A polymer onium acting as phase-transfer catalyst in halogen-exchange fluorination promoted by microwave

Article abstract of DOI:10.1016/j.jfluchem.2003.11.018

A polymerized quaternary ammonium salt polydiallyldimethylammonium chloride, exhibiting high stability to heat and base, was prepared and applied as phase-transfer catalyst (PTC) in halogen-exchange (Halex) fluorination of chloronitrobenzenes to give excellent yields of corresponding fluoronitrobenzenes. Dimethyl sulfoxide was found to be the best solvent when microwave was applied as heating resource.

Full text of DOI:10.1016/j.jfluchem.2003.11.018

Journal of Fluorine Chemistry 125 (2004) 701–704
A polymer onium acting as phase-transfer catalyst in
halogen-exchange fluorination promoted by microwave
*
¨
¨
Jun Luo , Chunxu Lu, Chun Cai, Wenchao Qu
School of Chemical Engineering, Nanjing University of Science & Technology, 210094 Nanjing, PR China
Received 22 January 2003; received in revised form 19 November 2003; accepted 28 November 2003
Available online 13 January 2004
Abstract
A polymerized quaternary ammonium salt polydiallyldimethylammonium chloride, exhibiting high stability to heat and base, was
prepared and applied as phase-transfer catalyst (PTC) in halogen-exchange (Halex) fluorination of chloronitrobenzenes to give excellent
yields of corresponding fluoronitrobenzenes. Dimethyl sulfoxide was found to be the best solvent when microwave was applied as heating
resource.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Fluoronitrobenzenes; Polydiallyldimethylammonium chloride; Halogen-exchange fluorination; Microwave; Solvent
1. Introduction
2. Results and discussion
Halogen-exchange (Halex) fluorination is an important
method to prepare fluorinated aromatics. To accelerate the
reaction between KF and substrates, phase-transfer cata-
lyst (PTC) is often used. But quaternary ammonium salts
containing b-H are prone to decompose to certain degree
under Halex fluorination conditions [1]. At the same time,
by-products derived from the decomposition would wor-
sen the reaction greatly. To solve these problems, Yoshida
grafted N-(2-ethylhexyl)-4-(N0,N0-dimethyl)aminopyridi-
nium bromide on polystyrene to get a polymer PTC with
a relative molecular weight of near 1000 gÁmolÀ1 [2].
Subsequently, he and co-workers developed divinylben-
zene across linked polystyrene supported tetraphenyl-
phosphonium bromide and its modified analogue by
replacing active hydrogen by methyl group [3]. The
two polymers would not decompose obviously even at
210 8C. Herein, we report another efficient and stable
polymer onium PTC to act as phase-transfer catalyst in
halogen-exchange fluorination under the irradiation of
microwave.
As we know, efficient and stable PTC should have big
relative molecular weight, stable structure and big polarity.
So, we think it may be a polymer with cyclic structure. For
the sake of convenience for preparation, polydiallyldi-
methylammonium chloride (1) was found to be the best
candidate. It was prepared by polymerization of diallyldi-
methylammonium chloride according to reference [4]
(Scheme 1).
As shown in Scheme 1, polymer 1 has a big density of
catalytic active component. We reason that it should have
high phase-transfer activity. At the same time, the cataly-
tically active part is a five-membered ring, which provide
it high stability. And fortunately these were proved by
our experimental results.
1 is a very hygroscopic yellowish polymer with an average
relative molecular weight of about 2 Â 105 gÁmolÀ1. It would
convert to a liquid when exposed to air for only 1 min.
Furthermore, it is almost unsolvable in some polar
aprotic solvents such as Me2SO, sulfolane, HCONMe2,
MeCONMe2 and N-methylpyrrolidone. So, the Halex reac-
tion is a solid (1)–solid (KF)–liquid (solution of substrates)
triphase procedure. To the best of our knowledge, it is the
first time that 1 is used as PTC in Halex reaction. 1 is
so hygroscopic that it is usually stored as solution in
water; so, it is necessary to to dry it thoroughly before
use. At first, its solution is dried at 100 8C in common oven,
* Corresponding author. Present address: Shanghai Institute of Organic
Chemistry, P.O. Box 29, 354 Feng Lin Lu, Shanghai 200032, China.
Tel.: þ86-21641633001250.
E-mail address: luoj87@163.com (J. Luo).
0022-1139/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2003.11.018

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J. Luo et al. / Journal of Fluorine Chemistry 125 (2004) 701–704
(NH4)2S2O8
Cl
NaOH
+
N
H
+
N
+
N
-
.
.
-
Cl
Cl
n
1
Cl
F
R1
2a,3a: R1=H, R2=NO2
2b,3b: R1=NO2, R2=H
2c,3c: R1=Cl, R2=NO2
R1
1, KF
MW, solvent
2d,3d: R1=NO2, R2=Cl
2
R2
2e,3e
: R1=NO2, R =NO2
R2
2
3
Scheme 1.
then grinded to fine powder and further dehydrated at
100 8C in vacuum for 10 h, and finally stored in desiccator.
To examine its stability to heat, 0.33 g of 1 was added in
20 ml Me2SO and refluxed for 6 h to find no obvious change.
2.19 g of KF was added in this system and refluxed for
another 6 h to find almost the same phenomenon. So, 1 is
very stable to heat and base. When 1 was used as phase-
transfer catalyst in the halogen-exchange fluorination of p-
chloronitrobenzene (PCNB), only 3 h was needed to com-
plete the reaction and 90.1% p-fluoronitrobenzene (PFNB)
was obtained. In a control experiment, only 86.8% PFNB
yielded after 6 h and some tar appeared when cetyltrimethy-
lammnoium bromide was used.
The application of microwave in organic synthesis was
found its beginning by Gedye et al. [5] for esterization and
have been developing very rapidly from then on. Some
authors reported that the fluorination by radioactive ele-
mental fluorine under microwave irradiation could be accel-
erated tremendously [6–8]. Kidwai et al. [9] got similar
results when using KF and (Me)4NF as fluorinating agent.
But they applied only in a very small scale and reacted in
sealed tubes. In this work, microwave irradiation was
applied to promote Halex fluorination in common scale.
To our surprise, sulfolane and N-methylpyrrolidone,
excellent solvents in traditional preparation, did not increase
the conversion of PCNB but rather decreased it (Table 1). In
fact, the final reaction mixture was viscous, filtered to get a
saturated solid, washed twice by benzene and changed to
brownish. The filtrate was distilled by steam to get nut-
brown mother liquids with considerable amount of tar or
dispersed black powder. It is known that superheating of
liquid is sure to occur when microwave was used as heating
resource [10], and so are these solvents according to this
work (Table 2). The boiling point increment at the same
conditions obviously followed linear plot against dielectric
constant with a correlation coefficient of 0.998 (Fig. 1).
The superheating phenomenon made excessively high
reaction temperature when sulfolane and N-methylpyrroli-
done, which would promote the decomposition, were used as
solvents. Some evidences were found in blank experiments
that sulfolane and N-methylpyrrolidone would darken their
color gradually to nut-brown and brown, respectively, when
refluxed in microwave field. The former changed to black
and obviously some fine black grit appeared at the bottom of
the vessel after 2 h. Anhydrous fluoride is such a strong base
in this system that it can extract proton from solvents, which
is a main factor for the formation of a series of by-products
[11]. To prove this, 3 g of KF was added in 20 ml of fresh
sulfolane and refluxed in 700 W microwave for 2 h. A black,
viscous and effluvial system was gotten and some one fifth
Table 1
Preparation of PFNB catalyzed by 1 in 350 W microwave
Solvent
Me2SO
Time
(h)
Yield
(%)
Conversion
(%)
2
1.5
92.5
90.3
99.7
99.0
HCONMe2
2
2
2
2
6.0
31.5
40.6
46.2
7.2
38.2
61.9
70.9
MeCONMe2
N-Methylpyrrolidone
Sulfolane
Table 2
Superheating phenomenon of some solvents (50 ml) heated by microwave
Solvents
Dielectric constant
bp in oil (8C)
bp in microwave (8C)
bp increment (8C)
HCONMe2
36.71
37.78
48.9
153
166
189
287
204
161
174
199
296
211
8
8
MeCONMe2
Me2SO
Sulfolane
10
9
43.3
N-Methylpyrrolidone
32.0
7

J. Luo et al. / Journal of Fluorine Chemistry 125 (2004) 701–704
703
Table 3
Preparation of Ar–F catalyzed by 1
Entry
a
Substrate (2)
Heating resource
350 W (MW)
Solvent
Me2SO
Time
2 h
Product (3)
Yield (%)
92.5
200 8C (oil bath)
Me2SO
Me2SO
3 h
3 h
90.1
60.5
b
c
300 W (MW)
200 8C (oil bath)
Me2SO
Me2SO
5 h
51.1
94.5
350 W (MW)
10 min
200 8C (oil bath)
Me2SO
1 h
88.6
d
e
350 W (MW)
190 8 (oil bath)
Me2SO
Me2SO
15 min
30 min
58.0
41.2
200 W (MW)
Acetonitrile
Acetonitrile
2 h
89.2
80.5
110 8C (oil bath)
10 h
of total mixture solidified to coke-like thing. It was obvious
that decomposition was reinforced by KF. It seemed that the
decomposition of solvents changed to be the main reaction at
elevated boiling point since much lower conversion of
substrates were resulted in comparison with Me2SO
(Table 1). In a word, Me2SO is the best solvent for micro-
wave-promoted Halex reaction.
enhancement by a great number of authors. And we have
also found the kinetic functions of common and microwave-
promoted fluorination following different reaction orders in
our further research. But, it is beyond the scope of this paper.
3. Conclusion
In order to identify its catalytic activity, 1 was used to
catalyze the preparation of a series of fluoronitrobenzenes
from corresponding chloronitrobenzenes (Table 3).
The results shown in Table 3 illustrated that 1 was an
efficient and stable PTC for Halex reaction, and the applica-
tion of microwave irradiation accelerated the reaction rate
for several times and the yields of products were also
elevated. At the same time, very small tar and light color
of reaction system were observed in experiments. ‘‘Non-heat
effect’’ has being presumed to be the reason for the rate
A stable polymerized quaternary ammonium salt poly-
diallyldimethylammonium chloride was developed to act as
efficient and stable phase-transfer catalyst in halogen-
exchange fluorination to prepare fluoronitrobenzenes with
good yields. The application of microwave enhanced the
reaction rate several times. Superheating phenomenon
showed that sulfolane and N-methylpyrrolidone, excellent
solvents for traditional fluorination, were not fit for micro-
wave-promoted fluorination any longer.
Me2SO
10
4. Experimental
4.1. General
9
sulfolane
Spray-dried KF was further dried in vacuum at 150 8C for
15 h before use. All solvents were subjected to re-distillation
and dried over 4A molecular sieve for at least 1 week.
Sanle WHL07S-1 chemistry-oriented microwave oven,
designed by the author, was made in Nanjing Sanle Micro-
wave Technology Development Co. Ltd., and the tempera-
ture was measured by Reytek1 infrared thermometer.
HCONMe2
8
7
MeCONMe2
N-methylpyrrolidone
30 32 34 36 38 40 42 44 46 48 50
Dielectric constant
Fig. 1.

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J. Luo et al. / Journal of Fluorine Chemistry 125 (2004) 701–704
Products were inspected by Varian 3700 gas chromatograph,
Varian Saturn 2000 GC/MS chromatograph and Bomenn
MB1543 FT-IR chromatograph (KBr). The IR and MS
spectra of products were identical with authentic standard
spectra.
and benzene was removed by simple distillation. Pure
products was obtained by rectification under reduced pres-
sure. Traditional heating reaction was all the same except for
being heated by oil bath.
4.3.2. Preparation of 2,4-dinitrofluorobenzene(3e) from
2,4-dinitrochlorobenzene (2e)
4.2. Preparation of polydiallyldimethylammonium
chloride (1)
4.05 g of 2,4-dinitrochlorobenzene, 0.37 g of 1, 1.75 g of
KF and 0.28 g of urotropine, acting as polymerizing inhi-
bitor, were added in 20 ml acetonitrile, heated by 200 W
microwave for 2 h, then cooled down to room temperature
and filtered. The filtrate was examined by GC directly.
Traditional reaction was all the same except that the mixture
was refluxed in oil bath.
To a mixture of 30 ml of dimethylamine (35.8 wt.%) and
37 ml of allyl chloride was added 22.4 g of solution of
NaOH (40%) at 15 8C with agitation; after 10 min, the
system was heated to 40 8C and maintained for 6 h. The
final mixture was filtered and 6 ml of water was added, and
finally distilled under reduced pressure to get rid of volatile
by-products. Consequently, the residue was diluted by water
to 48.4 g to get a solution of diallyldimethylammonium
chloride. At 60 8C in N2 atmosphere, 33 g of solution of
(NH4)2S2O8 (4%) in water was added evenly into the above
solution within 2 h and reacted for further 6 h. 1 was
recovered by a yield of 92.3%.
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