Mild and Robust Redox-Neutral Pd/C-Catalyzed Lignol β-O-4′ Bond Cleavage Through a Low-Energy-Barrier Pathway

Article abstract of DOI:10.1002/cssc.201500117

A Pd/C catalyzed redox neutral C - O bond cleavage of 2-aryloxy-1-arylethanols has been developed. The reactions are carried out at 80 °C, in air, using a green solvent system to yield the aryl ketones in near quantitative yields. Addition of catalytic amounts of a hydrogen source to the reaction mixture activates the catalyst to proceed through a low energy barrier pathway. Initial studies support a transfer hydrogenolysis reaction mechanism that proceeds through an initial dehydrogenation followed by an enol adsorption to Pd/C and a reductive C - O bond cleavage.

Full text of DOI:10.1002/cssc.201500117

DOI: 10.1002/cssc.201500117
Communications
Mild and Robust Redox-Neutral Pd/C-Catalyzed Lignol b-O-
4’ Bond Cleavage Through a Low-Energy-Barrier Pathway
Maxim V. Galkin, Christian Dahlstrand, and Joseph S. M. Samec*[a]
A Pd/C catalyzed redox neutral CÀO bond cleavage of 2-ary-
loxy-1-arylethanols has been developed. The reactions are car-
ried out at 808C, in air, using a green solvent system to yield
the aryl ketones in near quantitative yields. Addition of catalyt-
ic amounts of a hydrogen source to the reaction mixture acti-
vates the catalyst to proceed through a low energy barrier
pathway. Initial studies support a transfer hydrogenolysis reac-
tion mechanism that proceeds through an initial dehydrogena-
tion followed by an enol adsorption to Pd/C and a reductive
CÀO bond cleavage.
(Figure 1), besides a few excep-
tions.[4] Since the pioneering arti-
cles from the groups of Bergman[5]
and Toste[6] on the redox-neutral
depolymerization of lignin model
compounds,
many
research
groups have reported both reduc-
tive and oxidative methodologies
to depolymerize lignin by means
of hydrogen transfer. Bergman
used a homogeneous ruthenium-
based catalyst and achieved excel-
lent yields of acetophenone deriv-
atives by using various 2-aryloxy-
COVER
Lignin is a polyphenolic, highly cross-linked polymer that com-
prises up to 30% of the mass and more than 40% of the
energy in wood (Figure 1).[1] Although cellulose, hemicelluloses,
and tall oil produce high-value products, lignin is currently
burned to regenerate process chemicals and produce process
1-arylethanol model compounds and performing the reactions
at 1408C under inert conditions. (Scheme 1). However, the
methodology was limited to Schlenk techniques, an expensive
catalyst, and elevated reaction
temperatures.
A
recyclable heterogeneous
catalytic system has many ad-
vantages over homogeneous
systems, especially in a large-
scale industrial process. Rinaldi’s
group has reported that Raney
nickel reductively cleaves lignin,
utilizing isopropyl alcohol as
Figure 1. Representative structure of native soft wood lignin with the b-O-4’ ether linkage highlighted.
heat in pulp mills.[1] Aromatic compounds are of a high value,
both for fine-chemical production and as part of fuels. There is
a lack of both sources of aromatic hydrocarbons other than
fossil resources, and industrially applicable methods to pro-
duce such aromatic hydrocarbons from renewable feeds.[2]
Lignin is a potentially renewable source of aromatic com-
Scheme 1. Redox neutral and cleavages under reductive conditions of the b-
O-4’-ether linkage in 2-aryloxy-1-arylethanols to generate acetophenone.
pounds, and hence there is a need to develop efficient meth-
ods to convert lignin into high-value products.
Lignin depolymerization, by means of catalysis, is key to the
efficient utilization of lignin.[1] The b-O-4’ ether bond comprises
more than 50% of the monomer linkages in lignin. Also, CÀO
bonds are in general significantly weaker and more labile than
CÀC bonds.[3] Thus, the b-O-4’ ether linkage is a structural
motif that is commonly targeted in lignin depolymerization
mild hydrogen donor.[7] However, this chemistry is limited to
over-stoichiometric use of nickel (up to 600 wt%) and harsh re-
action conditions (160–2408C). Palladium-based catalysts have
received significant attention in development of lignin depoly-
merization methods. Palladium on solid supports is stable to
air and moisture in a wide range of pH values, which allows re-
actions in aqueous media and under ligand-free conditions.
Generally, palladium-based lignin chemistry requires hydrogen
at high pressures (above 30 bar), high temperatures (above
2008C), or both.[8] Processes that demand high hydrogen pres-
sures and that operate at relatively high temperatures are in-
[a] M. V. Galkin, Dr. C. Dahlstrand, Dr. J. S. M. Samec
Department of Chemistry, BMC
Uppsala University
Husargatan 3, 751 23, Uppsala (Sweden)
E-mail: joseph.samec@kemi.uu.se
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500117.
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compatible with current pulp mills, and this renders such pro-
cesses unsuitable for industrial applications.[9] Recently, the
group of Rauchfuss and our group reported the use of palladi-
um on carbon (Pd/C) in the catalytic transfer hydrogenolysis of
lignin models using mild hydrogen donors (Scheme 1).[10] Both
in Rauchfuss’s and in our report, an excess of hydrogen equiva-
lents was used in the transformation of substrate 1 to the aryl
ketone 2.
a reactive palladium surface. To prove our hypothesis, the pal-
ladium was activated by hydrogen gas, instead of NaBH4, to
remove oxides and then flushed with argon to remove the
excess of hydrogen from the system. The reaction proceeded
to generate products from cleaved b-O-4’ ethanolaryl ethers.
However, a longer reaction time was required. The addition of
H+, Na+, Li+, K+ salts, or alkali did not significantly affect the
outcome of the reaction (Supporting Information).[10c,11,13]
A
To our knowledge, there is no report of a heterogeneous
catalyst operating under redox-neutral conditions to cleave the
b-O-4’ ether linkage. Herein, we report a comparably mild,
robust, and efficient redox-neutral b-O-4’ ether linkage cleav-
age of 2-aryloxy-1-arylethanol ethers by recyclable and com-
mercially available Pd/C to generate aryl ketones and phenols
in very high to excellent yields.
possible explanation for the slower reaction is that the hydro-
gen concentration is difficult to control by hydrogen gas (vide
infra).
The effect of NaBH4 was investigated (Figure 2). Low reactivi-
ty was observed below 0.01 equivalents of NaBH4. Between
0.01 and 0.03 equivalents of NaBH4, an aerobic oxidation of the
Pd/C and different additives were screened for redox-neutral
cleavage of PhCH(OH)CH2-OPh (1a) to generate acetophenone
(2a) and phenol (3a) (Table 1). Without additive and under an
atmosphere of air, negligible amounts of benzylic alcohol oxi-
Table 1. Effect of additives for redox neutral cleavage of b-O-4’ ethanolar-
yl ether bond.
Entry
[H]-donor
Conversion [%]
2a [%]
4a [%]
1[b]
2[a,b]
3
No
No
5
11
45
76
0
100
100
0
0
11
25
73
0
99
99
0
5
0
16
3
0
0
Figure 2. Effect of NaBH4. Products versus NaBH4 equiv for the conversion of
1a after 30 min.
HCOOH
NH4HCO2
2-propanol
NaBH4
4
5[b]
6
benzylic alcohol to yield 4a was observed with cleavage of the
b-O-4’ ethanolaryl ether bond. A possible explanation is that
the palladium is activated, but the excess of oxygen present in
the reaction flask pushes the equilibrium toward dehydrogena-
tion product 4a.[14,12d,e] Thereby, the hydrogen adsorbed onto
the palladium from the dehydrogenation of 1a is consumed
by oxygen, instead of substrate 4a. The redox-neutral transfer
hydrogenolysis was observed between 0.05–0.08 equivalents
of NaBH4. Under these reaction conditions, NaBH4 efficiently re-
moves oxygen on palladium as well as the oxygen in the reac-
tion flask. According to the equation NaBH4 +(2+x)H2O=Na-
BO2·xH2O+4H2; the amount of hydrogen liberated in the reac-
tion (see Experimental Section) will be 0.188 mmol, and this
almost equals the oxygen content in the vial. Reduction of the
generated ketone was observed above 0.1 equivalents of
NaBH4. Above 0.3 equivalents of NaBH4 the reaction was inhib-
ited and resulted in formation of trace amounts of 1-phenyle-
thanol (5). To study the effect of a reaction performed with
excess hydrogen, a reaction was performed at 50 bar of hydro-
gen pressure (Scheme 2). Analysis of the products showed sig-
nificant suppression of the desired cleavage reaction. The che-
moselectivity had changed toward benzylic CÀO bond hydro-
genolysis, from which phenethoxybenzene (6) became the
major product. Notably, 6 has previously been reported to be
cumbersome to transform at various conditions,[15] however at
elevated temperatures CÀO bond cleavage has been reporte-
7[b]
8[b,d]
H2
0
0
[c]
NaBH4
1
Yields were determined by H NMR using mesitylene as internal standard.
[a] Reaction were run under argon atmosphere. [b] Reaction time is 6 h
instead of 1 h, time was not optimized. [c] Amount of H2 was not deter-
mined (Supporting Information). [d] Without Pd/C.
dation product 4a and no b-O-4’ ether linkage cleavage was
observed (entry 1). When performing the reaction in an inert
atmosphere, negligible amounts of 2a were observed
(entry 2). The addition of formic acid led to marginally better
results, where the desired 2a was generated in 25% yield
(entry 3). The addition of ammonium formate led to signifi-
cantly better conversion, where 2a was generated in 73%
yield (entry 4). Previous studies have shown that lower pH
values accelerate the disproportionation of benzylic alcohols
while higher pH values accelerate hydrogenolysis of the b-O-4’
ethanolaryl ether bond.[10c,11] The addition of
a catalytic
amount of sodium borohydride led to an efficient transforma-
tion of 1a and resulted in a quantitative yield of 2a (entry 6).
The palladium nanoparticles on the carbon support were
covered by a thin oxide layer at the beginning of the reac-
tion.[12] We hypothesized that the role of sodium borohydride
was to remove oxygen from the reaction medium to generate
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groups at the R3’ and R5’ positions gave the corre-
sponding aryl ketones and phenols in excellent
yields under the same reaction conditions (en-
tries 7,8). These bulky model substrates have previ-
ously been reported to be cumbersome to trans-
form.[5] b-O-4’ Ethanolaryl ether models having more
than two methoxy groups required extended reac-
tion times, up to 4 h (entries 4,6,8–10). Gratifyingly,
Scheme 2. The effect of hydrogen excess on chemoselectivity of b-O-4’ ether linkage hy-
drogenolysis.
d.[10a] Initial studies showed that NaBH4 operated as an appro-
priate hydrogen model source. Sodium borohydride has many
practical advantages over hydrogen gas, especially when small
amounts need to be handled.[16]
the redox-neutral transfer hydrogenolysis gave excellent yields
also for these models. Furthermore, phenolic substrates (R4 =
OH) were transformed under the present reaction conditions
to generate products 2d and 2e (entries 9 and 10) in excellent
yields. Unprotected phenols have previously been reported to
be difficult to transform.[18] The products were easily separated,
to obtain aryl ketones in excellent yields. This method allows
facile separation of the catalyst from the product by decanta-
tion of the organic phase, where the Pd/C is found mainly in
the water phase after the reaction. It should be noted that also
phenol, guaiacol, and syringol (3a–3c) could be isolated in
very good to excellent yields (entries 1,5,and7).
Pd/C catalyzed redox neutral cleavage of the b-O-4’ ethano-
laryl ether bond in lignin model compounds was investigated
(Table 2). b-O-4’ Ethanolaryl ethers, which mimic lignin from
Table 2. Redox neutral Pd/C catalyzed cleavage of b-O-4’ ethanolaryl
ethers substrates.
Pd/C is a heterogeneous catalyst on which a dimeric model
compound can easily adsorb and react. To determine the com-
patibility of the heterogeneous catalyst with a polymeric sub-
strate that mimics natural lignin in terms of both chemical and
physical properties (e.g., solubility); the redox-neutral transfer
hydrogenolysis reaction was performed on model polymer 1d’
(Scheme 3). The molecular weight of the model polymer,
Entry
Substrate
R3
R4
R3’
R5’
Yield [%]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1 f
1g
1h
1i
H
H
H
OMe
H
OMe
H
OMe
H
H
OMe
H
OMe
OMe
OMe
H
OMe
OH
OH
H
H
OMe
H
OMe
OMe
OMe
OMe
OMe
OMe
H
H
H
H
H
H
OMe
OMe
H
95/92[b]
98
94
95[a]
99[c]/95[b]
98[a]
97/95[b]
92[a]
97[a]
96[a]
10
1j
OMe
H
Average isolated yield of the corresponding ketone based on two dupli-
cate experiments. Conversion in all cases was above 99%, neglecting
amount of 4a–j was observed. [a] Reaction was run for 4 h instead of 1 h.
[b] Yield of phenol, guaiacol, and syringol. [c] EtOH/H2O was tested at
redox neutral conditions. Substrates 1e and 1d’ resulted in 98% and
97% yield of hydrogenolysis products respectively.
Scheme 3. Redox neutral depolymerization of polymeric b-O-4’ ethanolaryl
ether lignin model 1d’.
6 kDa, corresponds to the molecular weight of native lignin.
The model polymer underwent Pd/C-catalyzed b-O-4’ bond
cleavage reaction upon treatment at the same conditions as
the dimeric models. The corresponding 2d was isolated in
a yield of 99%.
different sources, were chosen as model substrates to investi-
gate the scope of the reaction. 1a and 1b, with no methoxy
substitution of the aryl, model the major lignin component in
the monocotyledons, such as switch grass.[1,17] Substitution of
the aryl group by a methoxy group at the R3 and R3’ positions
(1c–f) corresponds to the substitution pattern found in conifer
lignin, for example in pine and spruce.[1,17] Substitution of the
aryl group by two methoxy groups at the R3’ and R5’ positions
(1g and h) corresponds to the substitution pattern found in
lignin from the dicotyledons, for example birch and eucalyp-
tus.[1,17] The redox-neutral transfer hydrogenolysis of the b-O-4’
ethanolaryl ethers proceeded at 808C to yield the correspond-
ing acetophenones and phenols in excellent yields (92–99%
yield) within 60 min (entries 1–3,5,7). This is to our knowledge
both the mildest and fastest cleavage of b-O-4’ ethanolaryl
ether models ever reported, including results achieved through
homogeneous catalysis. Even bulky substrates with methoxy
Rauchfuss et al. recently proposed a reaction mechanism for
the transfer hydrogenolysis of b-O-4’ ethanolaryl ethers using
dioxane as hydrogen donor. The reaction was proposed to pro-
ceed through an insertion of palladium into the a-CÀH bond
of 1a (Scheme 4). The CÀH activated intermediate A trans-
Scheme 4. Reaction mechanism proposed by Rauchfuss et al. for the Pd/C
catalyzed transfer hydrogenolysis of the b-O-4’ ethanolaryl ether bond.
Adopted from original manuscript.[10a]
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formed into intermediate B and tautomerized to yield product
2a. Experimental data, including productive reactivity of
PhCH2CH2-OPh
(6),
PhCH(OMe)CH2-OPh
(7),
and
PhC(Me)(OH)CH2-OPh (8), and lower reactivity for Ph(C=O)CH2-
OPh (4), supported the proposed reaction mechanism. It
should be noted that these reactions proceeded at elevated
temperatures (1608C) compared to the present redox-neutral
transfer hydrogenolysis (808C).
Previously, we proposed an alternative mechanism for the
Pd/C-catalyzed transfer hydrogenolysis of b-O-4’ ethanolaryl
ethers by ammonium formate.[10c] We proposed a reaction
mechanism that proceeds through an initial dehydrogenation
to form Ph(C=O)CH2ÀOPh (4a) as a reactive intermediate.
Under transfer hydrogenolysis conditions using formate as re-
ducing agent, it was experimentally supported that 4a is an in-
termediate in the transfer hydro-
Figure 3. Formation of 4a and 2a thought the reaction.
genolysis of 1a.
We wanted to study the reac-
tion mechanism of the current
redox-neutral transfer hydroge-
nolysis of b-O-4’ ethanolaryl
ethers and determine whether
the reaction mechanism pro-
posed by Rauchfuss et al. was
operating under our conditions.
Running the reaction on models
6–8, that cannot undergo a dehy-
Scheme 5. Comparison of dimeric ketones reactivity 4k and 4a versus dimeric alcohols 1a and 1k respectively.
drogenation of the benzylic alco-
hol, gave no reaction using the
redox neutral reaction conditions [Equations (1)–(3)]. Thereby,
the transformation does not proceed through the same reac-
throughout the reaction (Figure 3). After a short induction
period, the concentration of 4a remains low and was observed
nearly until the end of the reaction, suggesting steady-state ki-
netics in which k1 !k2.
To verify the higher reactivity of 4a versus 1a in the reac-
tion, a cross-experiment was performed (Scheme 5). Fluoro la-
beling was used to introduce a reactivity difference, where the
ortho-fluoro substituted species 1k reacts 2.3 times faster than
1a (Supporting Information), and also for ease of analysis, by
19F NMR spectroscopy. Reaction of 4k with 1a as hydrogen
donor resulted in formation of 2a in 57% conversion and 2-
fluorophenol (3d) in 55% conversion. The selectivity towards
4k is thereby 96% and this is much higher than what is ex-
pected (70%) based on the rate difference. Under identical
conditions 4a with 1k as hydrogen donor were converted to
2a in 27% and 3d in 15%. Thereby, selectivity towards the
ketone is 44% and this is again much higher than the expect-
ed (30%) from rate difference. These cross-experiments show
that the dimeric ketone is more reactive than the dimeric alco-
hol under redox neutral reaction conditions at 808C.
To determine if the hydrogen in the b position participates
in overall transformation, the b position was blocked using
two methyl groups [substrate 1l, Equation (4)]. The reaction
was performed at 808C and resulted in a low dehydrogenation
of 1l, where no b-O-4’ bond cleavage products were observed
even after prolonged reaction times. However, at 1608C full
conversion of 1l to yield cleaved product 2 f was observed
[Equation (5)]. The experiments performed [Equations (1)–(5)
tion mechanism as proposed by Rauchfuss et al.. The results
from these reactions could be rationalized by that both the
proton of the hydroxyl group and the a-hydride at the benzyl-
ic position are participating in CÀO bond cleavage of the
ether.
A
plausible explanation is that the reaction proceeds
through an initial dehydrogenation to generate 4a and chemi-
cally adsorbed hydrogen. This adsorbed hydrogen is responsi-
ble for the following hydrogenolysis step. To determine if 4a is
an intermediate, the concentration of 4a was monitored
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quirements of hydrogen in the b position supports this [Equa-
tion (4)]. The basic medium may facilitate the keto–enol tauto-
merization and thereby also the adsorption of 4 to the palladi-
um surface to generate C.[13] Following a Horiuti–Polanyi-type
mechanism,[20] the atomic hydrogen may insert into either the
a or b position of intermediate C. In route I, atomic hydrogen
inserts into the b position to generate intermediate D. CÀO
bond cleavage generates intermediate E where the enol equiv-
alent is chemically adsorbed together with the aryloxide. Inter-
mediate E decomposes to products 2 and 3. This route gives
a similar intermediate as proposed by Rauchfuss et al., howev-
er, the reaction pathway is distinctly different. In route II,
atomic hydrogen inserts into the a position to give intermedi-
ate F. Palladium inserts into the CÀO bond to generate G, in
which a carbene and aryloxide are chemically adsorbed on pal-
ladium. Reductive elimination of aryloxide and the carbene
tautomerization lead to products 2 and 3. In an environment
with excess hydrogen a different chemoselectivity is observed,
in which a hydrogenolysis reaction of the benzylic alcohol
occurs to yield 6. At 808C, no further reaction takes place.
When the reaction temperature is increased to 1608C, a CÀO
ether bond cleavage is reported to occur by Rauchfuss et al.[10a]
Thereby, the present methodology proceeds through a unique
low energy barrier (12 kcalmolÀ1) pathway for the lignol CÀO
bond cleavage via an initial dehydrogenation.
and Scheme 5] support that both the hydrogens at a and b po-
sitions of b-O-4’ ethanolaryl ethers participate in the reaction
performed at 808C.
To gain more insight into the apparent low-energy-barrier
pathway in the current redox neutral reaction, the activation
energy was studied. The apparent activation energy for the
cleavage of 1a under redox neutral conditions showed a low
value (12 kcalmolÀ1
)
(Supporting Information; 1 kcal=
4.184 kJ). The obtained apparent activation energy value is sig-
nificantly lower than the corresponding calculated bond disso-
ciation energy of the ether bond in 1a (72.2 kcalmolÀ1) or 4a
(61.2 kcalmolÀ1) (Supporting Information). Therefore, further
studies are needed to determine the contribution to this value
from mass transfer processes and adsorption.
The following reaction mechanism is proposed for the redox
neutral reaction performed at 808C (Scheme 6): The oxide on
the palladium surface is removed by hydrogen. This is demon-
strated in Figure 2, where the efficacy of the reaction is depen-
dent on the amount of NaBH4. The reaction starts with a rever-
sible dehydrogenation of 1 to generate 4 and chemically ad-
sorbed hydrogen on Pd/C. This intermediate was proven to be
more reactive than the starting material (Scheme 5). The eno-
late form of 4 adsorbs to palladium to generate C.[19] The re-
We have demonstrated the first heterogeneous lignol CÀO
bond cleavage under redox-neutral reaction conditions. A low-
energy-barrier pathway is operating in the reaction of b-O-4’
ethanolaryl ethers. The key is to control the concentration of
oxygen and hydrogen on the palladium surface. By applying
catalytic amounts of hydrogen to the reaction mixture, the
chemically adsorbed oxygen on the palladium surface was re-
moved to generate an efficient catalyst. Excess hydrogen has
a detrimental effect on the system, in which formation of less
reactive dimeric intermediates (6) occurs. These intermediates
require harsher reaction conditions for CÀO bond cleavage. By
applying the present redox-neutral reaction conditions, the re-
action initially proceeds through a low-energy-barrier dehydro-
genation to yield 2a. This intermediate was proven to be more
reactive than the starting material (1a). The mild and robust
redox-neutral transfer hydrogenolysis was applied to several 2-
aryloxy-1-arylethanols that mimic the aryl substitution patterns
in lignin from different sources. The products were isolated in
very good to excellent yields in 1–4 h running the reactions at
808C.
Experimental Section
General procedure for b-O-4’ bond transfer hydrogenolysis: To
a suspension of Pd/C (50 mg, 0.023 mmol) in EtOAc (2.5 mL) and
substrate (0.47 mmol), freshly prepared NaBH4 water solution
(0.5 mL, 0.092m) was added. The suspension was stirred (600 rpm)
at 808C (oil bath temperature) for desired time. The reaction mix-
ture was cooled to RT and poured into a separatory funnel contain-
ing EtOAc (30 mL) and water (30 mL). The reaction vessel was
rinsed with 10 mL of EtOAc. The combined organic extracts were
washed with brine and dried over anhydrous Na2SO4, filtrated and
Scheme 6. Proposed reaction mechanisms for the redox neutral transfer hy-
drogenolysis of 1 (see Supporting Information). Metal atoms are depicted by
surface for clarity.
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Keywords: heterogeneous catalysis · lignin · palladium ·
reaction mechanisms · transfer hydrogenolysis
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Received: January 21, 2015
Revised: March 3, 2015
Published online on April 29, 2015
ChemSusChem 2015, 8, 2187 – 2192
www.chemsuschem.org
2192
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