Iodine-Mediated Electrochemical C(sp2)-H Amination: Switchable Synthesis of Indolines and Indoles

Article abstract of DOI:10.1021/acs.orglett.0c01821

A metal-free electrochemical intramolecular C(sp2)-H amination using iodine as a mediator was developed. This method enables a switchable synthesis of indoline and indole derivatives, respectively, from easily available 2-vinyl anilines.

Full text of DOI:10.1021/acs.orglett.0c01821

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Letter
Iodine-Mediated Electrochemical C(sp2)−H Amination: Switchable
Synthesis of Indolines and Indoles
Kangfei Hu, Yan Zhang, Zhenghong Zhou, Yu Yang, Zhenggen Zha,* and Zhiyong Wang*
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ABSTRACT: A metal-free electrochemical intramolecular C(sp2)−H amination using iodine as a mediator was developed. This
method enables a switchable synthesis of indoline and indole derivatives, respectively, from easily available 2-vinyl anilines.
ndolines and indoles are prevalent structures in bioactive
the oxidant (Scheme 1B). NIS-mediated amination reactions
for the synthesis of indolines and indoles were also reported.7
Other methods for the synthesis of indoles via C(sp2)−H
amination include sulfonium ion catalysis,8 selenium catalysis
with N-fluorobenzenesulfonimide (NFSI) oxidation,9 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) mediation,10
and 3,5-dichlorobenzoic acid catalysis in the presence of
dioxygen.11 Among these amination reactions, the metals and
stoichiometric oxidants are usually necessary to conduct the
reaction. Therefore, it is highly desirable to develop a more
efficient metal-free oxidation system.
Organic electrochemistry, employing an electron as the
reagent, has been discovered to be an efficient and environ-
mental friendly tool in organic synthesis.12 Recently, some
elegant methods for the synthesis of indolines and indoles were
reported in this field.13 For instance, the Vincent group
reported a directed electro-oxidative dearomatization of
indoles to synthesize indolines.14 Zeng and coworkers
developed the electrochemically catalyzed aminooxygenation
of styrenes for the synthesis of indolines.15 The Xu group
developed various methods to synthesize nitrogen-containing
heterocyclic compounds via an anodic oxidation.16 The Lei
group developed electrochemical methods for the construction
of a C(sp2)−N bond or a C(sp3)−N bond.17 Despite this
outstanding research, the selective synthesis of indolines and
1,2
Iintermediates, natural products, and medicines.
The
synthesis of indolines and indoles remains a huge interest in
the field of organic chemistry. Direct C(sp2)−H amination
shows a powerful and practical strategy for the synthesis of
indolines and indoles.3 The general methods involve metal
catalysis (Scheme 1A).4,5 To meet the requirement of the
particular purity in biological and medicinal chemistry,
significant contributions are based on metal-free oxidation
reactions.
Scheme 1. Synthesis of Indolines and Indoles
Received: May 30, 2020
Recently, the metal-free amination or diamination of 2-vinyl
aniline has attracted chemists’ attention. A series of hyper-
valent iodine-mediated intramolecular amination reactions
were reported.6 For instance, both Johnston’s efficient
synthesis of indolines and Muniz’s rapid preparation of indoles
are intramolecular aminations with hypervalent iodine(III) as
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A

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indoles in one reaction system still remains challenging. Our
group has been focusing on iodine-mediated chemical and
electrochemical research for a long time.18 Herein we report a
switchable synthesis of indolines and indoles from readily
available 2-vinyl anilines in the presence of Me4NI under
electrochemical conditions.
auxiliary electrolytes, such as NH4PF6, NH4BF4, NH4ClO4, and
nBu4NBF4 were examined in the reaction. It was found that
NH4PF6 gave the best result (entry 15). The control
experiments demonstrated that both Me4NI and the electricity
were essential for the transformation (entries 17 and 18).
Finally, the electric current was examined, and 5 mA showed
the best result with the highest yield of 96% (entry 19 vs
entries 15 and 20). The optimal conditions were determined,
as shown in entry 19.
Initially, we chose 2-vinyl aniline 1a as the model substrate
(Table 1). The reaction was performed in an undivided cell at
a
Table 1. Optimization of Reaction Conditions
Having established the optimal electrolytic protocols, the
scope of the amines was investigated under standard
conditions (Scheme 2). Various amines were employed in
a
Scheme 2. Substrate Scope of Amines
AE
b
entry
electrolyte
KI
NH4I
nBu4NI
nBu4NBr
Et4NI
Me4NI
NaI
(1.0 equiv)
solvent
yield (%)
1
2
3
4
5
6
7
8
9
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
26
trace
59
trace
57
82
66
72
Me4NI
Me4NI
CH3CN/DMF (5.5/0.5)
CH3CN/DMSO
(5.5/0.5)
76
c
10
Me4NI
Me4NI
Me4NI
Me4NI (0.2)
Me4NI (0.2)
Me4NI (0.2)
Me4NI (0.2)
CH3CN/H2O (5.5/0.5)
CH3CN/H2O (5/1)
CH3CN/H2O (4.5/1.5)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
CH3CN/H2O (5/1)
83
91
80
92
65
94
84
n.d.
n.d.
96
89
11
12
13
14
15
16
NH4BF4
nBu4NBF4
NH4PF6
NH4ClO4
NH4PF6
NH4PF6
NH4PF6
NH4PF6
a
Reaction conditions: 1a (0.3 mmol), Me4NI (0.06 mmol), NH4PF6
d
(0.3 mmol), amine (0.3 mmol), CH3CN/H2O (5 mL/1 mL); the
electrolysis was conducted in an undivided cell at 25 °C for 5 to 6 h.
The isolated yields after column chromatography are shown.
17
e
18
19
Me4NI (0.2)
Me4NI (0.2)
Me4NI (0.2)
f
g
20
a
the reaction to afford the desired products in good to excellent
yields. As for the anilines, both electron-withdrawing groups
and electron-donating groups at the para position of anilines
were well tolerated (3ab−an). The electronic properties of
substituted anilines had some influence on the yields. In
general, the electron-deficient anilines gave inferior results
compared with electron-rich anilines (3aj−am vs 3af−ai). 1-
Naphthylamine also performed well to give the desired product
in 80% yield (3ao). Moreover, the secondary anilines also
proceed smoothly in 80 and 88% yields, respectively (3ap,
3aq). The aliphatic amines, such as cyclohexylamine and
piperidine, also conducted well in excellent and moderate
yields (3as and 3at). Notably, benzylamine was also
successfully employed in the reaction to afford the target
product in 80% yield (3ar).
Subsequently, the scope of 2-vinyl anilines was explored
(Scheme 3). A wide range of 2-vinyl anilines underwent the
reaction efficiently to afford the corresponding indolines in
moderate to excellent yields. It was found that the electronic
effect had a great influence on the reaction. Moderate electron
density within a certain range gave high yields. The stronger
the electron-withdrawing group, the lower the yield (3bh−bl);
the stronger the electron-donating group, the lower the yield
(3bm−bp). The 4-halogen-2-vinyl anilines showed little
difference in yields with a variety of halogens (3ba, 3bb, and
Reaction conditions: 1a (0.3 mmol), electrolyte (1.0 equiv), PhNH2
(1.0 equiv), and solvent (6 mL); the electrolysis was conducted in an
undivided cell at room temperature, Pt−Pt (1.5 × 1.5 × 0.3 cm3), air,
b
I = 10 mA; AE = auxiliary electrolyte, n.d. = no detection. Isolated
yields after column chromatography. Entry 10: 5.5 mL of CH3CN
and 0.5 mL of H2O, entry 12: 4.5 mL of CH3CN and 1.5 mL of H2O,
c
d
and entries 13−20: Me4NI (0.2 equiv), AE (1.0 equiv). No Me4NI.
e
f
g
Without electricity. I = 5 mA. I = 15 mA.
a constant current of 10 mA in the presence of KI in CH3CN
at room temperature. After 3 h, the desired product indoline
was obtained with 26% isolated yield (entry 1, Table 1). Then,
we investigated different electrolytes, as shown in entries 1−7
of Table 1. Of them, Me4NI exhibited the best result in
comparison with iodide and bromide electrolytes (entries 1−
7). The results are perhaps relevant to the solubility,
conductivity, and redox potential of the corresponding
halogenated electrolytes (Table S3). Considering the poor
solubility and conductivity of Me4NI in CH3CN, other
solvents were added to the reaction (entries 8−10). H2O
was found to be the best choice (entry 10). Delightfully,
CH3CN/H2O (5/1) gave the best result with 91% isolated
yield (entry 11). The reaction also conducted well when the
stoichiometric Me4NI was decreased to a catalytic amount in
the presence of an auxiliary electrolyte. Afterward, various
B
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a
of potassium thiocyanate (KSCN) promoted the formation of
indole product (entry 1). Then, we conducted the reaction at
different temperature (entries 1−5). It was found that at 80
°C, the indole yield could be efficiently enhanced (entry 4).
Me4NI showed a better result as both an electrolyte and a
mediator compared with KI (entries 4 and 10). The optimal
conditions are listed in entry 4.
Scheme 3. Substrate Scope of 2-Vinyl Anilines
Then, we explored the scope of the reaction substrates for
the synthesis of indoles (Scheme 4). A series of 5-substituted
a
Scheme 4. Synthesis of Indoles
a
Reaction conditions: 1 (0.3 mmol), Me4NI (0.06 mmol), NH4PF6
(0.3 mmol), PhNH2 (0.3 mmol), CH3CN/H2O (5 mL/1 mL); the
electrolysis was conducted in an undivided cell at 25 °C for 5 to 6 h.
The isolated yields after column chromatography are shown.
3bd). However, the electronic and steric effects of the
substituent on the different positions of 2-vinyl anilines had
a significant influence on the yields (3bc−bf). For instance, 3-
bromo-2-vinyl aniline gave a low yield compared with 4-
bromo-2-vinyl aniline and 5-bromo-2-vinyl aniline, whereas 6-
bromo-2-vinyl aniline hardly performed the reaction and failed
to give the desired product. The disubstituted terminal alkene
also completed the reaction well (3bw).
a
Reaction conditions: 1 (0.3 mmol), Me4NI (0.3 mmol), KSCN (0.3
mmol), CH3CN/H2O (5 mL/1 mL); the electrolysis was conducted
in an undivided cell at 80 °C for 2 to 10 h. The isolated yields after
column chromatography are shown.
On the contrary, it was found that different additives could
change the distribution of the products (Table 2). We can
control the product distribution by the addition of different
additives (entry 4 and entries 6−9). For instance, the addition
indoles with various substituents, such as halogens (4b−h),
trifluoromethyl (4i), cyano (4j), carbonyl (4l, 4m), and alkyl
(4n−p), were obtained under the electrochemical conditions.
Other substituents like phenyl, methoxyl, and nitryl (4r, 4q,
and 4k), were also found to be compatible with the electrolytic
conditions. The different bromo positions of 2-vinyl aniline
were also investigated under the electrolytic conditions. It was
found that 4-, 5-, and 6-bromoindole products were obtained
(4d−f), whereas 7-bromoindole failed to be obtained, perhaps
due to the electronic and steric effect (4g). 4,5-Disubstituted
substrates also performed well to afford the desired products
under the reaction conditions (4h and 4s). Gratifyingly, the
reaction had a broad substrate scope. The reaction provided an
innovative and efficient electrochemical method for the rapid
synthesis of indoles.
a
Table 2. Optimization of Reaction Conditions
b
entry
electrolyte
additive
T (°C)
yield (%)
1
2
3
4
5
6
7
8
9
10
Me4NI
Me4NI
Me4NI
Me4NI
Me4NI
Me4NI
Me4NI
Me4NI
Me4NI
KI
KSCN
KSCN
KSCN
KSCN
KSCN
NH4SCN
NaSCN
KSCN
KOTf
25
40
70
80
90
80
80
80
80
80
43
70
83
92
90
trace
78
n.d.
n.d.
77
To prove the practicability and expansibility of the method,
gram-scale experiments were performed under standard
conditions. The desired products 3aa and 4a were obtained
smoothly in 87 and 90% yields, respectively (Scheme 5).
c
Scheme 5. Gram-Scale Experiments
KSCN
a
Reaction conditions: 1a (0.3 mmol), electrolyte (1.0 equiv), additive
(1.0 equiv) and solvent (CH3CN/H2O = 5 mL/1 mL); the
electrolysis was conducted in an undivided cell for 6 h, Pt−Pt (1.5
b
× 1.5 × 0.3 cm3), air, I = 10 mA, n.d. = no detection. Isolated yields
c
after column chromatography. Without electricity.
C
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To get insight into the reaction mechanism, control
experiments were performed (Scheme 6). First, the reaction
be caught by the hydroxyl anion, followed by elimination to
give indole 4a. Simultaneously, H2O is reduced to hydrogen
and hydroxyl anion at the cathode.
In conclusion, we developed a method for the switchable
synthesis of indolines and indoles by virtue of anodic oxidation
under metal-free conditions. This electrochemical intra-
molecular C(sp2)−H amination reaction can avoid the use of
a metal catalyst and a stoichiometric oxidant. The reaction can
be carried out smoothly under mild conditions, which provides
a rapid and efficient method for preparing functionalized
indolines and indoles.
Scheme 6. Control Experiments
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01821.
1H/13C NMR data and spectra of all products and
HRMS data of unknown products (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
Zhiyong Wang − Hefei National Laboratory for Physical Science
at Microscale, CAS Key Laboratory of Soft Matter Chemistry &
Center for Excellence in Molecular Synthesis of Chinese
Academy of Science, Collaborative Innovation Center of Suzhou
Nano Science and Technology & School of Chemistry and
Materials Science, University of Science and Technology of
China, Hefei, Anhui 230026, People’s Republic of China;
orcid.org/0000-0002-3400-2851; Email: zwang3@
ustc.edu.cn
was conducted with chemical oxidant I2, giving 20% yield and
76% starting material recovery (Scheme 6a). When the radical
inhibitor 2,2,6,6-tetramethylpiperidinooxy (TEMPO) or buty-
lated hydroxytoluene (BHT) was added to the reaction
mixture, the corresponding product 3aa could be obtained in
90 and 86% isolated yield, respectively (Scheme 6b). These
results implied that the reaction may not go through a radical
process. The radical clock experiment was also conducted
under the standard conditions, and the ring-reserved product
was obtained (Scheme 6c). The experimental results indicated
that the reaction proceed through an ionic process.
Zhenggen Zha − University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China;
Email: zgzha@ustc.edu.cn
On the basis of the previously described experimental results
and literature reports,19 a plausible mechanism was proposed
(Scheme 7). Iodide is oxidized to molecular iodine. The iodine
reacts with substrate 1a to afford the intermediate 5a; then, the
following nucleophilic addition leads to the formation of 5c
and further deprotonation gives 5d, followed by the
nucleophilic attack of PhNH2 to form indoline 3aa. In the
presence of potassium thiocyanate, the iodide 5d can be
substituted by thiocyanate to give 5e. Then, the α-proton can
Authors
Kangfei Hu − University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
Yan Zhang − University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
Zhenghong Zhou − University of Science and Technology of
China, Hefei, Anhui 230026, People’s Republic of China
Yu Yang − University of Science and Technology of China, Hefei,
Anhui 230026, People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01821
Scheme 7. Plausible Mechanism
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (nos. 21672200,
21472177, 21772185, and 21801233) and the assistance of
the product characterization from the National Demonstration
Center for Experimental Chemistry Education of University of
Science and Technology of China. This work was supported by
the Strategic Priority Research Program of the Chinese
Academy of Sciences (Grant No. XDB20000000).
D
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