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LETTER
Trichloroisocyanuric Acid/NaNO2/wet SiO2 as an Efficient System for the
Selective Dinitration of Phenols under Solvent-free Conditions
An
E
fficient System fo
o
r
the Selectiv
h
e
D
initratio
a
n
of Phenol
m
s
mad Ali Zolfigol,* Elaheh Madrakian, Ezat Ghaemi
Chemistry Department, College of Science, Bu-Ali Sina University, Hamadan 65174, Post Box No 4135, Iran
Fax +98(811)8272404; E-mail: Zolfi@basu.ac.ir
Received 23 July 2003
nomic advantage and environment protection.10 In contin-
Abstract: Dinitrophenols can be obtained via direct nitration of
phenols with trichloroisocyanuric acid, NaNO2 and wet SiO2 at
room temperature under solvent-free conditions with moderate to
high yields.
uation of our studies in this regard,11 we have found that
trichloroisocyanuric acid,12 a cheap commercially avail-
able reagent used primarily as a disinfectant and deodor-
ant, has found little application in organic chemistry so
far.13 Therefore, we were interested in using this reagent
for the dinitration of phenols when used in conjunction
with NaNO2 and wet SiO2 under solvent free conditions,
as shown in Scheme 1 and Scheme 2.
Key words: trichloroisocyanuric acid, dinitrophenols, solvent-free
Nitration of aromatic substrates is a widely studied reac-
tion of great industrial significance as many nitro-aromat-
ics are extensively utilised and act as chemical feedstocks
for a wide range of useful materials such as dyes, pharma-
ceuticals, perfumes, and plastics.1 Therefore, the nitration
of aromatic rings has received considerable attention of
late, due to unsolved problems pertaining to regioselectiv-
ity, overnitration and competitive oxidation of substrates.2
Nitration of phenol taken as a special case has been stud-
ied by various nitrating agents under different conditions.3
Very recently, two excellent procedures have been report-
ed for the nitration of phenols, one of them using silica-
polyethylenglycols/N2O4 complexes for their regioselec-
tive nitration,4 whereas the second one reported an ultra-
sonic method for the nitration of phenols.5
During the course of our studies concerning the utilization
of NO+ in functional groups transformations,11 we thought
that phenol (1) could be converted into the corresponding
4-nitrosoderivative 2 selectively by trichloroisocyanuric
acid (I), NaNO2 (II), and wet SiO2 (50% w/w) under solid
phase reaction via in situ generation of HNO2. In addition,
phenol nitrosation is known to be rapid and yields almost
entirely the para isomer which in turn can be readily con-
verted to 4-nitrophenol (the desired product which is pre-
cursor of acetaminophene) via a mild oxidation with:
HNO3,14 H2O2/Na2WO415 etc. (Scheme 1, path a). There-
fore, we decided to produce 4-nitrophenol via a nitrosa-
14–16
tion-oxidation strategy
in a one pot reaction under
mild and heterogeneous conditions. We chose trichloro-
isocyanuric acid (I)/NaNO2/wet SiO2 system for the nitro-
sation of phenol (Scheme 1, path a) but, in contrast to the
reported procedures in aqueous media,14–16 we observed
that apparently direct formation of 2,4-dinitrophenol (3)
occurred (Scheme 1, path b, Table 1). For this new system
the nitrous acid catalyzed mechanism (NAC) may be
proposed (see ref.17).
Recently, we have reported the applications of trichloro-
isocyanuric acid/NaNO2/wet SiO2 system for the selective
mononitration of phenols under mild and heterogeneous
conditions.3 Our goal, in undertaking this line of work,
was three-fold: a) to overcome the limitations and draw-
backs of the reported methods such as tedious work-up,6,7
strongly acidic media,8 oxidation ability of the reagents
and safety problems (storage, handling, using and also
presence of toxic transition metal cations such as Cr3+,
Hg2+, Cu2+, within molecular structure of the reagents),9
(b) to perform solvent-free organic synthesis which seems
to be a highly useful technique, especially for industry
possessing many advantages like: reduced pollution,
low costs, as well as simplicity in process and handling
(these factors are especially important in industry),10
(c) moreover, to develop an high-yielding one-pot syn-
thesis of dinitrophenols using a novel combination of
reagents.
Different kinds of 4-substituted phenols 5 were also sub-
jected to nitration in the presence of trichloroisocyanuric
acid (I), NaNO2 (II), and wet SiO2 (50% w/w) under sol-
vent free conditions (Scheme 2). The dinitration reactions
were performed under mild conditions at room tempera-
ture with moderate to good yields (Scheme 1 and
Scheme 2, Table 1) by simply placing the nitrating agents
and phenols 1 or 5 in a reaction vessel and efficiently
shaking the resulting mixture. Highly pure dinitrophenols
can be obtained by simple extraction and subsequent
evaporation of the solvent.
In addition, any reduction in the amount of liquid acids
needed and/or any simplification in handling procedures
would be highly convenient in terms of risk control, eco-
Therefore, a combination of sodium nitrite and trichloro-
isocyanuric acid (I) can act as solid nitrating agent which
can be readily weighed, handled and used for different
purposes in the presence of moist SiO2.11
SYNLETT 2003, No. 14, pp 2222–2224
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Advanced online publication: 07.10.2003
DOI: 10.1055/s-2003-42063; Art ID: G18203ST.pdf
© Georg Thieme Verlag Stuttgart · New York