Direct addition of fluorine to arylacetylenes

Article abstract of DOI:10.1016/j.jfluchem.2008.12.009

Under suitable conditions elemental fluorine can be added across the carbon-carbon triple bond of arylacetylenes forming tetrafluoroethane derivatives - ArCF₂CF₂R - in good yields.

Full text of DOI:10.1016/j.jfluchem.2008.12.009

Journal of Fluorine Chemistry 130 (2009) 332–335
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Direct addition of fluorine to arylacetylenes
*
Julia Gatenyo, Shlomo Rozen
School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel
A R T I C L E I N F O
A B S T R A C T
Article history:
Under suitable conditions elemental fluorine can be added across the carbon–carbon triple bond of
arylacetylenes forming tetrafluoroethane derivatives – ArCF2CF2R – in good yields.
ß 2008 Elsevier B.V. All rights reserved.
Received 14 October 2008
Received in revised form 15 December 2008
Accepted 18 December 2008
Available online 27 December 2008
Keywords:
F2
Acetylenes
Direct fluorination
Aryltetrafluoroethanes
1. Introduction
or in using it as a tool for constructing other fluorine containing
reagents [13,14]. In the past we have developed a methodology for
Today there is no need to elaborate on the importance of the
fluorine atom in organic chemistry, especially in the physical and
biological fields. There are numerous examples of fluorine
containing compounds which have found many applications in
medicinal and material chemistry [1,2]. Recently, it was reported
that the introduction of tetrafluoroethane bridges (CF2CF2) to the
structure of liquid crystals yields new classes of materials with
improved properties for applications in active matrix liquid crystal
display (LCDs) [3]. Furthermore, bridge-fluorinated octafluor-
o[2.2]paracyclophane was found to be a very useful chemical
vapor deposition (CVD) precursor of a family of thin film polymers
known as parylenes [4].
adding fluorine across double bonds [15], but adding it across triple
ones was more challenging. Compounds with the general formula
of ArCF2CF2Ar0 were achieved, for example, by preparing 1,2-
difluorostilbenes from tetrafluoroethylene and only then treated
with F2 to form the tetrafluoro derivatives. Direct addition of this
halogen to triple bonds was described to give ‘‘complicated
mixtures’’ [16]. Another indirect approach was to react Deoxo-
fluor1 with some benziles [17] or use combination of reagents
such as nitrosonium tetrafluoroborate (NO+BF4ꢀ) and pyridinium
polyhydrogen fluoride (PPHF) [18]. We report here of our attempts
to add F2 across triple bonds toward producing the tetrafluor-
oethanes derivatives in a direct one-step reaction.
Addition of halogens to
p centers is almost as old as chemistry
itself. The terms halogens in this context, however, was reserved
almost exclusively to Cl2, Br2 and occasionally also to XF (X = Br, I,
OH) [5–7] but almost never to F2. Merritt was the first to add F2 to
some double and triple bonds [8], but his technique was such that
nobody could repeat the experiments in the last 40 years. During
this period, many believed that using elemental fluorine for
organic synthesis is an exercise in futility since this halogen is so
reactive that inevitably the reactions were supposed to be
destructive. Slowly, but steadily, we and others have shown that
under the right conditions this element can become very potent,
and yet very selective, both in cases of employing it directly [9–12]
2. Results and discussion
When diluted fluorine (3–10% F2 in N2) was bubbled slowly
through a cold solution (ꢀ78 8C) of diphenylacetylene (1a) in a
mixture of CHCl3/CFCl3/EtOH as a solvent, 1,2-diphenyltetrafluoro-
ethane (2a) [19] was formed in 75% yield (Scheme 1).
Consequently, a series of substituted arylacetylenes with
either electron donating or withdrawing groups were allowed to
react with elemental fluorine under similar reaction conditions.
1-Methoxy-4-(phenylethynyl)benzene (1b) gave 1-methoxy-4-
(1,1,2,2-tetrafluoro-2-phenylethyl)benzene (2b) [18] in 65% yield
and 1-ethyl-4-(phenylethynyl)benzene (1c) was converted to the
corresponding 1-ethyl-4-(1,1,2,2-tetrafluoro-2-phenylethyl)ben-
zene (2c) in 70% yield. In both cases, despite the presence of
activated rings toward electrophilic attack, we have not found
* Corresponding author. Tel.: +972 3 6408378; fax: +972 3 6409293.
E-mail address: rozens@post.tau.ac.il (S. Rozen).
0022-1139/$ – see front matter ß 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2008.12.009

J. Gatenyo, S. Rozen / Journal of Fluorine Chemistry 130 (2009) 332–335
333
Scheme 2. The proposed mechanism of the addition of F2 to arylacetylenes.
bonding lowering the transition state and enabling the addition
reaction to occur. As shown in the past, such a chain of events is
essential for ionic reactions involving F2 [23].
The mechanism outlined above may serve as a guideline for the
addition reaction described in this work. Despite the high
probability that the creation of the tetrafluoro derivative is a
two-step reaction, we were not able to detect the difluoroolefin
intermediate. This is rationalized by the assumption that the
difluoroolefin reacts faster than the starting acetylene with F2 since
the resulting carbocation
a to the CF2 moiety should form a
Scheme 1. Adding fluorine to acetylenes.
relatively unstable ions which collapse fast. When two vicinal aryl
groups are present, the above carbocation is stabilized enough to
react with the counter fluoride ion to form the tetrafluoroethane
product (Scheme 2) [24]. When such stabilization is weakened, as
in mono aryl or lacking, as in alkylacetylenes, the unstable
carbocation may turn to other collapsing options such as fast
rearrangements. This may lead to creation of several secondary
carbocations along the chain, which apart from their own reactions
with F2, can also result in a host of additional unsaturated centers
all of which react fast with the halogen to produce many fluorine
containing products, lowering the yield of the reaction or
rendering it impractical.
aromatic substitution products as with other electrophilic regents
derived from F2 [20–22], indicating that the triple bond reacts
faster with F2 than the benzenoidic aromatic
p electrons. Halogen
possessing derivatives such as 1-chloro-4-(phenylethynyl)ben-
zene (1d) or 4-(phenylethynyl)-40-fluorobenzophenone (1e)
behaved similarly forming 1-chloro-4-(1,1,2,2-tetrafluoro-2-phe-
nylethyl)benzene (2d) and 1-(p-fluorobenzophenone)-2-phenyl-
1,1,2,2-tetrafluoroethane (2e) both in 70% yield without any
halogen exchange. Strong electron-withdrawing moieties such as
cyano, or nitro groups found in 4-(phenylethynyl)benzonitrile (1f)
and 1-nitro-4-(phenylethynyl)benzene (1g) did not alter the
reaction course either, and the fluorine attacked selectively the
carbon–carbon triple bond, producing the desired 4-(1,1,2,2-
tetrafluoro-2-phenylethyl)benzonitrile (2f) in 75% yield and 1-
nitro-4-(1,1,2,2-tetrafluoro-2-phenylethyl)benzene (2g) in 70%
yield. Disubstituted 1,2-bis(4-bromophenyl)ethyne (1h) was also
successfully fluorinated to the corresponding 1,2-bis(4-bromo-
phenyl)-1,1,2,2-tetrafluoroethane (2h) in 65% yield [17]. The
reaction is not limited to tolanes although the best results were
associated with this family. We found that monoarylacetylenes
such as 1-pentynylbenzene (1i) and methyl phenylpropiolate (1j)
did react with elemental fluorine to produce the desired (1,1,2,2-
tetrafluoropentyl)benzene (2i) and methyl 2,2,3,3-tetrafluoro-3-
phenylpropanoate (2j) products. The yields of these reactions were
somewhat lower (45% and 40%, respectively) and the by-products
formed made analytical purification of the tetrafluoro compounds
extremely difficult. Moving a step further, to dialkylacetylenes,
proved to be completely impractical as only minute amounts of the
desired tetrafluoro products were detected along with numerous
other by-products.
Although there is no profound effect on the reaction by the ring
substitution, we believe that a radical mechanism is not likely.
When the reaction was performed in the presence of radical chain
scavengers such as 1,3-dinitrobenzene or oxygen, the fluorine was
added across the triple bond with the same efficiency as when such
scavengers were not present. In addition, it is clear that the
presence of an alcohol such as ethanol, is mandatory since in its
absence a complicated mixture of fluorinating products was
obtained. Leaving us with the ionic reaction prospect means that
one of the roles of ethanol, or methanol for that matter, is to serve
as an acceptor for the developing fluoride anion through hydrogen
3. Experimental
1H NMR spectra were recorded using a 200 MHz spectrometer
with CDCl3 as a solvent and Me4Si as an internal standard. The 19
F
NMR spectra were measured at 188.1 MHz with CFCl3, serving as
an internal standard. The proton broad-band decoupled 13C NMR
spectra were recorded at either 50.2 MHz or at 100.5 MHz. Here
too, CDCl3 served as a solvent and Me4Si as an internal standard.
MS was measured under EI, or ESI-QqTOF conditions. In case these
methods could not detect the molecular ion, we have successfully
used the Amirav’s supersonic GC–MS developed in our depart-
ment. The main feature of this method is to provide electron
ionization while the sample is vibrationally cooled in a supersonic
molecular beam. This considerably enhances the relative abun-
dance of molecular ions [25,26]. Silica gel 60H (Merck) and
petroleum ether/ethyl acetate were used for flash chromatogra-
phy. As mentioned above we were unable to obtain analytically
pure samples of compounds 2i and 2j and used isotope abundance
analysis to confirm the structure. This analysis confirmed the
proposed elemental formulas as it ranked them at the first place
and hence as the best choice with very good matching factors of
better then 860 (out of 999) [27].
3.1. General fluorination procedure
Fluorine is a strong oxidant and corrosive material. In organic
chemistry, it is mostly used after dilution with nitrogen or helium.
Such dilution can be achieved by using either an appropriate
cooper or monel vacuum line constructed in a well-ventilated area
or simply purchasing prediluted fluorine. A detailed description for

334
J. Gatenyo, S. Rozen / Journal of Fluorine Chemistry 130 (2009) 332–335
simple setup had appeared in the past [28]. The reactions
themselves can be carried out in regular glassware. If elementary
precautions are taken, work with F2 is simple and we have had no
bad experiences working with it.
F = 24 Hz), 131.2, 137.5 ppm; 19F NMR ꢀ112.3 (2F, s), ꢀ112.3 ppm
(2F, s); The usual MS methods fail to show any molecular peak.
However, using Amirav’s method revealed a strong molecular ion
peak of m/z 288 (M)+. Anal. Calcd for C14H9F4Cl: C, 58.25; H, 3.14.
Found: C, 58.36; H, 2.88.
The reactions were usually carried out on scales of 1–5 mmol
acetylene derivatives, monitored by GC on a 3% SE-30 column, and
usually stopped when conversion reached about 95%. Fluorine, at a
concentrations of 3–10% in N2, was slowly passed through a cold
(ꢀ78 8C) and vigorously stirred solution of the arylacetylene
dissolved in 100 mL of CFCl3, 125 mL CHCl3 and 25 mL of EtOH. An
efficient mixing is achieved by using a vibromixer, which also
ensures a fine dispersion of the gas bubbles. The term ‘‘worked up
as usual’’ means stopping the reaction by pouring it into 200 mL
water, washing the organic layer with NaHCO3 solution followed
by water until neutral, drying the organic layer over MgSO4, and
finally evaporating the solvent. The crude product was usually
purified by vacuum flash chromatography using silica gel 60-H
(Merck) or by recrystallization.
1-(p-Fluorobenzophenone)-2-phenyl-1,1,2,2-tetrafluor-
oethane (2e) was prepared from 1e (0.54 g) as described above.
After the usual workup the crude reaction mixture was purified by
vacuum flash chromatography using 5% ethyl acetate in petroleum
ether as eluent. The tetrafluoro product was obtained as white
crystals in 70% yield: 0.48 g; mp: 174.9–175.7 8C; 1H NMR 7.19
(2H, t, J = 8 Hz), 7.45–7.51 (5H, m), 7.61 (2H, d, J = 8 Hz), 7.81 (2H, d,
J = 8 Hz), 7.84–7.88 ppm (2H, m); 13C NMR 115.9 (d, JC–F = 22 Hz),
116.4 (tt, 1JC–F = 252 Hz, 2JC–F = 36 Hz), 116.7 (tt, 1JC–F = 253 Hz, 2JC–
F = 36 Hz), 127.1 (t, JC–F = 6 Hz), 127.3 (t, JC–F = 6 Hz), 128.4, 129.5,
130.6 (t, JC–F = 25 Hz), 131.3, 132.9 (d, JC–F = 9 Hz), 133.3 (d, JC–
F = 3 Hz), 134.8 (t, JC–F = 25 Hz), 139.9, 165.8 (d, JC–F = 255 Hz),
194.5 ppm; 19F NMR ꢀ105.4 (1F, s), ꢀ111.9 (2F, s), ꢀ112.3 ppm
(2F, s); HRMS (ESI-Qq TOF) (m/z) calcd For C21H13F5O = 399.0778
(M + Na)+, found: 399.0764. Anal. Calcd for C21H13F5O: C, 67.02; H,
3.48; F, 25.24. Found: C, 66.95; H, 3.24; F, 25.15.
3.2. Preparation of starting materials
Substituted diarylacetylenes were prepared using known
procedures mainly by reacting cuprous phenyl acetylide with
variously substituted iodoarenes in refluxing pyridine [29].
1,2-Diphenyltetrafluoro ethane (2a) [19] was prepared from
1a (0.92 g) as described above. After the usual workup the crude
reaction mixture was purified by vacuum flash chromatography
using petroleum ether as eluent. The tetrafluoro product was
obtained as white crystals in 75% yield: 0.98 g; mp: 120.3–
121.0 8C; 1H NMR 7.40–7.48 ppm (10H, m); 13C NMR 116.8 (tt, 1JC–
4-(1,1,2,2-Tetrafluoro-2-phenylethyl)benzonitrile (2f) was
prepared from 1f (0.50 g) as described above. After the usual
workup the crude reaction mixture was purified by vacuum flash
chromatography using 1% ethyl acetate in petroleum ether as
eluent. The tetrafluoro product was obtained as white crystals in
75% yield: 0.51 g; mp: 113.1–113.9 8C; 1H NMR 7.43–7.75 ppm
(9H, m); 13C NMR 115.3, 115.9 (tt, JC–F = 253 Hz, JC–F = 35 Hz),
116.5 (tt, 1JC–F = 253 Hz, 2JC–F = 35 Hz), 117.9, 127.0 (t, JC–F = 5.7 Hz),
128.5, 130.2 (t, JC–F = 25 Hz), 131.5, 132.1, 135.6 ppm (t, JC–
F = 26 Hz); 19F NMR ꢀ112.0 (2F, s), ꢀ113.0 ppm (2F, s); HRMS
1
2
2
F = 253 Hz, JC–F = 36 Hz), 127.1 (t, JC–F = 4 Hz) 128.2, 131.1 ppm;
19F NMR ꢀ112.2 ppm (4F, s); MS (EI) (m/z) (M)+: 254.
(CI) (m/z) calcd for C
15H9F4N = 280.0749 (M + H)+, found:
1-Methoxy-4-(1,1,2,2-tetrafluoro-2-phenylethyl)benzene
(2b) [18] was prepared from 1b (0.30 g) as described above. After
the usual workup the crude reaction mixture was purified by
vacuum flash chromatography using 10% ethyl acetate in
petroleum ether as eluent. The tetrafluoro product was obtained
as white crystals in 65% yield: 0.26 g; mp: 38.9–39.2 8C; 1H NMR
3.84 (3H, s), 7.32–7.47 ppm (9H, m); 13C NMR 55.4, 113.5, 116.8 (tt,
1JC–F = 251 Hz, 2JC–F = 36 Hz), 116.9 (tt, 1JC–F = 251 Hz, 2JC–F = 36 Hz),
123.0 (t, JC–F = 25 Hz), 126.9 (t, JC–F = 6 Hz), 128.4, 128.9 (t, JC–
F = 6 Hz), 131.2, 131.7 (t, JC–F = 18 Hz), 161.7 ppm; 19F NMR ꢀ111.2
(2F, s), ꢀ112.3 ppm (2F, s); HRMS (CI) (m/z) calcd for
280.0742. Anal. Calcd for C15H9F4N: C, 64.52; H, 3.25; N, 5.02.
Found: C, 64.90; H, 3.09; N, 5.05.
1-Nitro-4-(1,1,2,2-tetrafluoro-2-phenylethyl)benzene (2g)
was prepared from 1g (1.10 g) as described above. After the usual
workup the crude reaction mixture was purified by vacuum flash
chromatography using petroleum ether as eluent. The tetrafluoro
product was obtained as white crystals in 70% yield: 1.03 g; mp:
91.3–92.0 8C; 1H NMR 7.47–7.54 (5 H, m), 7.68 (2 H, d, J = 8 Hz),
1
2
8.3 ppm (2 H, d, J = 9 Hz); 13C NMR 115.9 (tt, JC–F = 254 Hz, JC–
1
2
F = 37 Hz), 116.5 (tt, JC–F = 254 Hz, JC–F = 37 Hz), 123.5, 123.8,
127.0 (t, JC–F = 6 Hz), 128.5, 128.7, 129.4, 130.1 (t, JC–F = 25 Hz),
131.5, 132.0, 132.4, 137.2 (t, JC–F = 26 Hz) 149.7 ppm; 19F NMR
ꢀ111.9 (2F, s), ꢀ112.5 ppm (2F, s); The usual MS methods fail to
show any molecular peak. However, using Amirav’s method
revealed a strong molecular ion peak of m/z 299 (M)+. Anal. Calcd
for C14H9F4NO2: C, 56.20; H, 3.03; F, 25.40; N, 4.68. Found: C, 55.95;
H, 2.90; F, 25.90; N, 4.63.
C
15H12F4O = 285.0902 (M + H)+, found: 285.0903.
1-Ethyl-4-(1,1,2,2-tetrafluoro-2-phenylethyl)benzene (2c)
was prepared from 1c (0.50 g) as described above. After the usual
workup the crude reaction mixture was purified by vacuum flash
chromatography using petroleum ether as eluent. The tetrafluoro
product was obtained as white crystals in 70% yield: 0.48 g; mp:
69.3–70.1 8C; 1H NMR 1.25 (3H, t, J = 8 Hz), 2.69 (2H, q, J = 8 Hz),
7.22–7.24 (2H, m), 7.35–7.47 ppm (7H, m); 13C NMR 15.4, 28.8,
116.8 (tt, 1JC–F = 253 Hz, 2JC–F = 36 Hz), 116.9 (tt, 1JC–F = 252 Hz, 2JC–
F = 36 Hz), 127.1, 127.8, 128.2, 128.3 (t, JC–F = 25 Hz), 130.9, 131.2
(t, JC–F = 25 Hz) 147.5 ppm; 19F NMR ꢀ111.7 (2F, s), ꢀ112.2 ppm
(2F, s); The usual MS methods fail to show any molecular peak.
However, using Amirav’s method revealed a strong molecular ion
peak of m/z 282 (M)+. Anal. Calcd for C16H14F4: C, 68.08; H, 5.00; F,
26.92. Found: C, 67.92; H, 4.88; F, 27.15.
1-Chloro-4-(1,1,2,2-tetrafluoro-2-phenylethyl)benzene (2d)
was prepared from 1d (0.95 g) as described above. After the usual
workup the crude reaction mixture was purified by vacuum flash
chromatography using petroleum ether as eluent. The tetrafluoro
product was obtained as white crystals in 70% yield: 0.90 g; mp:
49.9–50.5 8C; 1H NMR 7.39–7.47 ppm (9H, m); 13C NMR 116.5 (tt,
1JC–F = 253 Hz, 2JC–F = 37 Hz), 116.6 (tt, 1JC–F = 253 Hz, 2JC–F = 37 Hz),
127.1 (t, JC–F = 6 Hz), 128.3, 128.6, 129.2 (t, JC–F = 24 Hz), 130.7 (t, JC–
1,2-Bis(4-bromophenyl)-1,1,2,2-tetrafluoroethane (2h) [17]
was prepared from 1h (0.80 g) as described above. After the usual
workup the crude reaction mixture was purified by vacuum flash
chromatography using 5% ethyl acetate in petroleum ether as
eluent. The tetrafluoro product was obtained as white solid in 65%
yield: 0.63 g; mp: 98–100 8C; 1H NMR 7.30 (4H, d, J = 9 Hz),
1
2
7.55 ppm (4H, d, J = 9 Hz); 13C NMR 116.0 (tt, JC–F = 252 Hz, JC–
F = 37 Hz), 125.8, 128.5, 129.2 (t, JC–F = 26 Hz), 131.4 ppm; 19F NMR
ꢀ112.2 (2F, s).
(1,1,2,2-Tetrafluoropentyl)benzene (2i) was prepared from 1i;
oil, 45% yield: 1H NMR 7.57–7.42 (5H, m), 2.00 (2H, t, J = 8 Hz), 1.62
(2H, sex, J = 8 Hz), 0.98 ppm (3H, t, J = 8 Hz); 13C NMR 14.0, 14.4 (t,
1
2
JC–F = 4 Hz), 32.7 (t, JC–F = 23 Hz), 116.8 (tt, JC–F = 251 Hz, JC–
1
2
F = 35 Hz), 119.1 (tt, JC–F = 251 Hz, JC–F = 35 Hz), 126.9 (t, JC–
F = 6 Hz), 128.4, 130.9 ppm; 19F NMR ꢀ110.7 (2F, s), ꢀ113.6 ppm
(2F, t); The usual MS methods fail to show any molecular peak.
However, using Amirav’s method revealed a strong molecular ion

J. Gatenyo, S. Rozen / Journal of Fluorine Chemistry 130 (2009) 332–335
335
peak of m/z 220 (M)+ with isotope abundance analysis matching
factor of 867 out of 999 [27].
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1-Methyl-(2,2,3,3-tetrafluoro-3-phenyl)propanoate (2j) was
prepared from 1j; oil, 40% yield: 1H NMR 7.57–7.37 (5H, m),
1
2
3.91 ppm (3H, s); 13C NMR 54.0, 109.4 (tt, JC–F = 260 Hz, JC–
F = 39 Hz), 126.7 (t, JC–F = 6 Hz), 128.6, 130.7, 131.6, 133.0,
154.5 ppm; 19F NMR ꢀ112.0 (2 F, s), ꢀ119.0 ppm (2F, s); The
usual MS methods fail to show any molecular peak. However, using
Amirav’s method revealed a strong molecular ion peak of m/z 236
(M)+ with isotope abundance analysis matching factor of 960 out of
999 [27].
Acknowledgment
This work was supported by the USA-Israel Binational Science
Foundation (BSF), Jerusalem, Israel.
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