Phosphane-free green protocol for selective nitro reduction with an iron-based catalyst

Article abstract of DOI:10.1002/chem.201003621

Iron phthalocyanine with iron sulfate has been successfully applied for high chemo- and regioselective reduction of aromatic nitro compounds to give the corresponding amines in a green solvent system without using any toxic ligand. The catalytic systems were also compatible with a large range of other reducible functional groups, such as keto, acid, amide, ester, halogen, lactone, nitrile, N-benzyl, O-benzyl, hydroxy, and heterocycles. In the present study, dinitro compounds have been regioselectively reduced to the corresponding amines with high yield. In most of the cases the conversion and selectivity was greater than 99% as determined by GC-MS analysis. Copyright

Full text of DOI:10.1002/chem.201003621

FULL PAPER
DOI: 10.1002/chem.201003621
Phosphane-Free Green Protocol for Selective Nitro Reduction
with an Iron-Based Catalyst
Upendra Sharma, Praveen Kumar Verma, Neeraj Kumar,* Vishal Kumar,
[
a]
Manju Bala, and Bikram Singh*
Abstract: Iron phthalocyanine with
iron sulfate has been successfully ap-
plied for high chemo- and regioselec-
tive reduction of aromatic nitro com-
pounds to give the corresponding
amines in a green solvent system with-
out using any toxic ligand. The catalyt-
ic systems were also compatible with a
large range of other reducible function-
al groups, such as keto, acid, amide,
ester, halogen, lactone, nitrile, N-
benzyl, O-benzyl, hydroxy, and hetero-
cycles. In the present study, dinitro
compounds have been regioselectively
reduced to the corresponding amines
with high yield. In most of the cases
the conversion and selectivity was
greater than 99% as determined by
GC-MS analysis.
Keywords:
chemoselectivity
·
hydrazine · iron · phthalocyanines ·
reduction · regioselectivity
Introduction
chemoselective method for nitro reduction is always in
demand. Recently, we have carried out the selective reduc-
tion of aromatic nitro compounds catalyzed by copper/
The need for new, mild and selective methods for functional
group transformations is a continuing driving force in syn-
thetic organic chemistry. Aromatic amines, which are widely
used as intermediates for the preparation of important
chemicals such as dyes, pharmaceuticals, and agricultural
chemicals, can be obtained easily by the introduction of a
nitro group followed either by catalytic hydrogenation or
by employing a variety of other reduction conditions. Selec-
tive reduction of the nitro group is a very important step in
the synthesis of pharmaceuticals such as sildenafil
[17]
cobalt phthalocyanines. We further thought to investigate
whether iron phthalocyanine might also be suitable for this
task because the abundant availability of iron makes it a
[18]
highly attractive candidate for catalysis. Use of iron with
water for carrying out any reaction is probably the most
ecofriendly method. Carrying out reactions in water as sol-
vent rather than organic solvents has been a long-standing
goal for chemists because water is a potentially cleaner and
cheaper reaction medium.
[1]
[
2]
[3]
(
Viagra), the antibiotic linezolid (Zyvox), and the HIV
protease inhibitor amprenavior (Agenerase). Numerous
methods have been developed to accomplish this transfor-
Previously, iron has been successfully applied in different
[
4]
forms for selective nitro reduction, such as Fe/NH Cl, Fe/
4
AcOH, Fe/HCl, Fe/CaCl2, FeCl –Zn/N,N-dimethylform-
3
[5]
mation, including sodium borohydride/catalyst, hydrazine/
amide (DMF)–H O, FeS/NH Cl/CH OH–H O, iron oxide/
2
4
III
3
2
[
6]
[7]
catalyst, metals such as tin or zinc, and a variety of other
hydroxide/NH NH ·H O, Fe -substituted hexagonal meso-
2 2 2
porous aluminophosphate/KOH/propan-2-ol, Fe –ethylene-
diaminetetraacetic acid (EDTA)Na /H /H O, Fe O –MgO/
II
catalytic systems such as Mo(CO) /1,8-diazabicyclo-
6
[8]
ACHTUNGTRENNUNG[ 5.4.0]undec-7-ene (DBU), Pd ACHTUNGERTNNUNG( OAc) /polymethylhydrosi-
2
2
2
2
2
3
[
9]
[10]
loxane (PMHS), Sm/I2, Sm/1,1’-dioctyl-4,4’-bipyridinium
NH NH ·H O, Au-Fe(OH) /CO–H O, and Fe ACHTUNGTREUNNNG( acac) /TMDS/
2 2 2 3 2 3
[
11]
[12]
[13]
[19]
dibromide, Sm/NH Cl, Cu nanoparticles/HCOONH4,
tetrahydrofuran (THF).
However, most of these iron-
4
[15]
[14]
[16]
S /NaHCO ,
HI,
and silane/oxorhenium complexes.
based catalytic methods require acid or base for pre-activa-
tion of the catalyst and lack the desired chemo- and regiose-
lectivity for nitro reduction. Beller and co-workers carried
out chemo- and regioselective reduction of aromatic nitro
compounds by using FeBr /P(cy) with PhSiH as a hydrogen
8
3
However, most of the synthetic methods lack the desired
chemoselectivity over other functional groups that are often
present in the substrates. Thus, an improved, ecofriendly,
2
3
3
[20]
source in toluene.
Although, both high selectivity and
[
a] U. Sharma, P. K. Verma, Dr. N. Kumar, V. Kumar, M. Bala,
Dr. B. Singh
Natural Plant Products Division
conversion were observed, the long reaction times and the
use of toxic P(cy) and toluene as solvent, limited the scope
3
Institute of Himalayan Bioresource Technology
of the reaction. Herein, we report the first iron(II) salt/iron
phthalocyanine (FePc) catalyzed reduction of nitroarenes to
the corresponding anilines in a mixture of water and ethanol
(
Council of Scientific & Industrial Research)
Palampur, Himachal Pradesh, 176 061 (India)
Fax : (+91)1894-230433
E-mail: neerajnpp@rediffmail.com
bikram_npp@rediffmail.com
(
1:1) (ethylene glycol in some cases) with hydrazine hydrate
as hydrogen source.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003621.
Chem. Eur. J. 2011, 17, 5903 – 5907
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5903

Results and Discussion
Table 2. Evaluation of different solvents for the reduction of 4-nitroben-
zonitrile.
To optimize the reaction conditions, the reaction of 4-nitro-
benzonitrile with different hydrogen sources in different sol-
vents with a range of iron metal salts and complexes was ini-
tially investigated.
[a]
Entry
Catalyst
Solvent
Yield [%]
As expected, no reaction occurred without catalyst
Table 1, entry 9) whereas, in the presence of FeSO ·7H O,
4 2
1
2
3
4
5
6
7
8
FePc
FePc
FePc
FePc
FePc
FePc
FePc
FePc/FeSO
methanol
water
ethanol
ethyl acetate
THF
ethylene glycol
water/ethanol (1:1)
ethylene glycol
25
43
43
2
(
K [Fe(CN) ]·3H O, or FePc/FeSO ·7H O (0.5 mol% each),
4
6
2
4
2
7
Table 1. Evaluation of different catalysts and hydrogen sources for the
reduction of 4-nitrobenzonitrile.
99
58
99
4
·7H
2
2
O
O
(
0.5 mol% each)
9
FePc/FeSO
4
·7H
water/ethanol (1:1)
99
(
0.5 mol% each)
[
a]
[a] Yield calculated on the basis of GC-MS analysis observed (Yield=
Conv. ꢁ selectivity/100).
Entry
Catalyst
Hydrogen source
Yield [%]
[
[
[
[
b]
b]
b]
b]
1
2
3
4
5
6
7
8
FeS
Fe powder
NH
NH
NH
NH
NH
NH
NH
NH
2
2
2
2
2
2
2
2
NH
NH
NH
NH
NH
NH
NH
NH
2
2
2
2
2
2
2
2
·H
·H
·H
·H
·H
·H
·H
·H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
82
79
52
Fe
FeCl
FeSO
[Fe(CN)
FePc
FePc/FeSO
0.5 mol% each)
no catalyst
2
O
3
strates were soluble, was used as solvent. After optimization,
several substituted aromatic nitro compounds were subject-
3
86
4
·7H
2
O
>99
>99
58
ed to three different methods [Method A: FeSO ·7H O as
K
4
6
] 3H
2
O
4
2
catalyst, Method B: FePc as catalyst, and Method C: FePc/
4
·7H
2
O
>99
FeSO ·7H O as catalyst] to produce the corresponding aro-
4
2
(
matic amines. The results obtained are presented in Table 3.
Of the three catalytic methods studied, Method C (FePc/
9
NH
NH
2
4
NH
Cl
2
·H
2
O
no product
no product
no product
no product
no product
no product
no product
10
11
12
13
14
15
FeSO
FeSO
FeSO
FeSO
FeSO
FeSO
4
·7H
4
·7H
4
·7H
4
·7H
4
·7H
4
·7H
2
2
2
2
2
2
O
FeSO ·7H O (0.5 mol% each)) was found to be more gener-
O
O
O
O
O
HCOOH
HCOONH
HCOOK
4
2
4
al and catalyzed the reduction of the nitro group efficiently
with high selectivity. 4-Chloro-, 2-chloro-, 4-bromo-, and 4-
iodonitrobenzene (Table 3, entries 3–6) were cleanly re-
duced to the corresponding amines without any dehalogena-
tion, which was often encountered with several previous hy-
[c]
NaBH
no source
4
[
a] Yield calculated on the basis of GC-MS analysis (Yield=Conv. ꢁ se-
lectivity/100). [b] Amide was also observed as a side product. [c] Amide
was observed as the main product with 91% yield.
[1]
drogenation procedures. However, although the selectivity
was high, full conversion was not observed except in the
case of 2-chloro and 4-iodonitrobenzene. Therefore, ethyl-
ene glycol was used as solvent in case of 4-bromonitroben-
zene to obtain full conversion.
more than 99% yield and selectivity was observed (Table 1,
entries 5, 6, and 8). FeSO ·7H O was selected for catalytic
4
2
applications because it is more readily available than
The methoxy, carboxylic acid, ester, nitrile, ketone, amide,
sulfonamide, hydroxy, and lactone functionalities present in
the aromatic ring also remained unaffected during reduction
of the corresponding nitrobenzenes by using this procedure
(Table 3, entries 7–14, 16, and 19). One of the major advan-
tages of the present method is its tolerance towards
-OCH C H and -NHCH C H groups, which were often
K [Fe(CN) ]·3H O. In the presence of other iron salts the
4
6
2
product was formed in low yield. When the effect of differ-
ent hydrogen sources was investigated, the results indicated
that efficient reduction to 4-aminobenzonitrile (more than
99% yield) took place with hydrazine hydrate. Hydrazine
hydrate was a hydrogen source of choice because it only
produces water and nitrogen gas as byproducts. The desired
reaction was not observed in the absence of hydrogen
source (Table 1, entry 15). Furthermore, formation of the
product was not observed with other hydrogen sources
2
6
5
2
6
5
either not studied or found to be affected by other methods
[20,21]
(Table 3, entries 17 and 18).
Excellent yield and selectiv-
ity were observed for both these cases using Method C. Var-
ious heterocyclic nitro arenes were converted into the corre-
sponding anilines without affecting the heterocyclic ring
(Table 3, entries 20–23). In case of 1-nitronaphthalene, more
than 99% selectivity and yield were obtained (Table 3,
entry 24). 2-Nitrofluorene was selectively (more than 99%)
reduced to the corresponding 2-aminofluorene with 68%
yield, which is significantly higher than obtained with the
(
Table 1, entries 10–13). In the case of NaBH as hydrogen
4
source, nitro reduction was not observed; in this case the
corresponding amide was formed as the major product
(
Table 1, entry 14).
Changes in the solvent used had a major effect on the
product yield (Table 2). Very good yields were obtained in
water, water/ethanol (1:1), and ethylene glycol. However,
because most of the nitro substrates were insoluble in water,
the mixture of water/ethanol (1:1), in which all the sub-
[20]
previously reported method (Table 3, entry 25).
Method C was not applicable to the reduction of 4-nitro-
toluene, 5-nitroindole, or 3-nitrostyrene (Table 3, entries 15,
5904
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 5903 – 5907

Iron Catalysis
FULL PAPER
II
Table 3. Fe -catalyzed reduction of nitroarenes to anilines.
21, and 27), however, 5-nitroindole was selectively
reduced to the corresponding amine by using meth-
ods A and B. 4-Nitrotoluene and 3-nitrostyrene
were reduced to corresponding amines by applying
Method B.
Method A
Conv./
selectivity [%]
Method B
Yield Conv./
Method C
Yield Conv./ Yield
Highly regioselective reduction of one nitro
group in the case of dinitro compounds is often a
S. No. Substrate
[
a]
[a]
[a]
selectivity [%]
selectivity [%]
[22]
problem with a number of methods.
In Meth-
[
b]
[b]
[b]
1
2
3
64/93
60
99/99
99/99
98
99
70/65
97/53
46
51
od A, although complete conversion was observed,
both of the nitro groups were reduced (Table 3, en-
tries 28–30). This is thus a good method for obtain-
ing diamine compounds. In case of Method B, more
than 99% regioselectivity was observed with good
to excellent yields (Table 3, entries 28–30). In case
of Method C, regioselectivity was observed in 1,3-
and 1,4-dinitrobenzene, but not in the case of 1,2-di-
nitrobenzene (Table 3, entries 28–30).
[
[
c]
71/99
69
57
b]
[b]
[b]
[b]
69/83
[
[
c]
75/71
53
89
99/99
81/99
98
80
b]
95/94
59/99
99/99
59
99
6
9
3/85
9/99
53
99
17/99
99/99
17
99
[
b]
4
[
[
c]
These methods were also applicable to more
complex substrates such as 4-chloro-2-nitrophenol
9
4/93
21/7
88
2
b]
[b]
[b]
5
6
7
8
99/99
99/99
99
99
99/99
99/99
99
99
(
Scheme 1, Table 3, entry 26), in which the nitro
[
b]
group was reduced selectively without affecting
other groups.
62/99
22/99
90/99
62
21
71
When these methods were applied on larger
scales (up to gram scale), with some important sub-
strates containing functional groups such as acid, ni-
trile, sulfonamide, hydroxy, O-benzyl, N-benzyl, and
lactone (Table 3, entries 8, 10, 14, and 16–19), good
isolated yields were obtained in all cases.
[
[
b]
d]
[b]
[d]
[b]
[d]
99/95
95/99
95
76
99/99
99/99
99
84
[20]
[23]
9
99/99
99
99/99
79/74
99
99/90
99/99
90
Toxicity
and stability
are always important
factors for the development of a new catalytic
method. In this respect, iron phthalocyanine is
highly stable towards moisture and temperature
99/99
99
94
58
51
99
92
[
d]
[d]
[d]
1
0
1
(
m.p. >3608C), it is environmentally friendly, and
no additional ligand is required. Furthermore, the
use of a green solvent makes the present method
useful for industrial purposes.
1
99/99
98
29/51
15
66/99
66
1
1
1
1
1
1
1
2
3
4
5
6
7
8
45/99
99/99
99/99
NR
45
99
80
–
NR
NR
–
–
99/99
99/99
99/99
NR
99
99
78
–
Conclusion
Ecofriendly methods have been developed for se-
lective nitro reduction with inexpensive iron-based
catalytic systems by using hydrazine hydrate as hy-
drogen source. The simple method and easy workup
procedure make these methods most suitable for se-
lective nitro reduction. Further studies on the
mechanism of the reaction and the catalytic applica-
tions of metallophthalocyanines are being carried
out in our laboratory.
[
d]
[d]
[d]
90/99
99/96
99/99
72
95
99
[
b]
[
d]
[d]
89/99
72
99/99
82
9
9
[
[
d]
d]
99/99
99
99
99/45
99/99
45
99
99/99
99/99
84
9
9
9/99
9/99
99
83
Experimental Section
99
43/99
43
99/99
99
86
[
d]
[d]
84
General procedure for the reduction of aromatic nitro com-
pounds: Hydrazine hydrate (2 equiv) was added to a mixture
of nitro compound (0.67 mmol) and catalyst (0.5 mol%) in
water/ethanol (1:1) or ethylene glycol (10 mL). The reaction
1
9
Chem. Eur. J. 2011, 17, 5903 – 5907
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5905

N. Kumar, B. Singh et al.
Table 3. (Continued)
S. No. Substrate
Acknowledgements
Method A
Conv./
selectivity [%]
Method B
Yield Conv./
Method C
Yield Conv./
The authors are grateful to the Director of the institute for
providing the necessary facility and encouragement. U.S. and
V.K. are also thankful to CSIR and UGC for fellowships.
IHBT Communication No. 2123.
Yield
[
a]
[a]
[a]
selectivity [%]
selectivity [%]
2
0
1
99/99
85/99
99
85
42/99
99/99
42
99
99/99
NR
99
–
2
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46/99
36/99
89/99
46
36
89
99/99
68/99
99/99
99
68
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[
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<1
99/99
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99
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99/99
99/99
99
99
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Scheme 1. Selective reduction of the nitro group in the presence of other
functional groups.
[
[
[
[
5
2
solution was heated to reflux at 1208C and the progress of the reaction
was monitored by TLC (silica gel; hexane/ethyl acetate) and GC-MS.
Upon completion, the reaction mixture was cooled to ambient tempera-
ture and extracted with ethyl acetate (3ꢁ5 mL). The combined organic
fractions were (washed with water in cases where ethylene glycol was
used as solvent) dried under reduced pressure. The crude product was an-
alyzed directly by GC-MS. To obtain the isolated yield, the product mix-
ture was subjected to column chromatography (silica 230–400; n-hexane/
ethyl acetate).
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Received: December 15, 2010
Published online: April 15, 2011
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