 (s, 1H), 8.55 (t, J = 8.0 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.31–8.26 (m, 3H), 8.10 (d, J = 8.0 Hz, 1H), 7.85–7.75 (m, 3H), 7.41–7.34 (m, 2H), 4.65 (t, J = 7.2 Hz, 2H), 2.08 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.55, 146.49, 144.78, 144.14, 143.17, 143.05, 132.17, 132.06, 130.01, 128.29, 127.73, 124.81, 124.08, 120.65, 116.06, 115.78, 115.30, 114.72, 114.30, 49.25, 22.50, 11.27, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1717. 4.5.8. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d] imidazol-3-ium chloride (1h) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (72 mg, 37%), mp = 312.3–313.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.66 (s, 1H), 9.19 (s, 1H), 8.55 (t, J = 8.0 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.30–8.28 (m, 3H), 8.08 (d, J = 8.0 Hz, 1H), 7.85–7.76 (m, 3H), 7.42–7.34 (m, 2H), 4.64 (t, J = 7.2 Hz, 2H), 2.08 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.56, 146.49, 144.80, 144.14, 143.17, 132.17, 130.01, 128.30, 127.73, 124.81, 124.08, 120.66, 116.06, 115.77, 115.30, 114.72, 114.30, 49.26, 22.51, 11.27, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1715. 4.5.9. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-methyl-1H-benzo[d] imidazol-3-ium iodide (1i) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (159 mg, 70%), mp = 235.3–235.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.49 (s, 1H), 8.61 (d, J = 7.6 Hz, 2H), 8.46 (t, J = 8.0 Hz, 1H), 8.38 (d, J = 8.0 Hz, 1H), 8.30 (d, J = 8.0 Hz, 1H), 8.22 (d, J = 8.0 Hz, 1H), 7.98 (d, J = 8.0 Hz, 1H), 7.89–7.78 (m, 3H), 7.53 (t, J = 8.0 Hz, 1H), 7.39 (t, J = 8.0 Hz, 1H), 4.27 (s, 3H), ppm; 13C NMR (100 MHz, DMSOd6): d 153.29, 145.46, 143.62, 143.54, 139.32, 138.47, 132.78, 129.92, 129.09, 128.15, 127.67, 126.56, 123.80, 122.17, 115.45, 114.95, 114.64, 114.47, 114.08, 34.31, ppm; HRMS (ESI): m/z calcd for C20H16N5 [M]+ 326.1400, found 326.1405.
4.5.5. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-methyl-1Hbenzo[d] imidazol-3-ium iodide (1e) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (152 mg, 67%), mp = 229.1–229.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.58 (s, 1H), 9.18 (s, 1H), 8.55 (t, J = 8.0 Hz, 1H), 8.42 (d, J = 8.0 Hz, 1H), 8.29–8.26 (m, 2H), 8.21 (d, J = 8.0 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.85–7.77 (m, 3H), 7.41–7.35 (m, 2H), 4.26 (s, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.63, 146.36, 144.78, 144.24, 143.61, 143.02, 132.82, 132.04, 129.78, 128.30, 127.71, 124.83, 124.08, 120.65, 115.79, 115.10, 114.65, 114.31, 34.41, ppm;
4.5.10. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium iodide (1j) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (161 mg, 67%), mp = 216.5–217.0 C; 1H NMR (400 MHz, DMSO-d6): d 10.57 (s, 1H), 8.61 (d, J = 7.6 Hz, 2H), 8.47 (t, J = 8.0 Hz, 1H), 8.40 (d, J = 8.0 Hz, 1H), 8.31 (t, J = 8.0 Hz, 2H), 7.97 (d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.86-7.77 (m, 2H), 7.52 (t, J = 8.0 Hz, 1H), 7.38 (t, J = 8.0 Hz, 1H), 4.65 (t, J = 7.2 Hz, 2H), 2.10 (hex, J = 7.2 Hz, 2H), 1.06 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 153.21, 145.59, 143.57, 143.06, 139.32, 138.49, 132.16, 130.16, 129.09, 128.19, 127.74, 126.58, 123.81, 122.19, 115.74, 114.99, 114.79, 114.45, 114.29, 49.28, 22.49, 11.30, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1716.
4.5.6. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d] imidazol-3-ium iodide (1f) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (135 mg, 56%), mp = 228.3–228.9 C; 1H NMR (400 MHz, DMSO-d6): d 10.60 (s, 1H), 9.19 (s, 1H), 8.56 (t, J = 8.0 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.31–8.27 (m, 3H), 8.06 (d, J = 8.0 Hz, 1H), 7.86–7.76 (m, 3H), 7.41–7.35 (m, 2H), 4.63 (t, J = 7.2 Hz, 2H), 2.06 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.58, 146.47, 144.80, 144.14, 143.14, 143.04, 132.18, 132.07, 130.03, 128.31, 127.74, 124.81, 124.09, 120.66, 116.04, 115.78, 115.29, 114.72, 114.31, 49.28, 22.50, 11.28, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1729.
4.5.12. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium bromide (1k) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (46 mg, 21%), mp = 238.5–238.8 C; 1H NMR (400 MHz, DMSO-d6): d 10.85 (s, 1H), 8.54 (s, 1H), 8.51 (d, J = 7.6 Hz, 1H), 8.42 (t, J = 8.0 Hz, 1H), 8.32 (d, J = 8.0 Hz, 2H), 8.18 (d, J = 8.0 Hz, 1H), 8.01 (d, J = 7.6 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.79 (t, J = 8.0 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 7.45 (t, J = 8.0 Hz, 1H), 7.31 (t, J = 8.0 Hz, 1H), 4.71 (t, J = 7.6 Hz, 2H), 2.10 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.2 Hz, 3H), ppm; 13 C NMR (100 MHz, DMSO-d6): d 153.00,145.57, 143.46, 142.99, 139.20, 138.35, 132.11, 129.96, 129.00, 128.09, 127.70, 126.48, 123.68, 122.10, 115.71, 114.88, 114.84, 114.28, 114.15, 49.23, 22.56,
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11.29, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1713.
114.28, 112.83, 109.58, 49.09, 22.62, 11.23, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1563.
4.5.13. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium chloride (1l) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (109 mg, 56%), mp = 246.3–247.7 C; 1H NMR (400 MHz, DMSO-d6): d 10.92 (s, 1H), 8.60–8.57 (m, 2H), 8.45 (t, J = 8.0 Hz, 1H), 8.39 (d, J = 8.0 Hz, 1H), 8.31 (d, J = 8.0 Hz, 1H), 8.26 (d, J = 8.0 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 7.84–7.75 (m, 2H), 7.51 (t, J = 7.2 Hz, 1H), 7.37 (t, J = 7.2 Hz, 1H), 4.68 (t, J = 7.6 Hz, 2H), 2.10 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 153.11, 145.70, 143.52, 143.31, 139.28, 138.46, 132.16, 130.11, 129.07, 128.12, 127.69, 126.56, 123.76, 122.17, 115.82, 114.95, 114.79, 114.37, 114.28, 49.17, 22.53, 11.28, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1715.
4.5.18. 3-propyl-1-(6-(3-propyl-1H-imidazol-3-ium-1-yl)pyridin-2yl)-1H-benzo[d]imidazol-3-ium iodide (1q) The reaction mixture was washed with ethyl acetate (3 30 ml) and dried under vacuum for 24 h to give product 1q: a white solid (270 mg, 90%), mp = 209.0–209.8 C; 1H NMR (400 MHz, DMSO-d6): d 10.77 (s, 1H), 10.28 (s, 1H), 8.74–8.71 (m, 2H), 8.59–8.57 (m, 1H), 8.34–8.31 (m, 3H), 8.21 (s, 1H), 7.87–7.82 (m, 2H), 4.66 (t, J = 7.2 Hz, 2H), 4.37 (t, J = 7.2 Hz, 2H), 2.09 (Hex, J = 7.2 Hz, 2H), 1.97 (Hex, J = 7.2 Hz, 2H), 1.03 (t, J = 7.2 Hz, 3H), 0.96 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 146.63, 146.15, 145.11, 143.25, 136.15, 132.12, 129.65, 128.70, 127.86, 124.29, 120.20, 118.23, 116.38, 115.64, 114.76, 51.67, 49.40, 23.16, 22.53, 11.28, 10.97, ppm; HRMS (ESI): m/z calcd for C21H25N5 [[M]2+/2] 173.6050, found 173.6050; calcd for C21H25N5I [[M]2++I ] 474.1149, found 474.1151. 4.5.19 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-2-methyl-3-propyl1H-benzo[d]imidazol-3-ium iodide (1r) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (74 mg, 33%), mp = 198.0–201.3 C; 1H NMR (400 MHz, DMSO-d6): d 8.59 (d, J = 2.8 Hz, 1H), 8.47 (t, J = 8.0 Hz, 1H), 8.25 (t, J = 8.4 Hz, 2H), 7.95 (s, 1H), 7.90–7.85 (m, 2H), 7.75 (t, J = 7.2 Hz, 1H), 7.68 (t, J = 8.0 Hz, 1H), 6.65 (dd, J = 2.8 Hz, J = 1.6 Hz, 1H), 4.62 (t, J = 7.2 Hz, 2H), 3.02 (s, 3H), 1.96 (hex, J = 7.2 Hz, 2H), 1.07 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 152.64, 151.36, 144.50, 144.22, 143.76, 131.35, 131.06, 128.28, 127.61, 127.21, 119.77, 114.46, 114.00, 113.90, 109.63, 47.44, 22.39, 12.58, 11.44, ppm.
4.5.14. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-methyl-1H-benzo[d] imidazol-3-ium iodide (1m) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (145 mg, 72%), mp = 223.4–224.1 C; 1H NMR (400 MHz, DMSO-d6): d 10.60 (s, 1H), 8.78 (s, 1H), 8.53 (d, J = 7.2 Hz, 1H), 8.44 (t, J = 8.0 Hz, 1H), 8.17 (t, J = 7.6 Hz, 2H), 8.00–7.97 (m, 2H), 7.84 (t, J = 7.6 Hz, 2H), 6.73 (s, 1H), 4.23 (s, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.64, 146.12, 144.26, 143.81, 143.57, 132.80, 129.53, 128.47, 127.62, 115.94, 114.56, 114.17, 113.05, 109.66, ppm; HRMS (ESI): m/z calcd for C16H14N5 [M]+ 276.1244, found 276.1244. 4.5.15. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium iodide (1n) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (125 mg, 58%), mp = 203.2–203.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.65 (s, 1H), 8.78 (d, J = 2.4 Hz, 1H), 8.55 (dd, J = 7.2 Hz, J = 2.4 Hz, 1H), 8.45 (t, J = 8.0 Hz, 1H), 8.28 (dd, J = 6.4 Hz, J = 2.4 Hz, 1H), 8.16 (d, J = 8.0 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H), 7.96 (d, J = 1.6 Hz, 1H), 7.87–7.80 (m, 2H), 6.72 (dd, J = 2.4 Hz, J = 1.6 Hz, 1H), 4.62 (t, J = 7.2 Hz, 2H), 2.07 (hex, J = 7.2 Hz, 2H), 1.03 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.61, 146.22, 144.18, 143.81, 143.07, 132.19, 129.76, 128.53, 127.70, 116.20, 114.70, 114.48, 113.06, 109.67, 49.30, 22.55, 11.28, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1562. 4.5.16. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium bromide (1o) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (71 mg, 37%), mp = 231.2–231.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.96 (s, 1H), 8.80 (d, J = 2.4 Hz, 1H), 8.53 (dd, J = 7.6 Hz, J = 1.6 Hz, 1H), 8.41 (t, J = 8.0 Hz, 1H), 8.29 (dd, J = 7.2 Hz, J = 1.6 Hz, 1H), 8.10 (dd, J = 8.0 Hz, J = 4.0 Hz, 2H), 7.92 (d, J = 1.6 Hz, 1H), 7.84– 7.76 (m, 2H), 6.68 (dd, J = 2.4 Hz, J = 1.6 Hz, 1H), 4.68 (t, J = 7.2 Hz, 2H), 2.07 (hex, J = 7.6 Hz, 2H), 1.02 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.72, 146.03, 143.74, 143.42, 142.80, 132.10, 129.70, 128.43, 128.19, 127.67, 116.02, 114.53, 114.20, 113.04, 109.33, 49.31, 22.70, 11.22, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1560. 4.5.17. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium chloride (1p) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (70 mg, 41%), mp = 223.1–224.8 C; 1H NMR (400 MHz, DMSO-d6): d 11.34 (s, 1H), 8.84 (d, J = 2.4 Hz, 1H), 8.54 (d, J = 8.0 Hz, 1H), 8.39 (t, J = 8.0 Hz, 1H), 8.28 (d, J = 8.0 Hz, 1H), 8.17 (d, J = 8.0 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H), 7.91 (s, 1H), 7.82–7.75 (m, 2H), 6.67 (s, 1H), 4.69 (t, J = 7.6 Hz, 2H), 2.07 (hex, J = 7.2 Hz, 2H), 1.01 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.46, 146.34, 144.05, 143.68, 143.46, 132.15, 129.61, 128.70, 128.43, 127.59, 116.27, 114.70,
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
“Fiddler crab-type” imidazolium salt as remote substituents tuning organocatalyst for the cycloaddition of epoxides with carbon dioxide Fei Chen, Dengtai Chen, Lei Shi, Ning Liu* , Bin Dai* School of Chemistry and Chemical Engineering, Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, North 4th Road, Shihezi, Xinjiang 832003, China
A R T I C L E I N F O
Article history: Received 8 August 2016 Received in revised form 28 August 2016 Accepted 6 October 2016 Available online xxx Keywords: Carbon dioxide fixation Cyclic carbonate Organocatalysis Imidazolium salt
A B S T R A C T
A series of unsymmetrical pyridine-bridged pincer-type imidazolium salts are synthesized and demonstrated as highly active catalyst. This catalyst is for the cycloaddition of carbon dioxide with a broad range of terminal epoxides without organic solvent, and co-catalyst. The performance of catalyst considerably depends on the remote structural feature bearing pyridine ring. In addition, bulky internal epoxide is efficiently converted to the desired product. The robustness of the catalytic system is shown by recycling the catalyst over seven consecutive times without significant loss of activity. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Carbon dioxide can serve as a renewable C1 building block for constructing C-C, C-O, C-N, and C-H bonds by reacting with epoxides [1], aziridines [2], amines [3], and terminal alkynes [4]. The 100% atom-economic conversion of CO2 and epoxides yielding cyclic carbonates is one of the most promising reaction routes for CO2 chemical utilization. Cyclic carbonates are considerably interesting and might be utilized as intermediates for preparation of fine chemicals and polymers. Some of the metal-based catalysts exhibit high catalytic activity in the cycloaddition reaction between CO2 and epoxides [5]. Furthermore, organocatalysts for the synthesis of cyclic carbonates could represent an attractive alternative [1e]. To date, various organocatalysts for the cycloaddition reaction between CO2 and epoxides, such as imidazolium salts [6], ammonium salts [7], phosphonium salts [8], and organic amines [9], have been developed. Among these organocatalysts, imidazolium salts are generally synthesized in simple one or two-step procedures by using abundant and inexpensive precursors. Accordingly, various imidazolium salts catalysts have been developed for the synthesis of cyclic carbonates from epoxides and CO2. The mechanistic studies of imidazolium salt-catalyzed the cycloaddition reaction proposed that the proton in the C2 position of the imidazolium ring activates the epoxide through hydrogen
bonding, which facilitates the nucleophilic attack for ring opening of the epoxide (Fig. 1, left) [10]. Research on the improvement of catalytic activity focused on tuning the acidity of the C2 proton by varying substituents R1 and/or R2 that directly bears an imidazolium ring (Fig. 1, left). Cokoja and co-workers [10] suggested that the acidity of imidazolium salts should be proper; significantly weak acidity leads to difficultly in activating the epoxides, whereas considerably strong acidity inhibits the insertion of CO2 because of strong hydrogen bonding contacts. However, the electronic influence of directly functionalized strategy on C2 proton is strong. Consequently, controlling the acidity of catalysts accurately is difficult. The pyridine ring is a typical electronic transfer structure. Thus, this ring is an alternative for the design of imidazolium salts when used as a bridge to tune the acidity of C2 proton by varying remote substituents bearing pyridine ring (Fig. 1, right). In our previous work, we developed a copper-catalyzed [11] and metal-free [12] selective C-N bond construction method, which was applied successfully to the synthesis of unsymmetrical pyridine-bridged pincer-type (fiddler crab-type) imidazolium salts. The resulting the “fiddler crab-type” imidazolium salts were investigated for their use as catalysts in the cycloaddition reaction of CO2 to epoxides. This catalyst design provided insight into the rational development of highly efficient catalysts. 2. Results and discussion
* Corresponding authors. E-mail addresses: [email protected] (N. Liu), [email protected] (B. Dai). http://dx.doi.org/10.1016/j.jcou.2016.10.005 2212-9820/ã 2016 Elsevier Ltd. All rights reserved.
In the present work, 18 “fiddler crab-type” imidazolium salts 1a–r bearing various N-heterocycle substituents through pyridine
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Fig. 1. Design of imidazolium salts.
ring as an electronic transfer bridge were initially prepared with good to excellent yield according to our developed method (Fig. 2) [11]. The molecular structure of imidazolium salts 1q was confirmed by X-ray crystallography (Fig. 2, 1q). The influence of structural features on catalytic behavior was investigated in detail using the cycloaddition of propylene oxide (PO) to CO2 as a model reaction, and the results are summarized in Table 1. They are efficient single-component, metal-free catalysts under solvent-free condition. The remote electronic effect of substituents bearing pyridine ring was initially studied under the same reaction conditions. The structure of substituents bearing pyridine ring, using benzimidazole salt as a catalytic active site, shows an obvious effect on the catalytic activity toward the synthesis of propylene carbonate (PC). The catalytic activities are in the following order: imidazolyl group > benzimidazolyl group > indazolyl group > pyrazolyl group (Table 1, entries 1–4; 5–8; 9–12; and 13–16). The preliminary experimental results first demonstrated that the remote substituents show evident effect on the catalytic performance of imidazolium salts catalyst for the cycloaddition of epoxides to CO2. The influence of alkyl chain lengths bearing cation of imidazole on the catalytic performance was evaluated. The catalytic activity of imidazolium salts with a long alkyl chain is stronger than those with a short alkyl chain (Table 1, entries 2, 6, 10, and 14; 1, 5, 9, and 13); this result is consistent with those reported by Cokoja group
[10b] who suggested that the steric effect of substituents bearing the cation of imidazolium salts is responsible for catalytic activity. The effect of the counter anions on catalytic performance was also investigated (Table 1). The activity order of anion is I > Br > Cl (Table 1, entries 2, 6, 10, and 14; 3, 7, 11, and 15; 4, 8, 12, and 16), which is consistent with results obtained in previous reports [6a,13]. The nucleophilic order of these anions is responsible for the results. Maginn and co-workers [14] suggested that the most significant effect on CO2 solubility in alkylimidazolium-based ionic liquids relies on the nature of the anion. These results indicated that the anion structure has a significant effect on CO2 solubility and capture. Catalysts 1b and 1q show nearly equal catalytic activity on the synthesis of PC (Table 1, entries 2, and 17). Catalyst 1b exhibits an obvious advantage over the synthetic procedure compared with catalyst 1q; therefore, 1b was chosen as the model catalyst for further investigation. The influence of reaction parameters on the reaction was optimized to achieve mild reaction conditions. As shown in Table 1, reaction rate for synthesis of PC was increased with prolonged reaction time. Thus, the optimal reaction time was 5 h (Table 1, entry 18). Pressure influenced the cycloaddition largely in terms of the reactivity when the CO2 pressure varied from 0.1 MPa to 1.0 MPa (Table 1, entries 18–20). The dependence of the PC yield on the catalyst amount was examined. The catalytic activity of 1b was sensitive to the amount of catalyst. A decrease in the molar ratio of 1b to PO, from 1 mol% to 0.1 mol% resulted in evident decrease of PC yield (Table 1, entries 2, 21, and 22). To explore the correlation between structural feature of our developed imidazolium salts and catalytic activity, the pH of two known benzimidazolium salts and four selected imidazolium salts, namely, 1b, 1j, 1m, and 1n, was determined in the mixture of dichloromethane and methanol. In the substitution of imidazolium salts bearing N-atom varying from methyl group to propyl group, the pH of benzimidazolium salts shows evident change from 3.59 to 4.67 (Table 2, entries 1, and 2). In contrast to the abovementioned results, the pH change range of our developed imidazolium salts 1m, and 1n was slow (Table 2, entries 5, and 6). Our developed catalysts are relatively accurate in controlling the acidity of C2 proton by remote substitution effect. As shown in Table 2 (entries 3, 4, and 5), the acidity of imidazolium salts is
Fig. 2. Imidazolium salts in this work.
F. Chen et al. / Journal of CO2 Utilization 16 (2016) 391–398 Table 1 Optimization of PC synthesisa .
entry
Catal. (mol%)
t (h)
Yieldb (%)
Selectivityb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19c 20d 21 22 23
1a (1.0) 1b (1.0) 1c (1.0) 1d (1.0) 1e (1.0) 1f (1.0) 1g (1.0) 1h (1.0) 1i (1.0) 1j (1.0) 1k (1.0) 1l (1.0) 1m (1.0) 1n (1.0) 1o (1.0) 1p (1.0) 1q (1.0) 1b (1.0) 1b (1.0) 1b (1.0) 1b (0.5) 1b (0.1) 1r (1.0)
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 5 5 5 5 5 3
69 88 43 31 57 79 72 45 44 54 46 12 45 63 41 33 90 98 67 36 78 38 42
97 99 93 91 97 96 95 92 96 95 96 97 93 96 95 95 98 99 99 98 98 95 98
a Conditions: unless otherwise specifically notified, all the reactions were carried out with PO (0.58 g, 10 mmol), catalyst (1.0 mol%), no solvent, 90 C, CO2 (1.0 MPa), 3 h. b Determined by GC using biphenyl as an internal standard. c CO2 (0.5 MPa). d CO2 (0.1 MPa).
Table 2 The pH of the imidazolium salts. Entry
1
2
Imidazolium salt
pH Product yield
3.59
4.67
3
4
5
6
1j
1b
1n
1m
4.02 54%
4.47 88%
4.80 63%
5.00 45%
Conditions: imidazolium salts (0.1 mmol) was added in CH2Cl2: MeOH = 10:1 (1 ml). The solution was stirred for 90 min at room temperature and then the pH value was determined.
crucial for its catalytic activity. Significantly weak acidity easily leads to difficultly in activating the epoxides, whereas considerably strong acidity inhibits the insertion of CO2 because of strong hydrogen bond contacts. Our developed method provides a tunable alternative to imidazolium salt design and synthesis. Various epoxides were subjected to cycloaddition in our developed catalytic system to survey the substrate scope. As shown in Table 3, both aliphatic and aromatic substrates were well tolerated to yield desired products 2a–g, and 2i with 78–99% yield under mild conditions. Internal epoxide, which is a challenging substrate because of its high hindrance effect, was also evaluated in the developed catalytic system. A satisfying product yield was obtained by prolonging the reaction time to 12 h (Table 3, 2h). Moreover, the current catalytic system is also effective for the
393
reaction of two enantiopure terminal epoxides, thereby affording the corresponding cyclic carbonates (S)-2j, and (S)-2e with 99% ee’s. In addition to the catalyst activity, the recyclability and stability potential also play an important role for possible industrial application. Catalyst 1b was used to evaluate reusability under optimized conditions. In each cycle, the catalyst was separated from the volatile organic products and starting materials by distillation under reduced pressure, and reused for subsequent recycles. As shown in Fig. 3, no evident decrease was observed in PC yield after eight runs, thereby indicating high stability and reusability of the catalyst 1b. The recycled the catalyst 1b was characterized by the NMR analysis, as shown in Fig. S1 (for details, see Supporting information), and there was no difference in the 1H NMR spectra between the fresh and the last-used catalysts, which provided evidence for the good structural stability of the our developed catalyst. The mechanism of the cycloaddition of epoxides with carbon dioxide was investigated widely in previous works [1e,10a]. To determine which substrate, whether CO2 or epoxides, preferentially binds to our developed catalysts, control experiments were designed by conducting the reaction in the presence of CO2 but in the absence of epoxides. However, CO2 adducts were not obtained from catalyst 1b under CO2 pressure of 1 MPa. D’Elia et al. [15] demonstrated that CO2 adducts formation is not relevant to the achievement of catalytic activity. Cokoja and co-workers [10] investigated the cycloaddition reaction mechanism using FTIR spectroscopy technology and proposed that imidazolium salts first activate the C-O bond of epoxide through the hydrogen interactions between the C2 proton of the imidazolium salt and the oxygen atom of epoxide. The polarization enhancement of the C-O bond of epoxide increases the electrophilicity of the epoxide carbon, thereby facilitating ring opening of the epoxide. Kleij and co-workers [16] synthesized an iron(III) amine triphenolate complex which is active for the cycloaddition reaction of CO2 to epoxides. UV–vis spectroscopy analysis showed that the dimeric iron(III) complex reacts with epoxide to form the monomeric complex. He and co-workers [17] reported a Zn-salen catalysts with multiple active sites including Lewis acidic zinc center, phenolic hydroxyl group amd protonated tertiary ammonium. The synergistic effects of multiple hydrogen bonding donors renders the reaction to perform smoothly at atmospheric CO2 pressure. These results showed that catalysts first interact with epoxide instead of CO2 to trigger the reaction. To investigate the hydrogen bonding between PO and 1b, UV– vis titration studies were carried out. The UV–vis spectroscopy of neat catalyst 1b (Fig. 4, black curve) exhibited evident hyperchromic effect (l = 297, and 362 nm) upon addition of titrant of PO to dichloromethane solution of catalyst 1b (Fig. 4). This UV hyperchromic effect can be ascribed to the intermolecular interaction between PO and 1b. We infer that the electrondonating effect of oxygen atom bearing epoxide promotes the conjugated effect of catalysts, which is responsible for the hyperchromic phenomenon. To determine the hydrogen bond interaction between PO and catalysts, catalyst 1r bearing methyl group at the C2 position of the imidazolium ring was evaluated in the same reaction conditions (Table 1, entry 23). The result showed that the alkylation of the C2 position leads to a decrease of PC yield, which might be caused by the lack of hydrogen bonding interaction between PO and catalyst 1r; this result is consistent with previous reports [10,18]. On the basis of previous works [1e,10a,17] and our preliminary experimental studies, we proposed a mechanistic cycle for the “fiddler crab-type” imidazolium salts-catalyzed cycloaddition of CO2 to epoxide as depicted in Fig. 5; this cycle consists of four steps. Starting from the intermediate (1) between catalyst 1b and
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Table 3 Scope of substratesa.
crucial for catalyst activity. The pH determination experiments demonstrates the use of a series of unsymmetrical pyridinebridged pincer-type imidazolium salts as tunable alternatives to the previously reported imidazolium salts. We anticipate that the correlation between the remote electronic effect and catalyst activity provides insight into the design of efficient catalysts. Further efforts on developing highly active catalyst under mild condition for CO2 utilization are underway in our laboratory. 4. Experimental section
Fig. 3. Recycling of 1b in the cycloaddition of CO2 to PO. Reaction condition: PO (0.58 g, 10 mmol), 1b (43.1 mg, 1.0 mol%), CO2 (1 MPa), no solvent, 90 C, 5 h.
epoxide through hydrogen bonding interaction, the rate-determining step occurs where the epoxide ring is opened by nucleophilic attack of the iodide to form an oxyanion species (2) Subsequently, the negatively charged oxygen atom attacks the electrophilic carbon atom of CO2 and generates the open-chain carbonate. (3) The negatively charged oxygen atom attacks the carbon atom through an intramolecular nucleophilic substitution to release the desired product and catalytically active iodide. 3. Conclusion A series of unsymmetrical pyridine-bridged pincer-type imidazolium salts are synthesized and demonstrated as an efficient and recyclable catalyst for the cycloaddition of CO2 to epoxides under metal-free, solvent-free, and co-catalyst-free conditions. Investigation on the influence of catalyst structure on the catalytic performance shows the remote effect of substitution is evident. UV–vis spectroscopy studies suggest that the catalyst selectively reacts with the epoxides through the hydrogen bonding interaction, thereby showing that the proton acidity of C2 position is
All reactions were carried out in Schlenk tubes under an atmosphere of nitrogen. All solvents and reagents were purchased from commercial sources used without additional purification. NMR spectra were recorded on a Bruke Avance III HD 400 spectrometer using TMS as internal standard (400 MHz for 1H NMR and 100 MHz for 13C NMR). Mass spectroscopy data of the compounds were collected on a Bruker ultrafleXtreme mass spectrometer. GC analysis were carried on a Shimadzu GC-2014 equipped with a packed column (GDX-301, 2 m 4 mm) using a flame ionization detector. pH analysis were performed on a HANNA pH-211 meter. All products were isolated by short chromatography on a silica gel (300–400 mesh) column. 4.1. General procedure for the synthesis of imidazolium salts A mixture of 2,6-dibromopyridine (0.5 mmol), amines (1.0 mmol), Cu (0.1 mmol), tetramethylethane-1,2-diamine (0.2 mmol) and K2CO3 (1.5 mmol) in DMSO (2 ml) was stirred for 30 min at room temperature, and then heated to 90 C for 24 h under nitrogen atmosphere. The solvent was concentrated under vacuum and the product of 2-bromo-6-substituent-pyridine was isolated by short chromatography. Thereafter, 2-bromo-6-substituent-pyridine (0.5 mmol), amines (0.75 mmol), Cu (0.1 mmol), N, N-dimethylethylenediamine (0.2 mmol) and K2CO3 (1.5 mmol) in DMSO (2 ml) was allowed to react under nitrogen atmosphere. The solvent was removed under reduced pressure and the remaining solid was isolated by short chromatography. The resultant 2,6disubstituent-pyridine (0.5 mmol), and haloalkanes (1 ml) was heated to the desired temperature and the reaction stirred for 8 h under air. The reaction mixture was added to brine (15 ml) and extracted three times with dichloromethane (3 15 ml). The desired products were isolated by short chromatography.
F. Chen et al. / Journal of CO2 Utilization 16 (2016) 391–398
395
Fig. 4. UV–vis spectra of 1b (5 10 5 M) titrated with propylene oxide (0–100, 120, and 140 equiv.) in dichloromethane at room temperature. The arrows indicate the increase/decrease of the major absorption bands upon increase of the amount of titrant.
brine (15 ml) and extracted three times with dichloromethane (3 15 ml). The solvent was removed under reduced pressure and the desired products were isolated by short chromatography. 4.3. General procedure for the recycling experiments PO (0.58 g, 10 mmol) and the catalyst 1b (43.1 mg, 1.0 mol%) were put into a 25 ml stainless steel reactor equipped with a magnetic stirrer. The reactor was pressurized with CO2 to 1.0 MPa and heated to 90 C for 5 h. After the reaction completion, the reactor was cooled to room temperature, and the remaining CO2 was removed slowly. The catalyst was separated from the volatile organic products and starting materials by distillation under reduced pressure, and reused for subsequent recycles. The volatile organic phase containing product were analyzed by GC using biphenyl as internal standard. 4.4. Preparation of the crystalline
Fig. 5. Proposed mechanism of “fiddler crab-type” catalyst for the cycloaddition of CO2 to epoxide.
The crystalline was prepared through the layer-to-layer diffusion method. The imidazolium salt 1q was added into THF solution, and layered with n-hexane. After 3 days, a crystal suitable for single-crystal X-ray diffraction was obtained. Crystallographic data of the structures have been deposited at the Cambridge Crystallographic Database Centre, Supplementary publication no.: CCDC 1484922 for imidazolium salt 1q. 4.5. Characterization data for imidazolium salts
4.2. General procedure for the synthesis of cyclic carbonates In a typical reaction, 10 mmol epoxide and 1.0 mol% of the catalyst were put into a 25 ml stainless steel reactor equipped with a magnetic stirrer. The reactor was pressurized with CO2 to 1.0 MPa and then heated to 90 C for the required time. After reaction, the reactor was cooled to room temperature and excess CO2 was carefully vented off. The mixture was added biphenyl as internal standard for GC to determine conversion, selectivity and yield. For the product purified procedure, the reaction mixture was added to
4.5.1. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-methyl-1H-benzo[d] imidazol-3-ium iodide (1a) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (145 mg, 72%), mp = 217.8–218.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.12 (s, 1H), 9.19 (s, 1H), 8.63 (s, 1H), 8.47 (t, J = 8.0 Hz, 1H), 8.30 (d, J = 8.0 Hz, 1H), 8.18 (d, J = 8.0 Hz, 1H), 8.03–7.99 (m, 2H), 7.84 (d, J = 8.0 Hz, 1H), 7.49–7.39 (m, 2H), 4.03 (s, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.23, 145.93, 144.82, 144.34, 143.16, 136.43, 131.86, 125.44, 125.03, 124.07, 120.64, 119.81, 115.55, 114.14,
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111.96, 37.00, ppm; HRMS (ESI): m/z calcd for C16H14N5 [M]+ 276.1244, found 276.1244. 4.5.2. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium iodide (1b) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (144 mg, 67%), mp = 168.0–168.4 C; 1H NMR (400 MHz, DMSO-d6): d 10.19 (s, 1H), 9.21 (s, 1H), 8.67 (s, 1H), 8.48 (t, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.19–8.14 (m, 2H), 8.02 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.49–7.39 (m, 2H), 4.32 (t, J = 7.2 Hz, 2H), 1.96 (hex, J = 7.2 Hz, 2H), 0.96 (t, J = 7.6 Hz, 3H), ppm; 13 C NMR (100 MHz, DMSO-d6): d 149.12, 145.97, 144.83, 144.30, 143.18, 135.90, 131.84, 125.03, 124.19, 124.05, 120.63, 120.09, 115.48, 114.06, 112.12, 51.61, 23.17, 10.96, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1558. 4.5.3. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium bromide (1c) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (90 mg, 47%), mp = 190.2–190.6 C; 1H NMR (400 MHz, DMSO-d6): d 10.25 (s, 1H), 9.22 (s, 1H), 8.68 (s, 1H), 8.48 (t, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.20–8.15 (m, 2H), 8.05 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.49-7.39 (m, 2H), 4.33 (t, J = 7.2 Hz, 2H), 1.96 (hex, J = 7.2 Hz, 2H), 0.96 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.13, 145.98, 144.84, 144.29, 143.22, 135.93, 131.86, 125.02, 124.19, 124.04, 120.64, 120.09, 115.48, 114.07, 112.13, 51.58, 23.18, 10.95, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1561. 4.5.4. 1-(6-(1H-imidazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium chloride (1d) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (49 mg, 32%), mp = 142.7–143.1 C; 1H NMR (400 MHz, DMSO-d6): d 10.46 (s, 1H), 9.24 (s, 1H), 8.72 (t, J = 2.0 Hz, 1H), 8.46 (t, J = 8.0 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.19 (d, J = 8.0 Hz, 2H), 8.12 (d, J = 8.0 Hz, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.48–7.38 (m, 2H), 4.34 (t, J = 7.2 Hz, 2H), 1.96 (hex, J = 7.2 Hz, 2H), 0.95 (t, J = 7.6 Hz, 3H), ppm; 13 C NMR (100 MHz, DMSO-d6): d 149.09, 145.99, 144.82, 144.27, 143.23, 136.08, 131.82, 125.02, 124.18, 124.01, 120.61, 120.04, 115.43, 114.06, 112.17, 51.53, 23.18, 10.94, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1557.
4.5.7. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d] imidazol-3-ium bromide (1g) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (163 mg, 75%), mp = 277.2–277.8 C; 1H NMR (400 MHz, DMSO-d6): d 10.70 (s, 1H), 9.19 (s, 1H), 8.55 (t, J = 8.0 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.31–8.26 (m, 3H), 8.10 (d, J = 8.0 Hz, 1H), 7.85–7.75 (m, 3H), 7.41–7.34 (m, 2H), 4.65 (t, J = 7.2 Hz, 2H), 2.08 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.55, 146.49, 144.78, 144.14, 143.17, 143.05, 132.17, 132.06, 130.01, 128.29, 127.73, 124.81, 124.08, 120.65, 116.06, 115.78, 115.30, 114.72, 114.30, 49.25, 22.50, 11.27, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1717. 4.5.8. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d] imidazol-3-ium chloride (1h) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (72 mg, 37%), mp = 312.3–313.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.66 (s, 1H), 9.19 (s, 1H), 8.55 (t, J = 8.0 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.30–8.28 (m, 3H), 8.08 (d, J = 8.0 Hz, 1H), 7.85–7.76 (m, 3H), 7.42–7.34 (m, 2H), 4.64 (t, J = 7.2 Hz, 2H), 2.08 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.56, 146.49, 144.80, 144.14, 143.17, 132.17, 130.01, 128.30, 127.73, 124.81, 124.08, 120.66, 116.06, 115.77, 115.30, 114.72, 114.30, 49.26, 22.51, 11.27, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1715. 4.5.9. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-methyl-1H-benzo[d] imidazol-3-ium iodide (1i) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (159 mg, 70%), mp = 235.3–235.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.49 (s, 1H), 8.61 (d, J = 7.6 Hz, 2H), 8.46 (t, J = 8.0 Hz, 1H), 8.38 (d, J = 8.0 Hz, 1H), 8.30 (d, J = 8.0 Hz, 1H), 8.22 (d, J = 8.0 Hz, 1H), 7.98 (d, J = 8.0 Hz, 1H), 7.89–7.78 (m, 3H), 7.53 (t, J = 8.0 Hz, 1H), 7.39 (t, J = 8.0 Hz, 1H), 4.27 (s, 3H), ppm; 13C NMR (100 MHz, DMSOd6): d 153.29, 145.46, 143.62, 143.54, 139.32, 138.47, 132.78, 129.92, 129.09, 128.15, 127.67, 126.56, 123.80, 122.17, 115.45, 114.95, 114.64, 114.47, 114.08, 34.31, ppm; HRMS (ESI): m/z calcd for C20H16N5 [M]+ 326.1400, found 326.1405.
4.5.5. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-methyl-1Hbenzo[d] imidazol-3-ium iodide (1e) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (152 mg, 67%), mp = 229.1–229.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.58 (s, 1H), 9.18 (s, 1H), 8.55 (t, J = 8.0 Hz, 1H), 8.42 (d, J = 8.0 Hz, 1H), 8.29–8.26 (m, 2H), 8.21 (d, J = 8.0 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.85–7.77 (m, 3H), 7.41–7.35 (m, 2H), 4.26 (s, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.63, 146.36, 144.78, 144.24, 143.61, 143.02, 132.82, 132.04, 129.78, 128.30, 127.71, 124.83, 124.08, 120.65, 115.79, 115.10, 114.65, 114.31, 34.41, ppm;
4.5.10. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium iodide (1j) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (161 mg, 67%), mp = 216.5–217.0 C; 1H NMR (400 MHz, DMSO-d6): d 10.57 (s, 1H), 8.61 (d, J = 7.6 Hz, 2H), 8.47 (t, J = 8.0 Hz, 1H), 8.40 (d, J = 8.0 Hz, 1H), 8.31 (t, J = 8.0 Hz, 2H), 7.97 (d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.86-7.77 (m, 2H), 7.52 (t, J = 8.0 Hz, 1H), 7.38 (t, J = 8.0 Hz, 1H), 4.65 (t, J = 7.2 Hz, 2H), 2.10 (hex, J = 7.2 Hz, 2H), 1.06 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 153.21, 145.59, 143.57, 143.06, 139.32, 138.49, 132.16, 130.16, 129.09, 128.19, 127.74, 126.58, 123.81, 122.19, 115.74, 114.99, 114.79, 114.45, 114.29, 49.28, 22.49, 11.30, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1716.
4.5.6. 1-(6-(1H-benzo[d]imidazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d] imidazol-3-ium iodide (1f) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (135 mg, 56%), mp = 228.3–228.9 C; 1H NMR (400 MHz, DMSO-d6): d 10.60 (s, 1H), 9.19 (s, 1H), 8.56 (t, J = 8.0 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.31–8.27 (m, 3H), 8.06 (d, J = 8.0 Hz, 1H), 7.86–7.76 (m, 3H), 7.41–7.35 (m, 2H), 4.63 (t, J = 7.2 Hz, 2H), 2.06 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.6 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 149.58, 146.47, 144.80, 144.14, 143.14, 143.04, 132.18, 132.07, 130.03, 128.31, 127.74, 124.81, 124.09, 120.66, 116.04, 115.78, 115.29, 114.72, 114.31, 49.28, 22.50, 11.28, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1729.
4.5.12. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium bromide (1k) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (46 mg, 21%), mp = 238.5–238.8 C; 1H NMR (400 MHz, DMSO-d6): d 10.85 (s, 1H), 8.54 (s, 1H), 8.51 (d, J = 7.6 Hz, 1H), 8.42 (t, J = 8.0 Hz, 1H), 8.32 (d, J = 8.0 Hz, 2H), 8.18 (d, J = 8.0 Hz, 1H), 8.01 (d, J = 7.6 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.79 (t, J = 8.0 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 7.45 (t, J = 8.0 Hz, 1H), 7.31 (t, J = 8.0 Hz, 1H), 4.71 (t, J = 7.6 Hz, 2H), 2.10 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.2 Hz, 3H), ppm; 13 C NMR (100 MHz, DMSO-d6): d 153.00,145.57, 143.46, 142.99, 139.20, 138.35, 132.11, 129.96, 129.00, 128.09, 127.70, 126.48, 123.68, 122.10, 115.71, 114.88, 114.84, 114.28, 114.15, 49.23, 22.56,
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11.29, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1713.
114.28, 112.83, 109.58, 49.09, 22.62, 11.23, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1563.
4.5.13. 1-(6-(1H-indazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium chloride (1l) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (109 mg, 56%), mp = 246.3–247.7 C; 1H NMR (400 MHz, DMSO-d6): d 10.92 (s, 1H), 8.60–8.57 (m, 2H), 8.45 (t, J = 8.0 Hz, 1H), 8.39 (d, J = 8.0 Hz, 1H), 8.31 (d, J = 8.0 Hz, 1H), 8.26 (d, J = 8.0 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 7.84–7.75 (m, 2H), 7.51 (t, J = 7.2 Hz, 1H), 7.37 (t, J = 7.2 Hz, 1H), 4.68 (t, J = 7.6 Hz, 2H), 2.10 (hex, J = 7.2 Hz, 2H), 1.05 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 153.11, 145.70, 143.52, 143.31, 139.28, 138.46, 132.16, 130.11, 129.07, 128.12, 127.69, 126.56, 123.76, 122.17, 115.82, 114.95, 114.79, 114.37, 114.28, 49.17, 22.53, 11.28, ppm; HRMS (ESI): m/z calcd for C22H20N5 [M]+ 354.1713, found 354.1715.
4.5.18. 3-propyl-1-(6-(3-propyl-1H-imidazol-3-ium-1-yl)pyridin-2yl)-1H-benzo[d]imidazol-3-ium iodide (1q) The reaction mixture was washed with ethyl acetate (3 30 ml) and dried under vacuum for 24 h to give product 1q: a white solid (270 mg, 90%), mp = 209.0–209.8 C; 1H NMR (400 MHz, DMSO-d6): d 10.77 (s, 1H), 10.28 (s, 1H), 8.74–8.71 (m, 2H), 8.59–8.57 (m, 1H), 8.34–8.31 (m, 3H), 8.21 (s, 1H), 7.87–7.82 (m, 2H), 4.66 (t, J = 7.2 Hz, 2H), 4.37 (t, J = 7.2 Hz, 2H), 2.09 (Hex, J = 7.2 Hz, 2H), 1.97 (Hex, J = 7.2 Hz, 2H), 1.03 (t, J = 7.2 Hz, 3H), 0.96 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 146.63, 146.15, 145.11, 143.25, 136.15, 132.12, 129.65, 128.70, 127.86, 124.29, 120.20, 118.23, 116.38, 115.64, 114.76, 51.67, 49.40, 23.16, 22.53, 11.28, 10.97, ppm; HRMS (ESI): m/z calcd for C21H25N5 [[M]2+/2] 173.6050, found 173.6050; calcd for C21H25N5I [[M]2++I ] 474.1149, found 474.1151. 4.5.19 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-2-methyl-3-propyl1H-benzo[d]imidazol-3-ium iodide (1r) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (74 mg, 33%), mp = 198.0–201.3 C; 1H NMR (400 MHz, DMSO-d6): d 8.59 (d, J = 2.8 Hz, 1H), 8.47 (t, J = 8.0 Hz, 1H), 8.25 (t, J = 8.4 Hz, 2H), 7.95 (s, 1H), 7.90–7.85 (m, 2H), 7.75 (t, J = 7.2 Hz, 1H), 7.68 (t, J = 8.0 Hz, 1H), 6.65 (dd, J = 2.8 Hz, J = 1.6 Hz, 1H), 4.62 (t, J = 7.2 Hz, 2H), 3.02 (s, 3H), 1.96 (hex, J = 7.2 Hz, 2H), 1.07 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 152.64, 151.36, 144.50, 144.22, 143.76, 131.35, 131.06, 128.28, 127.61, 127.21, 119.77, 114.46, 114.00, 113.90, 109.63, 47.44, 22.39, 12.58, 11.44, ppm.
4.5.14. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-methyl-1H-benzo[d] imidazol-3-ium iodide (1m) Purification by flash chromatography (DCM/MeOH = 5:1): a white solid (145 mg, 72%), mp = 223.4–224.1 C; 1H NMR (400 MHz, DMSO-d6): d 10.60 (s, 1H), 8.78 (s, 1H), 8.53 (d, J = 7.2 Hz, 1H), 8.44 (t, J = 8.0 Hz, 1H), 8.17 (t, J = 7.6 Hz, 2H), 8.00–7.97 (m, 2H), 7.84 (t, J = 7.6 Hz, 2H), 6.73 (s, 1H), 4.23 (s, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.64, 146.12, 144.26, 143.81, 143.57, 132.80, 129.53, 128.47, 127.62, 115.94, 114.56, 114.17, 113.05, 109.66, ppm; HRMS (ESI): m/z calcd for C16H14N5 [M]+ 276.1244, found 276.1244. 4.5.15. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium iodide (1n) Purification by flash chromatography (DCM/MeOH = 10:1): a yellow solid (125 mg, 58%), mp = 203.2–203.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.65 (s, 1H), 8.78 (d, J = 2.4 Hz, 1H), 8.55 (dd, J = 7.2 Hz, J = 2.4 Hz, 1H), 8.45 (t, J = 8.0 Hz, 1H), 8.28 (dd, J = 6.4 Hz, J = 2.4 Hz, 1H), 8.16 (d, J = 8.0 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H), 7.96 (d, J = 1.6 Hz, 1H), 7.87–7.80 (m, 2H), 6.72 (dd, J = 2.4 Hz, J = 1.6 Hz, 1H), 4.62 (t, J = 7.2 Hz, 2H), 2.07 (hex, J = 7.2 Hz, 2H), 1.03 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.61, 146.22, 144.18, 143.81, 143.07, 132.19, 129.76, 128.53, 127.70, 116.20, 114.70, 114.48, 113.06, 109.67, 49.30, 22.55, 11.28, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1562. 4.5.16. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium bromide (1o) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (71 mg, 37%), mp = 231.2–231.5 C; 1H NMR (400 MHz, DMSO-d6): d 10.96 (s, 1H), 8.80 (d, J = 2.4 Hz, 1H), 8.53 (dd, J = 7.6 Hz, J = 1.6 Hz, 1H), 8.41 (t, J = 8.0 Hz, 1H), 8.29 (dd, J = 7.2 Hz, J = 1.6 Hz, 1H), 8.10 (dd, J = 8.0 Hz, J = 4.0 Hz, 2H), 7.92 (d, J = 1.6 Hz, 1H), 7.84– 7.76 (m, 2H), 6.68 (dd, J = 2.4 Hz, J = 1.6 Hz, 1H), 4.68 (t, J = 7.2 Hz, 2H), 2.07 (hex, J = 7.6 Hz, 2H), 1.02 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.72, 146.03, 143.74, 143.42, 142.80, 132.10, 129.70, 128.43, 128.19, 127.67, 116.02, 114.53, 114.20, 113.04, 109.33, 49.31, 22.70, 11.22, ppm; HRMS (ESI): m/z calcd for C18H18N5 [M]+ 304.1557, found 304.1560. 4.5.17. 1-(6-(1H-pyrazol-1-yl)pyridin-2-yl)-3-propyl-1H-benzo[d] imidazol-3-ium chloride (1p) Purification by flash chromatography (DCM/MeOH = 10:1): a white solid (70 mg, 41%), mp = 223.1–224.8 C; 1H NMR (400 MHz, DMSO-d6): d 11.34 (s, 1H), 8.84 (d, J = 2.4 Hz, 1H), 8.54 (d, J = 8.0 Hz, 1H), 8.39 (t, J = 8.0 Hz, 1H), 8.28 (d, J = 8.0 Hz, 1H), 8.17 (d, J = 8.0 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H), 7.91 (s, 1H), 7.82–7.75 (m, 2H), 6.67 (s, 1H), 4.69 (t, J = 7.6 Hz, 2H), 2.07 (hex, J = 7.2 Hz, 2H), 1.01 (t, J = 7.2 Hz, 3H), ppm; 13C NMR (100 MHz, DMSO-d6): d 150.46, 146.34, 144.05, 143.68, 143.46, 132.15, 129.61, 128.70, 128.43, 127.59, 116.27, 114.70,
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