Fluorinated Johnson reagent for transfer-trifluoromethylation to carbon nucleophiles

Article abstract of DOI:10.1002/ejoc.200800419

A novel reagent, [(oxido)phenyl(trifluoromethyl)-λ⁴- sulfanylidene] dimethylammonium tetrafluoroborate has been developed for the electrophilic trifluoromethylation of carbon nucleophiles. The reagent was designed as a trifluorinated version of

Full text of DOI:10.1002/ejoc.200800419

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800419
Fluorinated Johnson Reagent for Transfer-Trifluoromethylation to Carbon
Nucleophiles
Shun Noritake,[a] Norio Shibata,*[a] Shuichi Nakamura,[a] Takeshi Toru,[a] and
Motoo Shiro[b]
Keywords: Fluorine / Trifluoromethylation / Electrophilic / Sulfur / C–C bond formation
A novel reagent, [(oxido)phenyl(trifluoromethyl)-λ4-sulfanyl-
idene]dimethylammonium tetrafluoroborate has been devel-
oped for the electrophilic trifluoromethylation of carbon nu-
cleophiles. The reagent was designed as a trifluorinated ver-
sion of a Johnson-type methyl-transfer reagent. The first
example of vinylogous trifluoromethylation of dicyanoalk-
ylidenes is also demonstrated.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
Since the discovery by Yagupolskii and co-workers that
diaryl(trifluoromethyl)sulfonium salts are effective for the
trifluoromethylation of thiophenolates,[1] the design and
synthesis of electrophilic trifluoromethylating reagents have
been extensively researched in both academic and industrial
fields, due to the significant unique features that trifluoro-
methylated compounds have in pharmaceuticals, agricul-
tural chemicals,[2] and functional materials. Among the se-
veral reagents developed,[3,4] S-(trifluoromethy1)dibenzothio-
phenium salts designed by Umemoto et al. have emerged as
one of the most powerful tools for this purpose.[3a–3d]
A
Figure 1. Electrophilic trifluoromethylating reagents.
series of Umemoto reagents are commercially available and
are effective for the trifluoromethylation of hetero-nucleo-
philes, however, yields for the trifluoromethylation of car-
bon nucleophiles are not satisfactory, with the exception of
the Cahard’s modification protocol.[5] Recently, Togni et al.
reported that the hypervalent iodine(III)–CF3 reagent is a
mild electrophilic trifluoromethylating reagent of carbon-
and heteroatom-centered nucleophiles (Figure 1).[4e–4g] This
new reagent has wide generality for effecting trifluoro-
methylation of a range of nucleophiles; however, more
effective reagents are required for the trifluoromethylation
of carbon nucleophiles, in particular β-keto esters.
carbon nucleophiles, in order to further develop the enan-
tioselective electrophilic trifluoromethylation reaction.[2,8]
We herein disclose the design, synthesis and evaluation of
[(oxido)phenyl(trifluoromethyl)-λ4-sulfanylidene]dimethyl-
ammonium tetrafluoroborate (1) for the electrophilic tri-
fluoromethylation of β-keto esters 2. We also demonstrate
the first vinylogous trifluoromethylation of dicyanoalkylid-
enes 4 using 1 (Scheme 1).
In connection with our interest in the enantiocontrolled
synthesis of organo-fluorine compounds,[6,7] we required
new reagents for the electrophilic trifluoromethylation of
Scheme 1. A novel reagent 1 for trifluoromethylation of carbon nu-
cleophiles.
[a] Department of Frontier Materials, Graduate School of Engi-
neering, Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555, Japan
Fax: +81-52-735-5442
Results and Discussion
E-mail: nozshiba@nitech.ac.jp
[b] Rigaku Corporation
Forty years ago, Johnson and co-workers introduced
[methyl(oxido)phenyl-λ4-sulfanylidene]dimethylammonium
tetrafluoroborate (6) for methylene transfer reactions such
3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan
Supporting information for this article is available on the
WWW under http://www.eurjoc.org/ or from the author.
Eur. J. Org. Chem. 2008, 3465–3468
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3465

S. Noritake, N. Shibata, S. Nakamura, T. Toru, M. Shiro
SHORT COMMUNICATION
as cyclopropanation and aziridine formation.[9] The methyl-
ene transfer from 6 over a wide range of substrates is pro-
moted by the cleavage of the good leaving group, N,N-di-
methylbenzenesulfinamide (7). Since then, 6 and related
compounds have gained substantial synthetic significance
as immensely powerful tools for methylene transfer.[9d,9e]
This success led us to investigate the synthesis of the trifluo-
rinated version of 6 (salt 1) as a novel electrophilic reagent
for trifluoromethylation. Transfer of the CF3 group from 1
to the substrates should proceed via the reaction with the
nucleophiles to give the trifluoromethylated adducts, dis-
placing 7 (Scheme 2).
Figure 2. X-ray crystallographic structure of 1.
Table 1. Trifluoromethylation of β-keto esters 2 with 1.[a]
Scheme 2. Johnson’s reagent 6 and the newly designed reagent 1.
The salt 1 was synthesized by the procedure shown in
Scheme 3. Phenyl trifluoromethyl sulfoxide 8 was synthe-
sized from the nucleophilic trifluoromethylation of methyl
phenylsulfinate with TMSCF3, the Friedel–Crafts reaction
of benzene with sodium trifluoromethanesulfinate or the
oxidation of trifluoromethyl phenyl sulfide.[1,3,10] Conver-
sion of sulfoxide 8 to sulfoximine 9 was done under conven-
tional NaN3/H2SO4 conditions.[11] The stepwise methyl-
ation of 9 with MeI/K2CO3 followed by the treatment with
methyl trifluoromethansulfonate gave the trifluoromethane-
sulfonate 11 as a viscous oil. Finally, 11 was successively
transformed into 1 by NaBF4 in MeOH to give colourless
crystals in high yield (Scheme 3). The salt 1 is a relatively
stable, easy to handle solid, and was characterized by
single-crystal X-ray crystallography, which shows that N1
adopts a nearly planar, sp2-like structure, as evidenced by
the sum of the dihedral angles (350.83°) between S–N1–C8
(119.6°), S–N1–C9 (117.29°) and C8–N1–C9 (113.94°). In
addition, the N1–S bond length is 1.5810 Å which is shorter
than the typical length of an N–S bond (1.656 Å)[12] and
similar to the length of a typical N=S double bond
(1.541 Å).[12] Therefore, a cation is presumed to be located
on N1 rather than on the sulfur atom (Figure 2).
Entry
2
Base[b]
Solvent
3
Yield[c] (%)
1
2
3
4
5
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
2f
DBU
TMG
TBD
P1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMF
toluene
MeCN
THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3f
93
87
71
88
77
80
49
74
81
0
13
74
89
90
65
80
86
76
79
63
37
60
71
24
54
75
P2
6
DBU
DBU
DBU
DBU
pyridine
Et3N
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
P1
DBU
DBU
DBU
P1
P2
DBU
7[d]
8
9
10[e]
11
12
13
14
15
16
17
18
19
20
21
22
23[f]
24
25
26[g]
2g
2h
2i
3g
3h
3i
2j
3j
2k
2l
3k
3l
2l
3l
2m
2m
2a
3m
3m
3a
Scheme 3. a) NaN3, 25% fuming H2SO4, 70 °C, 3 h, 81%; b) CH3I,
K2CO3, THF, reflux, 7 h, 97%; c) methyl trifluoromethansulfonate,
neat, room temp., 6 h, 93%; d) sat. NaBF4 aq., MeOH, room
temp., 13 h, 92%.
[a] The reaction of 2 with 1 (1.5 equiv.) was carried out in the pres-
ence base (1.2 equiv.) in solvent at room temp. For detailed reaction
conditions, see the Supporting Information. [b] TMG: tetrameth-
ylguanidine; TBD: 1,5,7-triazabicyclo[4.4.0]dec-5-ene; P1: phos-
phazene base P1-tBu; P2: phosphazene base P2-Et. [c] Isolated
yield. [d] The reaction time: 2 h. [e] The reaction time: 16 h. [f] The
reagent 1 (3.0 equiv.) was used. [g] The reaction was carried out in
the presence of nitrobenzene (1.0 equiv.).
To optimize the trifluoromethylation of carbon nucleo-
philes, we first examined the reaction of β-keto ester 2a with
1. The solvent and base were varied, and the results are
summarized in Table 1.
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Eur. J. Org. Chem. 2008, 3465–3468

Fluorinated Johnson Reagent for Transfer-Trifluoromethylation
We found that salt 1 reacts with indanone carboxylate 2a Table 2. Vinylogous trifluoromethylation of dicyanoalkylidenes 4.[a]
in the presence of DBU in CH2Cl2 to yield the trifluoro-
methylated compound 3a in a good yield of 93% (Table 1,
entry 1). Guanidine bases, such as TMG and TBD, and
phosphazene bases such as P1-tBu and P2-Et, were also
equally effective for the transfer trifluoromethylation from
1 to 2a in good to high yields (entries 2–5). The reaction
proceeded in a variety of solvents in the presence of DBU
(entries 6–9). On the other hand, no reaction was observed
in the case of weak bases such as triethylamine and pyridine
(entries 10 and 11). This finding is of interest because the
En-
try
4
Base
(equiv.)
1 (equiv.)
5
Yield[b] (%)
transfer trifluoromethylation was not hampered under
strong base conditions. Further studies focused on the vari-
ation of the substrates. Under similar conditions, β-keto
esters such as indanone, tetralone, and oxocyclopentan-
carboxylates 2b–l were examined to investigate the scope of
the reaction. The substitutions on the benzene ring and the
variation of ester moiety did not effect the yield and reac-
tion time, and were indeed good substrates to afford the
corresponding trifluoromethylated compounds 3b–l in
good to high yields within an hour (entries 12–23).
Although the yield of the trifluoromethylation of acyclic
ester 2m was low in the presence of P1 base; it was improved
to 54% when the reaction was carried out using the
phosphazene base, P2 (entries 24 and 25). To understand
the mechanism of this trifluoromethylation, we attempted
the trifluoromethylation of 2a in the presence of nitro-
benzene (entry 26). Nitrobenzene is known to act as a
radical scavenger resulting in the inhibition of a radical
reaction.[13] However, in our experiment, the desired prod-
uct was obtained independent of the presence of nitro-
benzene. Therefore, under typical conditions, the reaction
would be classified as an electrophilic trifluoromethylat-
ion. A free radical or single electron transfer (SET) mecha-
nism might be ruled out, although a detailed study is re-
quired.
1
2
4a
4a
4a
4a
4b
4c
4d
4e
4f
DBU (1.2)
DBU (2.0)
P1 (1.2)
P1 (2.0)
P1 (2.0)
P1 (2.0)
P1 (2.0)
P1 (1.2)
P1 (1.2)
P1 (2.0)
P1 (1.2)
P1 (2.0)
1.5
2.2
1.5
2.2
2.2
1.0
2.2
1.5
1.5
2.2
1.5
2.2
5a
5a
5a
5a
5b
5c
5d
5e
5f
53
52
77
92
86
3
4[c]
5
6[d]
7
63
77[e]
84
8
9
10
11
12
88
68
4g
4h
4i
5g
5h
5i
50 (79)[f]
79[g]
[a] The reaction of 4 with 1 (1.5–2.2 equiv.) was carried out in the
presence base (1.2–2.0 equiv.) in CH2Cl2 at room temp. For detailed
reaction conditions, see the Supporting Information. [b] Isolated
yield. [c] Reaction time: 15 min. [d] The reaction was carried out
using 2.0 equiv. of 4c. The yield was calculated from the reagent 1.
[e] A mixture of 5d and bis(trifluoromethylated) compound 5dd
(5d/5dd = 54:46). [f] Based on recovered 4h. [g] A mixture of 5i and
bis(trifluoromethylated) compound 5ii (5i/5ii = 76:24).
Conclusions
We were next interested in the vinylogous trifluoromethyl-
ation of dicyanoalkylidenes 4 under the same trifluoro-
methylation conditions, since a number of C–C bond for-
mation reactions has been developed for activated alkylid-
enes,[14] with the exception of trifluoromethylation. To
start our investigation, we examined the reaction of the
tetralone-derived dicyanoalkylidene 4a with 1 in CH2Cl2
using DBU or P1 as the base at room temperature. The
corresponding trifluoromethylated dicyanoalkylidene 5a
was obtained with both bases, but the P1 base showed a
better yield of 5a (Table 2, entries 1–4). The scope of the
vinylogous electrophilic trifluoromethylation reaction is
shown in Table 2. The reaction generally proceeded nicely
to provide the desired products in good to high yields (en-
tries 3–12). Substitution at the allylic position influenced
the reaction slightly. While mono-trifluoromethyaltion was
observed for the reaction of the tetralone-type (n = 2, six-
We have fathomed the potential of the newly synthesized
[(oxido)phenyl(trifluoromethyl)-λ4-sulfanylidene]dimethyl-
ammonium tetrafluoroborate (1) as a reagent for the tri-
fluoromethylation of carbon nucleophiles, including the
first example of vinylogous trifluoromethylation of dicya-
noalkylidenes. The enantioselective trifluoromethylation of
carbon nucleophiles using 1 is under investigation.
Experimental Section
Typical Procedure for the Trifluoromethylation of β-Keto Esters: To
a stirred solution of β-keto ester 2a (20 mg, 0.105 mmol) in CH2Cl2
(0.5 mL, 0.2 ) was added DBU (18.8 µL, 0.126 mmol), and stirred
for 15 min at room temperature. The reagent
1 (51.4 mg,
0.158 mmol) was added, and stirred for 15 min at room tempera-
ture. The solvent was evaporated, and the residue was purified by
membered ring) and benzosuberone-type (n = 3, seven- column chromatography on silica gel (benzene) to give 3a (25.1 mg,
93%) as a pale yellow solid.
membered ring) dicyanoalkylidenes 4a, 4b and 4g, a mix-
ture of mono- and di-trifluoromethylated products (5d and
5dd) was obtained in the reaction of the indanone-type 4d
(n = 1, five-membered ring).
Methyl 2-(Trifluoromethyl)-2,3-dihydro-1-oxo-1H-indene-2-carbox-
1
ylate (3a): H NMR (200 MHz, CDCl3): δ = 7.84 (d, J = 7.4 Hz,
1 H), 7.73–7.65 (m, 1 H), 7.54–7.41 (m, 2 H), 3.78 (s, 3 H), 3.75,
Eur. J. Org. Chem. 2008, 3465–3468
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3467

S. Noritake, N. Shibata, S. Nakamura, T. Toru, M. Shiro
SHORT COMMUNICATION
3.59 (AB quartet, J = 17.5 Hz, 2 H) ppm. 19F NMR (188 MHz,
CDCl3): δ = –69.2 (s, 3 F) ppm. 13C NMR (150.9 Hz, CDCl3): δ =
192.82, 165.60, 151.65, 136.28, 134.36, 128.50, 126.30, 125.60,
123.47 (q, J = 282 Hz), 63.05 (q, J = 26.3 Hz), 53.56, 34.16 ppm.
7009; e) N. Shibata, T. Ishimaru, E. Suzuki, K. L. Kirk, J. Org.
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IR (KBr): ν = 2968 (C–H), 1756 (COOMe), 1719 (CO), 1316–1163
˜
(CF3) cm–1. MS (APCI) m/z 257 (M– – H). HRMS (EI) Calcd for
C12H9F3O3[M]+: 258.0504, found: 258.0504.
Supporting Information (see also the footnote on the first page of
this article): Experimental procedures and spectroscopic data for
all compounds.
Acknowledgments
[7]
Support was provided from the Ministry of Education, Culture,
Sports, Science and Technology Japan by a Grant-in-Aid for Scien-
tific Research (B) (19390029) for Scientific Research on Priority
Areas “Advanced Molecular Transformations of Carbon Re-
sources” (19020024). We are grateful to Central Glass Co., Ltd. for
the gift of sodium trifluoromethanesulfinate.
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Received: April 29, 2008
Published Online: May 30, 2008
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Eur. J. Org. Chem. 2008, 3465–3468