Radical synthesis of tetrameric lignin model compound

Article abstract of DOI:10.1016/j.cclet.2015.05.011

Abstract The lack of suitable lignin model compound limits the understanding of the characteristics of lignin, and hence hinders the efficient utilization of this kind of bioresource. A tetramer phenolic lignin model compound composed of 5-5, α-O-4 and β-

Full text of DOI:10.1016/j.cclet.2015.05.011

G Model
CCLET 3315 1–3
Chinese Chemical Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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Radical synthesis of tetrameric lignin model compound
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Q1 Xin-Ping Ouyang, Yun Yang, Guo-Dian Zhu, Xue-Qing Qiu
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School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The lack of suitable lignin model compound limits the understanding of the characteristics of lignin, and
hence hinders the efficient utilization of this kind of bioresource. A tetramer phenolic lignin model
Received 12 March 2015
Received in revised form 15 April 2015
Accepted 20 April 2015
Available online xxx
compound composed of 5-5, a-O-4 and b-5 linkages was prepared by a two-step of free radical reaction
with hydrogen peroxide/horseradish peroxidase and S2O82ꢀ/Fe2+ as the initiator. Compared with
enzymatic process, this synthetic process gives a higher yield of 33.8% within a shorter time. HRMS and
13C NMR spectroscopy show that synthesized model compound contains phenylpropane structure
Keywords:
linked by 5-5,
a-O-4 and b-5 bonds, which can mimic some chemical characteristics of lignin.
Lignin
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Model compound
Tetramer
Published by Elsevier B.V. All rights reserved.
Radical synthesis
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1. Introduction
2. Experimental
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Lignin is the most abundant biopolymer containing aromatic
compound, which is consist of phenylpropane linked by -O-4,
O-4, -5, 5-5, 4-O-5 and bonds [1]. Increasing attentions have
2.1. Materials and methods
b
a-
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b
b-
b
Isoeugenol, ferrous sulfate heptahydrate, potassium persulfate 35
and acetone were purchased from Aladdin Co., China. All reagents 36
in this work were of analytical grade. High resolution mass 37
spectrum (HRMS) measurement was carried out on a maXis impact 38
(Bruker, Germany). 1H NMR and 13C NMR spectra were recorded 39
with a Bruker DRX-400 spectrometer (Bruker, Germany) with 40
been paid to its potential use as a renewable raw material.
However, compared to other biomasses, the utilization of lignin is
limited due to its complicated chemical structure and the
corresponding chemical reaction mechanism. Therefore, lignin
model compound used to address the characteristics of lignin is
considered to be the key to the efficient utilization of lignin.
Up to now, a lot of dimeric lignin model compounds have been
reported to apply for investigating the characteristics of lignin
[2,3]. Although the synthesis of trimmers [4], tetramers [5],
hexamers [6] and oligomers with higher molecular weight [7] are
dimethyl sulfoxide (DMSO) as solvent.
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42
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2.2. Synthesis of lignin model compound
4-(2,3-Dihydro-7-methoxy-3-methyl-5-propenyl-2-benzo-
reported, these lignin models are mostly composed of only
b
-O-4
furanyl)-2-methoxyphenol (2): isoeugenol (2.4630 g), phosphate 44
buffer solution (60 mL, pH 5.83), acetone (45 mL) and 10 g/L 45
horseradish peroxidase (4.5 mL) were added into a flask equipped 46
with a magnetic stirrer, and then 30% hydrogen peroxide 47
(1.8700 g) was dropwise added. After reaction at 50 8C for 2 h, 48
the crude product was extracted with ethyl acetate and dried 49
under vacuum. The resulting product was further purified by 50
column chromatography with silica gel using dichloromethane/n- 51
hexane (1/5, v/v) as the mobile phase. The purified compound 2 52
was obtained with the yield of 95.3% after evaporation under 53
linkage or two bond linkages which cannot exactly mimic the
characteristics of lignin macromolecule. Therefore, it is of great
significance to synthesize the models containing a variety of other
linkages. Shiba et al. [8] prepared a tetramer lignin model
compound composed of 5-5, b-5 and a-O-4 linkages with a yield
of 2 mol% (equal to mass percent of 8.0%) catalyzed by laccase at
30 8C for 24 h. Considering the limitation of prolonged time and
low yield, a radical synthesis of the tetramer phenolic lignin model
compound with hydrogen peroxide/horseradish peroxidase and
S2O82ꢀ/Fe2+ as the initiator is investigated in the present work to
shorten reaction time and improve yield.
vacuum. 1H NMR (400 MHz, DMSO-d6):
CH3), 1.83 (d, 3H, J = 4.0 Hz,
’-CH3), 3.81 (s, 6H, 2 ꢁ OCH3), 3.41 55
(m, 1H, H ), 5.11 (d, 1H, J = 8.0 Hz, H ), 6.16 (m, 1H, H ), 6.35 (d, 56
d
1.32 (d, 3H, J = 8.0 Hz,
g
-
54
g
’
b
b
a
1H, J = 12.0 Hz, H ), 6.87–7.12 (m, 5H, aromatics), 9.06 (s, 1H, 57
’
a
OH); HRMS calculated for C20H22O4 [M+H]+ 327.1591, found 58
327.1599. 59
*
Corresponding author.
E-mail address: xueqingqiu66@163.com (X.-Q. Qiu).
http://dx.doi.org/10.1016/j.cclet.2015.05.011
1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: X.-P. Ouyang, et al., Radical synthesis of tetrameric lignin model compound, Chin. Chem. Lett. (2015),
http://dx.doi.org/10.1016/j.cclet.2015.05.011

G Model
CCLET 3315 1–3
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X.-P. Ouyang et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
Scheme 1. Synthetic route for phenolic model compound.
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5,50-Bis[2,3-dihydro-7-methoxy-3-methyl-5-propenyl-2-ben-
zofuranyl]-3,30-dimethoxy-[1,10-biphenyl]-2,20-diol (3): compound
2 (0.3260 g), ferrous sulfate heptahydrate (0.0140 g) and potassium
persulfate (0.1360 g) were added into a flask equipped with a
magnetic stirrer, then deionized water (10 mL) and acetone (20 mL)
were added to make the solid dissolved. The mixture was stirred at
50–100 8C for 5–25 min. The target product 3 was purified with the
6.83–6.99 (m, 8H, aromatics), 8.52 (s, 2H, OH); HRMS calculated
for C40H42O8Na [M+Na]+ 673.2772, found 673.2778. The synthetic
route is presented in Scheme 1.
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3. Results and discussion
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3.1. Effect of process variables on the yield
same procedure as compound 2. 1H NMR (400 MHz, DMSO-d6):
1.31 (d, 6H, J = 8.0 Hz, -CH3), 1.81 (d, 6H, J = 4.0 Hz, ’-CH3), 3.78
d
g
g
The activation of potassium persulfate promoted by ferrous
sulfate to generate sulfate radical is commonly carried out in
aqueous media [9]. However, the addition of acetone increases the
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(s, 6H, OCH3), 3.82 (s, 6H, OCH3), 3.41 (m, 2H, H ), 5.09 (d, 2H,
b
J = 12.0 Hz, H ), 6.14 (m, 2H, H ), 6.34 (d, 2H, J = 12.0 Hz, H ),
’
b
’
a
a
Fig. 1. HRMS (a) and 13C NMR (b) spectrum of lignin model compound 3.
Please cite this article in press as: X.-P. Ouyang, et al., Radical synthesis of tetrameric lignin model compound, Chin. Chem. Lett. (2015),
http://dx.doi.org/10.1016/j.cclet.2015.05.011

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X.-P. Ouyang et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
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Table 1
mass of model compound 3 with sodium ion. This means that the 99
molecular weight of the synthesized model compound is 650 Da, 100
which is equal to that of C40H42O8. 13C NMR spectroscopy is a 101
powerful tool to elucidate complicated lignin structures, and the 102
peak of each carbon from 13C NMR in lignin model compound is 103
Effect of the ratio of solvent on the yield of compound 3.
Acetone/water (v/v)
Yield (%)
4/1
3/1
2/1
1/1
0/1
0
31.3
30.6
33.8
25.1
labeled in Fig. 1b, in which the peaks at
d
44.54,
d
92.55 and
d
104
Table 2
125.34 are assigned to C in
respectively.
b
-5, C in
a
-O-4, and C5 in 5-5, 105
b
a
The influence of temperature and time on the yield of compound 3.
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Temp
Time
Yield
(%)
4. Conclusion
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(8C)
(min)
50
60
70
80
90
100
80
80
80
80
10
10
10
10
10
10
5
20.6
23.7
25.8
33.8
18.3
18.5
19.3
30.2
29.2
28.7
A tetramer phenolic lignin model compound was prepared by a 108
two-step free radical reaction with hydrogen peroxide/horseradish 109
peroxidase and S2O82ꢀ/Fe2+ as the initiator. Compared with 110
enzymatic process, this synthetic process gives a higher yield of 111
33.8% within a shorter time. HRMS and 13C NMR spectroscopy 112
show that synthesized model compound contains phenylpropane 113
15
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structure linked by 5-5,
a-O-4 and b-5 bonds, which can mimic 114
some chemical characteristics of lignin.
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yield of model compound 3 (Table 1), which shows that a higher
yield is obtained when the volume ratio of acetone to water is 2:1.
This is because dimeric model compound 2 is water-insoluble, and
an appropriate ratio of acetone to water can make both inorganic
initiator and compound 2 dissolve in the reaction media to a large
degree. This can be demonstrated from Table 1 that the yield of
compound 3 is zero when using water as the solvent.
The influence of temperature on the yield is shown in Table 2. It
is found that the yield of model compound increases with the
increase of temperature when temperature is less than 80 8C,
whereas when temperature exceeds 80 8C, the yield decreases with
the increase of temperature. It is well known that a higher
temperature is favorable for generating sulfate radical and
therefore increasing the yield of model compound. However,
excessive temperature should produce more sulfate radicals which
can be destructed by the fast reaction with ferrous ion [10],
resulting in the decrease of the yield.
Acknowledgments
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This work is financially supported by the National Basic Q2117
Research Program of China (No. 2012CB215302) and the Major 118
Program of National Natural Science Foundation of China (No. 119
21376100).
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References
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3.2. Characterization of lignin model compound
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HRMS spectrum of compound 3 is shown in Fig. 1a. It shows
that the m/z of the target product is 673.2772, which is the adduct
Please cite this article in press as: X.-P. Ouyang, et al., Radical synthesis of tetrameric lignin model compound, Chin. Chem. Lett. (2015),
http://dx.doi.org/10.1016/j.cclet.2015.05.011