Functionalized β-Cyclodextrins Catalyzed Environment-Friendly Cycloaddition of Carbon Dioxide and Epoxides

Ammonium, imidazole, or pyridinium functionalized β-cyclodextrins (β-CDs) were used as efficient one-component bifunctional catalysts for the coupling reaction of carbon dioxide (CO2) and epoxide without the addition of solvent and metal. The influence of different catalysts and reaction parameters on the catalytic performance were examined in detail. Under optimal conditions, Im-CD1-I catalysts functionalized with imidazole groups were able to convert various epoxides into target products with high selectivity and good conversion rates. The one-component bifunctional catalysts can also be recovered easily by filtration and reused at least for five times with only slight decrease in catalytic performance. Finally, a possible process for hydroxyl group-assisted ring-opening of epoxide and functionalized group- induced activation of CO2 was presented.

Though significant improvements in catalyst species have been achieved, some drawbacks such as scarce active sites and low carrier still remain to be overcome for effective chemical conversion of CO 2 via its coupling reaction with epoxides [87,88]. Moreover, in many cases harsh reaction conditions (e.g., high pressure and/or high temperature) and participation of co-solvent/additive are generally required. Furthermore, accompanying issues of inherent corrosion, toxicity and environmental concerns associated with the use of metallic cations remain challenging. Therefore, it is very demanding to put forward efficient environment-friendly catalysts for the coupling reaction of carbon dioxide and epoxide.

Synthesis of Functionalized β-CDs
The ammonium, imidazole, and pyridinium functionalized β-CDs (Scheme 1) were exactly synthesized and characterized according to previously reported methods [92]. After synthesis and purification following the reported procedures, these functionalizedβ-CDs were directly employed to initiate CO 2 coupling reaction with epoxides.

General Procedure for Cyclic Carbonates Synthesis from Epoxides and CO2
Cycloaddition reaction of CO2 and epoxide was conducted in a 250 mL stainless steel autoclave. In a typical reaction, predetermined amounts of catalyst and epoxide were fed into the reactor, CO2 was then added into the reactor at certain pressure. The autoclave was sealed and then immersed into an oil bath at preset temperature with stirring. The Scheme 1. Structures of β-CD and various functionalized β-CDs.

General Procedure for Cyclic Carbonates Synthesis from Epoxides and CO 2
Cycloaddition reaction of CO 2 and epoxide was conducted in a 250 mL stainless steel autoclave. In a typical reaction, predetermined amounts of catalyst and epoxide were fed into the reactor, CO 2 was then added into the reactor at certain pressure. The autoclave was sealed and then immersed into an oil bath at preset temperature with stirring. The reactor was cooled down in an ice-water bath after predesigned time and the unreacted CO 2 was released slowly. The yield and selectivity are determined by 1 H NMR characterization.

Effect of Reaction Parameters with Am-CD1-I
Reaction conditions were screened for optimizing the catalytic activity based on the ammonium functionalized β-CD Am-CD1-I and the coupling reaction of CO 2 and propylene oxide (PO). The reaction conditions as collected in Figure 1 were standardized by observing the effect of reaction temperature, pressure, time, and catalyst loading on the yield of propylene carbonate (PC). not favorable for the cycloaddition because PO is another reactant [99,100]. As a result, a maximal PC yield was obtained. The PC yield increased steadily with reaction time until 7 h and the coupling reaction proceeded rapidly within the first 5 h, and no appreciable increment in PC yield was observed thereafter (Figure 1c). This might originate from a hampered interaction between the catalyst and reactant due to the formation of PC [101]. A more viscous reaction system after prolonged reaction time was another negative factor disfavoring the activation of CO2. Thus, the reaction time of 5 h was chosen to be optimal. Increasing of the catalyst loading from 0.14 mol% to 1 mol% led to rising catalytic activity ( Figure 1d). However, there was a decrease in the PC yield for the reaction conducted with 2 mol% catalyst, which may be from a hindered mass transfer due to excess catalyst. Thus, 1 mol% Am-CD1-I is optimum for this work and selected for subsequent research.

Effect of Reaction Parameters overIm-CD1-I
Inspired by the high performance of ammonium functionalized β-CD Am-CD1-I, the imidazole functionalized β-CD Im-CD1-I was also attempted to catalyze the cycloaddition of CO2 and PO. PC yields catalyzed by Im-CD1-I trended similarly to those in the case of using Am-CD1-I ( Figure 2). The reaction temperature also affected the PC yield and 110 °C is chosen to be optimal (Figure 2a). The PC formation with Im-CD1-I correlated with The reaction temperature was first investigated to test its effect on the PC yield. Figure 1a displays a strong effect of temperature on the PO conversion. A high reaction temperature is favorable for the synthesis of PC, indicating that the cycloaddition reaction was thermodynamically favorable [93,94]. To our satisfaction, low temperature appears insensitive to the PC selectivity in view of the fact that low temperature would be favorable for producing polycarbonate. Considering that polymerization of cyclic carbonates occurs at higher temperatures [95] and has adverse effect on the equipment, an optimal reaction temperature of 130 • C was selected for following studies. Figure 1b reflects the influence of the CO 2 pressure on the PC yield. Low pressures ranging from 0.5 to 1.0 MPa gave rise to increase in the PC yield. The PC yield decreased in high-pressure range (3.0-5.0 MPa) after a plateau from 1.0 to 3.0 MPa of CO 2 pressure. Such phenomenon is observed as well in other catalytic systems [96][97][98]. It could be explained that in the low-pressure region, the increase in CO 2 pressure enhanced PC yield due to higher CO 2 concentration in the liquid phase. However, much higher CO 2 pressure would lower the PC yield due to decreased PO concentration around the catalyst, this is not favorable for the cycloaddition because PO is another reactant [99,100]. As a result, a maximal PC yield was obtained.
The PC yield increased steadily with reaction time until 7 h and the coupling reaction proceeded rapidly within the first 5 h, and no appreciable increment in PC yield was observed thereafter (Figure 1c). This might originate from a hampered interaction between the catalyst and reactant due to the formation of PC [101]. A more viscous reaction system after prolonged reaction time was another negative factor disfavoring the activation of CO 2 . Thus, the reaction time of 5 h was chosen to be optimal. Increasing of the catalyst loading from 0.14 mol% to 1 mol% led to rising catalytic activity ( Figure 1d). However, there was a decrease in the PC yield for the reaction conducted with 2 mol% catalyst, which may be from a hindered mass transfer due to excess catalyst. Thus, 1 mol% Am-CD1-I is optimum for this work and selected for subsequent research.

Effect of Reaction Parameters over Im-CD1-I
Inspired by the high performance of ammonium functionalized β-CD Am-CD1-I, the imidazole functionalized β-CD Im-CD1-I was also attempted to catalyze the cycloaddition of CO 2 and PO. PC yields catalyzed by Im-CD1-I trended similarly to those in the case of using Am-CD1-I ( Figure 2). The reaction temperature also affected the PC yield and 110 • C is chosen to be optimal ( Figure 2a). The PC formation with Im-CD1-I correlated with CO 2 pressure ( Figure 2b). The peak PC yield appeared at 3 MPa and much higher CO 2 pressure resulted in decreased yield. Moreover, prolonged reaction time exceeding 3 h failed in further increasing PC yield (Figure 2c). The PC yield at the low catalyst loading (0.14-0.33 mol%) rose with increasing catalyst loading, realizing a drastic elevation in PC yield. However, a further increase in catalyst loading contributed little to PC yield ( Figure 2d). To sum up, the optimal condition of the cycloaddition over Im-CD1-I is 110 • C, 3 MPa, 3 h and 0.33 mol%. Comparison of the optimal conditions for Am-CD1-I and Im-CD1-I (130 °C, 1 MPa, 5 h,1 mol% Am-CD1-I vs. 110 °C, 3 MPa, 3 h, 0.33 mol% Im-CD1-I) found that ammonium functionalized β-CD Am-CD1-I required higher reaction temperature and more time, along with higher catalyst loading, while the imidazole functionalized β-CD Im-CD1-I Comparison of the optimal conditions for Am-CD1-I and Im-CD1-I (130 • C, 1 MPa, 5 h, 1 mol% Am-CD1-I vs. 110 • C, 3 MPa, 3 h, 0.33 mol% Im-CD1-I) found that ammonium functionalized β-CD Am-CD1-I required higher reaction temperature and more time, along with higher catalyst loading, while the imidazole functionalized β-CD Im-CD1-I only required relatively higher CO 2 pressure, which may be due to the different solubility of the two functional β-CDs in the reaction system. In general, the optimal condition for Im-CD1-I is milder than that forAm-CD1-I, reflecting the superiority of imidazole functionalized β-CDs catalysts although Im-CD1-I and Am-CD1-I are both metal-, solventand cocatalyst-free.

Catalytic Performances of Various Catalysts
Under optimal conditions, solvent-free synthesis of PC from CO 2 and PO catalyzed by ammonium, imidazole, and pyridinium functionalized β-CDs was investigated. As listed in Table 1, most of functionalized β-CDs afforded excellent selectivity for PC. The reason for such a high selectivity of this reaction is due to the tendency of the X − ion to attack the C with small site resistance during the nucleophilic attack on the epoxide to open its ring. The details will be shown in the description of the mechanism section. For various catalysts, the catalytic activities correlated with their structures. The mono-6-halide-β-CDs can convert PO in quantitative yields ( Table 1, entries 1-3), suggesting synergetic effect of rich hydroxyl groups and halide ions in the modified β-CDs. The synergetic effect of these two functional groups has been reported and testified using a DFT calculation by Zhang et al. [102]. Moreover, when β-CD was modified by ammonium, imidazole, and pyridinium, the catalyst performance was improved visibly. For the ammonium functionalized β-CDs (Table 1, entries 4-9), higher catalytic activities of Am-CD1-I and Am-CD2-I are attributed to bulky alkyl on the butyl amine group. The more-bulky butyl on the amine group might form a more flexible ion pair with I-, thus increasing its nucleophilicity and making it the most viable catalyst [103,104]. For both mono-6-halide-β-CDs and functionalized β-CDs, the activity for various halogen anions decreases in the order of I − > Br − > Cl − ( Table 1, entries 1-3,5-7,13-15) probably owing to the leaving ability and nucleophilicity of the anion [105,106].
To study the effect of imidazole functionalized β-CD structure on the catalytic activity, a milder reaction condition was conducted afterwards. A longer alkyl chain endowed imidazole functionalized β-CD with higher catalytic performance (Table 1, entries 16-18) because a long alkyl chain may weaken electrostatic interaction, thus enhancing the nucleophilicity of anion [107]. The ammonium, imidazole, and pyridinium functionalizedβ-CDs played quite well in coupling reaction between CO 2 and epoxides with much better performance compared with binary catalytic system β-CD/KI [90] or β-CD/TBAI [108], because our catalyst only required lower CO 2 pressure and catalyst loading without adding metal and additive.

Recycling Test
A series of reaction recycles using Am-CD1-I and Im-CD1-I as catalysts were performed to investigate the stability of the catalyst for the cycloaddition reaction of PO with CO 2 under each optimal condition. In each cycle, Am-CD1-I and Im-CD1-I were recovered via simple filtration, washed with acetone, dried in vacuo and directly reused for the next cycle. As Figure 3 presents, both Am-CD1-I and Im-CD1-I can be reused for at least 5 times without obvious loss in catalytic activity. In order to confirm the stability of Im-CD1-I (showing slight decrease after 5 times of recycling, Figure 3), the reused Im-CD1-I was characterized by FT-IR analysis. Strong characteristic bands assigned to the in-plane C-H deformation vibration and in-plane C-C and C-N stretching vibration of the imidazole ring (1629 and 1318 cm −1 ), along with the characteristic band of C-I bond at 604 cm −1 remain after reuse (Figure 4), indicating a very stable Im-CD1-I for this reaction. As shown in Figure 4, the structure of the catalyst was maintained after five times of reuse, which proved the stability and reusability of the synthesized catalyst, and the slight decrease in the catalytic effect after five times of use might be due to the partial loss of catalyst during the recycling process. imidazole ring (1629 and 1318 cm −1 ), along with the characteristic band of C-I bond at 604 cm −1 remain after reuse (Figure 4), indicating a very stable Im-CD1-I for this reaction. As shown in Figure 4, the structure of the catalyst was maintained after five times of reuse, which proved the stability and reusability of the synthesized catalyst, and the slight decrease in the catalytic effect after five times of use might be due to the partial loss of catalyst during the recycling process.

Cycloaddition of Various Epoxides and CO 2
To probe the prospect and versatility of as-synthesized functionalized β-CD catalyst, the cycloaddition reaction of CO 2 with various epoxides with Im-CD1-Iwas studied ( Table 2). Im-CD1-I worked well towards various epoxides possessing both electronwithdrawing and electron-donating substituents, forming respective cyclic carbonates with excellent selectivity and good yields. For isobutyl oxide (Table 2, entry 7) and cyclohexene oxide ( Table 2, entry 8), identical reaction conditions gave rise to relatively low yield possibly due to that a steric hindrance obstructed the nucleophilic attack of the epoxide while its coordination to the Lewis acid metal center benefited the yield [108][109][110][111]. The aliphatic substituted epoxides (including PO in Table 1) were transformed with CO 2 to desired products in good yields. Especially, the activated epoxide epichlorohydrin was converted by as-designed catalysts and transformed into respective cyclic carbonate in good yield in 3 h ( Table 2, entry 1). Surprisingly, aromatic substituted epoxide styrene oxide reacted with CO 2 in a yield of 100% (Table 2, entry 3). Furthermore, the glycidyl ethers were turned into corresponding carbonates in good yields from 75 to 98% (Table 2, entries 4-6). It is also noteworthy that the Im-CD1-I catalyzed the diepoxides to produce respective bicyclic carbonates as well (Table 2, entries 10,11), raw materials for synthesizing non-isocyanate polyurethanes (NIPUs) without using toxic phosgene or isocyanates via the reaction with polyfunctional primary amines [112][113][114]. With increasing aliphatic chain length, the addition of CO 2 was hindered because of chain folding or the fluidity of chains and the hindrance of methylene groups. Such phenomenon was also observed by Qin et al. [115][116][117].

Cycloaddition of Various Epoxides and CO2
To probe the prospect and versatility of as-synthesized functionalized β-CD catalyst, the cycloaddition reaction of CO2 with various epoxides with Im-CD1-Iwas studied ( Table  2). Im-CD1-I worked well towards various epoxides possessing both electronwithdrawing and electron-donating substituents, forming respective cyclic carbonates with excellent selectivity and good yields. For isobutyl oxide (Table 2, entry 7) and cyclohexene oxide ( Table 2, entry 8), identical reaction conditions gave rise to relatively low yield possibly due to that a steric hindrance obstructed the nucleophilic attack of the epoxide while its coordination to the Lewis acid metal center benefited the yield [108,109,110,111]. The aliphatic substituted epoxides (including PO in Table 1) were transformed with CO2 to desired products in good yields. Especially, the activated epoxide epichlorohydrin was converted by as-designed catalysts and transformed into respective cyclic carbonate in good yield in 3 h ( Table 2, entry 1). Surprisingly, aromatic substituted epoxide styrene oxide reacted with CO2 in a yield of 100% (Table 2, entry 3). Furthermore, the glycidyl ethers were turned into corresponding carbonates in good yields from 75 to 98% (Table 2, entries 4-6). It is also noteworthy that the Im-CD1-I catalyzed the diepoxides to produce respective bicyclic carbonates as well (Table 2, entries 10,11), raw materials for synthesizing non-isocyanate polyurethanes (NIPUs) without using toxic phosgene or isocyanates via the reaction with polyfunctional primary amines [112,113,114]. With increasing aliphatic chain length, the addition of CO2 was hindered because of chain folding or the fluidity of chains and the hindrance of methylene groups. Such phenomenon was also observed by Qin et al [115,116,117].

Proposed Mechanism
A mechanism is proposed for the functionalized β-CD catalyzed reaction as shown in Scheme 2 based on experimental results and literatures [118]. Firstly, the interaction

Proposed Mechanism
A mechanism is proposed for the functionalized β-CD catalyzed reaction as shown in Scheme 2 based on experimental results and literatures [118]. Firstly, the interaction between the epoxide oxygen and hydroxyl groups of Im-CD1-I promoted the polarization of the C-O bond in epoxide as reported in literature [119][120][121][122][123]. Simultaneously, CO 2 was activated by functionalized group, such as imidazole herein. Moreover, the imidazolium cations could also stabilize the metal-alkoxide bond through charge interactions, which would help explain the superior performance of imidazole, ammonium, or pyridinium functionalized β-CDs than mono-6-halide-β-CDs. Subsequently, the nucleophilic halide anion attacked the less hindered carbon atom of epoxide followed by ring opening step to form an intermediate of oxygen anion. The oxygen anion intermediate then reacted with activated CO 2 to form a carbonate anion, followed by an intramolecular ring-closure step to produce cyclic carbonate and regenerate the catalyst. According to this mechanism, the cooperative effect between the electrophile (hydrogen bond) and nucleophile (flexible halide anion) in the same catalyst molecules could effectively promote the coupling reaction in an eco-friendly mode without the introduction of metal, additive, and solvent [124,125].

Conclusions
A series of imidazole, ammonium, and pyridinium functionalized β-CDs were first employed as a one-component and recyclable catalyst for the coupling reaction between various epoxides and CO 2 without the addition of metal, cocatalyst, and solvent. Excellent selectivity and high cyclic carbonate yields are realized under mild conditions. As disclosed by the mechanism, the reaction proceeded smoothly owing to a synergistic effect from abundant hydroxyl groups of β-CD and the halide anion of functional groups. These green, biocompatible, and non-toxic catalysts derived from inexpensive environment-friendly starting material β-CD have great potential in industrial application for the conversion of CO 2 .