Generation of active ruthenium catalysts for hydroarylation of cc multiple bonds from isolated Ru(IV)=O species supported on CeO2

Article abstract of DOI:10.1246/bcsj.20180144

This paper reports that the treatment of Ru/CeO₂ with PPh₃ and HCHO in 2-methoxyalcohol was an efficient way to generate the catalytically active Ru species for hydroarylation of C-C multiple bonds. A variety of alkenes was applicable toward the title reactions. The solid Ru/CeO2 could be recycled for several times.

Full text of DOI:10.1246/bcsj.20180144

Generation of Active Ruthenium Catalysts for Hydroarylation of C-C
Multiple Bonds from Isolated Ru(IV)=O Species Supported on CeO2
Hiroki Miura,*1,2,3 Masahiro Nagao,4 Saburo Hosokawa,3,5 Tetsuya Shishido,1,2,3
Masashi Inoue,4 and Kenji Wada*6
1Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences,
Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
2Research Center for Hydrogen Energy-based Society, Tokyo Metropolitan University,
1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
3Elements Strategy Initiative for Catalysts & Batteries, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8245, Japan
4Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
5Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
6Department of Chemistry for Medicine, Graduate School of Medicine, Kagawa University,
1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793, Japan
E-mail: miura-hiroki@tmu.ac.jp
Received: May 18, 2018; Accepted: June 18, 2018; Web Released: July 7, 2018
Hiroki Miura
Hiroki Miura received his Ph.D. degree from Kyoto University in 2012. He was appointed research associate
at Aoyama Gakuin University in 2012. Since 2013, he has been an assistant professor at Tokyo Metropolitan
University (TMU). His main research interests include catalyst chemistry, organometallic chemistry, and
design and synthesis of alloy nanoparticles.
Kenji Wada
Kenji Wada received his Ph.D. in 1992 in Chemistry from Kyoto University. Then he worked at Kyoto
University as an assistant professor, a senior lecturer, and an associate professor. Since 2013, he has been
a professor of chemistry at Kagawa University, Japan. His main research interests are the design of solid
catalysts that enables a variety of organic transformations in an environmentally benign way.
Abstract
1. Introduction
This paper reports that the treatment of Ru/CeO2 with PPh3
and HCHO in 2-methoxyalcohol was an efficient way to gener-
ate the catalytically active Ru species for hydroarylation of C-C
multiple bonds. A variety of alkenes was applicable toward the
title reactions. The solid Ru/CeO2 could be recycled for several
times.
From the perspective of sustainable chemistry, the develop-
ment of greener and environmentally-benign catalytic process-
es for the synthesis of organic molecules has attracted con-
siderable attention in the 21st century.1 The use of supported
metal catalysts is promising for decreasing environmental load
because of simple preparation, high stability and facile recy-
clability, as well as minimal contamination of the products by
metallic species.2 However, highly selective organic trans-
formation with supported catalysts is still a challenge, since the
Keywords: Ruthenium
j
Hydroarylation
j
Solid catalyst
Bull. Chem. Soc. Jpn. 2018, 91, 1397–1401 | doi:10.1246/bcsj.20180144
© 2018 The Chemical Society of Japan | 1397

electronic and steric control of active metal species on the
surface of the solid support is much more difficult than with
homogeneous complex catalysts. Against this background, in
our previous studies, the properties of Ru/CeO2 catalysts
toward organic syntheses and the fine structure of Ru species
on CeO2 have been investigated.3a-3c The specific feature of
Ru/CeO2 is the formation of Ru(IV)-oxo species on the surface
of CeO2, and the Ru(IV)-oxo species could be transformed into
low-valent Ru species, which showed high activities for vari-
ous types of organic transformations.3 In contrast, other Ru
catalysts supported on SiO2, Al2O3, TiO2 or MgO did not show
any activities for the reactions due to the absence of Ru(IV)-
oxo species on the metal oxides. These results indicate that
different types of active Ru species could be generated from
Ru(IV)-oxo species on CeO2 through the appropriate choice of
additives or pretreatment conditions. On the other hand, transi-
tion metal-catalyzed C-C bond-forming reactions that involve
the cleavage and subsequent transformation of less-reactive
C-H bonds are central issues in modern synthetic chemistry.4
In particular, the direct addition of aromatic C-H bonds to
carbon-carbon multiple bonds (hydroarylation) is one of the
most straightforward and atom-efficient methods for the syn-
thesis of alkylated or alkenylated arenes.5 Since the pioneering
works with homogeneous Ru complexes by Murai, Kakiuchi,
et al.,6 various effective transition-metal-catalyzed hydroaryla-
tions of unsaturated molecules have been developed.7 As
described in our previous reports, Ru/CeO2 that have been
pretreated under a hydrogen atmosphere in the presence of
PPh3 (PPh3-modified Ru/CeO2) showed excellent catalytic
activities for the addition of aromatic C-H bonds to alkoxy-
vinylsilanes3b as well as the direct arylation of aromatic C-H
bonds with aryl chlorides.3c However, in our previous catalytic
system for hydroarylation, recycling of the catalysts was
unfeasible due to the irreversible deposition of siliceous resi-
dues derived from vinylsilanes on surface Ru species. More-
over, the catalytic system could not be applied to the reaction of
unsaturated compounds other than alkoxyvinylsilanes.3c
In this paper, we describe the transformation of Ru(IV)-oxo
species on CeO2 to active Ru catalysts for the hydroarylation of
unsaturated compounds. This catalytic system showed much
higher efficiency than our previous PPh3-modified Ru/CeO2
catalysts, and a variety of unsaturated compounds could be
used. Furthermore, the solid Ru catalysts could be easily
recycled and used several times without a significant loss of
activity.
10 °C min¹1 and maintained at 400 °C for 30 min to afford ceria
in an excellent ceramic yield. Zirconium oxide and magnesium
oxide were prepared from zirconium(IV) dinitrate oxide and
magnesium nitrate by a method similar to that used to obtain
ceria. Titania (JRC-TIO-4), £-alumina (JRC-ALO-8) and silica
(Cabosil) were used as received.
2.2 Physical and Analytical Measurements. The products
of catalytic runs were analyzed by GC-MS (Shimadzu GC-MS
Parvum 2, Zebron ZB-1 capillary column, i.d. 0.25 mm, length
30 m, at 50-250 °C) and gas chromatography (GL Sciences
GC353, Inertcap 17 capillary column, i.d. 0.25 mm, length
30 m at 50-250 °C). NMR spectra were recorded on a JEOL
JNM-EX-400 (FT, 400 MHz (1H), 100 MHz (13C)) instrument.
Chemical shifts (¤) are referenced to SiMe4.
The oxide catalysts were analyzed by FT-IR and XAFS.
Diffuse reflectance IR spectra were recorded using a Nicolet
Magna-IR 560 FT-IR spectrometer with DRIFT optical con-
figuration. Ru K-edge XAFS measurements were performed
at BL01B1 beam line at SPring-8 operated at 8 GeV using a
Si(311) two-crystal monochromator. XAFS spectra were taken
at room temperature. XANES and EXAFS were analyzed using
the REX2000 version 2.5 program (Rigaku). Leaching of
ruthenium species from the catalysts during the reaction was
investigated by ICP atomic emission spectroscopic analysis by
using a Shimadzu ICPS-1000III analyzer.
2.3 A typical Method for the Preparation of Ru/Support
Catalysts. Supported catalysts were prepared by the impreg-
nation method. 1.0 g of a support was added to a solution of
Ru(acac)3 (79.5 mg, 0.20 mmol) in 10 mL of THF in air at room
temperature. After impregnation, the resulting powder was
calcined in air for 30 min to afford the Ru(2.0 wt%)/Support
catalyst.
2.4 Preparation of 4PPh3-Ru/CeO2. Ru/CeO2 (125 mg,
0.025 mmol as Ru) was stirred in the presence of PPh3 (26.3
mg, 0.10 mmol) at 100 °C under a hydrogen atmosphere (1 bar)
on a hot stirrer with a cooling block for 20 min to give 4PPh3-
Ru/CeO2.
2.5 Preparation of (HCHO+4PPh3)-Ru/CeO2 Catalyst.
Ru/CeO2 (125 mg, 0.025 mmol as Ru) was stirred in the
presence of PPh3 (26.3 mg, 0.10 mmol) and 36% aq. HCHO
(0.25 mL) in 2-methoxyethanol (2 mL) at 140 °C under an
argon atmosphere on a hot stirrer with a cooling block for
30 min. The resulting solvents and HCHO were removed under
reduced pressure at room temperature to give (HCHO+4PPh3)-
Ru/CeO2. The treated catalyst was used for the catalytic
reaction without exposure to open air.
2. Experimental
3. Results and Discussion
2.1 Materials and Methods.
All manipulations were
performed under an argon atmosphere using standard Schlenk
techniques. Ruthenium(III) acetylacetonate (Aldrich), all of the
aromatic ketones, alkenes and alkynes (TCI) and potassium
hydroxide, cerium(III) nitrate hexahydrate, tetrahydrofuran
(THF; Wako) were obtained commercially and used without
further purification. Ceria was prepared by treating a solution
of cerium(III) nitrate hexahydrate (12.6 g, 29 mmol) in 400 mL
of deionized water with 40 mL of 3M KOH aqueous solution
with stirring for 1 h at room temperature. The resulting precip-
itates were collected by centrifugation and then dried overnight
at 80 °C. The product was heated in a box furnace at a rate of
3.1 Addition of Aromatic C-H Bonds to Alkenes by
Modified Ru/CeO2 Catalysts. The methods for the prepara-
tion of supported Ru catalysts are summarized in Scheme 1.
CeO2-supported Ru catalysts (Ru/CeO2) were prepared by
impregnation of a solution of Ru(acac)3, followed by calcina-
tion in air at 400 °C for 30 min. The Ru/CeO2 catalyst was
then heated at 100 °C for 20 min under a hydrogen atmosphere
(1 atm) without any solvent in the presence of PPh3 (4 molar
equivalents to Ru) to give a phosphine-modified Ru/CeO2 cata-
lyst (4PPh3-Ru/CeO2). On the other hand, Ru/CeO2 catalysts
were treated in 2-methoxyethanol in the presence of PPh3 with
1398 | Bull. Chem. Soc. Jpn. 2018, 91, 1397–1401 | doi:10.1246/bcsj.20180144
© 2018 The Chemical Society of Japan

with PPh3
Table 2. Hydroarylation of alkenes catalyzed by
(HCHO+4PPh3)-Ru/CeO2
4PPh3−Ru/CeO2
at 100 oC for 20 min
withoutany solvent
underH2
a
(HCHO+4PPh3)–Ru/CeO2
(0.025 mmol as Ru)
O
with PPh3, aq. HCHO
O
+
Ru/CeO2
(2.0 wt% as Ru)
O
R
(HCHO+4PPh3)−Ru/CeO2
(4PPh3)−Ru/CeO2
+
Toluene (1.0 mL), 140 o
C
at 140 oC for 30 min
in 2-methoxyethanol
underAr
R
H
R
1a
2
3
4
(0.50 mmol)
(1.5 mmol)
with PPh3
at 140 oC for 30 min
in 2-methoxyethanol
underAr
O
F
3aa, 4aa
85% (75 : 25)
3 h
3ab, 4ab
3ac, 4ac
3ad, 4ad
94% (70 : 30)
4 h
90% (75 : 25)
3 h
99% (74 : 26)
4 h
Scheme 1. Method for the preparation of supported Ru
catalysts.
Si(OEt)3
SiMe2Ph
SiMe3
3ae
99% (100 : 0)
0.5 h
3af
99% (100 : 0)
3 h
3ag
74% (100 : 0)
24 h
3ah
78%, 6 h
Table 1. Hydroarylation of styrene in the presence of
supported Ru catalystsa
aMolar ratio of 3 : 4 are shown in parentheses.
Table 3. Addition of aromatic compounds to styrene by
(HCHO+4PPh3)-Ru/CeO2 catalysts.a
(HCHO+4PPh3)−Ru/CeO2
Ar
(0.025 mmol as Ru)
+
Ar
Ar
H
Ph
+
R
Toluene (1.0 mL), 140 o
C
R
1
2a
(1.5 mmol)
3
4
(0.50 mmol)
O
O
O
O
O
S
3da, 4da
98% (75 : 25)
3 h
3ca, 4ca
75% (75 : 25)
6 h
3ea, 4ea
67% (77 : 23)
3 h
3ba, 4ba
99% (25 : 75)
3 h
aMolar ratio of 3: 4 are shown in parentheses.
or without formaldehyde (36% aqueous solution; aq. HCHO) at
140 °C for 30 min, followed by removal of the solvent under
reduced pressure to afford (HCHO+4PPh3)-Ru/CeO2 or
(4PPh3)-Ru/CeO2, respectively.
the amount of Ru species leached into the solution after the
reaction of 1a with 2a by (HCHO+4PPh3)-Ru/CeO2 catalyst
was estimated to be 7.4% of the Ru species in the fresh catalyst.
(HCHO+4PPh3)-Ru/CeO2 showed high catalytic activity
for the hydroarylation of a wide range of alkenes (Table 2).
Styrene derivatives 2b-2d efficiently coupled with 1a to afford
alkylated aromatic ketones in high yields. The linear selectiv-
ities of the products 3ab, 3ac and 3ad were around 75%. The
catalytic system also showed excellent activities in the reaction
of 1a with vinylsilanes. The reaction of triethoxyvinylsilane
(2e) went to completion within 30 min, although the previous
4PPh3-Ru/CeO2 catalysts needed 90 min to finish the reac-
tion.3c Furthermore, other vinylsilanes without an alkoxy sub-
stituent could be used in the present catalytic system and the
products 3af and 3ag were obtained in excellent yields.
Hydroarylation of 2-norbornene (2h) also occurred and led to
the product in a high yield, whereas 1-hexene 2i was not a good
substrate under the present reaction conditions.
The scope of aromatic compounds that could be used in this
catalytic system was also examined (Table 3). The addition of
aromatic ketones 1b and 1c to 2a took place in the presence
of (HCHO+4PPh3)-Ru/CeO2. The reactions of heterocyclic
ketones 1d and 1e gave the corresponding products 3da
and 3ea in high yields. However, ester and cyano groups did
not participate as an effective directing group for the C-H
functionalization by the present catalytic system.
In our previous study, the irreversible deposition of siliceous
residues derived from vinylsilanes on surface Ru species ham-
pered recycling of the solid catalysts.3c Therefore, efforts were
made to realize the addition of aromatic C-H bonds with sim-
ple unsaturated hydrocarbons. An attempt to add ¡-tetralone
(1a) to styrene (2a) in the presence of Ru/CeO2 and PPh3
(4 equivalents to Ru) resulted in no formation of the desired
adduct (Table 1, Entry 1). 4PPh3-Ru/CeO2 showed excellent
activity for the addition to triethoxyvinylsilane,3c while no
reaction of 2a also take place (Entry 2). On the other hand,
(HCHO+4PPh3)-Ru/CeO2 showed excellent activity to give
alkylated aromatic ketones 3a and 4a in a yield of 85% as
a mixture of regioisomers (linear:branch = 3:1) (Entry 3). In
contrast, (4PPh3)-Ru/CeO2, which was treated in alcohol in the
absence of HCHO, was not at all effective (Entry 4). The reac-
tion did not occur with Ru/CeO2 in the presence PPh3 and aq.
HCHO as external additives (Entry 5). These results indicated
that pretreatment in 2-methoxyethanol with aq. HCHO is essen-
tial for the transformation of Ru(IV)-oxo species to active Ru
catalysts for the hydroarylation of styrene. Other Ru catalysts
supported on SiO2, Al2O3, TiO2 or MgO treated with PPh3 and
HCHO in 2-methoxyethanol gave no products at all. Note that
Bull. Chem. Soc. Jpn. 2018, 91, 1397–1401 | doi:10.1246/bcsj.20180144
© 2018 The Chemical Society of Japan | 1399

Ru/CeO2
PPh3 (4 eq. to Ru)
and aq. HCHO
1. washed by Et2O
2. dried at 80 oC overnight
3. calcined at 400 oC
for 30 min
4. pretreated
at 140 oC for 30 min
Ph
in 2-methoxyethanol
Ph
O
(HCHO+4PPh3)−Ru/CeO2
Toluene, 140 oC, 3 h
O
O
+
Ph
+
S
S
S
1d
(1.0 mmol)
2a
3da
4da
(3.0 mmol)
total yield
1st use : 99% (3da : 4da = 75 : 25)
2nd use : 99% (3da : 4da = 76 : 24)
Scheme 2. Recycling of Ru/CeO2 catalysts.
(HCHO+4PPh3)−Ru/CeO2
Toluene, 140 oC, 6 h
O
O
+ Ph
Ph
H
Ph
Figure 1. DRIFT spectrum of (HCHO+4PPh3)-Ru/CeO2.
Ph
6
1a
5
(1.2 mmol)
(1.0 mmol)
1st use : 88%
2nd use : 85%
Scheme 3. Hydroarylation of alkyne by (HCHO+4PPh3)-
Ru/CeO2.
cleavage of a Si-O bond of alkoxyvinylsilanes, followed by
abstraction of ¢-hydride from the alkoxo ligand and decarbon-
ylation of the resulting aldehyde.8 In previous catalytic systems
that used untreated Ru/CeO2 together with PPh3 or PPh3-Ru/
CeO2 as catalysts, alkoxyvinylsilanes could act as a source of
CO ligand, which might explain the absence of any activity for
the reaction of alkenes except for alkoxyvinylsilanes. On the
other hand, in the present catalytic system, treatment with PPh3
and HCHO in alcohol is deduced to generate Ru(II) species
coordinated by CO and PPh3, which are transformed, at the
initial stage of the reactions, to active Ru(0) species that are
effective for the catalytic addition of aromatic C-H bonds to
unsaturated carbon-carbon bonds.7c
One major advantage of solid catalysts is their high recy-
clability. After the reaction, the present solid Ru catalyst was
washed with diethyl ether, dried in air at 80 °C, and then
calcined in air at 400 °C to be recovered as Ru/CeO2. The thus-
obtained solid was re-treated with PPh3 and HCHO in 2-
methoxyethanol to give the reproduced (HCHO+4PPh3)-Ru/
CeO2 for the next catalytic run (Scheme 2).
With the recycled catalysts, the reaction of 1d with 2a suc-
cessfully proceeded to give the corresponding products 3da and
4da in a high total yield. Unlike the reaction of vinylsilanes,
the surface state of the original Ru/CeO2 catalysts after the
reaction of unsaturated hydrocarbons was restored by washing
and re-calcination in air. Thus, the recycled catalysts showed
high activities for hydroarylation without a significant loss of
activity.
Multi-substituted alkenes are important organic molecules in
material science. Hydroarylation of alkynes is the most atom-
economical method for the synthesis of these compounds and
a (HCHO+4PPh3)-Ru/CeO2 catalyst was found to be effec-
tive for the hydroarylation of alkyne (Scheme 3). The reac-
tion of 1a with diphenylacetylene (5) in the presence of a
(HCHO+4PPh3)-Ru/CeO2 catalyst proceeded efficiently to
give triaryl-substituted alkene 6 in 88% yield. Furthermore, the
catalyst, recycled by the same method as described above, was
Ru/CeO2
(HCHO+4PPh3)−Ru/CeO2
RuHCl(CO)(PPh3)3
22100
22150
Photon energy (eV)
22200
Figure 2. XANES spectra of Ru/CeO2 before and after the
treatment.
To elucidate the state of the active Ru species in the present
catalytic system, (HCHO+4PPh3)-Ru/CeO2 was characterized
by spectroscopic analyses. The diffuse reflectance infrared
Fourier transform (DRIFT) spectrum of the Ru/CeO2 cata-
lyst after treatment with PPh3 and aq. HCHO is shown in
Figure 1. Whereas a peak due to a Ru=O bond was observed
at 980 cm in the spectrum of Ru/CeO2 as described in our
previous reports, this peak disappeared in the spectrum of
(HCHO+4PPh3)-Ru/CeO2. On the other hand, small peaks
¹1
that could be assigned to Ru-CO or Ru-H species were
¹1
observed at around 1900-2050 cm
.
Figure 2 shows the X-ray absorption near-edge structure
(XANES) spectra of Ru catalysts. The absorption-edge of the
(HCHO+4PPh3)-RuCeO2 catalyst shifted to a lower energy
than that of Ru/CeO2 before treatment and was very close to
that of a Ru(II) complex with PPh3 and CO ligand, whereas the
shape of the spectrum was significantly different. This result
indicates that the treatment of Ru(IV)=O on CeO2 with PPh3
and HCHO in 2-methoxyethanol produced Ru(II) species.
A recent mechanistic investigation of the alkylation of
aromatic C-H bonds with homogeneous Ru complex catalysts
revealed the importance of a CO ligand coordinating active
Ru(0) species.7e Hiraki et al. reported that the reaction of a low-
valent Ru complex with alkoxyvinylsilanes produced Ru-CO
species via the oxidative addition of Ru accompanying the
1400 | Bull. Chem. Soc. Jpn. 2018, 91, 1397–1401 | doi:10.1246/bcsj.20180144
© 2018 The Chemical Society of Japan

Hosokawa, M. Inoue, Chem.®Eur. J. 2010, 16, 4186. c) H. Miura,
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also effective for the hydroarylation of alkyne 5 to afford 6 in a
high yield.
4. Conclusion
In summary, a novel procedure for the generation of highly
active and recyclable catalysts that are effective for the addi-
tion of aromatic C-H bonds to unsaturated compounds was
described. The treatment of Ru(IV)-oxo species on CeO2 with
PPh3 and HCHO in 2-methoxyethanol transformed them to
active Ru species, which showed high activities for the
hydroarylation of various alkenes and alkynes. The present
catalytic system is very attractive from practical and environ-
mental points of view because of its simple handling, high
recyclability and low contamination of the products by metallic
species.
4
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This study was supported in part by Grants-in-Aid for
Young Scientists (B) (No. 26820353), Scientific Research (B)
(No. 17H03458) and Scientific Research on Innovative Areas
(No. 17H06443) commissioned by MEXT of Japan. The XAFS
experiment at SPring-8 was carried out under the approval
(proposal Nos. 2011A1598, 2015A1411 and 2015B1478) of
Japan Synchrotron Radiation Research Institute (JASRI).
5
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Bull. Chem. Soc. Jpn. 2018, 91, 1397–1401 | doi:10.1246/bcsj.20180144
© 2018 The Chemical Society of Japan | 1401