One-pot synthesis of benzimidazoles by simultaneous photocatalytic and catalytic reactions on Pt@TiO2 nanoparticles

Article abstract of DOI:10.1002/anie.200906573

"Chemical Equation Presented" Dual platinum action: A one-pot catalytic synthesis of benzimidazoles from the photoirradiation of an alcohol solution containing an ortho-arylenediamine and Pt@TiO₂ nanoparticles is described. This reaction proceeded by the platinumassisted photocatalytic oxidation of an alcohol and a catalytic dehydrogenation of the intermediates on the surface of platinum nanoparticles.

Full text of DOI:10.1002/anie.200906573

Communications
DOI: 10.1002/anie.200906573
Heterogeneous Catalysis
One-Pot Synthesis of Benzimidazoles by Simultaneous Photocatalytic
and Catalytic Reactions on Pt@TiO Nanoparticles**
2
Yasuhiro Shiraishi,* Yoshitsune Sugano, Shunsuke Tanaka, and Takayuki Hirai
Benzimidazole and its derivatives occupy pivotal positions in
the synthesis of natural products and pharmaceutical materi-
als. These compounds have been studied extensively because
[
1]
of their biological activities as bactericides, anticarcino-
[
2]
[3]
gens, and peptic ulcer agents. There is particular interest in
Scheme 1. One-pot synthesis of benzimidazole using a Pt@TiO2
catalyst under photoirradiation.
[
4]
their activity against several viruses such as HIV, herpes
HSV-1), and influenza. The classical method for benzi-
[
5]
[6]
(
midazole synthesis is the coupling of ortho-arylenediamines
[
7]
[10]
with carboxylic acids or their derivatives, which requires
strongly acidic conditions and high temperatures (> 2008C).
The other method is the oxidation of benzimidazoline
intermediates that are generated from the condensation of
TiO surface, and the catalytic dehydrogenation of benzi-
2
midazoline intermediates that are generated from the con-
densation of ortho-arylenediamines and aldehydes on the
platinum particle surface. Various catalytic systems that
enable one-pot organic transformations have been pro-
However, there are only two reports of one-pot
systems that combine photocatalysis and catalysis;
[
8]
ortho-arylenediamines and aldehydes. This method produ-
[11]
ces benzimidazoles at relatively low temperatures (approx
posed.
[
12]
1
008C), but requires unstable aldehydes as reactants and
both
stoichiometric or excess amounts of strong oxidants such as
DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone). Recent
advances in this method have allowed the use of molecular
oxygen as an oxidant, but these processes require homoge-
neous catalysts, such as metal triflates, and a free radical.
Alternative methods that proceed under mild reaction con-
ditions with stable reactants, such as alcohols and carboxylic
acids, and easy-to-handle heterogeneous catalysts are there-
fore desirable for economically and environmentally benign
benzimidazole production.
systems use a homogeneous photocatalyst (photoredox
catalyst) and an organocatalyst to effect the one-pot trans-
formations.
[9]
Pt(x)@TiO samples, containing different platinum load-
2
ings [x (wt%) = Pt/(TiO + Pt)100; x = 0.05, 0.1, 0.2, 0.5, 1.0],
2
[13]
were prepared by a conventional photodeposition method,
in which photoirradiation of an aqueous solution that contains
JRC-TIO-4 TiO particles (equivalent to Degussa P25) and
2
H PtCl afforded gray powders of the Pt@TiO catalyst. The
2
6
2
transmission electron microscopy (TEM) image of Pt-
Herein, we present a new strategy for the acid- and
oxidant-free synthesis of benzimidazoles using ortho-aryle-
nediamines and alcohols as the reactants at room temperature
under photoirradiation conditions (l > 300 nm; Scheme 1).
This process employs nanoparticles that comprise of a TiO2
(0.2)@TiO (Figure 1) shows a spherical platinum particle
2
semiconductor loaded with platinum (Pt@TiO ) as a hetero-
2
geneous catalyst. This catalyst promotes two different trans-
formations in one pot by employing both photocatalytic and
catalytic actions: the conversion of alcohols into aldehydes
through a platinum-assisted photocatalytic oxidation on the
[
*] Dr. Y. Shiraishi, Y. Sugano, Prof. T. Hirai
Research Center for Solar Energy Chemistry
and Division of Chemical Engineering
Graduate School of Engineering Science, Osaka University
Toyonaka 560-8531 (Japan)
Figure 1. a) TEM micrograph of Pt(0.2)@TiO and b) size distribution
2
of platinum particles on Pt(0.2)@TiO2.
Fax: (+81)6-6850-6273
E-mail: shiraish@cheng.es.osaka-u.ac.jp
Dr. S. Tanaka
with an average diameter of 2.0 nm. The platinum particle
size increased with an increase in the platinum loading on
Department of Chemical, Energy and Environmental Engineering
Kansai University (Japan)
TiO : where x = 0.5 and 1.0, the platinum particle size was
2
[
**] This work was supported by a Grant-in-Aid for Scientific Research
4
.0 nm and 9.3 nm, respectively (see the Supporting Informa-
(
No. 19760536) from the Ministry of Education, Culture, Sports,
Science and Technology (Japan) (MEXT).
tion, Figure S1). Furthermore, the catalysts with higher
platinum loading showed an increased absorbance at l >
300 nm owing to light scattering by the platinum particles
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906573.
[14]
1
656
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Chemie
(
see the Supporting Information, Figure S2), although the
band gap energies of the catalysts are similar (3.2–3.3 eV).
The catalytic activity of Pt@TiO2 for the synthesis of
benzimidazoles was investigated for the reaction of ortho-
phenylenediamine (1) with ethanol. The change in the
concentrations of 1 and the product, 2-methylbenzimidazole
(
(
2), of an EtOH solution containing 1 under photoirradiation
l > 300 nm) in a nitrogen atmosphere were determined over
time (Figure 2). Using pure TiO (Figure 2a), 24 h photo-
2
Scheme 2. Proposed mechanism for the one-pot synthesis of benzimi-
dazole promoted by a Pt@TiO catalyst under photoirradiation.
2
Figure 2. Time-dependent change in the concentrations of substrate
and products during photoirradiation of 1 in EtOH with a) TiO or
2
the aldehyde on Pt@TiO allows the efficient production of
intermediate 4.
The product (2) is formed by cyclization of 4 followed by
the oxidation of benzimidazoline intermediate 5 with the
2
b) Pt(0.2)@TiO catalyst. Reaction conditions: catalyst (10 mg), 1
2
(
1
20 mmol), EtOH (10 mL), nitrogen (1 atm), l>300 nm, 303 K, where
0 mg Pt(0.2)@TiO contains 0.10 mmol platinum. * =1, * =2,
2
&
=3, ~ =evolution of H (right hand axis).
2
+
[18]
[19]
release of H , where 4 and 5 are in equilibrium. Imine 4
readily reacts with a second aldehyde molecule to produce a
diimine intermediate (6), which leads to the formation of by-
irradiation was required to achieve > 90% consumption of 1,
affording 2 in only circa 60% yield. The major by-product was
[
20]
product 3.
Therefore, in the classical reaction between
1
13
[8,9]
determined by H and C NMR spectroscopy, and EI-MS
ortho-arylenediamines and aldehydes,
oxidants that pro-
analysis to be 1-(1-ethoxyethyl)-2-methyl-1H-benzimidazole
mote the rapid transformation of 5 into 2 are necessary for the
selective formation of 2 whilst suppressing the formation of
(3; see the Supporting Information, Figures S3–S5), which
was obtained in 5.2% yield after 36 h irradiation. In contrast,
by-products. In the Pt@TiO -catalyzed process, platinum
2
Pt(0.2)@TiO promoted the rapid and selective production of
particles promote the rapid and selective transformation of
5 into 2 via a catalytic dehydrogenation pathway (Scheme 2).
This mechanism is confirmed by the reaction of 1 with
equimolar amounts of acetaldehyde in the dark. Figure 3
shows the time-dependent change in the amounts of 1 and the
products at 303 K. The rate of decrease of 1 with Pt@TiO2
(Figure 3c) is similar to that obtained without the catalyst
2
2
; achieving > 99% consumption of 1 after only 2 h irradi-
ation to afford 2 in > 93% yield (Figure 2b). This indicates
that Pt@TiO promotes rapid and selective benzimidazole
2
production.
The formation of 2 was achieved by tandem photocata-
lytic and catalytic reactions on Pt@TiO (Scheme 2). The
2
reaction is initiated by the photoexcitation of TiO particles.
2
À
The excited TiO produces electron (e ) and positive hole
h ) pairs. The h oxidizes EtOH to acetaldehyde on the TiO
2
surface. Spontaneous condensation of the aldehyde with 1
produced a monoimine intermediate (4). The rapid con-
2
+
+
(
[
15]
[16]
sumption of 1 during the reaction with Pt@TiO (Figure 2b) is
2
À
because the platinum particles efficiently trap e on the
excited TiO and enhance the charge separation between e
À
2
+
[17]
and h ;
this trapping accelerates the oxidation of the
alcohol (and produces large amounts of aldehyde), and thus
facilitates the rapid condensation between 1 and the alde-
hyde. The amount of H produced after 12 h irradiation with
2
Pt@TiO was 486 mmol, whereas pure TiO produced only
2
2
À
Figure 3. Time-dependent change in the amount of 1 (*), 2 (*), and
3
mmol H (Figure 2). This indicates that the e formed on
2
H (~) during the reaction of 1 with 1 equivalent of acetaldehyde in
2
TiO was trapped by the platinum particles and consumed
2
the dark, a) without catalyst, b) with TiO , and c) with Pt(0.2)@TiO .
2
2
+
efficiently by the reduction of H that is formed during the
oxidation of the alcohol.
Reaction conditions: catalyst (10 mg), 1 (20 mmol), acetaldehyde
[
15]
These results suggest that the
(20 mmol), EtOH (10 mL), nitrogen (1 atm), 303 K, where 10 mg Pt-
enhanced photocatalytic transformation of the alcohol into
(0.2)@TiO contains 0.10 mmol Pt.
2
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1657

Communications
(
Figure 3a) and with pure TiO (Figure 3b), which indicates
when x = 0.5 and 1.0 the particle sizes were 4.0 and 9.3 nm,
respectively. The reaction of 1 with an equimolar amount of
acetaldehyde with Pt(0.2)@TiO2 (4 h, 303 K) in the dark
2
that the rate of condensation of 1 with the aldehyde is
comparable. The yields of 2 obtained without a catalyst and
with TiO2 only were low (< 40%), but the addition of
produced 2 in a 91% yield, along with 82% H formation (see
2
Pt@TiO afforded 2 in quantitative yield, which indicates that
the platinum particles successfully promote the transforma-
the Supporting Information, Table S1). In contrast, catalysts
where x = 0.5 or 1.0 showed lower yields of 2 (< 80%) and H2
(< 48%). This indicates that smaller platinum particles with a
< 4 nm diameter have higher dehydrogenation activities and
are responsible for efficient benzimidazole formation. TEM
2
tion of 5 into 2. In this case, a comparable amount of H gas to
2
2
was produced, whereas other systems did not produce H2.
This suggests that the platinum particles successfully catalyze
the dehydrogenation of 5. Therefore, the rapid transforma-
analysis of Pt(0.2)@TiO that had been recovered after a 12 h
2
tion of 5 into 2 in the Pt@TiO system leads to a shift of the
photoreaction for the synthesis of 2 (Figure 2b) revealed that
the platinum particle size scarcely changed during the
reaction (Supporting Information, Figure S6). Furthermore,
the catalyst was reusable at least three times without a loss of
activity (Supporting Information, Table S2).
This process is tolerant for the synthesis of various
substituted benzimidazoles. Photoirradiation of alcohol sol-
utions that contained various ortho-arylenediamines and the
2
equilibrium between 4 and 5 towards 5 (Scheme 2). Thus, this
allows the selective formation of 2 whilst suppressing the
formation of by-products. The high activity of platinum
particles for dehydrogenation at room temperature is prob-
ably due to the strong affinity of the amine nitrogens of 5 to
the platinum surface, as observed for the dehydrogenation of
[21]
ethylenediamine on the platinum surface.
The amount and size of platinum particles are important
factors in the rapid and selective benzimidazole production.
Figure 4 shows the conversion of 1 and the selectivity of 2
Pt(0.2)@TiO catalyst successfully afforded the corresponding
2
benzimidazoles (Table 1). Both 2-alkyl- and 2-aryl-substi-
tuted benzimidazoles were successfully produced in very high
yields. Furthermore, 5- and/or 6-substituted derivatives were
also produced successfully.
In conclusion, Pt@TiO enables efficient benzimidazole
2
production under photoirradiation conditions. This is pro-
moted by one-pot multiple catalytic transformations on
Pt@TiO , which involve a platinum-assisted photocatalytic
2
oxidation on TiO and a catalytic dehydrogenation on the
2
surface of the platinum particles. This process has significant
[24]
advantages when compared with other methods: 1) a cheap
and stable reactant (alcohol), 2) it does not require the use of
acids or oxidants, 3) the by-products formed are harmless
(only water and H2 form during the reaction), and 4) the
reaction proceeds under milder ambient conditions. There-
fore, this process has the potential to enable a more
sustainable benzimidazole synthesis. Organic transformations
using semiconductor photocatalysts has attracted much
recent attention but successful examples are still scarce.
The combination of photocatalytic and catalytic reactions
presented here may help to develop a new strategy towards
the development of photocatalysis-based organic synthesis.
Figure 4. The conversion of 1 (&) and selectivity of 2 (&) obtained
from the photoirradiation of an EtOH solution containing 1 with TiO2
[
25]
or Pt(x)@TiO catalysts for 4 h. Reaction conditions: EtOH (10 mL), 1
2
(
0.1 mmol), catalyst (10 mg), nitrogen (1 atm), l>300 nm, 303 K.
during the 4 h photoirradiation of an EtOH solution of 1 with
their respective catalysts. The conversion of 1 increases with
the platinum loading on TiO because larger amounts of
2
platinum allow an efficient charge separation on the photo-
excited TiO . The highest conversion was obtained with Experimental Section
2
JRC-TIO-4 TiO (anatase/rutile = 8:2) was supplied from the Catalyst
Pt(0.2)@TiO , and the catalysts with larger platinum loading
2
2
Society of Japan. The Pt(x)@TiO [x (wt%) = 0.05, 0.1, 0.2, 0.5, and
2
show lower conversion. This is because excess amounts of
[22]
1.0] catalysts were synthesized as follows: TiO
2
(0.1 g) and H
PtCl
2 6
platinum suppresses the incident light absorption by TiO2.
(
(
0.11, 0.21, 0.42, 1.06, and 2.12 mg) were added to a water/MeOH
24:1, v/v) mixture (10 mL) in a Pyrex glass tube (20 cm ) and purged
The selectivity of 2 is also affected by the amount of
platinum particles; the selectivity increases with an increase in
the platinum loading, but the catalysts with > 0.2 wt%
platinum showed lower selectivities (Figure 4). This is
because the larger sized platinum particles have a lower
3
with nitrogen gas. The tube was photoirradiated using a high-pressure
mercury lamp (300 W; Eikohsha Co. Ltd.; light intensity at 300–
À2
400 nm, 19.1 Wm ) under magnetic stirring at 303 K for 30 min. The
product was recovered by filtration, washed thoroughly with water,
and dried in vacuo at 353 K for 12 h. The platinum loadings were
determined by X-ray fluorescence spectrometry.
[
23]
dehydrogenation activity. As previously reported,
the
dehydrogenation activity of platinum particles increased
with a decrease in their size; in particular, particles with a
diameter of < 4 nm showed very high activity. The size of
platinum particles on Pt(0.2)@TiO2 was 2.0 nm, whereas
Photoreaction procedure: ortho-arylenediamine, alcohol, and
3
catalyst were added to a Pyrex glass tube (20 cm ). The tube was
purged with nitrogen gas and photoirradiated using a xenon lamp
À2
(2 kW; Ushio Inc.; light intensity, 18.2 Wm at 300–400 nm) under
1
658
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[
a]
Table 1: Synthesis of benzimidazoles from ortho-arylenediamines and alcohols.
Entry
Diamines
Amount [mmol]
Alcohol
t [h]
Catalyst
Diamine
Product
GC yield [%]
conversion [%]
1
2
3
4
5
6
7
8
20
20
20
20
20
20
20
20
10
10
4
4
TiO2
27
11
93
Pt(0.2)@TiO2 >99
TiO2 56
Pt(0.2)@TiO2 >99
TiO2 54
Pt(0.2)@TiO2 >99
TiO2 62
Pt(0.2)@TiO2 >99
TiO2 93
Pt(0.2)@TiO2 >99
TiO2 >99
4
34
4
>99
37
4
4
>99
43
4
4
95
[
[
b]
b]
9
0
24
24
1.8
89
1
[
[
c]
c]
1
1
1
2
10
10
12
12
57
82
Pt(0.2)@TiO2 >99
1
1
3
4
20
20
4
4
TiO2
43
23
94
Pt(0.2)@TiO2 >99
1
1
5
6
20
20
4
4
TiO2
63
11
>99
Pt(0.2)@TiO2 >99
1
1
7
8
20
20
4
4
TiO2
64
20
83
Pt(0.2)@TiO2 >99
[a] Reaction conditions: catalyst (10 mg), alcohol (10 mL), nitrogen (1 atm), l>300 nm; [b] catalyst (80 mg), alcohol (5 mL); [c] catalyst (5 mg),
alcohol (750 mmol), MeCN (5 mL).
magnetic stirring at 303 K. The reactant and product concentrations
were determined by GC equipped with FID or TCD.
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