Investigating HostâGuest Complexes in the Catalytic Synthesis of

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Investigating host-guest complexes in the catalytic synthesis of cyclic carbonates from styrene oxide and CO2 Anaïs Mirabaud, Jean-Christophe Mulatier, Alexandre Martinez, Jean-Pierre Dutasta, and Veronique DUFAUD ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01545 • Publication Date (Web): 06 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Investigating host-guest complexes in the catalytic synthesis of cyclic carbonates from styrene oxide and CO2 Anaïs Mirabaud,†,‡ Jean-Christophe Mulatier,† Alexandre Martinez,†,§ Jean-Pierre Dutasta,*,† and Véronique Dufaud*,‡ †

Laboratoire de Chimie, École Normale Supérieure de Lyon, CNRS, Université Claude Bernard

Lyon 1, 46 allée d’Italie, F-69364 Lyon, France ‡

Université de Lyon, Laboratoire de Chimie, Catalyse, Polymères, Procédés (C2P2), CNRS,

Université Claude Bernard Lyon 1, CPE Lyon, 43 Bd du 11 novembre 1918, F-69616 Villeurbanne cedex, France §

Aix Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313, F-13397 Marseille,

France

ABSTRACT: Combining tetraalkyl-ammonium halides and cavitand hosts as catalysts, we achieved the carboxylation of styrene oxide to produce cyclic styrene carbonate from CO2 in good to excellent yields. In a first approach and in the presence of tetraalkyl-ammonium chloride/tetraphosphonate cavitand catalytic systems, an enhanced activity of the catalytic

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reaction of styrene oxide was observed depending on the host-guest association. In a second set of experiments, using tetramethyl-ammonium halide/tetraphosphonate cavitand complexes as catalysts, we obtained the corresponding styrene carbonate with high yields, whereas used alone the tetramethyl-ammonium salts are inactive under similar mild experimental conditions. The reaction involves the strong binding properties of the cavitand hosts towards ammonium salts. It is emphasized that the structure of the host-guest association is crucial. This simple protocol offers a new approach toward the synthesis of cyclic carbonates and provides the first example of a host-guest catalyzed, metal free, reaction for CO2 valorization.

KEYWORDS:

CO2

valorization,

cyclic

carbonates,

epoxides,

host-guest

chemistry,

organocatalysis, supramolecular chemistry, cavitands.

1. INTRODUCTION The efficient transformation of carbon dioxide into valuable chemical compounds has recently attracted a lot of interest as CO2 is recognized to be environmentally benign, inexpensive, and an abundant renewable C1 building block.1-3 One of the promising methodologies for CO2 fixation is the coupling with epoxides to form five-membered cyclic carbonates, which are valuable chemicals currently used in numerous applications e.g. as electrolytes components in lithium batteries, polar aprotic solvents and intermediates for the production of plastics, pharmaceuticals and fine chemicals.4-10 Ethylene and propylene carbonates are currently manufactured using this technology in the presence of quaternary ammonium or phosphonium salts as the catalysts, but generally under high temperatures and pressures (temperature > 120°C, 40-80 bar of CO2).11-14 Milder conditions can be achieved when these catalysts are associated with transition metal


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complexes,15-17 but in this case, metal contamination as well as catalyst separation and disposal present both environmental and economic drawbacks. Thus, the development of new technologies and metal-free catalysts that operate at atmospheric pressure and mild temperature is a priority to minimize costs and greenhouse gas emissions and provide true viable alternatives for fossil fuel-based chemicals. Detailed studies of the carboxylation of epoxides to produce cyclic carbonates from CO2 have shown that the catalyst should provide a good nucleophile to open the epoxide ring and generate an alkoxide species, which then activates the CO2 molecule. Furthermore, this nucleophile should also be easily displaced by the resulting carbonate anion during the subsequent intramolecular cyclization step to yield the desired cyclic carbonate and regenerate the catalyst. In this regard, tetraalkyl-ammonium halides are good candidates as they provide both prerequisites in addition to be cheap and easily available. However, as mentioned above, in the absence of any Lewis18 or Brønsted acid activators,19,20 which may coordinate and so activate the epoxide and/or stabilize subsequent reaction intermediates by a similar process, poor reactivity is achieved when tetraalkyl-ammonium halides are used alone, at least under mild reaction conditions. In these bicomponent systems, catalytic enhancement is obtained by increasing the epoxide carbon electrophilicity towards nucleophilic attack either through multiple hydrogen-bonding provided by poly-hydroxyl or phenolic based compounds19 (e.g. pyrogallol)5 or by electrostatic interactions with electron deficient metal centers (Al, Zn, Mn, Cr, Co amongst other).21 Herein, we wish to report for the first time on the use of an alternative approach to activate tetraalkyl-ammonium halides based on host-guest chemistry. In this approach, we seek to improve catalytic reactivity by increasing the nucleophilicity of the halide rather than epoxide activation. Supramolecular systems capable of quaternary ammonium ions recognition are


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employed, liberating the halide for the initial epoxide ring-opening reaction. Bowl-shaped tetraphosphonate cavitands are well-recognized as efficient receptors for cationic species.22-27 Their improved affinity for ammonium guests has been linked to the strong donating power of the four inward (i) oriented P=O phosphoryl groups (4i configuration) towards the nested cationic species, the potential of these groups to form strong hydrogen bonds,28-30 and the cooperative effect of the aromatic cavity in forming cation-π and CH-π interactions. Firstly, as a proof of concept, tetraalkyl-ammonium chlorides Et4N+Cl–, n-Bu3MeN+Cl– and nBu4N+Cl– were investigated in the cycloaddition reaction of CO2 to styrene oxide (SO) to form cyclic carbonates (Figure 1). Indeed, these onium salts display a moderate catalytic activity for this reaction, and thus appear as relevant probes to address the effect of supramolecular encapsulation, i.e. inhibition or improvement of the catalytic activity. Among the molecular receptors capable of recognizing ammonium cations, we selected the tetraphosphonate cavitands 2a and 2b, which differ by their narrow rim substituents (Figure 1).

Figure 1. Synthesis of cyclic carbonate from styrene oxide and carbon dioxide and catalytic systems investigated.


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Comparative catalytic studies were undertaken, focusing on the expected effects of the bulkier quaternary ammonium cations on binding affinities and ammonium-chloride ion-pairing, together with the less expected changes induced by the C11 or C3 alkyl chains in hosts 2a and 2b, respectively. Secondly, to further explore the potential of such host-guest approach in the catalytic cycloaddition of CO2 to styrene oxide, we investigated tetra-methyl ammonium halides Me4N+X– (X = Cl, Br, I) (Figure 1). They were targeted as quaternary ammonium guests since they have never shown any activity in the coupling of CO2 and epoxides even under harsh reaction conditions.31 We thus anticipated that the Me4N+@cavitand inclusion complex would result in greater nucleophilicity of the anion and hence enhanced reactivity. 2. RESULTS AND DISCUSSION 2.1. Cavitands synthesis. The tetraphosphonate cavitands 2a, bearing C11 alkyl chains at the narrow rim, was prepared according to a published procedure.27 A new tetraphosphonate cavitand 2b bearing shorter propyl chains was also synthesized using the same two-step, gramscale procedure, which involves first the formation of the resorcin4arene 1b from resorcinol and butyraldehyde, followed by reaction with phenyldichloro-phosphine oxide in the presence of pyridine in refluxing toluene to afford the 4i stereoisomer 2b with 11% yield (Scheme 1). Full characterization of 1b and 2b has been displayed in Supporting Information (Figures S1-S5). In particular, the C4v symmetry of the 4i isomer 2b was unambiguously evidenced by the presence of a single

31

P NMR resonance at 8.0 ppm (Figure S5). Cavitands 2a and 2b only differ by the

nature of the alkyl chains, which modulate the solubility of the hosts, and can intervene in the formation of the host-guest complex by the possibility to trap the anionic counterion at the narrow rim (vide infra).


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Scheme 1. Synthesis of tetraphosphonate cavitand host 2b

2.2. Carboxylation reactions with tetraalkyl-ammonium chloride/cavitand catalysts. The effect of the addition of tetraphosphonate cavitand hosts 2a-b on the catalytic performance of various tetraalkyl-ammonium chlorides was investigated using the coupling of styrene oxide (SO) with CO2 to produce styrene carbonate (SC) as a model reaction. In coherence with the requirements of highly sustainable green methodologies, reactions were carried out at low CO2 pressure (1 bar) and catalyst loading (1 mol%) with, when present, a stoichiometric amount of the cavitand host with respect to the quaternary ammonium chlorides. A reaction temperature of 100 °C was found to be sufficient to provide measurable activity in all cases studied. Reactions were carried out in neat substrate (SO, 600 mg) in a test tube. After addition of substrate and catalyst components to the reactor, the solution was saturated with CO2, the tube was topped with a CO2 filled balloon and heated. Samples were withdrawn via syringe at regular intervals and analyzed by 1H NMR (see Supporting Information for details). Catalytic results are summarized in Table 1. Under these conditions and in the absence of cavitand host, the tetraalkyl-ammonium salts Et4N+Cl–, n-Bu4N+Cl– and n-Bu3MeN+Cl– exhibit similar activities for the conversion of styrene


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oxide to styrene carbonate with yields around 60% after 24 hours (Table 1). The addition of 1 equivalent of cavitand 2a to the reaction mixture resulted in lower yields, the strongest effect being observed for Et4N+Cl– with a drop of 19% yield, while for the more hindered cations nBu4N+Cl– and n-Bu3MeN+Cl– only a slight decrease in activity (≤5%) was observed. On the contrary, the addition of host 2b to the three ammonium chloride salts, leads to a significant gain in yield of 32% (Et4N+Cl–), 24% (n-Bu4N+Cl–) and 8% (n-Bu3MeN+Cl–) after the same reaction time, underlining the importance of the host involved in the catalytic reaction (Table 1).

Table 1. Effect of cavitands 2a-b on the tetraalkyl-ammonium chloride catalyzed coupling of CO2 and styrene oxidea Entry

Yieldb (%)

Catalyst No host

host 2a

host 2b

1

Et4N+Cl–

63

44

95

2

n-Bu4N+Cl–

61

56

85

3

n-Bu3MeN+Cl–

58

57

66

a

Conditions: styrene oxide (5.0 mmol), tetraalkyl-ammonium chloride (1 mol %), cavitand 2a/2b (1 mol %), 100 °C, 1 bar CO2. bYields were determined at 24 hours by 1H NMR of the crude reaction mixture using 2,4-dibromomesitylene (1 mmol) as an internal standard.

These data show that catalytic systems involving host-guest interactions are complex and several factors should be taken into account. (i) The ability of the host to break the ion-pair thus allowing the halide ion to interact more efficiently with the styrene oxide; (ii) The eventual trapping of the chloride anion by the cavitand either through the reformation of the ion-pair in the host-guest association, or by interacting with the alkyl chain substituents; (iii) The role of the reaction medium (SO under our solvent-free conditions) either in the complexation process of


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the ammonium salt or in the solvation of the free anion. These different factors not only may impact the binding strength of the host molecule towards ammonium cation but also the overall reactivity of the halide anion. As well known, host-guest association is highly depending on the host ability to break the ion pair, and on the size of the ammonium cation. In the presence of tetraphosphonate host, larger ammonium cations, in particular tri- or tetrabutyl derivatives probably interact less efficiently with the aromatic cavity of 2a-b leading to a moderate binding strength, and hence the consequences on the catalytic activity are more pronounced with Et4N+Cl–. However, we observe an increase of the reactivity with 2b whatever the ammonium used, proving that charge separation in the resulting ammonium chloride/2b assembly is sufficient to promote catalysis. Although the same binding ability towards ammonium cation is anticipated for cavitand host 2a with regard to 2b, a strong inhibiting effect was clearly evidenced in the presence of 2a. The drop in reactivity observed with 2a can be related to the structure of the supramolecular association, notably to the relative location of the ammonium and the counter-anion. Many solid-state structures of phosphonate cavitand/ammonium salt complexes have shown that the anion (essentially halide anions) can be located in-between the substituents at the narrow rim of the host.26,32 In our case, long C11 alkyl chains in 2a are particularly suited for chloride trapping. Such host-guest architecture will depend on the strength of the ammonium/chloride binding and will have consequently a dramatic effect on the ion-pair breaking, and therefore on the catalytic reactivity. Indeed, increasing the amount of 2a seems to inhibit the reaction as exemplified by the reaction performed with Et4N+Cl– and 2a in a 1:2 ratio where only 5% yield of styrene carbonate was obtained after 24 h reaction versus 44% when a 1:1 Et4N+Cl–/2a ratio was used. Nevertheless, the promising results obtained with 2b emphasize


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that this methodology is efficient and could be applied to other ammonium halide salts to improve reactivity. 2.3 Carboxylation reactions with Me4N+X–/2b catalysts. To further support the dramatic effect of the supramolecular approach exemplified above with the tetraalkyl-ammonium chloride/2b host-guest systems, we investigated the catalytic performance of various tetramethylammonium halides Me4N+X– (X = Cl, Br, I) using styrene oxide as model substrate, in the presence or not of tetraphosphonate cavitand 2b. The reaction was run at atmospheric pressure of CO2 under the same operating conditions as those described above for the tetra-alkyl ammonium chlorides. As aforementioned, the tetramethyl-ammonium halides have been shown to be very weak catalysts under much more rigorous conditions,31,33,34 stressing further, if successful, the high potential of such host-guest approach in the field of CO2 chemical conversion. Catalytic results are summarized in Table 2, and the time dependent reaction profiles are provided in Figure 2 with and without host 2b. Table 2. Effect of the addition of cavitand 2b on the tetramethyl-ammonium halide catalyzed coupling of CO2 and styrene oxidea,b Entry Me4N+X– X–

Yieldc (%) without host

with host 2b

rate constant (h-1) kd

1

Cl–

trace

25 (38)e

10 ± 0.7

2

Br–

trace

55 (95)e

42 ± 0.3

3

I–

trace

92 (100)e

98 ± 0.2

a

Conditions: styrene oxide (5.0 mmol), Me4NX (1 mol %), cavitand 2b (1 mol %), 100 °C, 1 bar CO2. bNo reaction occurred with only 2b as catalyst. cYields were determined at 24 hours by 1 H NMR of the crude reaction mixture using 2,4-dibromomesitylene (1 mmol) as an internal standard. dRate constant determined by fitting the data to pseudo first order (in substrate) kinetics. eYields measured after 48 hours of reaction.


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Under these reaction conditions, all three tetramethyl-ammonium halide catalysts (chloride, bromide, iodide) were shown to be practically inactive, with only trace of styrene carbonate observed after 48 hours at 100 °C (Table 2), which is entirely coherent with previous literature reports. When the molecular host cavitand 2b was used without an ammonium halide present, no product whatsoever was observed after 48 hours. The association of cavitand and tetramethylammonium halides produced active to very active catalysts, even under these relatively mild reaction conditions. The 1:1 mixture of cavitand 2b and tetramethyl ammonium chloride leads to 25% conversion after 24 hours of reaction and 38 % after 48 hours (Table 2, entry 1). As would be expected, the more nucleophilic bromide was more reactive (58 % and 95 % after 24 and 48 hours respectively; Table 2, entry 2) and the iodide the most active (92 % conversion at 24 hours, quantitative conversion at 48 hours; Table 2, entry 3). Clearly, the tetraphosphonate cavitand 2b efficiently captures the tetramethyl-ammonium cation and the resulting inclusion complex is less coordinating with respect to the halide anion, enhancing its nucleophilic activity. The strong affinity of host 2b for the Me4N+ cation was demonstrated by means of NMR titration experiments which exhibit an association constant Ka of 26100 M-2 for Me4N+Cl– (See Supporting Information, Figures S8-S10). Differences among the three halides follow classical trends, with the iodide providing both the best nucleophilic attributes for the initiation of the reaction (nucleophilic attack on the epoxide) as well as being a good leaving group for the final ring-closure step of the reaction. As mentioned, the reactions were followed at regular intervals (hourly for the first seven hours), and the time dependent reaction profiles for the Me4N+X–/2b systems are shown in Figure 2. Fitting the data to the pseudo first order rate equation ([product] = [reactant] 0*e(1-kt)), allowed us to determine the relative rate constants k which was found to be 10 ± 0.7 for the


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chloride system, 42 ± 0.3 for the bromide and 98 ± 0.2 for the iodide (Table 2). Of course, these results are consistent with the classical explanation of reactivity trends among halides mentioned above. 100 Me4NX Me4NX Me4NCl/2b Me4NCl + 2b Me4NBr/2b Me4NBr + 2b

80

Me4NI/2b Me4NI + 2b

60

Yield (%)

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40

20

0 0

10

20

30 Time (h)

40

50

60

Figure 2. Kinetic profile of tetramethyl-ammonium halides with and without cavitand host 2b. Conditions: Styrene oxide (5.0 mmol), Me4N+X– (X = Cl, Br, I, 1 mol %), cavitand 2b (1 mol %), 100 °C, 1 bar CO2. Finally, lower catalyst loadings were tested in order to attempt to achieve higher turnover. The most active system, a 1:1 mixture of Me4NI/2b was tested under the same conditions as above (where the substrate to catalyst ratio was 100:1) but with significantly lower catalyst loadings of 0.5 mol % and 0.1 mol % (substrate to catalyst ratios of 200:1 and 1000:1, respectively). Yields


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were measured at 48 hours of reaction as 86 % (TON = 172) and 27 % (TON = 266) respectively. Thus, lower loading can produce significant activity which clearly indicates that even small amounts of host activated ammonium halides can be useful as catalysts for the production of cyclic carbonates. 3. CONCLUSION In summary, we report on the unprecedented use of host-guest systems for the coupling of CO2 and epoxides to form cyclic carbonates, a novel approach never proposed in the field of CO2 chemical valorization. Tetraalkyl-ammonium halides were found to be good to excellent active catalysts when used in conjunction with tetraphosphonate host molecules, particularly suited for ammonium recognition. Interestingly, in presence of cavitand 2a, Et4N+Cl–, n-Bu3MeN+Cl– and n-Bu4N+Cl– salt catalysts lose some of their effectiveness, whereas enhanced activities were obtained when associated to cavitand 2b. This complex behavior is attributed to the dual properties of the host to recognize the cationic substrates and to interact with the counter-anion. In a second set of experiments, we showed that activation of completely unreactive Me4N+X– (X = Cl, Br, I) could be efficiently achieved through host-guest chemistry. Choosing tetraphosphonate cavitand 2b as adequate molecular receptor, allowed the reaction to operate at CO2 atmospheric pressure and relatively mild temperature (100°C). The Me4N+I–@2b inclusion complex showed the best performance in accord with the good leaving ability and high nucleophilicity of the iodide anion achieving up to 92% yield in styrene carbonate after 24 hours of reaction and full conversion at 48 hours. Further studies are underway to heterogeneize these dual catalytic systems towards greener and sustainable processes.

ASSOCIATED CONTENT


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Supporting Information Experimental procedures, 1H, 13C and 31P NMR spectra for all new compounds, kinetic profile of Me4NCl/2a, binding studies. This material is available free of charge on the ACS Publications website via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: *[email protected] *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We gratefully acknowledge financial support from the Région Rhône-Alpes, France (ARC project). REFERENCES (1) Carbon Dioxide as Chemical Feedstock; Aresta, M., ed.; Wiley-VCH: Weinheim, 2010. (2) Sakakura, T.; Choi, J. C.; Yasuda, H. Chem. Rev. 2007, 107, 2365-2387. (3) Omae, I. Catal. Today 2006, 115, 33-52. (4) North, M.; Pasquale, R.; Young, C. Green Chem. 2010, 12, 1514-1539. (5) Whiteoak, J.; Nova, A.; Ainara, M. F.; Kleij, A. W. ChemSusChem 2012, 5, 2032-2038.


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195x90mm (300 x 300 DPI)


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

254x190mm (96 x 96 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

184x60mm (300 x 300 DPI)


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

254x190mm (96 x 96 DPI)


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