A novel zinc based binary catalytic system for CO₂ utilization under mild conditions

Abstract

Herein we report the development of a novel binary catalytic system for the addition of CO₂ to epoxides based on zinc iodide and an imidazolium salt. The influence of the nature of the imidazolium ion and the anion has been studied in a test reaction. The optimized system exhibits excellent activity and selectivity. Furthermore, the reaction can be performed under very mild (30 °C) and solvent free conditions yielding the desired cyclic carbonates in up to 99%.

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Thomas Werner

Thomas Werner studied chemistry at the Technische Universität Berlin, Germany, and at the Northumbria University in Newcastle upon Tyne, UK. After receiving his Diploma from the Technische Universität Berlin in 2001 he moved together with Prof. J. Christoffers to the Universität Stuttgart, Germany, where he worked on cerium catalyzed transformations of β-dicarbonyl compounds in the presence of molecular oxygen. He completed his Ph.D. in 2004 and joined shortly after, the group of Prof. A. G. M. Barrett at the Imperial College London, UK, for postdoctoral work on biomimetic approaches to the antifungal agent, 15G256β. In 2006 he joined the Coating and Colorants business unit of Evonik (former Degussa) as the head of R&D laboratory. In 2008 he relocated to Leibniz-Institut für Katalyse e. V. in Rostock, Germany, where he is currently working towards his habilitation in the group of Prof. M. Beller. Since October 2015 he is a visiting Professor at the Ernst Moritz Arndt University of Greifswald. His research interests include phosphorus based organocatalysis, especially catalytic reactions based on P^III/P^V redox cycling and the utilization of carbon dioxide as a C1 building block.

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Introduction

Establishing a chemical industry based on carbon dioxide as a C1 building block is one scenario for the future to close the gap between decreasing fossil feedstocks and the increasing demand on chemical products.¹ Recently efforts have been greatly intensified to develop efficient methods for the synthesis of useful products from this nonhazardous, economically feasible and environmentally benign building block.² Particularly the atom economic conversion of epoxides and carbon dioxide into cyclic carbonates is one of the most promising routes for carbon dioxide fixation into value added products (Scheme 1).³ Cyclic carbonates are used in numerous applications ranging from polar aprotic solvents,⁴ electrolytes in lithium-ion batteries,⁵ synthetic building blocks,⁶ intermediates in fine chemical industry, industrial lubricants,⁷ components in oils and paints,^3e raw materials for polycarbonates and isocyanate free polyurethanes.⁸

Scheme 1 Synthesis and application of cyclic carbonates.

Numerous catalysts⁹ have been developed for the coupling of carbon dioxide to epoxides. Most frequently employed systems are based on transition metal catalysts,¹⁰ alkali metal salts,¹¹ carbenes,¹² quaternary ammonium¹³ or phosphonium salts,¹⁴ functional polymers,¹⁵ and ionic liquids.¹⁶ A variety of metal salts and complexes have been reported in combination with ionic liquids as binary systems.^9a,d,f,17 The Lewis acidic activation of the epoxide by the metal species and subsequent ring opening by the counter anion of the ionic liquid is well accepted in the literature. In those systems simple zinc salts are often used due to their low cost and high efficiency. Kim and co-workers reported imidazolium zinc tetrahalides as highly active catalysts.¹⁸ They also investigated the effect of the role of the Lewis acidic species in the presence of imidazolium salts. Subsequently the groups of Xia¹⁹ and Arai²⁰ studied similar systems and proposed a mechanism involving the activation of the epoxide by zinc followed by nucleophilic ring opening by the counter anion of the imidazolium salt. Thereafter a series of onium salts have been utilized in combination with zinc salts as catalysts.^9d,21 Most recently Sun et al. developed zinc based task specific ionic liquids for the coupling of carbon dioxide with epoxides to form cyclic carbonates.²² Even though great advances have been made in the development of ionic liquid zinc-based systems, in most cases, high reaction temperatures ≥100 °C and CO₂ pressures >1.0 MPa are needed. We developed one and two component catalyst systems for the synthesis of cyclic carbonates under mild conditions based on alkali metal and bifunctional onium salts.^11a,c,13a,14 In general, we observed the superior activity of iodides compared to other counter anions.

Thus we envisioned that a zinc iodide based binary catalytic system should be especially efficient even at temperatures well below 100 °C.

Results and discussion

Initially we tested various simple zinc salts as potential catalysts to set a benchmark. Since our aim was to develop a system, which efficiently catalyzes the addition of CO₂ under mild conditions, we set the initial screening parameters to 50 °C, 1.0 MPa CO₂ pressure and 1 h reaction time under solvent free conditions (Table 1). We choose the conversion of 1,2-epoxybutane (1a) with CO₂ to 1,2-butylene carbonate (2a) as test reaction. As expected without any catalyst the reaction does not proceed and no product formation was observed (entry 1). In the presence of simple zinc salts and absence of a co-catalyst no conversion was observed (except for ZnI₂ see below).²³ The co-catalyst 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPr·HCl) was also not active under these reaction conditions (entry 2). Subsequently, a combination of zinc salts and co-catalyst IPr·HCl were applied (entries 3–8). Thereby, zinc salts bearing non-nucleophilic anions showed only very low activity (entries 3–5). However, using zinc halides and IPr·HCl moderate to excellent yields were obtained (entries 6–8). The activity of the salts increased in the order Cl < Br < I, which has previously been observed for binary catalytic systems^11d,18 and bifunctional catalysts.^13a,14,24 Quantitative conversion of model substrate 1a was achieved with the combination of ZnI₂ and IPr·HCl, which was chosen for further investigations (entry 8). Notably this was the only salt in which 2a could be detected in traces even in the absence of IPr·HCl. Employing 1.0 mol% catalyst and co-catalyst, 99% of 2a was obtained (entry 9). Further reduction of the catalyst amount to 0.5 mol% led to reduction in yield to 82% of 2a (entry 10).

Table 1 Screening of various zinc salts in combination with IPr·HCl as a binary catalytic system for the addition of CO₂ to 1a as test reaction

Entry	ZnX2	ZnX2/IPr·HCl	Yield 2a^a/%
a Reaction conditions: 25 mmol epoxide 1a, 0.5–2.0 mol% catalyst and co-catalyst (1 : 1), T = 50 °C, p(CO2) = 1.0 MPa, t = 1 h. Yield determined by GC-FID with n-hexadecane as internal standard. b Only 2.0 mol% IPr·HCl was employed. c Yield in bracket was obtained in the absence of IPr·HCl.
1	—	—	0
2	—^b	2.0 mol%	0
3	Zn(OAc)2	2.0 mol%	3
4	Zn(BF4)2	2.0 mol%	0
5	Zn(acac)2	2.0 mol%	2
6	ZnCl2	2.0 mol%	13
7	ZnBr2	2.0 mol%	70
8	ZnI2	2.0 mol%	>99 (1)^c
9	ZnI2	1.0 mol%	>99
10	ZnI2	0.5 mol%	82

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Having identified zinc iodide as the most active zinc salt we turned our attention to investigate the influence of both the structure and counter anion of the imidazolium salt (Table 2). First we studied the impact of the anion by employing various 1-ethyl-3-methylimidazolium (EMIm) salts as co-catalysts in the test reaction (entries 1–6). As expected EMIm salts with non-nucleophilic counter anions showed only low conversions <10% (entries 1–3). However in the presence of [EMIm]Cl the desired product 2a was obtained in 75% yield (entry 4). Interestingly the respective bromide gave lower yields of only 62% of 2a (entry 5). In this series the best yield was obtained when [EMIm]I was utilized (entry 6). Subsequently we turned our attention to the impact of the imidazolium species (entries 7–8). Dimethylimidazolium chloride [DMIm]Cl gave 2a in 57% yield while 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride (IMes·HCl) gave the product in very good yield of 85% (entry 8). We also employed a thiazolium chloride, which showed only low conversion and yield, thus underlining the impact of the cationic structure (entry 9). Since iodide proved to be the most promising anion and IMes·HCl and IPr·HCl bearing the most active imidazolium structures, we decided to prepare the corresponding iodide salts IMes·HI and IPr·HI.

Table 2 Screening of different imidazolium salts in the model reaction

Entry	Co-catalyst	Yield 2a^a/%
a Reaction conditions: 25 mmol epoxide 1a, 0.5 mol% ZnI2, 0.5 mol% co-catalyst, T = 50 °C, p(CO2) = 1.0 MPa, t = 1 h. Yield determined by GC-FID with n-hexadecane as internal standard.
1	[EMIm][BF4]	1
2	[EMIm][PF6]	0
3	[EMIm][FeCl4]	9
4	[EMIm]Cl	75
5	[EMIm]Br	62
6	[EMIm]I	81
7	[DMIm]Cl	57
8	IMes·HCl	85
9		4

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Both salts can be prepared by anion exchange starting from the commercially available chlorides (Scheme 2).²⁵ In the presence of a large excess of NaI in acetone/water the desired products IMes·HI and IPr·HI could be isolated in 51% and 61% yield, respectively. Notably, after recrystallization IMes·HI could be obtained as yellow crystals suitable for X-ray crystallographic analysis (Fig. 1).

Scheme 2 Synthesis of imidazolium iodides IPr·HI and IMes·HI from chlorides by anion exchange.

Fig. 1 ORTEP representation of the molecular structure of IMes·HI in the crystal. Displacement ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity (see ESI† for crystallographic data).

We employed the prepared imidazolium salts IMes·HI and IPr·HI under the standard conditions of our test reaction (Table 3, entries 1 and 2). In both cases the yields were slightly improved compared to the respective chlorides. We further evaluated the influence of catalyst concentration, reaction temperature, CO₂ pressure and reaction time. Notably, the reaction can be performed even at 30 °C. Even though the amount of catalyst had to be increased to 1 mol% after 1 h, 1.0 MPa CO₂ pressure, the obtained yield for 2a was 80% (entries 3 and 4). A further experiment was performed with IPr·HI at room temperature, the yield dropped significantly to 33% (entry 5). At 30 °C the extension of the reaction time to 3 h and 5 h led to product yields of 96% and >99% respectively (entries 6 and 7). The reduction of the pressure to 0.5 MPa and 0.1 MPa led to slightly reduced product yields of 97% and 83% respectively (entries 8 and 9).

Table 3 Parameter optimization

Entry	ZnI2/co-catalyst (1 : 1)	Co-catalyst	T/°C	p(CO2)/MPa	t/h	Yield 2a^a/%
a Reaction conditions: 25 mmol epoxide 1a, T = 30–50 °C, p(CO2) = 0.1–1.0 MPa, t = 1–5 h. Yield determined by GC-FID with n-hexadecane as internal standard.
1	0.5 mol%	IMes·HI	50	1.0	1	85
2	0.5 mol%	IPr·HI	50	1.0	1	87
3	1.0 mol%	IMes·HI	30	1.0	1	80
4	1.0 mol%	IPr·HI	30	1.0	1	80
5	1.0 mol%	IPr·HI	25	1.0	1	33
6	1.0 mol%	IPr·HI	30	1.0	3	96
7	1.0 mol%	IPr·HI	30	1.0	5	>99
8	1.0 mol%	IPr·HI	30	0.5	5	97
9	1.0 mol%	IPr·HI	30	0.1	5	83

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Based on the results of parameter screening we determined the standard reaction conditions for the evaluation of the substrate scope (Table 4). Initially we employed 1 mol% zinc iodide in a 1 : 1 ratio with IPr·HI as a co-catalyst at 30 °C, 1.0 MPa CO₂ pressure for 5 h. Since some substrates gave only low to moderate yields, the catalyst amount was increased to 5 mol% which led to significantly improved yields in those cases. Alkyl functionalized cyclic carbonates 2a–2d could be obtained in yields of 83–99% (entries 1–8). Also aryl derivatives 2e and 2f could be isolated in excellent yields up to 93% (entries 9–12). Several glycidyl ethers 1g–1j could be smoothly converted leading to the respective products in yields up to 99% (entries 13–20). Interestingly the potential monomer 2k was obtained in 94% isolated yield as well as unsaturated substrate 1l was converted to 2l and isolated in 95% (entries 21–24). Other functionalized substrates bearing chloride 1m, bromide 1n and hydroxy 1o were also converted (entries 25–30). Moreover, internal epoxides 1p–1r were reacted with CO₂ under more drastic reaction conditions (entries 31–34). However, due to more steric hindrance^13a,14,26 significant lower yields up to 54% of their corresponding cyclic carbonates 2p–2r were determined.

Table 4 Substrate scope under optimized reaction conditions

Entry	ZnI2/IPr·HI (1 : 1)	Product		Yield 2 ^a/%
a Reaction conditions: epoxide 1, 1 or 5 mol% ZnI2, 1 or 5 mol% IPr·HI, T = 30 °C, p(CO2) = 1.0 MPa, t = 5 h. Isolated yield. b Yield determined by ^1H NMR. c T = 75 °C. d T = 75 °C, p(CO2) = 5.0 MPa.
1	1 mol%		2a	99
2	5 mol%			—
3	1 mol%		2b	94
4	5 mol%			—
5	1 mol%		2c	99
6	5 mol%			—
7	1 mol%		2d	6
8	5 mol%			83
9	1 mol%		2e	76^b
10	5 mol%			93
11	1 mol%		2f	30^b
12	5 mol%			87
13	1 mol%		2g	6
14	5 mol%			99
15	1 mol%		2h	9^b
16	5 mol%			72
17	1 mol%		2i	14^b
18	5 mol%			99
19	1 mol%		2j	64^b
20	5 mol%			85
21	1 mol%		2k	71^b
22	5 mol%			94
23	1 mol%		2l	5^b
24	5 mol%			95
25	1 mol%		2m	90
26	5 mol%			—
27	1 mol%		2n	38
28	5 mol%			—
29	1 mol%		2o	43^b
30	5 mol%			67
31	1 mol%		2p	18^b^,^c
32	1 mol%			36^d
33	1 mol%		2q	8^c
34	1 mol%		2r	54^d

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Proposed mechanism

We assume that Lewis acidic zinc halide coordinates to the oxygen atom to activate the epoxide. Similar activations of epoxides by Lewis acidic metal species have been reported previously.^{10f,11b,21b,27} The activation of the epoxide by the imidazolium cation via hydrogen bonding is also possible.^9a,16a We observed a significant downfield shift of the C2 proton in the ¹H NMR spectra when the epoxide was added to a mixture of ZnI₂ and IPr·HI.²³ Since no conversion is observed when one of the components is used alone under the reaction conditions, we assume a cooperative activation of the epoxide by the Lewis acidic zinc species and the imidazolium cation as a hydrogen donor. Subsequent ring opening by the iodide of the imidazolium salt through a nucleophilic attack on the less sterically hindered carbon atom forms an alkoxide. Insertion of carbon dioxide into the zinc–oxygen bond and subsequent ring closure yields the desired cyclic carbonate. This proposal is in accordance with the mechanisms of other zinc based binary systems (Scheme 3).^9a,18

Scheme 3 Possible reaction mechanism for the synthesis of cyclic carbonates catalyzed by ZnI₂ and IPr·HI.

Conclusion

We developed an efficient binary catalytic system based on zinc iodide and an imidazolium salt for the addition of CO₂ to epoxides. This system allows the conversion of various epoxides into the corresponding cyclic carbonates in yields up to 99% under very mild and solvent free conditions. We showed that in this system the iodide anion is a superior nucleophile compared to the usually employed anions, bromide and chloride, respectively. Besides the anion, the structure of the cation also has a large influence on the reaction outcome. Based on those results catalytic systems based on zinc iodide which efficiently catalyze the addition of CO₂ to epoxide under ambient conditions might emerge in the future.

Experimental

Coupling reaction of CO₂ with epoxides 1 (GP1)

A 45 mL stainless steel autoclave equipped with a magnetic stirrer was charged with zinc iodide (0.01 equiv.), IPr·HI (0.01 equiv.) and epoxide 1 (1.00 equiv.). The reactor was sealed and pressurized with 1.0 MPa of carbon dioxide until the equilibrium was reached. Subsequently the reactor was warmed to 30 °C for 5 hours. The autoclave was cooled with an ice bath below 20 °C and the CO₂ was slowly liberated from the vessel. The crude reaction mixture was extracted with dichloromethane and filtered over a silica gel plug. After the removal of all volatiles under reduced pressure, the cyclic carbonates 2 were obtained.

Coupling reaction of CO₂ with epoxides 1 (GP2)

A 45 mL stainless steel autoclave equipped with a magnetic stirrer was charged with zinc iodide (0.05 equiv.), IPr·HI (0.05 equiv.) and epoxide 1 (1.00 equiv.). The reactor was sealed and pressurized with 1.0 MPa of carbon dioxide until the equilibrium was reached. Subsequently the reactor was warmed to 30 °C for 5 hours. The autoclave was cooled with an ice bath below 20 °C and the CO₂ was slowly liberated from the vessel. The crude reaction mixture was extracted with dichloromethane and filtered over a silica gel plug. After the removal of all volatiles under reduced pressure, the cyclic carbonates 2 were obtained.

IPr·HI and IMes·HI were synthesized according to known procedures.²⁵

4-Ethyl-1,3-dioxolan-2-one (2a)^13a. According to GP1, 1,2-epoxybutane (1a, 1.83 g, 25.4 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (79.0 mg, 0.25 mmol) and IPr·HI (130 mg, 0.252 mmol). After workup the title compound 2a (2.92 g, 25.1 mmol, 99%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 1.00 (t, J = 7.44 Hz, 3H), 1.67–1.86 (m, 2H), 4.07 (t, J = 6.96 Hz, 8.37 Hz, 1H), 4.52 (t, J = 8.12 Hz, 1H), 4.61–4.70 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 8.4 (CH₃), 26.8 (CH₂), 68.9 (CH₂), 78.0 (CH), 155.1 (C=O) ppm.

4-Methyl-1,3-dioxolan-2-one (2b)^13a. According to GP1, 1,2-epoxypropane (1b, 593 mg, 10.2 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (32 mg, 0.10 mmol) and IPr·HI (52 mg, 0.10 mmol). After workup the title compound 2b (983 mg, 9.63 mmol, 94%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 1.42 (d, J = 6.24 Hz, 3H), 3.98 (dd, J = 8.43 Hz, 7.23 Hz, 1H), 4.51 (dd, J = 8.27 Hz, 7.85 Hz, 1H), 4.76–4.87 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 19.1 (CH₃), 70.5 (CH), 73.5 (CH₂), 155.0 (C=O) ppm.

4-Butyl-1,3-dioxolan-2-one (2c)^13a. According to GP2, 1,2-epoxyhexane (1c, 1.05 g, 10.5 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (158 mg, 0.495 mmol) and IPr·HI (255 mg, 0.494 mmol). After workup the title compound 2c (1.48 g, 10.3 mmol, 99%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 0.92 (t, J = 7.02 Hz, 3H), 1.30–1.50 (m, 4H), 1.60–1.86 (m, 2H), 4.06 (dd, J = 8.36 Hz, 7.19 Hz, 1H), 4.52 (t, J = 8.11 Hz, 1H), 4.65–4.75 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 13.7 (CH₃), 22.2 (CH₂), 26.4 (CH₂), 33.5 (CH₂), 69.3 (CH₂), 77.00 (CH), 155.0 (C=O) ppm.

4-Hexyl-1,3-dioxolan-2-one (2d)^13a. According to GP2, 1,2-epoxyoctane (1d, 1.32 g, 10.3 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (160 mg, 0.501 mmol) and IPr·HI (265 mg, 0.513 mmol). After workup the title compound 2d (1.46 g, 8.48 mmol, 83%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 0.88 (t, J = 6.83 Hz, 3H), 1.24–1.52 (m, 8H), 1.62–1.87 (m, 2H), 4.06 (dd, J = 8.35 Hz, 7.20 Hz, 1H), 4.52 (t, J = 8.09 Hz, 1H), 4.67–4.75 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 13.9 (CH₃), 22.4 (CH₂), 24.2 (CH₂), 28.7 (CH₂), 31.4 (CH₂), 33.8 (CH₂), 69.3 (CH₂), 77.0 (CH), 155.0 (C=O) ppm.

4-Phenyl-1,3-dioxolan-2-one (2e)^13a. According to GP2, styrene oxide (1e, 1.87 g, 15.6 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (242 mg, 0.758 mmol) and IPr·HI (391 mg, 0.757 mmol). After workup the title compound 2e (2.37 g, 14.8 mmol, 93%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 4.33 (dd, J = 8.76 Hz, 7.77 Hz, 1H), 4.80 (t, J = 8.42 Hz, 1H), 5.68 (t, ³J = 8.00 Hz, 1H), 7.33–7.49 (m, CH, 5H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 77.1 (CH₂), 77.9 (CH), 125.8 (2 × CH), 129.1 (2 × CH), 129.6 (CH), 135.7 (C), 154.8 (C=O) ppm.

4-(4-Chlorophenyl)-1,3-dioxolan-2-one (2f)²⁸. According to GP1, 4-chlorostyrene oxide (1f, 1.56 g, 10.1 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (160 mg, 0.501 mmol) and IPr·HI (269 mg, 0.521 mmol). After workup the title compound 2f (1.74 g, 8.78 mmol, 87%) was isolated as a colorless solid. ¹H NMR (300 MHz, CDCl₃): δ = 4.31 (dd, J = 8.62 Hz, 7.84 Hz, 1H), 4.81 (t, J = 8.42 Hz, 1H), 5.67 (t, J = 7.97 Hz, 1H), 7.28–7.33 (m, 2H), 7.40–7.45 (m, 2H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 70.9 (CH₂), 77.2 (CH), 127.2 (2 × CH), 129.4 (2 × CH), 134.2 (C), 135.7 (C), 154.5 (C=O) ppm.

4-(tert-Butoxymethyl)-1,3-dioxolan-2-one (2g)^13a. According to GP2, tert-butyl glycidyl ether (1g, 3.28 g, 25.2 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (397 mg, 1.24 mmol) and IPr·HI (647 mg, 1.25 mmol). After workup the title compound 2g (4.36 g, 25.0 mmol, 99%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 1.17 (s, 9H), 3.50 (dd, J = 10.43 Hz, 3.56 Hz, 1H), 3.60 (dd, J = 10.41 Hz, 4.32 Hz, 1H), 4.36 (dd, J = 8.25 Hz, 5.82 Hz, 1H), 4.46 (t, J = 8.22 Hz, 1H), 4.73–4.80 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 27.2 (3 × CH₃), 61.2 (CH₂), 66.4 (CH₂), 73.7 (C), 75.1 (CH), 155.1 (C=O) ppm.

4-(Iso-butoxymethyl)-1,3-dioxolan-2-one (2h)^13a. According to GP2, iso-butyl glycidyl ether (1h, 1.34 g, 10.3 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (162 mg, 0.508 mmol) and IPr·HI (264 mg, 0.511 mmol). After workup the title compound 2h (1.29 g, 7.41 mmol, 72%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 0.88 (d, J = 6.69 Hz, 6H), 1.80–1.93 (m, 1H), 3.25 (d, J = 6.63 Hz, 2H), 3.58 (dd, J = 11.07 Hz, 3.51 Hz, 1H), 3.66 (dd, J = 11.06 Hz, 3.71 Hz, 1H), 4.39 (dd, J = 8.28 Hz, 6.00 Hz, 1H), 4.48 (t, J = 8.30 Hz, 1H), 4.76–4.85 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 19.0 (2 × CH₃), 28.3 (CH), 66.2 (CH₂), 69.7 (CH₂), 75.1 (CH), 78.6 (CH₂), 155.0 (C=O) ppm.

4-(Iso-propoxymethyl)-1,3-dioxolan-2-one (2i)^13a. According to GP2, iso-propyl glycidyl ether (1i, 1.18 g, 10.2 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (161 mg, 0.504 mmol) and IPr·HI (263 mg, 0.510 mmol). After workup the title compound 2i (1.61 g, 10.1 mmol, 99%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 1.13 (d, J = 6.18 Hz, 6H), 3.53–3.67 (m, 3H), 4.35 (dd, J = 8.28 Hz, 6.00 Hz, 1H), 4.47 (t, J = 8.30 Hz, 1H), 4.74–4.81 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 21.6 (CH₃), 21.8 (CH₃), 66.3 (CH₂), 67.0 (CH₂), 72.8 (CH), 75.2 (CH), 155.0 (C=O) ppm.

4-Phenoxymethyl-1,3-dioxolan-2-one (2j)^13a. According to GP2, phenyl glycidyl ether (1j, 1.50 g, 10.0 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (159 mg, 0.498 mmol) and IPr·HI (259 mg, 0.501 mmol). After workup the title compound 2j (1.65 g, 8.50 mmol, 85%) was isolated as a colorless solid. ¹H NMR (300 MHz, CDCl₃): δ = 4.13 (dd, J = 10.67 Hz, 3.62 Hz, 1H), 4.24 (dd, J = 10.69 Hz, 3.96 Hz, 1H), 4.52 (dd, J = 8.53 Hz, 5.94 Hz, 1H), 4.61 (t, J = 8.42 Hz 1H), 4.99–5.06 (m, 1H), 6.89–6.94 (m, 2H), 6.99–7.05 (m, 1H), 7.28–7.35 (m, 2H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 66.1 (CH₂), 66.8 (CH₂), 74.1 (CH), 114.5 (2 × CH), 121.9 (CH), 129.6 (2 × CH), 154.7 (C=O), 157.7 (C) ppm.

(2-Oxo-1,3-dioxolan-4-yl)methyl methacrylate (2k)^13a. According to the GP2, glycidyl methacrylate (1k, 2.18 g, 15.3 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (239 mg, 0.749 mmol) and IPr·HI (389 mg, 0.753 mmol). After workup the title compound 2k (2.67 g, 14.4 mmol, 94%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 1.94 (t, J = 1.20 Hz, 3H), 4.32 (dd, J = 12.67 Hz, 3.72 Hz, 1H), 4.34 (dd, J = 8.94 Hz, 5.37 Hz, 1H), 4.43 (dd, J = 12.64 Hz, 3.12 Hz, 1H), 4.58 (t, J = 8.58 Hz, 1H), 4.95–5.02 (m, 1H), 5.65 (p, J = 1.46 Hz, 1H), 6.13–6.15 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 18.08 (CH₃), 63.38 (CH₂), 66.02 (CH₂), 73.78 (CH), 127.22 (CH₂), 135.07 (C), 154.43 (C=O), 166.56 (C=O) ppm.

4-(But-3-en-1-yl)-1,3-dioxolan-2-one (2l)^13a. According to GP2, 1,2-epoxy-5-hexene (1l, 997 mg, 10.2 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (161 mg, 0.504 mmol) and IPr·HI (261 mg, 0.505 mmol). After workup the title compound 2l (1.37 g, 9.65 mmol, 95%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 1.72–1.83 (m, 1H), 1.87–1.99 (m, 1H), 2.10–2.32 (m, 2H), 4.08 (dd, J = 8.39 Hz, 7.22 Hz, 1H), 4.53 (t, J = 8.15 Hz, 1H), 4.68–4.77 (m, 1H), 5.01–5.12 (m, 2H), 5.71–5.85 (m, 1H) ppm. 13C{1H} NMR (75 MHz, CDCl₃): δ = 28.6 (CH₂), 33.0 (CH₂), 69.3 (CH₂), 76.3 (CH), 116.3 (CH₂), 136.0 (CH), 154.9 (C=O) ppm.

4-(Chloromethyl)-1,3-dioxolan-2-one (2m)^13a. According to GP1, epichlorohydrin (1m, 930 mg, 10.0 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (32 mg, 0.10 mmol) and IPr·HI (53 mg, 0.10 mmol). After workup the title compound 2m (1.24 g, 9.07 mmol, 90%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 3.71 (dd, J = 12.28 Hz, 3.66 Hz, 1H), 3.81 (dd, J = 12.28 Hz, 4.86 Hz, 1H), 4.38 (dd, J = 8.87 Hz, 5.72 Hz, 1H), 4.58 (t, J = 8.60 Hz, 1H), 4.95–5.03 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 43.9 (CH₂Cl), 66.8 (CH₂), 74.3 (CH), 154.3 (C=O) ppm.

4-(Bromomethyl)-1,3-dioxolan-2-one (2n)^13a. According to the GP1, epibromohydrin (1n, 1.58 g, 11.5 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (35 mg, 0.11 mmol) and IPr·HI (58 mg, 0.11 mmol). After workup the title compound 2n (802 mg, 4.43 mmol, 38%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 3.59 (d, J = 5.19 Hz, 2H), 4.35 (dd, J = 8.89 Hz, 5.89 Hz, 1H), 4.60 (t, J = 8.53 Hz, 1H), 4.92–5.00 (m, 1H) ppm. ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 31.3 (CH₂Br), 68.1 (CH₂), 73.9 (CH), 154.1 (C=O) ppm.

4-(Hydroxymethyl)-1,3-dioxolan-2-one (2o)²⁸. According to GP1, glycidol (1o, 747 mg, 10.1 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (160 mg, 0.501 mmol) and IPr·HI (259 mg, 0.501 mmol). After filtration over silica with ethyl acetate the title compound 2o (797 mg, 6.75 mmol, 67%) was isolated as a colorless liquid. ¹H NMR (300 MHz, DMSO-d₆): δ = 3.50 (ddd, CH₂OH, J = 3.36, 5.67, 12.64 Hz, 1H), 3.66 (ddd, CH₂OH, J = 2.82, 5.46, 12.64 Hz, 1H), 4.28 (dd, OCH₂CH, J = 5.85, 8.14 Hz, 1H), 4.49 (t, OCH₂CH, J = 8.33 Hz, 1H), 4.76–4.83 (m, CH, 1H), 5.25 (t, J = 5.58 Hz, 1H, OH) ppm. ¹³C{¹H} NMR (75 MHz, DMSO-d₆): δ = 60.6 (CH₂), 65.9 (CH₂), 77.0 (CH), 155.2 (C=O) ppm.

cis-Cyclohexene carbonate (2p)²⁹. According to the GP1 at 75 °C and 5.0 MPa CO₂, cyclohexene oxide (1p, 2.58 g, 26.3 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (84 mg, 0.26 mmol) and IPr·HI (137 mg, 0.27 mmol). After workup the title compound 2p (1.34 g, 9.40 mmol, 36%) was isolated as a colorless solid. ¹H NMR (300 MHz, CDCl₃): δ = 1.35–1.47 (m, 2H), 1.55–1.67 (m, 2H), 1.84–1.91 (m, 4H), 4.64–4.71 (m, 2H) ppm; ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 19.1 (2 × CH₂), 26.7 (2 × CH₂), 75.7 (2 × CH), 155.3 ppm (C=O).

(3aR,7aS)-Tetrahydrofuro[3,4-d][1,3]dioxol-2-one (2q)²⁹. According to the GP1 at 75 °C, 3,4-epoxytetrahydrofuran (1q, 946 mg, 11.0 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (35 mg, 0.11 mmol) and IPr·HI (56 mg, 0.11 mmol). After workup the title compound 2q (110 mg, 0.857 mmol, 8%) was isolated as a colorless liquid. ¹H NMR (300 MHz, CDCl₃): δ = 3.53–3.58 (m, 2H), 4.23 (dd, J = 11.0 Hz, 1.3 Hz, 2H), 5.21 (dd, J = 2.1 Hz, 1.2 Hz, 2H) ppm; ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 72.9 (2 × CH₂), 80.0 (2 × CH), 154.4 ppm (C=O).

cis-Tetrahydro-8H-cyclopenta-[1,3]dioxol-2-one (2r)³⁰. According to GP1 at 75 °C and 5.0 MPa CO₂, cyclopentene oxide (1r, 845 mg, 10.0 mmol) was allowed to react with CO₂ in the presence of ZnI₂ (31 mg, 0.97 mmol) and IPr·HI (54 mg, 0.10 mmol). After workup the title compound 2r (694 mg, 5.41 mmol, 54%) was isolated as a colorless solid. ¹H NMR (300 MHz, CDCl₃): δ = 1.59–1.84 (m, 4H), 2.10–2.18 (m, 2H), 5.08–5.13 (m, 2H) ppm; ¹³C{¹H} NMR (75 MHz, CDCl₃): δ = 21.5 (CH₂), 33.1 (2 × CH₂), 81.8 (2 × CH), 154.4 ppm (C=O).

1,3-Bis(2,6-diisopropylphenyl)imidazolium iodide²⁵. 1,3-Bis(2,6-diisopropylphenyl)imidazolium chloride (4.33 g, 10.2 mmol) was dissolved in H₂O (108 mL). Another solution of NaI (7.03 g, 46.9 mmol) in acetone (130 mL) was prepared and slowly poured into the first solution. The resulting mixture was stirred for 1 h at 23 °C and was then cooled with an ice bath to 0 °C. The resulting precipitate was filtered off, washed with a mixture of cold H₂O/acetone (8 mL, 1 : 1, 0 °C) and dried under vacuum to yield the title compound (3.23 g, 6.24 mmol, 61%) as a colorless solid. ¹H NMR (300 MHz, DMSO-d₆): δ = 1.17 (d, J = 6.84 Hz, 12H), 1.27 (d, J = 6.78 Hz, 12H), 2.36 (pent, J = 6.79 Hz, 4H), 7.53–7.55 (m, 4H), 7.67–7.72 (m, 2H), 8.58 (d, J = 1.47 Hz, 2H), 10.18 (t, J = 1.47 Hz, 1H) ppm. ¹³C{¹H} NMR (75 MHz, DMSO-d₆): δ = 23.1 (4 × CH₃), 24.1 (4 × CH₃), 28.6 (4 × CH), 124.6 (4 × CH), 126.2 (2 × CH), 130.0 (2 × C), 131.8 (2 × CH), 139.2 (CH), 144.8 (4 × C) ppm; elemental analysis calcd (%) for C₂₇H₃₇IN₂ (516.51) C 62.79, H 7.22, I 24.57, N 5.42; found C 62.70, H 7.30, I 24.63, N 5.56.

1,3-Bis(2,4,6-trimethylphenyl)imidazolium iodide. 1,3-Bis(2,4,6-trimethylphenyl)imidazolium chloride (2.16 g, 6.34 mmol) was dissolved in H₂O (400 mL). Another solution of NaI (3.79 g, 25.3 mmol) in acetone (100 mL) was prepared and slowly poured into the first solution. The resulting mixture was stirred for 6 h at 23 °C. The solvent was removed under vacuum and the residue washed with Et₂O (10 mL). The title compound was obtained as yellow crystals (1.41 g, 3.26 mmol, 51%) from slow evaporation from CHCl₃. ¹H NMR (300 MHz, DMSO-d₆): δ = 2.13 (s, 12H), 2.36 (s, 6H), 7.22 (s, 4H), 8.30 (d, J = 1.47 Hz, 2H), 9.68 (t, J = 1.47 Hz, 1H) ppm. ¹³C{¹H} NMR (75 MHz, DMSO-d₆): δ = 16.9 (4 × CH₃), 20.6 (2 × CH₃), 124.8 (2 × CH), 129.3 (4 × CH), 130.9 (2 × C), 134.3 (4 × C), 138.4 (CH), 140.5 (2 × C) ppm; HRMS (ESI, pos): m/z calcd for C₂₁H₂₅N₂ [M⁺ − I] 305.20123; found 305.20125; HRMS (ESI, neg): m/z calcd for I [M⁻ − C₂₁H₂₅N₂] 126.90502; found 126.90473.

Acknowledgements

We wish to thank the Federal Ministry of Research and Education (BMBF) for financial support (Chemische Prozesse und stoffliche Nutzung von CO₂: Technologien für Nachhaltigkeit und Klimaschutz, grant 01 RC 1004A).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1435552. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qo00356c

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