Synthesis of 2-arylindoles by Suzuki coupling reaction of 3-bromoindoles with hindered benzoboronic acids

Article abstract of DOI:10.1016/j.tetlet.2017.01.014

A new synthetic method for 2-arylindoles has been developed, the process through Suzuki coupling reaction of 3-bromoindoles with hindered boronic acid catalyzed by Pd(OAc)₂/PCy₃, and a series of 2-arylindoles have been synthesized in moderate to high yields.

Full text of DOI:10.1016/j.tetlet.2017.01.014

Accepted Manuscript
Synthesis of 2-Arylindoles by Suzuki Coupling Reaction of 3-Bromoindoles
with Hindered Benzoboronic Acids
Guizhou Yue, Yao Wu, Caimei Wu, Zhongqiong Yin, Huabao Chen, Xianxiang
Wang, Zuming Zhang
PII:
S0040-4039(17)30026-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.01.014
TETL 48517
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 October 2016
3 January 2017
4 January 2017
Please cite this article as: Yue, G., Wu, Y., Wu, C., Yin, Z., Chen, H., Wang, X., Zhang, Z., Synthesis of 2-Arylindoles
by Suzuki Coupling Reaction of 3-Bromoindoles with Hindered Benzoboronic Acids, Tetrahedron Letters (2017),
doi: http://dx.doi.org/10.1016/j.tetlet.2017.01.014
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Synthesis of 2-Arylindoles by Suzuki
Coupling Reaction of 3-Bromoindoles with
Hindered Benzoboronic Acids
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Guizhou Yue, Yao Wu, Caimei Wu, Zhongqiong Yin, Huabao Chen, Xianxiang Wang, Zuming Zhang

1
Tetrahedron Letters
Synthesis of 2-Arylindoles by Suzuki Coupling Reaction of 3-Bromoindoles with
Hindered Benzoboronic Acids
Guizhou Yuea,b, , Yao Wua, Caimei Wuc , Zhongqiong Yind, Huabao Chenb, Xianxiang Wanga , Zuming
Zhanga
aCollege of Science, Sichuan Agricultural University, Ya’an, Sichuan, 625014, China
bCollege of Agricultural Sciences, Sichuan Agricultural University, Chengdu, Sichuan, 611130, China
cAnimal Nutrition Institute, Sichuan Agricultural University, Chengdu, 611130, China
dCollege of Veterinary Medicine, Sichuan Agricultural University, Chengdu, Sichuan, 611130, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A new synthetic method for 2-arylindoles has been developed, the process through Suzuki
coupling reaction of 3-bromoindoles with hindered boronic acid catalyzed by Pd(OAc)2/PCy3,
and a series of 2-arylindoles have been synthesized in moderate to high yields.
2009 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Suzuki coupling
1,2-migration
Indole
Palladium catatlysis
Boronic acid
Introduction
epipolythiodiketopiperazine alkaloids as potent anticancer
reagents,8 and compound II as a powerful KDR kinase inhibitor.9
In many synthetic methods and application of indole ring,10
palladium-catalyzed one had been researched widely and
thoroughly.11 Several elegant works for synthesis of 2-
subsitituted indoles,12 but the synthesis of 2-mesity or (2,6-
dimethyl)phenylindole derivatives is rarely reported.
Suzuki coupling reaction had been widely developed in
academic and industrial fields, since it was found in 1979.1
Among the numerous cross-coupling methods to construct biaryl
or substituted aromatic compounds, Suzuki coupling is one of the
most efficient and useful methods.2 Suzuki coupling has several
advantages, such as commercial availablility of most of boronic
acids and their esters, stability of boron reagents to heat, air, and
moisture, tolerance to a broad range of functional groups, low
toxicity, mild reaction conditions, as well as easy separation of
formed inorganic boron. Coupling of heteroaryl halides is
generally considered to be more challenging than aryl ones. In
particular, nitrogen-containing heterocycles, for instance,
pyridine, indole and quinoline, can displace some phosphine
ligands on Pd(II) complexes. In the past three decades, Buchwald
phoshines,3
bukly
trialkylphosphines,4
and
other
Figure 1. Selective indole alkaloids.
dialkylarylphosphines5 had been exploited successfully for
Suzuki coupling of heteroaryl halides.
Recently, Zhou’s group has reported the coupling of
heteroaryl chlorides or bromides with aryl or heteroaryl boranes
in the mild conditions13. In our case, 3-bromo-N-methylindole
reacted with mesitylboronic acid14 using Pd(OAc)2 as a catalyst,
we found that the product was 3-mesityl-N-methylindole (1'a) in
Indole nucleus is an important motif of a huge number of
biologically active alkaloids and unnatural coupounds, for
example, dragmacidin D (I) of antiviral activity against HSV-16
(Figure 1), isatisine A for anti-HIV activity,7 dimeric
93%
yield
using
t-
———
 Corresponding author. Tel.: +(86) 0835 2886189; e-mail: yueguizhou@sicau.edu.cn

2
Tetrahedron Letters
Scheme 1. Synthesis of 2-mesityl-N-methylinodle and 3-mesityl-N-methylindole from 3-bromo-N-methylindole.
/24h
Bu3P•HBF4 as ligand and t-BuONa as base, whereas the
employment of Cy3P and K2CO3 led to 2-mesityl-N-methylindole
(1a) as a single product in 96% GC yield via 1,2-migration of Pd
intermediate (Scheme 1). The relative configuration of these
products was confirmed easily by 1H NMR spectroscopy. The C3-
H shift of indole cycle is 6.34 ppm, while the C2-H one is 6.88
ppm. The 1D NOE and 1H-1H COSY experiments also
unambiguously confirmed the regioselectivity of coupling
reaction. To the best of our knowledge, this represents the first
synthesis of 2-arylindoles through Pd-catalytic 1,2-migration.
Herein, we reported the our preliminary results on this Suzuki
coupling under Pd(OAc)2/Cy3P catalytic system.
CsOH•H2O/t-BuOH/120oC
/24h
6
69
0.4+21
7
8
9
10
11
12
13
14
15
16
KOH/t-AmOH/100oC/6h
KOH/t-BuOH/100oC/6h
Cs2CO3/t-AmOH/120oC/24h
Cs2CO3/t-BuOH/120oC/24h
K3PO4/t-AmOH/120oC/24h
K3PO4/t-BuOH/120oC/24h
K2CO3/t-AmOH/100oC/6h
K2CO3/t-BuOH/100oC/6h
Na2CO3/t-AmOH/120oC/24h
Na2CO3/t-BuOH/120oC/24h
85
67
99
11+3
4+32
32+2
25+34
64+0
73+3
77+0.1
90+1
96+0
99+0
96
100
100
100
100
99
100
Results and Discussion
With the optimized conditions in hand, the scope of the
reaction with 3-bromo-N-subsitutedindoles bearing either
electron-donating or electron-withdrawing groups (Table 2) was
explored. As shown in Table 2, when the substituted groups in
nitrogen atom were electron-donating groups (Et, Pr, Hex, TBS,
Bn etc.), the results of the reaction were good and the yields
exceeded 70%. It is noted that the 3-bromo-N-alkylindoles,
especially propyl or hexyl, were found to be very unstable to
decompose in air and at the normal ambient temperature, in
which probably resulted in decreased yields. The solution of 3-
bromo-N-alkylindoles in t-AmOH or t-BuOH was found to be
relatively stable in low temperature, which can partially avoid
this problem. When the substituted groups were electron-
withdrawing ones, the reaction was slowly and the by-product
also occurred. For example, compound 1i was partly taken off p-
toluene sulfonyl (Ts) to obtain 1b under the reaction condition
(Table 2, Entry 9).
We chose 3-bromo-N-methylindole and mesitylboronic acid
as model substrates to search for the reaction to find the
optimized condition (Table 1). Initially we tested the model
reaction by using 5 mol% Pd(OAc)2 and 6 mmol% Cy3P loadings
when t-BuOK was used in tertiary amyl alcohol (t-AmOH) at
120oC for 24h, which gave high conversion (100 %), but very
low yield (Table 1, Entry 1). As shown in Table 1, the product is
almost 2-substituted indole in about 3% yield. t-BuONa as the
base delivered 2-substituted product in 26% yield. Decreasing
temperature (100oC) and shorting time (6h) could enhance yield,
but the selectivity was limited. The solvent was changed to t-
BuOH showed the same result. Other strong base, for example,
CsOH and KOH, were less active. Based on the above
experiments, it can be determined that the selectivity and yield
were almost affected by the basicity of base. Weaker base gave
better selectivity and higher yields. Na2CO3 showed the best
result that the product was almost 2-mesityl-N-methylindole
(Table 1, Entry 15 and 16). Furthermore, the reaction in t-BuOH
showed a slightly higher yield than t-AmOH.
Table 2. Synthesis of 1-substituted-2-mesitylindole
derivatives 1a-1j16
The steric hinderance of boronic acids were very important for
the 1,2-migration of the reaction. When the mesitylboronic acid
was replaced by less hindered boronic acids, for example,
PhB(OH)2, the product was 3-subsituted indole and not 2-
subsittuted one at all. Other palladium salts, such as Pd(PPh3)4,
PdCl2, Pd2(dba)3 were not so efficient catalysts compared with
Pd(OAc)2. When the loadings of Pd salt and ligand (2.5 mmol%
Pd and 3.0 mmol% Cy3P) were decreased, the reaction had low
yield. Finally, the optimal reaction condition for this Suzuki
coupling was established [3-bromo-N-methylindole (1.0 equiv.)
mesitylboronic acid (2.0 equiv.), and Na2CO3 (2.7 equiv.) in t-
Entry
1
Products
Isolated Yield (%)
96
o
BuOH at 120 C under catalyst Pd(OAc)2 (5 mol%) and Cy3P (6
2
3
4
5
6
71
90
85
83
73
mol%)].
Table 1. The condition optimization of model reaction.15
GC
Conversio
n (%)
100
GC Yield
(%)
1a+1'a
0.2+3.4
0+26
13+6
0+32
0.5+3
Entry
Change of conditions
1
2
3
4
5
t-BuOK/t-AmOH/120oC/24h
t-BuOK/t-BuOH/120oC/24h
t-BuONa/t-AmOH/100oC/6h
t-BuONa/t-BuOH/100oC/6h
CsOH•H2O/t-AmOH/120oC
100
93
100
86

3
7
8
70
71
65
68
Scheme 2. Synthesis of 2-mesitylbenzofuran 3.
The mechanism of 1,2-aryl migration of indole mainly focused
on the direct C-H activation by palladium-catalyzed, in which
presented that features an electrophilic palladium, accompanied
by a 1,2-migration of an intermediate palladium species.12e,19
Unlike the above mechanism, we proposed mechanism of our
Suzuki coupling which is shown in Figure 2. The first step of
catalytic cycle involves formation of an aryl-palladium (II)
intermediate (A) via the oxidative addition of substrate to a
palladium (0) species formed in situ. Next, intermediate A is
converted into the four-coordinate species (B) via the “oxo-
palladium” pathway2m and transmatalation. The effect of the aryl
steric hindrance and ligand compels the intermediate B to
proceed a 1,2-migration, which generates thermodynamically
more stable 2-palladium species (C)20. Finally, the reductive
elimination of this species provides the 1,2-migrated products
and Pd(0) species.
9
10
The coupling efficiency of hindered boronic acid was further
examined. The reaction of 2,6-dimethylphenyl boronic acid with
the above six 3-bromo-N-subsitutedindoles had been carried out
(Table 3). The reaction also gave the same result and provided
good to excellent yields, whether electron-withdrawing or
electron-donoring groups of nitrogen atom.
Table 3. Synthesis of 1-substituted -2-(2,6-dimethylphenyl)
indole derivatives 2a-f16
Entry
1
Products
Isolated Yields (%)
90
2
3
4
5
6
82
76
70
70
66
Figure 2. Proposed Catalytic Cycle.
Conclusion
A highly efficient catalytic system for direct C-2 arylation of
indoles has been developed. This process involves Suzuki
coupling reaction of 3-bromoindoles with hindered boronic acids
and furnished sixteen 2-substitutedindole derivatives with good
to excellent yields. Further exploration and application of this
reaction in organic synthesis is ongoing in our laboratory.
Supporting Information for this article is available online at
http://www.thieme-connect.com/products/ejournals/journal/10.
1055/s-00000083.
Acknowledgments
We thank the Science & Technology Department of Sichuan
Province (2012JY0118) and Sichuan Agricultural University for
the financial support. We also acknowledge Prof. (Steve) Zhou of
Nanyang Technological University for the helpful suggestion and
discussion.
The catalyst was also applied to other aromatic heterocycle.
The reaction of 3-bromobenzofuran17 with mesitylboronic acid
smoothly proceeded under the optimized condition, which
obtained 3 in 68% yield (Scheme 2). The 1H and 13C NMR of 3
was in agreement with the literatures18.
References and notes
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4
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filled glove box, Pd(OAc)2 (0.7 mg, 0.003 mmol), PCy3(1.0 mg,
0.0036 mmol), solvent (0.4 mL) and n-dodecane (5 µL) were
charged into a 10-mL reaction tube. After stirring for 15 min,
boronic acid (0.12 mmol), base (0.162 mmol) and bromide (0.06
mmol) were added successively. The mixture was vigorously
stirred in a preheated oil bath at 100 oC for 6 h and at 120 oC for
24h. At intervals, an aliquot of the reaction mixture was taken and
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16. Typical procedure for Suzuki coupling reactions: In an argon-
filled glove box, to a 25 mL Schlenk tube was charged
sequentially Pd(OAc)2 (5.6 mg, 0.025 mmol) and PCy3 (8.4 mg,
0.03 mmol), t-BuOH (2.8 mL) and n-dodecane (20 µL). After
stirring for 15 min, mesitylboronic acid or (2, 6-dimethylphenyl)
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H2O. The aqueous phase was further extracted with Et2O (10
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Supplementary Material
Supplementary material (general experimental details, lists of
spectral data, copies of 1H and 13C NMR spectra for all
compounds) can be found in the online version of this article.
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Highlights
1. A highly efficient catalytic system [Pd(OAc)2/PCy3] has
been developed.
2. All target products were obtained with good to excellent
yields.
3. The synthesis of 2-arylindole via Pd-catlytic 1, 2-
migration was firstly discovered.
4. The catalyst has been also applied to synthesize 2-
arylbenzofuran.