Efficient and selective reduction of aromatic nitro compounds to aromatic amines by NbCl5/indium system

Full text of DOI:10.5012/bkcs.2012.33.9.2851

Communications to the Editor
Bull. Korean Chem. Soc. 2012, Vol. 33, No. 9 2851
http://dx.doi.org/10.5012/bkcs.2012.33.9.2851
Efficient and Selective Reduction of Aromatic Nitro Compounds to Aromatic
Amines by NbCl5/Indium System
*
Byung Woo Yoo, Duckil Kim, Hyung Min Kim, and Sung Ho Kang
Department of Advanced Materials Chemistry, Korea University, Jochiwon, Chungnam 339-700, Korea
*
E-mail: bwyoo@korea.ac.kr
Received May 5, 2012, Accepted June 21, 2012
Key Words : Nitroarene, Amine, Niobium, Indium
Aromatic amines find applicability in diverse fields includ-
ing dyes, pharmaceuticals, agrochemicals, and photographic
over other labile substituents. The high yields of the re-
duction products and selectivity over other labile sub-
stituents demonstrate the efficiency of this new method.
Some control experiments confirmed that the reaction was
not initiated at all using niobium(V) chloride in the absence of
indium metal and was also not very effective with indium
metal without niobium(V) chloride. Thus, a combination of
indium and niobium(V) chloride is essential to carry out the
reduction. The scope of the reaction was examined using
various substituted aromatic nitro compounds and the results
were summarized in Table 1. In the course of this process,
several sensitive functional groups such as -OH, -Br, -Cl,
-COOCH3, -COCH3, -COOH, -OCH3, and -NH2 groups
remained unaffected. In the case of carbonyl substituted
aromatic nitro compound, the corresponding aniline was
obtained in high yield without any further reduction of
carbonyl group (entry 7). Furthermore, haloaromatic nitro
compounds (entries 4, 5, and 11) showed remarkable
1
materials. To date, there are a variety of methods, which can
be used to convert aromatic nitro compounds to their
2
corresponding amines. Some of them include Cp2TiCl2/In,
3
Al/NH4Cl, (NH4)2SO4/NaBH4, NiCl26H2O/In, HI, Sm/I2,
4
5
6
7
8
9
10
In/NH4Cl, B4H10/Pd/C, Co2(CO)8/H2O and In/HCl.
11
However, most methods still lack the desired chemoselec-
tivity when other reducible functional groups are present in
the nitroarene and often require long reaction times, or harsh
reaction conditions. Consequently, efficient and selective
methods for the reduction of aromatic nitro compounds
continue to be developed. It has been reported that NbCl5/Zn
system is used as a reagent for reducing sulfoxides, epoxides,
12
and amine N-oxides. Because of the close resemblance of
indium to zinc in several respects, including first ionization
potential, we considered that NbCl5/In system can be an
efficient reducing agent for the conversion of aromatic nitro
compounds to the corresponding amines. Recently, indium
metal has attracted much attention for its unique properties
such as low toxicity and high stability in water and air
Table 1. Reduction of various aromatic nitro compounds by NbCl5/
In system
13
compared with other metals. In continuation of our interest
Time (h)
(Yield %) (Yield %)
for NbCl5/In for NbCl5/Zn
Time (h)
in exploring the utility of metal-metal salt system in organic
a
14
synthesis, we would like to report an efficient and chemo-
Entry
Substrate
Product
selective method for the reduction of various aromatic nitro
compounds to the corresponding amines by treatment with
NbCl5/In system (eq. 1). The new reduction system was
generated by the addition of indium powder to a stirred
1
2
3
4
5
6
7
8
9
C6H5NO2
C6H5NH2
1.5(95)
3.0(88)
4.0(81)
2.0(92)
0.5(94)
2.5(90)
3.5(81)
4.5(78)
3.0(88)
1.5(90)
2.0(89)
3.0(91)
1.0(90)
4.5(76)
4.0(79)
p-CH3C6H4NO2
p-HOC6H4NO2
p-BrC6H4NO2
p-ClC6H4NO2
p-CH3C6H4NH2
p-HOC6H4NH2
p-BrC6H4NH2
p-ClC6H4NH2
(1)
p-CH3OOCC6H4NO2p-CH3OOCC6H4NH2 1.5(92)
o-CH3COC6H4NO2 o-CH3COC6H4NH2 1.5(93)
p-HOOCC6H4NO2 p-HOOCC6H4NH2 0.5(90)
p-CH3OC6H4NO2 p-CH3OC6H4NH2 3.5(82)
15
solution of niobium(V) chloride in THF under sonication.
The generation of low-valent niobium species was examined
at room temperature with an excess of indium metal. The
appearance of a black color was judged as the formation of
low-valent niobium complexes. Encouraged by the
convenient generation of low-valent niobium complexes
from reduction of NbCl5 with indium metal, we have
investigated the reactions of NbCl5/In system with various
aromatic nitro compounds and found that the reductions
generally proceeded with high yields and showed selectivity
10 p-NH2C6H4NO2 p-NH2C6H4NH2
4.0(80)
11
0.5(93)
0.5(90)
12
2.5(87)
3.0(80)
a
All isolated amines were fully characterized by spectral analysis.
b
Isolated yield.

2852 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 9
Communications to the Editor
References
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1
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(Table 1). Thus, we have been able to demonstrate the utility
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15. Typical procedure: Niobium(V) chloride (270 mg, 1.0 mmol),
indium powder (460 mg, 4.0 mmol) and THF (3 mL) were mixed
and the resulting mixture was sonicated for 30 min. A blak
solution of the complex was obtained. To this solution, nitro-
benzene (62 mg, 0.5 mmol) was added. The reaction mixture was
stirred for 1.5 h at room temperature. After the reaction was over,
as indicated by TLC, water was added and then it was extracted
with ether. The combined organic extract was washed with brine,
dried over anhydrous Na2SO4, and concentrated under reduced
pressure. The crude product was purified by silica gel column
chromatography (hexane:ethyl acetate = 2:1) to yield aniline (44
mg, 95%).
In conclusion, we have demonstrated that the NbCl5/In
system mediates an efficient and mild reduction of aromatic
nitro compounds to the corresponding amines. Although the
scope and limitations were not fully established, the present
method could be a practical alternative to the conventional
method. Further work on the application of the NbCl5/In
system is currently in progress.
Acknowledgments. This work was financially supported
by Korea University.