Chemoselective reduction of nitrobenzenes having other reducible groups over titanium(IV) oxide photocatalyst under protection-, gas-, and metal-free conditions

Article abstract of DOI:10.1016/j.tet.2014.04.067

m-Nitrostyrene was chemoselectively reduced to m-aminostyrene in an acetonitrile suspension containing a titanium(IV) oxide (TiO₂) photocatalyst and a suitable hole scavenger at room temperature under atmospheric pressure without the use of a precious metal or reducing gas. Addition of a small amount of water to acetonitrile increased the reaction rate, and the highest rate was obtained. Moreover, applicability of the photocatalytic system was investigated using various nitro compounds having other reducible groups (chloro, bromo, carboxyl, and acetyl groups), and only the nitro group of these compounds was chemoselectively reduced, resulting in the formation of corresponding amino compounds with high yields.

Full text of DOI:10.1016/j.tet.2014.04.067

Tetrahedron xxx (2014) 1e6
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Chemoselective reduction of nitrobenzenes having other reducible
groups over titanium(IV) oxide photocatalyst under protection-,
gas-, and metal-free conditions
*
Kazuya Imamura, Kousuke Nakanishi, Keiji Hashimoto, Hiroshi Kominami
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, Higashiosaka, Osaka 577-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
m-Nitrostyrene was chemoselectively reduced to m-aminostyrene in an acetonitrile suspension con-
taining a titanium(IV) oxide (TiO2) photocatalyst and a suitable hole scavenger at room temperature
under atmospheric pressure without the use of a precious metal or reducing gas. Addition of a small
amount of water to acetonitrile increased the reaction rate, and the highest rate was obtained. Moreover,
applicability of the photocatalytic system was investigated using various nitro compounds having other
reducible groups (chloro, bromo, carboxyl, and acetyl groups), and only the nitro group of these com-
pounds was chemoselectively reduced, resulting in the formation of corresponding amino compounds
with high yields.
Received 30 January 2014
Received in revised form 18 April 2014
Accepted 23 April 2014
Available online xxx
Keywords:
Titanium(IV) oxide
Photocatalyst
Ó 2014 Elsevier Ltd. All rights reserved.
Nitrobenzenes
Chemoselective reaction
Hydrogenation
1. Introduction
catalyst such as nickel (Ni) or copper (Cu) is the main method for
reduction of a nitro group.1 However, this method cannot be
Chemoselective conversion of a functional group is important
for organic synthesis, especially synthesis of fine chemicals. Lith-
ium aluminum hydride, a traditional reducing reagent in organic
chemistry, non-selectively reduces various groups such as carbonyl,
carboxyl, vinyl, and nitro groups. Therefore, a protecting group is
introduced into a compound by chemical modification of a func-
tional group in order to obtain chemoselectivity in a subsequent
chemical reaction. However, the yield of the product is decreased
due to introduction of the protecting group and the deprotection
process gives much waste. Therefore, a protection-free and catalytic
chemoselective reduction method is favorable in many reduction
processes.
Aminobenzenes are important compounds as intermediates of
agrochemicals, medicines, dyes, and various useful compounds,
and they are synthesized by reduction of corresponding nitroben-
zenes.1 Therefore, selective conversion of the nitro group in com-
pounds having other functional groups to an amino group is an
important technique. For example, when nitrobenzenes have other
reducible groups such as a vinyl group, it is difficult to selectively
reduce the nitro group. Catalytic hydrogenation over a metal
applied for selective reduction of the nitro group of nitrobenzenes
having a vinyl group because both the nitro and vinyl groups are
reduced. Chemoselective reduction of the nitro group is achieved
by using a large excess of a reducing agent such as tin (Sn),2 zinc
(Zn),3 iron (Fe)4 or sodium hydrosulfite.5 However, these reaction
systems give harmful wastes containing metal ions, and a catalytic
process without metal reducing agents is therefore favored. Al-
though catalytic chemoselective reduction of nitrobenzenes is im-
portant, there have been only a few reports on chemoselective
reduction of a nitro group in the presence of C]C bonds6e8 and
their yields were low. Recently, chemoselective reduction of m-
nitrostyrene (NS) to m-aminostyrene (AS) has been achieved by
many researchers.1,9e19 However, these (thermo)catalytic systems
require precious metals, high temperature, and high-pressure of
reducing reagents such as H2 and carbon monoxide (CO). Reduction
of NS to AS would be more attractive if reducing reagents other
than gaseous H2 and CO 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 pres-
sure. Since electrochemical (or electrocatalytic) reduction of nitro
compounds proceeds at room temperature and without high-
pressure of H2, electrochemical hydrogenation of nitrobenzenes
has been studied. However, these methods need a noble metal and
a large energy cost to apply sufficient voltage to the reaction
* Corresponding author. Tel.: þ81 6 6721 2332; fax: þ81 6 6727 2024; e-mail
addresses: hiro@apch.kindai.ac.jp, kominami@kindai.ac.jp (H. Kominami).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.04.067
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K. Imamura et al. / Tetrahedron xxx (2014) 1e6
systems. When a noble metal is used as an electrode, reduction of
protons to H2 occurs and competes with reduction of the substrate.
In addition, an electrolyte is indispensable.
For the above-mentioned reasons, photocatalytic methods are
preferable. When titanium(IV) oxide (TiO2) is irradiated by UV light,
charge separation occurs and thus-formed electrons in the con-
duction band and positive holes in the valence band cause
reduction and oxidation, respectively. Titanium is a common and
inexpensive element because it has the tenth largest Clarke num-
ber. Although various oxides of titanium exist, TiO2, in which the
oxidation state of titanium is þ4, is most stable under the atmo-
sphere and is easily formed without extra attention. Photocatalytic
reaction proceeds at room temperature and under atmospheric
pressure, and the TiO2 photocatalyst is easily separated from the
reaction mixture after the reaction. In addition, TiO2 has been used
for a long time as an indispensable inorganic material such as
a pigment and UV absorber because it is inexpensive and not toxic
for humans and the environment. Most of the applications of
photocatalysis are mineralization (or degradation) of toxic organic
compounds in air and water.20e22 Since photocatalytic reaction
satisfies almost all of the 12 proposed requirements for green
chemistry,23 organic synthesis of various compounds using pho-
tocatalysis has recently been studied by many researchers.21,24,25
However, less attention has been paid to the photocatalytic
reduction of organic compounds by photogenerated electrons
because a deaerated condition is needed to achieve reduction of
substrates.21 Photocatalytic reduction for organic synthesis can be
carried out in the presence of a large excess of an electron donor,
such as methanol, and in the absence of dioxygen (O2).26e29 The
purpose of the electron 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 re-
agents for photocatalytic reduction of organic compounds. Alcohols
such as methanol have been used as both a solvent and a hole
scavenger.26e29 However, since toxic aldehydes are formed as the
oxidized species of alcohols, a sacrificial reagent converting to
a non-toxic compound is preferable. We have reported photo-
catalytic reduction of nitrobenzenes to corresponding amino-
benzenes in an aqueous suspension of TiO2 in the presence of oxalic
acid or formic acid as a hole scavenger.30e32 These hole scavengers
are ‘greener’ sacrificial reagents because they are easily oxidized
into carbon dioxide (CO2) and thus-formed CO2 molecules are re-
moved from the liquid phase in the presence of organic acids.
In this study, we examined the photocatalytic reduction of
nitrobenzenes having other reducible groups using a simple pho-
tocatalyst, TiO2, in the presence of oxalic acid 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 were ob-
tained in high yields without using precious metals or high-
pressure gaseous reducing reagents.
Fig. 1. Time courses of the amount of NS remaining (squares), AS formed (circles), and
material balance (diamonds) in an acetonitrile suspension of TiO2 (50 mg) in the
presence of oxalic acid (200 mmol) as a hole scavenger under deaerated conditions.
photocatalytic chemoselective reduction of NS to AS in the pres-
ence of oxalic acid. The present photocatalytic reduction was ach-
ieved without precious metals, high-temperature or high-pressure
of H2 in contrast to (thermo)catalytic hydrogenation.
Scheme 1. Stoichiometry of photocatalytic reduction of NS to AS over TiO2 in the
presence of oxalic acid as a hole scavenger.
Reduction of a nitro group to an amino group consists of several
steps as shown in Eq. 1, i.e., nitro compounds are reduced to amino
compounds via nitroso compounds and hydroxylamines as
intermediates.
ꢀ
ꢀ
2eꢀ
R ꢀ NO2/R ꢀ NO2/e R ꢀ NHðOHÞ2/e R ꢀ NH2
(1)
Nitroso compounds and hydroxylamines were not detected
under the present conditions, indicating that the rate of reduction
of these intermediates was larger than that of NS or that in-
termediates were strongly adsorbed on the surface of TiO2. The
possibility of adsorption of intermediates can be discussed on the
basis of the value of material balance calculated by Eq. 2.
Summation of NS and AS
Initial amount of NS
Material balance ¼
ꢁ 100ð%Þ
(2)
The time course of material balance is also shown in Fig. 1. The
value of material balance was initially decreased by photo-
irradiation and then recovered with further photoirradiation, in-
dicating that the reduction intermediate(s) was adsorbed on the
TiO2 surface and released when it was reduced to AS. Recently,
photocatalytic cyanomethylation of aromatic compounds using
acetonitrile as reagent, in which acetonitrile was activated by
positive holes, has been reported.33 However, cyanomethylation
did not occur in the present system, indicating that hole scavenger
reacted with holes much faster than acetonitrile.
2. Results and discussion
2.1. Photocatalytic chemoselective reduction of NS to AS in an
acetonitrile suspension of TiO2
2.2. Intermolecular chemoselective reduction of nitroben-
zene and styrene
Fig. 1 shows time courses of the amounts of NS remaining and
AS formed in the photocatalytic reduction of NS in an acetonitrile
suspension of TiO2 under deaerated conditions. The amount of NS
monotonously decreased along with photoirradiation time and NS
was almost completely consumed after 6 h, while AS was obtained
in a high yield (86%). It is notable that there are few reports on
photocatalytic chemoselective reduction of a nitro group having
a C]C double bond. Scheme 1 shows the probable stoichiometry of
The high chemoselectivity of this method for reduction of NS
was further investigated in the intermolecular competitive reaction
of nitrobenzene and styrene. As expected, nitrobenzene (50
was reduced to give aniline (50 mol) with over >99%yield, while
no styrene (49 mol) was reduced as shown in Scheme 2. These
mmol)
m
m
results clearly demonstrate that the photocatalytic system showed
complete chemoselectivity for the nitro group in the presence of
inter- and intra-molecular vinyl groups. The selectivity between
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K. Imamura et al. / Tetrahedron xxx (2014) 1e6
3
a nitro group and a vinyl group can be explained electrochemically,
i.e., reduction potential of the conduction band of the TiO2 and
substrates. The half-wave reduction potential of styrene (ꢀ2.41 V)34
is more negative than the potential of TiO2 conduction band
(ꢀ0.3 V), while the half-wave reduction potential of nitrobenzene
(0.16 V)35 is more positive.36
being expanded 20-fold. A difference between the two emission
spectra was observed under 350 nm, i.e., emission at around
300e350 nm (painted area) was observed in the high-pressure
mercury arc, while there was almost no emission in the black
light. Fig. 2 also shows the absorption spectrum of NS in an ace-
tonitrile solution, in which strong photoabsorption was observed in
the UV range, especially at wavelengths shorter than 350 nm. These
results suggest that photochemical reaction(s) of NS is caused by
UV light with strong energy emitted from a high-pressure mercury
arc and can be avoided by using black light.
Scheme 2. Intermolecular competitive reaction of nitrobenzene and styrene in an
acetonitrile suspension of TiO2 in the presence of oxalic acid as a hole scavenger.
2.3. Blank reactions and effects of gaseous conditions and
light intensity on photocatalytic reduction of NS to AS
Effects of various reaction conditions on photocatalytic re-
duction of NS to AS were investigated and the results are summa-
rized in Table 1.
Table 1
Effects of various reaction conditions on photocatalytic chemoselective reduction of
NS to ASa
Entries
TiO2/mg
Light source
Reaction
time/h
Gas
phase
Conv./%
Sel./%
Fig. 2. Emission spectra of a high-pressure mercury arc (green line), black light (purple
line), and absorption spectrum of NS in acetonitrile.
1
2
3
4
5
6
50
50
d
50
50
d
Mercury arc
Dark
Mercury arc
Mercury arc
Black light
Black light
6
5
6
5
30
30
Ar
Ar
Ar
Air
Ar
Ar
99
Trace
25
47
86
87
d
Trace
13
98
0
2.4. Effect of hole scavengers
When photocatalytic reduction is applied to organic conver-
sions, hole scavengers are indispensable for donating electrons.
In addition, frequency of recombination depends on the rate of
oxidation of hole scavengers at holes. Therefore, a hole scav-
enger would strongly affect the yield in photocatalytic re-
duction. Table 2 shows the effects of various hole scavengers on
yield of AS.
2
a
Oxalic acid: 200 mmol.
Entry 1 is the result at 6-h photoirradiation in Fig. 1. As shown in
entries 2 and 3, photoirradiation and TiO2 were indispensable for
the formation of AS, indicating that NS was reduced to AS by TiO2
photocatalysis. No AS was formed in the absence of TiO2 (entry 3),
although NS was consumed. We also observed colorization of the
solution to light yellow after photoirradiation. This result indicates
that photochemical reaction(s) occurred to give a by-product(s).
The photochemical reaction(s) reduced the yield of AS in the
present system. In the presence of O2, the yield of AS was very low
and many by-products were detected in the reaction mixture (entry
4). Over TiO2 irradiated by UV light, electrons were injected into O2
to give active oxygen species, and these species induced degrada-
tion of NS and/or the product(s). Thus, since active oxygen species
decompose various organic compounds, almost all cases of
photocatalytic reduction are carried out under deaerated
conditions.21,24,25
Table 2
Effect of hole scavenger on photocatalytic reduction of NS to AS under photo-
irradiation of a high-pressure mercury arc
Entries Hole
Amounts of hole Irradiation Conv./% Sel./% Material
scavengers scavengers
time/h
balance/%
1
2
3
4
5
6
d
d
6
6
6
6
2
2
39
>99
95
>99
>99
>99
9
87
79
17
15
20
56
86
80
17
15
20
Oxalic acid 200
Oxalic acid 400
Formic acid 200
Methanol
Ethanol
m
m
m
mol
mol
mol
5 cm3
5 cm3
Since the yield of AS was decreased by the photochemical re-
action(s) in the absence of a TiO2 photocatalyst under irradiation of
strong light of a high-pressure mercury arc (entry 3), the effect of
light source was investigated in detail by the use of a black light as
another light source. Entries 5 and 6 show results when black light
was used as the light source with small UV light intensity. Although
longer photoirradiation was required, AS selectivity was much
higher (98%, entry 5) than that obtained under photoirradiation of
a high-pressure mercury arc (entry 1). Fruitless consumption of NS
was avoided when black light was used as the light source (entries
3 and 6). Suppression of undesired photochemical reaction(s) of NS
by using black light instead of a mercury arc as the light source is
attributed to the high AS selectivity. Fig. 2 shows emission spectra
of the high-pressure mercury arc and black light, the latter intensity
Entries 1e3 show the effect of amount of oxalic acid on pho-
tocatalytic chemoselective reduction of NS to AS. Oxalic acid was
essential for effective reduction of NS. As shown in entry 1, NS was
consumed and a small amount of AS was formed, while a large
amount of by-products was obtained even in the absence of oxalic
acid. In this case, NS probably worked as a hole scavenger, i.e.,
some NS was oxidized with positive holes and thus by-products
were formed, while other NS was reduced by exited electrons.
Excess of oxalic acid decreased the reaction rate (entry 3). The
decrease in reaction rate may be caused by strong adsorption of
oxalic acid because the carboxyl group of oxalic acid is strongly
adsorbed on the TiO2 surface. It has been reported that formic acid
was an effective hole scavenger for photocatalytic reduction of
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K. Imamura et al. / Tetrahedron xxx (2014) 1e6
nitro aromatics.31 However, many by-products were produced and
low selectivity was obtained in the present reaction (entry 4).
Photocatalytic reduction of NS in some alcohols was examined
because alcohols, mainly methanol, have often been used as sol-
vents in photocatalytic reduction of nitrobenzenes. In the present
reaction, alcohols also worked as hole scavengers as shown in
entries 5 and 6. After 2-h photoirradiation, NS was almost com-
pletely 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, AS selectivities were
much smaller than that obtained in the reaction system using
acetonitrile and oxalic acid as a solvent and hole scavenger, re-
spectively. These results indicate that the combination of aceto-
nitrile and oxalic acid is essential for efficient photocatalytic
reduction of NS.
Table 3
Photocatalytic chemoselective reduction of various nitrobenzenes to corresponding
aminobenzenes
Entries
Substrates
Products
Time/h
Conv./%
Sel./%
1
6
>99
88
2
3
2
2
>99
>99
>99
95
2.5. Applicability
Applicability of the photocatalytic chemoselective reduction
was investigated using various nitro compounds having other re-
ducible groups (chloro, bromo, carboxyl, and acetyl groups), and
Table 3 shows results of the photocatalytic chemoselective re-
duction of various nitrobenzenes in acetonitrile suspension of TiO2
particles under deaerated 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 for-
mation 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 protecting process, precious metals and H2. When o-
acetylnitrobenzene was used as substrate (entry 8), the selectivity
was low even at low conversion, suggesting that intermediate(s)
and/or o-acetylaminobenzene easily converted to by-product(s)
under photoirradiation.
4
2
>99
54
5
6
2
2
>99
>99
>99
>99
7
2
>99
88
2.6. Effect of amount of water contained in acetonitrile
solvent
8
9
2
2
>99
>99
15
91
The effect of water addition to solvents on yields of AS was in-
vestigated and the results are shown in Fig. 3.
When a small amount of water was added to acetonitrile, yields
of AS after 2-h photoirradiation increased and maximum yield was
obtained at the ratio of 10 vol %. A positive effect of addition of
water to acetonitrile was also reported in photocatalytic conver-
sions.37,33 Further addition of water, however, decreased the yield
of AS. Recently, photocatalytic hydroxylation of aromatic com-
pounds38,39 and hydration of alkenes40 in the presence of water
over metal-loaded TiO2 have been reported. However, these com-
pounds were not formed in the present system because bare TiO2
was used.
10
2
>99
88
To understand the effect of water on reduction of NS in the
presence of oxalic acid, another photocatalytic reaction using oxalic
acid, i.e., photocatalytic decomposition of oxalic acid along with H2
formation (Scheme 3) in a water/acetonitrile suspension of plati-
nized TiO2(PteTiO2) under deaerated conditions, was examined.
This reaction system is simple because only oxalic acid is dissolved
as a substrate in the liquid phase.
Since the oxalic acid decomposition was carried out under
deaerated conditions as in the NS system, the effect of oxygen on
the reaction can be eliminated. Therefore, hole trapping by oxalic
acid would be important in the oxalic acid decomposition system,
and the results are also shown in Fig. 3 (right axis). A tendency
similar to that in the NS system was observed in the oxalic acid
decomposition system and there was a clear correlation between
them as shown in Fig. 4, indicating that water added to acetonitrile
affected the two photocatalytic reactions in a similar way, though
11
12
2
2
>99
>99
98
>99
13
2
>99
98
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K. Imamura et al. / Tetrahedron xxx (2014) 1e6
5
Fig. 3. Effect of water of solvents (water/acetonitrile) on yields of AS formed by
photocatalytic reduction of NS over TiO2 for 2 h and photocatalytic H2 evolution from
oxalic acid over 1 wt %Pt/TiO2 for 0.5 h.
Fig. 5. Effect of water added in acetonitrile (50 cm3) on the amount of oxalic acid
(initially 200 mmol) adsorbed on TiO2 (1 g).
Scheme 3. Photocatalytic decomposition of oxalic acid along with H2 formation in
a suspension of PteTiO2 under deaerated condition.
Fig. 6. Effect of pH on photocatalytic chemoselective reduction of NS to AS in water/
acetonitrile suspensions (water/acetonitrile ratio¼0.1) of TiO2 for 30-min photo-
irradiation. NS: blue, AS: red. Other conditions were same for those described in Fig. 5.
2.7. AQE, durability, and effects of different kinds of TiO2
photocatalyst
The value of apparent quantum efficiency (AQE) at 366 nm
calculated from the ratio of the amount of AS and amount of pho-
tons irradiated using Eq. 3 reached 15%.
Fig. 4. Correlations between amount of AS formed and H2 evolved after 2-h and 30-
min photoirradiation. Values in the figure indicate the content of water in reaction
mixtures.
6 ꢁ amount of AS
AQE ¼
ꢁ 100:
(3)
number of incident photons
the reduced products (AS and H2) and photocatalysts (bare TiO2 and
platinized TiO2) are different.
For comparison, AQE for photocatalytic H2 formation from 2-
propanol (200 mol) in an aqueous suspension of platinized TiO2
These results suggest that hole trapping by oxalic acid was im-
portant in the NS reduction as well as in the oxalic acid de-
composition. Addition of water seems to have various effects on the
surface properties and suspending state (stability) of TiO2 particles
as well as on the stabilities, solubilities, and adsorption behaviors of
NS, AS, and oxalic acid. These factors would affect each other and
would change complexly depending on the amount of water added.
For example, a small amount of water would increase the stability
of TiO2 particles and the surfaces of TiO2 particles would become
more hydrophilic. Adsorption behavior of oxalic acid on the TiO2
surface was also investigated, and Fig. 5 shows the effect of water
on the amount of oxalic acid adsorbed on the TiO2 surface. The
amount of oxalic acid adsorbed on TiO2 decreased with increase in
water ratio of solvents. This tendency was different from that of the
reaction rate shown in Fig. 3. Even when pH of the reaction mixture
was changed, the reaction rate was not affected (Fig. 6). The ex-
perimental results suggest that various factors affecting photo-
catalytic efficiency including hole trapping by oxalic acid were
balanced most efficiently when water was added at the ratio of
10 vol % water containing acetonitrile, though the reason(s) for this
is not clear.
m
was also examined and was determined to be 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 H2 evolution.
The lager value of AQE in the present reaction than that of 2-
propanol dehydrogenation indicates that oxalic acid efficiently
works as a hole scavenger for photocatalytic reduction of NS to AS.
These results indicate that the combination of water/acetonitrile
and oxalic acid is effective for photocatalytic reduction of NS.
To examine the durability of the TiO2 photocatalyst in this re-
action system, TiO2 was used repeatedly. After reaction in 10 vol %
water containing acetonitrile for 2 h, TiO2 particles were recovered
by simple filtration from the reaction mixture and were re-used. As
summarized in Fig. 7, TiO2 photocatalysts were reusable without
notable loss of activity.
Table 4 shows effects of different kinds of TiO2 photocatalysts on
photocatalytic chemoselective reduction of NS to AS in water/ace-
tonitrile, and physical properties of TiO2 are shown in Table 4.
In all cases, NS was consumed and AS was obtained. Among these
samples, the anatase-type TiO2 sample (ST-01) and anatase-rutile
mixed TiO2 (P 25) exhibited higher conversion of NS. When P 25
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6
K. Imamura et al. / Tetrahedron xxx (2014) 1e6
performance liquid chromatography (Jasco, UV-2075Plus detector,
PU-2089Plus pump, equipped with an Inertsil ODS-3 column, elu-
ent: aqueous sodium borate buffer/acetonitrile¼50:50, flow rate:
0.5 cm3 minꢀ1 at room temperature) and GCeMS (GC-17A, GCMS-
QP5050 (Shimadzu); column: DB-1, 0.25 mm, 30m (J&W)).
To obtain apparent quantum efficiency (AQE), an UV light-
emitting diode (UV-LED, PJ-1505-2CA, CCS Inc, Kyoto,
927
m
W cmꢀ2, maximum energy at
l
¼366 nm) was also used as
a light source. A spectrum and light intensity of the UV-LED were
determined using a spectroradiometer USR-45D (Ushio, Tokyo).
Acknowledgements
Fig. 7. Durability of TiO2 in photocatalytic chemoselective reduction of NS to AS in
This work was partly supported by a Grant-in-Aid for Scientific
Research (No. 26630415) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT) of Japan. One of the
authors (H.K.) is grateful for financial support from the Faculty of
Science and Engineering, Kinki University, and another author
(K.I.) is grateful to the Japan Society for the Promotion of Science
(JSPS) for a Research Fellowship for young scientists (grant number:
12J09929).
10 vol % water/acetonitrile solution for 2-h irradiation.
Table 4
Effects of kinds of TiO2 on photocatalytic chemoselective reduction of NS to AS
TiO2
SBET/m2 gꢀ1
Crystal phase
Conv./%
Sel./%
P 25
ST-01
MT-150A
50
338
97
Anatase/rutile
Anatase
Rutile
>99
62
50
94
61
99
References and notes
was used, 99% of material balance was obtained, indicating that NS
was reduced to AS selectively. Lower material balance (76%) was
observed with formation of an unknown by-product when ST-01
was used. In addition, the color of ST-01 became light pale-yellow
after the reaction. As mentioned in Section 2.1 (about Fig. 1), the
intermediate(s) of reduction of NS may be strongly adsorbed on the
TiO2 surface. Using TiO2 with a large surface area, ST-01, the in-
termediate(s) that accumulated on the TiO2 surface mayreact to give
by-products, and thus the color of TiO2 surface was changed. When
rutile-type MT-150A was used as the TiO2 photocatalyst, good ma-
terial balance (>99%) was obtained as in the case of using P 25.
Conversion of NS, however, was lower than that of P 25.
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We succeeded in a photocatalytic chemoselective reduction of
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sponding aminobenzenes in an acetonitrile suspension of TiO2 in
the presence of oxalic acid as a hole scavenger. Addition of water to
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AS. This photocatalytic chemoselective method was applied to re-
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containing NS (50
mmol, SigmaeAldrich Japan, Tokyo) and oxalic
acid (200 mol, Wako Pure Chemical Industries, Osaka) in a test
m
tube. The tube was sealed with a rubber septum and then photo-
irradiated at a wavelength >300 nm by a high-pressure mercury arc
(Eiko-sha, 400 W) under argon (Ar) with magnetic stirring at 298 K.
After the reaction, the gas phase was analyzed by a gas chro-
matograph (Shimadzu, GC-8A equipped with MS-5A columns).
After the suspension had been filtered to remove the particles, the
amounts of NS and product(s) were determined by high-
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Please cite this article in press as: Imamura, K.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.04.067