Syntheses of cyclic carbonates with amidinium halide catalysts in reusable, reversible, room-temperature ionic liquids or acetonitrile

Abstract

In situ-prepared, reversible, room-temperature ionic liquids (RTILs), amidinium carbamates, have been used as media for the syntheses of cyclic carbonates by cycloaddition of CO₂ to epoxides. The amidinium carbamates were prepared by exposing equimolar mixtures of an easily synthesized amidine and a primary amine to CO₂ gas. For comparison purposes, amidinium dithiocarbamates were also employed as the RTILs in some experiments. Reaction between CO₂ and four epoxides in the RTILs, occurs in good yields (> 90% in many cases) at room temperature or 50 °C in the presence of an amidinium halide catalyst. Product and any unreacted epoxide were extracted easily upon addition of an immiscible, lower density liquid to an RTIL reaction mixture. This process was repeated three times with the same RTIL without any obvious decrease in catalytic activity; presumably, additional transformations could have been conducted. The influences of the type of catalyst, CO₂ pressure, reaction time, and temperature on the reaction yields have been investigated. The relatively mild reaction conditions and ease of separation of products, as well as the ability of the amidinium carbamates to be reused in the presence or absence of water make this an attractive alternative to other procedures for the efficient syntheses of cyclic carbonates.

---

Introduction

Chemical fixation of CO₂ and its use as an inexpensive, nontoxic, highly abundant, single carbon atom (C1 resource) building block are of great current interest due to the growing concern about the deleterious effects of greenhouse gases on our environment.^1,2 The syntheses of five-membered ring carbonates from CO₂ and epoxides (Scheme 1) is a promising methodology from the standpoint of resource utilization because this reaction has 100% atom economy and cyclic carbonate products are widely used for a variety of applications, such as polar aprotic solvents, precursors of polymeric materials, and intermediates in the syntheses of pharmaceuticals.^3,4,5 Numerous catalysts, including zeolites, metal halides, metal complexes and Lewis acids or bases have been developed and investigated to facilitate the reaction between an epoxide and a molecule of CO₂.⁶ However, problems associated with the separation of products from the solvents and catalysts after reaction and sensitivity of catalysts to water or air⁷ still need to be addressed. Also, the nature of the solvent can be a critical element in the ease of reaction.⁸

Scheme 1

Although ionic liquids, by themselves, are not necessarily ‘greener’ than other solvents, they can offer intrinsic advantages in catalytic reactions over many other organic solvents in reactions involving CO₂,⁹ especially in the steps leading to isolation of the products and to minimize byproducts such as polycarbonates¹⁰ or polyesters¹¹ in reactions involving epoxides as co-reactants. In addition, high temperatures (>100 °C),¹² high pressures of CO₂¹³ and co-catalyst systems¹⁴ are usually required to obtain good yields of cyclic carbonates from epoxides.

Previously, we developed a new class of thermally-reversible, room-temperature ionic liquids (RTILs).^15,16,17 They are made from easily synthesized, readily available materials and can be transformed reversibly to their non-ionic liquid states. The non-ionic liquids consist of equimolar mixtures of an N′-alkyl-N,N-dimethylacetamidine (L) and an aliphatic amine (M). When exposed to 1 atm of CO₂ gas, 1/1 (mole/mole) amidine/amine solutions become cationic–anionic pairs, amidinium carbamates (eqn (1)). Heating the ionic liquids in air to ca. 50 °C or bubbling N₂ gas through them at ambient temperatures for protracted periods displaces the CO₂ and re-establishes the non-ionic amidine/amine states. Utilizing similar chemistry, two classes of chiral RTILs^16,17 were also prepared using a chiral amino acid ester or a chiral amino alcohol as the amine moiety. The reversibility of these achiral and chiral RTILs offers significant potential benefits as solvents in the syntheses and separations of products which are soluble in low-polarity media and whose mechanisms of formation depend on catalysts soluble only in high-polarity media. Unlike many other ILs, the ones employed here (as well as the catalysts derived from the amidines) can be exposed to water without deleterious consequences.

Here, a prototype reaction, the synthesis of cyclic carbonates by cycloaddition of CO₂ to epoxides, utilizing the in situ-reversibility of the RTILs, is described (Scheme 1). The amidine (R′ = -(CH₂)₆H; C6) and amine components of the amidinium carbamate RTILs were prepared as described previously.¹⁵ For comparison purposes, amidinium dithiocarbamates, prepared by adding CS₂ to 1/1 (mole/mole) amidine/amine solutions (eqn (2)), were employed as the RTILs¹⁸ in some experiments. Reaction between a number of epoxides and CO₂ at 25–50 atm of pressure is found to occur in yields > 90% in the RTILs in many cases at ambient or slightly elevated temperatures in the presence of an amidinium halide or other Lewis acid/base catalyst; the yields are somewhat lower at both lower and higher CO₂ pressures, but for very different reasons. The postulated mechanism for formation of the cyclic carbonates is shown in Scheme 2, The cyclic carbonate products and any unreacted epoxide could be extracted easily upon addition of an immiscible, lower density liquid (e.g., an alkane or ether) to an RTIL reaction mixture. Then, bubbling of CO₂ through the amidinium carbamate (L-M-CO₂) was sufficient to remove traces of the extracting liquid so that it and the catalyst could be reused. The influence of the type of catalyst, CO₂ pressure, reaction time and temperature on the reaction yields are reported.

Scheme 2 Postulated mechanism for catalyzed syntheses of cyclic carbonates from epoxides and CO₂.¹⁹

The relatively mild reaction conditions, ability to use metal-free catalysts, ease of separation of products from the amidinium carbamate, and the ability to reuse the solvent and catalyst with little or no decrease of product yield make the procedures described here an attractive alternative to other methods for the syntheses of cyclic carbonates.

Experimental section

Materials

Epichlorohydrin (99%), 1,2-epoxy-3-phenoxypropane (99%), styrene oxide (97%), cyclohexene oxide (98%), tetraethylammonium bromide (98%) and lithium bromide (>99%) were purchased from Sigma–Aldrich and used as received. Carbon dioxide (CO₂) was supplied by GT & S with a purity > 99.9%. Sodium bromide (99.9%), zinc chloride (99.2%)and zinc iodide (99.9%) were from Fisher. Zinc bromide (99.9%) was from Baker. Hydrochloric acid (HCl, 99%, 37 wt% in water), hydroiodic acid (HI, 99.95%, 57 wt% in water), and hydrobromic acid (HBr, 99.9%, 48 wt% in water) are from Baker. All organic solvents were HPLC grade (Fisher or Aldrich). Acetonitrile was dried over 4A molecular sieves before being used. Carbon disulfide (anhydrous, 99.9%), biphenyl (99.5%), n-hexylamine (99%) were purchased from Aldrich. Chloroform-D (99.8% D) was purchased from Cambridge Isotope Laboratory. Silica gel for column chromatography was purchased from YB in San Clemente, CA. The aliphatic amidines were available from previous studies.¹⁶

Instrumentation

IR spectra were obtained on a Perkin-Elmer Spectrum One FTIR spectrometer interfaced to a PC, using an attenuated total reflection accessory or NaCl plates. NMR spectra were recorded on an Inova 400 MHz NMR spectrometer operating at 400 MHz (¹H), or 100.5 MHz (¹³C), respectively. ¹H and ¹³C NMR spectra were indirectly referenced to TMS using residual solvent signals as internal standards and CDCl₃ as the solvent. GC-MS was performed on a Varian Saturn 2100 instrument in the electron ionization (EI) mode using a DB-5 (15 m × 0.25 mm) column.

The high pressure stainless steel cell and the CO₂ compressor apparatus are described in detail in Supporting Information (Fig. S1†). The temperature variation throughout the cell at a nominal temperature of 50 °C is estimated to be ca. 1.5 °C based upon measurements of the internal and external temperatures of the cell at various points.

General experimental procedure

Procedure for the syntheses of cyclic carbonates

A typical reaction procedure is described for the synthesis of 4-phenoxymethyl-1,3-dioxolan-2-one (2b) from CO₂ and 1,2-epoxy-3-phenoxy propane (2a) (Scheme 1) using LiBr as a catalyst in the ionic liquid amidinium carbamate, C6-hexylamine-CO₂. Organic salts, amidinium halides (Cn-HX), were used as catalysts as well. In all cases, ¹H, ¹³C NMR and MS spectra of the products were consistent with those in the literature.²⁰

In an open glass vial with a 1 cm long Teflon stir bar, LiBr (1.2 mg, 1.4 × 10⁻² mmol) was added to a solution of 1,2-epoxy-3-phenoxypropane (2a) (0.10 g, 0.70 mmol) in 0.50 mL of C6-hexylamine-CO₂, prepared by bubbling CO₂ through a C6/hexylamine mixture for 10 min. The glass vial was placed in the stainless-steel, high-pressure cell. Then, CO₂ gas was introduced to a desired pressure and the cell temperature was raised. After stirring the reaction mixture for several hours, the pressure was brought slowly back to one atmosphere and diethyl ether (3.0 mL×3) was used to extract the products and any unreacted epoxide from the reaction mixture. NMR and GC-MS spectra of the product mixtures, after removing the diethyl ether, showed no discernible ionic liquid or its precursor amidine and amine components. In some cases, the products were isolated by combining the etherates and reducing them carefully to residue which was separated by column chromatography (silica, 50–75 μm) using 2 : 3 hexane:EtOAc as the eluent. Spectra of the isolated products are collected in Supporting Information.† Product yields were calculated from the weight of the residues (which were a combination of product and starting material only, whose proportion was calculated by integration of characteristic peaks in ¹H NMR spectra, or (as indicated) by the weight of isolated product after column chromatography. Some reactions were repeated and the reproducibility of the yields was ± 3%. (GC response factor calculations are shown in Fig. S2 of Supporting Information.†)

Syntheses of amidinium halides

The C6-HX salts were prepared by the addition of an acid to C6 amidine (eqn (3)). A detailed example is presented for the synthesis of C6-HBr. C6-HCl and C6-HI were prepared in analogous fashions using 37% aqueous HCl and 57% aqueous HI, respectively. To a solution of C6 amidine (0.34 g, 2.0 mmol) in a stirred round bottom flask, 48% aqueous HBr (0.69 g, 4.0 mmol HBr) was added dropwise. After stirring the reaction mixture for 2 h at room temperature, most of the water was removed on a rotary evaporator. The residue was placed in a vacuum oven at 250 Torr and 50 °C for 48 h before characterization;²¹ see ¹H NMR, ¹³C NMR, MS and IR spectra in Supporting Information†). Yield 0.79 g (95%) of a yellowish liquid; ¹H NMR (CDCl₃): 3.26 (t, 2H, -CH̲₂–N=); 3.07 (s, 6H, -N–(CH̲₃)₂); 2.15 (s, 3H, -N=C(CH̲₃)–N); 1.43 (m, 2H, -CH̲₂–CH₂–N=); 1.15–1.2 (m, 6H,CH₃–(CH̲₂)₃-); 0.72 (t, 3H, CH̲_3̲). ¹³C NMR (D₂O): 164.56; 44.31; 40.57; 37.95; 30.57; 29.09; 25.26; 21.78; 13.92 ppm; MS (m/z 172 M−Cl⁺); IR: 1646 cm⁻¹ (C=N⁺).

C6-HCl: Yield: 88% of a yellowish liquid; ¹H NMR (CDCl₃): 3.28 (t, 2H, -CH̲₂–N=); 3.09 (s, 6H, -N–(CH̲₃)₂); 2.16 (s, 3H, -N=C(CH̲₃)–N); 1.45 (m, 2H, -CH̲₂–CH₂–N=); 1.15–1.23 (m, 6H, CH₃–(CH̲₂)₃-); 0.73 (t, 3H, CH̲_3̲). ¹³C NMR (CDCl₃): 164.73; 44.48; 40.67; 37.88; 30.68; 29.21; 25.32; 21.86; 13.99 ppm; MS (m/z 172 M−Br⁺); IR: 1646 cm⁻¹ (C=N⁺).

C6-HI: Yield: 85% of a dark orange color solid, mp 33.5–34.8 °C; ¹H NMR (CDCl₃): 3.29 (t, 2H, -CH̲₂–N=); 3.10 (s, 6H, -N–(CH̲₃)₂); 2.17 (s, 3H, -N=C(CH̲₃)–N); 1.46 (m, 2H, -CH̲₂–CH₂–N=); 1.18–1.23 (m, 6H,CH₃–(CH̲₂)₃-); 0.74 (t, 3H, CH̲_3̲). ¹³C NMR (CDCl₃): 164.66; 44.49; 40.63; 37.82; 30.78; 29.28; 25.38; 21.88; 14.05 ppm; MS (m/z 172 M−I⁺); IR: 1646 cm⁻¹ (C=N⁺).

Solvent recycling test

The recycling procedure is illustrated in Scheme 3. The experimental steps in consecutive runs were the same as the initial run. Also, CO₂ at 1 atm was bubbled into the RTIL solvent for 10 min after each run to ensure that it consisted of only amidinium carbamate and to remove any residual ether; according to the ¹H NMR spectra, no ether remained after this procedure. Then, the same amount of 1,2-epoxy-3-phenoxypropane (2a) was added to the reaction mixture to start the next cycle.

Scheme 3 Diagram of the recycling procedure for reuse of RTIL/catalyst mixtures in the syntheses of cyclic carbonates from CO₂ and an epoxide.

Attempted polymerization of cyclohexene oxide

In CH ₃ CN: In an open glass vial with a 1 cm Teflon stir bar, C6-HBr (10 mg, 4.0 × 10⁻² mmol) was added to a solution of cyclohexene oxide (3a) (0.20 g, 20 mmol) in 0.25 mL of CH₃CN. The glass vial was placed in the stainless steel high pressure cell under 25 atm CO₂ and the temperature was increased to 50 °C. The work-up procedures are the same as described above.

In solvent-free conditions: The procedure is the same as in CH₃CN except that only cyclohexene oxide (3a) (0.80 g, 80 mmol) and C6-HBr (40 mg, 0.16 mmol) were employed.

Results and discussion

The postulated mechanism in Scheme 2 involves the ring opening of the epoxide by means of nucleophilic attack at the less hindered carbon atom by X⁻ (as Br⁻ in most of the reactions explored here) which leads to an oxy anion intermediate A. Then, CO₂ reacts with A to form anion B which forms the carbonate products upon ring closure.²²

Three mono-substituted epoxides – epichlorohydrin (1a), 1,2-epoxy-3-phenoxypropane (2a), and styrene oxide (3a) – and one disubstituted epoxide, cyclohexene oxide (4a), have been tested as substrates. Both hexane and diethyl ether, solvents in which the epoxides and cyclic carbonates are very soluble, are almost completely immiscible with our RTILs.¹⁷ Therefore, they were selected to extract products from the RTILs in our experiments. The reactions were conducted between room temperature and 50 °C, a much lower temperature range than reported for the syntheses of cyclic carbonates from epoxides and CO₂ in most procedures.⁶ However, the yields are lower than found in several other procedures to produce 2b.⁵ Also, Endo and coworkers reported recently that an amidine-mediated system, an N-methyltetrahydropyrimidine-CO₂ adduct (MTHP-CO₂), is capable of catalyzing cyclic carbonate syntheses at room temperature and 1 atm of CO₂ pressure in the presence of a high concentration of LiBr catalyst (25 mol%).^23,24

In an initial survey, the catalytic activities of various metal salt catalysts were evaluated in our RTILs (Table 1). When the anion is Br⁻, Li⁺ is the most efficient of the cations examined. Although Zn²⁺ is a stronger Lewis acid than Li⁺,²⁵ its catalytic activity is lower here. The type of anion is important to the yield as well. Of the catalysts with Zn²⁺ as the cation, the one with Br⁻ shows the highest catalytic activity. Polarity of the salts, their hard–soft/acidity–basicity,²⁶ and their solubility in the RTILs may also affect the reaction yields. Overall, the trends in Table 1 are consistent with literature results—the lithium cation is expected to have the highest catalytic activity.²⁷ Note that no reaction occurred in the absence of a catalyst; the RTIL does not serve as a catalyst. Also, no byproducts, such as polycarbonates or polyesters,²⁸ were detected by GC-MS or by ¹H NMR in the products isolated by column chromatography.

Table 1 Syntheses of cyclic carbonate (2b) from reaction of 1.40 mol L⁻¹1,2-epoxy-3-phenoxypropane (2a) in C6-hexylamine-CO₂ for 12 h at 50 °C under 40 atm CO₂ as a function of metal halide catalyst (2 mol%)

Catalyst	Yield (%)^a
a Yields by ^1H NMR spectroscopy or isolated (in parentheses).
LiBr	70 (65)
Et4NBr	62
NaBr	24
ZnBr2	67 (64)
ZnCl2	45
ZnI2	38
(No catalyst)	0

---

The yields of cycloaddition products using LiBr as catalyst and C6-hexylamine-CO₂ as solvent (Table 2) are again, somewhat lower than those reported using other reaction conditions.²⁹ At least in part, they can be attributed to the slower diffusion of bromide anion and other species in the RTILs (due to the relatively high local viscosity of RTILs, especially at room temperature), and the higher probability that intermediates A lose the X group than reacting with CO₂ to form the intermediates B (Scheme 2). Tetra-alkylammonium bromide salts, including the tetramethyl, tetraethyl and tetrapropyl, have also been investigated as catalysts supported on silica (without solvent).³⁰ All produced ≥ 96% yields of cyclic carbonates 1b, 2b and 3b.

Table 2 Dependence of substrate (1.40 mol L⁻¹) on reactivity in C6-hexylamine-CO₂ for 12 h at 50 °C under 25 atm CO₂ with 2 mol% LiBr as catalyst

Epoxide	Product	Yield (%)^a
a Yields were calculated by ^1H NMR spectroscopy. Isolated yields are shown in parentheses.
1a	1b	75
2a	2b	85 (84)
3a	3b	82 (79)
4a	4b	80

---

To extend further the versatility of this reaction, we explored the use of in situ prepared organic salts, amidinium halides (Cn-HX) to catalyze the reaction. This preparation is convenient for conducting the reactions and results in much higher yields (by ∼15%) of the products from 1a–4a than when metal halides are employed (Table 3). The enhancement of catalyst performance is attributed to the ability of the amidine itself to capture CO₂ molecules²³ and deliver them to the reactive site in a catalytic cycle (Scheme 2). Although the pK_a's of the protonated amidines used here are not known, the pK_a of protonated 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is ca. 12.³¹ Assuming reasonably a similar pK_a for our protonated amidines, they may form a hydrogen bond with the oxygen atom of an epoxide, thereby activating it for nucleophilic attack during the reaction. However, ¹H NMR spectra show no chemical shift change of protons after mixing C6-HX with styrene oxide.³² Note also that commercially available amidines, such as DBU, can be used as amidinium catalysts. As shown in Table 4, C6-HBr and C6-HI are better catalysts than C6-HCl in this system, indicating the importance of nucleophilicity on the reaction rate.³³ Given the estimated experimental error of the measurement of product yields (± 3%), it is difficult to differentiate the catalytic activities of C6-HBr and C6-HI.

Table 3 Syntheses of cyclic carbonates from reaction of 1.40 mol L⁻¹1a–4a with 2 mol% C6-HBr as catalyst at 50 °C and 25 atm CO₂ in C6-hexylamine-CO₂ (A) or in CH₃CN (B) for 8 h

a → b	Yield (%) (A)^a	Yield (%) (B)^a
a Yields determined by GC-MS using biphenyl as an internal standard.
1	94	91
2	98	96
3	99	96
4	91	90

---

Table 4 Syntheses of cyclic carbonate 2b from reaction of 1.40 mol L⁻¹2a with 2 mol% dry and ‘wet’ amidinium halide (C6-HX) catalysts^a in C6-hexylamine-CO₂ for 8 h at 50 °C under 25 atm CO₂

Catalyst	Yield (%) dry catalyst^b	Yield (%) wet catalyst^b
a See Experimental section for a description of the dry and wet catalysts. b Yields were calculated by GC-MS using biphenyl as an internal standard.
C6-HCl	90	82
C6-HBr	98	87
C6-HI	99	90

---

In addition, the amidinium halide, prepared by addition of C6 to an equimolar amount of an HX (X = Cl, Br, and I) as a concentrated aqueous solution, was used as the catalyst for the conversion of 2a to 2b (Table 4). The yields of 2b with ‘wet’ C6-HX were ca. 8–11% lower than with dry C6-HX, but were still > 80%; amidinium carbamate remains an effective medium and the C6-HX are effective catalysts even in the presence of water. Within experimental error, ± 3%, water has about the same impact on the catalytic activities of each of the C6-HX. Also, the trend of the ‘wet’ Cn-HX catalytic activities are the same as those of the dry catalysts: C6-HI or C6-HBr is a better catalyst than C6-HCl.

Although the presence of 3 wt% H₂O in a C6/leucine methyl ester mixture is known not to impede formation of the amidinium carbamate upon exposure to 1 atm of CO₂,¹⁶ reaction between wet DBU and CO₂ produces the amidinium bicarbonate.³⁴ Analogous reactions, resulting in amidinium bicarbonate salts from a Cn amidine and water, must occur. Bicarbonate, rather than CO₂, may be involved in the formation of the cyclic carbonates and it has been reported that 0.3 equivalents of water promoted the syntheses of cyclic carbonates from CO₂ and epoxides.³⁵ Also, a molecule of water and the anion of a Lewis base (PPh₃BuI) play a synergistic role in opening an epoxy ring and in stabilizing the carbonate intermediates:³⁵ at 125 °C under 20 atm of CO₂ for 1 h, the yields of cyclic carbonate from 3a were 95% in the presence of water and only 25% in its absence.

The influence of CO₂ pressure on reaction yields were compared using LiBr and C6-HBr as catalysts (Table 5). The yields of cyclic carbonate 2b increased with increasing pressure from 1 to 25 atm CO₂. However, the yields decreased as pressure was increased further to 60 atm. Similar trends have been found for cyclic carbonate syntheses in other media and explained as follows:³⁶ the initial increasing yields as pressure is changed from 1 to 25 atm is attributed to increasing CO₂ concentrations within the RTIL solvent; the decreasing yields yet higher pressures are presumably a consequence of dilution of the substrate and catalyst as they are partitioned between the RTIL and a pre-supercritical phase of CO₂³⁷ that moves some of the reactant into the total volume of the stainless steel cell (i.e., a gas-like phase).^38,39 In so doing, the concentration of epoxide in the vicinity of the catalyst (which is retained within the original liquid volume) is lowered, resulting in lower product yields. In the experiments conducted here, a small amount of liquid was observed outside the glass vial after the reactions, especially in runs conducted at higher CO₂ pressures.

Table 5 Influence of CO₂ pressure on the yields of cyclic carbonate 2b from reaction of 1.40 mol L⁻¹2a for 8 h at 50 °C in C6-hexylamine-CO₂ with 2 mol% LiBr (A) or C6-HBr (B)

Pressure (atm)	Yield (%) (A)^a	Yield (%) (B)^a
a Yields determined by ^1H NMR.
1	0	0
10	78	81
20	80	87
25	85	98
40	80	93
50	76	90
60	69	86

---

As expected, longer reaction times and higher temperatures resulted in increased yields of cyclic carbonate. For example, the yield of cyclic carbonate 3b during the first 5 h increased rapidly and gradually reached a plateau value after 12 h (Fig. 1). The specific reasons for the higher yields with C6-HBr than with LiBr were not investigated here. They may involve the relative solubilities of the two catalysts and how they are able to interact with the key intermediates A and B in Scheme 2. Similarly, the increased yields of cyclic carbonate 3b at higher temperatures (Fig. 2) are attributed to the lowered viscosity of the RTIL as well as an intrinsic rise in the rates of all the steps leading to product. Lower viscosity causes higher rates of diffusion of cations and ions, allowing more efficient capture of CO₂ by the critical intermediate A shown in Scheme 2 by making less probable the loss of the nucleophile (Xgroup) and reversion to epoxide.

Fig. 1 Influence of reaction time on the yield of cyclic carbonate 3b from 1.40 mol L⁻¹3a in C6-hexylamine-CO₂ at 50 °C and under 25 atm of CO₂ using 2 mol% of LiBr (○) or C6-HBr (●) as catalyst. Yields were calculated by ¹H NMR spectroscopy.

Fig. 2 Influence of temperature on the yield of cyclic carbonate 3b from 1.40 mol L⁻¹3a after 8 h, 25 atm of CO₂ in C6-hexylamine-CO₂ using 2 mol% of LiBr (○) or C6-HBr (●) as catalyst. Yields were calculated by ¹H NMR spectroscopy.

Amidinium dithiocarbamate RTILs, prepared by adding CS₂ to amidine/amine mixtures, usually have higher viscosities and better thermal stabilities than their CO₂ analogues, amidinium carbamates.¹⁸ The differences between the polarities of amidinium carbamate and amidinium dithiocarbamates are very small,^17,18 and apparently do not play an important role here. However, viscosity is important because lower viscosities increase the probability that intermediates A in Scheme 2 will encounter (and react with) a molecule of CO₂ rather than losing the Xgroup and returning to the epoxide. Thus, the amidinium carbamate solvent give higher product yields (Table 6).

Table 6 Comparison of yields of cyclic carbonates from reactions of 1.40 mol L⁻¹epoxide for 8 h at 50 °C under 25 atm CO₂ in C6-hexylamine-CO₂ (A) and in C6-hexylamine-CS₂ (B) RTILs with 2 mol% C6-HBr

Epoxide	Product	Yield (%) (A)^a	Yield (%) (B)^a
a Yields were calculated by ^1H NMR spectroscopy.
1a	1b	95	75
2a	2b	97	82
3a	3b	97	77
4a	4b	93	72

---

Furthermore, the recyclability of the RTIL/catalytic system under the optimum condition using the procedures outlined in Scheme 3 was explored. The results in Table 7 show that the efficiency of the LiBr catalyzed reactions decreased very slightly upon reuse, but still remained effective after 3 runs. Because lithium salts have a high affinity for carbonates⁴⁰ and are sparingly soluble in ether, some LiBr catalyst may have been lost during product separation (i.e., repeated extraction with diethyl ether) and be responsible for the small decreases in yields in the second and third runs. Although there is a trend toward slightly lower yields with C6-HBr as catalyst, all of them are very high and within the limits of experimental error, estimated to be ±3%.

Table 7 Recycling tests using C6-hexylamine-CO₂ and 2 mol% of LiBr (A) or C6-HBr (B) as catalyst for 8 h at 50 °C under 25 atm CO₂ to convert 1.40 mol L⁻¹2a and CO₂ to 2b

Run	Yield (%) (A)^a	Yield (%) (B)^a
a Yields were calculated by ^1H NMR spectroscopy.
1	85	98
2	82	95
3	78	94

---

Jessop and coworkers⁴¹ reported a switchable ionic liquid prepared from a secondary amine and CO₂, with Cr(salen)Cl as a catalyst that could be reused for the copolymerization of cyclohexene oxide (2a) and CO₂, but that the yield of polycarbonate was reduced by ca. 20% during the second run and the polydispersity increased from 1.06 to 1.10. By contrast, reaction of 2a and 25 atm CO₂ at 50 °C using C6-HBr as a catalyst in CH₃CN or under solvent-free conditions for 24 h led only to 2b in 55 (CH₃CN) or 42% yield (neat conditions); FT-IR and ¹H NMR spectra showed no peaks attributable to a polycarbonate.^10a In the latter case, the epoxide, as well as the resulting cyclic carbonate, act as the solvent.⁴² The lack of polymer as a competing product suggests that very high substrate concentrations may be possible without sacrificing the ‘cleanliness’ of reaction when amidinium bromides are used as the catalysts, and increasing the reaction time to 72 h and the temperature to 80 °C increased the reaction yield of 2b to ∼99%. Again, no evidence for polymer was found. We conjecture that the rate of ring closure to cyclic carbonate by intermediate B in Scheme 2 (when generated by amidinium bromide) must be much faster than the rate at which a second molecule of epoxide can intervene.

A zeolite-based, organic–inorganic hybrid catalyst has been reported to give 86–100% yields of cyclic carbonate 2b from neat 2a under 6.8 atm of CO₂ and at 120 °C for 8 h.⁴³ Also, bis(triphenylphosphine)immium (PPN) salts, [PPN]⁺Cl⁻, and PPN-manganese carbonylates, [PPN]⁺[Mn(CO)₄L]⁻ (L = CO, PPh₃), have been found to be good catalysts for cyclic carbonate syntheses under solvent-free conditions. No polycarbonate was produced in these reactions when they were conducted under 5 atm of CO₂ and at 100 °C.⁴⁴ The relatively low solubility of CO₂ in conventional solvents⁴² is a primary motivation to search for alternatives to the reaction between CO₂ and epoxides.⁴⁵ Supercritical CO₂ has also been found to be an effective solvent for cyclic carbonate formation, although very high pressures are necessary.⁴⁶ For example, in the presence of a Re(CO)₅Br catalyst, the reaction of epoxides with supercritical CO₂ without solvent at 110 °C afforded good yields.⁴⁷ Thus, our catalyst appears to be competitive with all of the others reported to date for the cycloadditions, and it is superior to most in terms of cost and required temperature.

Conclusions

We have described the syntheses of five-membered cyclic carbonates by the addition of CO₂ to epoxides using amidinium halides and other catalysts in a reversible room-temperature ionic liquid or acetonitrile. The conditions for performing the reactions are relatively mild when compared with most of the other methods in the literature, the yields are generally >90% at CO₂ pressures of several atmospheres, the products are easily separated from the reaction mixtures, and the RTIL/catalyst systems can be reused with the same or different epoxide substrates. Only the desired cyclic carbonates were produced upon reaction of the four epoxide substrates examined; the transformations are very clean under the reaction conditions. Notably, no metal catalyst is required to effect the reaction in our RTIL systems. Because the RTIL and catalysts can be reused and the temperatures and required CO₂ pressures are lower than in most other solvent/catalyst systems reported in the literature⁶—even 1 atm of CO₂ and room temperature lead to some product—this approach is an attractive alternative for preparing five-membered heterocycles and CO₂ fixation.

Future work will be directed to additional exploitation of these and other RTILs^16,17 developed by our group in the same and different reactions. For example, the RTILs may be able to effect syntheses of oxazolidinones from aziridines and CO₂,^48,49 and to prepare polycarbonates from epoxides and CO₂¹⁰ (under somewhat different experimental conditions than employed here). Also, we⁵⁰ are developing polymer-supported^6g,51 and optically-active amidinium halides as reusable catalysts to facilitate further the separation of products from the reaction media and to make chiral cyclic carbonates and oxazolidinones.^52,53

Acknowledgements

We thank the U. S. National Science Foundation for its financial support of this research. We are grateful to Prof. Rodrigo Cristiano, Mr. William Craig and Mr. Leon Der for their help in assembling and designing parts of the high pressure apparatus. We are very grateful to Profs. Pedro F. Aramendía and Laura Japas of the University of Buenos Aires (Argentina) for designing and constructing the stainless steel reaction cell and for invaluable technical advice. Prof. Christian Wolf and Mr. Peng Zhang are thanked for the use of their chiral column and HPLC.

Notes and References

1. (a) D. M. D'Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 2010, 49, 6058–6082; (b) C. A. Eckert, C. L. Liotta, D. Bush, J. S. Brown and J. P. Hallett, J. Phys. Chem. B, 2004, 108, 18108–18118; (c) M. Wei, G. T. Musie, D. H. Busch and B. Subramaniam, J. Am. Chem. Soc., 2002, 124, 2513–2517; (d) F. Jutz, J.-M. Andanson and A. Baiker, Chem. Rev., 2011, 111, 322–353; (e) S. N. Riduan and Y. Zhang, Dalton Trans., 2010, 39, 3347–3357; (f) W. Leitner, Coord. Chem. Rev., 1996, 153, 257–284; (g) J. Lu, M. J. Lazzaroni, J. P. Hallett, A. S. Bommarius, C. L. Liotta and C. A. Eckert, Ind. Eng. Chem. Res., 2004, 43, 1586–1590; (h) T. Yu, R. Cristiano and R. G. Weiss, Chem. Soc. Rev., 2010, 39, 1435–1447; (i) D. J. Darensbourg, Inorg. Chem., 2010, 49, 10765–10780.
2. M. M. Maroto-Valer, Ed. Carbon dioxide (CO₂) capture transport and industrial applicationsWoodhead Pub., Ltd:Oxford, 2010.
3. (a) A. Baba, H. Kashiwagi and H. Matsuda, Organometallics, 1987, 6, 137–140; (b) A. Baba, H. Kashiwagi and H. Matsuda, Tetrahedron Lett., 1985, 26, 1323–1324.
4. T. Sakakura, J-C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387.
5. D. J. Darensbourg, S. J. Lewis, J. L. Rodgers and J. C. Yarbrough, Inorg. Chem., 2003, 42, 581–589.
6. (a) R. L. Paddock and S. T. Nguyen, J. Am. Chem. Soc., 2001, 123, 11498–11499; (b) D. J. Darensbourg, C. C. Fang and J. L. Rodgers, Organometallics, 2004, 23, 924–927; (c) Y. M. Shen, W. L. Duan and M. Shi, J. Org. Chem., 2003, 68, 1559–1562; (d) H. W. Jing, S. K. Edulji, J. M. Gibbs, C. L. Stern, H. Y. Zhou and S. T. Nguyen, Inorg. Chem., 2004, 43, 4315–4325; (e) S. Liang, H. Liu, T. Jiang, J. Song, G. Yang and B. X. Han, Chem. Commun., 2011, 47, 2131–2133; (f) W. Clegg, R. W. Harrington, M. North and R. Pasquale, Chem.–Eur. J., 2010, 16, 6828–6843; (g) Y. Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu and K. Ding, Angew. Chem., Int. Ed., 2007, 46, 7255–7258; (h) M. Tu and R. J. Davis, J. Catal., 2001, 199, 85–91.
7. (a) M. Kihara, N. Hara and T. Endo, J. Org. Chem., 1993, 58, 6198–6202; (b) J. M. Sun, S. I. Fujita, F. Y. Zhao and M. Arai, Green Chem., 2004, 6, 613–615.
8. (a) M. Aresta, A. Dibenedetto, L. Gianfrate and C. Pastore, J. Mol. Catal. A: Chem., 2003, 204, 245–252; (b) R. Srivastava, D. Srinivas and P. Ratnasamy, J. Catal., 2005, 233, 1–15.
9. (a) H. Kawanami, A. Sasaki, K. Matsui and Y. Ikushima, Chem. Commun., 2003, 896–897; (b) A. L. Zhu, T. Jiang, B. X. Han, J. C. Zhang, Y. Xie and X. M. Ma, Green Chem., 2007, 9, 169–172; (c) H. Zhou, W.-Z. Zhang, C.-H. Liu, J.-P. Qu and X.-B. Lu, J. Org. Chem., 2008, 73, 8039–8044; (d) J.-L. Jiang, F. Gao, R. Hua and X. Qiu, J. Org. Chem., 2005, 40, 381–383; (e) S.-F. Yin and S. Shimada, Chem. Commun., 2009, 1136–1138; (f) W. J. Kruper and D. V. Dellar, J. Org. Chem., 1995, 60, 725–727; (g) Z. Yang, L.-N. He, C. Miao and S. Chanfreaua, Adv. Synth. Catal., 2010, 352, 2233–2240.
10. (a) D. J. Darensbourg, R. M. Mackiewicz, A. L. Phelps and D. R. Billodeaux, Acc. Chem. Res., 2004, 37, 836–844; (b) D. J. Darensbourg, M. W. Holtcamp, G. E. Struck, M. S. Zimmer, S. A. Niezgoda, P. Rainey, J. B. Robertson, J. D. Draper and J. H. Reibenspies, J. Am. Chem. Soc., 1999, 121, 107–116; (c) D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2003, 125, 11911–11924; (d) X.-B. Lu and Y. Wang, Angew. Chem., Int. Ed., 2004, 43, 3574–3577; (e) S. D. Thorat, P. J. Phillips, V. Semenov and A Gakh, J. Appl. Polym. Sci., 2003, 89, 1163–1176; (f) D. J. Darensbourg, A. I. Moncada, W. Choi and J. H. Reibenspies, J. Am. Chem. Soc., 2008, 130, 6523–6533.
11. F. Jutz, J-M. Andanson and A. Baiker, Chem. Rev., 2011, 111, 322–353.
12. (a) K. Yamaguchi, K. Ebitani, T. Yoshida, H. Yoshida and K. Kaneda, J. Am. Chem. Soc., 1999, 121, 4526–4527; (b) H. Kawanami, A. Sasaki, K. Matsui and Y. Ikushima, Chem. Commun., 2003, 896–897.
13. (a) S. Zhang, Y. Chen, F. Li, X. Lu, W. Dai and R. Mori, Catal. Today, 2006, 115, 61–69; (b) Y. X. Zhou, S. Q. Hu, X. M. Ma, S. G. Liang, T. Jiang and B. X. Han, J. Mol. Catal. A: Chem., 2008, 284, 52–57.
14. (a) F. Ono, K. Qiao, D. Tomida and C. Yokoyama, Appl. Catal., A, 2007, 333, 107–113; (b) S.-S. Wu, X.-W. Zhang, W.-L. Dai, S.-F. Yin, W.-S. Li, Y.-Q. Ren and C.-T. Au, Appl. Catal., A, 2008, 341, 106–111.
15. T. Yamada, P. J. Lukac, M. George and R. G. Weiss, Chem. Mater., 2007, 19, 967–969.
16. T. Yamada, P. J. Lukac, T. Yu and R. G. Weiss, Chem. Mater., 2007, 19, 4761–4768.
17. T. Yu, T. Yamada, G. C. Gaviola and R. G. Weiss, Chem. Mater., 2008, 20, 5337–5344.
18. T. Yu, T. Yamada and R. G. Weiss, Chem. Mater., 2010, 22, 5492–5499.
19. (a) D. J. Darensbourg and M. W. Holtcamp, Coord. Chem. Rev., 1996, 153, 155–174; (b) J. Sun, S. Zhang, W. Cheng and J. Ren, Tetrahedron Lett., 2008, 49, 3588–3591.
20. Y. Tsutsumi, K. Yamakawa, M. Yoshida, T. Ema and T. Sakai, Org. Lett., 2010, 12, 5728–5731.
21. In some reactions, the ‘wet’ (undried) catalysts were employed as well.
22. V. Calo, A. Nacci, A. Monopoli and A. Fanizzi, Org. Lett., 2002, 4, 2561–2563.
23. B. Barkakaty, K. Morino, A. Sudo and T. Endo, Green Chem., 2010, 12, 42–44.
24. Under the reaction conditions described in ref. 23 – 1 atm pressure of CO₂ and room temperature for 24 h – but with C6 amidine as the solvent, a ca. 22% yield of cyclic carbonate was obtained from cyclohexene oxide. Also, under the same conditions but with 25 mol% C6-HBr as catalyst in C6 amidine or DMF as solvent, the yields of cyclic carbonate were 28% or 20%, respectively. Experimental details are reported in the Supporting Information†.
25. J. March, Advanced Organic Chemistry, 4th ed., J. Wiley and Sons:New York, 1992.
26. (a) R. G. Pearson, J. Am. Chem. Soc., 1963, 85, 3533–3543; (b) P. K. Chattaraj, H. Lee and R. G. Parr, J. Am. Chem. Soc., 1991, 113, 1855–1856.
27. S-I. Fujita, M. Nishiura and M. Arai, Catal. Lett., 2010, 135, 263–268.
28. X-B. Lu, Y-J. Zhang, B. Liang, X. Li and H. Wang, J. Mol. Catal. A: Chem., 2004, 210, 31–34.
29. (a) M. Yoshida, M. Fujita, T. Ishii and M. Ihara, J. Am. Chem. Soc., 2003, 125, 4874–4881; (b) M. Yoshida and M. Ihara, Chem.–Eur. J., 2004, 10, 2886–2893; (c) S. Fukuoka, M. Kawamura, K. Komiya, M. Tojo, H. Hachiya, K. Hasegawa, M. Aminaka, H. Okamoto, I. Fukawa and S. Konno, Green Chem., 2003, 5, 497–507.
30. J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai and L.-N. He, J. Mol. Catal. A: Chem., 2006, 249, 143–148.
31. D. A. Evans, pK_a's of inorganic and oxoacids. http://evans.harvard.edu/pdf/evans_pKa_table.pdf.
32. Three ¹H NMR spectra were recorded: (1) 5.0 ×10⁻³ mol L⁻¹C6-HI in CDCl₃; (2) 0.5 × 10⁻³ M styrene oxide in CDCl₃; (3) 5.0 × 10⁻³ M C6-HI and 0.5 × 10⁻³ M styrene oxide in CDCl₃. The chemical shifts of the epoxide ring protons were carefully compared and no changes in the shifts were observed.
33. (a) V. Calo, A. Nacci, L. Lopez and A. Napola, Tetrahedron Lett., 2001, 42, 4701–4703; (b) V. Calo, A. Nacci, A. Monopoli, L. Lopez and A. Di Cosmo, Tetrahedron, 2001, 57, 6071–6077.
34. D. J. Heldebrant, P. G. Jessop, C. A. Thomas, C. A. Eckert and C. L. Liotta, J. Org. Chem., 2005, 70, 5335–5338.
35. J. Sun, J. Ren, S. Zhang and W. Cheng, Tetrahedron Lett., 2009, 50, 423–426.
36. (a) M. North, R. Pasquale and C. Young, Green Chem., 2010, 12, 1514–1539; (b) J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang and B. Han, Green Chem., 2009, 11, 1031–1036.
37. E. J. Beckman, J. Supercrit. Fluids, 2004, 28, 121–191.
38. J. Sun, S-I. Fujita, F. Zhao and M. Arai, Green Chem., 2004, 6, 613–616.
39. H. F. Zhang, B. X. Han, Z. S. Hou and Z. M Liu, Fluid Phase Equilib., 2001, 179, 131–138.
40. A. M. Belostotskii, E. Markevich and D. Aurbach, J. Coord. Chem., 2004, 57, 1047–1056.
41. L. Phan, J. R. Andreatta, L. K. Horvey, C. F. Edie, A.-L. Luco, A. Mirchandani, D. J. Darensbourg and P. G. Jessop, J. Org. Chem., 2008, 73, 127–132.
42. T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387.
43. R. Srivastava, D. Srinivas and P. Ratnasamy, Appl. Catal., A, 2005, 289, 128–134.
44. W. Sit, S. Ng, K. Kwong and C. Lau, J. Org. Chem., 2005, 70, 8583–8586.
45. T. Sakai, Y. Tsutsumi and T. Ema, Green Chem., 2008, 10, 337–341.
46. G. W. Coates and D. R. Moore, Angew. Chem., Int. Ed., 2004, 43, 6618–6639.
47. J.-Li. Jiang, F. Gao, R. Hua and X. Qiu, J. Org. Chem., 2005, 70, 381–383.
48. (a) A. W. Miller and S. T. Nguyen, Org. Lett., 2004, 6, 2301–2304; (b) Y.-M. Shen, W.-L. Duan and M. Shi, Eur. J. Org. Chem., 2004, 14, 3080–3089; (c) O. Ihata, Y. Kayaki and T. Ikariya, Angew. Chem., Int. Ed., 2004, 43, 717–719; (d) Y. Du, Y. Wu, A.-H. Liu and L.-N. He, J. Org. Chem., 2008, 73, 4709–4712.
49. (a) Z.-Z. Yang, L.-N. He, S.-Y. Peng and A.-H. Liu, Green Chem., 2010, 12, 1850–1855; (b) W.-Z. Li, and R. G. Weiss, unpublished results.
50. T. W. Farnsworth, T. Yu, and R. G. Weiss, unpublished results.
51. C. Qi, J. Ye, W. Zeng and H. Jiang, Adv. Synth. Catal., 2010, 352, 1925–1933.
52. (a) J. T. Groves and R. S. Myers, J. Am. Chem. Soc., 1983, 107, 5791–5796; (b) E. N. Jacobsen, Acc. Chem. Res., 2000, 33, 421–431; (c) L. E. Martinez, J. L. Leighton, D. H. Carsten and E. N. Jacobsen, J. Am. Chem. Soc., 1995, 117, 5897–5898; (d) E. N. Jacobsen, F. Kakiuchi, R. G. Konsler, J. F. Larrow and M. Tokunaga, Tetrahedron Lett., 1997, 38, 773–776.
53. Preliminary attempts to synthesize optically active cyclic carbonates using a chiral amidinium bromide catalyst, Nor-HBr, in CH₃CN have not been successful, thus far. The epoxides, 3a and 4a, produced high chemical yields of the corresponding cyclic carbonates, 3b and 4b, but neither with a discernible enantiomeric excess.
    .

---

Footnote

† Electronic supplementary information (ESI) available: Photographs of the high pressure CO₂ generator, reaction cell and temperature control setup, diagram of the layout of the high pressure instrument components, GC response factor calculations, ¹H NMR, ¹³C NMR, FT-IR, and MS spectra of cyclic carbonates and amidinium halide catalysts. See DOI: 10.1039/c1gc16027c

---