Ind. Eng. Chem. Res. 2004, 43, 8155-8162
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Amine-Catalyzed, One-Pot Coproduction of Dialkyl Carbonates and 1,2-Diols from Epoxides, Alcohols, and Carbon Dioxide Yasuhisa Kishimoto*,† and Ikuko Ogawa‡ Kansai Environmental Engineering Center Company Ltd., 11-20 3-Chome, Nakoji, Amagasaki, Hyogo 661-0974, Japan, and Environmental Research Center, The Kansai Electric Power Company, Inc., 11-20 3-Chome, Nakoji, Amagasaki, Hyogo 661-0974, Japan
Amine-catalyzed, one-pot coproduction of dimethyl carbonate (DMC) and 1,2-diols from epoxides, methanol, and carbon dioxide has been reevaluated and investigated in detail. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) serves as a desirable catalyst for the reaction. Heating of a mixture of glycidyl phenyl ether, methanol, a catalytic amount of DBU, and carbon dioxide at a total pressure of ca. 15 MPa at 150 °C gives DMC and the corresponding 1,2-diol in excellent yield (up to 97%). The influence of the reaction conditions on the DMC yield is discussed. The extension of this procedure to the synthesis of other symmetric dialkyl carbonates is also described. Introduction Recently, it has been well recognized that the efficient transformation of harmful wastes such as carbon dioxide (CO2) into useful chemicals is an important contribution to the preservation of the Earth. In particular, the utilization of CO2 as a carbon resource is important.1-3 CO2 is an attractive C1 building block in organic synthesis because it is abundant, inexpensive, nontoxic, and nonflammable. However, the inert nature of CO2 restricts the development of efficient catalytic processes for chemical fixation of CO2. As a result, chemical transformations of CO2 remain a significant synthetic challenge. Dimethyl carbonate (DMC, 4e) has received much attention as a solvent, octane enhancer, nontoxic carbonylation and methylation reagent, and precursor for polycarbonate resins.4 Although DMC is currently synthesized on an industrial scale by the oxidative carbonylation of methanol (non-phosgene route)5 or by the reaction of carbon monoxide with methyl nitrite,6 the synthesis of DMC using CO2 has been widely studied. Direct preparation of DMC from methanol and CO2 is one of the most preferable reactions. Several research groups have reported on the synthesis of DMC from methanol and CO2 in the presence of various activators.7 However, in all cases, the yield of DMC was far from satisfactory because of the reversible nature of the reaction. Very recently, Sakakura et al. reported on a new method for the direct synthesis of DMC from methanol and CO2.7e They employed molecular sieves as a water scavenger and dramatically improved the yield of DMC (up to 50% based on methanol) in comparison with conventional methods. Multistep syntheses of DMC from methanol and CO2 have also been studied.8-10 The transesterification of cyclic carbonates with methanol11 can be regarded as an indirect route to the preparation of DMC because cyclic carbonates can be * To whom correspondence should be addressed. Tel.: +81-6-6498-5720. Fax: +81-6-6498-5720. E-mail:
[email protected]. † Kansai Environmental Engineering Center Company Ltd. ‡ The Kansai Electric Power Company, Inc.
synthesized from epoxides and CO2 in high yields. This indirect route consists of two steps, i.e., (1) the production and isolation of cyclic carbonates and (2) the transesterification process. This two-step manipulation is one of the main drawbacks of this process. To overcome this drawback, the development of efficient and selective one-pot coproduction of DMC and 1,2-diols from epoxides, methanol, and CO2, in which all of the substrates are mixed at the start of the reaction, is desired (Scheme 1). However, methanol can react with epoxides in the presence of catalysts to form side reaction products. The catalyst selection and tuning of the reaction conditions are, therefore, crucial. More than 2 decades ago, researchers at Bayer AG reported on this approach and disclosed a variety of catalysts for this purpose.12 The catalysts include tetraalkylammonium halides, pyridinium halides, sulfonium salts, phosphonium halides, amines, alkaline metal salts (halides, hydroxides, and carbonates), tin compounds, thallium salts, and combinations of the above compounds. Among these, the NaI/Tl2CO3 catalyst system effected an excellent conversion of epoxides and gave the highest yields of DMC and 1,2-diols. However, the scope and limitations of the one-pot synthesis catalyzed by the above catalysts have not been completely explored. Recently, Arai et al. studied the onepot synthesis of dialkyl carbonates and 1,2-diols using MgO as the catalyst,13 but both the overall yield and selectivity of the desired products were very low (99.5% purity) was purchased from Nippon Sanso Corp. and was used without further purification. Methanol and ethanol were dried over Mg(OCH3)2 and Mg(OC2H5)2, respectively, distilled, and stored under a dinitrogen atmosphere. 1-Butanol was distilled over CaH2 and stored under a dinitrogen atmosphere. Other commercially available chemicals were used as received. Authentic samples of dibutyl carbonate (4g),15 4-phenoxymethyl-1,3-dioxolan-2-one (3a),16a 4-phenyl-1,3-dioxolan-2-one (3c),16a and 4-ethyl-1,3-dioxolan-2-one (3d)16b were prepared according to literature procedures. Commercially unavailable 1-methoxy-3-phenoxy-2-propanol (6ae), 1-ethoxy-3-phenoxy-2-propanol (6af), 1-butoxy3-phenoxy-2-propanol (6ag), 1,3-dimethoxy-2-propanol
(6be), 2-methoxy-1-phenylethanol (6ce), and 1-methoxy2-butanol (6de) were prepared from the corresponding epoxides and sodium alkoxides according to a literature method,17 with the modification that the reaction was carried out at room temperature for 18 h. Gas chromatography (GC) analysis was performed on a Shimadzu GC-14A gas chromatograph equipped with an flame ionization detector and a capillary column (Agilent Technologies HP-5, 0.25 mm i.d. × 30 m). GC-mass spectrometry analysis was carried out on an Agilent Technologies 6890 series GC system equipped with a capillary column (Agilent Technologies HP-5MS, 0.25 mm i.d. × 30 m) and a 5973 mass-selective detector. Typical Procedure for the One-Pot Coproduction of Dialkyl Carbonates (4) and 1,2-Diols (5) from Epoxides (1), Alcohols (2), and CO2. The experimental setup used in this study is schematically illustrated in Figure 1. All experiments were performed in a batch reactor. The reactor (4) was a stainless steel 316 vessel with a volume of 50 cm3. Stop valve V4 was first opened, and stop valve V5 and stopper S1 were closed. The reactor was evacuated through stop valve V4 and then filled with dinitrogen. Stopper S1 was then opened, and the desired amounts of alcohol (20-80 mmol), epoxide (1 mmol), and catalyst (2-10 mol % with respect to the epoxide) were charged into the reactor through stopper S1 with a syringe under a flow of dry dinitrogen. In the case of solid catalysts, the catalyst was placed in the reactor before evacuation and filling with dinitrogen. Stopper S1 and stop valve V4 were closed, and the reactor was connected quickly to a CO2 cylinder (1) through stop valves V2 and V4 at room temperature [outside of the oil bath (6)]. Stop valves V1, V2, and V4 were then opened, and CO2 was added to the reactor [by means of an HPLC pump (3), if necessary] to give a total pressure (Ptotal) of ca. 5-6 MPa. Stop valve V4 was then closed. The reactor was immersed in an oil bath (6) controlled at the desired reaction temperature. It usually required ca. 10 min for the temperature of the reactor to reach 150 °C. When the temperature of the reactor reached the desired temperature, stop valve V4 was opened again, and more CO2 was added to achieve the desired pressure. When the addition of CO2 was finished, stop valve V4 was closed. The time of the second addition of CO2 was considered to be the start of the reaction. After the desired reaction period, the vessel was removed from the oil bath, immersed in an ice bath for ca. 1 h, and allowed to cool to ca. 0 °C. CO2 was then slowly vented through stop
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Figure 1. Schematic diagram of the setup used in this study: (1) CO2 cylinder; (2) cooling unit; (3) high-pressure pump; (4) reactor; (5) stirring bar; (6) oil bath; (7) magnetic stirrer; (V1-V5) stop valves; (S1) stopper. Table 1. Catalytic One-Pot Coproduction of 4e and 5a from 1a, 2e, and CO2a
Figure 2. Illustration of the sapphire-window-equipped autoclave used in this study: (1) reactor; (2) window; (3) cartridge heater; (4) stirring bar; (5) magnetic stirrer; (V3′ and V4′) stop valves.
valve V5 at a temperature of ca. 0 °C. An aliquot of the remaining organic liquid was diluted with ether. After the diluted solution was treated with a small amount of dilute hydrochloric acid (ca. 0.5 N) to remove the amine catalyst, it was analyzed by GC. The conversion of epoxides (1) and yields of 4-substituted 1,3-dioxolan2-ones (3), dialkyl carbonates (4), 1,2-diols (5), and other byproducts were calculated on the basis of calibration curves, prepared by using ethylbenzene, o-xylene, or 1,2,3,4-tetrahydronaphthalene as an internal standard. The yields, as determined by the GC analysis, had ca. 3% experimental error. All of the yields in this paper are based on the starting epoxides (1). To visually inspect the reaction mixtures, the identical setup was used except a sapphire-window-equipped stainless steel autoclave (internal volume of ca. 30 mL) and four cartridge heaters were used instead of the reactor and oil bath, respectively. A schematic diagram of the sapphire-window-equipped autoclave is shown in Figure 2. Results 1. Amine-Catalyzed One-Pot Coproduction of 4e and 3-Phenoxy-1,2-propanediol (5a) from Glycidyl Phenyl Ether (1a), Methanol (2e) and CO2. 1.1. Catalyst. In the first part of this study, 1a was employed as an epoxide, and the one-pot coproduction of 4e and 5a from 1a, 2e, and CO2 was examined in the presence of various catalysts (eq 1). A mixture of 1a, 2e, the catalyst (10 mol % with respect to 1a), and CO2 was heated at 150 °C for 5 h. The alcohol-epoxide feed ratio ([alcohol]0/[epoxide]0, which is hereafter abbreviated as AER) and the total pressure (Ptotal) were
entry
catalyst (pKa)b
1 2 3 4 5 6 7 8 9 10
none TMG (13.54)c DBU (12)d triethylamine (10.72) DABCO (8.6) N,N-dimethylaniline (6.6)e DMAP (6.1) potassium tert-butoxide TBAB titanium isopropoxide
Ptotal, conv, MPa % 3a 16 15 16 16 16 17 17 15 15 16
32 92 100 80 97 17 98 88 100 15
5 9 4 27 13 3 6 12 84 0
yield, % 4e
5a
0 0 80 76 97 100 42 42 70 63 0.2 68 79 50 73 17 17 0 0
6ae 9 3 9 7 9 15 2 17
a Conditions: [1a] ) 1 mmol; [catalyst] ) 0.1 mmol; AER (2e/ 0 0 1a feed ratio in moles) ) 80; 150 °C, 5 h. b At 22-27 °C. From ref 18 except as noted. c This pKa value is for guanidine (from ref 19). d From ref 20. e This pK value is for N,N-diethylaniline. a
kept at 80 and 15-17 MPa, respectively. The catalysts of choice included amines such as DBU, 1,1,3,3-tetramethylguanidine (TMG), 1,4-diazabicyclo[2.2.2]octane (DABCO), 4-(dimethylamino)pyridine (DMAP), and N,Ndimethylaniline and metal alkoxides such as potassium tert-butoxide and titanium isopropoxide. Tetrabutylammonium bromide (TBAB) was also tested. Table 1 summarizes some typical experimental results.
Visual inspection of the reaction mixtures revealed that the reaction system was almost homogeneous at 150 °C at a Ptotal of above 15 MPa. All catalysts appeared to be soluble in the reaction mixture. The appearance and phase behavior of the reaction mixtures remained essentially unchanged. The absence of any catalyst did not give 4e at all (entry 1). An intermediate 3a and a side product 6ae were detected only in trace amounts. DBU effected the efficient and selective one-pot coproduction of 4e and 5a: 1a was consumed quantitatively to afford 4e in 97% yield and 5a in 100% yield with negligible formation of 6ae (entry 3). TMG also efficiently catalyzed the reaction to give 4e and 5a in ca. 80% yield, respectively (entry 2). Both DABCO and DMAP exhibited catalytic activity to give 4e in ca. 70% yield (entries 5 and 7). In contrast, triethylamine formed 4e in a much lower yield (ca. 40%; entry 4). In this case, a considerable amount
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of 3a remained intact, indicating the low transesterification activity of triethylamine. N,N-Dimethylaniline showed essentially no catalytic activity (entry 6). These results suggest that more highly basic amines18-20 give higher yields of 4e and 5a. Potassium tert-butoxide also catalyzed the reaction, giving 4e as a major product even at a temperature as high as 150 °C, although the amount of 6ae produced was considerable (15%; entry 8). TBAB is well-known to catalyze the cyclic carbonate formation from epoxides and CO216a as well as the transesterification of ethylene carbonates with 2e.11 However, under our conditions, TBAB did not catalyze efficiently the transesterification of intermediate 3a (entry 9). Titanium isopropoxide, which is an active catalyst for the transesterification of esters21 and has the potential to catalyze the direct synthesis of 4e from 2e and CO2,7a exhibited essentially no activity under our conditions and gave 6ae as the sole product in low yield (entry 10). In all experiments, neither 2-hydroxy-3-phenoxypropyl methyl carbonate nor 1-hydroxymethyl-2-phenoxyethyl methyl carbonate, an intermediate derived from the methanolysis of 3a, was detected.22 Texaco researchers reported that, in the polymer-supported ammonium halides catalyzed transesterification of ethylene carbonate with 2e, 2-hydroxyethyl methyl carbonate was constantly present through the course of the reaction and 4e was not formed before the yield of 2-hydroxyethyl methyl carbonate reached its maximum value.11 The difference between their observation and ours may stem from differences in the reaction conditions employed, i.e., the temperature (80 vs 150 °C) and AER (8 vs 80). Our reaction conditions are more favorable for transesterification. Another possibility is the difference in the steric bulkiness of the substituent (hydrogen vs phenoxymethyl group) located in the 4 position of 1,3-dioxolan-2-ones. 1.2. Effect of the AER. On the basis of the results obtained in the previous section, we chose DBU as a catalyst and examined the influence of reaction conditions on the one-pot coproduction of 4e and 5a from 1a, 2e, and CO2 in detail (eq 1). The results are shown in Table 2. The effect of the AER was investigated in the reaction with 10 mol % of DBU at 150 °C for 5 h at a Ptotal of 15-17 MPa. The range of AER was 20-80. In all cases, the conversion of 1a was quantitative, and 2-hydroxy3-phenoxypropyl methyl carbonate or 1-hydroxymethyl2-phenoxyethyl methyl carbonate was not detected. At an AER of 20, intermediate 3a was a major product (66% yield), not 4e (35% yield) (entry 1). In contrast, in the case of the AER of 40, 4e and 5a were obtained selectively in 88% and 93% yield, respectively (entry 4). The yield of 3a was only 3%. An increase in the AER to 80 similarly resulted in the clean formation of 4e and 5a, as was also shown in the previous section (entry 19). These AER effects are consistent with the reversible nature of the transesterification reaction (Scheme 1). An AER of 40 was sufficient to promote the transesterification cleanly under our conditions. Here, the time dependence of the reaction is briefly discussed. The data of entries 2-4 (AER ) 40) show the time course of the reaction; the transesterification of intermediate 3a started before 1a was consumed quantitatively, but the yield of the side product 6ae from 1a and 2e was constant (ca. 4%) throughout the reac-
Table 2. DBU-Catalyzed, One-Pot Coproduction of 4e and 5a from 1a, 2e, and CO2a entry
AERb
temp, °C
Ptotal, MPa
time, h
conv, %
3a
yield, % 4e 5a 6ae
1 2 3 4 5c 6d 7d 8 9 10 11 12 13 14 15 16 17 18 19 20 21
20 40 40 40 40 40 40 40 40 40 40 40 40 40 40 80 80 80 80 80 80
150 150 150 150 150 150 150 150 150 150 150 150 120 120 100 150 150 150 150 150 150
17 15 15 15 15 16 15 3 7 12 21 25 16 15 15 3 7 12 16 20 25
5 1 3 5 24 5 24 5 5 5 5 5 5 24 24 5 5 5 5 5 5
99 78 99 99 98 99 100 100 100 99 92 60 60 100 84 100 100 100 100 94 65
66 65 38 3 27 55 6 1 3 2 56 55 43 32 70 6 14 5 4 45 47
35 18 43 88 65 44 97 67 77 83 34 5 7 57 14 47 54 62 97 48 11
30 24 45 93 67 40 98 72 70 68 30 7 11 71 17 51 48 72 100 47 18
3 4 4 5 4 4 4 22 12 5 3 1 8 4 4 40 34 22 3 3 1
a Conditions: [1a] ) 1 mmol; [DBU] ) 0.1 mmol. b 2e/1a feed 0 0 ratio in moles. c [DBU]0 ) 0.02 mmol. d [DBU]0 ) 0.05 mmol.
tion. These results indicate the high selectivity of the transformation of 1a to 4e and 5a. 1.3. Effect of Ptotal. The influence of Ptotal on the reaction is shown in entries 4, 8-12 (AER ) 40), and 16-21 (AER ) 80) of Table 2. For clarity, the data are paraphrased in Figure 3. A parallel trend was observed for the AER of both 40 (Figure 3a) and 80 (Figure 3b), and some features can be highlighted. First, in a pressure region lower than 20 MPa, 1a was depleted within 5 h, whereas in the region of Ptotal higher than 20 MPa, the conversion of 1a was drastically reduced. Second, a plot of 4e yield against Ptotal showed a bellshaped curve. The most efficient and selective coproduction of 4e and 5a from 1a, 2e, and CO2 was accomplished at 15-17 MPa. The low yields of 4e at lower Ptotal’s were due to side reactions of 1a and 2e to 6ae. In particular, at the AER of 80 and at 3 MPa, the yield of 6ae reached as high as 40% (entry 16). On the other hand, the low yields of 4e at a Ptotal of above 20 MPa can be attributed to the low conversion of 1a and/ or a suppression of the transesterification of 3a. In this region, the selectivity of the transformation from 1a to 3a still remained high. This low conversion of 1a and the suppression of the transesterification of 3a can be attributed to the CO2-induced deactivation of DBU. This CO2 pressure dependence of the reaction profile is a remarkable feature of the DBU-catalyzed one-pot synthesis, compared to KI/K2CO3-catalyzed reactions.14 1.4. Effect of the Reaction Temperature. The effect of the reaction temperature (100-150 °C) is shown in entries 4 and 13-15 of Table 2. A higher temperature was favorable to an efficient reaction. At 150 °C, the reaction was complete within 5 h (entry 4). When the mixture was heated at 120 °C for 5 h, a considerable amount of 1a remained intact and the transesterification of 3a was negligible (entry 13). A prolonged reaction period of 24 h resulted in the quantitative conversion of 1a but the incomplete transesterification of 3a. Accordingly, the yield of 4e was only moderate (ca. 60%; entry 14). A further decrease in the reaction temperature to 100 °C led to a much slower reaction; almost all
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Figure 4. Photographs of the reactant mixture taken at an AER (2e/1a feed ratio in moles) of 40. The black object at the bottom of the reactor is a Teflon magnetic stirring bar. Conditions: (a) 25 °C, 6 MPa (before heating); (b) 70 °C, 11 MPa (during rising temperature); (c) 150 °C, 17 MPa (at the start of the reaction); (d) 100 °C, 15 MPa; (e) 120 °C, 15 MPa.
Figure 3. Total pressure dependence for the DBU-catalyzed, onepot coproduction of 4e and 5a from 1a, 2e, and CO2 at 150 °C: (a) for AER (2e/1a feed ratio in moles) ) 40; (b) for AER ) 80.
of 3a survived without undergoing the transesterification, and 4e was detected only in trace amounts (entry 15). 1.5. Catalyst Amount. A decrease in the catalyst loading retarded the reaction. For example, a reaction employing 5 mol % of DBU required a prolonged reaction time of 24 h for the quantitative conversion of 1a to 5a (and 4e) (entry 7 of Table 2), whereas the reaction with 10 mol % of DBU was complete within 5 h (entry 4 of Table 2). A further decrease in the catalyst loading to 2 mol % retarded the reaction drastically; almost all of 1a was consumed within 24 h, but the transesterification of 3a with 2e was slow, giving 4e in only moderate yield (entry 5 of Table 2). 1.6. Phase Behavior. To understand the phase behavior of the reaction, the reaction mixture was monitored. The composition of the mixture was identical with that for entry 4 of Table 2. Parts a-c of Figure 4 depict the phase behavior of the reaction mixture. At room temperature, the mixture gave a clear liquid-gas interface (Figure 4a). With increasing reaction temperature, the volume of the liquid phase gradually became reduced (Figure 4b). At 150 °C, where Ptotal reached 17 MPa, the reaction mixture formed a homogeneous phase (Figure 4c). The appearance of the reaction mixture was maintained over the course of the reaction. A similar phase transition was observed for the reaction for an AER of 80. Apparently, 3a dissolved in the CO2/2e medium under these conditions. At lower temperatures as high as 100 and 120 °C while maintaining Ptotal at
15 MPa, the reaction mixtures showed a similar phase behavior (parts d and e of Figure 4, respectively). Even when the temperature was maintained at 150 °C and Ptotal was lowered to 8 or 12 MPa, the reactant mixtures showed an analogous appearance (photographs not shown). These observations indicate that the most efficient coproduction of 4e and 5a proceeds in a homogeneous phase. 1.7. Concluding Remarks. Thus, the DBU-catalyzed one-pot coproduction of 4e and 5a from 1a, 2e, and CO2 could be conducted quantitatively and selectively by fine-tuning the reaction conditions. The DBUcatalyzed reaction is also characterized by a strong dependence on the CO2 pressure. Compared to the procedure by Bayer AG,12 the optimal reaction conditions reported here are harsher in terms of the AER value (20 vs 40-80), catalyst loading (
