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Remarkable synergistic effect between copper(I) and ionic liquids for promoting chemical fixation of CO2
Journal of CO₂ Utilization 22 (2017) 374–381
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Remarkable synergistic effect between copper(I) and ionic liquids for promoting chemical fixation of CO2
T
Yuling Zhaoa, Li Tiana, Jikuan Qiua, Zhiyong Lia, Huiyong Wanga, Guokai Cuia, Suojiang Zhangb, ⁎ Jianji Wanga, a Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan, PR China b Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: CO2 conversion Copper(I) Ionic liquids Propargylic alcohols DFT calculations Atmospheric pressure
Recently, efficient chemical conversion of CO2 into high value chemicals at room temperature and atmospheric pressure without using noble metal catalysts remains a great challenge. In this work, we find that the carboxylative cyclization of propargylic alcohols with CO2 can proceed at 60 °C and ambient pressure by using a combination of CuI and an ionic liquid (IL) 1,8-diazabicyclo-[5.4.0]-7-undecenium trifluoroethanol ([DBUH] [TFE]) as catalyst. By simple extraction after the reactions, a series of desired products have been obtained in good to excellent yields with this highly efficient catalyst system. Spectroscopic investigations and DFT calculations demonstrate that such a high efficiency originates from remarkable synergistic catalysis between Cu(I) and the IL on substrates. In addition, it is worth noting that this catalyst system also works well for the carboxylative cyclization of propargylic amines even at ambient temperature and pressure, with the highest turnover number among the reported base metals catalytic systems.
1. Introduction Carbon dioxide (CO2), as an alternative feedstock to fossil fuels, represents an ideal renewable and environmentally friendly C1 source [1,2]. Efficient transformation of CO2 into high-valued chemicals may benefit sustainable development of our society. For instance, direct catalytic coupling of CO2 with propargylic alcohols is a promising strategy to produce five-membered α-alkylidene cyclic carbonates [3], which can be used as building blocks in the formation of α-hydroxy ketones [4] and 5-methyleneoxazolidin-2-one derivatives [5], and have many applications in organic synthesis [6]. Therefore, numerous reports have been focused on this transformation in the last two decades [3,7–13]. Transition metal catalysts play a key role in CO2 conversion because of their efficient catalytic ability [14]. Various metal catalytic systems have been successfully developed for the synthesis of cyclic carbonates from CO2 and different substrates (Scheme 1). In 2007, Yamada and coworkers [15] reported a highly active Ag catalyst for this synthesis. Since the transformation of CO2 has been relying on the use of noble metal catalysts [16–29] (Scheme 1a), development of catalytic systems based on environmentally benign and earth-abundant base metals
⁎
becomes an important goal in homogeneous catalysis to replace the precious Ag and Pd catalysts [30]. Indeed, significant progress has been made in employing base metal catalysts in recent years. For example, Fe, Co, Cu and Zn catalytic systems have been developed to catalyze this reaction [31–36] (Scheme 1b). However, the majority of these reactions require stoichiometric amounts of bases such as DBU (1,8-diazabicyclo-[5.4.0]-7-undecenium), and most of the catalysts and bases cannot be recycled, thus leading to high costs for practical application. In addition, these catalysts suffer from some other drawbacks such as requirement of high CO2 pressure (10–140 atm) and harmful solvents. Based on the principles of sustainable chemistry, ionic liquids (ILs), a class of emerging green materials, are promising in this regard because of their unique structures, properties, and outstanding reusability. In this work, we report a highly efficient Cu(I)/IL catalytic system to afford the corresponding compounds under mild and solventfree reaction conditions (Scheme 1c). Here, the IL acts as a base, and its dosage is only 10 mol%. The turnover number (TON) reaches up to 4100 which is comparable with the result using Ag catalysts [25]. Moreover, the catalyst and base can be easily recovered from the reaction mixture and then reused without loss of activity. Spectroscopic investigations and DFT calculations demonstrate that such a high
Corresponding author. E-mail address: [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.jcou.2017.10.017 Received 12 July 2017; Received in revised form 30 September 2017; Accepted 17 October 2017 Available online 02 November 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
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Scheme 1. Carboxylative cyclization of propargyl alcohols with CO2 by different strategies.
2.4. Characterization of the products
efficiency originates from synergistic catalysis between Cu(I) and the IL on the substrates. Notably, this catalytic system also performs well for the cyclization of propargylic amines and CO2 with the highest turnover number of 860. 2. Experimental
1 H NMR and 13C NMR characterization of the products was conducted in CDCl3 or DMSO-d6 on a Bruker Avance III HD 600 spectrometer. The NMR data of the known products were found to be consistent with the previously reported experimental results.
2.1. Chemicals
2.5. Theoretical methods
CO2 was supplied by Beijing Analytical Instrument Factory with a purity of 99.999%. The deuterated solvents (DMSO-d6, CDCl3) were provided by Cambridge Isotope Laboratories, Inc. All chemicals were of analytical grade and used as received. The protic ionic liquids (PILs) [DBUH][TFE], [DBUH][Im], [DBUH][OAc] and [DBUH][2-OP] were prepared via the neutralization of corresponding bases and proton donors according to the reported procedures [10]. Terminal propargylic alcohols, propargylic amines, CuX, CuO and CuCN were purchased from J & K Scientific Ltd.
All quantum chemical calculations were performed with Gaussian 09 package using hybrid density functional theory B3LYP [37]. For O, N, C, F and H, the 6–311 + G(d) basis set was used, while the Cu and I atoms were described by the standard Lanl2DZ basis set [38]. Vibrational frequency calculations were performed at the same level of theory to verify that a local minimum has no imaginary frequency and each transition state has only one single imaginary frequency. To obtain the relative free energies, single-point energy calculations were carried out at the same level for all of the species studied. Intrinsic reaction coordinate (IRC) calculations were also performed to make sure that each calculated transition state connects two relevant minima.
2.2. Procedures for the preparation of the PILs The DBU based ILs were synthesized by neutralization. As an example, the procedure for the preparation of [DBUH][TFE] was described here. In a typical experiment, ethanol (50 mL), DBU (10 mmol) and trifluoroethanol (10 mmol) were loaded into a 100 mL flask placed in a water bath of 25 °C and the neutralization reaction was allowed to proceed for 12 h. The ionic liquid was then obtained by centrifuging and drying under vacuum at 60 °C overnight.
3. Results and discussion 3.1. Screening of the catalyst Considering the excellent catalytic ability of superbase-derived protic ILs in CO2 transformation, several simple DBU based ILs were selected, in this work, to catalyze the reaction, and the structures of these ILs were given in scheme S1 (Supporting information). Firstly, various Cu(I) catalysts and DBU based ILs were tested using 2-methyl-3butyn-2-ol (1a) as the substrate at 60 °C and 1 atm CO2, and the results were given in Table 1. It was shown that the reaction did not occur without any catalyst (Table 1, entry 1). Also, the individual component CuI and [DBUH][TFE] was not active under the given conditions (entries 2–3). Surprisingly, a 96% yield of the product was obtained using the combination of CuI and [DBUH][TFE] as catalyst. Then, various copper salts, including CuO, CuCl, CuBr and CuCN were respectively combined with [DBUH][TFE], and used as catalysts for the reaction. Unfortunately, only a moderate yield was achieved (entries 5–8), indicating that CuI was the best co-catalyst for the reaction. Next, the catalytic performances of ILs with different anions 2-methylimidazolide ([Im]−), pyridin-2-ol ([2-OP]−) and acetate ([OAc]−) were also studied (entries 9–11). The yield of 2a was found to be 79%, 36% and 16% respectively when CuI/[DBUH][Im], CuI/[DBUH][2-OP] and CuI/ [DBUH][OAc] were used as catalysts. These results suggest that anion
2.3. General procedures for the synthesis of α-alkylidene cyclic carbonates and oxazolidinones As an example, the procedures using 2-methyl-4-phenylbut-3-yn-2ol (1a) as the substrate was described here, and those for other substrates, similar procedures were used. In a typical experiment, propargylic alcohol 1a (1 mmol), CuI (5 mol%) and [DBUH][TFE] (10 mol %) were loaded in a 10 mL stainless-steel autoclave equipped with a magnetic stirring bar. The air in the reactor was replaced by CO2. Then the pressure of CO2 was kept at 1 atm using a balloon with CO2, and the reaction mixture was stirred at desired temperature (in this reaction, the temperature was 60 °C) for 12 h. After the completion of the reaction, the product was extracted by n-hexane and the solvent was removed under vacuum to afford the desired carbonate 2a (a colorless oil). The Cu/IL was recovered by heating under vacuum and reused in the next run. 375
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Table 1 Reaction of CO2 with 2-methyl-3-butyn-2-ol (1a) into 4,4-dimethyl-5-methylene-1,3-dioxolan-2-one(2a) in various catalyst systems.a
Table 3 TON of the reaction at 30 mmol scale.
Entry 1
Yield%b
Entry
Cat (mol%) Copper salt
Base
1 2 3 4 5 6 7 8 9 10 11 12
– CuI(5%) –c CuI(5%) CuO(5%) CuCl(5%) CuBr(5%) CuCN(5%) CuI(5%) CuI(5%) CuI(5%) CuI(5%)
–d –d [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][Im](10%) [DBUH][2-OP](10%) [DBUH][OAc](10%) DBU(10%)
2 c
0 0 0 96 42 54 77 50 79 36 16 0
3
Reaction condition CuI(5% mol), [DBUH][TFE] (10% mol), 60 °C, 1 atm, 18 h CuI(0.1% mol), [DBUH][TFE] (0.12% mol), 60 °C, 1 atm, 24 h CuI(0.01% mol), [DBUH][TFE] (0.012% mol), 60 °C, 30 atm, 180 h
Yield% 84
TON –
73
730
41
4100
temperature and the catalyst dosage, the reaction of propargyl alcohol 1a with CO2 catalyzed by CuI/[DBUH][TFE] was investigated (Table S1, Figs. S1 and S2, See Supporting information). It was found that under the optimized conditions, the yield of the desired product reached 96% (Table 1, entry 4). Thus, the following optimal reaction conditions were used for further investigations: CuI as co-catalyst (5% mol), [DBUH][TFE] as co-catalyst (10% mol), 12 h, 60 °C, and 1 atm.
a Reaction condition: 2-methylbut-3-yn-2-ol (1a, 1 mmol), catalyst (mol% based on 1a), CO2 (0.1 MPa), 12 h, 60 °C. b The yields were determined by 1H NMR spectroscopy using anisole as an internal standard. c Without copper salt. d Without IL.
3.2. Versatility of the catalyst With the optimized reaction conditions, we proceeded to explore the substrate scope of this transformation, and the yields of the target products were summarized in Table 2. It can be seen that propargylic alcohols with different alkyl substituents at the propargylic position were effective substrates to give the corresponding carbonates 2a–2 g in excellent yield. As for the propargylic alcohol with a cyclobutyl group, it could also afford a yield of 70% (2 h). Furthermore, to validate the effectiveness of this strategy, the reaction at a 30 mmol scale of 1a was investigated and the result was included in Table 3. As can be seen, 84% of 2a was obtained after 12 h under the optimized reaction conditions. To our delighted, a TON value of up to 4100 was achieved under specific reaction condition. In addition, it was found that this catalytic
of the ILs was crucial for the high catalytic activity. Yamada’s group [20] developed an AgOAc/DBU catalyst system in which DBU was an essential superbase used to activate the alcoholic hydroxyl for the formation of a carbonate intermediate. However, no reaction occurred using the DBU as the co-catalyst (entry 12), indicating that this reaction was significantly affected by the activity of the ionic liquids. From the above results, it is clear that CuI/[DBUH][TFE] is the most efficient catalyst system for the target reaction at atmospheric pressure. In addition, to study the effects of reaction time, reaction
Table 2 Scope of the propargylic alcohol substrates for the reaction under optimized conditions.a
a Reaction condition: propargylic alcohol (1 mmol), CuI (5 mol% based on 1), [DBUH][TFE] (10 mol% based on 1), CO2 (0.1 MPa), 12 h, 60 °C. The yields were isolated yield.
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Fig. 1. 1H NMR spectra (DMSO-d6, 25 °C) of propargylic alcohol (1a) (1 mmol) with and without CuI/[DBUH][TFE] (5% mol/10% mol) catalyst.
Fig. 2.
13
C NMR spectra (DMSO-d6, 25 °C) of propargylic alcohol (1a) (1 mmol) with and without CuI/[DBUH][TFE](5% mol/10% mol) catalyst.
the eOH group and the C^C triple bond at the same time. To verify this hypothesis, 1H NMR and 13C NMR spectra of propargylic alcohol (1a) with and without CuI/[DBUH][TFE] were studied. It was found that for the 1H NMR spectrum of 2-methyl-3-butyn-2-ol (1a) in the mixture of CuI/[DBUH][TFE], the signal at d = 5.296 ppm, assigned to the OH proton of 1a, shifted to d = 5.417 ppm (Fig. 1), and the peak became wider, which indicate the formation of a hydrogen bond between [DBUH][TFE] and 1a, resulting in activation of propargylic alcohol and thus enhancement of the nucleophilicity of the hydroxy group. In addition, changes in carbon peaks (Fig. 2) could be attributed to the interaction of the copper salt with the carbon–carbon triple bond, leading to the activation of propargylic alcohol [39]. The above analysis indicates that the hydroxyl group and carbon–carbon triple bond on the propargylic alcohol substrate were activated by [DBUH][TFE] and CuI, respectively. The mechanism of the detail reaction pathways were investigated by DFT calculations and the results were illustrated in Scheme 2. In the DFT calculations, the simplest
system could be easily recovered and reused more than 5 times without obvious loss of catalytic activity (Figs. S3, See Supporting information). Interestingly, the product could be obtained by simple extraction without other treatments. Moreover, the stability of the PIL before and after use was studied by thermo gravimetric and NMR spectrum analysis (see the Supporting information, Figs. S5 S6 and S7). It was found that thermal decomposition temperature of the PIL before and after use was 135 °C and 134 °C, respectively. In addition, no observable change was found in the 1H NMR spectrum of the IL before and after use. These results indicate that the IL used in this work was thermally and chemically stable within the reaction temperature range.
3.3. Reaction mechanism The catalytic mechanism of CuI/[DBUH][TFE] was investigated in detail by employing NMR spectroscopy and DFT calculations. As stated before, the aim for the design of Cu(I)/IL catalytic system is to activate 377
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Scheme 2. The mechanism for CuI/[DBUH][TFE] promoted cycloaddition of CO2 with propargylic alcohol substrates.
propargylic alcohol 1a was used as the representative substrate molecule to reduce the computing complexity. It can be seen from Scheme 2 that at the beginning, the cation and anion of the IL formed hydrogen bonds with the substrate. The hydrogen atom of the hydroxyl group was then captured by the anion to activate the substrate. In this step, structure 2 (Scheme 2) was formed via transition state TS1-2, which has a low free energy barrier of 13.9 kcal/mol (Fig. S4). From the optimized geometry of TS1-2 in Fig. 3, it is evident that the H(N) atom of the heterocyclic ring in [DBUH]+ formed a hydrogen bond with the O atom of the hydroxyl of 1a, whereas the N atoms of [TFE]− formed hydrogen bonds with the H atom of the hydroxyl of 1a. Thus, the hydroxyl hydrogen of the substrate was activated with the help of hydrogen bonding of [DBUH]+ and [TFE]− with substrate 1a. After the hydrogen atom of the hydroxyl group was captured by the IL, the CO2 electrophilic attack on the alkoxide was more favorable, leading to the formation of zwitterionic carbamate species (structure 3). Meanwhile, the carbon–carbon triple bond on the substrate was activated by CuI, and then the intermediate 4 was formed. These DFT calculations support the results obtained by NMR spectroscopy. After both eOH group and C^C triple bond of the substrate were activated by CuI/[DBUH][TFE], intramolecular cyclization of the substrate began to proceed, leading to the formation of product. The free energy profiles was shown in Fig. 4. In the intramolecular cyclization
Fig. 3. The optimized geometries of TS2-1 shown in Scheme 2: C atom (gray), H atom (white), O atom (red) and N atom (bule) were shown in different colors for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Free energy profiles of the reaction pathway.
Table 4 The synthesis of oxazolidinones from propargylic amines and atmospheric CO2 in CuI/[DBUH][TFE] catalytic system at room temperature.a
a
Reaction condition: propargylic amine (1 mmol), CuI (5 mol% based on 1), [DBUH] (10 mol% based on 1), CO2 (0.1 MPa), 8 h, 25 °C. The yields were isolated yield. Scheme 3. Evaluation of the catalytic activity.
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Acknowledgements
step, the attack of the O atom to the C^C bond occurred with the help of CuI catalyst, and a five-membered ring was formed via TS4-5. The free energy barrier of this step was 12.6 kcal/mol (Fig. 3). In the last demetallation step via transition state TS5–6, the target product was formed by robbing the hydrogen proton, and IL and CuI were regenerated. Depending on the source of the hydrogen proton, there are two potential routes for this intramolecular cyclization as shown in Scheme 2. For the route via transition state TS6-7, the [DBUH]+ cation provides the hydrogen proton with an energy barrier of 15.9 kcal/mol (Fig. 3). For the second route, the proton transfers from the trifluoroethanol molecule to the C]C bond via transition state TS6-7′ with an energy barrier of 19.4 kcal/mol (Fig. 3). Both routes could make the demetallation process easily, leading to the target product and the regeneration of the [DBUH][TFE]. However, by comparing the energy barrier of both routes, it is clear that the route via transition state TS6-7 has a lower energy barrier, and thus is more favorable than the other route. In addition, the result in Fig. 4 also suggests that the demetallation step with a free energy barrier of 15.9 kcal/mol is the rate-determining step. Meanwhile, as [DBUH][TFE] can react with CO2 and form carbonate intermediate (see Figs. S8 and S9 in Supporting information), the CO2-activated mechanism and free energy profiles are also considered for this reaction (Scheme S2 and Fig. S10). It is clear that the CO2activated mechanism has a higher overall energy barrier, and is not favorable than the hydroxyl-activated mechanism. Through the above analysis, it can be concluded that [DBUH][TFE] promoted deprotonation of the hydroxyl and the demetallation step by providing hydrogen protons, while the CuI promoted intramolecular cyclization steps by activating C^C triple bond of the substrate. As a result, the IL and CuI play a remarkable synergistic role in accelerating the reaction. The synthesis of oxazolidinones through the carboxylative cyclization of propargylic amines and CO2 is one of the most attractive synthetic methods [40–44]. To our knowledge, the catalytic system using base metals for this reaction under room temperature and atmospheric pressure has not been reported. To further display the effectiveness of the present catalytic system, the cyclization of propargylic amines with CO2 was further examined. It was found that the current reaction was efficiently performed to give a series of oxazolidinones with excellent yields even at ambient temperature and pressure (Table 4). In the case of 4a, the TON reached up to 860 (Scheme 3) which is the highest among the reported results to date.
This work was supported by the National Natural Science Foundation of China (Grant No. 21403060，21673068, and 21773058), the National Key Research and Development Program of China (2017YFA0403101), Program for Innovative Research Team in Science and Technology in University of Henan Province (16IRTSTHN002), and the 111 Project (No.D17007). This work was also supported by the High Performance Computing Center of Henan Normal University. References [1] T. Sakakura, J.C. Choi, H. Yasuda, Transformation of carbon dioxide, Chem. Rev. 107 (2007) 2365–2387. [2] S.N. Riduan, Y. Zhang, J.Y. Ying, Conversion of carbon dioxide into methanol with silanes over N©\heterocyclic carbene catalysts, Angew. Chem. Int. Ed. 121 (2009) 3372–3375. [3] Y.B. Wang, Y.M. Wang, W.Z. Zhang, X.B. Lu, Fast CO2 sequestration activation, and catalytic transformation using N-heterocyclic olefins, J. Am. Chem. Soc. 135 (2013) 11996–12003. [4] Y. Zhao, Z. Yang, B. Yu, H. Zhang, H. Xu, L. Hao, B. Han, Z. 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4. Conclusions In summary, we established an efficient strategy for expeditious chemical fixation of CO2 to generate various α-alkylidene cyclic carbonates at ambient pressure without using noble metal catalysts. Among the combinations of Cu(I) catalyst and various DBU based ILs investigated, the CuI/[DBUH][TFE] catalyst system was found to display the best catalytic performance for the reactions of CO2 with propargylic alcohols. After the completion of the reactions, the products were obtained in good to excellent yields by simple extraction separation, and the catalytic system could be easily regenerated without obvious loss in its activity. Based on NMR analysis and DFT calculations, it could be concluded that the ionic liquid promoted deprotonation of the hydroxyl and the demetallation step by providing hydrogen protons, while the CuI promoted intramolecular cyclization steps by activating C^C triple bond of the substrate. Additionally, the current protocol was also efficiently applied to the reaction of propargylic amines and CO2 with excellent yields even at ambient temperature and pressure. Considering the fact that the CuI/[DBUH][TFE] is a cheap, commercially available and recyclable catalyst system, we expect that more applications can be found in the transformation of CO2 under mild conditions.
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Y. Zhao et al. pressure, Chem. Commun. 52 (2016) 7830–7833. [24] J. Qiu, Y. Zhao, H. Wang, G. Cui, J. Wang, AgX@carbon (X = Br and I) as robust and efficient catalysts for the reaction of propargylic alcohols and CO2 to carbonates under ambient conditions, RSC Adv. 6 (2016) 54020–54026. [25] Q.W. Song, L.N. He, Robust silver(I) catalyst for the carboxylative cyclization of propargylic alcohols with carbon dioxide under ambient conditions, Adv. Synth. Catal. 358 (2016) 1251–1258. [26] Z.Z. Yang, Y. Zhao, H. Zhang, B. Yu, Z. Ma, G. Ji, Z. Liu, Fluorinated microporous organic polymers: design and applications in CO2 adsorption and conversion, Chem. Commun. 50 (2014) 13910–13913. [27] N.Y. Koji Iritani, Kiitiro Utimoto, Carboxylative coupling of propargylic alcohols with allyl chloride, J. Org. Chem. 51 (1986) 5499–5501. [28] Y. Yuan, Y. Xie, C. Zeng, D. Song, S. Chaemchuen, C. Chen, F. Verpoort, A simple and robust AgI/KOAc catalytic system for the carboxylative assembly of propargyl alcohols and carbon dioxide at atmospheric pressure, Catal. Sci. Technol. 7 (2017) 2935–2939. [29] Y. Yuan, Y. Xie, C. Zeng, D. Song, S. Chaemchuen, C. Chen, F. Verpoort, A recyclable AgI/OAc− catalytic system for the efficient synthesis of α-alkylidene cyclic carbonates: carbon dioxide conversion at atmospheric pressure, Green Chem. 19 (2017) 2936–2940. [30] X. Du, Z. Huang, Advances in base-metal-catalyzed alkene hydrosilylation, ACS Catal. 7 (2017) 1227–1243. [31] T.J. Kim, K.H. Kwon, S.C. Kwon, J.O. Baeg, S.C. Shim, D.H. Lee, Iron complexes of 1, 1, 1’-bis (diphenylphosphino) ferrocene (BPPF) as efficient catalysts in the synthesis of carbamates. X-ray crystal structure of (BPPF) Fe (CO)3, J. Org. Chem. 389 (1990) 205–217. [32] Y. Gu, F. Shi, Y. Deng, Ionic liquid as an efficient promoting medium for fixation of CO2: Clean synthesis of α-methylene cyclic carbonates from CO2 and propargyl alcohols catalyzed by metal salts under mild conditions, J. Org. Chem. 69 (2004) 391–394. [33] L. Ouyang, X. Tang, H. He, C. Qi, W. Xiong, Y. Ren, H. Jiang, Copper-promoted coupling of carbon dioxide and propargylic alcohols: expansion of substrate scope and trapping of vinyl copper intermediate, Adv. Synth. Catal. 357 (2015) 2556–2565. [34] H.S. Kim, J.W. Kim, S.C. Kwon, S.C. Shim, T.J. Kim, Catalytic formation of
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Remarkable synergistic effect between copper(I) and ionic liquids for promoting chemical fixation of CO2
T
Yuling Zhaoa, Li Tiana, Jikuan Qiua, Zhiyong Lia, Huiyong Wanga, Guokai Cuia, Suojiang Zhangb, ⁎ Jianji Wanga, a Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan, PR China b Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: CO2 conversion Copper(I) Ionic liquids Propargylic alcohols DFT calculations Atmospheric pressure
Recently, efficient chemical conversion of CO2 into high value chemicals at room temperature and atmospheric pressure without using noble metal catalysts remains a great challenge. In this work, we find that the carboxylative cyclization of propargylic alcohols with CO2 can proceed at 60 °C and ambient pressure by using a combination of CuI and an ionic liquid (IL) 1,8-diazabicyclo-[5.4.0]-7-undecenium trifluoroethanol ([DBUH] [TFE]) as catalyst. By simple extraction after the reactions, a series of desired products have been obtained in good to excellent yields with this highly efficient catalyst system. Spectroscopic investigations and DFT calculations demonstrate that such a high efficiency originates from remarkable synergistic catalysis between Cu(I) and the IL on substrates. In addition, it is worth noting that this catalyst system also works well for the carboxylative cyclization of propargylic amines even at ambient temperature and pressure, with the highest turnover number among the reported base metals catalytic systems.
1. Introduction Carbon dioxide (CO2), as an alternative feedstock to fossil fuels, represents an ideal renewable and environmentally friendly C1 source [1,2]. Efficient transformation of CO2 into high-valued chemicals may benefit sustainable development of our society. For instance, direct catalytic coupling of CO2 with propargylic alcohols is a promising strategy to produce five-membered α-alkylidene cyclic carbonates [3], which can be used as building blocks in the formation of α-hydroxy ketones [4] and 5-methyleneoxazolidin-2-one derivatives [5], and have many applications in organic synthesis [6]. Therefore, numerous reports have been focused on this transformation in the last two decades [3,7–13]. Transition metal catalysts play a key role in CO2 conversion because of their efficient catalytic ability [14]. Various metal catalytic systems have been successfully developed for the synthesis of cyclic carbonates from CO2 and different substrates (Scheme 1). In 2007, Yamada and coworkers [15] reported a highly active Ag catalyst for this synthesis. Since the transformation of CO2 has been relying on the use of noble metal catalysts [16–29] (Scheme 1a), development of catalytic systems based on environmentally benign and earth-abundant base metals
⁎
becomes an important goal in homogeneous catalysis to replace the precious Ag and Pd catalysts [30]. Indeed, significant progress has been made in employing base metal catalysts in recent years. For example, Fe, Co, Cu and Zn catalytic systems have been developed to catalyze this reaction [31–36] (Scheme 1b). However, the majority of these reactions require stoichiometric amounts of bases such as DBU (1,8-diazabicyclo-[5.4.0]-7-undecenium), and most of the catalysts and bases cannot be recycled, thus leading to high costs for practical application. In addition, these catalysts suffer from some other drawbacks such as requirement of high CO2 pressure (10–140 atm) and harmful solvents. Based on the principles of sustainable chemistry, ionic liquids (ILs), a class of emerging green materials, are promising in this regard because of their unique structures, properties, and outstanding reusability. In this work, we report a highly efficient Cu(I)/IL catalytic system to afford the corresponding compounds under mild and solventfree reaction conditions (Scheme 1c). Here, the IL acts as a base, and its dosage is only 10 mol%. The turnover number (TON) reaches up to 4100 which is comparable with the result using Ag catalysts [25]. Moreover, the catalyst and base can be easily recovered from the reaction mixture and then reused without loss of activity. Spectroscopic investigations and DFT calculations demonstrate that such a high
Corresponding author. E-mail address: [email protected] (J. Wang).
http://dx.doi.org/10.1016/j.jcou.2017.10.017 Received 12 July 2017; Received in revised form 30 September 2017; Accepted 17 October 2017 Available online 02 November 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
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Scheme 1. Carboxylative cyclization of propargyl alcohols with CO2 by different strategies.
2.4. Characterization of the products
efficiency originates from synergistic catalysis between Cu(I) and the IL on the substrates. Notably, this catalytic system also performs well for the cyclization of propargylic amines and CO2 with the highest turnover number of 860. 2. Experimental
1 H NMR and 13C NMR characterization of the products was conducted in CDCl3 or DMSO-d6 on a Bruker Avance III HD 600 spectrometer. The NMR data of the known products were found to be consistent with the previously reported experimental results.
2.1. Chemicals
2.5. Theoretical methods
CO2 was supplied by Beijing Analytical Instrument Factory with a purity of 99.999%. The deuterated solvents (DMSO-d6, CDCl3) were provided by Cambridge Isotope Laboratories, Inc. All chemicals were of analytical grade and used as received. The protic ionic liquids (PILs) [DBUH][TFE], [DBUH][Im], [DBUH][OAc] and [DBUH][2-OP] were prepared via the neutralization of corresponding bases and proton donors according to the reported procedures [10]. Terminal propargylic alcohols, propargylic amines, CuX, CuO and CuCN were purchased from J & K Scientific Ltd.
All quantum chemical calculations were performed with Gaussian 09 package using hybrid density functional theory B3LYP [37]. For O, N, C, F and H, the 6–311 + G(d) basis set was used, while the Cu and I atoms were described by the standard Lanl2DZ basis set [38]. Vibrational frequency calculations were performed at the same level of theory to verify that a local minimum has no imaginary frequency and each transition state has only one single imaginary frequency. To obtain the relative free energies, single-point energy calculations were carried out at the same level for all of the species studied. Intrinsic reaction coordinate (IRC) calculations were also performed to make sure that each calculated transition state connects two relevant minima.
2.2. Procedures for the preparation of the PILs The DBU based ILs were synthesized by neutralization. As an example, the procedure for the preparation of [DBUH][TFE] was described here. In a typical experiment, ethanol (50 mL), DBU (10 mmol) and trifluoroethanol (10 mmol) were loaded into a 100 mL flask placed in a water bath of 25 °C and the neutralization reaction was allowed to proceed for 12 h. The ionic liquid was then obtained by centrifuging and drying under vacuum at 60 °C overnight.
3. Results and discussion 3.1. Screening of the catalyst Considering the excellent catalytic ability of superbase-derived protic ILs in CO2 transformation, several simple DBU based ILs were selected, in this work, to catalyze the reaction, and the structures of these ILs were given in scheme S1 (Supporting information). Firstly, various Cu(I) catalysts and DBU based ILs were tested using 2-methyl-3butyn-2-ol (1a) as the substrate at 60 °C and 1 atm CO2, and the results were given in Table 1. It was shown that the reaction did not occur without any catalyst (Table 1, entry 1). Also, the individual component CuI and [DBUH][TFE] was not active under the given conditions (entries 2–3). Surprisingly, a 96% yield of the product was obtained using the combination of CuI and [DBUH][TFE] as catalyst. Then, various copper salts, including CuO, CuCl, CuBr and CuCN were respectively combined with [DBUH][TFE], and used as catalysts for the reaction. Unfortunately, only a moderate yield was achieved (entries 5–8), indicating that CuI was the best co-catalyst for the reaction. Next, the catalytic performances of ILs with different anions 2-methylimidazolide ([Im]−), pyridin-2-ol ([2-OP]−) and acetate ([OAc]−) were also studied (entries 9–11). The yield of 2a was found to be 79%, 36% and 16% respectively when CuI/[DBUH][Im], CuI/[DBUH][2-OP] and CuI/ [DBUH][OAc] were used as catalysts. These results suggest that anion
2.3. General procedures for the synthesis of α-alkylidene cyclic carbonates and oxazolidinones As an example, the procedures using 2-methyl-4-phenylbut-3-yn-2ol (1a) as the substrate was described here, and those for other substrates, similar procedures were used. In a typical experiment, propargylic alcohol 1a (1 mmol), CuI (5 mol%) and [DBUH][TFE] (10 mol %) were loaded in a 10 mL stainless-steel autoclave equipped with a magnetic stirring bar. The air in the reactor was replaced by CO2. Then the pressure of CO2 was kept at 1 atm using a balloon with CO2, and the reaction mixture was stirred at desired temperature (in this reaction, the temperature was 60 °C) for 12 h. After the completion of the reaction, the product was extracted by n-hexane and the solvent was removed under vacuum to afford the desired carbonate 2a (a colorless oil). The Cu/IL was recovered by heating under vacuum and reused in the next run. 375
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Table 1 Reaction of CO2 with 2-methyl-3-butyn-2-ol (1a) into 4,4-dimethyl-5-methylene-1,3-dioxolan-2-one(2a) in various catalyst systems.a
Table 3 TON of the reaction at 30 mmol scale.
Entry 1
Yield%b
Entry
Cat (mol%) Copper salt
Base
1 2 3 4 5 6 7 8 9 10 11 12
– CuI(5%) –c CuI(5%) CuO(5%) CuCl(5%) CuBr(5%) CuCN(5%) CuI(5%) CuI(5%) CuI(5%) CuI(5%)
–d –d [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][TFE](10%) [DBUH][Im](10%) [DBUH][2-OP](10%) [DBUH][OAc](10%) DBU(10%)
2 c
0 0 0 96 42 54 77 50 79 36 16 0
3
Reaction condition CuI(5% mol), [DBUH][TFE] (10% mol), 60 °C, 1 atm, 18 h CuI(0.1% mol), [DBUH][TFE] (0.12% mol), 60 °C, 1 atm, 24 h CuI(0.01% mol), [DBUH][TFE] (0.012% mol), 60 °C, 30 atm, 180 h
Yield% 84
TON –
73
730
41
4100
temperature and the catalyst dosage, the reaction of propargyl alcohol 1a with CO2 catalyzed by CuI/[DBUH][TFE] was investigated (Table S1, Figs. S1 and S2, See Supporting information). It was found that under the optimized conditions, the yield of the desired product reached 96% (Table 1, entry 4). Thus, the following optimal reaction conditions were used for further investigations: CuI as co-catalyst (5% mol), [DBUH][TFE] as co-catalyst (10% mol), 12 h, 60 °C, and 1 atm.
a Reaction condition: 2-methylbut-3-yn-2-ol (1a, 1 mmol), catalyst (mol% based on 1a), CO2 (0.1 MPa), 12 h, 60 °C. b The yields were determined by 1H NMR spectroscopy using anisole as an internal standard. c Without copper salt. d Without IL.
3.2. Versatility of the catalyst With the optimized reaction conditions, we proceeded to explore the substrate scope of this transformation, and the yields of the target products were summarized in Table 2. It can be seen that propargylic alcohols with different alkyl substituents at the propargylic position were effective substrates to give the corresponding carbonates 2a–2 g in excellent yield. As for the propargylic alcohol with a cyclobutyl group, it could also afford a yield of 70% (2 h). Furthermore, to validate the effectiveness of this strategy, the reaction at a 30 mmol scale of 1a was investigated and the result was included in Table 3. As can be seen, 84% of 2a was obtained after 12 h under the optimized reaction conditions. To our delighted, a TON value of up to 4100 was achieved under specific reaction condition. In addition, it was found that this catalytic
of the ILs was crucial for the high catalytic activity. Yamada’s group [20] developed an AgOAc/DBU catalyst system in which DBU was an essential superbase used to activate the alcoholic hydroxyl for the formation of a carbonate intermediate. However, no reaction occurred using the DBU as the co-catalyst (entry 12), indicating that this reaction was significantly affected by the activity of the ionic liquids. From the above results, it is clear that CuI/[DBUH][TFE] is the most efficient catalyst system for the target reaction at atmospheric pressure. In addition, to study the effects of reaction time, reaction
Table 2 Scope of the propargylic alcohol substrates for the reaction under optimized conditions.a
a Reaction condition: propargylic alcohol (1 mmol), CuI (5 mol% based on 1), [DBUH][TFE] (10 mol% based on 1), CO2 (0.1 MPa), 12 h, 60 °C. The yields were isolated yield.
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Fig. 1. 1H NMR spectra (DMSO-d6, 25 °C) of propargylic alcohol (1a) (1 mmol) with and without CuI/[DBUH][TFE] (5% mol/10% mol) catalyst.
Fig. 2.
13
C NMR spectra (DMSO-d6, 25 °C) of propargylic alcohol (1a) (1 mmol) with and without CuI/[DBUH][TFE](5% mol/10% mol) catalyst.
the eOH group and the C^C triple bond at the same time. To verify this hypothesis, 1H NMR and 13C NMR spectra of propargylic alcohol (1a) with and without CuI/[DBUH][TFE] were studied. It was found that for the 1H NMR spectrum of 2-methyl-3-butyn-2-ol (1a) in the mixture of CuI/[DBUH][TFE], the signal at d = 5.296 ppm, assigned to the OH proton of 1a, shifted to d = 5.417 ppm (Fig. 1), and the peak became wider, which indicate the formation of a hydrogen bond between [DBUH][TFE] and 1a, resulting in activation of propargylic alcohol and thus enhancement of the nucleophilicity of the hydroxy group. In addition, changes in carbon peaks (Fig. 2) could be attributed to the interaction of the copper salt with the carbon–carbon triple bond, leading to the activation of propargylic alcohol [39]. The above analysis indicates that the hydroxyl group and carbon–carbon triple bond on the propargylic alcohol substrate were activated by [DBUH][TFE] and CuI, respectively. The mechanism of the detail reaction pathways were investigated by DFT calculations and the results were illustrated in Scheme 2. In the DFT calculations, the simplest
system could be easily recovered and reused more than 5 times without obvious loss of catalytic activity (Figs. S3, See Supporting information). Interestingly, the product could be obtained by simple extraction without other treatments. Moreover, the stability of the PIL before and after use was studied by thermo gravimetric and NMR spectrum analysis (see the Supporting information, Figs. S5 S6 and S7). It was found that thermal decomposition temperature of the PIL before and after use was 135 °C and 134 °C, respectively. In addition, no observable change was found in the 1H NMR spectrum of the IL before and after use. These results indicate that the IL used in this work was thermally and chemically stable within the reaction temperature range.
3.3. Reaction mechanism The catalytic mechanism of CuI/[DBUH][TFE] was investigated in detail by employing NMR spectroscopy and DFT calculations. As stated before, the aim for the design of Cu(I)/IL catalytic system is to activate 377
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Scheme 2. The mechanism for CuI/[DBUH][TFE] promoted cycloaddition of CO2 with propargylic alcohol substrates.
propargylic alcohol 1a was used as the representative substrate molecule to reduce the computing complexity. It can be seen from Scheme 2 that at the beginning, the cation and anion of the IL formed hydrogen bonds with the substrate. The hydrogen atom of the hydroxyl group was then captured by the anion to activate the substrate. In this step, structure 2 (Scheme 2) was formed via transition state TS1-2, which has a low free energy barrier of 13.9 kcal/mol (Fig. S4). From the optimized geometry of TS1-2 in Fig. 3, it is evident that the H(N) atom of the heterocyclic ring in [DBUH]+ formed a hydrogen bond with the O atom of the hydroxyl of 1a, whereas the N atoms of [TFE]− formed hydrogen bonds with the H atom of the hydroxyl of 1a. Thus, the hydroxyl hydrogen of the substrate was activated with the help of hydrogen bonding of [DBUH]+ and [TFE]− with substrate 1a. After the hydrogen atom of the hydroxyl group was captured by the IL, the CO2 electrophilic attack on the alkoxide was more favorable, leading to the formation of zwitterionic carbamate species (structure 3). Meanwhile, the carbon–carbon triple bond on the substrate was activated by CuI, and then the intermediate 4 was formed. These DFT calculations support the results obtained by NMR spectroscopy. After both eOH group and C^C triple bond of the substrate were activated by CuI/[DBUH][TFE], intramolecular cyclization of the substrate began to proceed, leading to the formation of product. The free energy profiles was shown in Fig. 4. In the intramolecular cyclization
Fig. 3. The optimized geometries of TS2-1 shown in Scheme 2: C atom (gray), H atom (white), O atom (red) and N atom (bule) were shown in different colors for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Free energy profiles of the reaction pathway.
Table 4 The synthesis of oxazolidinones from propargylic amines and atmospheric CO2 in CuI/[DBUH][TFE] catalytic system at room temperature.a
a
Reaction condition: propargylic amine (1 mmol), CuI (5 mol% based on 1), [DBUH] (10 mol% based on 1), CO2 (0.1 MPa), 8 h, 25 °C. The yields were isolated yield. Scheme 3. Evaluation of the catalytic activity.
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Acknowledgements
step, the attack of the O atom to the C^C bond occurred with the help of CuI catalyst, and a five-membered ring was formed via TS4-5. The free energy barrier of this step was 12.6 kcal/mol (Fig. 3). In the last demetallation step via transition state TS5–6, the target product was formed by robbing the hydrogen proton, and IL and CuI were regenerated. Depending on the source of the hydrogen proton, there are two potential routes for this intramolecular cyclization as shown in Scheme 2. For the route via transition state TS6-7, the [DBUH]+ cation provides the hydrogen proton with an energy barrier of 15.9 kcal/mol (Fig. 3). For the second route, the proton transfers from the trifluoroethanol molecule to the C]C bond via transition state TS6-7′ with an energy barrier of 19.4 kcal/mol (Fig. 3). Both routes could make the demetallation process easily, leading to the target product and the regeneration of the [DBUH][TFE]. However, by comparing the energy barrier of both routes, it is clear that the route via transition state TS6-7 has a lower energy barrier, and thus is more favorable than the other route. In addition, the result in Fig. 4 also suggests that the demetallation step with a free energy barrier of 15.9 kcal/mol is the rate-determining step. Meanwhile, as [DBUH][TFE] can react with CO2 and form carbonate intermediate (see Figs. S8 and S9 in Supporting information), the CO2-activated mechanism and free energy profiles are also considered for this reaction (Scheme S2 and Fig. S10). It is clear that the CO2activated mechanism has a higher overall energy barrier, and is not favorable than the hydroxyl-activated mechanism. Through the above analysis, it can be concluded that [DBUH][TFE] promoted deprotonation of the hydroxyl and the demetallation step by providing hydrogen protons, while the CuI promoted intramolecular cyclization steps by activating C^C triple bond of the substrate. As a result, the IL and CuI play a remarkable synergistic role in accelerating the reaction. The synthesis of oxazolidinones through the carboxylative cyclization of propargylic amines and CO2 is one of the most attractive synthetic methods [40–44]. To our knowledge, the catalytic system using base metals for this reaction under room temperature and atmospheric pressure has not been reported. To further display the effectiveness of the present catalytic system, the cyclization of propargylic amines with CO2 was further examined. It was found that the current reaction was efficiently performed to give a series of oxazolidinones with excellent yields even at ambient temperature and pressure (Table 4). In the case of 4a, the TON reached up to 860 (Scheme 3) which is the highest among the reported results to date.
This work was supported by the National Natural Science Foundation of China (Grant No. 21403060，21673068, and 21773058), the National Key Research and Development Program of China (2017YFA0403101), Program for Innovative Research Team in Science and Technology in University of Henan Province (16IRTSTHN002), and the 111 Project (No.D17007). This work was also supported by the High Performance Computing Center of Henan Normal University. References [1] T. Sakakura, J.C. Choi, H. Yasuda, Transformation of carbon dioxide, Chem. Rev. 107 (2007) 2365–2387. [2] S.N. Riduan, Y. Zhang, J.Y. Ying, Conversion of carbon dioxide into methanol with silanes over N©\heterocyclic carbene catalysts, Angew. Chem. Int. Ed. 121 (2009) 3372–3375. [3] Y.B. Wang, Y.M. Wang, W.Z. Zhang, X.B. Lu, Fast CO2 sequestration activation, and catalytic transformation using N-heterocyclic olefins, J. Am. Chem. Soc. 135 (2013) 11996–12003. [4] Y. Zhao, Z. Yang, B. Yu, H. Zhang, H. Xu, L. Hao, B. Han, Z. 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4. Conclusions In summary, we established an efficient strategy for expeditious chemical fixation of CO2 to generate various α-alkylidene cyclic carbonates at ambient pressure without using noble metal catalysts. Among the combinations of Cu(I) catalyst and various DBU based ILs investigated, the CuI/[DBUH][TFE] catalyst system was found to display the best catalytic performance for the reactions of CO2 with propargylic alcohols. After the completion of the reactions, the products were obtained in good to excellent yields by simple extraction separation, and the catalytic system could be easily regenerated without obvious loss in its activity. Based on NMR analysis and DFT calculations, it could be concluded that the ionic liquid promoted deprotonation of the hydroxyl and the demetallation step by providing hydrogen protons, while the CuI promoted intramolecular cyclization steps by activating C^C triple bond of the substrate. Additionally, the current protocol was also efficiently applied to the reaction of propargylic amines and CO2 with excellent yields even at ambient temperature and pressure. Considering the fact that the CuI/[DBUH][TFE] is a cheap, commercially available and recyclable catalyst system, we expect that more applications can be found in the transformation of CO2 under mild conditions.
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