Reductive dimerization of dithianylium salts and fluorodesulfuration: A new synthetic approach to tetrafluoroethylene substructures

Article abstract of DOI:10.1016/j.jfluchem.2004.01.016

The reductive dimerization of dithianylium ions, followed by oxidative fluorodesulfuration, opens a convenient access to a variety of 1,2-disubstituted tetrafluoroethylene derivatives. A similar method leads to hexafluorodiacetyl, a potential building blo

Full text of DOI:10.1016/j.jfluchem.2004.01.016

Journal of Fluorine Chemistry 125 (2004) 1025–1029
Reductive dimerization of dithianylium salts and fluorodesulfuration:
a new synthetic approach to tetrafluoroethylene substructures
Peer Kirscha,*, Marc Lengesa, Andreas Ruhla, Dmitri V. Sevenardb,
b
¨
Gerd-Volker Roschenthaler
aMerck KGaA, Liquid Crystals Division, D-64271 Darmstadt, Germany
bInstitute for Inorganic and Physical Chemistry, University of Bremen, D-28359 Bremen, Germany
Received 27 May 2003; accepted 13 January 2004
Available online 11 March 2004
Abstract
The reductive dimerization of dithianylium ions, followed by oxidative fluorodesulfuration, opens a convenient access to a variety of
1,2-disubstituted tetrafluoroethylene derivatives. A similar method leads to hexafluorodiacetyl, a potential building block for fluorinated
heterocycles.
# 2004 Elsevier B.V. All rights reserved.
Keywords: Fluorodesulfuration; Dithianylium salts; Reductive dimerization; Tetrafluoroethylene bridge; Liquid crystals; Hexafluorodiacetyl
1. Introduction
2. Results and discussion
Recently, it was found that the introduction of highly
fluorinated bridges into the mesogenic core structure of
nematic liquid crystals [1] yields new classes of materials
with dramatically improved property profiles for their appli-
cation in active matrix LCDs [2]. For the convenient synth-
esis of such compounds, a need arose for versatile synthetic
building blocks, already containing those bridge structures
or their direct precursors.
In the same context, it was found that dithianylium salts
[3] are very useful synthetic precursors for a wide range of
gem-difluoromethylene derivatives [4]. Their synthesis is
easily accomplished from inexpensive starting materials,
such as carboxylic acid derivatives [5] or ketenedithioketals
[1a,4]. In one part of their series of communications on the
chemistry of dithianylium salts, Undheim and co-workers
also describe the reductive dimerization of tolyl dithiany-
lium tetrafluoroborate with zinc dust in acetonitrile [6].
Based on this report, we set out to improve the original
dimerization procedure and make use of the resulting
bis(1,3-dithian-2-yls) for the synthesis of tetrafluoroethylene
derivatives and other useful intermediates.
Probably due to the heterogeneous reaction system with
zinc as the reducing agent in acetonitrile, the reported
synthetic procedure for the reductive dimerization [6] suf-
fers from relatively poor reproducibility. Replacement of the
zinc by tetrakis(dimethylamino)ethylene (TDAE) resulted
in a significant increase of the dimerization yields and also in
a more convenient work-up procedure. The dimerization of
aromatic dithianylium triflates (1a–e) furnished the desired
products in 47–96% yield (Scheme 1).
In contrast to the aromatic compounds, all attempts to
dimerize aliphatic dithianylium salts failed completely. In
the resulting complex reaction mixtures, usually elimination
(e.g. compound 3 as the main product for the reaction of the
2-cyclohexyl-1,3-dithianylium salt 4 with TDAE) as well as
reduction products (e.g. compound 7 as the main product for
the reaction of the bicyclooctyldithianylium salt 6) were
identified as major components. On the other hand, a high
yield (72%) of the product 9 was obtained for the dimeriza-
tion of 2-trifluoromethyl-1,3-dithianylium triflate 8 [7]. In
addition, the precipitated TDAE2þ(CF3SO3À)2 salt (10) was
isolated in 80% yield.
For the reductive dimerization, two different, possible
reaction pathways have to be considered: In one pathway
(A), dithianyl radicals are generated which dimerize to the
observed reaction products. In the alternative pathway (B),
* Corresponding author. Tel.: þ49-6151-72-2218;
fax: þ49-6151-72-2593.
E-mail address: peer.kirsch@merck.de (P. Kirsch).
0022-1139/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2004.01.016

1026
P. Kirsch et al. / Journal of Fluorine Chemistry 125 (2004) 1025–1029
S+
1a, 2a: R = 4-MeOPh
1b, 2b: R = 4-n-H7C3Ph
1c, 2c: R = 4-(trans-4-n-pentylcyclohexyl)phenyl
1d, 2d: R = 4-FPh
1e, 2e: R = 3,4,5-F3Ph
S
S
R
R
S
R
a
S
S
-
CF3SO3
1a-e
2a-e
S
S+
S
S
H7C3
H C
H7C3
3 +
7
3
b
c
S
S
3
5
-
CF3SO3
4
S+
S
S
-
CF3SO3
H11C5
H11C5
a
S
7
6
N
N+
N
S+
S
S
S
F3C
-
(CF3SO3 )2
+
F3C
CF3
N+
d
S
S
-
CF3SO3
10
8
9
Scheme 1. Synthesis of 2a–e and 9, and failed attempts to dimerize the aliphatic dithianylium salts 4 and 6: (a) TDAE, CH3CN; À15 8C to room temp., 2 h
(2a: 47%, 2b: 70%, 2c: 96%, 2d: 91%, 2e: 80%, 7: yield not determined). (b) CF3SO3H, CH2Cl2; À70 8C (5 min), room temp. (30 min). (c) TDAE; À15 8C.
(d) TDAE, THF; À78 8C to room temp., 14 h (9: 72%, 10: 80%). Although the salt TDAE2þ(CF3SO3À)2 (10) is formed during each of the reactions, it was
only isolated after the reduction of 8.
one dithianylium cation is reduced directly to the dithianyl
anion, which recombines with a remaining cation.
The tentative mechanistic model depicted in Scheme 2
indicates a possible explanation why aliphatic dithianylium
salts are not dimerized under these conditions: they cannot
be expected to give long-lived radicals as required for
pathway A. On the other hand, the strongly basic 2-alkyl-
1,3-dithianyl anions formed in pathway B rapidly abstract
protons, either from the solvent or from dithianylium
cations, still present leading to elimination products (3) or
to reduced dithianes (5 and 7).
In contrast, for 2-aryl or 2-perfluoroalkyl-1,3-dithiany-
lium cations, the anions formed by two-electron reduction
with TDAE would be stabilized and much less basic, and
therefore less prone to cause such side-reactions.
The next step to our desired fluorinated bridge structures,
the oxidative fluorodesulfuration [8] of the bis(dithianyls),
showed a very strong dependency of the isolated yields on
the aromatic substitution pattern. Bis(2-aryl-1,3-dithian-
2-yls) with electron-donating substituents on the aromatic
moiety (such as 2c) are smoothly fluorodesulfurated to the
corresponding 1,2-diaryl tetrafluoroethylene derivatives
S
S
·
S
·
S
R
R
+ R
S
S
S+
S
S+
S
pathway A
R
S
S
R
R
+
R
TDAE TDAE2+
pathway B
S
-
S
S+
S
+ R
R´
R´´
S+
solvent-H
solvent-
S
S
H
R´
R´´
S
H S
S
R
Scheme 2. Mechanistic alternatives (pathwaya A and B) for the reductive dimerization of dithianylium salts with TDAE. Strongly basic 2-alkyl-1,3-dithianyl
anions resulting from the ionic pathway B might lead to undesired side reactions with unreacted dithianylium cations or the solvent.

P. Kirsch et al. / Journal of Fluorine Chemistry 125 (2004) 1025–1029
1027
F
F
X
X
F
F
2c-e
11: X = 4-(trans-4-pentylcyclohexyl)
12: X = 4-F
13: X = 3,4,5-F3
a
X
S
S
S
S
X
MeO
2a
F
F
a
Br
Br
F
F
14
OMe
Scheme 3. Oxidative fluorodesulfuration of 2a, 2c, 2d and 2e: a) DBH, HF-pyridine, CH2Cl2; À78 8C to room temp., 18 h (11: 31%, 12: 57%, 13: 15%, 14:
34%). Products 11 and 14 were obtained using 65% HF-pyridine, for 12 and 13, 70% HF-pyridine was used.
S
X
S
R
S
S
R
20
R
R
F
S+
F
F
19
SX
X+
F-
S
S
R
S
S
R
S
S
R
S
S
S
S+
R
R
S
R
R
R
X
R
S
S+
S
X
R
F-
X+
X+
S+
17
SX
F
S
F
S
21
16
15
18
SX
SX
F-
S+
SX
XS
XS
S
F
F
XS
S
F
F-
S
R
F
R
R
R
R
25
R
R
R
X+
- XS(CH2)3SX
SX
F
S+
S
SX
F
F
F
S
23
22
X
24
X+
F-
SX
SX
X
S+
F
F
F
R
R
R
R
F 27
- XS(CH2)3SX
F
F
F
F-
26
Scheme 4. Tentative mechanism for the fluorodesulfuration of bis(dithianyls). Xþ denotes a soft electrophile, such as a bromonium equivalent ‘‘Brþ’’ from
DBH. The reaction steps which might be sensitive to the aromatic substitution pattern are indicated by broad arrows.
with a 1,3-dibromo-5,5-dimethylhydanthoin (DBH)/65%
HF-pyridine system. In the case of the 4-methoxy derivative,
only the product from concomitant aromatic bromination
(14) was isolated.1 First attempts to fluorinate bis(dithianyls)
with more electron-demanding substituents (e.g. 2e) gave
only complex mixtures. Also, the application of some
alternative reaction systems (t-BuONO/65% HF-pyridine,
NIS/NEt3Á3HF or SO2Cl2/65% HF-pyridine) on 2d did not
yield the desired product 12 in significant quantities. Finally,
moderate to low yields of rather impure 12 and 13 could be
obtained using the more acidic 70% HF-pyridine as the
fluoride source. The main side products in the synthesis of 12
and 13 were a number of compounds containing bromine in
the bridge, according to mass spectroscopic analysis, whose
exact structure was not elucidated (Scheme 3).
The reason for this observed sensitivity of the oxidative
fluorodesulfuration to the aromatic substitution pattern
might be rationalized with a tentative mechanistic model
(Scheme 4), based on analogy to earlier proposals [1a,8b]; as
the degree of fluorination on the bis(dithianyl) system is
increasing during the course of the reaction sequence, the
remaining sulfur species are more and more deactivated
against further electrophilic attack by the halogenating agent
1 The competing aromatic bromination is probably due to ‘‘super-
electrophilic activation’’ of the bromination agent (DBH) by protonation of
one of its carbonyl groups in the strongly acidic reaction medium. Similar
effects are discussed in [9].

1028
P. Kirsch et al. / Journal of Fluorine Chemistry 125 (2004) 1025–1029
dried in vacuum. Yield: 91% of 2d as yellowish crystals,
1
O
S
S
m.p. 230 8C, dec. H NMR (250 MHz, CDCl3): d ¼ 1:76–
F3C
S
F3C
CF3
CF3
28
2.04 (m, 4H), 2.45–2.73 (m, 8H), 6.88 (dd, 4H, J ꢀ 9 Hz,
J ꢀ 9 Hz, ar-H), 7.50 (br. dd, 4H, J ꢀ 9 Hz, J ꢀ 9 Hz, ar-
H). MS (EI, 70 eV): m=e ð%Þ ¼ 426 [Mþ] (0.3), 320 (1), 246
(7), 213 (100), 139 (45). Anal. calcd. for C20H20F2S4: C,
56.3; H, 4.7; S, 30.1. Found: C, 56.2; H, 4.5; S, 29.8.
2a: Yield: 47%, yellowish crystals, m.p. 185–188 8C
(ethyl acetate/n-heptane 1:1). 1H NMR (250 MHz, CDCl3):
d ¼ 1:74À2:05 (m, 4H), 2.50–2.72 (m, 8H), 3.81 (s, 6H,
OCH3), 6.73 (d, 4H, J ¼ 8:8 Hz, ar-H), 7.44 (br. d, 4H, ar-
H). MS (EI, 70eV): m=e ð%Þ ¼ 450 [Mþ] (0.2), 344 (1), 270
(4), 238 (9), 225 (100), 151 (19). Anal. calcd. for
C22H26O2S4: C, 58.6; H, 5.8; S, 28.5. Found: C, 58.5; H,
5.5; S, 27.5.
2b: Yield: 70%, yellowish crystals, m.p. 126 8C (EtOH/
CH2Cl2 2:1; crystallization at À20 8C). 1H NMR (250 MHz,
CDCl3): d ¼ 0:93 (t, 6H, J ¼ 7:5 Hz), 1.55–2.03 (m, 12H),
2.50–2.72 (m, 8H), 6.95 (d, 4H, J ¼ 8:8 Hz, ar-H), 7.39 (br.
d, 4H, J ¼ 8:8 Hz, ar-H). MS (EI, 70eV): m=e ð%Þ ¼ 474
[Mþ] (2), 368 (5), 294 (3), 262 (7), 237 (100), 163 (25), 134
(16).
a
S
O
9
Scheme 5. Synthesis of hexafluorodiacetyl (28) by oxidation of 9: (a)
CrO3, oleum; 50 8C (21%).
DBH. With additional electron-withdrawing substituents at
the aryl moiety, the reaction stalls before completion, yield-
ing after work-up only a mixture of various reaction inter-
mediates and their hydrolysis products.
The steps which are most probably blocked, if the sub-
stituents R are too strongly electron-withdrawing (as in
the case of 3,4,5-trifluorophenyl), are the activations of
23 ! 24, and of 25 ! 26. A possible branch of the pathway,
leading from 18 to 19 seems unlikely, because the sulfur in
the dithiane ring should be oxidized much easier than the
alternative position which already has a deactivating a-
fluorine substituent. The improved but still unsatisfactory
reaction yields, which are achieved with a stronger acidic
fluoride source (70% HF-pyridine instead of 65% HF-pyr-
idine) can be explained by an increase of the electrophilic
brominating power of DBH by protonation at a carbonyl
group (Scheme 5) [9] (see Footnote 1).
Bis(2-trifluoromethyl-1,3-dithian-2-yl) 9 is a convenient
starting material for the preparation of hexafluorodiacetyl 28
[10], which again is a useful intermediate for the synthesis of
fluorinated heterocycles. Oxidation of 9 with CrO3/oleum at
50 8C furnishes 28 in 21% yield. The product was isolated
and purified by trap-to-trap distillation.
2c: Yield: 96%, yellowish solid, m.p. 181–182 8C, dec.
(chromatography with n-heptane on silica gel). H NMR
1
(250 MHz, CDCl3): d ¼ 0:9 (t, 6H, J ¼ 6:6 Hz), 0.95–1.52
(m, 26H), 1.74–2.00 (m, 12H), 2.37–2.70 (m, 10H), 6.93
(d, 4H, J ¼ 8:8 Hz, ar-H), 7.34 (br. m, 4H, ar-H). MS (EI,
70eV): m=e ð%Þ ¼ 694 [Mþ] (3), 588 (5), 482 (3), 347 (100),
274 (20), 147 (19).
2e: Yield: 80%, yellowish crystals, m.p. 2208C. 1H NMR
(250 MHz, CDCl3): d ¼ 1:83À2:00 (m, 4H), 2.46–2.58
(m, 4H), 2.70–2.79 (m, 4H), 7.32 (dd, 4H, J ¼ 9:7 Hz,
J ¼ 5:3 Hz, ar-H). MS (EI, 70eV): m=e ð%Þ ¼ 498 [Mþ]
(2), 392 (2), 318 (7), 286 (6), 249 (100), 175 (34). Anal.
calcd. for C20H16F6S4: C, 48.2; H, 3.2; S, 25.7. Found: C,
47.9; H, 2.9; S, 25.7.
3. Conclusion
Aromatic and perfluoroaliphatic dithianylium salts are
dimerized in high yields by reduction with tetrakis(dimethy-
lamino)ethylene (TDAE). A clean conversion of the result-
ing bis(dithianyls) to the corresponding tetrafluoroethylene
derivatives is achieved only if the starting material has
relatively electron-rich aromatic substituents. On the other
hand, the oxidation of the easily available bis(2-trifluoro-
methyl-1,3-dithianyl) 9 offers a convenient new access to
hexafluorodiacetyl and fluorinated heterocycles derived
therefrom.
Exemplary procedure for the fluorodesulfuration of 2c to
the 1,2-diaryltetrafluoroethylene 11: A solution of 2c (5.0 g,
7.1 mmol) in CH2Cl2 (50 ml) was cooled to À78 8C. 65%
HF in pyridine (10 ml, 370 mmol HF) was added dropwise,
followed by the slow addition of a suspension of DBH
(10.0 g, 35 mmol) in CH2Cl2 (100 ml) over a time span
of 30 min. After stirring for 1 h at À78 8C, the mixture was
allowed to warm up to 0 8C and poured into ice-cold 32%
aqueous NaOH (34 ml). The organic layer was separated,
and the aqueous phase was extracted twice with CH2Cl2
(30 ml each). The combined organic extracts were washed
with water, dried over Na2SO4, and evaporated to dryness
together with celite (20 g). The crude product was purified
by chromatography on silica gel in n-heptane/toluene
4:1. Subsequent crystallization from n-heptane yielded 11
4. Experimental
Exemplary procedure for the dimerization of the dithia-
nylium triflate 1d to the bis(1,3-dithian-2-yl) 2d: A solution
of 1d (33.3 g, 92 mmol) in acetonitrile (100 ml) was treated
at 0 8C with tetrakis(dimethylamino)ethylene (TDAE)
(10.2 g, 51 mmol). After stirring for 18 h, water (250 ml)
was added and, after additional 30 min, the precipitated
crude product was filtrated off, washed with water and
1
(1.6 g, 31%) as colorless crystals, m.p. 173 8C. H NMR
(250 MHz, CDCl3): d ¼ 0:89 (t, 6H, J ¼ 6:6 Hz), 0.96–1.52
(m, 26H), 1.70–1.95 (m, 8H), 2.40–2.57 (m, 2H), 7.23
(d, 4H, J ¼ 8:8 Hz), 7.39 (d, 4H, J ¼ 8:8 Hz). 19F NMR
(235 MHz, CDCl3;standardCFCl3):d ¼ À111:1(s,4F, CF2).

P. Kirsch et al. / Journal of Fluorine Chemistry 125 (2004) 1025–1029
1029
MS (EI, 70 eV): m=z ð%Þ ¼ 558 [Mþ] (3), 279 (100), 153
(7), 127 (15). HRMS for base peak (C18H25F2): calcd.
279.1923, found 279.1924; indicates breaking of the central
CF2-CF2 bond into two difluorobenzylic fragments as the
main fragmentation pathway.
12: Yield: 57%, waxy solid (purity 76% by HPLC; after
filtration in n-heptane over silica gel). 1H NMR (250 MHz,
CDCl3): d ¼ 7:1 (mc, 4H), 7.43 (mc, 4H). 19F NMR
(235 MHz, CDCl3; standard CFCl3): d ¼ À107:5 (mc, 2F,
ar-F), À109.5 (s, 4F, CF2). MS (EI, 70 eV): m=z ð%Þ ¼ 290
[Mþ] (3), 145 (100), 95 (10).
13: Yield: 15%, waxy solid, m.p. 118–120 8C (purity 81%
by GLC; after filtration in n-heptane over silica gel, and
subsequent crystallization from EtOH at À20 8C). 1H NMR
(250 MHz, CDCl3): d ¼ 7:24 (mc, 4H). 19F NMR
(235 MHz, CDCl3; standard CFCl3): d ¼ À110:4 (s, 4F,
CF2), À131.9 (mc, 4F, ar-3,5-F), À154.8 (mc, 2F, ar-4-F).
MS (EI, 70 eV): m=z ð%Þ ¼ 362 [Mþ] (4), 343 [Mþ-F] (3),
181 (100), 161 (6), 131 (11), 81 (13).
14: Yield: 34%, colorless crystals, m.p. 154.5 8C (flash-
chromatography in n-heptane/ethyl acetate 4:1 on silica gel,
crystallization fromn-heptane). 1H NMR(250 MHz, CDCl3):
d ¼ 3:95 (s, 6H, OCH3), 6.93 (d, 2H, J ¼ 9:3 Hz, ar-H), 7.38
(dd, 2H, J ¼ 9:3 Hz, J ¼ 3:1 Hz, ar-H), 7.67 (d, 2H, J ¼
3:1 Hz, ar-H). 19F NMR (235 MHz, CDCl3; standard CFCl3):
d ¼ À109:0 (s, 4F, CF2). MS (EI, 70 eV): m=z ð%Þ ¼ 470
[Mþ] (6), 235 (100), 141 (23), 126 (20), 113 (27).
(80), 200 [TDAEþ] (100), 185 [TDAE À CH3þ] (60), and
other fragments. MS (FAB negative, Xe, 8 keV, direct,
p-nitrobenzyl alcohol): m=e ð%Þ ¼ 149 [CF3SO3À] (100),
and other fragments. Anal. calcd. for C12H24F6N4S2O6: C,
28.9; H, 4.9; F, 22.9. Found: C, 29.0; H, 4.8; F, 22.9.
Synthesis of hexafluorodiacetyl 28 [10]: A mixture of
chromium trioxide (720 mg, 7.1 mmol), 30% oleum
(14 ml), and concentrated sulfuric acid (4 ml) was placed
under nitrogen in a 100 ml flask connected through a Vigreux
column to two traps in series, cooled to À100 8C. Compound
9 (0.64 g, 1.7 mmol) was added in small portions to the
magnetically stirred slurry maintained at 0 8C under a nitro-
gen atmosphere. The resulting mixture was stirred for 1 h at
50 8C. Then, nitrogen gas was allowed to flow through the
reaction vessel for ca. 1 h until all volatile products were
condensed in the two cooling traps. Their combined contents
were distilled trap-to-trap (first trap cooled to À40 8C, second
to À90 8C, third to À198 8C, at 0.01 Torr). Hexafluorodia-
cetyl (28) (65 mg, 21%) was condensed as a yellow liquid at
ambient temperature in the second trap. 19F NMR (188 MHz,
Et2O; standard CFCl3): d ¼ À77:1 (s).
References
[1] (a) P. Kirsch, M. Bremer, A. Taugerbeck, T. Wallmichrath, Angew.
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Chem. Int. Ed. 40 (2001) 1480–1484;
Synthesis of bis(2-trifluoromethyl-1,3-dithian-2-yl) 9: To
a well stirred suspension of 2-trifluoromethyl-1,3-dithiany-
lium triflate 8 (1.7 g, 5.0 mmol) in dried THF (25 ml),
prepared at À78 8C, TDAE (0.5 g, 2.5 mmol) was added
via cannula under nitrogen. The mixture was allowed to
warm up, and it was stirred at room temperature for 14 h.
Diethyl ether (15 ml) was added, and the precipitate of 10
(1.0 g, 80%) was filtered off. The filtrate was evaporated to
dryness, the solid residue was recrystallized from n-heptane
to afford 9 (0.68 g, 72%).
(b) P. Kirsch, M. Bremer, F. Huber, H. Lannert, A. Ruhl, M. Lieb, T.
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9:Colorlesscrystallinepowder, m.p. 144–145 8C. 1H NMR
(200 MHz, CDCl3): d ¼ 1:84À2:15 (m, 4H), 2.74–2.94 (m,
4H), 3.04–3.27 (m, 4H). 13C NMR (50 MHz, CDCl3): d ¼
21:4 (s, SCH2CH2CH2S), 28.9 (s, SCH2), 63.2 (m, CCF3),
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1
126.9 (q, JCÀF ¼ 287:6 Hz, CF3). 19F NMR (188 MHz,
¨
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CDCl3; standard CFCl3): d ¼ À63:4 (s). MS (EI, 70eV):
m=e ð%Þ ¼ 187 (100), 106 (34), and other fragments. MS
(DCI positive, NH3): m=e ð%Þ ¼ 392 [M þ NH4þ] (76), 375
[M þ Hþ] (6), 187 (100), 106 (24), and other fragments. MS
(DCI negative, NH3, 8 mA/s): m=e ð%Þ ¼ 374 [MÀ] (4), 195
(100), and other fragments. HRMS for MÀ (C10H12F6S4):
calcd. 373.9726, found 373.9739.
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(b) S.C. Sondej, J.A. Katzenellenbogen, J. Org. Chem. 51 (1986)
3508–3513;
(c) G.K.S. Prakash, D. Hoole, V.P. Reddy, G.A. Olah, Synlett (1993)
691–693;
(d) M. Kuroboshi, T. Hiyama, J. Fluorine Chem. 69 (1994) 127–128;
(e) R.D. Chambers, G. Sandford, M.E. Sparrowhawk, M.J. Atherton,
J. Chem. Soc., Perkin Trans. 1 (1996) 1941–1944;
(f) C. York, G.K.S. Prakash, G.A. Olah, Tetrahedron 52 (1996) 9–14.
[9] (a) G.A. Olah, Angew. Chem. 105 (1993) 805–827;
G.A. Olah, Angew. Chem. Int. Ed. 32 (1993) 767–788;
(b) G.A. Olah, Q. Wang, G. Sandford, G.K.S. Prakash, J. Org.
Chem. 58 (1993) 3194–3195.
1
10: Colorless crystalline powder, m.p. 235–236 8C. H
NMR (200 MHz, DMSO-d6): d ¼ 3:18 (s, 12H), 3.45 (s,
12H). 13C NMR (90 MHz, DMSO-d6): d ¼ 42:1 (s, CH3),
1
42.7 (s, CH3), 120.6 (q, JCÀF ¼ 322:2 Hz, CF3), 154.8 (s,
Cþ). 19F NMR (188 MHz, DMSO-d6; standard CFCl3):
d ¼ À77:3 (s). MS (FAB positive, Xe, 8 keV, direct, p-
[10] R.N. Haszeldine, R.E. Banks, T. Myerscough, J. Chem. Soc. (C)
(1971) 1951–1957.
þ
nitrobenzyl alcohol): m=e ð%Þ ¼ 349 [TDAE þ CF3SO3
]