Zeolite-assisted nitration of neat toluene and chlorobenzene with a nitrogen dioxide/molecular oxygen system. Remarkable enhancement of para-selectivity

Article abstract of DOI:10.1016/S0040-4039(01)00750-X

In the presence of molecular oxygen and HZSM-5, neat toluene reacted with liquid nitrogen dioxide in a regioselective manner at room temperature to yield mononitrotoluenes as the main product, where the para isomer predominated up to 90% and the ortho-para isomer ratio improved to 0.08. Chlorobenzene behaved similarly, giving a para-predominant nitration product.

Full text of DOI:10.1016/S0040-4039(01)00750-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4357–4359
Zeolite-assisted nitration of neat toluene and chlorobenzene with
a nitrogen dioxide/molecular oxygen system. Remarkable
enhancement of para-selectivity
Xinhua Peng,a Hitomi Suzukia,* and Chunxu Lub
aDepartment of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662-8501, Japan
bDepartment of Chemistry, School of Chemical Engineering, Nanjing University of Science and Technology, Xiao Ling Wei,
Nanjing 210094, China
Received 27 March 2001; revised 1 May 2001; accepted 2 May 2001
Abstract—In the presence of molecular oxygen and HZSM-5, neat toluene reacted with liquid nitrogen dioxide in a regioselective
manner at room temperature to yield mononitrotoluenes as the main product, where the para isomer predominated up to 90% and
the ortho–para isomer ratio improved to 0.08. Chlorobenzene behaved similarly, giving a para-predominant nitration product.
© 2001 Elsevier Science Ltd. All rights reserved.
Classical methodology for the preparative nitration of
toluene involves mixtures of nitric and sulfuric acids.
The reaction is not selective and gives rise to a mixture
of ca. 60% o-nitrotoluene, ca. 37% p-nitrotoluene and
less than 3% m-nitrotoluene, along with some oxidation
and/or polynitration products.1 Since p-nitrotoluene
has more market demand than the o-isomer, large
amounts of the unwanted products in general pose a
problem in industrial manufacture. Moreover, the dis-
posal of waste products and spent acids is environmen-
tally unfriendly and can be costly.
activate nitrogen dioxide has led to the Fe(III)-cata-
lyzed nitration of aromatic compounds with a NO2/O2
system.3 Rather unfortunately, the product composition
differs little from those of the Kyodai nitration and
conventional aromatic nitration.
Over the past few decades there has been a considerable
interest in improving the para-selectivity of toluene
nitration and many attempts have been made using the
catalysts supported on porous inorganic solids. Laszlo
has developed Claycop (Cu(NO3)2/montmorillonite-
K10) that selectively nitrates toluene in acetic anhy-
dride/CCl4 with an isomer distribution of ortho 23,
meta 1 and para 76%.4 Smith has shown that mordenite
and zeolite b can play an important role in the selective
mononitration of toluene, where the para-selectivity of
67% by using benzoyl nitrate5 and the ortho–para iso-
mer ratio of 0.23 by using acetyl nitrate6 have been
achieved. Toluene also has been nitrated with isopropyl
The Kyodai nitration using a NO2/O3 system has
demonstrated excellent conversion of a variety of aro-
matic compounds into the corresponding nitro deriva-
tives.2 The reaction bears the general appearance of
being an electrophilic process and is characterized by
some unique feature as operating under neutral condi-
tions. Further efforts to find an alternative means to
CH3
CH3
CH2NO2
CHO
CH3
NO2, O2
+
+
NO2
+
(NO2)2
HZSM-5, r.t.
1
2
3
4
5
6
ortho meta para
Scheme 1.
Keywords: nitration; regioselection; nitrogen oxides; arenes; nitro compounds; zeolites.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00750-X

4358
X. Peng et al. / Tetrahedron Letters 42 (2001) 4357–4359
nitrate in the presence of HZSM-5, producing a
product of isomeric composition ortho 5, meta 0 and
para 95%.7 Zeolite catalysts are attractive for being
inexpensive and reusable, and direct use of NO2 as the
source of nitro group has considerable potential in
industrial application. Thus, in search for the para-
selective methodology of aromatic nitration under neu-
tral conditions, we have investigated the reaction of
neat toluene and chlorobenzene with liquid NO2 in the
presence of a variety of zeolites by using each substrate
as its own solvent. We now report the activation of
nitrogen dioxide on the zeolite surface that results in
the para-selective nitration of these aromatic substrates.
cally and the representative results are summarized in
Table 1. Wider pore zeolite b and various types of
modified zeolites were also examined and some selected
results were included for comparison.
All zeolites were found to promote the reaction and the
isomer proportion always moved to the direction to
favor the para-isomer. HZSM-5 exhibited better para-
selectivity than zeolite b, but their difference was slight
as to the total product yield. Zeolites modified with
metal ion or heteropoly acid were not so effective to
accelerate the reaction and brought about complicated
variation in the product distribution. In contrast, zeo-
lites treated with methanesulfonic acid considerably
facilitated nuclear nitration and the isomer proportion
approached those of conventional nitration. Increasing
the amount of NO2 led to higher conversion, but was
indifferent to the ortho–para isomer ratio, while increas-
ing the amount of zeolites worked favorably for the
nuclear nitration as well as para-substitution. The rise
of reaction temperature from −10 to 10 to 50°C
improved both product yield and extent of para-substi-
tution considerably, the ortho–para isomer ratio turning
for the better from 0.33 to 0.15 to 0.07.8 The positive
effect of elevated temperature would probably be due
to the enhanced transport and interaction of the reac-
tants inside the zeolite channels.
Weakly activated toluene was quite reluctant to react
with NO2 alone, but passage of O2 into the reaction
mixture resulted in a certain degree of conversion into
nitrated compounds. Unfortunately, the reaction was
quite slow and unselective, giving a mixture of nitro-
toluenes (ortho- 1, meta- 2 and para- 3), phenyl-
nitromethane 4, benzaldehyde 5 and dinitrotoluenes 6
(Scheme 1). Trace amounts of a nitrophenol was also
detected. In the presence of oxygen and HZSM-5,
however, the reaction was promoted and the para-selec-
tivity was found to increase considerably. Thus, in the
hope of improving the para-selectivity further, the
NO2–O2–zeolite system has been investigated systemati-
Table 1. Regioselectivity of nitration of toluene with NO2–O2 over various zeolite catalystsa
Catalyst
Yieldb
Product distribution (mol%)
ortho–para
R/Sd
(mmol)
1
2
3
4
5
6
e
–
0.47 (1)
1.15 (5)
2.12 (11)
4.62 (24)
7.30 (46)
2.45 (14)
1.00 (6)
1.47 (8)
2.08 (11)
1.62 (8)
2.63 (14)
4.16 (22)
2.13 (13)
1.10 (6)
2.63 (14)
7
7
7
4
2
3
6
4
5
7
6
7
8
5
6
6
5
34
6
10
10
Trace
4
Trace
4
7
9
5
7
2
2
6
35
12
7
7
Trace
3
Trace
4
5
8
5
4
3
4
4
12
18
5
1
2
5
1
6
4
5
6
5
2
2
8
1.48
1.47
0.30
0.08
1.15
0.41
0.38
0.22
0.64
0.41
0.29
0.64
0.70
0.75
0.49
0.45
–
34
17
6
23
57
74
44
58
69
66
47
51
60
46
52
49
51
4.6
4.9
4.9
HZSM-5f
HZSM-5f,g
HZSM-5/MeSO3H
BiHZSM-5h
CuHZSM-5h
FeHZSM-5h
ZnHZSM-5h
P2O5HZSM-5i
Mo2O3P2O5HZSM-5i
Hb-25j
51
24
26
15
30
21
17
30
36
37
25
\99
13
\99
12
7.3
4.9
9.0
8.1
19
16
9.0
Hb-150j
FeHb-150h
NaMFI-90j
a All reactions were carried out by stirring a mixture of toluene (5.0 ml), liquid NO2 (0.5 ml), catalyst (2.0 g), 3 A molecular sieves (0.5 g), and
,
,
cyclododecane (internal standard) at room temperature for 22 h under oxygen, unless otherwise noted. Toluene was dried over 4 A molecular
sieves.
b Sum of products 1–6, estimated by GC. Numeral in parenthesis refers to total yield of nitrotoluenes based on NO2.
c Calculated from GC data.
d Ratio of ring and side-chain reaction products.
e Under exclusion of oxygen.
f HZSM-5 is a commercial product (Acros) of Si/Al ratio 1000. All catalysts except HZSM-5/MeSO3H were calcined in air at 500°C for 8 h prior
to use.
g Double amount of catalyst.
h Prepared as follows; a suspension of zeolite (5 g) in 0.2 M solution of a given salt in water (100 ml) was stirred for 24 h under reflux. The solid
was washed, dried at 110°C for 3 h, and calcined in air at 500°C for 8 h.
i Prepared by stirring HZSM-5 with aqueous phosphomolybdic acid (H3PMo12O40·nH2O) or 85% phosphoric acid for 48–68 h at room
temperature. Without washing, the solid was dried at 110°C for 5 h and calcined in air at 500°C for 8 h.
j A gift from the Catalytic Society of Japan.

X. Peng et al. / Tetrahedron Letters 42 (2001) 4357–4359
4359
Our results contrast to those reported by Smith, where
HZSM-5 was less effective than zeolite b and the ortho
and para isomers were obtained in comparable
amount.6,9 This difference may well be attributed to the
absence of intervention by chlorinated solvent
molecules, as well as the higher Si/Al ratio of our
zeolite samples (Si/Al 1000). Mixing of toluene with a
large excess of NO2 over HZSM-5 at room temperature
led to complete consumption of the substrate, leading
to the distribution of products 1–6 of 41:6:47:3:1:2 and
an ortho–para ratio of 0.87.
by Smith.9 In the absence of HZSM-5, the reaction was
quite sluggish and unselective, affording the nitrated
product in the ratio ortho:meta:para=32:15:53.
In conclusion, the present work has revealed that the
ortho-rich composition ordinarily expected from con-
ventional nitration of toluene can be reversed to the
para-dominant one by using a NO2–O2 system in the
presence of HZSM-5, the ortho–para ratio being
improved to 0.08 and the para-selectivity up to 90%.
The solvent-free nitration of toluene with NO2 is most
likely to occur in the interior of zeolite channels via the
electrophilic process involving the nitronium ion, partly
accompanied by an ordinary radical process that leads
to side-chain reaction products.
To our knowledge, no substantiated mechanisms have
been proposed to date for the nitration of arenes on the
surface of solid catalyst. High ortho–para preference
along with insignificant formation of m-isomer reflects
the general characteristic of electrophilic substitution
involving the nitronium ion. However, concurrent for-
mation of a slight amount of side-chain nitration and
oxidation products suggests a minor involvement of a
radical process.
Acknowledgements
X.P. thanks the Japan Society for the Promotion of
Science for the Fellowship (no. P99282).
In the presence of a radical scavenger such as m-di-
nitrobenzene, no obvious change was observed as to the
relative ratio of ring and side-chain substitution prod-
ucts as well as the ortho–para isomer ratio. The
Brønsted active sites resulting from the hydroxylic oxy-
gen atoms bridging between the silicon and aluminum
atoms in the zeolite framework are located over 90% of
total amounts in the interior of channels.7,10 They
would play a role similar to mineral acids inside the
channel system of HZSM-5, leading to the para-selec-
tive nitration.
References
1. (a) Olah, G. A.; Malhotra, R.; Narang, S. C. Nitration:
Methods and Mechanisms; VCH: New York, 1989; (b)
Schofield, K. Aromatic Nitration; Cambridge University
Press: Cambridge, 1980.
2. (a) Nonoyama, N.; Mori, T.; Suzuki, H. Zh. Org. Khim.
1998, 34, 1591–1601; (b) Suzuki, T.; Noyori, R. Chem-
tracts 1997, 10, 813–815; (c) Mori, T.; Suzuki, H. Synlett
1995, 383–392.
The side-chain reaction products 4 and 5 were probably
formed through the capture of the benzyl radical, partly
derived from toluene via the direct abstraction of
hydrogen by NO2 and partly via the electron transfer
between zeolite followed by proton release from alkyl
side-chain, by nitrogen dioxide and oxygen, respec-
tively. Nitrogen dioxide has been known to trap the
benzyl radical more efficiently than molecular oxygen,11
thus favoring phenylnitromethane 4 over benzaldehyde
5. Small amounts of dinitrotoluenes 6 were simulta-
neously formed in spite of the presence of a large excess
of toluene. Since none of the isomeric mononitrotolu-
enes reacted further to form dinitro derivatives under
the conditions employed, these dinitro compounds are
most likely to form directly from toluene via the addi-
tion–elimination sequence, as suggested previously.12
3. Suzuki, H.; Yonezawa, S.; Nonoyama, N.; Mori, T. J.
Chem. Soc., Perkin Trans. 1 1996, 2385–2389.
4. (a) Laszlo, P.; Vandormael, J. Chem. Lett. 1988, 1843–
1846; (b) Cornelis, A.; Delaude, L.; Gerstmans, A.; Las-
zlo, P. Tetrahedron Lett. 1988, 29, 5657–5660.
5. Smith, K.; Fry, K. Tetrahedron Lett. 1989, 30, 5333–
5336.
6. (a) Smith, K.; Musson, A.; DeBoos, G. A. Chem. Com-
mun. 1996, 469–470; (b) Smith, K.; Musson, A.; DeBoos,
G. A. J. Org. Chem. 1998, 63, 8448–8454.
7. Kwok, T. J.; Jayasuriya, K.; Damavarapu, R.; Brodman,
B. W. J. Org. Chem. 1994, 59, 4939–4942.
8. The reaction was carried out in a closed vessel by stirring
a mixture of toluene (1.0 ml), NO2 (0.1 ml), HZSM-5 (1.0
,
g), and 3 A molecular sieves (0.1 g) for 22 h under
oxygen.
9. Smith, K.; Almeer, S.; Black, S. J. Chem. Commun. 2000,
1571–1572.
10. Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid
Acids and Bases: Their Catalytic Properties; Elsevier:
Amsterdam, 1989; p. 143.
11. Goumri, A.; Elmaimouni, L.; Sawerysyn, J. P.; Devolder,
P. J. Phys. Chem. 1992, 96, 5395–5400.
12. Suzuki, H.; Mori, T. J. Chem. Soc., Perkin Trans. 2 1995,
41–44.
Under similar conditions, chlorobenzene also reacted in
a para-selective manner. Thus, on stirring a mixture of
chlorobenzene (5.0 ml), liquid NO2 (0.5 ml), HZSM-5
,
(2.0 g) and 3 A molecular sieves (0.5 g) at room
temperature for 22 h under oxygen, chloronitroben-
zenes (3.74 mmol) were obtained in the isomer ratio of
ortho 8, meta 2 and para 90. The ortho–para ratio 0.09
was better than those (0.25–0.40) previously reported
.