Rhodium-terpyridine catalyzed redox-neutral depolymerization of lignin in water

Article abstract of DOI:10.1039/c9gc03057c

Simple rhodium terpyridine complexes were found to be suitable catalysts for the redox neutral cleavage of lignin in water. Apart from cleaving lignin model compounds into ketones and phenols, the catalytic system could also be applied to depolymerize dioxasolv lignin and lignocellulose, affording aromatic ketones as the major monomer products. The (hemi)cellulose components in the lignocellulose sample remain almost intact during lignin depolymerization, providing an example of a "lignin-first" process under mild conditions. Mechanistic studies suggest that the reaction proceeds via a rhodium catalyzed hydrogen autotransfer process.

Full text of DOI:10.1039/c9gc03057c

Green Chemistry
View Article Online
View Journal | View Issue
COMMUNICATION
Rhodium-terpyridine catalyzed redox-neutral
depolymerization of lignin in water†
Cite this: Green Chem., 2020, 22, 33
a,b
b
a
a
a
Yuxuan Liu, Changzhi Li, * Wang Miao, Weijun Tang,
Dong Xue,
Received 30th August 2019,
Accepted 20th November 2019
a,c
b
a
Jianliang Xiao,
Tao Zhang
and Chao Wang
*
DOI: 10.1039/c9gc03057c
rsc.li/greenchem
1
9–21
Simple rhodium terpyridine complexes were found to be suitable lignin,
the following major issues have not been well
catalysts for the redox neutral cleavage of lignin in water. Apart resolved in most cases: (1) in a certain number of processes,
9
,10,17,25–29
12,13,21,30–37
from cleaving lignin model compounds into ketones and phenols, external oxidants
or reductants
are
the catalytic system could also be applied to depolymerize dioxa- required, which are often hazardous and generate waste; (2)
solv lignin and lignocellulose, affording aromatic ketones as the severe reaction conditions, such as high temperature and
9
,14,31
major monomer products. The (hemi)cellulose components in the pressure, are usually required;
lignocellulose sample remain almost intact during lignin depoly- alcohols, toluene, and dioxane,
(3) organic solvents, e.g.
1
8,22,23,31,38–41
are normally
merization, providing an example of a “lignin-first” process under used, which does not meet the green chemistry criteria. A
mild conditions. Mechanistic studies suggest that the reaction pro- lignin depolymerization process requiring no external oxidant/
ceeds via a rhodium catalyzed hydrogen autotransfer process.
reductant in aqueous media under mild conditions is desir-
able from the green chemistry viewpoint. Lignin contains a
Aromatic compounds are important building blocks for the
production of value-added chemicals for materials, fine chemi-
6,7,11
large number of hydroxyl groups in its sidechain,
which
have the potential to be dehydrogenated to provide hydrogen
for the hydrogenolysis of C–O bonds. Thus, the redox-neutral
cleavage or depolymerization of lignin could be realized, if a
catalyst can transfer the hydrogen from the alcohol moieties
in lignin to cleave its C–O bonds. This concept has
been recently demonstrated by several groups with both
1
–4
cals and pharmaceuticals. Currently, aromatic chemicals are
mainly produced from fossil resources. With diminishing
fossil resources and the growing worldwide concern over this
environmental problem, it is vital to find renewable
approaches for the production of aromatic chemicals. Lignin
is the largest aromatic heteropolymer in nature, accounting for
22,23,39,42–44
19,20,30,45–48
homogeneous
and heterogeneous
cata-
5
,6
1
5–30 wt% of biomass. By virtue of this feature, lignin is
lytic systems. Despite these progresses, homogeneous catalytic
systems with water as the solvent under mild conditions,
which could be used for real lignin depolymerization, are still
highly sought after. As native lignin is generally an insoluble
solid, homogeneous catalytic systems might be more suitable
for its depolymerization compared to heterogeneous catalytic
systems in view of its mass transfer efficiency. Recently, we
have demonstrated this hypothesis with a water soluble binuc-
considered to be a promising renewable source of aromatics
7
,8
that can alleviate the dependence on fossil resources.
However, due to the complicated amorphous structure and
highly heterogeneous nature of lignin, selective depolymeriza-
5
,6,9–11
tion of lignin remains a challenge.
One approach is to
develop an efficient catalytic system for the selective cleavage
of the relatively unreactive C–O bonds in lignin, particularly
1
2–24
the most abundant β-O-4 linkages.
Despite the various
24
lear Rh complex catalytic system.
elegant strategies reported for the chemical disassembly of
In this work, structurally tunable terpyridine mononuclear
Rh complexes are found to be efficient catalysts for the selective
cleavage of lignin model compounds into ketones and phenols
with water as the solvent under mild conditions. In addition,
this robust catalytic system can be used for depolymerizing real
lignin and raw woody biomass, affording aromatic ketones as
the major monomer products. Notably, the lignin component
in raw woody biomass could be selectively depolymerized,
leaving the (hemi)cellulose components intact, providing an
a
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an,
7
10119, China. E-mail: c.wang@snnu.edu.cn
b
CAS Key Laboratory of Science and Technology on Applied Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China.
E-mail: licz@dicp.ac.cn
c
Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, UK
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
49–51
c9gc03057c
example of a homogeneous “lignin-first”
process.
This journal is © The Royal Society of Chemistry 2020
Green Chem., 2020, 22, 33–38 | 33

View Article Online
Communication
Green Chemistry
Terpyridine ligands have found broad applications in obtained (Table 1, entry 1). Catalysts with electron-donating
coordination chemistry and molecular recognition. substituents showed better activity than the ones with elec-
Terpyridine metal complexes are oxidatively and thermally tron-withdrawing groups (1a vs. 1c, and 1g vs. 1h). During the
robust. However, examples of using terpyridine metal com- reaction, we observed that catalyst 1a is insoluble in water. As
5
2–57
plexes as catalysts are few.
Previously, we have shown that the model substrate 2a also has low solubility in water, the
a binuclear Rh complex with terpyridine (2,2′:6′,2″-terpyridine) cleavage reaction might take place in an “on water” manner,
2
4
ligand is an effective catalyst for dehydrogenation of alco- differing from our previous binuclear Rh catalytic system.
58
59
hols, dehydrogenative coupling reactions, borrowing hydro- We then increased the hydrophobicity of the catalysts by intro-
gen reactions and aqueous redox-neutral depolymerization ducing more hydrophobic groups, e.g. 2-naphthyl (1d) and
6
0
2
4
of lignin. Nevertheless, the structure of this binuclear Rh 9-anthracyl (1e). Although good conversion was obtained with
complex is difficult to modify, as the binuclear structure might 1d (Table 1, entry 4), low yield was observed with 1e (Table 1,
not be formed upon changing the substituents on terpyridine. entry 5), suggesting the requirement of a balanced hydropho-
Crabtree and co-workers reported that Ru and Ir terpyridine bicity and hydrophilicity. Taken together, the above results
complexes could catalyze borrowing hydrogen reactions suggest that ligands with electron-donating groups and conju-
5
4
between alcohols. We envisioned that structurally tunable gated groups with moderate hydrophobicity are good for this
simple Rh terpyridine complexes might be also employed as transformation. The amount of NaOH also affected the conver-
catalysts for hydrogen transfer reactions. We thus synthesized sion and yields of the products of the reaction (Table 1, entries
several Rh-terpyridine complexes (Table 1, 1a–1h) with 9 and 10). Full conversion of 2a into 4a (84% yield) and 5a
different electronic and hydrophilic properties and tested (87% yield) was achieved and a small amount of the dehydro-
them as catalysts for the redox-neutral cleavage of a lignin genated intermediate 3 was obtained, when the reaction was
model compound (2a) in water. Gratifyingly, when the reaction conducted with 1 mol% of catalyst 1a in the presence of 1.5
was performed with 1 mol% 1a and 1 equivalent of NaOH in equivalents of NaOH (lower amount of base compared with
2
4
water at 110 °C for 12 h, the majority of 2a was cleaved into 4a our previous binuclear Rh system) in water at 110 °C for 12 h
4-acetylanisole, 73% yield) and 5a (guaiacol, 77% yield), with under an Ar atmosphere (Table 1, entry 10).
(
a small amount of the dehydrogenation product 3 being
A variety of lignin model compounds were then tested to
probe the versatility of the catalyst (Table 2). It was found that
the methoxy groups at ortho-, meta- and para-positions on both
phenyl rings, which are commonly found in natural lignin, are
all tolerated under the optimized conditions (Table 2, entries
Table 1 Rh catalyzed redox-neutral cleavage of the C–O bond of a
a
lignin model substrate
1
–6). Full conversions were observed for these substrates.
However, low activities were observed for substrates with a
hydroxyl group on the α-phenyl ring; the starting substrates
could not be fully consumed (67% conversion for 2g and 79%
conversion for 2h) even with higher catalyst loading and pro-
longed reaction time (Table 2, entries 7 and 8). Delightfully,
substrates with the γ-OH functionality (2i and 2j) were fully
consumed, albeit affording relatively complex products
(Table 2, entries 9 and 10). As the structures of 2i and 2j are
closely related to that of real lignin, we then tested the catalytic
system for the depolymerization of real lignin samples.
b
Yield (%)
NaOH
(equivalent)
Conversion
(%)
Entry
Catalyst
3
4a
5a
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1a
1a
1
1
1
1
1
1
1
1
86
82
62
80
21
55
69
40
65
100
7
10
9
8
8
11
11
11
11
8
73
67
48
65
11
38
46
23
47
84
77
71
51
69
Dioxasolv lignin extracted from poplar wood was used to
test the feasibility of the depolymerization of real lignin
samples with the above developed catalytic system.
12 Delightfully, the lignin can be effectively converted into an oil
41
52
26
51 that the molecular weights of the major products are in the
87
sample under the optimized catalytic conditions, with a yield
of 90 wt%. MALDI-TOF analysis of the oil samples suggested
c
9
0
0.5
1.5
d
1
range of 50–400 Da (Scheme S1†), indicating monomers,
dimers and trimers to be the major products. Further analysis
by GC-MS and GC-FID indicated that aromatic ketones were
the dominant monomers with a yield of 10.8 wt% based on oil
and 9.7 wt% based on the starting lignin solid (Scheme S2 and
a
Reaction conditions: 2a (0.2 mmol), catalyst (0.002 mmol), NaOH Table S3†).
(0.2 mmol), H
2
O (1 mL), 110 °C, 12 h, under Ar. After the reaction,
The contents of the monomeric units and the percentage of
different linkages of lignin were characterized by Two-
Dimensional Heteronuclear Single Quantum Coherence NMR
hydrochloric acid (1 M) was used to acidify the solution to pH = ca. 1.
b
Yields were determined by GC-FID with diphenyl as the internal stan-
c
d
dard. NaOH (0.1 mmol). NaOH (0.3 mmol).
34 | Green Chem., 2020, 22, 33–38
This journal is © The Royal Society of Chemistry 2020

View Article Online
Green Chemistry
Communication
a
Table 2 Cleavage of different lignin model substrates
1
2
b
Entry
2
R
R
Time (h)
Yield (%)
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
2g
2h
2i
4-OMe
—
—
3,4-OMe
3,4-OMe
3,4-OMe
4-OH
2-OMe
2-OMe
—
2-OMe
2,6-OMe
3,5-OMe
2-OMe
2-OMe
12
12
12
12
12
12
60
60
60
4a, 84c
4b, 76
4b, 79
4c, 83
4c, 87
4c, 82
4d, 48
4e, 69
5a, 87
5a, 90
5b, 76
5a, 85
5c, 90
5d, 86
5a, 60
5a, 73
d
7
8
d
3-OMe, 4-OH
9
10
2j
60
a
2
Reaction conditions: 2 (0.4 mmol), 1a (0.004 mmol), NaOH (0.6 mmol), H O (1 mL), 110 °C, under Ar. After the reaction, hydrochloric acid
d
b
c
(1 M) was used to acidify the solution to pH = ca. 1. Isolated yields. Yield was determined by GC-FID. 1a (0.008 mmol).
(2D HSQC NMR). Fig. 1 (I) shows that β-O-4 aryl ethers (A lin- from the oil products via simple filtration. Enzymatic hydro-
kages) are dominant linkages (up to 87.7%), whilst the lysis of the solid residue yielded 72% of glucose and 20% of
number of β-5 (B linkages, 6.2%) and β–β (C linkages, 6.1%) is xylose, respectively (based on the weight of solid), which
very small in dioxasolv lignin. After the reaction, the 2D NMR suggests that the solid residue mainly consists of cellulose and
spectra show that most of the A linkages have disappeared hemicellulose. The above results suggest that the Rh catalytic
4
9–51
(
Fig. 1, I vs. III), indicating that the majority of the A linkages system is capable of “lignin-first” depolymerization
were cleaved by the catalytic system. The aromatic regions of lignocellulose.
the 2D-HSQC NMR spectra show that the S/S′ molar ratio has The oil product obtained from the raw wood powder was
decreased from 68.3 to 1.2 (Fig. 1, II vs. IV), suggestive of the further analyzed by GC-MS, GC-FID and 2D HSQC NMR. The
dehydrogenation of the C –OH group. The dehydrogenation of GC-MS and GC-FID analysis results indicate that the structures
the C –OH group is also supported by the presence of G′ , G′ of the aromatic monomers are similar to those obtained from
of
α
α
2
5
and G′ as shown in Fig. 1 (IV). These results suggest that the dioxasolv lignin oil (Scheme S2 vs. Scheme S4 and Table S3 vs.
6
depolymerization of real lignin could be achieved with the S5†). The total yield of the monomers is 16.7 wt% based on
aqueous redox-neutral catalytic system.
the oil and 2.5 wt% based on the starting raw poplar wood
To further demonstrate the versatility of this catalytic powder. Further, the 2D HSQC NMR analysis results show that
system, conversion of raw wood powder (poplar, 40 mesh, most of the A linkages (β-O-4) have disappeared in the side-
1
000 mg) without further pretreatment was conducted under chain region (Fig. 1, I vs. V) and certain amounts of S2,6′, G
2
′,
the standard conditions using 1a as the catalyst. Impressively, G ′ and G ′ have appeared in the aromatic region of the oil pro-
5
6
1
5 wt% of oil products (based on the starting raw wood ducts (Fig. 1, II vs. VI), which are in agreement with the results
powder) was obtained in water at 110 °C for 24 h, and 65 wt% of the dioxasolv lignin sample. The above results suggest that
6
1
(
651 mg) of a solid residue was recovered. Noting that the the Rh-terpyridine catalytic system is highly active and selec-
lignin content in poplar wood powder is determined to be tive not only for the cleavage of lignin model compounds and
6
2
1
8.6%, most of the lignin (80.6%) has been deconstructed real lignin depolymerization, but also for the woody biomass
into oil products. The solid residue could be easily separated depolymerization in a “lignin-first” manner, affording aro-
This journal is © The Royal Society of Chemistry 2020
Green Chem., 2020, 22, 33–38 | 35

View Article Online
Communication
Green Chemistry
a
Table 3 Mechanistic studies
b
Yield (%)
Entry
Additive
Conversion (%)
4a
5a
1
2
—
9
80
4
73
7
78
1-Phenylethan-1-ol
0.2 mmol)
(0.2 mmol)
(
3
H
2
26
16
24
a
Reaction conditions: 3a (0.2 mmol), 1a (0.002 mmol), NaOH
O (1 mL), 12 h, under Ar. After the reaction, hydro-
(
0.3 mmol), H
2
chloric acid (1 M) was used to acidify the solution to pH = ca. 1.
b
Yields were determined by GC-FID with diphenyl as the internal
standard.
Scheme 1 Deuterium labelling experiment.
our previous studies on binuclear Rh catalyzed lignin depoly-
2
4
merization. Thus, the key steps for this mononuclear Rh-ter-
pyridine complex catalyzed lignin cleavage might be similar to
those with the binuclear Rh complex.
Substrates 2i and 2j with the γ-OH functionality gave
complex products (Table 2, entries 9 and 10). Based on the
above mechanistic studies, a plausible mechanism for the for-
mation of these products as exemplified with 8 is proposed
Fig. 1 2D HSQC NMR spectra. I and II: dioxasolv poplar lignin; III and
IV: oil obtained from the dioxasolv poplar lignin sample; V and VI: oil
obtained from lignocellulose.
(
Scheme 2). 8 is first dehydrogenated to produce a Rh hydride
and 9; 9 could undergo a retro-aldol reaction to give 10 and
formaldehyde (the formation of formaldehyde is confirmed by
1
matic ketones as the major monomer products. As aromatic
H NMR studies, see section 8.2 of the ESI for details†) or de-
ketones were obtained as the major monomers in the depoly- hydration to afford 11; 10 could be cleaved by a Rh hydride to
merization of both real lignin and lignocellulose, the cleavage give 14 and 15; and 11 could be reduced by a Rh hydride to
might follow a mechanism similar to that for the model form 12, which could be further cleaved to give 13 and 14
substrates.
The mechanism for this Rh-terpyridine catalyzed redox
neutral depolymerization was then considered. Upon cleavage
of the model compound 2a, the dehydrogenated product 3a
was obtained. Based on our previous study and the literature,
3
a could serve as an intermediate for the cleavage reaction.
Indeed, 3a was cleaved into ketone and phenol products in the
presence of a hydrogen source (Table 3). These results suggest
that the cleavage was initiated by dehydrogenation of the
alcohol moiety into a ketone intermediate (with a weaker ether
C–O bond compared with 2a, 55.9 vs. 69.2 kcal mol− ), fol-
1 16
lowed by the reductive cleavage of the ether bond. Further deu-
terium labelling studies showed that the deuterium atom adja-
cent to the hydroxyl group in 2a′ was selectively transferred to
the α-methyl group of the ketone product 4a′ (Scheme 1), poss-
ibly via a Rh–D intermediate. These observations are similar to Scheme 2 A plausible mechanism for the cleavage of 8.
36 | Green Chem., 2020, 22, 33–38
This journal is © The Royal Society of Chemistry 2020

View Article Online
Green Chemistry
Communication
(
Scheme 2). Clearly, the hydrogen source in 8 is not sufficient
6 C. Li, X. Zhao, A. Wang, G. W. Huber and T. Zhang, Chem.
Rev., 2015, 115, 11559–11624.
7 C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and
M. Poliakoff, Science, 2012, 337, 695–699.
to cleave itself completely into 13, 14, and 15. Thus, a substan-
tial amount of 12 was produced. Various lines of evidence
suggest that 1a remains mononuclear for this reaction
(see section 8.3 in the ESI for details†).
8 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006,
1
06, 4044–4098.
Z. R. Zhang, J. L. Song and B. X. Han, Chem. Rev., 2017, 10,
834–6880.
9
6
Conclusions
1
0 H. Guo, D. M. Miles-Barrett, A. R. Neal, T. Zhang, C. Li and
In conclusion, simple and structurally modular Rh terpyridine
N. J. Westwood, Chem. Sci., 2018, 9, 702–711.
complexes are found to be effective catalysts for redox-neutral 11 J. A. Melero, J. Iglesias and A. Garcia, Energy Environ. Sci.,
cleavage of the C–O bonds of β-O-4 lignin model compounds 2012, 5, 7393–7420.
in water under mild conditions. The modularity of the catalyst 12 M. V. Galkin, C. Dahlstrand and J. S. M. Samec,
structure allows us to find a catalyst with optimal electronic ChemSusChem, 2015, 8, 2187–2192.
and hydrophobic properties for lignin cleavage, which requires 13 M. V. Galkin, S. Sawadjoon, V. Rohde, M. Dawange and
less base additives (1.5 equiv. vs. 4 equiv.) and behaves differ- J. S. M. Samec, ChemCatChem, 2014, 6, 179–184.
ently (on water vs. in water) compared with our previous binuc- 14 F. Gao, J. D. Webb, H. Sorek, D. E. Wemmer and
lear Rh catalytic system. The catalytic system could also be J. F. Hartwig, ACS Catal., 2016, 6, 7385–7392.
used for depolymerizing real lignin and raw poplar wood 15 S. H. Lim, K. Nahm, C. S. Ra, D. W. Cho, U. C. Yoon,
powder. The depolymerization of raw poplar wood powder pro-
J. A. Latham, D. Dunaway-Mariano and P. S. Mariano,
ceeds in a “lignin-first” manner giving high yields of lignin oil
J. Org. Chem., 2013, 78, 9431–9443.
products. The solid residue could be easily separated and enzy- 16 J. D. Nguyen, B. S. Matsuura and C. R. J. Stephenson, J. Am.
matically hydrolyzed to produce glucose and xylose.
Chem. Soc., 2014, 136, 1218–1221.
1
1
7 C. S. Lancefield, O. S. Ojo, F. Tran and N. J. Westwood,
Angew. Chem., Int. Ed., 2015, 54, 258–262.
8 A. Wu, B. O. Patrick, E. Chung and B. R. James, Dalton
Trans., 2012, 41, 11093–11106.
Conflicts of interest
There are no conflicts to declare.
19 N. Luo, M. Wang, H. Li, J. Zhang, T. Hou, H. Chen,
X. Zhang, J. Lu and F. Wang, ACS Catal., 2017, 7, 4571–
4580.
2
0 X. Wu, X. Fan, S. Xie, J. Lin, J. Cheng, Q. Zhang, L. Chen
and Y. Wang, Nat. Catal., 2018, 1, 772–780.
Acknowledgements
The authors acknowledge Xiaozan Dai and Prof. Zongbao Kent 21 A. Rahimi, A. Ulbrich, J. J. Coon and S. S. Stahl, Nature,
Zhao for performing the enzymatic hydrolysis reaction. This 2014, 515, 249–252.
research was supported by the 2017 Royal Society International 22 W. Huo, W. Li, M. Zhang, W. Fan, H. M. Chang and
Collaboration Award (IC170044), the National Natural Science H. Jameel, Catal. Lett., 2014, 144, 1159–1163.
Foundation of China (21773145, 21473109, 21690080, 23 J. M. Nichols, L. M. Bishop, R. G. Bergman and
1690083, and 21878288), the Science and Technology J. A. Ellman, J. Am. Chem. Soc., 2010, 132, 12554–12555.
Program of Shaanxi Province (2016KJXX-26), Projects for the 24 Y. Liu, C. Li, W. Miao, W. Tang, D. Xue, C. Li, B. Zhang,
2
Academic Leaders and Academic Backbones, Shaanxi Normal
University (16QNGG008), the 111 project (B14041), and the
DNL cooperation fund CAS (DNL180302).
J. Xiao, A. Wang, T. Zhang and C. Wang, ACS Catal., 2019,
9, 4441–4447.
25 S. K. Hanson, R. Wu and L. A. P. Silks, Angew. Chem., Int.
Ed., 2012, 51, 3410–3413.
26 B. Sedai, C. Díaz-Urrutia, R. T. Baker, R. Wu, L. A. P. Silks
and S. K. Hanson, ACS Catal., 2011, 1, 794–804.
Notes and references
2
7 Z. P. Cai, J. X. Long, Y. W. Li, L. Ye, B. L. Yin, L. J. France,
J. C. Dong, L. R. Zheng, H. Y. He, S. J. Liu, S. C. E. Tsang
and X. H. Li, Chem, 2019, 5, 2365–2377.
1
2
3
T. Dai, C. Li, L. Li, Z. K. Zhao, B. Zhang, Y. Cong and
A. Wang, Angew. Chem., Int. Ed., 2018, 57, 1808–1812.
A. Maneffa, P. Priecel and J. A. Lopez-Sanchez, 28 S. Son and F. D. Toste, Angew. Chem., Int. Ed., 2010, 49,
ChemSusChem, 2016, 9, 2736–2748. 3791–3794.
A. E. Settle, L. Berstis, N. A. Rorrer, Y. Roman-Leshkóv, 29 J. Ji, H. Guo, C. Li, Z. Qi, B. Zhang, T. Dai, M. Jiang, C. Ren,
G. T. Beckham, R. M. Richards and D. R. Vardon, Green
Chem., 2017, 19, 3468–3492.
Y. Xu, D. Liu and X. Liu, Appl. Catal., A, 2018, 552, 168–183.
A. Wang and T. Zhang, ChemCatChem, 2018, 10, 415–421.
30 M. V. Galkin and J. S. M. Samec, ChemSusChem, 2014, 7,
2154–2158.
4
5
J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius and 31 I. Klein, C. Marcum, H. Kenttamaa and M. M. Abu-Omar,
B. M. Weckhuysen, Chem. Rev., 2010, 110, 3552–3599. Green Chem., 2016, 18, 2399–2405.
This journal is © The Royal Society of Chemistry 2020
Green Chem., 2020, 22, 33–38 | 37

View Article Online
Communication
Green Chemistry
3
2 T. H. Parsell, B. C. Owen, I. Klein, T. M. Jarrell, 47 B. Zhang, C. Li, T. Dai, G. W. Huber, A. Wang and
C. L. Marcum, L. J. Haupert, L. M. Amundson, T. Zhang, RSC Adv., 2015, 5, 84967–84973.
H. I. Kenttamaa, F. Ribeiro, J. T. Miller and M. M. Abu- 48 J. W. Zhang, G. P. Lu and C. Cai, Green Chem., 2017, 19,
Omar, Chem. Sci., 2013, 4, 806–813. 4538–4543.
3 H. R. Wu, J. L. Song, C. Xie, C. Y. Wu, C. J. Chen and 49 Z. Cao, M. Dierks, M. T. Clough, I. B. Daltro de Castro and
B. X. Han, ACS Sustainable Chem. Eng., 2018, 6, 2872–2877. R. Rinaldi, Joule, 2018, 2, 1118–1133.
4 A. G. Sergeev and J. F. Hartwig, Science, 2011, 332, 439–443. 50 T. Renders, S. Van den Bosch, S. F. Koelewijn, W. Schutyser
3
3
3
5 A. L. Jongerius, P. C. A. Bruijnincx and B. M. Weckhuysen,
and B. F. Sels, Energy Environ. Sci, 2017, 10, 1551–1557.
51 R. Rinaldi, Joule, 2017, 1, 427–428.
Green Chem., 2013, 15, 3049–3056.
3
3
3
3
6 J. Zhang, J. Teo, X. Chen, H. Asakura, T. Tanaka, 52 S. Enthaler, B. Hagemann, G. Erre, K. Junge and M. Beller,
K. Teramura and N. Yan, ACS Catal., 2014, 4, 1574–1583. Chem. – Asian J., 2006, 1, 598–604.
7 C. Crestini, M. C. Caponi, D. S. Argyropoulos and 53 D. Gnanamgari and R. H. Crabtree, Organometallics, 2009,
R. Saladino, Bioorg. Med. Chem., 2006, 14, 5292–5302. 28, 922–924.
8 S. C. Chmely, S. Kim, P. N. Ciesielski, G. Jiménez-Osés, 54 D. Gnanamgari, C. H. Leung, N. D. Schley, S. T. Hilton and
R. S. Paton and G. T. Beckham, ACS Catal., 2013, 3, 963–974.
R. H. Crabtree, Org. Biomol. Chem., 2008, 6, 4442–
9 T. vom Stein, T. den Hartog, J. Buendia, S. Stoychev,
4445.
J. Mottweiler, C. Bolm, J. Klankermayer and W. Leitner, 55 D. D. Joarder, S. Gayen, R. Sarkar, R. Bhattacharya, S. Roy
Angew. Chem., Int. Ed., 2015, 54, 5859–5863. and D. K. Maiti, J. Org. Chem., 2019, 84, 8468–8480.
0 D. Weickmann and B. Plietker, ChemCatChem, 2013, 5, 56 J. Limburg, J. S. Vrettos, L. M. Liable-Sands,
4
4
4
2
170–2173.
1 X. Zhou, J. Mitra and T. B. Rauchfuss, ChemSusChem, 2014,
, 1623–1626.
2 R. Jastrzebski, S. Constant, C. S. Lancefield,
A. L. Rheingold, R. H. Crabtree and G. W. Brudvig, Science,
1999, 283, 1524–1527.
7
57 F. Shi, M. K. Tse and M. Beller, Chem. – Asian J., 2007, 2,
411–415.
N. J. Westwood, B. M. Weckhuysen and P. C. A. Bruijnincx, 58 X. Wang, C. Wang, Y. Liu and J. Xiao, Green Chem., 2016,
ChemSusChem, 2016, 9, 2074–2079. 18, 4605–4610.
3 C. S. Lancefield, L. W. Teunissen, B. M. Weckhuysen and 59 J. Cheng, M. Zhu, C. Wang, J. Li, X. Jiang, Y. Wei, W. Tang,
P. C. A. Bruijnincx, Green Chem., 2018, 20, 3214–3221. D. Xue and J. Xiao, Chem. Sci., 2016, 7, 4428–4434.
4 A. Wu, B. O. Patrick, E. Chung and B. R. James, Dalton 60 J. Li, Y. Liu, W. Tang, D. Xue, C. Li, J. Xiao and C. Wang,
Trans., 2012, 41, 11093–11106. Chem. – Eur. J., 2017, 23, 14445–14449.
5 M. V. Galkin, A. T. Smit, E. Subbotina, K. A. Artemenko, 61 The catalytic activity of 1a is superior to our previous
4
4
4
J. Bergquist, W. J. J. Huijgen and J. S. M. Samec,
ChemSusChem, 2016, 9, 3280–3287.
Rh–Rh complex for depolymerization of raw poplar wood
powder; see Table S4 in the ESI for details.†
4
6 R. G. Harms, I. I. E. Markovits, M. Drees, W. A. Herrmann, 62 Z. Sun, G. Bottari, A. Afanasenko, M. C. A. Stuart,
M. Cokoja and F. E. Kühn, ChemSusChem, 2014, 7, 429–
34.
P. J. Deuss, B. Fridrich and K. Barta, Nat. Catal., 2018, 1,
82–92.
4
38 | Green Chem., 2020, 22, 33–38
This journal is © The Royal Society of Chemistry 2020