Ruthenium-Catalyzed C-C bond cleavage in lignin model substrates

Article abstract of DOI:10.1002/anie.201410620

Ruthenium-triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox-neutral C-C bond cleavage of the β-O-4 lignin linkage of 1,3-dilignol model compounds. A mechanistic pathway involving a dehydrogenation-initiated retro-aldol reaction for the C-C bond cleavage was proposed in line with experimental data and DFT calculations.

Full text of DOI:10.1002/anie.201410620

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201410620
German Edition: DOI: 10.1002/ange.201410620
Lignin Valorization
À
Ruthenium-Catalyzed C C Bond Cleavage in Lignin Model
Substrates**
Thorsten vom Stein, Tim den Hartog, Julien Buendia, Spas Stoychev, Jakob Mottweiler,
Carsten Bolm, Jꢀrgen Klankermayer,* and Walter Leitner
Abstract: Ruthenium–triphos complexes exhibited unprece-
dented catalytic activity and selectivity in the redox-neutral
À
C C bond cleavage of the b-O-4 lignin linkage of 1,3-dilignol
model compounds. A mechanistic pathway involving a dehy-
À
drogenation-initiated retro-aldol reaction for the C C bond
cleavage was proposed in line with experimental data and DFT
calculations.
D
epleting fossil resources demand the development of
innovative catalytic concepts for the effective conversion of
renewable lignocellulosic feedstock into chemicals and trans-
portation fuels.[1,2] Although the valorization of the carbohy-
drate components of the lignocellulose raw material has
greatly advanced in recent years,[3] the catalytic conversion of
lignin still presents a major challenge. The use of the lignin
fraction of biomass is currently largely restricted to its caloric
value through incineration; consequently, roughly one-third
of lignocellulose remains chemically unexploited.[4] The
difficulties for catalytic valorization concepts are predom-
inantly related to the recalcitrant polymer structure of lignin,
Scheme 1. Selective fragmentation of models of the b-O-4 lignin link-
age (see text for details).
hold great potential for the selective fragmentation of
lignin.[4b] Several reductive,[6] oxidative,[7,8] and redox-neutral
methods[9] for the cleavage of the b-O-4 linkage in lignin
model compounds have been reported recently (Scheme 1).
À
À
with its complex connectivity through relatively stable C O
The cleavage of C C bonds is so far limited to oxidative
[4a]
and C C linkages. Within this multifaceted network the
procedures and radical pathways.[7e,h,8] The C O bond cleav-
À
À
most abundant unit is the b-O-4 linkage, represented by the
dilignol model structure in Scheme 1.[4,5] Hence, the efficient
catalytic cleavage of this specific linkage could pave the way
for selective lignin depolymerization and utilization.
Owing to their tunable reactivity and the accessibility of
the polymer for molecular catalysts, homogeneous catalysts
age of dilignol model compounds has been achieved in
a redox-neutral fashion, for example, by Toste and co-workers
with vanadium catalysts,[9e,f] by Kꢀhn, Cokoja, and co-workers
with rhenium catalysts,[9b] and by Stephenson and co-workers
by iridium photocatalysis.[9a]
The research groups of Bergman and Ellman initially
À
reported such a C O bond cleavage in 2-aryloxyethanols, the
[*] T. v. vom Stein, Dr. T. den Hartog, Dr. S. Stoychev,
Prof. Dr. J. Klankermayer, Prof. Dr. W. Leitner
Institut fꢀr Technische und Makromolekulare Chemie
RWTH Aachen University
elementary analogue of the b-O-4 motif, by intramolecular
transfer hydrogenolysis with a catalyst formed in situ from
[Ru(CO)(H)2(PPh3)3] and xantphos.[9c] Later, James and co-
workers elucidated the mechanistic aspects of this catalyst
system, including a substrate-dependent deactivation path-
way for more complex dilignol models that limits the
substrate scope of this catalyst.[9d]
Worringerweg 1, 52074 Aachen (Germany)
E-mail: jklankermayer@itmc.rwth-aachen.de
Homepage: www.itmc.rwth-aachen.de
Prof. Dr. W. Leitner
Recently, we and others have shown that ruthenium
complexes with triphos-type phosphine ligands are highly
active catalysts for the hydrogenation of challenging sub-
strates.[10] In the course of our studies, these systems were also
Max-Planck-Institut fꢀr Kohlenforschung
Dr. J. Buendia, J. Mottweiler, Prof. Dr. C. Bolm
Institut fꢀr Organische Chemie, RWTH Aachen University
[**] This research was performed as part of the Cluster of Excellence
“Tailor-Made Fuels from Biomass” (TMFB), which is funded by the
Excellence Initiative of the German federal and state governments
to promote science and research at German universities. We thank
the European Union (Marie Curie ITN “SuBiCat” PITN-GA-2013-
607044) for financial support. J.M. thanks the NRW Graduate
School BrenaRo for a predoctoral stipend. J.K. is grateful for the
computing time offered by the Rechenzentrum of RWTH Aachen
University.
À
found to exhibit very high activity for the C O bond cleavage
of 2-aryloxyethanols through intramolecular hydrogen trans-
fer.[11] Herein, we report that these complexes effectively
À
catalyze unprecedented redox-neutral C C bond cleav-
age[12–14] in more elaborate dilignol model compounds incor-
porating a 1,3-diol functionality. A hydrogen-transfer-initi-
ated retro-aldol mechanism[12,13] is proposed to account for
this novel pathway for the disintegration of linkages found
abundantly in lignin.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410620.
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[a,b]
[a,b]
À
À
Table 1: C O and C C bond cleavage of 1.
À
Table 2: Ligand screening for the selective C C bond cleavage of 1.
Entry Ligand T [8C] t [h] Yield of 3a [%][c] Yield of 3b [%][c]
Entry
Precursor
Yield of 2a [%][c]
Yield of 3a [%][c]
1
2
3
4
5
6
7
none
L3
L4
L1
L2
140
140
140
140
4
4
4
4
4
4
1
3
2
6
31
64
77
70
8
2
8
26
64
75
69
1
2
3
4
5
I
II
III
IV
V
28
14
5
26
4
4
21
24
15
66
140
L2
L2
160[d]
160[d]
[a] Only the products containing the phenolic ring are shown. For
a complete list of products from the reactions, see the Supporting
Information. [b] Reaction conditions: 1 (0.1 mmol), ortho-xylene
(0.5 mL). [c] The yield was determined by GC with dodecane as an
internal standard.
[a] For a complete list of products from the reactions, see the Supporting
Information. [b] Reaction conditions: [Ru(Cl)(H)(PPh3)3] (5 mol%),
1 (0.1 mmol), toluene (0.5 mL); the catalyst/L mixture was preheated for
2 h at 1408C. [c] The yield was determined by GC with dodecane as an
internal standard. [d] The reaction was carried out with ortho-xylene
(0.5 mL) as the solvent.
entry 1). The addition of the bidentate ligands xantphos (L3;
Table 2, entry 2) and DPPP (L4; entry 3) did not significantly
improve catalytic performance, and only minor amounts of
À
In initial experiments we investigated the activity of Ru
complexes bearing tripodal phosphine ligands in the trans-
formation of the complex lignin model substrate 1 (Table 1).
In line with previous findings,[11] [Ru(bdepp)(tmm)] (I)
the C C bond-cleavage products were observed. The use of
[Ru(Cl)(H)(PPh3)3] and the tridentate ligand bdepp (L1)
gave 3a and 3b in moderate yield (Table 2, entry 4). Gratify-
ingly, the catalyst formed in situ from [Ru(Cl)(H)(PPh3)3] and
triphos (L2) already showed high activity at 1408C, with
products 3a and 3b formed in 64% yield (Table 2, entry 5).
À
catalyzed the C O bond cleavage to give a mixture of
guaiacol (2a) and several products formed from the comple-
mentary part of the substrate in moderate yield (see the
Supporting Information). Also, small amounts of alcohol 3a
and 3,4-dimethoxybenzaldehyde (3b) were detected (Table 1,
entry 1). The formation of these products can be rationalized
formally by intramolecular hydrogen-atom transfer, associ-
À
When this catalytic system was used at 1608C, the C C
cleavage products 3a and 3b were obtained in (76 Æ 1)%
yield after 4 h, and 70% after 1 h (Table 2, entries 6 and 7,
respectively). These results demonstrate again the catalytic
potential of triphos–Ru complexes in challenging H-transfer
processes at high temperature.
To identify the position of the H-transfer that initiates the
C C bond cleavage, we studied derivatives of compound 1 as
substrates (Scheme 2). When the bis(methyl ether) substrate
4 or the secondary methyl ether substrate 5 was used, the
À
À
ated, however, with C C bond cleavage rather than C O
À
bond hydrogenolysis. C C bond cleavage became the major
pathway with the catalyst [Ru(tmm)(triphos)] (II)[11] to give
product 3a in 21% yield (Table 1, entry 2). Further screening
of complexes containing the triphos ligand revealed a strong
influence of the catalyst precursor. The acetate complex III[15]
gave product 3a in a similar yield of 24% (Table 1, entry 3),
whereas the use of [Ru(CO)(H)2(triphos)] (IV)[16] resulted in
À
À
À
C O and C C bond-cleavage products were each observed in
less than 2% yield. The reaction of substrate 6, incorporating
a secondary hydroxy and a primary methyl ether group, gave
À
À
À
À
only moderate activity for both C O and C C cleavage in
1 (Table 1, entry 4). Finally, the use of [Ru(CO)(Cl)(H)-
the C O bond-cleavage product 2a in 39% yield. This C O
bond-cleavage activity is reminiscent of the reactivity of our
catalyst system towards 2-phenoxy-1-phenylethan-1-ol.[11]
Overall, the reactivity of our catalyst towards substrates 4–6
indicates that both hydroxy functionalities need to be present
(triphos)] (V)[17] afforded the C C bond-cleavage product 3a
in 66% yield, whereas the C O bond-cleavage product 2a
was only detected in 4% yield (Table 1, entry 5).
À
À
À
À
As the ruthenium hydrido chloride precursor provided
a promising lead structure for the redox-neutral selective
for C C bond fragmentation to occur, whereas the C O
bond-cleavage pathway requires only the secondary alcohol
group. The results with substrates 7 and 8 finally prove that
the primary alcohol group[18] is essential to initiate the desired
À
C C bond cleavage of 1, catalyst systems formed in situ from
[Ru(Cl)(H)(PPh3)3] in combination with selected phosphine
ligands were investigated next (Table 2). The triphenylphos-
phine complex alone showed almost no activity (Table 2,
À
C C bond cleavage. In substrate 7 containing a primary and
a tertiary alcohol group, the C C bond was cleaved smoothly,
À
2
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Scheme 3. Ru complex obtained after the reaction.
À
C C cleavage of 1 with [Ru(CO)(Cl)(H)(triphos)] (V) as the
catalyst. This result strongly supports the hypothesis that VI is
À
a resting state of the Ru–triphos system for the catalytic C C
bond-cleavage of 1. The formation of complex VI by the
dehydrogenation of diol 1 is well in line with the known
dehydrogenation activity of related Ru catalysts.[21] In con-
trast to the catalytically inactive six-coordinated xantphos–
diol–Ru complexes reported by the James research group,[9d]
the Ru center of complex VI is five-coordinated, thus
providing a vacant coordination site. This feature may at
least in part explain the catalytic activity of the triphos system.
On the basis of the observed complex VI and the substrate
specificity depicted in Scheme 2, a combined hydrogen-
transfer/retro-aldol mechanism[22–24] can be proposed to
À
account for the C C bond cleavage (Scheme 4). Activation
of precatalyst VII by the association of substrate 1 could
result in complex VIII. Dehydrogenation of VIII subse-
À
quently gives IX, followed by retro-aldol-type C C bond
cleavage[25,26] to form X. The reaction of X with substrate
1 regenerates complex VIII with the liberation of aldehyde 3b
and enol 3a’, which can be hydrogenated under the reaction
conditions to give 3a. Alternatively, intramolecular hydrogen
transfer may also enable the formation of 3a and 3b, thus
regenerating IX directly.
Scheme 2. Reactivity of [Ru(Cl)(H)(PPh3)3]/triphos towards analogues
of substrate 1. The dotted bonds denote the bonds that are broken.
Reaction conditions: [Ru(Cl)(H)(PPh3)3] (5 mol%), substrate
(0.1 mmol), ortho-xylene (0.5 mL); the catalyst/L mixture was pre-
heated for 2 h at 1408C. For a complete list of products from the
reactions, see the Supporting Information.
The experimentally observed complex VI serves in our
proposed mechanism as an off-cycle resting state that is
À
whereas exclusively C O bond-cleavage products
À
were obtained from substrate 8, in which the C H
position of the primary alcohol was blocked.
Finally, the use of ketone substrate 9[19] gave only
À
À
traces of C C and some C O bond-fragmentation
products. These results led to the conclusion that
1) the secondary hydroxy group is required for both
À
À
the C C and the C O bond cleavage to occur, and
2) the primary alcohol group is essential for access
À
to the C C bond-cleavage pathway.
The important role of dehydrogenation at the
primary alcohol functionality was further corrobo-
rated by the characterization of complex VI in
solution after catalysis by multinuclear NMR
spectroscopy (Scheme 3; see also the Supporting
Information). When the [Ru(Cl)(H)(PPh3)3]/tri-
phos system was used, complex VI could be
assigned as a major reaction product alongside
dimeric
[(triphos)Ru(m-Cl)3Ru(triphos)]Cl,[20]
PPh3, and unreacted triphos ligand. The latter
À
complex is inactive for the C C bond cleavage of
À
Scheme 4. Proposed hydrogen-transfer-based retro-aldol mechanism for the C C
1. Consistently, VI was also formed in the catalytic bond cleavage.[22]
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connected to the ruthenium alkoxide IX by hydrogenation/
dehydrogenation. All three intermediates of the cycle VIII–X
were found as stationary points on the energetic hyperface by
DFT calculations, with VIII being the most stable component
(see the Supporting Information for details). The energetic
difference between VIII and the transition state for the
a moderate effect on the yield of 3b (10: 86%, 1: 75%, 11:
63%; Table 3, entries 1–3) unless significant steric constraint
was induced with the two ortho methoxy groups in substrate
12, in which case the yield of 3b was only 26% (entry 4).
Similarly, substituents on ring A only had a small effect on the
formation of 3a (13: 94%, 1: 77%, 14: 75%; Table 3,
entries 1, 5, and 6). However, the use of substrate 15 with
methoxy groups in the 2-, 4-, and 6-positions of ring B gave 3a
in only 14% yield, presumably because of steric hindrance
(Table 3, entry 7). Our results indicate that the [Ru(Cl)(H)-
(PPh3)3]/triphos catalyst is tolerant of various substitution
patterns of the aromatic rings in the model compounds of the
b-O-4 linkage of lignin, which is an important feature
considering the diversity of functionalized aryl rings present
in lignin.
À
crucial C C bond-cleavage step, TS[IX–X], was determined
to be only 16.6 kcalmolÀ1. Although our current results
cannot distinguish between the inter- and intramolecular
hydrogen-transfer paths for the formation of 3a, this energy
difference indicates that the overall catalytic cycle may well
be feasible under the given reaction conditions. Albeit
alternative pathways invoving dehydration cannot be
excluded, the experimental and computational data provide
a coherent picture at this stage.
Finally, the effect of substitution of the aromatic rings A
In conclusion, we have described a novel ruthenium-
À
À
and B on the C C bond-cleavage reaction was systematically
evaluated by using the catalyst prepared in situ with L2
catalyzed redox-neutral C C bond cleavage of 1,3-diol lignin
model compounds that mimic the important b-O-4 linkage. A
(Table 3). The substitution pattern of ring A had only
mechanistic pathway involving a dehydrogenation-initiated
À
retro-aldol reaction for the C C-bond cleavage was proposed
in line with experimental data and DFT calculations. Our Ru–
triphos-based system is one of the first homogeneous catalysts
that display this reactivity[12] and is by far the most selective
Table 3: Reactivity of [Ru(Cl)(H)(PPh3)3]/triphos towards substrates with
various aryl substitution patterns.[a,b]
À
known to date. Application of the catalytic protocol for C C
bond formation[27] is currently under investigation in our
laboratories.
À
Keywords: C C bond cleavage · cleavage reactions ·
homogeneous catalysis · lignin · ruthenium complexes
Entry
1
Substrate
Yield of
Yield of
3a [%][c]
3b [%][c]
1
77
–
75
86
variation of fragment A:
2
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11
12
–
–
63
26
4
variation of fragment B:
5
13
14
15
94
75
14
–
–
–
6
7
[a] For a complete list of products from the reactions, see the Supporting
Information. [b] Reaction conditions: [Ru(Cl)(H)(PPh3)3] (5 mol%),
substrate (0.1 mmol), ortho-xylene (0.5 mL); the catalyst/L mixture was
preheated for 2 h at 1408C. [c] The yield of fragment 3a or 3b was
determined by GC with dodecane as an internal standard.
4
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[24] On the basis of our current results we cannot exclude that in
intermediate IX the dehydrogenated substrate has dissociated,
after protonation, from the Ru catalyst before the retro-aldol
reaction takes place. However, since retro-aldol reactions are
well-known to be catalyzed by a base and thus occur readily from
sodium alkoxides (S. Searlies Jr., E. K. Ives, S. Nukina, J. Org.
Chem. 1959, 24, 1770), it seems more likely that the retro-aldol
step proceeds from the ruthenium alkoxide than from the
hydroxy derivative, that is, when the substrate is still coordinated
to the Ru catalyst.
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À
[12] For a related reaction involving homogeneous catalysis, see:
M. A. Andrews, S. A. Klaeren, J. Am. Chem. Soc. 1989, 111,
4131.
[13] For mechanistic studies on this type of mechanism of heteroge-
neous catalysis, see: a) K. Wang, M. C. Hawley, T. D. Furney,
Ind. Eng. Chem. Res. 1995, 34, 3766; b) D. G. Lahr, B. H. Shanks,
Ind. Eng. Chem. Res. 2003, 42, 5467; c) K. L. Deutsch, D. G.
Lahr, B. H. Shanks, Green Chem. 2012, 14, 1635; d) S. Wang, K.
Yin, Y. Zhang, H. Liu, ACS Catal. 2013, 3, 2112.
[14] For selected papers on similar reactivity by heterogeneous
catalysis, see: a) E. P. Maris, R. J. Davis, J. Catal. 2007, 249, 328;
b) J. Feng, H. Fu, J. Wang, R. Li, H. Chen, X. Li, Catal. Commun.
2008, 9, 1458; c) L. Zhao, J. H. Zhou, Z. J. Sui, X. G. Zhou,
Chem. Eng. Sci. 2010, 65, 30; d) J. Sun, H. Liu, Green Chem.
2011, 13, 135; e) Y. Liu, C. Luo, H. Liu, Angew. Chem. Int. Ed.
2012, 51, 3249; Angew. Chem. 2012, 124, 3303.
[25] For a related ruthenium-catalyzed C C bond-cleavage reaction,
see: T. Kondo, K. Kodoi, E. Nishinaga, T. Okada, Y. Morisaki, Y.
Watanabe, T. Mitsudo, J. Am. Chem. Soc. 1998, 120, 5587.
[26] For selected retro-aldol reactions, see: a) L. Giger, S. Caner, R.
Obexer, P. Kast, D. Baker, N. Ban, D. Hilvert, Nat. Chem. Biol.
2013, 9, 494; b) K. Murakami, H. Ohmiya, H. Yorimitsu, K.
Oshima, Tetrahedron Lett. 2008, 49, 2388.
À
[27] For a recent rhodium-catalyzed hydrogen-transfer-based C C
bond-forming reaction, see: D. Shen, D. L. Poole, C. C. Shotton,
A. F. Kornahrens, M. P. Healy, T. J. Donohoe, Angew. Chem. Int.
Ed. 2015, 54, 1642 – 1645; Angew. Chem. 2015, 127, 1662 – 1665.
Received: October 31, 2014
Revised: February 25, 2015
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Lignin Valorization
T. v. vom Stein, T. den Hartog, J. Buendia,
S. Stoychev, J. Mottweiler, C. Bolm,
J. Klankermayer,*
À
W. Leitner
&&&& — &&&&
Sele-C-C-tivity! Highly selective C C bond
(see scheme). Mechanistic investigations
indicated a hydrogen-transfer-based
retro-aldol pathway for this transforma-
tion.
cleavage of model compounds for the
lignin b-O-4 linkage was promoted by
a catalyst formed in situ from
À
Ruthenium-Catalyzed C C Bond
Cleavage in Lignin Model Substrates
[Ru(Cl)(H)(PPh3)3] and the ligand triphos
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!