Boronic Acids as Hydrogen Bond Donor Catalysts for Efficient

Letter pubs.acs.org/acscatalysis

Boronic Acids as Hydrogen Bond Donor Catalysts for Efficient Conversion of CO2 into Organic Carbonate in Water Jinquan Wang and Yugen Zhang* Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, 138669 Singapore S Supporting Information *

ABSTRACT: Boronic acids together with onium salts provide highly efficient organocatalysts for the conversion of CO2 with epoxides into various cyclic carbonates in H2O under mild conditions. The combination of experimental and computational studies allows further understanding of this active organocatalytic system. KEYWORDS: CO2 conversion, organic carbonate, boronic acid, hydrogen bond, water

C

promising method in the conversion of CO2 into useful organic compounds.2 Cyclic carbonates have wide applications in many fields. For example, cyclic carbonates can be used as excellent aprotic polar solvents and as intermediates in the production of pharmaceuticals, fine chemicals, and polymers.2,3 Various metal catalysts4 or organocatalysts5 have been successfully developed for the synthesis of cyclic carbonates from CO2 and epoxides. However, in most cases, high temperature, high pressure, high catalyst loading, or a combination thereof are required to make the reaction effective. As a result, the sustainability is significantly compromised by the large net output of CO2 due to heavy energy input. Recently, great efforts have been made to develop more efficient6 and sustainable7 catalyst systems that allow the reaction to be carried out under mild conditions. Although metal-containing catalysts are the dominating systems for the cyclic carbonates synthesis from CO2, the generally greener and low-cost organocatalytic systems have also attracted increasing attentions.7 Therefore, the development of new organocatalytic system that operates at low temperature, low CO2 pressure, low catalyst loading, and using a green solvent is an urgent task. Hydrogen-bond donors (HBDs) have emerged as remarkable metal-free catalysts for the activation of organic molecules.8 In the two-component catalyst systems for metalfree cyclic carbonate synthesis, nucleophiles such as KI, dimethylaminopyridine (DMAP), and tetrabutylammonium

arbon dioxide is an ideal renewable and environmentally friendly C1 source.1 The direct catalytic coupling of CO2 with epoxides into five-membered cyclic carbonates is a very Scheme 1. (a) CO2/Epoxide Coupling Reaction. (b) HBDs Used in This Study

Received: May 20, 2016 Revised: June 20, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acscatal.6b01422 ACS Catal. 2016, 6, 4871−4876

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ACS Catalysis Table 2. Solvent Screeninga

Table 1. Screening of HBDs for the Conversion of CO2 with Glycidyl Phenyl Ethera b

b

entry

HBDs

temperature (°C)

conversion (%)

yield (%)

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 27c 28d 29e

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 P18 B19 B20 B21 B22 B23 B24 B25 B26 B19 without B27

50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 25 50 50

55 62 41 38 39 13 36 28 49 77 28 16 70 56 73 35 52 68 86 90 89 74 89 85 70 68 82 30 60

55 62 41 38 39 13 36 28 49 77 28 16 70 56 73 35 52 68 86 90 89 74 89 85 70 68 82 30 60

entry

solvent

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11c 12d 13e 14e 15f 16g 17h

H2O ethanol dimethylformamide dichloromethane toluene 1.4-dioxane acetonitrile acetone tetrahydrofuran chloroform H2O H2O tetrahydrofuran H2O tetrahydrofuran H2O H2O

86 14 41 49 56 22 32 57 57 51 35 Trace 3 53 0 63 4

a

Reaction conditions: glycidyl phenyl ether (2 mmol), nTBAI (5 mol %), B19 (10 mol %), solvent (2 mL), CO2 (1 MPa), 50 °C, 4 h. bYield and conversion were determined by GC using biphenyl as the internal standard. cnTBAB, dnTBAC. eTriphenylboroxin (20.7 mg) without BA. fWithout BA. gnTBAI (2.5 mol %), BA (5 mol %). hnTBAI (1.25 mol %), BA (2.5 mol %).

a Reaction conditions: Glycidyl phenyl ether (2 mmol), nTBAI (5 mol %), HBDs (10 mol %), H2O (2 mL), CO2 (1 MPa), 4 h. bYield and conversion were determined by GC using biphenyl as the internal standard. c25 °C, 12 h. dWithout boronic acid. ePolystyrene-supported boronic acid.

Figure 1. Kinetic study of the B19 promoted conversion of CO2 with GPE. Reaction conditions: Glycidyl phenyl ether (2 mmol), nTBAI (5 mol %), B19 (10 mol %), H2O (2 mL), CO2 (1 MPa), 50 °C.

bromide/iodide (TBAB/TBAI) are used for opening the ring of epoxides. It was also proven recently that a Brønsted acid could activate epoxides through hydrogen-bond interaction.7,9 The synergistic effect of a nucleophile and a Brønsted acid makes ring opening of epoxide more efficient and allows the synthesis of cyclic carbonates at low temperature and low pressure.7 As part of our continuous effort to fix CO2 with epoxide,10 in this work, we report a new organocatalytic system comprising a commercially available boronic acid, a weak Lewis acid, acting as hydrogen-bonding catalyst together with nTBAI nucleophile for the activation of epoxides. Boronic acid is a very stable and relatively benign compound to humans.11 The synergistic effect between the two components of this new catalytic system makes the addition of CO2 to epoxide occur under very mild conditions. The combination of experimental and computational studies reveals more structure−property relationships of catalysts. Boronic acids can be regarded as “green” compounds due to their low toxicity and their ultimate degradation into the environmentally friendly boric acid.11 Boronic acids tend to exist as mixtures of oligomeric anhydrides, in particular, the cyclic six-membered boroxines under dry condition. With that, water was tested initially as a green and unique solvent for

boronic acid catalyst. The screening of various boronic acids was carried out with glycidyl phenyl ether (GPE) as a model substrate using nTBAI as a cocatalyst (Scheme 1, Table 1). Initially, a group of boronic acids with variation of the parasubstituent were tested as catalysts (entries 1−6). It was found that the aryl boronic acids with electron-donating substituents were generally observed better activities than the one with electron-withdrawing substituents. The boronic acids with variation of the ortho-substituent were also studied (entries 7−10). B10 showed much better activity (entry 10 vs 7−9) possibly because of that the pyrazole group would activate CO2 and promote the reaction,12 and the others showed lower activities compared with B1 (entries 7−9 vs 1). For boronic acids with variation of the meta-substituent, the aminosubstituted boronic acid demonstrated much higher activity than the other two electron-deficient ones (entry 13 vs 11−12). In the case of the heterocyclic aryl boronic acids (entries 14− 16), 3-pyridineboronic acid (B15) exhibited much higher activity than 4-pyridineboronic acid (B14) (entry 15 vs 14). However, the 3-thienylboronic acid (B16) showed much lower 4872

DOI: 10.1021/acscatal.6b01422 ACS Catal. 2016, 6, 4871−4876

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ACS Catalysis Table 3. Substrate Scopea

Reaction condition: epoxide (2 mol), nTBAI (5 mol %), B19 (10 mol %), H2O (2 mL), CO2 (1 MPa), 50 °C. bYield and conversion were determined by NMR. a

Scheme 2. Equilibriums of Boronic Acid with and without Water

yield than B15 (entry 16 vs 15). 1-Naphthylboronic acid and styrylboronic acid also showed moderate activities for this reaction (entries 17−18). Unexpectedly, the boronic acids with two substituents at ortho position were observed much higher activities in this study (Table 1, entries 19−25). It is worth noting that these boronic acids, especially the ones with two electron-donating substituents, showed better activities than the reported active catechol catalysts (entries 19−25 vs 26).13,7c Furthermore, the reaction performed very well using B19 as catalyst at room

Figure 2. Calculated natural bond orbital (NBO) charges on boron of catalysts in water and their activities (yields of carbonate in Table 1).

temperature (entry 27). It is also worth mentioning that the yield was only 30% without boronic acid under the same conditions (entry 28). Moreover, the products were automatically separated from the reaction system (water), making the 4873

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ACS Catalysis Scheme 3. Substitutions on 2,6-Position of Arylboronic Acids Affect Their Acid Strength

Scheme 4. Proposed Reaction Pathway for Cyclic Carbonate Formation Catalyzed by B19

Figure 3. Hydrogen bond interaction between B19 and propylene oxide (atom colors in geometries: H: light gray, C: dark gray, O: red, B: pink).

to monomer in THF is minor. This further confirmed that the monomeric boronic acid, which has hydrogen-bonding interaction with epoxide, plays the key role in this system. Expectedly, the conversion was decreased when the catalyst loading was reduced (entries 16−17). Additionally, the kinetics of conversion of CO2 with glycidyl phenyl ether using B19 as catalyst and nTBAI as cocatalyst was studied under identical reaction conditions. Figure 1 shows the dependence of the yield of cyclic carbonate on reaction time at 50 °C and 1.0 MPa of CO2. It can be seen that the yield increased rapidly in the first 4 h and then slowly increased in the remaining time period. The substrate scope was then screened using B19 as catalyst. As shown in Table 3, the current catalytic system is found to be effective for a variety of terminal epoxides (entries 1−7). Furthermore, epoxides functionalized with alkene or long hydrophobic chain are suitable substrates for this catalytic system (entries 5−7). Compared with other reported organocatalytic systems (Table S2 in the Supporting Information), boronic acid is indeed a very effective and promising organocatalyst under relatively mild conditions. It is interesting to find that water is the best solvent in this boronic acid catalyst system. As shown in Scheme 2, boronic acid tends to form a dimer or a trimer in dry conditions or in noncoordinating solvents (Figure S4−14). Boronic acid is also a weak Lewis acid in water. Both equilibriums would turn boronic acid to a weaker hydrogen-bond donor and a less active catalyst. For better understanding of this process, we calculated the charge distribution on boron for catalysts that results in a yield above 50% in Table 1. As shown in Figure 2, the catalyst activities are generally correlated to the positive charge on the boron atom. Catalysts with amine group (B10, B13, and B15) have higher activity because the amine could activate CO2 in the catalytic cycle. Interestingly, better catalysts with substitutions on 2, 6-position of arylboronic acids, located in the red circle in Figure 2, have higher positive charge on boron. Higher positive charge on boron increases the Lewis acidity of boronic acid, while substitution on 2, 6-position will apparently affect its Lewis acid strength due to the F-strain effect (Scheme 3). This phenomenon makes the catalysts in the red circle behave like a strong Brønsted acid, providing strong hydrogen

Figure 4. 1H NMR spectra in CDCl3 at 298 K. (A) B19, (B) B19 and propylene oxide (molar ratio = 1:2).

separation process very simple. In addition, to facilitate separation and recycling of boronic acid, a polystyrenesupported boronic acid was also tested, and moderate activity was observed under the same condition (entry 29). The recycling experiments showed the supported catalyst was stable at least eight times without obvious loss of the activity (Figure S1 in the Supporting Information). For a comparison, common organic solvents were also screened using B19 as catalyst under the same reaction conditions. Yields in other solvents were much lower than in water (Table 2, entry 1 vs 2−10), indicating that H2O is a unique solvent for this reaction system. Moreover, the optimum amount of H2O is around 2 mL under the reaction conditions (Figure S2 in the Supporting Information). Additionally, nTBAI as cocatalyst was better than nTBAB and nTBAC (entry 1 vs 11−12). The reaction almost did not occur with triphenylboroxine as catalyst in THF (entry 13), but 53% yield of cyclic carbonate was obtained when the reaction was carried in water with triphenylboroxine as catalyst (entry 15). It indicates that triphenylboroxine would present as monomeric phenyl boronic acid in water, but the equilibrium from trimer 4874

DOI: 10.1021/acscatal.6b01422 ACS Catal. 2016, 6, 4871−4876

ACS Catalysis bond donor and therefore demonstrating high catalytic activities. We next investigated the mechanism of the catalytic cycle and the role of hydrogen bond interaction through DFT study. All calculations were carried out using the B3PW91functional with the 6-311++G (d, p) basis set as implemented in Gaussian 09 program package. We found that there is a strong hydrogenbond interaction between B19 and epoxide in water (Figure 3). Under the hydrogen bond interaction with boronic acid, epoxide is activated with two C−O bond length enlarged from 1.445 and 1.448 Å to 1.452 and 1.458 Å. For comparison, we also calculated the hydrogen-bond interaction in other solvents. However, they are much weaker (Table S1 in the Supporting Information), indicating a different aspect of water as best solvent in this system. In the catalytic cycle, the first step is ringopening through the attack of a nucleophile on epoxide, which is generally considered to be the rate-determining step with high activation energy.4−7 However, it is found that the energy barrier of this step in the current catalytic system is quite low (ΔE = 18.82 kcal/mol) (Figure S3 in the Supporting Information). This low energy barrier allows the reaction to be performed under mild conditions. The hydrogen bond interaction was further proven by 1H NMR (Figure 4). A clear upfield shift (from δ = 4.77 to 5.30 ppm) of the B19 OH proton signal was observed after mixing with epoxide, indicating the hydrogen bond formation. Therefore, a hydrogen-bond-interaction-promoted catalytic cycle was proposed (Scheme 4). First, the epoxide ring is activated by the hydroxyl group of boronic acid via hydrogen bond interaction (step I). Then, the ring opens through nucleophilic attack by I− (step II), followed with CO2 insertion (step III). Consequently, cyclization via an intramolecular nucleophilic attack leads to cyclic carbonate and regeneration of the catalyst (step IV). In summary, boronic acid is a weak Lewis acid in water; however, due to the F-strain effect, 2,6-subtituted arylboronic acid behaves as a strong hydrogen-bonding donor. It has been demonstrated for the first time that boronic acid acts as a Brønsted acid type of catalyst in the reaction of CO2 with epoxide in water. Various epoxides were transformed into corresponding cyclic carbonates with excellent yields under environmentally benign conditions. The new class of hydrogen bond donor activators will expand the scope of metal-free catalysts in the synthesis of cyclic carbonates and beyond.

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ACKNOWLEDGMENTS

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REFERENCES

We are grateful to the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research (A*STAR), Singapore) for generous support of this project, and the A*STAR Computational Resource Centre through the use of its high-performance computing facilities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01422. General experimental and DFT computational methods, characterization of cyclic carbonates, supporting tables and figures, and also Cartesian coordinates for the optimized geometries of all intermediates and transition states (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4875

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