Cu(I)/Ionic Liquids Promote the Conversion of Carbon Dioxide into Oxazolidinones at Room Temperature

Recently, the efficient chemical fixation of carbon dioxide (CO2) into high value chemicals without using noble metal catalysts has become extremely appealing from the viewpoint of sustainable chemistry. In this work, a one-pot three component reaction of propargylic alcohols, anines and CO2 that can proceed in an atom economy and environmentally benign manner by combination of CuI and tetrabutylphosphonium imidazol ([P4444][Im]) as a catalyst was described. Catalysis studies indicate that this catalytic system is an effective catalyst for the conversion of CO2 into oxazolidinones at room temperature and ambient pressure without any solvent. The results provide a useful way to design novel noble metal-free catalyst systems for the transformation of CO2 into other valuable compounds.


Introduction
With the global consumption of fossil fuels, the increasing concentration of CO 2 in the atmosphere could have a significant impact on the global climate [1,2]. Therefore, the development of efficient strategies to reduce CO 2 emissions has becoms an imperative task for scientific researchers [3][4][5]. The chemical fixation and transformation of CO 2 into valuable chemicals is an ideal pathway for reducing CO 2 emissions and fully utilize this cheap C 1 source [6][7][8][9][10]. Thus, considerable efforts have been devoted to the exploration of efficient strategy for chemical utilization of CO 2 to produce high-value chemical commodities [11][12][13][14][15][16].
Oxazolidinones are a class of nitrogen-containing heterocyclic compounds which have been used as organic intermediates, chiral additives, antibacterial drugs and muscle relaxants, etc. [17][18][19][20]. The synthesis of oxazolidinones between propargyl alcohol, CO 2 and an amine is an important and atom-economic reaction for chemical utilization of CO 2 [12,21]. However, the biggest obstacle is the lack of an effective catalyst to facilitate its activation and conversion, since CO 2 is a highly oxidized and thermodynamically stable molecule [22]. In recent years, various metal catalysts [23][24][25][26][27][28] and metal-free catalysts [29,30] have been verified to be efficient for the reaction. Nevertheless, most of the catalytic systems usually required high temperatures, high pressures, toxic organic solvents and noble metals. Therefore, a cheap, green and highly active catalyst is urgently needed in order to make the reaction go smoothly at room temperature and atmospheric pressure.
Our previous work has revealed that metal salts combined with ionic liquid (IL) represent a new hybrid catalyst that can be regarded as one of the most promising and efficient catalysts for chemical transformations of CO 2 because of their remarkable synergistic catalysis mechanism [31]. For example, a Cu(I) salt/protic IL catalytic system can efficiently catalyze the cycloaddition of CO 2 with propargylic alcohols to produce α-alkylidene cyclic carbonates under mild conditions. Inspired by these works, we now report that Cu(I) salt and the IL tetrabutylphosphonium imidazol([P 4444 ][Im]) act as a catalyst for the three component reaction of terminal propargyl alcohols, CO 2 and anines (Scheme 1). The results show that the CuI/[P 4444 ][Im] catalyst system exhibits excellent catalytic performance for the reaction with a wide range of substrates under atmospheric pressure and room temperature conditions. In addition, the ionic liquid could be recovered and reused at least five times without an obvious loss of catalytic activity and selectivity. Our previous work has revealed that metal salts combined with ionic liquid (IL) represent a new hybrid catalyst that can be regarded as one of the most promising and efficient catalysts for chemical transformations of CO2 because of their remarkable synergistic catalysis mechanism [31]. For example, a Cu(I) salt/protic IL catalytic system can efficiently catalyze the cycloaddition of CO2 with propargylic alcohols to produce α-alkylidene cyclic carbonates under mild conditions. Inspired by these works, we now report that Cu(I) salt and the IL tetrabutylphosphonium imidazol ([P4444][Im]) act as a catalyst for the three component reaction of terminal propargyl alcohols, CO2 and anines (Scheme 1). The results show that the CuI/[P4444][Im] catalyst system exhibits excellent catalytic performance for the reaction with a wide range of substrates under atmospheric pressure and room temperature conditions. In addition, the ionic liquid could be recovered and reused at least five times without an obvious loss of catalytic activity and selectivity. Scheme 1. Cycloaddition reaction of propargyl alcohol and amines with CO2.

Result and Discussion
Owing to the unique catalytic performance of ILs in CO2 conversion [32][33][34], several quaternary phosphonium-based ILs and various copper (I) salts were selected to catalyze the reaction. Figure 1 shows the structures of the ILs. The synthesis of 1,3-oxazolidin-2-one 3a was carried out to systematically investigate the effect of various parameters on the reaction. The reaction was performed at 30 o C, 1 atm CO2, and 24 h, and the results are shown in Table 1 [5][6][7][8][9]. The significant differences in their catalytic activity may be ascribed to the different nucleophilicity of these ILs as a result of their different basicity (Table S1). A larger IL pKa resulted in an improved absorption capacity of CO2 in the reaction system, which led to stronger reactivity between CO2 with propargylic alcohol, amine and the catalyst, to afford a higher product yield [35]. The result indicated that the ionic liquid anions play an important role in this conversion. Next, a variety of copper (I) salts with [P4444][Im] were used for this reaction. The activity of these copper salts followed the order: CuI > CuBr > CuCl (Table 1, entries 4, 10-11). Owing to the fact that among the halogenated metal salts the iodide anion had a stronger dissociation capacity, the metal cation and

Result and Discussion
Owing to the unique catalytic performance of ILs in CO 2 conversion [32][33][34], several quaternary phosphonium-based ILs and various copper (I) salts were selected to catalyze the reaction. Figure 1 shows the structures of the ILs. Our previous work has revealed that metal salts combined with ionic liquid (IL) represent a new hybrid catalyst that can be regarded as one of the most promising and efficient catalysts for chemical transformations of CO2 because of their remarkable synergistic catalysis mechanism [31]. For example, a Cu(I) salt/protic IL catalytic system can efficiently catalyze the cycloaddition of CO2 with propargylic alcohols to produce α-alkylidene cyclic carbonates under mild conditions. Inspired by these works, we now report that Cu(I) salt and the IL tetrabutylphosphonium imidazol([P4444][Im]) act as a catalyst for the three component reaction of terminal propargyl alcohols, CO2 and anines (Scheme 1). The results show that the CuI/[P4444][Im] catalyst system exhibits excellent catalytic performance for the reaction with a wide range of substrates under atmospheric pressure and room temperature conditions. In addition, the ionic liquid could be recovered and reused at least five times without an obvious loss of catalytic activity and selectivity.

Result and Discussion
Owing to the unique catalytic performance of ILs in CO2 conversion [32][33][34], several quaternary phosphonium-based ILs and various copper (I) salts were selected to catalyze the reaction. Figure 1 shows the structures of the ILs. The synthesis of 1,3-oxazolidin-2-one 3a was carried out to systematically investigate the effect of various parameters on the reaction. The reaction was performed at 30 o C, 1 atm CO2, and 24 h, and the results are shown in Table 1 [5][6][7][8][9]. The significant differences in their catalytic activity may be ascribed to the different nucleophilicity of these ILs as a result of their different basicity (Table S1). A larger IL pKa resulted in an improved absorption capacity of CO2 in the reaction system, which led to stronger reactivity between CO2 with propargylic alcohol, amine and the catalyst, to afford a higher product yield [35]. The result indicated that the ionic liquid anions play an important role in this conversion. Next, a variety of copper (I) salts with [P4444][Im] were used for this reaction. The activity of these copper salts followed the order: CuI > CuBr > CuCl (Table 1, entries 4, 10-11). Owing to the fact that among the halogenated metal salts the iodide anion had a stronger dissociation capacity, the metal cation and  Figure 1. The structure and abbreviations of CO 2 -reactive ILs employed in this work.
The synthesis of 1,3-oxazolidin-2-one 3a was carried out to systematically investigate the effect of various parameters on the reaction. The reaction was performed at 30 o C, 1 atm CO 2 , and 24 h, and the results are shown in Table 1 [5][6][7][8][9]. The significant differences in their catalytic activity may be ascribed to the different nucleophilicity of these ILs as a result of their different basicity (Table S1). A larger IL pKa resulted in an improved absorption capacity of CO 2 in the reaction system, which led to stronger reactivity between CO 2 with propargylic alcohol, amine and the catalyst, to afford a higher product yield [35]. The result indicated that the ionic liquid anions play an important role in this conversion. Next, a variety of copper (I) salts with [P 4444 ][Im] were used for this reaction. The activity of these copper salts followed the order: CuI > CuBr > CuCl (Table 1, entries 4, 10-11). Owing to the fact that among the halogenated metal salts the iodide anion had a stronger dissociation capacity, the metal cation and iodide anion could efficiently activate the substrates and promote the reaction in high yields [36]. The result suggested that the dissociation capacity of the anions acted a significant role in determining the catalytic activity of the halogenated copper salts. In addition, Cu 2 O and CuCN could also catalyze the reaction, affording moderate yields (65% and 32%, respectively) ( Table 1, entries 12,13). Thus, CuI/[P 4444 ][Im] was the best catalyst to further investigate this reaction. Table 1. Reaction of 2-methyl-3-butyn-2-ol (1a), n-butylamine (2a) and CO 2 in various catalyst systems a . entries 12,13). Thus, CuI/[P4444][Im] was the best catalyst to further investigate this reaction.
Subsequently, the influence of catalyst amount on the yield of product was studied at 30 o C and 1 atm CO2 with a reaction time of 24 h. As seen in Figure S1, the yield of 3a was strongly dependent on the catalyst amount, and a maximum yield of 90% was obtained in the presence of 10 mol% CuI. It was shown that the yield of 3a will slowly decrease with further increase of the catalyst loading. Similar results were also obtained for the effect of IL amount in this reaction ( Figure S2). Also, the dependence of reaction time on the yield of 3a was studied and the results are shown in Figure 2. The reaction was carried out at 30 °C, 1atm CO2 using CuI (10 mol%) and [P4444][Im] (10mol%) as catalyst. It is shown that a yield of 92% could be obtained after 24h, and the yield of 3a does not improve with further prolonged reaction time. Moreover, the effect of CO2 pressure on the reaction was studied. As shown in Table S2, the yield of 3a increased significantly with increasing pressure. More CO2 could be dissolved in the liquid phase with further increasing pressure, which led to more contact between CO2 with the substrates and the catalyst, and thus a higher yield of product was obtained. From the above findings, the optimal reaction conditions for the reaction were: CuI/[P4444][Im] as the catalyst, 30 °C, 24 h, and 1 atm CO2. The reactions of CO2 with different kinds of propargylic alcohols and amines were evaluated under the optimized reaction conditions, and the results are listed in Table 2. It was shown that the coupling reaction of different alkyl substituted propargylic alcohols with CO2 and anines proceeded smoothly, producing the corresponding oxazolidinones in good yields under mild conditions. It was noted that a moderate yield of product was obtained for the propargylic alcohol with a cyclohexyl group (3i), indicating the steric hindrance of the substituent seemed to hamper the reaction. Furthermore, we also investigate the effect of amines with various substituents in this transformation. It was clearly shown that most of alkyl-substituted amines were efficient substrates to produce the corresponding oxazolidinones. However, when the R 3 group of amines was a phenyl group, no product was obtained (compound 3k), even after a prolonged reaction time of 36 h. This Subsequently, the influence of catalyst amount on the yield of product was studied at 30 o C and 1 atm CO 2 with a reaction time of 24 h. As seen in Figure S1, the yield of 3a was strongly dependent on the catalyst amount, and a maximum yield of 90% was obtained in the presence of 10 mol% CuI. It was shown that the yield of 3a will slowly decrease with further increase of the catalyst loading. Similar results were also obtained for the effect of IL amount in this reaction ( Figure S2). Also, the dependence of reaction time on the yield of 3a was studied and the results are shown in Figure 2. The reaction was carried out at 30 • C, 1atm CO 2 using CuI (10 mol%) and [P 4444 ][Im] (10mol%) as catalyst. It is shown that a yield of 92% could be obtained after 24h, and the yield of 3a does not improve with further prolonged reaction time. Moreover, the effect of CO 2 pressure on the reaction was studied. As shown in Table S2, the yield of 3a increased significantly with increasing pressure. More CO 2 could be dissolved in the liquid phase with further increasing pressure, which led to more contact between CO 2 with the substrates and the catalyst, and thus a higher yield of product was obtained. From the above findings, the optimal reaction conditions for the reaction were: CuI/[P 4444 ][Im] as the catalyst, 30 • C, 24 h, and 1 atm CO 2 .
Molecules 2019, 24, 1241 4 of 10 phenomenon can be attributed to both the weak N-nucleophilicity and steric hindrance of the substrates, which further hamper the reaction [37]. From the above analysis, the catalyst system reported here is more favorable for alkyl amines than aromatic amines.   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol The reactions of CO 2 with different kinds of propargylic alcohols and amines were evaluated under the optimized reaction conditions, and the results are listed in Table 2. It was shown that the coupling reaction of different alkyl substituted propargylic alcohols with CO 2 and anines proceeded smoothly, producing the corresponding oxazolidinones in good yields under mild conditions. It was noted that a moderate yield of product was obtained for the propargylic alcohol with a cyclohexyl group (3i), indicating the steric hindrance of the substituent seemed to hamper the reaction. Furthermore, we also investigate the effect of amines with various substituents in this transformation. It was clearly shown that most of alkyl-substituted amines were efficient substrates to produce the corresponding oxazolidinones. However, when the R 3 group of amines was a phenyl group, no product was obtained (compound 3k), even after a prolonged reaction time of 36 h. This phenomenon can be attributed to both the weak N-nucleophilicity and steric hindrance of the substrates, which further hamper the reaction [37]. From the above analysis, the catalyst system reported here is more favorable for alkyl amines than aromatic amines.
To gain insight into the reaction mechanism, the catalytic role of [P 4444 ][Im]/CuI with CO 2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P 4444 ][Im]/CuI with 1a and n-BuNH 2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P 4444 ][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol (Figure 4). These analysis suggested that the -OH and C≡C bond on the substrate 1a were synergistically activated by [P 4444 ][Im] and CuI. Moreover, several additional control experiments were performed and the results are shown in Scheme 2. It was shown that the CuI/[P 4444 ][Im] catalyst could efficiently catalyze the reaction of 1a with CO 2 to form cyclic carbonate 4a with a yield of 70%. In addition, 4a reacted with n-butylamine to afford 3a smoothly without any catalyst. Therefore, the reaction of propargylic alcohols, anines and CO 2 can be assumed to go through the cyclic carbonate pathway [37].
Based on the above results, a plausible mechanism for the reaction catalyzed by CuI/[P 4444 ][Im] as depicted in Scheme 3 is proposed. Firstly, the hydroxyl group of the propargyl alcohol is activated by IL [P 4444 ][Im], then it reacts with CO 2 to generate a zwitterionic carbamate species 1. Meanwhile, the C≡C triple bond of the propargyl alcohol was activated by CuI, and then the intermediate 2 was obtained. Then the intermediate cyclic carbonate 3 was formed by an intramolecular ring-closing reaction with the release of the catalyst. Finally, the cyclic carbonate undergoes a nucleophilic attack by a primary amine to generate the product oxazolidinone 5.    To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol   To gain insight into the reaction mechanism, the catalytic role of [P4444][Im]/CuI with CO2 and the substrates were elucidated by 1 H-NMR and 13 C-NMR spectroscopy. In the 1 H-NMR spectrum of the mixture of [P4444][Im]/CuI with 1a and n-BuNH2 (Figure 3), the OH signal became broad and shifted from 5.28 to 5.57, which indicated the formation of a hydrogen bond between [P4444][Im] and 1a [35]. In addition, the 13 C-NMR signal assigned to the C1, C2, and C3 sites could be attributed to the interaction between CuI with the C≡C bond, leading to the activation of propargylic alcohol In addition, 4a reacted with n-butylamine to afford 3a smoothly without any catalyst. Therefore, the reaction of propargylic alcohols, anines and CO2 can be assumed to go through the cyclic carbonate pathway [37].     30  35  40  45  50  55  60  65  70  75  80  85  90  95  100  105  110  115  ppm   10  15  20  25  30  35  40  45  50  55  60  65  70  75  80  85  90  95  100  105  110  115

Conclusions
In conclusion, we have developed an excellent and cost-competitive catalytic process catalyzed by the CuI/[P 4444 ][Im] system for the transformation of CO 2 to form oxazolidinones. The reaction can proceed efficiently under room temperature and atmospheric pressure conditions to give high yields of product. A wide range of propargylic alcohols and amines has been employed, which confirmed the versatility of the catalyst system for the synthesis of oxazolidinones. Preliminary mechanistic studies suggest that the reaction of propargylic alcohols, anines and CO 2 could involve the cyclic carbonate pathway. This work reveals the great potential of ionic liquids combined with metal salts as an efficient type of catalysts for CO 2 conversion under mild conditions.