Potassium Iodide–Tetraethylene Glycol Complex as a Practical

Letter pubs.acs.org/journal/ascecg

Potassium Iodide−Tetraethylene Glycol Complex as a Practical Catalyst for CO2 Fixation Reactions with Epoxides under Mild Conditions Shiho Kaneko and Seiji Shirakawa* Department of Environmental Science, Graduate School of Fisheries and Environmental Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan

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S Supporting Information *

ABSTRACT: A practical and efficient method for the synthesis of cyclic carbonates from epoxides and CO2 under mild reaction conditions was achieved via the use of a potassium iodide (KI)− tetraethylene glycol complex as a readily available and economical catalyst. The effects of glycols and alkali metal salts were investigated in the present work to clarify the importance of both KI and tetraethylene glycol. Scalability and reusability of this catalytic system were also demonstrated. KEYWORDS: CO2 fixation, Potassium iodide, Tetraethylene glycol, Epoxides, Cyclic carbonates, Recycle

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INTRODUCTION

Scheme 1. Synthesis of Cyclic Carbonates 2 under Mild Reaction Conditions

Carbon dioxide (CO2) is recognized as an ideal renewable C1 feedstock for chemical synthesis, because it is inexpensive, abundant, and nontoxic. Hence, the development of an efficient catalytic process for CO 2 transformations into useful compounds has become a topic of significant interest in the field of green and sustainable chemistry.1−6 Among CO2 transformations, the synthesis of cyclic carbonates 2 from epoxide 1 and CO2 has been most extensively studied, because of their high utility in the industrial process.7−12 Although numerous catalysts and catalytic systems have been developed to promote this reaction, harsh reaction conditions (pressure of >10 atm, reaction temperature of >100 °C) are often required to produce cyclic carbonates 2 in satisfactory yields.13−18 To overcome this problem and realize efficient CO2 fixation under mild reaction conditions, bifunctional quaternary onium iodide catalysts possessing a hydroxy group have recently been developed (Scheme 1).19−27 These bifunctional onium salts efficiently promote CO2 fixation under relatively mild conditions, and the importance of the iodide counteranion and the hydroxy group of the catalyst was clearly demonstrated in these reports. The design of these effective bifunctional onium salt catalysts piqued our interest in the catalytic ability of a potassium iodide (KI)−tetraethylene glycol complex as a readily available and economical catalyst.28−44 It is known that tetraethylene glycol forms a complex with potassium salts45−48 that significantly increases the nucleophilic ability of the iodide anion.49,50 Furthermore, the terminal hydroxy groups of tetraethylene glycol activate epoxide substrates via hydrogenbonding interactions (Scheme 1). Herein, we report a practical method for the synthesis of cyclic carbonates 2 in the presence © 2017 American Chemical Society

of a KI−tetraethylene glycol complex catalyst under mild reaction conditions (atmospheric pressure, 40 °C). Received: February 1, 2017 Revised: February 23, 2017 Published: February 27, 2017 2836

DOI: 10.1021/acssuschemeng.7b00324 ACS Sustainable Chem. Eng. 2017, 5, 2836−2840

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ACS Sustainable Chemistry & Engineering

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RESULTS AND DISCUSSION Our initial aim was to clarify the utility of a KI−tetraethylene glycol complex catalyst in the synthesis of a cyclic carbonate under mild conditions. The effects of glycols and alkali metal salts were investigated in the reaction of styrene oxide 1a as a model substrate (Scheme 2). When a mixture of styrene oxide

promotion of the reaction. In addition, the catalytic activity of an 18-crown-6 complex with KI, which is a known catalyst system for this reaction,41−44 was also compared with that of the tetraethylene glycol complex. The 18-crown-6 complex showed reactivity that was inferior to that of the tetraethylene glycol complex (24% vs 70% yields, respectively). The effect of alkali metal salts was also investigated, and potassium chloride (KCl) and potassium bromide (KBr) complexes with tetraethylene glycol showed quite low reactivities (∼0 and 4% yields, respectively). On the other hand, a sodium iodide (NaI) complex gave comparable results (68% yield) in the case of the KI complex. Similar trends were observed in previous reports using alkali metal halides, and the positive effect of iodide was explained by the ability of higher leaving groups (intermediate C in Scheme 3) and by the lower coordination ability of hydroxy groups (intermediate A in Scheme 3).11

Scheme 2. Effects of Glycols and Alkali-Metal Saltsa

Scheme 3. Assumed Catalytic Cycle

a

Reaction conditions: 1a (5.0 mmol), glycol (0.50 mmol, 10 mol %), alkali metal salt (0.50 mmol, 10 mol %), CO2 (1 atm, using a balloon), mesitylene (0.25 mmol, internal standard), room temperature (25 °C), 24 h. Yield was determined via 1H NMR analysis using mesitylene as an internal standard. bYield of isolated product 2a.

Based on the results in Scheme 2, the assumed catalytic cycle for the present reaction was proposed in Scheme 3, as follows.7−12 Epoxide 1 is activated via double hydrogen bonding with hydroxy groups of tetraethylene glycol (intermediate A). The activated epoxide then undergoes nucleophilic attack from an iodide anion to form intermediate B. The reactive alkoxide in intermediate B attacks CO2 to yield intermediate C. The intramolecular ring-closing of intermediate C affords cyclic carbonate 2 with the regeneration of the KI− tetraethylene glycol complex catalyst. With the important details of this reaction system in hand, the substrate scope of epoxides 1 was investigated using a KI− tetraethylene glycol complex catalyst (Scheme 4). Not only styrene oxide derivatives but also epoxides with a simple alkyl chain gave the corresponding cyclic carbonates (2a−2e) in high-to-quantitative yields under mild reaction conditions (atmospheric pressure, 40 °C). Epoxides containing various functional groups were also employed in the reaction to give corresponding products (2f−2j) in high-to-quantitative yields. The reaction with disubstituted epoxides such as a transstilbene oxide 3 was also examined (Scheme 5). The reaction proceeded slowly to give product 4, even at higher temperature (80 °C).

1a and KI (10 mol %) alone was stirred for 24 h at room temperature (25 °C) under a CO2 atmosphere (1 atm, using a balloon), almost no reaction was observed, mainly because of the low reactivity and solubility of KI under the reaction conditions. Then, the effect of glycols was investigated under the same reaction conditions. Although the reaction with simple ethylene glycol and KI gave the product 2a in only a low yield (6%), a KI−diethylene glycol catalyst moderately promoted the reaction to give product 2a in a 45% yield. Further enhancements of the reactivities were observed when KI complexes of triethylene glycol or tetraethylene glycol were employed as catalysts (67% and 70% yields, respectively). Note that the reactions with these catalysts at 40 °C gave product 2a in almost quantitative isolated yields (97% and 99%, respectively). To clarify the role of hydroxy groups in tetraethylene glycol, we also examined the reactions with tetraethylene glycol dimethyl ether and monomethyl ether. As expected, lower catalytic activities were observed in these reactions, and the reactions gave product 2a in lower yields (17% yield with dimethyl ether, 34% yield with monomethyl ether). These results clearly indicated that the two hydroxy groups of tetraethylene glycol were important for the efficient 2837

DOI: 10.1021/acssuschemeng.7b00324 ACS Sustainable Chem. Eng. 2017, 5, 2836−2840

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ACS Sustainable Chemistry & Engineering Scheme 4. Substrate Scopea

The present CO2 fixation reaction was conducted on a 50 mmol scale to demonstrate the practicability of the catalytic system using the KI−tetraethylene glycol complex (Scheme 7). Scheme 7. Larger-Scale Synthesis

A mixture of styrene oxide 1a (50 mmol), KI (5 mmol), and tetraethylene glycol (5 mmol) was stirred at 40 °C for 48 h under a CO2 atmosphere (1 atm, using a balloon), and the target product 2a was obtained in a quantitative yield (8.2 g). Practicability of the reaction system was further demonstrated in experiments for the recycling of the KI−tetraethylene glycol complex catalyst (Scheme 8). After the reaction of Scheme 8. Recycle Experiments

a

Reaction conditions: 1 (5.0 mmol), tetraethylene glycol (0.50 mmol, 10 mol %), KI (0.50 mmol, 10 mol %), CO2 (1 atm, using a balloon), 40 °C, 24 h. Yield of isolated product 2. bReaction temperature = 50 °C.

Scheme 5. Reaction with trans-Stilbene Oxide

To expand the utility of the present reaction system using the KI−tetraethylene glycol complex, enantiopure epoxides (1h and 1k) were submitted to obtain optically active cyclic carbonates (Scheme 6). To our delight, cyclic carbonates 2h and 2k were obtained in excellent yields with no loss of enantiomeric purity. epoxide 1l with a fresh KI−tetraethylene glycol complex catalyst, the conversion of 1l to product 2l was determined via 1 H NMR analysis. The product 2l was then isolated by distillation under reduced pressure. To the resultant residual catalyst in the reaction flask was added a new batch of substrate 1l, and the mixture was stirred at 40 °C for 24 h under a CO2 atmosphere (1 atm, using a balloon). This procedure was repeated, and after recycling the catalyst 10 times, we observed no significant loss of activity.

Scheme 6. Synthesis of Optically Active Carbonates

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CONCLUSIONS In summary, we successfully developed a practical method for the synthesis of industrially important cyclic carbonates 2 via a reaction of epoxides 1 and CO2 under mild reaction conditions, using a KI−tetraethylene glycol complex catalyst. The effects of 2838

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(3) Kielland, N.; Whiteoak, C. J.; Kleij, A. W. Stereoselective synthesis with carbon dioxide. Adv. Synth. Catal. 2013, 355, 2115− 2138. (4) Maeda, C.; Miyazaki, Y.; Ema, T. Recent progress in catalytic conversions of carbon dioxide. Catal. Sci. Technol. 2014, 4, 1482− 1497. (5) Fiorani, G.; Guo, W.; Kleij, A. W. Sustainable conversion of carbon dioxide: The advent of organocatalysis. Green Chem. 2015, 17, 1375−1389. (6) Klankermayer, J.; Wesselbaum, S.; Beydoun, K.; Leitner, W. Selective catalytic synthesis using the combination of carbon dioxide and hydrogen: Catalytic chess at the interface of energy and chemistry. Angew. Chem., Int. Ed. 2016, 55, 7296−7343. (7) Darensbourg, D. J.; Holtcamp, M. W. Catalysts for the reactions of epoxides and carbon dioxide. Coord. Chem. Rev. 1996, 153, 155− 174. (8) Decortes, A.; Castilla, A. M.; Kleij, A. W. Salen-complex-mediated formation of cyclic carbonates by cycloaddition of CO2 to epoxides. Angew. Chem., Int. Ed. 2010, 49, 9822−9837. (9) North, M.; Pasquale, R.; Young, C. Synthesis of cyclic carbonates from epoxides and CO2. Green Chem. 2010, 12, 1514−1539. (10) Xu, B.-H.; Wang, J.-Q.; Sun, J.; Huang, Y.; Zhang, J.-P.; Zhang, X.-P.; Zhang, S.-J. Fixation of CO2 into cyclic carbonates catalyzed by ionic liquids: A multi-scale approach. Green Chem. 2015, 17, 108−122. (11) Comerford, J. W.; Ingram, I. D. V.; North, M.; Wu, X. Sustainable metal-based catalysts for the synthesis of cyclic carbonates containing five-membered rings. Green Chem. 2015, 17, 1966−1987. (12) Cokoja, M.; Wilhelm, M. E.; Anthofer, M. H.; Herrmann, W. A.; Kühn, F. E. Synthesis of cyclic carbonates from epoxides and carbon dioxide by using organocatalysts. ChemSusChem 2015, 8, 2436−2454. (13) Aoyagi, N.; Furusho, Y.; Endo, T. Effective synthesis of cyclic carbonates from carbon dioxide and epoxides by phosphonium iodides as catalysts in alcoholic solvents. Tetrahedron Lett. 2013, 54, 7031− 7034 (For examples of synthesis of cyclic carbonates under mild reaction conditions (atmospheric CO2 pressure, mild temperature), see this work, as well as refs 14−18). (14) Hardman-Baldwin, A. M.; Mattson, A. E. Silanediol-catalyzed carbon dioxide fixation. ChemSusChem 2014, 7, 3275−3278. (15) Zhou, H.; Wang, G.-X.; Zhang, W.-Z.; Lu, X.-B. CO2 adducts of phosphorus ylides: Highly active organocatalysts for carbon dioxide transformation. ACS Catal. 2015, 5, 6773−6779. (16) Wang, L.; Zhang, G.; Kodama, K.; Hirose, T. An efficient metaland solvent-free organocatalytic system for chemical fixation of CO2 into cyclic carbonates under mild conditions. Green Chem. 2016, 18, 1229−1233. (17) Castro-Osma, J. A.; North, M.; Wu, X. Synthesis of cyclic carbonates catalysed by chromium and aluminium salphen complexes. Chem.Eur. J. 2016, 22, 2100−2107. (18) Castro-Osma, J. A.; Lamb, K. J.; North, M. Cr(salophen) complex catalyzed cyclic carbonate synthesis at ambient temperature and pressure. ACS Catal. 2016, 6, 5012−5025. (19) Sun, J.; Zhang, S.; Cheng, W.; Ren, J. Hydroxyl-functionalized ionic liquid: A novel efficient catalyst for chemical fixation of CO2 to cyclic carbonate. Tetrahedron Lett. 2008, 49, 3588−3591. (20) Tharun, J.; Hwang, Y.; Roshan, R.; Ahn, S.; Kathalikkattil, A. C.; Park, D.-W. A novel approach of utilizing quaternized chitosan as a catalyst for the eco-friendly cycloaddition of epoxides with CO2. Catal. Sci. Technol. 2012, 2, 1674−1680. (21) Werner, T.; Büttner, H. Phosphorus-based bifunctional organocatalysts for the addition of carbon dioxide and epoxides. ChemSusChem 2014, 7, 3268−3271. (22) Büttner, H.; Lau, K.; Spannenberg, A.; Werner, T. Bifunctional one-component catalysts for the addition of carbon dioxide to epoxides. ChemCatChem 2015, 7, 459−467. (23) Büttner, H.; Steinbauer, J.; Werner, T. Synthesis of cyclic carbonates from epoxides and carbon dioxide by using bifunctional one-component phosphorus-based organocatalysts. ChemSusChem 2015, 8, 2655−2669.

glycols and alkali-metal salts were carefully investigated to clarify the essential points for catalytic activity. The importance of both the hydroxy groups of tetraethylene glycol and the iodide of the alkali metal salt was clearly observed in the CO2 fixation reaction. Furthermore, the practicability of this reaction system was demonstrated on a larger scale in the synthesis and recycle experiments of the catalyst. This reaction system is one of the most economical processes documented for the synthesis of cyclic carbonates under mild reaction conditions.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Typical Procedure for the Reaction of Epoxide 1 with CO2 Catalyzed by KI−Tetraethylene Glycol Complex. A mixture of styrene oxide 1a (0.570 mL, 5.00 mmol), tetraethylene glycol (86.3 μL, 0.500 mmol, 10 mol %), and potassium iodide (83.0 mg, 0.500 mmol, 10 mol %) was heated at 40 °C for 24 h under a CO2 atmosphere (1 atm, using a balloon). After being cooled to room temperature, the reaction mixture was purified by flash column chromatography on silica gel (hexane/EtOAc = 10:1−3:1 as the eluent) to afford cyclic carbonate 2a (812 mg, 4.95 mmol; Rf = 0.24 in hexane/EtOAc = 3:1). Recycle Experiments. A mixture of epoxide 1l (0.435 mL, 5.00 mmol), tetraethylene glycol (86.3 μL, 0.500 mmol, 10 mol %), and potassium iodide (83.0 mg, 0.500 mmol, 10 mol %) was heated at 40 °C for 24 h under a CO2 atmosphere (1 atm, using a balloon). After cooled to room temperature, the reaction solution (5.0 μL) was added to an NMR tube, and diluted by CDCl3 (0.50 mL). The conversion of epoxide 1l to cyclic carbonate 2l was determined by 1H NMR analysis. The product 2l in reaction flask was isolated by distillation under reduced pressure (76−79 °C, 2.0 mmHg). To the resulting residual catalyst in reaction flask was added new batch of substrate 1l, and the mixture was stirred at 40 °C for 24 h under CO2 atmosphere (1 atm, using a balloon). This procedure was repeated. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00324. Experimental procedures, characterization data, HPLC and NMR charts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seiji Shirakawa: 0000-0003-1027-8922 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (C) from JSPS, the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”, and The Kurata Grants from The Hitachi Global Foundation.

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REFERENCES

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