C-Catalyzed Hydrogenolysis of Dibenzodioxocin Lignin Model

Letter pubs.acs.org/journal/ascecg

Pd/C-Catalyzed Hydrogenolysis of Dibenzodioxocin Lignin Model Compounds Using Silanes and Water as Hydrogen Source Elena Subbotina, Maxim V. Galkin, and Joseph S. M. Samec* Department of Organic Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden S Supporting Information *

ABSTRACT: A mild Pd/C-catalyzed hydrogenolysis of the C−O bond of model compounds representing the dibenzodioxocin motif in lignin using polymethylhydrosiloxane (PMHS) and water as hydrogen sources was developed. The efficiency of the reaction is highly dependent on both water concentration and the addition of a base. The results from mechanistic studies showed that the benzylic C−O bond is cleaved faster than the terminal C−O bond, which only cleaves upon the presence of the neighboring phenol. We propose a hydrogen bond formation between an oxygen atom of an ether group and a proton of a neighboring phenol under the employed mild reaction conditions, which facilitates cleavage of the C−O bond. KEYWORDS: Biomass, Lignin, Dibenzodioxocin, PMHS, Hydrogenolysis, Palladium

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INTRODUCTION Lignin is a phenolic polymer, which together with cellulose and hemicellulose forms the cell wall in plants and constitutes up to 35% of wood by mass and above 50% by energy.1 Formation of lignin in plants proceeds through an oxidative (radicalmediated) coupling between a monolignol and the growing oligomer or polymer of coniferyl, sinapyl, and p-coumaryl alcohols, which results in different interunit linkages: β-O-4′ (most abundant), α-O-4′, β-5′, β−β′, β-1′, 4-O-5′, 5−5′, and dibenzodioxocin (Table 1).2 Lignin is challenging to valorize due to the irregular structure of the polymer with different interunit linkages.4 To develop efficient procedures to transform lignin, reactions on model compounds that represent different interunit linkages in lignin have been studied. Several different model compounds have been synthesized that mimic the most abundant β-O-4′ bond, and a number of strategies have successfully been applied to cleave the C−O bond in these models.5−8 Model compounds representing other lignin moieties have also been reported in the literature, however, to a lesser extent.9−13 In contrast, there is a lack of model compounds representing 5-5′ and α-O-4′ motifs. It was shown that the α-O-4′ linkage is part of only cyclic structures such as phenylcoumaran or dibenzodioxocin, and the 5-5′ bond presents mainly as part of the dibenzodioxocin structure.3 The dibenzodioxocin motif represents 5-7% of linkages in softwood native lignin2 and is considered an important lignin branching point. Surprisingly, reactivity of this motif has not been thoroughly investigated. Studies of model compounds under acidic conditions demonstrated that they undergo rearrangement to cyclic oxepine derivatives.16,17 Thermal decomposition18 and photooxidation of model dibenzodioxocin compounds,19−21 as well © 2017 American Chemical Society

as behavior of this motif in lignin under kraft and soda pulping conditions,22,23 has also been reported. There are reports about the synthesis of dibenzodioxicin model compounds14,15 and computational studies of bond dissociation energies.24 In previous studies on the reactivity of the dibenzodioxocin motif, the harsh reaction conditions used lead to nonproductive (for valorization) isomerization or to formation of several products, including oligomeric compounds. Selective catalytic cleavage of the dibenzodioxocin motif under mild reaction conditions is challenging, and this motivated us to study the reactivity of the dibenzodioxicin moiety.

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RESULTS AND DISCUSSION Models 1 and 2 were synthesized in three steps according to the literature procedure (Figure 1).15 More advanced models containing additional functional groups presented in the lignin structure often show different reactivity in comparison to the more simplified model compounds. To have a better representation of lignin, model 3, with free phenyl and hydroxyl groups, was synthesized as a single diastereoisomer using a silver(I) oxide-catalyzed radical coupling of biphenyl compound and coniferyl alcohol (Figure 1).15 Catalytic cleavage of the β-O-4′ bond of lignin model compounds in the presence of Pd/C and hydride donors such as NaBH4 or HCOOH/NH3 have previously been established in our group.25,26 In the case of β-O-4′ bond model compounds used in these studies, the reaction can be performed under redox-neutral conditions with catalytic amount of reducing Received: February 10, 2017 Revised: March 26, 2017 Published: April 12, 2017 3726

DOI: 10.1021/acssuschemeng.7b00428 ACS Sustainable Chem. Eng. 2017, 5, 3726−3731

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ACS Sustainable Chemistry & Engineering Table 1. Relative Abundance of Lignin Interunit Linkages.3

abundance per 100 phenyl propane units linkage

β-O-4′ (%)

α-O-4′ (%)

β−β′ (%)

4-O-5′ (%)

dibenzodioxocin (%)

softwood hardwood

45−50 60−62

9−12 3−11

2−6 3−12

4−7 6−9

5−7 1−2

(polymethylhydrosiloxane) were reactive in the hydrogenolysis reaction of compound 1 (Table 2, entries 1−8).29 PMHS is an air- and water-stable nontoxic silane, which is very easy to handle. Moreover, it is cheap and generates as a byproduct in the silicon industry. This motivated us to use PMHS in the reductive cleavage of 1. By increasing the amount of PMHS to 8 equiv and elongating the reaction time to 16 h, full conversion was achieved (Table 2, entry 8). The effect of water was investigated. No conversion of the starting material was observed in the absence of water. To find the optimal reaction conditions, different ratios between EtOAc/H2O were tested (Figure 2). When the amount of water was increased to above 50 v%, unreproducible results were obtained. An optimal ratio between EtOAc/H2O of 4:1− 2:1 (v/v) for the hydrogenolysis reaction was found.30 The ratio EtOAc/H2O of 4:1 was chosen as the optimal ratio due to a higher homogeneity of the reaction mixture. Excess of water leads to poor solubility of the model compounds, which can be a reason for the lower yields. Introduction of an additional methoxy group in the ortho position of the aromatic ring of dibenzodioxocin (model 2) did not affect transformation. Full conversion was achieved under the same reaction conditions, and product 4 was isolated in 93% yield (Table 2). When a more advanced model 3 containing additional functional groups was subjected to the optimized reaction conditions, only 55% conversion was observed (Table 3, entry 1). An optimization of the reaction conditions for model compound 3 was performed. When EtOAc was exchanged for THF, 75% conversion was observed (Table 3, entry 2). However, full conversion could not be achieved even when the catalyst loading was increased or temperature was raised (Table 3, entries 3 and 4). Gratifyingly, the addition of a base (K2CO3 or NaOH) had a remarkable effect on the reaction, and compound 4 could be isolated in high yields (Table 3, entries 5−7).31 Since 2-methyltetrahydrofuran (Me-THF) is a sustainable solvent, which can be produced from biomass derived furfural32 and is less prone to peroxide formation than THF, the reaction was also performed in Me-THF, and product 4 was isolated in 94% yield (Table 3, entries 6 and 7). To study the reaction mechanism for the reductive cleavage of the eight-membered dibenzodioxocin compounds, it was decided to investigate the hydrogenolysis steps of both C−O bonds separately. Models 8 and 9 were synthesized

Figure 1. Synthesized dibenzodioxcin model compounds.

agent. At the same time, the reduction of dibenzodioxocin requires 2 equiv of hydride donor, and thus, environmentally friendly reducing agents are preferred for the reaction. With these procedures completed, an investigation of the Pd/C catalyzed hydrogenolysis of the C−O bonds in the dibenzodioxocin model compounds was initiated. Model compound 1 was subjected to Pd/C and NaBH4 in ethyl acetate and water. The reaction was completed within 2 h using 2 equiv of NaBH4 to generate products 4 and 5 in quantitative yields. Inspired by the good results, more suitable hydride donors were screened since NaBH4 has several drawbacks, such as safety, especially in water mixtures, toxicity, air and moisture sensitivity, and generating environmentally hazardous wastes. Silanes are known as efficient hydride donors in the reduction of C−O bonds.27,28 Therefore, silanes were evaluated as reducing agents for the hydrogenolysis of dibenzodioxocin model compounds (Table 2). Et3SiH was chosen in the initial studies, and the reactions were performed in an EtOAc/H2O mixture. Product 4 was obtained in 94% yield after 4 h (Table 2, entry 1). Evaluation of different silanes as hydride donors was conducted (Table 2). Interestingly, only Et3SiH and PMHS 3727

DOI: 10.1021/acssuschemeng.7b00428 ACS Sustainable Chem. Eng. 2017, 5, 3726−3731

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ACS Sustainable Chemistry & Engineering Table 2. Evaluation of Hydride Donors

entry

model

[H]

equiv

time (h)

yield of 4b and (5/6) (%)

1 2 3 4 5 6 7 8 9

1 1 1 1 1 1 1 1 2

Et3SiH (MeO)3SiH Ph3SiH (EtO)3SiH Ph3SiH PMHS PMHS PMHS PMHS

3 3 3 3 4 3 8 8 8

4 4 4 4 4 4 4 16 16

>95 (>95) traces traces traces 7 (7) 20 (20) 52 (52) >95 (>95) 93c (83c)

a Reaction conditions: model (0.03 mmol), Pd/C (5 mol %), EtOAc (0.2 mL), H2O (0.05 mL) at 80 °C for x h. bDetermined by 1H NMR with mesitylene as an internal standard; cIsolated yield (compound 6 has been isolated in 83% yield), but full conversion of the starting material has been achieved; yield of compound 5 was determined by 1H NMR with mesitylene as an internal standard and equal to the yield of compound 4.

O bond and thus facilitates the bond cleavage. Formation of such a hydrogen bond in the denzodioxocin structure has been proposed previously.33 It would be worthwhile to study the reactivity of the dibenzodioxocin motif in real lignin. However, such studies are difficult to carry out because of the significant changes in the lignin structure taking place during lignin isolation, such as a decrease in the ether bonds content and increase in the content of C−C bonds, even under mild organosolv pulping conditions.34 An alternative approach would be to mimic an early stage catalytic fractionation process.2,34,35 In such a fractionation process, lignin oligomers with native structure are released from the biomass matrix to rapidly react with a catalyst. The reaction has been performed with a ball milled spruce sample (Supporting Information), and comparison of 2D NMR spectra before and after the reaction clearly indicates significant decreasing of the signals intensity from main lignin interunit linkages. Nevertheless, due to the low intensity of the signals corresponding to the dibenzodioxocin structure, it is difficult to quantify the conversion of this motif. In order to do it, an experiment was designed where the reaction of dibenzodioxocin model compound 1 was performed in the presence of organosolv lignin isolated from pine using higher than natural proportions of the model (10% model in addition to 7% in lignin; 10 mg of model and 49 mg of organosolv lignin; Supporting Information). The reaction was run for 36 h at 80 °C in a THF/H2O = 4:1 mixture, and the reaction mixture was analyzed by NMR spectroscopy using the heteronuclear single quantum coherence (HSQC) experiment. According to the HSQC experiment,36 the cross signals corresponding to dibenzodioxocin model 1 have disappeared: α-CH (δH/δC 4.83/82.83 ppm) and β-CH2 (δH/δC 3.59; 4.35/75.31 ppm), and new cross signals corresponding to the product 4 have appeared: OMe (δH/δC 3.76/55.49 ppm), aromatic (δH/δC 6.50/122.79 and 6.71/110.50 ppm) (Figure 3). In addition to the observed cleavage of compound 1, the lignin was also transformed by hydrogenolysis reactions. Noteworthy, C−O bonds in benzylic alcohols and ethers

Figure 2. Evaluation of the optimal EtOAc/H2O ratio. Reaction conditions: model (0.03 mmol), Pd/C (5 mol %), EtOAc (0.2 mL), H2O (0.05 mL) at 80 °C for 4 h.

representing both possible intermediates (Table 4). Both 8 and 9 were separately subjected to the reaction conditions, and the efficiency of the hydrogenolysis was determined by measuring the initial conversions. Turnover frequencies (TOF) were calculated for the hydrogenolysis of models 8 and 9 as well as for dibenzodioxicin 1 (Table 4). The benzylic C−O bond in 9 was reductively cleaved at a significantly higher TOF than the terminal ether bond in 8, which is in accordance with BDE calculated for the dibenzodioxocin model compounds24 (Table 4, entries 2 and 3). The transformation of dibenzodioxicin 1 was slower than of model 8 and significantly slower than of model 9 (Table 4, entries 1−3). When reaction of 1 was monitored, intermediate 8 was detected in the reaction mixture by NMR spectroscopy, and no trace of model 9 was observed. Cleavage of the C−O bond in substrate 8 is a challenging reaction. For example, when the biphenyl moiety in compound 8 was exchanged to the corresponding guaiacyl moiety (compound 10), no reaction was observed under the employed reaction conditions (Table 4, entry 4).26 We propose that a protonation from the neighboring phenol participates in the weakening of the C− 3728

DOI: 10.1021/acssuschemeng.7b00428 ACS Sustainable Chem. Eng. 2017, 5, 3726−3731

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ACS Sustainable Chemistry & Engineering Table 3. Optimization of Reaction Conditions for the Reductive Cleavage of the Model 3

entry

PMHS (equiv)

time (h)

temp (°C)

solvent

additive (equiv)

Pd/C (mol %)

yield of 4b (%)

1 2 3 4 5 6 7

8 8 8 8 8 8 8

16 16 20 16 16 16 16

80 80 110 80 80 80 80

EtOAc/H2O THF/H2O THF/H2O THF/H2O THF/H2O Me-THF/H2O Me-THF/H2O

− − − − K2CO3 (1) K2CO3 (1.5) NaOH (1.5)

5 5 5 10 5 5 5

55 75 72 79 88c 94c 93c

Reaction conditions: model (0.1 mmol), Pd/C (5 mol %), solvent (0.5 mL), H2O (0.125 mL) at 80 °C for x h. bDetermined by 1H NMR with mesitylene as an internal standard; cIsolated yield. Compound 7 has been isolated in 88%yield. a

Table 4. TOF Numbers for Model Compounds 8−10 and Cyclic Model 1

entry

compound

TOF (h−1)

stand. deviation (h−1)

reaction time (min)

conversion (%)

1 2 3 4

1 8 9 10

30 99 1600 0

4.5 16 0 −

10 3 0.75 240

24 25 100 0

Reaction conditions: model (0.03 mmol), Pd/C (5 mol %), EtOAc (0.2 mL), H2O (0.05 mL) at 80 °C for 4 h. TOF = conversion × substrate/ catalyst/time (mol/mol/h).

a

Figure 3. 2D-NMR spectrum. Partial 13C−1H (HSQC) correlation spectra DMSO-d6 from (left) organosolv pine lignin and model compound 1 before the reaction and (right) same mixture after the reaction; p.p.m. parts per million.

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(7) Behling, R.; Valange, S.; Chatel, G. Heterogeneous catalytic oxidation for lignin valorization into valuable chemicals: what results? What limitations? What trends? Green Chem. 2016, 18, 1839−1854. (8) Mottweiler, J.; Puche, M.; Räuber, C.; Schmidt, T.; Concepción, P.; Corma, A.; Bolm, C. Copper- and Vanadium-Catalyzed Oxidative Cleavage of Lignin using Dioxygen. ChemSusChem 2015, 8, 2106− 2113. (9) Koyama, M. Hydrocracking of lignin-related model dimers. Bioresour. Technol. 1993, 44, 209−215. (10) Lahive, C. W.; Deuss, P. J.; Lancefield, C. S.; Sun, Z.; Cordes, D. B.; Young, C. M.; Tran, F.; Slawin, A. M. Z.; de Vries, J. G.; Kamer, P. C. J.; Westwood, N. J.; Barta, K. Advanced Model Compounds for Understanding Acid-Catalyzed Lignin Depolymerization: Identification of Renewable Aromatics and a Lignin-Derived Solvent. J. Am. Chem. Soc. 2016, 138, 8900−8911. (11) Alves, V.; Capanema, E.; Chen, C.-L.; Gratzl, J. Comparative studies on oxidation of lignin model compounds with hydrogen peroxide using Mn(IV)-Me3TACN and Mn(IV)-Me4DTNE as catalyst. J. Mol. Catal. A: Chem. 2003, 206, 37−51. (12) Yan, N.; Zhao, C.; Dyson, P. J.; Wang, C.; Liu, L.-t.; Kou, Y. Selective Degradation of Wood Lignin over Noble-Metal Catalysts in a Two-Step Process. ChemSusChem 2008, 1, 626−629. (13) Lancefield, C. S.; Westwood, N. J. The synthesis and analysis of advanced lignin model polymers. Green Chem. 2015, 17, 4980−4990. (14) Karhunen, P.; Rummakko, P.; Sipila, J.; Brunow, G.; Kilpeläinen, I. The Formation of dibenzodioxocin structures by oxidative coupling. A model reaction for linin biosynthesis. Tetrahedron Lett. 1995, 36, 4501−4504. (15) Karhunen, P.; Rummakko, P.; Pajunen, A.; Brunow, G. Synthesis and crystal structure determination of model compounds for the dibenzodioxocine structure occurring in wood lignins. J. Chem. Soc., Perkin Trans. 1 1996, 1, 2303−2308. (16) Argyropoulos, D.; Jurasek, L.; Kristofova, L.; Xia, Z.; Sun, Y.; Palus, E. Abundance and Reactivity of dibenzodioxocins in softwood lignin. J. Agric. Food Chem. 2002, 50, 658−666. (17) Pajunen, A.; Karhunen, P.; Brunow, G. 7-(4-Hydroxy-3methoxyphenyl)-6-hydroxymethyl-4,10-dimethoxy-2,8-dipropyl-cis6,7-dihydrodibenzo[b,d]oxepin-ll-ol. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1998, 54, 885−887. (18) Gardrat, C.; Ruggiero, R.; Rayez, M.; Rayez, J.; Castellan, A. Experimental and theoretical studies of the thermal degradation of a phenolic dibenzodioxocin lignin model. Wood Sci. Technol. 2013, 47, 27−41. (19) Gardrat, C.; Ruggiero, R.; Hoareau, W.; Damigo, L.; Nourmamode, A.; Grelier, S.; Castellan, A. Photochemical study of 4-(4,9-dimethoxy-2,11-n-dipropyl-6,7-dihydro-5,8-dioxa-dibenzo[a,c]cycloocten-6-yl)-2-methoxyphenol, a lignin model of phenolic dibenzodioxocin unit. J. Photochem. Photobiol., A 2005, 169, 261−269. (20) Machadoa, A.; De Paula, R.; Ruggiero, R.; Gardrat, C.; Castellan, A. Photophysics of dibenzodioxocins. J. Photochem. Photobiol., A 2006, 180, 165−174. (21) Gardrat, C.; Ruggiero, R.; Hoareau, W.; Nourmamode, A.; Grelier, S.; Siegmund, B.; Castellan, A. Photochemical study of an oethyl dibenzodioxocin molecule as a model for the photodegradation of non-phenolic lignin units of lignocellulosics. J. Photochem. Photobiol., A 2004, 167, 111−120. (22) Akim, L.; Colodette, J.; Argyropoulos, D. Factors limiting oxygen delignification of kraft pulp. Can. J. Chem. 2001, 79, 201−210. (23) Argyropoulos, D. Salient reactions in lignin during pulping and oxygen bleaching: an overview. J. Pulp. Pap. Sci. 2003, 29, 308−313. (24) Elder, T. Bond dissociation enthalpies of a dibenzodioxocin lignin model compound. Energy Fuels 2013, 27, 4785−4790. (25) Galkin, M. V.; Sawadjoon, S.; Rohde, V.; Dawange, M.; Samec, J. S. M. Mild Heterogeneous Palladium-Catalyzed Cleavage of β-O-4′Ether Linkages of Lignin Model Compounds and Native Lignin in Air. ChemCatChem 2014, 6, 179−184. (26) Galkin, M. V.; Dahlstrand, C.; Samec, J. S. M. Mild and Robust Redox-Neutral Pd/C-Catalyzed Lignol β-O-4′ Bond Cleavage

have been reductively cleaved where the ratio between alcohols and ethers (integral of 225 in starting material) has decreased to 69, whereas the aliphatic region has increased from 68 to 271 during the reaction (Figure 3). This is consistent with that nearly all benzyl alcohols and ether bonds (except aryl−aryl ethers) have been reductively cleaved during the reaction.

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CONCLUSIONS Novel models representing the dibenzodioxocin motif in lignin have been synthesized, and a protocol for a mild hydrogenolysis of C−O bonds has been established. High conversions could be achieved by Pd/C and hydride donors such as NaBH4, Et3SiH, and PMHS using mild reaction conditions and green solvents (Me-THF and EtOAc).37 Kinetic experiments show that the benzylic C−O bond cleaves much faster than the terminal ether bond. In addition, the corresponding intermediate was observed in the reaction mixture when a model of dibenzodioxocin was transformed. Performance of the reaction of a model compound in the presence of lignin showed applicability of the developed method to biomass valorization.

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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/acssuschemeng.7b00428. Experimental details and full characterization of all new compounds. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Joseph S. M. Samec: 0000-0001-8735-5397 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Swedish Energy Agency for financial support.

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