Regioselective mononitration of aromatic compounds by zeolite/dinitrogen tetroxide/air in a solvent-free system

Article abstract of DOI:10.1039/b108952h

Nitration of aromatic compounds using a zeolite catalyst and a combination of dinitrogen tetroxide and air in a sealed system leads to high yields and para-selectivities in a clean, solvent-free process.

Full text of DOI:10.1039/b108952h

Regioselective mononitration of aromatic compounds by
zeolite/dinitrogen tetroxide/air in a solvent-free system
Keith Smith,* Saeed Almeer and Christelle Peters
Centre for Clean Chemistry, Department of Chemistry, University of Wales Swansea, Swansea, UK
SA2 8PP. E-mail: k.smith@swansea.ac.uk
Received (in Cambridge, UK) 2nd October 2001, Accepted 22nd October 2001
First published as an Advance Article on the web 28th November 2001
Nitration of aromatic compounds using a zeolite catalyst
and a combination of dinitrogen tetroxide and air in a sealed
system leads to high yields and para-selectivities in a clean,
solvent-free process.
the previous conditions.18 By contrast, when the pressurising
gas was nitrogen, virtually no nitration occurred, demonstrating
the involvement of oxygen in the reaction.
It was not known whether the by-product was water, nitric
acid or a mixture of the two. Eqn. (1) (n = 0–6) covers the
various possible stoichiometries.
Electrophilic aromatic substitution reactions are of considerable
importance in the production of fine chemicals. However, many
traditional processes suffer serious disadvantages, including
low selectivity for the desired product and the requirement for
large quantities of mineral or Lewis acids as activators. Such
acids cause corrosion and generate large volumes of spent
reagents. Major efforts are therefore being made to develop
processes with lower environmental impact.
Inorganic solids offer significant benefits by providing
effective catalysis and, in some cases, enhanced selectivity.1
For example, in our own research we have utilised zeolites to
enhance the para-selectivity in chlorination,2 bromination,3
acylation4 and methanesulfonylation5 reactions of simple
aromatic substrates. In addition, such solids are easily re-
cycled.
Aromatic nitration is particularly important since nitro
compounds are versatile feedstocks for a range of industrial
products, including pharmaceuticals, agrochemicals, dyestuffs,
and explosives. Traditional nitration with a mixture of nitric and
sulfuric acids6 is notoriously unselective for nitration of
substituted compounds, and disposal of the spent liquors
presents a serious environmental concern.7 Consequently,
several alternative methods for aromatic nitrations have been
developed.8–15
At present, the best combination of high yield, high para-
selectivity and low solvent use is nitric acid, zeolite HBEA (Hb)
and acetic anhydride (for moderately active aromatics)12 or
trifluoroacetic anhydride (for deactivated aromatics).13 Even
these systems, however, produce carboxylic acid as by-
product.
An alternative approach to clean nitration, pioneered by
Suzuki et al., involves dinitrogen tetroxide and ozone,16 or
dinitrogen tetroxide with oxygen and a catalyst.17 The use of
oxygen rather than ozone was appealing and we demonstrated
that use of zeolite Hß as catalyst provided improved para-
selectivity and easier catalyst recovery.18 However, the method
still employed a large excess of dinitrogen tetroxide, a
halogenated solvent, cooling and a reaction time of two days.
Therefore, we continue to study the reaction in order to find
ways of overcoming the remaining disadvantages. A recent
publication from Suzuki et al. reveals that his group is also
trying to perfect this zeolite-catalysed approach,19 and we now
therefore disclose our own further findings.
24 ArH + (12 + 2n) N2O4 + (6 + n) O2 = 24 ArNO2 + 4n HNO3
(1)
+ (12 2 2n) H2O
A series of reactions involving different proportions of
chlorobenzene to dinitrogen tetroxide was allowed to react to
completion (2 days). The results suggested that 3 mol of ArH
react with 2 mol of N2O4 to give 3 mol of nitro product and 1
mol of HNO3 (eqn. (2), i.e. n = 2 in eqn. (1)).
(2)
3 ArH + 2 N2O4 + O2 = 3 ArNO2 + HNO3 + H2O
Various zeolites, with different pore structures, counter-
cations and Si:Al ratios, were examined under the new
conditions. All zeolites tested catalysed the reaction and
provided a modest increase in para-selectivity compared to
reaction in the absence of catalyst. Zeolite Hß was somewhat
faster and gave greater para-selectivity than the others, but the
differences were not major.
Zeolite Hß was tested with a range of other substrates (Table
1). All substrates tested gave good yields of nitration products,
with modest increases in the amounts of para-isomer in
comparison with traditional methods. The relative reactivities
did not appear to be consistent with a normal electrophilic
aromatic substitution mechanism.
In an attempt to obtain further information about the system,
we investigated the simple adsorption of N2O4 on zeolite Hß as
a function of temperature. The amount adsorbed dropped
steadily as the temperature was increased, but there was still
some adsorption even at 80 °C and the amount adsorbed at 20
°C was in the region of 1.53 mmol N2O4 per gram of zeolite. By
employing excess zeolite we were able to conduct a reaction in
ordinary glassware at 20 °C and near-atmospheric pressure,
which enabled direct observation of the reaction mixture.
Adsorption of N2O4 alone onto the zeolite imparted a pale
brown hue to the solid. However, when benzene was allowed
into the system, the colour of the adsorbed material immediately
became much darker. Following admission of oxygen the
reaction proceeded slowly to give nitrated product.
We speculate that the dark colour is indicative of an adsorbed
intermediate formed by interaction of the substrate with N2O4,
Table 1 Nitration of various substrates with N2O4, zeolite Hb and aira
Substrate
Yieldb (%)
p/o ratio
Our initial objective was to improve our previous procedure18
by avoiding the need for cooling, decreasing the excess of
dinitrogen tetroxide used and eliminating the solvent. These
features were dictated by the volatility of dinitrogen tetroxide
(bp 21 °C). Simply using an autoclave pressurised to 200 psi
with oxygen allowed a reaction with the desirable features to
proceed quickly and in high yield. Furthermore, when oxygen
was replaced by air the reaction was still efficient. With air it
required about 12 h to go to completion, still quicker than under
Toluene
Benzene
Fluorobenzene
Chlorobenzene
Bromobenzene
76
97
95
97
90
0.9
—
10.1
5.6
4.2
a Reactions were carried out with zeolite (3.0 g), substrate (33.0 mmol), and
N2O4 (1.4 ml, ca. 23 mmol), under 200 psi air pressure at room
temperature.b Calculated by quantitative GC.
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Chem. Commun., 2001, 2748–2749
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b108952h

and that its conversion into product requires another reactive
species to be formed more slowly between oxygen and the
excess N2O4. The relative reactivities of different substrates and
the small differences observed between the catalytic activities
of proton and sodium forms of the same zeolites do not seem to
be in line with an acid-catalysed electrophilic substitution
reaction under these conditions. We therefore suspect that the
reaction is radical in nature. Scheme 1 gives a tentative
mechanism that is consistent with the stoichiometry. A similar
overall result would be produced if NO2 radicals were to
abstract the first hydrogen atoms and the HNO2 produced were
then to react with oxygen to give HOONO2. However, in that
event nitration would be expected in the absence of oxygen.
In previous reports of nitrations of aromatic compounds using
dinitrogen tetroxide and zeolites in the absence of solvent, the
reactions have been conducted at elevated temperatures in the
gas phase.20,21 It was assumed that the stoichiometry was as
shown in eqn. (3) (c.f. eqn. (1) with n = 0). If that were correct,
it is likely that some of the product was formed by the action of
nitric acid on the substrate at the elevated temperatures.
Therefore, the reactions were likely to be complex and
conclusions about the mechanisms based on the overall results
need to be treated with caution.
ically. Therefore, only two remaining obstacles prevent this
reaction from fulfilling all the desired criteria. One is the
relatively low para-selectivity (though already better than for
traditional methods). The other is the production of a modest
amount of nitric acid as by-product, which can deactivate the
zeolite by adsorption or reaction, limits the efficiency of usage
of the dinitrogen tetroxide feed and could lead to plant
corrosion. We continue to search for ways to overcome these
remaining disadvantages.
The reaction of chlorobenzene illustrates the general proce-
dure for the nitration process. The zeolite (3 g) was placed in a
450 ml autoclave, followed by chlorobenzene (3.71 g, 33
mmol). Liquid dinitrogen tetroxide (ca. 1.4 ml, ca. 23 mmol)
was added quickly to the mixture, the autoclave was sealed and
pressurised to 200 psi with air, and the mixture was stirred at
room temperature for 14 h. The autoclave was then opened and
the product was extracted with dichloromethane (200 ml). The
extract was washed with water (50 ml), dried (MgSO4) and
concentrated under reduced pressure to give the product, which
was analysed by GC (hexadecane was added as internal
standard).
We thank Zeolyst International for gifts of zeolites and S. A.
thanks the Government of Qatar for a studentship.
(3)
4 ArH + 2 N2O4 + O2 = 4 ArNO2 + 2 H2O
Notes and references
In the recent work of the Suzuki group, superior selectivities
for production of para-isomers were achieved at low conversion
using H-ZSM-5 as the zeolite and the substrate as its own
solvent in the liquid phase.19 In the present work we have shown
how it is possible to carry out the reaction to give high yields,
with modest para-selectivities, without solvent, with only the
stoichiometric quantity of dinitrogen tetroxide, using air instead
of oxygen, and under mild conditions (ambient temperature and
a modest pressure). Under these conditions the reaction is rather
slow, but simply raising the temperature to 30–40 °C in the
sealed system can reduce the required reaction period dramat-
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Scheme 1 A speculative mechanism for the reaction. Any or all of the steps
involving reaction of the substrate–NO2 adduct with another radical could
proceed by either of two mechanisms: (i) direct hydrogen atom abstraction;
or (ii) electron abstraction to give a Wheland intermediate and an anion,
followed by proton abstraction from the Wheland intermediate by the anion.
Whether the zeolite would play an active part in catalysing any of the
processes or merely assist through stabilisation of intermediates by
adsorption is an open question.
21 N. F. Salakhutdinov, K. G. Ione, E. A. Kobzar and L. V. Malysheva,
Russ. J. Org. Chem., 1993, 29, 457.
Chem. Commun., 2001, 2748–2749
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