Copper-mediated intramolecular aza-Wacker-type cyclization of 2-alkenylanilines toward 3-aryl indoles

Article abstract of DOI:10.1016/j.tetlet.2016.12.053

A copper-mediated intramolecular aza-Wacker-type cyclization was developed for the direct and efficient synthesis of 3-aryl indoles using 2-alkenylanilines in moderate to good yields with good functional group compatibility. This strategy shows the high efficiency, operational simplicity as well as broad substrate scope.

Full text of DOI:10.1016/j.tetlet.2016.12.053

Accepted Manuscript
Copper-mediated intramolecular aza-Wacker-type cyclization of 2-alkenylani-
lines toward 3-aryl indoles
Rui Yang, Jin-Tao Yu, Song Sun, Qingheng Zheng, Jiang Cheng
PII:
S0040-4039(16)31700-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.12.053
TETL 48465
Reference:
Tetrahedron Letters
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Revised Date:
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19 December 2016
20 December 2016
Please cite this article as: Yang, R., Yu, J-T., Sun, S., Zheng, Q., Cheng, J., Copper-mediated intramolecular aza-
Wacker-type cyclization of 2-alkenylanilines toward 3-aryl indoles, Tetrahedron Letters (2016), doi: http://
dx.doi.org/10.1016/j.tetlet.2016.12.053
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Copper-mediated intramolecular aza-
Wacker-type cyclization of 2-alkenylanilines
toward 3-aryl indoles
Rui Yang, Jin-Tao Yu, Song Sun, Qingheng Zheng and Jiang Cheng*

1
Tetrahedron Letters
Copper-mediated intramolecular aza-Wacker-type cyclization of 2-alkenylanilines
toward 3-aryl indoles
Rui Yang, Jin-Tao Yu, Song Sun, Qingheng Zheng and Jiang Cheng*1
School of Petrochemical Engineering, Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, Jiangsu Province Key Laboratory of Fine
Petrochemical Engineering, Changzhou University, Changzhou 213164, P. R. China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A copper-mediated intramolecular aza-Wacker-type cyclization was developed for the direct and
efficient synthesis of 3-aryl indoles using 2-alkenylanilines in moderate to good yields with
good functional group compatibility. This strategy shows the high efficiency, operational
simplicity as well as broad substrate scope.
Available online
Keywords:
Copper-mediated
Intramolecular
Aza-Wacker-type cyclization
3-Aryl indoles
Over the past years, Wacker-type reaction has proven to be a
versatile approach for the synthesis of a variety of important
molecules, and has an extensive application in many fields
because of its unique properties.1 The metal-catalyzed oxidative
amination of olefins also called as aza-Wacker reaction, has been
developed for quite a long time, and has been the topic of
intensive investigations in recent years2. For example, in 2013,
Jiang3 and Chemler4 achieved the intermolecular oxidative
coupling of amines and alkenes catalyzed by palladium and
copper, respectively (Scheme 1, eqs 1 and 2). Recently, Youn5
demonstrated an aza-Wacker-type indoles synthesis of 2-
alkenylanilines, wherein catalytic amount of palladium was
involved (Scheme 1, eq 3). Very recently, Xu6 reported a copper-
catalyzed intramolecular oxidative amination of a variety of di-
and trisubstituted internal olefin moieties (Scheme 1, eq 4).
On the basis of our interests in the exploration of heterocycles
construction,15 Herein, we wish to report a new strategy for the
direct and efficient synthesis of 3-aryl indoles (Scheme 1, eq 5),
using 2-alkenylanilines, which are readily available by either the
reaction of terminal alkyne and aniline,16 or the reaction between
2-iodoaniline and phenacylamine N-Ts hydrazine.17
Scheme 1. Oxidative coupling of amines and alkenes
On the other hand, indole and its derivatives represent one of
the most important heterocycle compounds7, which are
ubiquitous in biologically significant natural compounds and
agrochemicals. Besides, they serve as important skeletons in
many synthetic drugs.8 Since Fischer indolization was achieved
over a century ago,9 numerous efficient synthetic routes have
been developed to construct such frameworks10 thanks to their
fantastic properties, including reductive cyclization of 2-
substituted nitroarenes,11 annulation of ortho-halogenated
anilines with alkynes,12 metal-catalyzed cyclization of o-
alkynylanilines,13 and other coupling/condensation cascade
reactions.14 Nevertheless, to develop both new reaction substrates
and methodologies to access such frameworks still remains
challenging and highly desirable in organic chemistry.
———
1* Corresponding author. E-mail: jiangcheng@cczu.edu.cn

2
Tetrahedron
Table 1. The optimization of reaction conditions.a
Entry
1
2
3
4
5
6
7
8
Copper (equiv)
CuCl2(0.2)
Oxidantb
BPO
BPO
BQ
K2S2O8
DCP
DTBP
TBHP
--
BPO
--
--
--
--
Yieldc (%)
12
43
37
<5
<1
<5
<5
20
<1
39
57
65
Cu(OAc)2(0.2)
Cu(OAc)2(0.2)
Cu(OAc)2(0.2)
Cu(OAc)2(0.2)
Cu(OAc)2(0.2)
Cu(OAc)2(0.2)
Cu(OAc)2(0.2)
--
Cu(OAc)2(0.5)
Cu(OAc)2(1.0)
Cu(OAc)2(1.5)
Cu(OAc)2(2.0)
CuI(2.0)
9
10
11
12
13
14
15
16
17
18
82, 16d, 23e, 28f, 30g
--
--
--
--
trace
36
Cu(OTf)2(2.0)
Cu(acac)2(2.0)
Cu(TFA)2(2.0)
Cu(OAc)2(2.0)
23
27
a
2-(1-phenylvinyl) aniline (0.2 mmol, 1 equiv), Cu(OAc)2 (2.0 equiv), in
--
<34h, 41i, 55j, <5k, 34l
DMSO (2 mL) at 120 oC under air for 12 h.
Based on the optimized reaction conditions, we first explored
the substrate scope and limitations of 2-(1-arylvinyl) anilines
(Scheme 2). As expected, a variety of 2-(1-arylvinyl) anilines
with different substituents on the both aromatic rings, including
electron-donating and electron-withdrawing groups could all
proceed well and generated the corresponding desired indole
derivatives in moderate to excellent yields (2a-2p, 40%-92%). A
variety of functional groups, such as fluoro, chloro, bromo,
methyl, methoxy and tert-butyl were well tolerated under these
standard conditions. Importantly, the good tolerance of bromo,
chloro and fluoro (2c, 2d, 2f, 2k and 2l), which could be easily
a
Reaction conditions: 1a (0.2 mmol), oxidant (4.0 equiv) at 120 oC, in
b
DMSO (2.0 mL) under air for 12 h in a sealed tube. BPO = benzoyl
peroxide, BQ = 1,4-benzoquinone, DCP = dicumyl peroxide, DTBP = di-tert-
c
butyl peroxide, TBHP = tert-butyl hydroperoxide (70% in water). Isolated
yields. DMF (2.0 mL). e 1,4-dioxane (2.0 mL). toluene (2.0 mL). xylene
d
f
g
(2.0 mL). h At 100 oC. i At 110 oC. j At 130 oC. k Under O2. l Under N2.
We commenced our study with the reaction 2-(1-phenylvinyl)
aniline (1a, 0.2 mmol) in the presence of BPO (BPO = benzoyl
peroxide, 4.0 equiv) and CuCl2 (0.2 equiv), in DMSO (2 mL)
o
under air at 120 C. To our delight, the desired 3-phenyl indole
subjected, provided handles
functionalizations
for potentially further
transformations.
product 2a was formed and isolated in 12% yield after 12 h
(Table 1, entry 1). Replacing CuCl2 with Cu(OAc)2 (0.2 equiv)
dramatically increased the yield to 43% (Table 1, entry 2).
Subsequently, the investigated co-oxidants including BQ, K2S2O8
and DCP, DTBP, TBHP were all inferior to BPO on the reaction
efficiency (Table 1, entries 3-7). Afterwards, two key blank
experiments were carried out that without BPO, 2a was still
obtained in 20% yield (Table 1, entry 8), while no reaction took
place in the absence of Cu(OAc)2 (Table 1, entry 9). This result
reminded us that BPO might not be necessary in this procedure
and encouraged us to further investigate the loading of Cu(OAc)2
without any co-oxidant (Table 1, entries 10-13). Gratifyingly, the
yield of 2a increased to 82% when 2.0 equivalent of Cu(OAc)2
was employed. With this exciting result, we subsequently
examined the solvent effects on the reaction efficiency, such as
DMF, 1,4-dioxane, toluene and xylene. However, no positive
result was brought out, generating 2a in 16%, 23%, 28% and
30%, respectively (Table 1, entry 13). The copper salts, such as
CuI, Cu(OTf)2, Cu(acac)2 and Cu(TFA)2 were investigated.
However, they were all inferior to Cu(OAc)2, generating 2a in
trace, 36%, 23% and 27% yields (Table 1, entries 14-17). Further
screening of the parameters including reaction atmosphere and
temperature (Table 1, entry 18) established the optimized
condition as follows: 2-(1-phenylvinyl) aniline (0.2 mmol, 1
whereby appropriate
However, when methyls were located on the 2-phenyl and 4-
position, negative effect was caused and the yield of 4,6-
dimethyl-3-phenyl-1H-indole decreased significantly to 48%
(2h), probably because of the steric hindrance. This result was
further confirmed by the phenomenon that the product 2p was
obtained only in 40% yield. In this transform the substrate with
naphthyl group was also a good reaction partner as 3-phenyl-1H-
benzo[g]indole was obtained in 92% yield (2j). Subsequently, the
compatibility of substituents on the nitrogen atom ring was
studied. Fortunately, the investigated methyl and isopropyl were
tolerated well, generating 2q, 2r and 2s in 92%, 96% and 41%
yields, respectively. Notably, the success on the attempts to
access 3-methyl-1H-indole (2t, 47%) and 2-methyl-3-phenyl-1H-
indole (2u, 33%) further broadened the scope the substrates and
the practicability of this strategy. The practicality of this strategy
was further improved by a 5.0 mmol scale reaction (66% of 2a,
Scheme 3)
Scheme 3. Gram-Scale Reaction of 1a
o
equiv), Cu(OAc)2 (2.0 equiv), in DMSO (2 mL) at 120 C under
air for 12 h (Table 1, entry 13).
Scheme 2. The Substrates Scope of 2-(1-Arylvinyl)anilinesa

3
(15KJA150001), Jiangsu Key Laboratory of Advanced
Scheme 4 Mechanistic Studies
Catalytic Materials &Technology (BM2012110) and Advanced
Catalysis and Green Manufacturing Collaborative Innovation
Center for financial supports.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at
References and notes
To investigate the mechanism of this reaction, experiments in
the presence of the radical-trapping reagents, such as 2,2,6,6-
Tetramethylpiperidinooxy (TEMPO) and 1,1-diphenylethylene,
were carried out under standard conditions (Scheme 4, eqs 1 and
2). However, the reactions could not be shut down even though
4.0 equivalent of radical-trapping reagents were used, affording
2a in 65% and 73% yields, respectively (Scheme 4, eqs 1 and 2).
Thus, it might rule out the possibility involving radical process.
Meanwhile, on the basis of our observations and the reports on
the synthesis of indoles,18 and the Wacker reaction,1,19 a proposal
was put forward that serves to account for this transform
(Scheme 5). Initially, 2-(1-arylvinyl)anilines 1a reacted with
copper to produce complex 3,20 followed by the generation of
1
2
3
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4
through metallization.21 Finally, the β-H
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2a.
Scheme 5 Proposed Mechanism
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4
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In summary, a copper-mediated intramolecular aza-Wacker
reaction has been successfully developed and applied for the
direct and efficient synthesis of a large variety of 3-aryl indoles
using 2-alkenylanilines as starting materials, which are readily
available by either the reaction of terminal alkyne and aniline, or
the reaction between 2-iodoaniline and phenacylamine N-Ts
hydrazine. In this procedure, a wide range of functional groups,
including methyl, methoxy and tert-butyl and halogens, were
well tolerated. Besides, the gram-scale reaction further improved
the practicality of this transform. It represents a facile pathway
leading to 3-aryl indole derivatives.
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Acknowledgments
We thank the National Natural Science Foundation of China
(nos. 21272028 and 21572025), “Innovation & Entrepreneurship
Talents” Introduction Plan of Jiangsu Province, the Key
University Science Research Project of Jiangsu Province
9
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5
Highlights
1. In this procedure, the starting materials are easy to be
prepared.
2. This strategy features high efficiency with operational
simplicity.
3. The yield in the procedure up to 96%