Chemoselective reduction of nitrobenzenes to aminobenzenes having reducible groups by a titanium(iv) oxide photocatalyst under gas- and metal-free conditions

Article abstract of DOI:10.1039/c2cc31524f

m-Nitrovinylbenzene was chemoselectively reduced to m-aminovinylbenzene in a suspension of a TiO₂ photocatalyst in the presence of a hole scavenger at room temperature under atmospheric pressure without the use of a precious metal or reducing gas, and nitrobenzenes having other reducible groups were also chemoselectively reduced to corresponding aminobenzenes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31524f

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COMMUNICATION
Chemoselective reduction of nitrobenzenes to aminobenzenes having
reducible groups by a titanium(IV) oxide photocatalyst under gas- and
metal-free conditionsw
Kazuya Imamura, Keiji Hashimoto and Hiroshi Kominami*
Received 29th February 2012, Accepted 15th March 2012
DOI: 10.1039/c2cc31524f
m-Nitrovinylbenzene was chemoselectively reduced to m-amino-
vinylbenzene in a suspension of a TiO photocatalyst in the
and inexpensive element because titanium has the tenth largest
Clarke number. Although various oxides of titanium exist,
2
presence of a hole scavenger at room temperature under atmo-
spheric pressure without the use of a precious metal or reducing
gas, and nitrobenzenes having other reducible groups were also
chemoselectively reduced to corresponding aminobenzenes.
2
TiO in which the oxidation state of titanium is +4 is most
stable under the atmosphere and is easily formed without any
extra effort. Photocatalytic reaction proceeds at room temperature
and under atmospheric pressure, and the TiO
easily separated from the reaction mixture after the reaction. In
addition, TiO has been used for a long time as an indispensable
2
photocatalyst is
Aminobenzenes synthesized by reduction of corresponding
nitrobenzenes are important compounds as intermediates of
2
inorganic material such as a pigment and UV absorber because it
is inexpensive and non-toxic for humans and the environment.
Most of the applications of photocatalysis are mineralization
(or degradation) of toxic organic compounds in air and
1
agrochemicals, medicines, dyes and various useful compounds.
However, it is difficult to selectively reduce the nitro group of
1
nitrobenzenes having other reducible groups such as a vinyl group.
5
water. Since photocatalytic reaction satisfies almost all of
6
the 12 proposed requirements for green chemistry, organic
Catalytic hydrogenation over a metal catalyst such as nickel (Ni) or
copper (Cu) is the main method for reduction of a nitro group.
However, this method cannot be applied for selective reduction of
the nitro group of nitrobenzenes having a vinyl group because both
nitro and vinyl groups are reduced. Chemoselective reduction of the
nitro group is achieved by using a large excess of a reducing agent
synthesis of various compounds using photocatalysis has
b,7
5
recently been studied by many researchers.
However, less
attention has been paid to the photocatalytic reduction of organic
compounds by photogenerated electrons because a deaerated
7
b
2
a
2b
2c
2d
condition is needed to achieve reduction of substrates. Photo-
catalytic reduction for organic synthesis can be carried out in the
such as tin (Sn), zinc (Zn), iron (Fe) or sodium hydrosulfite.
However, these reaction systems give harmful wastes. Although
catalytic chemoselective reduction of nitrobenzenes is important,
there have been only a few reports on chemoselective reduction of a
presence of a large excess of an electron donor, such as methanol,
8
and in the absence of dioxygen (O ). The purpose of the electron
2
3
donor is to scavenge holes and to reduce recombination of holes
and electrons in and/or on the particles. From the point of view of
environmentally-friendly production of chemicals, attention must
be paid to the choice of sacrificial reagents for photocatalytic
reduction of organic compounds. Alcohols such as methanol have
nitro group in the presence of CQC bonds and their yields were
low. Recently, chemoselective reduction of m-nitrovinylbenzene
(
NVB) to m-aminovinylbenzene (AVB) has been achieved by many
1,4
researchers. However, these (thermo)catalytic systems require
precious metals, high temperature and high pressure of reducing
8
been used as both a solvent and a hole scavenger. However, since
reagents such as hydrogen (H
Table S1, ESIw). Reduction of NVB to AVB would be more
attractive if reducing reagents other than gaseous H and CO
2
) and carbon monoxide (CO)
toxic aldehydes are formed as the oxidized species of alcohols, a
sacrificial reagent converting to a non-toxic compound is preferable.
We have reported photocatalytic reduction of nitrobenzenes to
(
2
are applied and the reaction is catalyzed by common elements
having a simple component and structure under mild conditions
such as room temperature and atmospheric pressure.
corresponding aminobenzenes in an aqueous suspension of TiO in
2
the presence of oxalic acid (OA) or formic acid (FA) as a hole
9
scavenger. These hole scavengers are ‘‘greener’’ sacrificial reagents
When titanium(IV) oxide (TiO ) is irradiated by UV light,
2
ꢀ
because they are easily oxidized to carbon dioxide (CO
thus-formed CO molecules are removed from the liquid phase
in the presence of organic acids.
2
) and
charge separation occurs and thus-formed electrons (e ) in the
+
conduction band and holes (h ) in the valence band cause
2
reduction and oxidation, respectively. Titanium is a common
In this study, we examined the photocatalytic reduction of
nitrobenzenes having other reducible groups using a simple
Department of Applied Chemistry, Faculty of Science and
Engineering, Kinki University, Kowakae, Higashiosaka,
Osaka 577-8502, Japan. E-mail: hiro@apch.kindai.ac.jp
w Electronic supplementary information (ESI) available: Experimental
section, Fig. S1–S3 and Table S1. See DOI: 10.1039/c2cc31524f
2
photocatalyst, TiO , in the presence of OA as a hole scavenger
at room temperature and atmospheric pressure, and we found
that only the nitro group was chemoselectively reduced to an
amino group and that aminobenzenes with reducible groups
4
356 Chem. Commun., 2012, 48, 4356–4358
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Table 1 Photocatalytic chemoselective reduction of various nitro-
benzenes to corresponding aminobenzenes
a
Reducible
Entry Position group: X Solvent
Time/ Conversion Selectivity
h
(%)
(%)
Fig. 1 Time courses of the amount of NVB remaining (squares) and
1
2
3
4
5
6
7
8
9
10
m-
m-
m-
m-
m-
m-
o-
m-
m-
p-
o-
m-
p-
o-
m-
p-
–CHQCH
–CHQCH 10% W–ACN 2
2
2
ACN
6
499
4b99
96
88
93
the amount of AVB formed (circles) in a 10% water–ACN suspension
b
of TiO
2
(50 mg) in the presence of OA (200 mmol) as a hole scavenger
–CHQCH
–CHQCH
–CHQCH
–CHQCH
–Cl
2
2
2
2
10% W—ACN 2
10% W–ACN 2
Methanol
Ethanol
ACN
ACN
10% W–ACN 1
ACN
ACN
ACN
ACN
ACN
ACN
ACN
95
85
c
c
96
under deaerated conditions.
d
2
2
2
2
499
499
499
499
499
499
499
499
4100
499
499
499
15
20
499
95
97
d
were obtained in high yields without using precious metals or
1
high-pressure gaseous reducing reagents.
0
–Cl
–Cl
–Cl
–Br
–Br
–Br
–COOH
–COOH
–COOH
Fig. 1 shows time courses of the amounts of NVB remaining
and AVB formed in the photocatalytic reduction of NVB in a
2
2
2
2
2
2
2
54
1
1
1
1
1
2
3
4
499
499
88
98
499
98
1
2
0% (v/v) water–ACN suspension of TiO . The amount of
NVB monotonously decreased along with photoirradiation
time and NVB was almost completely consumed after 2 h,
while AVB was obtained in a high yield (93%). The high yield
of AVB indicates that none of the reaction of the benzene ring,
reduction of the vinyl group or re-oxidation of the amino group
of AVB occurred in the present system. It is notable that there
are no reports on photocatalytic chemoselective reduction of a
nitro group having a CQC double bond. Eqn (1) shows the
probable stoichiometry of photocatalytic chemoselective
15
16
17
18
m-
p-
–COCH
–COCH
3
10% W–ACN 1.5 499
ACN 499
99
88
3
2
a
3
Substrate: 50 mmol, TiO
2
: 50 mg, solvent: 5 cm , oxalic acid:
200 mmol, light source: Hg arc (l 4 300 nm), temperature: 298 K,
atmosphere: Ar (1 atm), conversion and selectivity were based on
b
c
d
results of HPLC. Second use. Third use. No oxalic acid was
added because these solvents also work as hole scavengers.
1
1
reduction of NVB to AVB in the presence of OA.
ð1Þ
almost completely consumed in alcohols; however, many
unidentified by-products were formed probably due to side
reaction(s) of amino and vinyl groups with alcohols. As a
result, AVB selectivity was much smaller than that obtained in
the reaction system using 10% (v/v) water–ACN and OA as
solvent and hole scavenger, respectively. These results indicate
that the combination of 10% (v/v) water–ACN and OA is
effective for photocatalytic reduction of NVB.
The yield of the present photocatalytic system was as high as
that in thermocatalytic reduction of NVB to AVB systems using
1
,4a,c,d
4b,c,e
gold,
4d,g,h,k
precious metals such as platinum,
silver,
4c
4i
and rhodium. The present photocatalytic
ruthenium
reduction required no co-catalyst such as a precious metal
and occurred under conditions (298 K, atmospheric pressure
The effect of water content of solvents on yields of AVB was
investigated and the results are shown in Fig. 2. The yield of
AVB in water-free ACN was lower than that in ACN containing
a small amount of water, and longer photoirradiation was
required for complete consumption of NVB (Fig. S1, ESIw).
When a small amount of water was added to ACN, yields of
AVB after 2 h photoirradiation increased and maximum yield
was obtained at 10% (v/v) of water content. A positive effect of
addition of water to ACN was also reported in photocatalytic
and H
2
-free) milder than those of thermocatalytic systems
(
Table S1, ESIw). The value of AQE at 366 nm calculated from
the ratio of the amount of AVB and amount of photons
irradiated using eqn (2) reached 15%.
AQE = (6 ꢁ amount of AVB/number of incident photons)
ꢁ
100
(2)
For comparison, AQE for photocatalytic H2 formation
from 2-propanol (200 mmol) in an aqueous suspension of
platinized TiO2 was also examined and determined to be
12
epoxidation of alkenes, although the reason was not clarified.
Further addition of water, however, decreased the yield of AVB.
To understand the effect of water content on reduction of NVB
in the presence of OA, another photocatalytic reaction using OA,
4
.1% under the same irradiation conditions. This reaction has
often been used as a model reaction to evaluate the activity of a
photocatalyst for H evolution. The value of AQE in the present
i.e., photocatalytic decomposition of OA along with H
formation ((COOH) - H + 2CO ) in a water–ACN
suspension of platinized TiO (P 25) under deaerated conditions
2
2
reaction larger than that of 2-propanol dehydrogenation shows
that OA efficiently works as hole scavenger for photocatalytic
reduction of NVB to AVB.
2
2
2
2
was examined. This reaction system is simple because only OA is
dissolved as a substrate in the liquid phase. Since the OA
decomposition was carried out under deaerated conditions as well
as the NVB system, the effect of oxygen on the reaction can be
eliminated. Therefore, hole trapping by OA would be important
in the OA decomposition system, and the results are shown
Photocatalytic reduction of NVB in some alcohols was
examined because alcohols, mainly methanol, have often been
used as solvents in photocatalytic reduction of nitrobenzenes.
In this case, alcohols also work as hole scavengers. Results are
summarized in Table 1. After 2 h photoirradiation, NVB was
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4356–4358 4357

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of corresponding amino compounds with high yields. These
results indicate wide applicability of this photocatalytic
method for chemoselective reduction of a nitro group to an
amino group without using precious metals and H2.
We succeeded in a photocatalytic chemoselective reduction
of various nitro compounds having other reducible groups to
corresponding aminobenzenes in an ACN suspension of TiO
2
in the presence of OA. Addition of 10% (v/v) water to ACN
increased the reduction rate and improved the yield of AVB.
Fig. 2 Effect of water content of solvents (water–ACN) on yields of
AVB formed by photocatalytic reduction of NVB for 2 h.
Notes and references
in Fig. S2 (ESIw). A tendency similar to that in the NVB system
was observed in the OA decomposition system and there was a
clear correlation between them as shown in Fig. S3 (ESIw),
indicating that water added to ACN affected the two photo-
catalytic reactions in a similar way, though the reduced products
1 H.-U. Blaser, H. Steiner and M. Studer, ChemCatChem, 2009,
1, 210.
2
(a) J. Butera and J. Bagli, (American Home Products). WO Patent,
9
1
1/09023, 1991; (b) A. Burawoy and J. P. Critchley, Tetrahedron,
959, 5, 340; (c) M. Suchy, P. Winternitz and M. Zeller,
(
AVB and H ) and photocatalysts (bare TiO
2
2
and platinized
(Ciba-Geigy). WO Patent, 91/02278, 1991; (d) R. F. Kovar and
F. E. Armond, (US Air Force). US Patent, 3, 975, 1976, vol. 444.
(a) R. Braden, H. Knupfer and S. Hartung, US Pat., 4 002673 and
TiO ) are different. These results suggest that hole trapping by
2
3
4
OA was important in the NVB reduction as well as the OA
decomposition. Addition of water seems to have various effects
4
051177, 1977, Bayer AG; (b) A. Onopchenko, E. T. Sabourin
and C. M. Selwitz, J. Org. Chem., 1979, 44, 1233; (c) A. Saha and
B. Ranu, J. Org. Chem., 2008, 73, 6867.
(a) H.-U. Blaser and M. Studer, Appl. Catal., A, 1999, 189, 191;
on the surface properties and suspending state (stability) of TiO
2
particles as well as on the stabilities, solubilities and adsorption
behaviors of NVB and OA. These factors would affect each other
and change complexly depending on the amount of water added.
For example, a small amount of water would increase the
(
b) A. Corma and P. Serna, Science, 2006, 313, 332; (c) A. Corma,
P. Serna, P. Concepcion and J. J. Calvino, J. Am. Chem. Soc.,
008, 130, 8748; (d) P. Serna, P. Concepcion and A. Corma,
´
2
´
J. Catal., 2009, 265, 19; (e) K. Shimizu, Y. Miyamoto,
T. Kawasaki, T. Taji, Y. Tai and A. Satsuma, J. Phys. Chem. C,
2 2
stability of TiO particles and the surfaces of TiO particles
2
009, 113, 17803; (f) K. Shimizu, Y. Miyamoto and A. Satsuma,
would become more hydrophilic. The experimental results
suggest that various factors affecting photocatalytic efficiency
were highly balanced when 10% (v/v) water was added.
J. Catal., 2010, 270, 86; (g) K. Shimizu and A. Satsuma, J. Jpn.
Pet. Inst., 2011, 54, 347; (h) Y. Mikami, A. Noujima,
T. Mitsudome, T. Mizugaki, K. Jitsukawa and K. Kaneda, Chem.
Lett., 2010, 223; (i) K. Kaneda, H. Kuwahara and T. Imanaka,
J. Mol. Catal., 1994, 88, L267; (j) P. M. Reis and B. Royo,
Tetrahedron Lett., 2009, 50, 949; (k) T. Mitsudome, Y. Mikami,
M. Matoba, T. Mizugaki, K. Jitsukawa and K. Kaneda, Angew.
Chem., Int. Ed., 2012, 51, 136.
To examine the durability of the TiO
reaction system, TiO was used repeatedly. After reaction in
0% (v/v) water–ACN for 2 h, TiO particles were recovered by
2
photocatalyst in this
2
1
2
a simple filtration from the reaction mixture and were re-used. As
5
6
(a) T. Wakanabe, A. Kitamura, E. Kojima, C. Nakayama,
K. Hashimoto and A. Fujishima, in Photocatalytic Purification
and Treatment of Water and Air, ed. D. E. Olis and H. Al-Ekabi,
Elsevier, 1993, p. 747; (b) M. A. Fox and M. T. Dulay, Chem. Rev.,
shown in Table 1, TiO photocatalysts were reusable without
2
notable loss of activity.
The high chemoselectivity of this method for reduction of
NVB was further investigated in the intermolecular competitive
reaction of nitrobenzene (NB) and styrene (ST). As expected, NB
1
993, 93, 341; (c) M. R. Hoffmann, S. T. Martin, W. Choi and
D. W. Bahnemann, Chem. Rev., 1995, 95, 69.
P. T. Anastas and J. C. Warner, Green Chemistry, Theory and
Practice, Oxford University Press, 1998.
(50 mmol) was reduced to give aniline (AN, 50 mmol) with over
4
99% yield, while ST (49 mmol) was not reduced at all (eqn (3)).
7 (a) G. Palmisano, V. Augugliaro, M. Pagliarob and L. Palmisano,
Chem. Commun., 2007, 3425; (b) G. Palmisano, E. Garcıa-Lopez,
G. Marcı V. Loddo, S. Yurdakal, V. Augugliaro and
L. Palmisano, Chem. Commun., 2010, 46, 7074.
8 (a) F. Mahdavi, T. C. Bruton and Y. Li, J. Org. Chem., 1993,
8, 744; (b) J. L. Ferry and W. H. Glaze, Langmuir, 1998, 14, 3551;
c) O. V. Makarova, T. Rajh, M. C. Thurnauer, A. Martin,
P. A. Kemme and D. Cropek, Environ. Sci. Technol., 2000,
´
´
These results clearly demonstrate that the photocatalytic system
showed complete chemoselectivity for the nitro group in the
presence of inter- and intra-molecular vinyl groups.
´
,
5
(
ð3Þ
34, 4797; (d) H. Tada, T. Ishida, A. Takao and S. Ito, Langmuir,
004, 20, 7898.
2
9
(a) H. Kominami, S. Iwasaki, T. Maeda, K. Imamura,
K. Hashimoto, Y. Kera and B. Ohtani, Chem. Lett., 2009, 410;
(b) K. Imamura, S. Iwasaki, T. Maeda, K. Hashimoto, B. Ohtani
and H. Kominami, Phys. Chem. Chem. Phys., 2011, 13, 5114.
Applicability of the photocatalytic chemoselective reduction
was investigated using various nitro compounds having other
reducible groups (chloro, bromo, carboxyl and acetyl groups),
and Table 1 shows results of the photocatalytic chemoselective
reduction of various nitrobenzenes in ACN or a mixture of
1
0 Recently, it was reported that TiO
TiO microspheres exhibited tunable photocatalytic selectivity
2
films composed of flower-like
2
towards decomposition of azo dyes. Q. Xiang, J. Yu and
M. Jaroniec, Chem. Commun., 2011, 47, 4532.
2
water and ACN suspension of TiO particles under deaerated
1
1
1 No reaction occurred when zinc oxide (ZnO, Kanto Chemical,
Tokyo) was used as photocatalyst in the reaction system.
2 T. Ohno, Y. Masaki, S. Hirayama and M. Matsumura, J. Catal.,
2001, 204, 163.
conditions. Only the nitro group of these compounds was
chemoselectively reduced even in the presence of chloro,
bromo, carboxyl and acetyl groups, resulting in the formation
4
358 Chem. Commun., 2012, 48, 4356–4358
This journal is c The Royal Society of Chemistry 2012