Copper-Catalyzed Three-Component Reaction of Alkynes, TMSN3, and Ethers: Regiocontrollable Synthesis of N1- And N2-Oxyalkylated 1,2,3-Triazoles

Article abstract of DOI:10.1021/acs.orglett.9b02295

A new copper-catalyzed selective synthesis of N¹- and N²-oxyalkylated 1,2,3-triazoles has been developed through a three-component reaction of alkynes, TMSN₃, and ethers. Through this methodology, a series of N¹

Full text of DOI:10.1021/acs.orglett.9b02295

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Three-Component Reaction of Alkynes, TMSN3,
and Ethers: Regiocontrollable Synthesis of N1- and N2‑Oxyalkylated
1,2,3-Triazoles
Pengli Bao,† Huilan Yue,‡ Na Meng,† Xiaohui Zhao,*,‡ Jiangsheng Li,§ and Wei Wei*,†,‡
†School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165 Shandong, China
‡Qinghai Provincial Key Laboratory of Tibetan Medicine Research and Key Laboratory of Tibetan Medicine Research, Northwest
Institute of Plateau Biology, Chinese Academy of Sciences, Qinghai 810008, China
§School of Chemistry and Food Engineering, Changsha University of Science and Technology, Changsha 410114, China
S
* Supporting Information
ABSTRACT: A new copper-catalyzed selective synthesis of N1- and N2-oxyalkylated 1,2,3-triazoles has been developed
through a three-component reaction of alkynes, TMSN3, and ethers. Through this methodology, a series of N1- and N2-
oxyalkylated 1,2,3-triazoles could be efficiently and regioselectively obtained from simple and readily available starting materials
with favorable functional group tolerance.
s one of the most important classes of N-heterocyclic
compounds, N-substituted 1,2,3-triazoles have exhibited
allyl-1,2,3-triazoles via three-component reaction of alkynes,
allyl methyl carbonate, and TMSN3 (Scheme 1c).13 Fokin and
co-workers reported a copper-catalyzed three-component
reaction of alkynes, sodium azide, and formaldehyde leading
to 2-hydroxymethyl-2H-1,2,3-triazoles (Scheme 1d).14
Although some progress has been made, there is still a high
demand for the development of facile and efficient methods to
selectively construct N1- and N2-substituted 1,2,3-triazoles
from simple and readily available materials.
Cyclic ethers are privileged structural motifs in many
biologically active molecules and natural products, which
exhibited a broad spectrum of biological and pharmaceutical
properties.15 Recently, the direct alkylation of NH-1,2,3-
triazoles with cyclic ethers and vinyl ethers has been developed
for the construction of the N1- or N2-oxyalkylated azoles.16
However, the need of extra steps to prepare various NH-1,2,3-
triazoles and the formation of a mixture of the isomeric
products are the major drawbacks of this strategy. With our
continued interests in the synthesis of heterocyclic com-
pounds,17 here, we report an efficient and regiocontrollable
method for the synthesis of various N1- and N2-oxyalkylated
1,2,3-triazoles via copper-catalyzed three-component reaction
of alkynes, TMSN3, and ethers (Scheme 1e).
A
widespread applications in medicinal chemistry,1 organic
synthesis,2 and material science.3 Consequently, numerous
efforts have been paid to construct N-substituted 1,2,3-
triazoles.4−9 Representatively, N-substituted 1,2,3-triazoles are
synthesized by thermo- or copper/ruthenium-catalyzed cyclo-
addition of alkynes with organic azides (Scheme 1a and 1b).5
Alternative cycloaddition reactions of organic azides with nitro-
olefins,6 enaminones,7 and cinnamic acids8 as well as azide-free
synthetic methods9 have also been developed. Through these
methodologies, only N1-substituted 1,2,3-triazoles can be
obtained owing to the nature of organic azides. N2-Substituted
1,2,3-triazoles are extremely interesting compounds that
possess a wide range of biological activities such as antiviral,
antiarrhythmic, antiarthritic, and antiherpetic properties.10
Nevertheless, in comparison with N1-substituted 1,2,3-
triazoles, the strategies for the synthesis of N2-substituted
triazoles remained largely unexplored.11 Generally, N2-
substituted 1,2,3-triazoles are prepared by the N-2 alkylations
or N-2 arylations of the preformed NH-1,2,3-triazoles with
alkylation/arylation reagents.11 These reactions often require
harsh reaction conditions and suffer from low selectivities.
Multicomponent reactions (MCRs) are exceptionally attractive
tools for accomplishing the goal of step and energy economy in
the construction of structurally diverse compounds from
simple starting materials.12 In this context, the Yamamoto
group reported an elegant Pd/Cu cocatalyzed synthesis of 2-
Received: July 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02295
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Synthetic Approaches to N-Substituted 1,2,3-
Triazoles
Table 1. Optimization of the Reaction Conditions
catalyst
(mol %)
oxidant
(2.5 equiv)
4a yield
b
5a yield
b
entry
(%)
(%)
1
2
3
4
5
6
7
8
CuI (5)
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DTBP
H2O2
30
25
85
50
75
45
0
0
0
12
10
50
0
15
30
trace
trace
trace
30
0
0
0
10
15
37
0
0
trace
trace
trace
30
45
86
CuBr (5)
CuCl (5)
Cu(OAc)2 (5)
CuCl2 (5)
CuBr2 (5)
AgNO3 (5)
Pd(OAc)2 (5)
FeCl3(5)
CuCl (5)
CuCl (5)
CuCl (5)
CuCl (5)
CuCl (5)
CuCl(5)
CuCl (5)
CuCl (5)
CuCl (10)
CuCl (15)
CuCl (20)
CuCl (25)
9
10
11
12
13
14
15
16
BPO
DDQ
K2S2O8
TBHP(1eq)
TBHP(1.5eq)
TBHP
TBHP
TBHP
TBHP
TBHP
0
65
70
11
30
25
<5
<5
c
17
Initially, phenylacetylene (1a), TMSN3 (2a), and THF (3a)
were chosen as model substrates to optimize the reaction
conditions. When the model reaction was conducted at 80 °C
in the presence of CuI (5 mol %) and TBHP (2.5 equiv), the
N1-substituted triazole 4a and N2-substituted triazole 5a were
obtained in 30% and 15% yields, respectively (Table 1, entry
1). Subsequently, a series of metal catalysts (5 mol %) were
investigated to improve the yield and selectivity (Table 1,
entries 2−6). Among various metal catalysts examined, the
copper salt, especially CuCl, was found to be the best one to
selectively afford the N1-substituted triazole 4a with 85% yield
(Table 1, entry 3). None of the products were detected when
other metal salts such as AgNO3, Pd(OAc)2, and FeCl3 were
used (Table 1, entries 7−9). Then, the effect of other oxidants
was further screened. As shown in entries 10−12, lower yield
and selectivity were observed by using DTBP, H2O2, or BPO
as the oxidant. The reaction did not occur in the presence of
DDQ or K2S2O8 (entries 13 and 14). The decrease of TBHP
loading to 1 equiv or 1.5 equiv would lead to lower yield of 4a.
When other solvents such CH3CN were used in this reaction,
only 11% yield of 4a and a trace amount of product 5a were
obtained (Table 1, entry 17). To our delight, significantly
improved reaction efficiency of N2-substituted triazole 5a was
observed with the increase of CuCl loading (Table 1, entries
18−21), and 20 mol % of CuCl gave the best yield (86%) of 5a
(Table 1, entry 20). Further optimization of reaction
temperature found that 80 °C was the optimal choice, and
the decrease or increase of reaction temperature had a negative
effect on the yield of 4a and 5a (see Supporting Information).
With the optimized conditions in hand, the substrate scope
for the synthesis of various N1-substituted triazoles was first
investigated. As shown in Scheme 2, aromatic alkynes with a
methyl group at a different position of the benzene ring all
reacted well to give the desired products 4b−4d in good yields.
Other electron-rich groups such as tert-butyl and methoxy at
the para position of phenylacetylene were also compatible with
this reaction. Electron-withdrawing substituents including
fluorine, chloro, bromo, trifuoromethyl, and cyano were also
tolerated to afford the corresponding products 4g−4m, which
18
19
20
21
84
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), THF 3a (2
mL), catalyst (5−25 mol %), oxidant (0.5 mmol), 16 h, air, 80 °C.
b
Isolated yield based on 1a. TBHP (tert-butyl hydroperoxide, 70%
c
solution in water). THF (2 mmol), CH3CN (2 mL).
could be employed for further transformation. It should be
noted that the product 4n was selectively obtained in 60%
yield when 1,3-diethynylbenzene was used as the substrate.
Notably, heterocyclic aromatic alkynes such as 3-ethynylth-
iophene and 3-ethynylpyridine were also suitable substrates to
produce the desired products 4o and 4p in 72% and 67%
yields, respectively. Moreover, aliphatic alkynes such as
ethynylcyclohexane and hex-1-yne could also react smoothly
to afford the corresponding products 4q and 4r in good yields.
Furthermore, the scope of ether was also investigated under
the standard conditions. In addition to tetrahydrofuran, other
cyclic ethers such as 1,4-dioxane and tetrahydro-2H-pyran
were also tolerated in this process to give the desired products
in moderate yields. Linear ether such as diethyl ether was also a
suitable substrate but leading to the desired product 4u in
relatively lower yield.
After successful demonstration of the synthesis of N1-
substituted triazoles, the scope of the three-component
reactions of alkynes, TMSN3, and ethers to selectively
construct N2-substituted triazoles was studied (Scheme 3).
Similar to the scope for the construction of N1-substituted
triazoles, it was found that various aromatic alkynes containing
both electron-donating and electron-withdrawing groups
underwent construction smoothly to afford the desired
products 5b−5m in good yields. Heterocyclic aromatic alkyne
(i.g., 3-ethynylthiophene) could also provide the correspond-
ing product 5n in 77% yield. Aliphatic alkynes were also
suitable substrates to give the desired products 5o and 5p with
good yields. With respect to ethers, both cyclic ethers and
B
DOI: 10.1021/acs.orglett.9b02295
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Letter
Scheme 2. Scope for the Synthesis of N1-Oxyalkylated
Triazoles
Scheme 3. Scope for the Synthesis of N2-Oxyalkylated
a b
a b
,
,
Triazoles
a
Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), ether 3 (2 mL),
b
CuCl (5 mol %), TBHP (0.5 mmol), 16 h, air, 80 °C. Isolated yield
based on 1. 1a (5 mmol), 2a (10 mmol), and 3a (20 mL).
c
a
Conditions: 1 (0.2 mmol), 2 (0.4 mmol), 3 (2 mL), CuCl (20 mol
b
%), TBHP (0.5 mmol), 16 h, air, 80 °C. Isolated yield based on 1.
c
linear ethers were suitable for this protocol (5q−5t). In
general, cyclic ethers showed better activities than linear ethers.
Notably, when an asymmetric chain ether such as 2-
phenylethyl methyl ether was employed in this reaction
system, the desired product 5u was selectively obtained in
51% yield.
1a (5 mmol), 2a (10 mmol), and 3a (20 mL).
mol % and 20 mol %) in THF in the absence of TBHP, the
product 5f could be isolated in 50% and 71% yields,
respectively (Scheme 4f and 4g).
A series of control experiments were performed to probe the
possible reaction mechanism, and the results were demon-
strated in Scheme 4. First, when TEMPO was added in the
model reaction system, the reaction was suppressed, and the
TEMPO-trapped complex (TEMPO-THF) was observed by
LC-MS, confirming that a radical process should be involved in
the present system (Scheme 4a). Subsequently, when THF
reacted separately with TMSN3 under the standard reaction
conditions, oxyalkylated azide 8a was detected (Scheme 4b).
This result suggested oxyalkylated azide might be involved in
the reaction. Moreover, the 4-phenyl-1H-1,2,3-triazole 10a was
isolated in 17% yield when the model reaction was carried out
in the absence of TBHP (Scheme 4c). The reaction of 4-
phenyl-1H-1,2,3-triazole 10a with THF 3a in CuCl (5 mol %)
catalyzed conditions gave the products 4a and 5a in 46% and
23% yields, respectively (Scheme 4d). Furthermore, the
treatment of 4-phenyl-1H-1,2,3-triazole 10a with THF 3a
under the standard reaction conditions (CuCl, 20 mol %)
would produce the products 4a and 5a in 16% and 27% yields,
respectively (Scheme 4e). These results indicated that phenyl-
1H-1,2,3-triazole might also be an intermediate in this reaction.
In addition, when product 4f was directly stirred with CuCl (5
Based on the above experimental results and previous
reports,4,5,16,18 we propose possible mechanisms as described
in Scheme 5. One pathway for the formation of N1-
oxyalkylated 1,2,3-triazoles could be the initial copper-
catalyzed homolytic decomposition of TBHP which generates
radical species t-BuOO· and t-BuO·,18 which abstract a
hydrogen from α-C−H of THF (3a) to form alkoxyalkyl
radical 6a. Then, the α-alkoxyalkyl radical 6a is further oxidized
to generate α-alkoxyalkyl cation 7a. Next, the nucleophilic
attack of TMSN3 to 7a produces oxyalkylated azide 8a, which
undergoes the click reaction with phenylacetylene 1a to give
the desired product 4a (path A). Another probable pathway is
that phenylacetylene 1a first reacts with TMSN3 leading to 4-
phenyl-1H-1,2,3-triazole 10a, which further attacks the α-
alkoxyalkyl cation 7a to give the product 4a (path B). The
pathway for the formation of 5a should involve the copper-
promoted 1,2-shift of the ether group in 4a via a sequential
electronic transfer process.
In summary, we have developed a convenient and
regiocontrollable synthetic method to access N1- and N2-
oxyalkylated 1,2,3-triazoles via a copper-catalyzed three-
component reaction of alkynes, TMSN3, and ethers. Through
C
DOI: 10.1021/acs.orglett.9b02295
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of inexpensive copper salts as catalyst. This methodology
exhibits good regioselectivity, favorable functional group
tolerance, and a broad substrate scope. Further investigations
of the detailed reaction mechanism and synthetic application
are currently underway in our group.
Scheme 4. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02295.
Experimental details and compound characterization
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: weiweiqfnu@163.com.
*E-mail: xhzhao@nwipb.cas.cn.
ORCID
Huilan Yue: 0000-0003-4457-3233
Jiangsheng Li: 0000-0001-8359-4120
Wei Wei: 0000-0002-0015-7636
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation
of Shandong Province (ZR2018MB009), the International
Cooperation Project of Qinghai Province (2018-HZ-806 and
2018-HZ-815), the Qinghai Key Laboratory of Tibetan
Medicine Research (2017-ZJ-Y11), and the National Natural
Science Foundation of China (No. 21302109).
Scheme 5. Possible Reaction Mechanism
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