Aryltrifluoromethylsulfoxides: Sulfinylation of aromatics by triflinate salts in acid medium

Article abstract of DOI:10.1055/s-2001-12316

Direct sulfinylation reaction of simple aromatic compounds by triflinate salts in triflic acid led to aryltrifluoromethyl sulfoxides. The para isomer was the major one. The regioselectivity of the reaction was very high when the substituent on the aromatic ring was a halogen atom, a trifluoromethoxy group or an acetanilide function.

Full text of DOI:10.1055/s-2001-12316

550
LETTER
Aryltrifluoromethylsulfoxides: Sulfinylation of Aromatics by Triflinate Salts
in Acid Medium
Claude Wakselman,a Marc Tordeux,a Céline Freslon,a Laurent Saint-Jalmesb
aSIRCOB-CNRS, Bâtiment Lavoisier, Université de Versailles, 45 avenue des Etats-Unis, 78035 Versailles, France
bRhodia, Centre de Recherche, 85 avenue des Frères Perret, BP 62, 69192 Saint-Fons Cedex, France
Fax 33 1 39 25 44 52; E-mail: wakselma@chimie.uvsq.fr; tordeux@chimie.uvsq.fr; laurent.saint-jalmes@eu.rhodia.com
Received 17 January 2001
Abstract: Direct sulfinylation reaction of simple aromatic com-
pounds by triflinate salts in triflic acid led to aryltrifluoromethyl
sulfoxides. The para isomer was the major one. The regioselectivity
of the reaction was very high when the substituent on the aromatic
ring was a halogen atom, a trifluoromethoxy group or an acetanilide
function.
Key words: aryltrifluoromethylsulfoxides, sulfinylation, electro-
philic reactions
Scheme 1
Aryltrifluoromethylsulfoxides are used as precursors of
protonation should be much more difficult in the case of a
fluorinated acid, owing to the strong electron-withdraw-
ing effect of the trifluoromethyl group.14-16 Despite this ef-
fect, we have observed that the sulfinylation reaction of
substituted benzenes with triflinate salts occurred in a
strong acid medium and led to aryltrifluoromethylsulfox-
ides (Scheme 2).
S-(trifluoromethyl)-diphenylsulfonium triflates, which
can act as interesting electrophilic trifluoromethylating
agents.1-3 The trifluoromethanesulfinyl substituent is also
present in the structure of the potent insecticide
Fipronil.4,5 These fluorinated sulfoxides are usually pre-
pared by oxidation of the corresponding sulfides with a
peracid.2,3 This oxidation reaction is very sensitive to tem-
perature; a mixture of sulfoxide, sulfone and initial sulfide
is often obtained. Despite the number of preparations of
trifluoromethylsulfides, some regio isomers are not easily
available, specially when the aromatic ring is polysubsti-
tuted.6 On the other hand, the nucleophilic trifluoro-
methylation of arenesulfinyl halides by trifluoromethyltri-
methylsilane involves the use of the expensive tris(dime-
thylamino)sulfonium difluorotrimethylsilicate as a fluo-
ride anion source, though one example of aromatic
trifluoromethyl sulfoxide has been obtained from the con-
densation of an arenesulfinate ester with the Ruppert’s re-
agent in the presence of cesium fluoride.7,8 Obviously, the
direct trifluoromethanesulfinylation of aromatic sub-
strates is an interesting alternative path to these fluorinat-
ed sulfoxides.
Scheme 2
The reaction has been performed in trifluoromethane-
sulfonic (triflic) acid under argon at room temperature.17
Some triflic anhydride can be added in the medium in or-
der to remove traces of water. Either sodium triflinate or
its potassium analogue have been used.18,19 Two ortho/
para regio isomers can be formed in this sulfinylation re-
action. From toluene, an o/p = 25/75 mixture has been ob-
tained (Table, entry 7). When the benzene ring is
substituted by a halogen atom, the reaction is much more
selective. NMR analysis of the crude products showed
that the o/p ratios were 4/96 for fluorobenzene and 3/97
for chlorobenzene (entries 1, 2). Only para isomers were
formed from 1,3-dihalobenzenes (entries 4, 5, 6), from tri-
fluoromethoxybenzene (entry 3) or from acetanilide (en-
try 8). In the last case, the amide function was partly
hydrolyzed during the work up. The regioselectivity of
this trifluoromethanesulfinylation reaction can be com-
pared with the high selectivity of the arenesulfinylation of
aromatics towards the para position.20
Until now, electrophilic reactions of substrates with the
poorly stable trifluoromethanesulfinyl chloride, or with
the sodium trifluoromethanesulfinate (triflinate)/ phos-
phoryl chloride mixture, were limited to electron rich het-
erocyclic
compounds.9,10
A
more
electrophilic
intermediate appears necessary for the sulfinylation of
simple aromatic compounds. It has been reported that so-
dium arene- or alkanesulfinates react with aromatic sub-
strates in strong acid medium to give arylsulfoxides,
sulfonium salts or, in the case of benzene itself, polyaryl-
enesulfonium salts (Scheme 1).11-13
+
It is supposed that the electrophilic species RS(OH)2 is
formed by protonation of the sulfinic acid. Obviously, this
Synlett 2001, No. 4, 550–552 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Aryltrifluoromethylsulfoxides: Sulfinylation of Aromatics
551
Table Preparation and nmr characteristics of trifluoromethyl sulfoxides 3
a main isomer or single compound. b ortho/para ratio : 4/96. c ortho/para ratio: 3/97. d ortho/para ratio : 25/75. e mixture containing 30%
of 4-aminophenyltrifluoromethylsulfoxide.
Application of the procedure to benzene itself led to a methylsulfoxide, but with a low yield. Moreover, this con-
polymeric material. In that case, the replacement of triflic densation is very sensitive to the presence of water.
acid by concentrated sulfuric acid gave phenyltrifluoro-
Synlett 2001, No. 4, 550–552 ISSN 0936-5214 © Thieme Stuttgart · New York

552
C. Wakselman et al.
LETTER
(14) A comparison can be made with the corresponding
In summary, we have shown that simple aryltrifluorome-
thyl sulfoxides can be obtained directly from substituted
benzenes and triflinates salts in triflic acid medium.
sulfonylation of aromatics : methylarylsulfones are readily
formed from CH3S(O)2OS(O)2CF3 and various aromatics
without Friedel-Crafts catalysts, whereas the presence of
aluminium chloride is mandatory for the acylation of a narrow
range of mildly activated arenes by CF3S(O)2OS(O)2CF3 to
give aryltriflones.15,16
Acknowledgement
We thank Dr. Ximin Chen for her participation in this study during
her stay in Versailles.
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References and Notes
(17) Typical procedure : Preparation of 4-fluorophenyltrifluoro-
methyl sulfoxide (entry 1). In a 25 mL three-necked round
bottom flask, equipped with a condenser attached to a dry tube
(silica gel) and an argon inlet, was placed potassium triflinate
(1.72 g, 10 mmol). Under argon, triflic acid (15 g, 100 mmol)
was added. After stirring for 5 min, fluorobenzene (1.44g, 15
mmol) was introduced. Then, triflic anhydride (2.82 g, 10
mmol) was added to the medium. After 17 h stirring at room
temperature, the resulting mixture was hydrolyzed with ice-
water, neutralized by a NaHCO3 solution until pH 8, extracted
with diethyl ether and dried over MgSO4. After removing the
solvent, the residue was purified by column chromatography
on silica gel using pentane/ether (95/5) as eluent to give a
colorless oil. 1.56 g of product was obtained (yield: 74%).
Analysis (C7H4F4OS): % calcd. C, 39.5; H, 1.88; F, 35.76; S,
15.06; O, 7.53.% found C, 39.62; H, 1.9; F, 35.82; S, 15.11;
O, 7.4. Its NMR data are gathered in the Table (Bruker
AC300: solvent: CDCl3, references: TMS and CFCl3)
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Article Identifier:
1437-2096,E;2001,0,04,0550,0552,ftx,en;G00801ST.pdf
Synlett 2001, No. 4, 550–552 ISSN 0936-5214 © Thieme Stuttgart · New York