Reduction of nitroarenes using CO and H2O in the presence of a nanostructured cobalt oxide/Nitrogen-Doped Graphene (NGr) catalyst

Article abstract of DOI:10.1055/s-0034-1380003

The most common route to anilines is based on the reduction of the corresponding nitroarenes. In general, hydrogen is preferred as reducing agent and numerous catalytic systems are known to achieve such transformations. Besides, the use of CO/H₂O as hydrogen source offers interesting possibilities for reductions. Carbon monoxide is a cheap and abundant chemical used on industrial scale for a variety of transformations. Although the reduction of nitroarenes with CO/H₂O is known in the presence of noble-metal catalysts, earth-abundant inexpensive catalysts showing high selectivity have not yet been developed. In this respect, herein we present the use of a heterogeneous cobalt oxide catalyst (Co₃O₄/NGr@C), which is modified by nitrogen-doped graphene layers. Using this non-noble metal catalyst nitroarenes are reduced in high yields and good chemoselectivities.

Full text of DOI:10.1055/s-0034-1380003

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Reduction of Nitroarenes Using CO and H2O in the Presence of a
Nanostructured Cobalt Oxide/Nitrogen-Doped Graphene (NGr)
Catalyst
Felix A. Westerhaus1
Iván Sorribes1
Gerrit Wienhöfer
Kathrin Junge
Matthias Beller*
Leibniz-Institut für Katalyse e.V., Albert Einstein Str. 29a,
18059 Rostock, Germany
matthias.beller@catalysis.de
Received: 13.11.2014
Accepted after revision: 13.01.2015
Published online: 26.01.2015
ress has been achieved by Corma and coworkers applying
gold catalysts and more recently by our group developing
Co3O4- and Fe2O3-based nanostructured materials for a gen-
eral hydrogenation of nitroarenes.6,7
DOI: 10.1055/s-0034-1380003; Art ID: st-2014-r0939-c
Abstract The most common route to anilines is based on the reduc-
tion of the corresponding nitroarenes. In general, hydrogen is preferred
as reducing agent and numerous catalytic systems are known to
achieve such transformations. Besides, the use of CO/H2O as hydrogen
source offers interesting possibilities for reductions. Carbon monoxide
is a cheap and abundant chemical used on industrial scale for a variety
of transformations. Although the reduction of nitroarenes with CO/H2O
is known in the presence of noble-metal catalysts, earth-abundant inex-
pensive catalysts showing high selectivity have not yet been developed.
In this respect, herein we present the use of a heterogeneous cobalt
oxide catalyst (Co3O4/NGr@C), which is modified by nitrogen-doped
graphene layers. Using this non-noble metal catalyst nitroarenes are re-
duced in high yields and good chemoselectivities.
A less established methodology for the reduction of ni-
troarenes makes use of CO and H2O as the hydrogen
source.8 From an industrial point of view, this water-gas
shift reaction is interesting and might allow for an alterna-
tive selectivity control. The vast majority of heterogeneous
catalysts known for low-temperature water-gas shift reac-
tions are based on noble metals.5h,9,10
Recently, we have reported novel cobalt catalysts for hy-
drogenations of nitroarenes,7a reductive amination of car-
bonyl compounds,11 oxidation of alcohols to esters,12 and
epoxidation of alkenes.13 The catalyst used in all these
transformations predominantly consists of single core-shell
structured cobalt/cobalt oxide nanoparticles. Interestingly,
the metal particles are partly encapsulated by nitrogen-
enriched graphene layers, which influence the electronic
nature of the individual particles. The preparation of the
catalyst was carried out by impregnating in situ generated
phenanthroline-ligated cobalt (II) acetate complexes onto
commercially available carbon (Vulcan XC72R) and subse-
quent pyrolysis at 800 °C under inert conditions (Scheme
1).
The resulting heterogeneous system shows a remark-
able activity and selectivity for the reduction of nitroarenes
allowing the preparation of a wide range of functionalized
anilines. Motivated by these results, we decided to further
explore this catalyst using other reducing agents. To our de-
light, the reduction of the benchmark substrate (nitroben-
zene) proceeds efficiently under a carbon monoxide pres-
sure in the presence of water. Optimal results for the model
system were obtained after 24 hours at 125 °C and 60 bar
(CO/N2 = 1:1).14
Key words reduction, nitroarene, water-gas shift reaction, heteroge-
neous catalysis, cobalt
Aromatic amines are one of the most important build-
ing blocks in the preparation of pharmaceuticals, dyes, pig-
ments, agrochemicals, and polymers.2 Among the known
well-established methods for the synthesis of anilines, the
reduction of nitroarenes represents the most convenient
approach.3 In general, classical preparations are based on
stoichiometric methods which make use of reducing agents
such as iron, zinc, tin, or aluminum and the sulfide reduc-
tion (with H2S or NaSH as reducing agent).4 Compared to
these methods, the direct catalytic hydrogenation especial-
ly using heterogeneous catalysts offers advantages in terms
of cost and waste generation. However, a crucial issue for
the general applicability of this methodology is the chemo-
selectivity. Often functional groups such as halides, carbon-
yl, and unsaturated aliphatic groups are not well tolerated
using the known catalysts.5 In this regard, significant prog-
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Table 1 (continued)
NO2
8
>99
>99
93 (89)
Cl
NO2
Cl
9
99
95
Cl
NO2
NO2
10
>99
Cl
Cl
11
>99
>99
>99
87 (84)
93 (91)
95
Scheme 1 Preparation of the Co3O4/NGr@C catalyst
Br
Next, we demonstrated the general applicability of the
supported cobalt catalyst in the reduction of more than 25
different nitroarenes and heteroarenes (Table 1).15 Alkyl-
substituted nitroarenes in para position or bearing sterical-
ly demanding bulky substituents in ortho position showed
excellent conversion affording the corresponding anilines in
up to 99% yield (Table 1, entries 2–4). Moreover, naphtha-
lene- and fluorene-substituted nitroarenes are accessible in
excellent yields, too (Table 1, entries 5 and 6).
I
NO2
12
NO2
CF3
13
Br
NO2
14
>99
>99
>99
>99
91
83
87
83
93
F3C
Table 1 Co3O4/NGr@C-Catalyzed Reduction of Nitroarenesa
NO2
NO2
NO2
15
Entry
1
Substrate
Conv. (%)b Yield (%)b
MeO
NO2
>99
>99
96
91
16
MeS
NO2
2
3
17
O2N
NO2
O
>99
99
18
19
95
NO2
NO2
NO2
>99
>99
>99
65
99
96
4
5
6
89
86
99
92
O
O
NO2
NO2
20
21
>99
>99
Ph
NO2
NO2
NO2
NO2
22
>99
93
7
>99
85
F
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Table 1 (continued)
23
tion, nitroarenes bearing sensitive aldehydes and ketones
were also smoothly reduced with high selectivity affording
the corresponding anilines in 85% and 83% yield, respective-
ly (Table 1, entries 23 and 24). Gratifyingly, carboxylic acid
derivatives, such as esters and nitriles, are also well-tolerat-
ed (Table 1, entries 25 and 26). Indeed, 4-aminobenzoni-
trile, which is a common building block in the chemical in-
dustry,16 is afforded in quantitative yield. Due to the rele-
vance of N-heterocyclic anilines as intermediates in the
synthesis of pharmaceuticals and agrochemicals,17 we also
performed the reduction of different N-heteroaromatic ni-
tro compounds. Notably, the corresponding anilines could
be accessed in up to 93% yield (Table 1, entries 27 and 28).
It should be noted that most of the catalytic experi-
ments were performed on 0.5 mmol scale; however, it was
no problem to run similar experiments on gram scale
(Scheme 2). Obviously, there was no change in the yield
compared to the standard 0.5 mmol reaction.
NO2
>99
>99
>99
85c
H
O
NO2
24
83
O
NO2
25
93 (87)
MeO
O
NO2
26
27
>99
>99
99
55
N
S
NO2
Co3O4/NGr@C catalyst
NO2
NH2
(2 mol% Co)
N
60 bar (CO/N2 = 1:1)
THF–H2O (10:1)
125 °C, 24 h
8.1 mmol
95% yield
91% isolated yield
28
>99
93
N
Scheme 2 Reduction of nitrobenzene on gram scale
NO2
a Reaction conditions: substrate (0.5 mmol), catalyst (20 mg; 2 mol% Co),
THF (2 mL), H2O (0.2 mL), 60 bar (CO/N2 = 1:1), 125 °C, 24 h.
b Determined by GC using n-hexadecane as an internal standard; yield of
isolated product in parentheses.
Finally, the recycling of the catalyst was carried out up
to five times on the model reaction (Figure 1). After the sec-
ond recycling, a gradual decrease of reactivity of the cobalt
catalyst is observed. This loss of activity and the violet-col-
ored filtrate obtained after the first run suggests some co-
balt leaching from the supported material. To rule out that
the catalysis is not a result of these leached cobalt species, a
control experiment was carried out. During the reduction
of nitrobenzene under the optimized conditions the sup-
ported catalyst was removed from the reaction mixture by
filtration at 53% yield of aniline. Then, the filtrate contain-
ing the leached cobalt species was again reacted under oth-
erwise the same reaction conditions (24 h). As expected,
the reaction did not proceed any further. This result clearly
indicates that the reduction process is catalyzed by the
Co3O4/NGr@C-based materials.
c Determined by 1H NMR of the crude mixture after filtering off the hetero-
geneous catalyst and using DMSO as an external standard.
Interestingly, a broad range of industrially relevant ha-
lide-containing substrates were fully converted into the de-
sired haloanilines without any evidence of the undesired
dehalogenation side reactions (Table 1, entries 7–13). Even
aryl halides bearing labile bromides or iodides, which are
easily activated by noble-metal catalysts,5j are well tolerat-
ed (Table 1, entries 11–13). Notably, substrates containing
both electron-deficient, such as trifluoromethyl, and elec-
tron-rich groups, for example, methoxy and thiomethyl, are
equally well transformed into the corresponding anilines
(Table 1, entries 13–16). Moreover, 1,4-diaminobenzene is
easily accessible from the corresponding 1,4-dinitroben-
zene (Table 1, entry 17).
More challenging substrates bearing other reducible
groups were also investigated (Table 1, entries 18–28). In
general, unselective reduction of unsaturated bonds is
problematic in the presence of precious metal catalysts.5j To
our delight, using our cobalt catalyst not only internal, allyl-
ic, and terminal double bonds, but also alkynes are com-
pletely retained in the final anilines, which are afforded in
moderate to excellent yields (65–99%) from their corre-
sponding nitro compounds (Table 1, entries 19–22). In addi-
Figure 1 Recycling experiments of the reduction of nitrobenzene
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In conclusion, for the first time a non-noble metal-
based heterogeneous catalyst has been applied for the gen-
eral reduction of nitroarenes and heteroaromatic nitro
compounds using readily available CO/H2O as the hydrogen
source. This catalytic system shows good functional-group
tolerance towards other easily reducible moieties. Despite
some leaching of cobalt the catalyst can be reused several
times.
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Acknowledgment
This work was supported by the state of Mecklenburg-Vorpommern
and the BMBF.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1380003.
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Into a reaction glass vial fitted with a magnetic stirring bar and
a septum cap penetrated with a syringe needle was added the
Co3O4/NGr@C-catalyst (2 mol%, 3 wt% Co-phenanthroline on
carbon, 20 mg) followed by the nitro arene (0.5 mmol), the
internal standard (hexadecane, 100 μL), THF (2 mL), and H2O
(200 μL). The reaction vial was then placed into a 300 mL auto-
clave. The autoclave was flushed twice with nitrogen, pressur-
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ized with CO at 30 bar pressure. Finally, the autoclave was used
at 60 bar by adding nitrogen and placed into an aluminium
block, which was preheated at 125 °C. After 24 h the autoclave
was placed into a water bath and cooled to r.t. Finally, the
remaining gas was discharged, and the samples were removed
from the autoclave, diluted with EtOAc and analyzed by GC. To
determine the yield of isolated products, the general procedure
was scaled up by the factor of two, and no internal standard was
added. After the reaction was completed, the catalyst was fil-
tered off, and the filtrate was concentrated and purified by silica
gel column chromatography (n-heptane–EtOAc mixtures) to
give the corresponding anilines.
(15) Isolated products of Table 1 have been fully characterized by 1H
NMR and 13C NMR spectroscopy, and analyses are in agreement
with already published data (see Supporting Information).
3-Iodoaniline
Isolated yield 91%. 1H NMR (400 MHz, CDCl3): δ = 7.17–7.07 (m,
2 H), 6.91 (t, J = 7.9 Hz, 1 H), 6.73–6.64 (m, 1 H), 3.67 (br, 2 H).
13C NMR (101 MHz, CDCl3): δ = 147.73, 130.78, 127.46, 123.75,
114.30, 94.97. MS (EI): m/z (rel. int.) = 219.
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Chemistry – Organic Chemicals; McGraw-Hill: New York, 2005.
(17) Marson, C. M. Chem. Soc. Rev. 2011, 40, 5514.
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