Rapid and green reduction of aromatic/aliphatic nitro compounds to amines with NaBH4 and additive Ni2B in H2O

Article abstract of DOI:10.1007/s13738-014-0585-5

Abstract NaBH₄ with catalytic amounts of Ni₂B as an additive reagent reduced aromatic and aliphatic nitro compounds to the corresponding amines in high to excellent yields. Reduction reactions were carried out in H₂O within 3-30 min at room temperature or 75-80 °C. The catalytic activity of Ni₂B as an additive reagent was superior to using the in situ precipitated one.

Full text of DOI:10.1007/s13738-014-0585-5

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0585-5
ORIGINAL PAPER
Rapid and green reduction of aromatic/aliphatic nitro compounds
to amines with NaBH4 and additive Ni2B in H2O
Behzad Zeynizadeh · Mehdi Zabihzadeh
Received: 6 October 2014 / Accepted: 22 December 2014
© Iranian Chemical Society 2015
Abstract NaBH4 with catalytic amounts of Ni2B as an
additive reagent reduced aromatic and aliphatic nitro com-
pounds to the corresponding amines in high to excellent
yields. Reduction reactions were carried out in H2O within
3–30 min at room temperature or 75–80 °C. The catalytic
activity of Ni2B as an additive reagent was superior to
using the in situ precipitated one.
reduction of nitro compounds is one of the most important
and straightforward methods for the preparation of amines.
During the past decades, NaBH4 as a mild reducing agent
has brought about revolutionary changes in the reduction of
organofunctional groups in modern organic synthesis [9–12].
It is well known that the NaBH4 alone does not reduce nitro
compounds under ordinary conditions. However, the reducing
power of this reagent or its polymeric analog (BER: borohy-
dride exchange resin) undergoes a drastic change towards the
reduction of nitro groups by the combination with metal, metal
halides or other promoters. NaBH4 in the presence of Pd/C
[13], NiCl2 [14, 15], FeCl2 [16], CoCl2 [17, 18], TiCl4 [19],
CuCl2 [20, 21], CuBr·Me2S [22], SbF3 [23], SbCl3 [24], BiCl3
[24–26], ZrCl4 [27], SnCl2 [28], CuSO4 [29], (NH4)2SO4 [30],
Ni(OAc)2 [31, 32], Cu(acac)2 [33], Me3SiCl [34], Co(pyridyl)2
[35], Se [36], Sb [37], Raney nickel [38], nickel o-aminothio-
phenol Schiff base complexes [39] and bromoethanol-assisted
phthalocyanatoiron [40] and BER in the presence of (CuCl,
Cu(OAc)2, CoCl2 and PdCl2) [41] and Ni(OAc)2 [42] are
some of the combination systems which have been reported
for reduction of nitro compounds. In addition, PyZn(BH4)2
[43] and NaBH4/charcoal [44] have also been reported by our
research group for the efficient reduction of various aliphatic
and aromatic nitro compounds to the corresponding amines.
Along the outlined methodologies, herein, we wish to
introduce an easy and practical protocol for rapid reduction
of various aromatic and aliphatic nitro compounds to the
corresponding amines with NaBH4/Ni2B (additive) system
in H2O at room temperature or 75–80 °C (Scheme 1).
Graphical Abstract
NaBH4 (2.5 mmol), Ni2B (0.05 mmol)
NO2
NH2
H2O, r.t., 3 min, 100%
NaBH4 (2.5 mmol), Ni(OAc)2·4H2O (0.05 mmol)
H2O, r.t., 120 min, r.t., 55%
Keywords Amines · NaBH4 · Ni2B · Nitro · Reduction
Introduction
Amines are widely used as antioxidants and intermediates in
the production of many pharmaceuticals, polymers, dyestuffs,
agricultural chemicals, photographic and rubber materials,
chelating agents and other industrially important products
[1–5]. So, the particular immense interest has been devoted to
the preparation of amines by many ways [6–12]. Amines can
be prepared by reductive amination of carbonyl compounds,
alkylation of ammonia and reduction of azides, amides,
nitriles, oximes as well as nitro compounds. Among these,
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-014-0585-5) contains supplementary
material, which is available to authorized users.
Results and discussion
B. Zeynizadeh (*) · M. Zabihzadeh
Department of Chemistry, Faculty of Science, Urmia University,
5756151818 Urmia, Iran
A literature review shows that though the application of
NaBH4 in water or aqueous media has been reported for
e-mail: bzeynizadeh@gmail.com
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J IRAN CHEM SOC
NaBH4, Ni2B (additive)
H2O, r.t. or 70-80 oC
prepared Ni2B was isolated, dried under air atmosphere and
stored in a sealed bottle. The catalytic activity of this Ni2B
was studied through the reduction of nitrobenzene with
NaBH4 under different reaction conditions. The results of
this investigation are summarized in Table 1.
RNH2
RNO2
R= alkyl or aryl
Scheme 1 Reduction of nitro compounds with NaBH4/additive Ni2B
Entries 1–7 show that reduction of PhNO2 with NaBH4
in the absence or presence of additive Ni2B in protic and
aprotic solvents such as MeOH, EtOH, THF, CH3CN and
CH3CN/H2O did not take place. However, using H2O as
the sole solvent dramatically accelerated the rate of reduc-
tion (entries 8–12). These observations revealed that H2O
was the best solvent choice and using a molar equivalent of
1:2.5:0.05 for PhNO2, NaBH4 and Ni2B, respectively, was
the optimum for complete reduction of nitrobenzene. Sub-
sequently, aniline was obtained in 95 % yield within 3 min
at room temperature (Table 1, entry 11) (Scheme 2, path a).
To show the difference between catalytic activity of
Ni2B as an additive reagent and the in situ precipitated
one in the reaction mixture, we also performed reduction
of nitrobenzene with 2.5:0.05 molar equivalents of NaBH4
and Ni(OAc)2·4H2O in H2O at room temperature. In this
case, Ni2B as a fine black precipitate was also prepared,
however, reduction of PhNO2 encountered with low effi-
ciency and aniline was obtained in 55 % yield after 2 h
(Scheme 2, path b). These results definitely exhibited that
the catalytic activity of already prepared Ni2B as an addi-
tive reagent was superior to use it as the in situ precipita-
tion in the reaction mixture.
system
some reduction purposes [45–55], however, reduction of
nitro compounds with NaBH4 in H2O as the sole solvent
has not been reported yet. This subject and our ongo-
ing attention to the development of modified hydroborate
agents in organic synthesis [56–60] encouraged us to inves-
tigate the titled transformation in H2O as an easily available
and ecologically safe solvent.
In looking for the efficient and stable catalysts in
medium of H2O, we found that reduction of nitro com-
pounds with NaBH4 in the presence of NiCl2 [14, 15] or
Ni(OAc)2 [31, 32] in wet aprotic or alcoholic solvents
has been carried out through the catalysis of precipitated
fine black nickel boride (Ni2B). Moreover, the influence
and catalytic activity of already prepared Ni2B and using
it as an additive reagent to the reaction mixture have not
been investigated yet. Prompted by this idea, we decided
to study the reducing capability of NaBH4/Ni2B (additive)
system in H2O as a green solvent for the reduction of nitro
compounds.
The investigation was started by preliminary preparation
of Ni2B through the reaction of aqueous Ni(OAc)2·4H2O
and NaBH4 according to the reported procedure [61]. The
Next, synthetic utility of NaBH4/additive Ni2B system
in H2O was further studied by reduction of structurally
Table 1 Optimization
Entry
NaBH4 (mmol)
Ni2B (mmol)
Solvent (mL)
Time (min)
Conversion (%)a
experiments for reduction of
nitrobenzene to aniline with
NaBH4/Ni2B (additive) system
under different conditions
1
2
–
MeOH
H2O
120
120
180
120
120
120
180
240
100
3
0
0
2
2
–
3
2
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.05
0.05
0.05
MeOH
EtOH
5
4
2
0
5
2
THF
0
6
2
CH3CN
MeOH–H2O (1:1)
H2O
0
7
2
20
50
55
100
100
90
All reactions were carried out
with 1 mmol of nitrobenzene
in 2 mL solvent at room
temperature
8
2
9
3
H2O
10
11
12
3
H2O
a
Conversions less than 100 %
2.5
2
H2O
3
were determined on the basis of
recovered nitrobenzene
H2O
30
Scheme 2 Reduction of
Path a: NaBH4 (2.5 mmol), Ni2B (0.05 mmol)
H2O, r.t., 3 min, 100%
NO2
NH2
nitrobenzene with NaBH4 in the
presence of Ni2B as an addi-
tive reagent (Path a) or in situ
precipitated one (Path b)
Path b: NaBH4 (2.5 mmol), Ni(OAc)2·4H2O (0.05 mmol)
H2O, r.t., 120 min, r.t., 55%
1 3

J IRAN CHEM SOC
different nitroarenes to amines. Table 2 summarizes the
results of this investigation. As it is seen, all reactions were
carried out successfully in H2O with 2.5:0.05 molar equiva-
lents of NaBH4 and additive Ni2B at room temperature or
75–80 °C. The corresponding arylamines were obtained in
high to excellent yields within 5–30 min.
In conclusion, we have shown that NaBH4/additive Ni2B
system is an efficient protocol for rapid and green reduc-
tion of aliphatic and aromatic nitro compounds to the cor-
responding amines. Reduction reactions were carried out
in H2O at room temperature or 75–80 °C. Reduction of
dinitroarenes was also performed efficiently by this reduc-
ing system. Chemoselective reduction of nitro group in
the presence of carboxylic acid was achieved successfully.
By changing the solvent and decreasing the molar equiva-
lents of NaBH4, the selective reduction of carbonyl group
in the presence of nitro group was feasible. Simplicity of
the method, availability of the reagents, mild reaction con-
ditions, high yields and easy work-up procedure as well as
using H2O as an ecologically safe solvent are the advan-
tages which make this protocol a useful addition to the pre-
sent methodologies.
The study also showed that the chemoselective reduc-
tion of nitro group in the presence of carboxylic acid was
achieved successfully through the reduction of 2-nitroben-
zoic acid to anthranilic acid in 92 % yield (Table 2, entry
13). Molecules with the complexity of nitro and carbonyl
groups did not show any selectivity, and both of the func-
tional groups were reduced with the same reactivity. This
fact was shown in the reduction of nitrobenzaldehydes and
nitroacetophenones to their corresponding amino alcohols
(Table 2, entries 15–19). By changing the solvent from
H2O to a mixture of H2O-CH3CN (1:1 mL) and decreasing
the quantity of NaBH4 to 0.5–1.5 molar equivalents, how-
ever, the selective reduction of carbonyl group versus nitro
group was carried out successfully (Table 2, entries 14 and
20). Further examinations resulted that the present method
was also efficient for the reduction of dinitroarenes using
3.5:0.1 molar equivalents of NaBH4 and additive Ni2B,
respectively, in H2O at 75–80 °C (Table 2, entries 21–23).
Capability of NaBH4/additive Ni2B system in the reduc-
tion of aliphatic nitro compounds was also studied by the
reduction of 1-nitrohexane, 2-nitroheptane and nitrocy-
clohexane with 2.5:0.05 molar equivalents of NaBH4 and
Ni2B in H2O at room temperature. The results showed that
as aromatic nitro compounds, this protocol was also effi-
cient and the corresponding aliphatic amines were obtained
successfully in 90–94 % yield within 10–15 min (Table 2,
entries 24–26).
Experimental
General
All reagents and substrates were purchased from com-
mercial sources with high quality and they were used
1
without further purification. IR and H/13C NMR spectra
were recorded on Thermo Nicolet Nexus 670 FT-IR and
300 MHz Bruker spectrometers. The products were char-
acterized by 1H/13C NMR and IR spectra followed by com-
parison of the obtained data with authentic ones [74–79].
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.
Brown in 1970 demonstrated that the combination sys-
tem of NaBH4 and aqueous nickel salts (chloride, sulfate or
acetate) released hydrogen gas and various alkenes could
be hydrogenated through the catalysis of highly reactive
Ni2B [62] (Eq. 1).
Preparation of Ni2B [61]
In a two-necked and round-bottomed flask (100 mL), a
solution of Ni(OAc)2·4H2O (1.244 g, 5 mmol) in distilled
water (50 mL) was prepared and the solution was stirred
magnetically under N2 atmosphere. A solution of NaBH4
(10 mL, 1.0 M) in water was added to the prepared nickel
acetate solution by a syringe over 30 s. When the gas evolu-
tion was ceased, a second solution of NaBH4 (5 mL, 1.0 M)
was again added. The aqueous phase was decanted and the
residue fine black precipitate was washed twice with etha-
nol (25 mL). Drying the precipitate under air atmosphere
affords Ni2B in 85 % yield.
2Ni(OAc)2 + 4NaBH4 + 9H2O →
(1)
Ni2B + 4Na(OAc)2 + 3B(OH)3 + 12.5 H2 (g)
The literature review also shows that reduction of nitro
compounds to amines has been carried out by catalytic
hydrogenation in the presence of homogeneous [63] or
heterogeneous [64–73] catalysts. The exact mechanism of
NaBH4/additive Ni2B system in H2O is not clear. How-
ever, we think that due to similar characteristics of the
present protocol with Brown’s method and our obser-
vation to vigorous releasing of H2 gas on the surface of
additive Ni2B, the reduction of nitro compounds with
NaBH4/additive Ni2B system may take place through cat-
alytic hydrogenation as well as hydride transferring from
NaBH4.
A typical procedure for reduction of nitrobenzene to aniline
with NaBH4/additive Ni2B system
In a round-bottomed flask (10 mL) equipped with a mag-
netic stirrer, a mixture of nitrobenzene (0.123 g, 1 mmol)
and H2O (2 mL) was prepared. Ni2B (0.006 g, 0.05 mmol)
1 3

J IRAN CHEM SOC
Table 2 Reduction of aromatic
and aliphatic nitro compounds
with NaBH4/additive Ni2B
system
Molar Ratio
Time
(min)
Yield
(%)a
Entry
1
Substrate
NO2
Product
NH2
Condition
r.t.
Subs./NaBH4/Ni2B
1:2.5:0.05
3
95
2
H2N
NO2
H2N
NH2
1:2.5:0.05
r.t.
r.t.
10
93
NO2
NH2
3
1:2.5:0.05
1:2.5:0.05
1:2.5:0.05
6
93
96
94
H2N
H2N
CH2OH
CH2OH
4
5
r.t.
r.t.
10
12
NO2
NH2
CH2OH
CH2OH
O2N
O2N
H2N
H2N
6
7
CH2OH
NO2
CH2OH
NH2
1:2.5:0.05
1:2.5:0.05
r.t.
30
7
92
96
Br
Br
oil bath
NO2
NH2
8
9
1:2.5:0.05
1:2.5:0.05
oil bath
oil bath
8
5
94
92
OH
OH
HO
NO2
NO2
HO
NH2
NH2
10
1:2.5:0.05
r.t.
6
96
NO2
NH2
Me
Me
Me
Me
11
12
13
1:2.5:0.05
1:2.5:0.05
1:2.5:0.05
oil bath
oil bath
r.t.
20
15
10
94
96
92
Me
NO2
Me
NH2
NO2
NH2
CO2H
CO2H
NO2
NO2
14b
15
1:0.5:0.01
1:2.5:0.05
r.t.
r.t.
0.5
10
97
95
CH2OH
CHO
NH2
NO2
CH2OH
CHO
NH2
NO2
16
17
18
19
1:2.5:0.05
1:2.5:0.05
1:3.5:0.05
1:3.5:0.1
r.t.
8
10
4
97
93
94
92
HOH2C
HOH2C
OHC
OHC
NO2
NH2
OH
r.t.
O
oil bath
oil bath
O2N
O2N
H2N
H2N
O
OH
15
1 3

J IRAN CHEM SOC
Table 2 continued
Molar Ratio
Subs./NaBH4/Ni2B
Time
(min)
Yield
(%)a
Entry
Substrate
Product
Condition
O
OH
20b
21
1:1.5:0.05
1:3.5:0.1
1:3.5:0.1
r.t.
1
98
93
96
O2N
O2N
O2N
H2N
NO2
NH2
oil bath
oil bath
22
17
NO2
NH2
All reactions were carried out in
H2O (2 mL) at room tempera-
ture or under oil bath conditions
(75–80 °C)
22
O2N
O2N
H2N
Me
NO2
H2N
Me
NH2
1-Hexylamine
a
23
1:3.5:0.1
oil bath
15
93
Yields refer to isolated pure
products
b
The reaction was carried out
24
25
26
1-Nitrohexane
2-Nitroheptane
Nitrocyclohexane
1:2.5:0.05
1:2.5:0.05
1:2.5:0.05
r.t.
r.t.
r.t.
10
12
15
92
94
90
in a mixture of H2O–CH3CN
(1:1 mL) at room temperature
2-Aminoheptane
Cyclohexylamine
12. M. Hudlicky, Reductions in organic chemistry (Ellis Horwood,
Chichester, 1984)
was then added and the mixture was stirred for 5 min.
NaBH4 (0.095 g, 2.5 mmol) was also added and the result-
ing mixture was continued to stirring for 3 min at room
temperature. TLC monitored the progress of the reaction
(eluent, n-hexane/Et2O:5/3). After completion of the reac-
tion, aqueous solution of KOH (2 %, 5 mL) was added
and the mixture was stirred for 10 min. The mixture was
extracted with EtOAc (3 × 8 mL) and then dried over
anhydrous Na2SO4. Evaporation of the solvent affords the
pure liquid aniline in 95 % yield (0.088 g, Table 2, entry 1).
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