Article pubs.acs.org/IECR
Polyethylene Glycol Wrapped Potassium Bromide Assisted Chemical Fixation of Carbon Dioxide Subodh Kumar and Suman L. Jain* Chemical Sciences Division, CSIR-Indian Institute of Petroleum Mohkampur, Dehradun-248005, India S Supporting Information *
ABSTRACT: Polyethylene glycol (PEG 400) embedded potassium bromide i.e. [K+{PEG}Br−] was found to be an extremely simple, highly efficient and recyclable promoter/medium for the cycloaddition reaction of epoxides and aziridines to CO2 to give corresponding cyclic carbonates and oxazolidinones, respectively, under mild conditions without utilization of additional organic solvents or additive. The catalyst worked well for a wide variety of substrates and could be recovered easily and reused without significant loss of catalytic activity and selectivity.
1.0. INTRODUCTION Owing to the growing economical and environmental concerns, utilization of carbon dioxide for the production of value added chemicals has attracted considerable interest in the last few decades.1−3 One of the most promising ways that effectively utilizes CO2 is the synthesis of cyclic carbonates and oxazolidinones through the coupling reaction of carbon dioxide with epoxides and aziridines, respectively. These five membered cyclic compounds have many synthetic uses in various fields4,5 including in biologically active materials for pharmaceutical and agricultural uses.6−8 Numerous homogeneous and heterogeneous catalyst systems associated with metal complexes, organic bases or ammonium salts, and metal oxides, have been developed for this transformation.9−19 Nonetheless, toxic organic solvents and cocatalysts are generally required to achieve high yields, along with a limited substrate scope in those above-mentioned cases. Furthermore, nonrecyclability of the catalysts and difficult product separation are some other drawbacks associated with the state of the art procedures. Few methods reported the use of alkali halides in conjunction with macrocyclic molecules such as cucurbit[6]uril,20 β-cyclodextrin,21 crown-ethers,22 phosphacarbamate,23 and in ionic liquids;24,25 however, the expensive nature and tedious synthesis of ionic liquids limits the synthetic utility of these methods. The chemistry of polyethylene glycols and their uses in various fields including medicinal chemistry26 and catalysis27 and as alternative separation media28 and phase transfer catalysts29,30 in organic synthesis have been attracting increasing interest. One of the unique abilities of PEG (host) is to bind metal ions (guest) to form host−guest complexes.31 These complexes have been widely used in extraction purposes;32 albeit rarely for organic transformations. Recently, PEG-supported quaternary ammonium salt reported to be an efficient and recyclable homogeneous catalyst for chemical fixation of carbon dioxide.33,34 Herein, we wish to report an extremely simple, economically viable, and efficiently recyclable system for the reactions of carbon dioxide with epoxides and aziridines to give corresponding cyclic carbonates and carbamates, respectively, by using PEG400-embedded-KBr as a promoter without any additive. Furthermore, the catalyst is quite efficacious for a © 2013 American Chemical Society
wide scope of substrates under organic solvent-free conditions (Scheme 1). Scheme 1. Cycloaddition of Epoxides and Aziridines with CO2
2.0. EXPERIMENTAL SECTION 2.1. Materials and Method. All the epoxides and aziridines were either purchased or synthesized by known methods. The reactions were carried out in a 50 mL round bottomed flask under atmospheric pressure by purging of carbon dioxide in batch wise manner. Polyethylene glycol (PEG400) and KBr were purchased from Acros organics and used as received. All the products were confirmed by comparing their physical and spectral data with those of known compounds. 2.2. Synthesis of [K+{PEG}Br−] 2. In a round-bottomed flask (100 mL) equipped with a magnetic stirrer, KBr (0.95 g, 8 mmol) was added to a solution of PEG-400 (3.2 g, 8 mmol) in water (30 mL) and the mixture stirred for 4 h at room temperature. After completion, the obtained residue was extracted with dichloromethane and the combined organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure. The brownish yellow viscous oil was obtained which has been used as catalyst for cycloaddition. Received: Revised: Accepted: Published: 541
October 7, 2013 December 9, 2013 December 9, 2013 December 9, 2013 dx.doi.org/10.1021/ie4033439 | Ind. Eng. Chem. Res. 2014, 53, 541−546
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2.3. Typical Procedure for the Carboxylation of Epoxides and Aziridines. A 50 mL round-bottom flask was charged with catalyst [K+{PEG}Br−] (10 mol %) and substrate (2 mmol). CO2 was purged into the flask before equipping the CO2 balloon and then the mixture was stirred at 60 °C for 4 h continuously. The reaction progress was monitored by TLC using ethyl acetate:hexane (6:4) as eluent. At the end of the reaction, the reaction mixture was cooled using ice-bath and the CO2 balloon was removed. The product was extracted with diethyl ether. The combined ether layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure to give the residue. The residue was purified by column chromatography on silica gel (200−300 mesh, eluting with 8:1 to 1:1 petroleum ether/ethyl acetate to give desired products). All the products were characterized by comparing their physical and spectral data with those of authentic samples. Figure 2. ESI-MS Spectra of PEG Embedded KBr 2.
3.0. RESULTS AND DISCUSSION The required PEG-embedded-KBr e.g. [K+{PEG}Br−] 2 was easily obtained by the stirring of KBr and PEG400 1 in water at room temperature. The reaction mixture was extracted with dichloromethane and subjected to usual workup to give [K+{PEG}Br−] 2 as a brownish yellow viscous oil in quantitative yield (Scheme 2). The formation of host guest complex
2 (10 mol %) as a promoter and reaction media. Styrene oxide was chosen as the model substrate to investigate suitable reaction conditions for the desired reaction. The results are summarized in Table 1. In the absence of 2 or by using PEG400 Table 1. Results of Optimization Experimentsa
Scheme 2. Synthesis of [K+{PEG}Br−] 2
entry
promoter
cat amount (mol %)
temp (°C)
time
yield (%)b
1c 2d 3e 4f 5 6 7 8 9 10 11
PEG400 KBr PEG400+KBr 2 2 2 2 2 2 2
10 10 10 10 10 10 10 5 2 1
100 60 60 60 60 100 50 30 100 100 100
8 8 8 8 3 1 5 10 8 8 8
20 65 98 98 92 30 84 58 35
a
Reaction conditions: PEG400-embedded-KBr (1 mL), styrene oxide (2 mmol), CO2 1 atm. bThe yield of the styrene carbonate determined by GC. cIn the absence of PEG400-embedded-KBr. dPEG (1 mL) alone. eKBr (2 mmol). f1:1 mixture of PEG400 and KBr.
1 (2 mL) alone, the coupling reaction of styrene oxide with CO2 did not occur at all (Table 1, entries 1 and 2). KBr itself showed very poor efficiency in the carboxylation of styrene oxide with CO2 (Table 1, entry 3). A mixture of KBr with PEG-400 has higher efficiency than the unsupported KBr salt (Table 1, entry 4 vs 3). However, PEG400-embedded-KBr 2 exhibited the highest efficiency as compared to the simple physical mixture of KBr with PEG400 under the identical conditions (Table 1, entry 5 vs entries 3 and 4). The enhancement of efficiency of the PEG400-embedded-KBr is presumably attributed to the enhanced solubilizing power of the 2 for the substrate and carbon dioxide. The effect of the reaction temperature was evaluated for the carboxylation of styrene oxide with CO2 (Table 1, entry 5−8). At room temperature (30 °C) the reaction of styrene oxide with CO2 was found to be very slow and gave only 30% conversion of the substrate to desired carbonate (Table 1, entry 8). At 100 °C for 1 h, styrene oxide was completely transformed to styrene carbonate under atmospheric pressure. It is worth mentioning, however, that the developed protocol provided
Figure 1. UV−vis Spectra of PEG Embedded KBr 2.
between PEG and KBr was confirmed by UV−vis spectra analysis (Figure 1). As the guest’s (KBr) concentration increases, the absorbance peak of the host (PEG400) at 238 nm gradually reduced and a clear isosbestic point was observed at 294 nm. The formation of PEG400·KBr complex 2 was further confirmed by electron spray ionization (ESI) mass spectrometric detection as shown in Figure 2. The cycloaddition reaction of epoxides and aziridines with CO2 was conducted in a batch wise operation by using 542
dx.doi.org/10.1021/ie4033439 | Ind. Eng. Chem. Res. 2014, 53, 541−546
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reaction conditions. 1-Ethyl-2-phenylaziridine was chosen as the model substrate to investigate the suitable reaction conditions for the desired reaction. The reaction of 1-ethyl-2phenylaziridine did not proceed at all at room temperature under atmospheric pressure of CO2. At 60 °C under atmospheric pressure of CO2, the reaction was completed within 4 h and afforded 90% yield of the product (6a + 6b) showing higher regioselectivity toward 6a with 96% and 6b with 4% yield. The generality of this protocol was also examined by using a number of aziridines under identical reaction conditions. Various oxazolidinones were selectively formed in good yields; the results are summarized in Table 3. It is important to mention that the 1-methyl-2-phenylaziridine and 2-phenylaziridine, which are well-known to form self-oligomers, afforded quite good yield of the desired product under the described reaction conditions (Table 3, entries 1 and 2). The substrates bearing linear alkyl or benzyl groups at the nitrogen atom gave the corresponding oxazolidinones in good to excellent yields all with above 90% regioselectivities (entries 2−8). However, substrates with a branched or cyclic alkyl group at the nitrogen atom showed slow reaction rate with excellent regioselectivity (entries 9 −11), probably due to the steric effect. In addition, the aziridines with either electron-donating groups or electronwithdrawing groups on the C1-aryl ring could smoothly react with CO2 and afforded oxazolidinones in high yield and regioselectivity (entries 4−5). As a consequence, the activity and regioselectivity may greatly depend on the electron nature of the C1-aryl group and the steric hindrance of the substituent group at the nitrogen atom. To test the recyclability, the coupling reaction of CO2 with styrene oxide and 1-ethyl-2-phenylaziridine was performed with PEG400-embedded-KBr 2 to give corresponding cyclic carbonate and oxazolidinone respectively under described experimental conditions. After completion of the reaction, desired product was isolated by extraction with diethyl ether from the reaction mixture, and the remaining PEG400-embedded-KBr was reused for further reaction with a fresh charge of substrates and CO2. The recycling experiments revealed that the developed reagent PEG400-embedded-KBr could efficiently be recycled for several runs (six runs) without any loss of its original reactivity as shown in Figure 3.
quantitative conversion of styrene oxide to styrene carbonate even at 60 °C in a longer reaction time of 3 h. On the basis of these studies, we selected 60 °C as the optimum temperature for the present transformation. We also evaluated the effect of catalyst concentration by varying the amount of catalyst from 2 to 10 mol % under otherwise similar reaction conditions. The results are summarized in Table 1, entries 9−11. As shown, the lower amount of the catalyst provided poor yield of the desired product; therefore, we have selected 10 mol % as the optimum amount of the catalyst required for the present study. To broaden the scope of the coupling reaction, various epoxides were examined for the formation of cyclic carbonates using 2 under optimized reaction conditions, e.g., 60 °C and 1 atmospheric pressure of CO2. The results of these experiments are summarized in Table 2. All the epoxides were readily Table 2. PEG400-Embedded-KBr Assisted Synthesis of Cyclic Carbonatesa
a
Reaction conditions: substrate (2 mmol), 2 (10 mol %), CO2 1 atm, 60 °C. bDetermined by GC. cIsolated yield of the pure products after column chromatography.
converted to the corresponding cyclic carbonates, but reactivity was greatly affected by the substituent at the carbon atom of the epoxide. In case of aryl group containing epoxides, the yield of the corresponding carbonates was found to be lower (Table 2, entries 1−6) than the epoxides having alkyl or cyclic substituents (Table 2, entries 7, 8, and 10−12). To evaluate the versatility of the developed protocol, we studied the cycloaddition of aziridines with CO2 under identical
Figure 3. Results of recycling experiment. 543
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Table 3. PEG400-Embedded-KBr Assisted Synthesis of Cyclic Carbamatesa
Reaction conditions: substrate (2 mmol), 2 (10 mol %), CO2 1 atm, 60 °C. b,cMolar ratio of 6a:6b was determined by GC. dTotal isolated yield of 6a + 6b.
a
Scheme 3. Possible Mechanistic Pathway
reaction probably involves the coordination of the epoxide with reagent 2 through hydrogen bonding to give intermediate A.
Although the exact mechanism of the reaction is not clear, in analogy to the existing literature reports22 we assume that the 544
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In the next step ring-opening of the epoxide ring by bromide ion takes place as shown in Scheme 3. In the subsequent step an interaction between the oxygen anion of the opened epoxy ring and CO2 will form an alkylcarbonate anion B which subsequently undergoes to the intramolecular cyclization step to give corresponding carbonate along with the regeneration of the catalyst 2.
4.0. CONCLUSION In summary, we developed an efficient, economically viable, and recyclable approach for the cycloaddition of CO2 to epoxides and aziridines to give cyclic carbonate and cyclic carbamates respectively without any added organic solvents or additive. The developed method is applicable to various substrates and afforded high to excellent product yields with high selectivity. To our knowledge, this is the first report on PEG400-embedded-KBr for chemical fixation of CO2. We believe that the developed method is one of the most efficient systems to date for this carboxylation reaction under mild conditions. One of the salient features of this protocol would be the facile recovery and recycling of the PEG400-embedded-KBr. This process represents a pathway for the economically and environmentally benign chemical fixation of CO2 to afford value added chemicals.
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ASSOCIATED CONTENT
S Supporting Information *
1 H and 13C NMR spectra of the products. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: 91-135-2525788. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful to director IIP for his kind permission to publish these results. S.K. kindly acknowledges CSIR, New Delhi, for his research fellowship. DST, New Delhi, is kindly acknowledged for funding.
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REFERENCES
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