Selective conversion of levulinic and formic acids to γ-valerolactone with the shvo catalyst

Article abstract of DOI:10.1021/om400938h

The selective transfer hydrogenation of levulinic acid (LA) with formic acid (FA) to 4-hydroxyvaleric acid (4-HVA) and carbon dioxide followed by the intramolecular dehydration of 4-HVA to γ-valerolactone (GVL) are key steps of the conversion of carbohydrate-based biomass to GVL, which can be used for the production of both energy and carbon-based products. LA was converted to GVL in the presence of a small excess of FA and the Shvo catalysts {[2,5-Ph ₂-3,4-(Ar)₂(η⁵-C₄CO)] ₂H}Ru₂(CO)₄(μ-H)]} (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)). The reactions were performed at 100 C with yields higher than 99% after a few hours. The formation of 1,4-pentanediol and 2-methyltetrahydrofuran remained below detection limits. The only side products were water and carbon dioxide, as expected, which were easily removed and separated from the product GVL under reduced pressure. The Shvo catalyst 1c was recycled four times without losing catalytic activity, and the product GVL was isolated each time as a colorless liquid of 99.9% purity with only trace amounts of water present.

Full text of DOI:10.1021/om400938h

Article
pubs.acs.org/Organometallics
Selective Conversion of Levulinic and Formic Acids to
γ‑Valerolactone with the Shvo Catalyst
Viktoria Fabos,† Laszlo T. Mika,†,‡ and Istvan T. Horvath*,†,§
́
́
́
́
́
́
†Institute of Chemistry, Eotvos University, Paz
́ ́ ́
many Peter 1/A, H-1117 Budapest, Hungary
̈
̈
‡Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Budafoki
street 8, H-1111 Budapest, Hungary
§Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
ABSTRACT: The selective transfer hydrogenation of levulinic acid (LA) with formic acid (FA) to 4-hydroxyvaleric acid (4-
HVA) and carbon dioxide followed by the intramolecular dehydration of 4-HVA to γ-valerolactone (GVL) are key steps of the
conversion of carbohydrate-based biomass to GVL, which can be used for the production of both energy and carbon-based
products. LA was converted to GVL in the presence of a small excess of FA and the Shvo catalysts {[2,5-Ph2-3,4-(Ar)2(η5-
C4CO)]2H}Ru2(CO)4(μ-H)]} (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)). The reactions were performed at 100 °C with
yields higher than 99% after a few hours. The formation of 1,4-pentanediol and 2-methyltetrahydrofuran remained below
detection limits. The only side products were water and carbon dioxide, as expected, which were easily removed and separated
from the product GVL under reduced pressure. The Shvo catalyst 1c was recycled four times without losing catalytic activity, and
the product GVL was isolated each time as a colorless liquid of 99.9% purity with only trace amounts of water present.
GVL occurs in fruits, is nontoxic, has been used by the food
industry, and has very attractive physical and chemical
properties.6 Its high solubility in water facilitates biodegradation
and easy bioremediation. GVL’s low melting (−30 °C) and
high boiling (204 °C) points make it one of the best candidates
for a moveable liquid in pipes, trucks, and tankers. In addition,
it is a safe material for large-scale use due to its very low vapor
pressure, high flash point (96 °C), and high stability in water at
neutral pH and in air. Also, GVL can be used as a platform
chemical for the synthesis of 1,4-pentanediols,6,8a,9 2-
MeTHF,8a,9,10 alkyl-4-alkoxyvalerates and tetraalkylammonium
4-hydroxyvalerates,11 butenes,12 mixtures of alkanes,8a alkylval-
erates,13 4-hydroxypentane amides,14 and adipic acid via
pentenoic acids15 (Scheme 2).
he utilization of non-edible biomass as feedstock for the
Tproduction of platform chemicals is becoming one of the
enabling resources of sustainable development.1 Since carbohy-
drates are the most abundant components of biomass,2 the
acid-catalyzed conversion of fructose, glucose, and sucrose as
well as the oligomers and polymers of C5- and C6-
carbohydrates to 5-(hydroxymethyl)-2-furaldehyde (HMF)3
and/or levulinic acid (LA) and formic acid (FA)4 has been
the focus of intense research (Scheme 1). The hydrogenation of
LA to 4-hydroxyvaleric acid (4-HVA) opens the way to the
facile formation of γ-valerolactone (GVL),5 which has been
proposed as a sustainable liquid6 and the key molecule of a
GVL economy.7
Recently, numerous papers have been published on the
catalytic conversion of LA to GVL. While heterogeneous
supported Ru, Pd, Pt, Ir, and Re catalysts were used in the
presence of different additives in the temperature range 25−
265 °C,16 homogeneous systems have been almost exclusively
based on ruthenium-containing catalysts. The combination of a
triphosphine-modified ruthenium catalyst with NH4PF6 was
Scheme 1. Conversion of Carbohydrates to GVL
Received: September 19, 2013
Published: January 2, 2014
© 2014 American Chemical Society
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can be used for the transfer hydrogenation of LA to 4-HVA.8b
The latter approach could be an inherently safer strategy, as the
process is independent of hydrogen gas and readily usable even
in remote parts of the world.
Scheme 2. Conversion of GVL to Chemicals
Formic acid has been used for the transfer hydrogenation of a
broad range of aldehydes and ketones.21 We have reported the
first example for the transfer hydrogenation of LA with FA
using [(η6-C6Me6)Ru(bpy)(H2O)][SO4],22 resulting in 25%
GVL, 25% 1,4-pentanediol, and 1% 2-MeTHF at 70 °C after 18
h.8a Since the dehydration of 1,4-pentanediol leads to the
formation of 2-MeTHF, which can readily form peroxides
under air,23 we have continued our search for a much more
selective catalyst.
The Shvo catalyst precursors {[2,3,4,5-Ar4-(η5-C5O)]2H}-
Ru2(CO)4(μ-H) have been used for the hydrogenation of
various aldehydes and ketones and transfer hydrogenation of
ketones using alcohols, sodium formate, and formic acid as the
reducing agents.24−26 The selective conversion of LA to GVL
was achieved by using formic acid in the presence of {[2,5-Ph2-
3,4-(p-MeOPh)2(η5-C5O)]2H}Ru2(CO)4(μ-H)] (1a).8b Con-
cerning the currently accepted mechanism, the reversible
dissociation of {[2,5-Ph2-3,4-Ar2(η5-C5O)]2H}Ru2(CO)4(μ-
H)] (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)) leads to
the formation of two mononuclear forms of the active species
in solution: the hydroxycyclopentadienyl ruthenium hydride
complex {[2,5-Ph2-3,4-Ar2(η5-C5OH)]Ru(CO)2H} (2a−c)
and the coordinatively unsaturated {[2,5-Ph2-3,4-Ar2(η4-
C4CO)]Ru(CO)2} (3a−c) (Scheme 3).24,25
reported by Leitner for the selective conversion of LA to GVL,
1,4-pentanol, or 2-MeTHF.9 Although the water-soluble tris-m-
sulfonated triphenylphosphine (TPPTS) modified Ru catalyst
was active for the conversion of LA to GVL under biphasic
conditions, the catalyst activity decreased during recycling.17
The more basic water-soluble butylbis(m-sulfonatophenyl)-
phosphine-modified Ru catalyst was very efficient for the
conversion of neat LA to GVL.18 It should be noted that the
Ru-catalyzed continuous hydrogenation of LA was also tested
using molecular hydrogen or formic acid as a hydrogen
source.18b,c A “hydrogen free” domino process was recently
reported starting from furfural and involving two transfer
hydrogenation steps using 2-butanol as the hydrogen source.19
While this is an interesting approach, the origin of 2-butanol
and/or the fate and reuse of 2-butanone was not addressed. If
the 2-butanone has to be reduced back to 2-butanol, this
approach also needs molecular hydrogen.
Intermediate 2 has both acidic and hydridic functions, which
could readily transfer a proton from the hydroxide group and a
hydride from the ruthenium atom to the CO double bond of
LA, resulting in the formation of 4-HVA and 3. While 4-HVA
could readily undergo dehydration to give GVL, the
coordinatively unsaturated 3 could activate FA27 to yield 2
and CO2 or react with hydrogen (if available) to form 2. In
addition, the reversible dimerization of intermediate 3 could
result in {[2,5-Ph2-3,4-Ar2(η4-C4CO)]Ru(CO)2}2, another
dimeric resting form of the Shvo catalyst.25a
Since the products of the hydration of HMF are an
equimolar mixture of LA and FA,20 there are two possible
strategies for their conversion to GVL. The catalytic
decomposition of FA to CO2 and H2 opens the way to
catalytic hydrogenation of LA to 4-HVA, which can undergo
ring closure via dehydration to form GVL.8a Alternatively, FA
Scheme 3. Shvo Catalyst Precursors (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)) and Proposed Mechanism for Transfer
Hydrogenation of Levulinic Acid with Formic Acid
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We report here the use of the Shvo catalyst for the selective
conversion of LA to GVL using FA as the hydrogen source.
The formation of 1,4-pentanediol and 2-methyltetrahydrofuran,
the typical side products of the reduction of GVL with
molecular hydrogen, was not observed. The reactions were
performed with yields up to >99%, and the only byproducts
were water and carbon dioxide, which were easily eliminated.
The Shvo catalyst was recycled several times without losing
catalytic activity.
the intramolecular dehydration of the initial product 4-HVA to
GVL is a facile process, much faster than formate formation.
The formation of 2a could be also observed in the reaction of
1a with FA at 80 °C (Figure 2), as a very slow or no reaction
RESULTS AND DISCUSSION
■
The orange powders of the dinuclear ruthenium Shvo catalyst
precursors (1a−c) were stable in air for years. Their light
yellow solutions in d8-toluene were also stable for several days
at room temperature in air. When the temperature was raised to
80 °C in air, the solutions turned dark brown within minutes,
indicating rapid decomposition. The most likely mechanism of
the decomposition involves the dissociation of 1 to 2 and 3
followed by their reaction with oxygen. It should be noted that
the elimination of hydrogen from 2b could also lead to the
formation of 3b (Scheme 3), which opens the way to (1)
reaction with 2b to re-form 1b, (2) dimerization to {[2,5-Ph2-
3,4-Ar2(η4-C4CO)]Ru(CO)2}2, or (3) completion of the
decomposition by reaction with oxygen. In contrast, in the
presence of hydrogen or formic acid, the coordinatively
unsaturated 3b could react either with H2 to form 2b or with
HCOOH to yield 2b and CO2, respectively. Accordingly, when
the dimer 1b was placed under 13 bar of hydrogen at 21 °C for
5 h, only trace amounts of 2b were observable by NMR (Figure
1). When the reaction mixture was kept under 13 bar of
1
Figure 2. H and 13C{1H} NMR spectra of 1a treated with FA in
THF-d8 at 100 °C after 30 min.
was observed at lower temperatures. In addition, the formation
of small amounts of H2 and CO2 was also detectable by NMR,
confirming that the Shvo catalyst can catalyze the conversion of
FA to H2 and CO2. While this reaction was at least 10 times
slower than the transfer hydrogenation of LA with FA to 4-
HVA, it has an unexpected impact: a slight excess of FA must
be used for full conversion of LA.
Also, an in situ IR study has shown the rapid disappearance
of the bands of 1a at 2035, 2002, and 1973 cm−1 with a
shoulder at 1962 cm−1 and the simultaneous appearance of
bands at 2012 and 1953 cm−1 for 2a (Figure 3). Since only 2a
Figure 1. Treatment of {[2,5-Ph2-3,4-(p-MePh)2(η5-C4CO)]2H}-
Ru2(CO)4(μ-H) (1b) in d8-toluene with 13 bar of H2 at different
temperatures.
Figure 3. In situ IR spectra of the reaction of 1a with HCOOH in
THF at 80 °C.
was formed, we propose that in the presence of FA the
dissociation of 1a to 2a and 3a was followed by the activation of
FA by 3a, resulting in 2a via 4a (Scheme 3).
hydrogen at 80 °C for 24 and 36 h, the ratio between 1b and 2b
was practically the same, clearly demonstrating that 1b and H2
are in equilibrium with 2b.
After the tube was depressurized and the H2 was replaced
with N2, 2b was converted slowly to 1b at 21 °C and much
more quickly at higher temperatures.
It is important to note that the original procedure of the
transfer hydrogenation of ketones by Shvo used HCOONa and
H2O to prevent the formation of formates.27 In the case of
levulinic acid, neat formic acid could be used efficiently because
Since {[2,5-Ph2-3,4-(p-MeOPh)2(η5-C4CO)]2H}-
Ru2(CO)4(μ-H) (1a) was the most effective Shvo catalyst for
the reduction of ketones, 1a was used in the catalytic
experiments. The transfer hydrogenation of neat LA with FA
in the presence of the Shvo catalyst was performed by heating
12 mmol of LA and 20.4 mmol of FA in the presence of 0.01
mmol of 1a at 100 °C for 5 h. The only observable product was
GVL in up to 99.9% purity by NMR (Figure 4) and GC.
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from FA. The retarding effect of CO2 has been shown by
performing the reaction in a high-pressure sealed tube. While
full conversion was achieved in an open vessel at an FA/LA
ratio of 2 and an LA/1a ratio of 2400 at 100 °C after 12 h, the
sealed tube gave only 52% conversion of LA under the same
conditions.
In order to establish the influence of the substituents of the
phenyl ring of the cyclopentadienyl ring on the catalyst
performance, two other Shvo catalyst precursors, 1b,c (Figure
5), were used in the reduction of LA. Table 2 clearly shows that
Figure 4. Transfer hydrogenation of LA with FA (neat) in the
presence of 1a at 100 °C.
Although the dehydration of 4-HVA produces water, the latter
can easily be eliminated by distillation, as GVL does not form
an azeotropic mixture with water.6 Also, the other byproduct
formed from FA, carbon dioxide, does not cause any difficulties,
as it bubbles out of the solution.
The catalytic performance of the Shvo catalyst 1a was
evaluated using LA/FA as the solvent under various conditions.
The reaction does not take place in the absence of formic acid
or the catalyst, as expected (Table 1).
Table 1. Transfer Hydrogenation of 8.6 mmol of LA with FA
in the Presence of 1a at 100 °C for 8 h
Figure 5. Transfer hydrogenation of 8.6 mmol of LA with FA in the
presence of different Shvo catalysts at 100 °C, n(LA)/n(FA) = 2, and
n(LA)/n(Shvo catalyst) = 1200.
T (°C)
n(FA)/n(LA)
n(LA)/n(Shvo catalyst)
GVL (%)
100
100
60
0
400
0
0
1.5
1.0
1.0
1.0
1.0
1.5
1.5
1.5
2.0
2.0
2.5
3.5
0
400
0
Table 2. Transfer Hydrogenation of LA with FA in the
Presence of 1a−c at 100 °C for 1 h
80
400
20.0
94.3
70.1
40.3
99.9
99.9
99.9
52.0
97.2
96.3
100
100
1200
2400
400
catalyst precursor LA (mmol) FA (mmol) [LA]/[1a] GVL (%)
a
100
1a
1b
1c
1a
1b
1c
8.6
8.6
8.6
8.6
8.6
8.6
12.9
12.9
12.9
17.2
17.2
17.2
1200
1200
1200
1200
1200
1200
59.2
56.6
44.8
61.5
59.2
48.1
100
100
100
400
2400
2400
2400
2400
b
100
100
100
2400
a
b
Na formate was use instead of FA. In a sealed vessel after 12 h.
the highest conversion level was achieved when p-MeOPh was
used as the ligand on the cyclopentadienyl ring, after 1 h. This
observation is consistent with the trends observed previously by
Shvo23 and Williams.28 Although the reaction is slower using
the precursors 1b,c, the full conversion of LA was observed
after 4−5 h with all three catalyst precursors (Figure 5).
The recyclability of the Shvo catalyst 1c, which has been the
cheapest and easiest to prepare, was investigated in a batch
reactor. An 85.7 mmol portion of LA was treated with 172.7
mmol of FA in the presence of 87.4 mg (0.0805 mmol) of
{[2,3,4,5-Ph4-(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1c) at 95 °C
for 6 h. The rapid evolution of CO2 from the orange solution
was observed during the reduction (Figure 6a). The volatile
compounds were removed by vacuum transfer at 85 °C/1
mmHg, resulting in a clear and colorless distillate containing
99.9% of the product GVL, the side product water, and the
unreacted FA. The orange viscous residue (Figure 6b) was
mixed with 85.6 mmol of LA and 174.7 mmol of FA under
nitrogen, and the mixture was heated to 95 °C for 6 h.
No reaction was observed at 60 °C, as the conversion of 1a
to the catalytically active species requires at least 80 °C. While 1
equiv of FA with respect to LA was not enough for the full
conversion of LA, the variation of the FA concentration did not
have a significant influence between 1.5 and 2.5 equiv of FA to
LA but resulted in lower conversion in the case of a higher
excess of FA. The highest turnover number of 3400 was
achieved at an FA/LA ratio of 2 and an LA/1a ratio of 3600 at
100 °C. The use of aqueous sodium formate as the H donor
instead of formic acid resulted in much lower conversion.
It has to be emphasized that GVL does not react further with
FA or the catalyst to produce unwanted byproducts such as 1,4-
pentanediol and 2-methyltetrahydrofuran. This was confirmed
in a separate experiment by refluxing GVL with FA in the
presence of the precursor 1a for 20 h, wherein no hydro-
genation of GVL was observed.
It is also important to note that the reaction vessel was
opened to the air via a capillary tube to release the CO2 formed
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with a recycle delay of 80 s. Quantitative 13C NMR was carried out
using inverse-gated decoupling with a recycle delay of 50 s. High
pressure NMR experiments were performed using a single-crystal
sapphire NMR tube equipped with a titanium head.31
Synthesis of {[2,5-Ph2-3,4-(p-MeOPh)2(η5-C4CO)]2H}-
Ru2(CO)4(μ-H) (1a). A solution of 0.4 g (0.63 mmol) of Ru3(CO)12
and 0.83 g (1.88 mmol) of 3,4-bis(4-methoxyphenyl)-2,5-diphenylcy-
clopentadienone in 80 mL of methanol was refluxed for 40 h. After the
reaction mixture was cooled to room temperature, 0.96 g (85%) of an
orange solid was filtered, washed with water and methanol, and dried.
Synthesis of {[2,5-Ph2-3,4-(p-MePh)2(η5-C4CO)]2H}Ru2(CO)4(μ-
H) (1b). A solution of 0.4 g (0.63 mmol) of Ru3(CO)12 and 0.77 g
(1.88 mmol) of 3,4-bis(4-tolyl)-2,5-diphenylcyclopentadienone in 80
mL of methanol was refluxed for 40 h. After the reaction mixture was
cooled to room temperature, 0.96 g (85%) of an orange solid was
filtered, washed with water and methanol, and dried.
Figure 6. (a) Reaction mixture of LA, FA, and {[2,3,4,5-Ph4(η5-
C4CO)]2H}Ru2(CO)4(μ-H) (1c) during reaction and (b) orange
viscous residue of the catalyst before recycling.
Synthesis of {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1c). A
solution of 0.4 g (0.63 mmol) of Ru3(CO)12 and 0.72 g (1.88 mmol)
of 2,3,4,5-tetraphenylcyclopentadienone in 80 mL of methanol was
refluxed for 40 h. After the reaction mixture was cooled to room
temperature, 0.96 g (85%) of an orange solid was filtered, washed with
water and methanol, and dried.
After a similar workup of the initial run the yield was 99.7%
GVL. The yields of the next two recycling experiments were
99.6% and 97.8%, respectively (Table 3).
Treatment of {[2,5-Ph2-3,4-(p-MePh)2(η5-C4CO)]2H}-
Ru2(CO)4(μ-H) (1b) with H2. A solution of 67.5 mg (0.06 mmol)
of 1b in 1.5 mL of d8-toluene was placed in a high-pressure single-
crystal sapphire NMR tube at room temperature, and NMR spectra
were recorded. The tube was charged with 13 bar of H2 at room
temperature, and NMR spectra were recorded at room temperature
again.
Table 3. Transfer Hydrogenation of LA with FA in the
Presence of 1c at 95 °C after 6 h
LA
FA
(mmol)
conversn
(%)
TOF
(h−1
entry (mmol)
TON
)
∑TON
Treatment of {[2,5-Ph2-3,4-(p-MeOPh)2(η5-C4CO)]2H}-
Ru2(CO)4(μ-H) (1a) with Formic Acid. (a) For the NMR experiment
a solution of 50 mg (0,042 mmol) of 1a in 1.7 mL of THF-d8 was
placed into a high-pressure single-crystal sapphire NMR tube at room
temperature and 15 mg (0.31 mmol) of formic acid was added. After
the tube was sealed, it was heated at 100 °C for 30 min. The tube was
cooled to room temperature, and NMR spectra were recorded.
(b) For the in situ IR experiment a solution of 25 mg (0.021 mmol)
of 1a in 0.9 mL of THF was placed in a 50 mL three-necked flask
equipped with a reflux condenser and a SiComp probe of a ReactIR
1000 instrument. After the solution was heated to 80 °C for 20 min,
0.025 mL (0.55 mmol) of formic acid was added and IR spectra were
recorded.
Transfer Hydrogenation of Levulinic Acid with Formic Acid
in the Presence of {[2,5-Ph2-3,4-Ar2(η5-C5O)]2H}Ru2(CO)4(μ-H)]
(Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)). (a) A typical small-
scale experiment was performed by heating a mixture of 3 g (25.84
mmol) of levulinic acid, 1.78 g (38.8 mmol) of formic acid, and 0.022
mmol of 1a−c in a 10 mL glass tube in an oil bath at 100 °C. Samples
were taken at different times and analyzed by GC-MS or NMR.
(b) A typical larger scale experiment was performed by heating a
mixture of 15 g (129 mmol) of levulinic acid, 11.9 g (258 mmol) of
formic acid, and 0.36 mmol of 1a−c in a 150 mL Hasteloy-C reactor at
100 °C. Samples were taken at different times and analyzed by GC-MS
or NMR.
1
2
3
4
85.7
85.6
87.4
86.4
172.7
174.7
174.4
174.6
99.9
99.7
99.6
97.8
1065
1063
1085
1046
177
177
181
174
1065
2128
3213
4259
The combined distillates (55.1 g) were fractionated by
atmospheric distillation using 10 cm Sulzer EX-20 HC packing
to yield 35.5 g of GVL and 19.5 g of aqueous FA, as expected.
CONCLUSIONS
■
We have shown that levulinic acid (LA) and a small excess of
formic acid (FA) can be converted to γ-valerolactone (GVL) in
the presence of the Shvo catalysts {[2,5-Ph2-3,4-Ar2(η5-
C4CO)]2H}Ru2(CO)4(μ-H)]} (Ar = p-MeOPh (1a), p-MePh
(1b), Ph (1c)). The reactions were performed at 100 °C in an
open vessel with yields higher than 99% after a few hours. The
formation of 1,4-pentanediol and 2-methyltetrahydrofuran, the
typical side products of the reduction of GVL with molecular
hydrogen, was not observed. The only byproducts were water
and carbon dioxide, which were easily eliminated, and the Shvo
catalyst was recycled several times without losing catalytic
activity.
Transfer Hydrogenation of Levulinic Acid with Formic Acid
in the Presence of {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1c)
with Catalyst Recycling. An 87.4 mg portion of 1c was placed in a
250 mL Schlenk flask followed by the addition of 9.96 g (85.7 mmol)
of levulinic acid and 7.95 g (172.7 mmol) of formic acid. The Schlenk
tube was connected to a bubbler containing oil, and the mixture was
heated to 95 °C. After 1 h a clear homogeneous solution was observed.
After 6 h, the reaction mixture was cooled and all volatile compounds
were removed under reduced pressure (1 mmHg) at 85 °C, resulting
in 13.96 g of a clear solution as distillate and 0.431 g of an orange
viscous residue. Due to the carbon dioxide evolution, 3.55 g weight
loss was measured. The calculated mass balance was 99.8%. The
orange residue was mixed with the next dose of LA and FA (Table 3),
and the protocol was repeated.
EXPERIMENTAL SECTION
■
1,3-Diphenyl-2-propanone (99%, Aldrich), 4,4′-dimethoxybenzil
(98%, Aldrich), RuCl3 (Aldrich), H2SO4 (95−97%, Sigma-Aldrich),
concentrated HCl (37%, Sigma-Aldrich), ammonium hydroxide (25%,
Sigma-Aldrich), levulinic acid (98%, Aldrich), formic acid (98%-100%,
Merck), biphenyl (99.5%, Sigma-Aldrich), p-anisaldehyde (98%,
Aldrich), d8-toluene (>99.5+ atom % D, Sigma-Aldrich), d6-dimethyl
sulfoxide (DMSO-d6, >99.5+ atom % D, Sigma-Aldrich), and
deuterium oxide (>99.8 atom D %, Armar) were all used as received.
Literature methods were used for the synthesis of Ru3(CO)12,29 3,4-
bis(4-methoxyphenyl)-2,5-diphenylcyclopentadienone,30 and the Shvo
catalyst precursors.25a
NMR spectra were collected using a Bruker AV III 400 instrument
1
at 20 °C. Quantitative H NMR was carried out using 90° flip angle
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T.; Kaposy, N.; Mehdi, H. Hungarian Patent Application P0900276,
May 4, 2009.
(9) Geilen, F. M. A.; Engendahl, B.; Harwardt, A.; Marquardt, W.;
Klankermayer, J.; Leitner, W. Angew. Chem., Int. Ed. 2010, 49, 5510−
5514.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for I.T.H.: istvan.t.horvath@cityu.edu.hk.
Notes
(10) (a) Geilen, F. M. A.; Engendahl, B.; Holscher, M.;
̈
The authors declare no competing financial interest.
Klankermayer, J.; Leitner, W. J. Am. Chem. Soc. 2011, 133, 14349−
14358. (b) Du, X.-L.; Bi, Q.-Y.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-
N. Green Chem. 2012, 14, 935−939.
ACKNOWLEDGMENTS
■
(11) (a) Fegyverneki, D.; Orha, L.; Lan
Tetrahedron 2010, 66, 1078−1081. (b) Stradi, A.; Molnar
G.; Richter, F. U.; Mika, L. T. Green Chem. 2013, 15, 1857.
́
g, G.; Horvat
́
h, I. T.
We are grateful for the support of the Hungarian Scientific
Research Fund (OTKA-T047207 and OTKA-CNK 78065).
Portions of this work were funded by the City University of
Hong Kong under project number 9380047 and the Budapest
University of Technology and Economics under project
number KMR_12-1-2012-0066.
́
́
, M.; Dibo,
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