Efficient access to extended yagupolskii-umemoto-type reagents: Triflic acid catalyzed intramolecular cyclization of ortho- ethynylaryltrifluoromethylsulfanes

Article abstract of DOI:10.1002/anie.200905225

(Figure Presented) King of the ring: S-(trifluoromethyl)benzo[b]thiophenium salts 1, as analogues of Yagupolskii-Umemoto type reagents, were synthesized by novel triflic acid catalyzed intramolecular cyclization of ortho- ethynylaryltrifluoromethylsulfane

Full text of DOI:10.1002/anie.200905225

Communications
DOI: 10.1002/anie.200905225
À
C C Bond Formation
Efficient Access to Extended Yagupolskii–Umemoto-Type Reagents:
Triflic Acid Catalyzed Intramolecular Cyclization of ortho-
Ethynylaryltrifluoromethylsulfanes**
Andrej Matsnev, Shun Noritake, Yoshinori Nomura, Etsuko Tokunaga, Shuichi Nakamura, and
Norio Shibata*
In memory of Lev Moiseevich Yagupolskii
Trifluoromethylation is one of the most direct and straight-
forward strategies in the synthesis of fluorine-containing
organic compounds, which are sought after building blocks in
the development of pharmaceuticals and agrochemicals.[1]
The reagents involved in transferring a trifluoromethyl
group onto a target molecule are classified according to
their nucleophilic or electrophilic character. Whilst the
nucleophilic trifluoromethylation reaction is well known,
exemplified by the use of trifluoromethyltrimethylsilane
(Me3SiCF3), the development of electrophilic trifluorome-
thylation remains a challenge.[2] In 1984, Yagupolskii and co-
workers discovered that S-(trifluoromethyl)diarylsulfonium
salts are effective for the trifluoromethylation of thiopheno-
lates.[3] Since then, the design and synthesis of electrophilic
trifluoromethylating reagents has been extensively
researched.[4] Of the reagents that have been developed, the
S-(trifluoromethyl)dibenzothiophenium salts designed by
Umemoto[1b] and Thayer[1c] have emerged as among the
most powerful tools for this purpose, and are effective for the
and is effective for the trifluoromethylation of cyclic b-keto
esters and dicyanoalkylidenes. However, the reactivity
towards acyclic substrates is unsatisfactory and, as such,
more effective reagents are required for the trifluoromethy-
lation of carbon nucleophiles. In connection with our interest
in the synthesis of organofluorine compounds,[9] we herein
report a straightforward route to extended Yagupolskii–
Umemoto-type reagents, S-(trifluoromethyl)thiophenium
salts 1, through the triflic acid catalyzed intramolecular
cyclization of ortho-ethynylaryltrifluoromethylsulfanes
2
(Scheme 1). 2-Phenylthiophenium salt 1a and 2-cyclopro-
trifluoromethylation of
a wide range of nucleophiles.
Although several Umemoto reagents are commercially
available, their use, and those of related reagents, suffers
from their relatively complex synthesis. Magnier and co-
workers developed a more straightforward, efficient one-pot
synthesis of S-(trifluoromethyl)dibenzothiophenium salts.[5]
Recently, Togni et al. reported the electrophilic trifluorome-
thylation of carbon- or heteroatom-centered nucleophiles
using a mild hypervalent iodine(III)trifluoromethyl reagent.[6]
This reagent effects the trifluoromethylation of a range of
nucleophiles, and is now commercially available. In 2008, we
reported a fluorinated Johnson-type reagent for the electro-
philic trifluoromethylation of carbon-centered nucleo-
philes;[7] this reagent is also now commercially available[8]
Scheme 1. Triflic acid catalyzed synthesis of S-(trifluoromethyl)thiophe-
nium salts and their use in trifluoromethylations.
pylthiophenium salt 1j were selected to evaluate the scope of
the trifluoromethylation reaction of b-ketoesters and dicya-
noalkylidenes. The best results were obtained for trifluor-
omethylations using 1j that afforded a quaternary carbon
center, 3 and 4, even with substrates that have an unreactive
acyclic system.
Cyclization of ortho-substituted aryl alkynes has received
interest recently in areas ranging from material sciences to
pharmaceuticals. Metal salts or acids have been used to
induce the cyclization of alkynes to obtain five-membered
rings and heterocycles.[10] However, the synthesis of S-
(trifluoromethyl)benzo[b]thiophenium salts using these
methods has not been reported, despite their potential
usefulness as trifluoromethylating reagents and wide applic-
ability as building blocks for the synthesis of biologically
active compounds.[11,12] Although recent rapid progress in the
coinage-metal-catalyzed cyclization[13] of ortho-thio-substi-
tuted aryl alkynes has allowed easy access to benzothio-
[*] A. Matsnev, S. Noritake, Y. Nomura, E. Tokunaga, Dr. S. Nakamura,
Prof. N. Shibata
Department of Frontier Materials, Graduate School of Engineering,
Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
Fax: (+81)52-735-7543
E-mail: nozshiba@nitech.ac.jp
[**] Support was provided by KAKENHI (21390030). We also thank
TOSOH F-TECH INC. We are grateful to Central Glass Co., Ltd. for
the gift of trifluoromethanesulfinate.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905225.
572
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Chemie
phenes, this method is not suitable for the synthesis of
benzothiophenium salts. In 1994, Kitamura and co-workers
reported the cyclization of o-SPh-substituted aryl alkynes
using HBF4 or HClO4 that afforded cyclized 1-phenyl-1-
benzothiophenium salts.[14] However, under the same con-
ditions, o-SMe-substituted alkynes gave de-methylated ben-
zo[b]thiophenes, and S-methyl salts were not detected.
Electrophilic cyclization using reagents such as I2, Br2, and
PhSeCl also gave the corresponding benzo[b]thiophenes
through a demethylation pathway.
corresponding S-(trifluoromethyl)thiophenium salts in good
to excellent yields (64–94%; Table 1, entries 8–14). No CF3
migration, often observed for the cyclization to S-phenyl,
methyl, or related alkylthiophenium salts,[10–14] was observed
within the ring. Annulation of benzyl-substituted alkyne 1i
with TfOH gave a complex mixture of products, presumably
owing to the deprotonation/de-trifluoromethylation of 1i into
A (Table 1, entry 15; Scheme 2). Cyclopropyl-substituted
Initially, we attempted the cyclization reaction of 2a to 1a
using coinage metal salts. However, neither gold salts nor
copper salts afforded a trifluoromethyl-substituted, five-
membered ring under analogous literature conditions
(Table 1, entries 1–4). Therefore, we turned our attention to
Table 1: Intramolecular cyclization of ortho-ethynylaryltrifluoromethylsul-
fane to S-(trifluoromethyl)benzothiophenium salts.[a]
Scheme 2. De-trifluoromethylation of 1i and double cyclization to
afford 1l.
Entry
1
2
R
Additive (equiv)
1
Yield[b]
[%]
alkyne 2j was converted into the corresponding S-(trifluor-
omethyl)thiophenium salt 1j in good yield without any loss of
the trifluoromethyl group (80%; Table 1, entry 16). Depro-
tonation of the product was presumably impeded by the
sterically bulky cyclopropane moiety. Indeed, tert-butyl
alkyne 2k also gave a good result, giving the product in
64% yield (Table 1, entry 17). Moreover, even C2-symmetric
substrate 2l was successfully transformed into the corre-
sponding dicationic salt 1l through a double cyclization
reaction (Table 1, entry 18; Scheme 2). These are the first
reported examples of the preparation of S-(trifluoromethyl)-
benzothiophenium salts.
Following the synthesis of various S-(trifluoromethyl)th-
iophenium salts 1, the electrophilic trifluoromethylation of a
wide range of carbon nucleophiles to afford their correspond-
ing trifluoromethylated compounds with a quaternary carbon
center was examined. As many important biologically active
compounds contain quaternary carbon centers, this procedure
would be particularly attractive to both organic and medicinal
chemists. Salts 1a and 1j were selected to evaluate the
effectiveness of the S-(trifluoromethyl)thiophenium reagents
with aryl (1a) or alkyl (1j) substituents. Cyclic and acyclic b-
keto esters afforded their corresponding trifluoromethylated
compounds, which had a quaternary carbon center, in good to
high yields in the presence of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) or tert-butyliminotri(pyrrolidino)phosphorane
(P1) as the base (59–95%; Table 2, entries 1–25). Cyclo-
propyl-substituted reagent 1j afforded slightly improved
yields compared to when phenyl-substituted 1a was used
(Table 2, entries 1–6 versus 7–14). Cyclopropyl reagent 1j
presents a clear advantage of higher yields over both the
aromatic analogue (1a) and the commercially available
Umemoto reagent (S-(trifluoromethyl)dibenzothiophenium
tetrafluoroborate) in the trifluoromethylation of acyclic b-
keto esters (Table 2, entries 15–17). Good to high yields of
trifluoromethylated acyclic b-keto esters were obtained more
2a Ph
[Au(PPh3)2Cl]
(0.1)[c]
1a
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2a Ph
2a Ph
2a Ph
2a Ph
2a Ph
2a Ph
2b p-MeC6H4
2c m-BrC6H4
2d p-BrC6H4
2e p-MeOC6H4
2 f o-ClC6H4
2g o,p-di-FC6H3
2h p-IC6H4
2i CH2Ph
2j cyclopropyl
2k tBu
AuCl3 (0.1)[c]
CuCl2 (0.1)[c]
CuBr2 (0.1)[c]
HBF4 (2.5)[c]
HClO4 (2.5)[c]
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
TfOH (2.0)
1a
1a
1a
1a
1a
1a 79
1b 94
1c 87
1d 82
1e 73
1 f 64
1g 72
1h 78
0
0
0
0
0
1i
0[d]
1j 80
1k 64
1l[e] 68
2l p-C6H4-CꢀC-o-C6H4-
SCF3
[a] The cyclization of 2 into 1 was carried out in the presence of TfOH
(2 equiv) in solvent at À788C. For detailed reaction conditions, see the
Supporting Information. [b] Yield of isolated product. [c] For experimen-
tal details of unsuccessful reactions, see the Supporting Information.
[d,e] See Scheme 2.
the Brønsted acid catalyzed cyclization reaction; however,
neither HBF4 nor HClO4 were effective for this transforma-
tion (Table 1, entries 5 and 6). In contrast, triflic acid
(CF3SO3H, TfOH) was extremely efficient in affording the
desired product 2a in 79% yield (Table 1, entry 7).[15] The
scope of this procedure was then considered for a wide range
of ortho-substituted alkynes, and the cyclized product was
obtained efficiently in almost all cases (Table 1, entries 8–17).
The annulation of a broad range of o-SCF3-substituted aryl
alkynes that have functionalized aromatic rings afforded their
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Communications
Table 2: (Continued)
Table 2: Trifluoromethylation of b-keto esters 5 or dicyanoalkylidenes 6
with 1.[a]
Entry
5
Base
1
3 or 4 Yield[b] [%]
or (equiv)
6
16
17
18
19
20
21
22
23
24
25
R1 =Bn,
R2 =Et,
R3 =Me
R1 =Bn,
R2 =Et,
R3 =Me
R1 =Et,
R2 =Me,
R3 =Bn
R1 =tBu,
R2 =Me,
R3 =Et
5j P1 (2.0) 1j 3j
5j P1 (2.0) UR 3j
5k P1 (2.0) 1j 3k
5l P1 (2.0) 1j 3l
5m P1 (2.0) 1j 3m
5n P1 (2.0) 1j 3n
5o P1 (2.0) 1j 3o
5p P1 (2.0) 1j 3p
5q P1 (2.0) 1j 3q
5r P1 (2.0) 1j 3r
84
59
70
68 (86)[c]
R1 =Et,
R2 =Ph,
R3 =Me
R1 =Et,
R2 =Me,
R3 =Et
87
Entry
5
Base
1
3 or 4 Yield[b] [%]
or (equiv)
6
44 (69)[c]
1
2
R1 =Me,
5a DBU
1a 3a
1a 3b
1a 3c
1a 3d
1a 3e
1a 3 f
1j 3a
1j 3b
1j 3c
1j 3d
1j 3 f
1j 3g
1j 3h
76
82
80
79
59
80
92
89
78
91
82
95
92
R2 =R3 =H,
n=1
(1.2)
R1 =Et,
R2 =Ph,
R3 =Et
84
71
83
67
R1 =Bn,
R2 =R3 =H,
n=1
5b DBU
R1 =Et,
R2 =Ph,
R3 =Pr
(1.2)
3
R1 =tBu,
R2 =R3 =H,
n=1
5c DBU
R1 =Et,
R2 =Pr,
R3 =Bn
R1 =CHPh2,
R2 =Et,
R3 =Me
(1.2)
4
R1 =tBu,
R2 =H, R3 =Br,
n=1
5d DBU
(1.2)
5
R1 =Me,
R2 =R3 =OMe,
n=1
5e DBU
(1.2)
26
27
28
29
6a P1 (1.2) 1j 4a
6b P1 (1.2) 1j 4b
6c P1 (2.2) 1j 4c+7
89
90
6
R1 =Me,
R2 =R3 =H,
n=2
5 f DBU
75[c] (4c:7=1:24)
(1.2)
6d DBU
(1.2)
6e DBU
(1.2)
5a DBU
(1.2)
1j 4d
1j 4e
1b 3a
93
7
R1 =Me,
R2 =R3 =H,
n=1
5a DBU
30
31
91
83
(1.2)
R1 =Me,
R2 =R3 =H,
n=1
8
R1 =Bn,
R2 =R3 =H,
n=1
5b DBU
(1.2)
32
33
34
35
36
R1 =Me,
R2 =R3 =H,
n=1
5a DBU
1g 3a
1k 3a
TR 3a
78
88
17
9
R1 =tBu,
R2 =R3 =H,
n=1
5c DBU
(1.2)
(1.2)
R1 =Me,
R2 =R3 =H,
n=1
5a DBU
10
11
12
13
R1 =tBu,
R2 =H, R3 =Br,
n=1
5d DBU
(1.2)
(1.2)
R1 =Me,
R2 =R3 =H,
n=1
5a DBU
R1 =Me,
R2 =R3 =H,
n=2
5 f DBU
(1.2)
(2.0)
R1 =Bn,
R2 =Et,
5j P1 (2.0) TR 3j
trace
R1 =tBu,
R2 =R3 =H,
n=2
5g DBU
R3 =Me
R1 =Me,
R2 =R3 =H,
n=1
(2.0)
5a K2CO3 TR 3a
42[d]
[d]
R1 =Me,
R2 =H,
5h DBU
(2.0)
R3 =OMe,
n=2
[a] P1: tert-butyliminotri(pyrrolidino)phosphorane; UR: Umemoto
reagent, S-(trifluoromethyl)dibenzothiophenium tetrafluoroborate; TR:
Togni’s reagent (1-trifluoromethyl-3,3-dimethyl-1,2-benziodoxole). The
reaction of either substrate 5 or 6 with 1 (1.5 equiv) was carried out in
the presence of a base (1.2–2.0 equiv) in MeCN at À43–À458C. For
detailed reaction conditions, see the Supporting Information. [b] Yield of
isolated product. [c] 2.4 Equivalents of 1j used. Yield determined by H
NMR spectroscopy. [d] Reaction performed in the presence of nBu4NI in
MeCN. These data were taken from reference [6a,b].
DBU
5i
14
15
1j 3i
60
60
(2.0)
R1 =Bn,
R2 =Et,
R3 =Me
5j P1 (2.0) 1a 3j
1
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or less independent of the substrate structure (Table 2,
entries 18–25). The reagent 1j was also found to be effective
for the trifluoromethylation of dicyanoalkylidenes 6a–e,
under similar conditions, to afford allylic trifluoromethylated
compounds 4a–e and 7 (Table 2, entries 26–30). To under-
stand the higher yields obtained with 1j over 1a, and the
influence of substituents, reagents 1b, 1g, and 1k were also
evaluated and their results compared. The trifluoromethyla-
tion of 5a with 1b, 1g, and 1k also proceeded well to furnish
product 3a in 83%, 78%, and 88% yields, respectively
(Table 2, entries 31–33). These results indicate that reagents
containing alkyl substituents tend to afford the product in
slightly higher yields than those containing aryl substituents.
Although the reason for the improved yield is not clear, it is
presumably due to the stability of the reagents: aryl-
substituted reagents have better CF3 releasing ability owing
to the stability of the benzothiophene derivative by-product,
as a consequence of the conjugated structures, whereas alkyl-
substituted trifluoromethylated reagents are more stable than
their aryl-substituted counterparts. High reactivity might be
ineffective for this trifluoromethylation reaction because of
the competitive decomposition under basic conditions. A
brief comparison to Togniꢀs reagent, which is also commer-
cially available and is a competitor to the Umemoto reagent,
was also performed. Under our reaction conditions, the yields
of trifluoromethylated products 3a and 3j with Togni’s
reagent were lower than those using 1j (Table 2, entries 34
and 35, versus 7 and 16, respectively). However, these
comparisons might not be fair because a higher yield (42%)
was reported under different conditions (Table 2,
entry 36),[6a,b] and might be improved by further optimization.
One advantage of our new trifluoromethylation reagents 1
is the potential functionalization at the thiophene 2-position
with chiral groups that offers the possibility of enantioselec-
tive trifluoromethylation of prochiral substrates, one of the
important unresolved issues of fluoroorganic synthesis.
Therefore, chiral reagent 1m was designed and synthesized,
and its preparation is shown in Scheme 3. (1R,2S,4R)-2-
Ethynyl-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ol 8,[16] pre-
pared from (1R)-(+)-camphor, was treated with 2-iodophe-
nyl(trifluoromethyl)sulfane 9 under Sonogashira coupling
conditions to furnish 10 in 85% yield. Dehydration of 10
was achieved in the presence of SOCl2 to give enyne 2m in
68% yield. Our triflic acid catalyzed intramolecular cycliza-
tion was performed on 2m to afford the target chiral reagent
1m in 34% yield as a 1:1 mixture of diastereoisomers, which
are racemic at the sulfur atom (Scheme 3). Compound 1m
was characterized by spectroscopy and elemental analysis;
however, a satisfactory 1H NMR spectrum could not be
obtained owing to slight decomposition of 1m, especially in
CDCl3. The decomposition of 1m was minimized in CD3CN.
The trifluoromethylation of 5a by 1m was then performed in
the presence of DBU under the same reaction conditions to
furnish the trifluoromethylated product 3a in 43% yield,
regrettably as a racemate (Scheme 4).
Scheme 4. Attempted enantioselective trifluoromethylation of 5a by
chiral reagent 1m. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene.
In conclusion, we have described the straightforward
synthesis of S-(trifluoromethyl)thiophenium salts by the
triflic acid catalyzed intramolecular cyclization of ortho-
ethynylaryltrifluoromethylsulfanes without migration or loss
of the CF3 group. These salts are promising as extended
Yagupolskii–Umemoto reagents for the electrophilic trifluor-
omethylation of carbon-centered nucleophiles, including b-
keto esters and dicyanoalkylidenes. Furthermore, cyclo-
propyl-substituted reagent 1j has advantages over the Ume-
moto reagents.
Received: September 18, 2009
Revised: October 29, 2009
Published online: December 15, 2009
À
Keywords: C C bond formation · cyclization · fluorine ·
triflic acid · trifluoromethylation
.
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