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Effect of Benzyl Functionality on Microwave-Assisted Cleavage of C-C Bonds in Lignin Model Compounds #

#

Guodian Zhu, Meilu Lin, Di Fan, Wenshan Ning, Xinping Ouyang, Yong Qian, and Xueqing Qiu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12056 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 6, 2017

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The Journal of Physical Chemistry

Effect of Benzyl Functionality on Microwave-Assisted Cleavage of Cα-Cβ Bonds in Lignin Model Compounds Guodian Zhu,a Meilu Lin,a Di Fan,b Wenshan Ning,c Xinping Ouyang,*,a Yong Qiana and Xueqing Qiu*,a,d

a

School of Chemistry & Chemical Engineering, South China University of

Technology, Guangzhou 510640, China b

College of Food Science, South China Agricultural University, Guangzhou 510642,

China c

School of Chemistry & Environment, South China Normal University, Guangzhou

510006, China d

State Key Laboratory of Pulp & Paper Engineering, South China University of

Technology, Guangzhou 510640, China

Abstract:

The

effect

of

benzylic

alcohol

(Cα-OH)

pretreatments

on

microwave-assisted cleavage of Cα-Cβ bonds in lignin model compounds was investigated by combining experimental results with density functional theory (DFT) calculations at M06-2X/6-311++G(d,p) level. The oxidization of Cα-OH to benzylic ketone (Cα=O) facilitated the cleavage of Cα-Cβ bond due to a greater decomposition rate constant and a lower barrier for decomposition-determining step. The reduction

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of Cα-OH to benzylic hydrogen (Cα-H) resulted in that no Cα-Cβ bond was cleaved. Since the Cα-OH quickly reacted with methanol solvent, the etherification of Cα-OH to benzylic ether (Cα-OCH3) had no effect on the Cα-Cβ bond cleavage. The aromatic aldehyde would be obtained from the model compounds containing Cα-OH or Cα-OCH3, while the model compound containing Cα=O would produce aromatic ester. The research also revealed that the microwave-assisted cleavage of Cα-Cβ bonds followed a first-order kinetic and radical reaction mechanism.


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INTRODUCTION The small aromatic molecules are the important raw materials in the chemical industry. The nonrenewable fossil resources are consumed rapidly, and lignin that contributes 15-30% by weight and 40% by energy in lignocellulosic biomass can be depolymerized into small aromatic molecules used for the further preparation of advanced biofuels.1,2 Hence, lignin is considered to be a potential renewable feedstock for the production of liquid biofuels and small aromatic molecules.3,4 However, a heterogeneous and branched three-dimensional alkyl-aromatic structure of lignin results in that efficient utilization of lignin has received less attention, and that the conversion of lignin into small aromatic molecules is still a challenge.1,5 Although lignin has a complicated structure, it is mainly composed of three main lignin precursors (p-hydroxyphenyl, guaiacyl and syringyl units) linked via C-O or C-C bonds formed by radical coupling reactions.6,7 The C-O bond is vulnerable and the β-O-4 bond is predominant linkage in lignin, so the cleavage of β-O-4 bond is usually considered as the crux of lignin depolymerization. The β-O-4 lignin model compounds are usually used to represent the most common substructures of lignin to discuss the reaction behaviour of lignin depolymerization. Many fundamental studies focused on the cleavage of β-O-4 bond using β-O-4 lignin model compounds.8-12 In fact, it is also a good choice to depolymerize lignin to produce small aromatic molecules through the cleavage of phenyl side-chain C-C bonds, such as C1-Cα and Cα-Cβ bonds. Especially, the most vulnerable phenyl side-chain C-C bond is Cα-Cβ bond.13 Therefore, the cleavage of Cα-Cβ bonds in β-O-4 lignin model



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compounds has also received more attentions. The Cα-Cβ bonds in β-O-4 lignin model compounds would be catalytically cleaved with HNO3 and NaNO3, affording 35% of veratraldehyde and 6% of 4-nitroveratrole.14 The cleavage of Cα-Cβ bonds in lignin model compounds also occurred with Baeyer-Villiger oxidation.15 To more effectively cleave the Cα-Cβ bond, many groups focused on transition-metal ligand catalysts, such as various copper-based catalysts,16 iron-based catalyst Fe(TAML)Li17 and vanadium-based catalyst VO(acac)2.18 However, it is necessary to overcome more difficulties on the application of lignin depolymerization with such transition-metal ligand catalysts. Microwave irradiation heating is increasingly used to accelerate the cleavage of linkages in lignin19,20 and considered as a potential method for the conversion of lignin into small aromatic molecules. The lignin depolymerization under microwave irradiation has also received more attentions.3,21,22 We have demonstrated that microwave irradiation vastly facilitated the cleavage of Cα-Cβ bond (Figure S1). To facilitate the cleavage of β-O-4 bond, many of the most promising pretreatments of Cα-OH were developed. When the Cα-OH was oxidized to Cα=O, the cleavage of β-O-4 bond was improved in aqueous formic acid23 and photocatalytic system.24 In hydrogenolysis system, the reduction of Cα-OH to Cα-H would increase the cleavage reactivity of β-O-4 bond,25 and the etherification of Cα-OH to Cα-OCH3 was also favorable to cleave β-O-4 bond.26 Additionally, Lohr et al.27 reported that the acetylation of Cα,γ-OH to Cα,γ-OOCCH3 would promote more selective cleavage of β-O-4 bond. The acetyl bromide used in acetylation of Cα,γ-OH is hazardous.


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Therefore, fundamental understanding of how the oxidization, reduction and etherification of Cα-OH control the cleavage of Cα-Cβ bond is crucial to further develop better catalyst or pretreatment methods for producing small aromatic molecules from lignin. In the present work, the effect of Cα-OH, Cα=O, Cα-H and Cα-OCH3 on microwave-assisted cleavage of Cα-Cβ bonds in lignin model compounds was investigated with ferric sulfate as a catalyst. The density functional theory (DFT) calculations were widely used to investigate the cleavage mechanisms of various linkages in lignin.28-32 We also sought to understand the fundamental mechanisms of microwave-assisted cleavage of Cα-Cβ bonds combining with DFT calculations. According to the experimental results and DFT calculations, the mechanisms of microwave-assisted cleavage of Cα-Cβ bonds were proposed.

EXPERIMENTAL Materials. Methanol and ferric sulfate were purchased from Aladdin Chemistry Co. Ltd (China). Guaiacol, 4-methoxyl acetophenone, 4-methoxyl benzaldehyde, 4-methoxyl

benzoic

acid,

4-methoxyl

benzoic

acid

methyl

ester

and

1-(dimethoxymethyl)-4-methoxybenzene were purchased from Alfa Aesar Co. Inc. (China). All chemicals were used without further purification. Lignin model compounds a-d were prepared according to our previous methods,26 and the structures of all model compounds were identified by 1H NMR, 13C NMR and MS. 4′-methoxyl-2-(2′-methoxyphenoxy)-1-phenylethanol (compound a). 1H NMR (400



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MHz, DMSO-d6): 7.37 (d, J = 8.3 Hz, 2H, aromatics), 6.83-6.97 (m, 6H, aromatics), 5.48 (d, J = 4.6 Hz, 1H, α-OH), 4.85-4.89 (dd, J = 11.1, 4.8 Hz, 1H, α-CH), 3.91-3.98 (m, 2H, β-CH2), 3.75 (s, 6H, OCH3).

13

C NMR (400 MHz, DMSO-d6): 158.50,

149.12, 148.19, 134.55, 127.55, 121.02, 120.78, 113.78, 113.39, 112.50, 74.06, 70.49, 55.56, 55.01. MS (EI): Found for C16H18O4: m/z 274.05. 4′-methoxyl-2-(2′-methoxyphenoxy)-1-acetophenone (compound b). 1H NMR (400 MHz, CDCl3): 8.05 (d, J = 8.9 Hz, 2H, aromatics), 6.91-7.02 (m, 4H, aromatics), 6.87 (d, J = 3.6 Hz, 2H, aromatics), 5.30 (s, 2H, β-CH2), 3.91 (d, J = 2.7 Hz, 6H, OCH3). 13

C NMR (400 MHz, CDCl3): 193.17, 163.97, 149.78, 147.67, 130.53, 127.76, 122.35,

120.82, 114.79, 113.96, 112.23, 72.03, 55.94, 55.51. MS (EI): Found for C16H16O4, m/z 272.05. 4′-methoxyl-2-(2′-methoxyphenoxy)-1-phenylethane (compound c). 1H NMR (400 MHz, DMSO-d6): 7.25 (d, J = 8.0 Hz, 2H, aromatics), 6.96 (d, J = 7.3 Hz, 2H, aromatics), 6.83-6.92 (m, 4H, aromatics), 4.11 (t, J = 7.0 Hz, 2H, β-CH2), 3.74 (d, J = 5.9 Hz, 6H, OCH3), 2.97 (t, J = 7.0 Hz, 2H, α-CH2). 13C NMR (400 MHz, DMSO-d6): 157.80, 149.08, 148.03, 130.13, 129.92, 120.90, 120.76, 113.70, 113.44 112.42, 69.16, 55.56, 54.97, 34.19. MS (EI): Found for C16H18O3, m/z 258.10. 1-methoxyl-4′-methoxyl-2-(2′-methoxyphenoxy)-phenylethane (compound d). 1H NMR (400 MHz, DMSO-d6): 7.31 (d, J=7.9 Hz, 2H), 7.07-6.81 (m, 3H), 4.60 (d, J=7.7 Hz, 1H), 4.19 (t, J=9.3, 1H), 4.01 (d, J=10.6 Hz, 1H), 3.83 (d, J=12.6 Hz, 3H), 3.33 (s, 3H). 13C NMR (400 MHz, CDCl3-d): 159.55, 149.88, 148.47, 130.65, 128.26, 121.64, 120.84, 114.61, 113.97, 112.19, 81.80, 77.36, 77.04, 76.72, 73.89, 56.95,


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55.98, 55.30. MS (EI): Found for C17H20O4, m/z 288.10. Decomposition of model compounds. 0.10 mmol of lignin model compound a (b, c or d), 0.013 mmol of ferric sulfate, 9.5 mL of methanol and 0.5 mL of water were added into the 100 mL vessel of microwave reactor (Ethos One, Milestone Inc., Italy). The decomposition was carried out at 140 or 160 °C for 2-60 min under microwave irradiation with an initial power of 400 W. The controlled experiments without microwave irradiation were carried out in stainless steel autoclave (Beijing Century SenLong Experimental Apparetus Co., Ltd., China). 0.10 mmol of lignin model compound a, 0.013 mmol of ferric sulfate, 9.5 mL of methanol and 0.5 mL of water were added into the autoclave vessel. The reaction was carried out at 160 °C for 10 (or 60) min. After reaction, the reaction vessel was quickly immersed in ice water to room temperature. Then the mixture was filtered by 0.45 µm filter membrane, and the filtrate was used to analyze products by gas chromatography-mass spectrometry (GC-MS, GCMS-QP2010, Shimadzu Co., Japan) and high performance liquid chromatography (HPLC, LC-20A, Shimadzu Co., Japan). Analysis of products. The products were identified by comparing their mass spectra in GC-MS with National Institute of Standards and Technology library in the instrument, and comparing their retention time in GC-MS and HPLC spectra with those of authentic compounds. The products were quantified using HPLC equipped with UV detector (at 280 nm) and Zorbax SB-C18 reverse-phase column (4.6×250 mm, 5.0 µm). The column temperature was set at 40 °C. The mobile phase consisted



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of HPLC grade degassed water with 0.1% formic acid and methanol. The separation was performed by gradient elution at a total flow rate of 0.8 mL/min. The conversion of model compounds and yields of products were calculated by: % = / × 100%

(1)

where ni and nm were moles of product i and initially added compound (0.10 mmol), respectively. Calculation methods. All calculations were performed using the Gaussian 09 program.33 The free energy and enthalpy were calculated according to the published procedures.30 All subsequent optimizations were performed at the M06-2X/6-31G(d) level of DFT, followed by a calculation with the larger 6-311++G(d,p)

basis

sets.

All calculations were

performed using a

conductor-like polarizable continuum model (CPCM) methanol solvation.

RESULTS AND DISCUSSION Effect of benzyl functionality on decomposition. The effect of Cα-OH, Cα=O, Cα-H and Cα-OCH3 on bond length of Cα-Cβ bond, bond dissociation enthalpies (BDE) of Cα-Cβ bond and dipole moment of compound is summarized in Table 1. DFT calculations showed that the benzylic functional groups had little influence on bond length of Cα-Cβ bond. The BDE of Cα-Cβ bond in compound a containing Cα-OH was 75.95 kcal/mol, while it increased to 80.18 kcal/mol when the Cα-OH was oxidized to Cα=O (compound b). Both reduction and etherification of Cα-OH had a mild effect on the BDE of Cα-Cβ bonds (compounds c and d). Previously, we found that it was


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difficult to cleave the β-O-4 and Cα-Cβ bonds only through thermal decomposition under a relatively low temperature (below 230 °C).34 It indicated that the oxidization, reduction and etherification of Cα-OH were still difficult to cleave the chemical bonds in lignin model compounds under a mild thermal condition. However, an obvious effect on dipole moment was found when the Cα-OH was pretreated to Cα=O or Cα-H. It implied that the oxidization and reduction of Cα-OH might affect the decomposition of lignin model compounds into aromatic monomers under microwave irradiation, and the decomposition of compounds a, b, c and d are shown in Scheme 1.

Table 1. Effect of benzyl functionality on bond length of Cα-Cβ bond, BDE of Cα-Cβ bond and dipole moment of compound. Lignin model compounds

Benzylic group

Bond length BDE Dipole moment (pm) (kcal/mol) (Debye)

Cα-OH (a)

152.1

75.95

5.241

Cα=O (b)

151.8

80.18

6.240

Cα-H (c)

152.0

76.88

4.245

151.6

73.92

5.378

Cα-OCH3 (d) M06-2X/6-311++G(d,p)//M06-2X/6-31G(d) with CPCM methanol solvation



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O HO

O OH O O

O

O

O

d

O

a

+

O

1

O

O

2

+

HO O

O

4

O

1

O

2

O

O

O

O

5

5

O

+

O

O

O

O

3

b

O

OH O

O

+

+

O

+

O

O

6

H O O

No reaction

O

c

HO

+

O O

O O

O

d

O

1

O

2

OH O O

3

+

+

O

O

O O

O

4

5

Scheme 1. The decomposition of compounds a, b, c and d under microwave irradiation; Condition: 0.10 mmol of compound a (c, b or d), 9.5 mL of methanol, 0.5 mL of water and 0.013 mmol of ferric sulfate, 160 °C and 30 min.

When compounds a, b, c or d was subjected to the decomposition system with ferric sulfate as a catalyst at 160 °C for 30 min under microwave irradiation, it would be found that the benzyl functional groups had a considerable effect on decomposition. The decomposition of compound a yielded guaiacol (1), 4-methoxyl acetophenone (2), 4-methoxyl benzaldehyde (3), 4-methoxyl benzoic acid (4), 4-methoxyl benzoic acid methyl ester (5) and d. And the same aromatic monomer products would be obtained from compound d whose Cα-OH was etherified to Cα-OCH3. When the Cα-OH was


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oxidized to Cα=O, the decomposition of compound b was also found, affording monomer products 1, 2, 5 and 1-(dimethoxymethyl)-4-methoxybenzene (6). Surprisingly, no compound c whose Cα-OH was reduced to Cα-H was decomposed. Therefore, the pretreatments of Cα-OH significantly affected the decomposition and products distribution. The decomposition of compounds a, b and d would produce product 1, and the yield of product 1 was equivalent to the total yield of other corresponding aromatic monomer products. Hence, the yield of product 1 would be an indicator to monitor the decomposition. Figure 1 summaries the yield of product 1 from compounds a, b and d under the different reaction conditions.

40 Yield of product 1 (%)

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


30

Compound a Compound b Compound d

20

10

0

140 ℃ , 15 min

160 ℃ , 5 min

160 ℃ , 15 min

Figure 1. Decomposition reactivities of compounds a, b and d under microwave irradiation; Condition: 0.10 mmol of compound a (b or d), 9.5 mL of methanol, 0.5 mL of water and 0.013 mmol of ferric sulfate.



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As shown in Figure 1, treatments of compounds a, b and d at 140 °C for 15 min afforded product 1 in 7.5%, 9.2% and 7.8%, respectively. When the decomposition of compounds a, b and d was carried out at 160 °C for 5 or 15 min, compound b also yielded the highest yield of product 1. It indicated that the oxidization of Cα-OH would improve the decomposition. However, a similar yield of product 1 was obtained from compounds a and d under the same decomposition conditions. Microwave heating is generally considered to be responsible for dipole momentum and ionic polarization,35 so it is important for permanent dipole moment of reactant to absorb microwave energy under the same reaction condition. Although the dipole moment of compound b increased by approximate 19% (from 5.241 to 6.240 Debye, Table 1), no significant improvement of decomposition reactivity was found (Scheme 1). And no compound c was decomposed, though its dipole moment decreased by approximate 19% (from 5.241 to 4.245 Debye, Table 1). Hence, it was inclined to believe that a difference of decomposition resulted from the difference of conversion rout. Microwave heating would resulted in a different reaction rout,36 so the difference of decomposition would be considered to be caused by the microwave irradiation. Therefore, it was more important to address the conversion routs of compounds a, b and d under microwave irradiation.

Effect of reaction time on decomposition. To further explore how the Cα-OH, Cα=O and Cα-OCH3 affected the decomposition under microwave irradiation, the effect of reaction time on decomposition of compounds a and b at 160 °C was first


Page 13 of 30

investigated. The results are shown in Figure 2 and 3, respectively.

100

1 3 5

80

2 4 d

60 Molar yield /%

Molar 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 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


40 20

4 3 2 1 0 0

10 20 30 40 50 60 Reaction time /min

0 0

10

20

30

40

50

60

Reaction time /min Figure 2. Variation of yields of products for the decomposition of compound a over time at 160 °C. Condition: 0.10 mmol compound a, 0.013 mmol ferric sulfate, 9.5 mL methanol, 0.5 mL water and microwave irradiation.

The decomposition of compound a at 160 °C for 2 min yielded 1.5% of product 1, 3.2% of 3 and 95.5% of d as a main product. It indicated that the etherification of Cα-OH to Cα-OCH3 (product d) needed less than 2 min, and that only a small amount of compound a was decomposed into aromatic monomer products in initial stage of reaction. The yields of aromatic monomer products 1, 3, 4 and 5 increased and yield of product d decreased with the elongation of reaction time to 60 min from 2 min. Therefore, the Cα-OH of compound a first quickly reacted with methanol solvent to form etherified product d, and a further decomposition of product d occurred to produce aromatic monomer products. The Cα-OH of compound a was quickly



etherified to Cα-OCH3 with methanol to form product d, which resulted in that a similar yield was obtained from compounds a and d under the same decomposition condition (Figure 1). Actually, when ethanol or isopropanol was used as reaction solvent, it would be found that the etherification also occurred betwween Cα-OH and ethanol or isopropanol, followed by a further decomposition of the corresponding etherified products into aromatic monomer products. Therefore, the etherified product was the intermediate in alcohol solvents.

100

1 5 b

80

2 6

60 Molar yield /%

Molar 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 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

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40 20

5 4 3 2 1 0 0

10 20 30 40 50 60 Reaction time /min

0 0

10

20

30

40

50

60

Reaction time /min Figure 3. Variation of yields of products for the decomposition of compound b over time at 160 °C. Condition: 0.10 mmol compound b, 0.013 mmol ferric sulfate, 9.5 mL methanol, 0.5 mL water and microwave irradiation.

Different from compound a, the decomposition of compound b yielded products 1, 2, 5 and 6, especially for products 1 and 5 as the main aromatic monomer products. With the elongation of reaction time, the yield of compound b decreased, and yields


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of products 1, 5 and 6 increased. Therefore, compound b was directly decomposed into aromatic monomer products. Notably, compound a containing Cα-OH mainly yielded aromatic aldehyde and guaiacol (Figure 2), while aromatic ester and guaiacol were mainly produced from compound b containing Cα=O (Figure 3). It implied that aromatic aldehyde or ester would be selectively produced from lignin with different benzylic functional groups. Compound a was quickly etherified to product d (less than 2 min, Figure 2), and the complete decomposition of product d needed about 60 min. Hence, the decomposition of product d, a decomposition-determining reaction, was approximate to that of compound a. The fitting of compound b and product d was examined by each linear plot of ln(yield of b (or d)) against reaction time at 160 °C (Figure 4).

5

ln(yield of b (or d))

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


4 3 Compound b:

2

2

kb = 0.0729/min; R = 0.9914

1

Product d: 2

ka = 0.0627/min; R = 0.9880

0

10

20

30

Reaction time /min

40

50

Figure 4. First-order kinetic models of the decomposition of compound b and product d at 160 °C under microwave irradiation.



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The correlation coefficients R2 and rate constants k were obtained from first-order kinetic models. Figure 4 showed the plots of linearized form of the first-order models at 160 °C, with high value of the correlation coefficient R2 (0.9914 and 0.9880). It demonstrated that the decomposition of compound b and product d into aromatic monomer products followed a first-order kinetic mechanism. The decomposition rate constants of compound b and product d were 0.0729/min and 0.0627/min, respectively, which preferably illustrated why the oxidization of Cα-OH would facilitate the decomposition of compound b (Figure 1). It is well known that the β-O-4 bond is more vulnerable to be cleaved than Cα-Cβ bond. The reaction processes of the cleavage of β-O-4 and Cα-Cβ bonds in compounds d and b are shown in Scheme 2.

OH O O

O

Fast

O

O

d

O O O

b

Cα -C

O

a

O

d cle 4 bon β -O-

O

β

b on -O-4

Cα -C

β

d c le

bond c

e ava g

β

+

HO

e

+

HO

O O

leava ge

O

1

7 +

HO O

O O

1

3

O O

O

le ava ge

bond c

avag

2

1 +

HO

O

O O

O

1

5

Scheme 2. The reaction processes of β-O-4 and Cα-Cβ bonds cleavage.

To address the cleavage routes of Cα-Cβ bonds in compounds b and d, compounds 1-methoxy-4-(1-methoxyethyl)-benzene (7) and 2 were subjected to the same decomposition system, and the results are shown in Scheme 3. Scheme 3 showed that


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compound 7 was converted to 4-methoxystyrene (8) rather than product 3, which indicated that an elimination occurred between Cα-OCH3 and Cβ-H in compound 7 to form Cα=Cβ bond, and that the decomposition of product d happened through the Cα-Cβ bond cleavage. And none of products 3-5 were detected with product 2 as a reactant and products 3-5 were also stable under the same reaction condition. Hence, product 2 was believed to arise from the cleavage of β-O-4 bond. Notably, the yield of product 2 reached a plateau in about 5 min (Figure 2 and 3), which indicated that only a small amount of β-O-4 bonds was cleaved in the whole decomposition of compounds a and b. Therefore, it demonstrated that products 3 and 5 were derived from the corresponding cleavage of Cα-Cβ bonds rather than β-O-4 bonds.

Microwave, Fe2(SO4)3

O

7

O

O

8

O

O

2

O

3 OH

O

O

Microwave, Fe2(SO4)3

No reaction

O O

4

O

5

O

Scheme 3. The conversion of compounds 2-5 and 7.

Mechanisms of microwave-assisted cleavage of Cα-Cβ bonds. Based on the experimental results and DFT calculations, a mechanism with the free energy and enthalpy profile was proposed for microwave-assisted cleavage of Cα-Cβ bond in compound a (Figure 5).34



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H

H

A

H OH O

O

O CH3 O

O

O

O

TSa1

O

O

a1

a

+ H

O

+ CH3 OH

O O

O

+O

H + H O

+H+

Page 18 of 30

O

O

a2

－ H2O

H O H H O

－H+

O O

O

O

+ H2O

O

O

TSa2 O

O

a4

O

O

O

a3

O

d

－ CH3 OH, H2O

+ OH O

OH OH O

Cα -Cβ bond cleavage

OH

O

O

O

-H

a6

O

3

4

HO O

a5

O HO

+H

O

HO O

O

O O

a7

- HCHO

O

1

5

TSa2

B

a4

50

TSa1 33.08 (12.80)

0

a

d 8.57 (7.42)

0 (0)

70.58 (59.28)

49.93 (62.67)

∆G (∆H) [kcal/mol]

a3 5.26 (17.26)

a6+a7

-50

a1 -100

-150

a5

-95.02 (-103.69)

a2

-108.49 (-119.65)

-47.05 (-42.69)

1+3 -112.88 (-98.40)

-144.38 (-154.22) M06-2X/6-311++G(d,p)//M06-2X/6-31G(d) with CPCM methanol solvation

Reaction coordinate

Figure 5. Proposed mechanism (A) and reaction free energy and enthalpy profile (B) of microwave-assisted cleavage of Cα-Cβ bond in compound a.

In the proposed mechanism, an etherification of Cα-OH with methanol solvent quickly (less than 2 min, Figure 2) occurred to form product d. Compound a was protonated at the Cα-OH (a1) followed by the formation of product a2. Geometric optimizations aimed at a transition state (TS) of the formation from product a1 to a2.


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Actually, the etherification process was an acid-catalyzed reaction, and the acidic environment would promote the etherification.26 Subsequently, an elimination occurred between Cα-OCH3 and Cβ-H of product d to form alkenyl product a3. The complete decomposition of product d needed about 60 min (Figure 2), while only decomposing the same amount (0.10 mmol) of compound a3 into products 1 and 3 needed 5 min at 160 °C. Therefore, it suggested that the formation of alkenyl product occurred in a slow reaction step, and that the further decomposition of alkenyl product a3 was extremely fast. Due to the slow reaction from product d to a3, the corresponding TS was also calculated. After the compound a was decomposed, the pH of mixture was 1.6. And when pH at 1.3 of H2SO4 aqueous solution was added, product d was mainly obtained. Hence, a TSa2 of the decomposition-determining process from product d to a3 was given in Figure 5. And intrinsic reaction coordinate (IRC) calculation starting from TSa2 led to an alkenyl product a3, water and methanol. However, when 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), a common radical scavenger, was added to the decomposition system, compound a was mainly converted to product d, together with little product a3. It indicated that the radical scavenger hindered the elimination of product d. And only litter product d was obtained from compound a3 in the presence of TEMPO, which also showed that the radical scavenger also prevented product a3 decomposing into aromatic monomer products. The Cα=Cβ bond might be susceptible to a 7% lengthening in the free radical-rich environment,37 and stretching a bond beyond its 7% length range would result in the homolytic cleavage of this bond.38 Therefore, product a4 would be



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Page 20 of 30

produced from product a3 in the radical decomposition system, which resulted in that the further cleavage of Cα-Cβ bond in product a3 followed a radical reaction mechanism. For compound b containing Cα=O, a mechanism with the free energy and enthalpy profile of microwave-assisted its Cα-Cβ bond cleavage is also proposed in Figure 6.

O

A

H O

O

OH O

O

O

OH O

O

TSb O

O

b

O

O

b1

O

O

b2 + OH

+ CH3OH

O O

+ CH3OH

O O

O

O

OH

b7

- H2O

OH

b6

HO

Cα -Cβ bond cleavage

O

HO

OH

- H2O

OH

O

b5

O

b3

+H －H

O O O

6

5

B 0

b 0 (0)

45.26 (32.69)

+H

O

HO O

O

b4

O

－ HCHO

1

b2

TSb

50

HO

O

O

b1

∆G (∆H)

59.38 (61.33)

[kcal/mol]

10.38 (8.60)

b4+b5

-50

b3

-100

b4+b6

-36.96 -40.23 (-42.88) (-47.75)

-100.19 (-122.24)

b4+b7

-42.88 (-51.88)

1+5

-150

-115.53 (-113.35)

-200

1+6 M06-2X/6-311++G(d,p)//M06-2X/6-31G(d) with CPCM methanol solvation

Reaction coordinate

-214.67 (-227.80)

Figure 6. Proposed mechanism (A) and reaction free energy and enthalpy profile (B) of microwave-assisted cleavage Cα-Cβ bond in compound b.


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In the proposed mechanism, an enolate isomerization of compound b occurred to form product b1, and the corresponding TS of the isomerization from keto (b) to enol (b1) was calculated.39,40 Both the elimination of product d (Figure 5) and enolate isomerization of compound b (Figure 6) were slow reactions, but the TS calculation results showed that a barrier of the enolate isomerization of compound a was lower than that of the elimination of product d. It also preferably illustrated that the oxidization of Cα-OH would facilitate the decomposition of compound b (Figure 1). According to the decomposition mechanism of compound a (Figure 5), it would also be found that both the etherification of Cα-OH (a to d) and the addition of Cα=Cβ bond (a3 to a5) were fast reactions. The conversion of compound b was zero when TEMPO was added to the decomposition system, which demonstrated that the cleavage of Cα-Cβ bond in compound b also followed a radical reaction mechanism. Additionally, it also implied that an addition reaction of Cα=Cβ bond was easier to occur than the etherification of Cα-OH, which resulted in that no product b21 (Figure S2) was produced in the presence of TEMPO. Hence, the mechanism of Figure 6 was proposed, instead of another mechanism that an etherification of Cα-OH occurred first, followed by an addition of Cα=Cβ bond (Figure S2). The product b5 contained two Cα-OH, so the etherification might happen at the two Cα-OH and afforded product 6. Seemingly, the theoretical results (∆G = -214.67 kcal/mol, ∆H = -227.80 kcal/mol) implied that compound b was more inclined to yield product 6. However, the experimental results showed that product 5 was mainly produced from compound b, suggestting that a hydrogen free radical of product b6 was easier to be removed to



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Page 22 of 30

form product 5. Moreover, the cleavage of Cα-Cβ bond in compound d would be considered as a part of decomposition of compound a, so a mechanism of microwave-assisted cleavage of Cα-Cβ bond in compound d is shown in Figure S3. According to the proposed radical mechanisms of Cα-Cβ bonds cleavage (Figure 5, 6 and S3), it suggested that compound c with no Cα=Cβ bond was difficult to react with the hydroxyl free radical, which resulted in that no compound c was decomposed (Scheme 1).

CONCLUSIONS The oxidization of Cα-OH to Cα=O facilitated the cleavage of Cα-Cβ bond, while the etherification of Cα-OH to Cα-OCH3 had no effect on the cleavage reactivity of Cα-Cβ bond. No Cα-Cβ bond was cleaved for the reduction of Cα-OH. The microwave-assisted cleavage of Cα-Cβ bonds followed a first-order kinetic and radical reaction mechanism. This work implied that aromatic aldehyde or ester would be selectively produced from lignin with different benzylic functional groups under microwave irradiation.


Page 23 of 30

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ACKNOWLEDGMENTS This work is financially supported by the National Basic Research Program of China [No. 2012CB215302]; and the National Natural Science Fundation of China [No. 21576104].

ASSOCIATED CONTENT Supporting Information Supporting information includes the supplementary figures and data set information.

AUTHOR INFORMATION Corresponding authors *Tel.: +86-20-87114722, Fax: +86-20-87114721. Email: [email protected] (Xinping Ouyang). *Tel.: +86-20-87114722, Fax: +86-20-87114721. Email: [email protected] (Xueqing Qiu). Notes The authors declare no competing financial interest.



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