trifluoroacetic anhydride (acetic acid or its anhydride), and
â zeolite have been used for the two-step dinitration of
toluene, giving 2,4- and 2,6-dinitro isomers in a greater ratio
of 70:1.3b Unfortunately, these nitration methods are still
conducted in strong acid media involving inorganic and/or
organic acids, so there still remain many problems similar
to the conventional method based on mixed acid. In the
present work, the inorganic solid assisted conversion of
toluene to dinitrotoluenes according to the Kyodai method
has been investigated via two different approaches, one
involving the straightforward dinitration of toluene and the
other involving further nitration of o-nitrotoluene.
Table 2. Effect of Solvent Types on the Regioselectivity of
Kyodai Dinitration of Toluene in the Presence of Zeolite
HBEA-25a
product composition (%)b
substrate solvent
2,4-
2,6-
others
2,4-/2,6- ratioc
1
1
1d
1
1
2
n-C6H14
CCl4
CCl4
CH2Cl2
MeCN
n-C6H14
75
74
78
84
87
72
15
18
12
9.0
7.2
25
10
8.0
10
7.0
5.8
3.0
5.0
4.1
6.5
9.3
12
2.9
Clays are assemblies of the tetrahedral layers of SiO4
silicate units and of the octahedral layers of AlO6 aluminate
units and have interlamellar spacings of less than 1 nm.
Organic molecules would be adsorbed onto such limited
spaces and/or pores, modifying their reactivity according to
the surface properties of inorganic solids. A commercial
montmorillonite K10 (Aldrich) was first tested as a catalyst
for the regioselective dinitration of toluene. For comparison,
we have also carried out the double Kyodai nitration of
toluene in dichloromethane in the absence of any solid
catalyst and determined the isomer composition of the
product. As is apparent from Table 1, the Kyodai method
did not affect significantly the isomeric product ratio in
comparison to the traditional mixed acid method. Introduction
of K10 into the reaction mixture was found to bring only a
slight increase in the para-selection. Increasing the amount
of K10 improved the regioselectivity somewhat toward the
2,4-dinitro isomer.
a A gift from the Catalytic Society of Japan; calcined in air at 500 °C
for 8 h prior to use. Other conditions excepting the solvent were the same
as those given in Table 1. b Final product composition determined by GC.
c Calculated from GC peak areas. d Additional 2.0 g was added 2 h after
the start of the reaction.
Increasing the amount of solvent and catalyst favored the
reaction further toward the 2,4-dinitro isomer up to ca. 97%
high selectivity (Table 3), while decreasing the reaction
temperature made little difference in regioselectivity.
Table 3. Effect of Catalyst Amount on the Isomer Ratio in the
Kyodai Dinitration of Toluenea
product composition (%)b
HBEA-25 (g)
2,4-
2,6-
others
2,4-/2,6- ratioc
Zeolites are crystalline aluminosilicates with uniform pore
dimension. Zeolite HZSM-5 (Acros) represents a medium-
pore material with an elliptical pore size of 0.51-0.55 nm.
Table 1 shows the results from the Kyodai nitration of
toluene over HZSM-5; the presence or absence of HZSM-5
made only a slight difference under the conditions employed.
A minor variation in the composition of the final product
probably originated from the mononitration step, where the
reaction primarily occurs in a free solution.
0.5
1.0
2.0
3.0d
4.0
83
87
90
93
94
12
5.0
5.8
5.3
3.0
2.7
6.9
12
19
23
28
7.2
4.7
4.0
3.3
a All reactions were carried out in acetonitrile. Other conditions excepting
the catalyst amount were the same as those given in Table 1. b Final product
composition determined by GC. c Calculated from GC peak areas. d The
reaction was started using 1.0 g of catalyst and an additional 2.0 g was
added 2 h after the start.
Zeolite HBEA-25 (Su¨d-Chemie AG) with much wider
pores than HZSM-5 was then chosen for the study; the results
are summarized in Table 1. As can be seen from the table,
HBEA-25 is of outstanding catalytic characteristics in the
regioselective dinitration of toluene. Judging from the relative
variation of the isomer ratios of both mono- and dinitration
products, we may well take that HBEA-25 can exert more
influence on the dinitration step. It is in the second nitration
step that the much higher than normal 2,4-/2,6-dinitro isomer
ratio in overall reaction is generated.
The nature of organic solvent was found to exert consider-
able influence on the regioselectivity of dinitration. As Table
2 shows, of the four different solvents of varied polarity
employed, acetonitrile with a high dielectric constant ex-
hibited the best result. The 2,4-/2,6-isomer ratio was en-
hanced up to 12, along with a 94% combined yield of 2,4-
and 2,6-dinitrotoluenes based on toluene.
The spent HBEA-25 was easily recovered from the
reaction mixture by simple decantation or filtration and could
be reused without any loss of original activity. Even after
five times usage, there was observed little change in the
numerical values as to the yield and relative ratio of 2,4-/
2,6-dinitro isomers (Table 4).
As an extension of the present work, we have investigated
the double Kyodai nitration of chlorobenzene under similar
conditions. 1-Chloro-2,4-dinitrobenzene is of the greatest
market demand among the six possible isomers as an
intermediate in the manufacture of dyes, fungicides, rubber
chemicals, and highly energetic materials. Treatment of
chlorobenzene with excess of mixed acid at elevated tem-
perature produces 1-chloro-2,4-dinitrobenzene as the major
product, together with about 10% of the 2,6-dinitro isomer.7
(6) Gigante, B.; Prazeres, A. O.; Marcelo-Curto, M. J.; Cornelis, A.;
Laszlo, P. J. Org. Chem. 1995, 60, 3445-3447.
(7) Hoffman, E. J.; Dame, P. A. J. Am. Chem. Soc. 1919, 41, 1013-
1020.
Org. Lett., Vol. 3, No. 22, 2001
3433