One-step synthesis of 2,3-difluoronaphthalene by the gas-phase co-pyrolysis of styrene with chlorodifluoromethane

Article abstract of DOI:10.1007/s11172-019-2546-8

The gas-phase co-pyrolysis of styrene with CHClF₂ in a flow reactor at 550–650 °C gives 2,3-difluoronaphthalene in two parallel reaction channels. The main channel includes decomposition of CHClF₂ to difluorocarbene, whose subsequent

Full text of DOI:10.1007/s11172-019-2546-8

Russian Chemical Bulletin, International Edition, Vol. 68, No. 6, pp. 1232—1238, June, 2019
1232
Оne-step synthesis of 2,3-difluoronaphthalene by the gas-phase
co-pyrolysis of styrene with chlorodifluoromethane*
N. V. Volchkov, M. B. Lipkind, and O. M. Nefedov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. E-mail: volchkov@ioc.ac.ru
The gas-phase co-pyrolysis of styrene with CHClF2 in a flow reactor at 550—650 С gives
2,3-difluoronaphthalene in two parallel reaction channels. The main channel includes decom-
position of CHClF2 to difluorocarbene, whose subsequent cycloaddition to styrene affords
1,1-difluoro-2-phenylcyclopropane. This compound rearranges to 2-fluoroindene giving
finally difluoronaphthalene upon the second difluorocarbene cycloaddition and subsequent
aromatization of the resulting cycloadduct. The second reaction channel consists in aromatiza-
tion of 1,1,2,2-tetrafluoro-3-phenylcyclobutane, which is formed by the [2+2]-cycloaddition
of styrene to tetrafluoroethylene, a product of difluorocarbene dimerization.
Key words: 2,3-difluoronaphthalene, gas-phase pyrolysis, mechanism, difluorocarbene,
1,1-difluoro-2-phenylcyclopropane, 1,1,2,2-tetrafluoro-3-phenylcyclobutane, 2-fluoroindene.
In comparison with fluoro benzenes, which find use in
by pyrolysis of 1,1,2,2-tetrafluoro-3-phenylcyclobutane,
but its yield was not specified.
Several syntheses of functional derivatives of com-
pound 116—30 are mostly based on annulation of 1,2-di-
substituted 4,5-difluorobenzenes employing cycloaddition
or intramolecular cyclization reactions.
the synthesis of medicines, agrochemicals and technical
materials,1—15 fluorine-containing naphthalenes represent
a much less accessible and studied class of fluoro arenes.
However, in recent years, preparative syntheses of func-
tional derivatives of mono- and difluoronaphthalenes,
promising precursors for new biologically active com-
pounds and electronics, grew topical.16—38
At the same time, alternative preparations of functional
2,3-difluoronaphthalenes based on classical arene chem-
istry of direct introduction of functional groups into the
2,3-difluoronaphthalene structure were restricted to our
studies considered in our review.49 Prospects for the de-
velopment and practical application of this approach re-
quire the development of a commercially acceptable
method for the synthesis of compounds 1. From this point
of view, the above-mentioned one-step method for obtain-
ing compound 1 by co-pyrolysis of styrene (2) and chloro-
difluoromethane is attractive because of a simple procedure
and the availability of initial reagents. Earlier, we as-
sumed47—49 that this process is based on thermal reactions,
which include the fragmentation of chlorodifluoromethane
to difluorocarbene, the cyclopropanation with the parti-
cipation of difluorocarbene, and rearrangements of the
intermediate difluorocyclopropanes.
In contrast to monofluoronaphthalenes, which are
easily accessible by the Balz—Schiemann dediazofluorina-
tion of naphthylamines,39—41 difluoronaphthalenes re-
main commercially almost unavailable compounds till
now, although some of them can be obtained by dediazo-
fluorination of fluoronaphthylamines42 or by the direct
fluorination of naphthalenes using various fluorinating
reagents.43—45 2,3-Difluoronaphthalenes represent ones
of the least investigated difluoronaphthalenes because of
difficulties in their preparation by the classical procedures
for fluorine introduction into arenes, including dediazo-
fluorination, fluorine-chlorine exchange, and the methods
for direct fluorination. For instance, after the first synthe-
sis of 2,3-difluoronaphthalene (1) by the gas-phase co-
pyrolysis of CHClF2 and styrene (2), which was developed
in our laboratory in 1970,46—49 only two studies devoted
to the synthesis of 1 were published. In one,50 2,3-difluoro-
naphthalene was obtained with an yield of <20% by the
reaction of perchloryl fluoride with a product of metalation
of 2-fluoronaphthalene. In the other,51 it was produced
Here we report our detailed experimental study of the
mechanism of formation of compound 1 in co-pyrolysis
of styrene (2) and CHClF2, including establishing and
analyzing main reactions and their intermediate steps.
Results and Discussion
In accordance with the patented process,46 synthesis
of compound 1 was carried out in one step by pyrolysis of
* Dedicated to Academician of the Russian Academy of Sciences
O. N. Chupakhin on the occasion of his 85th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1232—1238, June, 2019.
1066-5285/19/6806-1232 © 2019 Springer Science+Business Media, Inc.

Synthesis of 2,3-difluoronaphthalene
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 6, June, 2019
1233
a mixture of styrene (2) and CHClF2 in a flow tube reac-
tor at 580—675 С and atmospheric pressure (Scheme 1).
It results in formation of pyrolysate that contains 10—35%
of difluoronaphthalene 1 depending on the reaction tem-
perature. Difluoronaphthalene can be isolated in the
preparative yield of 10—18% (based on the starting styrene)
or 17—25% (based on styrene consumed).
m (%)
80
70
60
50
40
30
20
10
3
4
2
Scheme 1
1
390
410
430
450
470
490 T/C
Fig. 1. Composition of products formed in the gas-phase pyrol-
ysis of 1,1-difluoro-2-phenylcyclopropane (3) depending on the
process temperature: 2,3-difluoronaphthalene (1), styrene (2),
2-fluoroindene (4).
It could be expected that the mechanism of the process
included thermal fragmentation of CHClF2 with formation
of difluorocarbene, a reaction characteristic of the gas-
phase pyrolysis of CHClF2 at 580—750 С.52 Difluoro-
carbene should easily undergo addition to styrene (2),
leading to 1,1-difluoro-2-phenylcyclopropane (3)53
(Scheme 2). As shown below, the latter can transform into
2-fluoroindene (4) under the gas-phase pyrolysis condi-
tions due to intramolecular cyclization and simultaneous
or subsequent dehydrofluorination, with formation of
intermediate 2,2-difluoroindane (5) in the latter case.
Further cycloaddition of difluorocarbene to indene 4
should afford difluoronaphthalene 1 via intermediate labile
trifluorobicyclo[3.1.0]hexene 6, which would thermally
rearrange with the ring expansion and dehydrohalogena-
tion, which is typical of reactions between dihalogencar-
benes and indenes.54—59
pyrolysis of styrene and CHClF2 in the temperature range
550—650 С (Fig. 2).
As turned out, 1,1-difluoro-2-phenylcyclopropane (3)
(prepared by the addition of difluorocarbene to styrene
(2) according to known procedure53) gives 4 and 2 as the
main products of its gas-phase pyrolysis at temperatures
from 395 to 500 С (Scheme 3, see Fig. 1), apparently,
due to cyclopropane ring opening, which is accompanied
by intramolecular cyclization with elimination of HF, and
due to typical of gem-difluorocyclopropane thermoly-
sis,60,61 reversible decomposition of 3 to difluorocarbene
and 2, respectively.
Simultaneously, formation of 1 is also detected in this
temperature range. Its fraction in the pyrolysate increased
from 0.2% (at 390 С) to 5—6% (at 500 С) with raising
the temperature, which can be explained by partial trap-
ping difluorocarbene formed from 3 by fluoroindene 4.
The key role of the latter as an intermediate in the forma-
tion of 1 is experimentally confirmed by the results of
This mechanism finds its experimental corroboration
in the results of the gas-phase pyrolysis of 1,1-difluoro-2-
phenylcyclopropane (3) at 395—500 С (Fig. 1) and of the
co-pyrolysis of indene 4 with chlorodifluoromethane, as
well as in the data on composition of the products of co-
Scheme 2

1234
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 6, June, 2019
Volchkov et al.
m (%)
Table 1. Yields of products of the gas-phase co-pyrolysis of 2 and
chlorodifluoromethane in a flow reactor at different temperatures
(the styrene flow rate is 0.50 g min–1, the CHСlF2 flow rate is
80
70
60
50
40
30
20
10
300 mL min–1
)
Т/С
Conversion
Yields (GLC) (%)a
Yield
2
1
2 (%)
1 (%)b
1
4
7
8
550
570
580
600
610
615
620
630
640
650
15
26
38
46
48
56
58
65
68
74
12.1 17.6
15.3 16.3
17.2 17.4
15.3 13.3
20.3 19.2
23.6 17.2
24.3 16.5
27.6 14.5
30.1 14.1
28.2 13.8
1.8
1.9
1.8
1.8
1.2
1.3
0.9
0.8
0.7
0.6
0.2
0.9
2.6
2.8
3.4
3.0
4.3
4.0
4.2
4.8
—
—
—
—
4
—
—
550
570
590
610
630
650 T/C
—
14.2
15.0
13.7
Fig. 2. Composition of products formed in the gas-phase co-
pyrolysis of 2 and CHClF2 depending on the process temperature:
2,3-difluoronaphthalene (1), styrene (2), 2-fluoroindene (4) (the
composition of the pyrolysate after its separation from the tar
and gaseous products is shown).
a The yields were calculated on the basis of the data of GLC with
an internal standard (1,2-dichlorobenzene) for mixtures obtained
by separation of gaseous products and tar from the raw pyrolysates.
b The yield of 1 isolated from pyrolysate by distillation, freezing-
out and recrystallization.
Scheme 3
5—17% is observed along with the simultaneous increase
in the content of compound 1 in the raw pyrolysate to
25—36%.
It is worth noting that 1,1,2,2-tetrafluoro-3-phenyl-
cyclobutane (7) (the yield in the mixture was 1—2%) and
β,β-difluorostyrene (8) (the yield in the mixture was
1—5%) were detected among the minor products of co-
pyrolysis of 2 and CHClF2 in the studied temperature
range. At the same time, the expected primary product of
cycloaddition of difluorocarbene to styrene, viz., phenyl-
difluorocyclopropane 3, was only detected in trace amounts
(<0.2%), obviously, because of its practically complete
conversion into 4 and 2 at temperatures above 550 С.
Processes occurring during the gas-phase interaction
of 2 with difluorocarbene at lower temperatures were
modeled by co-pyrolysis of 2 with perfluoropropylene
oxide, which undergoes thermal decomposition with for-
mation of difluorocarbene at 320 С already.61,62 This
co-pyrolysis in the flow reactor at 390 С results in a mix-
ture of 72% styrene (2), 22% phenyldifluorocyclopropane
3, 3% phenyltetrafluorocyclobutane 7 and 1% 2-fluoro-
indene (4) according to GLC and 19F NMR data (Scheme 5).
Thus, the obtained experimental data model the entire
sequence of the intermediate reactions shown in Scheme
2 in the step-by-step manner and confirm the implemen-
tation of this sequence leading to 2,3-difluoronaphthalene
(1) in the course of the gas-phase co-pyrolysis of (2) and
СHСlF2.
co-pyrolysis of 4 with CHClF2 at 630—640 С leading to
difluoronaphthalene 1 with the 45% yield (Scheme 4).
Scheme 4
The above experiments are supplemented by the data
on the composition of the products of co-pyrolysis of 2
with CHClF2 at 550—650 С (see Fig. 2 and Table 1).
As can be seen from the obtained data, co-pyrolysis of
styrene 2 and CHClF2 in the temperature range 550—580 С
results in the formation of 4 (according to GLC, the yield
in the mixture is 50—65% relative to reacted styrene) and
1 (according to GLC, the yield in the mixture is 5—10%) as
the main products. At higher temperatures (600—640 С),
a decrease in the fraction of 4 in the pyrolysis products to
At the same time, the described conditions of the py-
rolytic synthesis of 1 assume the possibility of an alterna-
tive pathway of its formation including dimerization of
diflurocarbene to terafluoroethylene, cycloaddition of the

Synthesis of 2,3-difluoronaphthalene
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 6, June, 2019
1235
Scheme 5
m (%)
80
60
7
40
1
8
20
2
510
530
550
570
590
610 T/C
Fig. 3. Composition of products formed in the gas-phase pyrol-
ysis of 1,1,2,2-tetrafluoro-3-phenylcyclobutane (7) depending
on the process temperature: 2,3-difluoronaphthalene (1), styrene
(2), β,β-difluorostyrene (8) (the composition of the pyrolysate
after its separation from the tar and gaseous products is shown).
latter to 2 with formation of 7, which can undergo ring
expansion and thermal aromatization affording 1 (see
Ref.51) according to Scheme 6.
The possible occurrence of these reactions is supported
not only by the above results of the co-pyrolysis of 2 with
perfluoropropylene oxide, but also by the easiness of for-
mation of phenyltetrafluorocyclobutane 7 during the gas-
phase co-pyrolysis of tetrafluoroethylene and 2 in a flow
tube reactor at 470—520 С with the 45—56% yields at
different temperatures and by the fact of formation of
2,3-difluoronaphthalene 1 upon pyrolysis of 7 at 580—620 С
(the yield of isolated product was 19% at 605 С). The
accompanying thermal transformations of 7 at 580—650 С
include its fragmentation to 2, β,β-difluorostyrene (8),
tetrafluoroethylene and vinylidene fluoride, total contribu-
tion of which in the overall transformations of 7 reaches
40—60% (Fig. 3).
In total, the obtained experimental results make it
possible to expect that the formation of difluoronaph-
thalene 1 in the course of co-pyrolysis of 2 and СHСlF2
can proceed by two parallel reaction channels: by the
carbenic pathway (see Scheme 2) via consecutive inter-
mediate formation of phenyldifluorocyclopropane 3 and
2-fluoroindene 4 with cyclopropanation of the latter with
difluorocarbene, and by an alternative pathway (see
Scheme 6) including formation and aromatization of
phenyltetrafluorocyclobutane 7. Apparently, the carbenic
pathway represents the predominant channel of formation
of difluoronaphthalene 1 in co-pyrolysis of 2 and СHСlF2.
The contribution of the alternative pathway via 7 can be
15—20% (relative to the total amount of formed difluoro-
naphthalene 1). This assessment follows from a compari-
son of the amounts of β,β-difluorostyrene (8) formed in
the co-pyrolysis of 2 with СHСlF2 and in the pyrolysis of
7 under similar condidtions. As seen from Fig. 3, pyrolysis
of 7 at 550—620 С affords approximately equal amounts
of difluorostyrene 8 and difluoronaphthalene 1. At the
same time, the amount of difluorostyrene 8 formed in
co-pyrolysis of 2 and СHСlF2 in the temperature range
550—650 С was 10—15% of the amount of difluoronaphth-
alene 1. In addition, as it is seen from the data on co-
pyrolysis of 2 with perfluoropropylene oxide (see Scheme 5),
Scheme 6

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Russ. Chem. Bull., Int. Ed., Vol. 68, No. 6, June, 2019
Volchkov et al.
more than 30 minor unidentified compounds in the amounts
from 0.02 to 8% (about 30% in total). 2,3-Difluoronaphthalene
(1) was isolated from this mixture by distillation, freezing-out
and recrystallization from ethanol in an amount of 101.51 g (15%
yield, 98% purity, m.p. 61—63 С).
the amounts of phenyldifluorocyclopropane 3 and 2-fluoro-
indene 4, the products of difluorocarbene trapping with
styrene, are 7—8 times greater than the amount of phenyl-
tetrafluorocyclobutane 7 resulting from the competitive
reaction of difluorocarbene dimerization followed by
cycloaddition of tetrafluoroethylene to 2.
The reported one-step method for synthesis of 1 from
available and cheap reactants opens up broad prospects
for preparation of new polyfunctional difluoronaphthal-
enes based on it. The results of our studies of the С—H
functionalization of 2,3-difluoronaphthalene (1) with the
use of classical reactions of electrophilic substitution or
metalation processes leading to regioselective formation
of 1-, 4-, 5- or 6-substituted 2,3-difluoronaphthalenes will
be published soon.
By the same procedure, 78.5 g of pyrolysate was obtained by
co-pyrolysis of 60 g (0.577 mol) of 2, which was supplied to
the reactor with a rate of 0.50 g min–1, and СHClF2 (300 mL min–1
)
at 580 С for 2 h. Subsequent steam distillation of the pyrolysate
and drying over СaCl2 gave 69.92 g of the product mixture, which,
according to GLC analysis, contained 57% of styrene (2), 20%
of fluoroindene (4), 10% of difluoronaphthalene (1), 2% of
β,β-difluorostyrene (8), and 2% of 1,1,2,2-tetrafluoro-3-phenyl-
cyclobutane (7). 2-Fluoroindene (4) (b.p. 62—64 С, 16 Torr)
was isolated by distillation of the mixture under reduced pressure
in an amount of 9.5 g.
Co-pyrolysis of 2 and СHClF2 at other temperatures in the
range of 550—650 С was carried out in a similar way. The
composition of pyrolysate obtained after steam distillation was
determined on the basis of data of GLC analysis and NMR
spectroscopy.
Experimental
Commercially available chlorodifluoromethane (99.9%),
tetrafluoroethylene (99.98%), hexafluoropropylene oxide (98.5%)
(Poly-Trade Company, RF), styrene (98.5%), CH2Cl2 (98%)
and Na2SO4 (Acros) were used as purchased. GLC analysis was
carried out using a Crystal-2000M chromatograph (a Macherey-
Nagel OPTIMA-1 capillary column, 30 m0.25 mm, helium as
a carrier gas, a FID detector). 1H and 19F NMR spectra were
recorded using a Bruker AС-200 spectrometer (200.1 and
188.3 MHz, respectively, solutions in CDCl3, Me4Si as an in-
ternal standard). GC/MS spectra were recorded with the use of
a Trace GC Ultra chromatograph (Termo Fisher Scientific, USA)
equipped with a Thermo TR-5ms SQC column (15 m0.25 mm)
and a mass detector Finnigan MAT DSQII (electron impact,
70 eV, 200 C, a quadrupole ion trap).
General gas-phase pyrolysis procedure. A tube quartz reactor
(of 350 mm in length and with inner diameter of 24 mm) filled
with quartz chips was used in all pyrolysis experiments. Reactor
was placed into a electrically heated tube furnace with the length
of the heated zone of 250 mm. The measured length of the ap-
proximately isothermal zone (with temperature variation of 5 С
from the specified temperature) was 80—90 mm. The temperature
was measured with a thermocouple placed in a special quartz
pocket located in the center of the reaction zone. When the reac-
tor was heated to a required temperature, a nitrogen flow was
passed through it. Then, the reactants were fed with a constant
flow rate controlled by flowmeters (for gases) or a liquid micro-
dosing pump Ismatec (for liquids).
Synthesis of 2,3-difluoronaphthalene (1) and 2-fluoroindene
(4) by co-pyrolysis of 2 and СHClF2. A gas flow of СHClF2 with
a constant flow rate of 300 mL min–1 was passed through the
tube reactor heated to 640 С for 14 h together with styrene (2),
which was fed with a rate of 0.50 g min–1. The obtained pyrol-
ysate was condensed in a water-cooled condenser and in two
successively connected traps cooled by a mixture of ice with salt,
neutralized with aqueous NH4OH, steam-distilled to separate it
from tar, gaseous components and inorganic products, washed
with water and dried over CaCl2. The product mixture in an
amount of 552.4 g was obtained, which, according to the GLC
analysis, contained 25% of styrene (2), 4% of 2-fluoroindene (4),
36% of 2,3-difluoronaphthalene (1), 5% of β,β-difluorostyrene
(8), 1% of 1,1,2,2-tetrafluoro-3-phenylcyclobutane (7), and also
2,3-Difluoronaphthalene (1). M.p. 61—63 С. 1H NMR, δ:
7.48—7.63 (m, 4 H); 7.78 (m, 2 H). 19F NMR, δ: –137.2 (m, 2 F).
MS (EI, 70 eV), m/z (Irel (%)): 164 [M]+ (100), 144 (8), 138 (7),
82 (6), 69 (3). Found (%): С, 72.78; H, 3.41. С10H6F2. Calculat-
ed (%): С, 73.17; H, 3.68.
2-Fluoroindene (4).63 1H NMR, δ: 3.51 (br.s, 2 H, CH2);
6.18 (br.s, 1 H); 7.19—7.43 (m, 4 H). 19F NMR, δ: –119.7 (br.s,
1 F, CF=СH). MS (EI, 70 eV), m/z (Irel (%)): 134 [M]+ (84),
133 (100), 107 (7), 67 (8).
Co-pyrolysis of 2-fluoroindene (4) and СHClF2. Chlorodi-
fluoromethane was passed through the tube reactor heated to
640 С with a constant flow rate of 150 mL min–1 for 30 min
together with 9.0 g of 2-fluoroindene 4 fed into the reactor with
a constant rate of 0.3 g min–1. The obtained pyrolysate (8.60 g)
was washed with water, dried over Na2SO4 and separated from
tar by filtering through a silica gel layer (5 cm). A mixture of
products in an amount of 7.22 g was obtained. According to the
GLC and NMR data, it contains 17% of fluoroindene (4) and
69% of 2,3-difluoronaphthalene (1).
Synthesis of 1,1-difluoro-2-phenylcyclopropane (3).53,63
Freshly distilled styrene (2) (260.2 g, 2.5 mol), chlorodifluoro-
methane (239.1 g, 2.75 mol), ethylene oxide (136.4 g, 3.1 mol)
and tetraethylammonium bromide (2.5 g) were placed in a 1000 mL
stainless steel autoclave cooled to –70 С. The vessel was sealed,
and the reaction mixture was stirred at 150 С for 5 h. After the
processing, the mixture was washed with water, dried over СaCl2
and distilled. Difluorocyclopropane 3 was isolated in an amount
of 95.5 g (b.p. 63—65 С, 26 Torr; 25% yield).
1,1-Difluoro-2-phenylcyclopropane (3).53 1H NMR, δ:
1.15—1.95 (m, 2 H, cyclopropyl moiety); 2.30—2.92 (m, 1 H,
cyclopropyl moiety); 7.12—7.33 (m, 5 H, phenyl moiety).
19F NMR, δ: –125.3 (d.m, 1 F, JF—F = 154 Hz, 4JF—H = 8.1 Hz);
–142.5 (d.m, 1 F, JF—F = 154 Hz). MS (EI, 70 eV), m/z (Irel (%)):
134 [M]+ (100), 134 (69), 133 (45), 91 (18).
Pyrolysis of 1,1-difluoro-2-phenylcyclopropane (3). A solution
of 50.0 g of 1,1-difluoro-2-phenylcyclopropane (3) in 150 g of
СН2Cl2 was added with a constant rate of 0.4 g min–1 to the
reactor heated to 450 С in a nitrogen flow (100 mL min–1) for
8 h. The pyrolysis products were condensed in a trap cooled by
a mixture of solid CO2 and propan-2-ol, washed with water and

Synthesis of 2,3-difluoronaphthalene
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 6, June, 2019
1237
dried over Na2SO4. After distillation of СН2Cl2, 39.0 g of a mixtu-
re was obtained. According to the GLC and NMR data, it con-
tains 36% of phenyldifluorocyclopropane 3, 23% of styrene (2),
32% of 2-fluoroindene (4), and 3% of difluoronaphthalene 1.
Pyrolysis of 3 at other temperatures in the studied range was
carried out in a similar manner and gave product mixtures,
compositions of which shown in Fig. 1 were obtained using GLC
analysis and NMR spectrosopy.
Co-pyrolysis of 2 and hexafluoropropylene oxide. Gas flows
of hexafluoropropylene oxide and nitrogen at constant flow rates
of 150 mL min–1 and 100 mL min–1, respectively, were passed
for 30 min through the tube reactor heated to 390 С. A 50%
solution of styrene 2 in dichloromethane was simultaneously fed
into the reactor by the micro-dosing pump with a rate of
0.50 g min–1. The pyrolysis products were condensed in a trap
cooled to –(15—20) С. After quick weighing, the obtained
mixture (14.3 g) was mixed with 30 g of CH2Cl2 cooled to –20 С
and was analyzed after addition of 0.5 g of 1,2-dichlorobenzene
(an internal standard) with the use of GLC and 19F NMR.
According to the analysis data, pyrolysate contained 4.7 g of
styrene (2), 2.44 g of phenyldifluorocyclopopane (3) (22% yield),
0.1 g of 2-fluoroindene (4) (1% yield) and 0.44 g of 1,1,2,2-tetra-
fluoro-3-phenylcyclobutane (7) (3% yield). Warming of the
cooled solution of the pyrolysate to the room temperature was
accompanied by its fast thickening because of exothermic poly-
merization of 2, likely, induced by trifluoroacetyl fluoride, which
was formed upon thermal decomposition of hexafluoropropyl-
ene oxide.
to the GLC data, contained 17% of phenyltetrafluorocyclobutane
(7), 7% of styrene (2), 27% of -difluorostyrene (8), 34% of
2,3-difluoronaphthalene (1), and more than 10 unidentified
compounds in the amounts of 0.03—8% (12% in total). Difluoro-
naphthalene 1 was isolated from this mixture by distilation,
freezing-out and recrystallization from ethanol in an amount of
7.53 g (98% purity; m.p. 61—63 С, 19% yield).
Pyrolysis of 7 at other temperatures in the studied range was
carried out in a similar manner and gave product mixtures,
compositions of which shown in Fig. 3 were determined using
GLC analysis and NMR spectrosopy.
β,β-Difluorostyrene (8).64,65 1H NMR, δ: 5.23 (dd, 1 Н,
J = 26.4 Hz, J = 3.7 Hz); 7.16—7.39 (m, 5 Н, phenyl moiety).
19F NMR, δ: –82.6 (dd, 1 F, J = 31.6 Hz, J = 26.4 Hz); –84.5
(d, 1 F, J = 31.6 Hz, J = 3.7 Hz). MS (EI, 70 eV), m/z
(Irel (%)):140 [M]+ (100), 114 (15), 89 (10), 51 (4).
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 16-03-
01098a) and the Presidium of the Russian Academy of
Sciences in the framework of Basic Research Program
No. 38 (Project No. 0080-2018-0007).
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Received December 29, 2018;
in revised form March 27, 2019;
accepted April 9, 2019