A Mechanistic Study on the Addition of CO2 to Epoxides

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A Mechanistic Study on the Addition of CO to Epoxides Catalyzed by Ammonium- and Phosphonium Salts: A Combined Spectroscopic and Kinetic Approach Johannes Steinbauer, Christoph Kubis, Ralf Ludwig, and Thomas Werner ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02093 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Sustainable Chemistry & Engineering

A Mechanistic Study on the Addition of CO2 to Epoxides Catalyzed by Ammonium- and Phosphonium Salts: A Combined Spectroscopic and Kinetic Approach †§

†§

‡

Johannes Steinbauer , Christoph Kubis , Ralf Ludwig* , Thomas Werner*

†

†

Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Straße 29a,

18059 Rostock, Germany. ‡

Institute for Chemistry, Department for Physical Chemistry, University of Rostock, Dr.-

Lorenz-Weg 2, 18059 Rostock, Germany. [email protected] [email protected]

ABSTRACT: Herein we report the evaluation of the frequently employed tetrabutyl ammonium and phosphonium halides as well as their bifunctional analogues as catalysts in cyclic carbonate synthesis under benchmarked conditions. The kinetic data of all catalysts were evaluated and the rate constants determined. Moreover, a systematic infrared spectroscopic study of the interactions between cation and anion of the catalysts as well as the interactions between the catalysts and the substrate were conducted. These experimental 1 ACS Paragon Plus Environment

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results were additionally supported by DFT calculations. The observed trends in the interaction between the onium cation and the anion are correlated to their catalytic activity. Moreover, these investigations revealed the mode of the substrate activation for the monofunctional and the bifunctional catalysts. Furthermore, the kinetic studies and in situ infrared experiments revealed a product inhibition of the bifunctional catalysts via the unexpected formation of catalyst-carbonate adducts. The interaction between the catalysts and the product was further studied by infrared spectroscopy. Finally, the rate and the equilibrium constants for the binfunctional catalysts were determined by a Michaelis-Menten model considering a reversible product inhibition.

KEYWORDS: carbon dioxide fixation, cyclic carbonates, homogeneous catalysis, infrared spectroscopy, kinetics

INTRODICTION The anthropogenic carbon dioxide concentration in the atmosphere has risen dramatically over the past two decades and exceeded 400 ppm in 2015 contributing largely to the climate change.1 However, CO2 is also considered as an abundant, non-toxic, non-flammable and inexpensive C1 building block.2-5 Thus, the utilization of CO2 for the synthesis of various valuable organic compounds has recently gained great attention.6-10 In this context the atom economic addition of CO2 to epoxides 1 to form cyclic carbonates 2 is an attractive and one of the most widely studied reactions.11-19 These compounds find various applications e.g. as synthetic building blocks,20-24 monomers in polymer synthesis,25-27 electrolytes in batteries28-30 and solvents31-32 while some of them have been commercialized even on an industrial scale.

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Even though the addition of CO2 to epoxides to yield the respective cyclic carbonates is an exothermic reaction the high activation barrier (e.g. for the conversion of propylene oxide with CO2) may account for the observed difficulty of the addition in the absence of a catalyst.33-35 Thus, numerous catalysts and catalytic systems have been developed to facilitate this reaction.11-19 Several homogeneous metal based catalysts which operate at room temperature and atmospheric pressure have been reported.36-42 These catalysts are commonly used in combination with a nucleophilic co-catalyst. Usually simple onium salts such as tetrabutyl ammonium halides are frequently used as nucleophilic organo(co-)catalysts in these binary systems. Interestingly, for those systems often no catalytic activity is observed in the absence of these (co-)catalysts while the onium salts alone are capable to facilitate the reaction. In turn the activity of simple onium salts can significantly be enhanced by adding a suitable co-catalyst. In this respect hydrogen bond donors (HBDs) such as phenolic derivatives,43 (amino)alcohols,44-46 silanol47 fluorinated alcohols,48-50 carboxylic51 and boronic acids,52 ionic liquids,53 phosphorous-modified bulk graphitic carbon nitride54 as well as biobased molecules such as chitosan,55-56 tannic acid,57 ascorbic acid58 and even water59 have been reported as suitable co-catalysts for the synthesis of cyclic carbonates. Moreover, highly efficient bifunctional catalysts where the HBD-function is tethered e.g. to an imidazolium,60-63 pyridinium,64 ammonium64-70 or phosphonium71-77 salt have been reported. The generally accepted route for the addition of CO2 to epoxides in the presence of HBDbased catalytic systems, which is supported by several theoretical studies,78-86 follows a threestep mechanism, i.e. ring-opening, CO2 insertion, and ring-closure (Scheme 1). The HBD interacts with the epoxide via hydrogen bonding, which (1) favors the activation of epoxide through polarization of the C–O–C bonds, facilitating the ring-opening reaction by the nucleophile and (2) stabilizes the intermediates and transition states. In general the understanding of the mechanism, the mode of activation as well as the reasons associated with 3 ACS Paragon Plus Environment


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low activity and catalyst deactivation is of crucial importance for the rational design of improved catalysts. Scheme 1. Generally proposed three-step mechanism for the cyclo addition of CO2 and epoxides catalyzed by a HBD catalyst and a nucleophilic (co-)catalyst.

Moreover, Kleij and co-workers recently demonstrated the vital importance of benchmarking to provide insights into the relative reactivity of binary catalytic systems.87 Thus, the understanding of the interaction between (bifunctional) onium catalysts and the substrate (and product), the influence of the anions as well as the reaction kinetics are of great interest. Even though there have been a few 1H NMR spectroscopic studies45, 47, 52, 88 on the interaction of HBDs with epoxides in binary catalytic systems a systematic infrared spectroscopic study on bifunctional onium salts has not been reported so far. Although, IR spectroscopy is an excellent tool to study the interaction between a hydrogen bond donor and acceptor and in general for in situ investigations regarding mechanistic and kinetic investigations.89-93 Herein, we report a benchmark study on the synthesis of cyclic carbonates in the presence of different bifunctional ammonium and phosphonium salt catalysts in comparison with their 4 ACS Paragon Plus Environment

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unfunctionalized analogs. The reaction kinetics, the relation of catalyst structure on the catalytic activity as well as the interaction of the catalysts with the substrate and product monitored by IR spectroscopy is reported. RESULTS AND DISCUSSION Even though there have been some studies on the catalytic performance of the monofunctional salts,59,

94-96

our initial objective was to compare the activity of commonly used tetrabutyl

ammonium 3 and phosphonium salts 4 (monofunctional catalysts) with their bifunctional analogs 5 and 6 (Scheme 2). Based on our previous work, we have chosen the conversion of 1,2-epoxybutane (1a) with CO2 under defined solvent-free conditions as the model reaction.9798

The reaction kinetics were studied under standard conditions (2 mol% catalyst, 90 °C, 6 h,

p(CO2)= 1.0 MPa) in the presence of ammonium 3 and phosphonium salts 4 as well as the respective bifunctional catalysts 5 and 6. Scheme 2. The conversion of 1,2-epoxybutane (1a) with CO2 under solvent-free conditions as model reaction for the IR spectroscopic investigations and benchmarking.

The results for the conversion of 1a using the monofunctional ammonium salt catalysts 3 as well as the monofunctional phosphonium salts 4 are depicted in Figure 1. The observed activity for the different ammonium salts 3 strongly depended on the nature of the anion and decreased in the order Cl– > Br– > I– (Figure 1a). Hence, under the standard reaction conditions [nBu4N]Cl (3a) proved to be the most active monofunctional ammonium salt. 5 ACS Paragon Plus Environment


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Figure 1. a) Impact of the anions (Cl–, Br– and I–) on the yield of 2a in the conversion of epoxide 1a with CO2 in the presence of tetra-n-butyl ammonium salts 3 and b) tetra-n-butyl phosphonium salts 4. The observed selectivities were >99%. Reaction conditions: 1,2epoxybutane (1a, 460 mmol), 2 mol% 3 or 4, p(CO2)= 1.0 MPa, 90 °C, 6 h. Even though no solvent was added the substrate 1a can be considered as an aprotic polar solvent and the order of activity correlates with the nucleophilicity of these anions in this kind of solvents, decreasing from Cl– > Br– > I–. This order of activity was also reported by others94-95 and furthermore observed in our previous work.65 For the conversion of epoxide 1a with CO2 under constant pressure a 1st order kinetics was assumed. With this assumption and the obtained kinetic data from Figure 1a we were able to determine the rate constants

for

the reaction in the presence of catalysts 3 (Table 1, entries 1–3, see SI-6).99 The experimental data from the kinetic evaluation fit to the proposed 1st order kinetics and the calculated rate constants for the monofunctional ammonium salt catalysts decreased in the order 0.178 h– 1

(3a) > 0.051 h–1 (3b) > 0.028 h–1 (3c).


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Table 1. Determined rate constants for the conversion of 1a using the onium salt catalysts 3 and 4 assuming a 1st order kinetics.

Entry

Catalyst

1 2 3 4 5 6

[nBu4N]Cl (3a) [nBu4N]Br (3b) [nBu4N]I (3c) [nBu4P]Cl (4a) [nBu4P]Br (4b) [nBu4P]I (4c)

/

(1st order kinetics)

0.178 0.051 0.028 0.291 0.063 0.066

After the evaluation of the monofunctional ammonium salt catalysts 3, we evaluated the results for the conversion of 1a using the monofunctional phosphonium salt catalysts 4 (Figure 1b). Interestingly, the monofunctional phosphonium salt catalysts 4 showed a higher activity compared to corresponding ammonium salts 3 (Figure 1a vs. 1b). The order of activity in dependence of the anion for the monofunctional phosphonium salt catalysts 4 was Cl– > I– ≈ Br– and differed to the order of the corresponding ammonium salts 3. As for catalysts 3 the kinetic data of the catalysts 4 fit to a 1st order kinetics (see SI-6).99 The calculated rate constants for the conversion of 1a by using the catalysts 4 decrease from 0.291 h–1 (4a) > 0.066 h–1 (4c) ≈ 0.063 h–1 (4b) (Table 1, entries 4–6). Subsequently, infrared spectroscopic experiments were performed to get insights into the interactions between the cations and the anions within the catalysts. Therefore, we dissolved 2 mol% of the catalysts 3 and 4 in chloroform (2 mol% in respect to chloroform), which is a polar aprotic solvent like epoxide 1a. However, in contrast to the epoxide we expected no significant interaction of the catalysts with this solvent. Sections of the obtained infrared spectra are shown in Figure 2. The bands of the deformation vibrations for the ammonium 3 (Figure 2a) and phosphonium salts 4 (Figure 2b) of the cations (1235–1248 cm–1) as well as the respective overtone (2436– 2463 cm–1) were observed. The values of the wavenumbers for the deformation vibrational bands in the ammonium salts were 1248 cm–1 (3a) > 1242 cm–1 (3b) > 1235 cm–1 (3c) (Figure 7 ACS Paragon Plus Environment


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2a). The same trend was found for the respective overtones. Remarkably, this correlates exactly to the catalytic activity which increased in the order 3a > 3b > 3c. In turn this might be accounted to an increasing interaction between the tetra-n-butyl cation and the respective anions (Cl– < Br– < I–) which is also reflected in the order of the deformation vibrational bands.

Figure 2. a) Infrared spectra of the monofunctional ammonium salts 3 and b) the monofunctional phosphonium salts 4 in chloroform at 45 °C. 2 mol% of the onium salts in regard to chloroform were dissolved. The same trend was observed for the monofunctional phosphonium salts 4 (Figure 2b). We turned our attention to the kinetic evaluation of the bifunctional analogues 5 and 6 which were also studied in the conversion of 1a under the standard reaction conditions. The kinetic data for the conversion of 1a using the bifunctional ammonium catalysts 5 are depicted in Figure 3a. Notably, the order of activity of the bifunctional catalysts 5 in respect to the anion is inversed (I– > Br– > Cl–) compared to the order of the monofunctional ammonium salt catalysts 3. This dependence on the nature of the halide anion can be explained by the inversed nucleophilicity of the anions in a polar protic environment which is provided by the


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introduction of the hydroxyl site in the catalyst. This switch in activity for hydrogen bond donor catalyst systems has been reported previously.47, 59, 65, 72-73, 100

Figure 3. a) Impact of the anion (Cl–, Br– and I–) on the yield of 2a in the conversion of epoxide 1a with CO2 in the presence of bifunctional ammonium salts 5 and b) bifunctional phosphonium salts 6. The observed selectivities were >99%. Reaction conditions: 1,2epoxybutane (1a, 460 mmol), 2 mol% 5 or 6, p(CO2)= 1.0 MPa, 90 °C, 6 h. As for the monofunctional catalysts we assumed a 1st order kinetics and calculated the rate constants for the catalysts 5 (Table 2, see SI-8).99 The obtained rate constants were 0.836 h– 1

(5c) > 0.420 h–1 (5b) > 0.247 h–1 (5a) reflecting the order of catalytic activity. Notably, for

the catalyst 5c the kinetic data did not exactly fit to a 1st order kinetics. However, these results indicate the higher activity of the bifunctional ammonium salt catalysts 5 compared to the monofunctional analogs 3 (Table 1, entries 1–3 vs. Table 2, entries 1–3).65 Finally, we turned our attention to the bifunctional phosphonium salts 6. The observed activity for these catalysts in relation to the halide anions was identical to the observed order of the ammonium salts 5 ( I– > Br– > Cl–, Figure 3a vs 3b). Based on the results shown in Figure 3 we determined the rate constants again assuming 1st order kinetics (Table 2, entries 4–6). The observed rate constants for the iodide 6c (0.828 h–1) and the bromide catalysts 6b (0.418 h–1) were similar to 9 ACS Paragon Plus Environment


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the values of the corresponding ammonium catalysts 5c (0.836 h–1) and 5b (0.420 h–1) respectively (Table 2, entries 2, 5, 3 and 6). Only the rate constant for the chloride-based catalyst 6a (0.147 h–1) was significantly lower compared to the corresponding ammonium salt 5a (0.247 h–1). Table 2 Determined rate constants for the conversion of 1a using the bifunctional onium salt catalyst 5 and 6 assuming a 1st order kinetics.

Entry

Catalyst

1 2 3 4 5 6

[nBu3N(CH2)2OH)]Cl (5a) [nBu3N(CH2)2OH)]Br (5b) [nBu3N(CH2)2OH)]I (5c) [nBu3P(CH2)2OH)]Cl (6a) [nBu3P(CH2)2OH)]Br (6b) [nBu3P(CH2)2OH)]I (6c)

/

(1st order kinetics)

0.247 0.420 0.836 0.147 0.418 0.828

Due to the observed switch in activity in respect to the anion of the bifunctional catalysts we assumed a considerable stronger interaction between the OH-functionalized cations and their respective anions. This interaction might also be the reason for the differences in activity within the series of bifunctional catalyst 5 and 6 respectively. To gain insights into these interactions we studied catalysts 5 and 6 by infrared spectroscopy in chloroform as a polar aprotic solvent.


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Figure 4. a) Infrared spectra of bifunctional ammonium salts 5 (2 mol%) in chloroform at 45 °C. b) Calculated vibrational spectra of the bifunctional ammonium salts 5 by DFT methods. c) Infrared spectra of the bifunctional phosphonium salts 6 (2 mol%) in chloroform at 45 °C. d) Calculated vibrational spectra of the bifunctional phosphonium salts 6 by DFT methods (see SI-12).99 Since the interactions between the OH-group and the halide anions are considered to be crucial for the catalytic activity we especially focused on the behavior of the OH-stretching vibration of the cation at wavenumbers between 3218–3330 cm–1 (Figure 4a and 4c). Additionally, the respective vibrational spectra of the catalysts using DFT methods have been calculated (Figure 4b and 4d). 11 ACS Paragon Plus Environment


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In the case of the bifunctional ammonium salts 5 the experimental (Figure 4a) as well as the calculated infrared spectra (Figure 4c) showed an increase in the wavenumbers for the OHstretching vibrations in dependence of the halides in the order Cl– < Br– < I–. This trend nicely correlates with the order of the catalytic activity which is observed for the catalysts 5. A higher wavenumber for the OH-stretching vibration indicates a strong O–H bond. In contrast an increasing interaction of the proton with the anion leads to a weakening of the O–H bond and thus to lower wavenumbers for the OH-stretching vibration. In the case of the chloride 5a the wavenumber of 3236 cm–1 indicates a stronger hydrogen bonding between the OH and the chloride compared to the iodide 5c (3421 cm–1, Figure 4a). A strong interaction between the halide and the OH group hampers the catalytic activity, since both the nucleophilicity of the halide as well as the ability of the OH group to activate the epoxide by hydrogen bonding are decreased. We performed the same infrared studies for the catalysts 6 and monitored the identical trends for the interactions between the OH group and the halide anions in the experimental infrared spectra (Figure 4c) as well as in the calculated spectra (Figure 4d). The bifunctional phosphonium salts 6 also showed the same correlation between the wavenumber in the infrared spectra and their catalytic activity in dependence of the anion. Additionally, bands for the deformation vibrations with corresponding overtones were identified for the bifunctional catalysts 5 and 6 (Figure S18 and S19) showing similar frequency shifts in dependence of the anion compared to the monofunctional salts 3 and 4 (Figure 2). Subsequently, further infrared spectroscopic investigations were conducted to get insights into the activation of the substrate 1a. For this purpose 2 mol% of the catalysts 3–6 were dissolved in 1a (2 mol% 3–6 in respect to 1a), which is the same catalyst concentration used for the kinetic experiments. The obtained infrared spectra showed changes in the region for the deformation vibrations of the cation (Figure S16–S19). In most cases these bands disappeared. This suggests that the cation is interacting with the substrate by non-directional 12 ACS Paragon Plus Environment

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electrostatic interaction rather than specific local interaction. Considering the OH-stretching vibrations for the bifunctional catalysts 5 and 6 dissolved in 1a we observed a shift of the vibrational bands to lower wavenumbers compared to the respective chloroform solutions (Figure 4 vs. 5). This indicates an interaction between the epoxide 1a and the OH group via hydrogen bonding leading to a weakening of the O–H bond and thus a shift to lower wavenumbers.

Figure 5. a) Infrared spectra of solutions of the bifunctional ammonium salts 5 (2 mol%) and b) the phosphonium salts 6 (2 mol%) in butylene oxide (1a) at 45 °C. This is excellent proof of the often proposed activation of epoxides 1 by HBD catalysts as well as the reason for the higher activity of the bifunctional catalysts compared to the monofunctional systems.18 We continue our investigations by monitoring the model reaction by in situ infrared spectroscopy using the bifunctional phosphonium salt 6c as the catalyst. The reaction was performed on a 173 mmol scale at 45 °C instead of 90 °C to enable the identification of the proposed intermediates. Figure 6a shows the overlaid in situ infrared spectra from 0 to 48 h reaction time. The band for the carbonyl stretching vibration at 1813 cm–1 and the respective overtone at 3576 cm–1 increased in intensity over time which correlates to the increase in yield of 2a (Figure S21).99 As observed before for the solution of



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6c in epoxide 1a the OH-stretching vibration band appeared at 3301 cm–1 (Figure 5b vs. Figure 6a).

Figure 6. a) Overlaid in situ infrared spectra and b) spectra series plotted over time for the conversion of butylene oxide (1a) with the phosphonium salt catalyst 6c. Reaction conditions: Butylene oxide (1a, 173 mmol), 2 mol% 6c, 45 °C, p(CO2)= 1.0 MPa, 0–48 h. A decrease in intensity of this band was monitored over time and a new band at 3484 cm–1 appeared which increased in intensity with increasing yield of butylene carbonate (2a, Figure 5b). The appearance of this new band in the IR spectra indicates the formation of a new species during the reaction. We assumed the formation of an adduct between the catalyst 6a and the product 2a via hydrogen bonding of the OH-moiety of the catalyst and the carbonyl group of the carbonate (Figure 7).


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Figure 7. Proposed adduct between the bifunctional catalyst 6a and butylene carbonate (2a). Due to this assumption, we subsequently took infrared spectra from a solution of the bifunctional phosphonium salts 6 (2 mol%) in the product 2a (Figure 8). The OH-stretching vibrations were observed at 3156 cm–1 for 6a, 3228 cm–1 for 6b and 3299 cm–1 for 6c. The position of these bands depended on the nature of the anion and increased in the order Cl– < Br– < I– (Figure 8a). This is the same trend and similar values were obtained for the wavenumbers as obtained in the epoxide (Figure 5b). Notably, for all three salts 6 in 2a an additional band at a wavenumber around 3484 cm–1 was detected in the infrared spectra which was assigned to the adduct between the catalyst and the carbonate. Interestingly, the position of these bands in the experimental spectra did not show a significant dependence on the nature of the anion (Figure 8a). This is in accordance with the calculated vibrational spectra which also showed no significant band shift of the respective adducts (Figure 8b).

Figure 8. a) Infrared spectra of bifunctional phosphonium salts 6 in butylene carbonate (2a) at 45 °C (a). 2 mol% of the phosphonium salts in regard to butylene carbonate (2a) were dissolved. b) Calculated vibrational spectra of the bifunctional phosphonium salts 6 in the presence of butylene carbonate (2a).



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Furthermore, a clear trend for the formation of the catalyst-carbonate adduct in dependence of the halide anion was observed. This is illustrated by an increasing intensity of the adduct band for the salts 6 in the order Cl– < Br– 99%. Reaction conditions: 1,2-Epoxybutane (1a, 460 mmol), 2 mol% 6, p(CO2)= 1.0 MPa, 90 °C, 6 h. The same method was successfully applied to the data for the bifunctional ammonium salt catalyst systems 5 (SI-8, Figures S8-S11). Besides the band for the OH group in the infrared spectrum of 5c dissolved in butylene carbonate (2a) (Figure 5) we found also an additional band at 3494 cm-1 which was assigned to a respective adduct 5c·2a (SI-10). In Table 3 the rate and equilibrium constants from the data analysis for the catalyst systems 5 and 6 are given. Especially for the iodide catalysts 6c and 5c the product inhibition is significantly effective (Table 3, Entries 3 and 6). The results from the kinetic analysis are in agreement with the aforementioned observed formation of the catalyst-carbonate adduct in the infrared spectroscopic measurements (Figure 8). Based on this data the intrinsic activity it can be stated that the iodide salts 5c and 6c show significant higher activity compared the chloride



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and bromide analogs. While the bifunctional phosphonium salt 6c shows better activity than the respective ammonium salt 6c. Table 3. Determined rate and equilibrium constants for the conversion of 1a using the onium salt catalysts 5 and 6 assuming a kinetic model including product inhibition.

Entry

1 2 3 4 5 6

Catalyst

/

[nBu3N(CH2)2OH)]Cl (5a) [nBu3N(CH2)2OH)]Br (5b) [nBu3N(CH2)2OH)]I (5c) [nBu3P(CH2)2OH)]Cl (6a) [nBu3P(CH2)2OH)]Br (6b) [nBu3P(CH2)2OH)]I (6c)

/

Kinh/ L mol1

(kinetic model Eq. 1)

(kinetic model Eq. 1)

0.25 0.48 5.43 0.17 0.65 8.91

0 0.03 0.82 0.07 0.11 1.48

For further investigations of the product inhibition effect we performed the model reaction under the standard conditions (90 °C, 6 h, p(CO2)= 1.0 MPa) using 2 mol% of 6c with methyl-tert-butyl ether (MTBE) and butylene carbonate (2a) as a solvent respectively (Figure 9). The oxygen in MTBE should interact less with the catalyst compared to the carbonyl group in the butylene carbonate (2a), due to sterical and electronical properties. Thus, as expected the reaction rate in MTBE was higher compared to the reaction rate in the carbonate 2a (Figure 10).


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Figure 10. Impact of the solvent on the yield of 2a in the conversion of epoxide 1a with CO2 in the presence of bifunctional phosphonium salt 6c. Reaction conditions: 1,2-Epoxybutane (1a, 460 mmol), 2 mol% 6c, in MTBE or butylene carbonate (2a), p(CO2)= 1.0 MPa, 90 °C, 6 h. Based on these results a mechanistic proposal for the addition of CO2 to epoxide 1a catalyzed by bifunctional onium salts 5 and 6 is shown in Scheme 3. In the first step the activation of 1a via hydrogen bonding forming the activated intermediate A is proposed. This activation was observed in the IR experiments. A nucleophilic ring opening leads to the alcoholate B and subsequent CO2 insertion to C. It is known that the ring opening is the rate determing step of the reaction. Even though B and C could not be detected in the in situ IR experiments in some cases the formation of the respective halohydrin species was detected previously by GC MS from the reaction mixture. This observation makes the suggestion of the formation of B feasible. Subsequent ring closure leads to the product 2a under liberation of the catalyst. However, after the product is formed the catalysts 5 and 6 are existent in an equilibrium with adduct D formed between the respective catalyst and the product a. Thus, hampering the activation of 1a via hydrogen bonding in the next catalytic cycle.



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Scheme 3. Mechanistic proposal in agreement with the results of infrared spectroscopic experiments.

CONCLUSION In conlcusion, we revealed that the conversion of epoxide 1a with CO2 in the presence of monofunctional ammonium 3 and phosphonium salts 4 under standard reaction conditions (2 mol% catalyst, p(CO2)= 1.0 MPa, 90 °C, 6 h) followed a 1st order kinetics. The rate constants were determined for the monofunctional salts 3 and 4 which proved that [nBu4P]Cl (4a) was the most active catalyst (

= 0.291 h–1). IR spectroscopic investigations showed that the

interaction between the cations and the anions depended on the nature of the anion and correlated with the observed wavenumber in the order Cl– > Br– > I– while the order of activity showed the same trend. Similar activity was observed for the phosphonium bromide 4b and chloride 4c. The infrared spectroscopic investigations indicate that the epoxide is 20 ACS Paragon Plus Environment

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activated by the onium cation. The observed superior activity of the chlorides 3a and 4a in the monofunctional onium salt series might be addressed to the fact that Cl– is the best nucleophile in polar aprotic solvents such as reactant 1a and product 2a respectively (under solvent-free conditions the substrate and product may be considered as solvent). Additionally, the weaker interaction between the chloride and the cation might allow a stronger activation of the epoxide by the cation. The kinetic studies on the bifunctional catalysts 5 and 6 showed the opposite trend in activity for the respective anion (I– > Br– > Cl–) compared to the monofunctional salts. This can be explained by the inversed nucleophilicity of the anions in a polar protic environment which is provided by the hydroxyl moiety in the bifunctional catalyst. The infrared spectroscopic investigations revealed the strongest interaction between the chloride and the OH group leading to low catalytic activity for 5a and 6a respectively. In contrast the iodides 5c and 6c showed the highest activity and lowest interaction between the OH group and the anion. Notably, the calculated infrared spectra of the catalyst showed the same trend in the interactions. The actual activation of the epoxide by hydrogen bonding was also observed when the reaction was monitored by in situ infrared measurements of the reaction mixture. Interestingly, a new species was observed which was identified as adduct between the catalyst and the product. The pronounced formation was observed for the catalysts 5c and 6c. Additionally, the initially assumed 1st order kinetics did not fit for these catalysts. Thus a Michaelis-Menten model including a reversible product inhibition which is valid for the case of a 1st order with respect to the substrate was assumed. A simultaneous integration-regression procedure led to a much better description of the experimental results. ASSOCIATED CONTENT Supporting Information 21 ACS Paragon Plus Environment


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General information regarding materials and procedures; synthesis of bifunctional catalysts; details about kinetic and infrared spectroscopic measurements; kinetic analysis of experimental data; additional infrared spectra; details about DFT calculations (PDF).

AUTHOR INFORMATION Corresponding Author *Ralf Ludwig. Institut für Chemie, Abteilung für Physikalische Chemie, Universität Rostock, Dr.-Lorenz-Weg 2, 18059 Rostock, Germany. E-mail: [email protected] *Thomas Werner. Leibniz Institut für Katalyse e.V. an der Universität Rostock, AlbertEinstein-Straße 29a, 18059 Rostock, Germany. E-mail: [email protected]. Author Contributions §

These authors contributed equally.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research project is part of the Leibniz ScienceCampus Phosphorus Research Rostock and is co-funded by the funding line strategic networks of the Leibniz Association. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research project is part of the Leibniz ScienceCampus Phosphorous Research Rostock and is co-funded by the funding line strategic networks of the Leibniz Association. 22 ACS Paragon Plus Environment

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TOC Graphic

A combination of kinetic studies, systematic IR spectroscopic investigations and DFT calculations reveal detailed mechanistic insight to the synthesis from cyclic carbonates from CO2 and epoxides in the presents of bifunctional catalysts.


A Mechanistic Study on the Addition of CO2 to Epoxides

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