Bismuth nanoparticles: an efficient catalyst for reductive coupling of nitroarenes to azo-compounds

Article abstract of DOI:10.1039/c5ra17994g

The synthesis of azoarenes from corresponding nitroarenes was developed by virtue of in situ bismuth nanoparticles. A series of aromatic azo compounds can be obtained under mild reaction conditions with excellent yields.

Full text of DOI:10.1039/c5ra17994g

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Bismuth nanoparticles: an efficient catalyst for reductive coupling
of nitroarenes to azo-compounds
Received 00th January 20xx,
Accepted 00th January 20xx
Kishore Pothula, Lin Tang, Zhenggen Zha, and Zhiyong Wang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
“Azo” is one of the very important organic functional groups
and particularly, aromatic azo-compounds have wide
applications in academic and industrial research, such as
radical reaction initiators, therapeutic and drug delivery agents,
chelating agents, pigments, organic dyes, food additives and
many others.6 Among the various methods reported in
literature, two methods are prominent for the synthesis of
aromatic azo compounds, (i) coupling of diazonium salts with
electron-rich aromatics7 (ii) metal based reductive coupling of
nitoraromatics by using Bi, Pt, Pd, Ru, In, Mg, Zn, Fe, Nb, Au,
Sm and others.8 Both the methods suffer from either safety
related concerns or tedious work-up process, which still need
to be addressed enough. Recently, oxidative coupling of
arylamines into azos were also reported,9 yet need to be
explored further. Previously Azo-arenes were prepared by
using stoichiometric amounts of Bi metal8a (Scheme 1), but the
method was not suitable for wider application such as
academic/industrial research.
The synthesis of azoarenes from corresponding nitroarenes was
developed by virtue of insitu bismuth nanoparticles. A series of
aromatic azocompounds can be obtained under mild reaction
conditions with excellent yields.
Rather than carbon nanostructures, in recent years
researchers focused on controlled synthesis and study of nano-
structures/properties of transition metals and binary alloys,
such as Au, Ag, Pt, Pd, Co, and Pt/Fe, have been proven to be
successful.1 In contrast, the syntheses of main group metal
nanoparticles are relatively not much studied. Bismuth is the
heaviest main group element, abundant, inexpensive and non-
toxic material. In addition to it, bulk bismuth exhibits
semimetal behaviour with a small band overlap and an
anisotropic electron effective mass. The above unique and
special characteristics of bismuth impressed researchers to
focus on its nano nature and followed by studying its
applications. Previous studies have found that as the size
decreases bismuth could transfer from a semimetal to a
semiconductor.2 over the past two decades, bismuth
nanostructures such as thin films, nanowires, nanolines,
nanobelts, nanopipes, nanotubes and others3 have been
extensively studied.
NO2
Previous work
Present work
0.1 eq of BiNPs,
1.0 eq NaBH4
Ball mill, 4.0 eq Bi
Though extensive research has been carried out on physical
characteristics such as semiconductor and thermoelectric
nature of bismuth, 4 much interest has not yet been directed
on exploration of its chemical properties such as employing
bismuth nanoparticles as a catalyst for the effective chemical
conversions. Our research group is interested in studying
“Nano-metals and exploring their applications in organic
synthesis”, as a course of intended above aim, we have taken
up the current objective of (i) synthesis of bismuth
nanoparticle (BiNPs) from reported literature5 with some
modifications and (ii) evaluation of its catalytic property in the
chemoselective conversion of aryl nitro to aryl azo compounds.
N
N
N
N
Scheme 1 Bismuth catalyzed synthesis of Azobenzene.
As our research group has been focusing on the heterogeneous
catalysis10 for a long time, herein we report an efficient method for
chemo selective preparation of aromatic azo compounds.
In
a typical procedure, 0.5 mmol BiCl3 was suspended in
demineralized water 30 ml. After stirring for about 15 min at room
temperature (RT, ~25-30 °C), the white precipitation was observed.
Then 0.75 mmol of zinc dust was added in portions and the reaction
mixture was further stirred for 1 hours. Black particles were
observed in the reaction. pH was adjusted to 3 with aqueous
hydrochloric acid. The reaction mixture was filtered and washed
with water followed by ethanol then dried under vaccum for 12
a.
Hefei National Laboratory for Physical Sciences at Microscale,CAS Key Laboratory
of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano
Science and Technology.
Deparment of Chemistry, University of Science and Technology of China, Hefei,
b.
230026, P. R. China. Fax: (+86) 551-360-3185; E-mail: zwang3@ustc.edu.cn
† Electronic Supplementary Information (ESI) available: Experimental Details and
characterization of products. See DOI: 10.1039/b000000x/
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hours at 50 °C to obtain BiNPs yield 92 %. The reported procedure
for BiNPs were Bamboo raft nano-tubes, here we got Nano-bismuth
Particles at pH 3.
Reaction conditions were optimized by examining the
influence/impact of mole ratio of catalyst and reducing agent in
various experiments and the results were depicted in Table 1. Apart
from the above conditions, the reaction was also conducted in
water at different temperatures (30, 50 and 80 °C), but the yield is
less than 20% (see the supporting information Table S1). This
indicates that water disfavored this reaction. During the
intermediate stage of the reaction, sample was analysed on GC-
Mass, finding that Nitro, Nitroso, Azoxy and Azo benzenes 10, 30, 9
and 42% respectively. To prove the conceptual bias, few control
experiments (Figure 2) were conducted by converting respective
intermediates (1a and 1b) into azo compound at given conditions.
In terms of these experimental results, the possible mechanism is
depicted in scheme 2.
The bismuth nanoparticles were characterized by X-ray
diffraction (XRD) and Transmission Electron Microscopy (TEM)
techniques. Figure 1a shows the typical powder XRD pattern of the
as-synthesized bismuth nanoparticle. The morphologies and
structures of the as-prepared bismuth nanoparticles were
investigated by TEM images Figure 1b. Diameters of these
nanoparticles varied from 3 to 8 nm.
N
10 mol % BiNPs
1 mmol NaBH4
N
O
N
2a (Yield 96 %)
1a (1 mmol)
Figure 1 (a) XRD and (b) TEM images of Bismuth
O
N
N
N
10 mol % BiNPs
1 mmol NaBH4
The catalytic activity of bismuth nanoparticles in reductive
coupling of nitroarenes in the presence of NaBH4 as reducing agent
and ethanol as a solvent was investigated. The reaction mixture of
nitrobenzene and bismuth nanoparticles was degassed and added
NaBH4/ethanol solution followed by stirring the reaction mixture
for 24 hours. Progress of the reaction mass was monitored by TLC
(mobile phase pet ether/ethyl acetate: 9/1). The catalyst was
filtered and the filtrate was evaporated under reduced pressure.
The obtained product was purified by column chromatography to
afford pure azo-benzene.
N
1b
(1 mmol)
2a (Yield 100 %)
NO2
OH
NH2
OH
10 mol % BiNPs
1 mmol NaBH4
Yield 98 %
1 mmol
Figure 2 Control experiments for reaction mechanism.
Table 1 Optimization of the reaction conditionsa
O
O
Ph
N
Bi Bi
A
1
NO2
N
aBH4
N
Catalyst, Reductant
Solvent
O
N
Bi Bi
Bi
Bi
Ph
Ph
N
N
N
Ph
Ph
B
Ar
N
O
O
2 a
1
2a
Catalyst (equiv.)
Entry
Reductant (equiv.)
Yield (%)b
O
N
Ph
N
O
traces
-
-
1
2
1
-
O
1a
1b
Bi Bi
Bi Bi
1
A
NaB
H4
3
4
5
6
7
8
B
0.5
0.5
1
87
80
92
92
65
68
0.5
N
aB
H4
O
0.2
Bi Bi
Bi Bi
A
0.2
B
1
0.1
1
O
N
0.05
0.05
0.05
Ph
N
O
1
Ph
N
Ph
9
2
68
1b
1a
Scheme 2 Plausible reaction mechanism
aReaction conditions: Nitro benzene (1 mmol), Catalyst BiNPs, Reductant
NaBH4, solvent 4 ml of ethanol, 25-30 °C, 24 h. bYield with reference to
nitrobenzene.
The conversion of the nitro group to the nitroso group is
most likely the key step of this overall transformation. First of
all, nitrobenzene converted to nitrosobenzene 1a which
1
reacts with another molecule of nitrosobenzene and gave
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Table 2 Synthesis of Azo compounds from Nitro Arenes a
azoxybenzene 1b which is further reduced to azo benzene 2a
In each intermediate step, bismuth oxide is formed and then
reconverts to bismuth nanoparticle by sodiumborohydrate
.
B
A
subsequently. Moreover, the electron donating substituent
would facilitate the nucleophilic attachment of the oxygen
atom of the nitro function onto the surface of the bismuth
nanoparticles in a push pull manner, while the electron-
withdrawing group would disfavour such process. When o-
Nitro phenol reacted under the conditions employed, it readily
converted to o-amino phenol (Figure 2) instead azo compound.
Perhaps the eletron-donation of the hydroxyl substituent
promted the reduction of the nitro to amino group while this
orth-substitution blocked the cross-coupling.
The generalization of the scheme is explained by proving its
efficiency on various examples (Table 2) with diversified
substitutions on aromatic ring. The reaction is general and can
be employed for the synthesis of mono substituted halo (2f –
2k), alkoxy (2l – 2n), alkyl (2b – 2e) and di substituted (2p – 2t
)
compounds. The present methodology was successfully
extended to the single step synthesis of long chain Bis-(4-butyl-
phenyl)-diazene (2e) from 4-butyl nitrobenzene. The reaction
was clean and the yield based on conversion was almost
quantitative. The unsatisfactory conversion was observed with
substituted alkoxy group (yield 62 – 68% for compounds 2l
–
2n), but it may also reflect some role played by alkoxy group in
the reduction of nitroarenes on the structured solid surface.
When two different nitroarenes (2-Methyl nitrobenzene and 2-
Chloro nitrobenzene) reacted together in the given conditions,
two symmetrical azo arenenes were formed in an approximate
ratio of 1:1. No unsymmetrical azo arenes were observed
during the reaction. All the compounds obtained were well
1
characterized by H and 13C NMR spectra and melting points.
Few unreported compounds (2r – 2t) were synthesized and
charecterised by NMR spectra, HRMS, IR Spectra and melting
points together.
As this is heterogeneous catalysis reaction, possibility of
recycling of catalyst and reusability followed by its impact on
yield was examined (Table 3). The Yield was good(85%) even
after the 5th run of bismuth nanoparticles.
Table 3 Recycling of BiNPs a
aReaction conditions: Nitro benzene (1 mmol) in 4 ml of ethanol, BiNPs
(10 mol%), Sodium borohydrate (1.0 equiv.), 25-30 °C, 24 h.
In summary a remarkable heterogeneous bismuth catalyst
system was developed successfully and the application of this
catalyst in coupling of aromatic nitro compounds was achieved
to afford the corresponding nitroarenes in excellent yields
under environmentally benign conditions. The reaction has a
broad scope of the reaction substrates, a variety of functional
groups can tolerate the reaction conditions well. Moreover,
a Reaction conditions: Nitro benzene (1 mmol) , BiNPs 10 Mole % (0.1
mol equiv), NaBH4 1 mol equiv in 4 ml of EtOH, 25 °C, 24 h.
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We are grateful to National Nature Science Foundation of
China (21432009, 2127222, 91213303, 21472177 and
CAS-TWAS (Chinese Academy of Sciences (CAS) and The
Academy of Sciences for the developing world (TWAS) for
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