Selective Depolymerisation of γ-Oxidised Lignin via NHC Catalysed Redox Esterification

Article abstract of DOI:10.1002/cctc.201900787

The development of new catalytic methods for the processing of lignocellulose-derived renewable feedstocks continues to gain momentum, despite the considerable challenges associated with the use of complex biopolymers such as lignin. Here, we report a new two-step depolymerisation method for lignin involving primary alcohol oxidation followed by a NHC-mediated redox esterification-depolymerisation step. The process takes advantage of the inherent structure of the β-aryl ether units present in the specific type of lignin that was used (butanosolv) and delivers 4 novel aromatic monomers which retain the C₃ side chain present in the lignin subunits. The utility of the major product is assessed suggesting that rapid access to a wide range of interesting chemical could be achieved from this renewable building block.

Full text of DOI:10.1002/cctc.201900787

Accepted Article
Title: Selective Depolymerisation of g(gamma)-Oxidised Lignin via
NHC Catalysed Redox Esterification
Authors: Ganyuan Xiao, Christopher Lancefield, and Nicholas James
Westwood
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10.1002/cctc.201900787
ChemCatChem
COMMUNICATION
Selective Depolymerisation of -Oxidised Lignin via NHC
Catalysed Redox Esterification
Ganyuan Xiao, Christopher S. Lancefield* and Nicholas J. Westwood*
Abstract: The development of new catalytic methods for the
processing of lignocellulose-derived renewable feedstocks continues
to gain momentum, despite the considerable challenges associated
with the use of complex biopolymers such as lignin. Here, we report
a new two-step depolymerisation method for lignin involving primary
alcohol oxidation followed by a NHC-mediated redox esterification-
depolymerisation step. The process takes advantage of the inherent
structure of the -aryl ether units present in the specific type of lignin
that was used (butanosolv) and delivers 4 novel aromatic monomers
which retain the C3 side chain present in the lignin subunits. The
utility of the major product is assessed suggesting that rapid access
to a wide range of interesting chemical could be achieved from this
renewable building block.
As a result, new pretreatment methods have been
developed which feature in situ stabilisation strategies to
preserve -aryl ether units, at the same time as providing high
quality carbohydrate streams.[17] We have recently reported on
the use of n-butanol (BuOH) as
a pretreatment solvent
(butanosolv) for this purpose.[18] This process provides
butanosolv lignins in high yield and with relatively high -aryl
ether content, but in contrast to native lignins which have an -OH
at the -position of the -aryl ether unit, they display extensive
incorporation of BuOH at this position. This significantly modifies
the properties of the lignin, for example making them much more
soluble in many common organic solvents, and therefore opens
up new possibilities for (catalytic) chemical transformations.
Lignocellulosic biorefineries, which do not utilise food crops,
have received a great deal of attention in recent years. This is
driven by the fact that they are potential sources of renewable
sugars that can be used as platform molecules in a renewable
chemical industry. A major co-product from such biorefineries is
lignin, an aromatic biopolymer which accounts for around 15-30
wt% of the initial feedstocks.[1] Furthermore, improvements in the
efficiency of pulp and paper mills, coupled with the advent of
new lignin isolation processes,[2–4] has resulted in the increased
production of industrial kraft lignin. As a result, there has been a
renewed interest in the isolation and valorisation of different
types of lignin. Various different strategies are currently being
explored to achieve this by many different research groups,[5]
with one significant approach being via depolymerisation of the
lignin to give aromatic monomers which could function as
renewable aromatic feedstocks for a future chemical industry.
Owing to the heterogenous structure of lignin, the production
of defined sets of aromatic compounds that can be readily
purified is particularly challenging. One general approach that
has recently been exploited to achieve this involves a two-step
process; the first step being a selective oxidation of either the
secondary benzylic (Scheme 1a)[6–11] or primary alcohols[12–14]
(Scheme 1b) in the most abundant -aryl ether (-O-4) linkage in
lignin. This activates the lignin towards controlled
depolymerisation in a second step (Schemes 1a and 1b). Due to
the fact that these processes require two suitably cleavable -
aryl ether units to be next to each other to release one monomer,
a key requisite for their success is the presence of large
amounts of -aryl ether linkages in the starting lignin.[15,16]
Unfortunately, even though -aryl ether linkages are the most
abundant linkage in native lignins, they are also the most
chemically labile and therefore frequently become depleted
during the lignin isolation (pretreatment) process.[1]
a) Secondary Alcohol Oxidation
O
MeO
O
OH
O
OH
O
OMe
or
C-OAr
Cleavage
MeO
O
OAr
OH
MeO
O
OAr
OH
[O]
O
OMe
OMe
MeO
O
OMe
b) Primary Alcohol Oxidation
OH
OH
O
OMe
C
-C
MeO
OAr
MeO
O
OAr
MeO
O
[O]
Cleavage
+
O
O
retro-aldol
O
OH
O
OMe
OMe
OMe
c) This Work
helps selectivity
blocks retro-aldol fragmentation
OBu
OBu
OBu O
C-OAr
Cleavage
MeO
OAr
OH
MeO
O
OAr
O
MeO
O
[O]
OBu
NHC/BuOH
O
OMe
OMe
OMe
Scheme 1. Previously reported two-step oxidative approaches to lignin
depolymerisation via a) secondary[6–11] or b) primary alcohol[12–14] oxidation of
native type -aryl ether linkages lignin compared to this work c) employing
selective oxidation and NHC catalysis for the depolymerisation of butanosolv
lignin -aryl ether linkages.
Inspired by previous pioneering work on the oxidation of the
primary alcohol in lignin by Bolm et al.[12] and the NHC catalysed
redox-esterification of -phenoxyaldehydes by Smith et al.,[19] we
proposed a new two-step depolymerisation route for butanosolv
lignins (Figure 1c). We believed that the incorporation of BuOH
into the -aryl ethers would be key to achieving this selective
transformation as it not only simplifies the selectivity challenges
in alcohol oxidation, but also prevents the usually observed
retro-aldol C-C fragmentation pathway following conversion of
the primary alcohol to the aldehyde.[12,20]
[a]
G. Xiao, Dr. C. S. Lancefield, Prof. Dr. N. J. Westwood
School of Chemistry and Biomedical Sciences Research Complex
University of St Andrews and EaStChem
North Haugh, St Andrews, Fife KY16 9ST, U.K.
E-mail: csl9@st-andrews.ac.uk; njw3@st-andrews.ac.uk
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900787
ChemCatChem
COMMUNICATION
Table 1. Optimisation of NHC catalysed redox esterification and cleavage of
butanosolv lignin -aryl ether γ-aldehyde models.
either K2CO3 (entry 2) or NEt3 (entry 3) proved detrimental to the
yield, however changing the solvent to the non-halogenated,
higher boiling point and potentially biomass derived THF was
well tolerated (entry 4). Increasing the reaction temperature to
65 °C allowed the reaction time to be reduced from 24 to 6 hours
(entries 5 and 6) but decreasing the catalyst loading resulted in
a significant drop in yield (entry 7), even after extended reaction
times (entry 8). Extending these reaction conditions to the other
model compounds 1b-1d showed that the methodology was
generally applicable, with excellent yields of 3a or 3b being
obtained in all cases (entries 9-11). We also found that the
reaction proceeded smoothly in 100% BuOH (entry 12)
indicating that THF was not necessary but is advantageous for
eventual lignin solubility. The conversion was also still excellent
using 100% DCM as the solvent but, in the absence of BuOH, a
more complex product mixture was obtained consisting
predominantly of rebound products (Scheme S4).[21]
.
Entry
Model
2
Co-
solvent[a]
Base[b]
Temp.
(°C)
Time
(hrs)
Yield of
3 (%)[c]
(mol%)
1
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1a
20
20
20
20
20
20
10
10
20
20
20
20
DCM
DCM
DCM
THF
THF
THF
THF
THF
THF
THF
THF
-
Cs2CO3
K2CO3
25
25
25
25
65
65
65
65
65
65
65
65
24
24
24
24
6
69
With optimised conditions identified for the NHC catalysed
cleavage of model compounds 1a-1d, we turned our focus to the
depolymerisation of a real lignin substrate. Thus, a beech
butanosolv lignin was prepared according to previous reports[18]
and the oxidation of the primary alcohol groups to aldehydes in
the -aryl ether linkages was attempted using the biphasic
TEMPO/NCS catalytic system developed by Einhorn et al.[22] In
contrast to our previous reports on the direct TEMPO-catalysed
oxidation of this alcohol to carboxylic acids,[23] attempts to
perform this reaction on the butanosolv lignin failed to provide
the expected aldehyde product. Reasoning that this was likely
due to competing and complicating oxidation of the abundant
phenol groups in lignin, we selectively methylated them using
the MeI/TBAF protocol reported by Lu and Ralph.[24] Once
methylated (Figure 1A), the lignin could be smoothly oxidised to
give the intended aldehyde, which was characterised by HSQC
NMR (Figure 1B). With the desired lignin-ox aldehyde in hand,
we transferred the optimal reaction conditions identified for the
NHC-catalysed reaction in our model studies to this substrate.
These also proved successful on lignin, with complete
disappearance of signals associated with -aryl ether -aldehyde
units and the appearance of new signals consistent with
formation of the intended cleavage products (Figure 1C) being
observed in the HSQC NMR spectrum obtained on analysis of
the crude reaction mixture. Additionally, cross peaks for resinol
(-) units were clearly observed in all the spectra (c.f. Figures
1A and 1B with 1C), indicating that they are preserved during
2
0
[d]
3
NEt3
30
4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
66
5
86
6
24
6
91
7
43
8
24
6
45
9
93
10
11
12
6
92 (3b)
90 (3b)
81
6
24
[a] BuOH:co-solvent = 3:1 [b] Pre-catalyst 2:Base = 1:0.9 [c] Isolated yields [d]
1 equivalent was used See Table S1-S4 for more detailed optimisation studies.
We started our investigations with the synthesis of a series
of oxidised dimeric model compounds (1a-d) (Scheme S1)
which represent the different types of units that would be found
in oxidised soft or hardwood butanosolv lignins (lignin-ox). The
NHC catalysed redox-esterification step was then investigated
using 1a-d and a previously reported triazolium based pre-
catalyst 2.[19] Typically, in these types of reactions the aryloxy
leaving group rebounds, undergoing acylation by the acyl
azolium intermediate, regenerating the NHC catalyst (Scheme
S2). In the case of lignin, this approach would result in
rearrangement but not depolymerisation of the polymeric chains,
therefore we proposed the use of an alcohol co-solvent (BuOH)
which would out-compete the phenoxy leaving group for
acylation by the acyl azolium intermediate resulting in
depolymerisation of the lignin and the release of monomeric
compounds (Scheme S3).
the
oxidation-depolymerisation
sequence.
Subsequent
purification of the crude products by column chromatography
gave 3a, 3b and 3c as an inseparable mixture (0.6wt%, 1.2wt%
and 0.6wt% yields respectively based on quantitative 1H NMR
analysis) and 3d (2.6wt%) (Scheme 2) providing us with a new
set of lignin-derived monomeric aromatic buildings blocks. It is
interesting to note that 3a and 3b are generated only from
(methylated) terminal -aryl ether units, while 3c and 3d derive
from internal units that have required two sequential -aryl ether
cleavages to be released. Based on semi-quantitative analysis
of the HSQC NMR data (Figure 1b), which can be used as a
guide but should not be viewed as quantiative, we estimate that
-aryl ether -aldehydes are present at a level of approximately
22 per 100 C9 units. Thus, assuming a polymer of infinite length,
Gratifyingly, our initial investigations proved very
encouraging. Reaction of 1a with 20 mol% 2 and 18 mol%
Cs2CO3 in BuOH/DCM (3:1) provided the intended cleavage
product 3a in 69% yield (Table 1, entry 1). Changing the base to
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10.1002/cctc.201900787
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the maximum molar yield of monomers arising from the selective
cleavage of such units would be approximately 5wt%, or about
6wt% accounting for incorporation of BuOH.[15,16] This suggests
that our depolymerisation method is actually rather efficient at
releasing the potentially available monomers from the lignin
polymer, although the overall yield (5wt%) appears moderate.
compounds with a catalytic amount of methanesulfonic acid in
toluene for just 10 minutes giving 4a or 4b at the same time as
releasing one equivalent of the BuOH that was introduced into
the lignin during the pretreatment process (Scheme 3 (reaction
a) and Figures S1 and S2). Alternatively, direct conversion of 3d
to sinapic acid (5b) was achieved under basic conditions
(Schemes 3 (reaction b) and S8). Similar compounds such as p-
coumaric and ferulic acid can be obtained directly by alkaline
hydrolysis of herbaceous biomasses or lignins,[25–28] however
sinapic acid derivatives cannot be isolated in the same way.
Thus, our methodology provides
accessing derivatives of these versatile renewable aromatic
building blocks. To highlight the potential value further
a complimentary way of
a
selection of interesting transformations reported in the literature
involving 5b are also summarised in Scheme 3. For example,
NHC-catalysed decarboxylation of 5b gives 4-vinylsyringol 6
(Scheme 3 (reaction c)) a potent antimutagenic compound,[29,30]
whilst palladium catalysed decarboxylative coupling with
arylboronic acids gives stilbenes such as 7 (Scheme 3 (reaction
d)).[31] 5b can also serve as the starting point for the synthesis of
more complex molecules such as the potential antioxidant dimer
8 and structures 9 and 10.[32-34]
Figure 1. Lignin 2D HSQC NMR spectra at each stage of the
depolymerisation process. A) Phenol methylated beech butanosolv lignin. B)
Beech butanosolv lignin-ox aldehyde. C) Crude mixture of depolymerised
products after NHC mediated redox esterification-depolymerisation. Full
spectra are available in Figures S3-S5. Whilst no apparent change in total -
O-4 content was observed during the methylation reaction, a small decrease
was observed after oxidation as assessed by integration of the corresponding
2D HSQC spectra.[21]
Scheme 3. Conversion of selected lignin depolymerisation products to
cinnamate-type esters and acids and examples of literature conversions of
sinapic acid (5b). Reaction conditions: This work: a) MsOH, d8-toluene, 90 °C,
10 min, 93% (4a), 91% (4b); b) KOtBu, i-PrOH, 120 °C, 24 h, 96% (5a), 94%
(5b); Literature reports using 5b: c) 1-Ethyl-3-methyl imidazolium acetate,
DMSO, 100 °C, 60 min, 100%[29,30]
Cu(OAc)2, LiOAc, DMF, 60 °C, 8 h, 89%[31]; e) (i): HCl, EtOH, reflux, 2d; (ii): H2,
Pd/C, EtOAc, 10 °C, 45 min; (iii) 1,4-Butanediol, CAL-B, 75 °C, 4-72 h, 87%[32]
; d) Phenylboronic acid, Pd(acac)2,
Scheme 2. Depolymerisation of beech butanosolv lignin-ox
. Given the
;
requirement for monomer production from lignin via approaches of this type
required that there are two adjacent -O-4 units, the yields of 3a, 3b, 3c and
3d could alternatively be stated as 10wt%, 20wt%, 10wt% and 43wt%
respectively, based on a theoretical monomer yield of ~5 mol% or ~6wt%
accounting for butanol incorporation.[15,16]
f) (i): Horseradish peroxidase–H2O2, aq. acetone; (ii) acetic anhydride/pyridine;
(iii) bromine, NH4OAc, AcOH, 50 °C, 6 h; (iv) AcCl–MeOH, 16 h; (v) Pd/C–
Et3N, H2, MeOH, 2d, 80%; (vi) DIBAL-H, toluene, 2 h; (vii) pyrrolidine, 3 min[33]
;
g) (i) FeCl3, EtOH, RT., 1h; (ii) 1N NaOH, RT., 5 min, 62%[34]
.
In summary, several different elegant methods have
previously been reported for the selective oxidation and
depolymerisation of lignins, either via the oxidation of the
secondary or primary alcohols. Here we have demonstrated, for
the first time, that the same general approach can be used for
the selective depolymerisation of butanosolv lignins, that contain
To demonstrate the potential utility of the produced
monomers, the transformation of 3b and 3d to their respective
cinnamate ester derivatives was investigated. We found this
could be achieved in near quantitative yield by heating the
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[11] W. Lan, J. B. de Bueren, J. S. Luterbacher, Angew. Chemie Int. Ed.
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feature, which precludes the use of previously reported
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primary alcohols after phenol methylation. This provides a
relatively stable -aryl ether -aldehyde containing lignin (lignin-
ox) which can be depolymerised selectively in the presence of
BuOH using an NHC catalyst generated from 2. The resulting
monomeric ester products could be purified by column
chromatography, giving 3d as the major compound from a
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This work was supported by the China Scholarship Council-St
Andrews PhD Fellowship (GX). C.S.L. thanks the Leverhulme
Trust Early Career Fellowship (ECF-2018-480) and the
University of St Andrews. We acknowledge the EPSRC UK
Mass Spectrometry Facility at Swansea and Mrs. C. Horsburgh
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essential for the NHC-catalysed reaction; (ii) that the co-solvent butanol
could be successfully replaced with ethanol leading to the analogous
major product S11 (ethyl not butyl ester); (iii) whilst semi-quantitative,
2D HSQC analysis of the lignin-ox suggested that a small decrease in
the total -O-4 content of the lignin occurred during the oxidation
reaction. See ESI for additional details. We thank the Reviewers for
suggesting these additional experiments.
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
Ganyuan Xiao, Christopher S.
Lancefield* and Nicholas J. Westwood*
Page No. – Page No.
Selective Depolymerisation of -
Oxidised Lignin via NHC Catalysed
Redox Esterification
Selective Lignin Depolymerisation: Using butanosolv lignins with high degrees of
butanol incorporation at the-position of the -aryl ether units facilitates oxidation
of the primary alcohols to stable aldehydes. Subsequent, depolymerisation via an
NHC catalysed redox esterification process is demonstrated which yields selectively
synthetically versatile aromatic monomers.
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