NH2OHâMediated Lignin Conversion to Isoxazole and Nitrile

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NH2OH-Mediated Lignin Conversion to Isoxazole and Nitrile Hongji Li, Min Wang, Huifang Liu, Nengchao Luo, Jianmin Lu, Chaofeng Zhang, and Feng Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04114 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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ACS Sustainable Chemistry & Engineering

NH2OH-Mediated Lignin Conversion to Isoxazole and Nitrile Hongji Li†, ‡, Min Wang†, Huifang Liu†, ‡, Nengchao Luo†, ‡, Jianmin Lu†, Chaofeng Zhang†, and Feng Wang*† †

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China. ‡

University of Chinese Academy of Sciences, No.19A Yuquan Road, Shijingshan

District, Beijing 100049, P. R. China. *Corresponding Author: Feng Wang E-mail: [email protected] Tel: +86-411-84379762; Fax: +86-411-84379798

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ABSTRACT: Conversion of lignin to aromatic compounds via C-C/C-O bond cleavage has been an attractive but challenging subject in recent years. We herein report the first protocol that converts lignin model and pre-oxidized lignin to isoxazole and aromatic nitrile. Isoxazole motif is constructed by β-hydroxyl ketone condensation with hydroxylamine. Magnesium oxide promotes oximation reaction and an intramolecular condensation. Aromatic nitriles and esters are obtained via Beckmann rearrangement or acidolysis reaction depending on selected additive. The hydroxylamine-mediated strategy works well for the pre-oxidized lignin conversion to aromatic isoxazole, nitrile and ester monomers with up to 7.6% yield. KEYWORDS: lignin • oxime • aromatic isoxazole • aromatic nitrile • hydroxylamine


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INTRODUCTION Lignin is the aromatic segment in lignocellulose, which has the potential of catering useful aromatic chemicals from biomass.1-5 Carbon-carbon and carbon-oxygen bonds linking methoxylated phenylpropanoid units together hamper lignin conversion into high value chemicals.6 Targeting at specific monomers, researchers have developed bond cleavage strategies and repolymerization preventing methods during pretreatment and reaction, and have gained new insights into lignin valorization.7-13 Employing hydrogenation, hydrogenolysis, oxidation and the combined methods, lignin models and native lignin can be converted to aromatic hydrocarbons, aldehydes, ketones, acids, and benzoquinones.14-29 However, little attention has been paid to the chemical complexity of highly value-added monomers, and products with nitrogen-containing group are greatly limited particularly from extracted lignin. Isoxazole derivatives are an important class of nitrogen-oxygen-containing heterocyclic compounds. Isoxazole structure exhibits diverse biological activities and pharmacological properties (e.g., leflunomide: anti-rheumatic drug; sulfamethoxazole: PABA antagonist; isoxaflutole: 4-hydroxyphenylpyruvate dioxygenase inhibitor), and is one of the core structures of pharmaceuticals.30-31 Aromatic nitriles are another class of vital synthetic scaffolds for pharmaceuticals, natural products, agricultural chemicals and materials.32-35 Synthesizing isoxazoles and nitriles from biomass is an interesting route. Herein we report a NH2OH-mediated strategy for lignin conversion. In previous works, the reaction with hydroxylamine hydrochloride was used to determine the



content of carbonyl groups in lignin.36-37 In our work, this strategy takes advantage of oximation-condensation reaction to cleave lignin C–O bond, and oximationBeckmann rearrangement to break C–C bond. Isoxazole and aromatic nitrile were obtained from lignin model and oxidized organosolv lignin (Scheme 1).

Scheme 1. Obtaining isoxazole and aromatic nitrile monomer from oxidized lignin. RESULTS AND DISCUSSION The pre-oxidation of β-O-4 linkage at benzyl alcohol to β-O-4 ketone has been previously developed,38-43 which enables the ketone to be a precursor for further conversion.44 We started with the model reaction between β-O-4 ketone 1a and hydroxylamine hydrochloride (Figure 1). About 5% yield of isoxazole and 20% yield of aromatic nitrile were obtained. The presence of additives can affect the isoxazole selectivity. MgO promoted the formation of isoxazole-ring with a yield of 74%. Other metal oxides (ZnO, MoO3 and ZrO2), Brönsted acids (Ac2O, oxalic acid and CF3COOH), Lewis acids [FeCl3, AlCl3, Yb(CF3SO3)3] and bases (Na2CO3, KOH and Et3N) were inferior to MgO with respect to the yield of isoxazole. In all cases, aromatic nitrile was generated in less than 30% yield. In the case of Na2CO3, FeCl3 and MoO3, 15-50% yields of aromatic ester were produced via an acidolysis reaction. Solvent screen tests indicated that alcohols were better than other polar solvents, and


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ethanol was the best solvent for generating isoxazole with 84% yield (Table S1). O

N O

OMe HONH2. HCl

O 2

1 MeO

Additive

OH 1a

+

1

+

1

O

1

MeO

MeO

MeO 2a

MeOH, N2, 120 oC

O

N

3a

4a

OMe HO +

2 5a

Figure 1. Conversion of lignin model using hydroxylamine with different additives. Conditions: lignin model (0.1 mmol), hydroxylamine hydrochloride (3 equiv), additive (2 equiv), in methanol (1 mL), N2 (1 atm), 120 °C, 12 h.

(a)

(b)

model 1a HONH 2 HCl + MgO

EtOH, N 2, r.t. 30 min

Mixture

model 1a + MgCl2 + HONH 2 HCl

EtOH, N2, 120oC 6h

EtOH, N2, 120oC 6h

2a, 72%

2a, 21%

model 1a (c) HONH 2 HCl + MeONa

(d) HONH 2 HCl + MeONa

Mixture EtOH, N 2, r.t. 30 min Mixture EtOH, N 2, r.t. 30 min

EtOH, N2, 120oC 6h

2a, 38%

model 1a, MgCl2 EtOH, N2, 120oC 6h

Scheme 2. MgO as a promoter for the formation of isoxazole.


2a, 61%


Isoxazole was obtained from the oxime formation followed by a formal intramolecular condensation. Control experiments reveal the role of MgO (Scheme 2). When hydroxylamine hydrochloride and MgO were mixed before adding the lignin model substrate, MgO reacted with hydrochloride to generate hydroxylamine and MgCl2 (eq a), and the yield of isoxazole was 72%. The isoxazole yield decreased to 21% or 38% when MgO was replaced with MgCl2 (eq b) or when HCl was consumed by MeONa to avoid MgCl2 formation (eq c). We thus deduce that the role of MgO is first to release NH2OH as an HCl-binding reagent, which promotes oximation reaction, and then to form MgCl2 which catalyzes the formation of isoxazole. Indeed, when MeONa was used to bind HCl, after which MgCl2 was added, the isoxazole yield was 61% (eq d), less than but close to the reaction with MgO as the additive. The time-on-stream profile of this reaction shows that isoxazole and guaiacol are instantaneously generated in the initial 8 min (Figure 2). An obvious lag period of formation of isoxazole in the initial 3 min was observed. In comparison, guaiacol was faster formed. This indicats the ring of isoxazole may be formed after Cβ–O cleavage. An intermediate, (Z)-(4-methoxyphenyl)(oxiran-2-yl)methanone oxime, was detected in the mixture (see analysis data in Figure S1-2).45 The use of the oxiran-2-yl(phenyl)methanone (6) as the substrate could generate a (Z)-oxiran oxime and an (E)-oxiran oxime (Scheme 3).46-47 Isoxazole with 39% yield was generated from condensation of oxime hydroxyl and oxiran on the same side in (Z)-oxiran oxime. The (E)-oxiran oxime may induce Beckmann rearrangement,48 and 41% yield of nitrile was obtained. Based on the above experiments, a possible reaction pathway


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is proposed (Scheme S3).

Figure 2. Time-on-stream profile for conversion of lignin model in the existence of HONH2•HCl/MgO. Conditions: 1a (0.1 mmol), hydroxylamine hydrochloride (3 equiv), MgO (2 equiv), in ethanol (1 mL), N2 (1 atm), 120 °C. N O

O O

MgO EtOH, N2, 120 o C 10 min

6

N

HONH2HCl + 2b, 39%

N

3b, 41%

OH

HO

O

O (Z)-oxime

N

(E)-oxime

Scheme 3. Possible intermediates in the formation of isoxazole and aromatic nitrile. Table 1. Bond dissociation energy of Cβ–O before and after oximation. substance O

bond dissociation energy OMe

O 192.6 kJ/mol MeO

HO N

OH

OMe

O 178.4 kJ/mol MeO

HO



Density functional theoretical (DFT) calculations showed that the formation of oxime reduced the Cβ–O bond dissociation energy of β-O-4 model by 14.2 kJ/mol (Table 1). Therefore, the pathway via oximation facilitates the breakage of β-O-4 linkage. This strategy is applicable to other model compounds, mimicking syringyl (S-) and p-hydroxy phenyl (H-) units in real lignin (Table 2). The methoxy group on Ar1 or Ar2 showed no obvious effect on C–O cleavage and isoxazole formation (63-93% yield). When diol model 1e was subjected to this reaction, no product from cleavage was obtained, which implied the necessity of oxidation at Cα-OH in this strategy. However, phenolic model gave low yields of isoxazole and nitrile (less than 20%) under this system, and the reason needs further investigation (Scheme S2).


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Table 2. Isoxazole formation from lignin β-O-4 models.a

a

Conditions: Substrate (0.1 mmol), hydroxylamine hydrochloride (3 equiv), MgO (2

equiv), in ethanol (1 mL), N2 (1 atm), 120 °C, 8-12 h. Next we investigated the conversion of the dioxasolv birch lignin. The oxidized birch lignin was prepared using DDQ/tBuONO/O2 system developed by Westwood et al (Figure S3).42 The β-O-4 ketone structure in the oxidized lignin was identified by 2D HSQC NMR spectra (Figure S4). Unfortunately, the HA/MgO additive gave no monomer (Figure S7), which implied the delicate difference between lignin model and real lignin structure. After further additive screening, phenolic isoxazoles, nitriles and esters were the major monomer products (Figure 3a). In the presence of HONH2·HCl (HA), the desired isoxazole was obtained with 27% selectivity, which amounted to 1.2 % yield from lignin. Size-exclusion chromatography (SEC) indicated



that the fraction with a low molecular weight obviously increased after reaction (Figure S8-10). The combination of HA and Yb(CF3SO3)3 (CFSOYb) decreased the monomer yield and the selectivity of isoxazole. The combination of HA and CF3SO3H (CFSO) or simple hydroxylamine-O-sulfonic acid (SA) increased the total yield of monomers (5.9% and 7.6%), while more phenolic esters (38-57% selectivity) were obtained via the acidolysis. Detailed information of monomers is shown in the gas chromatograph-mass spectrometry (GC-MS) analysis of reaction mixture using HA or SA (Figure 3b and 3c). The reaction mixture with HA was composed of syringyl (S-) monomers, which is in accordance with previous studies on birch lignin.7, 42 A small amount of guaiacyl (G-) and para-hydroxyl-phenyl (H-) monomers were detected in the reaction mixture with SA, which contributed to a higher yield of monomers.


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Figure 3. (a) Yield and product distribution of monomers for lignin depolymerization with different additive. Conditions: oxidized birch lignin (70 mg), HONH2·HCl (49 mg) or hydroxylamine-O-sulfonic acid (79 mg), additive Yb(CF3SO3)3 (49 mg) or additive CF3SO3H (47 µL), EtOH (2 mL), N2 (1 atm), 120 °C, 15 h, GC yield. (b) GC-MS analysis of reaction mixture of lignin with HONH2·HCl. (c) GC-MS analysis of reaction mixture of lignin with hydroxylamine-O-sulfonic acid. The formation of isoxazole structure was further identified by 2D HSQC NMR (Figure 4c, 4d compared with Figure 4b). The HSQC spectra of the reaction mixture using HA/MgO (Figure 4c) revealed the presence of the isoxazole motif, analogous to that observed in the monomer model (Figure 4a). Thereby, we deduce that the



isoxazole structure is built into the lignin network but not liberated due to adjacent inert linkages, 5-5’ linkage for example. The spectra of samples in the HA, CFSOYb/HA, CFSO/HA and SA also reveal the presence of isoxazole which can be attributed to the isoxazole monomers (Figure 4d and Figure S6).

Figure 4. 2D HSQC spectra of (a) isoxazole monomer, (b) oxidized birch lignin before reaction, (c) oxidized birch lignin after reaction with HA/MgO for 14 h, and (d) oxidized birch lignin after reaction with HA for 14 h. All spectra were obtained in DMSO-d6. CONCLUSIONS In conclusion, the NH2OH-mediated strategy has allowed us to build a new isoxazole or cyan motif during the breakage of the lignin, and synthesized aromatic isoxazole and aromatic nitriles from lignin for the first time. In the future, this strategy should


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be combined with others in order to liberate more isoxazole motif from native lignin.

EXPERIMENTAL SECTION

Chemicals and Materials. All chemicals were of analytical grade, purchased from Aladdin Chemicals and used without further puriﬁcation. The β-O-4 models and oxidized birch lignin were prepared by the literature procedures.40, 42

Catalytic Test. Typically, a pressure bottle was charged with lignin model (0.1 mmol), hydroxylamine hydrochloride (3 equiv), additive (2 equiv), solvent (1 mL), and the reaction was conducted at 120 °C under N2 atmosphere for 12 h. After cooling to room temperature, a solution of naphthalene (300 µL, 32 mg/mL DMF) was added. The mixture was filtered and the solution was analyzed by GC-MS.

A pressure bottle was charged with oxidized Birch lignin (70 mg), hydroxylamine hydrochloride (49 mg) or hydroxylamine-O-sulfonic acid (79 mg), MgO (9 mg), Yb(CF3SO3)3 (49 mg) or CF3SO3H (47 µL), solvent (2.5 mL), and the reaction was conducted at 120 °C under N2 atmosphere for 12 h. After cooling to room temperature, a solution of naphthalene (200 µL, 32 mg/mL DMF) was added. The mixture was filtered and the solution was analyzed by GC-MS.

ASSOCIATED CONTENT

Supporting Information

All data supporting the findings in this study are available within the article and



Supplementary Information file. For GC-MS, NMR spectra see Supplementary Pages 26-41.

AUTHOR INFORMATION Corresponding Author *E-mail for F.W.: [email protected]

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21603219, 21690082, 21690084), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300), the Dalian Excellent Youth Foundation (2015J12JH203), and the Department of Science and Technology of Liaoning province under contract of 2015020086-101.

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Aromatic isoxazoles and nitriles are firstly obtained from lignin conversion mediated by hydroxylamine.


NH2OHâMediated Lignin Conversion to Isoxazole and Nitrile

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NH2OHâMediated Lignin Conversion to Isoxazole and Nitrile