Chemical synthesis of β-O-4 type artificial lignin

Article abstract of DOI:10.1039/b518005h

An artificial lignin polymer containing only the β-O-4 substructure was synthesized. The procedure consists of two key steps: 1) polycondensation of a brominated monomer by aromatic Williamson reaction; and 2) subsequent reduction of the carbonyl polymer.

Full text of DOI:10.1039/b518005h

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Chemical synthesis of b-O-4 type artificial lignin†
Takao Kishimoto,* Yasumitsu Uraki and Makoto Ubukata
Received 19th December 2005, Accepted 7th February 2006
First published as an Advance Article on the web 27th February 2006
DOI: 10.1039/b518005h
An artificial lignin polymer containing only the b-O-4 substructure was synthesized. The procedure
consists of two key steps: 1) polycondensation of a brominated monomer by aromatic Williamson
reaction; and 2) subsequent reduction of the carbonyl polymer. 13C-NMR and HMQC spectra of the
polymer were consistent with b-O-4 substructures in milled wood lignin isolated from Japanese fir
wood. The weight average degree of polymerization (DPw) ranged from 19.5 to 30.6, which is
comparable to enzymatically synthesized artificial lignin from p-hydroxycinnamyl alcohols
(dehydrogenation polymer, DHP) and some isolated lignins. Using this new lignin model polymer, it
will now be possible to reinvestigate the properties and reactivity of the main lignin structure in terms
of its polymeric character.
Introduction
Lignin is the second most abundant biopolymer after cellulose. In-
creasing attention has been paid to its potential use as a renewable
raw material. However, despite much investigation in this area,
most lignins produced as byproducts in the pulp industry are not
used as raw materials and it is still a challenging issue to develop
new utilization methods of lignin. This is partly because lignin
is a complicated branched polymer.1,2 The b-O-4 linkage is both
Fig. 1 The b-O-4 linkage in lignin.
the most abundant and the most important substructure in lignin
(Fig. 1). It greatly affects the chemical and physical properties of
lignin. In order to elucidate the property and reactivity of this
complex biomacromolecule, b-O-4 type lignin model dimers are
often used. These small model compounds are quite simple and
are easy to characterize, but they cannot mimic the polymeric
nature of lignin. It is becoming more important to use larger
lignin model compounds. To address this problem, polymer-
supported lignin model compounds3 and a polymeric material
of lignin model dimer4 were synthesized. Unfortunately, their
structures were unlike native lignin polymer. Oligomeric lignin
model compounds were also synthesized by several groups.5–7
Recently, a b-O-4 type oligomer was synthesized by addition
polymerization,8 but the degree of polymerization was limited to
an oligomeric level in this case as well. Alternatively, synthetic
lignin or artificial lignin has been enzymatically synthesized from
p-hydroxycinnamyl alcohols (dehydrogenation polymer, DHP).9
DHP resembles native lignin in many respects, but its structure
is as complicated as isolated lignin. The preparation of syringyl
type DHP specifically linked with b-O-4 bonds was reported,10 but
here, the unusually high phenolic content suggests that the DHP
contains crosslinks between relatively short linear b-O-4 chains.
We recently reported a remarkably simple method for the
synthesis of lignin related polymers composed exclusively of the
b-O-4 substructure.11 Although these polymers are b-O-4 type
linear polymers with well-defined structures, they lack c -CH2OH
groups that are crucial for lignin model polymer. Despite several
attempts, introduction of the c -CH2OH into the intermediate
b-O-4 type carbonyl polymer failed11 due to the impracticality
of adding formaldehyde completely into each monomer unit of
the carbonyl polymer. To overcome this problem, we revised
our strategy in the present investigation to utilize a monomer
containing a three-carbon side chain instead of a two-carbon side
chain. We subsequently succeeded in synthesizing an artificial
lignin polymer composed exclusively of the b-O-4 substructure
with three-carbon side chains. This artificial lignin polymer has
a well-defined structure, and the structural changes caused by
various treatments are much easier to analyze than DHP. The
relationship between the structure and properties of lignin polymer
can now be better understood by using this lignin model polymer.
The properties of b-O-4 rich lignin in wood cell wall12 can also be
elucidated in more detail.
Results and discussion
Synthesis of artificial lignin polymer
The synthesis of the b-O-4 type artificial lignin consists of two
key steps: 1) polycondensation of a brominated three-carbon
side chain monomer by aromatic Williamson reaction; and 2)
subsequent reduction of the carbonyl polymer (Fig. 2). Inspired
by the literature,13 compound 3 was selected as the new monomer
from several possible compounds. A similar, but benzylated,
Laboratory of Wood Chemistry, Research Group of Bioorganic Chemistry,
Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido
University, Sapporo, 060-8589, Japan
† Electronic supplementary information (ESI) available: Synthesis and
spectral data for acetylated lignin model compounds. See DOI:
10.1039/b518005h
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Fig. 2 Synthesis of artificial lignin polymer 2. Reaction conditions: (a) Pd/C–H2–EtOH–0 ◦C, (b) CuBr2–HBr–EtOAc–reflux, (c) Cs2CO3–anhydrous
DMF–rt, (d) NaBH4–CH3OH–rt, Bn = benzyl, Et = ethyl.
compound had been used to synthesize a b-O-4 type lignin model
dimer. The coupling of the two aromatic moieties was carried out
in 70% yield.
Compound 1 was synthesized from 4-hydroxy-3-methoxyben-
zaldehyde (vanillin) in five steps.13 Debenzylation of compound
1 was carried out with palladium charcoal at 0 ◦C in 97.2%
yield without reduction of the carbonyl group at the a-position.
Bromination of compound 2 with CuBr2 gave compound 3 in
79.7% yield after purification on silica gel.
because the reaction products could not be separated from a
gelatinous precipitate of aluminium salts possibly due to the
phenolic nature of the polymer. The best results were obtained
with sodium borohydride in methanol at room temperature. A
suspension of polymer 1 in methanol gradually became a clear
solution and the reaction proceeded to completion. Polymer 2 was
obtained in 34–45% yield. While DMSO also dissolved polymer 1,
gelation occurred during reduction with borohydride and resulted
in incomplete reaction.
Polymerization of compound 3 was carried out with cesium
carbonate or potassium carbonate in anhydrous DMF. The reac-
tion conditions were similar to those for b-O-4 type lignin related
polymers with two-carbon side chains.11 Because of the highly
hygroscopic nature of cesium carbonate, the base was dried under
vacuum at 110 ◦C before use. After reaction at room temperature
for 24 h, polymer 1 was obtained in 93–97% yield. Contrary to the
b-O-4 type two-carbon side chain polymer,11 polymer 1 did not pre-
cipitate from solution during the polymerization of compound 3.
Reduction of polymer 1 was problematic. Reduction with
lithium aluminum hydride in anhydrous THF was unsuccessful
Structure of artificial lignin polymer 2
13C-NMR spectroscopy is a powerful tool to elucidate complicated
lignin structures, and most of the signals in lignin have been
assigned previously.14 The 13C-NMR spectrum of polymer 2 is
shown in Fig. 3. The signals were assigned by comparison with
phenolic and non-phenolic lignin model dimers (Fig. 4).15 Polymer
2 exists as a mixture of many isomers which have asymmetric
carbon atoms at a and b positions. The side chain signals at
59.8, 71.4 and 83.6 ppm were assigned to c , a and b carbons and
Fig. 3 13C-NMR spectra of b-O-4 type artificial lignin polymer 2 and Japanese fir MWL. 1ꢀ,4ꢀ: phenolic end units; e: erythro, t: threo; MWL: milled
wood lignin. The structure of non-phenolic end units is uncertain.
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Fig. 4 13C-NMR spectral data in DMSO-d6 for b-O-4 type lignin model
compounds.15
occur at almost the same positions as those for the erythro form
of veratrylglycerol b-guaiacyl ether (60.0, 71.5 and 83.6 ppm).
Small resonances at 70.8 and 84.4 ppm correspond to the threo
form of the b-O-4 substructure and correspond to those in the
model dimer (70.8 and 84.3 ppm). Aromatic carbons were also
assigned by comparison with lignin model dimers. Signals in
polymer 2 were compared with those in milled wood lignin (MWL)
obtained from Japanese fir (Abies sachallnensis MAST) wood.
Softwood lignin mostly consists of guaiacyl lignin, which has only
one methoxy group in each aromatic ring. MWL contains several
substructures other than the b-O-4 substructure, but the amount
of b-O-4 substructure is 41% in this specific sample.16 As illustrated
in Fig. 3, all signals in polymer 2 exist in MWL, and all correspond
to the b-O-4 substructure.
2D NMR spectra, especially HMQC and HSQC, are frequently
used to elucidate lignin structures.17–19 Signals of the main sub-
structure in lignin are well resolved and assigned. Fig. 5 shows
partial HMQC spectra of polymer 2 and Japanese fir MWL in
side chain regions. The crosssignals at dC/dH 71.4/4.75, 83.6/4.28
and 59.8/3.15–3.26, 3.36–3.71 ppm in the HMQC spectrum of
polymer 2 are assigned to a, b, and c -positions in the b-O-4 sub-
structure and are consistent with those in Japanese fir MWL. The
HMBC spectrum of polymer 2 is shown in Fig. 6. The distinct
crosspeak at dC/dH 146.7/4.28 ppm indicates the correlation
between Hb and C4 in polymer 2. These results clearly indicate
that polymer 2 has b-O-4 linkages between monomer units.
Fig. 5 HMQC spectra of b-O-4 type artificial lignin polymer 2 and
Japanese fir MWL in DMSO-d6.MWL: milled wood lignin.
analyses such as thioacidolysis and pyrolysis–gas chromatography
are necessary to clarify this point.
The number- and weight-average molecular weights of the
acetylated polymer 2 were determined by gel permeation chro-
matography (GPC), and were Mn = 3440 and Mw = 5990, respec-
tively. The number and weight average degrees of polymerization
were calculated to be DPn = 12.3 and DPw = 21.4, respectively.
The average molecular weight of polymer 2 was comparable
to enzymatically synthesized artificial lignin (DHP) and some
isolated lignins.16,21
Characterization of acetylated polymer 2
The structure of polymer 2 was further characterized by analysis
of the NMR spectrum of the acetyl derivative of polymer 2.
Acetylation was performed with acetic anhydride and pyridine at
room temperature. 1H and 13C-NMR spectral data are shown in the
1
Experimental section. The H-NMR spectrum of the acetylated
polymer 2 in CDCl3 is shown in Fig. 7. The signals were assigned
by comparison with b-O-4 type acetylated lignin model dimers.15,20
Aliphatic acetyl protons at 2.00 ppm are assigned to c -OAc in
erythro form anda, c -OAc in threo form. Aliphatic acetyl protons
at 2.06 ppm are a-OAc in erythro form. Acethyl protons at
2.29 ppm correspond to aromatic OAc in phenolic end units.20
The content of phenolic end units were roughly estimated to be
around 8–10% from signal area, although some signals seem to
overlap with acetyl protons. It is quite strange that there are no
distinct signals corresponding to non-phenolic end unit. It is likely
that some unexpected side-reactions took place at the end group,
when polymer 1 was treated with NaBH4. The structure of non-
phenolic end unit is uncertain at this moment. Further chemical
Effects of base and catalyst on the polymerization
Effects of base and catalyst on the polymerization are summarized
in Table 1. While both potassium carbonate and cesium carbonate
were effective, cesium iodide increased both the yield and degree
of polymerization and the DPw of polymer 2 reached 30.6. The
catalyst works better in these experiments than with the b-O-
4 type two-carbon side chain polymer.11 This is most likely
due to a lack of precipitate formation during polymerization
as was observed previously with experiments using the two-
carbon side chain monomer. The molecular weight distribution
of acetylated polymer 2 is shown in Fig. 8. Addition of cesium
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Table 1 Effect of base and catalyst on the polymerization of compound
3
Base/catalyst
Yielda (%)
Mn
Mw
DPn
DPw
K2CO3
Cs2CO3
Cs2CO3–CsI
32.8
35.4
43.5
3390
3440
4420
5460
5990
8570
12.1
12.3
15.8
19.5
21.4
30.6
a Overall yields of polymer 2 from compound 3.
Fig. 6 HMBC spectrum of b-O-4 type artificial lignin polymer 2 in
DMSO-d6.
Fig. 8 GPC profile of acetylated b-O-4 type artificial lignin polymer 2.
simpler than DHP which is structurally as complex as isolated
lignins. Cesium iodide effectively increased both yield and degree
of polymerization with the DPw of the polymer reaching 30.6. It
will now be possible to reinvestigate the properties and reactivity
of the main lignin structure by using this new lignin model polymer
instead of small molecular weight lignin model dimers.
Experimental
Fig. 7 1H-NMR spectrum of acetylated b-O-4 type artificial lignin
polymer 2 in CDCl3.
Measurements
1H- and 13C-NMR spectra were recorded with a JEOL JNM
EX-270 FT-NMR (270 MHz) or a Bruker AMX500 FT-NMR
(500 MHz) spectrometer. HMQC and HMBC NMR spectra
were recorded with a Bruker AMX500 FT-NMR (500 MHz)
spectrometer. Chemical shifts (d) and coupling constants are
reported in d-values (ppm) and Hz, respectively. Exponential
multiplication with line broadening of 5.0 and 2.0 Hz was applied
for processing of 13C-NMR spectra of MWL and polymer 2,
respectively. Average molecular weights were analyzed with acetyl
derivatives of polymer 2 by gel permeation chromatography (GPC)
in tetrahydrofuran. A HITACHI Liquid Chromatograph L-6200
with a UV detector, L-4000 (Hitachi, 280 nm), and a RI detector
(JASCO RI-2031 Plus) was used. Shodex GPC packed columns
iodide exhibited bimodal distribution, and a shoulder appeared at
a higher molecular weight. This peak suggests that the presence of
cesium iodide accelerated the coupling between growing polymer
chains, although further investigations are necessary in order to
clarify this point.
Conclusions
The artificial lignin polymer composed exclusively of the b-O-4
substructure was synthesized by a polycondensation method. The
molecular weight of the polymer was comparable to DHP and
some isolated lignins. Its structure is also well-defined and much
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KF-803l and 802 (30 cm × 8.0 mm) were connected in series
and molecular weight was calibrated with polystyrene standard
(tetrahydrofuran, flow-rate: 0.5 mL min−1, 40 ◦C).
Acetylation of polymer 2. Polymer 2 (30 mg) was acetylated
with acetic anhydride–pyridine (2 : 1, v/v, 3 mL) at room
temperature for 21 h. The reaction mixture was diluted with
ethyl acetate, washed with brine, dried over anhydrous Na2SO4
and concentrated to dryness in vacuo. The molecular weight of
acetylated polymer 2 was analyzed by GPC. Polymer 2 (acetate):
1H-NMR (CDCl3): d 2.00, 2.06, 2.29 (s, OCOCH3), 3.81 (s, OCH3),
3.95 (Cc –H, threo), 4.14 (Cc –H, erythro), 4.24 (Cc –H, threo), 4.37
(Cc –H, erythro), 4.67 (Cb–H), 6.00 (Ca–H, erythro), 6.05 (Ca–H,
threo), 6.80–7.08 (aromatics); 13C-NMR (CDCl3): erythro: d 20.8,
21.0 (OCOCH3 × 2), 55.9 (OCH3), 62.4 (c ), 73.9 (a), 79.7 (b),
111.9 (2), 118.3 (5), 119.7 (6), 131.3 (1), 147.0 (4), 150.6 (3), 169.5
(a-OCOCH3), 170.5 (c-OCOCH3); threo: d 63.1 (c ), 74.7 (a), 80.1
(b).
Compound 2. A stirred solution of compound 1 (2.835 g) in
ethanol (60 mL) was treated with 10% palladium carbon (300 mg)
under H2 at 0 ◦C for 2.5 h. The reaction mixture was filtered
and concentrated to dryness in vacuo. The product was purified
on a silica gel column with ethyl acetate–hexane (1 : 2, m/m)
to afford a syrup (2.00 g, 97.2%). 1H-NMR (CDCl3): d 1.26
(t, 3H, J = 7.0, OCH2CH3), 3.94 (s, 2H, Cb–H), 3.95 (s, 3H,
OCH3), 4.21 (q, 2H, J = 7.0, OCH2CH3), 6.95 (d, 1H, J =
8.1, C5–H), 7.50 (dd, 1H, J = 8.1, J = 1.8, C6–H), 7.54 (d, 1H, J =
1.8, C2–H).
Compound 3. To a stirred solution of compound 2 (2.00 g, 8.4
mmol) and CuBr2 (5.09 g, 22.8 mmol) in ethyl acetate (30 mL),
a small amount of 25% HBr in acetic acid was added via capillary.
The reaction mixture was refluxed for 6 h. The reaction mixture
was diluted with ethyl acetate, washed with brine, dried over
anhydrous Na2SO4 and concentrated to dryness in vacuo. The
product was purified on a silica gel column with chloroform to
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portionwise carefully at 0 ◦C. The reaction mixture was kept at
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133.1 (1ꢀ), 135.0 (1), 145.2 (4ꢀ), 146.7 (4), 148.9 (3); threo: d 70.8
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