Highly efficient synthesis of azos catalyzed by the common metal copper (0) through oxidative coupling reactions

Article abstract of DOI:10.1039/c4ra00749b

A facile and efficient approach to synthesize symmetric, asymmetric and bridged aromatic azo compounds (AAzos) from aromatic amines was developed by using red copper as catalyst. Despite numerous efforts towards the catalytic synthesis of symmetric and asymmetric AAzos derivatives, most reactions present certain drawbacks inhibiting their industrial applications, such as laborious multi-step processes, harsh reaction conditions and expensive reagents. And the synthesis of bridged azos had low yields before. With the presence of ammonium bromide as co-catalyst, pyridine as a ligand and molecular dioxygen as a sole oxidative reagent, red copper, a common and abundant metal in nature, exhibited unexpected catalytic activity towards the preparation of AAzos in high yields via one-step reaction, making this catalyst an attractive candidate for industrial and synthetic applications.

Full text of DOI:10.1039/c4ra00749b

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Highly efficient synthesis of azos catalyzed by the
common metal copper (0) through oxidative
coupling reactions†
Cite this: RSC Adv., 2014, 4, 16607
a
b
a
a
a
a
a
*
Jiaqing Wang, Jing He, Cong Zhi, Bin Luo, Xinming Li, Yue Pan, Xueqin Cao
a
*
and Hongwei Gu
A facile and efficient approach to synthesize symmetric, asymmetric and bridged aromatic azo compounds
(AAzos) from aromatic amines was developed by using red copper as catalyst. Despite numerous efforts
towards the catalytic synthesis of symmetric and asymmetric AAzos derivatives, most reactions present
certain drawbacks inhibiting their industrial applications, such as laborious multi-step processes, harsh
reaction conditions and expensive reagents. And the synthesis of bridged azos had low yields before.
With the presence of ammonium bromide as co-catalyst, pyridine as a ligand and molecular dioxygen as
a sole oxidative reagent, red copper, a common and abundant metal in nature, exhibited unexpected
catalytic activity towards the preparation of AAzos in high yields via one-step reaction, making this
catalyst an attractive candidate for industrial and synthetic applications.
Received 25th January 2014
Accepted 24th March 2014
DOI: 10.1039/c4ra00749b
www.rsc.org/advances
compounds via the reduction of nitroaromatics or oxidation
of aromatic amines. Sakai et al. described that AAzos were
Introduction
obtained from the catalytic hydrogenation of the corre-
sponding nitroaromatics in the InX3–Et3SiH reduction
system.12 Our group developed new strategies based on
Aromatic azo compounds (AAzos) are high-value materials and
are extensively used in the chemical industry as organic dyes,1
food additives,2 indicators,3 and drugs;4 they also have a wide
range of applications in electronics5 and optics,6 and as che-
mosensors7 and diagnostic probes.8 The traditional method for
synthesis of AAzos is based on the azo-coupling reactions with
electron-rich aromatic compounds by the formation of diazo-
nium salts,9 oxidation of hydrazines, reduction of azoxy-
benzenes, coupling of primary anilines with aromatic nitroso
compounds and reductive coupling of nitroaromatics. Addi-
tionally, the formation of asymmetric aromatic azo compounds
requires two step syntheses starting from anilines via diazo-
nium salt or nitrosobenzene. However, all these methods
present certain disadvantages. For example, there is a large
amount of environmentally unfriendly wastes generated from
nitrite salts or nitrosobenzene intermediates during the process
azo coupling.10
nanostructured Pd13 and Pt14 catalysts (120 C and 1 atm H2)
ꢀ
or ligand-free Pd(acac)2 (ref. 15) (70 ꢀC and 1 atm H2) for
green synthesis of AAzos from the corresponding nitro-
aromatics under mild conditions. Corma and coworkers
reported a novel catalytic system using an Au/TiO2 catalyst,
which promotes formation of AAzos through oxidation of the
corresponding amines under an initial 5 atm of O2 with high
selectivity and good yields.16 From the point of view from
green chemistry, green catalysts from common metals have
received great attentions in recent years.17 Jiao's group
reported an approach that the aromatic azo formation could
be achieved through a copper-catalyzed aerobic oxidative
dehydrogenative coupling process in which CuBr acts as
catalyst to facilitate the conversion of anilines to AAzos.18 Zhu
and Shi also demonstrated a method that primary and
secondary anilines can be coupled to corresponding azo
compounds using CuBr catalyst.19 Another Cu(I)-catalyzed
aerobic oxidative reaction of primary aromatic amines was
reported by Lu and Xi using CuCl as catalyst.20 It is worth
mentioning that although Cu(I) presents high catalytic
activity in various reactions, most of Cu(I)-compounds are
expensive, and the chemical instability of them also limits
their further applications. Therefore, it is highly desirable to
develop a facile, green and efficient method for the AAzos
formation.
In recent years, nanostructured materials have become
the focus of researches11 and on the basis of that, many new
methods have been developed for the generation of azo
aKey Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science & Collaborative Innovation Center of
Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China.
E-mail: hongwei@suda.edu.cn; xqcao@suda.edu.cn
bInstrument Department of China Nuclear Power Engineering CO., LTD, Beijing
100840, China
† Electronic supplementary information (ESI) available: Experimental procedures
and full spectroscopic data for aromatic azo products. See DOI:
10.1039/c4ra00749b
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increasing the amount of pyridine. Furthermore, this reaction
could be carried out under air atmosphere (Table 1, entries 12
and 13) at a relatively slow reaction rate.
Results and discussion
Herein, we report a novel, efficient and facile approach to
synthesize both symmetric and asymmetric AAzos compounds
from aromatic amines, by using a commercially available red
copper (copper (0), purity: 99.99%) as catalyst, ammonium
bromide and pyridine as co-catalysts and molecular oxygen as
sole oxidant. Compared with Cu(I), red copper is more inex-
pensive and available. More importantly, the catalyst, red
copper, is quite stable. In our catalytic system, the use of
dioxygen as an ideal oxidant is also green, environmentally
friendly and economical.
To demonstrate the high catalytic ability of red copper in the
preparation of azobenzene, we selected aniline as substrate for
the optimization of reaction conditions (Tables 1 and S1†). Table
1 shows the effect of different amount of red copper employed in
this reaction on the transformation of aniline to azobenzene.
The yields of azobenzene rose gradually with increasing the
amount of red copper (Table 1, entries 3, 5–11), and a complete
conversion was observed when 9 mg (0.14 mmol) or more red
copper were added. Similarly, ammonium bromide, co-catalyst
for this reaction, also exhibited catalytic effect on this
transformation.
The kinetic curve of the transformation of p-toluidine to
bis(4-methylphenyl)diazene, based on GC yields, is shown in
Fig. 1. The reaction was carried out by using p-toluidine
(5 mmol), red copper (ꢁ50 mg, 0.78 mmol), NH4Br (0.49 g,
5 mmol), pyridine (240 mL, 3.0 mmol), and toluene (10 mL) at
ꢀ
100 C under 1 atm of O2. In the rst 6 hours, the yield of the
products was rising gradually with time, and a complete
conversion to azobenzene was observed aer 6 hours, demon-
strating the high efficiency of our catalytic system. In addition,
when the reaction scale was expanded to 10 times larger by
using 1.07 g (10.0 mmol) of p-toluidine, an excellent yield
around 99% was still afforded aer 12 hours, which demon-
strated the high catalytic ability of the system. Optical images of
the reactant before and aer reaction was shown in Fig. 1
(insert). Red copper and NH4Br crystals served as heterogeneous
catalysts and the milky white colour reactant turned to dark red,
which clearly indicated the azo formation.
Fig. 2 showed the scanning electron microscopy images
(SEM) and high resolution SEM images of the red copper
With the increase of the amount of ammonium bromide, the
yields of azobenzene were also increased from 20 to 28% (Table
1, entries 1 and 2). Pyridine, which is a ligand to the formation
of Cu(I)–oxygen complex, acted as an important role in the
reactions. Only trace amount of azobenzene was achieved
without the presence of pyridine in the reaction mixtures (Table
1, entry 4), and the yields of azobenzene increased when
Table 1 The effect of catalysts, co-catalysts, pyridine and gas envi-
ronment on the oxidative coupling of anilinea
Fig. 1 Time–conversion plot for p-toluidine oxidation using red
copper as the catalyst. (Insert: optical images of AAzo formation before
and after reactions using red copper as the catalyst.)
Red copper
(mg)
NH4Br
(mmol)
Pyridine
(mmol)
Yieldb
(%)
Entry
1
2
3
4
5
6
7
8
3 (4.7 mmol%)
3 (4.7 mmol%)
3 (4.7 mmol%)
3 (4.7 mmol%)
4 (6.3 mmol%)
5 (7.9 mmol%)
6 (9.4 mmol%)
7 (11.0 mmol%)
8 (12.6 mmol%)
9 (14.2 mmol%)
10 (15.7 mmol%)
10 (15.7 mmol%)
10 (15.7 mmol%)
0.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.3
0.3
0.6
0
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
20
28
52
3
63
98
98
99
99
100
100
43
90
9
10
11
12c
13d
a
All reactions were carried out with 1 mmol aniline and 2 mL toluene at
100 ꢀC for 24 h under 1 atm of dioxygen. b GC yield. c 24 h under air. d 36
h under air.
Fig. 2 SEM (A and C) and high resolution SEM (B and D) images of red
copper before (A and B) and after reactions (C and D).
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catalysts before (A and B) and aer (C and D) reactions,
respectively. Obviously, the surface of copper became rough
compared with that of the starting copper catalysts, which
indicated the leakage of copper from the red copper catalysts.
The concentration of the copper ions in the reactant was
0.143 mg mLꢂ1, which was examined by atomic absorption
spectroscopy (AAS) analysis.
The proposed mechanism was shown in Fig. 3, which is very
similar with the CuBr–pyridine–O2 system shown in Jiao's
report.18 At 1 atm oxygen environment, the surface of copper can
be partially oxidized to form Cu2O, which can in situ react with
NH4Br to generate CuBr. Activated Cu–oxygen complex was
afforded while Cu(I) was chelated by a pyridine and followed by
oxygen oxidation. At this stage, molecular oxygen was activated,
which is the key catalysts for amine oxidation. Aromatic azos
were nally synthesized by active Cu–oxygen complex via an
oxidative coupling procedure.
In order to further verify the stability and high activity of
this catalytic system, we also prepared Cu nanoparticles
(NPs) (Fig. S1†) and tested their activity for the synthesis of
AAzos. The experimental results (Table S1,† entry 1) shown
that they rarely catalyzed the formation of AAzos from the
corresponding amines, due to high specic surface area of Cu
NPs which were easily oxidized into Cu(II) in the presence of
O2 and NH4Br. Under the optimized conditions for the
formation of azobenzene from aniline, other symmetric
azos were generated from their corresponding amino-
aromatic compounds (Fig. 4, Table S3†). Almost all the para-
substituted reactants were achieved in a quite quantitatively
conversion (Table S3,† entries 1, 4, 6, 8, 10 and 11). Slightly
differently, p-anisidine gave a relatively low yield (Table S3,†
entry 9) at ꢁ28%, due to its low solubility in toluene. Like
p-toluidine, both of m-toluidine and o-toluidine furnished
quite high yields (Table S3,† entries 2 and 3) at >98%. Other
ortho-substituted substrates (Table S3,† entries 5 and 7) gave
Fig. 4 Symmetric aromatic azos formation from different corre-
sponding aromatic amine compounds.
a
lower yield than the corresponding para-substituted
substrates (Table S3,† entries 4 and 6). It was interesting to
nd that Ullman-amination was not observed while using Cl
or Br containing aromatic amine as the starting materials
(Table S3,† entries 4–7).
Fig. 5 Asymmetric aromatic azos formation from different corre-
Fig. 3 Proposed mechanism of azo formation catalyzed by red
sponding aromatic amine compounds.
copper.
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With the purpose of testing the applicability of our catalytic group.21 As displayed in Fig. 6C, Z-bridged azobenzene showed a
system, we investigated its potential for generation of asym- distinctive absorption band at l z 400 nm, while E-bridged
metric azobenzene from corresponding aromatic amines, and azobenzene displayed a absorption peak at l z 500 nm.
the results were summarized in Fig. 5 (Table S4†). These results
In conclusion, we have successfully developed a facile and
demonstrated that asymmetric substituted azobenzenes con- efficient approach for synthesis of symmetric and asymmetric
taining various electro-withdrawing or donating groups can be aromatic azo compounds and a bridged azobenzene by using
constructed efficiently by this simple catalytic system in common red copper as catalyst under mild reaction conditions.
accepted yields.
Easy availability of red copper as catalyst, low reaction
In addition, we surprisingly found that our catalytic system temperature and high catalytic efficiency render this system as
was also suitable for the synthesis of bridged azobenzene (yield: attractive candidates for industrial and synthetic applications.
50%) from its corresponding precursor (Scheme 1). This data Further study of this efficient catalytic system is ongoing in our
has a signicant improvement compared with the results from laboratory to investigate wider applications.
previous reports.21
The bridged azobenzene can exhibit highly efficient revers-
ible Z–E photoisomerization with visible light through ultravi-
Acknowledgements
olet absorption. From Fig. 6, the solution of Z-bridged
H. W. G. acknowledges the nancial support from NSFC (no.
azobenzene was light yellow before irradiation (Fig. 6A).
21003092, no. 21373006), the Key Project of Chinese Ministry of
However, the solution turned to bright red colour (Fig. 6B) upon
Education (no. 211064), and the Priority Academic Program
UV irradiation at l ¼ 365 nm, due to the photoisomerization of
Development of Jiangsu Higher Education Institutions.
molecules from Z-bridged azobenzene to E-bridged azobenzene.
This result was consistent with the data reported from Temps
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