Highly selective transfer hydrogenation of functionalised nitroarenes using cobalt-based nanocatalysts

Article abstract of DOI:10.1039/c4gc00731j

Anilines are important feedstock for the synthesis of a variety of chemicals such as dyes, pigments, pharmaceuticals and agrochemicals. The chemoselective catalytic reduction of nitro compounds represents the most important and prevalent process for the manufacture of functionalized anilines. Consequently, the development of selective catalysts for the reduction of nitro compounds in the presence of other reducible groups is a major challenge and is crucial. In this regard, herein we show that the cobalt oxide (Co₃O₄-NGr@C) based nano-materials, prepared by the pyrolysis of cobalt-phenanthroline complexes on carbon constitute highly selective catalysts for the transfer hydrogenation of nitroarenes to anilines using formic acid as a hydrogen source. Applying these catalysts, a series of structurally diverse and functionalized nitroarenes have been reduced to anilines with unprecedented chemo-selectivity tolerating halides, olefins, aldehyde, ketone, ester, amide and nitrile functionalities.

Full text of DOI:10.1039/c4gc00731j

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Table of Contents (TOC) graphic
Highly Selective Transfer Hydrogenation of Functionalysed Nitroarenes Using Cobalt-
based Nanocatalysts
Rajenahally V. Jagadeesh, Debasis Banerjee, Percia Beatrice Arockiam, Henrik Junge, Kathrin Junge, Marga-Martina Pohl,
Jörg Radnik, Angelika Brückner, Matthias Beller*
The functionalized anilines have been synthesized by the transfer hydrogenation of nitroarenes using cobalt oxide-based nanocatalysts.

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ARTICLE
Highly Selective Transfer Hydrogenation of
Functionalysed Nitroarenes Using Cobalt-based
Nanocatalysts
Cite this: DOI: 10.1039/x0xx00000x
±
±
Rajenahally V. Jagadeesh , Debasis Banerjee , Percia Beatrice Arockiam, Henrik
Junge, Kathrin Junge, MargaꢀMartina Pohl, Jörg Radnik, Angelika Brückner, and
Matthias Beller*
Anilines are important feedstock for the synthesis of variety of chemicals such as dyes,
pigments, pharmaceuticals and agrochemicals. The chemoselective catalytic reduction of nitro
compounds represents the most important and prevalent process for the manufacture of
functionalized anilines. Consequently, the development of selective catalysts for the reduction
of nitro compounds in presence of other reducible groups is a major challenge and crucial. In
this regard, herein we show that the cobalt oxide (Co O ꢀNGr@C) based nanoꢀmaterials,
Received --- ----- 2014,
Accepted ---- 2014
3
4
prepared by the pyrolysis of cobaltꢀphenanthroline complexes on carbon constitute highly
selective catalysts for the transfer hydrogenation of nitroarenes to anilines using formic acid as
hydrogen source. Applying these catalysts, a series of structurally diverse and functionalized
nitroarenes have been reduced to anilines with unprecedented chemoꢀselectivity tolerating
halides, olefins, aldehyde, ketone, ester, amide and nitrile functionalities.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
can be easily recycled, which allows for costꢀeffective and
sustainable synthetic processes.
Structurally diverse anilines constitute major building blocks
and key intermediates for the production of pharmaceuticals,
drugs, dyes, pigments and agrochemicals. Generally, the
selective reduction of nitroarenes represent a straightforward
and versatile methodology for their synthesis.
Based on our experience using organometallic complexes in
6
the presence of nitrogen ligands, recently we developed novel
1
heterogeneous catalysts by supporting metalꢀnitrogen
7
complexes on solid supports through pyrolysis.
previous works,
In our
2
ꢀ4
Recently,
7a,b
we demonstrated that pyrolysis of cobaltꢀ
precious metalꢀbased nanomaterials have been used for the
reduction of nitro compounds using various reducing agents.
phenanthroline complexes on carbon leads to the formation of
nanoꢀscale Co O particles surrounded by a modified nitrogenꢀ
5
3
4
However, due to high price and toxicity of these precious
metals, the catalysts based on bioꢀrelevant metals or highly
preferable for this reaction. Particularly, anilines used in life
science applications are substituted with various functional
groups. Therefore, the chemoselective reduction of the nitro
group in presence of other reducible moieties is crucial. In this
respect, the development of highly selective and efficient
catalysts is essential. Advantageously, heterogeneous catalysts
doped graphene layer (NGr) (Co O ꢀNGr/C). Notably these
materials are active for both hydrogenation and oxidation 7
processes. Here, for the first time we show that these materials
represent excellent catalysts for the selective transfer
hydrogenation of all kinds of functionalized nitroarenes to
anilines using formic acid as hydrogen source.
3
4
7a
b,c
Complementary to reductions with molecular hydrogen are
8
transfer hydrogenations (CTH) using formic acidꢀEt N or
3
isopropanolꢀbase systems. Advantageously, CTH reactions do
not require any special experimental setup or the use of highꢀ
pressure equipment compared to conventional hydrogenation
9
methods. Notably, formic acid or formate, as hydrogen donors
are abundant, inexpensive and easy to handle.
.
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Results and Discussion
1
. Preparation and activity of Co O -NGr@C materials. As
3 4
7
a,b
developed previously,
nanoscale Co O4 catalysts were
3
prepared by pyrolysis of in situꢀgenerated phenanthrolineꢀ
cobalt (II)ꢀacetate on Vulcan XC72R carbon powder (see
Supporting Information). Preliminary catalytic experiments
were carried out applying the transfer hydrogenation of
nitrobenzene as a benchmark substrate. As seen from Table 1,
neither the homogeneous cobalt complex nor the supported
material was active (Table 1, entries 1ꢀ3). Similarly, pyrolysis
of simple cobalt acetate on carbon did not generate an active
catalyst (Table 1, entry 4). However, pyrolysis of the ligated
Co(OAc) on carbon at 800 °C for 2 h under argon atmosphere
2
led to an active catalytic material. Hence, the model reaction
proceeded with complete conversion and gave aniline in 96%
yield (Table 1, entry 5).
Table 1. Transfer hydrogenation of nitrobenzene with cobalt
a
catalysts .
Entry Catalyst
Pyrolysis
Conv. Yield
o
(
C, h, Ar)
(%)
2
(%)
<1
<1
<1
3
1
2
3
4
5
6
Co(OAc) .4H O
2 2
Co(OAc) ꢀPhen
2
2
Co(OAc) ꢀPhen/C
2
2
o
Co(OAc) .4H O/C 800 C, 2h,
5
2
2
o
Co(OAc) ꢀPhen/C 800 C, 2h,
>99
1
96
<1
2
o
Phen/C
800 C, 2h,
[
a] Reaction conditions: 1 mmol nitrobenzene, 20 mg catalysts (2
mol% Co), 3.5 mmol (3.5 equiv.) of HCOOH, 1.4 mmol of Et N
3
(
HCOOHꢀEt N (5:2) mixture), 5 mL THF, 15 h, conversions and
3
yields were determined by GC using nꢀhexadecane as internal
standard.
The active catalytic material, Co O ꢀNGr/C has been
3
4
7
a,b
characterized by TEM, XPS, EPR and XRD spectral analysis
see Supporting Information). All these studies revealed the
preferential formation of small (2ꢀ10 nm) Co O particles on a
(
3
4
nitrogenꢀdoped carbon surface. Besides, a few larger particles
of 20ꢀ80 nm and occasionally up to 800 nm size are also
present, which consist of a Co and/or CoO core and a Co O
3
4
shell. These cobalt oxide particles are in close contact to the N
atoms of the modified outermost layers of the graphite support.
2
.
Chemo-selective
transfer
hydrogenation
of
functionalized and challenging nitroarenes. The general
applicability of the transfer hydrogenation process using the
active Co O ꢀNGr/C has been demonstrated applying
3
4
structurally diverse functionalized nitro compounds under
optimized conditions. As seen from Schemes 1 and 2 more than
5
0 functionalized and challenging nitroarenes have been
reduced to the corresponding anilines in excellent yields and
selectivities.
2
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DOI: 10.1039/C4GC
A
00
R
7
T
3
I
1
C
J
LE
3
.
Synthesis of amino-heterocycles. Finally, transfer
hydrogenations of heterocyclic nitro compounds were performed.
The aminoꢀsubstituted Nꢀheterocycles, which are important
intermediates in pharmaceutical and agrochemical industries, were
also obtained in good to excellent yields (Scheme 3).
Scheme 2. Co O -N/C catalyzed transfer hydrogenation of nitro-
3
4
heterocycles.Reaction conditions: 1 mmol nitro compound, 20
mg catalyst (1 mol% Co). 3.5 mmol (3.5 equiv.) of HCOOH,
1
1
.4 mmol of Et N (HCOOHꢀEt N (5:2) mixture), 5 mL THF,
00 °C, isolated yields, *GC yields using nꢀhexadecane as
3 3
internal standard.
4
. Catalyst recycling. In a final set of catalytic experiments,
we examined the stability and reusability of the Co O ꢀNGr/C
3
4
catalysts. Indeed the catalyst has been easily recycled and
reused up to 6 cycles without significant loss of catalytic
activity. In addition, to prove the stability of the material, 20
mg of catalyst along with THF was heated at 100 °C. Then, the
hot suspension was quickly filtered. Performing standard
catalytic tests (1 mmol nitrobenzene) using the hot filtrate
Scheme 1. Co O -NGr/C-catalyzed transfer hydrogenation of
3
4
a
a
functionalized nitroarenes . Reaction conditions: 1 mmol
nitroarene, 20 mg catalyst (2 mol% Co). 3.5 mmol (3.5 equiv.)
of HCOOH, 1.4 mmol of Et N (HCOOHꢀEt N (5:2) mixture),
3
3
5
mL THF, 100 °C. Conversions and yields were determined by
b
GC using nꢀhexadecane as internal standard. Isolated yields.
c
a
solution and HCOOH/Et N (5:2) did not result in any activity
3
Same as at 110 °C.
and the formation of aniline was not observed. This clearly,
indicates that, the active metal species have not been leached
out from the support during the reaction and remains bound to
the support.
From a synthetic point of view it is important to note that this
cobaltꢀbased catalyst system is highly selective for the transfer
hydrogenation of nitro groups in the presence of many other
easily reducible moieties. Functional groups such as aldehyde,
ketone, nitrile, ester, amide, and olefins are well tolerated
without being significantly reduced. Notably, aminoꢀsubstituted
benzonitriles common building blocks for the chemical industry
are obtained in up to 93 % yield (Scheme 1). Interestingly,
various esterꢀ, amideꢀ and sulphonamide based anilines, which
constitute subunits of biologically and pharmacologically
important compounds are obtained in 89ꢀ98% yields (Scheme
100
90
80
7
0
0
6
50
0
1
2
3
4
5
6
1
). Likewise, halogenated anilines, which are used as
Recycle time
precursors for agrochemicals, were obtained in good to
excellent yields. Noteworthy, the sensitive iodoꢀnitroarenes
were hydrogenated to the corresponding anilines in high yields
Figure 1. Transfer hydrogenation of nitrobenzene to aniline:
Recycling of Co O -NGr@C. Reaction conditions: 5.0 mmol
nitrobenzene, 3.5 mmol (3.5 equiv.) of HCOOH, 1.4 mmol of
Et N (HCOOHꢀEt N (5:2) mixture), 100 mg catalyst (2 mol%
(
95ꢀ96 %), too, and even diꢀ, triꢀ and pentaꢀhalogenated
nitroarenes produced the corresponding halogenated anilines in
6ꢀ97 % (Scheme 1).
3
4
8
3
3
Co), 13ꢀ15h, 10 mL THF, yields were determined by GC using
nꢀhexadecane as internal standard.
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Experimental Section
3
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(
.
2
0 mg Co O ꢀNGr/Cꢀcatalyst (2 mol% Co) and 5 mL dry THF
3 4
2
1
were added to oven dried pressure tube (ACE). Then,
corresponding nitro compound (1 mmol), 3.5 mmol (3.5 equiv.)
of HCOOH, 1.4 mmol of Et N (HCOOHꢀEt N (5:2) mixture)
,
3
3
and 100 ꢁL nꢀhexadecane as internal standard were added
sequentially. The pressure tube was flushed with argon, fitted
with screw cap and the reaction was allowed to progress at 100
9
,
0
C for desired time. After completion of the reaction, the
,
reaction mixture was cooled to room temperature and diluted
with ethyl acetate. Then, catalyst was filtered off, and the
sample of the mixture was directly subjected to GC analysis.
Conversion and yields were determined by GCꢀFID (HP6890
with FID detector, column HP530 m x 250 mm x 0.25 ꢁm).
Quantitative and qualitative analysis of anilines were made by
GC, GC–MS analysis. For isolation of anilines, the reactions
were performed without adding nꢀhexadecane. After
completion of the reaction, the catalyst was filtered off, washed
with ethyl acetate and the filtrate containing reaction product
was concentrated. The corresponding aniline was purified by
column chromatography (silica; nꢀhexaneꢀethyl acetate
mixture). The collected fractions were dried over anhydrous
Na SO . Then, solvent was removed in vacuo and finally the
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2
4
product was dried.
Conclusions
5
6
7
For recent examples using nanomaterials see: (a) M. B. Gawande, H.
Guo, A.K. Rathi, P. S. Branco, Y. Chen, R.S. Varma and D.ꢀL. Peng,
In conclusion, stable, inexpensive and reusable cobalt oxideꢀ
based nanoꢀmaterials are found to be highly active and selective
catalysts for the chemoꢀselective transfer hydrogenation of
nitroarenes to anilines. The active Co O ꢀNGr/C nanocatalysts
RSC Advances, 2013,
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3
4
6
have been prepared by pyrolysis of aminoꢀligated cobalt (II)
acetate on commercial Vulcan XC72R. Excellent chemoꢀ
selectivity was demonstrated applying nitroarenes with olefin,
aldehyde, keto, ester, amide and nitrile functionalities.
S. Jeong, B. M. Kim, Catalysis Communications, 2014 , 45, 25; (e)
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Acknowledgements
(a) R. V. Jagadeesh, G. Wienhöfer, F. A. Westerhaus, A.ꢀE. Surkus,
H. Junge, K. Junge and M. Beller, Chem. Eur. J. 2011, 17, 14375;
This work has been supported by the State of Mecklenburgꢀ
Vorpommern, and the BMBF (Bundesministerium für Bildung
und Forschung).
(
b) D. Banerjee, R. V. Jagadeesh, H. Junge, K. Junge and M. Beller,
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