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Article

Salophen-Type Schiff Bases Functionalized with Pyridinium Halide Units as Metal-Free Catalysts for Synthesis of Cyclic Carbonates from Carbon Dioxide and Terminal Epoxides

by
Aleksandra Kawka
1,
Karol Bester
2,
Agnieszka Bukowska
2 and
Wiktor Bukowski
2,*
1
Doctoral School of the Rzeszów University of Technology, Powstańców Warszawy 12, 35-959 Rzeszów, Poland
2
Faculty of Chemistry, Rzeszow University of Technology, Powstańców Warszawy 6, 35-959 Rzeszów, Poland
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(10), 658; https://doi.org/10.3390/catal14100658
Submission received: 28 August 2024 / Revised: 15 September 2024 / Accepted: 18 September 2024 / Published: 24 September 2024
(This article belongs to the Special Issue Advanced Catalysis for Energy and Environmental Applications)
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Abstract

:
Objectives: Salophen-type Schiff bases functionalized with 4-(dimethylamino)pyridinium halide units are shown to be effective single-component catalysts for the synthesis of cyclic carbonates from terminal epoxides and carbon dioxide. Methods: Using one of such trifunctional organocatalysts, epichlorohydrin could be selectively converted to the target cyclic carbonate under 2 bar of CO2 at 120 °C. Results: Over 80% conversion of E3 was then observed when organocatalyst S3 was used in the amount of 0.5 mol% (TON = 156) and even the use of 0.05 mol% S3 guaranteed almost 50% conversion of E3 to C3 (TON = 893). Conclusions: The presence of tertiary amine units in the molecules of these homogeneous organocatalysts proved to be crucial for the catalytic activity of developed organocatalysts. However, their catalytic activity was also supported by the presence of acidic phenolic units and halide ions as Lewis bases. Some closely related compounds were found to be clearly less active or inactive catalytically under the applied reaction conditions.

Graphical Abstract

1. Introduction

The carbon dioxide concentration in the atmosphere has increased significantly in recent years [1]. A large amount of this greenhouse gas is generated from anthropogenic activities, such as deforestation or industrial processes [1]. The global challenge of reducing CO2 concentration in the atmosphere requires a comprehensive strategy that incorporates techniques such as carbon capture and storage (CCS) and carbon capture and utilization (CCU) [2,3].
The CCS method involves capturing CO2 from fossil fuel power stations and similar waste streams and its transport and sequestering in suitable locations such as depleted oil fields and deep oceans or aquifers. It can next be utilized to improve methane recovery from the oil, gas, and coal beds [4,5]. Currently, CCS appears to be the most developed strategy that reduces CO2 emissions to the atmosphere.
Although CCS treats carbon dioxide as waste, the CCU methods are gaining popularity due to the possibility of treating CO2 as a valuable feedstock for the production of fuels and chemicals [6,7]. An attractive option for CO2 transformation is its conversion to fuels, including the following processes: reduction to carbon monoxide [8] and hydrogenation of carbon dioxide to methane [9] and methanol [9,10]. Alternatively, CO2 can be treated as a renewable and environmentally friendly C1 feedstock for the synthesis of valuable chemicals, encompassing urea [11], dimethyl carbonate [12] and cyclic carbonates [13,14].
The synthesis of cyclic carbonates from CO2 and epoxides is progressively attracting the interest of researchers due to its 100% atom efficiency, its sustainability and the fact that this reaction often does not require solvents and is thermodynamically favorable [15,16]. Cyclic carbonates find applications as high-boiling polar aprotic solvents and electrolytes in lithium batteries [17,18], solvents for electroanalytic purposes and polymerization, and intermediates for the preparation of polycarbonates and isocyanate-free polyurethanes [18,19,20].
The conversion of CO2 to cyclic carbonates requires the presence of efficient catalysts. Numerous homogeneous and heterogeneous catalysts have been examined in CO2 cycloaddition to epoxides in the last two decades [21,22,23,24]. Among homogeneous systems, soluble complexes of Al [25,26], Co(III) [27], Cr(III) [28] and Zn [29] with salen-type multidentate ligands and also different organocatalytic systems [23,30,31,32,33,34,35,36] deserve to be distinguished. However, to effectively catalyze the cycloaddition, both metal complex catalysts and organocatalysts require the presence of a proper cocatalyst. Tertiary amines (e.g., DBU, DMAP), ammonium (e.g., Et4NCl, Et4NBr) and phosphonium salts (e.g., PPNCl, PPNN3) can play this role when metal complexes are applied as catalysts, while compounds with Lewis/Brønsted acid functionality are required when organocatalysts with Lewis base units are used. Therefore, in addition to developing different two-component catalytic systems, efforts are also being made to develop binary ones that contain Lewis/Brønsted acid and Lewis base functions within a single molecule. Many single-component organocatalytic systems based on imidazolium [30,31] and quaternary ammonium [23,32] and phosphonium salts [33], as well as pyrazolium [34] and pyridinium salts [35], were previously synthesized and examined in CO2 cycloaddition to epoxides. As potential single-component catalysts, metal complexes with Lewis base functionalities were also the subject of catalytic studies but not so often [36,37]. Recently, novel functionalized chromium(III) complexes based on salophen-type ligands that have pyridinium chloride units in their molecules were developed in our laboratory [38]. In an examination of the catalytic activity of these complexes in the cycloaddition of CO2 to phenyl glycidyl, some organocatalytic properties were also observed for the ligands used for their synthesis. Therefore, it seemed to be interesting to examine these features in more detail. Organocatalysts, although generally less active than metal complex catalysts, are considered to be much more ‘greener’ catalysts than metal complexes. The results of these investigations are presented in this article.

2. Results and Discussion

To examine the catalytic properties of S1–S7 (Scheme 1; Schiff bases with units of pyridinium halide) in the reaction of CO2 cycloaddition to epoxides, epichlorohydrin (E3) was selected as a model substrate. The effect of the organocatalyst structure and reaction temperature was first studied. A blank experiment without a catalyst was also performed under the same reaction conditions. No reaction between CO2 and epichlorohydrin was observed. Catalytic experiments were conducted using the following reaction conditions: 0.5 mol% S1–S8, 2 bar of CO2 and 80, 100, 120 or 140 °C. The conversion of E3 was evaluated after 2 h based on the content of this compound in post-reaction mixtures determined using 1H-NMR analysis (see Supplementary Materials). The obtained results are presented in Table 1.
On the basis of 1H-NMR spectra recorded for the particular post-reaction mixtures, it was concluded that all examined compounds are able to convert E3 selectively to C3 under the applied reaction conditions. However, the value of E3 conversion changed depending on the reaction temperature and structure of the used organocatalysts. As expected, the conversion of E3 increased with increasing reaction temperature, but these general trends were observed only in the temperature range of 80–120 °C. A further increase in reaction temperature to 140 °C did not always result in an improvement in E3 conversion. In some cases, a decrease in E3 conversion was noted. It is likely the result of a decrease in CO2 solubility in the reaction mixture with an increase in reaction temperature. A decrease in CO2 concentration in a liquid phase appears to have a much more negative influence on reaction rate than an advantageous effect of increasing reaction temperature on reaction kinetics. As a result, decreases in E3 conversion were observed for the most studied organocatalysts (Table 1).
It has been noted previously that above a certain critical value of reaction temperature, side reactions and/or organocatalyst degradation can occur. These factors can influence the selectivity of cyclic carbonate formation. For example, Han et al., who investigated the impact of reaction conditions on propylene carbonate formation, concluded that raising the temperature from 140 to 150 °C resulted in a reduction in reaction yield from 98% to 78% [39]. Wei-Li and Sheng-Lian et al. found that the critical temperature value for this reaction seems to be 120 °C. Above this temperature, the selectivity of propylene carbonate formation decreased significantly in favor of side reactions such as propylene oxide isomerization to acetone and its hydrolysis to diol [40]. In our case, the formation of side products in the reaction of CO2 cycloaddition to E3 in the presence of S1–S5 was not concluded based on 1H-NMR spectra of post-reaction mixtures even when the temperature of 140 °C was applied. However, the process of organocatalyst degradation could not be excluded.
To investigate the thermal stability of organocatalysts S1–S5, additional thermogravimetric analyses were conducted in the temperature range of 30 to 600 °C. Figure 1 presents the results that were obtained. For catalysts S1 and S3–S5, it was found that degradation begins significantly above the range of temperature used in the catalytic experiments. However, for catalyst S2, which incorporates two 2,6-dimethylpyridinium chloride units into its structure, the process of degradation starts around 140 °C, which consequently leads to a decrease in epoxide conversion observed for the experiment conducted at this temperature. Other symmetrically substituted compounds, S1 and S3, containing pyridinium chloride and 4-(dimethylamino)pyridinium chloride units, were found to be more stable. The degradation of S1 and S3 starts around 180 °C and 250 °C, respectively, with the maximum decomposition rates occurring at approximately 205 °C for S1 and around 315 °C for S3. In the case of asymmetrically substituted organocatalysts S4 and S5, which have a single 4-(dimethylamino)pyridinium chloride unit in their structure, the first of them begins to degrade at around 230 °C, and the second, S5, at around 280 °C, with maximum decomposition rates occurring at approximately 290 °C for S4 and around 355 °C for S5. Given the findings described above, it was concluded that the thermal stability of compounds S1–S5 appears to correlate only partly with their organocatalytic activity. The number of 4-(dimethylamino)pyridinium) chloride units probably plays a decisive effect on the final catalytic ability of developed polyfunctional organocatalysts. S3 contains two such groups, while S4 and S5 possess only one.
When the catalytic activity of derivatives with pyridinium chloride units (S1–S5) was compared in the examined range of temperature, the highest activity was found for S3, and the lowest for S2. At a temperature of 120 °C, which was selected as a reference temperature, the conversions of E3 for these organocatalysts were 83 and 47% with TOF values of 78 and 45 h−1, respectively (Table 1, Entries 13 and 12). Both organocatalysts incorporate two pyridinium salt units in their molecules. The first was derived from 4-(dimethylamine)pyridine (DMAP), and the second from 2,6-lutidine. On the basis of the results obtained, it could be concluded that the presence of two methyl groups in the neighborhood of pyridinium salt centers clearly negatively influences the activity of S2 as an organocatalyst. Furthermore, when the result obtained for S3 was compared to that obtained for S1 (Table 1, Entry 11), it could be concluded that the presence of additional nucleophilic centers, in the form of dimethylamine substituents at position 4 in the pyridinium salt unit, positively influences the catalytic activity of an organocatalyst. Compound S1, which contains two unsubstituted pyridinium chloride units in its structure, shows clearly lower activity (62% conversion of E3 with a TOF value of 56 h−1) than compound S3 with 4-(dimethylamine)pyridine units. Furthermore, by comparing the activity of S3 with the activity of S4 and S5, it was concluded that the number of 4-(dimethylamine)pyridine units also reflected the organocatalytic activity of salophens functionalized with onium salt moieties. The values of E3 conversion obtained for S4 and S5 were approximately 6% and 11% lower than the value obtained for S3 (TOF values of 71 and 68 h−1, respectively). Furthermore, the presence of the second t-butyl group at position 3 in one of the salicylaldehyde units appears to positively influence the organocatalytic activity of asymmetrically substituted salophen S4.
A difference in activity observed for compounds S1–S5 appears to clearly indicate a crucial role of tertiary amine units in the activation of substrates in CO2 cycloaddition to E3. Similar findings have previously been discussed when other single-component organocatalytic systems composed of tertiary amines, functionalized salen and salophen ligands, and also simple phenols with a tertiary aromatic amine unit built into their molecules were examined [41,42]. Two possible pathways of substrate activation by amine units include the interaction of an amine with CO2 and the ring-opening of a previously activated epoxide by amine. Different aliphatic monoamines and diamines have also previously been found as single-component active organocatalysts in CO2 cycloaddition to epoxides [15].
To learn how DMAP alone can act as an organocatalyst in the subject cycloaddition reaction, an additional experiment was carried out at 120 °C under 2 bar of CO2, using 0.5 mol% of this amine. A result comparable to that found for the least active organocatalyst studied (S2) was obtained (Table 1, Entries 24 and 25). The experiment proved an important role of tertiary amine groups in the activation of substrates in the reaction of CO2 cycloaddition.
It is well established that when onium salts are used as cocatalysts or main catalysts in CO2 cycloaddition to epoxides, the role of halide ions is to open an epoxide ring by forming halogen-substituted alkoxide intermediates [43,44]. The formed alkoxides then react with carbon dioxide to form a carbonate, and finally, a halide acts as a leaving group during the formation of a five-membered ring of carbonate as a consequence of intermolecular substitution reactions. However, the relative activity of a halide ion can vary from one catalyst system to another because of mutual overlapping effects of many factors, for instance, the nature of an acidic accomplice, if used, reaction conditions, halide ion nucleophilicity and the reactivity of this halogen as a leaving group, epoxide reactivity, and also the stability of tetraalkylammonium salts under the used reaction conditions. As a result, different orders of activity were observed when onium salts were used as a component of catalytic systems. For example, when quaternary ammonium salts were used as a single-component catalyst for CO2 cycloaddition to ethylene oxide and propylene oxide, the following order of catalytic activity was found: Cl− > Br− > I− [45]. However, when chitosan-derived quaternary ammonium salts were used as a single-component catalyst for the synthesis of propylene carbonate (PC) from propylene oxide (PO) and carbon dioxide, the order of activity was as follows: I− > Br− > Cl− [46]. Yet another order of activity, Br− > I− > Cl−, was found by North et al. [47] when they used tetrabutylammonium halides as a cocatalyst in the reaction of CO2 cycloaddition to styrene oxide in the presence of bimetallic aluminum salen.
To learn how a bifunctional pyridinium chloride with a tertiary amine unit built into its molecule, for example, one derived from DMAP and benzyl chloride, can influence the conversion of E3 in CO2 cycloaddition, another catalytic experiment was carried out using 0.5 mol% of [BzDMAP]Cl. In this case, a slightly higher conversion of E3 was obtained compared to that found for DMAP (Table 1, Entry 24 vs. 25). The experiment seems to show that a quaternary ammonium chloride plays a meaningful role in the activation of substrates in the reaction of CO2 cycloaddition in addition to a tertiary amine unit present in this pyridinium salt. The use of trifunctional organocatalysts, such as compounds S3–S5, makes the occurrence of additional hydrogen bond interactions possible between an epoxide ring with phenolic hydroxyl groups within salophen units, which support the organocatalytic activity of tertiary amine and pyridinium chlorite units. The possibility of hydrogen bond activation by phenolic groups of salophen molecules was previously described by the North group [41] and proved by a DFT study by Guo et al. [48]. Lastly, Hurtado et al. presented additional proof for hydrogen bond activation by applying 1H-NMR spectroscopy and computational studies [24]. However, in the case of unfunctionalized salophen-type organocatalysts, the most kinetically preferred pathway is based on a concerted and synergistic mechanism, where a phenolic group acts as a hydrogen donor and a phenolate fulfills the role of a nucleophile that supports epoxide ring-opening and CO2 addition.
To explore the effect of halide ions on the catalytic activity of salophens with pyridinium salt units in the reaction of CO2 cycloaddition to E3, organocatalysts S6 and S7 with 4-(dimethylamine)pyridine bromide and 4-(dimethylamine)pyridine iodide units in their molecules were also synthesized from the salicylaldehydes that were derived from 3-bromo-5-t-butylsalicylaldehyde and 3-iodo-5-t-butylsalicylaldehyde, respectively, and DMAP (Scheme S1, Supplementary Materials). When the results obtained for S6 and S7 (Table 1, Entries 16 and 17) were compared with those obtained for S3, it was concluded that an exchange of Cl− ions on Br− ions positively, but only slightly, influenced the activity of this type of polyfunctional organocatalysts, while the presence of I− ions rather resulted in decreasing their catalytic activity. The values of E3 conversion observed in the presence of S6 and S7 were 86 and 77%, respectively.
Unfunctionalized salophen, which was reported by the North group as an active organocatalyst for the subject reaction under the conditions of 1 mol% catalyst, 120 °C, 10 bar CO2 and 3.5 h [41], was also examined as an organocatalyst under the reaction conditions applied for compounds S1–S7. No catalytic activity for this compound was observed when its concentration was reduced to 0.5 mol% and the pressure of CO2 to 2 bar.
To examine the effect of carbon dioxide pressure and reaction time on CO2 cycloaddition to E3, first, a series of catalytic experiments were performed at 100 °C using 4 bar of CO2 and including all five organocatalysts with pyridinium chloride units. The values of E3 conversion to C3 were found to be comparable to that obtained at 120 °C under 2 bar of CO2 except for the reaction catalyzed by compound S2 (Table 2, Entries 1–5 vs. Table 1, Entries 11–15), for which the conversion of E3 turned out to be clearly lower compared to that obtained at 120 °C under 2 bar of CO2. The low thermal stability of S2 is probably responsible for this finding.
Furthermore, when the values of E3 conversion obtained for S1–S5 at 100 °C under 4 bar of CO2 were compared, a certain difference in the order of organocatalytic activity was revealed relative to the order found for experiments carried out at 120 °C under 2 bar of CO2: S2 < S1 < S4 < S5 < S3 vs. S2 < S1 < S5 < S4 < S3, respectively. When the pressure of CO2 was increased to 4 bar in the experiment that was carried out for organocatalyst S3 at 120 °C, the conversion of E3 increased in relation to that obtained under 2 bar only by 2% (to 85%) and was only 3% higher than that observed for this catalyst at 100 °C under 4 bar CO2 (Table 2, Entries 7 and 3). A further increase in CO2 pressure to 6 bar at 120 °C also did not result in a significant increase in E3 conversion, which amounted to 87% (Table 2, Entry 10). A much more significant improvement in the conversion of E3 was found when the reaction time was extended to 4 and next to 6 h at 120 °C under 2 bar of CO2. The conversion of E3 then increased above 90% and amounted to 91 and 95%, respectively.
The effect of the concentration of organocatalyst S3 on the conversion of E3 to C3 at 120 °C under a pressure of 2 bar of CO2 was also examined. The concentration of S3 was changed in the range of 0.05 to 0.5 mol%. The use of higher concentrations was limited by the solubility of S3 in E3. The results obtained for this series of experiments are presented in Table 3. As expected, the conversion of E3 decreased with a decrease in the concentration of S3. However, even when the organocatalyst concentration was significantly decreased, it was possible to selectively convert E3 to the target cyclic carbonate (C3). The reduction in catalyst concentration five times resulted in a decrease in E3 conversion to 56%, and the reduction in catalyst concentration ten times resulted in a decrease to 48%. The relatively high TOF values obtained for experiments with 0.1 and 0.05 mol% S3, which were 257 and 447, respectively, demonstrate the high catalytic activity of this organocatalyst in CO2 cycloaddition to E3 under applied reaction conditions.
When comparing the activity of organocatalyst S3 with the activity of other homogeneous organocatalytic systems examined previously in the cycloaddition of CO2 to epichlorohydrin, it was difficult to find examples of experiments performed under similar reaction conditions. In general, to achieve high yields of cyclic carbonate C3, the organocatalyst concentration and reaction time were increased when a lower CO2 pressure or lower reaction temperature was applied. On the other hand, when the organocatalyst concentration was reduced, the reaction temperature and CO2 pressure were increased. Focusing on salophen-type organocatalysts (Table 4, Entries 7 and 8), it is worth noting that the organocatalyst used by North et al. [41] provided a similar yield of cyclic carbonate to that provided by S3 (76 vs. 78%) after 3.5 h when it was used in 1 mol% at 120 °C under 10 bar CO2 pressure. In our work, a somewhat higher C3 yield was achieved using a twice lower concentration of S3 (0.5 mol%), clearly lower pressure (6 bar) and shorter reaction time (2 h). The organocatalyst proposed by Hurtado et al. (Table 4, Entry 8) required 9 h reaction time under 8 bar CO2 pressure at 100 °C to obtain a high conversion of E3 (92%; the yield of C3 was not presented in Ref. [24]).
Finally, a series of eight epoxides was applied to explore the scope of use of S3 as a catalyst in the reaction of CO2 cycloaddition. The following reaction conditions were used: 0.5 mol% organocatalyst, 120 °C, 6 bar CO2 and 2 h. Under these reaction conditions, compound S3 was found to show organocatalytic activity in relation to all terminal epoxides used which were selectively converted to the target cyclic carbonates (Table 5, Entries 1–6). No side products were found based on the 1H-NMR spectra of post-reaction mixtures. Two internal epoxides, cyclohexene oxide (E7) and limonene oxide (E8), were found to be completely unreactive under the conditions investigated (Table 4, Entries 7 and 8). High values of cyclic carbonate yield were obtained for E3 and phenyl glycidyl ether (E4)—79% and 89%, respectively. The other terminal epoxides examined showed moderate reactivity in the presence of S3. The cyclic carbonate yields of 29% for 1,2-epoxybutane (E2) and 52% for styrene oxide were obtained in two-hour experiments. Due to the low solubility of compound S3 in propylene oxide (E1) and in E2, in the case of these two epoxides, 150 μL of 2-Me-THF was added to a reaction mixture. This solvent was chosen based on research by North et al. [41]. Greater discrepancies between the values of epoxide conversion and the values of cyclic carbonate yields found for E1 and E2 can result from low boiling points of these epoxides which facilitate their loss during the processing of the post-reaction mixtures.

3. Materials and Methods

Phenols, epoxides, and other reagents and solvents used in this work were purchased from Sigma-Aldrich (Sigma Aldrich Chemie GmbH, München, Germany), Merck (Darmstadt, Germany), Acros (Thermo Fisher Scientific, Waltham, MA, USA), and Alfa Aesar (Thermo Fisher Scientific, Waltham, MA, USA) and applied as received, unless otherwise stated. Salicylaldehydes used as intermediates in the synthesis of salophen-type Schiff bases that were examined as organocatalysts in this work and functionalized salophen derivatives S1–S5 were prepared using procedures described previously [38]. Experimental details on the preparation and analyses of compounds S6–S7 and salicylaldehydes with pyridinium bromide or iodide units used for their syntheses are presented in the Supplementary Materials.

Catalytic Examination—General Procedure

A laboratory glass pressure reactor (Büchi Tinyclave, Uster, Switzerland, 10 mL) was applied to perform a catalytic examination. The reactor, equipped with a magnetic bar, was charged with an internal standard (biphenyl, 5 mol%), a catalyst (0.05–0.5 mol%) and an epoxide (0.6 mL), sealed, placed in an oil bath preheated to the required temperature (80–140 °C), and finally pressurized to the required carbon dioxide pressure (2–6 bar CO2). The reaction mixture was stirred for 2–6 h under the reaction conditions applied and a constant carbon dioxide supply. After the indicated time, the CO2 supply was cut off, and the oil bath was removed. The reactor was gently cooled to room temperature in a water bath and then in an ice bath. After that, the pressure was released by opening the pressure vessel, the reactor walls were rinsed with 5 mL of acetone and then the 1H-NMR analyses of post-reaction mixtures were performed to assess the epoxide conversion. The procedure of cyclic carbonate isolation from the obtained post-reaction mixtures is presented in Section S2.5 in the Supplementary Materials. The spectrometric data for isolated cyclic carbonates were consistent with those previously reported [38].

4. Conclusions

Salophen-type derivatives S1–S7 were tested as selective organocatalysts for the synthesis of cyclic carbonates in the reactions of CO2 cycloaddition to epoxides under metal- and solvent-free conditions and relatively mild reaction conditions. Using epichlorohydrin as a model epoxy compound, it was established that organocatalysts S3–S7, which in addition to one or two pyridinium halide units also have one or two tertiary amine units built into their molecules, have clearly higher catalytic activity than compounds S1 and S2 having only pyridinium salt units as Lewis base functional groups. Compounds S2 and S3, which are symmetrically substituted salophen derivatives, were found to be the least active and the most active, respectively, among organocatalysts that have units of pyridinium chlorides (TOF values of 64 and 40 h−1 for 2 h experiment at 120 °C under 2 bar CO2 pressure, respectively). As was established on the basis of the results of thermogravimetric analysis, the former starts to degrade above 120 °C. There was no significant improvement in catalytic activity when chloride ions (S3) were exchanged with bromide ones (S6), and organocatalyst S7, being a derivative of pyridinium iodide, was found to be less active than S3. Compound S3, as the most active of the polyfunctional organocatalysts developed, could also be used effectively in the cycloaddition reactions including six terminal epoxides that have EWG or EDG substituents. Moreover, when used in CO2 cycloaddition to epichlorohydrin, this catalyst demonstrated high catalytic activity, even when used at a concentration of 0.05 mol% at 120 °C under 2 bar of CO2 (with a TOF value of 436 h−1 after 2 h). Unfortunately, S3 was shown to be inactive in the cycloaddition of CO2 to internal epoxides, such as cyclohexene oxide and limonene oxide.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14100658/s1, Scheme S1: Synthesis of salicylaldehydes with pyridinium bromide and iodide units. Scheme S2: Synthesis of salophen Schiff bases S6 and S7. Figure S1: 1H-NMR spectrum (CDCl3, 500 MHz) of A4. Figure S2: 13C-NMR spectrum (CDCl3, 125 MHz) of A4. Figure S3: 1H-NMR spectrum (CDCl3, 500 MHz) of A5. Figure S4: 13C-NMR spectrum (CDCl3, 125 MHz) of A5. Figure S5: 1H-NMR spectrum (CDCl3, 500 MHz) of A9. Figure S6: 13C-NMR spectrum (CDCl3, 125 MHz) of A9. Figure S7: 1H-NMR spectrum (CDCl3, 500 MHz) of A10. Figure S8: 13C-NMR spectrum (CDCl3, 125 MHz) of A10. Figure S9: 1H-NMR spectrum (dmso-d6, 500 MHz) of S6. Figure S10: 13C-NMR spectrum (dmso-d6, 125 MHz) of S6. Figure S11: 1H-NMR spectrum (dmso-d6, 500 MHz) of S7. Figure S12: 13C-NMR spectrum (dmso-d6, 125 MHz) of S7. References [38,55,56,57,58] are cited in the Supplementary Materials.

Author Contributions

Methodology, investigation, writing—original draft preparation, A.K.; conceptualization, methodology, investigation, writing—original draft preparation, K.B.; methodology, investigation, visualization, writing—review and editing, A.B.; conceptualization, methodology, formal analysis, writing—original draft, writing—review and editing, supervision, W.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Minister of Science and Higher Education Republic of Poland within the program “Regional Excellence Initiative”, funding number: RID/SP/0032/2024/01.

Data Availability Statement

All data are given in the paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00658 sch001
Scheme 1. Functionalized salophen-type Schiff bases examined as organocatalysts.
Scheme 1. Functionalized salophen-type Schiff bases examined as organocatalysts.
Catalysts 14 00658 sch001
Catalysts 14 00658 g001
Figure 1. Results of thermal analysis.
Figure 1. Results of thermal analysis.
Catalysts 14 00658 g001
Table 1. Effect of reaction temperature and catalyst structure on CO2 cycloaddition to epichlorohydrin.
Table 1. Effect of reaction temperature and catalyst structure on CO2 cycloaddition to epichlorohydrin.
Catalysts 14 00658 i001
No.Cat.Temp.
[°C]		Conversion a
[%]		TON
[-]		TOF
[h−1]
1S180203618
2S2484
3S35210050
4S4367236
5S5377236
6S1100448442
7S2152814
8S37314070
9S46812864
10S56913266
11S11206211256
12S2479045
13S38315678
14S47714271
15S57213668
16S68615879
17S77713869
18S8000
19S1140599648
20S2487839
21S37813668
22S46711859
23S57012261
24DMAP120499246
25[BnDMAP]Cl529849
Reaction time—2 h; CO2 pressure—2 bar. a Determined by 1H-NMR analysis using biphenyl as an internal standard.
Table 2. Effect of CO2 pressure and reaction time on the conversion of E3 to C3.
Table 2. Effect of CO2 pressure and reaction time on the conversion of E3 to C3.
No.Cat.Time
[h]		pCO2
[bar]		Temp.
[°C]		Conversion a
[%]		TON
[-]		TOF
[h−1]
1S1241005610653
2S224152814
3S3248215678
4S4246813266
5S5247414472
6S3221208315678
7S3429116241
8S3629516628
9S3248515678
10S3268715879
a Determined by 1H-NMR using biphenyl as an internal standard.
Table 3. Effect of concentration of catalyst S3 on CO2 cycloaddition to epichlorohydrin.
Table 3. Effect of concentration of catalyst S3 on CO2 cycloaddition to epichlorohydrin.
No.Loading S3,
[mol%]		Conversion E3 a
[%]		TON
[-]		TOF
[h−1]
10.0548893447
20.156513257
30.266300150
40.58315678
The reactions were carried out for 2 h under 2 bar, at 120 °C. a Determined by 1H-NMR using biphenyl as an internal standard.
Table 4. Comparison of the activity of different organocatalytic systems in the reaction of CO2 cycloaddition to epichlorohydrin.
Table 4. Comparison of the activity of different organocatalytic systems in the reaction of CO2 cycloaddition to epichlorohydrin.
No.CatalystTemperature
[°C]		Pressure
[bar]		Time
[h]		Conversion
[%]		Yield
[%]		TONTOF
[h−1]		Ref.
1Catalysts 14 00658 i002
0.1 mol%		1201010979796797[49]
2Catalysts 14 00658 i003
8 mol% + 8 mol% nBu4NI		25120-97--[50]
3Catalysts 14 00658 i004
5 mol%		451216-95--[51]
4Catalysts 14 00658 i005
75 mol%		3016-94--[52]
5Catalysts 14 00658 i006
1 mol%		1201496---[53]
6Catalysts 14 00658 i007
0.2 mol%		1101049998.5495.2123.8[54]
7Catalysts 14 00658 i008
1 mol%		120103.58476--[41]
8Catalysts 14 00658 i009
0.5 mol%		1008992-18420.4[24]
9S31004282-15678This work
10S312062877815879This work
Table 5. Effect of epoxide structure.
Table 5. Effect of epoxide structure.
Catalysts 14 00658 i010
No.EpoxideEpoxide Ab’br.Conversion a
[%]		Cyclic CarbonateCyclic Carbonate Abbr.Yield b
[%]		TONTOF
[h−1]
1 cCatalysts 14 00658 i011E153Catalysts 14 00658 i012C1265628
2 cCatalysts 14 00658 i013E238Catalysts 14 00658 i014C2265829
3Catalysts 14 00658 i015E387Catalysts 14 00658 i016C37815879
4Catalysts 14 00658 i017E492Catalysts 14 00658 i018C48717889
5Catalysts 14 00658 i019E559Catalysts 14 00658 i020C55010754
6Catalysts 14 00658 i021E652Catalysts 14 00658 i022C6479648
7Catalysts 14 00658 i023E70Catalysts 14 00658 i024C7000
8Catalysts 14 00658 i025E80Catalysts 14 00658 i026C8000
The reactions were carried out for 2 h at 120 °C using catalyst S3 at CO2 pressure of 6 bar. a Values determined by a 1H-NMR method using biphenyl as an internal standard. b The yield of isolated cyclic carbonates. c Addition of 150 μL 2-MeTHF as a solvent.
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Kawka, A.; Bester, K.; Bukowska, A.; Bukowski, W. Salophen-Type Schiff Bases Functionalized with Pyridinium Halide Units as Metal-Free Catalysts for Synthesis of Cyclic Carbonates from Carbon Dioxide and Terminal Epoxides. Catalysts 2024, 14, 658. https://doi.org/10.3390/catal14100658

AMA Style

Kawka A, Bester K, Bukowska A, Bukowski W. Salophen-Type Schiff Bases Functionalized with Pyridinium Halide Units as Metal-Free Catalysts for Synthesis of Cyclic Carbonates from Carbon Dioxide and Terminal Epoxides. Catalysts. 2024; 14(10):658. https://doi.org/10.3390/catal14100658

Chicago/Turabian Style

Kawka, Aleksandra, Karol Bester, Agnieszka Bukowska, and Wiktor Bukowski. 2024. "Salophen-Type Schiff Bases Functionalized with Pyridinium Halide Units as Metal-Free Catalysts for Synthesis of Cyclic Carbonates from Carbon Dioxide and Terminal Epoxides" Catalysts 14, no. 10: 658. https://doi.org/10.3390/catal14100658

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