A Convenient Method for Catalytic Aromatic Pentafluoroethylation Using Potassium (Pentafluoroethyl)trimethoxyborate

Article abstract of DOI:10.1055/s-0036-1588131

A method for the copper-catalyzed pentafluoroethylation of aryl iodides using potassium (pentafluoroethyl)trimethoxyborate has been developed. The borate was found to be a convenient pentafluoroethyl source. In parallel, it was found that the solvent, as well as ligand, makes significant impact on the catalytic perfluoroalkylations.

Full text of DOI:10.1055/s-0036-1588131

SYNTHESIS
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Georg Thieme Verlag Stuttgart · New York
2016, 48, A–E
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paper
Synthesis
T. Sugiishi et al.
Paper
A Convenient Method for Catalytic Aromatic Pentafluoroethylation
Using Potassium (Pentafluoroethyl)trimethoxyborate
Tsuyuka Sugiishi
Daisuke Kawauchi
Mizuki Sato
CF3CF2B(OMe)3
CuI, phen
K
X
CF2CF3
R
R
THF, 60 °C, 24 h
Tatsuya Sakai
Hideki Amii*
X = I, R = NO2, CN, nBu, OMe, SiMe3
7 examples
73 to >95% yield
X = Br, Ar = quinolyl
Division of Molecular Science, Graduate School of Science
and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu,
Gunma, 376-8515, Japan
amii@gunma-u.ac.jp
4
Received: 02.11.2016
Accepted after revision: 06.12.2016
Published online: 22.12.2016
priate ligand, phen (1,10-phenanthroline) (Scheme 1, a). In
2
011, Gooßen and co-workers demonstrated that potassi-
5
DOI: 10.1055/s-0036-1588131; Art ID: ss-2016-f0761-op
um (trifluoromethyl)trialkoxyborates are useful reagents
in handling, and aryl iodides undergo trifluoromethylation
with a combination of copper–phen catalyst and potassium
trifluoromethyl)trialkoxyborates in moderate to high
yields without an additional base (Scheme 1, b). Various
synthetic methodologies are currently required for fluori-
Abstract A method for the copper-catalyzed pentafluoroethylation of
aryl iodides using potassium (pentafluoroethyl)trimethoxyborate has
been developed. The borate was found to be a convenient pentafluoro-
ethyl source. In parallel, it was found that the solvent, as well as ligand,
makes significant impact on the catalytic perfluoroalkylations.
(
6
7
nated compounds in pharmaceutical, agricultural, and ma-
Key words fluorine, copper, cross-coupling, pentafluoroethylation,
boron
terial sciences, so that not only the trifluoromethyl group
but also the pentafluoroethyl group attracts interest as a
8
functional group substituted in aromatic compounds. In
Transition-metal-catalyzed cross-coupling of organic
halides with organometallic reagents provides one of the
most powerful tools in synthetic organic chemistry.1
Among them, the Suzuki–Miyaura cross-coupling has been
widely known as a helpful and common C–C bond forma-
tion over past decades because of the high air- and mois-
ture-stability of organoboron compounds and the tolerance
fact, useful protocols for perfluoroalkylation with copper
reagents have been published, which include trifluoro-
methylations and also pentafluoroethylations. Fuchikami et
9
10
al. and Hartwig et al. used pentafluoroethylsilanes (R Si-
3
11
CF CF ) as a pentafluoroethyl source. Additionally, Grush-
2
3
in et al.12 utilized readily available 1H-fluoroethane, and
Mikami and co-workers1 utilized pentafluoropropanoate
as an economical pentafluoroethyl source. Dauglis et al.
succeeded in the development of copper-catalyzed aromat-
ic perfluoroalkylation with 1H-perfluoroalkane reagents by
3
2
of the reaction to functional groups. Although palladium is
the most famous and nickel is the original catalyst, other
transition-metal catalysts are also utilized in approachs to
the Suzuki–Miyaura reaction. Several copper-catalyzed re-
actions, through transmetalation with organoboron com-
pounds, have been developed by virtue of the available and
14
using DMPU solvent and (TMP) Zn base. In other catalytic
2
aromatic pentafluoroethylations, pentafluoropropanoate
15
was used by Beller et al. and a bis(pentafluoroethyl)zinc
3
16
cost-effective synthesis. On the contrary, copper-mediated
reagent was used by Mikami et al. In this report, we de-
cross-couplings might be improved by the use of organobo-
ron reagents to control the reactivity or for facile treatment.
Through our handling and synthesis of fluorinated com-
pounds, which tend to have volatile, hydrophobic, and elec-
tron-withdrawing properties, or whose transition states
are unstable, we have realized the importance of the opti-
mization of reaction conditions in each type of synthesis. In
scribe a study of pentafluoroethylation which proceeds us-
ing a catalytic amount of CuI and phen (Scheme 1, c).
Firstly, we explored the solvent effects on the synthesis
of 1-nitro-3-(pentafluoroethyl)benzene (2a) from 1-iodo-
3-nitrobenzene (1a, 1.0 equiv) and potassium (pentafluoro-
ethyl)trimethoxyborate (2.0 equiv) in the presence of CuI
(10 mol%) and phen (10 mol%) at 60 °C. The current penta-
fluoroethylation afforded desired compound 2a in low
yields in polar solvents such as DMSO and DMF (Table 1, en-
tries 1 and 2), instead of the higher yields in the trifluoro-
2009, we reported that the difficulty in generating the ac-
tive species effectively in the catalytic system for trifluoro-
methylation using Et Si-CF was overcome with an appro-
3
3
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Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

B
Synthesis
T. Sugiishi et al.
Paper
equivalents and 1.2 equivalents in THF did not decrease the
yield (95% and 88%, respectively) (Table 1, entry 5 vs entries
a) trifluoromethylation (previous work)
CuI (10 mol%)
phen (10 mol%)
KF (2.0 equiv)
6
and 7).
Next, under the optimized reaction condition (Table 1,
Ar
I
+
Et3Si CF3
2.0 equiv)
Ar CF3
NMP/DMF (1:1)
entry 6), several other aryl iodides 1b–f were employed for
(
the pentafluoroethylation as shown in Table 2. Aryl iodide
1b substituted with a nitro group at the para position pro-
b) trifluoromethylation (Gooßen et al.)
vided corresponding pentafluoroethylated arene 2b in 87%
yield. In the cases that the functional group at the para po-
sition was a cyano group, a butyl group, or a methoxy
group, the pentafluoroethylation proceeded and products
OR
B
CuI (20 mol%)
phen (20 mol%)
K
Ar
I
+
RO
CF3
Ar CF3
DMSO
OR
3.0 equiv)
(
2c–e were synthesized, respectively. Furthermore, 2-bro-
moquinoline was also applied to the pentafluoroethylation
c) pentafluoroethylation (this work)
to provide 2-(pentafluoroethyl)quinoline (2f) quantitative-
ly.
CuI (10 mol%)
phen (10 mol%)
OMe
K
Ar
I
+
MeO
B
CF2CF3
OMe
1.5 equiv)
Ar CF CF
2
3
THF
Table 2 Substrate Scopea
(
Scheme 1 Catalytic perfluoroalkylations with aryl iodides
CF3CF2B(OMe)3
K
(1.5 equiv)
CuI (10 mol%)
phen (10 mol%)
Ar
X
Ar CF2CF3
THF, 60 °C, 24 h
methylation reported by Gooßen et al.6,17 The formation of a
1
2
small amount of pentafluoroethylated benzene 2a was ob-
served in the case that the reaction was carried out in ace-
tonitrile (Table 1, entry 3). The use of diglyme was more ef-
fective for enhancement of the product 2a than the use of
other polar solvents (Table 1, entries 1–3 vs entry 4). In the
course of solvent optimization, we found the yield dramati-
cally increased to 91% by switching the solvent to THF,
which worked sluggishly in the corresponding trifluoro-
Entry
1
1
2
Yieldb (%) of 2
O2N
I
O2N
CF2CF3
CF2CF3
>95 (82)
87 (71)
1
a
2a
I
2
3
4
5
O2N
1b
O2N
6
methylation. Reducing the amount of potassium (pentaflu-
oroethyl)trimethoxyborate from 2.0 equivalents to 1.5
2b
I
CF2CF3
CF2CF3
CF2CF3
85 (62)
>95 (78)
91 (72)
NC
1c
NC
2c
Table 1 Screening of Reaction Conditionsa
I
CF3CF2B(OMe)3
X equiv)
K
nBu
1d
nBu
2d
(
O2N
I
O2N
CF2CF3
CuI (10 mol%)
phen (10 mol%)
I
solvent
0 °C, 24 h
1a
2a
6
MeO
MeO
1
e
2e
Entry
Solvent
[CF
3
CF
2
B(OMe)
3
]K (X equiv) Yield (%) of 2a
1
2
3
4
5
6
DMSO
DMF
2.0
2.0
2.0
2.0
2.0
1.5
1.2
3
9
6
>95 (89)
N
Br
N
CF2CF3
1f
2f
a
MeCN
diglyme
THF
5
Reaction conditions: aryl iodide 1 (0.30 mmol), potassium (pentafluoro-
ethyl)trimethoxyborate (0.45 mmol), CuI (0.03 mmol, 10 mol%), phen
27
91
>95
88
(
0.03 mmol, 10 mol%),THF (0.6 mL), 60 °C, 24 h.
b
19
6 6
Yield was determined by F NMR analysis using C F as an internal stan-
dard. The value in parentheses indicates the isolated yield of 2.
THF
7
THF
The present pentafluoroethylation does not need the
addition of a fluoride or a strong base, differing from the
synthesis directly using Me Si-CF CF . The reaction starting
a
Reaction conditions: 1a (0.30 mmol), potassium (pentafluoroethyl)trime-
thoxyborate, CuI (10 mol%), phen (10 mol%), solvent (0.6 mL), 60 °C, 24 h.
Yields were determined by F NMR analysis using C
dard.
19
6 6
F as an internal stan-
3
2
3
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

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Synthesis
T. Sugiishi et al.
Paper
from 1-iodo-4-(trimethylsilyl)benzene (1g) with potassium
pentafluoroethyl)trimethoxyborate provided TMS-intact
product 2g selectively (Scheme 2).
(Pentafluoroethyl)arenes 2; General Procedure
(
To a flame-dried Schlenk tube equipped with a stirrer bar, were add-
ed aryl iodide or bromide 1 (0.30 mmol), potassium (pentafluoroeth-
yl)trimethoxyborate (0.45 mmol), CuI (0.03 mmol), phen (0.03
mmol), and THF (0.6 mL) under a N2 atmosphere. The mixture was
stirred at 60 °C for 24 h and cooled to r.t. The mixture was diluted
CF3CF2B(OMe)3
K
(
1.5 equiv)
with Et O and washed with sat aq NH Cl. The aqueous layer was sub-
I
CF2CF3
2
4
CuI (10 mol%)
sequently extracted with Et O (3 × 20 mL). The organic layer was
phen (10 mol%)
2
19
dried (anhyd Na SO ), filtered, and concentrated in vacuo. The
F
Me3Si
Me3Si
2
4
THF, 60 °C, 24 h
1g
2g
NMR yield was determined using C6F6 as an internal standard. The
crude product was purified by column chromatography (silica gel).
73% (59%) yield
Scheme 2 Pentafluoroethylation of 1-iodo-4-(trimethylsilyl)benzene
19
1-Nitro-3-(pentafluoroethyl)benzene (2a)19
(
1g); yield was determined by F NMR analysis using C F as an internal
6 6
standard, and the isolated yield is given in parentheses
The crude product was purified by column chromatography (silica
gel, hexane/EtOAc 5:1) to afford 2a (59.4 mg, 82%) as a yellow oil.
1
H NMR (400 MHz, CDCl ): δ = 7.76 (t, J = 8.0 Hz, 1 H), 7.96 (d, J = 8.0
In conclusion, we have achieved the catalytic pentafluo-
roethylation of aryl iodides with the addition of 10 mol% of
copper(I) iodide and 10 mol% of phen using potassium
3
Hz, 1 H), 8.47 (d, J = 8.0 Hz, 1 H), 8.50 (s, 1 H).
19
F NMR (376 MHz, CDCl , C F ): δ = 46.8 (s, 2 F), 77.2 (s, 3 F).
3
6 6
+
(pentafluoroethyl)trimethoxyborate without the addition
MS (EI): m/z (%) = 241 (M , 20), 195 (15), 172 (67), 145 (100), 126
36), 95 (15), 75 (25).
(
of a strong base. THF was found to be the best solvent for
catalytic pentafluoroethylation.
1
-Nitro-4-(pentafluoroethyl)benzene (2b)
The crude product was purified by column chromatography (silica
gel, Et O) to afford 2b (50.3 mg, 71%) as a pale yellow oil.
2
All reactions were carried out under a N atmosphere in flame-dried
2
1H NMR (400 MHz, CDCl
): δ = 7.83 (d, J = 8.8 Hz, 2 H), 8.38 (d, J = 8.8
glassware. Syringes which used to transfer anhydrous solvents or re-
agents were purged with argon prior to use. Most commercially sup-
plied chemicals were purchased from commercial suppliers and used
without further purification. THF was dried by reflux over Na chips in
the presence of benzophenone as an indicator. Analytical TLC was
performed on aluminum silica gel 60 F254 (Merck) sheets, which
were visualized by the quenching of UV fluorescence (254 nm). Col-
umn chromatography was conducted on silica gel (Cica, 60–210
mesh, spherical, neutral). NMR spectra were acquired on a Jeol JNM-
3
Hz, 2 H).
19
F NMR (376 MHz, CDCl , C F ): δ = 46.4 (s, 2 F), 77.3 (s, 3 F).
3
6 6
+
MS (EI): m/z (%) = 241 (M , 20), 195 (13), 172 (40), 145 (100), 125
19), 114 (16).
(
Data is consistent with that reported in the literature.13b
1-Cyano-4-(pentafluoroethyl)benzene (2c)12b
1
19
ECS 400 (400 MHz for H and 376 MHz for F, respectively) spectrom-
The crude product was purified by column chromatography column
1
eters. H NMR spectra were recorded using TMS as an internal stan-
dard (δ = 0). F NMR spectra (376 MHz) were recorded using hexafluo-
robenzene (C F ) as an internal standard (δ = 0). GC-MS were record-
chromatography (silica gel, Et O) to afford 2c (39.9 mg, 62%) as a pale
2
19
yellow oil.
6
6
1
H NMR (400 MHz, CDCl ): δ = 7.75 (d, J = 8.4 Hz, 2 H), 7.83 (d, J = 8.4
3
ed on a Thermo TRACE DSQ.
Hz, 2 H).
19
F NMR (376 MHz, CDCl , C F ): δ = 45.9 (s, 2 F), 77.2 (s, 3 F).
3
6 6
Potassium (Pentafluoroethyl)trimethoxyborate; General Proce-
dure
5
MS (EI): m/z (%) = 221 (M , 11), 152 (100), 102 (15).
+
A two-necked round bottom flask, equipped with a stirrer bar and
containing KF (581 mg, 10 mmol), was flame dried under reduced
pressure. When the flask reached r.t., THF (20 mL) was added under a
N2 atmosphere. The mixture was stirred for 1 h, and then trifluoro-
methoxyborate (1.25 g, 1.34 mL, 12 mmol) and (pentafluoroeth-
yl)trimethylsilane (2.31 g, 2.08 mL, 12 mmol) were added and the
contents were mixed at r.t. for 48 h. The obtained colorless mixture,
1-Butyl-4-(pentafluoroethyl)benzene (2d)
The crude product was purified by column chromatography (silica
gel, Et O) to afford 2d (58.8 mg, 78%) as a yellow oil.
2
1
H NMR (400 MHz, CDCl ): δ = 0.94 (t, J = 7.2 Hz, 3 H), 1.36 (m, J = 7.2
3
Hz, 2 H), 1.58–1.66 (m, 2 H), 2.67 (t, J = 7.6 Hz, 2 H), 7.30 (d, J = 8.8 Hz,
2
H), 7.49 (d, J = 8.8 Hz, 2 H).
19
which indicated that KF was exhausted by F NMR analysis, was con-
centrated in vacuo to afford potassium (pentafluoroethyl)trimethoxy-
borate (2.19 g, 8.36 mmol, 84%) as a white solid.
1
19
F NMR (376 MHz, CDCl , C F ): δ = 47.2 (s, 2 F), 76.9 (s, 3 F).
3
6 6
+
MS (EI): m/z (%) = 252 (M , 26), 210 (64), 183 (21), 140 (66), 127 (17),
91 (100).
H NMR (400 MHz, D O): δ = 3.33 (s, 9 H).
2
Data is consistent with that reported in the literature.12b
19
2
F NMR (376 MHz, D O, C F ): δ = 29.6 (q, J = 18.4 Hz, 2 F), 81.0 (s, 3
2
6
6
BF
F).
1
-Methoxy-4-(pentafluoroethyl)benzene (2e)
1-Iodo-4-trimethylsilylbenzene (1g) was prepared according to the
18
The crude product was purified by column chromatography (silica
gel, Et O) to afford 2e (49.2 mg, 72%) as a yellow oil.
reported procedure.
2
1
H NMR(400 MHz, CDCl ): δ = 3.86 (s, 3 H), 6.99 (d, J = 8.8 Hz, 2 H),
3
7.52 (d, J = 8.8 Hz, 2 H).
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

D
Synthesis
T. Sugiishi et al.
Paper
1
9
F NMR (376 MHz, CDCl , C F ): δ = 47.9 (s, 2 F), 76.7 (s, 3 F).
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6 6
+
MS (EI): m/z (%) = 226 (M , 18), 157 (100), 114 (22).
Data is consistent with that reported in the literature.13b
2
-(Pentafluoroethyl)quinoline (2f)
The crude product was purified by column chromatography (silica
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H NMR (400 MHz, CDCl ): δ = 7.69 (ddd, J = 8.4, 8.0, 1.2 Hz, 1 H), 7.76
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F NMR (376 MHz, CDCl , C F ): δ = 46.6 (s, 2 F), 77.0 (s, 3 F).
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-(Pentafluoroethyl)-4-(trimethylsilyl)benzene (2g)
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7.64 (d, J = 8.0 Hz, 2 H).
19
F NMR (376 MHz, CDCl , C F ): δ = 45.0 (s, 2 F), 78.9 (s, 3 F).
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6 6
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Acknowledgment
The financial support of JSPS KAKENHI Grant No. JP16H01003 in Pre-
cisely Designed Catalysts with Customized Scaffolding, JP16H01129
in Middle Molecular Strategy, and Japan Science and Technology
Agency (JST) (ACT-C: Creation of Advanced Catalytic Transformation
for the Sustainable Manufacturing at Low Energy, Low Environmental
Load) is acknowledged. We would like to thank Prof. Hiroshi Sano
(Gunma University) for his useful suggestions.
2
015, 115, 765. (x) Nenajdenko, V. G.; Muzalevskiy, V. M.;
Supporting Information
Shastin, A. V. Chem. Rev. 2015, 115, 973. (y) Alonso, C.; Martinez
de Marigorta, E.; Rubiales, G.; Palacios, F. Chem. Rev. 2015, 115,
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588131.
1847. (z) Sugiishi, T.; Amii, H.; Aikawa, K.; Mikami, K. Beilstein J.
S
u
p
p
ortioIgnfrm oaitn
S
u
p
p
ortioIgnfrm oaitn
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(
11) Using (pentafluoroethyl)trimethylsilane, the pentafluoroethyla-
tion of aryl iodides 1 needs additional base such as KF. Our pre-
vious method[ for the synthesis of (pentafluoroethyl)arenes 2
was carried out (Scheme 3). The addition of Me Si-CF CF (2.0
4]
3
2
3
(2) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
equiv) to 1-iodo-3-nitrobenzene (1a, 1.0 equiv) in the presence
of CuI (10 mol%) and phen (10 mol%) at 60 °C formed 1-nitro-3-
(b) Miyaura, N. J. Organomet. Chem. 2002, 54, 653. (c) Saito, S.;
Oh-tani, S.; Miyaura, N. J. Org. Chem. 1997, 62, 8024.
(
pentafluoroethyl)benzene (2a) in only 56% yield (determined
(
3) For examples of copper-catalyzed Suzuki–Miyaura couplings,
see: (a) Li, J.-H.; Li, J.-L.; Wang, D.-P.; Pi, S.-F.; Xie, Y.-X.; Zhang,
M.-B.; Hu, X.-C. J. Org. Chem. 2007, 72, 2053. (b) Li, J.-H.; Li, J.-L.;
Xie, Y.-X. Synthesis 2007, 984. (c) Mao, J.; Guo, J.; Fang, S.-J. J. Tet-
rahedron 2008, 64, 3905. (d) Yang, C.-T.; Zhang, Z.-Q.; Liu, Y.-C.;
Liu, L. Angew. Chem. Int. Ed. 2011, 50, 3904. (e) Gurung, S. K.;
19
by F NMR analysis using CF CF OH as an internal standard).
3
2
(12) (a) Zanardi, A.; Novikov, M. A.; Martin, E.; Benet-Buchholz, J.;
Grushin, V. V. J. Am. Chem. Soc. 2011, 133, 20901.
(b) Lishchynskyi, A.; Grushin, V. V. J. Am. Chem. Soc. 2013, 135,
12584.
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

E
Synthesis
T. Sugiishi et al.
Paper
(13) (a) Serizawa, H.; Aikawa, K.; Mikami, K. Chem. Eur. J. 2013, 19,
17692. (b) Serizawa, H.; Aikawa, K.; Mikami, K. Org. Lett. 2014,
O2N
I
1
6, 3456.
14) Propov, I.; Lindeman, S.; Daugulis, O. J. Am. Chem. Soc. 2011, 133,
286.
+
Me3Si CF2CF3
(
(
2.0 equiv)
1a
9
CuI (10 mol%)
phen (10 mol%)
KF (2.0 equiv)
(15) Schareina, T.; Wu, X.-F.; Zapf, A.; Cotte, A.; Gotta, M.; Bellar, M.
2
O N
2 3
CF CF
Top. Catal. 2012, 55, 426.
(16) Aikawa, K.; Nakamura, Y.; Yokota, Y.; Toya, W.; Mikami, K.
Chem. Eur. J. 2015, 21, 96.
NMP/DMF (1:1)
60 °C, 24 h
2
a
(17) Gonda, Z.; Kovács, S.; Wéber, C.; Gáti, T.; Mészáros, A.; Kotschy,
A.; Novák, Z. Org. Lett. 2014, 16, 4268.
5
6% yield
Scheme 3
(18) Doszczak, L.; Kraft, P.; Weber, H.-P.; Bertermann, R.; Triller, A.;
Hatt, H.; Tacke, R. Angew. Chem. Int. Ed. 2007, 46, 3367.
(19) Sheppard, W. A. J. Am. Chem. Soc. 1965, 87, 2410.
(20) Utsumi, S.; Katagiri, T.; Uneyama, K. Tetrahedron 2012, 68, 1085.
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E