NaBH4/charcoal: A new synthetic method for mild and convenient reduction of nitroarenes

Article abstract of DOI:10.1080/00397910600764709

NaBH₄ in the presence of charcoal (0.4-0.8 g) reduces varieties of nitroarenes to their corresponding amines. Reduction reactions were carried out in a mixture of H₂O-THF (1:0.5 mL) at 50-60°C with high to excellent yields of products. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600764709

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NaBH4/Charcoal: A New Synthetic Method
for Mild and Convenient Reduction of
Nitroarenes
Behzad Zeynizadeh a & Davood Setamdideh a
a Department of Chemistry, Faculty of Sciences, Urmia University, Urmia,
Iran
Available online: 16 Feb 2007
To cite this article: Behzad Zeynizadeh & Davood Setamdideh (2006): NaBH4/Charcoal: A New Synthetic
Method for Mild and Convenient Reduction of Nitroarenes, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 36:18, 2699-2704
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Synthetic Communicationsw, 36: 2699–2704, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600764709
NaBH4/Charcoal: A New Synthetic
Method for Mild and Convenient
Reduction of Nitroarenes
Behzad Zeynizadeh and Davood Setamdideh
Department of Chemistry, Faculty of Sciences, Urmia University,
Urmia, Iran
Abstract: NaBH4 in the presence of charcoal (0.4–0.8 g) reduces varieties of nitro-
arenes to their corresponding amines. Reduction reactions were carried out in a
mixture of H2O-THF (1:0.5 mL) at 50–608C with high to excellent yields of products.
Keywords: Amines, charcoal, NaBH4, nitroarenes, reduction
Arylamines are widely used as antioxidants and intermediates for dyes, photo-
graphic materials, pharmaceuticals, and agricultural chemicals. So, reduction
of aromatic nitro compounds is one of the important methods for the prep-
aration of arylamines. During the past decades, the application of NaBH4 as
a mild reducing agent has brought about fundamental changes in the
reduction of functional groups in modern organic synthesis.[1] It is known
that solely sodium borohydride does not reduce nitro compounds under
ordinary conditions. However, the reducing power of this reagent or its
polymeric analogue (BER) undergoes a drastic change toward reduction of
nitro groups by the combination with metal halides or salts[2] such as
NaBH4/CoCl2, NaBH4/FeCl2, NaBH4/SnCl2, NaBH4/CuSO4, NaBH4/
Ni(OAc)2, NaBH4/NiCl2, NaBH4/BiCl3 or SbCl3, NaBH4/Co(pyridyl),
NaBH4/Cu(acac)2, and BER/Ni(OAc)2.
The application of carbon in its palladium-supported form to modify the
reducing capability of NaBH4 toward reduction of nitro compounds was first
Received in Poland January 18, 2006
Address correspondence to Behzad Zeynizadeh, Department of Chemistry, Faculty
of Sciences, Urmia University, Urmia 57159-165, Iran. E-mail: b.zeynizadeh@mail.
urmia.ac.ir
2699

2700
B. Zeynizadeh and D. Setamdideh
reported in a brief note at 1962.[3] In this report, the transformation of nitro
compounds to amines with sodium borohydride by adding it to a mixture of
palladium on carbon in alkaline water or methanol was investigated. The com-
bination system of NaBH4 and Pd/C was also reported for the reduction of this
functional group in THF at room temperature.[4]
In the reported methods, a mixture of carbon and palladium showed an
activity; however, our literature review did not show any application for the
use of carbon alone as a promoter in the reduction of functional groups with
NaBH4. This subject and our continuous efforts to explore the synthetic
utilities of modified hydroborate agents in organic synthesis[5] encouraged
us to investigate reduction of nitroarenes by the combination of sodium
borohydride and charcoal in a mixture of H2O-THF.
Our preliminary experiments showed that the reduction of nitrobenzene
as a model compound with 2 molar equiv of NaBH4 in the presence of
charcoal (0.4 g) in THF did not show any activity under reflux conditions.
However, we found that by adding a few drops of water to the reaction
mixture, the rate of reaction accelerated extremely at room temperature
with the evolution of hydrogen gas.
The reaction conditions were optimized, and they showed that using 4
molar equiv of NaBH4 per 1 molar equiv of substrate in the presence of
charcoal (0.4 g), in a mixture of H2O-THF (1:0.5 mL) at 50–608C is
optimal for the complete conversion.
To explore the synthetic utility of this combination system, we studied the
reduction of structurally different nitroarenes to their amines. Table 1 shows
the results of these transformations. Reduction reactions were completed
within 1–5 h with high to excellent yields of the corresponding amines. The
results show that selective reduction of the nitro group in the presence of
carboxylic acid can be achieved perfectly with the reduction of 2-nitrobenzoic
acid to anthranilic acid in 93% yield (entry 4). Chemoselective reduction of
carboxaldehyde or keto groups over nitro in a molecule was demonstrated
with the selective reduction of C55O in 2-nitrobenzaldehyde and 20-nitroace-
tophenone by the use of 1 molar equiv of NaBH4 in the presence of charcoal
(0.4–0.5 g) within 5–10 min at 50–608C (entries 10,12). This utility was also
effective for the reduction of dinitro or trinitroarenes using 8–12 molar equiv
of NaBH4 in the presence of 0.6–0.8 g of charcoal (entries 15–19).
The exact mechanism of this protocol is not clear; however, we think that
the following factors may play a role in the reductions: a) Charcoal is a very
fine porous material and the use of sodium borohydride in the presence of
water makes it finely disperse on the surface of charcoal and therefore more

Table 1. Reduction of nitroarenes with NaBH4/charcoal systema
Mp or Bp (8C)
Found
Reported[6]
Molar ratio
subs./NaBH4
Charcoal
(g)
Time
(h)
Yield
(%)b
Entry
Substrate
Product
1
2
Nitrobenzene
Aniline
1:4
1:4
1:4
1:5
1:4
1:4
1:4
1:4
1:4
1:1
1:5
1:1
1:5
0.4
0.4
0.4
0.5
0.4
0.4
0.4
0.4
0.4
0.4
0.5
0.5
0.5
2
95
96
85
93
95
95
95
90
95
90
85
87
90
183–184
143–145
65–67
146–148
188–190
175–177
61–63
84–85
49–51
70–71
84–85
—
184
143–145
64–66
144–148
188–190
174–177
60–63
83–85
48–50
70–72
83–85
—
4-Nitroaniline
3-Nitroaniline
2-Nitrobenzoic acid
4-Nitrophenol
2-Nitrophenol
1,4-Phenylenediamine
1,3-Phenylenediamine
Anthranilic acid
3
3
3
4
3
5
4-Aminophenol
2-Aminophenol
3
6
3
7
4-Nitrobenzyl alcohol 4-Aminobenzyl alcohol
2-Nitrobenzyl alcohol 2-Aminobenzyl alcohol
2
8
3
9
1-Nitronaphthalene
2-Nitrobenzaldehyde
2-Nitrobenzaldehyde
20-Nitroacetophenone
20-Nitroacetophenone
1-Naphthylamine
2
5 min
3
10
11
12
13
2-Nitrobenzyl alcohol
2-Aminobenzyl alcohol
1-(2-Nitrophenyl)-1-ethanol
1-(2-Aminophenyl)-1-ethanol
10 min
3
—
—
(continued)

Table 1. Continued
Mp or Bp (8C)
Molar ratio
subs./NaBH4
Charcoal
(g)
Time
(h)
Yield
(%)b
Entry
14
Substrate
Product
4-Bromoaniline
Found
Reported[6]
1-Bromo-4-
nitrobenzene
1:4
0.4
2
95
63–65
62–64
15
16
17
18
19d
1,3-Dinitrobenzene
2,4-Dinitrotoluene
2,4-Dinitrophenol
2,4-Dinitroaniline
Picric acid
1,3-Phenylenediamine
2,4-Diaminotoluene
2,4-Diaminophenol
1:8
1:8
1:8
1:8
1:12
0.6
0.6
0.6
0.6
0.8
1
3
5
5
4
97
94
97c
95
97
65–67
97–100
221–223
—
64–66
97–99
222 (dec.)
—
1,2,4-Triaminobenzene
2,4,6-Triaminophenol
—
—
aAll reactions were carried out in H2O-THF (1:0.5 mL) at 50–608C.
bYields refer to isolated pure products.
cThe product was isolated as a salt of dihydrochloride.
dThe reaction was carried out in H2O-THF (1:1.25 mL) at 50–608C.

NaBH4/Charcoal
2703
interaction with the substrate produces the efficiency in the reductions. b) In
our experiments, we saw that NaBH4 in the presence of charcoal in a
mixture of H2O-THF gently liberates hydrogen gas, therefore H2 in a
synergic sense with the hydride attack performs reductions. c) The hydrolysis
of the produced amino-borate intermediate by the presence of water in the
reaction mixture makes a promotion in the reductions.
In conclusion, we have shown that NaBH4 in the presence of charcoal is
an efficient protocol for the reduction of nitroarenes. Reduction reactions were
carried out in a mixture of H2O-THF (1:0.5 mL) at 50–608C under mild con-
ditions. The product amines were obtained in high to excellent yields.
Reduction of dinitro or trinitroarenes was also achieved successfully with
this protocol. The chemoselective reduction of carbonyl groups over nitro
groups was also achieved perfectly. The simplicity of the method,
cheapness and availability of the reagents, mild reaction conditions, high
yields of products, and the easy workup procedure are the advantages that
could make this protocol a useful addition to the present methodologies.
EXPERIMENTAL
General
All reagents and substrates were purchased from commercial sources, are of
1
the best quality, and were used without further purification. IR and H NMR
spectra were recorded on Thermo Nicolet Nexus 670 FT-IR and 300-MHz
Bruker spectrometers, respectively. The products were characterized by a
1
comparison with authentic samples (melting or boiling points) and their H
NMR or IR spectra. All yields refer to isolated pure products. TLC was
applied for the purity determination of substrates, products, and reaction
monitoring over silica-gel 60 F254 aluminum sheet.
Reduction of Nitrobenzene to Aniline with NaBH4/Charcoal
System: A Typical Procedure
In a round-bottomed flask (15 mL) equipped with magnetic stirrer and
condenser, a solution of nitrobenzene (0.123 g, 1 mmol) in a mixture of
H2O-THF (1:0.5 mL) was prepared. Charcoal (0.4 g) was then added, and
the reaction mixture was stirred with magnetic stirrer for 10 min at room temp-
erature. To the resulted mixture, NaBH4 (0.151 g, 4 mmol) was added portion-
wise for 2 h with stirring at 50–608C. TLC monitored the progress of the
reaction (eluent, CCl4/Et2O:5:2). At the end of the reaction, the reaction
mixture was simply filtered and dried over anhydrous sodium sulfate. Evapor-
ation of the solvent and a short column chromatography of the resulting crude

2704
B. Zeynizadeh and D. Setamdideh
material over silica gel (eluent; CCl4/Et2O: 5/3) gave the pure liquid aniline
(0.088 g, 95%, Table 1).
ACKNOWLEDGMENT
The authors gratefully appreciate the financial support of this work by the
research council of Urmia University.
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agent for reduction of carbonyl compounds. Bull. Korean Chem. Soc. 2003, 24,
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