Copper(i)-mediated carboamination of vinyl azides by aryldiazonium salts: Synthesis of: N 2-substituted 1,2,3-triazoles

Article abstract of DOI:10.1039/c7cc02391j

A copper(i)-mediated carboamination cascade reaction between vinyl azides and aryldiazonium salts is described. Functionally diverse N²-substituted 1,2,3-triazoles were obtained in moderate to good yields through novel difunctionalization of vinyl azides by aryldiazonium salt sources. This method has a wide scope, good functional-group tolerance and insensitivity to an ambient atmosphere.

Full text of DOI:10.1039/c7cc02391j

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DOI: 10.1039/C7CC02391J
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Copper(I)-Mediated Carboamination of Vinyl Azides by
Aryldiazonium Salts: Synthesis of N2-substituted 1,2,3-Triazoles
Zhenhua Liu,† Huimin Ji,† Wen Gao,* Guangyu Zhu, Lili Tong, Fengcai Lei and Bo Tang*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A copper(I)-mediated carboamination cascade reaction between because of their high inherent reactivity.1b,11 Among the multitude of
vinyl azides and aryldiazonium salts is described. Functionally transformations, one classical pathway by generating iminyl species
diverse N2-substituted 1,2,3-triazoles were obtained in moderate to has drawn considerable attention because of its versatile assembly
good through novel difunctionalization of vinyl azides by of critical chemical bonds (Figure 1 a). However, the reaction modes
aryldiazonium salt sources. This method has a wide scope, good have been limited to capturing monofunctional groups, such as
functional-group tolerance and insensitivity to ambient carbon, nitrogen or sulfur sources,11 rather than the successful and
atmosphere.
flexible 1,1-difunctionalization of aliphatic olefins.12 Additionally,
aryldiazonium salts have been successfully applied as aryl or
aryldiazenyl precursors for more than one century.13 However,
aryldiazonium salts acting as both aryl and aryldiazenyl reagents in
reported protocols are still rare.14 These underdevelopments have
severely restricted the widespread utilization of vinyl azides and
aryldiazonium salts in organic synthesis. Hence, the development of
a dual-functionalization at the β-carbon of a vinyl azide using an
arenediazonium salts is urgently needed.
1,2,3-triazoles1 have been ubiquitous in pharmacological
developments, chemical biology applications, and materials science
because of their privileged chemical structure and properties as five-
membered heterocycles. Recently, 1,2,3-triazoles have also received
significant interest as chelates,2 auxiliaries for C-H activation3 and
efficient carbene precursors.4 Traditionally, the synthesis of triazoles
has been affected by the classical Huisgen 1,3-dipolar cycloaddition
reaction,5 which suffers from slow kinetics and poor regioselectivity.
The metal-catalyzed azide-alkyne cycloaddition1,6, especially method
using Cu(I) catalysis6a,b overcame the deficiencies of this reaction for
preparing 1,4- and 1,5-substituted 1,2,3-triazoles. However, concise
and regioselective strategies7 for the synthesis of N2-substituted
1,2,3-triazoles are still rare, with most methods requiring
postmodification.8,9a,9b This has seriously hindered the widespread
application of these molecules, which have been used as critical
components in orexin receptor antagonist MK-4305,9a-b SYK kinase
inhibitors,9c and ALK5 inhibitors.9d
Because of our sustained interest in readily available vinyl
azides,1c,15 we envisioned that dual-functionalization at the β-carbon
of vinyl azides with aryldiazonium salts could be achieved for the
preparation of 1,2,3-triazoles. The initial experimental results
confirmed this hypothesis and showed an intramolecular
carboamination and ring-closure reaction between vinyl azide and
the aryldiazonium salt through a dual-functionalization process at
the β-carbon of the vinyl azide with a sole aryldiazonium salt source
(Figure 1 b). Here, we describe this new copper(I)-mediated,
powerful and general method for synthesizing diverse N2-substituted
1,2,3-triazoles with moderate to high yields.
Vinyl azides, particular electron-rich alkenes, were first prepared
one hundred years ago.10 These active synthons have undoubtedly
emerged as especial azides and enamine analogues for series of
nitrogen-containing molecules such as N-vinyl-1,2,3-triazoles,
pyrroles, pyridines, etc., albeit as a “sleeper” for a long period
College of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education,
Institute of Biomedical Sciences, Shandong Normal University, Jinan 250014, P.R.
China. E-mail: gaowen@sdnu.edu.cn
；tangb@sdnu.edu.cn
† Z. Liu. and H. Ji. contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Figure 1 Functionalizations of vinyl azides.
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Scheme 1 Scope of aryldiazonium salts tolerated. a
Table 1 Optimization of the reaction conditions.a
View Article Online
DOI: 10.1039/C7CC02391J
Entry
1
[M]
CuI
---
Sol.
Additive
---
Yieldb/%
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
15c
2
---
0
＜5c,d
56
43
25
0
3
CuI
CuI
CuBr
CuCl2
NiCl2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
---
4
---
5
---
6
---
7
---
8
---
65
0
9
DCE
---
10
11
12
13
14
15
Toluene
DMF
---
0
K2CO3
HOAc
AIBN
TEMPO
BHT
26
47
DMF
aReaction conditions: 1a (0.5 mmol), 2 (1.5 mmol), CuI (0.55 mmol),
BHT (0.25 mmol), DMF (1 mL), 70 °C, 6-10 h.
DMF
77
70
83
DMF
was further confirmed by single-crystal X-ray analysis (see the
Supporting Information).16 The reactions also proceeded well with
electron-deficient aryldiazonium salts (Cl, Br, COOMe), affording the
products (3g-3i) under the optimized conditions. These groups were
left intact under the reaction conditions for further synthetic
diversification. Those substrates with ortho-methyl substituent at
the aryl ring (2j-2k) were smoothly converted to yield 1,2,3-triazole
units (3j-3k) with slight difference yields. Other poly-substituted
electron-deficient or electron-rich triazole products (3l-3n) were
also delivered in good yields.
DMF
Reaction conditions: 1a (0.5 mmol), 2a (1.5 mmol.), [M] (0.55 eq.),
c
additive (0.5 eq.), solvent (1 mL), 70 °C, 6 h. bIsolated yield. 0.01
mmol CuI was used. dReaction temperature is 25 oC. DMSO =
dimethyl sulfoxide, DMF = dimethylformamide, DCE = 1,2-dichloroe-
thane, AIBN=2,2'-azobis(2-methylpro- pionitrile), TEMPO=2,2,6,6-
tetramethylpiperidinooxy
methylphenol.
and
BHT=2,6-di-tert-butyl-4-
To determine the optimized conditions, the reaction with model
substrates α-azido styrene (1a) and 4-methylphenyldiazonium
tetrafluoroborate (2a) was initially carried out in DMSO at 70 °C.
Encouragingly, the product 3a was obtained with a 15% yield (Table
1, entry 1). Notably, the success of the desired transformation
strongly depended on the metal catalyst and the reaction
temperature (entries 2 and 3). With the aim of increasing the yield of
product 3a, we further increased the amount of copper(I) catalyst,
which led to the formation of the product in a moderate yield (entry
4). Compared to other metal salts, CuI afforded the highest yield of
3a (entries 4-7). The solvent also dramatically influenced the reaction.
DMF gave 3a in best yield (entries 8-10). Additional additives were
also tested, including K2CO3, HOAc, AIBN, TEMPO and BHT (entries
11-15), with BHT being the most efficacious in enhancing reactivity.
Scheme 2 Scope of vinyl azides. a
Having optimized conditions, we turned our attention to
investigating the scope of substitution patterns (Schemes 1 and 2).
We first explored the substrate scope by varying the aryldiazonium
salts (2b-2n) (Scheme 1). Aryl rings with electron-donating
substituents (OMe, OPh, C2H5 and CH(CH3)2) afforded the
corresponding products (3b-3e) in good yields. The structure of 3f
2 | J. Name., 2012, 00, 1-3
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aReaction conditions: 1 (0.5 mmol), 2a (1.5 mmol), CuI (0.55 mmol),
BHT (0.25 mmol), DMF (1 mL), 70 °C, 6-10 h. b1 mmol Na2CO3 was
used instead of BHT.
To verify this hypothesis, the reaction was carried out in perence of
0.5 eq. phenol (PhOH) instend of BHT and gave a 50% yield (Scheme
3 a). This result implies that BHT may also offer a protection of Cu(I)
from oxidation of oxygen in air except maintaining a certain acidity
of reaction system. Furthermore, we introduced 2H-azirine (4) into
the reaction with 2a and obtained desired product 3a in 40% with 55%
recovery of 4 (Scheme 3 b). This result indicates that the reaction
may proceed via 4. We carried out the reaction with enamine
substitution 517 and delivered the aimed product 6 in 28% (Scheme 3
c). It suggests the process may include enamine intermidiate.
Encouraged by these results, we subsequently evaluated general
applicability of diverse vinyl azides (1a-1n) with aryldiazonium salts
2a. Overall, as shown in Scheme 2, vinyl azides with various
substituents on the benzene ring smoothly participated in the
difunctionalization and cyclization reaction, delivering the
corresponding products (4a-4n) in moderate to good yields. Vinyl
azides bearing electron-donating or -poor substituents on the arene
ring (1a-1h) afforded the homologous products(4a-4h) in mediate to
good yields. Fused and substituted vinyl azides were also accessed to
the products (4i-4j). Notably, aliphatic-substituted substrates could
also readily undergo cycloaddition to furnish the target molecules
(4k-4n) in moderate yields with shifty conditions.
On the basis of the preliminary data and literature reports,11,13,18
a
plausible mechanism is illustrated in Scheme 4. Initially, the aryl
cation 2a′′ was delivered from aryldiazonium cation 2a′ with the
release of molecular nitrogen in the acidic conditions. Copper-
chelated complex A from vinyl azide 1a was then attacked by aryl
cation 2a′′ to form iminyl metal ions B, which could further produce
iminyl copper(I) intermediate C with elimination of dinitrogen.
Alternatively, Consecutive C-N bond cleavage of 4′, which generates
from decomposition of vinyl azide 1a with help of CuI, gives
intermediate C in the presence of aryl cation 2a′′. This also explains
the prolonged reaction time was needed when 1a was replaced with
4 (Scheme 3 b). Subsequently, tautomerization of C provides
enamide intermediate D, which undergoes addition with
aryldiazonium cation 2a′ to afford complexes E.17 The copper-chelate
complex E could isomerize to the formation of intermediate F. Direct
reductive elimination of intermediate E proceeds and affords final
product 3a.
To gain insights into the mechanism of the reaction, we designed
series of control experiments in scheme 3. When the amount of BHT
was increased to 2.0 equiv, the yield of 3a sharply decreased to 11%
(Scheme 3 a). When 2.0 equiv of TEMPO was added instead of BHT,
the reaction generated 3a with a 30% yield (Scheme 3 a). These
results suggest that the transformation may involve a ionic process
rather than a radical pathway. Therefore, BHT may provide
appropriate acid medium for the decomposition of diazonium salts.
Scheme 3 Mechanistic investigations.
In conclusion, we developed
a flexible cross-coupling and
cyclization method through a novel dual-functionalization at the β-
carbon of vinyl azides with aryldiazonium salts as double roles, to
build structurally diverse N2-substituted-1,2,3-triazoles. This reaction
represents an innovative utilization of vinyl azides and reactive
arenediazonium salts and expands the toolbox for conveniently
forming three bonds (C-C, C-N and N-N) in one-pot. In addition to
tolerating diverse functional groups, the reaction offers the benefit
of proceeding under an ambient atmosphere, yielding polyfunctional
triazoles in moderate to good yields through readily available
substrates. A possible ion mechanism was also proposed. Additional
efforts to clearly understand this attractive transformation and the
synthesis of other heterocyclic scaffolds are currently ongoing in our
group.
Scheme 4 Proposed Mechanism.
This work was supported by 973 Program (2013CB933800),
National Natural Science Foundation of China (21390411, 21535004,
21227005, 21305081, 21605097) and Natural Science Foundation of
Shandong Province of China (ZR2016BQ01).
Notes and references
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For synthesis and applications of substituted 1,2,3-triazoles,
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(b) K. Banert in Organic Azides: Syntheses and Applications
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