Efficient and highly selective iron-catalyzed reduction of nitroarenes

Article abstract of DOI:10.1039/c1cc13728j

Pyrolysis of iron-phenanthroline complexes supported on carbon leads to highly selective catalysts for the reduction of structurally diverse nitroarenes to anilines in 90-99% yields. Excellent chemoselectivity for the nitro group reduction is demonstrated.

Full text of DOI:10.1039/c1cc13728j

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COMMUNICATION
Efficient and highly selective iron-catalyzed reduction of nitroarenesw
¨
Rajenahally V. Jagadeesh, Gerrit Wienhofer, Felix A. Westerhaus, Annette-Enrica Surkus,
Marga-Martina Pohl, Henrik Junge, Kathrin Junge and Matthias Beller*
Received 22nd June 2011, Accepted 27th July 2011
DOI: 10.1039/c1cc13728j
Pyrolysis of iron–phenanthroline complexes supported on carbon
leads to highly selective catalysts for the reduction of structurally
diverse nitroarenes to anilines in 90–99% yields. Excellent chemo-
selectivity for the nitro group reduction is demonstrated.
attracted by these materials. Here, for the first time we are
showing that such heterogeneous iron–nitrogen materials
represent excellent catalysts for the selective reduction of
aromatic nitro compounds to the corresponding amines.
An initial screening on the activity of a pyrolyzed carbon-
supported iron–phenanthroline complex for the transfer
hydrogenation22 of nitrobenzene revealed excellent activity
in the presence of hydrazine hydrate as a reducing agent.
Hydrazine hydrate is widely used in organic synthesis,11,17,18,23
because it produces only nitrogen and water as by-products.
For the preparation of the heterogeneous catalyst Fe–
phenanthroline/C,21a we mixed Fe(OAc)2 with 1,10-phenanthroline
(Fe : phenanthroline = 1 : 2) and commercially available vulcan
XC72R carbon powder and pyrolyzed this mixture at 800 1C
for 2 h.
Aromatic amines are central intermediates and key precursors
in the synthesis of dyes, pigments, agrochemicals, polymers,
herbicides, and pharmaceuticals.1 Regarding their synthesis
the selective reduction of functionalized nitro compounds
represents the most important and versatile methodology.
Apart from catalytic hydrogenations,2,3 various stoichiometric
reducing agents4–9 have also been successfully used to reduce
nitro compounds in the presence of metal catalysts based on
Rh,7 Pd,4,5,7 and Ru.8,9 Despite all these achievements the
development of novel protocols with high chemoselectivity
towards other reducible moieties is still desired.10
First catalytic experiments were carried out with nitro-
benzene as a model substrate. Obviously, there is no appreciable
reaction in the absence of catalyst (Table 1, entry 1). Notably,
using homogeneous Fe(OAc)2 with or without added phen-
anthroline only little conversion is seen (o10%; Table 1, entries
2 and 3). Similarly, carbon-supported catalysts based on Fe(OAc)2
alone, and with phenanthroline under non-pyrolyzed conditions
are not active for the reduction (Table 1, entries 4 and 5).
However, using pyrolyzed carbon-supported Fe(OAc)2 the
reaction took place with 30% conversion and 20% yield
(Table 2, entry 6). Conversion and yield could not be improved
Traditionally, nitro group reductions are carried out using
precious metal catalysts.4,5,7–10 However, these precious metal
complexes are expensive and usually air- and moisture-sensitive.
The limited availability of these metals makes it desirable to
search for more economical and environmentally friendly alter-
natives. A possible solution of this problem could be the increased
utilization of catalysts based on bio-relevant metals, such as
iron, copper, and cobalt.11–18 Especially, iron offers significant
advantages compared to precious metals,13 since it is the
second most abundant metal in the earth crust (4.7%). More-
over, iron compounds are relatively non-toxic, and inexpensive.
Nevertheless, there are only few reports available on iron-
catalyzed reduction of nitroarenes with various reducing
agents.14–18 To overcome the problem of catalyst separation, also
heterogeneous catalystshave been used for this transformation.10,19
Based on our background in homogeneous iron catalysis,20
recently we started a program to synthesize and apply novel
carbon-supported iron catalyst systems for selective redox
processes. Simple carbon-supported iron catalysts are known
to be less active in reduction reactions. On the other hand,
carbon-supported iron–nitrogen systems have been shown
to be interesting oxygen reduction catalysts.21 Hence, we got
Table 1 Reduction of nitrobenzene with different Fe-catalystsa,b
Pyrolysis
(1C, h, gas) C (%) Y (%)
Entry Carbon support Iron salt
L
1a
2a
3a
4b
5b
6b
7b
8b
—
—
—
—
Fe(OAc)2
Fe(OAc)2
—
—
L
—
L
—
L
—
—
—
—
—
2
5
10
8
10
30
1
2
3
5
5
20
99
3
Vulcan XC72R Fe(OAc)2
Vulcan XC72R Fe(OAc)2
Vulcan XC72R Fe(OAc)2
Vulcan XC72R Fe(OAc)2
—
800, 2, Ar
800, 2, Ar 100
5
Vulcan XC72R
—
—
Leibniz-Institut fur Katalyse e. V. an der Universitat Rostock,
¨
¨
Albert-Einstein-Str. 29a, 18059 Rostock, Germany.
E-mail: matthias.beller@catalysis.de; Fax: +49-381-1281-51113;
Tel: +49-381-1281-113
w Electronic supplementary information (ESI) available: Experimental
procedure and TEM images. See DOI: 10.1039/c1cc13728j
L = 1,10-phenanthroline; C = conv.; Y = yield.a Homogeneous
conditions: 0.5 mmol nitrobenzene, 2 mmol hydrazine hydrate,
b
0.025 mmol Fe(OAc)2, 0.05 mmol L. Heterogeneous conditions:
0.5 mmol PhNO2, 2 mmol hydrazine hydrate, 1 mol% Fe.
c
10972 Chem. Commun., 2011, 47, 10972–10974
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Table 2 Reduction of nitroarenes with Fe–phenanthroline/Ca
Table 2 (continued )
Yield Selectivity
(%) (%)
Entry Nitro compound
26
Aniline
Yield Selectivity
(%) (%)
99
99
Entry Nitro compound
Aniline
1
2
R=H 99
R=4-Br 99
99
R=4-Cl
96
R=4-I 99
R=4-F 90
99
99
99
96c
99
90
27
28
99
99
99
99
3
4
5
95
95
96
95c
6
R=2-Cl 99
99
29
99
98
99
98
30
R=3-Cl
7
8
9
10
11
12
13
99
99
99
99
99
99
99
99
R=3,5-dichloro 99
R=3,4,5-trichloro 99
R=4-Me 99
31b
R=4–ethyl 99
R=4-iPr 99
R=4-ter-butyl 99
32
95
95
14
93
99
33
34
35
36
97
93
99
96
97
94
99
96
15
16
17
99
99
99
99
99
99
37
38
96
99
96
99
18
95
96
19
20
94
95
90
95
95
90
39
40
99
99
99
99
41
99
98
99
98
21
9
9
42
43
44
22b
23b
98
98
99
99
99
99
94
95
95
96
95c
96
24b
25
98
99
98
99
45
46
96
97
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10972–10974 10973

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Table 2 (continued )
perform procedure is applicable with a series of structurally
diverse nitroarenes. Notably, the nitro group is chemoselectively
reduced in the presence of other sensitive groups such as CQC,
CRC, CRN, OH, ether, thioether and ester groups.
Yield Selectivity
(%) (%)
Entry Nitro compound
Aniline
47
48
a
92
90
93
98
Notes and references
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2 For reviews see: (a) H. U. Blaser, U. Siegrist, H. Steiner and M. Studer,
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3 For recent examples see: (a) M. Takasaki, Y. Motoyama, K. Higashi,
S. H. Yoon, I. Mochida and H. Nagashima, Org. Lett., 2008, 10,
Reaction conditions: 0.5 mmol nitro compound, 2 mmol hydrazine hydrate,
Fe–phenanthroline/C (1 mol% Fe), 100 1C, 10 h. b Reaction conditions:
0.5 mmol nitro compound, 4 mmol hydrazine hydrate, Fe–phenanthroline/C
(1 mol% Fe), 100 1C. c Scaled up by factor 10 and isolated yields are given.
with this catalyst even after 24 h reaction time. To our delight,
pyrolysis of the in situ-generated Fe(OAc)2–phenanthroline
complex supported on carbon led to a highly active system.
Here, nitrobenzene is completely reduced to aniline in 99%
yield (Table 1, entry 7). Notably, the pyrolyzed catalyst system
is highly stable and can be re-used.
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highly selectively reduced under the optimized conditions in
the presence of Fe–phenanthroline/C. Except for the fluorine-
substituted substrate (90%), excellent yields (99%) of the corres-
ponding haloanilines are achieved (Table 2, entries 2–9). Obviously,
in none of these cases significant amounts of dehalogenations
occurred. The protocol also works well for the reduction of other
substituted nitroarenes (Table 2, entries 10–16). Even, sterically
hindered 2,6-dimethylnitrobenzene selectively gave 93% aniline
(Table 2, entry 14). The more active 4-nitrobenzotrifluoride
and 5-bromo-2-nitrobenzotrifluoride are also completely reduced
to the corresponding anilines in excellent yield (99%, Table 2,
entries 15 and 16). Notably, nitro anilines are selectively reduced
to dianilines in 493% yield (Table 2, entries 17–20). Reduction of
1,4-dinitrobenzene proceeded selectively, affording 90% 4-nitro-
aniline (Table 2, entry 21). In contrast, complete reduction of
both the nitro groups of dinitrobenzenes was achieved using 4
additional equivalents of hydrazine hydrate to produce diamino-
benzenes in 94–98% yield (Table 2, entries 22–24). This makes it
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Next, we turned our interest towards the chemoselective reduc-
tion of nitroarenes in the presence of other reducible groups.
Notably, functional groups such as cyano, ether, thioether and
ester groups, and as well as more challenging C–C double and
triple bonds are not affected under these conditions (Table 2,
entries 28–40). In all these cases the nitro group was chemo-
selectively reduced to anilines in 93–99% yield. More specifically,
the reduction of cyano-substituted nitrobenzene, which is an
important transformation in organic chemistry, gave 99% of
4-cyanoaniline (Table 2, entry 35). Interestingly, the catalyst also
allows for the selective synthesis of amino heterocycles in 92–99%
yields from the corresponding heterocycles (Table 2, entries 41–48).
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the reduction of the model system. Indeed, Fe–phenanthroline/C
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ficant loss of catalytic activity (see the ESIw).
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In conclusion, an inexpensive and recyclable iron catalyst
system is introduced for the efficient reduction of nitro com-
pounds using hydrazine hydrate. Our convenient and easy to
c
10974 Chem. Commun., 2011, 47, 10972–10974
This journal is The Royal Society of Chemistry 2011