Electrophilic, catalytic alkylation of polyfluoroolefins by some fluoroalkanes

Article abstract of DOI:10.1016/S0022-1139(00)00408-5

New data are presented on the antimony pentafluoride catalyzed reaction of hydrofluorocarbons such as CH₃F, CH₂F₂, CH₃CHF₂ and R_fCH₂CH₂F with fluoroolefins. The condensation of CH₂F₂ and fluoroolefins CF₂=CFX (X=F, CF₃) proceeds under mild conditions producing the corresponding propanes FCH₂CFXCF₃ in moderate to high yield. Under similar conditions methyl fluoride reacts with tetrafluoroethylene giving CH₃CF₂CF₃. However, a complex mixture of products forms in the analogous reaction with hexafluoropropene. The structures of the products of the reactions of CH₃CHF₂ and tetrafluoroethylene and F-butylethylene were determined.

Full text of DOI:10.1016/S0022-1139(00)00408-5

Journal of Fluorine Chemistry 108 (2001) 15±20
Electrophilic, catalytic alkylation of poly¯uoroole®ns
by some ¯uoroalkanes$
G.G. Belen'kiia, V.A. Petrovb,*, P.R. Resnickc
aINEOS RAN, Vavilova 28, Moscow 117813, Russia
bDuPont Central Research and Development Experimental Station, P.O. Box 80328, Wilmington, DE 19880-0328, USA
cDuPont Fluoroproducts, 22828 NC Highway 87 West, Fayetteville, NC 28306, USA
Received 18 October 2000; accepted 6 November 2000
Abstract
New data are presented on the antimony penta¯uoride catalyzed reaction of hydro¯uorocarbons such as CH3F, CH2F2, CH3CHF2 and
RfCH2CH2F with ¯uoroole®ns. The condensation of CH2F2 and ¯uoroole®ns CF2=CFX (X ˆ F, CF3) proceeds under mild conditions
producing the corresponding propanes FCH2CFXCF3 in moderate to high yield. Under similar conditions methyl ¯uoride reacts with
tetra¯uoroethylene giving CH3CF2CF3. However, a complex mixture of products forms in the analogous reaction with hexa¯uoropropene.
The structures of the products of the reactions of CH3CHF2 and tetra¯uoroethylene and F-butylethylene were determined. # 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Poly¯uoroole®ns; Fluoroalkanes; Electrophilic; Alkylation; Di¯uormethane; Methyl ¯uoride
1. Introduction
(X ˆ H, Cl, Br, I) with ¯uoroethylenes [7]. In this
paper, we report new data on the antimony penta¯uoride
catalyzed reaction of hydro¯uorocarbons such as CH3F,
CH2F2, CH3CHF2 and RfCH2CH2F with ¯uoroole®ns.
Reaction of di¯uoromethane (1) in the presence of antimony
penta¯uoride with either tetra¯uoroethylene (TFE) or
hexa¯uoropropylene (HFP) at ambient or elevated tempera-
ture gave the corresponding dihydro¯uoroalkanes,
1,1,1,2,2,3-hexa¯uoropropane (2) and 2-tri¯uoromethyl-
1,1,1,2,3-penta¯uoropropane (3), respectively. Both reac-
tions proceed rapidly in the presence of 2±10 mol% anti-
mony penta¯uoride.
The electrophilic condensation of poly¯uoroalkanes with
¯uoroole®ns is an important synthetic route for the synthesis
of higher poly¯uoroalkanes. Some condensation reactions
involving simple alkyl ¯uorides or chlorides and ¯uoroole-
®ns are catalyzed effectively by a solution of antimony
penta¯uoride in anhydrous HF [1,2]. A key property of this
superacid system is the elimination of the pronounced
oxidizing power of antimony penta¯uoride. Additions of
chloro¯uoroalkanes to poly¯uoroethylenes catalyzed by
antimony penta¯uoride are limited to easily activated mole-
cules such as CClF2CCl2F [3] or CH3CF3 [4]. Aluminum
chloro¯uoride, ACF [5] is a potent Lewis acid catalyst which
has no oxidizing capability. However, ACF has a much
higher sensitivity to protic impurities. This requires parti-
cular care in excluding materials such as water or hydro-
carbons and also limits the range of substrates to highly
¯uorinated materials. Within these limits the range of mate-
rials compatible with ACF is broad [6]. A recent example is
the ACF catalyzed condensation of tri¯uoromethanes, CF3X
(1)
The reactions are exceptionally clean and only produce the
one to one adducts. No evidence for the formation of
compounds resulting from the further condensation of
alkanes 2 and 3 with a second mole of fluoroolefin was
found. The reactions could be scaled up to kilogram scale by
$ Publication no. 8134.
* Corresponding author. Tel.: ‡1-302-695-1958; fax: ‡1-302-695-8281.
E-mail address: viacheslav.a.petrov@usa.dupont.com (V.A. Petrov).
0022-1139/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 0 ) 0 0 4 0 8 - 5

16
G.G. Belen'kii et al. / Journal of Fluorine Chemistry 108 (2001) 15±20
penta¯uoropropane (11). Reaction of 10 with tri¯uoroethy-
lene at ambient temperature produced 1,1,1,2-tetra¯uoro-
propane (12) in moderate yield. In addition a small amount
of 1,1,1,2-tetra¯uoroethane (13), was formed by the Lewis
acid catalyzed addition of adventitious HF to tri¯uoroethy-
lene. No further condensation products from either 12 or 13
and tri¯uoroethylene were found.
(2)
(3)
Scheme 1.
adding the fluoroolefin to excess difluoromethane under
pressure at 35±508C (TFE) or at 60±808C (HFP). The
regiospecificity of the product with HFP is that expected
for an electrophilic reaction [8].
We believe that the probable mechanism involves the
formation of an ionized complex 4, between di¯uoro-
methane (1) and the Lewis acid followed by reaction with
the ¯uoroole®n to intermediate 5. Stabilization of 5, by
addition of ¯uoride from the counter ion leads to the ®nal
product and reformation of the catalyst. The mechanism is
similar to that proposed for the Lewis acid condensation of
¯uoroole®ns with CF3CH3 [4] and CF3X [7] (Scheme 1).
The reaction of di¯uoromethane (1) and 1,2-dichlorodi-
¯uoroethylene (6), resulted in extensive tar formation.
Surprisingly, 1,3-dichloro-2,2,3,3-tetra¯uoropropane (7),
formed in 32% yield, was the only product isolated. We
believe that the formation of 7 is the result of process
represented by Scheme 2.
Carbocation 8 formed by the reaction of 1 and 6 under-
goes two rearrangements: a 1,3-migration of ¯uorine [9]
followed by a 1,2-migration of chlorine [10]. This results
in formation of cation 9, which is converted into the
®nal product by addition of ¯uoride anion from the counter
anion.
A similar lack of higher condensation products was reported
in the reaction of tetrafluoroethylene and 1,1,1,2,3,3,3-hep-
tafluoropropane [11].
The reaction of methyl ¯uoride (10), and hexa¯uoropro-
pylene at 508C led to a complex mixture of products. The
major product was 1,1,1,2,3,3,3-hepta¯uoropropane (14). In
addition approximately equal amounts of three hydro¯uoro-
alkanes (15, 16, 17) were formed. The structures of the
alkanes were established by GC/MS and NMR spectroscopy.
The formation of 16 and 17 containing C2H5± and (CH3)3C±
‡
fragments most likely resulted from addition of C2H5 and
‡
(CH3)3C to hexa¯uoropropylene. These cations are well
known to form in the self-condensation reaction of methyl
¯uoride in magic acid [12]. The hydrogen ¯uoride formed in
this process will react with hexa¯uoropropylene to give
major product 14. However, it is possible that formation
of 14 could be the result of the protonation of a C±C bond of
17 by superacid followed by bond cleavage to give
‡
(CH3)3C and 14. Although this process is unknown at
present for hydro¯uorocarbons it has been reported for
hydrocarbons [13].
Methyl ¯uoride (10) reacted similarly with ¯uoroole®ns.
Reaction of 10 with tetra¯uoroethylene gave 1,1,1,2,2-
(4)
(5)
Tetra¯uoroethylene reacted with 1,1-di¯uoroethane (18)
in the presence of SbF5 to give the isomeric butanes 19a and
Scheme 2.

G.G. Belen'kii et al. / Journal of Fluorine Chemistry 108 (2001) 15±20
17
19b in moderate yield in addition to some higher boiling
materials of unknown structure.
(6)
A possible mechanism for the formation of 19a and 19b is
shown in Scheme 3. Reaction of a-¯uoroethyl cation (20)
[14] with tetra¯uoroethylene leads to the major product 19a.
Under the reaction conditions elimination of HF from 19a
produces ole®n 21. Readdition of HF to 21 yields the
isomeric butane 19b. The addition of HF to the carbon±
carbon double bond of per¯uoroalkylethylenes is known to
result in the formation of 1-per¯uoroalkyl-2-¯uoroethanes
[15,16]. The formation of higher boiling materials may well
be due to the self-condensation of 21.
Scheme 4.
The formation of the observed products is shown in
Scheme 4. The intermediate is believed to be
CH2=CHCF2CF3 (21) formed as shown in Scheme 3. Pro-
tonation of 21 may give cation 23a which may yield (19b) or
may yield the mesomeric cation 23b, which in turn will yield
FCH2CH=CFCF3 (23c) [16]. Under the reaction conditions
both carbocations 23a and 23b may be in equilibrium with
23c and 19b. Reaction of carbocation 23a with tetrafluor-
oethylene leads to the observed decafluorohexane 22a.
The reaction of carbocation 23b with tetrafluoroethylene
leads to the formation of the relatively stable a,a-difluor-
ocarbocation 23d. It may either add fluoride ion via path
``A'' to give the observed olefinic product 22b or undergo
an intramolecular cyclization via path ``B'' to yield cyclo-
butane 22c the minor product of the reaction. Intramole-
cular cyclization of perhalogenated carbocations resulting
in the formation of a cyclobutane was first reported by
Krespan and Dixon [9] and later also observed in the
reaction of hexafluoroisobutene and tetrafluoroethylene
[17].
Scheme 3.
Surprisingly when the above reaction of 18 and tetra-
¯uoroethylene was carried out in the presence of SbF5 with
HF as a solvent the major reaction products were hexane 22a
and hexene 22b. In addition a trace amount cyclobutane 22c,
identi®ed by GC/MS, as well as some higher boiling mate-
rials were formed.
The formation of products 25a and 25b in the reaction of
per¯uorobutylethylene 24 and TFE in a mixture of HF and
SbF5 support the mechanism proposed in Scheme 4. The
isolated products, octane 25a and octene 25b are analogous
to those found from the reaction of CH3CHF2 (18) and
tetra¯uoroethylene carried out in the presence of SbF5 with
HF as a solvent where (21) was the proposed intermediate
(Eq. (7) and Scheme 4).
(7)

18
G.G. Belen'kii et al. / Journal of Fluorine Chemistry 108 (2001) 15±20
in a 788C cold trap. Distillation of the crude product gave
75 g of 2, bp 0±18C. The yield of 2 was 80%.
The reaction was carried out using same amount of
reagents, but 5 g of SbF5. In this experiment compound 2
was isolated in 92% yield.
2.2. Reaction of difluoromethane (1) and hexafluoro-
propene (HFP)
(8)
A 250 ml stainless steel shaker tube was loaded with 45 g
of SbF5, 39 g of 1 and 66 g of HFP. The reaction vessel was
agitated for 8 h at 508C, a second portion of HFP (30 g)
added and the reaction vessel heated at 508C for another 8 h.
The gaseous products were bleed out of the reactor and
collected in a 788C cold trap. Distillation of the crude
product gave 80 g of 3, bp 22±248C. The yield of 3 was 90%.
Higher boiling compounds 25c±e were also products of this
reaction. Although these materials were not isolated GC/MS
and NMR spectra data strongly suggest that 25c has the
formula C14H6F22 and is the product of the condensation of
1 mol of tetrafluoroethylene with 2 mol of starting F-n-
butylethylene. Compounds 25d and 25e with the formulas
C12H6F18 and C12H5F17, respectively, are the result of the
dimerization of starting olefin 24. Formation of compounds
with similar composition has been reported in reaction of 24
in superacidic media [16]. The presence of significant
amounts of 25c±e (>40%, see Section 2) indicates that under
these conditions the rate of reaction of 24 with tetrafluor-
oethylene is comparable to the rate of self-condensation.
Interestingly, neither 25a nor 25b were formed when 24 was
reacted with tetrafluoroethylene in HF with BF3 as the
catalyst [16]. This is consistent with the use, in the present
work, of the more powerful Lewis acid, antimony penta-
fluoride, which serves to increase the effective concentra-
tions of the intermediate cations analogous to 23a and
23b.
2.3. Reaction of difluoromethane (1) and CFCl=CFCl
A 250 ml stainless steel shaker tube was loaded with 22 g
of SbF5, 36 g of 1 and 26 g of CFCl=CFCl. The reaction
vessel was agitated for 3 h at 208C. Crude product is
collected in cold trap ( 788C) while the reactor is heated
in hot water bath. Crude product is collected in cold trap
( 788C) while the reactor is heated in hot water bath.
Collected in cold trap product was washed with water, dried
over MgSO4 and distilled to give 12 g (36% yield) of
compound 7, bp 62±648C.
2.4. Reaction of methyl fluoride (10) and
tetrafluoroethylene (TFE)
A 400 ml Hastelloy shaker tube was loaded with 15 g of
SbF5, 35 g of 10 and 50 g of TFE. The reaction vessel was
agitated for 6 h at 258C and 10 h at 508C. The gaseous
products were bleed from the reactor and collected in a
788C cold trap. Distillation of the 40 ml crude reaction
mixture gave 45 g (67%) of CH3CF2CF3 (98% purity), bp
13 to 128C.
2. Experimental
1
19F and H NMR spectra were recorded on a QE-300
(General Electric, 200 MHz) or Brucker DRX-400 instru-
ments (400.5524 and 376.8485 MHz, respectively) using
CFCl3 as internal standard or chloroform-d or acetone-d6 as
a lock solvent. IR spectra are recorded on Perkin-Elmer
1600 FT spectrometer in a liquid ®lm. CF2H2, CH3F
(Aldrich), SbF5, CFH=CF2, CFCl=CFCl (PCR), CH3CF2H,
C4F9CH=CH2, TFE and HFP (DuPont) are commercially
available and were used without further puri®cation. Com-
pounds 13, 14 were identi®ed by comparison with authentic
samples. Compounds 2, 12 [18], 3, 15, 16 [19], 7, 11 [20], 16
[21], 19b [15] were identi®ed by comparison of their 19F and
1H NMR spectra data with their reported literature values.
2.5. Reaction of methyl fluoride (10) and
hexafluoropropene (HFP)
A mixture of 20 g of 10, 75 g of CF2=CFCF3 and 15 g of
SbF5 was heated for 14 h at 508C. Isolation gave 68 g of a
mixture containing 77% of 14, 11% of 15, 5% of 16 and 9%
(CF3)2CFC(CH3)3 (17) based on 1H and 19F NMR analysis.
The yield of 17 was 4%. Compound 17 was characterized in
the reaction mixture. 1H NMR: 1.12 (s). 19F NMR: 74.13
(6F, d; 6 Hz), 211.73 (1F, m). MS, m/e: 211.0364 (M±
2.1. Reaction of difluoromethane (1) and tetrafluoro-
ethylene (TFE)
‡
‡
CH3) , C6H6F7 , calc. 211.0358.
A 250 ml stainless steel shaker tube was loaded with 45 g
of SbF5, 39 g of 1 and 30 g of TFE. The reaction vessel was
agitated for 8 h at 508C, second portion of TFE (30 g) added
and the reaction vessel heated at 808C for another 20 h. The
gaseous products were bleed out of the reactor and collected
2.6. Reaction of methyl fluoride (10) and trifluoroethylene
A mixture of 10 g of 10, 24 g of CF2=CFH and 10 g of
SbF5 was heated for 14 h at 258C. Isolation gave 12 g of a

G.G. Belen'kii et al. / Journal of Fluorine Chemistry 108 (2001) 15±20
19
fraction, bp 0±28C, which contained 90% CF3CFHCH3 and
10% CF3CFH2 as determined by 19F NMR. The yield of
¯uoropropane was 38.6%.
was injected into reaction vessel, the organic layer was
separated and dried over MgSO4. The crude product
(30 g) was found to be a mixture of four major components,
which comprised >95% of the sample, 25a±d in the ratio of
11:47:15:27, respectively. Distillation of the crude reaction
mixture afforded 10 g of fraction, bp 105±1108C and 15 g of
residue. The NMR spectrum of the ®rst fraction showed it to
consist of a mixture of compounds 25a and 25b in the ratio
15:85. The 90% of the residue were found to be a mixture of
25c and 25d. The remaining 10% were not identi®ed.
25a: 19F NMR: 81.60 (3F, m), 86.01 (3F, s), 117.00
(2F, m), 119.51 (2F, m), 124.79 (2F, m), 126.51 (2F,
m). 1H NMR: 2.20 (m). GC/MS (m/z, major peaks): 327 (M±
2.7. Reaction of 1,1-difluoroethane (18) and
tetrafluoroethylene (TFE)
Compound 18 (53 g), antimony penta¯uoride (44 g) and
TFE (30 g) were heated in a 250 ml stainless steel shaker
tube at 40±508C for 10 h. The products were transferred to a
788C cold trap. The excess 18 was removed by distillation,
the residue washed with water, dried and distilled to give
20 g of a mixture, bp 23±278C. This mixture contained 89%
19a and 11% 19b based on NMR analysis. The total yield
was 40%. The residue (20 g) was not analyzed. Compound
19a was characterized in the mixture. 19F NMR: 84.0 (3F,
m; 10 Hz), 131.5 (2F, typical AB pattern; 230; 16; 7 Hz),
‡
‡
‡
HF2, C8H3F12 ), 277 (C7H3F10 ), 213 (C5HF8 ), 197
‡
‡
‡
‡
(C5H4F7 ), 177 (C5H3F6 , 100%), 157 (C5H2F5 ), 127
‡
‡
‡
(C4H3F4 ), 113 (C3HF4 ), 77 (C2H3F2 ), 69 (CF3 ), 64
‡
(C2H2F2 ).
1
196.5 (1F, m; 45; 17 Hz). H NMR: 1.7 (3H), 5.25 (1H).
25b: 19F NMR: 81.57 (3F, m), 86.66 (3F, s), 117.07
(2F, m), 119.51 (2F, m), 124.96 (1F, m), 127.97 (2F,
m). 1H NMR: 2.88 (2H, m), 5.52 (1H, dt; 31; 8 Hz). IR 1730
2.8. Reaction of 1,1-difluoroethane (18) and
tetrafluoroethylene (TFE) in HF
‡
‡
(w) cm 1. GC/MS (m/z, major peaks): 346 (M , C8H3F13 ),
‡
‡
327 (M±HF2, C8H3F12 ), 277 (C7H3F10 , 100%), 213
‡
‡
‡
‡
Compound 18 (11 g), antimony penta¯uoride (45 g), TFE
(40 g) and 80 g anhydrous HF were shaken in a 250 ml
stainless steel shaker tube at 208C for 8 h. The reactor was
heated in boiling water and the crude product collected in a
polyethylene wash bottle ®lled with water. The organic layer
was separated, washed with water, dried over MgSO4 and
distilled to give 27 g of a fraction, bp 46±578C. Based on GC
and NMR data the mixture contained 22a and 22b and small
amount of 22c in the ratio of 70:29:3. The yield of the
mixture was 50%. No attempt was made to separate com-
pounds 22a±c. The materials are characterized from the
mixture. Compound 22a: 19F NMR: 87.5 (3F, s), 121.0
(2F, s). 1H NMR: 2.5 (m). MS (m/z, relative intensities %):
(C5HF8 ), 157 (C5H2F5 ), 127 (C4H3F4 ), 113 (C3HF4 ),
‡
‡
77 (C2H3F2 ), 69 (CF3 ).
Acknowledgements
Authors would like to thank Dr. C.G. Krespan for helpful
discussions.
References
[1] G.G. Belen'kii, L.G.I. Savicheva, L.S. German, Izv. Akad. Nauk
SSSR. Ser. Khim. (1978) 1433.
‡
‡
247 (M±F, C6H4F9 , 0.1), 227 (C6H3F8 , 9.7), 197
‡
‡
‡
‡
[2] V.A. Petrov, G.G. Belen'kii, L.S. German, Izv. Akad. Nauk SSSR.
Ser. Khim. (1980) 1099.
(C5H4F7 , 18.9), 177 (C5H3F6 , 29.6), 157 (C5H2F5
‡
,
,
‡
6.7), 127 (C4H3F4 , 6.7), 77 (C2H3F2 , 100), 69 (CF3
[3] G.G. Belen'kii, L.S. German, in: M.E. Volpin (Ed.), Soviet Scientific
Reviews/Sec. B, Chemistry Reviews, Vol. 5, Harwood Academic
Publishers, GmbH, New York, 1984, p. 183.
68.4). Compound 22b: 19F NMR: 75.3 (3F, d; 10 Hz),
1
87.0 (3F, s), 119.0 (2F, t; 10 Hz), 131.5 (1F, m). H
NMR: 3.1 (2H, m; 7.5; 10 Hz), 5.8 (1H, dt; 30; 7.5 Hz).
Raman spectrum: 1728 cm 1. MS (m/z, relative intensities
[4] V.A. Petrov, G.G. Belen'kii, L.S. German, Izv. Akad. Nauk SSSR.
Ser. Khim. (1980) 2117.
‡
‡
‡
[5] A.C. Sievert, F.G. Weigert, C.G. Krespan, US Patent (to DuPont)
5157171 (1992).
%): 246 (M , C6H3F9 , 9.6), 227 (C6H3F8 , 8.5), 207
‡
‡
‡
(C6H2F7 , 2.3), 177 (C5H3F6 , 7), 157 (C5H2F5 , 0.7),
‡
[6] C.G. Krespan, V.A. Petrov, Chem. Rev. 96 (1996) 3269.
[7] V.A. Petrov, C.G. Krespan, J. Fluorine Chem. 102 (2000) 199.
[8] V.A. Petrov, V.V. Bardin, in: R.D. Chambers (Ed.), Topics in Current
Chemistry, Vol. 192, Springer, Berlin, 1997, pp. 39±95.
[9] C.G. Krespan, D.A. Dixon, J. Fluorine Chem. 77 (1996) 117.
[10] V.A. Petrov, C.G. Krespan, B.E. Smart, J. Fluorine Chem. 89 (1998)
125.
‡
‡
127 (C4H3F4 , 66), 113 (C3HF4 , 20.3), 77 (C2H3F2 ,
100), 69 (CF3 , 68.4). Compound 22c: MS (m/z, relative
‡
‡
‡
intensities %): 227 (M±F, C6H3F8 , 0.8), 207 (C6H2F7 ,
‡
1.6), 177 (C5H3F6 , 1.8), 157 (C5H2F5 , 1.9), 127
‡
‡
‡
‡
(C4H3F4 , 4.5), 113 (C3HF4 , 9.7), 77 (C2H3F2 , 13), 69
‡
‡
(CF3 , 10.6), 64 (C2H2F2 , 100).
[11] G.G. Belen'kii, V.A. Petrov, P.R. Resnick, PCT WO 98, 42645, 1998,
CA 129 (1998) 246869.
[12] G.A. Olah, J.R. De Member, R.H. Schlosberg, Y. Halpern, J. Am.
Chem. Soc. 94 (1972) 156.
2.9. Reaction of perfluorobutylethylene (24) and TFE
[13] G.A. Olah, G. Klopman, R.H. Schlosberg, J. Am. Chem. Soc. 91
(1969) 3261.
A 400 ml Hastelloy shaker tube was loaded with 10 g of
SbF5, 100 g of HF, 20 g of 24 and 20 g of TFE. The reaction
vessel was agitated for 12 h at 508C. After 100 ml of water
[14] G.A. Olah, D. Beil, R. Wesreman, J. Am. Chem. Soc. 95 (1973)
3387.

20
G.G. Belen'kii et al. / Journal of Fluorine Chemistry 108 (2001) 15±20
[15] M. Van Der Puy, T.R. Demmin, G.V. Bindu Madhavan, A.
Thenappan, J. Fluorine Chem. 76 (1996) 49.
[19] J. Burdon, T. Huckerby, R. Stephens, J. Fluorine Chem. 10 (1977)
523.
[16] V.A. Petrov, J. Org. Chem. 60 (1995) 3423.
[17] V.A. Petrov, US Patent (to Dupont) 6156943 (2000).
[18] J. Burdon, L. Gainer, R.L. Powell, J. Chem Soc., Perkin Trans. 2
(1996) 625.
[20] T. Tanuma, K. Ohnishi, H. Okamoto, T. Miyajima, S. Morikawa, J.
Fluorine Chem. 57 (1992) 259.
[21] M. Van Der Puy, A.J. Poss, P.J. Persichini, L.A.S. Ellis, J. Fluorine
Chem. 67 (1994) 215.