A polymer imidazole salt as phase-transfer catalyst in halex fluorination irradiated by microwave

Article abstract of DOI:10.1016/j.jfluchem.2007.02.002

A new imidazole polymer salt was synthesized in order to develop a high efficiency phase-transfer catalyst for multi-phase reactions. The polymer salt was prepared easily by co-polymerization of 1-1′-(1,4-butamethylene)bis(imidazole) and 1,2-dibromoethane, and has the properties of excellent chemical and thermal stability and high ionic conductivity. It was applied as phase-transfer catalyst in the fluorination of chloronitrobenzenes under the irradiation of microwave and gave excellent yields of corresponding fluoronitrobenzenes. In addition, the enhanced mechanism of microwave was studied and found "non-thermal" effect was a great factor.

Full text of DOI:10.1016/j.jfluchem.2007.02.002

Journal of Fluorine Chemistry 128 (2007) 608–611
www.elsevier.com/locate/fluor
A polymer imidazole salt as phase-transfer catalyst in halex
fluorination irradiated by microwave
*
Zheng Yong Liang, Chun Xu Lu¨ , Jun Luo, Li Bin Dong
School of Chemical Engineering, Nanjing University of Science and Technology, 200 Xiaoling Wei, Nanjing 210094, China
Received 8 December 2006; received in revised form 24 January 2007; accepted 2 February 2007
Available online 12 February 2007
Abstract
A new imidazole polymer salt was synthesized in order to develop a high efficiency phase-transfer catalyst for multi-phase reactions. The
polymer salt was prepared easily by co-polymerization of 1-10-(1,4-butamethylene)bis(imidazole) and 1,2-dibromoethane, and has the properties
of excellent chemical and thermal stability and high ionic conductivity. It was applied as phase-transfer catalyst in the fluorination of
chloronitrobenzenes under the irradiation of microwave and gave excellent yields of corresponding fluoronitrobenzenes. In addition, the enhanced
mechanism of microwave was studied and found ‘‘non-thermal’’ effect was a great factor.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Microwave chemistry; Polymer imidazole salt; Halogen-exchange fluorination; Fluoronitrobenzenes; Non-thermal effect
1. Introduction
hales fluorination [3]. Herein, we describe another stable and
effective polyalkylimidazolimu bromide 1 (Scheme 1) acts as
PTC in halex fluorination under the irradiation of microwave.
Halogen-exchange (halex) fluorination is an important
method to prepare fluorinated compounds. To accelerate the
reaction between KF and substrates, phase transfer-catalyst
(PTC) is often used. But quaternary ammonium salts containing
b-H are prone to take place Hofmann Elimination during halex
fluorination. At the same time, by-products derived from the
decomposition would worsen the reaction greatly. To improve
PTC’s catalytic activity under the condition of high tempera-
ture, quaternary ammonium or phosphonium salts are grafted
on polymer carrier and form a new kind of phase-transfer
catalyst named polymer PTC. Yoshida grafted N-(2-ethyl-
hexyl)-4-(N,N0-dimethyl)aminopyridinium bromide on poly-
styrene to get a polymer PTC (noted: Cat.A) [1]. Subsquently,
he and his co-workers developed divinylbenzene across linked
polystyrene supported tetraphenylphosphonium bromide and
its modified analogue by replacing active hydrogen by methyl
group (Note: Cat.B and Cat.C) [2]. The three polymers would
not decompose obviously even over 210 8C. Luo and his co-
workers also reported an effective and stable polydiallyldi-
methylammonium chloride (PDMDAAC) as PTC was used in
2. Results and discussion
As we know, efficient and stable PTC should have big
relative molecular weight, stable structure and big polarity. As
shown in Scheme 1, polymer 1 has a big density of catalytic
component, so it should have high phase-transfer activity. In
fact, polymer imidazole salts have received much attention
because of their remarkable properties including catalytic
activity, chemical and thermal stability and high ionic
conductivity [4]. In addition, polymer imidazole salts have
several beneficial features for phase transfer catalyst: (1)
reactivity of the monomer molecule can be controlled easily by
modification of the alkyl chain and reaction temperature, which
makes it possible to control PTC’s polymerization degree and
density of catalytic active component, (2) addition of
polymerization initiator is not required, which makes poly-
merization manageable. For the reasons above, polymer 1 is
very suitable for halex fluorination on high temperature. Its
decomposition temperature determined by TGA is about
340 8C. The TGA data of polymer 1 was demonstrated in Fig. 1.
Catalytic capability of different polymer catalysts on prepara-
tion of 4-fluoronitrobenzene (PFNB) via halex fluorination
* Corresponding author.
E-mail address: lcx781103@126.com (C.X. Lu¨).
0022-1139/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2007.02.002

Z.Y. Liang et al. / Journal of Fluorine Chemistry 128 (2007) 608–611
609
Scheme 1.
from 4-chloronitrobenzene (PCNB) with the conventional
heating was tested. The results were shown in Table 1. From
Table 1, it could be found that polymer 1 had excellent catalytic
effect than the other polymer catalyst for the sake of its higher
density of catalytic active center. [bmim]Br was unstable owing
to elimination or substitution processes under the condition of
high temperature and KF [5]. While polymer 1 was more stable
due to its greater steric hindrance effect.
The kinetics experiments were carried based on considera-
tions below: (1) to eliminate the influence of diffusion, all
reactions were carried at the same stirring speed. (2) To
eliminate the influence of PTC’s decomposition on fluorination
reaction rate during the process of reaction, all reactions were
carried without PTC. The kinetics curve on condition of
microwave irradiation and conventional heating were shown in
Figs. 2 and 3. From Fig. 2, we could find the reaction followed
pseudo-first order kinetics based on the C(PCNB) under the
conventional heating because the concentration of the Fꢀ was
Reaction rate can be accelerated remarkably in some
reactions under the irradiation of microwave, which was
attributed to ‘‘thermal effect’’ and ‘‘non-thermal effect’’ of
microwave [6]. The reason why ‘‘thermal effect’’ can accelerate
reaction rate is known as overheat. So, it cannot change kinetics
under the conditions of same reactant, catalyst and product
when compared to conventional heating while ‘‘non-thermal
effect’’ can change kinetic feature, reduce activation energy and
even change reaction order [7]. The promoting effect of ‘‘non-
thermal effect’’ is possibly attributed to an enhancement in the
polarity of the system from the ground state towards the
transition state [8]. It is consistent with the extension of anionic
charge distributing in transition state.
Owing to the defect of our microwave equipment without
temperature controller, we could not study ‘‘non-thermal
effect’’ of microwave through comparing the activation energy
(Ea) between microwave irradiation and conventional heating.
But based on the theoretical premise that microwave can
change kinetics feature, ‘‘non-thermal effect’’ can be revealed
from the change of kinetics curve under microwave and
convention heating conditions.
Fig. 2. First-order kinetic plot for fluorination of PCNB in Me2SO under the
irradiation of microwave.
Fig. 3. Second-order kinetic plot for fluorination of PCNB in Me2SO under the
irradiation of microwave.
Fig. 1. TGA data of polymer 1.

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Z.Y. Liang et al. / Journal of Fluorine Chemistry 128 (2007) 608–611
Table 1
Comparison of different polymer catalysts in halogen-exchange fluorination
Catalyst
Mw
PCNB
KF
Solvent
Temperature
Time
(h)
Yield of
Reference
(mmol)
(mmol)
(8C)
PFNB (%)
Polymer 1 (0.5 g)
Cat.A (2.43 g)
Cat.B (4.5 g)
28,400a
50
25
50
50
25
75
Me2SO (10 mL)
TMSO2 (15 g)
TMSO2 (30 g)
TMSO2 (30 g)
Me2SO (20 mL)
192
180
180
180
185
2
5
4
5
3
92.8
87.0
72.0
83.0
90.1
970
892
37.5
75
[1]
[2]
[2]
[3]
Cat.C (4.2 g)
840
75
PDMDAAC (0.33 g)
200,000
37.5
[bmim]Br (0.5 g)
219
50
75
Me2SO (10 mL)
192
2
78.4
a
Mw = 28,400; Mn = 24,483, D = 1.160.
Table 2
Fluorination of different substrates in Me2SO under irradiation of microwave
Entry
Reactant (2a–2f)
(substance C6H5 NO2)
KF (equiv.)
Power (W)
Temperature (8C)a
Time (h)
Product (3a–3f)
(substance C6H5 NO2)
Yield (%)b
1
4-Cl
1.5
1.5
3
300
300
250
250
250
200
202
203
203
206
204
90
1.5
2
4-F
94.2
87.8
70.2
87.4
84.4
88.3
2
2-Cl
2-F
3
2,4-Cl2
3,4-Cl2
2,5-Cl2
4-Cl, 3-NO2
2
2,4-F2
4
1.5
1.5
1.5
0.5
1
3-Cl, 4-F
5-Cl, 2F
4-F, 3-NO2
5
6c
4
a
Determined by Reytek infrared thermometer.
Determined by GC.
b
c
Reacted in 15 mL CH3CN.
essentially constant due to the excess of insoluble KF present
throughout the reaction. To our surprise, the relationship of
ln C(PCNB) and reaction time was not linear any more on
condition of microwave irradiation (Fig. 2). The reaction
followed pseudo-second order kinetics based on the C2(PCNB)
(see Fig. 3).
and the reaction time can be shortened more or less when
compared with conventional heating. The catalytic effect is
better than other kind of polymer phase transfer catalysts. In
addition, ‘‘non-thermal effect’’ of microwave was found in
fluorination through comparing macroscopic kinetics between
conventional heating and microwave heating.
Different substrates were halex fluorinated in Me2SO under
the irradiation of microwave. The results were shown in
Table 2. From Table 2, it could be found that all reactions were
performed well under the irradiation of microwave. The reason
why 3,4-dichloronitrobenzene could be promoted best is that
the ‘‘non-thermal effect’’ of microwave was most significant in
the fluorination. To our surprise, the fluorination reaction rate of
2,4-dichloronitrobenzene could be accelerated slightly. The
reason may be the limitation of mass transfer between inorganic
solid phase and organic liquid phase besides of reaction activity
of substrate, which is also the reason why more and more
scientists devoted themselves to research and development of
high efficiency phase-transfer catalyst.
4. Experimental
Thermal gravimetric analyses data were obtained by
SHIMADZU DTA-50 thermal analyzer. Molecule weight
was recorded on AGILENT 1100 gel permeation chromato-
graph. Reactions were carried in Sanle WHL07S-1 chemistry-
oriented microwave oven made in Nanjing Sanle Microwave
Technology and Development Co. Ltd. Yields were determined
by Agilent 4890 gas chromatograph.
4.1. Synthesis of polyalkylimidazolium bromide
1-10-(1,4-Butamethylene)bis(imidazole) was prepared by a
coupling reaction of 1 mol 1,4-dibromobutane with 2 mol
imidazole potassium salt in THF according to procedures
described in reference [9].
3. Conclusion
Polymer 1 is a new kind of polymer phase transfer catalyst,
which has the superiority of convenience for preparation, good
thermal stability and high catalytic activity. So it is very
suitable for halogen-exchange fluorination at high temperature.
Some fluorinitrobenzene derivatives were prepared via halo-
gen-exchange fluorination catalyzed by polymer 1 from
corresponding chloronitrobenzene derivatives under the irra-
diation of microwave. The yields of products under the
optimum reaction conditions were in the range of 70.2–94.2%,
A 0.1 mol 1,2-dibromoethane was added dropwise to a
stirred mixture of 0.1 mol 1-10-(1,4-butamethylene)bis(imida-
zole) and 50 mL toluene, then the mixture was heated and
maintained at 110 8C for 24 h. Cooled to room temperature and
filtered, yellow crude product could be obtained. The crude
product was wished several times with acetic ether and
chloroform successively and dried in vacuum at 100 8C for 36 h
before use.

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611
4.2. Preparation of fluorinated nitrobenzene derivates
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