Heteropolyacids as an efficient and reusable catalytic system for the regiospecific nitration of phenols with metal nitrates

Article abstract of DOI:10.4314/bcse.v26i1.17

Highly regiospecific mononitration of phenols and substituted phenols is accomplished employing a metal nitrate and a catalytic amount of heteropolyacid in acetonitrile. An xclusive ortho-selectivity was observed with excellent yields. A variety of metal nitrates were used to obtain o-nitrophenols exclusively in good to excellent yields. The use of heteropolyacid is key for the selectivity observed.

Full text of DOI:10.4314/bcse.v26i1.17

Bull. Chem. Soc. Ethiop. 2012, 26(1), 145-152.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v26i1.17
ISSN 1011-3924
 2012 Chemical Society of Ethiopia
SHORT COMMUNICATION
HETEROPOLYACIDS AS AN EFFICIENT AND REUSABLE CATALYTIC SYSTEM
FOR THE REGIOSPECIFIC NITRATION OF PHENOLS WITH METAL NITRATES
1
*
2*
Bita Baghernejad , Majid M. Heravi , Hossein A.Oskooie and Fatemeh F. Bamoharram
2
3
1
Department of Chemistry, School of Sciences, Payame Noor University (PNU), 19395-4697,
Iran
2
Department of Chemistry, School of Science, Azzahra University, Vanak, Tehran, Iran
3
Department of Chemistry, School of Sciences, Azad University Khorasan Branch, Mashhad,
Iran
(Received November 8, 2010; revised June 24, 2011)
ABSTRACT. Highly regiospecific mononitration of phenols and substituted phenols is accomplished
employing a metal nitrate and a catalytic amount of heteropolyacid in acetonitrile. An exclusive ortho-selectivity
was observed with excellent yields. A variety of metal nitrates were used to obtain o-nitrophenols exclusively in
good to excellent yields. The use of heteropolyacid is key for the selectivity observed.
KEY WORDS: Heteropolyacids, Phenol, Nitration, Regiospecific
INTRODUCTION
Aromatic nitro compounds are important starting materials for the manufacture of various
industrial products such as pharmaceuticals, dyes and plastics.
The nitration of aromatic is one of the oldest and most important processes in the chemical
industry for the production of intermediates. Extensive and well-documented reviews have been
published about the mechanism [1-3] and using various nitrating agents under different
conditions [4-25], most of them are not usually selective and cause environmental concerns
regarding the disposal of large volumes of mixed acids typically employed in this process [26,
2
7]. A mixture of concentrated nitric acid and sulfuric acid is used as the most common
nitrating reagent for the nitration of benzene, alkyl benzene, and less reactive aromatic
compounds [28, 29], highly reactive aromatic compounds, phenols and pyrroles, etc., require
mild nitration condition, thus a wide variety of nitration methods have been developed [24, 25,
3
0, 31]. Conventional methods for the nitration of aromatic compounds utilize a mixture of
nitric and sulfuric acids or nitronium tetrafluoroborate. However their use is always associated
with the formation of dinitro compounds, oxidized products and unspecified resinous tarry
materials from over-oxidation of the substrate. Furthermore, it is noteworthy that the typical
yield of direct nitration never exceeds 60% [26] because of the above mentioned side reactions
in most cases, which makes these existing processes uneconomical. On the other hand, the use
of metal nitrates like ceric ammonium nitrate [8], copper nitrate [32, 33] and vanadium nitrate
[
34], ferric nitrate [7] have contributed significantly, enhancing the scope of the nitration
process however, there are concerns regarding achieving efficient regioselectivity. All these
methods also suffer from other drawbacks, for example some of the nitrates must be prepared
from dinitrogen pentoxide, or they must either use the clay supported or absorbed on silica gel
prior to their use as nitrating agents. Recently, solid acids have attracted much attention in
organic synthesis owing to their easy work-up procedures, easy filtration, and minimization of
_
*
_________
Corresponding author. E-mail: bitabaghernejad@yahoo.com; mmh1331@yahoo.com

146
Bita Baghernejad et al.
cost and waste generation due to reuse and recycling of these catalysts [35]. The application of
heteropolyacids, HPAs, as catalytic materials is growing continuously in the catalytic field.
These compounds possess unique properties such as: well-defined structure, Brönsted acidity,
possibility to modify their acid-base and redox properties by changing their chemical
composition (substituted HPAs), ability to accept and release electrons, high proton mobility,
being environmentally benign and presenting fewer disposal problems [36]. Because of their
stronger acidity, they generally exhibit higher catalytic activity than conventional catalysts such
as mineral acids, ion exchange resins, mixed oxides, zeolites, etc [37]. They are used as
industrial catalysts for several liquid phase reactions [38], such as alcohol dehydration [39],
alkylation [40] and esterification [41]. Recently, H PW O was used on the nitration of
3
12 40
3 3 2
phenols by Fe(NO ) .9H O in low yields [42] and in our lab, we used nitric acid as a strongly
acidic media in the presence of HPAs to the nitration of phenols [43]. Because of our studies
towards the development of new routes to the synthesis of heterocyclic compounds [44-49] in
mild nitration condition, we decided to use metal nitrates as a safe nitrating reagent on the
nitration of phenols in the presence of a catalytic amount of various type of HPAs including,
H
yields (Table 2, 3).
14[NaP
5
W
30
O
110], H
4 5 2 6 2 18
[PMo11VO40], H [PMo10V O40] and H [P W O62] (Scheme 1) in good
OH
OH
M(NO ) .xH O
NO2
3
2
n
heteropoly acid
R
CH CN, reflux
R
3
Scheme 1
RESULTS AND DISCUSSION
Nitration of phenol in the presence of a catalytic amount of HPAs with a variety of metal
nitrates was screened and the results are summarized in Table 1. The use of a catalytic amount
of HPAs is key for the reaction to proceed with high regiospecificity and each of the metal
nitrates used afforded o-nitrophenols as a single regioisomer in good to excellent yields. Even
though the reaction proceeds with various metal nitrates, the time required for completion of the
3 4 2
reaction was found to be less and the yields are more effective with Th(NO ) .5H O. Phenols
were treated with a metal nitrate and HPAs in acetonitrile at reflux to afford o-nitrophenol as the
exclusive product in satisfactory to good yields (Scheme 1). The scope and generality of the
method is illustrated with several examples in Table 3 and all of them yielded the o-nitro
product exclusively in good to excellent yields. Phenols with electron donating groups (entries
2
, 6, 7 in Table 2) behaved very well to afford the o-nitrophenols as the exclusive products in
excellent isolated yields. Phenols with moderately deactivating groups (entries 3,4 in Table 2)
also yielded the o-nitrophenols in good yields.
In a systematic study and aimed work, the reaction has been examined with various type of
HPAs and the effects of the catalyst type, the reaction temperature and solvent have also been
studied.
Effect of the catalyst type. As shown in Table 2, among heteroployacids with Keggin, Preyssler
and Wells-Dawson structures used in present study, H [NaP W O110] shows higher activity
and the yield of product decreases in the following order:
14
5
30
H
5
14[NaP W
30
O
110] > H
[P W O62] > H [PMo10V O40] > H [PMo11VO40]
6 2 18 5 2 4
Bull. Chem. Soc. Ethiop. 2012, 26(1)

Short Communication
147
Table 1. Nitration of phenol with metal nitrates using various heteropolyacids under refluxing condition.
a
Entry M(NO
Th(NO
Th(NO
Th(NO
3
)
n
.xH
2
O
Catalyst
14[NaP
[P
[PMo10
[PMo11VO40
14[NaP
[P
[PMo10
[PMo11VO40
14[NaP
[P
[PMo10
[PMo11VO40
14[NaP
[P
[PMo10
[PMo11VO40
14[NaP
[P
[PMo10
[PMo11VO40
14[NaP
[P
[PMo10
[PMo11VO40
Time (min) o-nitro phenol (%)
1
2
3
4
5
3
)
4
.5H
4
.5H
4
.5H
2
O
O Th(NO
H
5
W
30
O
110
]
]
]
]
]
]
25
30
35
35
30
35
40
40
40
45
45
45
50
60
60
60
50
60
60
60
50
60
60
60
95
90
89
85
90
83
80
80
88
83
81
79
85
79
75
73
85
82
80
79
80
77
72
70
3
)
)
2
2
3
)
4
.5H
2
O
H
6
2
W
18
O
62
]
3
O
H
5
2 40
V O ]
H
H
4
]
Ni(NO
Ni(NO3)
Ni(NO
3
)
2
.6H
.6H
.6H
2
O
5
W
30
O
110
2
2
O Ni(NO
O
3
)
2
.6H
2
O
H
6
2
W
18
O
62
]
3
)
2
2
H
5
2 40
V O ]
H
H
4
]
Hg
2
Hg
2
Hg
2
Hg
2
(NO
(NO
(NO
(NO
3
)
3
)
3
)
3
)
2
2
2
2
.2H
.2H
.2H
.2H
2
O
O
O
O
5
W
30
O
110
2
H
6
2
W
18
O
62
]
2
H
5
2 40
V O ]
2
H
4
]
Bi(NO
Bi(NO
Bi(NO
Bi(NO
3
3
3
3
)
)
)
)
3
3
3
3
.5H
.5H
.5H
.5H
2
O
2
O
2
O
2
O
H
5
W
30
O
110
H
6
2
W
18
O
62
]
H
5
2 40
V O ]
H
H
4
]
Fe(NO
Fe(NO
Fe(NO
Fe(NO
3
3
3
3
)
)
)
)
3
3
3
3
.9H
.9H
.9H
.9H
2
O
O
O
O
5
W
30
O
110
2
2
2
H
6
2
W
18
O
62
]
H
5
2 40
V O ]
H
H
4
]
6
NaNO
NaNO
NaNO
NaNO
3
5
W
30
O
110
3
H
6
2
W
18
O
62
]
3
3
H
5
2 40
V O ]
H
4
]
a
Yields were analyzed by GC.
3 4 2 5 30
Table 2. Nitration of phenols with Th(NO ) .5H O and H14[NaP W O110].
a
Entry Substrate Product
Time
(
Yield (%)
o
o
min) 25 C 50 C
o
82 C
95
1
2
3
4
5
6
7
8
9
Phenol
o-Nitrophenol
25
10
60
60
25
20
10
25
25
75
80
70
70
70
81
80
72
71
85
90
79
80
80
88
86
80
80
4-Methoxy phenol 4-Methoxy-2-nitro phenol
98
4-Nitro phenol
4-Bromo phenol
1,3-Benzen diol
2-Methyl phenol
4-Methyl phenol
2-Naphthol
2,4-Dinitro phenol
88
4-Bromo-2-nitro phenol
2-Nitro-1,3-benzen diol
2-Methyl-2-nitro phenol
4-methyl-2-nitro phenol
1-Nitro-2-naphthol
87
87
98
98
93
1-Naphthol
2-Nitro-1-naphthol
93
a
Yields were analyzed by GC.
In acid-catalyzed reactions by heteropolyacids, several types of acid sites are present [50-
3]. They are including: proton sites in bulk heteropolyacids, Lewis acid sites in salts of them
metal counteractions), proton sites in acidic salts, proton sites generated by dissociation of
5
(
coordinated water and reduction of salts, and proton generated by partial hydrolysis of
polyanions. Generally reactions catalyzed by heteropolyacids may be represented by the
conventional mechanisms of Bronsted acid catalysis. The mechanism may include the
protonation of the substrate followed by the conversion of the ionic intermediate to yield the
reaction product [50, 51]. Misono and co-workers advanced two types of catalysis for
heterogeneous acid catalysis by heteropolyacids as surface type and bulk type [52, 53]. In
Bull. Chem. Soc. Ethiop. 2012, 26(1)

148
Bita Baghernejad et al.
surface type catalysis, the reactions occur on the surface of bulk or supported heteropoly
compounds and the catalytic activity usually depends on the surface acidity of heteropolyacid.
In this case, the reaction rate and yield is parallel to the number and the strength of the
accessible surface acid sites. The bulk type of mechanism is largely relevant to reactions of
polar substrates on bulk heteropoly compounds. These substrates are capable of absorbing into
the catalyst bulk, and thus all protons both in the bulk and on the surface of heteropolyacid, are
suggested to participate in the catalytic reaction Unfortunately, unlike the Keggin-type
heteropolyacids that have been widely used as acid and oxidation catalysts for organic
syntheses, the role of Preyssler catalyst is largely overlooked and there is not sufficient
information. We believe that the large anion of Preyssler with the larger number of tungsten
atoms, provides many ‘‘sites’’ on the oval shaped molecule that are likely to render the catalyst
effective. Also the larger number of protons in Preyssler catalyst may lower the activation
barrier for nitration reactions.
1
4-
Preyssler’s anion, [NaP
cyclic assembly of five PW
fivefold axis and 1.25 above the pseudo mirror plane that contains the five phosphorus atoms
51, 54]. Preyssler polyanion as a large anion can provide many ‘‘sites’’ on the oval shaped
molecule that are likely to render the catalyst effective. The Keggin anions have an assembly of
2 corner shared octahedral MoO from trimetallic groups [Mo 13] around a heteroatom
tetrahedron PO [54]. The introduction of vanadium(V) into the Keggin framework of
is beneficial for catalysis reactions [55]. Usually positional isomers are possible
5
W
30
O
110
]
, has an approximate D
5
h symmetry and consists of a
6
O22 units. A sodium ion is located within the polyanion on the
[
1
6
3
O
4
3
-
[
40
PMo12O ]
and coexist when two or more vanadium atoms are incorporated into the Keggin structure [56].
Studies on these isomers in catalytic reactions indicate that different isomers cause to show
different reactivities [57]. With respect to the catalytic performances for these catalysts and the
overall effects of all isomers, for synthesizing of them, we cannot control the reaction
conditions to synthesis of positional vanadium-substituted isomers separately, revealing the
relationship between the structures of H3+xPMo12↑xVxO40 (x = 1-2) and hence study of their
catalytic activity, is difficult. However, because the metal substitution may modify the energy
and composition of the LUMO and redox properties, for mentioned heteropolyacids with
different charges, the energy and composition of the LUMOs have significant effects on the
catalytic activity [58]. Substitution of vanadium ions into the molybdenum framework stabilize
the LUMOs because these orbitals derive, in part from vanadium d-orbitals which have been
assumed to be more stable than those of molybdenum and tungsten [59].
The abundance of different isomers may also play an important role in catalytic
performance. In addition, different positional Mo atom(s) substituted by the V atom(s) in
3
-
[
to exhibit varying catalytic performances.
PMo O ] may create different vanadium chemical environments, thus causing these catalysts
12 40
Effect of the solvent. Influences of solvents are the important parameter for the determination of
yield of the product. Table 3 shows the effect of various solvents on the nitratiom reaction. The
performance of various solvents are in the following the order:
acetonitrile > ethyl acetate > acetone > THF > chloroform > dichloromethane.
Acetonitrile shows better yield as compared with other solvents. In addition, the time
required for completion of the reaction was found to be less in acetonitrile. Because of large
negative charge of polyoxoanions, all of the HOMOs and LUMOs of them have very high
energy levels. These highly charged heteropolyanions do not exist in the gas phase and that the
external field generated by the solvent is crucial to stabilize them. With regard to the
heteropolyanions which are reducible easily and is required in catalytic reactions, the energy of
the LUMO must be sufficiently low to accept the electrons in catalytic reactions. The solvent
Bull. Chem. Soc. Ethiop. 2012, 26(1)

Short Communication
149
molecules can place these molecular orbitals at the appropriate level. As shown in Table 3, the
solvent effects change in parallel to the charges of the anions. The greater negative charge lead
to the greater solvent effects and finally the higher yields. It is suggested that the solvent effects
are dominated by the interactions of the polarized polyanions with the solvent, to place the
molecular orbitals at the appropriate level and or to lower the activation energy. Apparently this
effect is higher for acetonitrile.
3 4 2
Table 3. Synthesis of o-nitro phenol in the presence of different solvents with Th(NO ) .5H O using various
heteropolyacids under refluxing condition.
a
Entry
1
Solvent
Catalyst
14[NaP
Temperature (˚C) Time (min) Yield (%)
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Ethylacetate
Ethylacetate
Ethylacetate
Ethylacetate
Acetone
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
5
W
30
O
110
]
]
]
]
]
]
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
25
30
30
30
30
35
40
40
35
40
40
40
45
50
50
50
50
55
60
60
50
55
60
60
95
93
80
89
92
90
88
85
90
88
85
85
89
87
86
83
87
85
82
80
86
84
80
79
6
5
4
[P
[PMo10
[PMo11VO40
2
W
18
O
62
]
2 40
V O ]
]
2
3
4
5
6
14[NaP
5
W
30
O
110
6
5
4
[P
[PMo10
[PMo11VO40
2
W
18
O
62
]
2 40
V O ]
]
14[NaP
5
W
30
O
110
Acetone
Acetone
Acetone
6
5
4
[P
[PMo10
[PMo11VO40
2
W
18
O
62
]
2 40
V O ]
]
THF
14[NaP
5
W
30
O
110
THF
THF
6
5
4
[P
[PMo10
[PMo11VO40
2
W
18
O
62
]
2 40
V O ]
THF
]
CHCl
CHCl
CHCl
CHCl
3
3
3
3
14[NaP
5
W
30
O
110
6
5
4
[P
[PMo10
[PMo11VO40
2
W
18
O
62
]
2 40
V O ]
]
CH
CH
CH
CH
2
2
2
2
Cl
Cl
Cl
Cl
2
2
2
2
14[NaP
5
W
30
O
110
6
5
4
[P
[PMo10
[PMo11VO40
2
W
18
O
62
]
2 40
V O ]
]
a
Yields were analyzed by GC.
Effect of the reaction temperature. The temperature effect on the nitration of phenol in the
solvent of choice (acetonitrile) was also carefully investigated. The reaction was carried out in
o
110] and the temperature varied from room temperature (25 C to
the presence of H14[NaP W
5
30
O
o
2 C). The results are shown in Table 3. As can be seen in Table 3 the highest yield is obtained
8
o
at 82 C (reflux temperature), and the yields are lower when the reactions are carried out at
temperatures lower than reflux temperature. We believe that increasing in the temperature for
accelerating the reaction is apparently favorable.
CONCLUSIONS
In conclusion, the mono-nitration of phenols with a metal nitrate and HPAs has been achieved
with high regiospecificity and with excellent isolated yields. In general, exclusive ortho-
selectivity was observed for all the phenols subjected to this protocol. The high regiospecificity,
with excellent isolated yields and the use of inexpensive and easily accessible HPAs as the
catalyst makes this methodology of wide synthetic and commercial utility. This method offers
Bull. Chem. Soc. Ethiop. 2012, 26(1)

150
Bita Baghernejad et al.
some advantages in terms of simplicity of performance, easy work-up, use of inexpensive,
available and easy to handle catalyst and high yields of products and relatively short reaction
times.
EXPERIMENTAL
Chemical and apparatus. All the chemicals were obtained from Merck Company and used as
received. H14[NaP
[PMo11VO40], H
5
W
30
O
110] was prepared according to earlier works [44-49, 60, 61].
[PMo10 40] and H [P 62] were prepared according to the literatures
50]. The integrity of the synthesized heteropolyacids has been proven by comparing of spectral
H
4
5
V
2
O
6
2 18
W O
[
data with those reported in literature [51, 54, 62, 63]. All products are known compounds and
1
were characterized by m.p., IR, H NMR and GC/MS. Melting points were measured by using
1
the capillary tube method with an electrothermal 9200 apparatus. H NMR spectra were
recorded on a Bruker AQS AVANCE-300 MHz spectrometer using TMS as an internal standard
(
3
CDCl solution). IR spectra were recorded from KBr disk on the FT-IR Bruker Tensor 27
spectrometer. GC/MS analysis using an Agilent (Denver, CO, USA) 6890 GC system Hp-5
capillary 30 m × 530 µm × 1.5 µm nominal. Thin layer chromatography (TLC) on commercial
aluminium backed plates of silica gel, 60 F254 was used to monitor the progress of reactions.
Typical experimental procedure. To a stirred solution of the phenol (1 mmol) in acetonitrile (5
mL) was added metal nitrate (1 mmol) followed by a catalytic amount of HPAs (0.01 mmol) and
the reaction mixture was refluxed until all the phenol was consumed. The progress of the
reaction was monitored by TLC using petroleum ether:ethyl acetate (4:1) as eluent. After
completion of the reaction, the catalyst was filtered. The mixture was then washed with
saturated sodium hydrogen carbonate (10 mL) and the product extracted into dichloromethane
(2 × 5 mL). The combined organic extracts were washed with distilled water (10 mL), dried
over anhydrous MgSO and the solvent was removed by evaporation to afford the o-nitrophenol,
m.p. 91-92 °C (lit. [64] m.p. 92 °C ).
4
Reusability of the catalyst. The catalyst was recovered after the reaction and reused as a catalyst
in the nitration reactions. Several times recoveries had only slightly decreased the catalytic
activity, pointing to the stability and retention capability of this useful polyanion. At the end of
o
the reaction, the catalyst was filtered, washed with diethyl ether, dried at 130 C for 1 h, and
reused in another reaction. As shown in Table 4, the recycled catalyst was used for three
reactions without observation of appreciable lost in its catalytic activities.
Table 4. Reuse of the H14[NaP
5 30 3 4 2
W O110] for synthesis of o-nitro phenol with Th(NO ) .5H O.
a
Entry Time (min) Yield (%)
1
2
3
4
5
25
30
40
50
60
95
94
92
86
85
a
Yields were analyzed by GC.
Bull. Chem. Soc. Ethiop. 2012, 26(1)

Short Communication
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