Selective C4-F bond cleavage of pentafluorobenzene: Synthesis of N-tetrafluoroarylated heterocyclic compounds

Article abstract of DOI:10.1016/j.tetlet.2013.06.055

A simple aromatic nucleophilic monosubstitution reaction for the synthesis of N-tetrafluoroarylated heterocyclic compounds via selective C4-F bond cleavage of pentafluorobenzene with N-H containing heterocycles is demonstrated. This method is highly tolerant of a wide range of substrates to give the corresponding products in moderate to good yields. Additionally, this strategy is applied to synthesize other mono-, di-, and tri-fluoroarylated indole derivatives. Crown Copyright

Full text of DOI:10.1016/j.tetlet.2013.06.055

Tetrahedron Letters 54 (2013) 4649–4652
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective C4–F bond cleavage of pentafluorobenzene: synthesis
of N-tetrafluoroarylated heterocyclic compounds
⇑
Cuibo Liu, Huan Wang, Xing Xing, You Xu, Jun-An Ma, Bin Zhang
Department of Chemistry, School of Science, Tianjin University, The Co-Innovation Center of Chemistry and Chemistry Engineering of Tianjin and The Key Lab of
Systems Bioengineering, Ministry of Education, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 March 2013
Revised 1 June 2013
Accepted 13 June 2013
Available online 21 June 2013
A simple aromatic nucleophilic monosubstitution reaction for the synthesis of N-tetrafluoroarylated het-
erocyclic compounds via selective C4–F bond cleavage of pentafluorobenzene with N–H containing het-
erocycles is demonstrated. This method is highly tolerant of a wide range of substrates to give the
corresponding products in moderate to good yields. Additionally, this strategy is applied to synthesize
other mono-, di-, and tri-fluoroarylated indole derivatives.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords:
C–F cleavage
Polyfluoroarenes
N-heterocycles
Fluorine chemistry
The incorporation of fluorine or fluorine-containing groups into
organic molecule has received intensive attention due to the un-
ique effect of F-substituents in material science and the pharma-
ceutical industry.1 While pentafluorobenzene is considered to be
a representative class of such fluorinated compounds which could
be transformed to other fluorine-containing compounds through a
series of reactions.2 To date, elegant achievements of the function-
alization of pentafluorobenzene have been made on the direct ary-
lation, alkenylation, and alkylation.3 Yet, these pioneering and
intriguing works are mainly focused on the C–H activation of pen-
tafluorobenzene. The carbonꢀfluorine bond is known as the stron-
gest single bond, and its activation/cleavage has attracted an
increasing interest because such a process offers great potential
in the preparation of bioactive compounds. Despite the C–F cleav-
age reactions (intermolecular or intramolecular C–F cleavage) of
pentafluoroarenes and other fluoroarenes have emerged, most of
them were carried out in the presence of the transition-metal cat-
alysts.4 For instance, the Johnson group has successfully developed
an efficient method for selective C–F bond activation of tetrafluoro-
benzenes by Nickel with an analogous nitrogen donor.3f Well-de-
signed nickel-based catalytic strategy reported by the Radius
group enables us to produce new C–C bonds via selective C–F acti-
vation of polyfluoarenes.5 And recently, the Nakamura group has
made an outstanding advance on finding a diphosphine-assisted
transition metal catalyst, which allows us to create partially fluori-
nated arenes.4j Although these significant advances have been
achieved, the development of metal-free methods for the direct
cleavage of C–F bond of pentafluoroarenes is highly desirable.
N-arylated heterocycles are key intermediates in the synthesis
of many biologically-active compounds6 and great efforts have
been made for their constructions. The transition-metal catalyzed
Ullmann-type reaction is one of the traditional methods for the
preparation of N-arylated heterocyclic compounds,7 but usually
these protocols suffer from some drawbacks, such as higher reac-
tion temperature, long reaction time, tedious procedures for the
preparation of ligands,8 toxicity of some catalysts,9 and waste dis-
posal issues arising from the use of excess catalysts,10 which inhi-
bit their wide applications in the construction of complex
molecules. Apart from the transition-metal-catalyzed strategy,
nucleophilic aromatic substitution reactions,4h,11 or coupling with
organometallic reagents12 are two alternatives for the N-arylation
reactions. Yet, these efficient methods are suitable for substrates of
aryl chlorides, aryl bromides, or aryl iodides. At present, although
there are reports on the C–N bond formation via C–F bond cleavage
of aryl fluorides with N–H containing compounds,13 the C–N for-
mation of indoles with pentafluorobenzene via C4–F bond cleavage
based on the SNAr reaction still remains untouched. Moreover,
among N-arylated heterocycles, the N-fluoroaryl substituted struc-
ture-units, especially the indole derivatives, are considered to be
important building blocks in some pharmaceutical molecules.14
Thus, the search for simple and effective approaches to synthesize
N-fluoroaryl substituted indole derivatives is of special interest.
Herein, we report a milder (70 °C), rapid (mostly within two
hours), operational, and efficient protocol for the synthesis of N-
tetrafluoroarylated heterocyclic compounds via selective C4–F
⇑
Corresponding author. Tel.: +86 22 27406140; fax: +86 22 27403475.
E-mail address: bzhang@tju.edu.cn (B. Zhang).
0040-4039/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.055

4650
C. Liu et al. / Tetrahedron Letters 54 (2013) 4649–4652
bond cleavage of pentafluorobenzene with diverse N–H heterocy-
cles. With this method, a series of products were furnished in high
yields (up to 88%), which might contribute to the synthesis of some
pharmaceuticals.
der the selected conditions of base and solvent, a brief optimiza-
tion for the reaction temperature, time, and the loading of base
was carried out. Gratifyingly, when the reaction of 1a and 2a was
performed in the presence of 1.0 equiv of NaOH as base, 2.0 mL
of DMF as solvent at 70 °C for 1 h under Ar, product 3a was ob-
tained in 86% yield.
With the optimal conditions, we investigated the substrate
scope of N–H-containing heterocyclic compounds. Both electron-
donating groups such as methyl, ethyl, and methoxy (Table 1,
3b–f) and electron-withdrawing groups such as chloro, bromo,
and acetyl (Table 1, 3g–j) on the aryl ring of indoles were all toler-
ated under the selected conditions, and moderate to good yields of
the corresponding SNAr products were obtained. In addition, for
methyl-substituted substrates, good results could be obtained no
matter where the methyl group lay (Table 1, 3b, 3c, and 3e). During
this reaction, chloro and bromo groups are inactive (Table 1, 3h–j),
obviously different from the reported transition-metal catalyzed
coupling reaction.6–9 The presence of chloro or bromo groups in
the products provided a good opportunity for later creation of
Pentafluoroarenes are known to be electron-deficient com-
pounds.3 We speculated that electron-rich groups like organic an-
ions may attack pentafluoroarenes through the SNAr reaction to
produce new compounds. It has been reported that the N–H of in-
dole was able to form Nꢀ anion in basic medium.15 Initially, we
examined the reaction of pentafluorobenzene 1a with indole 2a
in DMSO at 90 °C under Ar atmosphere. Surprisingly, we found that
the presence of NaOH (2 equiv) resulted in the formation of prod-
uct 3a in 81% yield, which suggested that the cleavage of C4–F
bond occurred. No detectable amount of 3a was observed in the
absence of NaOH, thus indicating the reaction is an alkali-driven
process. Encouraged by this preliminary result, a survey of reaction
parameters including bases and solvents was conducted (Table S1
in the Supporting information). NaOH and DMF were found to be
the optimal. Other bases and solvents were less or not active. Un-
Table 1
N-polyfluoroarylated heterocyclic compounds derived from N–H-containing heterocycles with pentafluorobenzenea
X
N
Y
R
F
F
F
F
X
F
NaOH, DMF
70 °C, Ar, 0.5 - 2 h
R
Y
H
F
F
N
H
F
F
H
1a
2
3
H3CO
N
N
N
N
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
3a, 86%
3b, 88%
3c, 81%
3d, 80%
COCH3
Cl
N
N
N
F
F
N
F
F
F
F
F
F
F
F
F
F
F
F
F
3g, 66%
3e, 69%
3f, 71%
3h, 83%
Br
N
N
N
N
N
N
Cl
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
3i, 83%
3j, 84%
3k, 78%
3l, 76%
N
S
N
N
N
N
N
F
F
N
F
F
F
F
F
F
F
F
F
F
F
F
F
3m, 73%
3n, 82%
3o, 83%
3p, 67%b
COCH3
F
N
N
F
F
F
F
F
F
F
3q, 77%b
3r, 68%b
a
Reaction conditions: 1a (0.4 mmol), 2 (0.2 mmol), and NaOH (0.2 mmol) were added in DMF (2.0 mL) with stirring at 70 °C for 0.5–2 h. Yield: isolated yield.
b
The reaction was conducted in 6 h.

C. Liu et al. / Tetrahedron Letters 54 (2013) 4649–4652
4651
new bonds. The other N–H heterocycles such as pyrrole, imidazole,
piperazine, indazole, carbazole, and azaindole also served as suit-
able coupling partners in acceptable yields (Table 1, 3k–r). Further-
more, to demonstrate the synthetic application of this
methodology, a 0.5-g-scale synthesis of 3a could also be performed
in good yield (80%), indicating the good reliability of the process.
Note that most of the products were isolated in the form of white
solids. In some cases, in addition to the NMR, IR, and HRMS analy-
ses, the structure of products 3g was analyzed by the single-crystal
X-ray technique (see the Supporting information).
To gain some insight into the reaction mechanism of 1a with 2,
we carried out several control experiments (Scheme 1). To our de-
light, good result (83% yield of 3a1) was accessed when 2a reacted
with 2,3,4,5,6-pentafluorotoluene under the standard conditions,
thus suggesting that C–N forming reaction between 1a and N–H
heterocycles proceeded by direct C4–F cleavage of pentafluoroben-
zene. To determine whether the C4–F cleavage was caused by the
base only, 19F and 1H NMR spectra were examined (details see the
Supporting information, Figs. S1 and S2). The 19F and 1H NMR spec-
tra in Figures S1 and S2 clearly imply that the C4–F cleavage of 1a
is not directly attributed to the presence of the base, that is, the
nucleophilic attack of nitrogen is essential for this facile reaction.
In addition, NaN3 with electron-rich Nꢀ3 was found to react with
pentafluorobenzene to form 3a2 in the absence of base (Scheme 1),
further confirming that the nucleophilic attack of Nꢀ anion can in-
duce the cleavage of C4–F bond of 1a and then produce the SNAr
products. To further study the mechanism of 1a and 2, the reaction
system was examined by NMR spectroscopy, but unfortunately, all
the resonances in the NMR spectra originate from the reactants
and product, that is, no information about the intermediates is de-
tected in the time-dependent NMR spectra. This may be related to
the quick formation and transformation of the intermediates.
Based on our preliminary results, the reaction mechanism related
to the nucleophilic substitution process is proposed (see the Sup-
porting information, Scheme S1).
This metal-free method could afford the coupling products with
other pentafluoroarenes (Table 2). For example, 2,3,4,5,6-pentaflu-
orotoluene reacted with indoles to give the desired products in
good yields regardless of the substituents on the indoles (3a1,
3s–w). Octafluorotoluene and hexafluorobenzene also react with
2a and produced the corresponding products in good yields, de-
spite the later generation of a mixture of multiple coupling prod-
ucts (3x, 3y, and 3y0).
It was found that polyfluoroarene substrates could bear differ-
ent numbers of fluorine atoms. For example, 2,3,5,6-tetrachloro-
pyridine and 1,2,4-trifluorobenzene could smoothly undergo
direct tri- and difluoroarylation of 2a in good yields (Table 3, 4a,
4b). In addition, 1,4-difluorobenzene could also give the desired
product (Table 3, 4c), albeit in a slightly lower yield (36%). In all
cases, the removal of F atoms in polyfluoroarenes is observed (Ta-
bles 1–3), confirming that the C–F cleavage occurs in this interest-
ing reaction. However, fluorobenzene was unreactive under the
standard condition adopted in this communication, which may
be attributed to the electron-rich character of fluorobenzene. This
fact further shows that the electron-deficiency of the aryl ring is
important for this nucleophilic substitution reaction.
F
N
F
F
F
F
F
NaOH (0.2 mmol)
DMF (2.0 mL), 70 °C,
Ar, 2 h
F
N
H
F
CH3
3a1
F
CH3
(0.2 mmol)
(0.4 mmol)
Isolated Yield = 83%
F
F
F
DMF (2.0 mL), 70 °C
Ar, 2 h
F
N3
F
F
N3
F
F
3a2
F
N-Tetrafluoroarylated heterocyclic compound is also versatile
synthetic intermediate and could be readily transformed into other
tetrafluoroarylated compounds (Scheme 2). In fact, 5 could not be
obtained in high yield through the direct reaction of 1-(1H-indol-3-
(0.2 mmol)
(0.4 mmol)
Isolated Yield = 47%
Scheme 1. Control experiments.
Table 2
N-Tetrafluoroarylation of indolesa
F
F
F
NaOH, DMF
70 °C, Ar, 0.5 - 2 h
R
R
R1
F
N
N
H
F
ArF
H3CO
N
N
N
Cl
N
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
CH3
CH3
CH3
CH3
3s, 83%
3t, 78%
3u, 76%
3v, 78%
F
F
F
F
F
F
N
F
N
N
F
F
N
F
N
F
F
F
F
F
F
CH3
CF3
3y
3y'
3w, 71%
3x, 83%
85%, 3y : 3y' = 1 : 3.7
a
Reaction conditions: indoles (0.2 mmol), NaOH (0.2 mmol), and pentafluoroarenes (0.4 mmol) were added in DMF (2.0 mL) with stirring at 70 °C for 0.5–2 h. Yield:
isolated yield.

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C. Liu et al. / Tetrahedron Letters 54 (2013) 4649–4652
Table 3
References and notes
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a
Reaction conditions: 2a (0.2 mmol), NaOH (0.2 mmol), and polyfluoroarenes
(0.4 mmol) were added in DMF (2.0 mL) with stirring at 70 °C for 0.5–2 h. Yield:
isolated yield.
b
The reaction was conducted with KOH (0.2 mmol) as base stirring at 90 °C, 24 h.
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NaBH4
N
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Scheme 2. Further synthetic transformation of the product.
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mation indicated that our protocol provides a reliable approach for
the synthesis of diverse tetrafluoroarylated compounds.
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In summary, we developed a simple and effective aromatic
nucleophilic monosubstitution approach for the N-tetrafluoroary-
lation of N–H containing heterocycles with pentafluorobenzene
via selective C4–F bond cleavage. Preliminary mechanistic analyses
suggested that the nucleophilic attack of electron-rich Nꢀ anion
was significant for C4–F cleavage of pentafluorobenzene and C–N
bond formation. The strategy is highly tolerant of substrates of
both polyfluoroarenes and N–H heterocycles, and is able to give
the corresponding products in moderate to good yields within
0.5–2 h. This effective protocol might be a promising alternative
to the existing methods and be helpful to the synthesis of the N-
fluoroaryl substituted pharmaceutical structural units. Meanwhile,
the products could be further transformed into other organofluo-
rine compounds, which are otherwise difficult to be accessed. Ef-
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the mechanism are underway.
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Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (Nos. 20901057 and 11074185) and
Innovation Foundation of Tianjin University.
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.2013.06.055.