Reduction of nitroarenes to aromatic amines with nanosized activated metallic iron powder in water

Article abstract of DOI:10.1055/s-2003-41021

A practical reduction of nitroarenes with nanosized activated metallic iron powder in water at 210°C (near-critical water) has been developed. The reduction generates the corresponding aromatic amines in excellent yields.

Full text of DOI:10.1055/s-2003-41021

PAPER
2001
Reduction of Nitroarenes to Aromatic Amines with Nanosized Activated
Metallic Iron Powder in Water
n Powder ng,*a,b Pinhua Li, Zongtao Wu, Jincan Yan, Min Wang, Yanbin Ding
a
a
a
a
a
R
L
eduction of
N
i
e
troarenes
w
i
ith Nanosize
W
d
A
ctivated
M
etallic Ir
o
a
a
Department of Chemistry, Huaibei Coal Teachers College, Huaibei, Anhui 235000, P. R. China
Fax +86(561)3090518; E-mail: leiwang@hbcnc.edu.cn
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, P. R. China
Received 8 May 2003; revised 16 June 2003
tion and the most classic and practical reductants are zinc,
tin, or iron in the presence of an acid. However, most of
Abstract: A practical reduction of nitroarenes with nanosized acti-
vated metallic iron powder in water at 210 °C (near-critical water)
has been developed. The reduction generates the corresponding ar-
omatic amines in excellent yields.
7
them lack the chemoselectivity over other functional
groups and reduction of aromatic nitro compounds often
8
yield a mixture of products. In addition, the reactions are
Key words: nanosize, iron, nitroarenes, aromatic amines, reduction
performed in organic solvents or in the presence of acids,
which pose waste-handling problems. Poliakoff reported
a selective reduction of nitroarenes to anilines using me-
tallic zinc in water at 250 °C (near-critical water) in high
yields, but only 10% yield of aniline was observed from
the reduction of nitrobenzene using iron powder under the
Organic reactions carried out in water have received con-
siderable attention in recent years. Unfortunately most
1
organic compounds are poorly soluble in water at ambient
temperature. Nonetheless, the remarkable properties of
water near its critical point (T = 374 °C, P = 221 bar)
9
reaction conditions. It is well known that the reactivity of
c
c
0
2+
metallic zinc (E Zn /Zn = –0.763 V) is superior to that
have promoted researchers to use it instead of organic sol-
vents or ambient temperature water in organic synthesis.
As temperature increases, water becomes more compati-
ble as a medium for reaction of organics. For example, as
temperature rises from 25 to 300 °C, the density of water
0
2+
of iron (E Fe /Fe = –0.440 V), while metallic iron is
cheap and commercially available. To perform the reduc-
tion of nitro compounds to amines with iron powder, the
reaction should be carried out at higher temperature (>250
°C) owing to the low reactivity of iron metal.
3
decreases from 0.997 to 0.713 g/cm , its dielectric con-
Nanostructured materials have many magnetic, electron-
ic, optical, biological, mechanical, and structural applica-
stant (hydrogen bonding structure) decreases from 78.85
to 19.96, and its solubility parameter decreases from 23.4
10
3
1,2
tions because of their unique properties. On account of
their ultrahigh surface areas and controlled loose struc-
ture, nanosized palladium and nickel powders exhibit a
very high catalytic activity in Heck reaction, Sonogashira
reaction, Suzuki coupling, Stille coupling, Negishi reac-
tion, Kumada reaction, hydrogenation of olefins, and am-
ination reactions.11 Most recently, copper, silver, gold,
and palladium nanoparticles catalyzed reduction of aro-
to 14.5 (cal/cm ). Over the same temperature range, the
dissociation constant of water, K increases by 3 orders of
w
magnitude from ambient to near-critical conditions
–
11
(
K = 10 at 275 °C), providing hydronium and hydrox-
w
ide ions that can act as modest acid or base in chemical re-
2
actions. Although much of supercritical water research
has been focused on the total oxidation of organic com-
3
pounds and geochemical modeling, there are increasing
matic nitro compounds to the corresponding amino deriv-
numbers of papers which suggest that near-critical water
is an excellent solvent for organic reactions because or-
ganic reactions in it offer many advantages over those in
traditional organic solvents. For example, it is environ-
mentally benign and separation of products from the reac-
atives have been also explored.1
1a,12
Here we wish to
report the reduction of nitroarenes by using nanosized ac-
tivated metallic iron powder in water at 210 °C (near-crit-
ical water) to the corresponding aromatic amines in good
yields (Scheme 1).
4
tion mixture is simplified.
Aromatic amines, widely used as important intermediates
in the synthesis of chemicals such as dyes, antioxidants,
photographic, pharmaceutical and agricultural chemicals,
can be obtained easily by the reduction of corresponding
Fe powder (nm)
NO2
NH2
H2O (210 °C)
R
R
aromatic nitro compounds using catalytic hydrogenation5 Scheme 1
and a variety of other reduction conditions. Many reduc-
6
tive agents have been recommended for this transforma-
First of all, we investigated the reaction conditions for the
reduction of nitroarenes with metallic iron powder in hot
water. p-Nitrotoluene was chosen as a model compound
for this study. The results are summarized in Table 1.
Synthesis 2003, No. 13, Print: 18 09 2003. Web: 19 08 2003.
Art Id.1437-210X,E;2003,0,13,2001,2004,ftx,en;F04704SS.pdf.
DOI: 10.1055/s-2003-41021
©
Georg Thieme Verlag Stuttgart · New York

2
002
L. Wang et al.
PAPER
Table 1 Reaction Conditions for the Reduction of p-Nitrotoluene with Iron Metal in Watera
Entry
Iron Powder
Fe/p-Nitrotoluene Temp (°C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
commercially available iron metal powder (ca. 325 mesh)
commercially available iron metal powder (ca. 325 mesh)
commercially available iron metal powder (ca. 325 mesh)
activated iron metal powder (ca. 325 mesh)
activated iron metal powder (ca. 325 mesh)
activated iron metal powder (ca. 325 mesh)
activated micron iron metal powder
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
1:1
2:1
4:1
3:1
3:1
3:1
250
275
300
250
275
300
250
275
275
250
230
210
190
170
210
210
210
210
210
210
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
3
2
12
38
68
41
74
76
70
81
84
90
93
95
89
64
51
73
96
78
95
95c
activated micron iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
nanosized activated iron metal powder
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
a
b
c
Reaction conditions: p-nitrotoluene (1 mmol), tap water (10 mL) in a high T/p batch reactor system.
Isolated yields.
Distilled water (10 mL) instead of tap water (10 mL) was used.
As seen from Table 1, the particle size of metallic iron only a moderate yield of p-toluidine was achieved at 300
powder has a strong effect on the reduction of p-nitrotol- °C because of the side reaction (entry 3, Table 1). The re-
uene in hot water. When the commercially available iron actant could not be completely reduced to the correspond-
metal powder (ca. 325 mesh, 45 mm) was used for the re- ing amine with activated micron iron metal powder in
duction of p-nitrotoluene in water at 250 °C, only 12% water at less than 250 °C. Excellent yields of product were
yield of p-toluidine was obtained (entry 1, Table 1). How- observed by using nanosized activated iron metal powder
ever, activated micron iron metal powder was used, a at temperature range of 190–250 °C, but the reaction was
good yield of p-toluidine was obtained (entry 7, Table 1) not complete at a lower temperature of 170 °C (entry 14,
at 250 °C. As nanosized activated iron metal powder was Table 1). The ratio of nanosized iron metal to p-nitrotolu-
used for this transformation, it is surprising that an excel- ene was also examined. The results showed that when the
lent yield of p-toluidine was formed (entries 11–13, ratio of iron metal to p-nitrotoluene is less than 2:1, the re-
Table 1) even at lower reaction temperature (190–230 duction was not complete (entries 15 and 16, Table 1) and
°
C). The reaction temperature drops significantly as the when the ratio equals or is more than 3:1, satisfactory re-
particle size of iron powder used in the reduction is re- sults were obtained (entries 12 and 17, Table 1). The ex-
duced to nanometer size. It may be due to the larger spe- periments also indicated that oxidized iron powder
cific surface areas and higher activity of ultrafine iron surface blocked the reduction (entry 1 vs entry 4, Table 1).
particles. The reaction temperature also plays an impor- We also investigated the effect of reaction time on the re-
tant role in the reaction. It is evident that p-nitrotoluene duction of p-nitrotoluene and the results indicated that the
could not be completely reduced to p-toluidine with com- reaction was not over when the reaction time is less than
mercially available iron metal powder (ca. 325 mesh, 45 2 hours (entry 18, Table 1). However, no increase of yield
mm) in water at less than 275 °C (entry 2, Table 1), and was observed when the reaction time was prolonged (en-
Synthesis 2003, No. 13, 2001–2004 © Thieme Stuttgart · New York

PAPER
Reduction of Nitroarenes with Nanosized Activated Metallic Iron Powder
2003
try 19, Table 1). It is noteworthy that tap water does not It is well known that Fe reacts with H O at high tempera-
2
affect the effectiveness of the nanosized iron metal-medi- ture to form H and Fe O . During our investigation, we
2
3
4
ated reduction of nitroarenes (entry 20 vs entry 12, found that hydrogen gas was liberated and a brown pre-
Table 1). The optimized reaction conditions for the reduc- cipitate of Fe O was formed when nanosized activated
3
4
tion of p-nitrotoluene with nanosized iron metal in water iron powder in water at 210 °C. Although a study of the
were found to be iron (3 equiv), p-nitrotoluene (1 equiv), detailed reaction mechanism has not been undertaken, it is
H O (10 mL) at 210 °C for 2 hours.
suggested that the reaction process that takes place is the
2
hydrogenation of the nitro group by H rather than direct
reduction of the nitro group by iron. A more detailed
study about the reaction mechanism is in progress in our
laboratory.
2
9
A variety of nitroarenes were reduced to the correspond-
ing anilines with nanosized activated metallic iron powder
in water at 210 °C. The reaction was carried out simply by
stirring substrates in water with nanosized activated me-
tallic iron powder under nitrogen without any other organ- In conclusion, a reliable and practical procedure for the
ics and additive for 2 hours. The results are summarized preparation of aromatic amines has been developed,
in Table 2. Substituent effects were also investigated. The which involves the use of nanosized activated metallic
results revealed that the electronic characteristics of gen- iron powder as reductive agent for the corresponding nitro
eral electron-donating groups (such as MeO, Me) or gen- compounds in water at 210 °C. The advantages of the
eral electron-withdrawing groups (such as MeCO, Cl, Br, method are its simple operation, and it is environmentally
CO Et) on the aromatic ring were relatively insensitive to benign. The present method is a new way of using nano-
2
the reduction. However, an iodo group on the aromatic sized iron powder directly in organic synthesis.
ring underwent reductive elimination of the iodine in a
competitive process, but the chloro and bromo groups re-
mained unchanged. The reactivity of halogen atoms in the
Iron metal powder (ca. 325 mesh), activated micron iron metal (<10
micron) powder, and nanosized activated iron metal powder were
aromatic ring is I > Br > Cl, which is consistent with the purchased from Aldrich Chemical Co. Other reagents were received
expected reactivity. Aliphatic nitro compounds inhibited from commercial supply without purification prior to use. Activa-
tion of iron metal powder (ca. 325 mesh) was according to the gen-
eral method. Products were purified by flash column
chromatography.
the reduction, as well as trans-b-nitrostyrene, where the
nitro group is attached to the carbon–carbon double bond.
Table 2 Reduction of Nitro Compounds with Iron Metal in Watera
Reduction of Nitroarenes with Nanosized Activated Iron Pow-
der in Water; General Procedure
Entry
Nitro compounds
Amines
Yield
b
Nitroarene (1 mmol) and nanosized activated metallic iron powder
168 mg, 3 mmol) were added to a high temperature and pressure
(
%)
(
1
2
3
4
5
6
7
8
9
PhNO2
PhNH2
95
96
91
94
92
94
95
95
90
91
89
52c
stainless steel autoclave reactor charged with tap water (10 mL) un-
der N with stirring. The reactor was heated at 210 °C for 2 h. After
2
p-MeC H NO
p-MeC H NH
6
4
2
6
4
2
cooling, Et O (2 × 10 mL) was added to extract the products. After
2
the organic layer was dried (Na SO ), the solvents were evaporated
2
4
m-MeOC H NO
m-MeOC H NH
6 4 2
6
4
2
under reduced pressure. The product was purified by flash chroma-
tography to yield the amine (Table 2).
m-MeC H NO
m-MeC H NH
6 4 2
6
4
2
All the products are commercially available and their analytical and
spectral data were in agreement with the corresponding authentic
samples.
p-MeCOC H NO
p-MeCOC H NH
6 4 2
6
4
2
p-EtO CC H NO
p-EtO CC H NH
2 6 4 2
2
6
4
2
p-FC H NO
p-FC H NH
6 4 2
6
4
2
Acknowledgments
p-ClC H NO
p-ClC H NH
6 4 2
6
4
2
We wish to thank the National Natural Science Foundation of China
(No. 20172018), the Excellent Scientist Foundation of Anhui Pro-
p-BrC H NO
p-BrC H NH
6 4 2
6
4
2
vince, China (No. 2001040), the Natural Science Foundation of the
Education Department of Anhui Province, China (No.
1
1
1
0
1
2
m-ClC H NO
m-ClC H NH
6 4 2
6
4
2
2
002kj254zd), the Scientific Research Foundation for the Returned
Overseas Chinese Scholars, State Education Ministry, China (No.
002247), and the Excellent Young Teachers Program of MOE,
o-ClC H NO
o-ClC H NH
6 4 2
6
4
2
2
p-IC H NO
p-IC H NH
6
4
2
6
4
2
China (No. 2024) for support of this research.
c
PhNH2
37
1
3
a-C H NO
a-C H NH
95
1
0
7
2
10
7
2
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Synthesis 2003, No. 13, 2001–2004 © Thieme Stuttgart · New York