Phosphonium ionic liquids as highly thermal stable and efficient phase transfer catalysts for solid-liquid Halex reactions

Article abstract of DOI:10.1016/j.cattod.2012.02.063

Trihexyl (tetradecyl) phosphonium tetrafluoroborate was found to be an active catalyst for the introduction of fluoride by nucleophilic aromatic substitution. With a decomposition temperature above 300 °C, this ionic liquid is suitable for reactions at high temperatures. The addition of 1 mol% of the ionic liquid relative to KF increased the initial rate for the fluorination of 1,2-dichloro-4-nitrobenzene six-fold compared to the uncatalysed reaction. The highest reaction rate was observed at 20 mol% ionic liquid relative to KF. Ease of handling without the need for stringent drying conditions and reuseability make this ionic liquid a useful phase transfer catalyst.

Full text of DOI:10.1016/j.cattod.2012.02.063

Catalysis Today 198 (2012) 300–304
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Catalysis Today
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Phosphonium ionic liquids as highly thermal stable and efficient phase transfer
catalysts for solid–liquid Halex reactions
Ao Fan, Gaik-Khuan Chuah, Stephan Jaenicke∗
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Trihexyl (tetradecyl) phosphonium tetrafluoroborate was found to be an active catalyst for the introduc-
tion of fluoride by nucleophilic aromatic substitution. With a decomposition temperature above 300 ◦C,
this ionic liquid is suitable for reactions at high temperatures. The addition of 1 mol% of the ionic liquid
relative to KF increased the initial rate for the fluorination of 1,2-dichloro-4-nitrobenzene six-fold com-
pared to the uncatalysed reaction. The highest reaction rate was observed at 20 mol% ionic liquid relative
to KF. Ease of handling without the need for stringent drying conditions and reuseability make this ionic
liquid a useful phase transfer catalyst.
Received 19 January 2012
Accepted 7 February 2012
Available online 1 May 2012
Keywords:
Ionic liquids
Phase transfer catalyst
Phosphonium ion
Halex reaction
KF
© 2012 Elsevier B.V. All rights reserved.
Chloronitrobenzenes
1. Introduction
In recent years, ionic liquids (IL) have attracted considerable
interest as solvents for a wide range of organic and inorganic mate-
Fluoroaromatics are important building blocks in pharma-
ceuticals and agrochemicals due to their lipophilicity, broader
spectrum of activity, and lower toxicity. These compounds fre-
quently aso show enhanced chemical and physical properties over
their non-fluorinated analogs [1]. An important method to pre-
pare fluoroaromatic compounds is by halogen-exchange (Halex)
where a chloro- or bromo-group in the precursor is substituted by
fluoride [2,3]. A number of metal fluorides can be used as nucle-
ophilic fluorinating agents, but potassium fluoride (KF) is preferred
because of its ready availability and low cost [3–5]. In comparison,
sodium fluoride is of lower reactivity than KF while cesium flu-
oride, although more reactive than KF, is too expensive for large
scale use. Potassium fluoride is reasonably soluble in protic sol-
vents aided by the formation of strong hydrogen bonds between
H and F−. However, this reduces the nucleophilicity of F− so that
only highly electrophilic molecules can be substituted. For this
reason, Halex reactions are best carried out in polar aprotic sol-
vents where the nucleophilicity of F− is higher, although at the
expense of solubility. At ambient temperature, KF dissolves in sol-
vents such as dimethylsulfoxide (DMSO), tetramethylenesulfone,
sulfolane, N,N-dimethylacetamide (DMAC), N-methylpyrolidinone
(NMP) and benzonitrile only at the millimole level [3,6]. To achieve
sufficient solubility, temperatures in the range of 240–300 ◦C are
therefore required [4].
rials [6–11]. Ionic liquids are also useful as phase transfer catalysts.
The use of phase transfer catalysts has led to rate enhancements
of 107 for the Halex reaction [12]. Various phase transfer cata-
lysts including tetraalkylammonium, phosphonium, guanidinium,
and carbophosphazenium and bis(phosphoranylidene) ammonium
salts have been reported [12–19]. The Halex reaction has to be car-
ried out at high temperatures, but the conductivity of the ionic
liquids was found to be lower after the reaction, indicating degra-
dation of the catalyst [17]. Hence, the discovery of highly efficient
and thermally stable ionic liquids remains important. Phospho-
nium salts are much more thermally stable than the corresponding
ammonium salts. Furthermore, acidic protons are absent in the
phosphonium cation, unlike the imidazolium cation, so that car-
bene formation can be avoided [10]. In this work, we investigate
three phosphonium ionic liquids (IL), namely, triisobutyl (methyl)
phosphonium tosylate 1, tributyl (methyl) phosphonium methyl
sulfate 2 and trihexyl (tetradecyl) phosphonium tetrafluorobo-
rate 3 (Scheme 1), for their performance as transfer catalyst in
the Halex reaction. The fluorination of 1,2-dichloro-4-nitrobenzene
to 1-chloro, 2-fluoro-4-nitrobenzene was used as a test reaction
(Scheme 2).
2. Experimental
Spray-dried KF (Sigma–Aldrich) was dried under vacuum
for 6 h before use. Dimethylsulfoxide (DMSO) was dried using
molecular sieve 4A. All other reagents (4-chloro-nitrobenzene,
∗
Corresponding author. Tel.: +65 6516 2918, fax: +65 67791691.
E-mail address: chmsj@nus.edu.sg (S. Jaenicke).
1,3-dichloro-4-nitrobenzene,
1,2-dichloro-4-nitrobenzene,
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.02.063

A. Fan et al. / Catalysis Today 198 (2012) 300–304
301
Scheme 1. Phosphonium ionic liquids.
Table 1
Conductivity of phosphonium ionic liquids after heating followed by cooling.
Temp. (◦C)
Conductivity (mS cm−1
)
IL-1
IL-2
IL-3
Heating
Cooling
Heating
Cooling
Heating
Cooling
Scheme 2. Halex reaction of 1,2-dichloro-4-nitrobenzene.
70
100
120
140
160
180
1.58
3.52
6.45
9.45
12.76
17.30
1.56
3.54
6.44
9.45
12.76
17.31
0.68
1.33
1.96
2.68
3.53
4.04
0.67
1.32
1.95
2.65
3.56
4.04
0.16
0.29
0.43
0.56
0.77
1.01
0.16
0.29
0.42
0.57
0.76
1.01
2, 6-dichloro-benzaldehyde, tetramethyl ammonium chloride,
triisobutyl (methyl) phosphonium tosylate (CYPHOS IL106),
tributyl (methyl) phosphonium methyl sulfate (CYPHOS IL 108),
trihexyl (tetradecyl) phosphonium tetrafluoroborate (CYPHOS IL
111), sodium hydroxide pellets, ethanol) were used as received.
The ionic liquids were gifts from Cytec Industries Inc.
Note: As IL-3 is a solid at room temperature, all conductivity measurements were
started at 70 ◦C.
3. Results and discussion
The thermal stability of the ionic liquids was assessed by con-
ductivity measurements and thermogravimetry. A Jenway 4510
conductivity meter with a Pt/glass cell was used to measure the
conductivity of the bulk ionic liquids during heating and cooling
cycles in the range of 70–180 ◦C under nitrogen. The weight loss
during heating from 30 to 650 ◦C at 10 ◦C min−1 in nitrogen was
determined using a Mettler TA instrument 2050.
In a typical Halex reaction, dry dimethylsulfoxide (10 ml) was
preheated in a screw-capped Teflon reactor at 180 ◦C before adding
the phase transfer catalyst (0.86 mmol), KF (0.5 g) and 1,2-dichloro-
4-nitrobenzene (6 mmol). The reaction was carried out under
nitrogen with stirring. Samples were withdrawn periodically dur-
ing the reaction, filtered to remove the solid salts and analyzed by
gas chromatography (HP-5). Compounds were identified by com-
parison with authentic samples and by gas chromatography–mass
spectrometry (Shimadzu QP 5000).
The reuse of the ionic liquid was tested. After the reaction, the
solid KF was removed by suction filtration. Hexane (30 ml) was
added to the DMSO-containing filtrate, forming two phases. The
product was extracted into the hexane phase and removed. Another
portion of hexane was added to fully extract the products from
the DMSO phase. Fresh substrate and KF was then added to the
DMSO-phosphonium ionic liquid to start a new cycle of reaction.
The use of ionic liquids as reaction medium was also investi-
gated. In this case, 10 ml of the ionic liquid was used instead of
DMSO. For analysis, water was added to dissolve the ionic liquid
(the ionic liquid and water are miscible) and the aqueous phase
was extracted several times with hexane. The organic phase was
dried, filtered and analyzed by gas chromatography.
3.1. Thermal stability of phosphonium ionic liquids
The conductivity of the ionic liquids at 70 ◦C decreased in the
order: tributyl (methyl) phosphonium methyl sulfate 1 > triisobutyl
(methyl) phosphonium tosylate 2 > trihexyl (tetradecyl) phospho-
nium tetrafluoroborate 3 (Table 1). This is in accordance with the
increasing size of the cations and/or anions. Upon heating to 180 ◦C,
the conductivity increased. Tributyl (methyl) phosphonium methyl
sulfate 1 showed the biggest increase in conductivity. Upon cool-
ing, the conductivity returned to the initial value, suggesting that
the ionic liquids were stable to short-term temperature increase
up to 180 ◦C. However, extended heating at 180 ◦C for 5 h led to a
pronounced decrease in conductivity for IL-1 and IL-2 (Table 2). In
contrast, the conductivity of IL-3 decreased only slightly from 0.16
to 0.14 mS cm−1
.
The loss in conductivity correlates with thermogravimetric
results (Table 3). All three ionic liquids lost weight when they
were held at a constant temperature of 180 ◦C for 4 h. The small-
est weight loss, 0.97%, was observed for IL-3 while IL-1 suffered
a decrease of 6.09% from its starting weight. This weight loss is
Table 2
Conductivity of phosphonium ILs after heating at 180 ◦C for 5 h.
Ionic liquid
Conductivity (mS cm−1
Before heating
)
% change
After heating
IL 1
IL 2
IL 3
1.58
0.68
0.16
0.79
0.38
0.14
50.0
44.1
12.5

302
A. Fan et al. / Catalysis Today 198 (2012) 300–304
Table 3
Weight loss of ionic liquids during thermogravimetric analyses.
Dynamic TGA – T (onset
decomposition) (◦C)
Static TGA – weight loss at
180 ◦C after 4 h (%)
TMAC
IL 1
IL 2
270
282
296
329
−1.62
−6.09
−1.35
−0.97
IL 3
Fig. 3. Composition versus time for fluorination of 1,3-dichloro-4-nitrobenzene (ꢀ)
to. (ꢀ) 1-Chloro-3-fluoro-4-nitrobenzene (ꢁ) 1-fluoro-3-chloro-4-nitrobenzene and
(ꢀ) 1,3-difluoro-4-nitrobenzene. Reaction conditions: 1,3 dichloro-4-nitrobenzene
(6 mmol), KF (0.5 g, 8.6 mmol), IL 3 (0.86 mmol), DMSO (10 ml), 180 ◦C, N2 protection.
catalyst: the initial rate doubled, and close to 100% conversion was
obtained within 30 min.
The fluorination of 1,3-dichloro-4-nitrobenzene catalyzed by
IL-3 was also investigated. With this substrate, halogen exchange
can occur at the ortho and para chloro atoms. In the initial
phase of the reaction, the mono-substituted products, 1-chloro-
3-fluoro-4-nitrobenzene and 1-fluoro-3-chloro-4-nitrobenzene,
were preferentially formed (Fig. 3). Fluorination at the ortho-
position was faster than at the exchange with the para-chlorine due
to an enhanced inductive effect brought about by the proximity to
the nitro group [5]. After 50 min, the conversion reached 97% but
substantial amounts of the monofluorinated nitrobenzenes (40%)
were still present. Further fluorination occurred with longer reac-
tion times so that after 90 min, only 1,3-difluoro-4-nitrobenzene
was present.
Fig. 1. TGA profiles of ionic liquids on heating in air.
higher than that that observed with the non-phosphonium ionic
liquid, tetramethyl ammonium chloride (TMAC). Under dynamic
TGA where the temperature is continuously increased, the onset
of complete decomposition occurred at 270 ◦C for TMAC while
the phosphonium ionic liquids had higher onset temperatures of
296–329 ◦C (Fig. 1). IL-3 was the most thermally stable with an
onset temperature 59 ◦C higher than TMAC.
The catalytic activity of IL-3 was next tested for the Halex
reaction of monochloro-nitrobenzenes (Table 4). Compared to
dichloro-nitrobenzenes which are activated due to inductive effects
of the halogens [5], monosubstituted nitrobenzenes are less reac-
tive. For the fluorination of 1-chloro-4-nitrobenzene, 97% yield of
1-fluoro-4-nitrobenzene was obtained after 9 h. The fluorination of
1-chloro-2-nitrobenzene was even slower and the yield to 1-fluoro-
2-nitrobenzene was only 55% after 9 h. The presence of a nitro group
ortho to the chloro atom in 1-chloro-2-nitrobenzene poses steric
hindrance to fluorination. The resonance effect of the nitro group is
reduced as it cannot assume a position coplanar with the ring due
to the bulky chlorine atom at the ortho position [5]. Additionally,
it has been proposed that the incoming F− ion encounters an elec-
trostatic repulsion from the oxygen of an adjacent nitro group [20].
This was substantiated by studies showing an increase in substitu-
tion at the para- rather the ortho-position as the solvent polarity
increased.
3.2. Phosphonium ionic liquids in Halex reactions
The fluorination of 1,2-dichloro-4-nitrobenzene was carried out
using DMSO as the solvent. After 60 min, the conversion reached
74% giving 1-fluoro, 2-chloro-4-nitrobenzene as the only product
(Fig. 2). The preferential fluorination at the para-position in this
compound is attributed to inductive and mesomeric effects [2].
Addition of TMAC as catalyst increased the rate of reaction so that
after 90 min, 95% conversion was reached. A further increase in ini-
tial rate of reaction was observed with the use of phosphonium
ionic liquids 1 and 2, both of which showed very similar activ-
ity. The best result was obtained with IL-3 as the phase transfer
3.3. Effect of solvents
The effect of solvents on the Halex reaction was investigated
using IL-3. The rate of reaction was highest when polar aprotic
Table 4
Halex reaction of different aromatic substrates.
Substrates
Time (h)
Yield (%)
1,3-Dichloro-4-nitrobenzene
1-Chloro-2-nitrobenzene
1-Chloro-4-nitrobenzene
1.8
9
9
100
55
97
Fig. 2. Ionic liquids as phase transfer catalyst for the Halex reaction of KF with
1,2-dichloro-4-nitrobenzene. (ꢀ) uncatalyzed (×) TMAC (ꢀ) IL 1 (ꢁ) IL 2 (ꢀ) IL 3.
Reaction conditions: 1,2 dichloro-4-nitrobenzene (6 mmol), KF (0.5 g, 8.6 mmol), IL
(0.86 mmol), DMSO (10 ml), 180 ◦C, N2.
Reaction conditions: substrate (6 mmol), KF (0.5 g, 8.6 mmol), IL 3 (0.86 mmol),
DMSO (10 ml), 180 ◦C, N2.

A. Fan et al. / Catalysis Today 198 (2012) 300–304
303
Table 5
Dielectric constant and boiling point of different solvents [21].
Solvent
Boiling point (◦C)
Dielectric constant
Dipole moment (Debye)
Initial rate (mmol min−1
)
DMSO
Sulpholane
Nitrobenzene
Benzonitrile
1,2-Dichlorobenzene
189
287
211
191
181
47.2
43.3
34.8
26.0
9.8
3.96
4.80
3.99
4.18
2.14
0.536
0.271
0.003
0.005
0.006
Fig. 5. Initial reaction rate for fluorination of 1,2-dichloro-4-nitrobenzene vs mol%
phosphonium IL-3. Reaction conditions: 1,2 dichloro-4-nitrobenzene (6 mmol), KF
(0.5 g, 8.6 mmol), DMSO (10 ml), 180 ◦C, N2.
Fig. 4. Effects of different solvents on the Halex reaction of 1,2-dichloro-4-
nitrobenzene. Reaction conditions: 1,2 dichloro-4-nitrobenzene (6 mmol), KF (0.5 g,
8.6 mmol), IL 3 (0.86 mmol), solvent (10 ml),180 ◦C, N2.
solvents were used. In DMSO and using 10% of phosphonium IL-3,
the conversion reached 98% after only 30 min (Fig. 4). In sulpholane,
the initial rate was about half that in DMSO and after 3.5 h, the
conversion was 89%. The initial rate of reaction was up to a hun-
dredfold slower in solvents such as nitrobenzene, benzonitrile and
1,2-dichlorobenzene. Despite long reaction times of more than 20 h,
the conversion did not exceed 60%. The rate of reaction roughly
correlates with the dielectric constant of the solvent [21] (Table 5).
A solvent with high dielectric constant is able to shield the F− ion
from the K+ cation more effectively than one with a lower dielectric
constant, thus minimizing the ionic interaction. Theoretical calcu-
lations show that ion pairing and solvation are factors that limit the
nucleophilicity of the fluoride ion. In order to have mild reaction
conditions, highly soluble, weakly ion-paired fluoride salts must
be used in relatively nonpolar media [22].
the surface of KF at high concentration of the ionic liquid can also
contribute to the low activity. For the reaction to occur, the ionic
liquid must first react with solid KF to form the quaternary phos-
phonium fluoride and potassium borofluoride (Scheme 3). Transfer
of the F− ions into the organic liquid phase enables the substrate
(RCl) to be fluorinated. As shown by Arrad and Sasson [23], traces
of water are essential for the Halex reaction because the exchange
of F− ions from the solid KF to the ionic liquid can only take place
via free dissolved anions. If water was stringently excluded, no F−
exchange occurs. A higher concentration of the ionic liquid leads to
the formation of more quaternary phosphonium fluoride ion pairs,
Q+F−. Due to the longer distance between the ions, Q+F− is a bet-
ter nucleophile and hydrogen acceptor than KF, as the F− anions
are more “naked”. However, water will preferentially associate to
the Q+F− rather than to the surface of KF, causing the exchange
reaction between the ionic liquid and KF to stop. Hence, the rate of
reaction goes through a maximum with increasing concentration
of the ionic liquid.
The structure of the quaternary phosphonium ion with three
medium length ligands and one long hydrophobic tail may be
partly responsible for the good performance of IL-3 compared to the
other two phosphonium ionic liquids, which contained one short
and three medium ligands. The size of the ligands determines the
distance of approach between the F− and the phosphorous, and
thereby the cohesive strength of the ion pair. This has to be suffi-
cient to allow for the extraction of F− from the crystal lattice of KF,
but weak enough to yield a highly nucleophilic fluoride species for
the halogen exchange reaction.
3.4. Optimization of ionic liquid/KF ratio
It was noted that the addition of small amounts of the phos-
phonium IL-3 was sufficient to cause a noticeable change in the
reaction rate. Indeed, the initial reaction rate increased by about
sixfold compared to the uncatalyzed reaction even at 1 mol% IL-3 to
KF (Fig. 5). The rate increased with IL-3 concentration and reached
a maximum at 20 mol% IL-3. The lower rate of reaction at 50 mol%
IL-3 can be attributed to diffusion limitation as the reaction mixture
was very viscous. Indeed, using the phosphonium ionic liquids as
solvents led to very low rates of reactions (Table 6). The conversion
after 400–450 min was only 4–21%. The removal of free water from
Table 6
Phosphonium ionic liquids as solvent for the Halex reaction of 1,2-dichloro-4-
nitrobenzene.
3.5. Effect of temperature
In the presence of IL-3, the reaction can be carried out to
>99% conversion even at temperatures as low as 140 ◦C (Fig. 6).
Although a longer reaction time was required, 300 min as compared
to 120 min at 160 ◦C and 30 min at 180 ◦C, the lower temperature
limits the decomposition of the phase transfer catalyst and allows a
prolonged lifetime. The activation energy calculated from an Arrhe-
nius plot was 83.2 kJ mol−1. The relatively high value shows that the
Solvent
Time (min)
Conversion (%)
Selectivity (%)
DMSO
IL 1
IL 2
120
450
400
420
82
4
12
21
100
100
100
100
IL 3
Reaction conditions: 1,2 dichloro-4-nitrobenzene (6 mmol), KF (0.5 g, 8.6 mmol),
solvent (10 ml), 180 ◦C, N2.

304
A. Fan et al. / Catalysis Today 198 (2012) 300–304
Scheme 3. Ionic exchange reaction between IL-3 and KF and phase transfer catalysis.
4. Conclusions
The phosphonium ionic liquids are powerful phase transfer cat-
alysts for the Halex reaction. A moderate loading of 20 mol% led to
a maximum in the initial reaction rate. While triisobutyl (methyl)
phosphonium tosylate and tributyl (methyl) phosphonium methyl
sulfate were stable to short temperature excursions to 180 ◦C, their
conductivity decreased significantly after prolonged heating for 5 h.
In comparison, trihexyl (tetradecyl) phosphonium tetrafluorobo-
rate showed the best thermal stability and activity for the Halex
fluorination of 1,2-dichloro-4-nitrobenzene and chloronitroben-
zenes. The combination of three flexible medium-length ligands
and one long hydrophobic tail on the phosphorous seems to be ben-
eficial for the complexing ability of the phase transfer catalyst. The
rate of reaction was highest with dimethylsulfoxide as the solvent.
Due to its good stability at high temperatures, trihexyl (tetradecyl)
phosphonium tetrafluoroborate can be reused without significant
loss in activity.
Fig. 6. Effect of temperature on the Halex reaction of 1,2-dichloro-4-nitrobenzene.
Reaction conditions: 1,2 dichloro-4-nitrobenzene (6 mmol), KF (0.5 g, 8.6 mmol), IL
3 (0.86 mmol), DMSO (10 ml), 180 ◦C, N2.
Acknowledgement
Financial support from National University of Singapore under
Grants Nos. 143-000-374-112 and 143-000-418-112 is gratefully
acknowledged.
References
[1] S.G. Dimagno, H. Sun, Current Topics in Medicinal Chemistry
1473–1482.
6 (2006)
[2] J.H. Clark, D. Wails, T.W. Bastock, Aromatic Fluorination, CRC Press, Boca Raton,
1996.
[3] D.J. Adams, J.H. Clark, Chemical Society Reviews 28 (1999) 225–231.
[4] A. Pleschke, A. Marhold, M. Schneider, A. Kolomeitsev, G.-V. Roschenthaler,
Journal of Fluorine Chemistry 125 (2004) 1031–1038.
[5] B. Langlois, L. Gilbert, G. Forat, Industrial Chemistry Library 8 (1996) 244–292.
[6] Y. Sasson, S. Negussie, M. Royz, N. Mushkin, Chemical Communications (1996)
297–298.
[7] J. Knifton, Journal of Molecular Catalysis 43 (1987) 65–77.
[8] D. Kaufmann, M. Nouroozian, H. Henze, Synlett (1996) 1091–1092.
[9] D. Gerritsma, A. Robertson, J. McNulty, A. Capretta, Tetrahedron Letters 45
(2004) 7629–7631.
Fig. 7. Reusability of IL-3 for Halex reaction of 1,2-dichloro-4-nitrobenzene. (ꢀ)
Fresh (ꢀ) reused once (ꢁ) reused twice and (ꢀ) uncatalyzed.
[10] J. McNulty, A. Capretta, J. Wilson, J. Dyck, G. Adjabeng, A. Robertson, Chemical
Communications (2002) 1986–1987.
[11] J. McNulty, S. Cheekoori, J. Nair, V. Larichev, A. Capretta, A. Robertson, Tetrahe-
dron Letters 46 (2005) 3641–3664.
rate-determining step in the reaction involves a chemical reaction
rather than a mass transfer limited step.
[12] R.A. Jones, Quaternary Ammonium Salts: Their Use in Phase-Transfer Catalysis,
Academic Press, London, 2001.
[13] J.H. Clark, D.J. Macquarrie, Tetrahedron Letters 28 (1987) 111–114.
[14] Y. Yoshida, Y. Kimura, Chemistry Letters (1988) 1355–1358.
[15] Y. Yoshida, Y. Kimura, Journal of Fluorine Chemistry 44 (1989) 291–298.
[16] M.-A. Lacour, M. Zablocka, C. Duhayon, J.-P. Majoral, M. Taillefer, Advanced
Synthesis & Catalysis 350 (2008) 2677–2682.
[17] C.B. Murray, G. Sandford, S.R. Korn, Journal Fluorine Chemistry 123 (2003)
81–84.
[18] Y.F. Hu, J. Luo, C.X. Lü, Chinese Chemical Letters 21 (2010) 151–154.
[19] J. Luo, C. Lü, C. Cai, W. Qü, Journal of Fluorine Chemistry 125 (2004) 701–704.
[20] J.F. Bunnett, R.J. Morath, Journal of the American Chemical Society 77 (1955)
5051–5055.
3.6. Reusability of trihexyl (tetradecyl) phosphonium
tetrafluoroborate
After the reaction, the products were extracted from the ionic
liquid catalyst–DMSO mixture with hexane, and the mixture
was used for another batch Halex reaction of 1,2-dichloro-
4-nitrobenzene at 180 ◦C. The subsequent cycles indicated a
somewhat lower activity but the conversion was within 5% of the
fresh catalyst after 30 min (Fig. 7). Moreover, the reaction rate was
still much higher than the uncatalyzed reaction, indicating the good
stability of IL-3 when subjected to high temperature (180 ◦C) for
short time periods (30 min).
[21] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 2nd ed., VCH,
Weinheim, 1988.
[22] H. Sun, S.G. DiMagno, Angewandte Chemie International Edition 45 (2006)
2720–2725.
[23] Arrad, Y. Sasson, Chemical Communications (1988) 148–149.