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Palladium-catalyzed reduction of nitroaromatic compounds to the corresponding anilines.

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Full Paper
Received: 4 October 2009
Revised: 16 February 2010
Accepted: 16 February 2010
Published online in Wiley Interscience: 6 April 2010
(www.interscience.com) DOI 10.1002/aoc.1645
Palladium-catalyzed reduction of
nitroaromatic compounds to the
corresponding anilines
Maryam Mirza-Aghayana∗ , Rabah Boukherroubb, Mahshid Rahimifarda
and Mohammad Bolourtchiana
Reduction of a variety of nitroaromatic compounds with triethylsilane in the presence of catalytic amounts of palladium chloride
c 2010 John Wiley & Sons, Ltd.
in ethanol resulted in the formation of the corresponding anilines in excellent yields. Copyright Keywords: palladium (II) chloride; triethylsilane; reduction; nitroaromatic compounds; anilines
Introduction
Appl. Organometal. Chem. 2010, 24, 477–480
Scheme 1. Reduction of nitroaromatic compounds using Et3 SiH in the
presence of PdCl2 catalyst in ethanol.
to yield the corresponding anilines in high yields. The reduction
is most likely due to the reaction of the nitroaromatic compounds
with molecular hydrogen generated in situ by the spontaneous
reaction of ethanol and triethylsilane catalyzed by palladium
dichloride.
Results and Discussion
The aim of the present study is to evaluate the efficiency of
the Et3 SiH–EtOH system in the presence of catalytic amounts
of PdCl2 for the reduction of nitroaromatic compounds under
mild conditions (Scheme 1). The reduction reaction requires the
use of an inert atmosphere and anhydrous solvent. In a typical
reaction, PdCl2 (10 mol%) is added at room temperature to a
stirred mixture of a nitroaromatic compound (1 equiv.) and Et3 SiH
Ł
Correspondence to: Maryam Mirza-Aghayan, Chemistry and Chemical Engineering Research Center of Iran PO Box 14335-186, Tehran, Iran.
E-mail: m.mirzaaghayan@ccerci.ac.ir
a Chemistry and Chemical Engineering Research Center of Iran (CCERCI), PO Box
14335-186, Tehran, Iran
b Institut de Recherche Interdisciplinaire (IRI, USR 3078), and Institut
d’Electronique, de Microélectronique et de Nanotechnologie (IEMN, UMR 8520),
Cité Scientifique, Avenue Poincaré – B.P 60069, 59652 Villeneuve d’Ascq, France
c 2010 John Wiley & Sons, Ltd.
Copyright 477
Aromatic amines are important intermediates in the fine chemical,
dyes and pigment industries.[1] Amines are extensively distributed
in nature and display a wide range of biological activities.[2 – 4]
Several methods are available in the literature for the synthesis of
amines.[5 – 7] Some of these techniques have significant limitations
regarding the safety and handling considerations.[8 – 10] Reduction
of nitroaromatic compounds by catalytic hydrogenation[11 – 15] is
probably one of the best-known methods to produce aromatic
amines, although various synthetic methods have appeared
in the literature.[16,17] Many novel reducing agents have been
reported in the literature, such as the Al–NiCl2 –THF system,[18]
decaborane in methanol,[19] indium–ammonium chloride in
ethanol,[20] hydrazine hydrate/ferric oxide–magnesium oxide,
N,N-dimethyl-hydrazine in the presence of catalytic FeCl3 .6H2 O
in methanol,[21,22] diethyl chlorophosphite,[23] samarium iodine
in methanol[24] and water-soluble palladium catalyst[25] for the
preparation of aromatic amines.
On the other hand, organosilicon reagents have been widely
applied for the reduction of functional groups in the presence
of catalytic amounts of a catalyst.[26 – 30] In recent years, we
have investigated the efficiency of Et3 SiH–PdCl2 system for the
hydrogenation of 1-alkenes,[31,32] transformation of alcohols to
their corresponding silyl ethers and cleavage of triethylsilyl ethers
to the parent alcohols[33] and the selective hydrogenation of
the carbon–carbon double bond of α,β-unsaturated ketones to
the corresponding saturated ketones under mild conditions.[34]
More recently, we have shown the versatility of the Et3 SiH–PdCl2
system for the reduction of aromatic ketones and aldehydes,[35]
and benzyl alcohol derivatives to the corresponding methylene
compounds.[36]
This paper is a continuation of our previous work on
the exploitation of the Et3 SiH–PdCl2 system for the chemical
transformation of organic functional groups. Herein, we report
our preliminary results, which account for the versatility of the
PdCl2 –Et3 SiH couple in the reduction of nitroaromatic compounds
to the corresponding amines under mild conditions. Direct
reaction of different nitroaromatic compounds with Et3 SiH in the
presence of 10 mol% PdCl2 in ethanol occurs at room temperature
M. Mirza-Aghayan et al.
Table 1. Reduction of nitroarenes with Et3 SiH–PdCl2 in ethanol
Entry
Nitroarene
1
2
3
4
5
6
8
10
11
4
10
100
[37, 38a]
4
10
100
[37, 38a]
4
30
100
[37, 38a]
5
10
100
[37, 38b]
4
4
30
60
87
100
[37, 38c]
15
15
10
60
180
60b
80
91
98
10
7
45
60b
100
99
12
7
120
60b
86
85
NH2
10
6
30
60b
100
100
NH2
15
7
10
60
60b
60b
84
78
94
5
30
100
NH2
Me
NO2
Me
NH2
HO
NO2
HO
NH2
H2N
NO2
H2N
NH2
Me
OHC
NO2
OHC
HOH2C
NH2
Me
NH2
Me
HOH2C
NO2
H3COC
NO2
NO2
Me
[37]
[37, 38a]
[37]
NH2
Me
Me
Me
NH2
[37, 38a]
[37]
[38d,e]
N
Me
b
Reference
NO2
N
a
Yielda
(%)
NH2
NO2
9
Time
(min)
NO2
NO2
7
Et3 SiH time
(equiv.) (min)
Product
Me
Determined by GC/MS analysis.
The reaction was conducted in reflux of solvent.
478
in dry ethanol (5 ml). An exothermic reaction takes place in the first
5 min and then the temperature decreases to room temperature.
The resulting mixture is further kept under stirring for the time
indicated in Table 1 prior to GC/MS analysis. The obtained results
are summarized in Table 1.
A wide variety of nitroaromatic compounds are investigated in
this work. The results indicate that the nitroarene compounds can
be readily converted to the corresponding amines in excellent
yields. For example, the reaction of nitrobenzene (entry 1, Table 1)
with 4 equiv. of Et3 SiH in ethanol gave 100% of aniline after 10 min.
Similarly, 1-nitronaphthalene (entry 2, Table 1) was reduced to
naphthylamine in only 10 min by this method in excellent
yield (100%). Furthermore, 4-nitrotoluene (entry 3, Table 1), 4nitrophenol (entry 4, Table 1) and 4-nitroaniline (entry 5, Table 1)
were effectively reduced to p-toluidine, 4-aminophenol and 1,4phenylenediamine within 30, 10 and 60 min, respectively in 100%
yield.
We next examined the reduction of substituted nitroaromatic
compounds bearing a reducible group such as carbonyl or alcohol
(-CH2 -OH) function (entries 6–10, Table 1). The results show that
www.interscience.wiley.com/journal/aoc
these compounds can be readily converted to the corresponding
anilines in excellent yields. A concomitant reduction of the
carbonyl (CO) and hydroxyl (OH) groups to CH2 and H groups,
respectively, was observed. Under these conditions, the reduction
reaction requires an excess of triethylsilane or reflux of solvent. For
example, the reduction of substituted nitroaromatic compounds
such as 3- and 4-nitrobenzaldehyde (entries 6 and 7, Table 1)
using 15 and 10 equiv. of Et3 SiH gave 80% m-toluidine and 100%
p-toluidine after 60 and 45 min at room temperature, respectively.
Decreasing the amount of Et3 SiH requires a higher temperature
to achieve high reaction yields. Indeed, the reaction of 3- and
4-nitrobenzaldehyde with 10 and 7 equiv. of Et3 SiH yielded
98% m-toluidine and 99% p-toluidine after 60 min in reflux of
ethanol, respectively. On the other hand, the reaction of 3- and
4-nitrobenzyl alcohol (entries 8 and 9, Table 1) with 12 and 10
equiv. of Et3 SiH afforded 86% m-toluidine and 100% p-toluidine
after 120 and 30 min at room temperature, respectively. While the
reduction of the same compounds using 7 and 6 equiv. of Et3 SiH
gave 85% m-toluidine and 100% p-toluidine after 60 min in reflux
of ethanol, respectively (entries 8 and 9, Table 1). Furthermore, the
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 477–480
Palladium-catalyzed reduction of nitroaromatic compounds
Table 2. Comparison of our method with other procedures
Entry
Nitroarene
Product
1
C6 H5 NO2
C6 H5 NH2
2
3
4
1-Nitronaphthalene
p-NO2 C6 H5 Me
p-NO2 C6 H5 OH
1-Aminonaphthalene
p-NH2 C6 H5 Me
p-NH2 C6 H5 OH
Appl. Organometal. Chem. 2010, 24, 477–480
Time
Yield (%)
This work
Baker’s yeast, NaOH
H2 O–MeOH, 70–80 Ž C
FeCl3 –In
H2 O–MeOH-Sonication
Room temperature
10 min
100
20 h
55[39a]
1.5 h
94[39b]
This work
Baker’s yeast, NaOH
H2 O–MeOH, 70–80 Ž C
Sm, I2 (cat.)
MeOH, reflux
Sm (4 equiv.)–NH4 Cl (20 equiv.)
MeOH-Sonication
Room temperature
10 min
100
7 h
95[39a]
8 h
64[24]
10 min
88[39c]
This work
FeCl3 –In
H2 O–MeOH-Sonication
Room temperature
30 min
100
1.5 h
91[39b]
This work
Baker’s yeast, NaOH
H2 O–MeOH, 70–80 Ž C
FeCl3 –In
H2 O–MeOH-Sonication
Room temperature
10 min
100
24 h
18[39a]
2 h
86[39b]
temperature under sonication for 30 min. To this solution,
a nitroarene was then added and stirred for 2 h at room
temperature. Using this procedure, nitrobenzene (entry 1, Table 2),
4-nitrotoluene (entry 3, Table 2) and 4-nitrophenol (entry 4,
Table 2) were reduced to aniline, p-toluidine and 4-aminophenol
within 90, 90 and 120 min in 94, 91 and 86% yield, respectively.[39b]
Banik et al.[24] studied the reduction of 1-nitronaphthalene (entry
2, Table 2) in refluxing of methanol using samarium and catalytic
amounts of iodine. 1-Aminonaphthalene was obtained after 8 h in
only 64% yield. Another report showed the reduction of the
same compound (entry 2, Table 2) using 4 equiv. samarium
metal and 20 equiv. NH4 Cl under sonication conditions in 88%
yield after 10 min.[39c] In comparison with other procedures,
the Et3 SiH–PdCl2 system in ethanol reduces nitroaromatic
compounds in higher yields and shorter reaction times under mild
conditions. The comparative results are summarized in Table 2.
The mechanism of this reduction is not very clear at this stage.
Based on previous reports,[31,34,40] it is assumed that molecular
hydrogen generated in situ by the reaction of Et3 SiH and ethanol
in the presence of PdCl2 catalyst is responsible of the reduction of
the nitro group. The reaction of molecular hydrogen (H2 ) with the
nitro function leads to the formation of the corresponding amines.
In conclusion, we developed an efficient and simple method for
the synthesis of aromatic amines by the reduction of nitroaromatic
compounds using an Et3 SiH–PdCl2 system in ethanol. The utility
of this methodology will make this simple technique an attractive
addition to the range of procedures already known for this
general transformation. The obvious advantages over the previous
methods[24,39] are: short reduction times, mild conditions, use
of catalytic amount of catalyst and high reduction yields. This
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
479
reduction of 40 -nitroacetophenone with 15 equiv. of Et3 SiH yielded
84% 4-ethylaniline after 60 min at room temperature (entry 10,
Table 1). Decreasing the concentration of Et3 SiH (10 equiv.) while
operating in reflux of ethanol led to an increase of the reaction
yield to 94%.
We have next examined the reduction of heterocyclic nitroaromatic compounds. The reaction of 2,6-dimethyl-3-nitropyridine
with 5 equiv. of Et3 SiH in ethanol at room temperature yielded
100% of 2,6-dimethylpyridin-3-amine only after 30 min (entry 11,
Table 1). However, only the starting material was recovered when
2-nitrothiophene was reacted with 4 equiv. of Et3 SiH in ethanol at
room temperature or with 8 equiv. of Et3 SiH in reflux of ethanol
after 6 h.
This method of reduction appears to be limited to nitroaromatic
compounds. Indeed, when the reaction conditions described
above were applied to an aliphatic nitro compound such as
nitroethane or 1-nitropropane, only the starting material was
recovered after 1 h reaction with 4 equiv. Et3 SiH in ethanol at
room temperature or with 8 equiv. Et3 SiH in reflux of ethanol after
5 h.
It should be noted that long reaction times were reported in the
literature for the reduction of nitro aromatic compounds.[39] For
example Baik et al. investigated the reduction of nitrobenzene
(entry 1, Table 2), 1-nitronaphthalene (entry 2, Table 2) and
4-nitrophenol (entry 4, Table 2) using Baker’s yeast in a basic
solution at 80 Ž C. The corresponding aromatic amines were
obtained only after 20, 7 and 24 h in 55, 95 and 18% yield,
respectively.[39a] Yoo et al. performed the reduction of nitroarenes
with FeCl3 ž 6H2 O–indium powder.[39b] Firstly, FeCl3 ž 6H2 O, indium
powder and H2 O–MeOH were mixed and stirred at room
Conditions
M. Mirza-Aghayan et al.
procedure will therefore be of general use especially in cases
where rapid and mild reduction conditions are required.
Experimental
All manipulations were carried out under an argon atmosphere.
The nitroaromatic compounds and triethylsilane were obtained
from Aldrich and used without further purification. Ethanol was
distilled and stored under argon. GC/MS analysis was performed
on a Fison GC 8000 series Trio 1000 gas chromatograph equipped
with a capillary column CP Sil.5 CB, 60 m ð 0.25 mm i.d. 1 H NMR
spectra were recorded on a Bruker 80 spectrometer using TMS as
internal standard.
General Procedure for the Reduction of Nitroaromatic
Compounds
To a solution of a nitroaromatic compound (0.2 g, 1 equiv.) and
triethylsilane (amount indicated in Table 1) in 5 ml of ethanol was
added a catalytic amount of palladium (II) chloride (10 mol%)
under an argon atmosphere. The resulting mixture was kept under
stirring for the time indicated in Table 1 prior to GC/MS analysis. The
solvent was evaporated and then water was added and decanted.
The aqueous phase was extracted with diethylether. The organic
phase was dried over MgSO4 and the solvent was evaporated
under reduced pressure. Pure products for entries 6, 8 and 10 were
isolated by column chromatography using hexane–ethylacetate
(9 : 1) as eluent. The products were characterized using 1 H NMR
and mass spectrometry.[37,38]
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