CNT-CuO catalyzed C–N bond formation for N-arylation of 2-phenylindoles

Article abstract of DOI:10.1016/j.jorganchem.2019.120970

Carbon nanotube–copper oxide (CNT–CuO) nanocomposites were prepared by depositing CuO nanoparticles onto functionalized CNT surfaces. The structure and elemental content of CNT–CuO were characterized using transmission electron microscopy, X-ray diffraction, and Auger electron spectroscopy. The prepared CNT-CuO was subsequently employed as a catalyst for the coupling reaction of 2-phenylindole with aryl iodides to provide the desired N-aryl 2-phenylindoles in good yields.

Full text of DOI:10.1016/j.jorganchem.2019.120970

Journal Pre-proof
CNT-CuO catalyzed C–N bond formation for N-arylation of 2-phenylindoles
Jeongah Lim, Ji Dang Kim, Hyun Chul Choi, Sunwoo Lee
PII:
S0022-328X(19)30413-9
DOI:
https://doi.org/10.1016/j.jorganchem.2019.120970
JOM 120970
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 20 June 2019
Revised Date: 24 September 2019
Accepted Date: 3 October 2019
Please cite this article as: J. Lim, J.D. Kim, H.C. Choi, S. Lee, CNT-CuO catalyzed C–N bond
formation for N-arylation of 2-phenylindoles, Journal of Organometallic Chemistry (2019), doi: https://
doi.org/10.1016/j.jorganchem.2019.120970.
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CNT-CuO catalyzed C–N bond formation for
N-arylation of 2-phenylindoles
Jeongah Lim, Ji Dang Kim, Hyun Chul Choi,* Sunwoo Lee*
Department of Chemistry, Chonnam National University, 77 Yongbong-rog, Buk-gu, Gwangju
61186, Republic of Korea
Corresponding author. Tel: +82-62-530-3492 (H. C. Choi), +82-62-530-3385 (S. Lee), Fax:
+82-62-530-3389,
E-mail : chc12@chonnam.ac.kr (H. C. Choi). sunwoo@chonnam.ac.kr(S. Lee).

Abstract
Carbon nanotube–copper oxide (CNT–CuO) nanocomposites were prepared by depositing
CuO nanoparticles onto functionalized CNT surfaces. The structure and elemental content of
CNT–CuO were characterized using transmission electron microscopy, X-ray diffraction, and
Auger electron spectroscopy. The prepared CNT-CuO was subsequently employed as a
catalyst for the coupling reaction of 2-phenylindole with aryl iodides to provide the desired
N-aryl 2-phenylindoles in good yields.
Keywords : CuO-CNT; C-N bond formation; arylamination; 2-phenylindole; recyclable
catalyst

Indole is one of the most important heterocycles in pharmaceutical chemistry, leading to the
development of many marketed drugs and promising drug candidates comprising an indole
nucleus.[1] Owing to their important applications, many examples of the N-arylation of
indoles have been reported over the past decades.[2] Among them, Buchwald-Hartwig aryl
amination, which is the palladium-catalyzed coupling reaction of aryl halides and amine, is
the most powerful method for the synthesis of N-arylated heterocyclic compounds including
indoles.[3] Although this method provides a variety of arylated amines in good yields and
high functional group tolerance, it presents several limitations including the high cost of the
palladium catalyst and requirement of a ligand. To address these issues, impressive progress
has been made in the aryl amination methodology during the last decades. For example,
inexpensive first-row metals such as cobalt,[4] zinc,[5] nickel,[6] and copper[7] have been
employed as catalysts. However, the use of such homogeneous catalytic systems lead to
environmental problems. To address these issues, heterogeneous catalytic systems are
continuously being developed and applied to metal-catalyzed aryl aminations.[8] Notably,
heterogeneous catalytic systems are considered as green chemical systems as they address
waste issues derived from the purification process.[9] However, to the best of our knowledge,
a heterogeneous copper catalytic system for the N-arylation of 2-phenylindoles has not been
reported to date. The 1,2-diaryl-1H-indole moiety is a key structure of bioactive molecules
such as antiestrogen,[10] and antiprion[11] compounds. Thus, 2-arylindoles have been readily
prepared via Pd-catalyzed arylation.[12,13] However, the employed palladium catalyst is
expensive and this synthetic process usually requires a ligand.
To address these issues, we focused on the development of an environmentally friendly
method for the N-arylation of 2-phenylindoles, paying particular attention to two factors: the
employment of a copper catalyst known to be insensitive to catalyst poisoning caused by the
strong coordination ability of the heteroatom and the development of a heterogeneous copper

catalyst. Herein, we report the preparation of a recyclable copper catalyst and its application
to C–N bond formation of aryl iodide and 2-phenylindole.
To achieve our goals, we propose a simple and effective process for the preparation of
carbon nanotube–copper oxide (CNT–CuO) nanocomposites using functionalized CNTs as an
efficient recyclable catalyst for the C–N coupling reaction.
Figure 1. (a) Transmission electron micrograph of the pristine carbon nanotubes (CNTs)
and (b) transmission electron micrograph of the prepared carbon nanotube–copper oxide
(CNT–CuO), with the corresponding energy-dispersive X-ray (EDX) spectrum (inset).
Figure 1(a) illustrates the overall morphology of the pristine CNTs: the multi-walled
tubular structures comprise outer diameters in the range 5–10 nm and lengths of several
hundreds of nanometers. All the tubes present clean surfaces. Figure 1(b) displays a typical
transmission electron micrograph of a CNT–CuO catalyst in which the nanoparticles are
densely dispersed on the surfaces of the individual CNTs and isolated nanoparticles are not
observed (average particle size, ~4.6 nm). The corresponding energy-dispersive X-ray (EDX)
spectrum presented peaks corresponding to the C, O, and Cu elements [Fig. 1(b)], signaling
the presence of Cu-based nanoparticles on the CNTs.

Figure 2. (a) Powder X-ray diffraction (XRD) patterns of the functionalized carbon
nanotube (CNT), CNT–copper oxide (CuO), and commercial CuO and (b) Auger spectrum of
CNT–CuO.
Figure 2(a) displays the powder X-ray diffraction (XRD) patterns of the functionalized
CNT (black), CNT–CuO (red), and commercial CuO (blue, monoclinic structure). The
pristine CNTs presented two peaks at 25.7 and 43° corresponding to the (002) and (101)
planes of graphite, respectively. For the CNT–CuO sample, distinct diffraction peaks
appeared at 2θ = 35.6 and 38.7°, respectively, corresponding to the (002) and (111) reflection
planes of monoclinic CuO. These observations reconfirmed that the monoclinic CuO
nanoparticles were effectively deposited onto the functionalized CNT surface. The Cu content
(~9.2%) of CNT-CuO was estimated from the Auger peak areas [Fig. 2(b)] and corrected
according to the tabulated sensitivity factors.
We next evaluated the CNT-CuO catalytic activity in the C-N coupling reaction. We selected

2-phenylindole and iodobenzene as the standard substrates and employed 5 mol% CNT-CuO
based on the copper content.
Table 1. Optimal condition for the coupling reaction with 1 and 2a using carbon nanotube–
copper oxide (CNT–CuO).a
Entry
Base
K3PO4
Solvent
CH3CN
dioxane
toluene
DMF
Temp (oC)
80
Yield (%)b
1
2
-
K3PO4
80
-
3
K3PO4
120
120
120
120
120
120
120
120
120
8
4
K3PO4
30
37
42
41
81
77
4
5
K3PO4
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
dioxane
6
Cs2CO3
NaOH
7
8
NaOtBu
KOtBu
NaHMDS
KHMDS
9
10
11
10
aReaction conditions: 1 (0.3 mmol) and 2a (0.3 mmol) were reacted with a base (0.45 mmol)
b
in the solvent (1.0 mL). Yields were determined via gas chromatography with an internal
standard.
To evaluate the solvent conditions, the reaction was first conducted with K3PO4 under
various solvents. The results revealed that CH3CN and 1,4-dioxane were not suitable solvents
(entries 1 and 2). When the reaction was conducted with toluene, DMF, and DMSO, the

desired product 3a was formed in 8%, 30%, and 37% yields, respectively (entries 3–5,
respectively). Selecting the best solvent (DMSO), we next evaluated different bases at 120 °C.
The reactions with Cs2CO3 and NaOH afforded 3a in 42% and 41% yields, respectively
(entries 6 and 7). Among the strong bases, NaOtBu afforded the best result, producing the
desired product in 81% yield (entry 8), while the other bases produced inferior results (entries
9–11).
With the optimal conditions in hand, we next evaluated various substituted aryl iodides
for the coupling reaction of 2-phenylindole. As expected, 3a was successfully isolated in 80%
yield from the reaction with 1 and iodobenzene. Methyl substituted aryl iodides such as para-
and meta-tolyl iodides afforded the product in 71% and 55% yields, respectively. The alkoxy
substituted aryl iodides presented good yields in the formation of the corresponding indoles
3d and 3e. Thus, 4-iodo-1,1’-biphenyl and 2-iodo-9,9-dimethyl-9H-fluorene furnished the
corresponding 3f and 3g in 64% and 95% yields, respectively, while aryl iodides comprising
a halogen group produced 3h, 3i, and 3j in acceptable yields. The 4-nitro- and 4-acetyl-
substituted aryl iodides coupled with 1 to afford 3k and 3l, respectively, in good yields.
Cs2CO3 was employed as a base for aryl iodides comprising base-sensitive ester and nitrile
functional groups; these reactions afforded the desired product 3m and 3n in 45% and 42%
yields, respectively. 2-Iodothiophene and 4-iodophenyl methyl sulphide also provided the
desired product 3o and 3p with moderate yields. In most cases, we found that 2-phenyl indole
still remained even though aryl iodides were added more than one equivalents.
Scheme 1. Coupling reactions of 2-phenylindole with aryl iodidea

R
H
I
cat. Cu-CNT
N
N
R
Ph
Ph
+
NaOt-Bu (1.5 equiv)
DMSO, 120 oC
3
2
1
Me
OMe
Me
N
N
N
N
Ph
Ph
Ph
Ph
3b (71%)
3c (55%)
3d (65%)
Cl
3a (80%)
Ph
O
O
N
N
N
N
Ph
Ph
Ph
Ph
3h (64%)
3f (64%)
CF3
3g (95%)
3e (71%)
O
Me
F
NO2
N
N
N
N
Ph
Ph
Ph
Ph
3j (51%)
CN
3i (42%)
3l (64%)
SMe
3k (79%)
O
OMe
S
N
N
N
N
Ph
Ph
Ph
Ph
3p (45%)
3n (42%)
3m (45%)
3o (42%)
aReaction conditions: 1 (2.0 mmol), 2 (2.0 mmol), and CNT-CuO (5 mol%) were reacted
with NaOt-Bu (3.0 mmol) in DMSO (6.0 mL) at 120 °C for 12 h. The numbers in parenthesis
represent the isolated yields.
We expanded this method to the coupling reactions of iodobenzene with the substituted 2-
arylindoles, indole and benzimidazole. The results are summarized Scheme 2. 2-(4-

Fluorophenyl)indole, 2-(4-chlorophenyl)indole, 6-phenyl-5H-[1,3]dioxolo[4,5-f]indole, 3-
ethyl-2-phenylindole, and 2,3-diphenylindole afforded the corresponding products 3q, 3r, 3s
3t, and 3u in 65%, 70%, 77%, 71%, and 70% yields, respectively. In addition, when indole
and benzimidazole reacted with iodobenzene, the desired product 3v and 3u were formed in
62% and 66% yields, respectively.
Scheme 2. Coupling reactions of substituted 2-arylindolesa
H
I
cat. Cu-CNT
N
N
R1
R1
+
NaOt-Bu (1.5 equiv)
DMSO, 120 oC
R2
R2
1
2
3
N
O
N
N
F
Cl
O
3s (77%)
3q (65%)
3r (70%)
N
N
N
N
N
3t (71%)
3v (62%)
3u (70%)
3w (66%)
aReaction conditions: 1 (2.0 mmol), 2 (2.0 mmol), and CNT-CuO (5 mol%) were reacted
with NaOt-Bu (3.0 mmol) in DMSO (6.0 mL) at 120 °C for 12 h. The numbers in parenthesis
represent the isolated yields.
The potential recyclability of CNT-CuO was next explored in the cross-coupling reaction
of iodobenzene and indole. The reaction was conducted in DMSO solvent at 120 °C for 12 h.

o
After cooling to 25 C, the reaction mixture was worked up with water and the organic
products were extracted with ethyl acetate. Subsequently, the yield was determined by gas
chromatography. The precipitated CNT-CuO catalyst in the water phase was collected by
filtration and subsequently washed with water and ethyl acetate. The recovered CNT-CuO
was transferred to a new reaction vial containing fresh 2-phenylindole and iodobenzene. This
process was repeated until the product yield began to decrease significantly. The results
revealed that the CNT-CuO catalyst system afforded the product in 80% yield for five recycle.
[1st run: 80%, 2nd run: 79%, 3rd run: 82%, 4th run: 78%; 5th run: 80%]
Based on the previous report and our results [14], we next proposed the reaction
pathway illustrated in Scheme 2. CuO-CNT might be reduced to Cu(I) which is the active
catalyst species in this reaction system because Cu(I) species was found in the XRD analysis
after the completion of reaction [See Supporting Information]. Thus, aryl iodide was
oxidatively added to the copper catalyst to form the arylated copper complex. The 2-
phenylindole reacted with the base and subsequently substituted the iodide of the arylated
copper complex A to afford the copper complex B. Finally, reductive elimination afforded the
desired product. It was noteworthy that Cu(I) species was found in the XRD analysis after the
completion of reaction [See Supporting Information].
Scheme 2. Proposed reaction pathway.

Ar
N
Ar
I
Cu(I)-CNT
Ph
Ar
Cu(III)-CNT
Ar
Cu(III)-CNT
N
I
Ph
H
N
Ph
NaOt-Bu +
In summary, we prepared a CNT–CuO nanocomposite and used it as a catalyst for the
cross-coupling reaction of aryl iodide and 2-phenylindole. The best yield was observed when
5 mol% CNT-CuO was employed with NaOt-Bu as the base. A variety of aryl iodide
compounds were coupled with 2-phenylindole to afford the corresponding N-arylated 2-
phenylindoles in good yields. The results revealed that the synthesized catalyst could be
reused fivefold and showed good activity.
Experimental Section
Preparation of CNT-CuO
Functionalized CNTs were prepared according to the previously reported surface
thiolation method.[15] The acid-treated CNTs were subsequently dispersed in tetrahydrofuran
(THF), after which a NaSH aqueous solution was added to produce thiol groups on their
surfaces. In this study, a CNT–CuO catalyst can be facile prepared by thermal oxidation of
Cu nanoparticles deposited onto functionalized CNTs. In brief, Cu nanoparticles were first
synthesized by a sol-gel method. Copper chloride (CuCl2) and sodium borohydride (NaBH4)
were used as the metal precursor and reducing agent, respectively. The obtained Cu
nanoparticles were added to the functionalized CNT solution under 24 h stirring. Finally, the

CNT–CuO catalysts were obtained by centrifugation, washed with DI water, and dried in air
at 150 °C overnight.
The transmission electron micrographs were obtained using an FEI TECNAI-F20
transmission electron microscope equipped with a CM-200 EDX at 200 kV. The powder
XRD patterns obtained from the CNT–CuO nanoparticles were recorded using an X’Pert-
PRO high-resolution X-ray diffractometer (PANalytical, Almelo, Netherlands) with Cu Kα (λ
= 1.5406 Å) radiation. Auger electron spectroscopy (AES) was performed using a PHI 700
scanning Auger nanoprobe with an incident electron energy of 10 keV.
CNT-CuO catalyzed N-arylation of 2-phenylindole
CNT-CuO (0.01 mmol), aryl iodide (0.2 mmol), 2-phenylindole (0.2 mmol), and NaOt-Bu
(0.3 mmol) in DMSO were placed in a reaction vial and stirred at 120 °C for 12 h. The
reaction mixture was washed with water and extracted with EtOAc. The organic layer was
dried with MgSO4 and evaporated. The crude reaction mixture was purified by column
chromatography on silica gel.
Acknowledgement
This research was supported by the National Research Foundation of Korea (NRF) grant
funded
by
the
Korea
government
(NRF-2018R1D1A1B07047022,
NRF-
2012R1A1A2044286 and NRF-2015R1A4A1041036). The spectral and HRMS data were
obtained from the Korea Basic Science Institute, Gwangju center and Daegu center.
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Highlights
ò Carbon nanotube-copper oxide (CNT-CuO) nanocomposites were prepared by
depositing CuO nanoparticles onto functionalized CNT surfaces
ò CNT-CuO was employed as a catalyst for N-arylation of 2-phenyl indoles and
showed good yields.
ò CNT-CuO catalyst was reused for five times with good activity