Highly Selective Oxidation and Depolymerization of α,γ-Diol-Protected Lignin

Article abstract of DOI:10.1002/anie.201811630

Lignin oxidation offers a potential sustainable pathway to oxygenated aromatic molecules. However, current methods that use real lignin tend to have low selectivity and a yield that is limited by lignin degradation during its extraction. We developed stoichiometric and catalytic oxidation methods using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as oxidant/catalyst to selectively deprotect the acetal and oxidize the α-OH into a ketone. The oxidized lignin was then depolymerized using a formic acid/sodium formate system to produce aromatic monomers with a 36 mol % (in the case of stoichiometric oxidation) and 31 mol % (in the case of catalytic oxidation) yield (based on the original Klason lignin). The selectivity to a single product reached 80 % (syringyl propane dione, and 10–13 % to guaiacyl propane dione). These high yields of monomers and unprecedented selectivity are attributed to the preservation of the lignin structure by the acetal.

Full text of DOI:10.1002/anie.201811630

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Accepted Article
Title: Highly Selective Oxidation and Depolymerization of α,γ-Diol
Protected Lignin
Authors: Wu Lan, Jean Behaghel de Bueren, and Jeremy Luterbacher
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Highly Selective Oxidation and Depolymerization of α,γ-Diol Protected
Lignin
Wu Lan, Jean Behaghel de Bueren, Jeremy S. Luterbacher*
Abstract: Lignin oxidation offers a potential pathway to sustainable
oxygenated aromatic molecules. However, current methods that use real
lignin tend to have low selectivity and a yield that is limited by lignin
degradation during its extraction. Here, we developed stoichiometric and
catalytic oxidation methods using 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) as oxidant/catalyst to selectively deprotect the acetal
and oxidize the α-OH into a ketone. The oxidized lignin was then de-
polymerized using a formic acid/sodium formate system to produce aromatic
monomers with a 36 mol% (in the case of stoichiometric oxidation) and 31
mol% (in the case of catalytic oxidation) yield (based on the original Klason
lignin). The selectivity to a single product reached 80% (syringyl propane
dione, and 10-13% to guaiacyl propane dione). These high yields of
monomers and unprecedented selectivities are attributed to the preservation
of the lignin structure by the acetal.
the yield was that the lignin extraction conditions either extracted a small
amount of the original lignin, or caused irreversible condensation of the
β–O–4 interunits to form interunit C–C linkages, which then cannot be
broken selectively and limit attainable yields[11]. As we demonstrate here,
selectivity was likely also limited by modification of the lignin structure
during extraction. Recently, we reported a lignin extraction method
using formaldehyde as a α,γ-diol-protection reagent to preserve the β–
O–4 interunit linkage and produce aromatic monomers at near-
theoretical yield (based on ether cleavage) in the subsequent
hydrogenolysis[12]. In further work, we demonstrated that a careful
choice of the protection group and hydrogenolysis catalyst could lead to
high product selectivity[13]. Considering the high lignin extraction yield
and our ability to preserve the lignin structure, this acetal protected
lignin represented an ideal lignin source for the two-step oxidation and
depolymerization required to produce oxygenated aromatic monomers.
The challenge in using this substrate was to develop an oxidation method
that was adapted to the modified chemical structure of protected lignin
compared to unprotected lignin. Here, we developed a method to both
selectively deprotect the acetal and oxidize the α-OH of the acetal
protected lignin in a single step (Figure 1). The resulting oxidized lignin
could then be depolymerized using HCOOH/NaOOCH to produce
phenyl propane diones at high yield and selectivity.
Lignin is a heterogeneous aromatic biopolymer that is present in
most plant tissue. It is predominately formed by radical coupling
between coniferyl, sinapyl, and p-hydroxyphenyl alcohol resulting in
different interunit linkages formed by C-C and C-O bonds[1]. The β-aryl
ether bond (also known as β–O–4 linkage) that features a benzylic
secondary hydroxyl at Cα position and a primary hydroxyl at Cγ position
is the most abundant interunit linkage, and a relatively delicate portion
To develop the initial deprotection and oxidation step, we explored
previously reported methods, including stoichiometric oxidation using
DDQ in DCM; and catalytic oxidation systems, such as
O2/TEMPO/HCl/HNO3, O2/DDQ/tBuONO, and O2/DDQ/NHIP/NaNO2
using propylidene acetal protected veratrylglycerol-β-guaiacyl ether
(VG) as a model compound (Table 1). None of these methods led to high
yield (ranging from 18-33 mol%) of oxidized VG (VGox, 2a, Table 1).
Presumably, the oxidation of acetal protected VG occurs in two distinct
stages: first, the acetal is deprotected, and second, the α-OH is oxidized.
Interestingly, DDQ has been reported as an effective catalyst for acetal
deprotection in wet organic solvents. With water, DDQ has been
reported to act similarly to a Lewis acid—increasing the proton
concentration and thus promoting acetal removal[14]. Based on these
reports, we performed a stoichiometric oxidation using DDQ in wet
organic solvents (with 5% H2O). Reactions in acetic acid (AcOH),
dichloromethane (DCM), and acetonitrile (MeCN) gave promising
yields of VGox ranging from 82 to 92 mol%, as shown in Tables 1 and
S1. In most cases, the presence of H2O was critical to the high yield of
VGox, even in the case of DCM, where water was not miscible with the
system (Table S1).
Given the high yields obtained with a stoichiometric oxidation, we
further investigated the catalytic oxidation using DDQ combined with
different co-catalysts in the same solvent systems under O2 (Table S2),
the most successful of which was HNO3. The yields of VGox were
generally higher when reactions were performed in DCM/H2O than
those in AcOH/H2O or MeCN/H2O (Table S2). Specifically, the
O2/DDQ/ HNO3 catalytic oxidation system in DCM/H2O generated 82
to 94 mol% VGox depending on the loading of DDQ (Table 1). tBuONO
had been reported as a good co-catalyst to selectively oxidize the
benzylic alcohol in lignin model compounds with DDQ[6], but, in our
of the lignin polymer[2]
depolymerization processes in the pulp and paper industry and
biorefineries are based on the cleavage of these β–O–4 bonds[3]
Recently, several groups have explored two-step lignin
.
Most delignification and lignin
.
a
depolymerization strategy pioneered by Stahl and coworkers[4] based on
the selective oxidation of the α-OH into a ketone, which reduces the
bond dissociation enthalpy of the associated β-aryl ether bond[5]. This
lower energy facilitates the cleavage of the β–O–4 bonds in downstream
processes. Several studies reported successful methods that selectively
oxidize the α-OH on β–O–4 dimeric model compounds and real lignin
using different oxidation systems including O2/4-amino-2,2,6,6-
tetramethylpiperidinyloxy (4-AcNH-TEMPO)/HCl/HNO3[4], O2/DDQ/
tBuONO[6], O2/DDQ/N-hydroxyphthalimide (NHIP)/NaNO2[7], and [4-
AcNH-TEMPO]BF4[8]. Various reactions were investigated to cleave the
Cα–Cβ bond and Cβ–O4' ether bond of the oxidized lignin or lignin
models to produce aromatic monomers, including those using
NaOH/H2O2[4], Zn /NH4Cl[6], NiMo sulfide/H2[7], HCOOH/NaOOCH[9]
,
and [Ir(ppy)2(dtbbpy)]PF6/ visible light[8]. However, all of these methods
led to either low yields of monomers (i.e. < 6.0 wt%)[6] or poor product
selectivity (limited to 26% for a single product )[10] when they were
applied to extracted lignin. One of the most important factors limiting
[*]
Dr. W. Lan, J. B. Bueren, Prof. J. S. Luterbacher
Laboratory of Sustainable and Catalytic Processing, Institute of
Chemical Sciences and Engineering. École polytechnique fédérale
de Lausanne (EPFL). CH-1015 Lausanne (Switzerland)
E-mail: jeremy.luterbacher@epfl.ch
[b]
Supporting information for this article is given via a link at the end of
the document
1
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10.1002/anie.201811630
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case, the yield was not as good as that obtained using HNO3 (Table S2).
The major function of HNO3 was its participation in the NO/NO2 redox
cycle, in which the dihydroquinone (reduction product of DDQ) could
be oxidized back to DDQ and complete the catalytic cycle[15]. Another
possible function was, as a Brønsted acid, to enhance the acetal cleavage
and thereby promote the whole deprotection and oxidation process.
Once again, when using DCM, the presence of H2O was necessary to
reach a high yield of VGox (Table S3), which is consistent with previous
observation for acetal deprotection[14]. The presence of O2, DDQ, and
HNO3 was necessary for efficient catalytic oxidation, and their
respective concentrations influenced the result (see Table S3 and our
proposed reaction mechanism for the whole process in Scheme S2,
further discussion on the effect of each reaction component is also
provided in section 5 of the SI). We also tested this catalytic oxidation
system using other dimeric β–O–4 model compounds with different
substitutions of methoxyl group on the aromatic ring and/or different
acetal protection groups. Good to excellent yields (81-95%) of the
corresponding oxidized products were achieved in all cases,
demonstrating the full applicability of this method to α,γ-diol-protected
lignin (Table 2).
4 structure by the protection method before oxidation. In comparison,
other methods likely suffer from a certain degree of structure
modification during extraction. Additionally, the in situ generated
deprotected β–O–4 units were rapidly converted to α–ketones, which
limited condensation and other structural modifications resulting in
higher yield and product selectivity.
The mass balance of the original birch Klason lignin during the
extraction, oxidation, and depolymerization processes was showed in
Figure 4. First, 237 mg of PA-lignin were extracted from 1.0 g of
biomass (containing 20 wt% of Klason lignin, the difference being the
presence of the protection group and some impurities) (Figure 4A). After
oxidation, 189 mg of ligninox were obtained by stoichiometric oxidation.
The reduced weight of lignin was largely due to removal of the
protection group. The resulting ligninox was depolymerized to produce
72 mg of aromatic monomers, representing 38% of the oxidized ligninox,
and 36% of the original Klason lignin. Using the catalytic oxidation, 193
mg ligninox were obtained and depolymerized and generated 63 mg of
aromatic monomers, representing 32% of the oxidized ligninox and 31%
of the original Klason lignin.
Previous work reported that 51 wt% of monomers was achieved
when using enzymatically extracted lignin as a feeding source for
oxidation and depolymerization. This is so far the highest monomeric
aromatics yield for lignin oxidative depolymerization[9]. However, this
lignin was produced by extensive ball milling and several enzymatic
treatments, and, as such, is not considered industrially scalable. When
the same method was applied on chemically isolated lignin, such as that
obtained after a mild acid-catalyzed extraction of lignin, the monomer
yield decreased to 41 wt%[10]. However, this treatment led to a very low
lignin extraction yield (35%; this was not reported in the literature, but
the procedure was reproduced here). Furthermore, the most easily
Following the study on model compounds, we successfully used both
the stoichiometric and catalytic oxidation conditions on the real
propylidene acetal protected lignin (PA-lignin) (see Figure 2, S3 and
Section
stoichiometrically and catalytically oxidized lignin were depolymerized
using the HCOOH/NaOOCH system as reported in the literature[9]
4 in SI for a detailed discussion). Next, both the
,
which allowed us to determine the effect of the oxidation on the final
monomer yields. The ligninox samples that were oxidized with 0.5, 1.0,
and 1.5 weight equivalents of DDQ, respectively, were depolymerized
and the yields were compared (Figure 3), which revealed that aromatic
monomer yields increased along with the amount of DDQ, reaching 36%
from birch lignin and 52% from F5H poplar lignin (a genetically
modified poplar with high syringyl content)[12]. The increase in yield
follows the increase in oxidation measured by HSQC NMR, which can
be explained by protected and unoxidized β–O–4 units undergoing
condensation, respectively. Lignin condensation easily occurs in acidic
conditions by dehydration of the α-OH followed by condensation of the
resulting unsaturation with a neighboring aromatic group[16]. When the
α-OH is oxidized, it can no longer dehydrate, which limits lignin
condensation. Our best yield was close to the yield obtained from
hydrogenolysis of the PA-protected lignin (42%), which is near the
maximum attainable yield based on complete ether cleavage (45-
50%)[17]. This suggests that most of the β–O–4 units were oxidized and
depolymerized. The difference in yield could likely be explained by the
few groups of unoxidized units that remained (Figure 2B). These groups
could have undergone condensation or have not depolymerized for the
reasons discussed above. In the case of catalytic oxidation, 0.2 and 0.06
weight equivalents of DDQ and HNO3 were used in oxidation process
and the monomer yield was 31% after depolymerization (Figure 3). This
lower yield compared to the stoichiometrically oxidized lignin can once
again be explained by incomplete oxidation and, in this case, incomplete
deprotection of the β–O–4 structure as shown in the HSQC spectrum
(Figure 2).
extracted lignin fractions tend to give higher monomer yields[12]
.
Considering this low extraction yield, the overall yield based on the
original Klason lignin was about 14%. We also used our oxidation and
depolymerization procedure on this lignin and achieved 38 % based on
the extracted lignin (13% overall) with similar product selectivity (Table
S4, entry 4). A longer extraction time increased the extracted lignin yield
to over 50% but lowered the over-all monomer yield and resulted in
worse product selectivity (Table S4, entry 5) likely due to increased
degradation.
Recent studies have shown that efficiently isolating native-like
lignin was key to achieving a high overall monomer yield in the
following process[11, 18]. In an alkaline aerobic oxidation study, a
substrate treated with anhydrous ammonia followed by alkaline
extraction (70 wt% of the total lignin was recovered in solids with a 30
wt% lignin content) generated yields corresponding to 22 wt% of the
original Klason lignin with selectivity lower than 33% to a single
product[18]. In another study, GVL lignin from corn stover was
oxidatively depolymerized over Au@Li–Al layered double hydroxide
and generated 40 wt% of aromatic monomers based on the extracted
oxidized lignin. The most abundant product was syringaldehyde with
only a 28% selectivity. Furthermore, the extracted lignin yield was not
reported, so the overall monomers yield was unknown[19]. Overall, using
both stoichiometric and catalytic oxidation followed by
depolymerization, we were able to obtain overall lignin monomer yields
by oxidative polymerization that were significantly higher than these
other methods, with typically 2-3-fold increases in the selectivity to a
single product. These dramatic selectivity improvements were likely due
to our preservation of the native lignin structure using acetals followed
by immediate oxidation after deprotection, which minimized structural
changes. Yield was maximized by achieving almost complete extraction
In all cases, the product distributions were similar, with 8 - 16% G-
diketone; 76 - 82% S-diketone; and less than 10% of syringic acid,
vanillic acid, 4-hydroxybenzoic acid, and syringaldehyde (Table S4).
The selectivity of the diketones reached 90-93% under the optimal
oxidation conditions (TableS4). The significantly higher selectivity to
the two diketone products (>90%) and syringyl diketone (>80%)
compared to previous studies using wild wood (~40% and 25%,
respectively)[9-10] is likely due to the complete preservation of the β–O–
2
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[12]
L. Shuai, M. T. Amiri, Y. M. Questell-Santiago, F.
of this stabilized lignin thanks to the use of a protection group on the
1,3-diol moiety.
Heroguel, Y. D. Li, H. Kim, R. Meilan, C. Chapple, J.
Ralph, J. S. Luterbacher, Science 2016, 354, 329-333.
W. Lan, M. T. Amiri, C. M. Hunston, J. S. Luterbacher,
Angew Chem Int Edit 2018, 57, 1356-1360.
A. Oku, M. Kinugasa, T. Kamada, Chem Lett 1993, 165-
168.
a) A. E. Wendlandt, S. S. Stahl, Angew Chem Int Edit 2015,
54, 14638-14658; b) C. Aellig, C. Girard, I. Hermans,
Angew Chem Int Edit 2011, 50, 12355-12360.
a) M. R. Sturgeon, S. Kim, K. Lawrence, R. S. Paton, S. C.
Chmely, M. Nimlos, T. D. Foust, G. T. Beckham, Acs
Sustain Chem Eng 2014, 2, 472-485; b) K. Shimada, S.
Hosoya, T. Ikeda, J Wood Chem Technol 1997, 17, 57-72.
a) Q. Song, F. Wang, J. Y. Cai, Y. H. Wang, J. J. Zhang, W.
Q. Yu, J. Xu, Energ Environ Sci 2013, 6, 994-1007; b) S.
Van den Bosch, W. Schutyser, S. F. Koelewijn, T. Renders,
C. M. Courtin, B. F. Sels, Chem Commun 2015, 51, 13158-
13161; c) C. Z. Li, M. Y. Zheng, A. Q. Wang, T. Zhang,
Energ Environ Sci 2012, 5, 6383-6390.
In summary, we developed a method to selectively deprotect
propylidene acetal-stabilized lignin while simultaneously oxidizing the
α-OH. The oxidized lignin was then depolymerized by
HCOOH/NaOOCH to produce aromatic monomers. We attribute the
high overall aromatic monomer yields based on the original Klason
lignin (36 mol% for the stoichiometric oxidation strategy and 31 mol%
for the catalytic oxidation strategy), and the remarkable selectivity
(>90% to syringyl and guaiacyl propane dione) to the preservation of the
lignin structure during extraction, which occurs even when the
extraction conditions are harsh enough for near-complete lignin
recovery. The high yield of products from the original lignin maximizes
carbon recovery from the plant. Carbon recovery has important
implications for biomass conversion due to the important energetic cost
of fixing atmospheric CO2 by photosynthesis. At the same time, high
selectivity to 1-2 dione products could jumpstart the development of
valorization processes for oxygenated lignin-derived aromatic
[13]
[14]
[15]
[16]
[17]
[18]
[19]
W. Schutyser, J. S. Kruger, A. M. Robinson, R. Katahira, D.
G. Brandner, N. S. Cleveland, A. Mittal, D. J. Peterson, R.
Meilan, Y. Román-Leshkov, G. T. Beckham, Green Chem
2018.
Y. Song, J. K. Mobley, A. H. Motagamwala, M. Isaacs, J. A.
Dumesic, J. Ralph, A. F. Lee, K. Wilson, M. Crocker, Chem
Sci 2018.
molecules, which has so far focused on reduced-lignin monomers[11]
.
Acknowledgements
This work was supported by the Swiss Competence Center for Energy Research:
Biomass for a Swiss Energy Future through the Swiss Commission for Technology and
Innovation grant KTI.2014.0116 as well as EPFL. The authors thank Dr. Graham R.
Dick (EPFL, Switzerland) for providing acetal protected lignin samples and Dr.
Aurélien Bornet for help with NMR experiments.
Keywords: biomass • homogeneous catalysis • oxidation •
depolymerization • lignin
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3
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Table 1. Selective oxidation of propylidene acetal protected veratrylglycerol-β-guaiacyl.
Solvent
Catalyst/[Oxidant]
Co-catalyst
Condition
Conv.
Yield
Methods reported for VG
DCM
150 mol% DDQ
20 mol% DDQ
10 mol% TEMPO
23 mol% DDQ
-
25 °C, 24h
80 °C, 15h
80 °C, 20h
80 °C, 3 h
89%
48%
29%
54%
25%
33%
18%
29%
2-Methoxyethanola
MeCN/H2O 10:1a
MeCNa
20 mol% tBuONO
10 mol% HNO3, 10 mol% HCl
33 mol% NHPI, 60 mol% NaNO2
Methods developed in this work
Stoichiometric oxidation
AcOH/H2O 95:5
AcOH/H2O 95:5
DCM/H2O 95:5
Catalytic oxidationa
DCM/H2O 95:5
DCM/H2O 95:5
DCM/H2O 95:5
150 mol% DDQ
200 mol% DDQ
150 mol% DDQ
-
-
-
60 °C, 24h
60 °C, 24h
80 °C, 15h
88%
93%
92%
82%b
92%c
88%
10 mol% DDQ
15 mol% DDQ
20 mol% DDQ
20 mol% HNO3
20 mol% HNO3
20 mol% HNO3
80 °C, 15h
80 °C, 15h
80 °C, 15h
84%
93%
95%
82%
91%
94%
aReactions performed under 2 bar of O2. bThe yield included 2a and acetylated (at the γ position) 2a with a ratio of 78:22. cThe yield included 2a
and acetylated (at γ position) 2a with a ratio of 76:24. A detailed product breakdown is available in Table S1 of the supplemental information.
Table 2. Catalytic oxidation of different lignin β–O–4 model compounds.
2a'
2b (90%, 81%)
2c (92%, 85%)
(82%, 81%)a;
2a"
(98%, 95%)
aNumbers represent conversion and yield, respectively.
4
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Figure 1. Oxidation of protected lignin followed by depolymerization to produce syringyl and guaiacyl propane diones as the two major products.
MeO
γ
HO
A PA-lignin
S/G
HO
OMe
β
O
α
Cβ
Aγ
OMe
S/G
O
PAγ
MeO
OMe
Aα
A
β–O–4
Cγ
PAβ
MeO
γ
O
1
PAα
S/G
δ
C
Cα
Aβ
O
β
O
α
(ppm)
OMe
S/G
O
S2/6
MeO
OMe
PA1
S'2/6
G2
G5
PA
β–O–4
G6
OH OH
HO MeO
γ
S/G
7
6
5
4
3
O
O
O
β
B Stoichiometric oxidized lignin
OMe
S/G
O
OMe
MeO
OMe
A'γ+A"γ
Aγ
Acetylated
A'γ+A"γ
A'
β–O–4α-ox
O
OH
A"β A'β
Aα
PAβ
PAγ
S/G-G
HO MeO
γ
PAα
S/G
OMe
O
O
S/G-S A"β A'β
δ
C
O
β
(ppm)
S/G
O
MeO
OMe
S2/6
S'2/6
PA1
A"
β–O–4α-ox
G'2
G'5
OMe
G'6
O
O
S/G
γ
OMe
7
6
5
4
3
β
MeO
O
α
S/G
O
C Catalytic oxidized lignin
C
β–β
OMe
MeO
A'γ+A"γ
PAβ
HO
HO
2
6
5
6
A"β
S/G-G
2
PAγ
OMe MeO
OMe
PAα
O
O
δ
C
S/G-S A"β A'β
(ppm)
S
G
Guaiacyl
Syringyl
S2/6
S'2/6
G'2
O
PA1
O
6
2
6
5
2
MeO
OMe
OMe
O
O
S'
G'
Guaiacyl
7
6
5
4
3
Syringyl
δH (ppm)
Figure 2. HSQC spectra of propylidene acetal protected lignin isolated from birch and its oxidation products. A Original propylidene acetal
protected lignin. B Oxidation products of protected lignin resulting from stoichiometric and C catalytic oxidation. Reaction conditions: B, 150 wt%
DDQ in acetic acid at 60 °C for 24 h; C, 20 wt% DDQ/6 wt% HNO3 in DCM with 0.5 % H2O, 2 bar O2, at 80 °C for 24h.
5
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A Depolymerization of oxidized lignin
O
O
O
MeO
MeO
HO
Lignin
OH
S/G
O
O
O
HO
HO
HO
O
HCOOH
NaOOCH
OMe
OMe
OMe
OMe
S/G
O
O
O
O
MeO
HO
MeO
HO
MeO
Lignin
OMe
H
OH
OH
HO
OMe
OMe
60
50
40
30
B
Monomer
yields [wt%]
20
10
0
50 wt%
DDQ
100 wt% 150 wt% 20 wt% DDQ 150 wt% 20 wt% DDQ
DDQ
DDQ 6 wt% HNO3
DDQ 6 wt% HNO3
Brich
F5H Poplar
Figure 3. Depolymerization of oxidized lignin in HCOOH/NaOOCH. A Reaction scheme. B Monomer yields and product distributions. Reaction
condition of depolymerization were: 100 wt% NaOOCH, HCOOH:H2O 10:1 v/v, 110 °C for 48 h. Yield was reported on a Klason lignin basis. The
yield calculation is explained in more detail in Section 3 of the Supplemental Information. Detailed data is given in Table S4.
6
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10.1002/anie.201811630
Angewandte Chemie International Edition
COMMUNICATION
A
36%
26%
Stiochiometric
189mgb
38%
72mg
36% of
Klason
lignin
20%
Klason
lignin
Polymeric material
238 mga
Soluble productsc
Aromatic monomers
42%
26%
1000 mg
Birch
Catalytic
193mgb
32%
31% of
63mg
Klason lignin
B
Extracted lignin
yield
Stoichiometric oxidation
Propylidene acetal
protected lignin
Catalytic oxidation
Monomer yield
(oxidized lignin
basis)
Mild acid-catalyzed
lignin extraction
Extracted for 45 min
+
stoichiometric
Monomer yield
(original Klason
lignin basis)
Extracted for 150 min
oxidation
Direct
hydrogenolysis
0
10 20 30 40 50 60 70 80 90 100
Yield [wt%]
Figure 4. A Mass balance of the original birch Klason lignin following the extraction, oxidation, and depolymerization processes. Arrow widths are
proportional to the amount of material produced. B comparison of the overall monomer yields to that of using lignin extracted using a mild acid
catalyzed process. Reaction conditions are indicated in the caption of Figure 2. aThe extracted lignin contained the additional protection group. bThe
weight loss was mainly due to the protection group removal. cProducts that are soluble in EtOAc but not detectable in GC, which have been referred
to as dimeric and trimeric compounds[9-10]. The monomers were quantified by GC (see Section 3 in the Supporting Information); the soluble products
were calculated as the mass of total products extracted by EtOAc minus the mass of monomers; the polymeric material was considered to be the
remainder.
7
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10.1002/anie.201811630
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
O
High overall monomer yield (31-36%)
High selectivity (90% to diketones)
Wu Lan, Jean Behaghel de Bueren, Jeremy
S. Luterbacher*
R
H
O
R
O
MeO
HO
O
OH
OMe
O
O
HO
O
R
O
OMe
O
OMe
O
O
[O]
Page No. – Page No.
O
O
O
OMe
O
OMe
O
R
MeO
MeO
Extraction
OH
O
O
MeO
OMe
Highly Selective Oxidation and
Depolymerization of α,γ-Diol Protected
Lignin
MeO
OMe
O
O
HO
O
O
OMe
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone is able to selectively deprotect the acetal and
oxidize the α-OH of acetal protected lignin. The resultant oxidized lignin generated syringyl
and guaiacyl propane diketons in high monomer yield (31~36% on the base of original Klason
ligin) and high selectivity (90%) in formiac acid/sodium formate, thanks to the nearly
completely extraction of lignin and preservation of β–O–4 linkages using aldehyde
stabilization extraction method.
8
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