Branched polyfluorinated triflate - An easily available polyfluoroalkylating agent

Article abstract of DOI:10.1016/j.jfluchem.2005.12.031

A novel branched polyfluoroalkyl triflate was prepared from readily available (perfluorohexyl)propyl iodide in five steps with high overall yield. The reactivity of the triflate has been tested in model reactions with methyl gallate (O-nucleophile) and be

Full text of DOI:10.1016/j.jfluchem.2005.12.031

Journal of Fluorine Chemistry 127 (2006) 386–390
www.elsevier.com/locate/fluor
Branched polyfluorinated triflate—An easily available
polyfluoroalkylating agent
a,b,
a,b,
a
a
*
*
, Martin Havlık , Bohumil Dolensky ,
´
´ˇ ˇ´
, Tomas Brıza
a
´
´
Robert Kaplanek
Zdenek Kejık , Pavel Martasek , Vladimır Kral
b
a
ˇ
´
´
´
´
a
´
Department of Analytical Chemistry, Institute of Chemical Technology in Prague, Technicka 5, Prague 6, 16628, Czech Republic
b
ˇ ´
First Faculty of Medicine, Charles University in Prague, Katerinska 32, Prague 2, 12108, Czech Republic
Received 28 October 2005; received in revised form 20 December 2005; accepted 30 December 2005
Available online 7 March 2006
Abstract
A novel branched polyfluoroalkyl triflate was prepared from readily available (perfluorohexyl)propyl iodide in five steps with high overall yield.
The reactivity of the triflate has been tested in model reactions with methyl gallate (O-nucleophile) and benzylamine (N-nucleophile). The
reactions yielded good amounts of the desired polyfluorinated products.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Fluorinated branched triflate; Fluorous building block; Perfluoroalkylating agent; Fluorous tag; Electrophilic fluorinated reagent
1. Introduction
necessary. Our aim was to develop a synthetic route for the
primary triflate which could transfer two polyfluorinated chains
in one step.
Highly fluorinated compounds (generally called fluorous
compounds [1–3]) display properties which are utilized in
catalysis, organic syntheses, separation techniques, the
biomedicinal field, electronics and material chemistry [4].
Substitution of compounds with nucleophilic functional groups
(–OH, –SH, –NH2) can be achieved using electrophilic
polyfluoroalkylating compounds such as iodides [5–7],
acylhalogenides [8,9], anhydrides [10–12], tosylates [13–15],
mesylates [13,14], triflates [16–19] or epoxides [20–22].
One of the most reactive classes of polyfluoroalkylating
agents are polyfluorinated triflates. Those with ethylene or
propylene spacers have been synthesized and utilized success-
fully for the construction of various highly fluorinated
compounds, such as fluorophilic cyclopentadienes [16,17],
alkynes [19] or polyfluorinated benzyloxamines [18]. In all
cases, using triflates was necessary because other agents (i.e.
iodides) failed due to low reactivity. Known polyfluorinated
triflates are able to transfer only one polyfluoroalkylated chain.
For the connection of two chains, 2 mol of the triflate are
We used a known method for the preparation of monoacid
with two polyfluorinated chains [23]. In the next step, the
carboxylic group was converted into the corresponding alcohol.
The final step included a reaction with trifluoromethanesulfonyl
anhydride to obtain the desired primary branched polyfluori-
nated triflate with two polyfluorinated chains. The triflate is
sufficiently insulated (five carbon spacer) from the electron
withdrawing fluorinated chain in order to preserve its high
reactivity.
2. Results and discussion
Our synthetic strategy was based on the substitution of
diethyl malonate with two polyfluorinated chains [23]. The
diethyl-malonate was deprotonated with NaH and then reacted
with 3-(perfluoroalkyl)propyl iodide in anhydrous DMF and
diethyl bis(polyfluoroalkyl)malonate 1 was obtained at 98%
yield. The basic hydrolysis of polyfluorinated malonate 1 by
aqueous–ethanolic potassium hydroxide produced fluorinated
dicarboxylic acid 2 (89% yield), which after pyrolysis at 180 8C
yielded the corresponding product of decarboxylation, fluori-
nated monoacid 3 at almost quantitative yield (98%). This acid
(3) was treated with borane–THF complex in anhydrous THF
* Corresponding authors. Fax: +420 224 310 352.
´
E-mail addresses: robert.kaplanek@vscht.cz (R. Kaplanek),
ˇ´
ftor@seznam.cz (T. Brıza).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.12.031

´
R. Kaplanek et al. / Journal of Fluorine Chemistry 127 (2006) 386–390
387
Scheme 1. Preparation of branched polyfluorinated triflate.
Scheme 2. Modification of gallate with fluorinated chains.
Scheme 3. Reaction of benzylamine with triflate 5.
and 2,2-[bis(polyfluoroalkyl)]ethan-1-ol (4) was obtained in
high yield (97%). The alcohol 4 was converted into
corresponding triflate under same conditions as we used for
preparation of [2-(perfluoroalkyl)ethyl]triflates [16]. Thus, the
alcohol was reacted with trifluoromethanesulfonic acid
anhydride in the presence of pyridine in anhydrous dichlor-
omethane at low temperatures. The desired branched triflate
was obtained in excellent yield (97%). The overall yield of the
bis(polyfluoroalkylated) triflate 5 was high (80%) (Scheme 1).
To demonstrate the reactivity of our new triflate 5, we carried
out two reactions leading to potentional fluorinated blocks for
further building of larger fluorinated units. The modification of
gallate with three fluorinated chains in one step was published
recently [24–26]. Gallate was transformed into the correspond-
ing sodium salt which was reacted with triflate 5 under reflux.
The reaction afforded the corresponding gallate 6 substituted
with six fluorinated chains (Scheme 2).
We tested the reactivity of a primary amino group toward the
triflate 5. We chose a reactive amine—benzylamine. By means
of triflate 5, we connected four fluorinated chains in one step.
Benzylamine bearing only two fluorinated chains was
synthesized by this point [27].
The reaction was carried out in acetonitrile with potassium
carbonate as a base. The triflate was used in excess (2.5
equivalents versus amino group). The mixture was heated to
80 8C for 5 h and produced the corresponding tertiary amine 7
in yield 89% (Scheme 3).
The gallate 6 (61.6% F) and amine 7 (61.0% F) can be
regarded as fluorophilic compounds, because their content of
fluorine is over 60% [28].
3. Conclusions
We developed a facile method for the preparation of
branched triflate from commercially available compounds
with an overall yield of 80%. Its reactivity was tested in
The fluorinated gallate 6 could be used as a potential
precursor for the construction of larger fluorous tags [23].

´
R. Kaplanek et al. / Journal of Fluorine Chemistry 127 (2006) 386–390
388
reactions with benzylamine and methyl gallate. This triflate
was found to be a suitable polyfluoroalkylating agent for
construction of highly fluorinated structures (e.g. fluorous tags
or dendrones).
4.3. Preparation of diacid 2
The diester 1 (3.525 g, 4 mmol) was dissolved in 96%
ethanol (25 mL) and KOH (50% aqueous solution, 25 mL) was
added. The reaction mixture was heated at 80 8C overnight.
After cooling, the reaction mixture was acidified with 36%
aqueous solution HCl (pH ꢀ 2). A brown precipitate was
filtered off, washed with water (250 mL), dissolved in ethyl
acetate and dried over Na2SO4. Ethyl acetate was removed
under reduced pressure and crude diacid 2 was obtained
(3.167 g) as a brown solid. Crude product was recrystallized
from hot toluene to give 2 (2.276 g, 69%) as a pale yellow
powder. The next crop of 2 (0.654 g, 20%) was obtained by
concentration of mother liquor.
4. Experimental
4.1. General
NMR spectra were recorded on a Varian Gemini 300 HC
(FT, 1H at 300 MHz, 13C at 75 MHz, 19F at 281 MHz)
instrument using TMS and CFCl3 as the internal standards.
Chemical shifts are quoted in ppm (d, scale; s, singlet; bs, broad
singlet; d, doublet; t, triplet; q, quadruplet; m, multiplet),
coupling constants J in Hertz, solvents CDCl3, CD3OD. Mass
spectrometry analyses were performed on a Hewlett–Packard
GC HP5890 instrument. IR spectrometry analyses were
performed on a Bruker IFS 66v/S instrument.
1H NMR (CD3OD) d 1.62 (m, 4H), 2.05 (m, 4H), 2.31 (tt,
3
3
4H, JHH = 7.6 Hz, JHF = 19.6 Hz); 13C NMR (CD3OD) d
16.8 (s, 2C), 31.9 (t, 2C, 2JCF = 22 Hz), 33.4 (s, 2C), 58.3 (s,
1C), 174.6 (s, 2C); 19F NMR (CD3OD) d À81.7 (t, 6F,
3JFF = 10 Hz), À114.5 (m, 4F), À122.5 (m, 4F), À123.4 (m,
4F), À124.1 (m, 4F), À126.8 (m, 4F).
The chemicals used were as follows: benzylamine, diethyl
malonate, 3-(perfluorohexyl)propyl iodide, sodium hydride
(60% suspension in oil), trifluoromethanesulfonic acid anhy-
dride, methyl gallate (all Aldrich). Silica gel (60–100 mm,
Merck). Anhydrous dichloromethane, pyridine and N,N-
dimethylformamide were purchased from Fluka. Other solvents
were purchased from Penta and dried according to standard
procedures.
IR (KBr) (cmÀ1) 3440, 1704, 1237, 1209, 1143.
MS: for C21H14F26O4 calculated: 824, found: (M-H) 823;
mp = 139–141 8C.
Anal. calcd. for C21H14F26O4: C, 30.60; H, 1.71. Found: C,
30.18; H, 1.72.
4.2. Preparation of diester 1
4.4. Preparation of monoacid 3
To stirred suspension of NaH (60% suspension in oil;
270 mg, 6.75 mmol; washed with petroleum ether before use)
in anhydrous DMF (15 mL) under argon atmosphere the
solution of diethyl malonate (360 mg, 2.25 mmol) in anhydrous
DMF (7 mL) was added. After stirring for 1 h at room
temperature, 3-(perfluorohexyl)propyl iodide (2.490 g,
5.1 mmol) in anhydrous DMF (7 mL) was added dropwise.
The mixture was heated to 80 8C and stirred for 12 h. After
cooling, water (150 mL) was added and the mixture was
extracted with diethyl ether (3 Â 150 mL). The organic fraction
was washed with brine (3 Â 100 mL) and dried over Na2SO4.
Diethylether was removed under reduced pressure. The crude
product was obtained as a brown oil (1.943 g, 98%) and used
for the next step without further purification.
The diacid 2 (recrystallized, 884 mg, 1.07 mmol) was heated
at 190 8C for 45 min. After cooling, the crude monoacid was
dissolved in chloroform and filtered (diacid 2 is almost
insoluble in CHCl3). Chloroform was removed under reduced
pressure and light brown waxy product was obtained (815 mg,
98%).
1H NMR (CDCl3) d 1.69 (m, 8H), 2.09 (m, 4H), 2.45 (m,
1H); 13C NMR (CDCl3) d 18.1 (s, 2C), 30.6 (t, 2C,
2JCF = 21 Hz), 31.3 (s, 2C), 44.9 (s, 1C), 108.5–121.2 (m,
12C), 181.3 (s, 1C); 19F NMR (CDCl3) d À81.5 (t, 6F,
3JFF = 10 Hz), À114.9 (m, 4F), À122.5 (m, 4F), À123.5 (m,
4F), À124.1 (m, 4F), À126.8 (m, 4F).
IR (CDCl3) (cmÀ1) 1709, 1241, 1210, 1145.
MS: for C20H14F26O2 calculated: 780, found: 780, (M-H)
779; mp = 44–45 8C.
3
1H NMR (CDCl3) d 1.25 (t, 6H, JHH = 7.0 Hz), 1.55 (m,
4H), 1.97 (t, 4H, 3JHH = 8.5 Hz), 2.19 (m, 4H), 4.21 (q, 4H,
3JHH = 7.0 Hz); 13C NMR (CDCl3) d 13.9 (s, 2C), 15.5 (s,
Anal. calcd. for C20H14F26O2: C, 30.79; H, 1.81. Found: C,
30.80; H, 1.85.
2
2C), 31.0 (t, 2C, JCF = 21 Hz), 32.2 (s, 2C), 57.2 (s, 1C),
61.5 (s, 2C), 108.5–121.2 (m, 12C), 170.8 (s, 2C); 19F NMR
4.5. Preparation of alcohol 4
3
(CDCl3) d À81.8 (t, 6F, JFF = 10.4 Hz), À115.2 (m, 4F),
À123.0 (m, 4F), À124.0 (m, 4F), À124.7 (m, 4F), À127.3
(m, 4F).
The monoacid 3 (810 mg, 1.04 mmol) was dissolved in
anhydrous THF (2 mL). Under argon atmosphere, borane–THF
complex (1 M solution in THF, 5.2 mL, 5.2 mmol) was added
dropwise. Reaction mixture was refluxed for 16 h. After
cooling down, 20% aqueous solution HCl (10 mL) was
carefully added to destroy excess of borane. Water (100 mL)
was added and mixture was extracted with diethyl ether
IR (CDCl3) (cmÀ1) 1727, 1243, 1237, 1207, 1146.
MS: for C25H22F26O4 calculated: 880, found: (M) 880, (M-
H) 879.
Anal. calcd. for C25H22F26O4: C, 34.11; H, 2.52. Found: C,
33.93; H, 2.53.

´
R. Kaplanek et al. / Journal of Fluorine Chemistry 127 (2006) 386–390
389
(3 Â 150 mL). Organic fractions were washed with water
(2 Â 100 mL) and brine (100 mL) and dried over Na2SO4.
Diethylether was removed under reduced pressure and product
was obtained as a white waxy solid (771 mg, 97%).
pressure and, the oily residue was purified by column
chromatography on silica gel (eluent:hexan/dichloromethane,
1:1). Product 6 was obtained as a pale yellow oil (112 mg,
42%).
1H NMR (CDCl3) d 1.40 (m, 5H), 1.61 (m, 4H), 2.03 (tt, 4H,
1H NMR (CDCl3) d 1.66 (m, 24H), 1.88 (m, 3H), 2.06 (m,
12H), 3.90 (s, 3H), 3.91 (d, 2H, 3JHH = 4.7 Hz) 3.96 (d, 4H,
3JHH = 4.7 Hz), 7.28 (s, 2H); 13C NMR (CDCl3) d 17.7 (s,
3
3JHH = 7.3 Hz, JHF = 18.5 Hz), 2.69 (bs, 1H), 3.55 (d, 2H,
3JHH = 4.4 Hz); 13C NMR (CDCl3) d 17.6 (s, 2C), 30.4 (s,
2
2
2C), 31.1 (t, 2C, JCF = 21 Hz), 40.2 (s, 1C), 64.4 (s, 1C),
6C), 29.7 (s, 6C), 30.9 (t, 6C, JCF = 21 Hz), 38.2 (s, 2C),
108.5–121.2 (m, 12C); 19F NMR (CDCl3) d À82.8 (t, 6F,
3JFF = 10.4 Hz), À116.1 (m, 4F), À123.6 (m, 4F), À124.6
(m, 4F), À125.3 (m, 4F), À128.0 (m, 4F).
IR (CDCl3) (cmÀ1) 1241, 1215, 1145.
39.0 (s, 1C), 52.2 (s, 1C), 70.8 (s, 2C), 75.1 (s, 1C), 104.0–
120.2 (m, 36C), 107.8 (s, 2C), 125.3 (s, 1C), 141.8 (s, 1C),
152.5 (s, 2C), 166.6 (s, 1C); 19F NMR (CDCl3) d À81.7
(m,18F), À115.1 (m, 12F), À122.6 (m, 12F), À123.6 (m,
12F), À124.3 (m, 12F), À126.9 (m, 12F).
MS: for C20H16F26O calculated: 766, found: (M-HF) 746;
mp = 48–49 8C.
Anal. calcd. for C20H16F26O: C, 31.35; H, 2.10. Found: C,
IR (CDCl3) (cmÀ1) 2927, 1716, 1241, 1209, 1145.
MS: for C68H50F78O5 (MH) calculated: 2429, found: 2429.
Anal. calcd. for C68H50F78O5: C, 33.62; H, 2.07. Found: C,
33.65; H, 2.08.
31.14; H, 2.18.
4.6. Preparation of triflate 5
4.8. Substitution of benzylamine
The solution of trifluoromethanesulfonic acid anhydride
(723 mg, 2.56 mmol) in anhydrous dichloromethane (20 mL)
was cooled to À15 8C in an ethanol/dry ice bath. Under argon
atmosphere, the solution of alcohol 4 (1310 mg, 1.71 mmol)
and pyridine (135 mg, 1.71 mmol) in anhydrous dichloro-
methane (15 mL) was slowly added while stirring. Then the
mixture was slowly warmed to room temperature. The reaction
mixture was evaporated to dryness and dark oily residue was
purified by column chromatography on silica gel (eluent:di-
chloromethane). The solvent was removed under reduced
pressure and triflate 5 was obtained as a light brown oil
(1490 mg, 97%).
A mixture of triflate 5 (100 mg, 0.11 mmol), benzylamine
(6 mg, 0.056 mmol), anhydrous potassium carbonate (17 mg,
0.12 mmol) and acetonitrile (2 mL) was stirred under argon at
80 8C for 5 h. After cooling, water (10 mL) was added and the
mixture was extracted with chloroform (2 Â 50 mL). Organic
fractions were dried over MgSO4. Chloroform was removed
under reduced pressure and the oily residue was purified by
column chromatography on silica gel (eluent:dichloro-
methane). Product 7 was obtained as a viscous pale yellow
oil (80 mg, 89%).
1H NMR (CDCl3) d 1.20–1.60 (m, 18H), 2.05 (m, 8H), 2.23
(d, 4H, 3JHH = 6.9 Hz), 3.48 (s, 2H), 7.26 (m, 5H); 13C NMR
1H NMR (CDCl3) d 1.51 (m, 4H), 1.65 (m, 4H), 1.87 (m,
1H), 2.10 (heptet, 4H, 3JHH = 7.0 Hz, 3JHF = 18.2 Hz), 4.49
2
(CDCl3) d 17.2 (s, 4C), 31.1 (t, 4C, JCF = 21 Hz), 31.5 (s,
3
(d, 2H, JHH = 5.0 Hz); 13C NMR (CDCl3) d 17.5 (s, 2C),
4C), 35.9 (s, 2C), 59.7 (s, 2C), 60.2 (s, 1C), 104.0–122 (m,
24C), 127.2 (s, 1C), 128.2 (s, 2C), 129.1 (s, 2C), 139.4 (s,
29.7 (s, 2C), 30.9 (t, 2C, 2JCF = 21 Hz), 38.1 (s, 1C), 78.2 (s,
1
1C), 108.5–121.2 (m, 12C), 118.0 (q, JCF = 317 Hz; 19F
3
1C); 19F NMR (CDCl3) d À81.5 (m, 12F, JFF = 10 Hz),
NMR (CDCl3) d À75.1 (s, 3F), À81.4 (t, 6F, 3JFF = 10 Hz),
114.8 (m, 4F), À122.4 (m, 4F), À123.4 (m, 4F), À124.1 (m,
4F), À126.8 (m, 4F).
À114.9 (m, 8F), À122.5 (m, 8F), À123.4 (m, 8F), À124.2
(m, 8F), À126.9 (m, 8F).
IR (CDCl3) (cmÀ1) 2857, 1243, 1192, 1145.
MS: for C47H37F52N (MH) calculated: 1603, found: (M-H)
1602.
IR (neat) (cmÀ1) 2961, 1418, 1245, 1207, 1145.
MS: for C21H15F29O3S calculated: 898, found: (M-CF3SO3)
749.
Anal. calcd. for C47H37F52N: C, 35.20; H, 2.33. Found: C,
35.07; H, 2.39.
Anal. calcd. for C21H15F29O3S: C, 28.08; H, 1.68. Found: C,
27.97; H, 1.68.
Acknowledgement
4.7. Substitution of gallate
The authors thank the grant agency of Czech Academy of
Science for financial support of this project, Grant No. KJB 401
280 501 and MSM 604 613 73 07.
A mixture of sodium hydride (7.8 mg, 0.33 mmol), methyl
gallate (20 mg, 0.11 mmol) and THF (2 mL) was stirred under
argon at room temperature for 30 min. The solution of triflate 5
(430 mg, 0.48 mmol) in THF (2 mL) was added and the
reaction mixture was heated to 60 8C for 2 h. After cooling,
water (20 mL) was added and the mixture extracted with
dichloromethane (2 Â 20 mL). Organic fractions were dried
over MgSO4. Dichloromethane was removed under reduced
References
´ ´
[1] I.T. Horvath, J. Rabai, Science 266 (1994) 72–75.
[2] J.A. Gladysz, Science 266 (1994) 55–56.
[3] J.A. Gladysz, D.P. Curran, Tetrahedron 58 (20) (2002) 3823–3825.

´
R. Kaplanek et al. / Journal of Fluorine Chemistry 127 (2006) 386–390
390
ˇ
[17] T. Brıza, J. Kvıcala, O. Paleta, J. Cermak, Tetrahedron 28 (2002) 3841–
ˇ´
´ˇ
´
[4] J.A. Gladysz, D.P. Curran, I.T. Horvath, Handbook of Fluorous Chemistry,
first ed., Wiley–VCH, Weinheim, 2004.
¨
[5] H. Trabelsi, F. Szonyi, S.J. Geribaldi, J. Fluorine Chem. 107 (2001) 177–
3846.
[18] N. Koshti, G.V. Reddy, H. Jacobs, A. Gopalan, Synth. Commun. 32 (2002)
3779–3790.
ˇ´ ´ˇ
[19] T. Brıza, J. Kvıcala, O. Paleta, Collect. Czech. Chem. Commun. 68 (2003)
181.
[6] N.O. Brace, L.W. Marshall, C.J. Pinson, G. van Wingerden, J. Org. Chem.
48 (1984) 2361–2368.
1039–1045.
´ ´
[20] V. Cırkva, B. Ameduri, B. Boutevin, O. Paleta, J. Fluorine Chem. 84
[7] K. Werner, H. Blank, A. Gisser, E. Manhart, J. Fluorine Chem. 16 (1980)
193–197.
(1997) 53–61.
[8] A.I. Cooper, J.D. Londono, G. Wignall, J.B. McClain, E.T. Samulski, J.S.
Lin, et al. Nature 2 (389) (1997) 368–371.
[21] H. Plenkiewicz, W. Dmowski, J. Fluorine Chem. 45 (1989) 389–400.
¨
[22] C. Coudures, R. Pastor, S. Szonyi, A. Cambon, J. Fluorine Chem. 24
[9] E.L.V. Goetheer, M.W.P.L. Baars, L.J.P. van den Broeke, E.W. Meijer,
J.T.F. Keurentjes, Ind. Eng. Chem. Res. 39 (2000) 4634–4640.
[10] S. Clark, J. Am. Chem. Soc. 75 (1953) 6305–6306.
[11] M. Kuroboshi, T. Hiyama, Tetrahedron Lett. 35 (23) (1994) 3983–3984.
[12] Y.A. Serguchev, V.G. Davydova, G.A. Stetsyuk, D.P. Blendonogii, I.P.
Beletskaya, Zh. Org. Khim. 19 (4) (1983) 820–824.
(1984) 105–115.
[23] J. Loiseau, E. Fouquet, R.H. Fish, J.M. Vincent, J.B. Verlhac, J. Fluorine
Chem. 108 (2001) 195–197.
[24] G. Johansson, V. Percec, G. Ungar, J.P. Zhou, Macromolecules 29 (1996)
646–660.
[25] M.W. Markowicz, R. Dembinski, Org. Lett. 4 (22) (2002) 3785–3787.
[26] V. Percec, M. Glodde, G. Johansson, V.S.K. Balagurusamy, P.A. Heiney,
Angew. Chem. Int. Ed. 42 (2003) 4338–4342.
[27] C. Rocaboy, W. Bauer, J.A. Gladysz, Eur. J. Org. Chem. 14 (2000) 2621–
2628.
[13] D. Prescher, T. Thiele, R. Ruhmann, J. Fluorine Chem. 79 (1996) 145–148.
[14] S. Elshani, E. Kobzar, A.R. Bartsch, Tetrahedron 56 (2000) 3291–3301.
´
[15] F. De Campo, D. Lastecoueres, J.M. Vincent, J.B. Verlhac, J. Org. Chem.
64 (1999) 4969–4971.
ˇ
[16] T. Brıza, J. Kvıcala, P. Mysık, O. Paleta, J. Cermak, Synlett 5 (2001) 685–
ˇ´
´ˇ
´
´
´
[28] L.E. Kiss, I. Kovesdi, J. Rabai, J. Fluorine Chem. 108 (2001) 95–
¨
687.
109.