2,2',2''-Terpyridine-Catalyzed Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide under Solvent-Free Conditions

An efficient coupling reaction of epoxides with CO2 affording cyclic carbonates with the use of 2,2',2''-terpyridine as catalyst under solvent-free conditions has been developed.

On the other hand, recently, the catalytic reactions based on the use of small organic molecules as catalysts have been greatly developed with the significant advantages of metal-free procedure, lack of sensitivity to moisture and oxygen, affordability, low cost and low toxicity [20,21]. Recently, the reaction between amine and CO 2 is considered to be one of the methods for the storage of CO 2 [22], and it has been also disclosed that the quaternary ammonium salts [23] and amines [24] are the efficient catalysts for the reaction of epoxides with CO 2 to give cyclic carbonates. In addition, our previous work revealed that N,N-dimethylformamide (DMF) could catalyze the same reaction OPEN ACCESS efficiently [25]. In continuation of our interest in developing efficient catalytic systems using simple and cheap organic compounds as catalysts in the coupling reaction of epoxides with CO 2 , we therefore have investigated the catalytic activity of nitrogen-containing organic compounds such as amines, anilines, amides and pyridines, and found that 2,2',2''-terpyridine is an excellent organocatalyst to provide an alternative metal-free catalytic system for the conversion of CO 2 to cyclic carbonates. Scheme 1. Synthesis of cyclic carbonates from epoxides and CO 2 . Table 1 concludes the catalytic activity of a variety of nitrogen-containing organic compounds in the coupling reaction of 1-chloro-2,3-epoxypropane (1a) with CO 2 (3.0 MPa of initial pressure) in an autoclave in the presence of 10 mol % of organocatalysts at 130 °C for 20 h under solvent-free conditions. In general, all the used nitrogen-containing compounds showed the catalytic activity to promote the coupling reaction to give the desirable product of 4-cholromethyl- [1,3]dioxolan-2-one (2a) in fair to good yields. As was observed for the formation of 2a, trialkyl tertiary amines of Et 3 N and Bu 3 N showed modest catalytic activity to give 2a in 48% and 53% yields, respectively (entries 1,2). Although TEMED (N,N,N',N'-tetramethylethylenediamine), DBU (1,8-diazabicyclo[5.4.0]-undec-7-ene) and DABCO (1,4-diazabicyclo[2.2.2]octane) contains two nitrogen atoms, they displayed the considerable different catalytic activities, and DABCO showed higher catalytic activity due possible to its higher nucleophilicity resulting from the alkyl groups not to disturb the lone pairs (entries 3,4 vs. entry 5). In addition, TBD (1,5,7-Triazabicyclo[4.4.0]dec-5-ene), which has three different nitrogen atoms showed moderate catalytic activity to give 2a in 56% yield (entry 6). In the cases of anilines, the catalytic activity was enhanced by increasing the number of methyl groups to nitrogen atom (entries 7-9). However, unexpectedly, benzamide, N,N-dimethylbenzamide and 2,3,4,5,6-pentafluorobenzamide had similar catalytic activities to catalyze the coupling reaction to give 2a in good yields (entries 10-12). When six-membered N-heterocyclic compounds such as pyridine, 2-methylpyridine and 2-(dimethylamino)pyridine were used, 2a could be obtained in modest to good yields (entries 13-15). 2-Phenylpyrimidine with two nitrogen atoms catalyzed the reaction to afford 2a in 79% yield (entry 16), and the best yield in product 2a was obtained in the presence of 2,2',2''-terpyridine (entry 17). The reason that 2,2',2''-terpyridine had high catalytic activity might be resulted from its unique structure and synergy effects of three pyridyl groups bonded by position 2.

Results and Discussion
In addition, in order to optimize the catalytic system using 2,2',2''-terpyridine as catalyst, the effect of reaction temperature, catalyst dosage, reaction time and pressure of CO 2 were also investigated. As shown in Table 2, the formation of 2a was greatly affected by reaction temperature, and when the reaction was performed at 110 °C, the yield of 2a was greatly decreased to 72% (entry 1). However, at 130 °C, 2a could be obtained in similar yields by either using less amount of catalyst (5.0 mol % (entry 2) or 1.0 mol % (entry 3) vs. entry 16 of Table 1), or decreasing the reaction time from 20 to 10 h (entry 5), although further decreasing the catalyst dosage (0.5 mol % of catalyst, entry 4) and the reaction time (entry 6) or decreasing the pressure of CO 2 resulted in the decrease of product yield (entry 7). In addition, increase of CO 2 pressure could not improve the yield of 2a (entry 8). Therefore, the reaction conditions indicated in entry 5 of Table 2 are selected as the optimized conditions for the reactions of a variety of epoxides with CO 2 .
To examine the substrate scope of the present catalytic system, several representative epoxides were subjected to the optimized reaction conditions as shown in Scheme 2, and found that the monoalkyl-and monoaryl-substituted epoxides could undergo the coupling reaction with CO 2 giving the corresponding cyclic carbonates in good to high yields. It is worth noting that vinyl-substituted epoxide underwent the reaction smoothly to give the vinyl cyclic carbonate, which is expected to be a useful monomer for synthesis of functional polymer. However, unfortunately, the internal epoxide such as 2,3-epoxybutane and 1,2-epoxycyclohexane showed very low reactivity due possible to their steric hindrance. The proposed mechanism for the coupling reaction of epoxides with CO 2 to produce cyclic carbonates catalyzed by 2,2',2''-terpyridine is shown in Scheme 3. It involves the nucleophilic addition of nitrogen atom(s) of 2,2',2''-terpyridine to CO 2 , and the ring-opening reaction of epoxide with nucleophilic intermediate via C-O bond cleavage, followed by intramolecular nucleophilic addition to construct the five-membered ring, and finally, to give cyclic carbonate and regenerate catalyst. All the steps are the traditional and well-known transformation. Table 2. Effect of reaction conditions on the formation of 4-cholromethyl- [1,3]dioxolan-2-one (2a) using 2,2',2''-terpyridine as catalyst a .

A Typical Experiment for the Synthesis of 4-Chloromethyl-[1,3]dioxolan-2-one (2a)
1-Chloro-2,3-epoxypropane (1a) (462.6 mg, 5.0 mmol) and 2,2',2''-terpyridine (0.05 mmol, 1.0 mol %) were charged in a 25 mL-autoclave, and then CO 2 was introduced at an initial pressure of 3.0 MPa at room temperature, and the mixture was heated at 130 °C with stirring for 10 h. After the reaction, the autoclave was cooled to room temperature, CO 2 was released slowly. To the obtained reaction mixture, 1.0 mmol of octadecane (as an internal standard material for GC analysis) and 4.0 mL of CH 2 Cl 2 were added with stirring, and then the resulting mixture was analyzed by GC and GC-MS. 2a was obtained in 90% yield (614.4 mg, 4.5 mmol) by Kugelrohr distillation. 2a-c was isolated by Kugelrohr distillation, and 2d-f were purified by flash column chromatography on silica gel with petroleum ether as eluent.
All the products (2a-f) were known compounds and identified by their 1 H-NMR, 13 C-NMR and GC-MS. As exampled, the characterization data of 2a is reported as follow: 1

Conclusions
In summary, 2,2',2''-terpyridine was proven to be an efficient organocatalyst for the coupling reaction of epoxides with CO 2 to afford cyclic carbonates under solvent-free and metal-free conditions. The present process represents a simple and green catalytic system for the activation and conversion of CO 2 into valuable organic compounds.