Iridium-catalysed primary alcohol oxidation and hydrogen shuttling for the depolymerisation of lignin

Article abstract of DOI:10.1039/c8gc01366g

Lignin is a potentially abundant renewable resource for the production of aromatic chemicals, however its selective depolymerisation is challenging. Here, we report a new catalytic system for the depolymerisation of lignin to novel, non-phenolic monoaromatic products based on the selective β-O-4 primary alcohol dehydrogenation with a Cp?Ir-bipyridonate catalyst complex under basic conditions. We show that this system is capable of promoting the depolymerisation of model compounds and isolated lignins via a sequence of selective primary alcohol dehydrogenation, retro-aldol (C_α-C_β) bond cleavage and in situ stabilisation of the aldehyde products by transfer (de)hydrogenation to alcohols and carboxylic acids. This method was found to give good to excellent yields of cleavage products with both etherified and free-phenolic lignin model compounds and could be applied to real lignin to generate a range of novel non-phenolic monomers including diols and di-acids. We additionally show, by using the same catalyst in a convergent, one-pot procedure, that these products can be selectively channelled towards a single di-acid product, giving much simpler product mixtures as a result.

Full text of DOI:10.1039/c8gc01366g

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Iridium-catalysed primary alcohol oxidation and hy-
drogen shuttling for the depolymerisation of lignin†
Christopher S. Lancefield,∗a Lucas W. Teunissen,a Bert M. Weckhuysena and Pieter C.
A. Bruijnincx∗ab
Lignin is a potentially abundant renewable resource for the
production of aromatic chemicals, however its selective
tively depolymerising this material to monoaromatic chemicals
which can then either be directly used in chemical applications
or undergo further upgrading to bulk or fine chemicals. Achiev-
ing this has, however, proved challenging for a number of rea-
sons. For example, the lignin structure is highly complex and
heterogeneous, depending strongly on both the botanical origin
and, if isolated, the fractionation method used.2,4 Furthermore,
lignin and its derived monomers have a tendency to undergo re-
polymerisation reactions which can severely impact the success of
many depolymerisation approaches.5,6
depolymerisation is challenging. Here, we report a new
catalytic system for the depolymerisation of lignin to
novel, non-phenolic monoaromatic products based on the
selective β-O-4 primary alcohol dehydrogenation with a
Cp∗Ir-bipyridonate catalyst complex under basic condi-
tions. We show that this system is capable of promoting
the depolymerisation of model compounds and isolated
lignins via a sequence of selective primary alcohol dehy-
drogenation, retro-aldol (Cα -Cβ ) bond cleavage and and
in-situ stabilisation of the aldehyde products by transfer
(de)hydrogenation to alcohols and carboxylic acids. This
method was found to give good to excellent yields of
cleavage products with both etherified and free-phenolic
lignin model compounds and could be applied to real
lignin to generate a range of novel non-phenolic monomers
including diols and di-acids. We additionally show, by
using the same catalyst in a convergent, one-pot procedure,
that these products can be selectively channelled towards
a single di-acid product, giving much simpler product
mixtures as a result.
Recently, a number of methods have been developed which
overcome some of these key challenges. One general approach
has been to carry out the lignin depolymerisation reaction in-situ
during the lignocellulosic pretreatment process, so called ‘lignin-
first’ approaches.7–12 In this way, the most deleterious lignin re-
condensation reactions, which occur during other pretreatment
and lignin isolation processes, are avoided as released reactive
lignin fragements are immediately converted to more stable com-
pounds. Despite significant recent progress13–17 there are cur-
rently no ‘lignin-first’ biorefineries in commercial operation and so
the valorisation of isolated lignins, such as those being produced
now, by depolymerisation still warrants attention. Furthermore,
new pretreatment processes are being developed which allow ac-
cess to high quality, high β-O-4 content lignins which are much
more amenable to further upgrading than most of the current
generation technical lignins.18–23
Lignin is the most abundant natural source of renewable aro-
matic carbon on earth and is now well recognised as a feedstock
with great potential for the future production of biobased aro-
matic chemicals.1–3 One of the keys to achieving lignin valorisa-
tion will be the development of new methods capable of selec-
In this respect, depolymerisation processes which again make
use of in-situ stabilisation of reactive intermediates5,6,24,25
and two-step selective lignin oxidation-depolymerisation strate-
gies26–30 (Scheme 1a) have proved very effective for producing
monoaromatic products from isolated lignins. Most of these de-
polymerisation methods target cleavage of the β-aryl ether (C-O)
bonds in the most abundant β-O-4 linkages, generating pheno-
lic monomers as the major products. However, by targeting al-
ternative depolymerisation pathways (i.e. C-C cleavage) it may
be possible to improve lignin-derived product structure and func-
aInorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht
University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands; bOrganic Chem-
istry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Uni-
versiteitsweg 99, 3584 CG Utrecht, The Netherlands; E-mail: c.s.lancefield@uu.nl,
p.c.a.bruijnincx@uu.nl
† Electronic Supplementary Information (ESI) available: Experimental procedures,
characterisation data, additional optimisation and control experiments, copies of 1
H
NMR and 13C NMR spectra of all new compounds and additional lignin NMR spectra.
See DOI: 10.1039/b000000x/
1–7 | 1

Green Chemistry
Page 2 of 8
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a) Previous Work - 2° Alcohol
O
rectly on lignin, we decided to start our investigations using full,
dimeric lignin model compounds (e.g. 1) which provide many of
the same structural and chemical features as lignin but allow for
significantly more straightforward analysis to be carried out. We
were, however, aware from the outset that the approach would
have to translate to real lignin meaning, in particular, that the
solvent chosen must be able to dissolve lignin and the catalyst
should, ideally, be water and air tolerant. With this in mind
the Cp∗Ir based catalysts bearing bipyridonate (10 and 14) or
6,6’-dihydroxy-2,2’-bipyridine ligands (15) (Fig. 1) were identi-
fied as promising candidates for primary alcohol dehydrogena-
tion. These catalysts are capable of performing (acceptorless) al-
cohol dehydrogenation and ketone/aldehyde hydrogenation reac-
tions in organic (14) and/or aqueous (10 and 15) solvents,35–38
and 14 has previously been shown to be active towards accep-
torless secondary alcohol dehydrogenation of lignin and lignin
model compounds34 (Scheme 1a). In general, these catalysts
have been found not to be capable of producing aldehydes from
aliphatic primary alcohols due to preferential re-hydrogenation of
the products. Nevertheless, if the aldehyde intermediates can be
intercepted, e.g. during lactone formation, good reactivity can be
achieved.39 Furthermore, Fujita et al. have shown that anionic
complex 10, which can be formed from (inactive) 14 or 15 at
high pH (pH 7.6-13) displays excellent activity in the dehydro-
genation of methanol to hydrogen and CO2, suggesting a high pH
can be used to increase reactivity towards primary alcohols.40
Building on this, we believed these catalysts would serve as a
good starting point for our work, leading us to propose a new
catalytic approach for the depolymerisation of lignin consisting
of anionic iridium complex 10 in a 1,4-dioxane/water solution,
at pH ∼11. These conditions were designed to 1) efficiently dis-
solve lignin, 2) promote primary over secondary alcohol dehydro-
genation, 3) catalyse the retro-aldol reaction of 6 to give 7 and 9
(Scheme 2a) and 4) provide in-situ product stabilisation (Scheme
Rahimi et al. (2013)
5 wt% AcNH-TEMPO, HCl/HNO3, O2
O
OH
OMe
O
OMe
MeO
OMe
Lancefield et al. (2015)
10 wt% DDQ, tBuONO, O2
α
β
β
O
O
OMe
HO
3
+
γ
γ
MeO
OH
Stephenson et al. (2017)
10 mol% NHPI, 2,6-lutidine, 0.64 V vs. Fe+/Fe
MeO
OH
or
OMe
OMe
5
O
1
2
Zhu et al. (2016)
1 mol% Cp*Ir-bipyridonate
Models & Lignin
MeO
OH
OMe
4
O
OMe
b) Previous Work - 1° Alcohol
Stein et al. (2015)
5 mol% [Ru(CO)(H)2(triphos)]
O
+
160 °C, 4 hours
OH
α
OMe
MeO
HO
β
O
Models Only
OMe
γ
7
8
MeO
OH
OMe
OH
OMe
O
OMe
OMe
10 mol% TEMPO
130 mol% PhI(OAc)2
20 mol%
DL-proline
1
O
O
O
HO
+
+
rt, 7 hours
rt, 5 hours
MeO
O
MeO
OMe
OMe
6
7
9
5
Dabral et al. (2017)
Models & Lignin
-
c) This Work
OH
O
Ir
N
O
N
OH
OMe
10
O
OMe
α
β
O
O
O
active catalyst (formed in-situ)
+
H/OH
γ
1-2 mol%
MeO
OH
MeO
HO
base, 130 °C, 1 hour
Models & Lignin
OMe
OMe
1
7 + 11 (acid)
+ 12 (alcohol)
8 + 13 (acid)
Scheme 1 Examples of lignin depolymerisation strategies utilising a) sec-
onday alcohol26,27,29,34 and b) primary alcohol31,32 oxidation to activate
the β-O-4 linkage towards cleavage compared to c) this work.
tional group diversity, allowing, for example, the production of
non-phenolic compounds directly from lignin. Indeed, methods
which target an alternative cleavage of the β-O-4 linkages by scis-
sion of the Cα -Cβ bond for lignin depolymerisation have recently
gained interest. For example, Stein et al. reported a Ru-triphos
system capable of cleaving this bond in lignin model compounds
such as 131 whilst Dabral et al. reported a two-step organocat-
alytic approach working with model compounds and real lignin
(Scheme 1b).32 In the former case, the reaction is overall redox
neutral, proceeding through a hydrogen-transfer initiated retro-
aldol pathway, with 7 and 8 being the main products. In the
latter case, a stoichiometric co-oxidant such as PhI(OAc)2 (or a
polymer supported version), together with 10 mol% TEMPO is
required to generate intermediate aldehyde 6, to which 20 mol%
proline is then added catalysing the retro-aldol reaction to give
7 and 9. However, due to the ability of proline to further catal-
yse the decomposition of 9, this process actually gives phenol 5
as the second major product. Model reactions with phenolic sub-
strates were found to give complex reaction products due to non-
selective oxidation reactions, limiting the scope of this reaction
to etherified units. Additionally, Sannami et al. have shown that
the primary alcohol moiety in β-O-4 models can be selectively
oxidised to carboxylic acids, without C-C bond cleavage, under
TEMPO-mediated electro-oxidation conditions.33
-
2+
OH
N
OH2
N
OH2
N
O
O
OH
Ir
Ir
Ir
O
N
O
N
HO
N
10
high pH
water soluble
14
neutral pH
organic soluble
15
low pH
water soluble
Fig. 1 Iridium bipyridine and bipyridonate complexes active in alcohol de-
hydrogenation and ketone/aldehyde hydrogenation reactions in organic
and aqueous solvents.
a) Bond Cleavage
OH
OMe
OH
OMe
O
Ir Cat.
OMe
α
α
β
O
β
O
O
Here we report a new approach using a highly efficient iridium-
based catalytic system for the oxidant-free cleavage of the Cα -Cβ
bond in β-O-4 lignin model compounds and real lignin (Scheme
1c), with in-situ stabilisation of the resulting aldehyde products
through (de)hydrogenation to the corresponding alcohols and
carboxylic acids. Additionally, the relatively low catalyst load-
ings, short reaction times, absence of stoichiometric oxidants and
water tolerance set our system apart from previous examples.
Due to the complexity of developing any new reaction di-
H
O
+
γ
MeO
H
OH
MeO
MeO
O
IrH2 Cat.
OMe
OMe
OMe
1
6
7
9
b) Product Stabilisation
IrH2 Cat.
-
Ir Cat.
OH
O
OH
O
OH
OH
O
H2O
Ir
Ir Cat.:
10
N
O
N
R
H
R
R
R
OH
IrH2 Cat.
Scheme 2 Proposed route for iridium catalysed β-O-4 cleavage and
product stabilisation.
2 |
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Table 1 Results of the iridium catalysed cleavage of β-O-4 lignin model compounds
OH
O
OH
OMe
OH
O
O
OMe
HO
OMe
O
Ir Cat.
OH
O
O
A
B
A
A
A
+
+
+
+
Base (Na2CO3)
dioxane-water (1:1)
130 °C, 1 hour
B
B
R1O
R2
R1O
R1O
R1O
HO
R2
R2
OMe
OMe
OMe
OMe
1: R1 = Me, R2 = H
18: R1 = R2 = H
A1
A2
A3
B1
B2
19: R1 = Me, R2 = OMe
Pre-catalyst to 10
Product Yields (%)
Entry
Model
Base (eq.)
Conversion (%)
Structure
14
mol%
A1
A2
43
31
0
28
38
5
11
33
2
67
22
A3
12
26
0
13
24
6
23
33
0
Total A
78
B1
56
70
0
41
67
7
B2
20
25
0
15
28
0
12
42
0
44
10
Total B
76
95
0
56
95
7
49
88
0
82
41
1
2
1
1
1
2
2
2
2
1
1
2
2
2
2
1
1
0
0.2
0.5
1
84
100
5
23
28
0
12
25
4
14
13
0
14
85
3
1
14
0
4
1
14
58
53
5
1
14
96
87
6a
7b
8
1
14
34
15
1
14
1
79
100
21
100
70
48
37
46
0
38
31
1
16+17
17
1
79
9
10
11
1
1
2
18
19
16+17
16+17
1
0
8
1
19
69c
49
1
a
b
in 100% 1,4-dioxane. in 100% H2O. c including 1% 4-methylguaiacol. Conversions and yields determined by quantitative 1H NMR analysis using
1,3,5-trimethoxybenzene as an internal standard. Every effort has been made to ensure the accuracy of these measurements however small errors (ca.
<5%) are expected. Blank reaction runs with A1-3 and B1-2, in the absence of catalyst, indicated that all the products are stable under the reaction
conditions. See Tables S1-5†for more extensive optimisation studies and Figures S1-3†for time course profiles.
2b).
Gratifyingly, our initial reaction of the etherified dimeric lignin
activity and selectivity observed in either water or 1,4-dioxane
alone (Entry 6-7).
model compound 1 with 1 mol% of catalyst 14, which gener-
ates 10 in-situ, in a 1:1 mixture of 1,4-dioxane/water using 1
equivalent of Na2CO3 as base in a closed system, resulted in 79%
conversion of 1 after 1 h at 130 ◦C (Table 1, Entry 1). Quan-
titative 1H NMR analysis of the reaction mixture revealed five
main products, all resulting from the intended primary alcohol
dehydrogenation/retro-aldol pathway. For the A ring, the ex-
pected benzaldehyde product (A2) was formed in 41% yield, to-
gether with benzyl alcohol A1 and benzoic acid A3 in 19% and
18% yields respectively. Following the reaction with respect to
time (Figures S1-3†) suggested that A2 is gradually converted to
A1 and A3 during the reaction (Figure S2†). Indeed, a control re-
action using only A2 as a substrate revealed that this is indeed the
case and that this is due to the action of the catalyst, rather than
base-catalysed disproportionation reactions.41 Furthermore, two
products were identified as originating from the B ring, namely
primary alcohol B1 and acid B2, which were formed in 63%
and 22% yields respectively. The high yields of B ring fragments
and lack of any observable B ring aldehydes, or aldol products,
suggests that these aldehydes are short lived species and never
present in appreciable quantities.
Interestingly, we found that simple addition of the catalyst com-
ponents 6,6’-dihydroxy-2,2’-bipyridine (16) and [Cp∗IrCl2]2 (17)
to the reaction mixture resulted in an active system with simi-
lar activity to the preformed pre-catalyst complex 14 (Entry 8).
This indicates that the active catalyst complex 10 readily forms
under these reaction conditions and, to the best of our knowl-
edge, this is the first example of such in-situ preparation of these
catalysts. This proposal was supported by the fact that omission
of ligand 16 resulted in greatly reduced activity (Entry 9). En-
couragingly, we also found that this system can effectively cleave
phenolic model 18, which, as mentioned previously, was found to
be a challenging substrate for organocatalytic approaches32 with
totals yield of 69% and 82% of the A and B ring products, re-
spectively. Interestingly, in this case, whilst phenolic A2 (vanillin)
was formed in good yield of 67%, only relatively small amounts
of benzoic acid A3 was formed (1%), whilst no benzyl alcohol
A1 was observed. Instead, small amounts of 4-methylguaiacol
(1%, See A4, Table 2 for structure) could be detected, which we
believe arises from phenolic A1, via reduction of an intermediate
quinone methide formed under the reaction conditions due to the
high pH. One explanation for the relatively low yields of A1 (or
A4) and A3, compared to the neutral products obtained from the
etherified model compound substrates, is that phenolic aldehyde
A2 will be deprotonated under the reaction conditions. This re-
sults in a delocalised negative charge on the phenolic aldehyde
and, therefore, an electrostatic repulsion will exist between this
substrate and the anionic catalyst complex 10, potentially lower-
ing the reactivity. Finally, model compound 19 with a sterically
bulkier syringyl B ring was tested. This model gave similar prod-
ucts but displayed somewhat reduced overall reactivity compared
Increasing the catalyst loading to 2 mol% improved conversion
to 100% after 1 h, with an accompanied improvement in the total
yields of the A and B ring fragments to 85% and 95%, respectively
(Entry 2). Sufficient base was found to be vital for good activity,
with at least 0.5 equivalents of Na2CO3 being required (Entry
3-5). Additional experiments showed that the exact nature of
the base was less important with other soluble carbonates and
sodium hydroxide also being suitable (Table S1†). In contrast,
the solvent mixture was found to be very important, with reduced
1–7 | 3

Green Chemistry
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to 1 (Entry 11), consistent with the increased steric demands of
Polysytrene
Standards (Mw)
this substrate.
Having successfully developed and applied our catalytic sys-
tem on model compounds, producing excellent yields of up to
87% and 95% respectively of A and B ring products, we turned
our attention towards real lignin. Treatment of a softwood en-
zyme lignin with approximately 2 mol% (based on total lignin)42
of in-situ formed catalyst (from 2.6wt% 16 and 5wt% 17) re-
sulted in the production of a lignin oil (82wt%) which had a
significantly lower molecular weight than the starting lignin (Oil
Native Lignin
Residue (Insoluble)
Oil (Soluble)
Mw = 1698, Mn = 612 vs. Enzyme Lignin Mw = 8326, Mn
=
1303), as determined by GPC analysis (Fig. 2). Additionally, a
small amount of extracting solvent insoluble (EtOAc/MeCN, 1:1),
higher molecular weight, material (18wt%) was also recovered
from the reaction. Detailed 2D HSQC NMR analysis of these frac-
tions revealed major structural changes had occurred in the lignin
(Fig. 3). Analysis of the linkage region (Fig. 3 A-C) showed that
cross peaks corresponding to β-O-4 units (A) had almost com-
pletely disappeared following the reaction and new cross peaks
consistent with both types of expected cleavage products were
present. For example, cross peaks for phenoxyethanol (PE), phe-
noxy acetic acid (PA) and benzyl alcohol (BAlc) groups (c.f. A and
B rings products from model compounds) were all clearly visible.
Additionally, analysis of the aromatic regions (Fig. 3 D-E) showed
the presence of new cross peaks which could be assigned to the
formation of benzaldehyde (BAld) and benzoic acid (BA) groups,
all completely in line with the chemistry seen for the model com-
pounds. Of the other lignin linkages, phenylcoumaran (β-5) units
(B) were also almost completely absent in the lignin products,
indicating that these units are also processed by our catalytic
system, whilst resinol (β-β) units (C) appeared to be relatively
inert, presumably because they possess no free alcohol groups.
Cinnamyl alcohol (X) and cinnamaldehyde (J) end groups were
also absent in the products but dihydroconiferyl alcohol groups
(DHCA) were still present. A number of additional new cross
peaks were also identified in the HSQC spectra which we could
assign to products from other minor reaction pathways in lignin.
For example, reduced β-O-4 units (Ar), phenylcoumaran deriva-
tives (B’) and arylpropanoic acid groups (PrA) were all present
in the products. The formation of all these units can be read-
ily explained based on the interplay of (de)hydrogenation and
base catalysis during the reaction (Scheme 3). In the case of Ar
units, we propose that the most likely route for their formation is
via reduction of intermediate enal structures (Scheme 3a), how-
ever direct formation and reduction of β-O-4 derived quinone me-
thides, without any oxidation step, is another possible route. For
B’ units, we believe the most likely route for their formation in-
volves an initial double dehydrogenation of the β-5 unit, followed
by base-catalysed ring opening43 and subsequent hydrogenation
of the conjugated double bond (Scheme 3b). This was proposed
over the potentially competing aldehyde pathway based on the
absence of any detectable B” units in the products (Scheme 3b).
A general reaction pathway for the conversion of primary alcohols
to acids (e.g. DHCA to PrA groups), via hydration of intermediate
aldehydes, is also shown based on previous proposals for related
iridium catalyst complexes (Scheme 3c).44
10
15
20
time (min)
25
Fig. 2 GPC analysis of softwood enzyme lignin before (green, Mw
8326, Mn = 1303) and after reaction: residue (blue, Mw = 9615, Mn
1308) and oil (orange, Mw = 1698, Mn = 612). The yield of residue was
18wt% from which an oil yield of 82wt% was calculated (by difference).
Elution times for polystyrene standards (grey) are added for reference.
=
=
a)
OH
OMe
OH
O
OMe
O
OMe
A1-4
(from end groups)
+
L1-6
(from internal units)
Major
Pathway
O
O
O
Ir Cat.
+
A
L
A
L
A
L
H2
O
O
HO
HO
OMe
HO
HO
OMe
Base
OMe
β-O-4 (A)
H2
O
OMe
OMe
O
Ir Cat.
A
L
A
L
+2H2
O
HO
HO
HO
Minor
Pathway
OMe
OMe
Ar
b)
O
O
HO
O
OH
OH
OH
Base
Ir Cat.
-2H2
Ir Cat.
O
O
O
O
+H2
O
O
MeO
OH
MeO
OH
O
O
+H2O
OMe
OMe
OMe
OMe
OMe
OMe
β-5 (B)
B'
Ir Cat.
-H2
O
+H2
Ir Cat.
H2
O
O
H
H
OH
Base
Ir Cat.
O
OMe
O
O
+2H2
O
MeO
OH
MeO
OH
O
OMe
OMe
B''
OMe
NOT OBSERVED
c)
OH
OH
OH
O
H2O
Ir Cat.
Ir Cat.
-H2
OH
O
H2
O
O
O
O
OMe
DHCA
OMe
OMe
OMe
PrA
Scheme 3 Proposed routes for iridium catalysed processing of some of
the major units in lignin. a) β-O-4 b) β-5 and c) dihydrocinnamyl alcohol
units.
GC-FID analysis of the reaction mixture showed that all the
expected monomeric products were present in the lignin oil to
different degrees (Table 2, Entry 1). Additionally, we could group
these products into those obtained from β-O-4 end groups (A1-
4), requiring only a single bond cleavage to be released, and
those obtained from internal units (L1-6), requiring the cleav-
age of two adjacent β-O-4 linkages to be released. Overall the
total yield of identified monomeric products was 3.8wt%, with
the most abundant products being vanillin (A2), L1, L3, L4 and
L6. To put this into perspective, based on a β-O-4 content of
38 per 100 C9 units and a degree of (C5) condensation of 34%
4 |
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B) Reaction Oil
A) Enzyme Lignin
C) Reaction Residue
D) Enzyme Lignin
DHCA
DHCA
G2
110
115
120
125
PrA
PrA
J2
DHCA
DHCA
30
40
50
60
70
80
DHCA
DHCA
30
40
50
60
70
80
30
40
50
60
70
80
Ar
B’
Arα
G5/6
PrA
PrA
J6
B’
J
H2/6
B’
B’
E) Reaction Oil
BAld/BA2
110
115
120
125
B
C
Methoxy
C
C
Methoxy
Methoxy
PE
PE
DHCA
DHCA
A
X
B
BAlc
BAlc
BA6
DHCA
Ar
PE
Ar
PA
PA
BAld6
H2/6
PE
A
F) Reaction Residue
BAld/BA2
Units/100 C9
A: 3.6
B: 0.6
Units/100 C9
A: 7.7
B: 1.3
C
C
C
110
115
120
125
C: 1.9
PE: 21
C: 3.8
PE: 21
Ar
Ar
BAlc: 8.2
PA: 6.5
Ar: 4.2
B’: 6.6
BAlc: 7.7
PA: 5.0
Ar: 6.1
B’: 7.5
Units/100 C9
A: 38
B: 10
BA6
A
C
C
C
H2/6
7.0
C: 2.5
B
13C
1H
ppm
7.5
6.5
6
5
4
3
2
6
5
4
3
2
6
5
4
3
2
phenylcoumaran ( -5)
guiaicyl unit
-aryl ether ( -O-4)
resinol (
-
)
cinnamyl alcohol
cinnamaldehyde
dihydroconiferyl alcohol
reduced -aryl ether
phenylcoumaran derivative
phenoxyacetic acid
phenoxyethanol
benzyl alcohols
benzaldehydes
benzoic acids
p-hydroxyphenyl
propanoic acids
Fig. 3 1H-13C HSQC NMR analysis of the linkage (A-C) and aromatic regions (D-F) of softwood enzyme lignin (A and D), reaction oil (B and E) and
residue (C and F). Contours are colour coded and labelled to match the assigned structures as shown. Reaction conditions: ∼ 2 mol% cat. (5wt%
[Cp∗IrCl2]2, 2.6wt% 6,6’-dihydroxy-2,2’-bipyridine), 1,4-dioxane/water (1:1), Na2CO3, 130 ◦C, 1 hour (Table 2, Entry 1). Unit quantities were estimated
based on volume integrals of the characteristic cross peaks for each unit relative to the total integral for the aromatic G and H cross peaks. These
values are intended for qualitative comparisons and are not rigorously quantitative. For the enzyme lignin this analysis gave identical results to using
only the G2 and H2/6 cross peaks, i.e. the per 100 C9 values were equivalent, hence the use of this descriptor was maintained but may not strictly be
correct. See Figure S4†for HSQC-TOCSY analysis, and Figures S5-6†for more detailed assignment of B’, PrA and Ar units.
for softwood lignin,45 the estimated theoretical maximum yield
of β-O-4 derived monomers from this softwood lignin is approx-
imately 9.5wt%.46, 47 Moreover, products such as L1-6 have not
previously been reported from lignin, meaning this system allows
us to expand the scope of lignin depolymerisation reactions into
new classes of structurally interesting compounds. For example,
L3 has previously been investigated for the synthesis of bio-based
polyesters,48 and it is easy to imagine similar uses for many of
these compounds as bio-renewable aromatic buildings blocks.
(organosolv) lignin that also had a high β-O-4 content (Entry
6).49 This lignin performed slightly better in the reaction, giving
improved yields of the same monomeric products compared to
the enzyme lignin. In particular, the small increase in yield is
predominantly due to an increase in the amount of end group
derived vanillin (L2). Given the rather large range of products
generated in this reaction, we considered whether it would be
possible to further process the products so that they converge to
a single product, L6 in this case. Indeed, we found that simply
further refluxing the crude mixture of products open to air for 16
h, led to a significant convergence of the products giving L6 as
the major product in an improved 2.4wt% yield (Entry 7), with
a concurrent disappearance or significant reduction in the yields
of most of the other products (L1-5). The selectivity towards
L6 could be further enhanced by performing the second reflux
step in 100% water, although overall lower yields were obtained
(Entry 8). It should be noted that these are not extensively
Replacing the 1,4-dioxane solvent with diglyme was well
tolerated (Entry 4), however omitting the organic solvent
altogether severely reduced monomer yields (Entry 5), consistent
with our model studies. Interestingly, based on HSQC NMR
analysis, this appears to be due to a switch in selectivity to
unproductive reaction pathways for the β-O-4 linkages linkages
and concomitant low monomer yields, rather than poor catalyst
activity (Figure S7†). We also tested our system on a dioxasolv
1–7 | 5

Green Chemistry
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Table 2 Results of the iridium catalysed depolymerisation of softwood lignins
OH
OMe
HO
OH
O
O
OH
O
O
OH
O
O
O
OH
HO
OH
OH
A
L
A1
A2
A3
A4
L1
L2
L3
L4
L5
L6
HO
HO
OMe
O
HO
HO
HO
O
O
O
O
O
O
OH
OMe
OMe
OMe
OMe
OMe
OMe
OMe
O
OMe
O
OMe
O
OMe
OH
OH
OH
OH
OH
OH
Pre-Catalyst
Time
Product Yields (wt%)c
L1 L2 L3
Entry
Lignina
Solvent
(wt% of 17)b
(h)
1
A1
A2
1.1
0.38 0.04
1.0
0.82 0.10
0.00 0.17
1.4
A3
A4
L4
L5
L6
Total
3.8
1
2
EL
EL
Dioxane/Water
Dioxane/Water
Dioxane/Water
Diglyme/Water
Water
Dioxane/Water
Dioxane/Water
Dioxane/Water
5
1.5
5
0.02
0.01
0.03
0.03
0.00
0.04
0.12 0.15
0.53 0.07
0.44 0.42 0.05 0.89
0.03 0.00 0.01 0.12
0.44 0.46 0.06 0.77
0.39 0.34
0.22 0.00
0.54 0.58 0.05 0.84
1
0.05 0.01 0.00
0.65
3.8
3
EL
2
0.15 0.36
0.34 0.15
0.58 0.44 0.09
0.42 0.00 0.00
0.34 0.11
4
EL
5
2
0.05 0.58
0.10 0.42
3.4
5
EL
5
2
1.5
6
Dioxasolv
Dioxasolv
Dioxasolv
5
2
0.13 0.56
0.54 0.28
4.6
7d
8e
5
5
1+16 0.00
1+16 0.00
1.4
0.00 0.55
0.00 0.19
0.18 0.00 0.11
0.01 0.00 0.01
0.00 0.00 0.00
2.4
1.8
4.8
2.7
b
EL = enzyme lignin, Dioxasolv = lignin extracted with dioxane/water (8:2), 0.1 M HCl, 1 hour, reflux. 5wt% of 17 corresponds to ∼2mol% loading
c
of Ir based on total lignin. 1.5wt% corresponding to ∼2mol% loading of Ir based on β-O-4 content. from GC-FID analysis of silylated products using
d
1,3,5-trimethoxybenzene as internal standard and experimentally determined response factors. Crude reaction mixture further refluxed open to air
e
for 16 h. Crude reaction mixture concentrated, redissolved in the same volume of water and refluxed open to air for 16 h.
optimised conditions, however they do elegantly demonstrate an
Acknowledgements
ability to tune and simplify the complex mixtures of products
The authors would like to gratefully acknowledge Dr. Hans Wienk
obtained directly from lignin depolymerisation reactions.
and the NMR facility of Utrecht University. The CatchBio program
is also acknowledged for financial support.
Conclusions
Notes and references
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A new Ir catalysed approach for the selective cleavage of the Cα-Cβ bond in lignin β-O-4 units,
allowing access to novel and tuneable monomeric product mixtures.