Effective C-O bond cleavage of lignin β-O-4 model compounds: A new RuHCl(CO)(PPh3)3/KOH catalytic system

Article abstract of DOI:10.1007/s10562-014-1264-y

Base, especially KOH, can be used in place of xantphos for the Ruthenium-complex-catalyzed C-O bond cleavage of β-O-4 lignin model compounds. In the presence of KOH, RuHCl(CO)(PPh₃)₃ and Ru(H)₂(CO)(PPh₃)₃ show the best catalytic effect among several ruthenium-complexes studied.

Full text of DOI:10.1007/s10562-014-1264-y

Catal Lett
DOI 10.1007/s10562-014-1264-y
Effective C–O Bond Cleavage of Lignin b-O-4 Model Compounds:
A New RuHCl(CO)(PPh ) /KOH Catalytic System
3
3
Wei Huo
Wei Fan
•
Wenzhi Li
Hou-min Chang
•
Minjian Zhang
•
•
•
Hasan Jameel
Received: 18 February 2014 / Accepted: 22 April 2014
Ó Springer Science+Business Media New York 2014
Abstract Base, especially KOH, can be used in place of
xantphos for the Ruthenium-complex-catalyzed C–O bond
cleavage of b-O-4 lignin model compounds. In the pre-
which b-O-4 linkage comprises some 50–60 % of the lig-
nin structures [4]. Lignin comprises a large mass propor-
tion (15–30 % by weight) and energy content (up to 40 %)
of the lignocellulose [5], and it is recalcitrant to chemical
and biological conversion [6]. As a result, a large effort is
underway to develop mild, less-energy-intensive and eco-
friendly methods to convert lignin into small molecules
that can be upgraded into a fuel stream or chemicals [7].
Studies on degradation of lignin and lignin models
include bacterial degradation [8], heterogeneous Co(salen)-
catalyzed oxidation using H O under microwave radiation
sence of KOH, RuHCl(CO)(PPh3)3 and Ru(H) (CO)
2
(
PPh3)3 show the best catalytic effect among several
ruthenium-complexes studied.
Keywords Lignin Á Homogeneous catalysis Á KOH Á
b-O-4 cleavage
2
2
1
Introduction
[9], hydrolytic cleavage in an ionic liquid in the presence of
FeCl , CuCl , or AlCl [10] depolymerization in alcohol
3
2
3
At the present time, lignocellulosic biomass is valued as a
source for the production of renewable chemicals and fuels
in light of diminishing fossil fuel reserves and increasing
demand for energy [1, 2]. Lignin is a complicated three
dimensional polymer linked by ether bonds and C–C bonds
such as a-O-4, b-O-4, b-1, b-5 and 5–5 types [3], among
over nickel-based catalysts [11], and vanadium-catalyzed
nonoxidative degradation [12, 13].
In 2010 Nichols et al. [14] reported a Ru(H) (CO)
2
(PPh ) /xantphos catalyst that is capable of effectively
3
3
degrading b-O-4 models as well as a synthetic b-O-4
linked polymer under N as shown below (Fig. 1). The
2
catalytic system was studied in detail by Wu et al. [4], and
a plausible mechanism was proposed. This research is
meaningful because of the large mass proportion of b-O-4
linkage in lignin structure. It is also the first reported
ruthenium-catalyzed dehydrogenation and then hydroge-
nation to create mixtures of monofunctional carbonyl and
phenolic products. However, the studies conclude that
addition of a wide bite angle phosphine ligand [15], namely
xantphos, to the Ru(H) (CO)(PPh3) system is necessary
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-014-1264-y) contains supplementary
material, which is available to authorized users.
W. Huo Á W. Li (&) Á M. Zhang
Key Laboratory for Biomass Clean Energy of Anhui Province,
University of Science and Technology of China, Hefei 230026,
People’s Republic of China
e-mail: liwenzhi@ustc.edu.cn
2
3
for efficient reactivity, which leads to inapplicability of the
catalytic system for practical uses because of the high
price.
W. Fan
Chemical Engineering, University of Massachusetts, 159
Goessmann Lab, 686 North Pleasant Street, Amherst,
MA 01103-3110, USA
In this study, we report that RuHCl(CO)(PPh ) and
3
3
Ru(H) (CO)(PPh ) in the presence of some bases such as
2 3 3
H. Chang Á H. Jameel
KOH, can perform well without the addition of xantphos.
The major advantage of using an inorganic base such as
Department of Forest Biomaterials, North Carolina State
University, Raleigh, NC 27695-8005, USA
123

W. Huo et al.
KOH lies in its low cost relative to the organic ligand and
its ability to be recycled because of its insolubility in the
organic reaction media.
b-O-4 structure as shown in Fig. 2 [16, 17]. The formation
of 1-phenylethyl alcohol (C) is hard to rationalize as it
involves reductive cleavage. This needs to be clarified by
further studies. Nevertheless, it is clear that KOH can be
used in place of xantphos in the catalytic degradation of 2.
Whether KOH could transform RuHCl(CO)(PPh ) to
2
Results and Discussions
3
3
another Ru-complex and eventually contribute to the deg-
radation of 2 is another important question. In the
RuCl (PPh ) -catalyzed transfer-hydrogenation of ketones,
Table 1 summarizes the results of a series of reaction of
-(2-methoxyphenoxy)-1-phenylethanol (2) under various
conditions. In the absence of KOH (entry 3), no products
2
2
3 3
addition of a catalytic amount of base dramatically accel-
erates the rate [18, 19]. The experimental results indicate
that the role of the base is to transform the unreactive
RuCl (PPh ) into the catalytically active Ru(H) (PPh )
3 3
were observed, which is consistent with earlier studies [4,
1
4]. However, addition of KOH to the RuHCl(CO)(PPh3)3
system greatly increases catalytic activity (entry 2), prov-
ing the role of base in the catalytic degradation of 2. The
same treatment to Ru(H) (CO)(PPh ) system is also
2
3 3
2
(Fig. 3). However, in the present study, RuHCl(CO)
(PPh ) is as effective, if not more, as Ru(H) (CO)(PPh )
3 3
2
3 3
3 3
2
proved to be effective(entry 4). When KOH is used in the
absence of a ruthenium catalyst (entry 1), consumption and
yields of products were shown but at a relative low level. A
small amount of 1-phenylethyl alcohol was detected. The
formation of acetophenone (A) and guaiacol (B) can be
explained by the well-known base catalyzed hydrolysis of
(entry 2 and entry 4, Table 1). Therefore, KOH may play
H
OMe
O
OMe
O
O
KOH
OH
+
Toluene,125
KOH,Toluene,125
O
R''
OH
R''
2 3 3
Ru(H) (CO)(PPh ) (1mol%)
Ph-xantphos (1mol%)
OH
O
OH
OH
O
+
toluene (0.4M)
KOH
R
R
1
35 , 4h
R'
R'
Toluene,125
OH-
Fig. 1 The reaction studied by Nichols et al. [14], Ru(H)
PPh /xantphos catalyst under N
2
(CO)
Fig. 2 Alkali catalyzed hydrolysis reaction of 2-(2-methoxyphen-
oxy)-1-phenylethanol
(
3
)
3
2
Table 1 C–O bond cleavage of 2-(2-methoxyphenoxy)-1-phenylethanol catalyzed by RuHCl(CO)(PPh
3
)
3
with and without KOH
OH
OMe
O
OMe
OH
Ru-source(2.5mol%)
toluene(2ml)
O
OH
+
+
1
25
2
. 0.8mmol
A
B
C
a
b
b
Entry
1
Ru-source (2.5 mol%)
KOH (50 mol%)
Cons. (%)
Yield %
Time (h)
A
B
C
None
Y
60
12
15
13
15
38
62
80
71
0
6
10
15
15
16
0
12
24
36
48
12
24
36
48
36
48
5
6
7
7
2
0
9
11
7
2
RuHCl(CO)(PPh
)
3 3
Y
50
15
39
75
65
0
7
9
9
4
3
5
0
0
0
0
3
4
a
RuHCl(CO)(PPh
)
3 3
N
Y
0
Ru(H)
2
(CO)(PPh
3
)
3
97
63
16
0
Reactions were run in a Schlenk flask under an atmosphere of purified argon equipped with condenser
Yields and consumptions were determined by GC relative to an internal standard
b
1
23

Effective C–O Bond Cleavage of Lignin b-O-4 Model Compounds
other roles in addition to the transformation of RuHCl
Wu and coworkers showed that in the hydrogenolysis of
2-(2-methoxyphenoxy)-1-phenyl-1,3-propanediol with a
Ru(H) (CO)(PPh ) /xantphos catalyst [4], three products
(
CO)(PPh ) into Ru(H) (CO)(PPh ) . We will show that
3 3 2 3 3
KOH is also effective in other related ruthenium catalytic
systems (vide infra).
2
3 3
were formed, guaiacol, 3-hydroxy-2-(2-methoxyphenoxy)-
1-phenyl-1-propanone and 2-(2-methoxyphenoxy)-1-phe-
nyl-ethanone. Guaiacol was formed in less than 15 % yield
and without the concurrent formation of acetophenone. The
authors convincingly showed that guaiacol was formed
with concurrent formation of an inactive ruthenium catalyst
complex. While 2-(2-methoxyphenoxy)-1-phenyl-ethanone
is apparently formed via reverse aldol reaction from
3-hydroxy-2-(2-methoxyphenoxy)-1-phenyl-1-propanone
[4], the formation of the latter clearly indicates that the
catalytic system is capable of dehydrogenating the benzyl
alcohol to the corresponding ketone. The latter is also
capable of forming an inactive complex with the catalytic
system [4], therefore limiting the yield of guaiacol.
We also investigated the effectiveness of several other
bases on the degradation of 2 with RuHCl(CO)(PPh )
3
3
t
including NaOH, KO Bu, K CO , and NEt , among which
2
3
3
t
KOH is the best one followed by KO Bu and NaOH,
K CO and NEt3 are the least functioning bases. The
2
3
results suggest basicity, steric factors and compatibility
with the solvent may all contribute to their effectiveness.
The substrates in Table 2 model the substitution patterns
found in lignin. Increasing the substitution on Ar’ has a
negative influence on both consumption and yield, while
methoxylation of Ar has relatively little effect on the
reaction. These results (entries 1–5) are in agreement with
those of Nichols et al. [14]. As for substrate 6, we found
notable yield decrease of Ar’OH and absence of ArCOCH3,
at the same time, a new product was detected. GC–MS
analysis identified its structure as 2-Methoxy-4-vinyl-phe-
nol, indicating that the OH on Ar could not only influence
the yield but also the mechanism of the reaction. It is
conceivable that ArOH may coordinate with ruthenium
center, generating an inactive complex.
We also applied this catalytic system to hydrogenolysis
of 2-(2-methoxyphenoxy)-1-(3,4-dimethoxy)phenyl-1,3-
propanediol (7), The result showed 4 staple products, all
having relatively low yields (Table 3). Nevertheless, the
consumption and yield of guaiacol are higher than those
reported by Nichols et al. [4]. In our catalytic system with
KOH, both a- and c-OH of the starting compound can
produce guaiacol (B) via epoxide formation [16, 17], giving
an extra amount of guaiacol. The rearrangement of epoxide
followed by reverse aldol reaction then gives either ace-
toveratrone (A) or veratryl aldehyde (C). The starting
compound will also produce guaiacol via complexation
with the catalyst, resulting in an inactive catalyst [4].
In order to test the influence of KOH on the catalytic
effect of other ruthenium-complex, we investigated
2 3 3
Fig. 3 The role of base in the RuCl (PPh ) -catalyzed transfer-
hydrogenation of ketones
Table 2 C–O bond cleavage of various 2-aryloxy-1-arylethanols
RuHCl(CO)(PPh
3
)
3
(2.5mol%)
O
OH
KOH(50mol%)
+
Ar'OH
O
Ar
Ar'
Ar
toluene(2ml)
1
25
0
.8mmol
A
B
a
b
b
Substrate
Ar
Ar’
Cons. (%)
Yield %
Time (h)
A
B
1
2
3
4
5
6
a
C
C
C
6
6
6
H
H
H
5
5
5
C
6
H
5
99
93
41
91
95
89
86
80
30
80
66
84
75
4
12
36
24
12
24
24
2-(CH
3
O)-C
6 4
H
2,6-(CH
3
O)
2 6 3
-C H
4-(CH
3,4-(CH
3-(OH)-4-(CH
3
O)-C
6
H
4
C
H
6 5
77
45
14
3
O)
2
-C
6
H
3
2-(CH
2-(CH
3
O)-C
6
H
H
4
4
c
3
O)
2
6
-C H
3
3
O)-C
6
ND
Reactions were run in a Schlenk flask under an atmosphere of purified argon equipped with condenser
Yields and consumptions were determined by GC relative to an internal standard
Not detected
b
c
123

W. Huo et al.
Table 3 C–O bond cleavage of 2-(2-methoxyphenoxy)-1-(3,4-dimethoxy)phenyl-1,3-propanediol
O
OH
O
O
OH
OMe
RuHCl(CO)(PPh
3
)
3
(2.5mol%)
OMe
+
O
H
KOH(100mol%)
+
+
Toluene(2ml)
MeO
HO
OMe
MeO
MeO
MeO
125
OMe
OMe
OMe
7
. 0.4mmol
A
B
C
D
b
b
Ru-source (2.5 mol %)
Cons. (%)
95
Yield %
Time (h)
A
B
C
D
a
c
c
c
c
RuHCl(CO)(PPh
3
)
3
4
6
6
4
18
26
26
53
5
N
N
N
N
12
24
36
48
9
8
9
4
9
1
7
8
11
a
Reactions were run in a Schlenk flask under an atmosphere of purified argon equipped with condenser
Yields and consumptions were determined by GC relative to an internal standard
Minor and not quantitated
b
c
Table 4 C–O Bond Cleavage of 2-(2-methoxyphenoxy)-1-phenylethanol catalyzed by other Ru-complex with and without KOH
OH
OMe
O
OMe
Ru-source(2.5%)
toluene(2ml)
O
OH
+
1
25
2. 0.8mmol
A
B
a
b
b
Entry
Ru-source (2.5 mol %)
KOH
50 mol %)
Cons. (%)
Yield %
Time (h)
(
A
B
1
2
3
4
5
6
7
a
RuH
2
(CO)(PPh
3
)
3
Y
N
Y
N
Y
N
Y
N
Y
N
Y
N
Y
N
97
30
79
26
82
10
79
10
54
8
63
12
57
0
16
13
34
0
48
48
48
48
48
48
48
48
48
48
48
48
48
48
RuCl
RuCl
2 3 3
(PPh )
2
(COD)
62
0
24
0
Ru(methallyl)
2
(COD)
56
2
26
2
RuClCp(PPh
3
)
3
24
0
16
0
RuH
2 3
(PPh )
4
56
21
56
0
22
0
11
3
Ru
3
(CO12
)
34
0
24
0
Reactions were run in a Schlenk flask under an atmosphere of purified argon equipped with condenser
Yields and consumptions were determined by GC relative to an internal standard
b
catalytic cleavage of 2 by other ruthenium-complexes. The
catalytic system is effective. No better results were
obtained in comparison with RuHCl(CO)(PPh ) . It seems
results are shown in Table 4. While the effectiveness of
different catalytic system varies to a great extent, it is clear
that KOH could dramatically increase the catalytic effect of
all ruthenium-complexes. Without KOH, none of the
3
3
that the ease of selective substitution of phosphine ligand
-
trans to H in cis,mer-[RuHCl(CO)(PPh ) ] by alkoxy from
3
3
2 provide clear evidence for the kinetic trans effect of the
1
23

Effective C–O Bond Cleavage of Lignin b-O-4 Model Compounds
OH
Acknowledgments The research project is financially supported by
the National Natural Science Foundation of China (51176184), the
National Basic Research Program of China (No. 2012CB215302), the
National High-tech R&D Program (2012AA101808-2), and by the
Fundamental Research Funds for the Central Universities
KOH (minor)
O
OAr'
Ar'OH
+
Ar
Ar
[A]
-
H
2
O
[
Ru]-H
(
WK2090130009).
Ar
[
Ru]
[E]
O
OAr'
Ar
+H
2
[
B]
[
Ru]-H
O
O
KOH
OAr'
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[
Ru]-OAr'
Ar
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Fig. 4 Plausible mechanism for C–O bond cleavage
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In conclusion, we found that base, especially KOH, can
be used in place of xantphos in catalytic cleavage of C–O
bond of 2-aryloxy-1-arylethanols using several ruthenium
based complexes. Among them, RuHCl(CO)(PPh ) and
2
2
3
3
Ru(H) (CO)(PPh ) show the best catalytic effect.
2 3 3
123