Base initiated aromatization/CO bond formation: A new entry to O-pyrazole polyfluoroarylated ethers

Article abstract of DOI:10.1016/j.tetlet.2014.09.129

A base initiated intermolecular S_NAr reaction of pyrazolones with polyfluoroarenes was developed. The process involved the isomerization aromatization of pyrazolone followed by the CO bond formation via the selective CF bond cleavage. With this strategy, a wide range of O-pyrazole polyfluoroarylated ethers bearing diverse functional groups were synthesized in mild to good yields. Additionally, our method was also applied to the isoxazol substrates.

Full text of DOI:10.1016/j.tetlet.2014.09.129

Tetrahedron Letters 55 (2014) 6534–6537
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Base initiated aromatization/CAO bond formation: a new entry
to O-pyrazole polyfluoroarylated ethers
a,b,c,
Xiangyang Tang a, Jing Chang a, Cuibo Liu a,b, , Bin Zhang
⇑
⇑
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, PR China
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
c The Key Lab of Systems Bioengineering, Ministry of Education, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A base initiated intermolecular SNAr reaction of pyrazolones with polyfluoroarenes was developed. The
process involved the isomerization aromatization of pyrazolone followed by the CAO bond formation
via the selective CAF bond cleavage. With this strategy, a wide range of O-pyrazole polyfluoroarylated
ethers bearing diverse functional groups were synthesized in mild to good yields. Additionally, our
method was also applied to the isoxazol substrates.
Received 16 August 2014
Revised 23 September 2014
Accepted 29 September 2014
Available online 5 October 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Polyfluoroarenes
CAF bond cleavage
SNAr reaction
Pyrazolones
The creation of diverse molecules via carbonAfluorine bond
cleavage of fluorine-containing compounds, especially polyfluoro-
arenes, has been considered to be a powerful tool in synthetic
chemistry. Many pioneering and interesting works have been
achieved by using this method.1 Among various reports, the transi-
tion-metal catalyzed strategy was in general regarded as an effi-
cient option toward the cleavage of CAF bond.2 However, the
intrinsic defects, such as high cost of some metal catalysts, envi-
ronment-unfriendly, and contamination of the final products by
heavy metals arouse the motivation to search for more benign
alternatives. Thus, significant progress has been made on the
development of metal-free nucleophilic aromatic substitution
(SNAr) reaction.3 Due to the simpler, milder, and metal-free condi-
tions, the SNAr reaction has gradually become a fundamental
procedure, and numerous new bonds including CAC,4 CAO,5
CAN,6 CAS,7 and CAP8 bonds were fabricated with increasing
levels of efficiency, practicality, and reliability. For example,
Sandford and co-workers reported annelation reactions of penta-
fluorobenzonitrile with different types of difunctional nitrogen
centered nucleophiles to synthesize various [5,6]- and [6,6]-ring-
fused systems.9 Just recently, the Cao group has successfully devel-
oped an efficient method for the synthesis of 2-polyfluoroaryl ary-
lacetates via a one-pot three-component VNSAr–SNAr reaction of
polyfluoroarenes, ethyl 2-chloropropionate, and ortho substituted
nitrobenzenes. It was found that the strong electron-withdrawing
group in polyfluoroarenes was essential for the reaction effice-
ney.10 Very recently, our group has reported a one-pot two-step
sequential transformation for
a
series of O-2,3,5,6-tetra-
fluorobenzonitrile substituted oxime ethers via an in situ formed
oxygen centered nucleophile intemediate.11 In order to implement
this protocol of CAF bond cleavage of polyfluoroarenes effectively
and expand the substrate scope for fruitful novel compounds, seek-
ing for appropriate and practical nucleophiles was of high
importance.
Pyrazolone frameworks were a class of important subunits, and
compounds bearing such moiety frequently exhibited wide appli-
cations in pharmaceutical and agrochemical chemistry.12 Specially,
pyrazolone was selected as a key intermediate in organic synthesis
owing to its special structure and property as an ambident nucle-
ophile: the carbon anion and oxygen anion nucleophile sites. To
date, the usefulness of pyrazolone as nucleophiles has been exten-
sively explored for the construction of a variety of pyrazolone
derivatives. Yet, most of the examples were mainly focused on
the carba-Michael addition reactions by employing pyrazolone as
a carbon nucleophile.13 Relatively fewer examples have been doc-
umented for organic transformations by using pyrazolone as an
oxygen nucleophile.14 The Zhao group made a great advance on
finding a new catalytic mode of the MDO-catalyzed tandem
Michael addition–cyclization reaction between 3-methyl-2-pyraz-
olin-5-one and benzylidenemalononitriles. Mechanism investiga-
tion hinted that the cyclization process was associated with the
⇑
Corresponding authors. Tel.: +86 22 27406140; fax: +86 22 27403475.
E-mail addresses: chemliucuibo@163.com (C. Liu), bzhang@tju.edu.cn (B. Zhang).
http://dx.doi.org/10.1016/j.tetlet.2014.09.129
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

X. Tang et al. / Tetrahedron Letters 55 (2014) 6534–6537
6535
tBuO
O
Table 1
Screening results for the SNAr reaction of pyrazolone 1a with pentafluorobenzonitrile
2aa
O
F
CN
H3C
N
O
F
O
F
F
F
F
N
O
N
N
N
CH3
Base, Solvent
RT
O
F
N
N
F
CN
O
N
N
F
Fenpyroximat
O
O
NH
H3C
1a
2a
3a
O
HO
O
N
O
Entry
Base
Solvent
Temp
Yieldb (%)
N
O
N
O
HN
N
O
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KOH
K3PO4
K2CO3
NaOtBu
NaH
DMSO
DMF
CH3CN
DCE
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
41
63
73
Trace
—
Trace
Trace
85
68
74
HN
OCF3
P2Y12 antagonist
P2Y1 receptor antagonist
H2O
H
O
O
Dioxane
Toluene
THF
THF
THF
THF
THF
THF
THF
F
F
OEt
O
F
O
N
OH
NC
F
F
F
F
OH
78
49
56
66
F
F
O
O
F
F
DABCO
O
CN
a
Inhibitor of PARP1
STAT3 pathway inhibitor
Reaction conditions: 1a (0.2 mmol), 2a (0.26 mmol), base (0.22 mmol), solvent
(2.0 ml), at room temperature.
b
Isolated yield. DMSO = dimethyl sulfoxide, DMF = N,N-dimethylformamide,
Figure 1. Several examples of pyrazolone frameworks and 2,3,5,6-tetra-
DCE = dichloroethane, THF = tetrahydrofuran.
fluorobenzonitrile-containing pharmaceutical and bioactive compounds.
nucleophilic attack of oxygen anion to cyano.15 In this context, all
events were the nucleophilic addition reactions with respect to
pyrazolone. However, the SNAr reaction of pyrazolone with poly-
fluoroarenes remained untouched until now.
only be of actual synthetic utility if one could control the selectiv-
ity for the sole product. Literatures illustrated that pyrazolone
could transform into the enolate isomer under basic conditions.
We had previously reported inorganic base promoted SNAr reac-
tion of phenols or benzyl alcohols with pentafluorobenzene.16
Encouraged by these transformations, we conceived that a CAO
bond forming reaction based on SNAr protocol might precede the
CAC bond formation with a similar reaction system. To test the
speculation, the model reaction of 3-methyl-1-phenylpyrazolone
1a and pentafluorobenzonitrile 2a in the presence of 110 mol %
Cs2CO3 in DMSO at room temperature was performed. Delightedly,
the reaction actually occurred, affording the product 3a in 41%
yield (Table 1, entry 1). The structure of product was determined
by NMR, IR and HRMS analysis, as well as the X-ray analysis of
3c (Fig. 2) and confirmed that pyrazolone was as an oxygen nucle-
ophile in this reaction. The CAC bond forming product was not
detected. Next, we turned our attention to the screening of sol-
vents. The primary results revealed that THF was most promising
for this reaction, giving rise to the desired product in 85% yield
Given the importance of the pyrazolone skeletons and the
2,3,5,6-tetrafluorobenzonitrile-containing compounds in pharma-
ceutical and agricultural fields (Fig. 1), it was of great synthetic
interest to develop an efficient and simple method to access these
valuable compounds. Herein, we reported, for the first time, a base
initiated aromatization/CAO bond formation reaction using pyraz-
olone and pentafluorobenzonitrile as the starting materials via the
selective CAF bond cleavage (Scheme 1). This SNAr strategy enabled
us to obtain a variety of O-pyrazole polyfluoroarylated ethers as the
only products in mild to good yields. Additionally, our method was
also applied to the isoxazol substrates for efficient transformations.
Furthermore, we provided a new way for the preparation of polyflu-
oroarylated ethers.
Since there were two reaction sites in pyrazolone, the SNAr
reaction with polyfluoroarenes might conduct in two modes to
form a mixture. From the synthetic point of view, method could
F
R3
R1
F
F
favourable product
N
O
N
R2
R1
F
F
F
F
F
3
Cs2CO3, THF
RT
F
R3
N
O
N
R2
1
R1
F
F
F
F
N
N
R3
disfavourable product
2
R2
O
3'
Scheme 1. SNAr reaction of pyrazolones with polyfluoroarenes.

6536
X. Tang et al. / Tetrahedron Letters 55 (2014) 6534–6537
F
R3
F
F
F
F
N
O
N
F
Cs2CO3, THF
RT
F
R3
N
O
N
F
F
3t, R3 = H, Yield = 57%
3u
3v
1a
2b-2d
, R3 = CF3, Yield = 74%
, R3 = NO2, Yield = 41%
Scheme 2. Substrate scope of the SNAr reaction of 3-methyl-1-phenylpyrazolone
1a with other polyfluoroarenes.17
where the substituents lay. The Naphthyl substrated pyrazolones
were amenable to the standard conditions to produce the products
in acceptable yields (Table 2, 3m and 3n). Interestingly, heterocy-
cle-substituted pyrazolones were also viable substrates in this
reaction, which afforded the desired products in 44–73% yields
(Table 2, 3o–q). Next, examination of the substitution pattern at
the N-aryl moiety determined that the existence of 4-fluorophenyl
and benzyl as the protection groups did not hamper the reaction,
and 71% and 57% yields of the products were generated (Table 2,
3r and 3s).
The other polyfluoroarenes were also selected as the partners
for this SNAr reaction (Scheme 2).17 For example, when employed
pentafluorobenzene as the electrophile, elevating the temperature
to 90 °C and using DMSO as the solvent were essential for this
transformation. Note that, a negative result was obtained when
1a reacted with pentafluoronitrobenzene.
To test the generality of our approach and expand its applica-
tions in constructing new compounds, we treated the isoxazol sub-
strates to the optimal conditions. As depicted in Table 3, the SNAr
reaction took place regardless of different types of the reactants,
furnishing O-isoxazol polyfluoroarylated ethers in 37–65% yields.
These exciting results further demonstrated that this strategy
was of great synthetic potential for fabricating new polyfluoroary-
lated ethers.
In summary, we have developed a base initiated intermolecular
SNAr reaction of pyrazolones with polyfluoroarenes via the selec-
tive CAF bond cleavage. The process involved the isomerization
aromatization of pyrazolone followed by the CAO bond formation.
With this strategy, a wide range of the O-pyrazole polyfluoroarylat-
ed ethers bearing diverse functional groups were synthesized in
mild to good yields. Additionally, our method was also applied to
the isoxazol substrates. Furthermore, we expected this approach
to be used for assembling valuable pharmaceutical and bioactive
molecules in the future.
Figure 2. X-ray structure of 3c (Hatoms omitted for clarity).
(Table 1, entry 8). The others were less effective or even not useful
(Table 1, entries 2–7). This proved that the solvent played an
important role on the reaction efficiency. Further optimization on
the bases led to a discovery that only Cs2CO3 was optical, and the
rest were inferior to Cs2CO3 (Table 1, entries 9–14). Elevating the
temperature to 60 °C or decreasing the temperature to 0 °C implied
no improvement on the effectiveness of this SNAr reaction.
After having established the optimized reaction conditions, the
scope of the reaction was explored with respect to the nucleo-
philes. Results were listed in Table 2. Firstly, we investigated the
substitution effect at the 3-position of pyrazolone, and found that
a good yield of the product 3b was also created when replacing the
methyl group with a larger n-propyl group. A number of 3-aryl
substituted pyrazolones had also been explored. When the aro-
matic ring bearing electron-donating or -withdrawing groups, the
aromatization/CAO bond formation reaction proceeded smoothly
to deliver the terminal products 3c–3l in 59–88% yields no matter
Table 2
Substrate scope of the SNAr reaction of pyrazolones with pentafluorobenzonitrile 2aa
F
R1
F
F
CN
F
R1
F
Cs2CO3, THF
RT
F
CN
N
O
N
N
O
N
R2
R2
F
F
F
1a-1s
2a
3a-3s
Entry
R1
R2
Product 3
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Me
n-Propyl
C6H5
4-MeC6H4
4-MeOC6H4
4-FC6H4
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4-FC6H4
Benzyl
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
85
75
88
70b
83b
80b
63
80b
64
59
70
79
64
63b
70
44
73
71
57
Table 3
Substrate scope of the SNAr reaction of the isoxazol substrates with penta-
fluorobenzonitrile 2aa
4-ClC6H4
F
F
F
F
4-BrC6H4
3-MeC6H4
3-MeOC6H4
3-ClC6H4
2-BrC6H4
1-Naphthyl
2-Naphthyl
2-Furanyl
R4
CN
F
R4
Cs2CO3, THF
RT
F
F
CN
N
O
N
O
O
O
F
F
1w-1z
2a
3w-3z
Entry
R4
Product 3
Yield (%)
2-Thiopheneyl
2-Pyridyl
C6H5
1
2
3
4
4-MeC6H4
4-MeOC6H4
4-ClC6H4
3w
3x
3y
3z
42
51
37
65
Me
1-Naphthyl
a
Reaction conditions: 1 (0.2 mmol), 2a (0.26 mmol), base (0.22 mmol), solvent
(2.0 ml), at room temperature.
a
Reaction conditions: isoxazol (0.2 mmol), 2a (0.26 mmol), base (0.22 mmol),
solvent (2.0 ml), at room temperature.
b
2.0 ml DMSO was used as solvent.

X. Tang et al. / Tetrahedron Letters 55 (2014) 6534–6537
6537
1038; (h) Liu, C. B.; Wang, H.; Xing, X.; Xu, Y.; Ma, J.-A.; Zhang, B. Tetrahedron
Lett. 2013, 54, 4649; (i) Cao, L.; Liu, C.; Tang, X.; Yin, X.; Zhang, B. Tetrahedron
Lett. 2014, 55, 5033.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (No. 21422104), New Century Excel-
lent Talents in University of Ministry of Education of China
(NCET-12-0391) and Innovation Foundation of Tianjin University.
4. Vaidyanathaswamy, R.; Radha, K.; Dharani, M.; Raguraman, T. S.; Anand, R. J.
Fluorine Chem. 2012, 144, 33.
5. (a) James, J. H.; Peach, M. E.; Williams, C. R. J. Fluorine Chem. 1985, 27, 91; (b)
Brooke, G. M. J. Fluorine Chem. 1997, 86, 1; (c) Adonin, N. Yu.; Bardin, V. V. J.
Fluorine Chem. 2013, 153, 165.
6. (a) Diness, F.; Begtrup, M. Org. Lett. 2014, 16, 3130; (b) Xiong, Y.; Zhang, X. X.;
Huang, T.; Cao, S. J. Org. Chem. 2014, 79, 6395; (c) Leyva, E.; Leyva, S.;
Moctezuma, E.; González-Balderas, R. M.; Loera, D. D. J. Fluorine Chem. 2013,
156, 164; (d) óBrien, A. G.; Horváth, Z.; Lévesque, F.; Lee, J. W.; Seidel-
Morgenstern, A.; Seeberger, P. H. Angew. Chem. Int. Ed. 2012, 51, 7028; (e)
Sandford, G.; Tadeusiak, A.; Yufit, D. S.; Howard, J. A. K. J. Fluorine Chem. 2007,
128, 1216; (f) Jitchati, R.; Batsanov, A. S.; Bryce, M. R. Tetrahedron 2009, 65, 855.
7. (a) Stephens, D. E.; Chavez, G.; Valdes, M.; Dovalina, M.; Arman, H. D.; Larionov,
O. V. Org. Biomol. Chem. 2014, 12, 6190; (b) Zhou, Q. Z.; Zhang, B.; Gu, H. N.;
Zhong, A. G.; Du, T. Q.; Zhou, Q.; Ye, Y. Y.; Jin, Z. N.; Chen, H. J.; Jiang, H. J.; Chen,
R. E. Lett. Org. Chem. 2012, 9, 175; (c) Peach, M. E.; Smith, K. C. J. Fluorine Chem.
1985, 27, 105.
8. Gupta, O. D.; Kirchmeier, R. L.; Shreeve, J. M. J. Fluorine Chem. 1999, 97, 223.
9. Cargilla, M. R.; Linton, K. E.; Sandford, G.; Yufit, D. S.; Howard, J. A. K.
Tetrahedron 2010, 66, 2356.
10. Xiong, Y.; Wu, J. J.; Xiao, S. H.; Cao, S. Chin. J. Chem. 2012, 30, 2747.
11. Liu, C. B.; Yin, X. G.; Chang, J.; Tang, X. Y.; Zhang, B. J. Fluorine Chem. 2014, 165,
101.
12. For selected examples, see: (a) Pfefferkorn, J. A.; Choi, C.; Winters, T.; Kennedy,
R.; Chi, L. G.; Perrin, L. A.; Lu, G. N.; Ping, Y.-W.; McClanahan, T.; Schroeder, R.;
Leininger, M. T.; Geyer, A.; Schefzick, S.; Atherton, J. Bioorg. Med. Chem. Lett.
2008, 18, 3338; (b) Londershausen, M. Pestic. Sci. 1996, 48, 269; (c) Zech, G.;
Hessler, G.; Evers, A.; Weiss, T.; Florian, P.; Just, M.; Czech, J.; Czechtizky, W.;
Görlitzer, J.; Ruf, S.; Kohlmann, M.; Nazaré, M. J. Med. Chem. 2012, 55, 8615; (d)
Ma, R.; Zhu, J.; Liu, J.; Chen, L.; Shen, X.; Jiang, H.; Li, J. Molecules 2010, 15, 3593;
(e) Mariappan, G.; Saha, B. P.; Bhuyan, N. R.; Bharti, P. R.; Kumar, D. J. Adv.
Pharm. Technol. Res. 2010, 1, 260; (f) Chande, M. S.; Barve, P. A.; Suryanarayan,
V. J. Heterocycl. Chem. 2007, 44, 49; (g) Nagashima, S. Anal. Chem. 1983, 55,
2086; (h) Alba, A. N.; Zea, A.; Valero, G.; Calbet, T.; Font-Bardía, M.; Mazzanti,
A.; Moyano, A.; Rios, R. Eur. J. Org. Chem. 2011, 1318.
13. (a) Yang, Z. G.; Wang, Z.; Bai, S.; Liu, X.; Lin, L.; Feng, X. Org. Lett. 2011, 13, 596;
(b) Liao, Y.-H.; Chen, W.-B.; Wu, Z.-J.; Du, X.-L.; Cun, L.-F.; Zhang, X.-M.; Yuan,
W.-C. Adv. Synth. Catal. 2010, 352, 827; (c) Radi, M.; Bernardo, V.; Bechi, B.;
Castagnolo, D.; Pagano, M.; Botta, M. Tetrahedron Lett. 2009, 50, 6572; (d)
Gogoi, S.; Zhao, C.-G.; Ding, D. Org. Lett. 2009, 11, 2249; (e) Li, F.; Sun, L.; Teng,
Y. O.; Yu, P.; Zhao, J. C.-G.; Ma, J.-A. Chem. Eur. J. 2012, 18, 14255; (f) Wu, J.-W.;
Li, F.; Zheng, Y.; Nie, J. Tetrahedron Lett. 2012, 53, 4828.
Supplementary data
Supplementary data (experimental details and NMR spectra)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2014.09.129.
References and notes
1. (a) Amii, H.; Uneyama, K. Chem. Rev. 2009, 109, 2119; (b) Diness, F.; Fairlie, D. P.
Angew. Chem. Int. Ed. 2012, 51, 8012; (c) Zhang, L. J.; Zhang, W.; Liu, J.; Hu, J. B. J.
Org. Chem. 2009, 74, 2850; (d) Jasch, H.; Höfling, S. B.; Heinrich, M. R. J. Org.
Chem. 2012, 72, 1520; (e) Clot, E.; Eisenstein, O.; Jasim, N.; Macgregor, S. A.;
Mcgrady, J. E.; Perutz, R. N. Acc. Chem. Res. 2011, 44, 333; (f) Choi, J.; Wang, D.
Y.; Kundu, S.; Choliy, Y.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. Science
2011, 332, 1545; (g) Allemann, O.; Duttwyler, S.; Romanto, P.; Baldridge, K. K.;
Siegel, J. S. Science 2011, 332, 574; (h) Cano, R.; Ramón, D. J.; Yus, M. J. Org.
Chem. 2011, 76, 654.
2. (a) Zhan, J. H.; Lv, H. B.; Yu, Y.; Zhang, J.-L. Adv. Synth. Catal. 2012, 354, 1529; (b)
Zhang, J.; Wu, J. J.; Xiong, Y.; Cao, S. Chem. Commun. 2012, 8553; (c) Sun, L. B.;
Rong, M. G.; Kong, D. D.; Bai, Z. H.; Yuan, Y. F.; Weng, Z. Q. J. Fluorine Chem.
2013, 150, 117; (d) Barrio, P.; Castarlenas, R.; Esteruelas, M. A.; Lledos, A.;
Maseras, F.; Onate, E.; Tomas, J. Organometallics 2001, 20, 442; (e) Vela, J.;
Smith, J. M.; Yu, Y.; Ketterer, N. A.; Flaschenriem, C. J.; Lachicotte, R. J.; Holland,
P. L. J. Am. Chem. Soc. 2005, 127, 7857; (f) Sun, A.; Love, J. L. Dalton Trans. 2010,
39, 10362; (g) Nakamura, Y.; Yoshikai, N.; Ilies, L.; Nakamura, E. Org. Lett. 2012,
14, 3316; (h) Yu, D.; Shen, Q. L.; Lu, L. J. Org. Chem. 2012, 77, 1798; (i) Doster, M.
E.; Johnson, S. A. Angew. Chem. Int. Ed. 2009, 48, 2185; (j) Schaub, T.; Backes, M.;
Radius, U. J. Am. Chem. Soc. 2006, 128, 15964; (k) Jones, W. D. Dalton Trans.
2003, 3991; (l) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. Rev. 1994,
94, 373; (m) Buckley, H. L.; Wang, T.; Tran, O.; Love, J. A. Organometallics 2009,
28, 2356; (n) Yoshikai, N.; Matsuda, H.; Nakamura, E. J. Am. Chem. Soc. 2009,
131, 9590.
3. (a) Brooke, G. M.; Burdon, J.; Stacey, M.; Tatlow, J. C. J. Chem. Soc. 1960, 1768;
(b) Birchall, J. M.; Haszeldine, R. N.; Parkinson, A. R. J. Chem. Soc. 1962, 4966; (c)
Chow, W. S.; Chan, T. H. Tetrahedron Lett. 2009, 50, 1286; (d) Fujii, S.; Maki, Y.;
Kimoto, H. J. Fluorine Chem. 1989, 43, 131; (e) Otsuka, M.; Endo, K.; Shibata, T.
Chem. Commun. 2010, 336; (f) Cargill, M. R.; Sandford, G.; Tomlinson, D. J.;
Hollfelder, N.; Pleis, F.; Nelles, G.; Kilickiran, P. J. Fluorine Chem. 2011, 132, 829;
(g) Singh, R.; Allam, B. K.; Raghuvanshi, D. S.; Singh, K. N. Tetrahedron 2013, 69,
14. Gogoi, S.; Zhao, C.-G. Tetrahedron Lett. 2009, 50, 2252.
15. Muramulla, S.; Zhao, C.-G. Tetrahedron Lett. 2011, 52, 3905.
16. Liu, C. B.; Cao, L. M.; Yin, X. G.; Xu, H. L.; Zhang, B. J. Fluorine Chem. 2013, 156, 51.
17. Reaction conditions: 1a (0.2 mmol), polyfluoroarenes (0.26 mmol), Cs2CO3
(0.22 mmol), THF (2.0 ml), at room temperature.