Highly modulated bisindoles: ligands for copper-catalyzed Sonogashira reaction

Article abstract of DOI:10.1002/aoc.3510

Bisindoles (BIMs) were modulated as powerful N,N′ donor ligands for the copper-catalyzed Sonogashira reaction. Ligand screening experiments on 11 BIM compounds found that 3,3′-(4-chlorophenyl)methylenebis(1-methyl-1H-indole) (10%) efficiently accelerated CuCl (5%)-catalyzed cross-coupling of aryl iodides with terminal alkynes. A wide range of substituted aryl iodides and/or alkyl- and aryl-substituted terminal alkynes were examined, leading to the corresponding coupling products with yields up to 99%. An efficient and scalable protocol for the synthesis of BIM ligands on a gram scale, with extremely low catalyst loading of o-ClC₆H₄NH₃ ⁺Cl^?, was also developed with a reaction time of 20?min with yields up to 93%. This novel N,N′ ligand was air-stable, easily available and highly modulated with low copper loading. Copyright

Full text of DOI:10.1002/aoc.3510

Full paper
Received: 1 March 2016
Revised: 15 April 2016
Accepted: 16 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3510
Highly modulated bisindoles: ligands for
copper-catalyzed Sonogashira reaction
†
†
Xiu Wang , Zhenhua Wang , Ya Wu, Yanlong Luo, Guofang Zhang, Yajun Jian,
Huaming Sun, Weiqiang Zhang* and Ziwei Gao*
Bisindoles (BIMs) were modulated as powerful N,N′ donor ligands for the copper-catalyzed Sonogashira reaction. Ligand screening
experiments on 11 BIM compounds found that 3,3′-(4-chlorophenyl)methylenebis(1-methyl-1H-indole) (10%) efficiently acceler-
ated CuCl (5%)-catalyzed cross-coupling of aryl iodides with terminal alkynes. A wide range of substituted aryl iodides and/or
alkyl- and aryl-substituted terminal alkynes were examined, leading to the corresponding coupling products with yields up to
9
9%. An efficient and scalable protocol for the synthesis of BIM ligands on a gram scale, with extremely low catalyst loading of
+
À
o-ClC H NH Cl , was also developed with a reaction time of 20 min with yields up to 93%. This novel N,N′ ligand was
6
4
3
air-stable, easily available and highly modulated with low copper loading. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: copper catalysis; bisindoles; Sonogashira reaction; N,N′ ligand
perspective of coordination chemistry, BIMs can supply two hard
and strongly basic N-donor centers (Fig. 1), making BIMs versatile li-
Introduction
The Sonogashira reaction is one of the most important and widely
used sp –sp carbon–carbon bond formation reactions in organic
synthesis, frequently employed in the synthesis of natural
gands for hard metal ions. Also, tuning the conjugate aromatic
backbones of BIMs could finely improve the activity of copper cen-
ters by stabilizing catalytic active species. Thereby, BIMs are sup-
posed to be highly modulated ligands for copper-catalyzed
Sonogashira reactions in terms of their unique structure. Unlike
many primary amines, those compounds are extremely stable.
Therefore, it is promising to explore BIMs in terms of accelerating
copper-catalyzed cross-couplings of aryl iodides and terminal al-
2
[
1]
[2]
[3]
products, biologically active molecules, heterocycles and mo-
[
4]
lecular nanostructures. Various attempts have been made to
achieve the Sonogashira reaction in the absence of the expensive
noble metal palladium, and copper has proved an excellent
alternative. With intensive study of the copper-based catalyst
system, it is found that the coupling efficiency is greatly affected
by ligands. Since Miura and co-workers
CuI/PPh -catalyzed cross-couplings of aryl iodides and
arylacetylenes, hard donors as green and efficient promoters have
dominated the copper-catalyzed coupling reactions instead of
the classic phosphine ligands. The combination of 10 mol% copper
salts with a variety of ligands including bidentate N^O, N^N and
[
5]
kynes. To the best of our knowledge, reports of a robust, reliable
[
6]
[9–14]
introduced the
approach
to obtain gram-scale amounts of those compounds
3
are still scarce. We are also interested in developing a reliable
pathway to produce BIMs.
[
7]
Herein, we report the combination of BIMs (10 mol%) and CuCl
(5 mol%) to catalyze the cross-couplings of a wide range of vari-
ously substituted aryl iodides and both alkyl- and aryl-substituted
terminal alkynes in good to excellent yields. Undoubtedly, the
newly developed strategy, employing lower ligand and copper salt
loading, greatly arms the ligand arsenal for promoting copper-
catalyzed Sonogashira reactions, and further verifying a hard donor
as promising ligand for accelerating these reactions. We also de-
velop an efficient gram-scale approach for the synthesis of BIMs
[
7r]
O^O ligands, such as 8-hydroxyquinoline (20 mol%),
dimethylglycine (30 mol%),
20 mol%),
), rac-1,1′-bi-2-naphthol (20 mol%),
N,N-
1,4-diazabicyco[2.2.2]octane
N,N-dibenzyl-1,1′-binaphthyl-2,2′-diamine (20 mol
[
7o]
[7e]
(
%
[7l]
[7h]
diphenylpropane-1,3-
[7m]
[7i]
dione (30 mol%)
and salicylic acid (20 mol%), has ensured
the robust and straightforward construction of C≡C bond, making
this an indispensable methodology for the synthesis of complex
molecular architectures of C≡C. Considering the coordination sites
of the efficient ligands mentioned above, it was found that multiple
hard donor atoms and π-electron-rich aromatic moieties greatly fa-
cilitate the proceeding of cross-coupling reactions. Therefore, ex-
ploring highly modulated ligands is the key for the development
of copper-catalyzed Sonogashira reactions.
*
Correspondence to: Weiqiang Zhang or Ziwei Gao, Key Laboratory of Applied
Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and
Chemical Engineering, Shaanxi Normal University, 199 South Chang’an Road,
Xi’an, China. E-mail: zwq@snnu.edu.cn
†
These authors contributed equally to this paper
In recent years, bisindoles (BIMs) have been extensively studied
[
8]
mainly due to their frequent presence as core structural motifs
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, 199
South Chang’an Road, Xi’an, China
of natural products, pharmaceuticals and agrochemicals. However,
their potential application as ligands has been ignored. From the
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Wang X. et al.
Figure 1. Representative structures of N,N ligands.
+
3
À
via a catalyst of 1 mol% o-ClC H NH Cl , not only fulfilling our de-
6
4
mands, but also supplying a viable gram-scale approach for the
synthesis of these derivatives.
Scheme 1. Effect of ligands on CuI-catalyzed Sonogashira couplings.
Experimental
for this transformation. Notably, electron-rich aromatic functional
groups enhanced the activity of BIM ligands, and L gave 58% yield.
The methylation of the nitrogen donors of L greatly facilitated the
3
General remarks
4
All reactions were conducted using a standard Schlenk line. Aryl ha-
lides and alkynes were purchased from Aldrich or Alfa Aesar. CuCl
coupling efficiency with 83% yield. Such improvement might result
from less ligand arylation, and therefore leading to faster cross-
[
15]
(99.999%) was purchased from Aldrich. NMR spectra were collected
coupling reactions.
The ligand screening found that neither
in CDCl with a Bruker AVANCE 400 NMR spectrometer (400 MHz)
3
electron-withdrawing groups nor electron-donating groups further
using tetramethylsilane as the internal reference.
increased the yields of cross-coupling product. L , L and L bear-
5
6
7
ing electron-donating groups of OMe were less active than L
and L11 with electron-withdrawing groups of F, Br and NO . The
yields of desired product with these ligands ranged from 28 to
9%. Interestingly, the ligand bearing p-chlorobenzyl (L ) efficiently
9
Typical procedure for Cu/L -catalyzed Sonogashira reaction of
alkynes with aryl iodides
2
9
7
9
A mixture of aryl halide (0.5 mmol), alkyne (0.6 mmol), CuCl (5 mol
catalyzed the reaction with 87% isolated yield, whilst L10 bearing
m-chlorobenzyl group gave only 59% yield. This observation was
consistent with the ‘chlorobenzene accelerating effect’ in Pd-
%
3
), L
9 2 3
(10 mol%), K CO (1 mmol) and dimethylformamide (DMF;
mL) in a Schlenk tube was stirred under a nitrogen atmosphere
at 130 °C for 24 h. After that, the mixture was poured into water
[16]
catalyzed C≡C cross-coupling reactions. To sum up, the function-
(
(
50 mL). The mixture was then extracted with ethyl acetate
3 × 15 mL). The organic phase was washed with saturated sodium
ality of the BIM ligand system plays a vital role in the rate of Cu(I)-
2
catalyzed cross-coupling reactions of Csp–Csp bonds.
chloride (15 mL). After drying with anhydrous MgSO and evaporat-
4
Next, the reaction conditions were systematically optimized with
ing the solvent, the residue obtained was purified via silica gel chro-
matography to afford the desired product.
the best BIM ligand, L (Table 1). Since copper sources with different
9
valences and counter ions might form different complexes with N,N′
À
À
donor ligand, we screened Cu(I)/Cu(II) precatalysts, with acac , OAc ,
À
À
À
Typical procedure for synthesis of bis(indolyl)methanes
Cl , Br and I as counter ions. Cu(I) halides were more active than
Cu(II) salts (entries 1–6). Amounts of 5 mol% CuCl and 10 mol% L9
afforded the product with 92% yield (entry 6). It is worth pointing
out that the activity of the catalyst is highly dependent on the
CuCl/ligand molar ratio. Reducing the amount of L9 deactivated
the Cu catalyst system. While with Cu/L changed to 1:1, the yield fell
markedly to 45% (entries 6–8). In another set of experiments (entries
A reaction tube (50 mL) with a magnetic stir bar was charged with
indole (2.34 g, 20.0 mmol), aldehyde (1.06 g, 10.0 mmol), o-
chloroaniline (16.5 mg, 0.1 mmol) and CH CN (10 mL). The reaction
3
mixture was sealed and stirred at room temperature for 20 min. On
completion, the solvent was removed under reduced pressure, and
the residue obtained was purified via silica gel chromatography to
afford the desired product.
6
, 9–11), the effect of base was examined: K CO was the best choice.
2 3
Finally, the solvent effect was studied, showing that only in DMF did
the reaction proceed smoothly (entries 6, 12–14).
The scope and limitation of BIM/Cu catalyst system were exten-
sively explored with functionalized aryl iodides and terminal al-
kynes (Table 2). The current catalytic system efficiently coupled
aryl halides and terminal alkynes bearing electron-rich, electron-
neutral and electron-deficient groups. The cross-coupling of
iodobenzene and phenylacetylene gave the desired product in
99% yield (entry 1). Methoxyl, dimethyl and methyl groups at ortho
position of iodobenzene yielded the corresponding coupling
Results and discussion
Initially, the cross-coupling reaction of 1-iodo-4-methoxybenzene
and phenylacetylene was selected as a model system to optimize
the reaction conditions. Various BIMs as ligands were examined,
as shown in Scheme 1. Indole (L
1
) and 3,3′-(3-methylbutane-1,1-
diyl)bis(1H-indole) (L ) afforded 30 and 43% yields, respectively,
2
demonstrating that simple indole derivatives were not effective
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

BIMs as efficient ligands for copper-catalyzed Sonogashira reaction
Table 1. Optimization of reaction conditions
products (entries 2–4). Substituents at ortho and para positions of
iodobenzene gave superior results to those at meta position (en-
tries 4–6). Para-substituted aryl iodides afforded the desired prod-
ucts in 92–96% yields (entries 6–8), while aryl iodides with strong
electron-deficient para-substituted phenylacetylenes (p-NO
CF ) led to reduced yields of coupling reactions of 60–80% (entries
3
2
or p-
a
b
Entry
[Cu]
Base
Solvent
Yield (%)
1
2
3
4
5
6
Cu(acac)
Cu(OAc)
CuCl
2
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
PO
Et
3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
43
45
64
87
54
92
51
45
56
88
4
9 and 10). Various alkynes, both aryl and aliphatic alkynes, could
smoothly couple with aryl iodides, and the yields exceeded 83%
(entries 11–18). Unfortunately, the coupling reactions using
bromobenzene and chlorobenzene with phenylacetylene gave 20
and 5% yields of cross-coupling products (entries 19 and 20).
After successfully developing a BIM/CuCl catalyst system for the
Sonogashira reaction, a reliable approach for the synthesis of BIMs
and derivatives would be highly desirable since there is no scalable
method available at present. In light of our previous work on
Brønsted acid-catalyzed Mannich reaction, we established ani-
line hydrochloride-catalyzed condensation between indole and
benzaldehyde (Table S1 in the supporting information). To our de-
light, o-chloroaniline hydrochloride was so effective that as low as
1% catalyst catalyzed the formation of BIMs with 99% isolated yield
in just 20 min (entry 6).
2
3
2
Á2H
2
O
3
CuI
3
CuBr
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
3
3
c
7
3
d
8
3
9
Cs
2
3
[17]
10
11
12
13
14
K
3
4
3
N
K
2
K
2
K
2
CO
CO
CO
3
58
5
3
CH CN
3
3
Toluene
3
a
All reactions were performed with 1-iodo-4-methoxybenzene
Finally, we prepared 11 examples of BIMs on a large scale
(
(
0.5 mmol), phenylacetylene (0.6 mmol), [Cu] (0.025 mmol), L
9
(Table 3). Amounts of 20 eq. of indole and 10 eq. of aldehyde with
0.05 mmol) and base (1 mmol) in 3 mL solvent for 24 h.
b
1 mol% o-chloroaniline hydrochloride at room temperature in
20 min were used as the typical selection. As evident from
Table 3, a yield of 93% was obtained in the reaction of 20 mmol
of indole with 10 mmol of benzaldehyde (entry 1), which showed
that the gram-scale synthesis had a small to minimal effect on effi-
ciency compared to the results of micro-reaction (99%; Table S1 in
the supporting information, entry 6). 1-Methylindole was also an
excellent substrate for the reaction, resulting in 89% yield (Table 3
Isolated yields obtained after purification by column chromatography.
c
[
Cu] (0.025 mmol), L
9
(0.025 mmol).
d
[
Cu] (0.05 mmol), L (0.05 mmol).
9
Table 2. CuCl-catalyzed Sonogashira reaction of aryl iodides with ter-
minal alkynes
,
entry 2). The generality of the reaction was further extended by
variation of aldehydes. Aldehydes with ortho, meta and para
a
1
R
2
R
b
Entry
Yield (%)
Table 3. o-Chloroaniline hydrochloride-catalyzed gram-scale synthesis
of BIMs
1
2
3
4
5
6
7
8
9
H
C
C
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
H
5
5
5
5
5
5
5
5
5
5
99
85
89
90
77
96
92
94
60
80
95
86
90
96
83
89
90
93
20
5
o-OCH
3
o-CH
o-CH
m-CH
p-CH
3
, o-CH
3
3
3
3
p-OCH
p-Cl
3
a
1
R
2
R
3
R
4
b
Entry
R in 2
Ligand
Yield (%)
p-NO
2
10
11
12
13
14
15
16
17
18
19
20
p-CF
H
3
1
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C
C
6
H
H
5
L
L
L
L
L
L
L
3
4
5
6
7
8
9
93
89
90
93
90
88
89
86
86
80
70
m-NH
p-OMeC
p-C
p-C
p-OCH
n-C
n-C
n-C
2
C
6
H
4
2
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
6
5
H
6
H
4
4
3
o-OMeC
m-OMeC
p-OMeC
p-FC
6 4
H
H
2
H
5
C
6
H
4
6 4
H
H
5
H
11
C
6
H
4
5
6 4
H
p-CH
H
3
3
C
6
H
4
6
6
H
4
4
H
9
7
p-ClC
o-ClC
6
H
H
4
4
H
5
H
H
11
8
6
L
10
L
11
L
12
H
6
13
9
p-BrC
p-NO
6
H
4
H (X = Br)
H (X = Cl)
C
C
6
H
5
10
11
2
C
6
H
4
6
H
5
isoamyl
L
2
a
a
All reactions were performed with aryl iodide (0.5 mmol), alkyne
Reaction conditions: substituted indole (20 mmol), substituted
aldehyde (10 mmol), o-chloroaniline hydrochloride (0.1 mmol), and
MeCN (10 mL) at room temperature.
9 2 3
(0.6 mmol), CuCl (0.025 mmol), L (0.05 mmol), K CO (1 mmol), DMF
(
3 mL), 24 h.
b
b
Isolated yields obtained after purification by column chromatography.
Isolated yield.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Wang X. et al.
4
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doles quantitatively, demonstrating that the steric nature of the
substituents has a slight effect on the catalytic reaction. Aldehydes
containing various electronic properties at the para position could
be readily converted into the desired adducts in 80–90% yields (en-
tries 5–7, 9 and 10). In particular, the method is also compatible
with aliphatic aldehydes such as isovaler aldehyde (entry 11) and
heterocyclic aldehydes.
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