Mild and efficient nitration of aromatic compounds mediated by transitionmetal complexes

Article abstract of DOI:10.1080/00397911.2010.515432

Aromatic compounds were efficiently nitrated under facile reaction conditions by employing 69% nitric acid catalyzed by transition-metal complexes such as [Co(NH₃)₅Cl]Cl₂, [Cu(NH ₃)₄]SO₄, Mn(acac)₃, [Ni(NH ₃)₆]Cl₂, [Ni(en)₃]S ₂O₃, and Hg[Co(SCN)₄]. The reaction was completed smoothly at room temperature and afforded corresponding mono-nitro derivatives in quantitative yield. This new method offers efficient and facile regioselective mononitration of aromatic compounds. Taylor & Francis Group, LLC.

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Mild and Efficient Nitration of Aromatic
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Complexes
a
a
a
b
Abdulla , S. Amina , Y. Arun Kumar , M. Arifuddin & K. C.
c
Rajanna
a
Department of Chemistry, Muffakhamjah College of Engineering
and Technology, Hyderabad, India
b
National Institute of Pharmaceutical Education and Research
(
NIPER Hyderabad), Hyderabad, India
c
Nizam College, Osmania University, Hyderabad, India
Version of record first published: 29 Jun 2011.
To cite this article: Abdulla , S. Amina , Y. Arun Kumar , M. Arifuddin & K. C. Rajanna (2011): Mild
and Efficient Nitration of Aromatic Compounds Mediated by Transition-Metal Complexes, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
41:19, 2946-2951
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1
Synthetic Communications , 41: 2946–2951, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.515432
MILD AND EFFICIENT NITRATION OF AROMATIC
COMPOUNDS MEDIATED BY TRANSITION-
METAL COMPLEXES
1
Abdulla, S. Amina, Y. Arun Kumar, M. Arifuddin, and
1
1
2
3
K. C. Rajanna
1
Department of Chemistry, Muffakhamjah College of Engineering and
Technology, Hyderabad, India
2
National Institute of Pharmaceutical Education and Research (NIPER
Hyderabad), Hyderabad, India
3
Nizam College, Osmania University, Hyderabad, India
GRAPHICAL ABSTRACT
Abstract Aromatic compounds were efficiently nitrated under facile reaction conditions
by employing 69% nitric acid catalyzed by transition-metal complexes such as
[
and Hg[Co(SCN)
Co(NH
3
)
5
Cl]Cl
2
,
[Cu(NH
3 4 4 3 3 6 2 3 2 3
) ]SO , Mn(acac) , [Ni(NH ) ]Cl , [Ni(en) ]S O ,
4
]. The reaction was completed smoothly at room temperature and
afforded corresponding mono-nitro derivatives in quantitative yield. This new method offers
efficient and facile regioselective mononitration of aromatic compounds.
Keywords Aromatic nitration; nitric acid; regioselective; transition-metal complexes
INTRODUCTION
Nitration of organic compounds has long been a very active and rewarding
[
1]
area of research and is the subject of a large body of literature. Nitro aromatic
compounds are extensively utilized as chemical feedstocks for the synthesis of a wide
[
2]
range of useful materials such as dyes, pharmaceuticals, perfumes, and plastics.
The classical nitration procedure requires the use of a potent mixture of concentrated
nitric and sulfuric acids, yielding large amounts of waste streams of inorganic acids,
which are environmentally unfriendly and expensive to dispose of. In addition, the
synthetic procedure is not very selective and often leads to overnitration or
Received May 7, 2010.
Address correspondence to K. C. Rajanna, Nizam College, Osmania University, Hyderabad 500
94, India. E-mail: kcrajannaou@rediffmail.com
0
2
946

NITRATION OF AROMATIC COMPOUNDS
2947
Scheme 1. Nitration of aromatic compounds mediated by transition-metal complexes.
by-products. The obvious disadvantages associated with these procedures prompted
extensive research in finding new nitration methods such as the use of solid acid cata-
[
lyst, different sources of NO2, organic nitrating agents, metal nitrates, Lewis
3]
[4]
[5]
[6]
[
acids to replace sulfuric acid such as inorganic acidic salt (oxone), and rare-earth
7]
[
8]
metals. With chemists under increasing pressure to perform atom-economical pro-
cesses with minimal or no environmentally unfriendly by-products, development of a
novel catalyst system that facilitate aromatic nitrations in this manner should be of
[9]
great importance. In continuation of our work on environmentally friendly aro-
matic nitration using metal ions,
[10]
we found that transition-metal complexes are
efficient catalysts for the nitration of a wide range of aromatic compounds using stoi-
chiometric quantities of nitric acid.
It is very well known that transition metals and their complexes efficiently
accelerate a wide variety of organic reactions and have been successfully applied
[
11]
[12]
[13]
for catalytic epoxidation,
transition-metal complexes recently have been used for the nitration of aromatics,
oxidation,
and hydrogenation.
Although some
[
14]
[
15]
surprisingly, the Werner type of transition-metal complexes has not been explored
for aromatic nitration reactions so far. In continuation of our earlier work on
nitration reactions, here we report the mild and efficient nitration of an aromatic
compound mediated by transition-metal complexes (Scheme 1).
[16]
RESULTS AND DISCUSSION
A few Werner-type transition-metal complexes such as [Co(NH ) Cl]Cl ,
3
5
2
[
chosen as the best candidates to promote the nitration of aromatics with nitric acid
Cu(NH ) ]SO , Mn(acac) , [Ni(NH ) ]Cl , [Ni(en) ]S O , and Hg[Co(SCN) ] were
3 4 4 3 3 6 2 3 2 3 4
[
17]
because it is easy to prepare these complexes. We have investigated the nitration
of various aromatics by these complexes with nitric acid in dichloromethane at room
temperature. To optimize the reaction condition, phenol was selected as a promising
candidate for plausible nitration. The phenol (0.1 mol) and [Co(NH )5Cl]Cl2
3
(0.1 mol) were taken in dichloromethane, and nitric acid was added to this mixture
and stirred at room temperature. Thin layer chromatography (TLC) monitored the
progress of the reaction. The products were isolated and analyzed by comparison
with their characteristic spectroscopic data as reported in the literature. The
nitration reactions performed with the other chosen transition-metal complexes
revealed that all the Werner-type transition-metal complexes are quite effective in
promoting the nitration of aromatics. Furthermore, the present nitration procedure
was found to be applicable to a range of aromatics compounds as depicted in
Tables 1 and 2. It is interesting to note that the present nitration work is under

2948
ABDULLA ET AL.
Table 1. Nitration of aromatic compounds mediated by transition-metal complexes
Co(NH
3
)
5
Cl]Cl
2
[Cu(NH
3
)
4
SO
4
Mn(acac)
3
Time
(h)
Yield
(%)
Time Yield Time Yield
(%)
a,b
No.
Substrate
Phenol
O-Cresol
Products
(h)
(%)
(h)
1
2
3
4
5
6
7
8
9
4-Nitrophenol
4-Nitro-2-methylphenol
2-Nitroacetanilide
2
2
85
85
85
85
82
82
82
85
82
82
82
80
82
82
80
2.5
2.5
2.5
2
82
85
85
89
85
85
85
86
84
84
85
89
89
84
82
2.5
2.5
2.5
2
82
84
85
87
83
80
80
85
83
85
85
89
89
82
80
Acetanilide
Naphthalene
Toluene
2
1-Nitronaphthalene
4-Nitrotoluene
2
3
3
3
Chlorobenzene
Bromobenzene
Salicylic acid
4-Nitrochlorobenzene
4-Nitrobromobenzene
5-Nitrosalicylic acid
2
2
2.5
2.5
2.5
2
2
2
2.5
2
2.5
2
4-Methylacetanilide 4-Methyl-2-nitroacetanilide
1
1
1
1
1
1
0
1
2
3
4
5
Benzaldehyde
Acetophenone
Benzophenone
Benzoic acid
Benzonitrile
Benzamide
3-Nitrobenzaldehyde
3-Nitroacetophenone
3-Nitrobenzophenone
3-Nitrobenzoic acid
3-Nitrobenzonitrile
3-Nitrobenzamide
2
3
2.5
2
2
2
2
2.5
2.5
2.5
3
2.5
2.5
2.5
3
3
2.5
3
a
b
All the products were characterized by the usual spectral and analytical methods.
Isolated yields.
heterogeneous conditions and therefore transition-metal complexes are easily
separated from the reaction mixture by simple workup. Furthermore, it is also
observed that nitration of aromatics proceeded rapidly and efficiently with the
Table 2. Nitration of aromatic compounds mediated by transition-metal complexes
[
Ni(NH
3
)
6
]Cl
2
Ni[(en)
3
]S
2
O
3
4
Hg[Co(SCN) ]
Time Yield Time Yield Time Yield
(%)
a,b
No.
Substrate
Phenol
O-Cresol
Products
(h)
(%)
(h)
(%)
(h)
1
2
3
4
5
6
7
8
9
4-Nitrophenol
4-Nitro-2-methylphenol
2-Nitroacetanilide
2
2
80
82
82
80
82
80
80
80
82
80
82
82
80
80
82
2.5
2.5
2.5
2
82
80
82
80
83
82
82
82
84
82
82
80
80
80
82
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
72
74
65
77
73
70
70
75
73
65
75
69
69
72
70
Acetanilide
Naphthalene
Toluene
2
1-Nitronaphthalene
4-Nitrotoluene
2
3
3
Chlorobenzene
Bromobenzene
Salicylic acid
4-Nitrochlorobenzene
4-Nitrobromobenzene
5-Nitrosalicylic acid
2
2
2
2
2.5
2
2.5
2
4-Methylacetanilide 4-Methyl-2-nitroacetanilide
1
1
1
1
1
1
0
1
2
3
4
5
Benzaldehyde
Acetophenone
Benzophenone
Benzoic acid
Benzonitrile
Benzamide
3-Nitrobenzaldehyde
3-Nitroacetophenone
3-Nitrobenzophenone
3-Nitrobenzoic acid
3-Nitrobenzonitrile
3-Nitrobenzamide
2
3
2
2
2
2.5
2.5
2.5
3
3
2.5
3
a
b
All the products were characterized by the usual spectral and analytical methods.
Isolated yields.

NITRATION OF AROMATIC COMPOUNDS
2949
present system without any overoxidation. In all the cases, nitration products we are
obtained in quantitative yield within short times at room temperature. The reaction
conditions are extremely mild as evidenced by the formation of 3-nitrobenzaldehyde
from benzaldehyde without oxidation of aldehyde functionality. The reaction is car-
ried out in various solvents such as ethyl acetate, ethanol, acetonitrile, and dichlor-
omethane. Among all the solvents tried, the dichloromethane was found to be more
suitable as it gives good yields of the product.
The structure of the nitrating complex and plausible mechanism are not clear at
present. It is believed that the nitration is proceeding via the binding of nitric acid to
the metal part of the transition-metal complex, which in turn releases a proton. The
proton thus released further reacts with another molecule of nitric acid and liberates
þ
þ
the nitronium ion (NO ). Nitro aromatics are produced when this NO ion attacks
2
2
the aromatic ring. Further work to explore this mechanism of the reaction is in
progress.
CONCLUSION
In summary, the use of the transition-metal complex–nitric acid system is
simple, fast, and mild for the nitration of aromatic ring. The reaction is tolerant
to a range of functional groups and gives quantitative yields. Thus, this new method
offers efficient and facile regioselective mononitration of aromatic compounds.
EXPERIMENTAL
The general procedure is as follows: To a solution of phenol (1 mmol) in
dichloromethane, 69% of nitric acid (1 mmol) and 1 mmol of transition-metal
complex ([Co(NH ) Cl]Cl ) were added, and the reaction mixture was stirred at
3
5
2
room temperature for 2 h. The progress of the reaction was monitored by TLC. After
the completion of reaction as indicated by TLC, the reaction mixture was treated
with sodium carbonate solution. The reaction mixture was extracted with dichloro-
methane. The organic layer was separated, dried over sodium sulfate, and evapo-
rated to give the crude product. The crude product was purified over silica gel to
get 4-nitrophenol as the product in 85% yield.
ACKNOWLEDGMENT
The authors are thankful to the principal and management of Muffakhamjah
College of Engineering and Technology for their constant encouragement, support,
and permission to carry out the research work.
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