Dehydrogenative synthesis of benzimidazoles under mild conditions with supported iridium catalysts

Article abstract of DOI:10.1039/c5cy01601k

Solid supported iridium catalysts, which show excellent activity for the dehydrogenative synthesis of benzimidazoles from primary alcohols and phenylenediamine derivatives under mild reaction conditions, were developed. Among the catalysts examined, the titania-supported iridium catalyst showed the highest activity, and benzimidazole derivatives were selectively produced at temperatures of 80°C or higher. For example, the reaction of phenylenediamine (1a) with benzyl alcohol (2a) in the presence of Ir(1.0 wt%)/TiO₂ (1.0 mol%, towards 1a) at 120°C selectively gave 2-phenylbenzimidazole (3aa) in 80% yield after 6 h and over 90% yield after 18 h. A high turnover number (TON) of more than 800 was achieved in the reaction at a higher substrate-to-catalyst ratio. Reduction of the catalysts under a hydrogen atmosphere at around 500 to 600°C is essential for their significant activity. Characterization of the catalysts by gas adsorption, XRD, XPS, H₂-TPR, TEM, and XAFS techniques revealed that highly-dispersed nanoparticles of the iridium(0) species (ca. 1 to 2 nm in diameter) formed on TiO₂ are responsible for the catalysis. The catalysts could be recycled without a significant loss of activity, and the leaching of the iridium species into the solution during the catalytic runs was negligible, which suggests that the present catalysts would be excellent from both environmental and practical perspectives.

Full text of DOI:10.1039/c5cy01601k

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DOI: 10.1039/C5CY01601K
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ARTICLE
Dehydrogenative Synthesis of Benzimidazoles under Mild
Conditions with Supported Iridium Catalysts
Keisuke Tateyama,a Kenji Wada,*,b Hiroki Miura,a,c,d Saburo Hosokawa,d Ryu Abea and Masashi
Inouea
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Solid supported iridium catalysts, which show excellent activity for the dehydrogenative synthesis of benzimidazoles from
primary alcohols and phenylenediamine derivatives under mild reaction conditions, were developed. Among the catalysts
examined, titania-supported iridium catalyst showed the highest activity, and benzimidazole derivatives were selectively
produced at temperatures of 80 oC or higher: for example, the reaction of phenylenediamine (1a) with benzyl alcohol (2a)
www.rsc.org/
o
in the presence of Ir (1.0 wt%)TiO2 (1.0 mol%, towards 1a) at 120 C selectively gave 2-phenylbenzimidazole (3aa) in 80%
yield after 6 h and over 90% yield after 18 h. A high turnover number (TON) of more than 800 was achieved in the reaction
at a higher substrate-to-catalyst ratio. Reduction of the catalysts under a hydrogen atmosphere at around 500 to 600 oC is
essential for their significant activity. Characterization of the catalysts by gas adsorption , XRD, XPS, H2-TPR, TEM, and XAFS
revealed that highly-dispersed nanoparticles of iridium(0) species (ca. 1 to 2 nm in diameter) formed on TiO2 are
responsible for the catalysis. The catalysts could be recycled without a significant loss of activity, and the leaching of
iridium species into the solution during the catalytic runs was negligible, which suggests that the present catalysts would
be excellent from both environmental and practical perspectives.
inotropic, fungicidal, anthelmintical, and antiviral activities.[6]
Among the methods used for the synthesis of benzimidazole
derivatives,[7] dehydrogenation between primary alcohols and
Introduction
The development of novel catalysts that can realize
derivatives of 1,2-phenylenediamine[8-10] is one of the most
efficient synthetic routes, since the only byproducts are
molecular hydrogen and water. An initial example catalyzed
by ruthenium phosphine complexes required harsh conditions
environmentally-benign organic syntheses is an important
[1,2]
topic.
The use of heterogeneous catalysts, especially
supported catalysts, is attractive from both environmental and
practical points of view, since these solid catalysts are
advantageous in terms of ease of preparation and handling,
high thermal and chemical stability, excellent reusability or
recyclability, and minimal contamination of the products by
heavy metals. Therefore, the development of solid catalysts
that are applicable for various reactions has attracted much
attention.[2,3] We previously developed Ru/CeO2 catalysts that
are effective for organic transformations[4,5] including the
synthesis of indole via the dehydrogenative N-
heterocyclization of 2-(2-aminophenyl)ethanol.[5]
o
such as a high reaction temperature of over 200 C.[8a] Resent
successful examples of homogeneous catalysis, which are
operated at lower temperatures, require a stoichiomeric
amounts of hydrogen acceptors[8b] or strong bases.[8c,e,f] On the
other hand, the development of solid catalysts are limited.[9,10]
o
Oxidative reactions at 80 C in O2 or air in the presence of a
supported ruthenium catalyst have been shown to give
benzimidazole.[9a] A photo-assisted dehydrogenative route
with a Pt/TiO2 catalyst has recently been demonstrated.[9b]
Very recently, dehydrogenative reactions catalyzed by a simple
Pt/TiO2 have been reported, which are operated in refluxing
On the other hand, benzimidazoles are important
compounds, particularly in pharmaceutical chemistry, because
of their antiarrhythminic, antihistaminic, antiulcer, anticancer,
o
mesitylene (b.p. 164.7 C).[10b] However, the development of
simple solid catalysts that can realize the dehydrogenative
reaction under much mild conditions without any oxidant, UV-
irradiation, or strong bases are highly desirable because of the
advantages in atom efficiency, operating costs, scalability, and
safety.
a Department of Energy and Hydrocarbon Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
b Department of Chemistry for Medicine, Graduate School of Medicine,
Kagawa University, 1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793 (Japan)
Phone +81-87-891-2249, Fax +81-87-891-2249,
Here, we report the development of very simple solid
catalysts that are effective for the selective synthesis of
benzimidazole derivatives via a dehydrogenative route under
e-mail: wadaken@med.kagawa-u.ac.jp
c Department of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1
Minami-Osawa, Hachioji-shi, Tokyo 192-0397 (Japan)
d Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-
30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245 (Japan)
o
significantly mild conditions, i.e. at 80–120 C. Among the
†Electronic Supplementary Informaꢀon (ESI) available: [Characterization of the
catalysts and effects of the reaction conditions]. See DOI: 10.1039/x0xx00000x
catalysts examined, titania-supported iridium ones are the
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was less than 1 x 10-8 Torr (1 Torr; 133.3 N m-2). The spectra
most effective. These catalysts could be recycled without a
significant loss of activity, and the leaching of iridium species
was negligible, which indicates that the present catalysts are
advantageous from both environmental and practical
perspectives.
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were measured at room temperature using Mg K radiation
DOI: 10.1039/C5CY01601K
(1254 eV) generated by an X-ray tube operating at 15 kV, 400
W. The electron take-off angle was set at 45 deg. Binding
energies were referenced to the C 1s level of residual graphitic
carbon.[11] Temperature-programmed reduction (TPR) was
carried out with a flow-type reactor. Hydrogen (1.9 vol.% in Ar;
atmospheric pressure; 30 cm3 min-1) was passed through a
reaction tube containing the catalyst. The tube was heated
with an electric furnace at 2 °C min-1, and the amount of H2
consumed was monitored with a TC detector on a Shimadzu
8AIT gas chromatograph. Transmission electron microscope
(TEM) photographs were taken using a JEOL JEM-200EX by the
Hanaichi Ultrastructure Research Institute, Okazaki, Japan.
Iridium K-edge XAFS measurements of the supported catalysts
were performed at the BL01B1 in the SPring-8 operated at 8
GeV using a Si(311) two-crystal monochromator. The Ir/TiO2
catalyst was pressed into a pellet (10 mm in diameter) and
mounted on an in situ measurement cell equipped with a
heating unit and aluminum windows. After heating the
samples in a flow of H2(5.0 vol%)/He (50 cm3/min) at 150 oC or
Experimental
Materials
Tris(acetylacetonato)iridium(III) (Ir(acac)3, Wako), Di-µ-chloro-
bis[chloro(pentamethylcyclopentadienyl)iridium(III)
([Cp*IrCl2]2,
(Aldrich),
Aldrich),
tris(acetylacetonato)rhodium(III)
(Nacalai
tris(acetylacetonato)ruthenium(III)
(Aldrich),
Tesque),
bis(acetylacetonato)palladium(II)
tetrahydrofuran (THF, dehydrated, stabilizer-free, Wako), 1,2-
phenylenediamine (Wako), mesitylene (Nacalai Tesque),
biphenyl (Wako), silica (Cabot, Cab-O-Sil), and alumina
(Sumitomo Chemical Co., Ltd, AKP-G015; JRC-ALO-8
equivalent) were obtained commercially and used without
further purification. Titania (JRC-TIO-4) and zirconia (JRC-ZRO-
3) were obtained from the Catalysis Society of Japan. Ceria
and magnesia were prepared by the precipitation method with
the use of cerium(III) nitrate hexahydrate and magnesium
nitrate hexahydrate, respectively.[4f]
o
500 C for 30 min, XAFS spectra were taken in a He flow.
Leaching of ruthenium species from the catalysts during the
reaction was investigated by an inductively coupled plasma
atomic emission spectroscopy (ICP-AES) using a Thermo
Scientific iCAP6300 analyzer.
Preparation of an Ir/support catalyst
Supported catalysts were prepared by the impregnation
method. To a solution of Ir(acac)3 (51 mg, 0.104 mmol) in 10
cm3 of methanol was added 1.0 g of a support in air at 80 oC.
Physical and Analytical Measurements
The products of the catalytic reactions were analyzed by GC-
MS (Shimadzu GC-MS Parvum 2, Zebron ZB-1 capillary column,
o
o
250 C) and gas liquid
After impregnation, the resulting powder was dried at 80 C in
i.d. 0.25 mm, length 30 m, at 50
chromatography (GLC, Shimadzu GC-14APF, Zebron ZB-1701
capillary column, i.d. 0.25 mm, length 30 m, at 50
250 oC). The
−
air for a day and calcined in air at 300 oC for 30 min. The thus-
prepared calcined catalysts (ca. 150 mg) were reduced in a H2
(2.0 vol%)/Ar flow (180 cm3 min-1) at the prescribed
−
amount of evolved hydrogen gas was measured by gas
chromatography (GC, Shimadzu GC8AlT, Porapak-Q, i.d. 3 mm,
length 2 m, at 70 C). The composition of titania-supported
o
temperature (ramping rate; 10 C/min) for 30 min by the use
o
of a conventional flow reactor to give Ir(2.0 wt%)/support-XX
catalysts, where XX is the reduction temperature in degrees
Celsius. For titania-supported catalysts, JRC-TIO-4 was used as
a support unless otherwise noted. Supported Ru-, Rh-, and Pd-
catalysts were prepared using Ru(acac)3, Rh(acac)3 and
Pd(acac)2, respectively.
iridium catalysts after calcination were confirmed by a
wavelength dispersive X-ray fluorescence spectrometer
(Panalytical Axios system equipped with a 4 kW rhodium
anode X-ray tube). Nitrogen adsorption/desorption isotherms
of the catalysts were obtained with a computer-controlled
automatic gas sorption system (Quantachrome NOVA 4200e).
o
Samples were degassed at 300 C for 2 h just before the
General procedure for the synthesis of 2-phenylbenzimidazole
(3aa) from 1,2-phenylenediamine (1a) and benzyl alcohol (2a) by
the Ir/TiO2 catalyst
measurements. The amount of carbon monoxide adsorbed
was measured using a semi-automatic pulse titration analyzer
(Ohkura Riken BP-2) at 298 K. Before measurement, a sample
o
was reduced at 500 C for 30 min in a flow of H2 (5 vol%)/Ar at
atmospheric pressure. The sample was then exposed to air at
All of the reactions were performed using hot stirrers
equipped with cooling blocks to reflux the solution. The Ir(2.0
wt%)/support-XX catalyst (96 mg, 0.010 mmol as Ir) was
mounted in a glass Schlenk tube (20 cm3): Since the catalyst
was exposed to open air at room temperature during
o
room temperature, and reduced again at 150 C for 1 h. The
dispersion (D(%); percentage of metal atoms exposed to the
surface) of the supported metal was evaluated from the
amount of chemisorbed carbon monoxide and adsorption
stoichiometry (CO/surface metal atom = 1). X-ray powder
diffraction (XRD) analysis was performed using Cu Kα radiation
and a carbon monochromator (Rigaku Ultima IV). X-ray
photoelectron spectroscopy (XPS) of the catalysts was
performed using an ULVAC-PHI 5500MT system equipped with
a hemispherical energy analyzer. Samples were mounted on
indium foil and then transferred to an XPS analysis chamber
under Ar. The residual gas pressure during data acquisition
o
mounting, the catalyst was again heated at 150 C for 1 h in
the Schelenk tube filled with H2 gas (1 atm) just before use,
and subjected to the catalytic run without any contact with air
or moisture. A typical reaction procedure is as follows: 1,2-
phenylenediamine (1a) (1.0 mmol), benzyl alcohol (2a) (2.0
mmol), and mesitylene (1.0 cm3) were added to the Schlenk
tube containing the reduced catalyst. The reaction mixture was
o
stirred at 120 C for 18 h under an argon atmosphere, and
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then rapidly cooled in an ice bath. The solid catalyst was the presence[12] or absence[13,14] of hydrogen acceptors, but
View Article Online
DOI: 10.1039/C5CY01601K
did not show catalytic activity under the present conditions
removed by passing the mixture through a 0.45
µ
m
polytetrafluoroethylene (PTFE) filter (Millipore Millex LH), and (entry 13). The results clearly show that the present titania-
the products were quantified by GC using biphenyl as an supported iridium catalysts are extremely effective.
internal standard. For isolation of the products, the remaining
solution was concentrated under reduced pressure, and the
Table 1 Effects of metallic species and supports on the activity of the
catalysts
products were then purified by Kugelrohr distillation.
Hot filtration tests
Entry
1
2
3
4
5
6
7
8
Catalysta
Yield of 3aa (%)b
A 20 cm3 Schlenk tube was charged with 1a (1.0 mmol), 2a (2.0
mmol), and the Ir(2.0 wt%)/TiO2-500 catalyst (0.010 mmol as
Ir) in mesitylene (1.0 cm3) together with an internal standard
(biphenyl, ca. 30 mg) under an argon atmosphere. After the
reaction was allowed to proceed for 3 h at 120 oC, the mixture
None
Trace
97
81
73
19
5
74
69
13
13
1
Ir(2.0 wt%)/TiO2 -500
Ir(2.0 wt%)/TiO2 -500c
Pd(2.0 wt%)/TiO2-500
Rh(2.0 wt%)/TiO2-500
Ru(2.0 wt%)/TiO2-500
Ir(2.0 wt%)/ZrO2-500
Ir(2.0 wt%)/Al2O3-500
Ir(2.0 wt%)/CeO2-500
Ir(2.0 wt%)/MgO-500
Ir(2.0 wt%)/SiO2-500
Ir(acac)3
was filtered through a 0.45 µm PTFE syringe filter (Millipore
Millex LH) into another preheated Schlenk tube. The filtrate
o
was stirred at 120 C, and the conversion of 1a was followed
9
10
11
12
13
14
by GLC.
0
6
d
[Cp*IrCl2]2
[Cp*IrCl2]2 + NaHCO3
e
trace
Results and Discussion
aFor supported Ir catalysts, Ir(acac)3 was used as an Ir precursor. For the
preparation of titania-supported catalysts, JRC-TIO-4 was used.
bDetermined by GLC. cPrepared using [Cp*IrCl2]2 as an Ir source. d160 oC, 18 h.
eNaHCO3 0.020 mmol.
Effect of the catalysts on the synthesis of 2-phenylbenzimidazole
The effects of metal oxide-supported transition metal catalysts
on the synthesis of benzimidazole
(
3aa
)
from 1,2-
phenylenediamine (1a) and benzyl alcohol (2a) were examined
o
at 120 C for 18 h (eq. 1). Entries 2–6 in Table 1 compare the
The reduction of supported iridium catalysts under suitable
conditions is essential for the present reaction. The effects of
the reduction conditions for the Ir/TiO2 catalysts on the yield
of 3aa are shown in entries 1–6 in Table 2. While the catalyst
without reduction showed very low activity (entry 1), 3aa was
obtained in a yield of over 90% after the reactions for 18 h
effects of various transition metal catalysts (2.0 wt%)
supported on titania. Among these catalysts, the iridium
catalyst showed the highest activity: 3aa was obtained in 97%
yield by the reaction in the presence of Ir (2.0 wt%)/TiO2-500.
Only a trace amount of benzaldehyde and benzyl benzoate
was observed, indicating an excellent selectivity of the present
catalysis. Note that the formation of a stoichiometric amount
of H2 (2.1 mmol) was confirmed by GC. The use of [Cp*IrCl2]2
instead of Ir(acac)3 as a source of the titania-supported iridium
catalyst slightly decreased the activity of the resulting catalyst
(entry 3). While a titania-supported palladium catalyst showed
moderate activity, ruthenium and rhodium catalysts were not
very effective.
o
with Ir/TiO2 catalysts reduced above 150 C. To examine the
effects of the reduction temperature more precisely, the yields
of 3aa in the reactions for 6 h were compared. With an
increase in the reduction temperature of the catalyst to above
300 oC, the yield of 3aa markedly increased. The Ir/TiO2
catalyst reduced at 500 oC or 600 oC showed enhanced activity:
As shown below, the spectroscopic studies have revealed that
the reduction of surface iridium species are promoted with
increasing reduction temperature, and low-valent iridium
species are considered to be responsible for the excellent
activity. The reaction conditions were optimized by the use of
the catalyst reduced at 500 oC in the following study.
Catalyst
(0.010 mmol as M)
NH2
N
HO
+
(1)
mesitylene (1.0 cm3),
under Ar, 120 oC, 18 h,
- H2O, 2H2
N
H
3aa
NH2
1a
(1.0 mmol)
2a
(2.0 mmol)
Table 2 also shows the effects of the iridium loadings. As
shown in entries 6, 8, 9, the optimum loading was found to be
around 1.0 wt% as iridium metal: Catalysts with higher and
lower levels of iridium loading showed lower activities. The
present result suggests that the formation of surface iridium
clusters of a suitable size is important for the present
dehydrogenative synthesis of 2-phenylbenzimidazoles (vide
infra). In the previous study on the related reactions, namely
the direct synthesis of quinolines from nitroarenes and
aliphatic alcohols, Ir/TiO2 catalysts with ultralow iridium
loading (ca. 0.05 wt%) showed excellent activities, and the
formation of surface iridium sub-nano-sized clusters was
reported to be responsible.[3a]
As shown in entries 2, 7–11 in Table 1, the activity of the
solid iridium catalyst is significantly affected by the support: Of
the supports examined, titania exhibited the highest
performance (entry 2). The alumina- and zirconia-supported
catalysts gave 3aa in moderate yields, whereas ceria-,
magnesia-, and silica-supported catalysts were ineffective.
Ir(acac)3, which was used as an iridium precursor of the
supported iridium catalysts, did not give 3aa at all (entry 12).
[Cp*IrCl2]2 with or without a suitable base such as NaHCO3 has
been reported to be an effective catalyst for the
dehydrogenative oxidation of alcohols and related reactions in
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Table 2 Effects of the reduction temperature and loading level of the Ir/TiO2
catalysts on the reaction of 1a with 2aa
As shown in Table 4, the reaction of equimolaVrieawmAroticulenOtsnlionef
DOI: 10.1039/C5CY01601K
1a and 2a proceeded smoothly to give 3aa in 94% yield (entry
2), which indicates that the present catalysis prevents the
formation of unnecessary wastes. Whereas the reaction of 1a
with two equivalents of 2a in the absence of mesitylene
resulted in a low yield of 3aa (entry 3), the reaction without
solvent gave 3aa in high yield when a large excess of 2a was
added (entry 4), indicating that the use of solvent is not
essential for the present catalysis. The reaction also proceeded
smoothly at a higher substrate to catalyst ratio: The reaction of
1a (5.0 mmol) and 2a (10 mmol) in the presence of Ir(2.0
wt%)/TiO2-500 (0.010 mmol as Ir) in 1.0 cm3 of mesitylene at
150 oC for 18 h gave 3aa in 87% GLC yield (TON 430, entry 5). A
higher TON of 810 was achieved by the reaction with the use
of the Ir(1.0 mol%)/TiO2-500 catalyst (0.0050 mmol as Ir, entry
6).
Entry
Catalystb
Yield of
Yield of
3aa (%) after
3aa (%) after
18 hc
9
6 hc
-
d
1
2
3
4
5
6
7
8
9
Ir(2.0 wt%)/TiO2
Ir(2.0 wt%)/TiO2-150e
Ir(2.0 wt%)/TiO2-200
Ir(2.0 wt%)/TiO2-300
Ir(2.0 wt%)/TiO2-400
Ir(2.0 wt%)/TiO2-500
Ir(2.0 wt%)/TiO2-600
Ir(0.50 wt%)/TiO2-500
Ir(1.0 wt%)/TiO2-500
90
97
98
99
97
-
35
41
37
43
55
60
77
80
-
91
a1a 1.0 mmol, 2a 2.0 mmol, catalyst 0.010 mmol as Ir, mesitylene 1.0 cm3,
under Ar, at 120 oC. bIr/TiO2 catalysts were prepared using Ir(acac)3 and JRC-
TIO-4. cDetermined by GLC. dWithout the reduction of the catalyst. eThe
catalyst was heated in H2 (1 atm) at 150 oC for 1 h just before the catalytic
run.
The effect of the reaction temperature on Ir(2.0 wt%)/TiO2-
500-catalyzed reactions is shown in Table 3. The present
supported iridium catalysts realized the synthesis of 3aa at
significantly lower temperatures than those required for the
Ru complex-catalyzed reactions:[8a] the reactions were
Scheme 1 Recycle use of Ir/TiO2 catalyst.
o
completed within 18 h at 120 C with Ir(2.0 wt%)/TiO2-500. At
100 oC, 3aa was obtained in a satisfactory yield after a
o
prolonged reaction. Remarkably, even the reaction at 80 C
gave 3aa in a moderate yield after 72 h.
Table 3 Effects of the reaction temperature on the reaction of 1a with 2a
catalyzed by Ir(2.0 wt%)/TiO2-500a
Entry
Temp. (oC)
120
120
110
100
100
100
90
Reaction time (h)
Yield of 3aa (%)b
The present Ir/TiO2 catalysts could be recycled without a
significant loss of activity. After the reaction was performed at
1
2
3
4
5
6
7
8
6
55
97
67
51
66
92
56
48
47
25
18
18
18
40
72
72
72
72
72
o
120 C for 18 h using the Ir(2.0 wt%)/TiO2-500 catalyst (see
Table 1, entry 2), the solid catalyst was separated from the
reaction mixture by centrifugation and washed three times
o
with THF (10 cm3). The resulting solid was calcined at 300 C
for 4 h and then reduced under a hydrogen atmosphere. The
thus-recycled catalyst afforded 3aa in 94% yield (Scheme 1),
which shows that the present catalyst exhibits excellent
recyclability.
80
80
70
9c
10
a1a 1.0 mmol, 2a 2.0 mmol, Ir(2.0 wt%)/TiO2-500 catalyst (prepared using
Ir(acac)3 and JRC-TIO-4) 0.010 mmol as Ir, mesitylene 1.0 cm3, under Ar.
bDetermined by GLC. cIr(1.0wt%)/TiO2-500 catalyst was used.
To investigate whether the reaction actually proceeds on
the surface of the solid catalyst,[15,16] the catalysts was
removed by hot filtration through a PTFE filter (pore size 0.45
μm) after the reaction had been allowed to proceed for 3 h
(Fig. 1): Because of poor solubility of 3aa in mesitylene,
changes of the yield of 3aa were not monitored. The further
conversion of 1a was completely stopped after removal of the
solid catalyst, indicating that the solid catalyst acts
heterogeneously. ICP-AES analysis also revealed that the
leaching of iridium species into the reaction mixture after the
catalytic run shown in Table 1, entry 2, was very slight:
0.000022 mmol (0.22 mol% of the iridium species in the fresh
catalyst). These results indicate that the reaction of 1a with 2a
requires the presence of solid catalysts.
Table 4 Effects of the amounts of subtrates and mesitylene on the reaction
of 1a with 2a catalyzed by Ir(2.0 wt%)/TiO2-500a
Entry 1a (mmol) 2a (mmol) Mesitylene (cm3) Yield of 3aa (%)b
1
2
3
1.0
1.0
1.0
1.0
5.0
5.0
2.0
1.0
2.0
5.0
10
1.0
1.0
-
97
94
39
84
87
81
4
-
5c
6d
1.0
1.0
10
aIr(2.0 wt%)/TiO2-500 catalyst (prepared using Ir(acac)3 and JRC-TIO-4) 0.010
mmol as Ir, under Ar, at 120 oC for 18 h. bDetermined by GLC. cThe reaction
at 150 oC for 18 h. dIr(1.0 wt%)/TiO2-500 (0.0050 mmol as Ir) was used as a
catalyst, at 150 oC for 18 h.
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alcohol
(
2g
)
gave the corresponding bVeienwziAmrticidleaOznolinlee
DOI: 10.1039/C5CY01601K
quantitatively. The present results indicate that these catalysts
can be used with a wide range of substrates.
100
80
60
40
20
0
Characterization of supported iridium catalysts
To investigate the factors that govern the activity of the
catalysts in the present reactions, we characterized the iridium
catalysts by various methods. The results of a nitrogen gas
adsorption study and XRD study of the fresh catalysts are
summarized in Table S1 and Figure S1 of the Supporting
Information. There is no significant correlation between the
surface areas of the supports and the activities of the
supported iridium catalysts.
Hot filtration
0
5
10
15
20
Time (h)
The surface compositions of the freshly-calcined catalysts
were investigated by an XPS study, and the results are
summarized in Table 6. For titania-supported catalysts
overlapping of Ti 3s peak (62.0 eV) on the Ir 4f peaks has been
considered. The surface ratio of iridium species greatly
depended on the support. Remarkably, the surface atomic
ratio of iridium species of the Ir/TiO2 catalyst was significantly
greater than those of other catalysts, which suggests the
formation of highly dispersed iridium species. The surface
atomic composition was not changed by reduction under a
hydrogen atmosphere (entries 1–3). The surface iridium ratios
of 0.5 wt%- and 1.0 wt%-loaded catalysts were as those
expected from the loading levels (entries 4, 5), which indicates
the high dispersion of surface iridium species. On the other
hand, the 5.0 wt%-loaded catalyst showed a surface iridium
ratio similar to that of the 2.0 wt%-loaded catalyst (data not
shown), suggesting the formation of aggregated surface
iridium species. For 2.0 wt%-loaded catalysts (entries 1, 6–10),
the order of the catalytic activities generally depended on the
surface iridium compositions, except for the case of Ir/CeO2,
which showed a lower activity than Ir/ZrO2 and Ir/Al2O3
despite a higher surface iridium ratio (entry 8).
Fig. 1 Time-course of the conversion of 1a over Ir(2.0 wt%)/TiO2-500 (a)
without filtration of the catalyst and (b) with removal of the catalyst by hot
filtration after 3 h. Reaction conditions: 1a 1.0 mmol, 2a 2.0 mmol, Ir
catalyst 0.010 mmol as Ir, mesitylene 1.0 cm3, under Ar, 120 oC.
Table 5. Scope of substrates for the reactions catalyzed by Ir(2.0 wt%)/TiO2-
500a
N
N
N
H
N
H
MeO
3ba
>99%
3ca
97%
N
N
OMe
N
H
N
H
3ab
3ac
87%
>99%
N
N
N
NH
3ae
99% (150 oC)
N
H
3ad
54% (150 oC)
N
N
CH2 CH3
8
NH
Fig. 2 shows Ir 4f XP spectra of freshly-calcined Ir(2.0
wt%)/TiO2 and Ir(2.0 wt%)/TiO2-500. The Ir 4f5/2 binding
energies reported for IrO2 (62.0 eV) and metallic Ir (60.7 eV)[11]
are also shown by vertical lines. As shown in Fig. 2a, the
spectrum of freshly-calcined Ir(2.0 wt%)/TiO2 shows a sharp
4f7/2 peak at around 62 eV, and a curve-fitting analysis
suggests that 94% of surface iridium species are high-valent
species such as Ir(IV), while the remaining 6% are considered
to be Ir(0). On the other hand, Ir(2.0 wt%)/TiO2-500 catalyst
showed a broader band at 60 to 62 eV, and the ratio of Ir(0) to
total iridium species further increased to 53%, as shown in Fig.
2b, suggesting that low-valent iridium species such as Ir(0) is
responsible for the catalytic activity.
N
H
Cl
3af
3ag
> 99% (150 oC)
99%
a1 1.0 mmol, 2 2.0 mmol, Ir(2.0 wt%)/TiO2-500 catalyst (prepared using
Ir(acac)3 and JRC-TIO-4) 0.010 mmol as Ir, mesitylene 1.0 cm3, under Ar, at
120 oC for 18 h. GLC yields are shown.
The Ir/TiO2 catalyst could be used in the reactions of
various primary alcohols and derivatives of 1,2-
phenylenediamine, as shown in Table 5. 4-Methylbenzene-1,2-
diamine (1b) and 4-methoxylbenzene-1,2-diamine (1c) could
be used in the reaction. Similarly, the reaction of 2a with
benzyl alcohols bearing an electron-donating substituent
proceeded smoothly to quantitatively afford the
corresponding 2-arylbenzimidazoles. On the other hand, the
reaction with 4-(trifluoromethyl)benzyl alcohol did not
proceed well (data not shown). The reactions with pyridin-2-
ylmethanol (2d) required a higher reaction temperature, 150
The most of the iridium species on other oxide supports
were also reduced to Ir(0) species after reduction, as shown by
the Ir 4f XP spectra of reduced catalysts (see Fig. S2), while
before reduction, the iridium species on all of the metal oxide
supports were fully oxidized. The results of the H2-TPR study
indicate that a major part of iridium species on TiO2 reduced
oC, to give 3ad in a moderate yield. From 2-phenylethanol (2e
)
o
below 100 C under a hydrogen atmosphere (see Fig. S3).
and 2-(4-chlorophenyl)ethanol (2f), the desired products 3ae
o
and 3af were obtained quantitatively at 150 C. An aliphatic
Iridium species on other supports were also fully reduced
under a H2 flow below 500 oC.
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Table 6 Surface atomic composition of supported iridium catalysts
estimated by XPS analysis
reduced iridium species exposed on the surface govern the
View Article Online
DOI: 10.1039/C5CY01601K
catalytic activity.
Entry
1
Catalyst
Ir (%) M (%) C (%)
O(%)
58.4
Ir(2.0 wt%)/TiO2
1.1
1.0
1.1
0.2
0.5
24.6
24.7
24.6
25.7
23.6
16.0
16.8
16.0
15.5
18.0
i) IrO2
Ir(2.0 wt%)/TiO2-150
Ir(2.0 wt%)/TiO2-500
Ir(0.50 wt%)/TiO2-500
Ir(1.0 wt%)/TiO2-500
57.5
58.4
58.5
57.9
2
ii) Ir/TiO2-150
iii) Ir/TiO2-500
iv) Ir metal
3
4
5
6
7
8
9
10
Ir(2.0 wt%)/ZrO2
Ir(2.0 wt%)/Al2O3
Ir(2.0 wt%)/CeO2
Ir(2.0 wt%)/MgO
Ir(2.0 wt%)/SiO2
0.6
0.5
0.9
0.3
0.1
25.5
31.8
24.3
34.7
32.9
14.2
8.6
15.6
8.6
59.8
59.2
59.2
56.5
66.5
0.5
Photon energy (eV)
Fig. 3 Ir L3-edge XANES spectra of Ir/TiO2 catalysts
(a)
(b)
Fig. 2 Ir 4f spectra of (a) Ir(2.0 wt%)/TiO2 and (b) Ir(2.0 wt%)/TiO2-500.
To further examine the oxidation states of iridium species
on ttania after the reduction, the X-ray absorption near-edge
structure (XANES) spectra were recorded just after the
reduction of the catalysts without any contact with air and
moisture. As discussed earlier,[17] the Ir LIII-edge XANES is
characterized by the white-line features, and the white line
intensities are closely related to the oxidation state of iridium
species. As shown in Fig. 3, the intensitiy of the white line of
Ir(2.0 wt%)/TiO2-500 was much smaller than that of Ir(2.0
wt%)/TiO2-150, which indicate that the reduction of both bulk
and surface iridium species further proceeded with increasing
reduction temperature. Further detailed structural analyses of
these catalysts by means of XAFS spectra are in progress.
Fig. 4 TEM images of (a) Ir(2.0 wt%)/TiO2-500 and (b) Ir(1.0 wt%)/TiO2-500.
Possible reaction mechanisms
As shown in Scheme 2, the present reaction is considered to
be initiated by the dehydrogenative oxidation of alcohol by the
metallic catalyst to form corresponding aldehyde and metal
hydride species. Note that the dehydrogenation of benzyl
alcohol 2a proceeded in the presence of Ir(2.0 wt%)/TiO2-500
o
at 120 C to give benzaldehyde (4a) together with a trace
amount of benzyl benzoate.[18] Interestingly, the
dehydrogenation of 2a to 4a after 3 h proceeded significantly
slower than the reaction of 1a and 2a (see Fig. S4). To
examined the possibility of the re-hydrogenation of
benzaldehyde 4a to benzyl alcohol 2a, 2.0 mmol of 4a was
treated with Ir(2.0 wt%)/TiO2-500 under H2(2.0 vol%)/Ar at 120
oC for 18 h: This reaction, however, resulted in the formation
of a very small amount of 2a (8% yield, eq. 2), which rules out
the contribution by re-hydrogenation of 4a. On the other hand,
the presence of 1.0 mmol of 4a almost completely inhibited
the dehydrogenation of 2a to 4a (see Fig. S5), which indicates
that benzaldehyde 4a had a strong poisoning effect on the
Fig. 4 shows TEM images of the Ir/TiO2-500 catalysts. As
reported previously by He at al.[3a] and Yoshida et al.,[3b,c] the
formation of very small iridium particles of around 1 to 2 nm
was confirmed. The formation of such small iridium particles is
also considered to contribute to the excellent activity of the
Ir/TiO2 catalysts. Note that the amounts of CO chemisorbed
on Ir(2.0 wt%)/TiO2-500 and Ir(1.0 wt%)/TiO2-500 were 1.44
cm3g-1 (D = 62%) and 0.77 cm3g-1 (D = 66%), respectively. These
results suggest that zero-valent iridium species are responsible
for the present reactions, and that the amount of fully-
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dehydrogenative reaction. In the present synthesis of Further application of the present catalysts to a wider range of
View Article Online
benzimidazoles, the rapid transformation of 4a to imine in the dehydrogenative reactions is now in progDreOsIs: .10.1039/C5CY01601K
presence of 1a would prevent the poisoning of the catalyst.
Acknowledgements
This research was supported by a Grant-in-Aid for Scientific
Research (No. 24360333) from the Ministry of Education,
Sports, Culture, Science and Technology, Japan, and the JST
ALCA program. This work has been performed with the
approval of SPring-8 (Proposal No. 2012B1421)..
The successive reaction of the thus-formed aldehyde with
1,2-phenylenediamine proceeds even in the absence of any
catalysts:[19] Heating of a mixture of 1a (1.0 mmol) and 4a (2.0
Notes and references
o
mmol) at 120 C for 18 h in the absence of catalysts gave 3aa
in 28% yield, and the main by-product was N-
benzylbenzimidazole. Since only small amounts of aldehydes
were detected by GC analysis during the catalytic runs, the
iridium-catalyzed dehydrogenation of alcohols is considered to
be rate-determining. The formation of a stoichiometric
amount of hydrogen (vide supra) also supports the mechanism
shown in Scheme 2.
1
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supported iridium catalysts that were reduced at around 500
to 600 oC showed the highest activity and realized the
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−
nanoparticles of less than 2 nm in diameter were formed on
the surface of titania, which would be responsible for the
excellent catalytic activity. These solid catalysts are recyclable,
and the leaching of iridium species was negligible. Due to
these characteristic features, the present Ir/TiO2 catalysts are
quite attractive as highly efficient heterogeneous catalysts
from synthetic, industrial, and environmental perspectives.
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Catalysis Science & Technology
View Article Online
DOI: 10.1039/C5CY01601K
Dehydrogenative Synthesis of Benzimidazoles under Mild
Conditions with Supported Iridium Catalysts
Keisuke Tateyama,a Kenji Wada,*,b Hiroki Miura,a,c,d Saburo Hosokawa,d Ryu Abea and
Masashi Inouea
a Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
b Department of Chemistry for Medicine, Graduate School of Medicine, Kagawa University, 1750-1
Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793 (Japan) Phone +81-87-891-2249, Fax +81-87-891-2249,
e-mail: wadaken@med.kagawa-u.ac.jp
c Department of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa,
Hachioji-shi, Tokyo 192-0397 (Japan)
d Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara,
Nishikyo-ku, Kyoto 615-8245 (Japan)
Text for Graphical Abstract
Recyclable Ir/TiO2 catalysts enable the synthesis of benzimidazoles from primary alcohols and
phenylenediamines under mild conditions, i.e. at 120 oC.