Highly chemo-and regioselective transfer reduction of aromatic nitro compounds using ammonium formate catalyzed by supported gold nanoparticles

Article abstract of DOI:10.1002/adsc.201000621

A highly chemo-and regioselective reduction of a wide diversity of aromatic nitro compounds to the corresponding amines has been achieved by a combination of gold nanoparticles supported on titania and ammonium formate (HCOONH ₄) in ethanol at room temperature. Furthermore, a direct and mild route to formanilides from aromatic nitro compounds bearing different functional groups by reductive N-formylation using the gold-mediated transfer reduction protocol is also established.

Full text of DOI:10.1002/adsc.201000621

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DOI: 10.1002/adsc.201000621
Highly Chemo- and Regioselective Transfer Reduction of
Aromatic Nitro Compounds using Ammonium Formate
Catalyzed by Supported Gold Nanoparticles
Xia-Bing Lou,a Lin He,a Yue Qian,a Yong-Mei Liu,a Yong Cao,a,*
and Kang-Nian Fana
a
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University,
Shanghai 200433, Peopleꢀs Republic of China
Fax : (+86)-21-6564-3774; e-mail: yongcao@fudan.edu.cn
Received: August 5, 2010; Revised: November 23, 2010; Published online: February 9, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000621.
Abstract: A highly chemo- and regioselective re-
duction of a wide diversity of aromatic nitro com-
pounds to the corresponding amines has been
achieved by a combination of gold nanoparticles
supported on titania and ammonium formate
(HCOONH4) in ethanol at room temperature. Fur-
thermore, a direct and mild route to formanilides
from aromatic nitro compounds bearing different
functional groups by reductive N-formylation using
the gold-mediated transfer reduction protocol is
also established.
the catalysts.[7] More recently, a facile selective reduc-
tion of nitro compounds catalyzed by HCOONH4-Cu
nanoparticles (NPs) has also appeared.[8] One critical
limitation of these processes, however, is the pyro-
phoric or air-sensitive nature of the involved catalysts,
which presents additional handling problems. More-
over, these catalysts generally lack the desired chemo-
selectivity over other functional groups that are often
present in the substrates such as acid, aldehyde, ester,
alkene, halide and nitrile moieties and also, the de-
sired regioselectivity in the reduction of dinitro com-
pounds. In addition, the reduction of aromatic nitro
compounds often stops at an intermediate stage,
yielding hydroxylamine, hydrazine, and azoarene
products.[9] In this sense, there is a strong incentive to
develop new simple, efficient, easily-handled proce-
dures that can allow the highly chemo- and regiose-
lective reduction of the nitro group under mild condi-
tions.
Keywords: ammonium formate; chemoselectivity;
gold; heterogeneous catalysis; N-formylation; nitro
group reduction
The reduction of aromatic nitro compounds to the
In recent years, supported gold NPs have emerged
corresponding amines is an important step in the in- as excellent catalysts for a broad range of organic
dustrial synthesis of dyes, biologically active com- transformations under mild conditions.[10] Gold was
pounds, pharmaceuticals, rubber chemicals, and agri- originally considered catalytically inactive. However,
cultural chemicals.[1] A wide variety of methods have when finely divided as small particles (ꢀ5 nm) and
been developed for this purpose. As opposed to the associated with a suitable support, Au could be active
commonly used reduction processes, which involve and highly selective in a variety of hydrogen-involving
hazardous Fe/HCl[2] or Sn/HCl[3] or molecular hydro- reactions.[11] Notably, the seminal work by Corma and
gen (H2), catalytic transfer hydrogenation (CTH)[4] Serna has shown that TiO2-supported Au NPs are
employing cheap and easily accessible hydrogen highly selective for the hydrogenation of a nitro
donors, e.g., ammonium formate (HCOONH4), is sim- group by H2 in the presence of other reducible func-
pler, safer, highly selective, and ecofriendly.[5] Further- tions.[12] Despite its efficiency, this method suffers
more, unlike conventional hydrogenation methods, from some drawbacks or limitations such as the need
CTH reactions do not require any elaborate experi- of relatively high temperature/pressure, complicated
mental set-up or high-pressure reactors.[6] Practically, equipment, and unfavourably low hydrogen-delivery
almost all relevant results have been achieved using rate. Very recently, we have described a facile Au-cat-
palladium supported on activated charcoal (Pd/C) as alyzed, CO/H2O-mediated method that circumvented
Adv. Synth. Catal. 2011, 353, 281 – 286
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281

Xia-Bing Lou et al.
Selectivity[b] [%]
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Table 1. Reduction of nitrobenzene to aniline under CTH conditions.[a]
Entry
Catalyst
Solvent
Hydrogen donor
Conversion[b] [%]
1
Au/TiO2
Au/TiO2
TiO2
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
2-propanol
ethanol
THF
HCOONH4
HCOONH4
HCOONH4
–
HCOOK
HCOOH
2-propanol
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
HCOONH4
>99
>99
–
–
37
–
>99
>99
–
–
84
–
2[c]
3
4
5
6
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/CeO2
Au/ZnO
Au/HAP
Au/SiO2
Au/Al2O3
Pd/C
7
–
–
8[d]
9
>99
35
7
98
59
71
45
92
>99
52
>99
56
59
61
97
–
10
11
12
13[e]
14
15
16
17
18
19
20
DCM
DMSO
CH3CN
H2O
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
10
54
>99
71
24
59
13
23
59
–
Ru/TiO2
[a]
Reaction conditions: PhNO2 (1 mmol), catalyst (metal: 1 mol%), solvent (10 mL), hydrogen donor (5 mmol), N2 protec-
tion, 258C, 3 h.
[b]
[c]
[d]
[e]
Conversion and selectivity were determined by GC (internal standard: n-decane) and GC-MS.
Results for the third run.
Reaction at 08C for 30 h.
12 h.
the difficult H2 activation thus enabling more efficient and 4). Other hydrogen sources such as HCOOK
and selective nitro reduction.[13] In continuation of showed much lower activity, whereas HCOOH and 2-
our interest in the use of supported Au as greener cat- propanol were not reactive (Table 1, entries 5, 6, and
alysts to facilitate new functional transformations 7). As shown in the Supporting Information
under mild conditions, we wish to report here an effi- (Table S2), the turnover frequency (TOF) of the cata-
cient, convenient and high-yield HCOONH4-mediat- lyst was dramatically improved by increasing the reac-
ed protocol using safe and easily-handled supported tion temperature. An excellent yield was also ob-
Au NPs as a robust catalyst for the reduction of aryl tained even at 08C although a longer reaction time
nitro compounds at ambient temperature. Our results was required (Table 1, entry 8). Among the solvents
have shown that a wide diversity of the functionalized tested, high yields of 2 were obtained using ethanol as
nitro compounds could be selectively reduced to the solvent (Table 1, entry 1). In contrast, low yields were
corresponding amines under mild conditions while the obtained using tetrahydrofuran (THF), dichlorome-
other reducible functions remained intact. To the best thane (DCM), dimethyl sulfoxide (DMSO) and aceto-
of our knowledge, this study also represents the first nitrile as solvents (Table 1, entries 9–12). Interestingly,
direct one-pot conversion of aryl nitro compounds to the Au/TiO2-catalyzed reduction can also be carried
formanilides by reductive N-formylation using gold out in water (Table 1, entry 13).
catalysts.
Next, we investigated the influence of the inorganic
First of all, a mixture of solid Au/TiO2 (supplied by support on the performance of Au NPs in more
Mintek), nitrobenzene (1), and HCOONH4 in ethanol detail. As observed with other gold-catalyzed process-
was stirred at room temperature under an N2 atmos- es, the choice of the solid support had a significant in-
phere for 3 h (Table 1, entry 1). Selective reduction of fluence on the catalytic activity. Among the tested
1 occurred to quantitatively yield aniline (2) without supports, TiO2 was the best. CeO2 was also effective
any by-products such as hydroxylamine, azo or azoxy (Table 1, entry 14), whereas Au/ZnO, Au/hydroxyapa-
compounds. The reaction was not initiated at all using tite (Au/HAP), Au/SiO2 and Au/Al2O3 resulted in low
HCOONH4 in the absence of Au/TiO2. No conversion yields of 2 (Table 1, entries 15–18). Moreover, the use
of 1 was observed with Au/TiO2 without HCOONH4. of other metal particles with catalytic potential, such
Thus, a combination of Au/TiO2 and HCOONH4 is es- as Pd and Ru, resulted in significantly lower or no
sential to carry out the reduction (Table 1, entries 3 yield of 2 for the reduction of 1 under similar reaction
282
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Highly Chemo- and Regioselective Transfer Reduction of Aromatic Nitro Compounds
conditions (Table 1, entries 19 and 20). These results
clearly indicate that TiO2-supported Au NPs, a versa-
tile material that has shown its high activity in a varie-
[11,12]
ty of reduction reactions involving H2
or CO/
H2O[13] as the hydrogen source, has the optimum ac-
tivity for the reduction of 1 by using HCOONH4 as
the reductant. Thus, under optimized conditions, the
average TOF and the TON reached up to 33.3 hÀ1 and
100, respectively (Table 1, entry 1). These values are
one or two orders of magnitude greater than those in
previously reported catalyst systems, such as
HCOONH4-Cu nanoparticles (TOF: 0.1 hÀ1, TON:
20, reaction at 1208C).[8]
To determine whether Au/TiO2 worked as a hetero-
geneous catalyst, Au/TiO2 was removed from the re-
action mixture by simple filtration at 50% conversion
of 1. Continued stirring of the filtrate under similar
conditions did not give any products. The absence of
Au ions in the filtrate was verified using ICP analysis
Scheme 1. Proposed reaction pathways for reduction of aro-
matic nitro compounds by the HCOONH4-Au/TiO2 system.
(detection limit: 0.10 ppm). These results clearly dem- posed a more straightforward route (path 3) involving
onstrate that reduction took place only on the Au the direct formation of the intermediate hydroxyl-
NPs deposited on TiO2. Furthermore, the Au/TiO2
amine for Au-catalyzed nitro hydrogenation by H2.[14]
ACHTUNGTRENNUNG
catalyst was recoverable by simple filtration after the To obtain an insight into the reaction pathways in-
transfer reduction without any loss in their activity or volved in the HCOONH4-mediated nitro reduction,
selectivity after several uses (Table 1, entry 2). TEM we carried out a series of experiments using the
images of Au/TiO2 after the reuse revealed that the above intermediates. When phenyl hydroxylamine,
average diameter and size distribution of the gold obtained by a different route, was subjected to reduc-
NPs were similar to those of the fresh Au/TiO2 (see tion by this procedure, aniline was obtained in 60%
Supporting Information, Figure S1), and that no ag- yield (Supporting Information, Table S3, entry 1).
gregation of the used gold NPs was apparent. In addi- However, when nitrosobenzene, azobenzene and hy-
tion, XPS analysis of Au/TiO2 showed virtually no drazobenzene, also obtained separately, were reduced
change in the surface Au/Ti ratio (see Supporting In- under identical reaction conditions, aniline was
formation, Figure S2, Table S1), corroborating the formed only in substantially lower yields of 3%, 12%
observation that the gold NPs after reuse were the and 3%, respectively (Supporting Information,
same size after recycling compared to the original Table S3, entries 2–4). Therefore, it seems that the
sample. These results are consistent with the retention present HCOONH4-Au/TiO2-mediated nitro reduc-
of the catalytic activities of Au/TiO2 during recycling tion occurs mainly through path 3.
experiments.
With these findings in hand, we then extended our
Given the observed high activity and selectivity, it studies to various kinds of nitro compounds to estab-
is of interest to clarify the catalytic origin of the pres- lish the scope of the CTH reaction with the
ent HCOONH4-Au/TiO2 system. In line with our HCOONH4-Au/TiO2 system. Table 2 shows that the
recent proposal for gold-catalyzed nitro reduction catalyst system was surprisingly versatile. Various
using CO/H2O as the reductant, a facile transfer dehy- structurally diverse aryl nitro compounds, regardless
À
drogenation from HCOONH4 leading to crucial Au
of the presence of electron-withdrawing or electron-
H formation could be the key factor for achieving donating functions (Table 2, entries 2–14), could be
rapid nitro reduction under mild conditions.[13] On the selectively reduced to the corresponding amine com-
other hand, it is known that the reduction of aromatic pounds. Unlike the catalytic hydrogenation of nitro
nitro compounds can proceed by two different routes compounds with halogens (Table 2, entries 2–5) on
(Scheme 1) in which a couple of consecutive elemen- other metal surfaces, the substrates were reduced
tal steps are involved.[14] In path 1, the nitro com- cleanly to the corresponding anilines without any de-
pound is first reduced to a nitroso compound which halogenation over Au/TiO2. In the transformations of
further proceeds to provide the hydroxylamine and fi- chloronitroaromatics, the lower reaction rate of o-
nally gives rise to amine. In path 2, the initially chloronitrobenzene relative to the m- and p-ana-
formed nitroso compound may condense with hydrox- logues indicated a steric effect (Table 2, entries 2–4).
ylamine to produce the azoxy compound, which then Strikingly, all of the reducible functions such as
may undergo further reduction to azo, hydrazo, and alkene, aldehyde, nitrile, ketone and ester as well as
finally to amine. Recently, Corma et al. have pro- O-benzyl moieties remained intact during the reduc-
Adv. Synth. Catal. 2011, 353, 281 – 286
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Xia-Bing Lou et al.
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Table 2. Transfer reduction of aromatic nitro compounds to
Table 2. (Continued)
the corresponding amines by HCOONH4-Au/TiO2.[a]
Entry
Substrate
Time [h]
Yield [%][b]
99 (96)
Entry
1
Substrate
Time [h]
3
Yield [%][b]
99 (95)
18
3.5
19
20
0.25
88 (85)[f]
2
3
4
5
6
6
99 (92)
98 (95)
90 (80)
99 (96)
98 (93)
3.5
8
0.4
3
91 (87)[f]
75 (71)[f]
21
[a]
Reaction conditions: substrate (1 mmol), Au/TiO2 catalyst
(Au: 1 mol%), ethanol (10 mL), HCOONH4 (5 mmol),
N2 protection, 258C.
Yields were determined by GC analysis; values in paren-
thesis are the yields of the isolated products. The struc-
tures of the products were established from their GC-
mass spectra.
3
[b]
3.5
[c]
[d]
[e]
7
5
99 (83)
94 (77)
Methanol (10 mL) as solvent.
CH3CN (10 mL) as solvent.
508C.[f] Yield for nitroaniline.
8[c,e]
1.7
9[d]
10
3
90 (82)
99 (91)
98 (94)
tion process (Table 2, entries 6, 9–14). 6-Nitroquino-
line was reduced to 6-aminoquinoline (an important
intermediate in synthesizing several dyes) which indi-
cated that the heterocyclic ring would not be affected
under these CTH conditions (Table 2, entry 17). The
relatively higher reduction rate of 1-nitroanthroqui-
none (Table 2, entry 16) to 1-aminoanthroquinone
(another important intermediate in the dye industry)
showed that the electron-withdrawing group might fa-
cilitate the reduction process.
The origin of the high chemoselectivity for the spe-
cific reduction of nitro group was confirmed by an in-
termolecular competitive reaction between nitroben-
zene and other unsaturated substrates, such as sty-
rene, acetophenone, benzaldehyde, and benzonitrile
under the conditions described above (see Supporting
Information). The result showed that while the yield
of aniline was >99% (GC yield), all other unsaturat-
ed substrates exhibited no conversion at all. There-
fore, the higher intrinsic rate for the reduction of the
nitro group than that of the other reducible functions
over the HCOONH4-Au/TiO2 system is responsible
for the high chemoselectivity observed.
2.5
3
11[c]
12
2
3
3
65 (61)
90 (81)
99 (96)
13[d]
14
15
5.2
99 (85)
16[c]
17[e]
284
1.7
3
96 (92)
99 (89)
The catalyst also shows promise for regioselective
reduction of dinitrobenzenes with high to excellent
yields (Table 2, entries 19–21). Many conventional
procedures involving hydride reducing agents, hydro-
genation, or indium failed to give such a high regiose-
lectivity.[9,15] Recently, the regioselective reduction of
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Highly Chemo- and Regioselective Transfer Reduction of Aromatic Nitro Compounds
Table 3. Reductive N-formylation of aryl nitro compounds
under catalytic transfer hydrogenation conditions.[a]
o-, m-, and p-dinitrobenzenes has been carried out by
using silane/oxo-rhenium complexes.[15] However, the
use of a toxic silane as hydrogen source, costly metal
complexes as catalyst, long reaction times (24–38 h),
and low yields (58–76%) limit the scope of the
method. Very recently, we have demonstrated that a
more efficient regioselective reduction of dinitroben-
zenes could be achieved with Au/TiO2 in the presence
of CO/H2O.[13] Nevertheless this method was incon-
venient to some extent due to the fact that the toxic
CO gas is highly dangerous and difficult to handle.
Thus, we have been able to further proceed along
these green lines. Notably, the convenient and safer
chemo- and regioselective reduction of the nitro com-
pounds can be achieved with the HCOONH4-Au/TiO2
system under mild CTH conditions.
Entry
1
Substrate
Time [h]
4.5
Yield [%][b]
94 (87)
2
3
6
7
94 (85)
95 (90)
Finally, taking into account that HCOONH4 in an
aprotic solvent like acetonitrile has previously been
reported as a formylating agent,[17] we envisioned that
the present HCOONH4-Au/TiO2 system could afford
a green and efficient protocol for the direct synthesis
of formanilides from nitro compounds. Formanilides
are widely used in the synthesis of biologically active
compounds as well as catalyzing several organic trans-
formations as Lewis bases.[18] Various methods have
been reported for the selective reduction of aryl nitro
compounds to anilines as well as the N-formylation of
anilines with different formylating agents.[19] However,
the direct conversion of aryl nitro compounds to for-
manilides would be an ideal choice for this class of
important compounds. The efficacy of the one-step
gold-catalyzed reductive N-formylation methodology
can be seen from Table 3. A variety of aromatic nitro
compounds having different sensitive functional
groups can be converted into the corresponding for-
manilides in high yields following the procedure as
4
5
6
7
5
92 (83)
95 (88)
97 (93)
70 (66)
3
2.5
8
8
9
77 (71)
[a]
Reaction conditions: substrate (1 mmol), Au/TiO2 catalyst
(Au: 1 mol%), HCOONH4 (7 mmol), N2 protection, in
CH3CN (3 mL) under reflux conditions.
Yields were determined by GC analysis; values in paren-
thesis are the yields of the isolated products.
[b]
depicted in Table 3. While Baskaran and co-work- valuable transformation, HCOONH4 serves two dis-
ers[20] have reported an efficient one-step synthesis of tinct functions: as hydrogen source for nitro reduction
formanilides from aromatic nitro compounds with and as a formylating agent. The mild reaction condi-
HCOONH4-Pd/C, this system requires prolonged re- tions and associated low cost of ammonium formate
action times (up to 10–24 h) and a large excess of will greatly facilitate the synthesis of functionalized
HCOONH4 to achieve high yields of the products amines and formanilides in both laboratory and indus-
(>90%). In contrast, the present Au-based system trial production.
has the following significant advantages: 1) only
7 equivalents of HCOONH4 was employed; 2) the re-
action was much more efficient even at much lower
metal loadings; and 3) the reaction uses the non-pyro-
Experimental Section
phoric Au/TiO2 catalyst that is easy to handle.
General Procedure for the Reduction of Aromatic
Nitro Compounds
In summary, we have developed an efficient and
highly chemo- and regioselective gold-catalyzed,
HCOONH4-mediated CTH methodology for the re-
duction of a great variety of multifunctionalized aro-
matic nitro compounds to the corresponding amines.
Moreover, this gold-based CTH system could also ac-
complish the direct conversion of aromatic nitro com-
pounds bearing different functional groups to the cor-
The mixture of nitro compound (1 mmol), metal catalysts
(metal: 1 mol%), ethanol (10 mL), and anhydrous
HCOONH4 (5 mmol) was put into a flask (100 mL) fitted
with a gas inlet tube for introducing N2 by bubbling
(10 mLminÀ1). The reaction mixture was magnetically
stirred (1000 rpm) at 258C. The conversion and product se-
lectivity were periodically determined by GC analysis [Shi-
responding N-formanilides chemoselectively. In this mazu GC-17A equipped with an HP-FFAP column (30 mꢂ
Adv. Synth. Catal. 2011, 353, 281 – 286
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285

Xia-Bing Lou et al.
COMMUNICATIONS
0.25 mm) and a flame ionization detector (FID)] using n-
decane as an internal standard. After completion of the re-
duction, the reaction mixture was filtered and the catalyst
was washed thoroughly with ethanol. Then the filtrate was
concentrated under vacuum and extracted by CH2Cl2 (3ꢂ
3 mL). The combined CH2Cl2 fractions were concentrated
under reduced pressure using a rotatory evaporator. The
crude product was purified by column chromatography
[silica 200–300; petroleum ether (60~908C)/ethyl acetate
mixture] to afford the amine product. All the products were
characterized by GC-MS and further confirmed by the com-
parison of their GC retention time, mass with those of au-
thentic samples.
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equipped with a reflux condenser. The resulting mixture was
allowed to reflux with vigorous stirring under a nitrogen at-
mosphere. After completion of the reaction, the reaction
mixture was filtered and the catalyst was washed thoroughly
with CH3CN. Then the filtrate was concentrated and dried
under reduced pressure using a rotatory evaporator. The
crude product was purified by column chromatography
[silica 200–300; petroleum ether (60~908C)/ethyl acetate
mixture] to afford the product. All the products were char-
acterized by GC-MS and further confirmed by the compari-
son of their GC retention time, mass with those of authentic
samples.
Acknowledgements
Financial support by the National Natural Science Founda-
tion of China (20721063, 20873026 and 21073042), New Cen-
tury Excellent Talents in the University of China (NCET-09-
0305), and the State Key Basic Research Program of PRC
(2009CB623506), FDUROP (09018) as well as Science &
Technology Commission of Shanghai Municipality
(08DZ2270500) is kindly acknowledged.
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