A highly active nano-palladium catalyst for the preparation of aromatic azos under mild conditions

Article abstract of DOI:10.1021/ol202362f

A worm-like Pd nanocatalyst has been prepared and used in the preparation of azo compounds from nitroaromatics under mild reaction conditions. This highly dispersible nano-Pd catalyst shows high activity toward the synthesis of both symmetric aromatic azo compounds and a range of asymmetric aromatic azo compounds.

Full text of DOI:10.1021/ol202362f

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5640–5643
A Highly Active Nano-Palladium Catalyst
for the Preparation of Aromatic Azos
under Mild Conditions
Lei Hu, Xueqing Cao, Linyan Shi, Fenqiang Qi, Zhiqiang Guo, Jianmei Lu,*
and Hongwei Gu*
College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou, China 215123
hongwei@suda.edu.cn; lujm@suda.edu.cn
Received August 31, 2011
ABSTRACT
A worm-like Pd nanocatalyst has been prepared and used in the preparation of azo compounds from nitroaromatics under mild reaction
conditions. This highly dispersible nano-Pd catalyst shows high activity toward the synthesis of both symmetric aromatic azo compounds and a
range of asymmetric aromatic azo compounds.
Aromatic azo (AAzo) compounds are key raw materials
and are widely used in the synthesis of organic dyes, food
additives, indicators, and drugs.1 The industrial produc-
tion of AAzo entails the coupling of diazonium salts with
electron-rich aromatic compounds.2 This process requires
stoichiometric amounts of nitrite salts to form the diazo-
nium salt and generates significant amounts of inorganic
waste. Therefore, a sustainable and industrially applic-
able process for highly selective AAzo formation is of
fundamental and practical interest. In the past, many
efforts have been made to use the catalytic oxidation
of amines3 in AAzo synthesis; however, these approaches
always require environmentally unfriendly transition me-
tals such as lead catalysts. Recently, Merino4 reviews
various synthetic methods on AAzo compounds. Jiao’s
group5 demonstrated a novel approach to the synthesis of
AAzo compounds from the corresponding amines under
mild conditions (60 °C and 1 atm O2) using CuBr and
pyridine as a homogeneous catalyst. Corma and Garcı
´
a6
reported a novel catalytic route applying a Au/TiO2
catalyst for the oxidation of amines at 5 atm of initial
oxygen pressure. Furthermore, they found the Au/TiO2
catalyst could act as a hydrogenation catalyst for the
formation of the amines at 9 atm of hydrogen. The AAzo
(1) (a) Stick, R. V.; Mocerino, M. J. Chem. Educ. 1996, 73, 540.
(b) Hunger, K. Industrial Dyes: Chemistry, properties, Applications;
Wiley-VCH: Weinheim, 2003. (c) Kuwabara, T.; Nakajima, H.; Nanasawa,
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1965, 30, 1203. (b) Wenkert, K.; Wickberg, B. J. Am. Chem. Soc. 1962,
84, 4914. (c) Werkurt, E.; Angell, E. C. Synth. Commun. 1988, 18, 1331.
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ꢀ
M.; Ueno, A. Anal. Chem. 1999, 71, 2844. (d) Orban, M.; Kurin-Cso1rgei,
K.; Zhabotinsky, A. M.; Epstein, I. R. J. Am. Chem. Soc. 1998, 120,
1146. (e) Hoult, J. R. S. Drugs 1986, 32, 18.
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Educ. 1975, 52, 195.
r
10.1021/ol202362f
Published on Web 09/22/2011
2011 American Chemical Society

compounds in these reactions were thus obtained from the
corresponding nitroaromatics through a two-step, one-pot
reaction. The nitroaromatic compound was reduced to
aniline and then oxidized toform the AAzo. In this process,
the AAzo is formed as an unstable intermediate from the
hydrogenation of a nitroaromatic compound and is then
quickly reduced to an amine. To isolate this unstable
intermediate, transition metals7 or a complicated reductive
system8 have been used to obtain the AAzo from the
corresponding nitroaromatic compound.
spectrum of the catalyst at the palladium 3d level indicated
that most of the palladium was present in the reduced form
(Figure S2).
Numerous studies have been devoted to the properties
which affect the catalytic activity and selectivity of nano-
catalysts. The shape of the nanomaterials has been studied
most widely and many groups have reported that nano-
wires or multipots have much higher catalytic activity than
nanoparticles.9 Palladium has long been one of the most
useful catalysts in hydrogenation and coupling reactions,
and many efforts have been put into developing novel Pd
based nanomaterials with higher catalytic activity.10 Here-
in, we have synthesized a worm-like Pd nanomaterial
and used it as the catalyst for the preparation of AAzo
compounds from nitroaromatics under mild reaction
conditions.
Figure 1. (A, B) TEM, (C) SAED, and (D) high resolution TEM
images of the worm-like Pd.
The worm-like Pd catalyst was prepared following a
reported methodology with minor modifications.9d,11 Pd-
(CH3COO)2, n-hexadecyl trimethyl ammonium bromide,
and 1-dodecylamine were dissolved in toluene. Reduction
was then performed by the dropwise addition of a freshly
prepared NaBH4 solution followed by stirring for 1 h at rt.
The worm-like Pd was isolated by precipitation with
ethanol and centrifuging at 8000 rpm for 10 min. Figure
1 A and B show the transmission electron microscopy
(TEM) images of this worm-like Pdcatalyst. The diameter
of the Pd catalyst was about 3.5 nm with a narrow size
distribution. Selected area electron diffraction (SAED)
indicates the worm-like Pd has a face-centered cubic
(FCC) phase (Figure 1C) which is confirmed by powder
X-ray diffraction (Figure S1). Figure 1D shows the high-
resolution TEM image of the worm-like Pd and the lattice
fringe with an interplanar spacing of 0.23 nm, assigned to
the (111) plane of FCC Pd. The X-ray photoelectron
Table 1. Azobenzene Formation from Nitrobenzene in Differ-
ent Solventsa
selectivity (%)b
temp
conv
(%)b
entry
solvent
(°C)
Azo-
Azoxy-
aniline
1
o-xylene
m-xylene
p-xylene
toluene
120
120
120
100
100
100
80
92.8
100
100
100
99
24.4
80.0
87.4
33.4
52.6
9.9
58.8
1.4
8.3
18.6
12.6
10.6
3.5
2
3c
4
0
56.0
40.6
78.7
67.1
77.7
61.9
83.7
5
n-heptane
dioxane
(7) (a) Srinivasa, G. R.; Abiraj, K.; Gowda, D. C. Synth. Commun.
2003, 33, 4221. (b) Ohe, K.; Uemura, S.; Sugita, N.; Masuda, H.; Taga,
T. J. Org. Chem. 1989, 54, 4169. (c) Srinivasa, G. R.; Abiraj, K.; Channe
Gowda, D. Tetrahedron Lett. 2003, 44, 5835.
(8) (a) Sakai, N.; Fujii, K.; Nabeshima, S.; Ikeda, R.; Konakahara, T.
Chem. Commun. 2010, 46, 3173. (b) Zhu, H.; Ke, X.; Yang, X.; Sarina,
S.; Liu, H. Angew. Chem., Int. Ed. 2010, 49, 9657.
6
32.6
>99
>99
40.4
86.5
5.3
7
ethanol
27.9
2.2
4.8
8
2-propanol
acetonitrile
H2O
80
18.9
24.3
15.1
9
80
4.0
10
80
1.2
(9) (a) Hu, B.; Ding, K.; Wu, T.; Zhou, X.; Fan, H.; Jiang, T.; Wang,
Q.; Han, B. Chem. Commun. 2010, 46, 8552. (b) Qin, G. W.; Pei, W.; Ma,
X.; Xu, X.; Ren, Y.; Sun, W.; Zuo, L. J. Phys. Chem. C 2010, 114, 6909.
(c) Hong, H.; Hu, L.; Li, M.; Zheng, J.; Sun, X.; Lu, X.; Cao, X.; Lu, J.;
Gu, H. Chem.;Eur. J. 2011, 17, 8726. (d) Hu, L.; Shi, L.; Hong, H.; Li,
M.; Bao, Q.; Tang, J.; Ge, J.; Lu, J.; Cao, X.; Gu, H. Chem. Commun.
2010, 46, 8591. (e) Li, M.; Hu, L.; Cao, X.; Hong, H.; Lu, J.; Gu, H.
Chem.;Eur. J. 2011, 17, 2763.
a All reactions were carried out with 1 mg of nano-Pd catalyst,
1 mmol of nitrobenzene, 1 equiv of KOH, and 2 mL of solvent at the
appropriate temperature for 24 h under 1 atm of hydrogen. b GC yield.
c 7 h. All reactions were exposed to air at 120 °C for 2 h.
To demonstrate the high catalytic activity of this Pd
nanocatalyst in the preparation of AAzo compounds,
nitrobenzene was selected as the substrate for the optimi-
zation of conditions. Table 1 shows the reduction and
coupling of nitrobenzene to form azobenzene (Azo-) and
azoxybenzene (Azoxy-). Comparison of 10 commonly
(10) (a) Huang, X.; Zheng, N. J. Am. Chem. Soc. 2009, 131, 4602.
(b) Yuan, Q.; Zhuang, J.; Wang, X. Chem. Commun. 2009, 43, 6613.
(c) Wang, J.; Chen, Y.; Liu, H.; Li, R.; Sun, X. Electrochem. Commun.
€
2010, 12, 219. (d) Teng, X.; Han, W. -Q.; Ku, W.; Hucker, M. Angew.
Chem., Int. Ed. 2008, 47, 2055. (e) Liang, H. -W.; Liu, S.; Gong, J. -Y.;
Wang, S. -B.; Wang, L.; Yu, S.-H. Adv. Mater. 2009, 21, 1850.
(11) Yang, J.; Sargent, E. H.; Kelley, S. O.; Ying, J. Y. Nat. Mater.
2009, 8, 683.
Org. Lett., Vol. 13, No. 20, 2011
5641

used solvents showed p-xylene to be the best solvent for
the formation of Azo- (87.4% yield, Table 1, entry 3).
Nonpolar solvents like xylene, toluene, and heptane
also encourage Azo- formation (Table 1, entries 1ꢀ2
and 4ꢀ5). Azoxy- can be reduced to Azo- by prolonging
the reaction time in these solvents (Table S1, entries
1ꢀ4). The polar solvents encourage Azoxy- formation
(Table 1, entries 6ꢀ10) and hinder further reduction to
Azo- (Table S1, entries 5ꢀ9).
nitroaromatic compounds also formed the corresponding
AAzo compounds in useful yields (Table 3, entries 7ꢀ10).
Furthermore, asymmetrical AAzo were successfully
synthesized with good yields while using two different
substituted nitrobenzenes as reactants (Table 4, entries
1ꢀ3).
Table 3. Aromatic Azos Formation from Different Nitro-
Compoundsa
Table 2. Azobenzene Formation from Nitrobenzene Using
Different Basesa
selectivity (%)b
base
temp conv
entry
(equiv)
(h)
(%)b Azo- Azoxy- aniline
1
none
24
24
24
24
24
18
13
7
100
0
0
28.8
60.6
39.8
0
92.4
7.0
2
NaOH (1)
(CH3)3CONa (1)
K2CO3 (1)
(C2H5)3N (1)
KOH (0.25)
KOH (0.5)
KOH (1)
100 64.2
100 28.6
3
10.2
56.6
>99
21.7
8.8
4
47.3
22.4
3.6
0
5
6
100 74.1
100 80.1
100 87.4
100 85.8
100 76.8
100 75.4
0
7
0
8
0
12.6
14.2
20.5
22.2
9
KOH (2)
6
0
10
11
KOH (2)
3
0
KOH (4)
1.5
0
a All reactions were carried out with 1 mg of nano-Pd catalyst,
1 mmol of nitrobenzene, base, and 2 mL of p-xylene at 120 °C for the
appropriate time under 1 atm of hydrogen. b GC yield. All reactions were
exposed to air at 120 °C for 2 h.
Further investigation into the reaction parameters re-
vealed that the catalytic activity and selectivity were sensi-
tive to the base employed in this reaction. Table 2 shows
the results of this reaction with and without a base. With-
out a base, aniline was the main product and no Azo- was
detected by GC (Table 2, entry 1). Five different bases were
used to investigate the selectivity of this reaction. KOH
was the most effective for the formation of azobenzene
(Table 2, entries 2ꢀ5, 8). Further optimization revealed
that superior yields (87.4%) of Azo- were achieved when 1
equiv of KOH was used for 6 h at 120 °C (Table 2, entry 8).
When an excess of KOH was used, the azobenzene was
obtained faster, but with less selectivity (Table 2, entries
9ꢀ11). Decreasing the amountof KOH to 0.5 or0.25equiv
resulted in Azo- only being observed after longer reaction
times (24 h) (Table 2, entries 6 and 7).
Having optimized the catalytic system and reaction
conditions for the formation of azobenzene from nitro-
benzene, other nitroaromatic compounds were investi-
gated as substrates. Three nitrotoluenes were investigated
with p-nitrotoluene giving the highest yield and o-nitroto-
luene requiring a higher initial hydrogen pressure to obtain
the corresponding AAzo (4 atm) (Table 3, entries 1ꢀ3).
Electron-rich nitroaromatic compounds gave the corre-
sponding AAzo in good to excellent yields (66ꢀ83%) in
these conditions (Table 3, entries 4ꢀ6). The electron-poor
a All reactions were carried out with 1 mg of nano-Pd catalyst,
1 mmol of nitroaromatic, 1 mmol of KOH, and 2 mL of p-xylene at
120 °C for 24 h under 1 atm of hydrogen. b Isolated yield. c 4 atm of H2.
d 4 atm of H2, 40 °C, CHCl3 as the solvent. e 80 °C, water as the solvent.
All reactions were exposed to air at 120 °C for 2 h.
The transformation from nitrobenzene to azobenzene
was investigated over the time of the reaction based on GC
yields (Figure 2). The amount of azoxybenzene was in-
creased to a maximum at 1.5 h with the nitrobenzene being
quickly consumed in the reaction. As the reaction pro-
ceeded, more azoxybenzene was reduced to azobenzene,
with the quantity of aniline remaining the same. Using this
catalyst system, excellent yields of azobenzene can be
achieved in 7 h.
(12) (a) Pizzolatti, M. G.; Yunes, R. A. J. Chem. Soc., Perkin Trans. 2
1990, 759. (b) Khan, F. A.; Dash, J.; Sudheer, C.; Gupta, R. K. Tetra-
hedron Lett. 2003, 44, 7783. (c) Zhao, F.; Ikushima, Y.; Arai, M. J. Catal.
2004, 224, 479. (d) Agrawal, A.; Tratnyek, P. G. Environ. Sci. Technol.
1996, 30, 153. (e) Shernyakin, M. M.; Maimind, V. I.; Vaichunaite, B. K.
Russ. Chem. Bull. 1957, 6, 1284. (f)Russell, G. A.; Geels, E. J.; Smentowski,
F. J.; Chang, K. -Y.; Reynolds, J.; Kaupp, G. J. Am. Chem. Soc. 1967, 89,
3821.
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Org. Lett., Vol. 13, No. 20, 2011

further reduced to azobenzene and then hydrazobenzene.
This product spontaneously oxidizes to azobenzene in air.
As a result, azobenzene is the only observed coupling
product via GC analysis. In addition, a serial experiment
detailing the transformations of azoxybenzene, azoben-
zene, and hydroazobenzene was performed. Hydrogena-
tion of azoxybenzene can form azobenzene in high yield
(Table S3, entry A) which is then reduced to form hydra-
zobenzene by extending the reactiontime(Table S3, entries
B). Hydrazobenzene can be quantitatively converted back
to azobenzene while heating in air for 2 h at 120 °C. Even in
the absence of the Pd catalyst, hydrazobenzene was con-
verted to azobenzene in a yield of 94.7% in 12 h at 120 °C.
The oxidization of azobenzene to azoxybenzene was found
to be less facile (Table S3, entry C). Overall, azobenzene is
the major product from these transformations.
Table 4. Formation of Asymmetric Azobenzenes from an
Equimolar Mixture of Nitroaromaticsa
entry
R1
R2
time (h)
yield (%)b
1
2
3
COCH3
CH3
H
24
36
36
55.3
42.4
70.1
OCH3
H
COOH
a All reactions were carried out with 1 mg of nano-Pd catalyst,
0.5 mmol of nitroaromatics, 1 mmol of KOH, and 2 mL of p-xylene at
120 °C for the appropriate time under 1 atm of hydrogen. b Isolated
yield. All reactions were exposed to air at 120 °C for 2 h.
Scheme 1. Proposed Mechanism for the Hydrogenation of
Nitrobenzene
In conclusion, we have developed a novel worm-like Pd
nanomaterial which shows high catalytic activity toward
the formation of AAzo directly from the corresponding
nitroaromatic compounds. The reaction is simple and
efficient and does not need harmful transition metals
which makes this AAzo preparation much easier and more
environmentally friendly. Further study of this catalytic
system for wider application is under investigation in our
laboratory.
Figure 2. Timeꢀconversion plot for nitrobenzene hydrogena-
tion using nano-Pd as the catalyst.
The proposed mechanism for the azobenzene formation
from nitrobenzene is shown in Scheme 1. Hydrogen is first
adsorbed on the surface of nano-Pd and nitrobenzene is
reduced to nitrosobenzene, which in turn is quickly con-
verted to N-hydroxybenzenamine. The N-hydroxybenze-
namine can be further reduced to aniline under acidic or
neutral conditions and acts as a sink toward AAzo forma-
tion. However, when KOH is added, the N-hydroxyben-
zenamine primarily couples to nitrosobenzene to form N,
N0-dihydroxy-diphenylhydrazine.12 This dihydroxy inter-
mediate is then dehydrated to give azoxybenzene. It is
possible to interrupt the reaction at this point if azoxyben-
zene is the desired product. Azoxybenzene can also be
Acknowledgment. H.W.G. acknowledges financial sup-
port from the National Natural Science Foundation of
China (No. 21003092), the Key Project of Chinese Minis-
try of Education (No. 211064). L.H. acknowledges finan-
cial support from scientific innovation research of college
graduate in Jangsu province (CXZZ11_0102).
Supporting Information Available. Experimental pro-
cedures and full spectroscopic data for all compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 20, 2011
5643