Gold-catalyzed direct hydrogenative coupling of nitroarenes to synthesize aromatic azo compounds

Article abstract of DOI:10.1002/anie.201404543

The azo linkage is a prominent chemical motif which has found numerous applications in materials science, pharmaceuticals, and agrochemicals. Described herein is a sustainable heterogeneous-gold-catalyzed synthesis of azo arenes. Available nitroarenes are deoxygenated and linked selectively by the formation of N-N bonds using molecular H₂ without any external additives. As a result of a unique and remarkable synergy between the metal and support, a facile surface-mediated condensation of nitroso and hydroxylamine intermediates is enabled, and the desired transformation proceeds in a highly selective manner under mild reaction conditions. The protocol tolerates a large variety of functional groups and offers a general and versatile method for the environmentally friendly synthesis of symmetric or asymmetric aromatic azo compounds.

Full text of DOI:10.1002/anie.201404543

Angewandte
Chemie
DOI: 10.1002/anie.201404543
Supported Catalysts
Gold-Catalyzed Direct Hydrogenative Coupling of Nitroarenes To
Synthesize Aromatic Azo Compounds**
Xiang Liu, Hai-Qian Li, Sen Ye, Yong-Mei Liu, He-Yong He, and Yong Cao*
Abstract: The azo linkage is a prominent chemical motif which
has found numerous applications in materials science, phar-
maceuticals, and agrochemicals. Described herein is a sustain-
able heterogeneous-gold-catalyzed synthesis of azo arenes.
Available nitroarenes are deoxygenated and linked selectively
this approach has delivered many useful transformations,[5]
the development of an inherently clean and efficient process
for step-economic construction of complex molecules, which
are often elusive through classical catalytic systems, has
remained a formidable challenge.
=
=
by the formation of N N bonds using molecular H2 without
The azo moiety (-N N- bond) is a common functionality
any external additives. As a result of a unique and remarkable
synergy between the metal and support, a facile surface-
mediated condensation of nitroso and hydroxylamine inter-
mediates is enabled, and the desired transformation proceeds
in a highly selective manner under mild reaction conditions.
The protocol tolerates a large variety of functional groups and
offers a general and versatile method for the environmentally
friendly synthesis of symmetric or asymmetric aromatic azo
compounds.
in dyes and pigments (represents the most widely used and
structurally diverse class of synthetic organic colorants by
far),[6a] pharmaceuticals, food additives, and in many modern
material science applications, including liquid crystal dis-
plays,[6b] optical storage media,[6c] and radical reaction initia-
tors. The high value of the azo motif is reflected in the myriad
strategies for its construction, the majority of which are the
coupling reaction of diazonium salts with electron-rich
aromatic compounds.[7] Nonetheless, it is widely appreciated
that there remains a need for azo syntheses which are more
concise, more selective, more versatile, and complementary to
conventional routes.[7] In response to this need, much effort
has been devoted to direct reductive coupling of nitroarenes,
an approach which is of great importance because of the
availability of substrates, the tolerance of a wide range of
functionalities, and a simple one-pot procedure.[8–10] Although
extensively studied, delivery of an efficient protocol using
molecular H2 as the ideal green reductant remains nontrivial.
In particular, the inherent complexity of the multistep nitro
reduction networks (see Scheme S1 in the Supporting Infor-
mation)[11] must be tackled. Moreover, the desired level of
selectivity is mostly achieved by the introduction of copious
amounts of a base to suppress the unwanted side reactions
catalyzed by PGMs.[12]
Thanks to the discovery that supported gold nanoparticles
(NPs) are capable of uniquely facilitating the direct nitro-
nitroso-hydroxylamine pathway, it is now possible for target-
specific synthesis of substituted anilines and related deriva-
tives by using catalytic nitro-group hydrogenation in a highly
regio- and chemoselective manner.[13] Based on the fact that
the azo compounds can be selectively synthesized through
aerobic oxidative coupling of anilines,[7] an innovative inte-
grated two-step process based on an initial nitro hydro-
genation and subsequent oxidation have been proposed to
access azo compounds directly from nitroarenes.[14] One
major limitation of this procedure, however, is that it is
potentially dangerous when applied to large-scale commercial
applications. An industrial-friendly process for one-step nitro
to azo conversion is thus of tremendous fundamental, as well
as practical interest.[15] In view of the likely intermediacy of
the azo compound in the selective hydrogenation of nitro-
arenes, and considering that the selectivity of the nitro
reduction might be tuned in favor of azo products by rational
regulation of the synergy between the metal and support in
Catalysis by supported gold has been intensively investi-
gated in recent years, thus providing distinct reactivity,
activity, and selectivity, all of which complement traditional
platinum-group-metal (PGM) catalysis.[1] Gold has long been
regarded to be catalytically inert. However, ever since the
breakthrough discovery by Hutchings[2a] and Haruta et al.[2b]
in the 1980s, it has become clear that gold, in its nanoscale
form, is an active and often more selective catalyst than
PGMs for a variety of reactions.[3] The activity of gold is
typically attributed to a size effect, although in many cases the
nature of the support also accounts for the superior effective-
ness of the gold-based catalysts.[4] Recent advances in the
understanding of catalysis by various gold-based nanostruc-
tures, and in particular the interplay between gold and the
underlying support, have enabled new strategies to maximize
the synthetic utility of particular gold catalysts and expand
their catalytic applications in green synthetic chemistry.[5] In
this regard, an integrated catalyst design, which relies on the
synergy of the gold and support originating from the mutually
beneficial cooperation between different active domains, has
proven particularly effective toward these goals. Although
[*] X. Liu, H. Q. Li, S. Ye, Dr. Y. M. Liu, Prof. Dr. H. Y. He, Prof. Dr. Y. Cao
Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Fudan University
Shanghai 200433 (P. R. China)
E-mail: yongcao@fudan.edu.cn
[**] This work was supported by the National Natural Science
Foundation of China (21073042, 21273044), the State Key Basic
Research Program of PRC (2009CB623506), the Research Fund for
the Doctoral Program of Higher Education (2012007000011), and
Science & Technology Commission of Shanghai Municipality
(08DZ2270500).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404543.
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the gold catalyst system, we were motivated to explore gold
deposited on functional support with tailored surface redox
and acid–base properties as a catalyst for the direct hydro-
genative coupling of nitro compounds.
inspection of the support reveals that a prominent surface
density of medium-strong acidic sites is also indispensable for
the superior performance of the Au/Mg4Al HT sample. In
support of this view, the addition of either benzoic acid or
pyridine greatly inhibited the hydrogenative coupling activity
of the Au/Mg4Al HT sample (see Scheme S2). These data,
together with the set of catalytic data reported in Table 1,
strongly suggest that the unique trifunctional character of the
Au/Mg4Al HT catalyst is essential for facilitating the con-
version of 1a into 2a in a highly selective manner.
Encouraged by these promising results, we initiated
a systematic study of a set of Mg4Al-HT-supported gold
samples with gold loadings ranging from 0.1 to 2.1 wt%.
Much to our delight, when the gold loading was decreased to
0.11 wt%, the selectivity for 2a increased significantly. Thus,
an excellent yield of 2a (98%) was obtained at 508C after
4.5 hours when using a 0.11 wt% Au/Mg4Al HT (Table 1,
entry 8). Particularly noteworthy is that, at a temperature as
low as 258C, the 0.11 wt% Au/Mg4Al HT can also transform
1a into 2a with an extremely high selectivity, and a 1a
conversion of up to 99% can be readily obtained (see
Table S3, entry 8). Subsequent investigation of the various
reaction parameters revealed that the solvent has a significant
influence on the chemoselectivity of the reaction and toluene
was the solvent of choice for the desired 2a (see Table S3,
entries 1–5). Increasing the H2 pressure generally resulted in
higher reaction rates but slightly lowered the selectivity for
azobenzene. Therefore, a 2 MPa of H2 pressure was enough to
obtain the optimal reaction activity.
We commenced our study by subjecting nitrobenzene (1a)
to gold NPs (average diameter of approximately 2.5 nm)
deposited on various inorganic oxide materials in the absence
of any external additives under mild hydrogenation condi-
tions, that is, 2 MPa of H2 pressure in toluene at 508C. This
initial screening identified gold on Mg-Al hydrotalcite (Mg/
Al = 2) as a possible catalyst which could deliver appreciable
levels of hydrogenative coupling activity, thus furnishing the
desired azobenzene 2a in a yield of approximately 34% (see
Table S1, entry 1). Consistent with most of the previous work
on gold-catalyzed nitro reductions,[13] the observation of
aniline (4a) as the sole or predominant product provides
strong support for the operation of a highly selective and
direct nitro-nitroso-hydroxylamine pathway (Scheme S1)
over gold deposited on all other carriers (see Table S1).
Further evaluation of a series of gold catalysts, with identical
an gold loading (1.0 wt%), deposited on Mg-Al hydrotalcites
(HTs) with different Mg/Al molar ratios (Mg/Al = 2–5, see
the Supporting Information for preparation details) revealed
that an impressive 2a yield of about 65% can be attained with
the Au/Mg4Al HT material (Table 1, entry 3).
Table 1: Hydrogenative coupling of nitrobenzene to azobenzene over
Au/MgAl HT catalysts at 508C.[a]
To further confirm the effectiveness of the present system,
a 100 mmol scale hydrogenation of 1a was carried out. The
hydrogenative coupling reaction proceeds efficiently, and the
corresponding azo compound could be obtained in 94% yield
(see Scheme S3). After the reaction, the 0.11 wt% Au/Mg4Al
HT sample was easily separated by filtration using ethanol as
a solvent. Analysis by inductively coupled plasma (ICP)
confirmed that no gold was present in the filtrate (limit of
detection: < 7 ppb), thus indicating that the observed catal-
ysis is truly heterogeneous. Furthermore, the recovered Au/
Mg4Al HT can also be reused at least four times without an
appreciable loss of activity on this scale, for which the total
turnover number (TON) approached 92000 (Scheme S3).[16]
Compared to the palladium-[12a,c] and platinum-based[12b]
nanorods which require inorganic bases (such as KOH) to
catalyze the hydrogenative coupling of nitroarenes, Au/
Mg4Al HT has a TON which is at least two orders of
magnitude greater than that of the reported catalysts under
additive-free reaction conditions. Transmission electron mi-
croscopy (TEM), X-ray photoelectron spectroscopy (XPS),
and X-ray diffraction (XRD) confirmed almost no change in
the dispersion of the gold or metallic state of gold before and
after reuse. These results are in good agreement with an
excellent retention of the activity of this catalyst.
Entry Metal
Mg/Al
Au size
t
Conv.
Sel. [%][d]
[wt%][b] [mol/mol] [nm][c]
[h] [%][d] 2a 3a 4a 5a
1
2
3
4
5
6
7
8
0.89
1.02
1.08
0.85
2.10
0.53
0.26
0.11
2
3
4
5
4
4
4
4
2.1
2.1
2.2
2.2
2.0
1.9
1.9
1.9
3.5 99
3.5 100
3.5 100
3.5 35
3.0 100
4.5 97
4.5 99
4.5 100
34 <1 64
61 <1 37 <1
1
65
10
3
8
31
82
1
0
0
27 <1 72
73
88 <1 11
98
2
24 <1
0
0
1
1
[a] Reaction conditions: nitrobenzene (1 mmol), catalyst (0.5 mol% Au),
H2 (2 MPa), toluene (5 mL), 508C. [b] Metal weight loadings determined
by ICP analysis. [c] Evaluated by TEM. [d] Conversion and selectivity
measured by HPLC analysis.
The dramatically improved hydrogenative coupling of 1a
over Au/Mg4Al-HT, under the reaction conditions described
above, clearly reflects the altered reaction pathway caused by
the intimate interaction of the gold NPs with the underlying
Mg-Al HT support. A preliminary analysis of the basic
properties of the HT materials (see Table S2) indicates that
a higher population of strong basic sites on the underlying
supports is crucial for the desired reaction pathway. Further
Preliminary mechanistic analysis of this hydrogenation
reaction was performed by monitoring the conversion of 1a
into 2a using high performance liquid chromatography. The
kinetic time course revealed the formation and disappearance
of the intermediate azoxybenzene 3a (see Figure S1), and is
consistent with the overall Harber mechanism as depicted in
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Table 2: Au/Mg4Al-HT-catalyzed hydrogenative coupling of nitroarenes to give symmetric azo compounds.[a]
experiment that the
condensation of nitro-
sobenzene and phenyl-
hydroxylamine occurs
spontaneously and rap-
idly, with a rate (r3) that
is much faster than
those of all the above-
mentioned processes.
These kinetic experi-
ments indicate that the
initial reaction rates for
each step of the overall
Entry Product
1
t [h] Yield [%][b]
Entry
11
Product
t [h] Yield [%][b]
4.5
5
98 (94)
87 (82)
18 97 (93)
2
3
12
13
36 95 (90)
25 62 (58)
sequential
decreases
reaction
on the
order: r3 @ r2 > r1 > r4,
thus revealing that the
rate-determining step
of the overall reaction
in the present Au/HT-
catalyzed system is
hydrogenation of azox-
ybenzene.
11
80 (74)
4
5
12
7
90 (85)
14
15
10 97 (91)
24 98 (92)
>99 (97)
Phenylhydroxya-
mine and azobenzene
hydrogenation experi-
ments also revealed
why such high selectiv-
6
7
9
95 (90)
79 (75)
16
17
30 87 (81)
33 85 (79)
ity
was
observed.
15
Under otherwise iden-
tical reaction condi-
tions, the hydrogena-
tive conversion of phe-
nylhydroxylamine over
8
24
98 (93)
18
30 80 (75)
0.11 wt%
Au/Mg4Al
HT closely resembles
that for the correspond-
ing H2-free transforma-
tion under an N2 atmos-
phere (see Table S4,
entries 1 and 2). Con-
versely, aniline has
been identified as a pre-
9
35
24
97 (91)
88 (82)
19
20
10 12
10
9
n.d.
dominant
product
when subjecting phe-
[a] Reaction conditions: nitroarene (1 mmol), catalyst (0.5 mol% Au), H2 (2 MPa), toluene (5 mL), 508C. [b] Yield
measured by HPLC analysis. Numbers within parentheses refer to yields of isolated products. n.d.=not detected.
nylhydroxylamine
hydrogenation
to
over
other supported gold
systems (Table S4).
Scheme S1. Note that the hydrogenative intermediates, nitro-
sobenzene (5a) and phenylhydroxylamine (6a), are not
detected because of the strong interaction between these
compounds and the catalyst surface,[11c,13d] and their fast
condensation. To gain further insight into the pathway, the
initial reaction rate for the hydrogenation of nitrobenzene
along with 5a and azoxybenzene were measured, and the
corresponding rates were r1 = 0.52 mmolgÀ1 hÀ1, r2 =
11.52 mmolgÀ1 hÀ1, and r4 = 0.14 mmolgÀ1 hÀ1, respectively
(see Scheme S4). Additionally, we confirmed in a separate
These findings, together with the lack of aniline formation
during the 1a to 2a conversion reveal that both the
disproportionation of phenylhydroxylamine and the reduc-
tion of the latter to aniline are practically inhibited over
0.11 wt% Au/Mg4Al HT during the coupling process. More-
over, it was found that the hydrogenation of azobenzene
proceeded sluggishly in the presence of 0.11 wt% Au/Mg4Al
HT (see Scheme S5). Taken together, all these results suggest
that the 0.11 wt% Au/Mg4Al HT catalyst biases the con-
densation to give the desired azobenzene via the intermediacy
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of azoxybenzene by suppressing the rate of both hydro-
genation of phenylhydroxylamine and azobenzene to aniline.
Furthermore, additional studies focused on the comparison
with other noble metals showed that the presence of gold is
indispensable for hydrogenative coupling of 1a to 2a (see
Table S5). These results confirm that the relatively low
hydrogen delivery capacity of a low loading of gold on
Mg4Al HT is one of the key aspects for maintaining a low and
steady concentration of phenylhydroxylamine, and allows the
reaction to proceed in a selective manner.
To demonstrate the general applicability of the novel
catalyst and the scope of the process, a wide range of simple
and readily available nitroarenes were tested without further
optimization (Table 2). We were pleased to find that various
functionalized nitroarenes, especially electron-deficient
(entries 4–18) substrates, are converted smoothly and selec-
tively into the desired products with good to excellent yields.
The catalytic hydrogenation of electron-rich nitroarenes, such
as 3-nitroaniline or 1-methoxy-3-nitrobenzene, was found to
be problematic as no or a low yield of the azo product was
formed (entries 19 and 20). However, the electron-donating
methyl-substituted nitrobenzenes, 1-methyl-3-nitrobenzene
and 1-methyl-4-nitrobenzene, were fully converted and gave
the corresponding azos in good yields after 5 and 11 hours,
respectively, of reaction (entries 2 and 3). Furthermore, steric
properties of the substituent affect the reductive coupling
significantly. Specifically, nitroarenes containing a substituent
on the aromatic ring require a longer reaction time (entries 1–
3). The yields of m-substituted azo arenes were somewhat
higher than those for the corresponding o- or p-substituted
nitroarenes (entries 2, 3, 5, 6, 10, and 11).
acceptable processes, which make use of more readily
available and easily handled feedstocks, is highly desired. It
should be noted at this point that upon mixing the two
different nitroarenes preferential self-coupling may occur as
an unwanted side reaction, particularly for the intrinsically
more reactive partner. Despite such challenges, one may
assume that it is possible to achieve a high selectivity for
desired cross-coupling, provided that one partner proved
much more reactive than the other. This selectivity is
especially good when the less reactive partner is present in
excess (see Table S6), wherein the lack of reactivity in the
reduction with this partner favored the preferential trapping
of the reactive-partner-generated phenylhydroxyamine with
the surface-abundant nitroso species derived from the less
reactive partner. Indeed, we were able to obtain seven
asymmetrically substituted azobenzenes with potential appli-
cations in dyes,[6a] liquid crystals,[6b] or optical storage
media,[6c] in high selectivity and conversion (Table 3) using
the Au/Mg4Al HT catalyst. To the best of our knowledge,
Table 3: Au/Mg4 Al-HT-catalyzed hydrogenative coupling of nitroarenes
to unsymmetrical azo compounds.[a]
Entry Substrates
Products
t [h] Yield [%][b]
R1
R2
1
H
o-Cl
7
75 (70)
It is noteworthy that halogenated nitroarenes can be
employed as selective hydrogenative coupling reagents. Both
mono- and dihalogenated nitroarenes were tolerated, thus
leading to the corresponding chloro-, bromo-, fluoro-, or
dihalogenated azo compounds without any dehalogenation
(Table 2, entries 5–13), a side reaction often encountered with
other procedures,[17] including catalytic hydrogenation. In
addition to halogen substituents, sensitive reducible function
groups such as olefin, nitrile, ketone, as well as ester moieties
were well tolerated without undergoing reduction to any
substantial extent during the reduction process (entries 4 and
16–18). Moreover, this Au/Mg4Al HT reduction system was
also applicable to dinitroarenes to obtain the corresponding
azo compound in excellent yield (entry 15), thus demonstrat-
ing the versatility of the present methodology for azo
compound synthesis.
Given that the surface-mediated condensation of nitroso
and hydroxylamine intermediates is a central step in the
formation of the azo linkage, we have explored the possibility
of the gold-catalyzed cross-coupling to prepare more indus-
trially important asymmetric aromatic azo dyes by using
a mixture of two different nitroarenes of very different
reactivity. These azo dyes are traditionally synthesized
through coupling of the diazonium salt of the electron-poor
partner with the electron-rich arene.[7] The problems inherent
in this procedure are the use of expensive and very unstable
starting materials and the release of large amounts of toxic
waste. Thus, the development of more environmentally
2
3
4
H
H
H
p-Cl
6.5 87 (80)
m-Cl
m-Br
5
6
92 (87)
90 (83)
5
6
7
H
p-COCH3
9
7
8
79 (71)
89 (83)
90 (82)
m-CH3 p-Cl
m-CH3 m-Br
[a] Reaction conditions: R1-C6H4NO2 (0.5 mmol), R2-C6H4NO2
(1.5 mmol), catalyst (1 mol% Au bases on R1-C6H4NO2), H2 (2 MPa),
toluene (5 mL), 508C. [b] Yield measured by HPLC analysis based on
R1-C6H4NO2 consumption. Numbers within parentheses refer to yields of
isolated products. Meanwhile, the R2-C6H4NO2 that was present in
threefold excess also gives appreciable amounts of hydrogenative
byproducts including anilines as well as self-coupling azo and azoxy
derivatives.
4
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In summary, aromatic azo compounds can be selectively
and directly synthesized from hydrogenative coupling of
nitroarenes without any external base or additives. This green
synthetic reaction, mediated by a robust hydrotalcite-sup-
ported gold catalyst, proceeds chemoselectively in a highly
efficient manner under mild reaction conditions using H2 as
a clean reductant. The new catalytic protocol exhibits a broad
substrate scope, thus providing a variety of symmetrical and
asymmetrical aromatic azo compounds in good to excellent
yields. The catalytic system described here promotes a sophis-
ticated multistep process by controlled manipulation of the
reaction pathways, and may present a new strategy toward the
development of versatile multitask, heterogeneous catalysts,
and also contribute to the design of catalytic systems for
accessing higher levels of complexity.
Received: April 22, 2014
Published online: && &&, &&&&
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[16] The design of active and recyclable catalysts capable of
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Keywords: arenes · azo compounds · gold · reduction ·
supported catalysts
.
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Supported Catalysts
X. Liu, H. Q. Li, S. Ye, Y. M. Liu, H. Y. He,
Y. Cao*
&&&& — &&&&
Gold-Catalyzed Direct Hydrogenative
Coupling of Nitroarenes To Synthesize
Aromatic Azo Compounds
Game of rates: A general approach for the
chemoselective hydrogenative coupling
of nitroarenes to give the corresponding
azo compounds, using a heterogeneous
gold catalyst, has been developed. As
a result of the remarkable synergy
between the metal and support, a facile
condensation of transient nitroso and
hydroxylamine intermediates proceeds.
The desired transformation is highly
selective under mild reaction conditions.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!