CO2 is anticipated to have applications as a non-toxic, inexpensive, abundant and renewable C1 feedstock. For this reason, much effort has been devoted to developing effective processes for the synthesis of desirable, economically competitive products from CO2. As an example, cyclic carbonates, which are used as raw materials for polycarbonates, as electrolytes in lithium-ion secondary batteries, and as polar aprotic solvents, have been synthesized from CO2 and epoxides with high atom efficiency. However, the addition of CO2 to epoxides for the preparation of cyclic carbonates in a typical synthesis procedure is generally conducted at relatively high pressures and reaction temperatures in the presence of metal halides or metal complexes acting as catalysts [1-8]. Therefore, devising a simple and low energy alternative method for the preparation of cyclic carbonates from epoxides and CO2 remains a challenge for organic chemists.
Recent studies have demonstrated that electrocatalysis is a promising method for the formation of cyclic carbonates from epoxides and CO2, and that this technique can be performed at temperatures below 100 ℃ and at atmospheric pressure [9-16]. In electrochemical carboxylation systems reported to date, efforts have been focused on the electrochemical activation of CO2 either by direct (at the electrode) or indirect (through heterogeneous or homogeneous catalysis) reduction. However, the electrogenerated CO2•- radicals acting as intermediates in the electrochemical carboxylation can be more readily converted into a number of side products (such as CO and CO32-) before diffusing from the electrode surface into the solution to react with epoxides. Indeed, the low Faradaic efficiency of these processes based on the electrochemical reduction of CO2 has become the greatest obstacle to establishing industrial processes.
It has been reported that the synthesis of cyclic carbonates from epoxides and CO2 involves three steps, consisting of ring-opening,CO2 insertion and ring-closing reactions, and that the activation energy for each step is in the order ring-opening > ring-closing > insertion [17]. Based on this, the present work focused on lowering the activation energy of the ring-opening reaction by developing a catalytic system composed of two successive steps; an electrochemical reaction followed by carboxylation of the epoxide. Specifically, a magnesium salt acting as a Lewis acid catalyst for the ring-opening of epoxides was conveniently prepared in a N2 atmosphere by the sacrificial metal anode method, after which the cooperative action of the electrogenerated magnesium (EGM) salt and 1-butyl-3-methyl- imidazolium bromide (BMIMBr) was used to promote the reaction of epoxides with CO2 to effectively generate cyclic carbonates under atmospheric CO2 pressure and at ambient temperature.
BMIMBr and 1-butyl-2, 3-methyl-imidazolium bromide (BMMIMBr) were synthesized according to a previously reported procedure [18] and dried under vacuum at 80 ℃ for 24 h prior to use. Tetrabutylammonium bromide (TBABr) was obtained from the Aladdin Chemistry Co.,Ltd. (China) and dried under vacuum at 80 ℃ for 12 h prior to use. Acetonitrile (MeCN, reagent grade) was purchased from the Shanghai Lingfeng Chemical Reagent Co.,Ltd. (China) and dried over 4 molecular sieves before use. Other reagents were used as received.
Styrene oxide (1a) was used for catalytic studies. A typical galvanostatic electrolysis at a current density of 7.14 mA/cm2 was carried out in a MeCN/BMIMBr (20 mL, 0.2 mol/L) solution under a N2 atmosphere in a single compartment cell equipped with a sacrificial magnesium (Mg) rod anode and a 316 stainless steel (Ss) disk cathode (28 cm2) until an electric charge of 1 F per mole of the starting substrate had passed through the cell. The electrolyzed solution was subsequently transferred to a 50-mL three-neck round bottom flask fitted with a straight condenser tube, followed by the addition of 0.05 mol/L 1a. The resulting mixture was stirred for 8 h at 50 ℃ under atmospheric pressure CO2. Following the carboxylation reaction, the solvent was removed under reduced pressure and the residue was mixed with distilled water (20 mL) and then extracted with three 25-mL aliquots of diethyl ether, after which the combined organic layers were dried over anhydrous MgSO4. Subsequent to evaporation of the ether, the crude product was purified by chromatography on a silica gel column using light petroleum ether-ethyl acetate as the eluent. The 2a yield was determined by gas chromatography with a flame ionization detector (GC-FID) and 2b-2e yields are reported as isolated yields based on the starting epoxides.
The obtained products were characterized by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. FTIR spectra were acquired with a TENSOR27 spectrometer and NMR spectra were recorded with a Bruker DRX-400 spectrometer using CDCl3 as the solvent and TMS as the internal standard.
4-Phenyl-1, 3-dioxolan-2-one (2a). νmax (film)/cm-1: 1794 (C=O); 1H NMR (400 MHz,CDCl3) δ: 7.27-7.48 (m, 5H), 5.61 (t,J= 8.0 Hz, 1H), 4.73 (t,J = 8.4 Hz, 1H), 4.27 (t,J = 8.3 Hz, 1H); 13C NMR (400 MHz,CDCl3) δ: 71.21, 78.04, 125.92, 129.26, 129.77, 135.80, 154.90.
4-(Phenoxymethyl)-1, 3-dioxolan-2-one (2b). νmax (film)/ cm-1: 1783 (C=O); 1H NMR (400 MHz,CDCl3) δ: 7.23 (m, 2H), 6.99-6.79 (m, 3H), 5.01-4.90 (m, 1H), 4.58-4.40 (m, 2H), 4.11 (m, 2H); 13C NMR (400 MHz,CDCl3) δ: 66.25, 66.85, 74.19, 144.61, 121.99, 129.72, 154.78, 157.77.
4-Methyl-1, 3-dioxolan-2-one (2c). νmax (film)/cm-1: 1792 (C=O); 1H NMR (400 MHz,CDCl3) δ: 4.79-4.55 (m, 1H), 4.40 (d,J = 8.2 Hz, 1H), 3.98-3.67 (m, 1H), 1.29 (d,J = 6.3 Hz, 3H); 13C NMR (400 MHz,CDCl3) δ: 19.20, 70.72, 73.76, 155.21.
4-Ethyl-1, 3-dioxolan-2-one (2d). νmax (film)/cm-1: 1798 (C=O); 1H NMR (400 MHz,CDCl3) δ: 1.04 (t,J = 8 Hz, 3H), 1.73-1.86 (m, 2H), 4.10 (t,J = 8 Hz, 1H), 4.54 (t,J = 8 Hz, 1H), 4.67 (q,J = 7 Hz, 1H); 13C NMR (400 MHz,CDCl3) δ: 9.2, 27.6, 69.8, 78.8, 155.9.
4-Hexyl-1, 3-dioxolan-2-one (2e). νmax (film)/cm-1: 1801 (C=O); 1H NMR (400 MHz,CDCl3) δ: 4.61 (m, 1H), 4.44 (t,J = 8.1 Hz, 1H), 4.11-3.90 (m, 1H), 1.64-1.52 (m, 1H), 1.20 (t,J = 18.2 Hz, 8H), 0.79 (t,J = 6.7 Hz, 4H); 13C NMR (400 MHz,CDCl3) δ: 13.99, 22.45, 24.32, 28.79, 29.68, 31.51, 33.85, 155.20.
Styrene oxide (1a) was chosen as a model substrate to verify the effectiveness of the proposed catalytic system for the synthesis of cyclic carbonates. Experiments were conducted to investigate the effects of the catalysts (EGM and BMIMBr) as well as the carboxylation temperature on the yields of 2a, with the carboxylation results summarized in Table 1.
EGM acting as a Lewis acid is capable of weakening the C-O bond of an epoxide to lower the activation energy of the rate-determining step in the carboxylation, which is the epoxide ring-opening. However, excess EGM can also strongly bind with the intermediate anion formed after the CO2 insertion reaction to prevent the completion of the ring-closing reaction, which may decrease the selectivity of the carboxylation. To achieve a desirable yield of the cyclic carbonate, the effect of the EGM concentration was investigated, expressing the concentration using the charge (Q) passed through the cell during the electrolysis step. As shown in Table 1 (entries 1-5), the yield of 2a increased significantly from 65.1% to 90.7%, as the charge increased from 0.5 to 1 F/mol, while the selectivity of the carboxylation was maintained at approximately 94%. Further increasing the charge improved the conversion of 1a but dramatically lowered the selectivity. These results show that the concentration of EGM obtained after a charge consumption of 1 F/mol is adequate for the synthesis of cyclic carbonates from epoxides and CO2.
It is noteworthy that a small amount of 1-butyl-3-methyl- imidazolium carbene (NHC) might be generated from the electrochemical reduction of the 1-butyl-3-methyl-imidazolium cation (BMIM+) during the preparation of the EGM. NHC has been reported to be an effective organic catalyst for the synthesis of cyclic carbonates from the coupling reaction of CO2 and epoxides under a CO2 pressure of 2 MPa and at 120 ℃ [19]. Therefore, it is necessary to ascertain whether NHC also catalyzes the synthesis of cyclic carbonates in the present reaction system. With this in mind, we replaced the BMIM+ cation with the 1-butyl-2, 3-methyl-imidazolium cation (BMMIM+), which cannot generate NHC. Following this substitution, it was found that the yield of 2a remained almost the same, at 89.4% (Table 1, entry 6). In addition,NHC was prepared in a two-compartment cell under a N2 atmosphere to further identify the role of this compound in the carboxylation. It is important to note that, in the absence of EGM, we observed little cyclic carbonate formation (5.4%) and only recovered unreacted 1a (Table 1, entry 7). These results clearly demonstrate that NHC has little influence on the synthesis of cyclic carbonates from epoxides and CO2 in the current catalytic system.
To evaluate the advantage conferred by EGM, an equivalent amount of MgBr2 was used as the Lewis acid catalyst for the carboxylation instead of EGM, in the absence of an applied current. Using this catalyst, only 35.5% of the 1a could be transformed to 2a when the other reaction conditions remained unchanged (Table 1, entry 8). Based on this remarkable difference in catalytic activity between EGM and MgBr2, we believe that the anions paired with the EGM in the acetonitrile solution may include species generated from the electrochemical reduction of the solvent or the cation of the supporting electrolyte, in addition to Br-. In general, reductive species with larger ionic radii will exert a weaker electrostatic attraction towards EGM than that of Br-, thus greatly improving the electrophilicity of the EGM and promoting the carboxylation of 1a. To prove the importance of the reductive species, we also evaluated the catalytic activity of EGM obtained under a CO2 atmosphere in a single compartment cell. The data show that the carboxylation yield dropped dramatically from 90.7% to 54.2% when the electrochemical reduction of the solvent or the cation of the supporting electrolyte was replaced by the reduction of CO2 at the cathode (Table 1, entry 9). Based on the results shown above, it can be concluded that the use of EGM is essential for the carboxylation to proceed, and that the catalytic activity of EGM under a N2 atmosphere is higher than that under a CO2 atmosphere.
We subsequently examined the supporting electrolyte cation moiety, which can be expected to affect the strength of ion pairing between EGM and Br-. The carboxylation results showed that the catalytic activity in the presence of the imidazolium-based cation is superior to that obtained when using the tetrabutylammonium cation (Table 1, entry 10). Taking previously reported DFT calculations into account [20, 21], these results can be explained by comparing the tetrahedral conformation of the quaternary ammonium cation with the planar geometry of the imidazolium cation. The imidazolium ion, having less steric hindrance, may form a tighter ion pair with Br-, thus decreasing the electrostatic attraction between EGM and Br- and so increasing the catalytic activity of the EGM for the ring-opening reaction. Temperature also appears to be a crucial factor for the carboxylation of the epoxide. Increasing the temperature will be helpful in terms of obtaining the activation energy required for the carboxylation, although higher temperatures will also decrease the solubility of CO2 in the acetonitrile. The effects of temperature on the carboxylation were investigated over the range 30 to 70 ℃, and the results are presented in Table 1 (entries 3 and 11-14), showing that the optimal temperature is 50 ℃.
Having identified the optimized conditions, we proceeded to screen the applicability of this system to a range of substrate types (Table 2). The corresponding cyclic carbonate was obtained from all substrates, including both electron rich and poor aromatic systems, and a range of aliphatic epoxides were also effectively converted to the corresponding cyclic carbonate. It should be noted that the lowest 2c yield in the current catalytic system may have partly resulted from the volatilization of 1c (b.p. 34 ℃) during the carboxylation reaction.
EGM obtained under a N2 atmosphere was found to exhibit excellent electrocatalytic activity for the synthesis of cyclic carbonates from epoxides and CO2 in acetonitrile in the presence of BMIMBr as a cocatalyst. The results show that the concentration of EGM obtained after 1 F/mol charge consumption is adequate for the synthesis of cyclic carbonates from epoxides and CO2. Comparison of the catalytic activity of three Lewis acids,EGM under a N2 atmosphere,EGM under a CO2 atmosphere and MgBr2, indicates that the significant catalytic activity of EGM under a N2 atmosphere may be related to the reductive species generated from the electrochemical reduction of the solvent or the supporting electrolyte cation. The stronger electrostatic attraction between the imidazolium cation and the bromine anion in acetonitrile solution will also enhance the catalytic activity of EGM. This study thus demonstrates a low energy alternative for the electrochemically catalyzed synthesis of cyclic carbonates from epoxides and CO2 under mild conditions.
2016,
Vol. 37 Issue (7): 1076-1080 
PDF (523 KB)
2016,
Vol. 37 Issue (7): 1076-1080 PDF (523 KB)