Deoxygenative coupling of nitroarenes for the synthesis of aromatic azo compounds with CO using supported gold catalysts

Article abstract of DOI:10.1039/c5cc03134f

A facile and efficient catalytic system based on a mesostructured ceria-supported gold (Au/meso-CeO₂) catalyst was developed for the synthesis of various aromatic azo compounds by the reductive coupling of the corresponding nitroaromatics, using CO as the sole deoxygenative reagent, under additive-free and mild reaction conditions.

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DOI: 10.1039/C5CC03134F
The graphical abstract for
“Deoxygenative coupling of nitroarenes to synthesize aromatic azo
compounds with CO using supported gold catalysts”
Hai-Qian Li, Xiang Liu, Qi Zhang, Shu-Shuang Li, Yong-Mei Liu, He-Yong He
and Yong Cao *
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Fudan University, Shanghai 200433, P. R. China
A simple ligand- and additive-free gold catalyst system that enables the CO-mediated
reductive coupling of nitroarenes has been identified.

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COMMUNICATION
Deoxygenative coupling of nitroarenes to synthesize aromatic azo
compounds with CO using supported gold catalysts †
Hai-Qian Li, Xiang Liu, Qi Zhang, Shu-Shuang Li, Yong-Mei Liu, He-Yong He and Yong Cao*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
produced in multiton quantities as a byproduct of the steel indu-
stry and thus represents an abundant source for chemical reduc-
50 tion.9 , 10 However, most of such reductions proceed by the
water- gas shift reaction (CO + H2O = H2 + CO2) and therefore
utilize hydrogen as the terminal reductant.11 While conceptuall-
y more attractive in the context of without using an external
hydrogen source, the potential of using CO as the sole reductant
55 to obtain aromatic azo compounds from corresponding nitroar-
enes remains largely unexplored. In an early research on the
interaction of CO with nitrocompounds in the absence of a
catalyst, the conversion of nitroarenes into azo derivatives with
yields up to 97.5% was achievable, but an exceptionally high
60 CO pressure (3000 atm) was required.12 A subsequent follow-
up study in the 1960s showed that the reductive coupling of
nitroarenes can take place in anhydrous benzene at 200 oC and a
CO pressure of 200 atm by employing Fe(CO)5 as the catalyst.13
However, this system is not adequately effective and not very
65 selective since only moderate yields of the corresponding azo
derivatives in the range of 40-80% can be attained.
There is considerable current interest in developing more be-
nign organic synthesis facilitated by supported gold nanoparti-
cles (NPs), which have emerged over the last decade as a pro-
70 mising new class of green catalytic materials to meet the ever-
increasing demand for clean and resource-efficient chemical
synthesis.14 As part of our continuing investigation into reduc-
tion chemistry by supported Au NPs, we recently reported the
outstanding catalytic ability of Au NPs for the deoxygenation
75 of nitroaromatics to anilines as well as epoxides to their corres-
ponding olefin compounds using CO/H2O as a reducing reag-
ent.11b,11e In such deoxygenation, a Au-hydride species was ge-
nerated in situ from the reaction of H2O with CO, which led to
the highly chemoselective reduction. Being aware of the advan-
80 tages of adopting the Au-based deoxygenation protocols and
also utilizing the excellent catalytic activity of supported gold
toward CO activation and related transformations,14 we envisa-
ged that the selective deoxygenation of nitroaromatics accom-
panied by the formation of the azo linkage in the reduced prod-
85 ucts would be plausible via gold-catalyzed deoxygenative cou-
plings under CO atmosphere and anhydrous conditions.
A facile and efficient catalytic system based on mesostructured
ceria-supported gold (Au/meso-CeO2) catalyst was developed
for the synthesis of various aromatic azo compounds by
reductive coupling of the corresponding nitroaromatics, using
10 CO as the sole deoxygenative reagent, under additive-free and
mild reaction conditions.
Azo compounds constitute one of the largest classes of industri-
ally synthesized organic compounds and are important interme-
diates for dyes, food additives, radical reaction initiators, and
15 therapeutic agents because of their widespread and diverse bio-
logical activities.1 Especially, those with aromatic substituen-
ts are known to serve as useful precursors to various products
that have found numerous applications in material science.2
Despite many known methods,3 there is an ongoing interest in
20 the development of convenient and general protocols for the
synthesis of these compounds. Hence, novel approaches such as
copper-catalyzed oxidation of aromatic amines and a two-step,
one-pot conversion of nitro compounds via the intermediacy of
the corresponding anilines have been recently developed.4 Des-
25 pite all these achievements, the direct reductive coupling of re-
adily available nitroarenes represents the most straightforwar-
d route to this class of compounds. In this resgard, H2 gas is un-
doubtly one of the most attractive and popular means to effect
reduction, thereby leading to a number of catalytic hydrogena-
30 tive coupling procedures of nitroarenes.5 However, it should be
noted that the use of H2 is not perfectly atom-economical as it
might seem, given that current industrial production of H2 relies
overwhelmingly on fossil fuels and the fact the associated emi-
ssions have led to a net increase in atmosphere CO2 levels.6
35 Moreover, the necessity of special handling of highly flamma-
ble H2 gas and/or the requirement of external base additives to
achieve an acceptable level of selectivity have diminished the
ultimate appeal and utility of these procedures.
So far, much less attention has been paid to the direct reduc-
40 tive coupling of nitroarenes to give azo compounds by exploi-
ting carbon monoxide (CO) as an effective reagent. As one of
the cheapest C1 sources in the chemical industry, CO is now
heavily used in a number of highly selective syntheses of a wide
range of chemicals and fuels.7 Aside from being a very useful
45 C1 building block, CO is also known to act as a reductant.8 The
use of CO instead of hydrogen gas or other hydrogen donar
agents is particularly interesting in light of the fact that CO is
Herein we report the first demonstration of the efficient cata-
lytic reductive coupling of nitroarenes under mild CO pressures,
facilitated by a Au catalyst supported on mesostructured CeO2
90 (meso-CeO2) featuring high redox capacities. The present cata-
This journal is © The Royal Society of Chemistry [year]
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lytic system is highly effective in that turnover numbers (TONs)
of up to ~ 2000 can be achieved, and is workable on a large
ith a set of Au/CeO2 samples with varied ceria specific surface
area ranging from 50 to 140 m2g-1 (entries 8-10). To our delight,
scale. Even under atmospheric conditions, the catalytic activity 40 it was found that an excellent yield of 2a of about 99% can be
of the meso-CeO2 supported Au is much greater than those of
previously reported systems requiring high CO pressures.13 In
addition, this new process works under mild conditions and
enables the selective deoxygenative coupling of a wide range of
attained with a catalyst composed of gold deposited on meso-
5
CeO2 (SBET ~ 139 m2g-1, Figure S2) (entry 10).
In a scaled-up reaction at gram quantity (1a, 20 mmol), the
Au/meso-CeO2 catalyst provided a high isolated yield (96%) of
functionalized nitroarenes while leaving other reducible or 45 the corresponding azo product (see Scheme S1). In this case, a
thermally labile functional groups intact.
To assess the viability of the proposed reaction, we first exp-
lored the conversion of nitrobenzene (1a) under mild process-
TON as high as 2000 was achieved, a value which is signifi-
cantly higher than those obtained with previously reported sys-
tem requiring high CO pressure (Fe(CO)513: TON 20). These
results indicated that Au/meso-CeO2 constituted a practical
10
o
ing conditions, i.e., 5 atm of CO pressure in toluene at 150 C.
We initially examined the reduction of 1a over a benchmark 50 catalyst for the deoxygenative coupling of nitroarenes produce
Au/TiO2 catalyst (average Au particle size ~ 2.8 nm, Figure
azobenzenes amenable to gram-operations. After the reaction,
the Au/meso-CeO2 sample could be easily recovered by
filtration using toluene as a solvent. Inductively Coupled Plas-
ma (ICP) analysis confirmed that no Au was present in the
15 S1a), which has been widely studied and proven to be highly
active for a large variety of functional group transformations.11b,
11e-11g
It is not surprising that aniline (4a) formation competes
with the desired reductive coupling in standard nondry toluene 55 filtrate (detection limit: <7 ppb), indicating that the observed
solvent (Table 1, entry 1). However, the use of freshly distilled
20 toluene minimises the formation of 4a (entry 2), appreciable
formation of azobenzene (2a) and azoxybenzene (3a) with the
overall yield being a modest 25% can be attained. This result
catalysis is truly heterogeneous. Furthermore, the recovered
Au/meso-CeO2 can also be reused at least five times without an
appreciable loss of activity(see ESI†), thus further demonstra-
ting the utility of Au/ meso-CeO2 for achieving reductive coup-
was encouraging and led us to further examine a series of gold 60 ling of nitro compounds, with catalyst loadings that are orders
deposited on other mineral supports such as Fe2O3, Al2O3, ZnO,
25 ZrO2 and hydrotalcite (HT, layered double Mg-Al hydroxide).
These catalysts, however, were not found to be particularly
active, and in most cases the unwanted 3a being obained pre-
of magnitude lower than that have previously been employed
for this transformation under additive-free conditions. In addi-
tion, the X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS) confirmed almost no change in the disper-
dominantly (entries 3-7). Interestingly, when applying a 65 sion of the Au or metallic state of Au before and after reuse (see
commercial ultrafine ceria (Evonik Adnano 50) supported gold
30 catalyst (Au/CeO2-50, average Au particle size ~3.3 nm, Figure
S1b), appreciable levels of reductive coupling activity was
observed, thus furnishing the desired 2a in a favorable selecti-
ESI†). These results are in good agreement with an excellent
retention of the activity of this catalyst.
Consistent with the case identified for catalytic deoxygena-
tion of nitroarenes to anilines using CO/H2O,11b,11e-11g we found
vity of approximately 58% at full 1a conversion (Table 1, entry 70 that gold is exceedingly active for reductive coupling of nitro-
8). A systematic study of the catalyst was then accomplished w-
arenes with CO compared with other noble metals (Table
S1,entry 1-5). Subsequent experiments focused on the effect of
the reaction temprature revealed that under otherwise identical
reaction conditions the Au/meso-CeO2 exhibited dramatically
75 diminished production of 2a at 120 oC (Table 1, entry 11).
Upon solvent screening it was found that the reaction catalyzed
by Au/meso-CeO2 proceeded efficiently in a variety of solvents,
with the highest rate reached in anhydrous toluene (Table S2).
Furthermore, it is revealed that the reaction can proceed smoo-
80 thly even at atmospheric pressure, although a longer reaction
time was required (entry 12). The reaction also proceed under
an inert atmosphere of N2, although the reaction rate decreased
significantly with 3a being formed as the major product (entry
13). In contrast, when replacing CO with H2, 4a is formed pre-
85 ferentially even at low 1a conversion levels (Table S3).15 We
also tested other gold-based precursors or materials such as
HAuCl4, Au2O3 and bulk Au0 powder (mean particle size, ca.
150 nm), but all of them gave no conversion (Table S1). At this
juncture, it is interesting to note that Au-free meso-CeO2 can
90 deliver appreciable reduction of 1a, despite the fact that only
the intermediate product of 3a was formed predominantly
(entry 14). Taken together, these results show that the combina-
tion of CO and Au NPs strongly associated with meso-CeO2 is
essential for achieving a high catalytic activity for selective
95 reductive coupling of 1a into 2a under mild conditions.
35 Table 1 Deoxygenative coupling of nitrobenzene to azobenzene with
CO in the presence of various catalysts.a
Entry
Catalyst
Time
(h)
5
5
5
5
5
5
5
5
5
5
5
11
5
5
Conv.
(%)b
27
26
17
28
37
8
12
100
100
100
100
100
8
Sel. (%)b
3a
45
64
81
10
18
83
88
40
15
0
2a
24
34
17
86
77
14
11
58
84
99
14
98
8
4a
31
2
2
4
5
3
1
2
1
1
1
2
0
0
1c
2
3
4
5
6
7
8
9
10
11d
12e
13f
14g
Au/TiO2
Au/TiO2
Au/Fe2O3
Au/Al2O3
Au/ZnO
Au/ZrO2
Au/HT
Au/CeO2-50
Au/CeO2-90
Au/meso-CeO2
Au/meso-CeO2
Au/meso-CeO2
Au/meso-CeO2
meso-CeO2
85
0
92
96
67
4
a Reaction conditions: 2.0 mmol 1a, anhydrous toluene (5 mL) and Au
0.5 mol% at 150 oC under 5 atm of CO. b GC analysis using tert-butyl-
c
d
benzene as an internal standard. As-received toluene from Aldrich.
120 oC. e 1 atm of CO. f 5atm of N2 instead of CO. g 250 mg meso-CeO2
was used.
2
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Table 2 Au/meso-CeO2 catalyzed deoxygenative coupling of nitroare-
nes to give symmetric azo compounds.a
Time
(h)
Yield.
(%)b
Entry
1
Products
5
99 (95)
74 (70)
2
3
4
33
15
28
91 (86)
85 (79)
Scheme 1 Proposed mechanism for deoxygenative coupling of nitro-
benzene with CO in the presence of Au/meso-CeO2 as catalyst.
tuent on the aromatic ring mostly require a longer reaction time.
15 The yields of m-substituted azo arenes were somewhat higher
than those for the corresponding o- or p-substituted nitroarenes
(Table 2, entries 3-7). The nitroaromatics bearing electron-
withdrawing groups are converted smoothly and selectively
into the desired products with good to excellent yields (74-91%)
20 (entries 2-9). Electron-donating substituted nitroaromatics,
such as 1-methyl-4-nitrobenzene and 1-methyl-3-nitrobenzene,
also formed the corresponding azo compounds in good yields
after 22 and 8 hours, respectively, of reaction (entries 10, 11).
Furthermore, asymmetrical aromatic azo compounds, having a
25 high value as key intermediates for prodrugs and liquid crystals,
could also be synthesized with moderate yields upon using two
different substituted nitroarenes as the reactants (Table S5).16
Taking into account the previous mechanisctic investigation
on related systems17 and our experimental observations, a pro-
30 posed surface-redox reaction pathway involving Ce4+/Ce3+ cou-
ple through cooperation between gold and the CeO2 support is
depicted in Scheme 1. The first step involves fast CO-induced
catalyst reduction via abstraction of the lattice oxygen at the Au
/CeO2 perimeter interface to produce an oxygen vacancy. Sub-
35 sequent deoxygenative coupling of the nitro group of 1a occur-
ed on the resultant vacancy sites provides the azoxybenzene,
which then undergoes a further deoxygenation to give the azo-
benzene species,15, 18 thereby completing the catalytic cycle.
This redox-coupling route is well supported by the following
40 results: 1) An evaluation of the material redox behavior by CO-
temperature-programmed reduction (CO-TPR) measurements
revealed that the reactive removal of surface oxygen from Au/
CeO2 occurred at much lower temperatures than other Au-based
catalysts (Figure S5), thus verifying the distinguished deoxyge-
45 nation activities of the Au/CeO2 samples. 2) By monitoring the
deoxygenation of 1a with reaction time, we established that the
reaction proceeds via the intermediacy of azoxy compound, 19
indicative of a sequential deoxygenation pathway (Figures S6).
5
6
7
5
91 (85)
74 (68)
86 (81)
28
16
8
20
86 (79)
9
15
22
8
79 (73)
86 (79)
92 (85)
10
11
a Reaction conditions: 2 mmol substrate, anhydrous toluene (5 mL),
Au/meso-CeO2 (Au 0.5 mol% ) at 150 C under 5 atm of CO. b GC
o
yield using tert-butylbenzene as an internal standard. Numbers with-
in parentheses refer to yields of isolated products.
5
With these promising results in hand, a variety of simple and
readily available nitrobenzenes were further examined to dem-
onstrate the general applicability of this novel CO-mediated
catalytic system (Table 2). Both steric and electronic propert-
ies of the substituents in nitroarenes affect the coupling reac-
10 tion significantly. Generally, nitrobenzenes containing a substi-
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S7), thus underscoring the importance of the greater surface
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ive coupling via a redox-mediated pathway.
6
7
5
In conclusion, we have successfully developed a facile and
efficient gold-catalyzed approach for the synthesis of aromatic
10 azo compounds directly from reductive coupling of the corresp-
onding nitroarenes by using CO as the sole reductant. The reac-
tion is very mild, general, scalable, and tolerant of various
functionalities. This reductive coupling method, using a robust
and reusable gold catalyst with inexpensive and abundatly
15 available CO as the appealing reductant under mild conditions,
can make a significant contribution not only to reveal the syn-
thetic potential of supported gold catalysts but also to establish
a more benign and industrially viable process.
8
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Financial support from the National Natural Science Found-
20 ation of China (21273044, 21473035), Research Fund for the
Doctoral Program of Higher Education (20120071110011) and
Science & Technology Commission of Shanghai Municipality
(08DZ2270500 and 12ZR1401500) is greatly appreciated.
Notes and references
25 Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Collaborative Innovation Center of
Chemistry for Energy Materials, Fudan University, Shanghai 200433, P. R.
China. Fax: (+86-21) 65643774; E-mail: yongcao@fudan.edu.cn
† Electronic Supplementary Information (ESI) available: Chemicals and
30 materials, catalyst characterization, catalyst preparation and activity test.
See DOI: 10.1039/b000000x/
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ESI† for details.
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