Mild heterogeneous palladium-catalyzed cleavage of β-o-4'-ether linkages of lignin model compounds and native lignin in air

Article abstract of DOI:10.1002/cctc.201300540

A mild and robust heterogeneous palladium-catalyzed C - O bond cleavage of 2-aryloxy-1-arylethanols using formic acid as reducing agent in air was developed. The cleaved products were isolated in 92-98 % yield; and by slightly varying the reaction conditions, a ketone, an alcohol, or an alkane can be generated in near-quantitative yield. This reaction is applicable to cleaving the β-O-4'-ether bond found in lignin polymers of different origin. The reaction was performed on a lignin polymer model to generate either the monomeric aryl ketone or alkane in a quantitative yield. Moderate depolymerization was achieved with native lignin at similar reaction conditions. Mechanistic studies under kinetic control indicate that an initial palladium-catalyzed dehydrogenation of the alcohol is followed by insertion of palladium to an enol equivalent. A palladium-formato complex reductively cleaves the palladium-enolate complex to generate the ketone. Copyright

Full text of DOI:10.1002/cctc.201300540

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DOI: 10.1002/cctc.201300540
Mild Heterogeneous Palladium-Catalyzed Cleavage of
b-O-4’-Ether Linkages of Lignin Model Compounds and
Native Lignin in Air
Maxim V. Galkin, Supaporn Sawadjoon, Volker Rohde, Monali Dawange, and Joseph
S. M. Samec*[a]
A mild and robust heterogeneous palladium-catalyzed CÀO
bond cleavage of 2-aryloxy-1-arylethanols using formic acid as
reducing agent in air was developed. The cleaved products
were isolated in 92–98% yield; and by slightly varying the re-
action conditions, a ketone, an alcohol, or an alkane can be
generated in near-quantitative yield. This reaction is applicable
to cleaving the b-O-4’-ether bond found in lignin polymers of
different origin. The reaction was performed on a lignin poly-
mer model to generate either the monomeric aryl ketone or
alkane in a quantitative yield. Moderate depolymerization was
achieved with native lignin at similar reaction conditions.
Mechanistic studies under kinetic control indicate that an initial
palladium-catalyzed dehydrogenation of the alcohol is fol-
lowed by insertion of palladium to an enol equivalent. A palla-
dium–formato complex reductively cleaves the palladium–eno-
late complex to generate the ketone.
Introduction
Cellulose, hemicellulose, and tall
oil are used to produce high-
value products[1] such as pulp,
ethanol, and soap, but the lignin
part of wood is currently used to
recycle process chemicals in the
pulping mills and is burnt to Figure 1. b-O-4’-glycerolaryl ether in lignin and the b-O-4’-ethanolaryl ether in the models (blue).
generate heat and electricity to
a low value.[2] Taking into ac-
count that the lignin polymer
constitutes up to 40% of the energy content in lignocellulose,
it would be worthwhile to upgrade lignin to more valuable
products.[3] To upgrade lignin into useful substrates for fine
and bulk chemicals, sustainable, selective, and preferably cata-
lytic procedures must be developed.[4] Catalytic processes for
both oxidative and reductive lignin depolymerization have
been reported.[2,5]
are used to model the b-O-4’-glycerolaryl ethers and were
chosen in this study too.[2,8]
Here we describe a selective palladium-on-carbon (Pd/C) cat-
alyzed CÀO bond cleavage of the b-O-4’-ether bond. The
method is notable as a mild and robust procedure in which
the catalysis can be performed at 808C in air without using
specialized equipment. By slightly modifying the reaction con-
ditions, the benzylic ketone, alcohol, or alkane can be selec-
tively generated in above 95% yield even for models having
ortho-methoxy substitution in both 2- and 6-positions of the
aryloxy group. These contributions compare favorably with
previously reports concerning the cleavage of the b-O-4’-ether
bond of lignin model compounds.[5]
The lignin polymer is composed of methoxy-substituted
phenolic subunits of irregular structural complexity and is not
optically active.[6] The most abundant structural motif in lignin
is the b-O-4’-linkage comprising around 50–60% of native
lignin (Figure 1). This bond is cleaved in, for example, the Kraft
process but not in the sulfite process producing pulp.[7] In
many studies, the b-O-4’-ethanolaryl ethers (marked in blue)
Results and Discussion
[a] M. V. Galkin,+ S. Sawadjoon,+ V. Rohde, M. Dawange, Dr. J. S. M. Samec
Department of Chemistry, BMC
We have previously shown that Pd/C is reactive in the transfer
hydrogenolysis of benzyl alcohols in which a hydride from
formic acid substitutes the hydroxyl group to generate alkyl
aryls.[9] Noteworthy, we found that a catalytic amount of base
was required to inhibit a competing disproportionation reac-
tion. Using the same reaction conditions to the b-O-4’-ethano-
laryl ether (1a) generated the expected phenethoxy benzene
Uppsala University
Husargatan 3, 751 23, Uppsala (Sweden)
E-mail: joseph.samec@kemi.uu.se
[+] These authors contributed equally to this work
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300540.
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ethylamine, diethylamine, ammonia, and polymer-supported
amine all generated 3a in above 95% yield (Table 1, entries 9–
11). Notably, water and a cosolvent are required for the desired
reactivity. Green solvents for example, ethyl acetate, tert-butyl
methyl ether, ethanol, and methanol all work as cosolvents in
the transformation. In the absence of formic acid, the reaction
of 1a proceeded in a redox neutral pathway albeit to a lower
conversion to afford 3a (20%) after 1 h (Table 1, entry 12).[5]
The reaction was faster and more selective to generate 3a
under an atmosphere of air than under argon under which
only 40% yield of 3a was obtained (Table 1, entry 13, and Sup-
porting Information).
Scheme 1. Additional base yield cleaved products 3a and 4a in favor of 2a.
(2a) in 80% yield (Scheme 1). The remaining 20% correspond-
ed to acetophenone (3a) and phenol (4a). These findings en-
couraged us to optimize the reaction conditions towards the
cleaved products. We found that the yield of the cleaved prod-
ucts increased with the amount of base.[10] A stoichiometric
amount of base promoted the cleavage of the b-O-4’-linkage
to generate 3a and 4a in favor of 2a.
The optimized reaction conditions were applied to different
2-aryloxy-1-arylethanols with substitution of the two aryl
groups in R1–R4-positions (Table 2). This substitution pattern
models native lignin from different sources.[2] Model 1a, with
In Table 1, the effects of different sources of metals, hydro-
gen donors, and bases are summarized for the catalytic cleav-
age of the b-O-4’ bond in 2-phenoxy-1-phenylethanol. Only
Pd/C was reactive in the transfer hydrogenolysis using formic
Table 2. CÀO bond cleavage in various lignin models.
Entry
1
R1
R2
R3
R4
3
Yield 3 [%][a]
Table 1. Optimization of the b-O-4’ bond cleavage in 1a.
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
H
MeO
H
MeO
H
MeO
HO
HO
HO
H
H
H
H
H
H
MeO
H
MeO
H
H
H
H
H
MeO
H
H
H
H
H
MeO
H
3a
3b
3a
3b
3a
3c
3d
3e
3d
3a
3c
98
97
95
98
97
96
98[b]
95[b]
92[b]
96
MeO
MeO
H
MeO
MeO
MeO
MeO
MeO
F
Entry
M/C
H donor
Base
Yield 3a [%][a]
1
2
3
4
5
6
7
8
Pd/C
Rh/C
Ir/C
HCOOH
HCOOH
HCOOH
HCOOH
HCOOH
2-propanol
H2
HCOOH
HCOOH
HCOOH
HCOOH
–
NH3
NH3/–
NH3/–
NH3/–
NH3/–
NH3/–
p-(allylamine)
–
NH2Et
>95
0
9
10
11
0
0
0
0
0
0
1j
1k
Re/C
Ni/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
MeO
MeO
F
H
95
[a] Reactions were performed on a 0.5 mmol scale in air for 3 h. Isolated
yields of 3 were determined as an average of 2 runs. [b] Yields deter-
1
mined by H NMR spectroscopy.
9
>95
>95
>95
20
40[b]
10
11
12
13
NHEt2
p-(allylamine)
p-(allylamine)
p-(allylamine)
no methoxy substitution of the aryl is one component in, for
example, switch grass.[2] Substitution of the aryl group by a me-
thoxy group in R3-position (1b) corresponds to the substitu-
tion pattern found in soft wood, for example, pine and
spruce.[2] Substitution of the aryl group by two methoxy
groups in R3 and R4-positions (1c) corresponds to the substitu-
tion pattern in hard wood, for example, birch and eucalyptus.[2]
Gratifyingly, the methoxy substitution in the R3- and R4-posi-
tions of the aryl group did not affect catalysis negatively
(Table 2, entries 2–6). Substrate 1c, in which the phenol is sub-
stituted by two methoxy groups in R3- and R4-positions has
previously been reported as cumbersome to cleave.[5] In all
cases, the aryl ketones (3a–d) were isolated in excellent yields.
Using more natural models with hydroxyl groups in R1-position
of the aryl also generated the corresponding ketones in excel-
lent yields (Table 2, entries 7–9). To extend the generality of the
reaction to other transformations in organic synthesis and also
study the effect of an electron-deficient aryl, substrates 1j and
1k were used in the reaction. Theses substrates generated the
HCOOH
[a] The reactions were performed on a 0.1 mmol scale under air for 1 h.
Yields were determined by 1H NMR spectroscopy using mesitylene as an
internal standard. Phenol 4a was also isolated in the same yield as 3a in
all reactions. [b] Reaction proceeded under argon atmosphere.
acid as hydrogen donor to generate 3a and 4a in high yields
(Table 1, entries 1–5).[11] With Rh/C a disproportionation reac-
tion was observed to generate 2a in 30% yield if the reaction
was performed in the absence of a base (Table 1, entry 2). Ir/C,
Re/C, and Ni/C did not exhibit any reactivity in the presence or
absence of base (Table 1, entries 3–5). Exchanging the hydro-
gen donor from formic acid to isopropanol did not give the
desired reaction (Table 1, entry 6). Running the reaction in hy-
drogen gas was unsuccessful (Table 1, entry 7). In the absence
of base a disproportionation reaction occurred to generate 2a
and the corresponding ketone. Different bases could be used;
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corresponding ketones (3a, c) in excellent yields (Table 2, en-
tries 10–11).
To expand the methodology, the reaction was performed on
model 1 f’ having the b-O-4’-glycerolaryl ether. Initial results
gave poor conversion and low reproducibility. A major side re-
action was the disproportionation without cleavage. After
some optimization steps, moderate conversion to the desired
products was achieved by using a one-pot two-step proce-
dure. Under the modified reaction conditions, complete con-
sumption of dilignol 1 f’ was observed after heating the reac-
tion mixture for 24 h in the presence of Pd/C and ammonium
formate. The intermediates formed were reduced by the addi-
tion of formic acid to yield the mixture of phenylpropanol 3c’’
and propylbenzene 3c’ in an overall yield of 71% (Scheme 2).
Scheme 4. Transformation of model polymer (p-1d) to generate either 3d
or 6d.
poorly soluble polymer. With the addition of formic acid after
3 h, 6d could be generated in high yield.
Encouraged by the results on the artificial lignin model poly-
mer, we next performed the reaction on organosolv lignin
from Pinus sylvestris. To achieve an efficient transformation, the
reaction was performed by a two-step one-pot procedure. Or-
ganosolv lignin, Pd/C, and ammonium formate were heated in
ethyl acetate and water for 24 h, after which formic acid was
added to the reaction mixture and the reaction was continued
for 12 h. A control experiment, in which Pd/C was absent, was
also performed. The reaction mixture was analyzed by gel per-
meation chromatography (GPC) (Figure 2), 2D NMR spectros-
copy (Figure 3), and GC–MS (Supporting Information). The GPC
Scheme 2. One-pot procedure to generate 3c’ and 3c’’ from 1 f’.
With slightly modified reaction conditions, the reaction can
selectively be directed to ketone (3b), alcohol (5b), or alkane
(6b, Scheme 3). Using an equimolar amount of formate, the re-
action proceeded to afford 3b in 98% yield. Using 2.5 equiva-
lents of formate, the reaction proceeded to generate 5b in
98% yield. The second equivalent of formate reduces the
ketone to the alcohol.[12] Addition of formic acid in a second
step generated the transfer-hydrogenolysis product 6b in
above 95% conversion.[9]
Figure 2. GPC chromatograms depicting the molecular weight distribution
of organosolv lignin, that of the corresponding control experiment (THF
soluble part), and the postreaction product mixtures.
analysis revealed a moderate change towards lower molecular
weight fragments. The NMR data showed that 73% of the b-O-
4’-glycerolaryl ether bonds were degraded by the present
methodology. In addition, resinol and aryl coumaran ether link-
ages were also cleaved. GC–MS spectra identified monomeric
compounds including coniferyl dihydroalcohol, coumaryl dihy-
droalcohol, and derivatives thereof. Interestingly, in the control
reaction, without Pd/C, GC–MC analysis revealed only the pres-
ence of oxidation products such as vanilic acid. None of the
compounds detected in the catalyzed reaction were observed
in the control reaction. The experiment without Pd/C displayed
Scheme 3. Substrate 1b can selectively be transformed to products 3b, 5b,
and 6b.
Owing to the heterogeneous nature of the catalyst, we were
curious whether the catalytic system would be applicable to
a model polymer (p-1d). Gratifyingly, the catalytic system was
efficient and depolymerized the polymer (5 kDa) of similar mo-
lecular weight to native lignin to generate 3d in a quantitative
yield (Scheme 4). Noteworthy, the reaction parameters did not
need to be modified for efficient depolymerization of the
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Figure 3. 2D HSQC NMR spectra of an isolated Pinus sylvestris lignin before and after reaction.
seemingly similar GPC chromatograms. However, in the chro-
matogram only the soluble part of the material is shown and
the THF-insoluble residue formed in the control experiment
was not suitable for analysis.
Size distribution data of the catalyst obtained by TEM
(Figure 4) revealed that a fraction of 2–3 nm sized particles do-
minated in the sample (Supporting Information). The surface
area determined by BET quantification methods was
813 m2 gÀ1 with a total pore volume of 0.635 m3 gÀ1.
Figure 5. Effect of the stirring rate on the initial reaction rate.
sigmoidal shape of the initial rate, supported that the Pd/C is
responsible for the catalysis.
Different inhibition experiments were performed (Figure 6).
Addition of mercury (25 equiv. to palladium) to the reaction
mixture inhibited the reaction and only traces of the product
were observed. Addition of one equivalent of triphenylphos-
phine to palladium also inhibited the catalysis and only trace
Figure 4. TEM image of Pd/C. Scale bar=20 nm.
The heterogeneous nature of the catalyst was determined
by different techniques.[13] The effect of agitation on the initial
reaction rate was studied. At stirring speeds below 500 rpm,
diffusion control and not kinetic control was operating and,
therefore, the kinetic experiments were performed by using
stirring speeds above 500 rpm (Figure 5). High reproducibility
in the kinetic runs, together with the absence of an observed
Figure 6. Effect of inhibitors on the reaction.
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amounts of product were observed. However, if one equivalent
of polymer-bound triphenylphosphine was added to the reac-
tion mixture, no effect on the reaction outcome was observed
as compared to the blank reaction. These findings supported
that heterogeneous Pd/C is the active catalyst in the cleavage
of the b-O-4’-ethanolaryl ether bond.
If the reaction of 1 was carefully monitored by taking ali-
quots from the reaction mixture at a low conversion, the mon-
omeric ketone and phenol (3 and 4) were the only products
observed in the presence of formate. The reaction may pro-
ceed either by an initial dehydrogenation step to generate the
2-phenoxy-1-phenylethanone 9 followed by an ether bond
cleavage[5a] or by the ether bond cleavage preceding the dehy-
Kinetic isotope effects of the CÀO bond cleavage of 1b
drogenation step. A possibility to distinguish between these were investigated by selective deuteration of the a- and b-po-
two different pathways would be to use substrates in which sitions of 1b as well as by using formic acid deuterated in the
the dehydrogenation to generate the ketone is not possible. If hydride position (Table 3).[15] A primary kinetic isotope effect
1-phenoxy-2-phenylpropan-2-ol (7) was used as the substrate,
no reaction occurred [Eq. (1)]. Likewise, no reaction occurred if
Table 3. Kinetic deuterium isotope effect on the cleavage of substrates
1b, 1b-aD, and 1b-bD2.
(1-methoxy-2-phenoxyethyl)benzene (8) was used as the sub-
strate [Eq. (2)]. If dehydrogenation product 9 was used as the
substrate, a fast ether bond cleavage generating the mono-
meric ketone was observed [Eq. (3)].
Entry
Run
Kinetic isotope effect
1
2
3
k
1b/k1b-aD
1.65Æ0.07
1.11Æ0.01
1.72Æ0.13
k1b/k1b-bD2
kHCOOH/kDCOOH
(1.65) was observed if 1b-aD deuterated in the a-position was
used, in agreement with a slow dehydrogenation step. A sec-
ondary kinetic isotope effect (1.11) was observed if substrate
1b-bD2 deuterated in the b-position was used in agreement
with a fast keto–enol tautomerization step.[16] A primary kinetic
isotope effect (1.72) was observed if formic acid, deuterated in
the hydride position, was used in agreement with a slow hy-
drogen transfer from formate to Pd/C.
The proposed reaction mechanism is shown in Scheme 5.[17]
These experiments supported
a pathway proceeding Substrate 1 is dissociatively chemisorbed to palladium to pro-
through an initial palladium-catalyzed dehydrogenation, fol- duce 9 and hydrogen adsorbed to palladium.[18] According to
lowed by CÀO ether bond cleavage. The observation that the the kinetic isotope effect (k1b/k1b-aD =1.65), this occurs in a slow
transformation is more efficient in air than in argon atmos- reaction step.
phere is also supportive of this hypothesis (Supporting Infor-
mation). The second step in the reaction mechanism may be
generation of an enol equivalent followed by insertion of palla-
dium.[14] A possible way to evaluate this hypothesis is to selec-
tively block the b-position by substituting the hydrogen atoms
with alkyl groups. If 2-methyl-2-phenoxy-1-phenylpropan-1-ol
(10) was used as the substrate, no reaction occurred [Eq. (4)].
To exclude that this was caused by steric hindrance, 2-phen-
oxy-1-phenylpropan-1-ol (11) was used as the substrate
[Eq. (5)]. This substrate generated the expected propiophe-
none (3l) in 1 h exhibiting similar reactivity to substrate 1a.
These results are in agreement with that the reaction proceeds
through a palladium–enolate intermediate.
Scheme 5. Proposed mechanism for the transformation of 1 to 3 and 4.
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The observation that no CÀO bond cleavage occurred in hy-
drogen gas supports this hypothesis. Hydrogen desorption
from palladium is facilitated by molecular oxygen in air, to gen-
erate water (see the Supporting Information).[19] Ketone 9 is in
fast equilibrium with its enol tautomer, which adsorbs to palla-
dium to generate intermediate A. The observation that sub-
strate 10 was unreactive in the transformation is supportive of
this hypothesis. A formate ion adsorbs to palladium to gener-
ate intermediate B that may decompose to generate a palladi-
um hydride. Either the formato complex or a generated palla-
dium hydride promotes the CÀO bond cleavage of the ether in
a slow reaction step according to the kinetic isotope effect
(kHCOOH/kDCOOH =1.72) to generate 3, which desorbs from palla-
dium. For further reduction, the role of the second equivalent
of formate is to reduce the ketone to the alcohol.[12] The mech-
anism for the transfer hydrogenolysis has previously been
reported.[9]
Acknowledgements
This research is funded by the Swedish Energy Agency.
Keywords: heterogeneous catalysis · cleavage reactions
green chemistry · palladium · polymers · lignin
·
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Conclusions
A robust, mild, and efficient palladium-catalyzed cleavage of
the b-O-4’-ether bond of model lignin compounds has been
developed. Dimer models of lignin sources were efficiently
transformed to the corresponding aryl ketones and phenols in
excellent yield. By slightly modifying the reaction conditions,
a ketone, alcohol, or alkane can selectively be generated in ex-
cellent yield. Also polymers with the b-O-4’-ether linkage were
efficiently cleaved to generate either the aryl ketone or alkane
in excellent yield. Degradation experiments with organosolv
lignin revealed a moderate shift toward lower molecular
weight species upon catalytic treatment. Initial mechanistic
studies indicate an initial dehydrogenation step of the sub-
strate followed by Pd insertion to give a palladium–enolate
complex.
Experimental Section
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General procedure for cleavage of lignin model compound (1b):
Pd/C (5 wt%) (0.027 g, 2.5 mol%), NH4HCO2 (0.032 g, 0.5 mmol),
and compound 1b (0.137 g, 0.5 mmol) were added to a 5 mL vial.
The vial was sealed and 2.4 mL of methyl tert-butyl ether and
0.6 mL of water were added by a syringe. Another needle was in-
serted through a septum to release pressure during the solvent ad-
dition. The needle was removed and the vial was placed in a pre-
heated oil bath at 808C and stirred with a stirring speed of
1000 rpm for 3 h. The vial was uncapped and left in the bath for
additional 30 min. The vial was cooled to RT and the reaction mix-
ture was filtrated through a celite pad, using diethyl ether (10 mL)
as eluent. Diethyl ether (50 mL) and 10% K2CO3 solution (10 mL)
were added to the filtrate. The mixture was extracted with diethyl
ether (2ꢁ30 mL). Organic fractions were combined and dried over
anhydrous MgSO4. Solvent was removed in vacuo to give com-
pound 3b as a white solid (0.073 g, 0.49 mmol) in 97% yield. To
isolate the guaiacol, the aqueous layer was acidified with 6m HCl
solution (pH 2–3) and extracted with diethyl ether (2ꢁ50 mL). Or-
ganic fractions were combined and dried over anhydrous MgSO4.
The solvent was removed in vacuo, to give compound 4b as
a transparent oil (0.059 g, 0.48 mmol) in 95% yield.
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Received: July 5, 2013
Revised: August 27, 2013
Published online on November 4, 2013
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