Fluorination of fluoro-cyclobutene with high-valency metal fluoride

Article abstract of DOI:10.1016/j.jfluchem.2005.10.007

Fluorinations of 1,4,4-trifluorocyclobutene and 3,3,4,4- tetrafluorocyclobutene using high-valency metal fluorides such as CoF ₃, MnF₃, AgF₂, CeF₄ and KCoF ₄, and elemental fluorine were examined. In these reactions with CoF₃ and MnF₃, vic-difluorination proceeded mainly. While, 1,4,4-trifluorocyclobutene yielded 3,3,4,4-tetrafluorocyclobutene, and 3,3,4,4-tetrafluorocyclobutene yielded 1,3,3,4,4-pentafluorocyclobutene mainly in the case of AgF₂. The further fluorinated products were increased under severer conditions. Also, the plausible reaction mechanism was suggested.

Full text of DOI:10.1016/j.jfluchem.2005.10.007

Journal of Fluorine Chemistry 127 (2006) 79–84
www.elsevier.com/locate/fluor
Fluorination of fluoro-cyclobutene with high-valency metal fluoride
*
Junji Mizukado , Yasuhisa Matsukawa, Heng-dao Quan,
Masanori Tamura, Akira Sekiya
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5-2, 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8565, Japan
Received 22 September 2005; received in revised form 13 October 2005; accepted 14 October 2005
Available online 28 November 2005
Abstract
Fluorinations of 1,4,4-trifluorocyclobutene and 3,3,4,4-tetrafluorocyclobutene using high-valency metal fluorides such as CoF3, MnF3, AgF2,
CeF4 and KCoF4, and elemental fluorine were examined. In these reactions with CoF3 and MnF3, vic-difluorination proceeded mainly. While,
1,4,4-trifluorocyclobutene yielded 3,3,4,4-tetrafluorocyclobutene, and 3,3,4,4-tetrafluorocyclobutene yielded 1,3,3,4,4-pentafluorocyclobutene
mainly in the case of AgF2. The further fluorinated products were increased under severer conditions. Also, the plausible reaction mechanism was
suggested.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Fluoro-cyclobutene; High-valency metal fluoride; Fluorination; vic-Difluorination
1. Introduction
tetrafluorocyclobutane [12]. Also, fluoro-cyclobutenes could be
prepared easily from the corresponding cyclobutanes by
dehalogenation or dehydrohalogenation.
vic-Difluorination of alkenes is one of the convenient
methods to introduce fluorine atom into organic compounds [1],
and utilized for the preparation of organofluorine compounds
such as biologically active compounds [2–4]. Alkyl perfluor-
oalkyl ethers could be also prepared by vic-difluorination of
corresponding perfluorovinyl ethers using high-valency metal
fluorides [5]. High-valency metal fluorides such as cobalt
trifluoride (CoF3), manganese trifluoride (MnF3), silver
difluoride (AgF2), cerium tetrafluoride (CeF4) and potassium
tetrafluorocobaltate (KCoF4) have been reported to be a mild
vic-difluorinating agent of various alkenes compared with
elemental fluorine [6–8].
In this paper, the fluorinations of fluoro-cyclobutenes such as
1,4,4-trifluorocyclobutene and 3,3,4,4-tetrafluorocyclobutene
with high-valency metal fluorides and elemental fluorine were
examined. The vic-difluorination proceeded mainly in the
reactions with CoF3 and MnF3. In contrast, the reaction using
AgF2 proceeded in the different manner, and the unexpected
cyclobutenes were formed mainly. And also, the reaction
mechanism was discussed.
2. Results and discussion
Fluoro-cyclobutanes could be conveniently prepared by
[2 + 2] cyclization reaction of fluorinated olefins such as
tetrafluoroethylene (TFE) [9], chlorotrifluoroethylene (CTFE)
[10] and 1,1-dichloro-2,2-difluoroethylene [11]. Furthermore,
various fluoro-cyclobutanes could be prepared by the cross-
coupling cyclization reaction of two kinds of alkenes [10,12].
For example, the reaction of TFE and ethylene afforded 1,1,2,2-
2.1. Fluorinations of 1,4,4-trifluorocyclobutene
First, the reactions of 1,4,4-trifluorocyclobutene (1) with
high-valency metal fluorides and elemental fluorine were
examined (Scheme 1 and Table 1). In the reactions with CoF3
or MnF3 at 100 8C, the vic-difluorination product 2 was a main
product along with the further fluorinated products 3–7
(Table 1, Entries 1–4). CoF3 was more reactive toward 1
than MnF3. In the reaction with CoF3 at 100 8C for
3 h, an unexpected product 8 was obtained in 23% yield
(Table 1, Entry 1). The reactions using MnF3 at 150 8C gave 5
* Corresponding author. Tel.: +81 29 861 2927; fax: +81 29 861 4771.
E-mail address: mizukado-junji@aist.go.jp (J. Mizukado).
0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.10.007

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J. Mizukado et al. / Journal of Fluorine Chemistry 127 (2006) 79–84
Scheme 1. Fluorination of 1,4,4-trifluorocyclobutene with high-valency metal fluoride.
Table 1
Fluorination of 1,4,4-trifluorocyclobutene with high-valency metal fluoridea
Entry
MFn
Temperature (8C)
Time (h)
Conv. of 1 (%)
Yield from 1 (%)b
2
3
4
5
6
7
8
9
1
2
CoF3
CoF3
MnF3
MnF3
MnF3
AgF2
AgF2
AgF2
CeF4
KCoF4
KCoF4
F2
100
100
3
24
24
72
6
>99
>99
57
68
44
47
70
9
2
2
2
0
6
0
trace
23
2
0
2
0
0
0
13
0
8
15
1
3
3
100
0
0
2
4
100
84
trace
3
trace
1
trace
16
0
0
2
5
150
>99
>99
>99
80
trace
55
0
5
0
6
100
3
5
0
0
0
94
29
63
0
trace
7
100
24
1
9
7
7
trace
trace
0
10
0
1
30
2
8
150
13
0
0
trace
0
9
200
1
10
0
0
0
0
3
0
0
0
10
11c
12d
200
24
144
15
48
25
44
24
0
0
0
0
14
3
0
200
>99
71
0
trace
5
0
0
0
ꢀ120 ! rt
3
3
trace
trace
0
a
Reaction conditions: 1 (1.0 mmol) and MFn (10 mmol) were heated in stainless-steel cylinder (volume: 50 ml).
b
c
d
Yields were determined by NMR and GC–MS.
CH2FCF2CF2CHF2 was obtained 32% yield.
F2: 1.0 mmol.
as a main product, which was produced by the fluorination of 2
to produce 6, one is the fluorination of 3 or 4, and the other is vic-
difluorination of 9. To consider the results of the reaction for 1 h,
the latter might be a predominant one. Using KCoF4 or CeF4, 8
was recovered in the reactions at lower than 200 8C, and the
complex mixture of products except for 3, 4, 6 and 9 were
obtainedinthereactionathigher temperature(Table2, Entries11
and12). Thereactionusingelementalfluorine atlow temperature
gave the similar result to CoF3 and MnF3, and the ratio of the
isomers 3 and 4 was almost same (Table 2, Entry 13).
(Table 1, Entry 5). This result means the –CHF– group of 2 was
fluorinated easier than –CH2– group in the case of MnF3.
Employing AgF2 as a high-valency metal fluoride, the
distribution of the products was much different from the
results using CoF3 or MnF3, and cyclobutenes 8 and 9 were
obtained mainly (Table 1, Entries 6–8). The reaction using AgF2
at 100 8C for 3 h afforded 8 in 94% yield. The reaction
using KCoF4 at 200 8C proceeded very slowly to afford 2, 8
and CH2FCF2CF2CHF2 (Table 1, Entries 10 and 11) [13].
CH2FCF2CF2CHF2 was considered to generate via the ring-
opening fluorination of 2. 2 was obtained in low yield by the
reaction with elemental fluorine (Table 1, Entry 12).
2.3. Reaction path and mechanism
In the fluorinations of 1,4,4-trifluorocyclobutene (1) and
3,3,4,4-tetrafluorocyclobutene(8) with high-valency metalfluor-
ides, the unexpected cyclobutenes were obtained especially in
the reaction using AgF2. At first, the reaction path of the flu-
orination of 8 would be discussed in Scheme 3, because the react-
ions of 1 with AgF2 contained the path of the fluorination of 8.
The vic-difluorination (path a) was a predominant and an
exclusive path inthecase of CoF3 andMnF3, respectively. While,
pathbtoform cyclobutene9waspredominantinthecaseofAgF2
(Scheme 3). The fluorinations of 3 and 4 with CoF3, MnF3 or
AgF2 at the same condition as the fluorinations of 8 did not afford
9, thus the dehydrofluorination path of 3 and 4 to form 9 was
not possible under this condition. The reasonable reaction
2.2. Fluorinations of 3,3,4,4-tetrafluorocyclobutene
Next, the fluorinations of 3,3,4,4-tetrafluorocyclobutene (8)
were carried out (Scheme 2 and Table 2). Using CoF3 and MnF3
as a high-valency metal fluoride, vic-difluorination proceeded to
afford 3 (trans-isomer) and 4 (cis-isomer), and 6 was obtained by
the further fluorination of 3 and 4 (Table 2, Entries 1–8). In the
reactions with CoF3 or MnF3, trans-isomer 3 was predominant
over cis-isomer 4. Employing AgF2, 6 was a main product in the
reaction at 100 8C for 24 h, while 9 was main product at 100 8C
for1 h(Table2, Entries9and10). Therewere twoplausiblepaths

J. Mizukado et al. / Journal of Fluorine Chemistry 127 (2006) 79–84
81
Scheme 2. Fluorination of 3,3,4,4-tetrafluorocylcobutene with high-valency metal fluoride.
Table 2
Fluorination of 3,3,4,4-tetrafluorocylcobutene with high-valency metal fluoridea
Entry
MFn
Temperature (8C)
Time (h)
Conv. of 8 (%)
Yield from 8 (%)b
3
4
6
9
1
2
CoF3
CoF3
CoF3
CoF3
MnF3
MnF3
MnF3
MnF3
AgF2
AgF2
KCoF4
CeF4
F2
100
100
1
24
0.25
1
17
>99
65
11
55
37
53
18
56
26
54
6
5
0
0
0
15
25
25
8
22
2
3
150
0
4
150
>99
27
17
0
0
5
100
1
0
6
100
24
0.5
1
>99
42
28
16
11
10
11
0
8
0
7
150
0
0
8
150
>99
77
25
22
73
0
0
9
100
1
37
0
10
11
12
13c
100
24
1
>99
34
8
250
0
0
250
1
11
0
0
0
0
ꢀ120 ! rt
24
84
37
33
10
0
a
Reaction conditions: 8 (1.0 mmol) and MFn (10 mmol) were heated in stainless-steel cylinder (volume: 50 ml).
Yields were determined by NMR and GC–MS.
F2: 1.0 mmol.
b
c
mechanism would be proposed in Scheme 4. It had been
as CoF3 and MnF3, the vic-difluorination proceeded mainly.
While, the fluorination using AgF2 proceeded in different
manner, and afforded the cyclobutenes mainly. The fluorination
with high-valency metal fluorides was considered to be
proceeded via the cationic intermediate. The quenching of
the cationic intermediate by fluoride ion leading to vic-
difluorination took place mainly in the cases of CoF3, MnF3 and
KCoF4. The elimination of proton from the cationic
intermediate to form cyclobutene proceeded in the case of
AgF2.
suggested that the fluorination with a high-valency metal fluoride
involves the initial oxidation of the substrate [14]. In the
fluorination of 8, the SET occurred at first, and the generated
radical cationic intermediate was quenched by fluoride ion to
afford the radical intermediate. This radical was oxidated further
byhigh-valency metalfluoride toafford thecationic intermediate
10. If 10 was quenched by fluoride ion, 3 and 4 would be
obtained. The elimination of proton from 10 would afford 9.
The reaction path of the fluorination of 1 was summarized in
Scheme 5. The vic-difluorination (path c) was a predominant
path in the case of CoF3, MnF3, and KCoF4, and path d to from
cyclobutene 8 was predominant in the case of AgF2. The further
fluorination of 8 was same as mentioned above. The plausible
mechanism was shown in Scheme 6, which was similar to the
mechanism of the fluorination of 8 in Scheme 4.
Inbothcaseofthereactionsof1and8withhigh-valencymetal
fluoride, the quenching of the cationic intermediate by fluoride
ion was predominant in the cases of CoF3 and MnF3, and the
elimination of proton from cationic intermediate was predomi-
nant in the case of AgF2. The reason for the difference had not
been clear, however, it was supposed the selectivity depended on
the fluorination ability of the radical anion of high-valency metal
fluoride, which was counterpart of cationic intermediate.
4. Experimental
4.1. General
High-valency metal fluorides (CoF3, MnF3, AgF2, CeF4
and KCoF4) were treated with elemental fluorine at 250 8C
for 6 h prior to use. Elemental fluorine was passed through
NaF to remove hydrogen fluoride. 1H (300 MHz) and 19F
NMR (282 MHz) were measured with JEOL ECA-300S
using TMS and CFCl3 as an internal standard and CDCl3 as a
solvent. MS (EI, 70 eV) spectra were measured using
the Shimazu GCMS-QP2010 system equipped with GC-
2010.
3. Conclusion
4.2. Preparation of 1,4,4-trifluorocyclobutene (1)
In the fluorination of 1,4,4-trifluorocyclobutene and 3,3,4,4-
tetrafluorocyclobutene with high-valency metal fluorides such
1,4,4-trifluorocyclobutene was prepared by dechlorination
of 1,2-dichloro-2,3,3-trifluoroethylene, which was prepared by

82
J. Mizukado et al. / Journal of Fluorine Chemistry 127 (2006) 79–84
Scheme 3. Reaction paths of fluorination of 8.
Scheme 4. Reaction mechanism of fluorination of 8.
Scheme 5. Reaction paths of fluorination of 1.
Scheme 6. Reaction mechanism of fluorination of 1.
cross-coupling cyclobutanation of chlorotrifluoroethylene and
vinyl chloride [15].
line, and 98.1 mmol (98%) of 1,4,4-trifluorocyclobutene was
isolated at a ꢀ196 8C trap.
1,2-dichloro-2,3,3-trifluorocyclobutane (100 mmol), zinc
dust (250 mmol) and diethyleneglycol (50 ml) was introduced
to a stainless-steel reactor (volume: 150 ml) equipped with a
stop valve, and heated at 150 8C. After 12 h, the products were
fractionated through traps at ꢀ78 and ꢀ196 8C with a vacuum
4.3. 1,4,4-Trifluorocyclobutene (1) [16]
1H NMR: d 2.63 (2H, m, CH2), 5.57 (1H, m, CH); 19F
NMR: d ꢀ106.1 (1F, m, CF), ꢀ113.7 (2F, m, CF2). MS: m/z,

J. Mizukado et al. / Journal of Fluorine Chemistry 127 (2006) 79–84
83
108 (M+), 89 (M+ ꢀ F), 88, 82, 75, 69, 64 (CF2CH2 ), 58, 57,
ꢀ118.6 (1F, d of m, J = 220.1 Hz, CF2), ꢀ135.1 (1F, d of m,
J = 220.1 Hz, CF2), ꢀ202.6 (1F, d of m, J = 52.6 Hz, CHF).
+
39, 31.
MS: m/z, 100 (CF2CF2 ), 95, 82 (CF2CHF+), 77, 75, 69, 64
+
+
(CF2CH2 ), 57, 51, 46 (CHFCH2 ), 31.
+
4.4. Preparation of 3,3,4,4-tetrafluorocyclobutene (8)
3,3,4,4-Tetrafluorocyclobutene was prepared by dehydro-
chlorination of 1-chloro-2,2,3,3-tetrafluorocyclobutane [17],
which was prepared by cross-coupling cyclobutanation of
tetrafluoroethylene and vinyl chloride [12].
4.9. Trans-1,1,2,2,3,4-hexafluorocyclobutane (3) [19]
1H NMR: d 5.25 (d of m, J = 47.6 Hz, CHF); 19F NMR: d
ꢀ122.0 (2F, d of m, J = 230.0 Hz, CF2), ꢀ134.3 (2F, d of m,
J = 230.0 Hz, CF2), ꢀ208.6 (2F, d of m, J = 47.6 Hz, CHF).
1-Chloro-2,2,3,3-tetrafluorocyclobutane (50 mmol), sodium
hydroxide (125 mmol) and calcium chloride (50 mmol) were
introduced to a stainless-steel reactor (volume: 50 ml) equipped
with a stop valve, and heated at 50 8C. After 62 h, the products
were fractionated through traps at ꢀ60, ꢀ120 and ꢀ196 8C
with a vacuum line, and 45.7 mmol (91% yield) of 3,3,4,4-
tetrafluorocyclobutene was isolated at a ꢀ120 8C trap.
MS: m/z, 100 (CF2CF2 ), 95, 82 (CF2CHF+), 69, 64
+
(CHFCHF+), 51, 31.
4.10. Cis-1,1,2,2,3,4-hexafluorocyclobutane (4)
1H NMR: d 5.13 (m, CHF); 19F NMR: d ꢀ119.4 (2F, d of m,
J = 235.1 Hz, CF2), ꢀ134.0 (2F, d of m, J = 235.1 Hz, CF2),
+
4.5. 3,3,4,4-Tetrafluorocyclobutene (8) [16,18]
ꢀ223.8 (2F, m, CHF). MS: m/z, 100 (CF2CF2 ), 95, 82
(CF2CHF+), 69, 64 (CHFCHF+), 51, 31.
1H NMR: d 6.85 (m, CH); 19F NMR: d ꢀ111.1 (m, CF2). MS:
+
m/z, 126 (M+), 107 (M+ ꢀ F), 100 (CF2CF2 ), 76, 75, 57, 31.
4.11. 1,1,2,2,3,3-Hexafluorocyclobutane (5)
4.6. A typical experimental procedure for the reaction of
1,4,4-trifluorocyclobutene (1) or 3,3,4,4-
tetrafluorocyclobutene (8) with high-valency metal fluoride
(Table 1, Entry 1)
1H NMR: d 3.15 (quin of t, J = 12.0 Hz, 3.7 Hz, CH2); 19F
NMR: d ꢀ117.6 (4F, t, J = 12.0 Hz, CF2CH2), ꢀ131.5 (2F, t,
J = 3.7 Hz, CF2CF2CF2). MS: m/z, 145 (M+ ꢀ F), 100
+
+
(CF2CF2 ), 95, 75, 69, 64 (CF2CH2 ), 50, 31.
CoF3 (10 mmol) was placed in a stainless-steel reactor
(volume: 50 ml) equipped with a stop valve. 1 (1.0 mmol) was
introduced to the reactor at ꢀ196 8C with a vacuum line. The
reactor was warmed up to 100 8C and stirred for 3 h. After the
reaction was completed, the crude products were fractionated
through traps at ꢀ120 and ꢀ196 8C with the vacuum line. The
structures and the ratio of the products were determined by GC–
4.12. 1,1,2,2,3,3,4-Heptafluorocyclobutane (6)
1H NMR: d 5.31 (m, CHF); 19F NMR: d ꢀ123.3 (2F, d of m,
J = 234.4 Hz, CF2), ꢀ130.7 (1F, d of m, J = 231.0 Hz, CF2),
ꢀ134.9 (1F, d of m, J = 231.0 Hz, CF2), ꢀ136.2 (2F, d of m,
J = 234.4 Hz, CF2), ꢀ218.6 (1F, d of m, J = 48.4 Hz, CHF).
MS: m/z, 131, 113, 100 (CF2CF2 ), 93, 82 (CF2CHF+), 75, 69,
+
1
MS, H NMR and 19F NMR spectra.
51, 31.
4.7. A typical experimental procedure for the reaction of
1,4,4-trifluorocyclobutene (1) or 3,3,4,4-
tetrafluorocyclobutene (8) with elemental fluorine (Table 1,
Entry 12)
4.13. Perfluorocyclobutane (7) [20,21]
19F NMR: d ꢀ134.0 (s, CF2). MS: m/z, 131, 100 (CF2CF2 ),
+
93, 69, 50, 31.
1 (1.0 mmol) and elemental fluorine (1.0 mmol) was
introduced into a stainless-steel reactor (volume: 50 ml)
equipped with a stop valve at ꢀ196 8C with a vacuum line.
The reactor was placed in the ethanol slush (ꢀ120 8C), and
warmed to ambient temperature gradually for 15 h. After the
reaction, products were treated with NaF to remove hydrogen
fluoride and fractionated through traps at ꢀ120 and ꢀ196 8C
with the vacuum line. The structures and the ratio of the
products were determined by GC–MS, 1H NMR and 19F NMR
spectra.
4.14. 2,3,3,4,4-Pentafluorocyclobutene (9)
1H NMR: d 5.93 (m, CH); 19F NMR: d ꢀ104.2 (1F, m, CF),
ꢀ113.7 (2F, m, CF2), ꢀ118.4 (2F, m, CF2). MS: m/z, 144 (M+),
+
125 (M+ ꢀ F), 100 (CF2CF2 ), 94, 93, 75, 69, 31.
References
[1] S. Rozen, M. Brand, J. Org. Chem. 51 (1986) 3607–3611.
[2] A. Toyota, J. Chiba, Y. Sugita, M. Sato, C. Kaneko, Chem. Pharm. Bull. 42
(1994) 459–461.
[3] M. Sato, T. Hirokawa, H. Hattori, A. Toyota, C. Kaneko, Tetrahedron
Asymmetry 5 (1994) 975–980.
4.8. 1,1,2,2,3-Pentafluorocyclobutane (2)
[4] A. Toyota, M. Aizawa, C. Habutani, N. Katagiri, C. Kaneko, Tetrahedron
51 (1995) 8783–8797.
1H NMR: d 2.86 (2H, m, CH2), 5.09 (1H, d of m,
J = 52.6 Hz, CHF); 19F NMR: d ꢀ113.6 (1F, d of m,
J = 216.9 Hz, CF2), ꢀ115.9 (1F, d of m, J = 216.9 Hz, CF2),
[5] M. Tamura, S. Takubo, H.D. Quan, A. Sekiya, Synlett 3 (2000) 343–344.
[6] G. Fuller, M. Stacey, J.C. Tatlow, C.R. Thomas, Tetrahedron 18 (1962)
123–133.

84
J. Mizukado et al. / Journal of Fluorine Chemistry 127 (2006) 79–84
[7] D.A. Rausch, R.A. Davis, D.W. Osborne, J. Org. Chem. 28 (1963)
[14] J. Burdon, I.W. Parsons, J.C. Tatlow, Tetrahedron 28 (1972) 43–52.
[15] J.D. Park, T.S. Croft, G. Groppelli, J. Fluorine Chem. 5 (1975) 293–306.
[16] N.C. Craig, H. Kim, P. Lorencak, S.P. Sibley, R.L. Kuczkowski, J. Mol.
Struct. 223 (1990) 45–61.
494–497.
[8] (a) J.C. Tatlow, J. Burdon, P.L. Coe, Ger. Offen. (1963) 1925836;
(b) J.C. Tatlow, J. Burdon, P.L. Coe, Chem. Abstr. 72 (1970) 78104t.
[9] J. Harmon, U.S. Patent 2,404,374 (1946).
[17] J.D. Park, L.H. Wilson, J.R. Lacher, J. Org. Chem. 28 (1963) 1008–1012.
[18] G. Fuller, J.C. Tatlow, J. Chem. Soc. (1961) 3198–3203.
[19] M.V.D. Puy, R.K. Belter, R.J. Borowski, L.A.S. Ellis, P.J. Persichini III,
A.J. Poss, T.P. Rygas, H.S. Tung, J. Fluorine Chem. 71 (1995) 59–63.
[20] V.W. Gash, D.J. Bauer, J. Org. Chem. 31 (1966) 3602–3607.
[21] V.G. Barabanov, G.V. Volkov, K.A. V’yunov, V.P. Makoveenko, J. Gen.
Chem. USSR (Engl. Transl.) 56 (1986) 997–1001.
[10] J.D. Park, H.V. Holler, J.R. Lacher, J. Org. Chem. 25 (1960) 990–993.
[11] A.L. Henne, R.P. Ruh, J. Am. Chem. Soc. 69 (1947) 279–281.
[12] D.D. Coffman, P.L. Barrick, R.D. Cramer, M.S. Raasch, J. Am. Chem.
Soc. 71 (1949) 490–496.
[13] J. Burdon, S.T. Ezmirly, T.N. Huckerby, J. Fluorine Chem. 40 (1988)
283–318.