Highly selective synthesis of 1-polyfluoroaryl-1,2,3-triazoles via a one-pot three-component reaction

Article abstract of DOI:10.1016/j.tetlet.2014.07.041

A series of 1-polyfluoroaryl-1,2,3-triazoles were synthesized in moderate to good yields by an efficient copper-catalyzed one-pot three-component reaction of polyfluoroarenes, sodium azide, and terminal alkynes. The method involves a polyfluoroarylated azide intermediate produced in situ by the selective CF bond cleavage of polyfluoroarene with sodium azide.

Full text of DOI:10.1016/j.tetlet.2014.07.041

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly selective synthesis of 1-polyfluoroaryl-1,2,3-triazoles
via a one-pot three-component reaction
Liming Cao a,b, Cuibo Liu a,b, Xiangyang Tang a, Xuguang Yin a,b, Bin Zhang a,b,
⇑
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 1-polyfluoroaryl-1,2,3-triazoles were synthesized in moderate to good yields by an efficient
copper-catalyzed one-pot three-component reaction of polyfluoroarenes, sodium azide, and terminal
alkynes. The method involves a polyfluoroarylated azide intermediate produced in situ by the selective
CAF bond cleavage of polyfluoroarene with sodium azide.
Received 5 May 2014
Revised 8 July 2014
Accepted 10 July 2014
Available online xxxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
CAF cleavage
Polyfluoroarenes
Cycloaddition
Multicomponent reactions
1,4-Disubstituted 1,2,3-triazoles
1,2,3-Triazole scaffolds are ubiquitous structural moieties in the
synthetic chemistry, which find widespread applications in
medicinal chemistry and materials science.1 Owning to their vast
importance, much attention has been devoted to the synthesis of
1,2,3-triazoles and their derivatives. To date, there are a large num-
ber of well-designed studies on the construction of 1,2,3-triazoles.
Among the strategies presented, the transition-metal catalyzed or
mediated 1,3-dipolar cycloadditions of organic azides with alkynes
represent one of the most useful approaches.2 However, usually
this methodology leads to a mixture of 1,4-disubstituted- and
1,5-disubstituted-1,2,3-triazoles. Later, copper-catalyzed azide–
alkyne cycloaddition (CuAAC) reported by the Sharpless3 and Mel-
del4 groups has proved to be an outstanding procedure for the
access to 1,4-disubstituted 1,2,3-triazoles. This reaction occurs
reliably under mild conditions, displaying superb substrate scope
and exquisite selectivity. However, the organic azide molecules
utilized in the reaction were generally prepared beforehand, which
increased the operational complexity of the reaction procedure. It
is known that the low molecular weight organic azides tend to
be explosive and are difficult to handle. Therefore, it is highly
desirable to find a facile method that avoids handling the neat
hazardous organic azides for the synthesis of the 1,2,3-triazoles.
One-pot three-component cascade reaction catalyzed by a
transition-metal was considered to be an efficient strategy in the
organic chemistry, especially for the construction of complex
organic compounds.5 Recently, intensive attention has been
devoted to the methodology involving the in situ generation of
required azides in the Cu(I)-catalyzed azide–alkyne 1,3-dipolar
cycloaddition reaction.6 For instance, the Fokin group reported a
microwave-enhanced, fast, and efficient three-component reaction
for the generation of 1,4-disubstituted-1,2,3-triazoles from corre-
sponding alkyl halides, sodium azide, and alkynes.7 Alonso and
co-workers reported an interesting multicomponent click synthe-
sis of 1,2,3-triazoles from epoxides in water catalyzed by copper
nanoparticles on activated carbon.8 Well-designed Cu(II)–b-cycl-
dextrin complex-based catalytic strategy reported by Kaboudin
enables us to produce the 1,2,3-triazoles via a one-pot reaction of
an aryl boronic acid with sodium azide in water.9 Kumar and
co-workers developed an efficient and simple tandem protocol
for the synthesis of structurally complex and novel 1,2,3-triazole-
fused imidazo-[1,2-a]pyridines via CuAAC, Ullmann-type CAN cou-
pling.10 Although these elegant advances have been achieved, the
organic azide intermediates were mainly prepared by the coupling
reaction of aryl amines, halides, and boronic acids with an azide
source such as NaN3, TMSCN, and TfN3.11 Consequently, the devel-
opment of in situ generation methods of organic azides to create
1,2,3-triazole compounds remains an interesting challenge.
Polyfluoroarenes are a class of electron-deficient compounds
which are prone to be attacked by the electron-rich atoms or groups
to form the new carbon–carbon bonds or carbon–heteroatom
bonds via the selective CAF bond cleavage.12 Our group has
⇑
Corresponding author. Tel.: +86 22 27406140; fax: +86 22 27403475.
E-mail address: bzhang@tju.edu.cn (B. Zhang).
http://dx.doi.org/10.1016/j.tetlet.2014.07.041
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Cao, L.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.07.041

2
L. Cao et al. / Tetrahedron Letters xxx (2014) xxx–xxx
F
F
F
F
F
F
F
.
N
Cu(OAc)2 H2O
N
+
+ NaN3
NC
F
NC
N
R
CH3CN, RT
R
F
1
2
3
Scheme 1. Synthesis of 1-polyfluoroaryl-1,2,3-triazole via a one-pot three-component reaction.
previously reported the base-promoted SNAr reaction of NAH and
OAH containing compounds with polyfluoroarenes via the selec-
tive CAF bond cleavage.13 The interesting results encouraged us
to expand the application of the SNAr protocol to fabricate the
novel polyfluoroarylated compounds. Given the importance of
the 1,2,3-triazoles and the ongoing interest of the CAF bond
cleavage strategy, herein, we reported the first copper-catalyzed
one-pot three-component tandem reaction of terminal alkynes,
sodium azide, and polyfluoroarenes at room temperature. This
strategy enables a practical one-step access to a variety of 1-poly-
fluoroaryl-1,2,3-triazoles, which are important fluorine-containing
compounds and the polyfluoroaryl skeletons were prevalent in a
large number of drug molecules and material compounds.14 Addi-
tionally, our method involves a polyfluoroarylated azide interme-
diate produced in situ by the selective CAF bond cleavage of
polyfluoroarene with sodium azide (Scheme 1).15
Based on our previous work, we speculated that the sodium
azide may directly react with polyfluoroarenes to give the polyflu-
oroarylated azide intermediate in suitable conditions, which was
then followed by the domino reaction with terminal alkynes to
generate the 1-polyfluoroaryl-1,2,3-triazoles in one-pot. In order
to validate the feasibility of the one-pot three-component process,
we treated 1.0 equiv of sodium azide, 1.1 equiv of penta-
fluorobenzonitrile 1a, and 1.5 equiv of phenylacetylene 2a utilizing
10 mol % Cu(OAc)2ÁH2O as the catalyst without a reducing agent in
0.5 mL DMSO stirred at room temperature for 8 h. Delightedly, we
observed that the desired product was obtained in 57% isolated
yield (Table 1, entry 1). This satisfied result inspired us to search
the suitable conditions for this interesting reaction. After the
preliminary screening of solvent (Table 1 entries 1–6), we found
only CH3CN was critical for the reaction efficiency, which gave
92% yield of the product (Table 1, entry 6). Other solvents such
t
as DMF, CH3OH, THF, and even the mixed solvents of BuOH/H2O
(1:1) led to a lower yield (Table 1, entries 1–5). Under otherwise
identical conditions, we then investigated the other copper cata-
lysts on the efficiency of this reaction. It was revealed that the
selected Cu(I) sources were less favorable than Cu(OAc)2ÁH2O in
this reaction (Table 1, entries 7–9), and the other Cu(II) sources
were also ineffective or even delivered a trace amount of 3a, which
were used with the association of sodium ascorbate as the reduc-
ing agent (Table 1, entries 10–13). The above screening results
illustrated that the solvent and the catalyst played an important
role in this one-pot three-component cascade transformation.
Increase or decrease in the loading of the catalyst led to unsatisfac-
tory results (Table 1, entries 14 and 15). Hence, the optimal condi-
tions include employing CH3CN as the solvent in combination with
10 mol % Cu(OAc)2ÁH2O as the catalyst.
With the optimized conditions in hand, we continued to inves-
tigate the generality and substrate scope of this facile one-pot
three-component reaction. The results are summarized in Table 2.
Table 1
Optimization of reaction conditions for the synthesis of 3aa
F
F
F
F
F
F
N
.
N
Cu(OAc)2 H2O
CH CN, 25
NC
N
NaN3
NC
F
+
+
ഒ
3
F
F
1a
2a
3a
Entry
Solvent
Catalyst
Yieldb (%)
1
2
3
4
5
6
7
DMSO
DMF
THF
CH3OH
CH3CN
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
CuCl
57
45
60
65
92
40
83
tBuOH/H2O (1:1)
CH3CN
8
CH3CN
CuBr
74
9
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CuI
85
10
11
12
13
14d
15e
CuSO4Á5H2O/NaAscc
Trace
Trace
42
91
85
Cu(NO3)2Á3H2O/NaAsc
CuCl2Á2H2O/NaAsc
Cu(acac)2/NaAsc
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
87
a
Reaction conditions: pentafluorobenzonitrile (0.22 mmol), phenylacetylene (0.30 mmol), sodium azide (0.2 mmol), catalyst (10 mol %), solvent (0.5 mL), room temper-
ature, 8 h.
b
Isolated yields.
c
d
e
NaAsc = sodium ascorbate (20 mol %).
Cu(OAc)2ÁH2O (5 mol %).
Cu(OAc)2ÁH2O (20 mol %).
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L. Cao et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 2
One-pot three-component synthesis of 1-polyfluoroaryl-1,2,3-triazolesa
F
F
F
F
F
F
.
N
Cu(OAc)2 H2O
N
+
R
+
NaN3
NC
F
NC
N
CH3CN, RT
R
F
F
1a
2
3
F
F
F
N
F
F
F
F
F
F
F
F
N
N
N
N
N
N
N
NC
N
N
NC
NC
NC
OCH3
F
OCH3
3b, 89%
3c, 86%
3a, 92%
F
F
F
F
F
F
F
F
F
N
N
N
N
N
N
N
NC
NC
N
OPh
F
F
F
H3CO
Cl
3d, 83%
3f, 80%
3e, 78%
F
F
F
F
F
F
F
N
N
N
N
N
N
N
N
NC
NC
NC
N
F
F
F
F
F
3g, 87%
3h, 85%
3i, 83%
F
F
F
F
F
F
F
F
N
N
N
N
N
N
NC
N
N
NC
NC
N
COOEt
F
F
F
F
3j, 82%
3k, 83%b
3l, 72%
F
F
F
F
F
F
F
N
F
F
F
F
N
N
N
N
N
NC
N
N
N
NC
O
NC
OH
OH
F
CHO
3m, 71%
3n, 74%
3o, 73%
F
F
F
F
F
F
F
N
N
N
N
NC
N
NC
N
S
N
F
3p, 76%
3q, 70%
a Reaction conditions: pentafluorobenzonitrile (0.22 mmol), alkyne (0.30 mmol), sodium azide (0.2 mmol),
Cu(OAc)2ÁH2O (10 mol %), CH3CN (0.5 mL), room temperature, 8 h.
b Cu(acac)2/NaAsc.
Various alkynes including the aromatic alkynes and the aliphatic
alkynes smoothly reacted with pentafluorobenzonitrile and
sodium azide under the optimized conditions to form the desired
1-ployfluoroaryl-4-substituted-1,2,3-triazoles (Table 2, 3a–q). Aro-
matic alkynes with electron-donating groups or with electron-
withdrawing groups on the aryl ring were all tolerated under the
optimal conditions, affording the corresponding products in good
yields (Table 2, 3b–j). The high efficiency of this reaction is fully
achieved for the substrates with the same substituent at different
positions, (e.g. methoxy, Table 2, 3b–d). Interestingly, the aliphatic
alkynes were also suitable in this reaction, which gave rise to the
products in acceptable to moderate yields (Table 2, 3k–q).
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4
L. Cao et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 3
One-pot three-component synthesis of 1-polyfluoroaryl-1,2,3-triazolesa
R
R
N
N
.
Cu(OAc)2 H2O
X
N
+
Ph
+ NaN3
X
CH3CN, RT
F5
F4
2a
3
1
X = C, N
F
F
F
F
F
F
F
F
F
N
N
N
N
N
N
F3C
N
N
F
N
H3COOC
N
F
F
3r, 88%
3s, 90%
3t, 86%
F
F
F
F
N
N
N
3u, 65%b
a Reaction conditions: polyfluoroarene (0.22 mmol), phenylacetylene (0.30 mmol), sodium azide (0.2 mmol),
Cu(OAc)2ÁH2O (10 mol %), CH3CN (0.5 mL), room temperature, 8 h.
b Reaction conditions: pentafluorobenzene (0.22 mmol), phenylacetylene (0.30 mmol), sodium azide (0.2 mmol),
Cu(OAc)2ÁH2O (10 mol %), DMF (0.5 mL), 70 °C, 8 h.
Although the 1-ethynylcyclohex-1-ene was slightly incompatible
with the standard conditions, replacing Cu(OAc)2ÁH2O with
Cu(acac)2 and addition of NaAsc gave a promising yield (Table 2,
3k). Furthermore, the successful production of 3m, 3n, and 3o in
mild yields with the aldehyde and hydroxy groups provided a good
opportunity for further formation of oxygen–carbon or oxygen
heteroatom bonds by transition-metal-catalyzed coupling and
other reactions.
as perfluorotoluene, perfluoropyridine, methyl 2,3,4,5,6-penta-
fluorobenzoate, and pentafluorobenzene (Table 3). Our optimal
conditions were well tolerated with the polyfluoroarene substrates
bearing electron withdrawing groups. The reaction of perfluorotolu-
ene, perfluoropyridine, and methyl 2,3,4,5,6-pentafluorobenzoate
with phenylacetylene produced the corresponding 1-ployfluoroa-
ryl-4-substituted-1,2,3-triazoles in good yields (Table 3, 3r–t).
Unfortunately, the standard conditions were less suitable for penta-
fluorobenzene, and no detectable amount of the product was
observed. Slightly elevating the reaction temperature to 70 °C and
To further explore the generality of the one-pot synthetic
strategy, we applied this system to the other polyfluoroarenes such
F
F
R1
N
N
N
Cu(II)Ln
Cu(I)Ln
F
R2
R2
F
R2
+ H+
F
F
R2
H
R1
N
N
N
F
F
R2
CuLn
IV
- H+
F
F
F
F
R1
F
R1
R2
CuLn
N
I
F
N
N
F
F
CuLn
F
R2
NaN3
F
III
F
N
F
R1
F
N
N
F
N
N
F
R1
N
CuLn
R2
F
F
II
Scheme 2. Proposed mechanism.
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L. Cao et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5
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using DMF as the solvent were essential to furnish the product in
65% yield (Table 3, 3u).
Based on the mechanism studies by the reported litera-
tures,1e,3,16 a possible mechanism for the synthesis of 1-polyfluoro-
aryl-1,2,3-triazoles is proposed as shown in Scheme 2. Firstly, the
copper(II) catalyst is reduced to the copper(I) reagent via the
alkyne oxidative homocoupling (OHC) reaction.17 Subsequently,
the copper(I) reagent combines terminal alkyne to form the cop-
per(I) acetylide I. Then the polyfluoroarylated azide intermediate,
which is generated in situ by the selective CAF bond cleavage of
polyfluoroarene with sodium azide, is coordinated to the copper
center, forming intermediate II. The coordinated polyfluoroarylat-
ed azide (see Fig. S1, ESI ) then undergoes a cyclization to give a
six-membered metallacycle III, which subsequently rearranges to
copper(I) triazolide IV. Finally, protonation of copper(I) triazolide
IV results in the desired product and regenerates the copper(I) cat-
alyst for the next catalytic cycle.
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In summary, we have successfully developed a simple and effi-
cient approach for the synthesis of 1-polyfluoroaryl-1,2,3-triazoles
via
a one-pot three-component tandem reaction. With this
method, a series of the corresponding products were synthesized
in moderate to excellent yields. The reaction shows high generality
and functional group tolerance. Most significantly, a polyfluoroary-
lated azide intermediate was produced via the selective CAF bond
cleavage of polyfluoroarene with sodium azide, avoiding the direct
utilization of organic azide, which provided an important comple-
mentary method for the synthesis of organic azide followed by the
domino reaction with alkynes to form the 1,2,3-triazole
compounds.
Acknowledgments
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This work was financially supported by the National Natural
Science Foundation of China (No. 20901057 and No. 21373149)
and New Century Excellent Talents in University of Ministry of
Education of China (NCET-12-0391).
Supplementary data
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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.2014.07.041.
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Please cite this article in press as: Cao, L.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.07.041