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Molecular Catalysis 450 (2018) 87–94

Contents lists available at ScienceDirect

Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat

Multisite activation of epoxides by recyclable CaI2/N-methyldiethanolamine catalyst for CO2 fixation: A facile access to cyclic carbonates under mild conditions ⁎

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Tian-Xiang Zhao, Yi-Yang Zhang, Jian Liang, Ping Li, Xing-Bang Hu , You-Ting Wu

School of Chemistry and Chemical Engineering, Separation Engineering Research Center, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University, Nanjing 210093, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 conversion Calcium-catalyzed Cyclic carbonates Multisite-activation C1 chemistry

The conversion of CO2 into valuable products has become the inherent part of green and sustainable chemistry. Here we demonstrated an efficient synthesis of cyclic carbonates from epoxides and CO2 under mild conditions (at 50 °C for 6 h under a CO2 atmosphere) via the use of CaI2/N-methyldiethanolamine (MDEA) system as the catalyst. The excellent conversions and selectivities for a broad range of epoxides are achieved under solvent-free conditions even at atmospheric CO2 pressure and in the presence of water. The catalyst can be simply recycled and reused without obvious loss in its activity. A plausible multisite-activation mechanism is proposed involving hydrogen bond site-assisted and Ca2+-promoted ring opening path, in combination with the activation of epoxides by iodide ion.

1. Introduction Carbon dioxide (CO2) is an attractive C1 feed-stock from the viewpoint of green and sustainable chemistry. In the field of CO2 chemistry, the conversion of CO2 into useful products has been the hot topic in both academia and industry [1–4]. Among various reaction paths, the fixation of CO2 to cyclic carbonates has been paid much attention, ascribing to the unique material properties of cyclic carbonates [5–15]. These carbonates can be used as aprotic polar solvents,5 sources for polymer synthesis [6,7], and substitutes for highly toxic phosgene or carbon monoxide in organic syntheses [8,9]. To be atom-economic, it is highly desirable to use CO2 as feedstock for the synthesis of cyclic carbonates. However, it is well known that the utilization of CO2 encounters major challenges originating from its high thermodynamic stability and kinetic inertness [1–4]. A number of effective catalysts have been therefore designed for the activation of both CO2 and epoxides. The representative metal complexe catalysts were prepared using Fe [16,17], Al [18], V [19], Zn [20], Cr [21], Mn [22], Nb [23], and Co [24], whilst organocatalysts e.g [25–33]. N-heterocyclic carbenes (NHC) [25], ionic liquids [26–28], and hydrogen bond donors [29–32] such as pentaerythritol [29,30], hydroxypyridin [31], and phenol derivatives [32] were also developed. The most representative of these metals complexes such as aluminum (amino triphenolate) [18,33] and aluminum porphyrin [34] complexes with turnover

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Corresponding authors. E-mail addresses: [email protected] (X.-B. Hu), [email protected] (Y.-T. Wu).

https://doi.org/10.1016/j.mcat.2018.03.010 Received 28 January 2018; Received in revised form 8 March 2018; Accepted 10 March 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.

numbers (TONs) of approximately 112 000 and turnover frequencies (TOFs) near 36000 h−1 have been achieved by lengthening reaction time and using bis-(triphenylphosphine)iminium (PPN-I) as a co-catalyst [18]. However, the addition of tetrabutylammonium bromide (TBAB) or tetrabutylammonium iodide (TBAI), high temperature (> 100 °C) and high pressure (> 1 MPa) are often required to produce the cyclic carbonates with satisfactory yields. In particular, air-sensitive (NHC) [25], and/or of complex structure that limit their applications in large-scale synthesis. Recently, several phosphonium salt catalysts functionalized with a hydroxy group have been developed [35–37] allowing the conversion even under relatively mild conditions. Hence, it is highly desired to develop readily available, environment-friendly, and cheap catalysts for the synthesis of cyclic carbonates under mild conditions. Alkali metal or alkaline earth salts are readily available promising catalysts in the synthesis of cyclic carbonate from CO2 and epoxides [38–45]. The most frequently used salt is potassium iodide (KI), it is usually applied in combination with crown ether [38,39] or ethylene glycol derivatives [40] as ligands. These catalytic systems are efficient under moderate temperatures (< 100 °C) and atmospheric pressure for the conversion of various substrates with yields (above 85%). In contrast, there are few reports on calcium-catalyzed CO2 conversion to cyclic carbonates. As shown in several representative examples, Troev’s group [41] found that CaCl2 showed a pronounced promoting effect in


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Scheme 1. The different strategy between literature and this work in CaI2-catalyzed CO2 fixation to cyclic carbonates.

times in HPLC. For recycle of catalyst, the mixture of epoxide 1j (10 mmol), CaI2 (1 mmol), and MDEA (1 mmol) was heated at 50 °C for 6 h under a CO2 atomsphere (balloon). After cooled to room temperature, diethyl ether (CaI2/MDEA complexe is insoluble in diethyl ether) (3 × 5 mL) was added into the reactor to precipitate the catalyst and to extract the unreacted substrate 1j and the generated product 2j. After the removal of diethyl ether, the product 2j purified using column chromatography with EtOAc/n-hexane as the eluent to afford the isolated yields of cyclic carbonate 2j. Particularly, the catalyst of CaI2/ MDEA complexe is insoluble in diethyl ether, which was dried in vacuum to remove residual diethyl ether and directly used for the next run under same conditions.

the presence of tetraalkylammonium or phosphonium halide, albeit with a high temperature of 170 °C under pressurized conditions. Bai et al. [42] reported a Ca2+-BINOL complexe along with tetraalkylammonium salts as co-catalysts to promote the cycloaddition reaction of CO2 with terminal epoxides under 1.2 MPa at 120 °C. In 2016, He’s group found that CaBr2 could be used as catalyst for the synthesis of cyclic carbonates [43], however, DBU was required as co-catalyst and the reaction had to be carried out at 100 °C and in DMF. Although these results are encouraging, efficient and facile Ca-based catalysts for the synthesis of cyclic carbonates under mild conditions and atmospheric pressure is still a challenge [41–43]. Recently, Werner et al. [44,45] reported that CaI2 coordinating with 18-crown-6 ether or poly(ethylene glycol) dimethyl ether (Scheme 1) promoted the cycloaddition of CO2 with epoxides. These systems showed outstanding activity in the conversion of terminal epoxides (even at room temperature), as well as for the conversion of internal epoxides under relatively mild conditions. The potential of calcium catalysts in ring-opening reactions also demonstrated clearly. On the other hand, the oxygen atom of the epoxide can also be activated by hydrogen bond interactions with the hydroxy group chain ends. Inspired by these previous findings [44–48], we argue that a complexe containing both CaI2 and hydrogen bond donor should provide multisites for the activation of epoxides and CO2. Herein, a series of amines especially alkanolamines are proposed and employed as ligands in combination with CaI2 to provide versatile catalysis and donor of hydrogen bond (Scheme 1). These new catalyst systems are found to be highly effective for conversion of CO2, which offers a new solution for low-temperature/pressure and solvent-free synthesis of cyclic carbonates from terminal epoxides and CO2.

2.3. Spectroscopic studies

2. Experimental

FTIR spectra were recorded on a Nicolet (iS50) FTIR spectrometer with a resolution of 1 cm−1 in the range from (4000–400) cm−1. The spectrometer possesses auto-align energy optimization and a dynamically aligned interferometer. It is fitted with a ATR module for the measurement of determinand. The interactions amang MDEA, CaI2 and 1b also were investigated by NMR spectra. The NMR spectra were recorded on a Bruker AV 400 or 500 spectrometer. The NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, m = multiplet, br = broad signal. Chemical shifts are given in ppm and are referenced to SiMe4 (1H, 13C). Typically, the 20 μL of MDEA was added to 0.5 mL of CDCl3 (0.34 M in CDCl3), and then the 1H NMR of the sample was measured at 25 °C on a Bruker AV 400 spectrometer. As a contrast, the equimolar 1b (15 μL) and MDEA (20 μL) were also added to 0.5 mL of CDCl3 (0.34 M in CDCl3), and the 1H NMR of the mixture also was measured under the same conditions.

2.1. Materials

3. Results and discussion

All solvents and chemicals are analytically pure agents purchased from Adamas Chemical and are used without further purification unless otherwise indicated. The CaI2 was dried under vacuum at 50 °C for 24 h. The deuterated solvents were purchased from TCI Chemical Company. CO2 gas (99.9 and 15.0 vt.%) was purchased from Nanjing Tianze Gas Center, Nanjing.

In the ongoing search of N-methyldiethanolamine (MDEA) based absorbents for CO2 capture [49–53], we have been dedicated to the simultaneous conversion of CO2 [54,55]. To our delight, an experiment in which MDEA/CaI2/epoxy propane (PO) mixture was used to absorb CO2 resulting unexpectedly in remarkable amount of propylene carbonate (PC) at 50 °C (black line in Fig. 1). The cycloaddition of CO2 with epoxy propane was evidenced from the pressure decay of CO2 that differs largely from the phenomena of other two parallel control experiments (blue and red lines in Fig. 1). Experimental results suggested that CaI2/MDEA served as the efficient catalyst in the reaction. Fig. 1 To find out an optimum catalytic system, the screening of various commercially available ligands was carried out with styrene oxide (1 g) as a benchmark substrate and CaI2 as catalyst to produce styrene carbonate (2 g) (Scheme 2). The promoting effects of L1 and L2 are poor, and the yield of 2 g are only 5% and 47%. L3 (79%) and L4 (92%) as the ligands lead to satisfactory yields. As we expected, the combination of CaI2 with hydrogen bond donors (L5–L7) can result in good to excellent yield of 2 g under the mild conditions. A quantitative conversion was obtained when using L5 as the ligand, others also afforded considerable yields of 2 g (≥80%, L6 and L7). In the same experimental conditions reported by Werner [44], a yield of 78% (2 g) was obtained. To clarify the role of hydroxy groups in alkanolamines,

2.2. Synthesis of cyclic carbonates A Schlenk flask (10 mL) was bubbled with CO2 to replace air, and then styrene oxide (10 mmol), CaI2 (1 mmol), and MDEA (1 mmol) were added successively. The flask was heated at 50 °C for 6 h under a CO2 atmosphere (balloon). After being cooled to room temperature, the desired products were obtained in the corresponding isolated yields after purification by flash chromatography on silica gel (hexane/EtOAc as eluent). The NMR information of products between the experimental and literature data was found to be satisfactory. Particularly, for lowboiling epoxide of 1a, the reaction was carried out in a stainless steel autoclave, and CO2 with a 0.5 MPa pressure is necessary to provide enough CO2. Optical purities of commercially available substrates (R)1g was determined by a Shimadzu HPLC (LA-20A) analysis meter, and the product of (R)-2g was confirmed by comparison of the retention 88


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Table 1 The optimization of the reaction conditions.

Fig. 1. Plots of reaction pressure vs reaction time. PO + CaI2 + MDEA experiment, black line: PO (10 mmol), CaI2 (1 mmol), MDEA (1 mmol), CO2 (5.0 bar); control experiment (A), red line: PC (10 mmol), CaI2 (1 mmol), MDEA (1 mmol), CO2 (5.0 bar); and control experiment (B), blue line: PO (10 mmol), MDEA (1 mmol), CO2 (5.0 bar). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Entry

Catalyst

Ligand

T (°C)

t (h)

Yield of 2f (%)

1 2 3 4 5 6 7a 8b 9c 10d 11e 12f 13 14 15 16 17g 18h

CaCl2 CaBr2 CaI2 Ca(OAc)2 CaC2O4 Ca(OTf)2 CaI2 CaI2 CaI2 CaI2 CaI2 CaI2 CaI2 – CaI2 TBAI CaI2 CaI2

L5 L5 L5 L5 L5 L5 L5 L5 L5 L5 L5 L5 L5 L5 – L5 L5 L5 + L6

50 50 50 50 50 50 50 50 50 50 50 50 25 50 50 50 50 50

10 10 10 10 10 10 6 6 6 6 6 6 6 10 10 10 24 6

0 56 99 0 0 0 99 99 71 90 96 95 43 0 0 52 88 99

Reaction conditions: 1 g (10 mmol), CO2 (balloon), catalyst (1.0 mmol), L5 (1.0 mmol). L5: N-methyldiethanolaMine. L6: diethanolamine. TBAI: tetrabutylammonium iodide. The yield of 2 g was determined by 1H NMR. aIn situ-generated CaI2/MEDA complexe. b Pre-prepared CaI2/MEDA complexe. cCaI2 (0.1 mmol), L5 (0.1 mmol). dCaI2 (0.5 mmol), L5 (0.5 mmol) e CaI2 (0.5 mmol), L5 (1.0 mmol). fCaI2 (1.0 mmol), L5 (0.5 mmol) gMixture gas of 15% CO2 and 85% N2 in volume instead of pure CO2. hCaI2 (1.0 mmol) + L5 (1.0 mmol) + L6 (0.1 mmol).

the N-methyl substituted tertiary amines of L8 (84%) and L9 (78%) were observed to have lower activity, indicating that hydroxy can cooperate with CaI2 to promote the cycloaddition (L5 vs L8 and L9). We have added the L13 and L14 which are similar to the MDEA as the control ligands, the yield of the product was 44% use of L13 as a ligand, meanwhile, the hydroxyl group protected analogue of L14 also led to an only 71% yield of the product. The activities of these catalysts are significantly lower than CaI2/MDEA system, indicating that the hydroxyl group can promote the reaction. It is well known that the reaction of primary or secondary amines with CO2 are readily achievable, ligands L10-L12 are therefore used to activate CO2 and promote the reaction. However, the systems have increased viscosities, resulting in the inferior activity of these ligands consistent with L5. The effect of coordinating atoms shows that the amines bearing the coordinating N atom is better than that of O atom (L11 vs L12). In addition, we also tested the activity of CaI2/ether systems (L15 vs 16), and the CaI2/ tetraethylene glycol (L16) system showed better catalytic activity. The results can also be supported as reported work by Shirakawa [40].

Table 1 Our afterwards efforts focused on the screening of several calcium salts using MDEA as the optimal ligand (Table 1). CaCl2 and CaBr2 show quite low activities (Entries 1 and 2), consistent with the observation in previous reports using alkali metal halides [44]. CaI2 is found to be the best catalyst since the highly-delocalized nucleophilic electrons of iodide ion have the positive effect on the reaction. Notably, some weak nucleophilic anions such as AcO−, C2O42−, and TfO− show no catalytic activity under the same conditions (Entries 4–6). The optimum time for the reaction is confirmed to be 6 h by NMR tracking (Entry 7, Fig. S1). The pre-prepared CaI2/MDEA complexe also exhibits the same catalytic activity with the in situ-generated catalyst (Entry 8 vs 7). The effects of the composition of the catalyst system (Entries 9–12) and temperature (Entry 13) on the yield of the product had also Scheme 2. Effect of ligand on the reaction of CO2 with styrene oxide (1 g). Reaction conditions: 1 g (10 mmol), CaI2 (1 mmol), ligand (1 mmol), CO2 (balloon), 50 °C, 10 h, solvent-free. The 1H NMR yields of styrene carbonate (2 g) are given in parentheses. a1 g (2.0 g, 16.7 mmol), CaI2 (246 mg, 0.84 mmol), L5 (100 mg, 0.84 mmol), CO2 (10 bar), 23 °C, 24 h, solvent-free, isolated yield is given.

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system (mixed amines are usually used as chemical absorbents for CO2 capture in industry [52,53]). Almost the same reaction equilibrium time of 6 h may be due to the inhibition of the reaction rate by the concentrations of the substrate and product. Fig. 2 To gain insight into activation of epoxide by CaI2/MEDA system, we performed several 1H NMR experiments (Fig. 3) (See supporting information). When an equivalent amount of 1,2-epoxybutane (1b) was added into MDEA, the hydroxy protons of MDEA were observed with a clear upfield shift from 4.16 to 3.79 ppm (Fig. 3A), meanwhile, the stretching vibration of hydroxy (vOH) on MDEA shifted high frequency from (3343–3378) cm−1 (Fig. 3C). The intermolecular hydrogen bond in MDEA was broken gradually, and the new hydrogen between MDEA and 1b was formed. These results suggested that MDEA can activate epoxide by hydrogen bonding interaction [40]. Especially, using CaI2 alone cannot initiate the reaction in absence of MDEA (Entry 13, Table 1), probably due to the low solubility of CaI2 in the epoxide. Amines are known as a kind of excellent ligands for Ca2+ [56]. For example, complexes of tetramethylethylenediamine (L4) or triethylenetetramine (L11) with CaI2 have been reported by Westerhausen et al. in 2010 [57]. The result encouraged us to study the interaction between CaI2 and MDEA. As shown in 1H NMR of MDEA (Fig. 3B), a specific signal at 4.34 ppm assigned to hydroxy protons of MDEA in DMSO-d6 has a downfield shift to 4.48 ppm by the addition of equivalent CaI2 into MDEA. This is due to the deshielding effect of oxygen by Ca2+ characterizing the complexation of Ca2+ with MDEA and thus leading to the increasing affinity and nucleophilicity of iodide ions [44,45,58]. It is interesting to find that adding 1b in the mixture of MDEA and CaI2 results in a further downfield shift to 4.70 ppm for the hydroxy protons of MDEA, suggesting that 1b interacting with Ca2+ enhances further the deshielding effect of O atom induced by Ca2+. Meanwhile, the chemical shifts of methyl (N-CH3) moved continuously to upfield from

Fig. 2. 1H NMR monitoring of the reaction of styrene oxide (10 mmol) with CO2 (balloon) at 50 °C under solvent-free condition.

been investigated accordingly. Interestingly, the reaction cannot be carried out in the absence of CaI2 or MDEA (Entries 14 and 15). These results suggest that MDEA and CaI2 are synergistic in the reaction. When TBAI was used instead of CaI2 (Entry 16), only a 52% yield of 2g was obtained, indicating that Ca2+ played important role in the activation of epoxide. It is particularly worth mentioning that a considerable yield (88%) of 2g is acquired by using a mixture of gas (CO2/N2) under the optimal conditions (Entry 17), which provides a viable strategy for CaI2-MDEA catalyzed synthesis of cyclic carbonate with diluted CO2. In addition, the rate of reaction can be effectively accelerated by adding ligand L6 as an activator of MDEA (Entry 18, Fig. 2) in the first 3 h due to the increased solubility of CO2 in the mixed-amine

Fig. 3. (A): 1H NMR of MDEA, 1,2-epoxybutane (1b), and mixture of MDEA and 1b (1:1 of molar ratio) in CDCl3 (0.34 M); (B): 1H NMR of MDEA, MDEA + CaI2 (1:1 of molar ratio) and MDEA + CaI2 + 1b (1:1:1 of molar ratio) in DMSO-d6 (0.34 M); and (C): FTIR of MDEA, 1,2-epoxybutane (1b), and mixture of MDEA and 1b with different molar ratio.

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Scheme 3. Substrate scope of the conversion of epoxides with CO2 into cyclic carbonates. Reaction conditions: epoxide (10 mmol), CaI2 (1 mmol), L5 (1 mmol), CO2 (balloon), 50 °C, 6 h, solvent-free. Isolated yields of cyclic carbonates are given. aThe reaction was carried out in a stainless steel autoclave because of epoxy propane with low-boiling, and CO2 with a 0.5 MPa pressure is necessary to provide enough CO2. b The reaction was carried out for 24 h.

this protocol was also examined (Scheme 3). The substrates include chain and cyclic epoxides, aromatic and non-aromatic epoxides, monoand di-epoxides. Except for substrate 1e, this catalyst is found to be very efficient for mono-substituted terminal epoxides and the reactions proceed smoothly leading to appreciable yield of desired products (up to 99%). However, a yield only 25% of 2e was obtained. It may be due to the nucleophilic substitution reaction of 1e with MDEA (MDEA is the good nucleophilic reagent for the halide group in 1e), so that the activity of the catalyst is thus reduced remarkably. Additionally, cyclohexene oxide 1l was also examined as the substrate in this coupling process. The cyclic carbonate 2l obtained was exclusively the cis isomer, as confirmed by 1H NMR and 13C NMR [44,46]. Because of the high hindrance created from the ring of 1l, the cis-cyclohexene carbonate 2l is produced with yield of only 31% even when the reaction time was prolonged to 24 h. On the basis of previous reports [40–48] and the catalytic and NMR results in this paper, a mechanism consisting of multisite activation of epoxide by the CaI2/MDEA system and subsequent ring opening by the iodide was proposed and illustrated in Scheme 4. Firstly, epoxide is activated via the synergistic interaction of the hydroxy of MDEA and Ca2+, resulting in the polarization of the CeO bond in epoxide that facilitates the ring opening by nucleophilic attack of the iodine (Int. 1) [59]. The formed iodine alkoxide Int. 2 can be stabilized by the coordination with calcium ion and the hydrogen bonding interaction with the hydroxy of MDEA [44,45]. The intramolecular ring-closing of Int. 3 affords cyclic carbonate accompanying with the regeneration of CaI2/ MDEA. To prove the applicability of the present catalytic system, the

Scheme 4. Proposed catalytic mechanism.

(2.19–2.13) ppm. Combining the results obtained in Fig. 3, it is reasoned that the substrate can interact with both Ca2+ and hydroxy of MDEA. This kind of multisite interactions should be responsible for the high activity of MDEA/CaI2 system. Fig. 3 Under the optimal reaction conditions, the substrate scope of 91


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Scheme 5. (a) 100 mmol scaled-up synthesis of styrene carbonate. (b) Effect of water on the synthesis of 2j. (c) synthesis of optically active carbonates.

leads to an overall retention of the stereochemistry [60–62]. The recyclability of catalyst plays an important role for the potential application in industry. Under optimized conditions, the separation and reusability of CaI2/MDEA system was examined (See supporting information). After the reaction, diethyl ether (CaI2/MDEA complex is insoluble in diethyl ether) was added into the reactor to precipitate the catalysts and to extract the unreacted substrate 1j and generated product 2j. Then, the recovered catalyst CaI2/MDEA was dried under vacuum to remove residual diethyl ether and directly used for the next run under the identical conditions. It was found that there was an acceptable decrease in the activity of CaI2/MDEA system from (98–92) % after six running (Fig. 4). As a result, CaI2/MDEA system represents a reusable and practicable catalytic system for the cycloaddition of CO2 with epoxides.

4. Conclusions Fig. 4. Catalyst recycling in synthesis of 2j, and the tsolated yields of cyclic carbonates are given.

In summary, we had successfully developed a readily available, environmentally friendly, and recyclable catalyst (CaI2/MDEA) with high efficiency in the coupling of carbon dioxide with epoxides at mild temperatures and pressures under solvent-free condition. A multisite activation of epoxides involving the hydroxy-assisting and the Ca2+promoting ring opening path was demonstrated, in combination with the activation of epoxides by iodide ions. This catalyst system was easy to be reused and the large-scale reaction performed quite well even in the presence of water. Compared to those complexes and expensive catalytic systems, CaI2/MDEA catalyst system is more attractive for conversion of CO2 into cyclic carbonate, which shows a potential and sustainable route to obtain cyclic carbonates from CO2 capture and its subsequent conversion.

reaction was carried out on a 100 mmol scale (Scheme 5a). A mixture of styrene oxide (100 mmol), CaI2 (10 mmol), and MDEA (10 mmol) was stirred at 50 °C for 24 h under a CO2 atmosphere. The desired product styrene carbonate (2 g) was obtained in a 96% 1H NMR yield. MDEA aqueous solutions have been usually used as excellent CO2 absorbents [49,51]. In addition, the CO2 source used for industry is usually moisture containing. Hence, it is necessary to check the influence of water on the synthesis of carbonate. Two parallel experiments were performed by adding water of equivalent amount with the catalyst (10 mol%) or the substrate (100 mol%) before the reaction to the reaction system (1j as substrate), and the yields of 2j are still larger than 96% and keep nearly unchanged (Scheme 5b). The experimental results provide a possibility for CO2 capture using aqueous MDEA and subsequent transformation into cyclic carbonate in situ. In addition, enantiopure (R)-1g was used to obtain optically active cyclic carbonates (Scheme 5c). To our delight, (R)-1g was converted into the corresponding enantiomerically (R)-2g with a yield of 97% (99% ee). It suggests that the reaction proceeds via two consecutive SN2 steps which

Acknowledgements This work was supported by National Natural Science Foundation of China (No.21376115, and 21676134) and the Fundamental Research Funds for the Central Universities (020514380106). 92


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Appendix A. Supplementary data

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