CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402477
Phosphorus-based Bifunctional Organocatalysts for the Addition of Carbon Dioxide and Epoxides Thomas Werner* and Hendrik Bttner[a] Bifunctional phosphonium salts were synthesized and employed as organocatalysts for the atom efficient synthesis of cyclic carbonates from CO2 and epoxides for the first time. These catalysts were obtained in high yields by a modular, straightforward one-step synthesis. The hydrogen-bond donating alcohol function in the side chain leads to a synergistic effect accelerating the catalytic reaction. The desired cyclic carbonates are obtained in high yields and selectivity under solvent-free reaction conditions without the use of any co-catalyst. Under optimized reaction conditions various epoxides were converted to the corresponding cyclic carbonates in excellent yields. The products were obtained analytically pure after simple filtration over a silica gel pad. This protocol is even applicable for a multigram reaction scale. Moreover, the catalysts could be easily recovered and reused up to five times.
In the past two decades transformations employing carbon dioxide as a readily available, inexpensive, nontoxic, and abundant carbon source have been studied extensively.[1] However, carbon dioxide is the end product of thermal combustion and its thermodynamic stability as well as its kinetic inertia are challenging for CO2 utilization.[2] Hence, the development of suitable catalysts creating value-added products from carbon dioxide is of particular interest. The atom-efficient addition of CO2 and epoxides produces cyclic carbonates and provides efficient carbon dioxide fixation as well as the production of valuable outputs.[3] Cyclic carbonates serve as polar green solvents and display some outstanding properties such as a high boiling point, being odorless, and low toxicity.[4] In addition, carbonates are applied as electrolytes in lithium-ion batteries,[5] as monomers in polymerization reactions,[6] and as intermediates for fine chemicals.[7] There are numerous reports on catalysts for the coupling of CO2 with epoxides. Especially organocatalysts based on pyridinium,[8] imidazolium,[9] ammonium,[10] and phosphonium salts[11] as well as carbenes[12] have been studied extensively as both single catalysts and co-catalysts. However, under metalfree conditions these catalysts usually require harsh reaction conditions: temperatures > 100 8C and/or pressures > 2.0 MPa. There are very few examples of catalytic systems that operate
[a] Dr. T. Werner, H. Bttner Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert-Einstein-Strasse 29a, 18059 Rostock (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402477.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
efficiently below 100 8C, and such systems often require co-catalysts, solvents, and/or multistep catalyst preparation. There have been several reports on synergistic effects in catalytic systems.[13] Especially in the presence of hydrogen-bond donors such as alcohols,[9a, 13e, 14] carboxylic acids,[15] silanols,[13d] chitosan,[16] cellulose,[17] b-cyclodextrin,[18] amino acids,[19] and amino alcohols[20] the addition of CO2 and epoxides to the corresponding cyclic carbonate was significantly accelerated. We envisioned that bifunctional phosphonium salts bearing hydrogen-donating alcohol functions should display a similar effect. To the best of our knowledge the application of bifunctional catalysts based on phosphorus derivatives has not been described so far. Thus, we established a straightforward synthesis of tunable bifunctional phosphonium salts 3 (Scheme 1). The simple change of either the phosphine 1 or the halo derivative 2 leads to a series of air-stable and structurally diverse catalysts 3.[21]
Scheme 1. Synthesis of bifunctional phosphonium catalysts 3.
These catalysts can activate the epoxide in dual mode via hydrogen-bond donation of the alcohol moiety and electrostatic interactions of the phosphonium salt center. This should enable an easier opening of the epoxide by the counter anion of the salts 3.[21] Thus, it allows to perform the reaction under mild conditions. In this context the bifunctional organocatalysts 3 were employed in the model reaction of butylene oxide (4 a) and CO2 producing 5 a to deduce structure–activity relationships as well as the most active catalysts 3 (Table 1). While trimethylphosphine-based bifunctional catalyst 3 a–c were present, only poor yields of cyclic carbonate 5 a were observed (entries 1–3). However, the yield increased from Cl < Br < I , emphasizing the impact of the halide. Tri-n-butyl substituted phosphonium salts 3 d–f showed significantly higher yields in the model reaction than the methyl-substituted analogues (entries 4–6). Amongst these, again the iodide salt tri-n-butyl-(2hydroxyethyl) phosphonium iodide (3 f) gave the best result with a yield of 95 % of cyclic carbonate 5 a. Additionally, we were interested in effects concerning the distance between nucleophilic and electrophilic groups. When the linkage between electrophilic and nucleophilic center was extended to propyl to form 3 g, the catalytic activity decreased slightly (entry 7). To emphasize the catalytic activity of the bifunctional catalyst we employed simple monofunctional tetra-n-butyl phosphonium salts 6 a–c (entries 8–10). In the presence of the best monoChemSusChem 2014, 7, 3268 – 3271
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Table 1. Model reaction and screening results for the catalysts 3 and 6.
Entry
Yield 5 a [%][a]
Catalyst
1
3a
0
2
3b
2
3
3c
21
4
3d
57
5
3e
86
6
3f
95
7
3g
78
8
6a
36 25
9
6b
10
6c
19
11
7a
51
12
7b
24
13
7c
19
[a] Yields were determined by GC and hexadecane as internal standard.
functional catalyst 6 a only a poor yield of 36 % was observed. The utilization of the corresponding simple ammonium salt 7 a led to a somewhat improved yield while 7 b and 7 c gave results comparable to their phosphonium counterparts (entries 11–13). In contrast to bifunctional organocatalyst 3 d–f the yield increased from I < Br < Cl and followed the order of nucleophilicity. When bifunctional organocatalysts 3 were employed the order turned to Cl < Br < I , which presumably can be explained by stronger hydrogen-bond interactions of the alcohol moiety and the chloride compared to the bromide and iodide. Nevertheless, these results confirmed the acceleration of the CO2 addition to epoxide 4 a due to synergistic effects. Catalyst 3 f was identified as the most active one and chosen for optimizing reaction conditions of the model reaction producing 1,2-butylene carbonate (5 a) from 4 a and CO2 (Table 2).
Table 2. Optimization of the reaction conditions employing catalyst 3 f.[a] Entry
T [8C]
p(CO2)[b] [MPa]
t [h]
Yield 5 a[c] [%]
1 2 3 4 5 6 7 8 9 10
60 60 80 90 90 90 90 90 90 90
1.0 1.0 1.0 1.0 1.0 1.0 0.3 0.5 2.0 3.0
2 24 2 2 1 3 3 3 3 3
45 97[d] 85 95 85 98 89 96 96 97
[a] Reaction conditions: 4 a (2.00 g, 27.7 mmol), 2 mol % 3 f (208 mg, 0.556 mmol). [b] Initial pressure; after reaching the desired temperature p(CO2) = const. [c] Yields were determined by GC-FID. [d] Isolated yield.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
First, the effect of the reaction temperature was examined at p(CO2) = 1.0 MPa and 2 h (entries 1–4). The yield of 1,2-butylene carbonate (5 a) increased significantly with increasing temperature from 60–90 8C. Even at a low temperature of 60 8C a yield of 97 % of 5 a was observed when prolonging the reaction time to 24 h (entry 2). The coupling reaction was most sensitive to the temperature and 90 8C was found to be optimal for the reaction. Shortening the reaction time to 1 h gave also a good yield of 85 % but for full conversion 3 h was necessary and 98 % of 5 a was obtained (entries 5–6). The effect of the initial CO2 pressure was also investigated in the range of 0.3–2.0 MPa (entries 7–10). Even at a low pressure of 0.3 MPa a good yield of 89 % of 5 a was observed. However, to preferably obtain quantitative conversion the reaction conditions were set to 1.0 MPa, 90 8C and 3 h reaction to evaluate the substrate scope (Table 3).
Table 3. Substrate scope for catalyst 3 f under optimized reaction conditions.[a] Entry
Yield [%][b]
Substrate
Product
1
4a
5a
97
2
4b
5b
99
3
4c
5c
99
4
4d
5d
97
5
4e
5e
99
6
4f
5f
93
7
4g
5g
97
8
4h
5h
98
9
4i
5i
99
10
4j
5j
17 (69)[c]
[a] Reaction conditions: epoxide 4 (2.00 g), 3 f (2 mol %), 90 8C, 3 h p(CO2) = 1.0 MPa. [b] Isolated yields. [c] 120 8C, 6 h, p(CO2) = 4.0 MPa.
Tri-n-butyl-(2-hydroxyethyl) phosphonium iodide (3 f) catalyzed effectively the coupling reaction of terminal alkyl- and phenyl-substituted epoxide 4 a–d to the corresponding cyclic carbonate 5 a–d in quantitative yields (Table 3; entries 1–4). The catalytic system was applicable for electron-withdrawing substituted epoxide 4 e and tolerated even the olefin side chain in epoxide 4 f. The corresponding carbonates 5 e–f were produced in excellent yields (entries 5–6). Furthermore, several glycidyl ether and ester derivatives 4 g–i were converted with CO2 (entries 7–9). Noteworthy, even the methyl methacrylateChemSusChem 2014, 7, 3268 – 3271
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CHEMSUSCHEM COMMUNICATIONS substituted carbonate 4 i was obtained in 99 % yield although it is sensitive to a polymerization side reaction.[22] Internal epoxides are known to be challenging substrates in this reaction. Nevertheless, cyclohexene oxide (4 j) could be converted in 17 % to the carbonate 5 j under optimized reaction conditions. Under more drastic conditions of 120 8C, 4.0 MPa, and 6 h the yield was increased up to 69 %. In all reactions cyclic carbonates were formed exclusively and no side products were observed. It is worth mentioning that the catalyst 3 f could be easily removed by filtration over silica gel to obtain analytically pure carbonates 5. In addition, we were interested in the catalytic activity of the organocatalyst 3 f even in multigram scale. Hence, the model reaction of 1,2-epoxybutane (4 a) with carbon dioxide was scaled up by a factor 20 and performed in a 100 mL stainless steel reactor and monitored by in situ ATR-FTIR spectroscopy (Figure 1).
Figure 1. Conversion of 1,2-epoxybutane (4 a) with CO2 followed by in situ ATR-FTIR spectroscopy. Reaction conditions: 1,2-epoxybutane (4 a, 38.7 g, 0.537 mol), 1 mol % 3 f (2.81 g, 7.51 mmol) 90 8C, p(CO2) = 1.0 MPa.
From offline ATR-FTIR spectroscopy we determined specific bands of 1,2-epoxybutane (4 a) and 1,2-butylene carbonate (5 a). According to these measurements, we were able to monitor the reaction by reference to the intensity of the carbonyl vibration band at 1790 cm 1 as well as the bending vibrations at 1190 and 1060 cm 1. The rapid increases of the intensity correlate with the results from optimizing reaction conditions after 1 h at 90 8C and 1.0 MPa (Table 2, entry 4). In the same time period the intensity of the corresponding substrate bands at 904 and 833 cm 1 decreased. A maximum of the intensity of 1,2-butylene carbonate (5 a) bands was observed after 255 min and the reaction was stopped. After work up 84 % of cyclic carbonate 5 a was isolated. We were also interested in the recycling of 3 f (Figure 2). The catalyst was separated from the product 5 a by Kugelrohr distillation and reused in five consecutive runs. In the first three runs no loss of activity was observed and 5 a was obtained in excellent yields up to 97 %. In the following two runs the yield decreased to 76 and 65 %, respectively. We assume partial decomposition of the catalyst 3 f due to the thermal stress during Kugelrohr distillation. Tri-n-butylphosphine and the corresponding phosphine oxide were identified by GC-MS as the major decomposition products. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Recycling of the catalyst 3 f in the coupling reaction of 4 a and CO2 under optimized reaction conditions: 90 8C, 3 h, p(CO2) = 1.0 MPa.
In conclusion, we present a straightforward route to tunable phosphorus-based bifunctional organocatalysts 3. The prepared salts are employed in a model reaction of CO2 with 1,2-butylene oxide (4 a). We determine structure–activity relationships and identify the most active catalyst. The catalyst activity is significantly influenced by the substituents at the phosphorus center and the anion. A synergistic effect of the catalyst shows a superior catalytic activity compared to simple tetra-n-butyl phosphonium salts 6. In the presence of the most active organocatalyst 3 f full conversion of several epoxides is achieved under solvent-free reaction conditions at 90 8C and 1.0 MPa within 3 h. In general, the corresponding products are obtained in excellent yields and can be easily isolated by filtration over silica. Notably, even at a low temperature of 60 8C an almost quantitative yield of 97 % is isolated. The protocol is even applicable on a multigram scale and can be easily monitored by using in situ ATR-FTIR spectroscopy. When examining the reusability of the catalyst 3 f in five consecutive runs, excellent yields up to 97 % of 1,2-butylene carbonate (5 a) are obtained in the first three runs.
Experimental Section In a typical coupling reaction a 45 mL stainless steel reactor was charged with catalyst 3 f (208 mg, 0.556 mmol) and 1,2-butylene oxide (4 a, 2.00 g, 27.7 mmol). The reactor was purged once with CO2, pressurized to p(CO2) = 1.0 MPa and heated to 90 8C for 3 h. After reaching the desired reaction temperature the pressure was kept constant. Subsequently, the reactor was cooled to ambient temperature with an ice bath and CO2 was released slowly. The reaction mixture was filtered over silica gel (CH2Cl2) and all volatiles were removed under reduced pressure. The product 5 a was obtained as a colorless oil (3.15 g, 27.2 mmol, 97 %).
Acknowledgements We wish to thank the Federal Ministry of Research and Education (BMBF) for financial support (Chemische Prozesse und stoffliche Nutzung von CO2 : Technologien fr Nachhaltigkeit und Klimaschutz, grant 01 RC 1004A) as well as Prof. Beller for support and advice.
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CHEMSUSCHEM COMMUNICATIONS Keywords: carbon dioxide fixation · coupling reactions · carbonates · organocatalysis · phosphorus [1] a) I. Omae, Coord. Chem. Rev. 2012, 256, 1384 – 1405; b) M. Peters, B. Kçhler, W. Kuckshinrichs, W. Leitner, P. Markewitz, T. E. Mller, ChemSusChem 2011, 4, 1216 – 1240; c) T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107, 2365 – 2387; d) X. Yin, J. R. Moss, Coord. Chem. Rev. 1999, 181, 27 – 59. [2] a) M. Aresta, Carbon Dioxide as Chemical Feedstock, Wiley-VCH, Weinheim, 2010; b) M. Aresta, A. Dibenedetto, Dalton Trans. 2007, 2975 – 2992. [3] a) X.-B. Lu, D. J. Darensbourg, Chem. Soc. Rev. 2012, 41, 1462 – 1484; b) M. North, R. Pasquale, C. Young, Green Chem. 2010, 12, 1514 – 1539; c) A. Decortes, A. M. Castilla, A. W. Kleij, Angew. Chem. Int. Ed. 2010, 49, 9822 – 9837; Angew. Chem. 2010, 122, 10016 – 10032; d) T. Sakakura, K. Kohno, Chem. Commun. 2009, 1312 – 1330. [4] a) B. Schffner, F. Schffner, S. P. Verevkin, A. Bçrner, Chem. Rev. 2010, 110, 4554 – 4581; b) J. Bayardon, J. Holz, B. Schffner, V. Andrushko, S. Verevkin, A. Preetz, A. Bçrner, Angew. Chem. Int. Ed. 2007, 46, 5971 – 5974; Angew. Chem. 2007, 119, 6075 – 6078; c) J. Bayardon, J. Holz, B. Schffner, V. Andrushko, S. Verevkin, A. Preetz, A. Bçrner, Angew. Chem. 2007, 119, 6075 – 6078. [5] a) O. Crowther, D. Keeny, D. M. Moureau, B. Meyer, M. Salomon, M. Hendrickson, J. Power Sources 2012, 202, 347 – 351; b) W. H. Meyer, Adv. Mater. 1998, 10, 439 – 448. [6] J. H. Clements, Ind. Eng. Chem. Res. 2003, 42, 663 – 674. [7] A.-A. G. Shaikh, S. Sivaram, Chem. Rev. 1996, 96, 951 – 976. [8] a) K. Motokura, S. Itagaki, Y. Iwasawa, A. Miyaji, T. Baba, Green Chem. 2009, 11, 1876 – 1880; b) W.-L. Wong, P.-H. Chan, Z.-Y. Zhou, K.-H. Lee, K.-C. Cheung, K.-Y. Wong, ChemSusChem 2008, 1, 67 – 70. [9] a) J. Sun, S. Zhang, W. Cheng, J. Ren, Tetrahedron Lett. 2008, 49, 3588 – 3591; b) Y. J. Kim, R. S. Varma, J. Org. Chem. 2005, 70, 7882 – 7891; c) J. Sun, S.-I. Fujita, M. Arai, J. Organomet. Chem. 2005, 690, 3490 – 3497; d) H. Kawanami, A. Sasaki, K. Matsui, Y. Ikushima, Chem. Commun. 2003, 896 – 897; e) H. S. Kim, J. J. Kim, H. Kim, H. G. Jang, J. Catal. 2003, 220, 44 – 46. [10] a) C. J. Whiteoak, A. Nova, F. Maseras, A. W. Kleij, ChemSusChem 2012, 5, 2032 – 2038; b) A. Decortes, M. Martinez Belmonte, J. Benet-Buchholz, A. W. Kleij, Chem. Commun. 2010, 46, 4580 – 4582; c) M. M. Dharman, J.I. Yu, J.-Y. Ahn, D.-W. Park, Green Chem. 2009, 11, 1754 – 1757; d) J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, Green Chem. 2009, 11, 1031 – 1036; e) J. Melndez, M. North, P. Villuendas, Chem. Commun. 2009, 2577 – 2579; f) M. North, R. Pasquale, Angew. Chem. Int. Ed. 2009, 48, 2946 – 2948; Angew. Chem. 2009, 121, 2990 – 2992; g) J. Sun, S.-I. Fujita, F. Zhao, M. Arai, Appl. Catal. A 2005, 287, 221 – 226; h) X.-B. Lu, B. Liang, Y.-J. Zhang, Y.-Z. Tian, Y.-M. Wang, C.-X. Bai, H. Wang, R. Zhang, J. Am. Chem. Soc. 2004, 126, 3732 – 3733; i) V. Cal, A. Nacci, A. Monopoli, A. Fanizzi, Org. Lett. 2002, 4, 2561 – 2563; j) T. Nishikubo, A. Kameyama, J. Yamashita, T. Fukumitsu, C. Maejima, M. Tomoi, J. Polym. Sci. Polym. Chem. 1995, 33, 1011 – 1017.
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Received: May 28, 2014 Revised: June 23, 2014 Published online on October 10, 2014
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Phosphorus-based Bifunctional Organocatalysts for the Addition of Carbon Dioxide and Epoxides Thomas Werner* and Hendrik Bttner[a] Bifunctional phosphonium salts were synthesized and employed as organocatalysts for the atom efficient synthesis of cyclic carbonates from CO2 and epoxides for the first time. These catalysts were obtained in high yields by a modular, straightforward one-step synthesis. The hydrogen-bond donating alcohol function in the side chain leads to a synergistic effect accelerating the catalytic reaction. The desired cyclic carbonates are obtained in high yields and selectivity under solvent-free reaction conditions without the use of any co-catalyst. Under optimized reaction conditions various epoxides were converted to the corresponding cyclic carbonates in excellent yields. The products were obtained analytically pure after simple filtration over a silica gel pad. This protocol is even applicable for a multigram reaction scale. Moreover, the catalysts could be easily recovered and reused up to five times.
In the past two decades transformations employing carbon dioxide as a readily available, inexpensive, nontoxic, and abundant carbon source have been studied extensively.[1] However, carbon dioxide is the end product of thermal combustion and its thermodynamic stability as well as its kinetic inertia are challenging for CO2 utilization.[2] Hence, the development of suitable catalysts creating value-added products from carbon dioxide is of particular interest. The atom-efficient addition of CO2 and epoxides produces cyclic carbonates and provides efficient carbon dioxide fixation as well as the production of valuable outputs.[3] Cyclic carbonates serve as polar green solvents and display some outstanding properties such as a high boiling point, being odorless, and low toxicity.[4] In addition, carbonates are applied as electrolytes in lithium-ion batteries,[5] as monomers in polymerization reactions,[6] and as intermediates for fine chemicals.[7] There are numerous reports on catalysts for the coupling of CO2 with epoxides. Especially organocatalysts based on pyridinium,[8] imidazolium,[9] ammonium,[10] and phosphonium salts[11] as well as carbenes[12] have been studied extensively as both single catalysts and co-catalysts. However, under metalfree conditions these catalysts usually require harsh reaction conditions: temperatures > 100 8C and/or pressures > 2.0 MPa. There are very few examples of catalytic systems that operate
[a] Dr. T. Werner, H. Bttner Leibniz-Institut fr Katalyse e.V. an der Universitt Rostock Albert-Einstein-Strasse 29a, 18059 Rostock (Germany) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402477.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
efficiently below 100 8C, and such systems often require co-catalysts, solvents, and/or multistep catalyst preparation. There have been several reports on synergistic effects in catalytic systems.[13] Especially in the presence of hydrogen-bond donors such as alcohols,[9a, 13e, 14] carboxylic acids,[15] silanols,[13d] chitosan,[16] cellulose,[17] b-cyclodextrin,[18] amino acids,[19] and amino alcohols[20] the addition of CO2 and epoxides to the corresponding cyclic carbonate was significantly accelerated. We envisioned that bifunctional phosphonium salts bearing hydrogen-donating alcohol functions should display a similar effect. To the best of our knowledge the application of bifunctional catalysts based on phosphorus derivatives has not been described so far. Thus, we established a straightforward synthesis of tunable bifunctional phosphonium salts 3 (Scheme 1). The simple change of either the phosphine 1 or the halo derivative 2 leads to a series of air-stable and structurally diverse catalysts 3.[21]
Scheme 1. Synthesis of bifunctional phosphonium catalysts 3.
These catalysts can activate the epoxide in dual mode via hydrogen-bond donation of the alcohol moiety and electrostatic interactions of the phosphonium salt center. This should enable an easier opening of the epoxide by the counter anion of the salts 3.[21] Thus, it allows to perform the reaction under mild conditions. In this context the bifunctional organocatalysts 3 were employed in the model reaction of butylene oxide (4 a) and CO2 producing 5 a to deduce structure–activity relationships as well as the most active catalysts 3 (Table 1). While trimethylphosphine-based bifunctional catalyst 3 a–c were present, only poor yields of cyclic carbonate 5 a were observed (entries 1–3). However, the yield increased from Cl < Br < I , emphasizing the impact of the halide. Tri-n-butyl substituted phosphonium salts 3 d–f showed significantly higher yields in the model reaction than the methyl-substituted analogues (entries 4–6). Amongst these, again the iodide salt tri-n-butyl-(2hydroxyethyl) phosphonium iodide (3 f) gave the best result with a yield of 95 % of cyclic carbonate 5 a. Additionally, we were interested in effects concerning the distance between nucleophilic and electrophilic groups. When the linkage between electrophilic and nucleophilic center was extended to propyl to form 3 g, the catalytic activity decreased slightly (entry 7). To emphasize the catalytic activity of the bifunctional catalyst we employed simple monofunctional tetra-n-butyl phosphonium salts 6 a–c (entries 8–10). In the presence of the best monoChemSusChem 2014, 7, 3268 – 3271
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Table 1. Model reaction and screening results for the catalysts 3 and 6.
Entry
Yield 5 a [%][a]
Catalyst
1
3a
0
2
3b
2
3
3c
21
4
3d
57
5
3e
86
6
3f
95
7
3g
78
8
6a
36 25
9
6b
10
6c
19
11
7a
51
12
7b
24
13
7c
19
[a] Yields were determined by GC and hexadecane as internal standard.
functional catalyst 6 a only a poor yield of 36 % was observed. The utilization of the corresponding simple ammonium salt 7 a led to a somewhat improved yield while 7 b and 7 c gave results comparable to their phosphonium counterparts (entries 11–13). In contrast to bifunctional organocatalyst 3 d–f the yield increased from I < Br < Cl and followed the order of nucleophilicity. When bifunctional organocatalysts 3 were employed the order turned to Cl < Br < I , which presumably can be explained by stronger hydrogen-bond interactions of the alcohol moiety and the chloride compared to the bromide and iodide. Nevertheless, these results confirmed the acceleration of the CO2 addition to epoxide 4 a due to synergistic effects. Catalyst 3 f was identified as the most active one and chosen for optimizing reaction conditions of the model reaction producing 1,2-butylene carbonate (5 a) from 4 a and CO2 (Table 2).
Table 2. Optimization of the reaction conditions employing catalyst 3 f.[a] Entry
T [8C]
p(CO2)[b] [MPa]
t [h]
Yield 5 a[c] [%]
1 2 3 4 5 6 7 8 9 10
60 60 80 90 90 90 90 90 90 90
1.0 1.0 1.0 1.0 1.0 1.0 0.3 0.5 2.0 3.0
2 24 2 2 1 3 3 3 3 3
45 97[d] 85 95 85 98 89 96 96 97
[a] Reaction conditions: 4 a (2.00 g, 27.7 mmol), 2 mol % 3 f (208 mg, 0.556 mmol). [b] Initial pressure; after reaching the desired temperature p(CO2) = const. [c] Yields were determined by GC-FID. [d] Isolated yield.
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First, the effect of the reaction temperature was examined at p(CO2) = 1.0 MPa and 2 h (entries 1–4). The yield of 1,2-butylene carbonate (5 a) increased significantly with increasing temperature from 60–90 8C. Even at a low temperature of 60 8C a yield of 97 % of 5 a was observed when prolonging the reaction time to 24 h (entry 2). The coupling reaction was most sensitive to the temperature and 90 8C was found to be optimal for the reaction. Shortening the reaction time to 1 h gave also a good yield of 85 % but for full conversion 3 h was necessary and 98 % of 5 a was obtained (entries 5–6). The effect of the initial CO2 pressure was also investigated in the range of 0.3–2.0 MPa (entries 7–10). Even at a low pressure of 0.3 MPa a good yield of 89 % of 5 a was observed. However, to preferably obtain quantitative conversion the reaction conditions were set to 1.0 MPa, 90 8C and 3 h reaction to evaluate the substrate scope (Table 3).
Table 3. Substrate scope for catalyst 3 f under optimized reaction conditions.[a] Entry
Yield [%][b]
Substrate
Product
1
4a
5a
97
2
4b
5b
99
3
4c
5c
99
4
4d
5d
97
5
4e
5e
99
6
4f
5f
93
7
4g
5g
97
8
4h
5h
98
9
4i
5i
99
10
4j
5j
17 (69)[c]
[a] Reaction conditions: epoxide 4 (2.00 g), 3 f (2 mol %), 90 8C, 3 h p(CO2) = 1.0 MPa. [b] Isolated yields. [c] 120 8C, 6 h, p(CO2) = 4.0 MPa.
Tri-n-butyl-(2-hydroxyethyl) phosphonium iodide (3 f) catalyzed effectively the coupling reaction of terminal alkyl- and phenyl-substituted epoxide 4 a–d to the corresponding cyclic carbonate 5 a–d in quantitative yields (Table 3; entries 1–4). The catalytic system was applicable for electron-withdrawing substituted epoxide 4 e and tolerated even the olefin side chain in epoxide 4 f. The corresponding carbonates 5 e–f were produced in excellent yields (entries 5–6). Furthermore, several glycidyl ether and ester derivatives 4 g–i were converted with CO2 (entries 7–9). Noteworthy, even the methyl methacrylateChemSusChem 2014, 7, 3268 – 3271
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CHEMSUSCHEM COMMUNICATIONS substituted carbonate 4 i was obtained in 99 % yield although it is sensitive to a polymerization side reaction.[22] Internal epoxides are known to be challenging substrates in this reaction. Nevertheless, cyclohexene oxide (4 j) could be converted in 17 % to the carbonate 5 j under optimized reaction conditions. Under more drastic conditions of 120 8C, 4.0 MPa, and 6 h the yield was increased up to 69 %. In all reactions cyclic carbonates were formed exclusively and no side products were observed. It is worth mentioning that the catalyst 3 f could be easily removed by filtration over silica gel to obtain analytically pure carbonates 5. In addition, we were interested in the catalytic activity of the organocatalyst 3 f even in multigram scale. Hence, the model reaction of 1,2-epoxybutane (4 a) with carbon dioxide was scaled up by a factor 20 and performed in a 100 mL stainless steel reactor and monitored by in situ ATR-FTIR spectroscopy (Figure 1).
Figure 1. Conversion of 1,2-epoxybutane (4 a) with CO2 followed by in situ ATR-FTIR spectroscopy. Reaction conditions: 1,2-epoxybutane (4 a, 38.7 g, 0.537 mol), 1 mol % 3 f (2.81 g, 7.51 mmol) 90 8C, p(CO2) = 1.0 MPa.
From offline ATR-FTIR spectroscopy we determined specific bands of 1,2-epoxybutane (4 a) and 1,2-butylene carbonate (5 a). According to these measurements, we were able to monitor the reaction by reference to the intensity of the carbonyl vibration band at 1790 cm 1 as well as the bending vibrations at 1190 and 1060 cm 1. The rapid increases of the intensity correlate with the results from optimizing reaction conditions after 1 h at 90 8C and 1.0 MPa (Table 2, entry 4). In the same time period the intensity of the corresponding substrate bands at 904 and 833 cm 1 decreased. A maximum of the intensity of 1,2-butylene carbonate (5 a) bands was observed after 255 min and the reaction was stopped. After work up 84 % of cyclic carbonate 5 a was isolated. We were also interested in the recycling of 3 f (Figure 2). The catalyst was separated from the product 5 a by Kugelrohr distillation and reused in five consecutive runs. In the first three runs no loss of activity was observed and 5 a was obtained in excellent yields up to 97 %. In the following two runs the yield decreased to 76 and 65 %, respectively. We assume partial decomposition of the catalyst 3 f due to the thermal stress during Kugelrohr distillation. Tri-n-butylphosphine and the corresponding phosphine oxide were identified by GC-MS as the major decomposition products. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Recycling of the catalyst 3 f in the coupling reaction of 4 a and CO2 under optimized reaction conditions: 90 8C, 3 h, p(CO2) = 1.0 MPa.
In conclusion, we present a straightforward route to tunable phosphorus-based bifunctional organocatalysts 3. The prepared salts are employed in a model reaction of CO2 with 1,2-butylene oxide (4 a). We determine structure–activity relationships and identify the most active catalyst. The catalyst activity is significantly influenced by the substituents at the phosphorus center and the anion. A synergistic effect of the catalyst shows a superior catalytic activity compared to simple tetra-n-butyl phosphonium salts 6. In the presence of the most active organocatalyst 3 f full conversion of several epoxides is achieved under solvent-free reaction conditions at 90 8C and 1.0 MPa within 3 h. In general, the corresponding products are obtained in excellent yields and can be easily isolated by filtration over silica. Notably, even at a low temperature of 60 8C an almost quantitative yield of 97 % is isolated. The protocol is even applicable on a multigram scale and can be easily monitored by using in situ ATR-FTIR spectroscopy. When examining the reusability of the catalyst 3 f in five consecutive runs, excellent yields up to 97 % of 1,2-butylene carbonate (5 a) are obtained in the first three runs.
Experimental Section In a typical coupling reaction a 45 mL stainless steel reactor was charged with catalyst 3 f (208 mg, 0.556 mmol) and 1,2-butylene oxide (4 a, 2.00 g, 27.7 mmol). The reactor was purged once with CO2, pressurized to p(CO2) = 1.0 MPa and heated to 90 8C for 3 h. After reaching the desired reaction temperature the pressure was kept constant. Subsequently, the reactor was cooled to ambient temperature with an ice bath and CO2 was released slowly. The reaction mixture was filtered over silica gel (CH2Cl2) and all volatiles were removed under reduced pressure. The product 5 a was obtained as a colorless oil (3.15 g, 27.2 mmol, 97 %).
Acknowledgements We wish to thank the Federal Ministry of Research and Education (BMBF) for financial support (Chemische Prozesse und stoffliche Nutzung von CO2 : Technologien fr Nachhaltigkeit und Klimaschutz, grant 01 RC 1004A) as well as Prof. Beller for support and advice.
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Received: May 28, 2014 Revised: June 23, 2014 Published online on October 10, 2014
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