Synthesis of lignin model compound containing a β-O-4 linkage

Article abstract of DOI:10.1515/znb-2016-0201

Development of catalysts for efficient conversion of lignin polymer to value-added materials requires appropriately-functionalized lignin model compounds. The predominant structural feature of lignin biopolymer is an extensive network of β-O-4 linkages. A

Full text of DOI:10.1515/znb-2016-0201

Z. Naturforsch. 2017; aop
ASsymanMtuhkhetasr,iMsuhoamfmlaidgZnahiener,mMuohadmmealdcSaoeemd*panoduWonlfdgancgoVonelttear*ining
a β-O-4 linkage
DOI 10.1515/znb-2016-0201
the pulp and paper industry, where it is burnt for heat
supply [13, 14].
Received September 7, 2016; accepted October 27, 2016
Lignin, an amorphous biopolymer, contains mono-
mers including p-coumaryl, coniferyl, and sinapyl alco-
hols linked by various C–O (e.g. β-O-4, α-O-4 and 4-O-5)
and C–C (e.g. β-5 and 5-5) bonds as shown in Figure 1. The
β-O-4 linkage is the predominant substructure in lignin
and accounts for 50%–60% of all its C–O linkages [10–12].
Selective cleavage of these linkages, especially the β-O-4
linkage, is a key step for the valorisation of lignin to aro-
matic compounds [15–17]. Toward this end, an efficient
synthesis and large-scale availability of dilignol model
compounds containing a β-O-4 linkage is crucial for the
development of robust catalytic methods for the transfor-
Abstract: Development of catalysts for efficient conver-
sion of lignin polymer to value-added materials requires
appropriately-functionalized lignin model compounds.
The predominant structural feature of lignin biopolymer
is an extensive network of β-O-4 linkages. Access to large
amounts of a model compound containing the β-O-4 link-
age is crucial for the valorisation of lignin biopolymer to
aromatic raw materials. Starting from commercially avail-
able vanillin, synthesis of dilignol model compound, con-
taining a β-O-4 linkage, is accomplished in good overall
yield.
Keywords: aromatic raw material; bio-waste; lignin; mation of lignin to useful chemicals. While continuing our
model compounds; β-O-4 linkage.
efforts to utilize synthetic tools for investigating biologi-
cal problems [18–21], we are now extending our endeav-
ors towards designing greener and sustainable energy
sources and the development of efficient catalysts.
1 Introduction
Various methods for the synthesis of dilignol model
compounds with β-O-4 linkage have been reported [22–25].
However, most of those methods are either low yielding,
laborious, or require special equipment. In this article we
report a convenient synthesis of monomeric β-O-4 model
Current efforts toward developing sustainable resources
for energy sector and chemical industry have highlighted
the potential of non-edible lignocellulose biomass (LCB)
as a renewable source of both fuels and chemicals [1–3].
Out of the three major components of LCB, i.e. cellulose,
hemicellulose, and lignin, extensive efforts have been
devoted to catalytic transformation of the former two con-
stituents into fuels and chemicals [4–9]. However, only
limited studies have been dedicated to the conversion
of lignin to value-added products despite the fact that it
contains almost 30% of the organic carbon on earth and
compound,
1-(4-hydroxy-3-methoxyphenyl)-2-phenoxy-
propane-1,3-diol (1), from commercially available starting
material vanillin (2). The method is adaptable to synthe-
size oligomeric lignin model compounds containing β-O-4
linkages.
a potential feedstock for renewable aromatic chemicals 2 Results and discussion
[10–12]. Lignin is, hitherto, treated as a waste product in
The initial step involves protection of the phenolic OH of
2 to the benzylated (Bn) derivative 3, which was accom-
plished by treating 2 with K2CO3 as a base in DMF [26].
The generated phenoxide ion was subsequently reacted
with benzyl bromide to afford benzyl-protected vanil-
lin 3 in 95% yield. At this point, we also sought to use a
benzoyl (Bz) group as an alternative phenol protection.
Main incentive behind this replacement was to accom-
plish the late-stage deprotection and ester reduction by
LiAlH4 in a single step, thus reducing the number of steps
from 6 to 5. However, our efforts toward the synthesis of
*Corresponding authors: Muhammad Saeed, Department of
Chemistry, SBA School of Science and Engineering, Lahore
University of Management Sciences, Lahore 54792, Pakistan,
e-mail: muhammad-saeed@lums.edu.pk; and Wolfgang Voelter,
Interfakultäres Institut für Biochemie, Eberhard Karls Universität
Tübingen, Hoppe-Seyler Straße 4, 72076 Tübingen, Germany,
e-mail: wolfgang.voelter@uni-tuebingen.de
Asma Mukhtar and Muhammad Zaheer: Department of Chemistry,
SBA School of Science and Engineering, Lahore University of
Management Sciences, Lahore 54792, Pakistan
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ꢀA. Mukhtar et al.: Synthesis of β-O-4 linkage
2ꢀ
O
O
OH
O
β
-5
O
HO
HO
OH
β
-O-4
O
O
OH
O
MeO
OMe O
β
-
β
OH
HO
OH
5-5
O
MeO
O
O
OH
HO
OCH3
OH
OH
4-O-5
O
O
HO
OH
OH
Aromatic
Raw
O
HO
Material
O
β-O-4 model compound (1)
Fig. 1:ꢁRepresentative structure of lignin biopolymer showing various types of linkages: Blue circles: C–O linkages, and green circles: C–C
linkages. The structure of β-O-4 lignin model compound 1 is shown in the box.
the Bz-protected vanillin derivative were unsuccessful
To introduce the 1,2-dioxygenated functional group
because of poor yield and instability of the protective of the target dilignol (1), we envisioned 1,2-halohydrin as
group. Therefore, we used Bn protection for our synthetic a suitable precursor. The correct regiochemistry of the
route.
hydroxyl group and bromine atom was accomplished
Elaboration of the aliphatic chain was accomplished by using the reaction conditions reported by Bar [27].
via a Wittig reaction on substrate 3a by using (2-methoxy- Briefly, the trans cinnamate (6a) was stirred with diphe-
2-oxoethyl)triphenylphosphonium bromide (5) in pres- nylthiourea and NBS in a mixture of CH3CNꢀ: H2O (4 : 1) at
ence of sodium ethoxide. The phosphonium bromide 5 0°C for 2 h. Water was added to quench the reaction, and
was synthesized by reacting a solution of 2-bromoacetate then the product was extracted with ethyl acetate. The
(4) with triphenylphosphine in ethyl acetate [26]. A white crude residue was finally loaded on a silica gel column
precipitate, thus formed, was deprotonated by sodium eth- and eluted with ~ 50% ethyl acetate in hexane to afford
oxide in ethanol and the resulting ylide was reacted in situ the desired compound 7 as a brownish gummy mate-
with the protected aldehyde 3 to obtain the trans cinna- rial in 94.5% yield. The IR spectrum of the compound
mate derivative 6a as the major (70%) product (Scheme 1) showed a broad peak at 3497 cm−ꢀ1, indicating the pres-
1
after purification by column chromatography. The H ence of the OH group. The structure of the bromohy-
1
NMR spectrum of the compound indicated two doublets, drin 7 was further confirmed with the help of H NMR
one signal at downfield chemical shift (δꢀ=ꢀ7.60 ppm) and and MS spectra. The absence of the two trans olefinic
the other one at δꢀ=ꢀ6.56 ppm with a coupling constant of protons in the cinnamate derivative 6a and appear-
J = 16 Hz, indicating a trans configuration of the olefinic ances of a doublet of doublet at δꢀ=ꢀ4.79 ppm (Jꢀ=ꢀ5.3
protons. A minor product 6b (20%) having a cis configu- and 9.6 Hz) and a doublet at δꢀ=ꢀ4.52 ppm (Jꢀ=ꢀ9.6 Hz)
ration (6b) was also formed. This was confirmed by the indicated the formation of bromohydrin 7 as previously
presence of two olefinic protons resonating at δꢀ=ꢀ6.91 and reported [27]. The formation of bromohydrin 7 resulted
5.87 ppm, respectively, with a mutual coupling constant in the generation of two stereogenic centers in the ali-
of J = 12.9 Hz.
phatic chain, and we expected the formation of 7 as a
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A. Mukhtar et al.: Synthesis of β-O-4 linkageꢀ
ꢀ3
Scheme 1:ꢁSynthesis of racemic dilignol ( )-1 from vanillin (2) [(i) K2CO3, BnBr or BzCl, DMF, r.t., 5 h; (ii) PPh3, EtOAc, r.t., 17 h; (iii) 5,
NaOC2H5, EtOH, r.t., to 55°C, 1.5 h; (iv) NBS, diphenylthiourea, acetonitrile:water (4ꢂ:ꢂ1), 0°C, 2 h; (v) KH, phenol, DMF, 0°C for 2 h and then to
r.t., 8 h; (vi) LiAlH4, diethyl ether, r.t., 1 h; (vii) H2/Pd, ethyl acetate, r.t., 5 h].
racemic mixture. However, since only the trans cinna- with ethyl acetate. Presence of a broad hydroxyl peak at
mate (6a) was used in this reaction, bromohydrin 7 was 3364 cm− 1 and absence of the C=O stretching at 1738 cm− 1
obtained in diastereomerically pure form. The natural indicated the reduction of the ester functionality to a
β-O-4 linkages of lignin polymer are chiral in nature, we primary alcohol. The 1H NMR of the product showed two
envisioned that the racemic dilignol model compound, doublets of doublet for the methylene protons, reso-
prepared from racemic 7, would have no stereochemi- nating at δꢀ=ꢀ4.03 and 3.88 ppm along with the –CH2Ph
cal effect on the outcome of the valorisation of lignin peaks at δꢀ=ꢀ5.02 ppm confirming the reduction of the
polymer by the metal-based achiral catalysts, currently ester group.
being developed in our laboratory.
The final step in the synthetic route was removal of
To avoid the potential intramolecular elimination the benzyl protective group. This was accomplished by
reaction or the formation of epoxide by intramolecular stirring 9 in the presence of 5% Pd/C and H2 gas, afford-
displacement of Br by the –OH functionality, the phenol ing the final product as a white solid in 92% yield. It is
was reacted with KH in DMF, subsequently mixed with the interesting to note that the benzylic hydroxy (–OH) group
DMF solution of bromohydrin 7 to afford the phenyl ether survived under the milder reducing conditions as reported
1
8 in 92% yield. The yield of the reaction was initially very previously [28–31]. The H NMR spectrum of the product
low owing to the loss of the product during work up using showed the disappearance of the benzylic methylene
dichloromethane, probably because of its polar nature. protons signaling at δ = 5.02 ppm as well as the benzene
The IR spectrum of the product showed a broad OH peak protons.
1
at 3390 cm−ꢀ1. The H NMR spectrum of the compound
The synthetic strategy proposed in this article can
showed multiple resonances between δꢀ=ꢀ7.65–6.74 ppm be adapted to accomplish oligomeric lignin models con-
amounting to 8 H, indicating the presence of an addi- taining only the β-O-4 linkages. Since they mimic the
tional benzene ring in the structure. Due to the combined polymeric nature of the lignin molecule, the oligomeric
anisotropic effects of the benzene ring and the carbonyl models are becoming more and more practicable than the
group the C–H protons in the alkyl chain are shifted ~ monomeric or dimeric models in designing and discover-
0.6 ppm upfield from the corresponding resonances of the ing catalytic methods for converting lignin to aromatic
C–H protons in the starting bromohydrin 7.
compounds. Toward this end, methyl cinnamate 6a can
The ester functionality of 8 was reduced to the serve as the starting point. Thus, removal of the protective
primary alcohol by using LiAlH4 in diethyl ether for benzyl group from 6a by hydrogenolysis at low tempera-
30 min at room temperature to furnish 9 as a colorless ture can be followed by the bromohydrin step, which in
solid in 85% yield. The reaction mixture was diluted presence of a base could afford the oligomeric model com-
with water and the product extracted several times pound containing β-O-4 linkages.
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4ꢀ
light brown solid. After recrystallization in methanol, the
title compound 3a was obtained as a white crystal in 95%
3 Conclusions
In conclusion, we are reporting here a convenient syn- yield (1.15 g). Spectroscopic data matched with the previ-
thesis of dilignol model compound containing a β-O-4 ously reported values [26].
linkage in six steps with overall yield of 50% starting from
the commercially available vanillin. The model β-O-4
linkage will be used for the development of catalytic reac- 4.3 Methyl 3-(4-(benzyloxy)-3-
tions yielding a diverse range of aromatic raw material by
the valorisation of lignin biopolymer.
methoxyphenyl)acrylate (6)
To a stirred solution of PPh3 (1.7 g, 6.7 mmol) in ethyl
acetate, a solution of methyl 2-bromoacetate (0.62 μL,
6.5 mmol) in ethyl acetate was added at room tem-
4 Experimental section
perature. After 17 h, a white precipitate was collected
by filtration, washed three times with ether and dried
4.1 General
in air to furnish (2-methoxy-2-oxoethyl)triphenylphos-
phonium bromide (4, 2.49 g) as a white solid in almost
All melting points were determined with a Stuart SMP 3 quantitative yield. The phosphonium bromide 4 (2.0 g,
instrument and are uncorrected. IR spectra were recorded 4.81 mmol) was added to a suspension of sodium ethox-
on a Bruker, Alpha-FT instrument. GC-MS Spectra were ide (340 mg) in ethanol (10 mL) followed by the addition
1
taken on Thermo Scientific TRACE 1300 ISQ. H NMR of 3a (1.15 g, 4.75 mmol) at room temperature for 25 min.
spectra were recorded in [D6]DMSO using TMS as an inter- The mixture was stirred at 50–55°C for 1 h. The progress
nal standard on an Avance-300 MHz spectrometer operat- of the reaction was monitored via TLC. After completion
ing at a frequency of 300 MHz. Chemical shifts are given of the reaction, water (10 mL) was added to the reac-
in parts per million (δ scale) and the coupling constants tion mixture, and the product was extracted four times
are given in hertz. Multiplicity of the resonance peaks is with diethyl ether (20 mL). The organic layers were com-
indicated as singlet (s), broad singlet (bs), doublet (d), bined, dried over anhydrous sodium sulfate and evapo-
triplet (t), quartet (q), and multiplet (m). Elemental analy- rated under reduced pressure to furnish a solid residue.
sis was performed using a CHNS-932 LECO instrument. Further purification by column chromatography (ethyl
Column chromatography was performed on silica gel acetate : hexane 3ꢀ:ꢀ7) afforded two methyl cinnamate
Sigma Aldrich, 60 Å, 230–400 mesh particle size. Routine derivatives as a 3.5ꢀ:ꢀ1 mixture of E- and Z-isomers,
monitoring of the reactions was done on Merck TLC Silica respectively.
gel 60 F254. Silica gel-G plates (Merck) were used for TLC
analysis with a mixture of ethyl acetate and hexane as
eluent. The starting materials vanillin, benzyl bromide, 4.3.1 Methyl (E)-3-(4-(benzyloxy)-3-methoxyphenyl)
triphenyl phosphine, methyl 2-bromoacetate, N-bromo-
acrylate (6a)
succinimide (NBS), and lithium aluminum hydride were
purchased from Sigma Aldrich and were used without Yield: 990 mg (70%). –M.p. 56–58°C. – IR (powder):
1
further purification.
νꢀ=ꢀ2922, 1716 (C=O), 1380, 1227, 920 cm−ꢀ1. – H NMR
(300.132 MHz, [D6]DMSO): δꢀ=ꢀ7.60 (d, J = 16.0 Hz, 1 H,
olefinic proton), 7.46–7.33 (m, 6 H, Ph), 7.22 (dd, Jꢀ=ꢀ1.6,
8.3 Hz, 1 H, Ph-H), 7.05 (d, Jꢀ=ꢀ8.3 Hz, 1 H, Ph-H), 6.56 (d,
J = 16 Hz, 1 H, olefinic proton), 5.13 (s, 2 H, Ph-CH2-), 3.77
(s, 3 H, CH3), 3.67 (s, 3 H, CH3). – C18H18O4 (298.34): calcd.
4.2 4-(Benzyloxy)-3-methoxybenzaldehyde
(3a)
A stirring solution of vanillin (2, 1 g, 5 mmol) in 20 mL C 72.47, H 6.08; found C 72.59, H 6.12.
DMF was charged with solid K2CO3 (8 g) at room tempera-
ture. To the resulting suspension benzyl bromide (1 mL)
was added drop wise and further stirred for 1 h. After com- 4.3.2 Methyl (Z)-3-(4-(benzyloxy)-3-methoxyphenyl)
pletion of the reaction (TLC control) water was added and
the product extracted with dichloromethane (10 mL) three
acrylate (6b)
times. The organic layer was dried with anhydrous sodium Yield: 283 mg (20%); –M.p. 57–60°C. – IR (powder):
sulfate and the solvent removed at low pressure to yield a νꢀ=ꢀ2922, 1716 (C=O), 1380, 1227, 920 cm−ꢀ1. – H NMR
1
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A. Mukhtar et al.: Synthesis of β-O-4 linkageꢀ
ꢀ5
(300.132 MHz, [D6]DMSO): δꢀ=ꢀ7.64 (d, J = 1.7 Hz, 1 H, Ph-H), 5.02 (s, 2 H, Ph-CH2-), 4.86 (d, Jꢀ=ꢀ5.1 Hz, 1 H, H-α), 4.63 (m, 1
7.46–7.31 (m, 5 H, Ph), 7.23 (dd, Jꢀ=ꢀ1.7, 8.4 Hz, 1 H, Ph-H), 7.05 H, H-β), 4.03 (dd, Jꢀ=ꢀ11.1, 4.2 Hz, 1 H, H-γ), 3.88 (dd, Jꢀ=ꢀ11.1,
(d, Jꢀ=ꢀ8.4 Hz, 1 H, Ph-H), 6.91 (d, J = 12.9 Hz, 1 H, olefinic 6.3 Hz, 1 H, H-γ), 3.74 (s, 3 H, CH3) ppm. – C24H24O6 (408.16):
H), 5.87 (d, Jꢀ=ꢀ12.9 Hz, 1 H, olefinic H), 5.13 (s, 2 H, Ph-CH2-), calcd. C 70.58, H 5.92; found C 70.87, H 5.72.
3.77 (s, 3 H, CH3), 3.67 (s, 3 H, CH3). – C18H18O4 (298.34):
calcd. C 72.47, H 6.08; found C 72.52, H 6.07.
4.6 3-(4-(Benzyloxy)-3-methoxyphenyl)-
3-phenoxypropane-1,2-diol (9)
4.4 Methyl 3-(4-(benzyloxy)-3-
8 (200 mg, 0.5 mmol) was dissolved in diethyl ether, fol-
lowed by the addition of LiAlH4 (19 mg, 0.5 mmol). The
methoxyphenyl)-3-bromo-2-
hydroxypropanoate (7)
mixture was stirred for 30 min. TLC analysis indicated the
A well-stirred solution of methyl cinnamate derivative consumption of 8 after 30 min. The reaction was quenched
(6a, 990 mg, 3.32 mmol) in a mixture of acetonitrileꢀ:ꢀwater with water and the product extracted with ethyl acetate
(4ꢀ:ꢀ1, mL) was charged with NBS (717 mg, 4.03 mmol) and to furnish the title compound as a colorless solid. Yield
diphenylthioura (7.65 mg) at 0°C. The reaction mixture 162 mg (85%); – M.p. 180°C. – IR (powder): νꢀ=ꢀ3364 (OH),
was stirred at 0°C for 2 h. After completion of the reac- 1510, 1453, 1260 cm− 1. – 1H NMR (300.132 MHz, [D6]DMSO):
tion, the solvent was evaporated under reduced pressure δꢀ=ꢀ7.42–6.69 (m, 13 H, 3ꢀ×ꢀPh-H), 5.02 (s, 2 H, Ph-CH2-), 4.86
and the crude mixture was subjected to column chroma- (d, Jꢀ=ꢀ5.1 Hz, 1 H, H-α), 4.63 (m, 1 H, H-β), 4.03 (dd, Jꢀ=ꢀ11.1,
tography (ethyl acetateꢀ:ꢀhexane 4.5ꢀ:ꢀ5.5) to furnish the 4.2 Hz, 1 H, H-γ), 3.88 (dd, Jꢀ=ꢀ11.1, 6.3 Hz, 1 H, H-γ), 3.74 (s,
title compound as a brown gummy material. Yield: 1.24 g 3 H, CH3) ppm. – C23H24O5 (408.16): calcd. C 72.61, H 6.36;
(94.5%). – IR (film): νꢀ=ꢀ3497 (OH), 2966, 2847, 1728 (C=O), found C 72.87, H 6.72.
1590, 1378 cm−ꢀ1. –ꢀ1H NMR (300.132 MHz, [D6]DMSO):
δꢀ=ꢀ7.45–7.32 (m, 5 H, Ph), 7.07 (d, Jꢀ=ꢀ1.6 Hz, 1 H, Ph), 6.97 Acknowledgments: An internal Faculty Initiative Fund-
(d, J = 8.20 Hz, 1 H, Ph), 6.89 (dd, Jꢀ=ꢀ1.6, 8.20 Hz, 1 H, Ph), ing (FIF), awarded to MS by LUMS, was used to support
5.07 (s, 2 H, Ph-CH2-), 4.79 (dd, Jꢀ=ꢀ5.3, 9.6 Hz, 1 H, H-α), 4.52 this work. The authors wish to acknowledge the support
(d, J = 9.6 Hz, 1 H, H-β), 3.81 (d, Jꢀ=ꢀ5.3 Hz, 1 H, OH), 3.78 (s, provided by Prof. Amnon Kohen at University of Iowa for
3 H, CH3), 3.74 (s, 3 H, CH3). – C18H19BrO5 (394.02): calcd. C assisting in obtaining NMR spectra of the compounds.
54.70, H 4.85, Br 20.22; found C 54.87, H 4.72, Br 20.62.
Two undergraduate students, Hassan Shahzad, and Huda
Zahid participated in this study as internees and assisted
AM in executing the syntheses.
4.5 Methyl 3-(4-(benzyloxy)-3-
methoxyphenyl)-2-hydroxy-3-
phenoxypro-panoate (8)
References
Potassium hydride (91 mg, 2.28 mmol) was suspended in
2 mL of DMF at 0°C under argon and stirred for 15 min. A
solution of phenol (238 mg, 2.53 mmol) in 1 mL DMF was
added under argon to the suspension and further stirred
for 2 h at 0°C. Finally, a solution of bromohydrin deriva-
tive 7 (500 mg, 1.26 mmol) in 0.5 mL DMF was added to the
mixture, and stirring was continued at room temperature
for 8 h. The progress of the reaction was monitored by TLC
until 7 was completely consumed. The reaction mixture
was plunged into 5 mL cold water and then extracted with
ethyl acetate (3ꢀ×). The organic layers were combined,
dried over anhydrous Na2SO4 and evaporated under
reduced pressure to furnish the title compound as a white
crystal. Yield: 473 mg (92%); – M.p. 147°C. – IR (powder):
νꢀ=ꢀ3390 (OH), 1738 (C=O), 1503, 1390, 1203 cm−ꢀ1. – 1H NMR
(300.132 MHz, [D6]DMSO): δꢀ=ꢀ7.42–6.69 (m, 13 H, 3ꢀ×ꢀPh-H),
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Z. Naturforsch.ꢀ
ꢀ2017 | Volume x | Issue x(b)
Graphical synopsis
Asma Mukhtar, Muhammad Zaheer,
Muhammad Saeed and Wolfgang
Voelter
Synthesis of lignin model compound
containing a β-O-4 linkage
DOI 10.1515/znb-2016-0201
Z. Naturforsch. 2017; x(x)b: xxx–xxx
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