786
Chemistry Letters Vol.37, No.7 (2008)
Extended Pummerer Reaction of Arylketene Dithioacetal Monoxides
with Aromatic Compounds by Means of Trifluoromethanesulfonic Anhydride
Suguru Yoshida, Hideki Yorimitsu,ꢀ and Koichiro Oshimaꢀ
Department of Material Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510
(Received May 8, 2008; CL-080474;
E-mail: yori@orgrxn.mbox.media.kyoto-u.ac.jp, oshima@orgrxn.mbox.media.kyoto-u.ac.jp)
Extended Pummerer reaction of arylketene dithioacetal
Me
TfO
monoxides with aromatic compounds by means of trifluoro-
methanesulfonic anhydride proceeded in moderate to good
yields. In the case of intramolecular cyclization of (2-arylphen-
yl)ketene dithioacetal monoxides, phenanthrenes were obtained
via ring-closure and skeletal rearrangement.
O
S
S
Tf2O
Me
S
S
S
S
Ph
Ph
OTf
Ph
8a
2 OTf
S
5a
7a
Me
−TfOH
S
−TfOH
S
S
Ph
9a
H
The Pummerer reaction is regarded as an important synthet-
ic method for preparation of ꢀ-substituted sulfides.1 Particularly,
the Pummerer rearrangement has been utilized widely as a
transformation from alkyl sulfoxides to aldehydes or ketones
for synthesis of many natural products.2 However, extended
Pummerer reactions using alkenyl or aryl sulfoxides are still
limited.3 Especially, there are few reports on nucleophilic attack
to cationic species generated from alkenyl sulfoxides via
cleavage of the S–O bond.
Recently, we have reported the synthesis of benzo[b]thio-
phenes 4 by cyclization of arylketene dithioacetal monoxides 1
under Pummerer-like conditions (Scheme 1).4 The reaction
should involve stabilized cationic intermediate 2 generated from
arylketene dithioacetal monoxides with trifluoromethanesulfon-
ic anhydride.5 Here, we report intermolecular nucleophilic attack
to a similar cationic intermediate by aromatic compounds.
First, 2-phenylmethylene-1,3-dithiane 1-oxide (5a) was
treated with trifluoromethanesulfonic anhydride in toluene, as
a nucleophile as well as solvent, at ꢁ78 ꢂC (Scheme 2). The
mixture was warmed to 25 ꢂC, and stirred for 12 h. Extractive
work up followed by silica-gel column purification afforded
2-[phenyl(4-tolyl)methylene]-1,3-dithiane (6a) in 80% yield. A
plausible reaction mechanism is as follows. At first, sulfoxide
5a gave intermediate 7a upon treatment with trifluoromethane-
sulfonic anhydride. Then, dicationic intermediate 8a could be
generated after cleavage of the S–O bond.6 Friedel–Crafts-type
nucleophilic attack to dicationic intermediate 8a followed by
deprotonation would afford desired product 6a.
Ph
OTf
6a
Scheme 2.
1.5 equiv of toluene and 1.5 equiv of trifluoromethanesulfonic
anhydride in several solvents. Although reactions in CH2Cl2,
CH3CN, THF, or diethyl ether gave complex mixtures, the
use of CH3NO2 provided a good result due to the high polarity
of the solvent (eq 1). Stabilization of cationic intermediates by
CH3NO2 could allow toluene to attack intermediate 8a intermo-
lecularly. When acetic anhydride, trifluoroacetic anhydride, or
trifluoromethanesulfonic acid was used instead of trifluoro-
methanesulfonic anhydride, no desired product was obtained.
Me
O
Tf2O (1.5 equiv)
S
Me
S
+
S
S
ð1Þ
CH3NO2
Ph
25 oC, 1 h
Ph
(1.5 equiv)
6a
5a
88%
We examined the scope of the extended Pummerer reaction
(Table 1). Nucleophilic attack of benzene or chlorobenzene to
sulfoxide 5a with the aid of trifluoromethanesulfonic anhydride
proceeded in moderate yields, although the nucleophiles were
used as solvent (Entries 2 and 3). In the case of anisole or naph-
thalene, the reaction proceeded in high yields and the products
were obtained as mixtures of regioisomers (Entries 4 and 5).
Treatment of mesitylene or p-chloroanisole gave desired multi-
substituted benzene derivatives in good yields (Entries 6 and 7).
Substituents on olefins affected the reaction significantly.
Treatment of a mixture of 2-methylene- or 2-propylidene-1,3-
dithiane 1-oxide and toluene with trifluoromethanesulfonic
anhydride in CH3NO2 gave a complex mixture. In the case of
2-arylmethylene-1,3-dithiane 1-oxide, yields of 6 were lowered
by the substituents on the aromatic rings (eq 2). Although the
reaction of 2-(4-chlorophenyl)methylene-1,3-dithiane 1-oxide
(5b) afforded desired product 6h in good yield, 2-(4-trifluoro-
methylphenyl)methylene-1,3-dithiane 1-oxide (5c), a more
electron-deficient substrate, afforded a complex mixture which
did not contain 6i. 2-(4-Methoxyphenyl)methylene-1,3-dithiane
1-oxide (5d) gave desired product 6j in low yield due to some
unidentified side reactions.
Then, we have examined reactions of sulfoxide 5a with
O
Tf2O
SMe
SMe
SMe
SMe
R
R
R'
1
R'
2
2 OTf
Me
OH
H2N
S
S
SMe
OTf
SMe
R
R
OH
MeHN
R'
R'
3
4
Scheme 1.
Copyright Ó 2008 The Chemical Society of Japan
Yoshida, Suguru
