Highly chemoselective reduction of nitroaromatic compounds using a hydrotalcite-supported silver-nanoparticle catalyst under a CO atmosphere

Article abstract of DOI:10.1246/cl.2010.223

Hydrotalcite-supported silver nanoparticles (Ag/HT) acted as an effective heterogeneous catalyst for the reduction of nitroaromatic compounds using CO/H₂O as a reducing reagent. The Ag/HT catalyst system showed complete chemoselectivity for nitro groups in the presence of inter- and intra-molecular olefinic functionalities. Moreover, Ag/HT was reusable without any loss of its activity or selectivity.

Full text of DOI:10.1246/cl.2010.223

doi:10.1246/cl.2010.223
Published on the web February 6, 2010
223
Highly Chemoselective Reduction of Nitroaromatic Compounds Using a Hydrotalcite-supported
Silver-nanoparticle Catalyst under a CO Atmosphere
1
1
1
1
1
1,2
Yusuke Mikami, Akifumi Noujima, Takato Mitsudome, Tomoo Mizugaki, Koichiro Jitsukawa, and Kiyotomi Kaneda*
1
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2
(
Received December 4, 2009; CL-091077; E-mail: kaneda@cheng.es.osaka-u.ac.jp)
Hydrotalcite-supported silver nanoparticles (Ag/HT) acted
as an effective heterogeneous catalyst for the reduction of
solution was stirred for 1 h in air at room temperature. The
resulting slurry was filtered, washed with deionized water, and
dried in vacuo at room temperature to yield HT-supported AgI
ions as a gray powder. Next, the obtained solid was reduced with
KBH4 at room temperature for 1 h to give Ag/HT. The X-ray
diffraction (XRD) patterns of Ag/HT were similar to those of
the parent HT. The fraction of Ag atoms in the Ag/HT com-
posite was determined by elemental analysis to be 2.0 wt % as a
nitroaromatic compounds using CO/H O as a reducing reagent.
2
The Ag/HT catalyst system showed complete chemoselectivity
for nitro groups in the presence of inter- and intra-molecular
olefinic functionalities. Moreover, Ag/HT was reusable without
any loss of its activity or selectivity.
3
dark green powder. k -Weighted Ag K-edge extended X-ray
Aniline derivatives are valuable intermediates in the
production of agrochemicals, pharmaceuticals, and dyes.
absorption fine structure (EXAFS) study, and transmission
electron microscopy (TEM) image of Ag/HT, showed that Ag
NPs with a mean diameter of 9.5 nm were formed on Ag/HT.
1
1
4
Although the reduction of aromatic nitro compounds is the
most straightforward method for the synthesis of the corre-
We examined the catalysis of Ag/HT for the reduction of 3-
nitrostyrene (1) under 9 atm of CO at 150 °C in the presence of
water. 3-Vinylaniline (2) was formed in over 99% yield in 3 h
without reduction products of the C=C double bond such as
3-ethylaniline (3) or 3-ethyl-1-nitrobenzene (Table 1, Entry 1).
Among the solvents tested, DMA provided the highest yield of
2. THF and toluene were also good solvents giving 2 with 99%
selectivity in moderate yields, while 1,4-dioxane, DMF, and 1,2-
dichloroethane were poor solvents (Entries 4­8).
The reductions of 1 using Ag NPs on various inorganic
materials showed that HT was the best support and that Ag/
Al2O3 also had high activity and chemoselectivity toward 2
(Entry 13). On the other hand, Ag/TiO2, Ag/SiO2, Ag/MgO,
and AgHAP gave low conversions of 1 (Entries 14­17).
Individual use of the bulk Ag metal, the catalyst precursor of
AgNO3 or the parent HT did not give any products (Entries 18­
20).
Figure 1 shows time courses for the reductions of 1 using
Ag/HT and other HT-supported metal NPs: Au/HT, Pd/HT, Pt/
HT, and Rh/HT. Although the activity of the Au/HT catalyst
was higher than that of Ag/HT, an over reduction to 3 was
observed, resulting in lower selectivity toward 2. Pt/HT and Rh/
HT functioned as good catalysts giving 2 in moderate yields
with high selectivity, while Pd/HT did not exhibit high activity
or chemoselectivity. These results indicate that Ag/HT gives the
highest yield of 2. Notably, the C=C bond of product 2 was
maintained intact even after the complete conversion of 1.
This chemoselectivity of Ag/HT toward 2 in the reduction
of 1 is greater than those of previously reported catalyst systems
such as Au/TiO2 with H2 (98.5% conversion with 96%
2
sponding anilines, it is difficult to reduce only the nitro
functionality of mother nitro compounds in the presence of other
3
reducible functionalities. The chemoselective reduction of nitro
compounds bearing C=C bonds to the corresponding anilines
has been achieved using a large excess of stoichiometric
4
reducing agents such as Fe, Sn, Zn, and NaS2O4. However,
these reaction systems have suffered from the production of
harmful wastes, the need for neutralization of acid additives as a
hydrogen source, and from their low atom efficiencies. From
practical and environmental perspectives, some efficient cata-
lysts for the chemoselective reduction of nitroaromatics with
C=C double bonds have been developed.5 To date, TiO2-
supported gold NPs (Au/TiO ) may be the best heterogeneous
2
catalyst for the chemoselective reduction of the nitro function-
ality under an H2 atmosphere. Thus, reduction of 3-nitrostyrene
using Au/TiO2 gave the desired 3-vinylaniline in 98.5%
conversion with 95.9% selectivity.6
Recently, we have found that supported silver nanoparticles
Ag NPs) can act as efficient heterogeneous catalysts for
(
versatile functional transformations under liquid-phase condi-
7
tions such as dehydrogenation of alcohols, oxidation of silanes
8
9
to silanols using water, and hydration of nitriles. These Ag NP
1
0
catalysts showed remarkably high and unique activities,
although Ag NP catalysts have generally been considered to
have low activities for many organic reactions except for the
gas-phase epoxidation of ethylene. Herein, we report that Ag
NPs, supported on an inorganic material of hydrotalcite (Ag/
HT), function as an effective heterogeneous catalyst for the
quantitative chemoselective reduction of various nitroaromatics
to corresponding anilines with a >99% selectivity in the
presence of olefinic bonds, using CO/H2O as the hydrogen
6
15
selectivity), Au/Al O with H2 (100% with 89%), Au
2
3
1
6
nanoparticle/polyelectrolyte with NaBH4 (99% with 85%),
Sm with 1,1¤-dioctyl-4,4¤-bipyridinium dibromide (82% with
100%), Cu nanoparticles with HCOONH (76% with 59%),
Rh6(CO)16/TMPDA with CO (91% with 100%), or PdCl2/
BINAS with CO (50% with 100%).
11,12
source.
This methodology completely suppresses the reduc-
1
7
18
tion of C=C double bonds during the reduction of nitro
functionalities.
4
1
9
2
0
Hydrotalcite [HT, Mg6Al2(OH)16CO3] was synthesized as
previously described.13 HT (2.0 g) was added to 150 mL of an
The high chemoselectivity of Ag/HT for nitro function-
alities was further investigated in the intermolecular competitive
¹
3
aqueous solution of AgNO3 (6.7 © 10 M), and then the mixed
Chem. Lett. 2010, 39, 223­225
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

2
24
Table 1. Reduction of 3-nitrostyrene under a CO atmosphere in
NO2
NH2
NH2
a
the presence of H O
+
2
NO2
NH2
NH2
1
2
3
+
1
2
3
100
Conversion Selectivity/%b
Entry Catalyst
Solvent
b
/
%
2
3
8
6
4
0
0
0
1
2
3
4
5
6
7
8
9
0
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
DMA
DMA
DMA
>99
99
95
58
42
10
8
>99
>99
>99
>99
>99
>99
>99
®
0
90
75
40
>99
>99
>99
>99
>99
>99
®
0
0
0
0
0
0
0
c
d
THF
Ag/HT
Toluene
1,4-Dioxane
DMF
1,2-DCE
DMA
Au/HT
Pd/HT
Rh/HT
Pt/HT
0
®
>99
9
24
59
0
0
0
0
0
0
®
®
e
99
35
42
30
90
42
17
3
2
8
1
0
f
1
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
g
1
1
1
1
1
1
1
1
1
2
a
1
20
0
2h
3
4
5
6
7
8
9
Ag/Al2O3
Ag/TiO2
Ag/SiO2
0
1
2
3
4
5
Ag/MgO
Ag/HAP
Ag powder
AgNO3
Time/ h
DMA
DMA
DMA
Figure 1. Kinetic plots showing the yield of 2 (solid lines) and
(wavy lines) for the reduction of 1 using HT-supported metal
NPs: Ag/HT, Au/HT, Pd/HT, Rh/HT, and Pt/HT. Reaction
conditons: catalyst (14 mol %), 1 (0.25 mmol), DMA (5 mL),
H2O (0.1 mL), CO (9 atm), 150 °C. Determined by LC using an
internal standard.
3
i
0
HT
®
Reaction conditions: 1 (0.25 mmol), catalyst (Ag: 14 mol %),
b
a
DMA (5 mL), H2O (0.10 mL), CO (9 atm), 150 °C, 3 h. Deter-
mined by LC using an internal standard. 1st reuse. 2nd reuse.
H2 (9 atm), 1 h. H2 (9 atm), 80 °C, 1 h. H2 (4 atm), 150 °C,
c
d
e
f
g
NO2
Ag/HT (Ag: 0.02mmol)
NH2
h
i
1
h. H2 (9 atm), 150 °C, 10 min. HT (0.2 g).
+
+
DMA (5 mL), H2O (0.1 mL)
4
(0.5 mmol) 5 (0.5 mmol)
6
7
CO (9 atm), 150 °C, 3 h
>
99% yield Not detected
reaction of nitrobenzene (4) and styrene (5) (Scheme 1).
Interestingly, 4 was reduced to give aniline (6) with over 99%
yield but 5 was not reduced at all. These results clearly
demonstrated that the Ag/HT catalyst system using CO/H2O
showed complete chemoselectivity for nitro functionalities in
the presence of inter- and intra-molecular olefinic functional-
ities. This is in sharp contrast with a previously reported
Scheme 1. Intermolecular competitive reaction of nitrobenzene
and styrene using Ag/HT in the presence of CO/H O.
2
Moreover, when H2 was used as a reducing agent instead of
CO/H2O, selective reduction of 1 to 2 did not occur (Table 1,
Au/TiO catalyst system in which reduction of 5 did occur
Entries 9­12). These results rule out the participation of H in
the Ag/HT-catalyzed reduction reaction described above. It is
likely that in situ silver hydride species generated from the
2
2
21
in the competitive reduction of 4 and 5. Furthermore, the
Ag/HT catalyst was applicable to the reduction of various nitro
compounds bearing C=C double bonds, such as 4-nitrostyrene,
reaction of H O with CO at the surface of AgNPs is an active
2
4
-nitrostilbene, 1-nitro-4-propenylbenzene, and 5-nitroindole,
species that leads to highly chemoselective reduction of nitro-
aromatics with suppression of the reduction of the olefinic
bond.
where the corresponding anilines were obtained in high yields
with over 99% selectivity.
2
3
After the reduction of 1, Ag/HT was recovered by a simple
filtration from the reaction mixture, and was reusable without
any loss of its activity or selectivity (Table 1, Entries 2 and 3).
TEM and XAFS analyses of the used Ag/HT catalyst revealed
that the size and oxidation state of the used Ag NPs on HT did
not differ from those of the fresh Ag/HT and no apparent
aggregation of used Ag NPs was observed. These data are
consistent with the retention of the catalytic activity of Ag/HT
during the recycling experiments.22
In conclusion, we found that the Ag/HT catalyst showed
complete chemoselective reduction of nitroaromatic compounds
in the presence of a C=C double bond to give the corresponding
anilines in high yields using CO/H O as a reducing reagent.
2
This methodology would be a powerful tool for the synthesis of
functionalized anilines.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. The work was also supported
by Grant-in-Aid for Scientific Research on Priority Areas
In separate experiments under identical conditions without
1
, H2 was not generated from the water-gas shift reaction.
Chem. Lett. 2010, 39, 223­225
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

2
25
(
No. 18065016, “Chemistry of Concerto Catalysis”) from
11 We first discovered the catalytic chemoselective reduction of
nitro functionalities using CO/H2O as a hydrogen source
using Rh carbonyl clusters. See Ref. 19.
12 Very recently, Cao et al. also reported that highly chemo-
selective hydrogenation of nitro compounds under CO/H2O
condition using Au catalyst, see: L. He, L.-C. Wang, H. Sun,
J. Ni, Y. Cao, H.-Y. He, K.-N. Fan, Angew. Chem., Int. Ed.
2009, 48, 9538.
13 F. Cavani, F. Trifiró, A. Vaccari, Catal. Today 1991, 11, 173.
14 See Supporting Information. Supporting Information is
available electronically on the CSJ-Journal Web site, http://
www.csj.jp/journals/chem-lett/index.html.
Ministry of Education, Culture, Sports, Science and Technology,
Japan. TEM experiments were carried out by using a facility in
the Research Center for Ultrahigh Voltage Electron Microscopy,
Osaka University. We thank Dr. Uruga, Dr. Tanida, Dr. Honma,
Dr. Taniguchi, and Dr. Hirayama (SPring-8) for XAFS measure-
ments. One of the authors Y.M. thanks the JSPS Research
Fellowships for Young Scientists. Y. M. express his special
thanks for The Global COE (center of excellence) Program
“
Global Education and Research Center for Bio-Environmental
Chemistry” of Osaka University.
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22 To assay potential leaching of the active species into the
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¹
2
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¹
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¹
2
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Chem. Lett. 2010, 39, 223­225
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/