Palladium-Catalyzed Hydrolytic Cleavage of Aromatic C?O Bonds

Article abstract of DOI:10.1002/anie.201611076

Metallic palladium surfaces are highly selective in promoting the reductive hydrolysis of aromatic ethers in aqueous phase at relatively mild temperatures and pressures of H₂. At quantitative conversions, the selectivity to hydrolysis products of PhOR ethers was observed to range from 50 % (R=Ph) to greater than 90 % (R=n-C₄H₉, cyclohexyl, and PhCH₂CH₂). By analysis of the evolution of products with and without incorporation of H₂¹⁸O, the pathway was concluded to be initiated by palladium metal catalyzed partial hydrogenation of the phenyl group to an enol ether. Water then rapidly adds to the enol ether to form a hemiacetal, which then undergoes elimination to cyclohexanone and phenol/alkanol products. A remarkable feature of the reaction is that the stronger Ph?O bond is cleaved rather than the weaker aliphatic O?R bond.

Full text of DOI:10.1002/anie.201611076

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Chemie
International Edition: DOI: 10.1002/anie.201611076
German Edition: DOI: 10.1002/ange.201611076
Heterogeneous Catalysis Hot Paper
ꢀ
Palladium-Catalyzed Hydrolytic Cleavage of Aromatic C O Bonds
Meng Wang, Hui Shi, Donald M. Camaioni, and Johannes A. Lercher*
Abstract: Metallic palladium surfaces are highly selective in
promoting the reductive hydrolysis of aromatic ethers in
aqueous phase at relatively mild temperatures and pressures
observed to promote hydrolysis in the presence of H ,
2
[
10a]
although to a lesser extent than hydrogenolysis.
In
previous work, our group hypothesized that hydrolysis
occurs along the same reaction path as hydrogenolysis, with
cleavage of the ether bond by Ni and subsequent addition of
of H . At quantitative conversions, the selectivity to hydrolysis
2
products of PhOR ethers was observed to range from 50%
[
10a]
(
R = Ph) to greater than 90% (R = n-C H , cyclohexyl, and
HC and OHC (from water dissociation).
4
9
PhCH CH ). By analysis of the evolution of products with and
Herein, we report that supported Pd catalysts are active
and highly selective towards ether hydrolysis (> 80% at
complete conversion) at relatively mild temperatures (ca.
2008C) in aqueous phase with pressurized H2 (typically
40 bar). As opposed to conventional acid-catalyzed hydrolysis
2
2
1
8
without incorporation of H2 O, the pathway was concluded to
be initiated by palladium metal catalyzed partial hydrogena-
tion of the phenyl group to an enol ether. Water then rapidly
adds to the enol ether to form a hemiacetal, which then
undergoes elimination to cyclohexanone and phenol/alkanol
products. A remarkable feature of the reaction is that the
stronger PhꢀO bond is cleaved rather than the weaker aliphatic
OꢀR bond.
(ArOR + H O!ArOH + ROH), we show that hydrolytic
2
aryl ether cleavage on Pd occurs by a hitherto unconsidered
mechanism requiring H addition followed by water attack.
2
Hereafter, we define this pathway as “reductive hydrolysis”
(
C H OR + nH + H O!C H(6+2n)O + ROH). Prior work
6
5
2
2
6
C
atalytic cleavage of CꢀO bonds in aromatic ethers is an
with Pd catalysts has not identified this reaction owing to
the use of non-aqueous or water–alcohol solvents (the
[
12]
[13]
important step for the conversion of oxygen-rich lignocellu-
losic plant biomass to deoxygenated fuels and commercial
alcohol serving as hydrogen donor instead of H ), which
2
[1]
chemicals and is challenging because of the strength and
disfavor reductive hydrolysis, making the hydrogenation and/
or hydrolysis steps uncompetitive with hydrogenolysis.
Diphenyl ether was first tested as the simplest diaryl ether
model, which also contains one of the strongest structural
links in lignin, the 4-O-5 type linkage (bond dissociation
[
2]
stability of these linkages. Cleavage of CꢀO bonds can occur
[
3]
[4]
through oxidation, transfer hydrogenation, hydrogenoly-
[
5]
[6]
[7]
sis, hydrolysis/solvolysis, and radical-mediated pathways,
[
1c,8]
among others.
Hydrogenolytic cleavage of strong aryl
ꢀ
1
[1a,14]
CꢀO bonds over heterogeneous metal catalysts requires high
energy: 314 kJmol ).
The pathways for CꢀO bond
temperatures and H pressures and occurs along with arene
cleavage of diphenyl ethers have been broadly classified
into hydrogenation, hydrogenolysis, and hydrolysis (reductive
or non-reductive). It should be noted that not all hydro-
genation events are counted towards the “hydrogenation”
category. Here, “hydrogenation” is limited to reactions that
saturate the aromatic rings without changing the molecular
backbone (i.e., cyclohexyl phenyl ether and dicyclohexyl
ether). The kinetic primary products from hydrogenolysis are
benzene and phenol (1:1) whereas two phenol molecules can
be generated from non-reductive hydrolysis of one ether
2
[
1a,2]
reduction.
used homogeneous nickel complexes
In a recent breakthrough, Hartwig and Sergeev
[5a]
in the presence of
NaOtBu base to catalyze the selective cleavage of aryl CꢀO
bonds under relatively mild conditions in m-xylene as the
solvent without hydrogenating the arene rings and cleaving
aliphatic CꢀO bonds. The reaction could also be accom-
[
9]
plished using Ni nanoparticles. Supported Ni or NiM (M =
Ru, Rh, Au, or Pd) bimetallic catalysts can catalyze this
cleavage at significantly higher rates in water, but always lead
[
5b,10]
[11]
also to some ring saturation.
molecule by conventional hydrolysis or the path proposed
[
10a]
Hydrolysis of aromatic CꢀO bonds is known to be
challenging, requiring harsh conditions such as using water
near or above its supercritical point or strong acids/bases at
for Ni.
First, we evaluated three different supported metal
catalysts (Pd/C, Pt/C, Ni/SiO ) in water at 40 bar H
2
2
[
11]
high temperatures.
Supported Ni catalysts have been
(Table 1, entries 1, 5, and 6). The dominant pathways were
reductive hydrolysis for Pd (80–88%) and hydrogenolysis for
Pt (40%) and Ni (60%) at quantitative conversions of
diphenyl ether (selectivities given as %carbon unless noted
otherwise). The strong preference for reductive hydrolysis
and the small extent of hydrogenolysis with the Pd catalyst
were confirmed to result from intrinsic characteristics of Pd
by using other supported metallic Pd catalysts (e.g., Pd/Al O ,
[*] M. Wang, Dr. H. Shi, Dr. D. M. Camaioni, Prof. Dr. J. A. Lercher
Institute for Integrated Catalysis
Pacific Northwest National Laboratory
P.O. Box 999, Richland, WA 99352 (USA)
Prof. Dr. J. A. Lercher
Department of Chemistry and Catalysis Research Institute
TU Mꢀnchen
Lichtenbergstrasse 4, 85748 Garching (Germany)
E-mail: Johannes.Lercher@ch.tum.de
2
3
entry 2). The product distribution was independent of the
amount of Pd/C over a wide range of diphenyl ether/Pd ratios
(
100–44000). Adding phosphoric acid together with Pd/C
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201611076.
only marginally changed the reactivity and selectivity of the
reaction (see the Supporting Information, Figure S1), indicat-
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1
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[
a]
[11b]
Table 1: Reactions of diphenyl ether.
non-reductive hydrolysis.
As
[
b]
phenol was hardly hydrogenated to
cyclohexanone in the presence of
diphenyl ether (Figure 1A and
Table S1), we dismissed the possi-
bility that hydrolysis of diphenyl
ether first forms phenol, half of
which is then hydrogenated on the
Pd surface to cyclohexanone. The
yields of these products increased
linearly for conversions of up to
20%, with constant selectivities of
Entry Catalyst
Solvent/
atmosphere
t [h]
Carbon selectivity [%]
Hydrogenolysis Hydrolysis Hydrogenation
1
2
5 wt% Pd/C (10.0 mg) H O/H2
5 wt% Pd/Al O3
2
12
2
4
88
80
10
16
2
H O/H2
2
2
(
10.0 mg)
3
4
5
6
5 wt% Pd/C (10.0 mg) decalin/H2
5 wt% Pd/C (10.0 mg) H O/N2
5 wt% Pt/C (10.0 mg) H O/H2
6
12
0.5 40
3 60
3
0
–
0
30
38
97
0
30
2
2
2
64 wt% Ni/SiO2
100 mg)
H O/H2
2
(
[
2
a] Reaction conditions: Diphenyl ether (1.70 g), catalyst, solvent (80 mL), H (40 bar) or N (4 bar),
2
2
008C, stirring at 700 rpm. [b] Calculated at >92% conversion: hydrogenolysis=2ꢁ(cyclohexene+
benzene); hydrolysis=(phenol+cyclohexanone+cyclohexanol)ꢀhydrogenolysis; hydrogenation=
5
0% cyclohexyl phenyl ether, 25%
phenol, and 25% cyclohexanone.
Thus the initial selectivities toward
(
phenyl cyclohexyl ether+dicyclohexyl ether).
reductive hydrolysis and hydroge-
ing that acid-catalyzed pathways do not contribute to the
observed reductive hydrolysis of diphenyl ether on metals at
nation were nearly 50% and 50%, respectively (Figure S3B).
As the reaction proceeded, the yield of cyclohexyl phenyl
ether increased to a maximum of 17% and then decreased to
zero at 100% conversion (Figure 1A). The selectivities to
cyclohexanone, cyclohexanol, and dicyclohexyl ether
increased at the expense of phenol and cyclohexyl phenyl
ether. At 900 min, the selectivities to the hydrogenation and
reductive hydrolysis products became 47% cyclohexanone,
25% cyclohexanol, 17% phenol, and 8% dicyclohexyl ether.
The amount of hydrogenolysis products (cyclohexane and
benzene) remained low (2–3%) during the
2
008C.
The temporal evolution of products during the conversion
of diphenyl ether on Pd/C in water was investigated at 1908C
Figures 1A and S2A). Cyclohexyl phenyl ether, phenol, and
(
cyclohexanone were the only primary kinetic products.
Importantly, phenol and cyclohexanone were initially
formed in a 1:1 yield ratio (Figure S3A) instead of the two
phenol molecules that would be expected from conventional,
entire course of the reaction.
The conversion pathways of cyclohexyl
phenyl ether were explored independently
(Figure 1B). The major products were
cyclohexanol, cyclohexanone, and dicyclo-
hexyl ether during the entire 540 min
reaction, with negligible amounts of
phenol, benzene, and cyclohexane. The
initial conversion rate of cyclohexyl
ꢀ1
phenyl ether (TOF = 0.53 s ) was lower
ꢀ1
than that of diphenyl ether (TOF = 1.9 s ).
The selectivities towards reductive hydrol-
ysis and hydrogenation were relatively
constant at 87% and 13%, respectively.
Under the same conditions, however, no
reactivity of dicyclohexyl ether was
observed. The pathways for CꢀO bond
cleavage of diphenyl ether are summarized
in Figure 1C, accounting for the remark-
able increase in reductive hydrolysis with
reaction time (Figure S3B). As for
diphenyl ether, the primary products from
palladium-mediated reductive hydrolysis
of cyclohexyl phenyl ether were cyclohex-
anone and cyclohexanol (1:1), in contrast
Figure 1. A,B) Product distributions for the reactions of diphenyl ether (A) and cyclohexyl
phenyl ether (B) over Pd/C as a function of conversion. Reaction conditions for (A): Diphenyl to cyclohexanol and phenol from conven-
ꢀ
7
ether (1.70 g, 0.010 mol), 0.2 wt% Pd/C (40.0 mg, 2.3ꢁ10 mol of Pdsurface, prepared by
tional hydrolysis.
To better understand the high selectiv-
ity for ether hydrolysis on Pd/C, we also
diluting 5 wt% Pd/C with activated carbon), H O (80 mL), H (40 bar), 1908C, stirring at
00 rpm, 0–900 min. Reaction conditions for (B): Cyclohexyl phenyl ether (0.18 g, 0.001 mol),
.2 wt% Pd/C (30.0 mg, 1.7ꢁ10 mol of Pdsurface), H O (80 mL), H (40 bar), 1908C, stirring
at 700 rpm, 0–540 min. The corresponding yield–time plots are shown in Figure S2.
C) Reaction pathways and selectivities for diphenyl ether. The TOF of cyclohexyl phenyl ether
was calculated from separate experiments with cyclohexyl phenyl ether.
2
2
7
0
ꢀ
7
2
2
performed the reactions in decalin (H2
atmosphere) and in water (N2 atmos-
phere). When decalin was used as the
2
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solvent, hydrogenation of the aromatic rings accounted for
7% selectivity. C oxygenates were produced in exactly the
9
6
same quantity as the sum of benzene and cyclohexane,
apparently as a result of the lack of water to initiate hydrolysis
pathways.
Reactions did not occur in water under 4 bar N even after
2
1
2 h (Table 1, entry 4). In a control experiment (Figure S1),
where the reaction mixture (ether, Pd/C, water) was first
heated at 2008C for 12 h under N atmosphere and then
2
reacted for another 10 min under H , the conversion and
2
product distribution were identical to those obtained without
the first 12 h under N . In another control experiment with
2
a reversed order of operation, where the mixture was first
reacted at 2008C for 10 min under H and then heated for
2
another 12 h under N , hardly any reactions took place while
2
under N atmosphere (Figure S1). Neither hydrogenation nor
2
hydrogenolysis occurred in the absence of H . The fact that
2
Figure 2. Postulated mechanistic pathway for the reductive hydrolysis
of aryl ethers on Pd surfaces. R=phenyl, cyclohexyl, phenylethyl, and
n-butyl in this work.
even hydrolysis did not proceed in the absence of H suggests
2
that water does not directly attack the aryl CꢀO bond of
diphenyl ether to initiate hydrolysis. The results of these
control experiments further indicate that no reactive inter-
mediate forms and accumulates in the absence of H , and that
The hydrolysis of enol ethers in aqueous phase is known to
occur even at ambient temperature by rate-limiting proto-
nation and fast water addition to form a hemiacetal; in the
next, fast step, the hemiacetal eliminates RꢀOH (one of the
2
the reactive intermediate for hydrolysis is also not any of the
detectable products. Most likely, hydrolysis of the ether bond
has to follow partial hydrogenation of diphenyl ether to
undetectable reactive species that are presents in amounts
below the detection limit.
One such partially hydrogenated, highly reactive inter-
mediate could be cyclohex-1-enyl phenyl ether, which was
used as a starting material under a variety of conditions
stable primary products) and forms an enol, which quickly
[15]
tautomerizes to a ketone. In the case of diphenyl ether, this
mechanistic framework predicts equimolar formation of
phenol (RꢀOH) and cyclohexanone along the initial reduc-
tive hydrolysis pathway whereas for cyclohexyl phenyl ether,
(
Table S2). In contrast to diphenyl ether, this enol aryl ether
cyclohexanol (RꢀOH) and cyclohexanone will be the stable
did not undergo hydrogenation, but was hydrolyzed rapidly,
forming equimolar amounts of cyclohexanone and phenol
primary products from reductive hydrolysis, which is fully
consistent with the experimental observations discussed
above (Figures 1 and S1). Without partial hydrogenation of
the aromatic ring, acid-catalyzed hydrolysis of the aryl CꢀO
[
Eq. (1)] with a total selectivity of 99.9% under all conditions
tested (with or without Pd/C, H or N ). The reaction already
2
2
proceeded considerably in water at 1008C without any
external catalyst, but was significantly promoted by the
presence of Pd/C (Table S2). At 1908C, hydrolysis of cyclo-
hex-1-enyl phenyl ether was also much faster, requiring no
bond cannot occur at these temperatures (e.g., 2008C).
On metals such as Pd, olefinic moieties are highly reactive
under hydrogenating conditions, preventing direct chromato-
graphic and spectroscopic observation of the partially hydro-
genated intermediates (excepting cyclohexanone). Therefore,
additional experiments were performed to provide evidence
for the postulated reductive hydrolysis pathway.
catalyst or H , than the conversion of diphenyl ether.
2
As shown in Figure 2, the mechanism predicts that the
initially formed cyclohexanone should contain oxygen exclu-
sively from water while phenol should contain oxygen solely
from the ether. This was confirmed by isotope labeling
1
8
Taken together, the above results led us to propose
a novel pathway for the reductive hydrolysis of diphenyl ether
on Pd in aqueous phase (R = Ph, Figure 2). Stepwise hydro-
gen addition events first occur at one of the aromatic rings
forming two types of ether intermediates. In one, the ether
oxygen atom is connected to a vinylic carbon atom (e.g.,
cyclohex-1-enyl phenyl ether), while in the other one, the
ether oxygen atom is connected to an alkyl carbon atom (e.g.,
cyclohex-3-enyl phenyl ether). In principle, these intermedi-
ates can be further hydrogenated to stable ether products
experiments with H2 O and unlabeled diphenyl ether. After
1
8
30 min at 1908C (< 5% conversion; Table 2, entry 1) no
had been incorporated into the phenol whereas the cyclo-
hexanone contained > 90% O. However, the observation of
O-labeled cyclohexanone is not sufficient proof for our
hypothesis as subjecting cyclohexanone directly to the same
O
1
8
1
8
1
8
conditions also led to the incorporation of O, presumably via
[
16]
rapid equilibration with the geminal diol (Figure S4). On
the other hand, the phenol being completely unlabeled is
entirely consistent with the proposed mechanism. If, as an
alternative mechanism, ether cleavage preceded H O addi-
2
tion or OHC addition (by dissociative water adsorption) to
1
8
(
cyclohexyl phenyl ether and dicyclohexyl ether). However,
1
8
as shown above, enol ether intermediates almost exclusively
undergo hydrolysis.
the phenyl and phenoxy fragments, half of the initially formed
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Table 2: Initial product selectivities of the Pd/C-catalyzed transformation
of various aryl ethers in H2 O.
bond, rather than the weaker aliphatic CꢀO bond, in contrast
1
8
[a]
to the cleavage pattern observed for aryl alkyl ethers on Ni/
[10a]
SiO2 in aqueous phase.
This should not be seen as
a violation of the bond dissociation energy; instead, it is due
to partial hydrogenation of the aromatic ring, which leads to
enol ether moieties that are much more reactive towards
water attack.
Entry
R group
Product selectivity [mol%]
1
2
3
4
5
In conclusion, we have demonstrated that Pd catalysts are
active and highly selective (up to 90% at quantitative
conversion) towards the reductive hydrolysis of diaryl and
aryl alkyl ethers in aqueous phase under relatively mild
conditions and lead to very limited hydrogenolysis. We have
identified a novel pathway for the Pd-catalyzed reductive
hydrolysis of aryl ethers, which is initiated by partial hydro-
genation of the arene ring to enol ether intermediates that are
highly susceptible to water attack. This pathway contrasts the
often postulated acid-catalyzed ether cleavage pathway,
[
b]
1
2
3
4
phenyl
31
46
32
45
26
46
32
48
41
8
5
1
–
3
–
1
–
3
–
cyclohexyl
phenylethyl
n-butyl
[
c]
7
[
a] Reaction conditions: Ether (50.0 mg), 0.2 wt% Pd/C (1.0 mg, pre-
pared by diluting 5 wt% Pd/C with activated carbon), H2 O (1.0 mL), H2
40 bar at room temperature), 1908C, 0.5 h. [b] As 2 and 5 were identical
in this case, the selectivity to phenol from hydrogenolysis was inferred
from that of 4. [c] The selectivity towards hydrogenation of the R group
1
8
(
(
phenylethyl) was 25%.
which does not require H . It is also distinct from metal-
2
mediated direct ether cleavage (without direct H participa-
tion) followed by recombination of the fragments with surface
2
1
8
phenol would contain O, in contradiction with the observa-
tion.
HC and COH radicals from water dissociation at the metal
[
10a]
Additionally, we investigated the H pressure dependence
surface.
We currently think that the reason for why Pd is
2
of the hydrogenation and reductive hydrolysis pathways with
diphenyl ether. The product selectivities did not change with
H2 pressure. The rates of hydrogenation and reductive
hydrolysis were both shown to be first order with respect to
the H pressure (Figure S5). An identical H pressure depend-
ency for hydrogenation and reductive hydrolysis is possible as
the conversion is proposed to be initiated by a series of
common hydrogen addition steps prior to the branching of the
pathways (Figure 2). The disparate activation energies mea-
sured for the two pathways (Figure S6) clearly reject the
better at hydrolysis than Ni or Pt is related to the activity of
the metals to catalyze hydrogen addition to C=C bonds. Ni is
the slowest in converting diphenyl ether, giving little hydro-
genation product, whereas Pt is fastest at converting diphenyl
ether and gives the largest amount of dicyclohexyl ether.
Future work will be directed towards exploring the effects of
ring substituents and understanding the origin of the prefer-
ence for reductive hydrolysis pathways on Pd.
2
2
possibility of a common rate-determining step (RDS). Addi- Acknowledgements
tional insight comes from the constant reaction order in H2
despite eightfold variation in H pressure (10–80 bar). Con-
This work was supported by the U.S. Department of Energy,
2
sidering that the adsorption of ethers and phenol onto a Pd
Office of Science, Office of Basic Energy Sciences, Division of
Chemical Sciences, Geosciences, and Biosciences. Portions of
the work were performed at the William R. Wiley Environ-
mental Molecular Science Laboratory, a national scientific
user facility sponsored by the DOEꢀs Office of Biological and
Environmental Research located at Pacific Northwest
National Laboratory, a multi-program national laboratory
operated for DOE by Battelle Memorial Institute.
[
12a,17]
surface is stronger than for H adatoms,
we conclude that
the (sub)surface H coverage is relatively low on Pd under the
reaction conditions. At low H coverages, the kinetic observa-
tions, namely reductive hydrolysis and hydrogenation path-
ways showing the same first-order dependence on H but
2
different activation energies, are consistent with a mechanistic
scenario in which the second H addition is the RDS for the
hydrogenation pathway while the RDS for reductive hydrol-
ysis occurs after the addition of at least two hydrogen atoms
(
see the Supporting Information for detailed derivations of Conflict of interest
the rate expressions). DFT calculations are being undertaken
to validate this hypothesis.
The authors declare no conflict of interest.
To explore the generality of the novel mechanistic frame-
work, we analyzed reactions of aryl ethers (ArꢀOꢀR)
Keywords: aryl ethers · hydrogenation · palladium ·
reductive hydrolysis · selective cleavage
containing R groups other than phenyl, namely cyclohexyl,
1
8
2
-phenylethyl, and n-butyl, all in H2 O at conversions < 5%
to minimize the impact of secondary reactions (Table 2,
entries 2–4). All of these ether substrates produced
1
8
16
[
O]cyclohexanone and R OH as the initial products in
a molar ratio of nearly 1:1, confirming that the mechanism
depicted in Figure 2 also applies to these aryl ethers.
Remarkably, reductive hydrolysis of ethers with CaromaticꢀOꢀ
Caliphatic linkages always occurred at the stronger aryl CꢀO
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2
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Manuscript received: November 11, 2016
Revised: December 22, 2016
Final Article published: && &&, &&&&
2
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Heterogeneous Catalysis
Selective hydrolysis: Palladium catalyzes
the reductive hydrolytic cleavage of aro-
matic ether CꢀO bonds with high selec-
M. Wang, H. Shi, D. M. Camaioni,
J. A. Lercher*
&&& — &&&
tivities by a hitherto unconsidered mech-
anism involving partial hydrogenation of
the phenyl group to an enol ether. Rapid
water addition provides the correspond-
ing hemiacetal, which then undergoes
elimination to cyclohexanone and
Palladium-Catalyzed Hydrolytic Cleavage
of Aromatic CꢀO Bonds
phenol/alkanol products.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!