Copper-catalyzed [3 + 2] cycloaddition/oxidation reactions between nitro-olefins and organic azides: Highly regioselective synthesis of NO2-substituted 1,2,3-triazoles

Article abstract of DOI:10.1021/ol503687w

A new copper-catalyzed [3 + 2] cycloaddition/oxidation reaction of nitro-olefins with organic azides has been developed to afford 1,4(-NO₂),5-trisubstituted 1,2,3-triazoles. This reaction sequence has a broad substrate scope and affords NO₂-substituted 1,2,3-triazoles with high regioselectivities and in good to excellent yields. The involved oxidative process overcomes the elimination of HNO₂ for general cycloaddition of nitro-olefins with organic azides, which shows a high atom economy and potential applications.

Full text of DOI:10.1021/ol503687w

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed [3 + 2] Cycloaddition/Oxidation Reactions
between Nitro-olefins and Organic Azides: Highly Regioselective
Synthesis of NO2‑Substituted 1,2,3-Triazoles
Yunfeng Chen,* Gang Nie, Qi Zhang, Shan Ma, Huan Li, and Qinquan Hu
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, China
S
* Supporting Information
ABSTRACT: A new copper-catalyzed [3 + 2] cycloaddition/
oxidation reaction of nitro-olefins with organic azides has been
developed to afford 1,4(-NO2),5-trisubstituted 1,2,3-triazoles.
This reaction sequence has a broad substrate scope and affords
NO2-substituted 1,2,3-triazoles with high regioselectivities and in
good to excellent yields. The involved oxidative process
overcomes the elimination of HNO2 for general cycloaddition of nitro-olefins with organic azides, which shows a high atom
economy and potential applications.
1,2,3-Triazoles are an important class of heterocyclic compounds
with wide applications in medicinal,1 material,2 and synthetic
organic chemistry.3 For example, some 1,2,3-triazoles show a
broad spectrum of biological activities such as antifungal,
antiviral, antiallergic, anti-HIV, and antimicrobial activities.4
Because of their reliability, the diverse syntheses of 1,2,3-triazoles
have gained much attention. The most popular methods for
these kinds of compounds are based on copper(I)-5 and
ruthenium(II)-mediated6 regioselective azide−alkyne cyclo-
addition reactions, which give 1,4- or 1,5-disubstituted 1,2,3-
triazoles, respectively. Recently, some fully substituted 1,2,3-
triazoles were also synthesized by different approaches, such as
1,4,5-trisubstituted 1,2,3-triazoles, which were obtained by
metal-catalyzed cycloaddition reactions of organic azides with
internal alkynes7 and by organocatalytic cycloaddition reactions
or multicomponent reactions of organic azides with activated
enones, keto esters, nitriles, or aldehydes.8 At the same time,
other 1,2,3-triazoles were also synthesized by post-functionaliza-
tion of NH-1,2,3-triazoles or 1,4-disubstituted 1,2,3-triazoles.9
Nitro-olefins also serve as an important partner with azides to
form the 1,2,3-triazoles (Scheme 1).10 Cyclization reactions of
nitro-olefins with NaN3 can give the NH-1,2,3-triazoles (Scheme
1a).10a−c TMSN3 as a nitrogen source with tetra-n-butylammo-
nium fluoride is an alternative method for the formation of NH-
1,2,3-triazoles (Scheme 1b).9d In general, organic azides react
with nitro-olefins to give the N-substituted 1,2,3-triazoles with
low efficiency (Scheme 1c).10e Interestingly, Wang et al. reported
that nitro-olefins could also react with organic azides in the
presence of Ce(OTf)3 to give 1,5-disubstituted 1,2,3-triazoles
(Scheme 1d).10f For these four cases, the formation of 1,2,3-
triazoles involved an elimination of HNO2 process. Nitrogen-
group-substituted 1,2,3-triazoles are very important for their
applications; most of them were synthesized by the click reaction
of electron-deficient ynamides with organic azides;11 even so, the
diverse nitrogen-group-substituted 1,2,3-triazoles are still scarce.
The NO2 group is a good nitrogen source, which is widely used
for synthesis of N-containing compounds.12 We propose that, if
the dehydrogenation process is involved instead of the
elimination of HNO2 between the cyclization of nitro-olefins
with organic azides, it can give the NO2-substituted 1,2,3-
triazoles. The challenge for this design is to inhibit the
elimination of HNO2 effectively. Herein we present a copper-
catalyzed highly selective synthesis of NO2-substituted 1,2,3-
triazoles from nitro-olefins and organic azides, which involved
the loss of two hydrogens instead of HNO2 for nitro-olefins
(Scheme 1e).
Scheme 1. Syntheses of 1,2,3-Triazoles from Nitro-olefins
The reaction conditions were tested by using a model reaction
between nitrostyrene (1a) and benzyl azide (2a), including
different metal salts, solvents, and additives, and the results are
shown in Table 1. The Cu(I) catalysts were first tested including
CuI and CuCl because the organic azides can coordinate with
copper complexes, especially the Cu(I) complex. Conversions
were up to 60% using dimethylformamide (DMF) as solvent
under air atmosphere; however, the yields for the proposed
oxidative cycloaddition product 3a were less than 10%, and the
Received: December 22, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ol503687w
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope for the Cycloaddition Reaction of
Nitro-olefins 1 and Azides 2
a
concn
(%)
yield of 3a/4a
b
entry
catalyst
CuI
solvent
(%)
1
2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
67
64
66
78
80
73
81
86
90
23
67
76
95
95
7/23
5/22
CuCl
3
CuO
35/19
23/41
28/43
27/36
56/17
77/4
4
CuCl2
5
CuBr2
6
Cu(OAc)2
Cu(NO3)2
CuSO4
7
8
9
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
83/trace
trace/20
16/34
68/trace
85/<5
90/trace
10
11
12
toluene
MeOH
DMSO
DMF
c
13
14
15
DMF/AcOH
(10%)
Cu(OTf)2
DMF/AcOH
(25%)
100
96/0
16
17
18
19
FeCl3
DMF
DMF
DMF
<5
38
37
68
−/−
PdCl2
trace/22
trace/30
trace/27
Pd(OAc)2
PdCl2(PPh3)2 DMF
a
Reactions conditions: nitrostyrene (1a) (1 mmol), benzyl azide (2a)
a
Reactions conditions: 1 (1 equiv), 2 (1.2−1.5 equiv), and Cu(OTf)2
(1.2 mmol), and catalyst (0.05 mmol), air (1 atm), solvents (5 mL for
b
(5 mol %), air (1 atm), DMF/AcOH (4:1, v/v, 0.2 M), 14−20 h.
Yields of isolated products are given.
each, 0.2 M), 110 °C, 14 h. Yields of products purified by column
c
chromatography on silica gel. O2 (1 atm) was used.
major product was 4a, which formed via elimination of HNO2.
We then tested the Cu(II) catalyst; it is gratifying that the
expected 3a was obtained with 35% separated yield by using CuO
as catalyst (entry 3), and in this case, 4a was the minor product
with 19% yields. Other Cu(II) salts were tested for improving the
conversion and selectivity, including CuCl2, CuBr2, Cu(NO3)2,
Cu(OAc)2, CuSO4, and Cu(OTf)2 (entries 4−9). By using
CuCl2, CuBr2, and Cu(OAc)2, the conversions were increased to
a certain extent; however, the yields of 3a were not modified,
while the yields of 4a were increased. When Cu(NO3)2 and
CuSO4 were used, the yields of 3a were increased obviously, and
the formation of 4a was inhibited greatly when CuSO4 was used.
When more acidic Cu(OTf)2 was chosen as catalyst, the
conversion increased to 90% and the yields of 3a increased to
83%. Other solvents were also tested for the optimization
conditions, but they were not better than DMF. Because there is
an oxidative process for the formation of 3a, the reaction was
tested at the oxygen atmosphere (entry 13); however, no
improvement was observed in the conversion and yield. By
contrast with different Cu(II) salts, we proposed that the acidities
of the catalysts may be crucial for this transformation, so AcOH
was introduced for this reaction system. The yield reached up to
90% if 10% AcOH was introduced, and when 25% AcOH was
added, it gave 100% conversion and the yield was up to 96%
(entry 15). Other metal salts were also tested for this reaction,
including FeCl3 and Pd(II) salts, and they did not give satisfying
results.
donating functional groups, such as methoxy (3d, 3h, and 3r),
methyl substituents (3e, 3j, 3l, and 3q), and electron-
withdrawing groups, such as halogens moieties (3b, 3c, 3f, 3g,
3i, 3m, 3o, and 3p), on the aromatic rings of the nitro-olefins
were compatible with this transformation. The structure of 3b
was identified by X-ray single-crystal analysis. Furthermore, acid-
sensitive furanyl-substituted nitro-olefins were also suitable
substrates to react with azides (3s and 3t), and yields (78 and
75%) were somewhat lower than those of other nitro-olefins; we
ascribe this to the decomposition of the furanyl groups. In
addition, different organic azides, including aliphatic and
aromatic azides, were also suitable substrates, and the
corresponding products were obtained in good to excellent
yields. The structure of 3p was also identified by X-ray single-
crystal analysis (see Supporting Information). However, the
reaction for nitro-olefins with the aromatic azides provided the
corresponding products in slightly diminished yields in
comparison with benzyl azides; furthermore, excess azides (1.5
equiv) were needed for full conversions of nitro-olefins (see
Supporting Information).
This strategy is a direct method for synthesis of NO2-
substituted 1,2,3-triazoles via introduction of an oxidative
dehydrogenative process instead of the elimination of HNO2,
which shows higher atom economy. More importantly, the
corresponding NO2-substituted 1,2,3-triazoles are difficult to
obtain by nitration of C4(5)−H 1,2,3-triazoles.13
To get insight into the mechanism of the reaction, some
control experiments were done (Scheme 3). The Cu/TEMPO
(2,2,6,6-tetramethylpiperidine-N-oxyl) system was widely used
for aerobic oxidation reactions,14 and TEMPO was first
With the optimized reaction conditions in hand, we studied
the scope and limitations of this transformation with a wide range
of nitro-olefins and organic azides (Scheme 2). Both electron-
B
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Org. Lett. XXXX, XXX, XXX−XXX

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Letter
tends to form 1,5-disubstituted 1,2,3-triazoles via elimination of
HNO2 under the same conditions. The Cu(II) can be
regenerated from Cu(I) species with oxygen under acidic
condition. The Cu(I) species react with oxygen to generate
[Cu(II)−O−O•] or [Cu(II)−O−OH]16 then dissociates to
form H2O2 and regenerates the Cu(II) catalyst.
Scheme 3. Control Experiments for the Cyclization of Nitro-
olefins with BnN3
In conclusion, we report a copper-catalyzed [3 + 2]
cycloaddition/oxidation reaction of organic azides with nitro-
olefins. The reactions give 1,4(-NO2),5-trisubstituted 1,2,3-
triazoles with high selectivity in good to excellent yields, and it
proceeds from simple building blocks and tolerates a broad range
of functional groups as well as readily available reagents. The
potential viability and generality of this reaction for the
construction of various 4-amino 1,2,3-triazoles may be
anticipated (reduction of the NO2 group); derivatives such as
4-amino 1,2,3-triazoles have already been found to increase
potency as antibacterial agents.17 The detailed reaction
mechanism and synthetic applications of this reaction to
synthesize other diverse 1,2,3-triazoles and other NO2-
substituted heterocycles together with biological activities are
under investigation.
ASSOCIATED CONTENT
■
introduced for this reaction; however, it was found that
formation of 3a was totally inhibited. In this case, 4a via
elimination of HNO2 also formed with 58% yield. When AcOH
was used, 3a still cannot be detected, but the elimination of
HNO2 may be inhibited to a certain extent (only 36% yield for
4a). We proposed that the formation of 3a should involve a
radical process: TEMPO served as a radical inhibitor,15 while
formation of 4a was not related to the oxidation process.
Furthermore, the reaction was run under Ar atmosphere using 2
equiv of CAN (ceric ammonium nitrate) as oxidant or a
stoichiometric amount (2.5 equiv) of Cu(OTf)2, and 3a was
normally formed with 90 and 95% separated yields, respectively.
So the air just served as an oxidant to promote the regeneration
of the Cu(II) catalyst. We also checked the alkyl-substituted
nitro-olefin 1b under standard conditions. The reaction gave
100% conversion; however, the corresponding product was 1,5-
disubstituted 1,2,3-triazole 4b.
S
* Supporting Information
General experimental procedure, characterization data of the
compounds, and CIF data for 3b and 3p. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chyfch@hotmail.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21002076), the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, and Wuhan Youth
Chenguang Program of Science and Technology
(201271031374) for financial support.
According to these control experiments, a plausible mecha-
nism for this Cu(II)-catalyzed oxidative [3 + 2] cycloaddition
reaction is outlined in Scheme 4. The first step of the reaction is
Scheme 4. Plausible Mechanim for Cu(II)-Catalyzed [3 + 2]
Cycloaddition/Oxidation Reaction
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DOI: 10.1021/ol503687w
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