Fe3O4 nanoparticles: A conveniently reusable catalyst for the reduction of nitroarenes using hydrazine hydrate

Article abstract of DOI:10.1002/asia.201100311

A magnetic personality: Commercially available Fe₃O₄ nanoparticles were utilized for efficient nitroarene reductions, and could be recycled up to 10 times using magnetic separation, whilst retaining activity (99 % aniline yield in each case without any side-products). Excellent chemoselectivity for reduction of the nitro versus other functional groups, such as halogen, ester, O-benzyl, and N-Cbz groups, was observed.

Full text of DOI:10.1002/asia.201100311

DOI: 10.1002/asia.201100311
Fe O Nanoparticles: A Conveniently Reusable Catalyst for the Reduction of
3
4
Nitroarenes Using Hydrazine Hydrate
[
a]
Seyoung Kim, Eunsuk Kim, and B. Moon Kim*
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
Reduction of the nitro group is one of the most funda-
mental methods for the preparation of amines, which serve
as valuable intermediates for many agrochemicals, pharma-
source because it obviates the use of autoclaves, acids, and
the production of large amounts of waste. Reduction with
hydrazine hydrate produces harmless by-products, such as
[1]
[17]
ceuticals, dyes, and pigments. Over the past several de-
cades, much effort has been concentrated on achieving effi-
cient and selective nitro-group reduction, and many reaction
conditions have been developed for reduction of nitro com-
pounds. The reduction of nitroarenes employing stoichio-
metric reagents are often associated with unwanted side-
nitrogen gas and water.
Since the first report of hydra-
[18]
zine-mediated nitro-group reduction over 80 years ago,
many heterogeneous catalytic systems that couple iron with
[17,19]
hydrazine have been reported.
Recently, we reported
the use of heterobimetallic nanocrystals consisting of rhodi-
um and Fe O as an efficient catalytic system for nitroarene
3
4
[2]
[20]
products, such as hydroxylamine. Therefore, the catalytic
hydrogenation of nitroarenes has been actively pursued
using many readily available metal catalysts, such as copper,
gold, iron, palladium, platinum, and rhodium immobilized
reduction. However, to the best of our knowledge, there
is no report of nitroarene reduction using commercially
available, easily recyclable superparamagnetic Fe O nano-
3
4
particle catalysts in the presence of hydrazine hydrate.
Magnetically separable catalysts are very attractive heter-
[
3,4]
on solid supports,
and several hydrogen sources have
[5]
[6]
[21]
been utilized, including hydrogen gas, silanes, and hydra-
zine derivatives. In contrast to the costly metal catalysts
mentioned above, iron is one of the most economical and
environmentally friendly metal catalysts. Since the tradi-
tional nitro group reduction using Fe/HCl system (Bꢀchamp
process) has been introduced, many iron-mediated catalyt-
ogeneous catalysts owing to their facile recycling process.
[7]
Among them, magnetic nanoparticle catalysts have attracted
much attention because they show good dispersibility and
[8]
[22]
allow easy recovery after the reaction. Herein, we report
the utilization of commercially available Fe O nanoparticles
3
4
[9]
(Aldrich,<50 nm) towards efficient and selective nitro-
group reduction.
[10]
ic systems have been developed. Homogeneous iron salts
have also been used with hydrogen gas or ammonium for-
mate for the nitro-reduction reaction. Recently, the groups
First, various hydrogen sources were tested under a varie-
ty of reaction conditions employing Fe O nanoparticles and
3
4
[11]
[12]
of Lemaire and Beller reported the use of organosilane
reagents in the presence of homogeneous iron catalysts.
However, heterogeneous iron catalysts have mostly been
used for nitro-group reduction; iron powders were used with
the results are summarized in Table 1. Reaction with only
the catalyst without any hydrogen source did not proceed at
all (Table 1, Entry 1). Reactions employing 1 atm hydrogen
gas (balloon) in the presence of Fe O did not proceed re-
3
4
[
13]
[14]
various hydrogen sources, such as NH Cl,
SnCl2,
gardless of the presence of PPh (Table 1, entries 2 and 3).
4
3
[15]
[16]
AcOH, and HCl. Hydrazine is an attractive hydrogen
Reactions at high hydrogen pressure (20 atm) proceeded
rather sluggishly, giving 84% yield after 48 hours (Table 1,
entry 4). When ammonium formate was used as a hydrogen
source, no reduction of the nitro group was observed
[
a] Dr. S. Kim, E. Kim, Prof. Dr. B. M. Kim
Department of Chemistry
[4h]
College of Natural Sciences
Seoul National University
Seoul, 151-747 (Republic of Korea)
Fax : (+82)2-875-7505
(Table 1, entries 5 and 6).
drate was used, the reduction proceeded smoothly and only
.5 equivalents of hydrazine was enough to complete the re-
However, when hydrazine hy-
1
duction without leaving any trace of incomplete reduction
products, such as nitroso- or N-hydroxylamino-benzene
(Table 1, entry 7; see the Supporting Information for GC
E-mail: kimbm@snu.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100311.
Chem. Asian J. 2011, 6, 1921 – 1925
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1921

COMMUNICATION
[
a]
[a]
Table 1. Optimization of nitrobenzene reduction conditions.
Table 2. Reaction dependence on the amount of catalyst.
[
b]
Entry
Hydrogen source
No additive
Solvent
t [h]
Yield [%]
[
b]
Entry
Amount of Fe
3
O
4
[mol%]
t [h]
Yield [%]
1
2
3
4
5
6
7
8
9
EtOH
EtOH
EtOH
EtOH
24
20
20
48
20
20
0
0
0
84
0
H
2
(1 atm)
(1 atm), PPh (10 mol%)
(20 atm)
CH (4.5 equiv)
CH (4.5 equiv)
1
2
3
4
5
0
20
10
5
24
1
1.5
3
0
99
99
99
11
H
2
3
H
2
[c]
NH
4
O
2
NH
4
O
2
0
1
6.5
[
c]
[c]
N
N
N
N
N
N
N
N
N
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
4
4
4
4
4
4
4
4
4
·H
·H
·H
·H
·H
·H
·H
·H
·H
2
2
2
2
2
2
2
2
2
O (1.5 equiv)
O (3.0 equiv)
O (4.0 equiv)
O (4.0 equiv)
O (4.0 equiv)
O (4.0 equiv)
O (4.0 equiv)
O (4.0 equiv)
O (4.0 equiv)
EtOH
EtOH
EtOH
EtOH
3(6)
65(99)
[
4
a] All the reactions were carried out with 1.0 mmol of nitrobenzene and
.0 equiv of hydrazine hydrate in 6.0 mL of ethanol at 808C. [b] Yields
3
1.5
1.5
89
99
[
d]
were determined by GC analysis using anisole as an internal standard.
10
11
12
13
14
15
6
83
26
97
99
14
Toluene 1.5
THF
DME
DMF
1.5
1.5
1.5
1.5
ucts were observed from the reactions of compounds con-
taining other reducible functional groups, such as halogens
2
H O
(
chloride or iodide; Table 3, entries 3 and 4, respectively)
[
6
a] All reactions were carried out with 1.0 mmol of nitrobenzene in
.0 mL of solvent at 808C unless otherwise noted. [b] Yields were deter-
and O-benzyl- or N-Cbz-protecting groups (Table 3, en-
tries 7 and 8, respectively). Based on the fact that hydrazine
mined by GC analysis using anisole as an internal standard. [c] Yield
were obtained after 6 h. [d] Reaction was performed at RT.
[23]
hydrate is known to be a two-electron reducing agent,
a
plausible mechanism for the reduction of the nitro group in-
[19i,24]
volving the Fe O catalyst is given in Scheme 1.
It is in-
3
4
traces of selected runs). When the number of equivalents of
hydrazine was increased gradually, the reactions proceeded
faster (Table 1, entries 7–9) and, with 4.0 equivalents of hy-
drazine hydrate, the reaction was complete in 1.5 hours
(
Table 1, entry 9). Temperature was also an important
factor: a reaction at room temperature produced only 6%
yield of the desired product (Table 1, entry 10). When the
reaction was performed in a non-polar solvent, such as tolu-
ene at 808C (Table 1, entry 11), the product was obtained in
slightly lower yield. Reaction in refluxing tetrahydrofuran
produced even lower yield of the product, presumably
owing to the lower reaction temperature (Table 1, entry 12).
When polar aprotic solvents, such as 1,2-dimethoxyethane
Scheme 1. Proposed mechanism of the hydrazine-mediated nitro-reduc-
tion.
teresting to note that under the Fe O -catalyzed nitro reduc-
3
4
(
DME) and N,N-dimethylformamide (DMF), were used
tion conditions, we detected no nitroso- nor hydroxylamine
intermediates by GC analysis (see the Supporting Informa-
tion). However, in the case of compounds containing a
double bond, reduction of the nitro group proceeded along
with the double-bond reduction (Table 3, entry 11). This
result indicates the formation of a diimide from hydrazine
hydrate, which is responsible for reducing the double bonds
(
Table 1, entries 13 and 14), nearly quantitative yields of ani-
line were obtained; therefore, the employment of a protic
solvent is not a necessity. However, in consideration of cost
and ease of solvent removal after the reaction, we chose eth-
anol as an optimal solvent for the nitroarene reduction. Re-
action in water proceeded very slowly (Table 1, entry 15).
To evaluate the catalytic activity of Fe O for nitro-group
reduction, various amounts of Fe O were used. The reac-
tion rate appeared to be dependent upon the amount of
Fe O , and at least 5 mol% of catalyst was required for a
practical reduction rate (Table 2, entries 1–4). Reaction
using only 1 mol% Fe O proceeded very slowly (Table 2,
entry 5).
To examine the scope of substrates for the Fe O -nanopar-
ticle-catalyzed reduction, various nitro-group-containing
compounds containing other functional groups, such as
ester, amide, and halogen groups, have been examined
under the standard reaction conditions and corresponding
aminoarenes were obtained in quantitative yields within
[23]
together with the nitro group through a two-electron pro-
3
4
[25]
cess, as shown in Scheme 1. In the reductions of aliphatic
nitro compounds (Table 3, entries 12 and 13), the reactivity
was noticeably lower than those of aromatic arenes, and
only moderate yields of the desired products accompanied
by several side-products were obtained after prolonged reac-
tion times.
3
4
3
4
3
4
To investigate the origin of the different reactivity of ni-
troalkanes compared to nitroarenes, we investigated a set of
reactions including both nitroarenes and nitroalkanes. Inter-
estingly, as shown in Figure 1, it was found that the reduc-
tion of nitrobenzene in the presence of nitrohexane
(Figure 1, line d) was considerably slower than the reaction
of nitrobenzene alone (Figure 1, line a). This may indicate
3
4
2
hours (Table 3). It is of particular note that no side-prod-
1922
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1921 – 1925

Fe O Nanoparticles as a Reusable Catalyst for the Reduction of Nitroarenes
3
4
[
a]
3 4
Table 3. Substrate scope of Fe O catalyzed nitro group reductions.
[
b]
Entry
1
Substrate
Product
t [h] Yield [%]
1
99
2
3
4
1.5
1
99
99
99
3 4
Figure 1. Reaction rate of 20 mol% Fe O catalyst in nitrobenzene reduc-
1
tion. a) nitrobenzene b) Nitromethane was added after 25 min. c) Nitro-
hexane was added after 25 min. d) Nitrohexane was added at the start of
the reaction.
5
6
1.5
1
99
99
examined the recycling capacity of the Fe O catalyst, taking
3
4
the reduction of nitrobenzene as a test reaction. After com-
pletion of the reaction, the catalyst was collected with an ex-
ternal magnet and the solution was decanted (Figure 2). The
7
8
2
2
99
99
9
2
2
99
99
45
1
1
0
1
[
[
c]
12
4
1
0
8
38
49
1
1
2
3
d]
3 4
Figure 2. Magnetic separation of Fe O catalyst after the reaction.
40
47
[
a] Reaction conditions: 1.0 mmol of nitro compound, 6.0 equiv of hydra-
remaining catalyst was washed with EtOH and reused for
the next reaction. As summarized in Table 4, the catalyst
showed consistent activity until the 5th cycle. Slightly dimin-
ished reactivity was observed for the 6th run, and prolong-
ing the reaction time secured quantitative conversions for
the 7th and 8th cycles. However, the yield obtained from the
9th run dropped to 88%, even after a 7 hour reaction time.
Addition of an extra 5 mol% catalyst for the 9th run in-
zine monohydrate, 6.0 mL of ethanol at 808C. [b] All of the yields were
determined by GC analysis using anisole as an internal standard. [c] 55%
of product was ethyl 3-(4-aminophenyl) propanoate. [d] 10 equiv of hy-
drazine monohydrate was used. Bn=benzyl, CBz=carboxybenzyl.
that the nitro groups of nitroalkanes bind more strongly to
the Fe O nanoparticles then do the nitroarenes so that the
3
4
catalytic reaction on nitrobenzene is slowed down. When ni-
troalkanes were added during the reduction of nitrobenzene,
it was evident that the reactions were slowed down from the
moment the nitroalkanes were added (Figure 1, lines b and
c). It is intriguing that there was a different amount of inhib-
ition depending upon the nitroalkane added; namely, the re-
action rate was affected less by nitromethane then by nitro-
hexane. The higher level of inhibition by nitrohexane might
be due to the bulkier hexyl group surrounding the Fe O
[
a]
Table 4. Recycling Test of Fe O catalyst in nitrobenzene reductions.
3
4
Catalyst
Fe
(10 mol%)
Run
1
2
3
4
5
6
7
8
9
10
[b]
[c]
3
O
4
yield [%]
99 99 99 99 95 87 99 99 88 99
t [h]
yield [%]
2
2
2
2
2
2
3
4
7
2
[
b]
Fe
3
O
4
99 99 99 99 99 99 99 99 99 99
(
20 mol%)
t [h]
1
1
1
1
1
1
1
1
1
1
3
4
[
a] Reaction conditions: 1.0 mmol of nitrobenzene, 4.0 equiv of hydrazine
monohydrate, Fe catalyst in 6.0 mL of ethanol at 808C. [b] All the
yields were determined by GC analysis using anisole as an internal stan-
dard. [c] 5 mol% of additional Fe was added before the reaction.
nanoparticles, thereby reducing the catalytic activity.
With the optimized reaction conditions in hand (10 mol%
3
O
4
of Fe O , 4.0 equiv hydrazine hydrate, 808C in EtOH), we
3 4
O
3
4
Chem. Asian J. 2011, 6, 1921 – 1925
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1923

B. M. Kim et al.
COMMUNICATION
creased the yield to 99%. However, when 20 mol% Fe O4
catalyst was used from the first cycle, all of the reactions
were complete in an hour with no loss of reactivity up to the
layer was decanted, and analyzed directly with gas chromatography. The
remaining catalyst was washed with ethanol (2ꢂ5 mL), dried under
vacuum, and reused for the next cycle of the reaction.
3
10th cycle.
Figure 3 shows TEM images of Fe O catalyst before use
3
4
and after the 10th run with 20 mol% of Fe O . Although
slight aggregation of the nanoparticles was observed, they
3
4
Acknowledgements
The authors thank the generous financial support of the NRF grant
funded by the MEST (No. R01-2008-000-20332-0).
Keywords: catalyst recycling
· iron oxide · magnetic
properties · nanoparticles · reduction
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3 4
O nanoparticle before use (left) and after
1
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3
4
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A General Procedure for the Recycling of Fe
Reduction
3 4
O Catalyst in Nitrobenzene
3 4
A magnetic stirrer bar, a reflux condenser, Fe O (purchased from Al-
drich,46 mg, 0.20 mmol), and ethanol (6.0 mL) were added to an oven-
dried, two-necked 25 mL round-bottom flask and the mixture was soni-
cated in an ultrasound bath for 1 min under argon. Nitrobenzene
(
1.0 mmol), anisole (internal standard, 1.0 mmol), and hydrazine mono-
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mixture was stirred at 808C under an argon atmosphere until the reaction
was completed. After magnetic separation of the catalyst, the organic
1924
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1921 – 1925

Fe O Nanoparticles as a Reusable Catalyst for the Reduction of Nitroarenes
3
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Received: March 29, 2011
Hyeon, Chem. Commun. 2011, 47, 3601–3603.
Published online: July 8, 2011
Chem. Asian J. 2011, 6, 1921 – 1925
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