Reduction of nitro-aryl compounds with zinc in the presence of poly[N-(2-aminoethyl)acrylamido]-trimethylammonium chloride as a phase-transfer catalyst

Article abstract of DOI:10.1081/SCC-200054221

The new polymeric phase-transfer catalyst, poly[N-(2-aminoethyl)acrylamido] trimethylammonium chloride resin, catalyzes the reduction of nitroarenes to the corresponding aromatic amines with zinc powder in water and in high yields under mild conditions. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200054221

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Reduction of Nitro‐Aryl Compounds
with Zinc in the Presence of
Poly[N‐(2‐aminoethyl)acrylamido]trimethylammo
Chloride as a Phase‐Transfer Catalyst
a
b
Hossen Mahdavi & Bahman Tamami
a
Chemistry Department, Tehran University, Tehran, Iran
b
Chemistry Department, Shiraz University, Shiraz, Iran
Version of record first published: 16 Aug 2006.
To cite this article: Hossen Mahdavi & Bahman Tamami (2005): Reduction of Nitro‐Aryl Compounds with
Zinc in the Presence of Poly[N‐(2‐aminoethyl)acrylamido]trimethylammonium Chloride as a Phase‐Transfer
Catalyst, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 35:8, 1121-1127
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Synthetic Communications , 35: 1121–1127, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200054221
Reduction of Nitro-Aryl Compounds
with Zinc in the Presence of
Poly[N-(2-aminoethyl)acrylamido]-
trimethylammonium Chloride as a
Phase-Transfer Catalyst
Hossen Mahdavi
Chemistry Department, Tehran University, Tehran, Iran
Bahman Tamami
Chemistry Department, Shiraz University, Shiraz, Iran
Abstract: The new polymeric phase-transfer catalyst, poly[N-(2-aminoethyl)acrylami-
do]trimethylammonium chloride resin, catalyzes the reduction of nitroarenes to the
corresponding aromatic amines with zinc powder in water and in high yields under
mild conditions.
Keywords: Phase transfer catalyst, nitro reduction, zinc, polyacrylamide
INTRODUCTION
The reduction of aromatic nitro compounds to anilines is a very important
synthetic transformation because the nitro group is often used to activate
the aromatic nucleus to nucleophilic substitution but the amino group is
often used further for derivation of valuable products such as pharmaceuticals.
Also, reduction of nitroarenes leading to aromatic amines is an important key
step in the industrial synthesis of dyes, medicinal supplies, and agricultural
Received in the U.K. November 3, 2004
Address correspondence to Hossen Mahdavi, Department of Chemistry, Faculty of
Science, University of Tehran, PO Box 14155-6455, Tehran, Iran. E-mail: hmahdavi@
khayam.ut.ac.ir

1
122
H. Mahdavi and B. Tamami
chemicals. Furthermore, the primary aromatic amines are readily converted
into diazonium salts, which can be substituted for many other functional groups.
A variety of methods for the reduction of nitro groups has been
[
1]
[2]
developed. The methods employed generally are catalytic hydrogenation,
using Raney Ni, Pd/C, and PtO ; or dissolving metal reduction, for example,
2
[5]
[
3]
[4]
[6]
[7]
Fe/HCl, Sn/HCl, Zn/HCl, Zn/aq./NaOH/EtOH, Zn/NH3, and
Zn/CaCl /EtOH. In addition, recently chemoselective reduction of nitro
groups using metallic reducing reagents such as Sm, In,
[
Al/NH Cl/MeOH,
[
8]
2
[9]
[10]
[11]
B H ,
10 14
12]
[13]
Sm/I (cat.)/THF/NH NH ,
NH NH (hydrate)/
2 2
4
2
2
[15]
2
[
14]
[16]
Fe O /MgO,
Raney Ni/N H /HCOOH,
2
FeCl /Zn/DMF/water,
3
2
3
4
[18]
[
17]
[19]
AlH /AlCl /hydrazine, TiCl3, and NaBH /BiCl , have been reported.
3
3
4
3
However, these reactions require expensive and/or moisture-sensitive
reagents, organic solvents, and, for catalytic hydrogenation, a high-pressure
hydrogen reactor is necessary to promote the reduction.
On the other hand, recently, in view of human health and environmental
concerns, much attention has been paid to “Green Chemistry,” which is a
chemical methodology to decrease or eliminate the use or generation of
[
20]
chemicals.
synthetic organic reactions in water, an environmentally friendly reaction
In this area of investigation, there is growing interest in
[
21]
[22]
medium.
reduction of nitro compounds that involves using zinc powder and NH Cl
Recently, Tsukinoki and Tsuzuki reported
a method for
4
in water without any organic solvent.
In a series of reports, we have introduced the use of a quaternized amino
functionalized cross-linked polyacrylamide as an efficient polymeric phase-
[
23]
[24]
transfer catalyst (PTC) for the synthesis of halohydrins,
[
azidohydrins,
chemoselective reduction of aldehydes and ketones by
25]
thiocyanohydrins,
sodium borohydride,
[
26]
[27]
synthesis of nitroalcohols,
and also synthesis of
Herein, we report the conversion of nitroarenes to
[
28]
b-dihydroxysulfides.
their corresponding aromatic amines with zinc powder in water, in the
presence of quaternized amino functionalized cross-linked polyacrylamide.
RESULTS AND DISCUSSION
Polyacrylamide cross-linked with divinylbenzene (2%) was prepared by free-
radical solution polymerization of the monomer mixture in ethanol using
benzoyl peroxide as an initiator. Poly[N-(2-aminoethyl)acrylamide] was
obtained by the transamidation reaction of cross-linked polyacrylamide with
excess ethylene-diamine. Poly[N-(2-aminoethyl)acrylamido]trimethylammo-
nium chloride was prepared by the reaction of poly[N-(2-aminoethyl)acryla-
mide] with an excess of methyl iodide in DMF at room temperature and
[23]
subsequent exchange of the iodide anion with chloride.
To develop the synthetic utility of this functional polymer, the reductions
of different nitroarenes with zinc metal in water were examined (Scheme 1).

Reduction of Nitro-Aryl Compounds
1123
Scheme 1.
The conversion of nitrobenzene to aniline in the presence of poly[N-(2-
aminoethyl)acrylamido]trimethylammonium chloride as a phase-transfer
catalyst was investigated. The optimum molar ratio of the polymeric catalyst
to nitro compound was found to be 0.15 (Table 1). The effects of different
solvents were investigated by carrying out the reactions in methanol, ethanol,
ethyl acetate, acetonitrile, and water (Table 2). Water was found to be the
best solvent for this purpose. On the basis of these results, we carried out
reductions of different nitroarenes carrying electron-donating or -withdrawing
groups with zinc powder in water at room temperature in the presence of
0
.15 molar equivalents of the polymeric phase-transfer catalyst, and the
corresponding aromatic amines were obtained in high yields (Table 3).
The reduction of nitroarenes took place smoothly and chemoselectively to
afford the corresponding amines. These results demonstrate that reduction-
sensitive substituents such as esters, amides, and halide groups were
unaffected during this reaction. In addition, the sterically hindered
2,6-dimethylnitrobenzene was readily reduced to the 2,6-dimethylaniline in
very high yield (93%) by this method (Table 3, entry 4).
Table 4 shows the comparison between Zn/NH Cl/H O reduction
4
2
[22]
system reported by Tsukinoki and Tsuzuki and our system (Zn/quaternized
polymer/H O). As shown, in the presence of our catalyst, the reduction takes
2
place under milder reaction conditions and is slightly faster. In addition, the
workup is easier, and the polymeric catalyst can be readily recovered by
simple filtration and reused at least five times.
In conclusion, this method introduces an environmentally friendly route
for the synthesis of anilines by the chemoselective reduction of nitroarenes
Table 1. Effect of the molar ratio of the polymeric PTC on the
reduction of nitrobenzene with zinc metal in water
Molar ratio
of PTC
Entry
Time (h)
Conversion (%)
1
2
3
4
5
6
7
—
0.05
0.1
2
5
100
100
100
100
100
100
1.5
1
0.15
0.2
0.3
0.3
0.25
0.25
0.3
0.4

1
124
H. Mahdavi and B. Tamami
Table 2. Effect of solvent on the reduction of nitrobenzene with
a
zinc metal using the polymeric PTC
b
Conversion
(%)
Entry
Solvent
Time (h)
1
2
3
4
5
CH OH
2
69
47
3
3
3
CH
CH
CH
COOC
2
OH
2
2
H
5
2
0
25
CH CN
3
2
0.3
H
2
O
100
a
Molar ratio of the polymeric catalyst to styrene oxide 0.15 : 1.
By GC.
b
with zinc metal in water. During the reduction nitroso compounds, hydroxy-
lamines, hydrazine derivatives, and diazo compounds were not observed.
EXPERIMENTAL
Material and Technique
Acrylamide (Fluka) was recrystallized from chloroform. Divinylbenzene
(
55%) (Fluka) was washed with sodium hydroxide solution (1%,
1
0 ꢀ 2 mL) and water (20 ꢀ 3 mL) to remove the inhibitor. Benzoyl
[24]
peroxide (Fluka) was purified.
Other reagents and solvents were used
without further purification. Gas chromatography was recorded on a
Shimadzu GC 14-A. IR spectra were run on a Perkin Elmer IR-157-G
spectrophotometer. NMR spectra were recorded on a Bruker Avance DPX
instrument (250 MHz). Mass spectra were carried out on a GC + MS
HP-5989 (70 eV) mass spectrometer. All products are known or commercially
available and were characterized by comparison of their IR and NMR spectra
and physical data with those of the authentic samples. All yields refer to the
isolated products.
Preparation of Poly[N-(2-aminoethyl)acrylamido]-
trimethylammonium Iodide
[23–26]
The resin was prepared as described in the our previous papers.
capacity of quaternized resin was 3.06 mmol/g of resin.
The
For conversion of the iodide form of resin to other halide forms, the resin
5 g) was added to the metal halide aqueous solution (2 M, 100 mL), and the
mixture was stirred for 12 h. The polymer sample was filtered, washed
(

Reduction of Nitro-Aryl Compounds
1125
Table 3. Reduction of nitro compound with zinc in water in the presence of
a
poly[N-(2-aminoethyl)acrylamido]trimethylammonium chloride resin as PTC
Time
(h)
Yield
b
(%)
Entry
1
Substrate
Product
0.3
0.3
90
95
2
c
3
0.3
93
4
5
0.3
0.6
92
93
6
0.4
0.4
0.3
94
91
92
c
7
8
9
0.3
0.7
0.2
91
89
94
1
1
0
1
a
b
c
All of the reactions were carried out at room temperature.
Yield refers to isolated product.
Done by the recovered catalyst.
successively with distilled water (ꢀ200 mL) and ether, and dried at 508C
under reduced pressure.
Procedures for the Reduction of Nitro Compounds Using Zinc
Metal in Water under PTC (General)
To a mixture of nitro compound (1 mmol) and water (15 mL), poly[N-(2-ami-
noethyl)acrylamido]trimethylammonium chloride) (0.1 g) and zinc powder

1
126
H. Mahdavi and B. Tamami
Table 4. Reduction of nitroarenes with zinc metal in the presence of the representa-
tive catalysts
Time
(h)
Isolated
yield (%)
Entry
Substrate
C H NO
Catalyst/condition
1
2
3
4
5
6
Zn/NH Cl/H O/808C
0.5
0.3
0.5
0.3
0.5
0.3
84
90
94
95
90
95
6
6
5
5
6
2
2
4
2
a
C
H
NO
Zn/QPA /H
Zn/NH Cl/H O/808C
Zn/QPA/H O/rt
2
O/rt
2-MeC
H
4
NO
2
4
2
2-MeC H NO
6
4
2
2
4-BrC
4-BrC
6
H
H
4
NO
NO
2
Zn/NH O/808C
4
Cl/H
2
6
4
2
Zn/QPA/H O/rt
2
a
QPA: poly[N-(2-aminoethyl)acrylamido]trimethylammonium chloride.
(
0.5 g, 7 mmol) were added. The reaction mixture was stirred at room
temperature for the length of time shown in Table 3. The progress of the
reaction was monitored by TLC using THF/n-hexane (1 : 5) as eluent and
GC. On completion, the polymeric catalyst and insoluble material were
removed by filtration. The filtrate was extracted with ethyl acetate
(
Na SO , and evaporated under reduced pressure to give the corresponding
3 ꢀ 20 mL). The extract was washed with water, dried with anhydrous
2
4
amino product.
Conversion of p-Nitro-Toluene to p-Amino-Toluene under PTC
p-Nitro-toluene (0.137 g, 1 mmol), zinc powder (0.5 g, 7 mmol), and poly[N-
(
2-aminoethyl)acrylamido]trimethylammonium chloride (0.1 g) were added
to water (15 mL). The reaction mixture was stirred at room temperature for
.3 h. The polymer and insoluble material were removed by filtration. The
0
filtrate was extracted with ethyl acetate and then washed with water, dried
with anhydrous Na SO , and evaporated under reduced pressure. p-Amino-
2
4
toluene was obtained as the sole product in 92% yield. The corresponding
data related to p-amino-toluene are as follows:
2
IR (neat, cm ): n 3421, 3338, 3223, 3094, 2913, 2860, 1878, 1624, 1580,
1
1
516, 1463, 1281, 819; H NMR d 6.95–6.99 (2H, m), 6.58–6.61 (2H, m),
1
þ
3.49 (2H, s), 2.22 (3H, s); MS: m/z (%): 107.0 (M , 83), 106.0 (100), 79.0
(
10), 77.0 (13), 51.0 (4).
ACKNOWLEDGMENT
We are thankful to the Tehran University Research Council and the Shiraz
University Research Council.

Reduction of Nitro-Aryl Compounds
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