Palladium-catalyzed reduction of nitroaromatic compounds to the corresponding anilines

Article abstract of DOI:10.1002/aoc.1645

Reduction of a variety of nitroaromaticcompoundswith triethylsilane in thepresence of catalytic amounts ofpalladium chloride in ethanol resulted in the formation of the corresponding anilines in excellent yields. Copyright

Full text of DOI:10.1002/aoc.1645

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
a∗
b
a
Maryam Mirza-Aghayan , Rabah Boukherroub , Mahshid Rahimifard
a
and Mohammad Bolourtchian
Reductionofavarietyofnitroaromaticcompoundswithtriethylsilaneinthepresenceofcatalyticamountsofpalladiumchloride
in ethanol resulted in the formation of the corresponding anilines in excellent yields. Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Keywords: palladium (II) chloride; triethylsilane; reduction; nitroaromatic compounds; anilines
Introduction
Aromatic amines are important intermediates in the fine chemical,
[
1]
dyes and pigment industries. 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
[
8–10]
regarding the safety and handling considerations.
of nitroaromatic compounds by catalytic hydrogenation[
Reduction
11–15]
is
probably one of the best-known methods to produce aromatic
amines, although various synthetic methods have appeared
Scheme 1. Reduction of nitroaromatic compounds using Et3SiH in the
presence of PdCl2 catalyst in ethanol.
in the literature.[
16,17]
Many novel reducing agents have been
[
18]
reported in the literature, such as the Al–NiCl2 –THF system,
[
19]
decaborane in methanol,
indium–ammonium chloride in
hydrazine hydrate/ferric oxide–magnesium oxide,
N,N-dimethyl-hydrazine in the presence of catalytic FeCl3.6H2O
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.
ethanol,[
20]
[
21,22]
[23]
in methanol,
diethyl chlorophosphite,
samarium iodine
in methanol[ and water-soluble palladium catalyst
24]
[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
[
26–30]
Results and Discussion
of catalytic amounts of a catalyst.
In recent years, we
have investigated the efficiency of Et3SiH–PdCl2 system for the
The aim of the present study is to evaluate the efficiency of
the Et3SiH–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 Et3SiH
[
31,32]
hydrogenation of 1-alkenes,
transformation of alcohols to
their corresponding silyl ethers and cleavage of triethylsilyl ethers
[
33]
to the parent alcohols
and the selective hydrogenation of
the carbon–carbon double bond of α,β-unsaturated ketones to
the corresponding saturated ketones under mild conditions.[
More recently, we have shown the versatility of the Et3SiH–PdCl2
system for the reduction of aromatic ketones and aldehydes,[
and benzyl alcohol derivatives to the corresponding methylene
compounds.[
34]
35]
36]
Ł
Correspondence to: Maryam Mirza-Aghayan, Chemistry and Chemical Engi-
This paper is a continuation of our previous work on
the exploitation of the Et3SiH–PdCl2 system for the chemical
transformation of organic functional groups. Herein, we report
our preliminary results, which account for the versatility of the
PdCl2 –Et3SiHcoupleinthereductionofnitroaromaticcompounds
to the corresponding amines under mild conditions. Direct
reaction of different nitroaromatic compounds with Et3SiH in the
presence of 10 mol% PdCl2 in ethanol occurs at room temperature
neering 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,deMicro e´ lectroniqueetdeNanotechnologie(IEMN,UMR8520),
Cit e´ Scientifique, AvenuePoincar e´ – B.P60069, 59652Villeneuved’Ascq, France
Appl. Organometal. Chem. 2010, 24, 477–480
Copyright ꢀc 2010 John Wiley & Sons, Ltd.

M. Mirza-Aghayan et al.
Table 1. Reduction of nitroarenes with Et3SiH–PdCl2 in ethanol
Et3SiH time
(equiv.) (min)
Time
(min)
Yielda
(%)
Entry
1
Nitroarene
Product
NH2
Reference
[37, 38a]
4
10
100
NO2
2
4
10
100
[37, 38a]
NO2
NH2
3
4
5
4
5
30
10
100
100
[37, 38a]
[37, 38b]
[37, 38c]
Me
HO
NO2
Me
HO
NH2
NO2
NO2
NH2
NH2
4
4
30
60
87
H N
H N
2
2
100
[
37]
6
15
60
80
91
98
OHC
OHC
Me
1
5
0
180
60b
NO2
NH2
1
7
8
10
7
45
60b
100
99
[37, 38a]
NO2
Me
NH2
[37]
12
7
120
60b
86
85
HOH C
Me
2
NO2
NH2
9
10
6
30
100
100
[37, 38a]
HOH C
NO2
Me
Me
NH2
2
60b
[37]
1
0
15
7
60
84
78
94
H COC
NO2
3
60b
NH2
10
60b
11
5
30
100
[38d,e]
Me
NO2
Me
NH2
N
N
Me
Me
a
Determined by GC/MS analysis.
The reaction was conducted in reflux of solvent.
b
in dry ethanol (5 ml). An exothermic reaction takes place in the first
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 Et3SiH 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), 4-
nitrophenol (entry 4, Table 1) and 4-nitroaniline (entry 5, Table 1)
were effectively reduced to p-toluidine, 4-aminophenol and 1,4-
phenylenediamine within 30, 10 and 60 min, respectively in 100%
yield.
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 Et3SiH gave 80% m-toluidine and 100%
p-toluidine after 60 and 45 min at room temperature, respectively.
Decreasing the amount of Et3SiH requires a higher temperature
to achieve high reaction yields. Indeed, the reaction of 3- and
4-nitrobenzaldehyde with 10 and 7 equiv. of Et3SiH 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 Et3SiH 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 Et3SiH
gave 85% m-toluidine and 100% p-toluidine after 60 min in reflux
of ethanol, respectively (entries 8 and 9, Table 1). Furthermore, the
5
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
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 477–480

Palladium-catalyzed reduction of nitroaromatic compounds
Table 2. Comparison of our method with other procedures
Entry
1
Nitroarene
C6H5NO2
Product
Conditions
Time
Yield (%)
100
C6H5 NH2
This work
Baker’s yeast, NaOH
10 min
Ž
55[39a]
H2O–MeOH, 70–80 C
FeCl3 –In
20 h
H2O–MeOH-Sonication
Room temperature
1.5 h
94[39b]
100
2
1-Nitronaphthalene
1-Aminonaphthalene
This work
10 min
Baker’s yeast, NaOH
Ž
95[39a]
64[24]
H2O–MeOH, 70–80 C
7 h
8 h
Sm, I2 (cat.)
MeOH, reflux
Sm (4 equiv.)–NH4Cl (20 equiv.)
MeOH-Sonication
Room temperature
10 min
88[39c]
3
4
p-NO2C6H5Me
p-NO2C6H5OH
p-NH2C6H5Me
p-NH2C6H5OH
This work
30 min
1.5 h
100
FeCl3 –In
H2O–MeOH-Sonication
Room temperature
91[39b]
This work
10 min
24 h
2 h
100
Baker’s yeast, NaOH
Ž
18[39a]
86[39b]
H2O–MeOH, 70–80 C
FeCl3 –In
H2O–MeOH-Sonication
Room temperature
0
reductionof4 -nitroacetophenonewith15equiv. ofEt3SiHyielded
4% 4-ethylaniline after 60 min at room temperature (entry 10,
temperature under sonication for 30 min. To this solution,
a nitroarene was then added and stirred for 2 h at room
temperature. Usingthisprocedure, nitrobenzene(entry1, Table 2),
4-nitrotoluene (entry 3, Table 2) and 4-nitrophenol (entry 4,
Table 2) were reduced to aniline, p-toluidine and 4-aminophenol
8
Table 1). Decreasing the concentration of Et3SiH (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 nitroaro-
matic compounds. The reaction of 2,6-dimethyl-3-nitropyridine
with 5 equiv. of Et3SiH in ethanol at room temperature yielded
00% of 2,6-dimethylpyridin-3-amine only after 30 min (entry 11,
Table 1). However, only the starting material was recovered when
-nitrothiophene was reacted with 4 equiv. of Et3SiH in ethanol at
[
39b]
within 90, 90 and 120 min in 94, 91 and 86% yield, respectively.
Banik et al.[ 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
24]
1
2
room temperature or with 8 equiv. of Et3SiH in reflux of ethanol
after 6 h.
metal and 20 equiv. NH4Cl under sonication conditions in 88%
yield after 10 min.[
39c]
In comparison with other procedures,
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. Et3SiH in ethanol at
room temperature or with 8 equiv. Et3SiH in reflux of ethanol after
the Et3SiH–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 Et3SiH 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 Et3SiH–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
5
h.
It should be noted that long reaction times were reported in the
literature for the reduction of nitro aromatic compounds.[ For
39]
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
Ž
solution at 80 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
generaltransformation. Theobviousadvantagesovertheprevious
[
39b]
[24,39]
with FeCl3ž6H2O–indium powder.
powder and H2O–MeOH were mixed and stirred at room
Firstly, FeCl3ž6H2O, indium
methods
are: short reduction times, mild conditions, use
of catalytic amount of catalyst and high reduction yields. This
Appl. Organometal. Chem. 2010, 24, 477–480
Copyright ꢀc 2010 John Wiley & Sons, Ltd.
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M. Mirza-Aghayan et al.
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[
[
[
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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
stirringforthetimeindicatedinTable 1priortoGC/MSanalysis.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
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Copyright ꢀc 2010 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 477–480