Synthesis, structure and reactivity of a trifluoromethyl sulfide anionic salt stabilized with tetrakis(dimethylamino)ethylene dication (TDAE2+)

Article abstract of DOI:10.1039/b002252g

A synthesis of the stable ionic trifluoromethanethiolate salts, TDAE²⁺2SCF₃^- (1) and TDAE₂₊SCF₃^-Cl^- (2) was accomplished in quantitative yields via reduction of bis(trifluoromethyl) disulfide and trifluoromethanesulfenyl chloride using tetrakis(dimethylamino) ethylene (TDAE). The salts were found to be stable up to their melting points. The trifluoromethanethiolate salt [TDAE₂₊2SCF₃^- (1)] was for the first time characterized by X-ray structural analysis (orthorhombic, Pbca, a = 1434.0(1), b = 1178.1(1), c = 2203.2(1) pm) and cyclic voltammetry. Several reactions show the synthetic utility of this new reagent.

Full text of DOI:10.1039/b002252g

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Synthesis, structure and reactivity of a trifluoromethyl sulfide
anionic salt stabilized with tetrakis(dimethylamino)ethylene
dication (TDAE2؉)
1
Alexander Kolomeitsev,*a Maurice Médebielle,*b Peer Kirsch,c Enno Lorkd and
Gerd-Volker Röschenthalerd
a Institute of Organic Chemistry, Ukrainian National Academy of Sciences, Murmanskaya 6,
253660 Kiev, Ukraine. E-mail: alex@chemie.uni-bremen.de
b Université Paris7 Denis Diderot, Electrochimie Moléculaire, UMR CNRS 7591.
Case Courrier 7107. 2, place Jussieu, F-75251 Paris Cedex 05, France.
E-mail: mauricem@paris7.jussieu.fr
c MERCK KGaA, Liquid Crystals Division, Frankfurter Strasse 250, D-64293 Darmstadt,
Germany
d Institute of Inorganic & Physical Chemistry, University of Bremen, Leobener Strasse,
D-28334 Bremen, Germany
Received (in Cambridge, UK) 21st March 2000, Accepted 7th June 2000
Published on the Web 3rd July 2000
A synthesis of the stable ionic trifluoromethanethiolate
trifluoromethyl sulfides in high yield.10 The only example of
Ϫ
Ϫ
salts, TDAE2؉2SCF3Ϫ (1) and TDAE2؉SCF3 ClϪ (2) was
a stable ionic trifluoromethanethiolate salt is TASϩSCF3 ,
accomplished in quantitative yields via reduction of
bis(trifluoromethyl) disulfide and trifluoromethanesulfenyl
chloride using tetrakis(dimethylamino) ethylene (TDAE).
The salts were found to be stable up to their melting points.
The trifluoromethanethiolate salt [TDAE2؉2SCF3Ϫ (1)] was
for the first time characterized by X-ray structural analysis
(orthorhombic, Pbca, a ؍ 1434.0(1), b ؍ 1178.1(1), c ؍
2203.2(1) pm) and cyclic voltammetry. Several reactions
show the synthetic utility of this new reagent.
synthesized by Middleton et al. from thiocarbonyl fluoride
and tris(dimethylamino)sulfonium difluorotrimethylsiliconate
(TASF).11 Meanwhile, it has been reported that potassium and
tetramethylammonium trifluoromethanethiolates decompose
even at 0 ЊC to form a range of SCF3 containing by-products.12
We have found that stable ionic trifluoromethanethiolates are
produced in quantitative yield via reaction of (CF3S)2 or
CF3SCl with TDAE in different organic solvents (e.g. diethyl
ether, monoglyme, THF, CH3CN).13 In a typical experiment, to
a Ϫ20 ЊC solution of one equivalent of (CF3S)2 in monoglyme,
one equivalent of TDAE was added dropwise under nitrogen;
immediately a copious precipitate was formed. After addition
of TDAE was complete, the solution was stirred vigorously and
warmed up to room temperature over 0.5 hours. The insoluble
solid was filtered, washed with diethyl ether and the trifluoro-
methanethiolate 1 was isolated in high purity (Scheme 1).
Introduction
Tetrakis(dimethylamino)ethylene (TDAE) has an ionization
potential of 6.13 eV and a reducing power comparable to that
of zinc.1 There are a limited number of reports on the use of
TDAE in organofluorine synthesis.2 Médebielle et al.2,3
have recently found that TDAE is an effective reductant
of bromodifluoromethylated heterocycles and chlorodifluoro-
methylated ketones; in such a way difluoromethyl heterocyclic
and α,α-difluoroacetyl anions were generated and trapped in
situ with several aldehydes and ketones. This methodology is
useful for obtaining heterocyclic difluoromethylated alcohols as
well as 2,2-difluoro-3-hydroxy ketone derivatives in reasonable
yields. To the best of our knowledge there are no reports on the
application of TDAE to reduction of fluorinated organo-
element derivatives. Chambers et al. have shown that fluorinated
cycloalkenes, such as perfluorobicyclopentylidene and per-
fluorobicyclobutylidene have low reduction potentials (close to
Ϫ1.05 V vs. SCE)4 and therefore they have been efficiently
reduced with TDAE to produce tertiary stable anions5 but the
isolation and X-ray structure of these anions have not been
reported. Similarly the TDAE salt of perfluoro-2-methyl-
pentanoyl anion has been produced in situ and its NMR spec-
trum recorded but it was not isolated.6–8 Herein we report the
utility of TDAE for the generation of stable perfluoroalkylated
sulfur anions.
Scheme 1
Compound 1 can be stored at ambient temperature under
nitrogen without decomposition. It is unstable in both air and
moisture. The resonance of the main fluorinated decomposition
product (19F NMR δ Ϫ44.4; 13C NMR δ 128.9 (1JCF = 315.5 Hz)
could be assigned to bis(trifluoromethyl) disulfide.14
Ϫ
Ϫ
(TDAE)2ϩ[SCF3]2 1 and (TDAE)2ϩSCF3 ClϪ 2 are therm-
ally stable colorless solids with a considerable ionic character
proven by the results of X-ray structural investigation for
(TDAE)2ϩ[SCF3]2Ϫ (see Fig. 1). The F3CSϪ anion is well separ-
ated from the [TDAE]2ϩ cation (distance -CH3 ؒ ؒ ؒ F3CSϪ 234.8
or 255.9 pm). The structural parameters of the dication are
similar to those of the cation in the corresponding bromide,15
chloride15 or hexafluorophosphate.16 The trifluoromethylthio
group with its not unusual C–S single bond lengths [C(3)–
S(1) = 172.0(5) pm and C(4)–S(2) = 169.2(5) pm] is sufficiently
chemically stabilized and can impart high lipophilicity and
exert a strong electron withdrawing effect as a substituent.
Results and discussion
Recently, stable covalent Hg(SCF3)2 and CuSCF3 have been syn-
thesized by reduction of (CF3S)2 with Hg(0) or Cu(0) respec-
tively.9 The disulfide has been also reduced at low temperature
by diamidoalkyl phosphites to form the corresponding alkyl
DOI: 10.1039/b002252g
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This journal is © The Royal Society of Chemistry 2000
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The addition of DMF or N-methylpyrrolidine (NMP) as a
cosolvent accelerated the nucleophilic substitution reactions.
Compound 1 has a comparable reactivity to the only previously
Ϫ
known stable anionic trifluoromethanethiolate salt TASϩSCF3
prepared from toxic difluorophosgene and expensive TASF.11
Despite the toxicity of CF3SSCF3 (or CF3SCl), our approach
has the advantage of using the commercially available and
cheap TDAE reagent.
Conclusion
We have demonstrated the utility of TDAE as an efficient
reductant for organofluorine compounds; the stable ionic salt
(TDAE)2ϩ[SCF3]2Ϫ was obtained in nearly quantitative yield via
reduction of CF3SSCF3 by two single-electron transfer steps,
and was characterized by X-ray analysis and cyclic voltam-
metry. It was shown to be a stable and viable source of CF3SϪ
for a series of transformations.
Fig. 1 Molecular structure of (TDAE)2ϩ[SCF3]2Ϫ 1 (thermal ellipsoids
with 50% probability). Selected bond distances (pm) and angles (Њ) for
the anion: S(1)–C(3) 172.0(5), S(1)–C(4) 189.7(3), C(3)–F(3) 135.1(5);
F(3)–C(3)–S(1) 115.6(3), F(3)–C(3)–F(1) 102.5(4).
Experimental
NMR spectra were obtained on a Bruker AC 80 instrument
operating at 75.39 MHz (19F, internal standard CCl3F) and a
Bruker DPX-200 spectrometer operating at 200.13 MHz for 1H
and 188.31 MHz for 19F. GCMS spectra were obtained on a
Varian 3700 gas chromatograph connected to a Finnigan MAT
8200 instrument. All reactions and manipulations were con-
ducted under an atmosphere of dry nitrogen. Cyclic voltam-
metry was performed using a ‘home-made’ potentiostat19 with
a positive feedback ohmic drop compensation and a Tacussel
GSTP4 signal generator. The working electrode was a glassy
carbon (Tokai Corp.) disc (3 mm diameter) and the reference
electrode a saturated calomel electrode (SCE). The supporting
electrolyte was tetrabutylammonium hexafluorophosphate
(NBu4PF6; Fluka puriss.). Bis(trifluoromethyl) disulfide20 was
kindly provided by Professor Alois Haas (Ruhr University,
Bochum, Germany). All reactions with air-sensitive com-
pounds were run under nitrogen atmosphere. The X-ray struc-
tural study was carried out on a Siemens P4 diffractometer
using graphite monochromated MoKα radiation (λ = 71.073
pm).21
Ϫ
In CH3CN, electrochemical reduction of (TDAE)2ϩ[SCF3]2
occurs in two reversible one-electron reduction steps, to
(TDAE) and TDAE at Ϫ0.602 V vs. SCE and Ϫ0.747 V vs.
ϩ
᝽
SCE [standard potential, EЊ = (Epc ϩ Epa)/2]. In CH3CN the
irreversible oxidation (up to scan rate close to 1000 V sϪ1
)
of (SCF3)Ϫ occurs at ϩ0.61 V vs. SCE (peak potential Ep at
0.2 V sϪ1) leading to the formation of the dimer CF3SSCF3.
However, in DMF, a ‘two-electron reduction wave’ is observed
at Ϫ0.645 V vs. SCE [standard potential, EЊ = (Epc ϩ Epa)/2].
The superposition of the two one-electron steps points out
that considerable conformational changes take place during
reduction,2 and this effect is enhanced in DMF. Similar observ-
ations have been observed for the oxidation of TDAE2ϩ and
reduction of (TDAE)2ϩBrϪ
.
2
Dmowski and Haas17 generated trifluoromethanethiolate ion
by reversible addition of fluoride ion to thiocarbonyl fluoride
or its trimer in aprotic solvents and it may also be generated
in situ by the reaction of thiophosgene and potassium fluoride
in acetonitrile.12,18 The ionic character of salt 1 was proven by
reactions with activated fluoroaromatics and benzyl halides,
which lead to near quantitative formation of the corresponding
trifluoromethylthio derivatives (Scheme 2). The main charac-
teristics of 1 (mp 136–137 ЊC) are its thermal stability, high
solubility in polar organic solvents and high nucleophilicity,
which is illustrated by the conversions in Scheme 2.
[(Me2N)2CC(NMe2)]2؉[SCF3]؊2 1
To a stirred solution of 2.02 g (10 mmol) of bis(trifluoromethyl)
disulfide [CAUTION (CF3S)2 is very toxic! It has to be handled
with great care in a well ventilated fumehood!] in monoglyme
(15 ml) was added portionwise at Ϫ20 ЊC, under nitrogen, 2.00
g (10 mmol) of tetrakis(dimethylamino)ethylene within 10 min.
The immediately formed colorless solid 1 was filtered and
washed with 10 ml diethyl ether and dried in vacuo at 0.01 mbar.
Yield 3.95 g (98.3%), mp 136–137 ЊC (decomp.); 19F NMR
(CD3CN): δ = Ϫ7.8 (s); 1H NMR (CD3CN): δ = 3.04 (s), 3.33 (s).
[(Me2N)2CC(NMe2)]2؉[SCF3]؊2 and electrophiles: a typical
procedure
To a cooled solution (0 ЊC) of 1 (4.82 g, 12 mmol) in 15 ml of
CH3CN–DMF (10:1), was added 20 mmol of benzyl chloride
(benzyl bromide) in one portion. The resulting stirred solution
was left to reach ambient temperature within 0.5 h, hydrolyzed
with water and extracted with Et2O. The organic extracts were
dried over MgSO4 and final purification by distillation under
reduced pressure gave pure benzyl trifluoromethyl sulfide 3.
Yield 3.65 g (19 mmol, 95%), bp 65–67 ЊC (17 mbar); 1H NMR:
δ = 4.20 (2H, CH2, s), 7.31–7.62 (5H, H-arom, m); 19F NMR:
δ = Ϫ42.5 (s).11
The same protocol gave 4-(trifluoromethylthio)tetrafluoro-
pyridine 4 [yield 80% (92% 19F NMR yield), bp 75–76 ЊC (100
mbar), purity 99.5% (GC-MS, Mϩ, 251); 19F NMR: δ = Ϫ41.5
(s, SCF3), Ϫ91.7 (2F, m), Ϫ135.0 (2F, m)];17 2,4-dinitrophenyl
1
trifluoromethyl sulfide 5 [yield 95%, mp 46–47 ЊC; H NMR:
δ = 9.11 (1H, d, J 2.4, 3-H), 8.54 (1H, dd, J 2.4 and 8.8, 5-H),
8.04 (1H, d, J 8.8, 6-H); 19F NMR: δ = Ϫ41.7 (s, SCF3); 13C
Scheme 2
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10 A. A. Kolomeitsev, K. Yu. Chabanenko, Yu. L. Yagupolskii and
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147.3 (aromatic C)].9,18
11 W. J. Middleton, 1976, US Patent N3 940 402.
12 D. J. Adams, J. H. Clark, P. A. Heath, L. B. Hansen, V. C. Sanders
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14 W. Tyrra and D. Naumann, J. Fluorine Chem., 1989, 45, 401.
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Acknowledgements
Financial suppport by the Deutsche Forschungsgemeinschaft,
CNRS (to M. M.), Royal Society of Chemistry Journals Grants
for International Authors (to A. K.), INTAS Foundation
(Grant N 95-UA-0005) is gratefully acknowledged.
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21 Crystal data for 1, colorless crystals, C12H24F6N4S2, M = 402.47,
orthorhombic, Pbca, T = 173(2) K, a = 1434.0(1), b = 1178.1(1),
c = 2203.3(2) pm, V = 3721.42(2), µ = 0.345 mmϪ1, α = β = γ = 90Њ,
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0.0609). CCDC reference number 207/445. See http://www.rsc.org/
suppdata/p1/b0/b002252g/ for crystallographic files in .cif format.
J. Chem. Soc., Perkin Trans. 1, 2000, 2183–2185
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