Synergism in catalytic halogen-exchange fluorination of 4-chloronitro- and dichlorotetrafluorobenzenes

Article abstract of DOI:10.1023/A:1023447908449

Synergism was studied in catalytic halogen-exchange fluorination of 4-chloronitro- and dichlorotetrafluorobenzenes. The synergistic effect in the fluorination of chloroaromatic compounds with potassium fluoride was mainly attributed to separation of the activation functions of catalysts with respect to substrate molecules.

Full text of DOI:10.1023/A:1023447908449

Russian Chemical Bulletin, International Edition, Vol. 52, No. 2, pp. 487—491, February, 2003
487
Synergism in catalytic halogenꢀexchange fluorination of
4ꢀchloronitroꢀ and dichlorotetrafluorobenzenes
ꢀ
A. I. Shipilov, A. B. Bykova, L. I. Elokhova, and S. M. Igumnov
Perm Branch of the Federal State Unitary Enterprise
Russian Scientific Center “Applied Chemistry”,
41 ul. Voronezhskaya, 614034 Perm, Russian Federation.
Fax: +7 (342 2) 53 0150. Eꢀmail: shipilov@perm.ru
Synergism was studied in catalytic halogenꢀexchange fluorination of 4ꢀchloronitroꢀ and
dichlorotetrafluorobenzenes. The synergistic effect in the fluorination of chloroaromatic comꢀ
pounds with potassium fluoride was mainly attributed to separation of the activation functions
of catalysts with respect to substrate molecules.
Key words: fluorodechlorination, halogenꢀexchange fluorination, potassium fluoride,
dichlorotetrafluorobenzene, chloronitrobenzene, phaseꢀtransfer catalysis, synergism.
Scheme 1
Industrially, fluoroaromatic compounds are mainly
produced by fluorodechlorination of chloroaromatic comꢀ
pounds with alkali metal (especially potassium) fluorides.1
Successful fluorodechlorination of chloroaromatic comꢀ
pounds with potassium fluoride (noncatalytic process)
depends on, at least, two key factors: (1) the state of the
crystal structure of potassium fluoride (primarily, its texꢀ
ture), which is obviously associated with the concentraꢀ
tion of "active" potassium fluoride, and (2) the inherent
reactivity of a substrate.2—4 Recently, this process has
been significantly intensified by using phaseꢀtransfer cataꢀ
lysts.5—7 In this case, the reaction conditions are milder,
and no solvent is required. A great number of structurꢀ
ally different compounds belonging to various classes
were studied as catalysts of fluorodechlorination.8—10
Guanidinium derivatives, quaternary phosphonium salts,
ammonium salts, cryptates, crown ethers, and polyethers
are usually most efficient catalysts for these processes.9,10
The effect of phaseꢀtransfer catalysts on fluoroꢀ
dechlorination of aromatic compounds was mainly atꢀ
tributed to the enhanced reactivity of alkali metal fluoride
due to either the formation of loose ion pairs5 or more
efficient coordination of a substrate at the potassium fluoꢀ
ride surface.4
According to our investigations,10 fluorodechlorination
of trichlorotrifluorobenzenes with potassium fluoride in
the absence of solvents can be catalyzed not only through
an increase in the concentration of active potassium fluoꢀ
ride, but also by involving a catalyst in stabilization of an
intermediate σꢀcomplex (Scheme 1); in the latter case,
the catalytic effect is usually significantly greater.
In a particular situation, either of these effects can
dominate, depending on the catalyst nature. Fluoroꢀ
dechlorination of haloarenes with potassium fluoride in
the presence of onium catalysts such as hexaethylꢀ
guanidinium chloride (1) or tetrakis(diethylamino)phosꢀ
phonium bromide (Et2N)4PBr (2) accelerates mainly beꢀ
cause of better stabilization of an intermediate σꢀcomplex
(Meisenheimer complex), while acceleration of this reacꢀ
tion with polyethers (18ꢀcrownꢀ6 (3) or tetraethylene glyꢀ
col dimethyl ether (tetraglyme) (4)) predominantly results
from an increase in the running concentration of active
potassium fluoride.10 We believe that this increase is not
directly attributable to better solubility of active potassium
fluoride in the organic phase; most probably, it is due to
fatal destruction of the crystal structure of the fluoride
surface to give active centers (primarily, as loose ion pairs).
Classification of catalysts according to the activation
mechanism in arene fluorodechlorination is fairly conꢀ
ventional and, first of all, indicates domination of one or
another mechanism through which a particular catalyst is
involved in halogen exchange in an aromatic molecule.
Taking into account that catalysts can accelerate
fluorodechlorination of chloroaromatic substrates at difꢀ
ferent steps of the process—on the one hand, by increasꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 464—468, February, 2003.
1066ꢀ5285/03/5202ꢀ487 $25.00 © 2003 Plenum Publishing Corporation

488
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
Shipilov et al.
ing the concentration of active potassium fluoride, on the
other hand, by lowering the activation energy of the reacꢀ
tion through stabilization of an intermediate σꢀcomꢀ
plex—synergism (i.e., nonadditive enhancement of the
catalytic activity) should be expected for a combination
of different catalysts (differing in nature and properties).
Synergistic effects in catalysis are known to arise not
only from the formation of new active centers (intermediꢀ
ates), but also from separation of the activation functions
of catalysts with respect to substrate molecules in time
and space, i.e., for kinetic (dynamic) reasons. Because of
this, synergism is a result of the functional organization of
the whole catalytic system.11
polyether were 5 mol %); for its combination with
18ꢀcrownꢀ6 3, the effect was smaller; the poorest results
were obtained for a system containing diglyme 5 (see
Table 1, entries 7, 5, 4). When the mole fractions of hexaꢀ
ethylguanidinium chloride and polyethers are 2.5 mol %,
the efficiency order is somewhat different: 4 ≈ 5 > 3 (see
Table 1, entries 12, 14, 9). A variation in the efficiency of
the polyethers with the concentration of catalyst 1 can be
due either to their competitive ionꢀdipole interactions
with hexaethylguanidinium or potassium cations or to
diffusion (instead of kinetic) control of the process at
increased concentrations of catalyst 1. Apparently, the
same reasons are responsible for insignificant changes in
the conversion of dichlorotetrafluorobenzene when the
mole fractions of catalysts 1 and 5 are increased from 2.5
to 5 mol % (see Table 1, entries 14, 4).
Results and Discussion
The reactions of a mixture of isomeric dichlorotetraꢀ
fluorobenzenes and 4ꢀchloronitrobenzene with KF were
studied in the presence of catalytic amounts of hexaꢀ
ethylguanidinium chloride, (Et2N)4PBr (which efficiently
stabilize a σꢀcomplex10), 18ꢀcrownꢀ6, tetraethylene glyꢀ
col and diethylene glycol dimethyl ethers (which effecꢀ
tively activate potassium fluoride10,12), and their mixtures
(see Tables 1, 2).
Close results were obtained in the study of fluoroꢀ
dechlorination of 4ꢀchloronitrobenzene (Table 2). The
conversion of 4ꢀchloronitrobenzene in the presence of
tetraglyme 4 alone (0.025 mol) did not exceed 2.5% over
5 h. Synergism arose in a catalytic system of (Et2N)4PBr 2
and tetraglyme 4 (see Table 2). With an increase in the
tetraglyme concentration, the synergistic effect monoꢀ
tonically grew throughout the range studied to a maxiꢀ
mum value at a (Et2N)4PBr : tetraglyme ratio of 1 : 8.
In fluorination of 4ꢀchloronitrobenzene with potasꢀ
sium fluoride in the presence of equal concentrations of
(Et2N)4PBr 2 and tetraglyme 4, the synergistic effect is
slightly weaker than that obtained in the fluorination of
C6F4Cl2 in the presence of hexaethylguanidinium chloꢀ
ride 1 and tetraglyme 4 (see Table 1, entry 12; Table 2,
entry 3). This can be due to different reactivities of the
substrates13 and different efficiencies of onium catalysts
(i.e., the ability of stabilize a σꢀcomplex)10 (Scheme 2).
Fluorodechlorination of dichlorotetrafluorobenzene
with KF in the presence of catalytic amounts of diglyme 5,
tetraglyme 4, or crown ether 3 under the chosen condiꢀ
tions was found to be very slow (after 6 h, only trace
amounts of chloropentafluorobenzene were detected). The
fluorination efficiency in the presence of hexaethylꢀ
guanidinium chloride 1 over 6 h was ∼10 or ∼20%, deꢀ
pending on the mole fraction of the catalyst (2.5 or 5 mol %
of the substrate, respectively) (Table 1). As expected, the
catalytic system became substantially more active when
hexaethylguanidinium chloride is combined with polyꢀ
ethers. The synergistic effect was greater for the lower
mole fraction of catalyst 1 (2.5 mol %). In this case (for
an equimolar ratio of hexaethylguanidinium chloride 1 to
a polyether), the conversion of dichlorotetrafluorobenzene
was virtually doubled (see Table 1, entries 1, 9, 12, 14).
When the content of catalyst 1 was 5 mol %, the maxiꢀ
mum synergistic effect results in the increase in the yield
of fluorination products at best by ∼7%; i.e., the converꢀ
sion gain was only ∼35% (see Table 1, entries 2, 7). The
reduced synergistic effect at the higher concentration of
hexaethylguanidinium chloride 1 is probably due to the
fact that this catalyst not only stabilizes a σꢀcomplex but
also efficiently increases the concentration of active poꢀ
tassium fluoride, thus reducing the demand in a polyether
for activation of potassium fluoride. It was noted that the
polyethers studied contribute differently to the acceleraꢀ
tion of fluorodechlorination of dichlorotetrafluorobenꢀ
zene. A combination of hexaethylguanidinium chloride 1
with tetraglyme 4 was most efficient, all other factors
being equal (the mole fractions of catalyst 1 and a
Scheme 2
R2 = H, n = 4, R1 = 4ꢀNO2;
R2 = F, n = 4, R1 = Cl
Similar synergistic effects in exchange fluorodechloriꢀ
nation in the presence of two different catalysts were noted
while studying reactions of other substrates and catalysts.

Synergism in catalytic "halex" fluorination
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
489
Table 1. Effects of the nature of the catalyst and the reaction time on the percentage of the products in the fluorodechlorination of
dichlorotetrafluorobenzene (219 g, 1 mol) with potassium fluoride (58 g, 1 mol)
Comꢀ
pound
Reaction time/h
Comꢀ
pound
Reaction time/h
1
2
3
4
5
6
1
2
3
4
5
6
Entry 1. Catalyst 1 (6.58 g (0.025 mol))
Entry 8. Catalysts 1 (6.58 g (0.025 mol))
+ 3 (3.3 g (0.0125 mol))
C6F6
C6F5Cl
0.06
0.55
0.07
2.47
0.07
4.54
0.08
6.30
0.09
7.57
0.09
8.67
90.11
1.13
C6F6
C6F5Cl
0.07
1.60
0.07
3.84
0.08
6.16
0.09
8.35
0.10
9.76
0.11
11.08
87.82
0.99
C6F4Cl2 98.96
96.70
0.75
94.52
0.87
92.75
0.87
91.3
1.05
C6F3Cl3
0.43
C6F4Cl2 97.83
95.14
0.97
92.84
0.92
90.59
0.97
89.13
1.01
C6F3Cl3
0.51
Entry 2. Catalyst 1 (13.16 g (0.05 mol))
Entry 9. Catalysts 1 (6.58 g (0.025 mol))
+ 3 (6.6 g (0.025 mol))
C6F6
C6F5Cl
C6F4Cl2 93.69
C6F3Cl3
0.07
5.41
0.1
0.11
13.0
85.62
1.27
0.14
15.41
83.18
1.27
0.16
17.15
81.39
1.3
0.18
18.48
79.91
1.43
10.62
88.12
1.16
C6F6
C6F5Cl
0.05
2.25
0.05
4.66
0.06
8.47
0.09
10.62
88.46
0.83
0.08
12.61
86.47
0.84
0.09
13.90
85.11
0.90
0.83
C6F4Cl2 97.07
94.53
0.76
90.59
0.88
Entry 3. Catalysts 1 (13.16 g (0.05 mol)
+ 5 (1.34 g (0.01 mol))
C6F3Cl3
0.63
Entry 10. Catalysts 1 (6.58 g (0.025 mol))
+ 3 (13.2 g (0.05 mol))
C6F6
C6F5Cl
C6F4Cl2 91.50
C6F3Cl3
0.08
7.53
0.09
11.74
87.10
1.07
0.11
14.07
84.55
1.27
0.13
16.24
82.34
1.29
0.15
17.46
80.90
1.49
0.18
19.25
79.05
1.52
C6F6
C6F5Cl
0.06
7.10
0.07
11.08
87.73
1.12
0.09
14.21
84.60
1.10
0.10
16.46
82.37
1.07
0.13
18.16
80.66
1.06
0.16
19.92
78.63
1.29
0.89
C6F4Cl2 92.40
Entry 4. Catalysts 1 (13.16 g (0.05 mol))
+ 5 (6.7 g (0.05 mol))
C6F3Cl3
0.44
Entry 11. Catalysts 1 (6.58 g (0.025 mol))
+ 4 (2.78 g (0.0125 mol))
C6F6
C6F5Cl
C6F4Cl2 92.50
C6F3Cl3
0.07
6.66
0.08
10.82
88.01
1.09
0.13
14.03
84.56
1.29
0.14
15.34
83.28
1.24
0.16
17.07
81.45
1.32
0.20
18.76
79.65
1.39
C6F6
C6F5Cl
0.06
7.10
0.07
11.08
88.29
0.56
0.09
14.21
85.05
0.65
0.11
16.47
82.71
0.71
0.14
18.17
80.89
0.82
0.16
19.90
79.28
0.66
0.76
C6F4Cl2 92.35
Entry 5. Catalysts 1 (13.16 g (0.05 mol))
+ 3 (13.2 g (0.05 mol))
C6F3Cl3
0.49
Entry 12. Catalysts 1 (6.58 g (0.025 mol)
+ 4 (5.56 g (0.025 mol))
C6F6
C6F5Cl
C6F4Cl2 93.06
C6F3Cl3
0.06
5.94
0.10
11.74
86.89
1.27
0.11
14.85
83.69
1.35
0.14
17.58
80.99
1.29
0.18
18.87
79.65
1.30
0.19
20.88
77.63
1.30
C6F6
C6F5Cl
0.06
6.96
0.08
11.78
87.48
0.60
0.11
15.43
83.78
0.69
0.14
18.05
81.27
0.55
0.15
19.80
79.41
0.64
0.17
21.23
77.89
0.71
0.94
C6F4Cl2 92.43
Entry 6. Catalysts 1 (13.16 g (0.05 mol))
+ 4 (5.56 g (0.025 mol))
C6F3Cl3
0.55
Entry 13. Catalysts 1 (6.58 g (0.025 mol))
+ 4 (11.12 g (0.05 mol))
C6F6
C6F5Cl
C6F4Cl2 91.35
C6F3Cl3
0.05
7.41
0.10
12.96
85.53
1.42
0.10
15.52
82.76
1.65
0.13
18.06
80.19
1.62
0.15
19.72
78.49
1.64
0.17
20.99
77.11
1.73
C6F6
C6F5Cl
0.05
6.15
0.09
10.12
88.91
0.89
0.12
13.46
85.46
0.96
0.12
15.64
83.17
1.07
0.12
17.28
81.64
0.96
0.14
18.98
79.92
0.96
1.19
C6F4Cl2 93.08
Entry 7. Catalysts 1 (13.16 g (0.05 mol))
+ 4 (11.12 g (0.05 mol))
C6F3Cl3
0.71
Entry 14. Catalysts 1 (6.58 g (0.025 mol))
+ 5 (3.32 g (0.025 mol))
C6F6
C6F5Cl
C6F4Cl2 89.33
C6F3Cl3 0.98
0.08
9.6
0.17
16.01
83.11
0.71
0.19
19.44
79.66
0.71
0.23
20.00
78.72
1.05
0.30
24.53
73.83
1.34
0.33
25.82
73.16
0.69
C6F6
C6F5Cl
0.06
8.34
0.07
12.01
87.32
0.60
0.12
17.64
81.84
0.40
0.15
19.40
80.25
0.20
0.14
19.80
79.54
0.52
0.16
21.55
77.82
0.47
C6F4Cl2 91.60
C6F3Cl3
—
Note. Chlorodefluorination of C6F4Cl2 yielded C6F3Cl3.10
For example, a combination of tetraphenylphosphonium
bromide or chloride with 18ꢀcrownꢀ6 or polyethylene glyꢀ
col dimethyl ethers is more active in fluorodechlorination
of chlorobenzaldehydes than the phenylphosphonium salt
alone.14,15
Apparently, here the synergism in catalytic
fluorodechlorination is also associated with separation
of the activation functions of catalysts with respect
to substrate molecules (the onium catalyst, namely,
tetraphenylphosphonium halide, activates an aroꢀ

490
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Shipilov et al.
Table 2. Effects of the nature of the catalyst and the reaction
time on the percentage of the products in the fluorodechlorinaꢀ
tion of 4ꢀchloronitrobenzene (157.56 g, 1 mol) with potassium
fluoride (58 g, 1 mol)
of chloroarenes with potassium fluoride. As shown earꢀ
lier,6,10,12 the use of catalysts in fluorodechlorination of
chloroarenes either increases the concentration of active
potassium fluoride or enhances the reactivity of a subꢀ
strate (by means of stabilization of an intermediate
σꢀcomplex), while a combination of catalysts with differꢀ
ent mechanisms of involvement in fluorodechlorination
results in the observed synergism (i.e., nonadditive inꢀ
crease in the activity of the whole catalytic system). An
alternative version, where the catalytic activity increases
because of the mutual influence of catalysts (e.g., as a
result of specific solvation of an onium salt in a polyether,
which can change the anion transfer activity), seems to be
unlikely. In such a case, synergism arising in the fluorinaꢀ
tion of hexachlorobenzene16 from a combination of an
onium catalyst and potassium fluoride supported on an
inert carrier (CaF2, BaF2) finds no explanation. Certainly,
fine details of the mechanism of synergism in catalytic
halogenꢀexchange fluorination in a liquid—solid system
remain unclear so far and further, including kinetic, inꢀ
vestigations are required.
Comꢀ
pound
Reaction time/h
0.5
1
2
3
4
5
Entry 1. Catalyst 2 (9.98 g (0.025 mol))
C6H4FNO2
C6H4ClNO2
3.66
6.53
9.55 10.26 10.40 10.22
96.34 93.46 90.46 89.74 89.56 89.78
Entry 2. Catalysts 2 (9.98 g (0.025 mol))
+ 4 (2.78 g (0.0125 mol))
C6H4FNO2
C6H4ClNO2
6.97 11.35 12.09 13.43 15.23 17.25
92.77 88.65 87.90 86.57 84.77 83.75
Entry 3. Catalysts 2 (9.98 g (0.025 mol))
+ 4 (5.56 g (0.025 mol))
C6H4FNO2
C6H4ClNO2
5.28
8.78 15.28 15.54 15.64 18.08
94.72 91.21 84.72 84.46 84.34 81.90
Entry 4. Catalysts 2 (9.98 g (0.025 mol))
+ 4 (11.12 g (0.05 mol))
C6H4FNO2
C6H4ClNO2
7.34 13.78 16.03 18.47 18.79 18.33
92.70 86.22 83.97 81.53 81.21 81.67
Experimental
Entry 5. Catalysts 2 (9.98 g (0.025 mol))
+ 4 (44.48 g (0.2 mol))
19
F NMR spectra were recorded on a Bruker WPꢀ80 instruꢀ
ment (75.398 MHz) in deuteroacetone with hexafluorobenzene
as the internal standard. The course of the reaction was moniꢀ
tored by chromatography using a Hewlett Packard 5890 chroꢀ
matograph (katharometer as a detector, capillary column
30 m × 0.25 mm, SEꢀ54 phase, isothermal regime (100 °C),
helium as a carrier gas (20 mL min–1). Mass spectra were reꢀ
corded on a Finnigan Mat ITDꢀ800 instrument (ionization
chamber temperature 200 °C, ionizing voltage 70 eV).
Materials. Potassium fluoride, diethylene glycol and tetraꢀ
ethylene glycol dimethyl ethers, 18ꢀcrownꢀ6, 4ꢀchloronitroꢀ
benzene (highꢀpurity grade), (Et2N)4PBr, and hexaethylꢀ
guanidinium chloride were conditioned according to the known
procedure.10 Dichlorotetrafluorobenzene was isolated by fracꢀ
tional distillation (fraction with b.p. 152—153 °C) from a prodꢀ
uct obtained in the catalytic fluorodechlorination of hexachloroꢀ
benzene. The dichlorotetrafluorobenzene obtained was a
mixture of isomeric 1,3ꢀdichloroꢀ2,4,5,6ꢀtetrafluorobenzene
(94.5%), 1,2ꢀdichloroꢀ3,4,5,6ꢀtetrafluorobenzene (5%), and
1,4ꢀdichloroꢀ2,3,5,6ꢀtetrafluorobenzene (0.5%).
Fluorodechlorination of chlorobenzenes. Given amounts of
C6Cl2F4 or 4ꢀClꢀC6H4NO2, KF, and a catalyst (or a mixture of
catalysts) were heated with stirring at 140 °C. The reaction
mixture was periodically sampled and analyzed by GLC for
conversion degree.
The 19F NMR spectra of all starting reagents and reaction
products were identical with the literature data.19 Their mass
spectra contained the corresponding molecular ions.
C6H4FNO2
C6H4ClNO2
9.13 14.42 20.23 25.91 27.79 29.70
90.87 85.58 79.77 74.09 72.21 70.30
matic molecule, while a polyether activates potassium
fluoride).
This agrees with data obtained in the study of the
catalytic activity of tetraphenylphosphonium bromide.6
The replacement rate for chlorine in chloronitropyridine
was determined in different aprotic solvents in the presꢀ
ence and absence of tetraphenylphosphonium bromide.
It was concluded from the resulting dependence6 that
Ph4PBr not only increases the concentration of fluoride
ions in the organic phase, but also helps the solvent to
stabilize an intermediate Meisenheimer complex. At the
same time, other researchers12 who have studied the cataꢀ
lytic activity of crown ethers in fluorodechlorination reꢀ
actions believe that replacement of a chlorine atom by
fluorine accelerates mainly because of higher nucleophiꢀ
licity of potassium fluoride.
A similar synergistic effect was also observed in fluoroꢀ
dechlorination of hexachlorobenzene with potassium fluoꢀ
ride supported on calcium (or barium) fluoride in the
presence of (Et2N)4PBr.16 In this case, the concentration
of active KF probably increases because of its support on
CaF2 or BaF2, while (Et2N)4PBr activates the aromatic
substrate.
References
The experimental results presented here, as well as the
published literature data,17,18 fit well the proposed mechaꢀ
nism for synergism in the catalytic fluorodechlorination
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Synergism in catalytic "halex" fluorination
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Received May 31, 2002;
in revised form October 18, 2002