Sage Journals: Discover world-class research

Abstract

Hydroxyl- and carboxyl-functionalized imidazolium halides are used as efficient bifunctional organocatalysts for the synthesis of cyclic carbonates from CO₂ and epoxides under mild reaction conditions. Control experiments suggest that the cycloaddition reaction is realized by the combination of the nucleophilic halide anions with hydroxyl and carboxyl groups as hydrogen bond donors. Moreover, the bifunctional organocatalysts can be easily recycled five times by simple filtration; however, a loss of activity was observed.

Keywords

carbon dioxide cyclic carbonate epoxide hydrogen bond donor organocatalysis

A family of pyridine-bridged bifunctional organocatalysts were synthesized that have proved to be an efficient and recyclable catalyst for the cycloaddition of CO₂ with epoxides in mild reaction conditions.

Introduction

Carbon dioxide (CO₂) holds great potential as an abundant, inexpensive, nonflammable, nontoxic, and renewable C1 building block.^1–9 The synthesis of cyclic carbonates from CO₂ and epoxides is a 100% atom economical process which has been extensively studied due to their high utility as industrial intermediates for fine chemicals,^10–12 green solvents,¹³ monomers for polymer synthesis,^14–16 and electrolysis for lithium ion batteries.^17–19 A wide variety of catalytic systems for the coupling reaction of CO₂ with epoxides have been developed over the past decade to achieve these important types of compound.^20–32 Imidazolium-based compounds represent a class of organocatalysts for the synthesis of cyclic carbonates from CO₂ and epoxides,^33–39 because most of the imidazolium catalysts are synthesized via simple procedures using low-cost, readily available raw materials. We have recently reported a family of imidazolium salts for the cycloaddition reaction of epoxides with CO₂ at 90 °C under 1 MPa of CO₂.⁴⁰ However, our previously reported imidazolium-based catalytic system suffers from harsh reaction conditions.⁴⁰ Recent developments in the substrate activation strategy by introducing hydroxyl,^41–50 carboxyl,^51–57 and amino group^58–62 to imidazolium salts as hydrogen bond donors has demonstrated that the design of the catalysts plays an important role in tuning the catalytic activity. We have developed a method for the synthesis of unsymmetrical pyridine-bridged pincer-type ligand,⁶³ which has been applied in palladium-catalyzed the Suzuki cross-coupling reactions⁶⁴ and iron-catalyzed and organocatalyzed the cycloaddition of epoxides with CO₂.^{40,57,65–67} Bowen et al. reported that CO₂ is capable of binding to the nitrogen atom of a quinoline to form the quinolone–CO₂ anionic complex.⁶⁸ Inspired by previously reported work, we became interested in designing bifunctional imidazolium salts by introducing a hydrogen bond donor, such as COOH or OH (Figure 1). In this work, we present efficient bifunctional organocatalysts composed of a nucleophilic site and a Brønsted acidic site for the synthesis of cyclic carbonates. It was demonstrated that the organocatalysts were highly effective for the cycloaddition of epoxides with CO₂ at 60 °C under 0.5 MPa of CO₂ pressure.

Figure 1.

Design concept for the catalyst.

Results and discussion

The 10 bifunctional organocatalysts 1a–j were synthesized according to procedures reported previously.⁶³ At the beginning of the reaction, 2,6-dibromopyridine reacts with benzimidazole or imidazole to generate a 2-monosubstituted product, which is further converted into the 2,6-disubstituted pyridine via Suzuki coupling. Finally, the resultant disubstituted product is reacted with the corresponding n-propyl halide to afford the organocatalysts (Scheme 1).

Scheme 1.

The bifunctional organocatalysts used this work.

The ability of organocatalysts 1a–j to catalyze the cycloaddition of propylene oxide (PO) with CO₂ to form propylene carbonate (PC) was evaluated (Table 1, entries 1–10). First, the influence of the halide counteranions with catalysts on the catalytic activity was investigated. The formation of products showed a dependence on the nature of the anions, and the activity order is I⁻ > Br⁻ ≈ Cl⁻ (Table 1, entries 1–3). This suggests that the halide leaving ability is crucial for excellent performance because iodides possess a stronger leaving ability. We selected an iodide ion (I⁻) as the representative counteranion for investigating the effect of the position of the hydroxyl substituents on the aromatic ring on the catalytic activity (Table 1, entries 1, 4, and 5). The results indicated that the electronic nature and steric hindrance due to the hydroxyl group on the aromatic ring are important for the catalytic activity. The best catalytic efficiency was obtained when the hydroxyl group was located at the para-position of the aromatic ring (Table 1, entry 1). The hydroxyl group facilitates nucleophilic attack for ring opening of the epoxide through hydrogen bonding with the epoxide. We assume that the carboxyl group would be advantageous since stronger acidity could be provided compared with the hydroxyl group. This led us to the assumption that the carboxyl-functionalized imidazolium halides would be more catalytically active than their hydroxyl-functionalized counterparts because the carboxyl group possesses stronger hydrogen bond interactions. However, the carboxyl-functionalized catalysts showed slightly lower activity than those of hydroxyl-functionalized imidazolium halides (Table 1, entries 1 vs 6). In order to understand the possible causes behind these unexpected results, we investigated the reaction process. The results indicated that the carboxyl-functionalized imidazolium iodide 1f (0.0085 mmol mL⁻¹) displayed poor solubility in PO solution compared with the hydroxyl-functionalized iodide 1a (0.021 mmol mL⁻¹). We assume that the solubility difference could be responsible for the results.

Table 1.

Optimization of propylene carbonate synthesis^a.


Entry	Cat. (mol%)	T (°C)	Time (h)	Yield (%)^b
1	1a (1.0)	90	3	90
2	1b (1.0)	90	3	67
3	1c (1.0)	90	3	63
4	1d (1.0)	90	3	78
5	1e (1.0)	90	3	39
6	1f (1.0)	90	3	81
7	1g (1.0)	90	3	84
8	1h (1.0)	90	3	77
9	1i (1.0)	90	3	36
10	1j (1.0)	90	3	78
11	1a (1.0)	60	3	23
12	1a (2.0)	60	7	75
13	1a (2.0)	60	24	96
14	1a (2.0)	rt	24	16

PO: propylene oxide; GC: gas chromatography.

Conditions: PO (10.0 mmol), catalyst (indicated in Table 1), no solvent, CO₂ (0.5 MPa).

Determined by GC analyses using biphenyl as an internal standard.

The type of imidazolium halide was also investigated, but there was no evident difference between benzimidazole salts and imidazolium salts, and a similar reactivities were observed (Table 1, entries 1 and 4–6 vs 7–10). An attempt to decrease the reaction temperature from 90 °C to 60 °C resulted in a low yield (Table 1, entries 1 vs 11), but excellent results were obtained when the loading of the catalyst 1a was increased from 1.0 mol% to 2.0 mol% with prolonging the reaction time to 24 h (Table 1, entries 12 and 13). Further lowering of the reaction temperature to room temperature, however, gave a poor result (Table 1, entry 14).

With optimized reaction conditions in hand (Table 1, entry 13), the substrate scope of epoxides for the cycloaddition of CO₂ was investigated (Table 2). Epoxides bearing electron-donating substituents, such as PO (2a), 2-ethyloxirane (2b), 1,2-epoxyhexane (2c), and epoxyoctane (2d), were smoothly converted into the desired products in good to excellent yields. Epoxides bearing electron-withdrawing substituents also reacted smoothly with CO₂ to give the corresponding cyclic carbonates 3e and 3f. In addition, aliphatic and aromatic substrates were transformed with CO₂ into the desired products 3h–j in yields of 84%–94%.

Table 2.

Scope of the substrates^a.

NMR: nuclear magnetic resonance.

Conditions: epoxide (10 mmol), 1a (0.2 mmol), no solvent, 60 °C, CO₂ (0.5 MPa), 24 h, isolated yield.

90 °C.

The selectivity refers to the ratio between cycloaddition product and polymerization product.

The d.r. values were determined by ¹H and ¹³C NMR spectroscopy.

120 °C.

The challenging substrate 2,2-dimethyloxirane was also converted into the corresponding cyclic carbonates 3k with 87% yield. Internal epoxides are challenging substrates because of their highly hindered nature and tendency to polymerize. An elevated reaction temperature was required to achieve the high conversion of internal epoxides, such as cyclohexene oxides 2l and 2m as well as bicyclic epoxide 2o, into the desired products 3l, 3m, and 3o. 2,3-Dimethyloxirane 2n and bicyclic epoxide 2p show lower reactivity even if a higher reaction temperature is employed.

Besides carboxyl and hydroxyl groups working as hydrogen donors, the proton at the C2 position of the imidazolium ring can also enhance the catalytic activity through the interaction with the epoxides.^69,70 In order to prove whether the hydrogen interaction occurs through the hydroxyl group or through the proton at the C2 position of the imidazolium ring, two control experiments were carried out with two designed catalysts (Scheme 2). To explore the function of the hydroxyl group on this reaction, a catalyst was synthesized by etherification of the phenolic hydroxyl group (1k, in Scheme 2, equation (1)). In the absence of hydroxyl protons, an evident reduction in the catalytic activity was observed (Scheme 2, equation (1)). In addition, the effect of the C2 proton in the imidazolium ring was investigated. When the C2 proton was protected by a methyl group (1l, in Scheme 2, equation (2)), the activity of the catalyst 1l was slightly affected (91% yield in Scheme 2, equation (2)) in comparison with the catalyst 1a (96% yield in Table 1, entry 13). The results indicate that the hydroxyl proton bearing aromatic ring activates epoxides more easily than the proton at the C2 position of the imidazolium ring.

Scheme 2.

Exploration of the hydrogen bond donor in the reaction.

Based on the above investigations, a mechanism for the organocatalytic cycloaddition of epoxides with CO₂ using bifunctional imidazolium salts has been proposed (Figure 2). In the first step, the hydroxyl proton on the catalyst 1a could coordinate to the epoxide through a hydrogen bond interaction, which results in polarization of the C–O bond. Subsequently, the epoxide ring is opened by nucleophilic attack by the iodide to form intermediate (II). Second, the negatively charged oxygen atom can nucleophilically attack the electrophilic carbon atom of CO₂, which leads to the formation of intermediate (III). The resulting open-ring intermediate (III) undergoes intramolecular nucleophilic attack, simultaneously releasing the catalytically active iodide and the cyclic carbonate product.

Figure 2.

Proposed mechanism for bifunctional organocatalysis.

The recycling procedure for the catalysts is often through energy-intensive distillation of the cyclic carbonates. The separation of catalysts by precipitation is a sustainable strategy compared with distillation of the products. The recyclability of catalyst 1a was investigated (Figures 3 and 4). During recycling experiments, the catalyst can be precipitated from the product phase via the addition of ethyl acetate (Figure 3). The recycled catalyst is directly used without further purification, and another batch of starting materials is added and the catalyst reused for subsequent recycles. However, a loss of catalytic activity was observed during these recycling studies (Figure 4), which was mainly attributed to leaching of the catalyst.

Figure 3.

Recycling of the bifunctional organocatalyst.

Figure 4.

Recycling experiments. Reaction conditions: PO (10.0 mmol), no solvent, 60 °C, CO₂ (0.5 MPa), 24 h, yield was determined by GC analysis using biphenyl as an internal standard.

Conclusion

In summary, a family of pyridine-bridged bifunctional organocatalysts has been prepared and the utility of these catalysts for the synthesis of cyclic carbonates from CO₂ and epoxides under mild reaction conditions was demonstrated. The one-component catalyst displayed a broad substrate scope and various functionalized terminal epoxides as well as internal epoxides could be converted into corresponding cyclic carbonates with good to excellent yields. Control experiments indicate that the cooperative role of the hydroxyl groups with the nucleophilic halide anions is crucial for the excellent performance.

Experimental section

The bifunctional organocatalysts 1a–l; general procedure

The bifunctional catalysts were prepared according to procedures reported previously.⁶³ A mixture of 2,6-dibromopyridine (5 mmol), imidazole or benzimidazole (10 mmol), CuI (1 mmol), tetramethylethane-1,2-diamine (TMEDA, 2 mmol), and K₂CO₃ (15 mmol) in dimethyl sulfoxide (DMSO; 20 mL) was stirred for 30 min at room temperature and then heated to 90 °C for 24 h under a nitrogen atmosphere. Thereafter, the 2-bromo-6-substituted pyridine (5 mmol), arylboronic acid (7.5 mmol), PdCl₂ (0.2 mmol), and K₂CO₃ (10 mmol) in dimethylformamide (DMF)/H₂O (10 mL/10 mL) were allowed to react at 90 °C for 12 h under air. The resulting 2,6-disubstituted pyridine (5 mmol) and n-propyl halide (10 mL) were heated to 100 °C and reacted for 8 h under an air atmosphere. The solvent was concentrated under vacuum and the product organocatalyst 1a–l was isolated by column chromatography on silica gel (dichloromethane (DCM)/methanol).

Data for 1-(6-(4-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium iodide 1a are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 30:1). Yellowish solid (1.09 g, 80%), m.p. = 229.8 °C–230.7 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.55 (s, 1H), 9.99 (s, 1H), 8.51 (d, J = 8.0 Hz, 1H), 8.27 (t, J = 8.0 Hz, 2H), 8.15 (d, J = 8.0, 1H), 8.10 (d, J = 8.0 Hz, 2H), 7.90 (d, J = 8.0 Hz, 1H), 7.85–7.78 (m, 2H), 6.95 (d, J = 8.0 Hz, 2H), 4.60 (t, J = 8.0 Hz, 2H), 2.06 (sext, J = 7.2 Hz, 2H), 1.02 (t, J = 8.0 Hz, 3H); ¹³C NMR (100 MHz, DMSO-d₆): δ 159.55, 156.54, 146.95, 142.33, 141.27, 131.72, 129.52, 128.54, 127.85, 127.72, 127.10, 119.92, 115.87, 115.65, 114.16, 114.08, 48.70, 22.09, 10.80; high-resolution mass spectrometry (HRMS) (Matrix-Assisted Laser Desorption (MALDI): m/z [M – I]⁺ calcd for C₂₁H₂₀N₃O: 330.1601; found: 330.1607.

Data for 1-(6-(4-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium bromide 1b are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 30:1): a yellowish solid (1.09 g, 89%), m.p. = 246.5 °C–247.3 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.58 (s, 1H), 9.99 (s, 1H), 8.51 (d, J = 8.0 Hz, 1H), 8.27 (t, J = 8.0 Hz, 2H), 8.16–8.09 (m, 3H), 7.91 (d, J = 8.0 Hz, 1H), 7.85–7.78 (m, 2H), 6.96 (d, J = 8.0 Hz, 2H), 4.61 (t, J = 7.2 Hz, 2H), 2.06 (sext, J = 7.2 Hz, 2H), 1.02 (t, J = 8.0 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO): δ 159.56, 156.53, 146.97, 142.36, 141.26, 131.72, 129.51, 128.52, 127.84, 127.71, 127.09, 119.90, 115.86, 115.66, 114.15, 114.07, 48.67, 22.09, 10.79, ppm; HRMS (MALDI): m/z [M – Br]⁺ calcd for C₂₁H₂₀N₃O: 330.1601; found: 330.1609.

Data for 1-(6-(4-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium chloride 1c are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 50:1): a yellowish solid (864 mg, 77%), m.p. = 226.5 °C–227.2 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.70 (s, 1H), 10.00 (s, 1H), 8.51–7.81 (m, 9H), 6.97 (s, 2H), 4.63 (s, 2H), 2.06 (s, 2H), 1.02 (s, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 159.55, 156.47, 147.00, 142.33, 141.22, 131.70, 129.46, 128.49, 127.81, 127.69, 127.06, 119.84, 115.86, 115.69, 114.15, 114.03, 48.64, 22.10, 10.78, ppm; HRMS (MALDI): m/z [M – Cl]⁺ calcd for C₂₁H₂₀N₃O: 330.1601; found: 330.1598.

Data for 1-(6-(3-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium iodide 1d are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 100:1): a yellowish solid (1.16 g, 84%), m.p. = 236.2 °C–237.0 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.57 (s, 1H), 9.75 (s, 1H), 8.54 (d, J = 8.0 Hz, 1H), 8.34 (t, J = 8.0 Hz, 1H), 8.27 (d, J = 8.0 Hz, 1H), 8.20 (d, J = 8.0 Hz, 1H), 8.00 (d, J = 8.0 Hz, 1H), 7.85–7.79 (m, 2H), 7.63 (t, J = 8.0 Hz, 2H), 7.39 (t, J = 8.0 Hz, 1H), 6.95 (d, J = 8.0 Hz, 1H), 4.61 (t, J = 7.2 Hz, 2H), 2.06 (sext, J = 7.2 Hz, 2H), 1.02 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 158.00, 156.30, 147.15, 142.38, 141.50, 138.25, 131.70, 130.18, 129.51, 127.83, 127.12, 121.10, 117.68, 117.17, 115.77, 115.43, 114.16, 113.55, 48.72, 22.06, 10.79, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₂₁H₂₀N₃O: 330.1601; found: 330.1606.

Data for 1-(6-(2-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium iodide 1e are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 30:1): a yellowish solid (1.206 g, 88%), m.p. = 254.5 °C–255.1 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.59 (s, 1H), 10.55 (s, 1H), 8.49–8.47 (m, 1H), 8.33 (d, J = 5.2 Hz, 2H), 8.28–8.26 (m, 1H), 8.01–7.96 (m, 2H), 7.81–7.76 (m, 2H), 7.35 (t, J = 6.0 Hz, 1H), 7.06–6.99 (m, 2H), 4.62 (t, J = 8.0 Hz, 2H), 2.06 (sext, J = 7.2 Hz, 2H), 1.02 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 156.04, 155.80, 146.28, 142.27, 140.71, 131.64, 131.17, 129.94, 129.53, 127.72, 127.07, 124.53, 123.13, 119.60, 116.94, 115.52, 114.95, 114.14, 48.71, 22.03, 10.78, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₂₁H₂₀N₃O: 330.1601; found: 330.1597.

Data for 1-(6-(4-carboxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium iodide 1f are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 30:1): a yellowish solid (1.321 g, 91%), m.p. = 236.3 °C–236.7 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 13.18 (s, 1H), 10.61 (s, 1H), 8.54 (d, J = 8.0 Hz, 1H), 8.44–8.35 (m, 4H), 8.28 (d, J = 8.0 Hz, 1H), 8.15–8.09 (m, 3H), 7.86–7.79 (m, 2H), 4.62 (t, J = 8.0 Hz, 2H), 2.07 (sext, J = 7.2 Hz, 2H), 1.03 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 166.91, 155.16, 147.29, 142.51, 141.85, 140.73, 132.00, 131.72, 130.03, 129.51, 127.96, 127.15, 121.87, 116.38, 115.69, 114.19, 48.75, 22.09, 10.80, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₂₂H₂₀N₃O₂: 358.1550; found: 358.1544.

Data for 1-(6-(4-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-imidazol-3-ium iodide 1g are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 30:1): a yellowish solid (1.014 g, 83%), m.p. = 192.0 °C–192.8 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.20 (s, 1H), 9.96 (s, 1H), 8.65 (s, 1H), 8.20–8.14 (m, 3H), 8.08 (t, J = 8.0 Hz, 2H), 7.85 (d, J = 8.0 Hz, 1H), 6.92 (d, J = 8.0 Hz, 2H), 4.30 (t, J = 8.0 Hz, 2H), 1.94 (sext, J = 7.2 Hz, 2H), 0.94 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 159.61, 155.98, 146.00, 141.26, 134.93, 128.66, 127.38, 123.55, 119.71, 119.32, 115.69, 110.85, 51.02, 22.78, 10.50, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₁₇H₁₈N₃O: 280.1444; found: 280.1441.

Data for 1-(6-(3-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-imidazol-3-ium iodide 1h are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 100:1): a yellowish solid (1.007 g, 80%), m.p. = 190.4 °C–191.0 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.19 (s, 1H), 9.67 (s, 1H), 8.63 (s, 1H), 8.26 (t, J = 8.0 Hz, 1H), 8.12 (d, J = 8.0 Hz, 2H), 7.95 (d, J = 8.0 Hz, 1H), 7.69 (d, J = 8.0 Hz, 1H), 7.62 (s, 1H), 7.35 (t, J = 8.0 Hz, 1H), 6.94 (d, J = 8.0 Hz, 1H), 4.30 (t, J = 7.2 Hz, 2H),1.94 (sext, J = 7.2 Hz, 2H), 0.94 (t, J = 8.0 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 157.92, 155.89, 146.11, 141.49, 137.94, 135.02, 129.96, 123.55, 121.02, 119.35, 117.80, 117.16, 113.75, 112.42, 51.00, 22.75, 10.47, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₁₇H₁₈N₃O: 280.1444; found: 280.1445.

Data for 1-(6-(2-hydroxyphenyl)pyridin-2-yl)-3-propyl-1H-imidazol-3-ium iodide 1i are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 100:1): a yellowish solid (1.102 g, 90%), m.p. = 154.1 °C–154.9 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.40 (s, 1H), 10.14 (s, 1H), 8.61 (t, J = 2.0 Hz, 1H), 8.31 (d, J = 8.0 Hz, 1H), 8.22 (t, J = 8.0 Hz, 1H), 8.10–8.06 (m, 2H), 7.91 (d, J = 8.0 Hz, 1H), 7.33 (t, J = 8.0 Hz, 1H), 7.04–6.97 (m, 2H), 4.30 (t, J = 8.0 Hz, 2H), 1.93 (sext, J = 7.2 Hz, 2H), 0.93 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 156.01, 155.16, 145.72, 140.59, 134.96, 131.17, 130.41, 124.86, 123.52, 123.11, 119.48, 119.42, 116.78, 111.78, 50.96, 22.74, 10.46, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₁₇H₁₈N₃O: 280.1444; found: 280.1439.

Data for 1-(6-(4-carboxyphenyl)pyridin-2-yl)-3-propyl-1H-imidazol-3-ium iodide 1j are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 50:1): a yellowish solid (873 mg, 67%), m.p. = 213.7 °C–214.4 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.42 (s, 1H), 8.72 (s, 1H), 8.45 (s, 1H), 8.32–8.23 (m, 4H), 8.13 (s, 1H), 8.06 (d, J = 8.0 Hz, 3H), 4.32 (t, J = 7.2 Hz, 2H), 1.94 (sext, J = 7.2 Hz, 2H), 0.94 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 167.10, 154.50, 146.26, 141.80, 140.13, 135.07, 132.40, 129.76, 127.14, 123.61, 121.58, 119.39, 113.21, 51.04, 22.78, 10.49, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₁₈H₁₈N₃O₂: 308.1394; found: 308.1390.

Data for 1-(6-(4-methoxyphenyl)pyridin-2-yl)-3-propyl-1H-benzo[d]imidazol-3-ium iodide 1k are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 30:1): a yellowish solid (1.278 g, 90%), m.p. = 247.5 °C–248.2 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.59 (s, 1H), 8.51 (d, J = 8.0 Hz, 1H), 8.33–8.26 (m, 2H), 8.20 (d, J = 8.0 Hz, 3H), 7.95 (d, J = 8.0 Hz, 1H), 7.85–7.78 (m, 2H), 7.13 (d, J = 8.0 Hz, 2H), 4.62 (t, J = 7.2 Hz, 2H), 3.85 (s, 3H), 2.06 (sext, J = 7.2 Hz, 2H), 1.03 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 160.94, 156.07, 147.00, 142.30, 141.36, 131.69, 129.45, 129.23, 128.41, 127.81, 127.09, 120.23, 115.63, 114.48, 114.16, 55.40, 48.70, 22.07, 10.78, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₂₂H₂₂N₃O: 344.1757; found: 344.1762.

Data for 1-(6-(4-hydroxyphenyl)pyridin-2-yl)-2-methyl-3-propyl-1H-benzo[d]imidazol-3-ium iodide 1l are as follows: Purification by column chromatography on silica gel (DCM/MeOH = 50:1): a yellowish solid (1.296 g, 92%), m.p. = 232.8 °C–233.5 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 10.18 (s, 1H), 8.30–8.20 (m, 3H), 8.01 (d, J = 8.0 Hz, 2H), 7.77–7.64 (m, 4H), 6.95 (d, J = 8.0 Hz, 2H), 4.60 (t, J = 7.2 Hz, 2H), 2.96 (s, 3H), 1.94 (sext, J = 7.2 Hz, 2H), 1.05 (t, J = 7.2 Hz, 3H), ppm; ¹³C NMR (100 MHz, DMSO-d₆): δ 159.73, 157.30, 151.88, 145.46, 141.28, 130.94, 130.70, 128.41, 127.53, 127.02, 126.61, 120.92, 118.85, 115.86, 113.46, 113.38, 46.83, 21.89, 11.86, 10.94, ppm; HRMS (MALDI): m/z [M – I]⁺ calcd for C₂₂H₂₂N₃O: 344.1757; found: 344.1760.

Cyclic carbonates 3a–p; general procedure

PO (587 mg, 10.0 mmol) and catalyst 1a (91.4 mg, 0.2 mmol) were successively put into a 25 mL stainless steel reactor that was equipped with a magnetic stir bar. The reactor was pressurized with CO₂ to 0.5 MPa and the contents reacted at 60 °C for 24 h. After the reaction was complete, the reactor was cooled to room temperature and excess CO₂ was carefully vented off. The reaction mixture was added to brine (50 mL) and extracted with dichloromethane (3 × 50 mL). The solvent was removed under reduced pressure and the product was isolated by column chromatography on silica gel (petroleum ether/ethyl acetate).

4-Methyl-1,3-dioxolan-2-one (3a): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 2:1) gave a colorless oil (918 mg, 91%). ¹H NMR (400 MHz, CDCl₃): δ 4.79–4.73 (m, 1H), 4.48–4.43 (m, 1H), 3.94–3.89 (m, 1H), 1.36–1.34 (m, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 155.16, 73.76, 70.68, 19.03.

4-Ethyl-1,3-dioxolan-2-one (3b): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 2:1) gave a yellow oil (1107 mg, 95%). ¹H NMR (400 MHz, CDCl₃): δ 4.67–4.59 (m, 1H), 4.51–4.46 (m, 1H), 4.06–4.01 (m, 1H), 1.78–1.65 (m, 2H), 0.99–0.94 (m, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 155.21, 78.11, 69.06, 26.79, 8.38.

4-Butyl-1,3-dioxolan-2-one (3c): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a colorless oil (1345 mg, 93%). ¹H NMR (400 MHz, CDCl₃): δ 4.73–4.66 (m, 1H), 4.54–4.50 (m, 1H), 4.08–4.04 (m, 1H), 1.83–1.64 (m, 2H), 1.47–1.27 (m, 4H), 0.93–0.89 (m, 3H); ¹³C NMR (101 MHz, CDCl₃): δ 155.2, 77.2, 69.5, 33.6, 26.5, 22.3, 13.8.

4-Hexyl-1,3-dioxolan-2-one (3d): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a colorless oil (1652 mg, 96%). ¹H NMR (400 MHz, CDCl₃): δ 4.73–4.66 (m, 1H), 4.52 (t, J = 8.0 Hz, 1H), 4.06 (dd, J = 8.4 Hz, J = 7.2 Hz, 1H), 1.82–1.74 (m, 1H), 1.71–1.63 (m, 1H), 1.49–1.28 (m, 8H), 0.87 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 155.15, 77.15, 69.43, 33.76, 31.46, 28.73, 24.26, 22.39, 13.91.

4-(Chloromethyl)-1,3-dioxolan-2-one (3e): Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a yellow oil (1293 mg, 95%). ¹H NMR (400 MHz, CDCl₃): δ 5.02–4.96 (m, 1H), 4.61(t, J = 8.0 Hz, 1H), 4.42 (dd, J = 8.8 Hz, J = 5.6 Hz, 1H), 3.83–3.73 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 154.54, 74.54, 66.99, 44.20.

4-(Chloromethyl)-4-methyl-1,3-dioxolan-2-one (3f): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 2:1) gave a yellow oil (1456 mg, 97%). ¹H NMR (400 MHz, CDCl₃): δ 4.54 (d, J = 8.4 Hz, 1H), 4.11 (d, J = 8.4 Hz, 1H), 3.77 (d, J = 12.4 Hz, 1H), 3.53 (d, J = 12.4 Hz, 1H), 1.45 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 155.48, 84.04, 71.50, 65.80, 21.29.

4-((Allyloxy)methyl)-1,3-dioxolan-2-one (3g): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a colorless oil (1450 mg, 92%). ¹H NMR (400 MHz, CDCl₃): δ 5.92–5.83 (m, 1H), 5.31–5.21 (m, 2H), 4.86–4.81 (m, 1H), 4.51 (t, J = 8.0 Hz, 1H), 4.41 (dd, J = 8.0 Hz, 6.0 Hz, 1H), 4.07–4.04 (m, 2H), 3.72–3.60 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 154.95, 133.67, 117.94, 75.04, 72.61, 68.85, 66.29.

4-(Phenoxymethyl)-1,3-dioxolan-2-one (3h): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 2:1) gave a white solid (1825 mg, 94%), m.p. = 100.9 °C–101.2 °C. ¹H NMR (400 MHz, CDCl₃): δ 7.33 (t, J = 8.0 Hz, 2H), 7.04 (t, J = 8.0 Hz, 1H), 6.94 (d, J = 8.0 Hz, 2H), 5.08–5.02 (m, 1H), 4.64 (t, J = 8.4 Hz, 1H), 4.56 (dd, J = 8.8 Hz, J = 6.0 Hz, 1H), 4.26 (dd, J = 10.4 Hz, J = 4.4 Hz, 1H), 4.17 (dd, J = 10.8 Hz, J = 3.6 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 157.77, 154.71, 129.71, 121.99, 114.63, 74.18, 66.90, 66.25.

4-(Butoxymethyl)-1,3-dioxolan-2-one (3i): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a colorless oil (1637 mg, 92%). ¹H NMR (400 MHz, CDCl₃): δ 4.84–4.78 (m, 1H), 4.50 (t, J = 8.0 Hz, 1H), 4.39 (dd, J = 8.0 Hz, 6.0 Hz, 1H), 3.70–3.59 (m, 2H), 3.51 (t, J = 6.4 Hz, 2H), 1.56 (quint, J = 6.4 Hz, 2H), 1.36 (sext, J = 8.0 Hz, 2H), 0.92 (t, J = 8.0 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 154.98, 75.10, 71.91, 69.64, 66.32, 31.52, 19.15, 13.83.

4-Phenyl-1,3-dioxolan-2-one (3j): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 10:1) gave a yellow solid (1386 mg, 84%), m.p. = 50.0 °C–51.4 °C. ¹H NMR (400 MHz, CDCl₃): δ 7.48–7.43 (m, 3H), 7.38–7.36 (m, 2H), 5.69 (t, J = 8.0 Hz, 1H), 4.81 (t, J = 8.0 Hz, 1H), 4.34 (t, J = 8.0 Hz, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 154.95, 135.87, 129.72, 129.23, 125.94, 78.05, 71.21.

4,4-Dimethyl-1,3-dioxolan-2-one (3k): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a colorless oil (1010 mg, 87%). ¹H NMR (400 MHz, CDCl₃): δ 4.13 (s, 2H), 1.50 (s, 6H). ¹³C NMR (100 MHz, CDCl₃): δ 154.64, 81.81, 75.37, 25.95.

Hexahydrobenzo[d][1,3]dioxol-2-one (3l): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a colorless oil (1240 mg, 87%). ¹H NMR (400 MHz, CDCl₃): δ 4.67–4.62 (m, 2H), 1.88–1.74 (m, 4H), 1.57–1.48 (m, 2H), 1.41–1.31 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 155.38, 75.78, 26.62, 19.03.

5-Vinylhexahydrobenzo[d][1,3]dioxol-2-one (3m): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) give a colorless oil (1386 mg, 83%). ¹H NMR (400 MHz, CDCl₃): δ 5.75–5.65 (m, 2H), 5.03–4.94 (m, 4H), 4.76–4.60 (m, 4H), 2.31–2.08 (m, 5H), 1.98 (s, 1H), 1.79–1.51 (m, 5H), 1.36–1.10 (m, 3H); ¹³C NMR (100 MHz, mixture of diastereoisomers, CDCl₃): δ 155.13, 155.10, 141.03, 140.93, 114.14, 113.84, 75.97, 75.62, 75.57, 75.08, 36.21, 33.81, 33.47, 31.55, 26.60, 25.69, 25.58, 24.97.

(4R,5R)-4,5-dimethyl-1,3-dioxolan-2-one (3n): Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a colorless oil (151 mg, 13%). ¹H NMR (400 MHz, CDCl₃) δ: 4.35–4.28 (m, 2H), 1.46–1.42 (m, 6H); ¹³C NMR(100 MHz, CDCl₃) δ: 154.61, 80.01, 18.51.

Tetrahydro-4H-cyclopenta[d][1,3]dioxol-2-one (3o): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a white solid (987.2 mg, 77%). ¹H NMR (400 MHz, CDCl₃): δ 5.14–5.10 (m, 2H), 2.18–2.13 (m, 2H), 1.85–1.63 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ 155.47, 81.85, 33.19, 21.57.

Tetrahydrofuro[3,4-d][1,3]dioxol-2-one (3p): Purification by column chromatography on silica gel (petroleum ether/EtOAc = 5:1) gave a white solid (144.3 mg, 11%). ¹H NMR (400 MHz, CDCl₃): δ 5.22–5.21 (m, 2H), 4.28–4.25 (m, 2H), 3.59–3.55 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 154.39, 80.04, 73.02.

Recycling experiments; typical procedure

PO (10.0 mmol) and catalyst 1a (0.2 mmol) were successively put into a 25 mL stainless steel reactor equipped with a magnetic stir bar. The reactor was pressurized with CO₂ to 0.5 MPa and the contents reacted at 60 °C for 24 h. After the reaction was complete, the reactor was cooled to room temperature and excess CO₂ was carefully vented off. The catalyst could easily be precipitated from the cyclic carbonate upon addition of ethyl acetate (5 mL). After filtration, the recycled catalyst was directly used without further purification, and another batch of PO (10.0 mmol) was added and the experiment repeated for subsequent cycles under the same reaction conditions. The solvent was removed under reduced pressure and the products were isolated by column chromatography on silica gel.

Analytical methods

The NMR spectra were recorded on a Bruker Avance III HD 400 spectrometer using tetramethylsilane (TMS) as an internal standard (400 MHz for ¹H NMR and 100 MHz for ¹³C NMR). Mass spectroscopy data were collected on a Bruker ultrafleXtreme mass spectrometer.

Supplemental Material

supporting_information – Supplemental material for Pyridine-bridged bifunctional organocatalysts for the synthesis of cyclic carbonates from carbon dioxide

Supplemental material, supporting_information for Pyridine-bridged bifunctional organocatalysts for the synthesis of cyclic carbonates from carbon dioxide by Quan-Yao Liu, Lei Shi and Ning Liu in Journal of Chemical Research

Footnotes

Acknowledgements

The authors thank Dr Cheng Guo (Cancer Institute, The Second Affiliated Hospital, Zhejiang University School of Medicine) for high-resolution mass spectrometry analysis.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Ning Liu

Supplemental material

Supplemental material for this article is available online.

References

Artz

Müller

Thenert

, et al. Chem Rev 2017; 118: 434.

Sanz-Perez

Murdock

Didas

, et al. Chem Rev 2016; 116: 11840.

Federsel

Jackstell

Beller

. Angew Chem Int Ed 2010; 49: 6254.

Decortes

Castilla

Kleij

. Angew Chem Int Ed 2010; 49: 9822.

Aresta

Dibenedetto

Angelini

. Chem Rev 2014; 114: 1709.

Cokoja

Bruckmeier

Rieger

, et al. Angew Chem Int Ed 2011; 50: 8510.

Sakakura

Choi

Yasuda

. Chem Rev 2007; 107: 2365.

Maeda

Miyazaki

Ema

. Catal Sci Technol 2014; 4: 1482.

North

Styring

. Faraday Discuss 2015; 183: 489.

10.

Cai

Guo

Martinez-Rodriguez

, et al. J Am Chem Soc 2016; 138: 14194.

11.

Xie

Guo

Cai

, et al. Org Lett 2017; 19: 6388.

12.

Gomez

Guo

Kleij

. Org Lett 2016; 18: 6042.

13.

Riduan

Zhang

. Dalton Trans 2010; 39: 3347.

14.

Darensbourg

. Chem Rev 2007; 107: 2388.

15.

Darensbourg

Wilson

. Green Chem 2012; 14: 2665.

16.

Guillaume

Carpentier

. Catal Sci Technol 2012; 2: 898.

17.

Matsumoto

Kakehashi

Ouchi

, et al. Macromolecules 2016; 49: 9441.

18.

Chai

Liu

Zhang

, et al. ACS Appl Mater Interfaces 2017; 9: 17897.

19.

Ren

Zhang

Engelhard

, et al. ACS Energy Lett 2017; 3: 14.

20.

Shaikh

Pornpraprom

D’Elia

. ACS Catal 2017; 8: 419.

21.

Martín

Fiorani

Kleij

. ACS Catal 2015; 5: 1353.

22.

Alves

Grignard

Mereau

, et al. Catal Sci Technol 2017; 7: 2651.

23.

Kleij

Rintjema

. Synthesis 2016; 48: 3863.

24.

Comerford

Ingram

IDV

North

, et al. Green Chem 2015; 17: 1966.

25.

North

Pasquale

Young

. Green Chem 2010; 12: 1514.

26.

Wang

Miao

Dou

, et al. Curr Org Chem 2011; 15: 621.

27.

Song

Zhou

. Green Chem 2017; 19: 3707.

28.

Bobbink

Dyson

. J Catal 2016; 343: 52.

29.

Buttner

Longwitz

Steinbauer

, et al. Top Curr Chem 2017; 375: 50.

30.

Wang

Sun

, et al. Green Chem 2015; 17: 108.

31.

Cokoja

Wilhelm

Anthofer

, et al. ChemSusChem 2015; 8: 2436.

32.

O’Brien

Kitselman

, et al. Catal Sci Technol 2014; 4: 1513.

33.

Wang

Zhang

, et al. J Am Chem Soc 2013; 135: 11996.

34.

Wang

Sun

Zhou

, et al. Green Chem 2014; 16: 2266.

35.

Buttner

Steinbauer

Werner

. ChemSusChem 2015; 8: 2655.

36.

Calabrese

Liotta

Carbonell

, et al. ChemSusChem 2017; 10: 1202.

37.

Tharun

Bhin

Roshan

, et al. Green Chem 2016; 18: 2479.

38.

Zhang

Huang

Jing

, et al. Green Chem 2009; 11: 935.

39.

Zhang

Xiao

, et al. J CO₂ Util 2017; 17: 37.

40.

Chen

Shi

, et al. J CO₂ Util 2016; 16: 391.

41.

Whiteoak

Nova

Maseras

, et al. ChemSusChem 2012; 5: 2032.

42.

Whiteoak

Henseler

Ayats

, et al. Green Chem 2014; 16: 1552.

43.

Toda

Komiyama

Kikuchi

, et al. ACS Catal 2016; 6: 6906.

44.

Arayachukiat

Kongtes

Barthel

, et al. ACS Sustainable Chem Eng 2017; 5: 6392.

45.

Anthofer

Wilhelm

Cokoja

, et al. ChemCatChem 2015; 7: 94.

46.

Zhong

Chen

, et al. ChemSusChem 2017; 10: 4855.

47.

Steinbauer

Longwitz

Frank

, et al. Green Chem 2017; 19: 4435.

48.

Wang

Leong

Zhang

. Green Chem 2014; 16: 4515.

49.

Liu

Suematsu

Maruoka

, et al. Green Chem 2016; 18: 4611.

50.

Liu

Liang

, et al. J CO₂ Util 2016; 16: 384.

51.

Zhou

Wang

Zhang

, et al. ACS Catal 2015; 5: 6773.

52.

Goodrich

Gunaratne

HQN

Jacquemin

, et al. ACS Sustainable Chem Eng 2017; 5: 5635.

53.

Desens

Werner

. Adv Synth Catal 2016; 358: 622.

54.

Sun

Han

Cheng

, et al. ChemSusChem 2011; 4: 502.

55.

Wang

Sun

Zhou

, et al. Green Chem 2015; 17: 4009.

56.

Liu

Song

Zhang

, et al. Catal Today 2016; 263: 69.

57.

Liu

Xie

Wang

, et al. ACS Catal 2018; 8: 9945.

58.

Sopeña

Martin

Escudero-Adán

, et al. ACS Catal 2017; 7: 3532.

59.

Kumatabara

Okada

Shirakawa

. ACS Sustainable Chem Eng 2017; 5: 7295.

60.

Liu

Song

, et al. Angew Chem Int Ed 2012; 51: 11306.

61.

Yao

Cheng

, et al. Green Chem 2017; 19: 2957.

62.

Ahmed

Sakthivel

. J CO₂ Util 2017; 22: 392.

63.

Wang

Liu

Dai

, et al. Eur J Org Chem 2014; 2014: 6493.

64.

Tao

Guo

Liu

, et al. Organometallics 2017; 36: 4432.

65.

Chen

Liu

Dai

. ACS Sustainable Chem Eng 2017; 5: 9065.

66.

Chen

Wang

, et al. J CO₂ Util 2018; 28: 181.

67.

Xie

Guo

Shi

, et al. Org Biomol Chem 2019; 17: 3497.

68.

Graham

Buytendyk

Wang

, et al. J Chem Phys 2015; 142: 234307.

69.

Anthofer

Wilhelm

Cokoja

, et al. Catal Sci Technol 2014; 4: 1749.

70.

Girard

Simon

Zanatta

, et al. Green Chem 2014; 16: 2815.

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

0.00 MB

3.33 MB