PEG-N2O4: An efficient nitrating agent for the selective mono- and dinitration of phenols under mild conditions

Article abstract of DOI:10.1080/00397910802136706

N2O4 was easily impregnated on polyethyleneglycol to give a stable reagent. The polyethyleneglycol-N2O4 (PEG-N2O4) system was used as an effective nitrating agent for the nitration of phenols. Mono- and dinitrophenols can be obtained via direct nitration of phenols in the presence of PEG-N2O4 at room temperature in moderate to high yields. Copyright Taylor & Francis Group, LLC.

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PEG-N O : An Efficient
2
4
Nitrating Agent for the
Selective Mono- and Dinitration
of Phenols Under Mild
Conditions
a
a
Mohammad Ali Zolfigol , Elaheh Madrakian , Ezat
a
b
Ghaemi & Khodabakhsh Niknam
a
Faculty of Chemistry, Bu-Ali Sina University,
Hamedan, Iran
b
Department of Chemistry, Faculty of Scinces,
Persian Gulf University, Bushehr, Iran
Version of record first published: 12 Sep 2008.
To cite this article: Mohammad Ali Zolfigol, Elaheh Madrakian, Ezat Ghaemi &
Khodabakhsh Niknam (2008): PEG-N O : An Efficient Nitrating Agent for the Selective
2
4
Mono- and Dinitration of Phenols Under Mild Conditions, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
38:19, 3366-3374
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1
Synthetic Communications , 38: 3366–3374, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802136706
PEG-N O : An Efficient Nitrating Agent for the
2
4
Selective Mono- and Dinitration of Phenols
Under Mild Conditions
1
1
Mohammad Ali Zolfigol, Elaheh Madrakian, Ezat Ghaemi, and
1
2
Khodabakhsh Niknam
Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
1
2
Department of Chemistry, Faculty of Scinces, Persian Gulf University,
Bushehr, Iran
Abstract: N O was easily impregnated on polyethyleneglycol to give a stable
2
4
2 4 2 4
reagent. The polyethyleneglycol-N O (PEG-N O ) system was used as an effec-
tive nitrating agent for the nitration of phenols. Mono- and dinitrophenols can be
obtained via direct nitration of phenols in the presence of PEG-N O at room
temperature in moderate to high yields.
2
4
2 4
Keywords: Dinitrophenols, nitration, nitrophenols, PEG-N O
INTRODUCTION
Immobilization of reactive species on a polymer support could provide
many important advantages over analogous homogeneous systems; for
example, separation of the support from the reaction mixture can be
achieved by simple filtration, aiding isolation and purification proce-
dures. Also, excess of polymeric reagent can be readily employed without
[1,2]
incurring a penalty in workup.
Dinitrogen tetra-oxide as an important industrial chemical is com-
mercially supplied at a low price in a cylinder and can be used directly
ꢀ
or in the form of a liquid (bp 21 C) collected by transfer distillation into
Received March 14, 2008.
Address correspondence to Mohammad Ali Zolfigol, Faculty of Chemistry,
Bu-Ali Sina University, Hamedan 65174, Iran. E-mail: zolfi@basu.ac.ir,
mzolfigol@yahoo.com
3
366

PEG-N
O
2 4
, Nitrating Agent for Phenols
3367
an ice-cooled vessel. Apart from difficulties in handling the poisonous
and corrosive dinitrogen tetroxide, the biggest disadvantage of utilizing
gaseous N O in organic reactions is its high reactivity, which usually
2
4
causes undesired side reactions. To overcome these problems, the reac-
tions usually were performed at low temperature, and careful control of
[3]
reaction conditions was needed. In recent years, some reports have been
published on the use of N O complexes of organic and inorganic
2
4
[4]
compounds such as 18-crown-6 ether=N O , polyvinylpyrolidone=
2
4
[
5]
[6]
[7]
[8]
N O , M(NO ) ꢁyN O , activated charcoal=N O , N O =SiO ,
2
4
3 x
2
9]
4
2
4
2
4
2
[
[10]
silica acetate-N O ,
(
dioxane-NO2)
calix[4]arene=N O ,
dimethyl formamide
DMF)-NO , and (tributyl phosphate-NO , polyethylene glycol-NO ,
2
4
2
4
2
2
2
[
11]
as useful reagents that solve the previously mentioned
limitations in handling and increased selectivity of organic reactions. In
continuation of our investigation on the preparation of N O complexes,
herein we report the application of N O -impregnated polyethyleneglycol
2
4
2
4
as a cheap reagent made from available sources. Polyethyleneglycols
PEGs) are important components of crown ethers, cryptands, and other
ion receptors, and their capability of binding with various ions has
(
[12]
been widely investigated.
This reagent, polyethyleneglycol-N O
2 4
(
of polyethylene glycol in cold CH Cl (Fig. 1).
PEG-N O ), was easily prepared by bubbling N O gas to a solution
2
2
4
4
2
2
Nitration of organic substrates is a widely studied reaction of great
industrial significance because many nitro-compounds are extensively
utilized as intermediates for the manufacture of plastics, dyes, perfumes,
drugs, and pharmaceuticals. O-Nitrophenol is an important starting
[
13]
material used in multistep synthesis of valuable compounds. Nitration
in manufacturing processes requires the use of a corrosive liquid-phase
nitric acid–sulfuric acid mixture, responsible for the generation of large
amounts of waste that is costly to treat. In addition, nitration reactions
usually are not selective and are the cause of environmental concerns
regarding the disposal of the large excess of mixed acids employed
in these processes. Thus, the utility of these processes are generally
[
13]
low.
Besides mixed acids, several other nitrating agents, including
Figure 1. Proposed interaction of PEG-N O complex.
2
4

3368
M. A. Zolfigol et al.
[
14]
concentrated nitric acid, acid anhydrides or triflates,
peroxy
have been utilized
in many investigations. Catalytic nitration of aromatic hydrocarbons
[
15,16]
[17]
[18]
nitrite,
metal nitrates,
and nitrogen oxides,
[
19]
with concentrated nitric acid in the presence of solid acids
received attention with regards to regioselectivity. In some cases, the
reaction has been performed in expensive media such as ionic liquids
With regards to nitration of phenols, concen-
has
[
20]
[21]
and microemulsions.
trated nitric acid or mixed acids are promising; their use is always asso-
ciated with the formation of dinitro compounds, oxidized products, and
unspecified resinous tarry materials resulting from the overoxidation of
the substrate. The typical yield of direct nitration never exceeds 60%
because of the previously mentioned side reactions in most of the cases,
making these existing processes not economical. Therefore, it is worth-
while to consider an alternative highly selective nitration process based
þ
upon the in situ generation of NO using a mild nitrating agent such as
PEG-N O . Herein, we report that the PEG-N O complex is an
2
4
2
4
efficient nitating agent both for the nitration of phenols into the corre-
sponding mono- and dinitrates under nonaqueous conditions at room
temperature.
RESULTS AND DISCUSSION
Phenol (1) was subjected to a nitration reaction in the presence of
PEG-N O as a nitrating agent in nonaqueous conditions (Scheme 1).
2
4
The mononitration reaction was performed by placing phenol (1 mmol)
and PEG-N O (0.4 g) in dichloromethane (solvent) at room temperature
2
4
with good yield. The dinitration product was achieved directly by the
reaction of phenol (1 mmol) and PEG-N O (1.0 g) in dichloromethane
at room temperature.
2
4
Because of the successful result from the mono- and dinitration of
phenol, a good range of substituted phenols (5) were reacted in both con-
ditions (Scheme 2). These reactions were performed for both substituted
Scheme 1. Mono- and dinitration of phenol in the presence of PEG-N O .
2
4

PEG-N
O
2 4
, Nitrating Agent for Phenols
3369
Scheme 2. Mono- and dinitration of 4-substituted phenols in the presence of
PEG-N
2
4
O .
with an electron-withdrawing group (e.g., cyano, fluro, or carboxylate
derivatives group) on the aromatic ring or an electron-donating group
(e.g., CH , or OCH3 groups) in short reaction times with good to
3
excellent yields (Table 1).
The present mono- and dinitration reactions can be readily carried
out by placing a stiochemometric amount of nitrating agent (PEG-
N O ) and phenols (1 or 5) in a reaction vessel in dichloromethane as
2
4
solvent and efficiently stirring the solution at room temperature. Highly
pure mono- and dinitrophenols can be obtained after column chromato-
graphy on a short column of silica gel.
The PEG-N O in CH Cl can act as a relatively efficient reagent
2
4
2
2
þ
as a source for the delivery of nitrosonium ion (NO ) under mild
4]
conditions. The following mechanism for the nitration reaction via in
situ generation of NO may be suggested (Scheme 3).
In conclusion, we have reported a mild and efficient reagent for the
nitration of phenols in the presence of PEG-N O in good to excellent
[
þ
[19]
2
4
yields and short reaction times. In contrast to other acids, no special
precautions are needed for storage and handling of this compound,
and it can be stored in the refrigerator for months without losing its
activity.

3370
M. A. Zolfigol et al.
a
b
Table 1. Nitration of phenols to their corresponding mono and dinitration
derivatives with PEG-N in dichloromethane at room temperature
2 4
O
c
Entry
Substrate
Product
2 (4), 3
Time (min)
Yield (%)
b,d
29 (68), 50
1
2
3
4
5
6
7
8
9
1
20 (90)
15 (90)
15 (90)
15 (100)
20 (90)
25 (95)
20 (110)
20 (110)
20 (110)
15 (100)
25 (110)
25 (90)
25 (110)
20 (100)
25 (115)
e
b,d
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
6a (7a)
85 (45)
97 (73)
95 (70)
85 (71)
90 (60)
87 (68)
85 (62)
95 (62)
90 (72)
92 (62)
68 (20)
80 (92)
87 (72)
83 (57)
b,d
6b (7b)
6c (7c)
6d (7d)
6e (7e)
6f (7f)
6g (7 g)
6h (7 h)
6i (7i)
6j (7j)
6k (7k)
6l (7 l)
6m (7 m)
6n (7n)
b,d
b,d
b,d
b,d
b,d
b,d
b,d
1
1
1
1
1
1
0
1
2
3
4
5
b,d
b,d
b,d
b,d
b,d
a
b
c
PEG-N
PEG-N O –substrate (1.0 g:1 mmol).
Isolated yields.
2 4
O –substrate (0.4 g:1 mmol).
2
4
d
The yields and times of dinitration are written in parentheses.
Cl was used as eluent for column
chromatography.
e
Because of volability of the product, CH
2
2
Scheme 3. Proposed mechanism for the nitration of phenols in the presence of
PEG-N O .
2
4

PEG-N
O
2 4
, Nitrating Agent for Phenols
3371
EXPERIMENTAL SECTION
General
Chemicals were purchased from Merck; polyethylene glycol 2000 (mw
800–2200 g=mol) was also purchased from Merck. Yields refer to
isolated pure products. The nitration products were characterized by
1
1
comparison of their spectral (IR and H NMR) and physical data with
[
14–19]
those authentic samples or those reported in literature.
NO -N O gas is exceedingly toxic. Thus, all operations involving
Caution:
2
2
4
N O must be conducted in an efficient hood.
2
4
Preparation of Polyethyleneglycol-N O
2
4
Into a 250-mL one-necked round-bottomed flask was charged with 45.0 g
of lead(II) nitrate. (The lead(II) nitrate had been crushed to a fine powder
ꢀ
with mortar and pestle and then dried in an oven at 120 C for three
days). The flask was equipped with a 35-cm column and connected to
[22]
a trap through a polyethylene tube.
gas inlet tube.
The trap was connected to the
The lead(II) nitrate was heated with a Bunsen burner. Brownish-red
NO -N O gas evolved, and it was bubbled slowly through the cold PEG
2
2
4
(
10.0 g) in methylene chloride solution (25 mL) for 30 min. After a few
minutes, a deep red color appeared. The temperature was kept colder
ꢀ
than 0 C. The red solution was allowed to stir another 60 min, and then
dry N gas was bubbled through the solution to extrude the excess of
2
NO gas. The solvent was evaporated. The obtained solid was dried
2
under vacuum to give PEG-supported N O as a cream-colored powder
4
2
(14.0 g). The capacity of the reagent was determined to be 2.5 mmol of
N O per gram of the polymer. The reagent could be stored in the
refrigerator for several months without losing its weight or activity.
2
4
Mononitration of Phenol (1) with PEG-N O : A Typical Procedure
4
2
A solution of compound 1 (0.188 g, 2 mmol) and PEG-N O (0.8 g) in
2
4
CH Cl (8 mL) was magnetically stirred at room temperature. After com-
2
2
pletion of the reaction (30 min), the mixture was passed through a short
column of silica gel and washed with dichloromethane as eluent to separ-
ꢀ
ate PEG. The dichloromethane was removed by water bath (35–40 C)
and simple distillation. The residue is a mixture of 2- and 4-nitrophenol.
ꢀ
4
(
-Nitrophenol (2) is insoluble in n-pentane: 0.08 g, 29%, mp 110–113 C
ꢀ
[16]
lit. mp 114 C). The n-pentane fraction was evaporated by water bath

3372
M. A. Zolfigol et al.
ꢀ
ꢀ
ꢀ
[16]
(35–40 C) to give 2-nitrophenol (3), 0.14 g, 50%, mp 44–45 C (lit.
mp 44 C) (Table 1, Scheme 1).
Mononitration of 4-Substituted Phenols (5) with PEG-N O :
4
2
A Typical Procedure
A solution of compound 5 (2 mmol) and PEG-N O (0.8 g) in CH Cl
2
2
4
2
(8 mL) was magnetically stirred at room temperature for the time speci-
fied in Table 1. After completion of the reaction, the reaction mixture
was passed through a short column of silica gel and washed with dichloro-
methane as eluent to separate PEG. The dichloromethane was removed
ꢀ
by water bath (35–40 C) and simple distillation to give the pure product.
Dinitration of Phenols (1 or 5) with PEG-N O : A Typical Procedure
4
2
A solution of compound 1 or 5 (2 mmol) and PEG-N O (2.0 g) in
2
4
CH Cl (10 mL) was magnetically stirred at room temperature for the
2
2
time specified in Table 1. After completion of the reaction, the reaction
mixture was purified by column chromatography on a column of silica
gel using chloroform as eluent. The chloroform was removed by rotary
evaporator to give the pure product.
ACKNOWLEDGMENT
Financial support for this work by the Research Affairs Program of
Bu-Ali Sina University, Hamedan, Iran, and the Center of Excellence of
Developing of Chemical Methods (CEDCM) is gratefully acknowledged.
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