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N-Heterocyclic Carbene Promoted Decarboxylation of Lignin-derived Aromatic Acids Dajiang Liu, Jian Sun, Blake A. Simmons, and Seema Singh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03612 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Sustainable Chemistry & Engineering

N-Heterocyclic Carbene Promoted Decarboxylation of Ligninderived Aromatic Acids Dajiang Liu,a,b† Jian Sun,a,b† Blake A. Simmons,a,c and Seema Singha,b,d* a

Deconstruction Division, Joint BioEnergy Institute, 5885 Hollis Street, Emeryville, California 94608, USA. b Biological and Engineering Sciences Center, Sandia National Laboratories, 7011 East Avenue, Livermore, California 94551, USA. c Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA. d Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota 55108, USA * Corresponding author: E-mail: [email protected]. † D.J. Liu and J. Sun contribute equally to this work.

ABSTRACT: Decarboxylation is an important reaction in organic synthesis and drug discovery, which is typically catalyzed by strong bases or metal based catalysts bearing low yield and selectivity. For the first time, we demonstrated a new strategy of decarboxylation of hydroxyl cinnamic acids such as p-coumaric acid, ferulic acid, sinapinic acid and caffeic acid in the presence of N-heterocyclic carbene (NHC) precursors (i.e., 1-ethyl-3-methyl imidazolium acetate [C2C1Im][OAc]), achieving high yields and selectivities up to 100% under relatively mild conditions. [C2C1Im][OAc] showed excellent recyclability as organocatalysis during three-times recycling using biphasic reaction system. Mechanistic study revealed that the decarboxylation was catalyzed by NHCs that was in situ generated by self-deprotonation of [C2C1Im][OAc]. Our demonstrated route is especially appealing for the production of lignin derived renewable aromatics.

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KEYWORDS: N-heterocyclic carbenes, Decarboxylation, Vinyphenol, Organocatalysis, Ionic liquids

INTRODUCTION Decarboxylation is a fundamental and important reaction in organic synthesis1-2 and drug discovery,3-4 which is generally rendered by biocatalysis,5-8 metal,6, 9-18 acid,19 base20-22 and light.23-24 Also decarboxylation is one of the major strategies to defunctionalize biomass,25-27

which

is

normally

over-functionalized.

Compared

with

hydro-

deoxygenation, it does not require the expense of hydrogen.28 The decarboxylation of renewable hydroxylated cinnamic acid derivatives (e.g., coumaric acid 1a, ferulic acid 2a and sinapic acid 3a, see Table 1) leads to the production of functional styrene derivatives, which have been applied widely in polymer, pharmaceutical and food industries.9-10, 29 Utilization of biomass as raw materials to produce useful chemical compounds can significantly reduce greenhouse gas emission and liberate us from the reliance on fossil resource.30 As the main constituent of lignocellulosic biomass,

31-32

however, lignin is

nearly underuse in industry, especially for valuable compounds production.33 Hydroxylated cinnamic acid derivatives are lignin-derived feedstock34-35 and thus the decarboxylation of hydroxylated cinnamic acid derivatives would be a promising way for lignin valorization but challenging in high selectivity and yield due to the rapid polymerization of substrate. Cadot et al. found that moderate to high isolated yields (31– 96%) towards the styrene derivatives were obtained from biosourced carboxylic acids by copper catalyzed decarboxylation, and to obtain reasonable yields for some selected substrates reaction temperatures >170°C are required.9 However, p-coumaric acid and caffeic acid that did not give the expected compounds under the reaction conditions due

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to rapid polymerization.9 Nomura et al. reported rapid base-catalyzed decarboxylation of substituted cinnamic acids via microwave heating with the styrene product yields varying from 16 to 75%.21 Takeshima et al. tested the decarboxylation of ferulic acid into a monosubstituted styrene derivative by using triethylamine.36 Kumar et al. developed a metal-free protocol for decarboxylation of substituted alfa-phenylcinnamic acid derivatives in basic aqueous media under microwave irradiation with low product yields (22-39%).37 Other metal-catalyzed decarboxylations for the formation of C-C bonds have also been reported. To our knowledge, one of the promising catalysts for decarboxylation of aromatic carboxylic acids is the transmission metal/NHCs complex. Nolan et al. demonstrated a NHC-gold complex, [Au(IPr)(OH)] (IPr = 1,3-bis(2,6-diisopropyl)phenyl-imidazol-2-ylidene), could efficiently catalyze the transformation of carboxylic acid to the corresponding decarboxylated gold-aryl complex in toluene.38 Sabater et al. applied [RuCl2(cymene)]2 for the decarboxylation of carboxylate-functionalized pyridyltriazolium salt in tetrahydrofuran/acetonitrile.39 With their applications in organic synthesis40-41 and polymerization,41-42 NHC-catalyzed reactions feature unique properties such as mild conditions and high product selectivity due to the strong nucleophilicity and basicity of NHCs.43-44 Theoretically, NHC is the ideal catalysts for decarboxylation reaction due to its strong basicity, which possibly led to decarboxylation rendered at mild conditions. However, there is no report on NHC-catalyzed decarboxylation, presumably because the carboxylic acid will readily deactivate the isolated NHCs by forming salts. As previously reported, imidazolium based ionic liquids could generate NHC

45-47

for

chemisorption of CO2,48-49 and organocatalysis.50-53 Chen et al. found the generated NHC was 17mol% in [C2C1Im][OAc] (Scheme 1) and could be stabilized by acetic acid,50 but

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afforded similar catalytic activity as isolated NHCs (See examples: benzoin condensation50 and conjugate addition of α, β-unsaturated aldehydes to chalcones54). In this work, we adopted an air and moisture-stable carbene- precursor ionic liquid (1-ethyl3-methylimidazolium acetate, [C2C1Im][OAc]) for the decarboxylation of cinnamic acid derivatives. High yields and selectivities up to 100% were achieved in DMSO under metal-free and relative mild conditions. Mechanistic study revealed that NHC that was in situ generated by self-deprotonation of [C2C1Im][OAc] probably plays a crucial role on catalyzing the decarboxylation reaction. The ionic liquid [C2C1Im][OAc] showed excellent recyclability during three-times recycling. This work opens up avenues for exploring environmentally benign and efficient technology for the production of highvalue chemicals from lignin derived renewable feedstock.

MATERIAL AND METHODS Materials. All the aromatic acids were purchased from Alfa Aesar and used without further purification. 1-Ethyl-3-methylimidzolium acetate ([C2C1Im][OAc], 98% purity) was purchased from BASF. 1,3-Dimethylimidazolium-2-carboxylate was synthesized according to literature procedure as reported. 55 Nuclear magnetic resonance (NMR) analysis. NMR spectra were recorded on a Bruker 500 MHz NMR facilities (FT 500 MHz, 1H) or a Bruker 600 MHz spectrometer. Chemical shifts for 1H spectra were referenced to internal NMR solvent residual resonances and are reported as parts per million relative to SiMe4. Typical procedures of decarboxylation by [C2C1Im][OAc]. For the typical decarboxylation rendered by in-situ NMR experiment, ferulic acid (20 mg, 0.10 mmol)

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and [C2C1Im][OAc] (18 mg, 0.10 mmol, 1 mol equiv. to ferulic acid) were premixed in DMSO-d6. The solution was transferred to the NMR tube and heated in preheated oil bath. For the recycle experiment of [C2C1Im][OAc], 0.1 g (0.51 mmol) ferulic acid and 90 mg [C2C1Im][OAc] (1 mol equiv. to ferulic acid) was loaded into a glass pressure tube reactor, to which 5 mL EtOAc was added. The mixture was heated at 100 °C for 1 h. After the reaction, the top organic layer was decanted, and the bottom ionic liquid layer ([C2C1Im][OAc]) was further loaded with ferulic acid and EtOAc. [C2C1Im][OAc] was recycled for 3 times with 2 repeats. The yield and purity of 2-methoxy-4-vinylphenol were measured after solvent removal under vacuum.

RESULTS AND DISCUSSIONS To demonstrate the potential application for decarboxylation of lignin-derived monolignols, we examined the efficiency on potential aromatic acids. All the aromatic acids tested are listed in Table 1. Starting with ferulic acid 1a, we observed a fast decarboxylation in the presence of equal molar of [C2C1Im][OAc] by in-situ NMR experiments in DMSO-d6 (Entry 1, Table 1). We observed from 1H NMR that only 19% yield of 1b was obtained at 100 °C, but elevated temperature (120 °C and 140 °C) afforded higher yield of 1b at 67 to 94% respectively. The decarboxylation was rendered by [C2C1Im][OAc], since there was no degradation of 1a in DMSO solution up to 160 °C (yield of 1b is 0)(control, data was not shown in Table 1). As shown in Table 1, for the decarboxylation of 2a, the yield of resulting 2b at 100 °C was significantly higher than previous 1b. 82% yield of 2b was produced at 100 °C in 1 h, and higher temperature (120 °C) led to the quantitative yield of 2b. For 3a,

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the decarboxylation showed to be even more efficient: 100% yield of 3b was produced at 100 °C within 1 h. It is worth noting that as the increase of electro-donation group (EDG, i.e., -OMe) from coumaric acid 1a to ferulic acid 2a and to sinapic acid 3a, the decarboxylation occurred more readily. Djakovitch et al. employed Cu(OH)2 and 1,10phenanthroline as catalysis for decarboxylation of 2a, and obtained up to 96% yield of 2b at 140 °C for 1 h (Table entry 2).9 In their methods, however, the decarboxylation only led to low yields and polymerization of vinyl phenols from coumaric acid 1a, sinapic acid 3a and trans-caffeic acid 5a (Table 1, entries 1, 3 and 5).9 Nomura et al. employed a microwave-assisted decarboxylation of 2a, achieving 62% yield of 2b in ethylene glycol in the presence of amine-based catalysis within 3 min (Table 1 entry 2).21 The operating temperature in this case was around 200 °C, but for the decarboxylation of 1a and 5a, the activities (yields of 63 and 30, respectively) were much lower than those (yields of 94 and 97, respectively) obtained in our present work (Table 1, entries 1, and 5). Similarly under microwave conditions, Sinha et al. used 1-hexyl-3-methylimidazolium bromide ([C6C1Im]Br) for the decarboxylation of 2a, but bases such as NaHCO3 and NaOH were necessary to improve the yield of 2b to 69% (Table 1, entry 2).56 In previous decarboxylation examples using acids or bases,9,

21

the side reactions

associated with decarboxylation of ferulic acid typically ended up with the polymerization of final product 2b due to the instability of 2b upon elevated temperature in the acid or base condition, which were interestingly not observed in our carbenecatalyzed system. In contrast, a clean formation 2b was observed by NMR, in quantitative yield and selectivity at 120 °C after 1 h, even under 5 mol% loading of [C2C1Im][OAc] (or 0.85 mol% loading of carbene). [C2C1Im][OAc] has thus for the first

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time been reported as a new type of neutral and metal-free organocatalysis which could cleave the hydroxylated cinnamic acids in high yield and selectivity. In order to gain mechanistic understanding of decarboxylation in our case using [C2C1Im][OAc], we used ferulic acid 2a as the model compound. In Scheme 2, carbene 10 was generated from self-decarboxylation of [C2C1Im][OAc] and was stabilized by acetic acid. As a strong base, carbene 10 will form an ionic complex 12 with 2 mol. equivalence of 2a via two reactions: The deprotonation of carboxylic acid resulted in 11, and 12 was yielded through further deprotonation of phenolic -OH. The complex 12 readily formed at room temperature upon instant mixing of [C2C1Im][OAc] and 2a, indicated by the swift color change from colorless to yellow (Figure S1, ESI) and the observation of 1H NMR peak shifts. As noted in Figure 1 (1H NMR spectra a and b), 1H NMR peaks associated with aromatic (7.28, 7.08 ppm) and α, β protons (7.49, 6.36 ppm) were largely shifted to the upfield (7.10, 6.91ppm for aromatic protons, and 7.23, 6.33 ppm for α, β protons) when

[C2C1Im][OAc] and 2a were mixed initially at room

temperature. This could be attributed to the increased electron density of the conjugated bonds after the deprotonation of 2a by carbene 10. At elevated temperature, the decarboxylation of 12 led to the formation of final product 2b (indicated by the disappearance of α, β protons (7.23, 6.33 ppm) of 2a and appearance of three vinyl peaks (6.58, 5.58, 5.01 ppm) of 2b, 1H NMR spectra b and c). Meanwhile, carbene 10 was released from the catalytic cycle, evidenced by the formation of carbene-CO2 complex 13 at the end of the reaction (7.63, 7.55, 4.46 ppm, 1H NMR spectrum c). Our proposed

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mechanism is similar to F. Himo’s theoretical proof of enzyme rendered decarboxylation of phenolic carboxylic acids, where the first step is deprotonation of carboxylic acids.7 Since the formation of 13 is reversible (the reverse reaction leads to carbene 10), we were interested in whether 13 could act as a precatalyst for decarboxylation. Similar carbene-CO2 complex 1,3-dimethylimidazolium-2-carboxylate 14 (Scheme 3) was previously reported to be stable in aqueous solution to catalyze the hydrolysis of cyclic carbonates to form vicinal diols due to its self-generated carbene.55 Therefore, to test our hypothesis, we used 14 for catalyzing the decarboxylation of 2a. As anticipated, quantitative yield and selectivity of 2b were achieved in the presence of 14 at 120 °C in 1 h. To demonstrate the broad applicability of our approach, we extended our discovery to other carboxylic acids. We first tested cinnamic acid 4a (no phenolic –OH group), but no decarboxylation was observed. We also used 3-(4-hydroxyphenyl)propionic acid 8a (no conjugated carboxylic acid), and there was still no observation of decarboxylation even at 140 °C. It is indicated that [C2C1Im][OAc] selectively targets the decarboxylation of molecules bearing conjugated -COOH and –OH. For complex 6a and 7a, in which substrates do not have –OH but have -OMe (EDG) and –NO2 (EWG) instead, no decarboxylation was observed. In contrast, if there are two phenolic –OH groups (e.g., caffeic acid 5a), the decarboxylation occurred more readily than its counterpart with one phenolic –OH group (coumaric acid 1a), producing 5b in 97% yield at 120 °C within 1 h. Interestingly, [C2C1Im][OAc] could also be applied to the decarboxylation of aromatic carboxylic acids, exemplified by vanillic acid 9a. The decarboxylation at 140°C resulted into 50% yield of guaicol 9b. Although not as efficient as ferulic acid decarboxylation,

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the decarboxylation of vanillic acid provides another example of decarboxylation of molecules bearing conjugated -COOH and -OH. Since the quantitative yield of 2b was obtained after decarboxylation in DMSO solution, we questioned if we can separate the product and recycle [C2C1Im][OAc]. Herein, we demonstrated a preliminary study using semi-continuous extraction of 2b. Briefly, ferulic acid 2a was initially dissolved into EtOAc, and was transferred to [C2C1Im][OAc] and heated to 120 °C for 1 h. The product 2b was extracted using EtOAc and [C2C1Im][OAc] was directly recycled for next batch reaction. The 1H NMR of the products extracted by EtOAc phase (no purification) showed a 100% conversion of ferulic acid using recycled [C2C1Im][OAc] for three times (Figure 2). Only some residual [C2C1Im][OAc] and solvent peaks was observed due to the partial solubility of [C2C1Im][OAc] in EtOAc. It is also noted that the interaction between [C2C1Im][OAc] and 2b can be observed based on the 1H NMR peak changes: The peaks at 6.82 ppm assigned to C5 and C6 protons of 2b is gradually splitting from 1st to 2nd and 3rd recycle of [C2C1Im][OAc] (as indicated from a to b to c in the 1H NMR spectra), presumably due to the decreased interaction between [C2C1Im][OAc] and phenolic group of 2b with the lower concentration of [C2C1Im][OAc]. Although there was loss of [C2C1Im][OAc] (due to partial solubility in organic solvent) during the recycling, no deactivation or side product was observed. Therefore, [C2C1Im][OAc] and 2b showed to be stable in our system. The solvent screening to further minimize [C2C1Im][OAc] solubility in organic solvent is under further investigation.

CONCLUSION

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In summary, we have discovered a novel NHC organocatalysis for decarboxylation of hydroxylated cinnamic acids (and other substrates with similar functionality) in quantitative yield and selectivity for the first time. The self-deprotonation of [C2C1Im][OAc] led to NHCs, which catalyzed the decarboxylation of hydroxylated cinnamic acids as Lewis bases. The NMR experiments demonstrated the “clean” decarboxylation reaction, and there was no degradation or polymerization of final products. Moreover, [C2C1Im][OAc] showed excellent recyclability as NHC precursor using biphasic (EtOAc as organic phase) reaction system, generating consistent quantitative yield of 2-methoxy-4-vinylphenol from the decarboxylation of ferulic acid. This approach is broadly applicable and can enable economically viable industrial decaroxylation

reactions

including

production

of

renewable

aromatics

from

depolymerized lignin. SUPPORTING INFORMATION Pictures of Ferulic acid, [C2C1Im][OAc] and their mixture; 1 H NMR spectra.

ACKNOWLEDGEMENTS This work conducted by the Joint BioEnergy Institute was supported by the Office of Science, Office of Biological and Environmental Research of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

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Figures and Table

Scheme 1. Formation of NHCs in [C2C1Im][OAc] by self-deprotonation of acetic acid.50

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Figure 1. In-situ 1H NMR spectra of ferulic acid decarboxylation catalyzed by carbene precursor [C2C1Im][OAc] (4-10 ppm region was shown). [C2C1Im][OAc] was mixed with ferulic acid in DMSO-d6 in 1:1 molar ratio, 1H NMR spectra was monitored at a) ferulic acid; b) Initial mixing of ferulic acid and [C2C1Im][OAc] at room temperature; and c) mixture was heated at 120 ºC for 1 h.

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Scheme 2. Proposed mechanism of ferulic acid decarboxylation rendered by [C2C1Im][OAc].

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Scheme 3. Formation of NHCs in 1,3-dimethylimidazolium-2-carboxylate.55

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Figure 2. 1H NMR overlay (DMSO-d6) of products extracted by EtOAc in biphasic system of 2a decarboxylation to 2b catalyzed by [C2C1Im][OAc]: a) 1st run; b) 2nd run; and c) 3rd run. The red lines indicate the peaks associated with residue [C2C1Im][OAc] in EtOAc (9.31, 7.75, 4.19, 3.85 ppm), the green lines indicate the peaks associated with 2b (7.01, 6.82, 6.59, 5.60, 3.77 ppm). There was no unconverted 2a observed.

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Table 1. Decarboxylation of aromatic acids catalyzed by [C2C1Im][OAc].[a]

Entry

Solvent [b]

Aromatic acid

T/t (°C/min)

Product

100/60 1

DMSO 1a PEG-6000

120/60

Yield (%)[c] 19

1b

67

140/60

94

130/90

-9 1b

1a DMF

6321

200/2 1b

1a DMSO

100/60

DMSO

120/60

PEG-6000

140/60

82

2 2a

2b

969 2b

2a EG

6221

200/2.5 2b

2a

6956

[hmim]Br/H2O 140/4 2b

2a 3

100

DMSO

100/60

100 3b

3a PEG-6000

319

130/70 3b

3a

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4

DMSO

140/60

DMSO

120/60

4a 5

PEG-6000

3021

200/1.5 5b

5a DMSO

140/60

tr. 6b

6a DMSO

140/60

tr. 7b

7a DMSO

140/60

tr. 8b

8a 9

-9 5b

DMF

8

97

130/30

5a

7

tr.

5b

5a

6

4b

DMSO

140/60

9a

9b

50

[a]: a: Aromatic acids; b: Corresponding decarboxylation products. For example, 2a represents ferulic acid, while 2b represent its corresponding decarboxylation product 2methoxy-4-vinylphenol. [b] DMSO: dimethyl sulfoxide; PEG-6000: poly(ethylene glycol) with molecular weight of 6000; DMF: dimethylformamide; EG: ethylene glycol; [hmim]Br: 1-hexyl-3-methylimidazolium bromide. [c] NMR yield.

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For Table of Contents Use Only:

Synopsis: Efficient decarboxylation of lignin-derived aromatic acids was promoted by NHeterocyclic Carbene, which is in-situ generated by self-deprotonation of 1-ethyl-3methyl imidazolium acetate [C2C1Im][OAc].

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N-Heterocyclic Carbene Promoted ... - ACS Publications

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