Copper nanoparticle decorated organically modified montmorillonite (OMMT): An efficient catalyst for the N-arylation of indoles and similar heterocycles

Article abstract of DOI:10.1016/j.catcom.2014.10.030

A highly active ligand-free and efficient copper nanoparticle decorated OMMT catalyzed N-arylation of NH-heterocycles, i.e. indole, imidazole, benzimidazole, pyrrole, carbazole has been demonstrated.

Full text of DOI:10.1016/j.catcom.2014.10.030

Copper nanoparticles decorated Or ganically Modifed Montmorillonite
OMMT): An effcient catalyst for the N-arylation of indoles and similar
(
heter ocycles
Dhrubajyoti Talukdar , Gautam Das, Suman Thakur , Niranjan Karak,
Ashim J. Thakur
PII:
DOI:
Refer ence:
S1566-7367(14)00449-X
doi: 10.1016/j.catcom.2014 .10.030
CATCOM 4118
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
24 August 2014
24 October 2014
29 October 2014
Please cite this article as: Dhrubajyoti Talukdar, Gautam Das, Suman Thakur, Ni-
ranjan Karak, Ashim J. Thakur, Copper nanoparticles decorated Or ganically Modifed
Montmorillonite (OMMT): An effcient catalyst for the N-arylation of indoles and similar
heter ocycles, Catalysis Communications (2014), doi: 10.1016/j.catcom.20 14.10.030
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ACCEPTED MANUSCRIPT
Copper nanoparticles decorated Organically Modified Montmorillonite (OMMT): An efficient
catalyst for the N-arylation of indoles and similar heterocycles
a
b
a
a
a*
Dhrubajyoti Talukdar, Gautam Das, Suman Thakur, Niranjan Karak, Ashim J. Thakur
a
Department of Chemical Sciences
Tezpur University, Napaam-784028, India
b
Department of Material Science and Engineering
Yonsei University, South Korea
Abstract: A highly active ligand free and efficient copper nanoparticles decorated OMMT
catalyzed N-arylation of NH-heterocycles, i.e. indole, imidazole, benzimidazole, pyrrole,
carbazole has been demonstrated.
Keywords: Copper nanoparticles, N-arylation, OMMT, arylhalide, indole, imidazole
1
. Introduction
N-arylated heterocycles are important building blocks in organic synthesis as well as in
pharmaceutical and agrochemical industries [1-3]. Often, transition metal catalysts have been
employed for this purpose via C-N bond formation [4-7]. Over the past decades, Pd and Cu
complexes have been introduced effectively for C-N coupling reactions [8-10]. Generally, Cu
catalysts are preferred over the noble metal (often Pd) catalysts from the industrial,
environmental and economic point of view [11-14]. Sometimes, traditional Cu catalysts require
ligands such as 2-oxocyclohexanecarboxylate, 2-amino-pyrimidinenes-4, 6-diol, amino acids etc.
which on scale up lead to the problem of waste disposal [15-17]. A number of copper catalysts
1

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have been reported over the last decades [18-23]. Both Cu(I) and Cu(II) nanoparticles (NPs)
have been reported as catalysts for C-N coupling reactions [24,25]. Buchwald et al. reported
bulky Cu O/4,7-dimethoxy-1,10-phenanthroline catalyst for coupling of imidazoles with
2
arylbromide [26]. The process demands the use of harmful solvents (PrCN and NMP) and also
the 4,7-dimethoxy-1,10-phenanthroline is very expensive one. Nano CuO has been found to be a
highly active catalyst for N-arylation reaction; however the reaction is limited to activated
aryliodide only [27]. Li et al. has reported a general and efficient method for the N-arylation of
heterocycles using Cu O/1,10-phenanthroline catalyst system under solvent free condition [28].
2
Although there are several reports for N-arylation, still there are scopes for further development
in terms of superior catalyst, simple and greener reaction conditions, versatility of the reaction
etc. Inspired by the activity of both Cu(I) and Cu(II) NPs in C-N coupling reaction, we have
developed a Cu decorated nanoclay catalyst system for the C-N coupling of indole with
arylbromide (Scheme 1). The developed catalyst has various advantages over the bare Cu(I) and
Cu(II) NPs or the other Cu reagents like good recoverability, reusability, high activity, short
reaction time. Furthermore, imidazole, benzimidazole, pyrrole, and carbazoles etc. can also be
effectively N-arylated under the standard reaction condition.
<Scheme 1: N-arylation of indoles catalyzed by Cu clay nanohybrid>
2
. Experimental
2
.1. Materials and Methods
Organically Modified Montmorilonite (OMMT), (octadecylamine modified, 25-30%) was
purchased from Aldrich. All other chemicals were purchased from Merck and were used as
2

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received without further purification. X-ray diffraction (XRD) data were collected with Rigaku
X-Ray diffractometer (Miniflex) using Cu Kα radiation. The morphology of the catalyst was
analyzed by Scanning Electron Microscopy (JSM-639LV, JEOL) and Transmission Electron
Microscopy (JEOL 2100X, 200kV). NMR spectra were obtained from JEOL JNM ECS 400
MHz spectrophotometer in CDCl with TMS as the internal standard. Nicolet (Impact 410) FT-
3
IR spectrophotometer was used to record the IR spectra of the samples in KBr pallets. Thermal
stability of the catalyst was determined using TGA (SHIMADZU-TGA50 Thermal analyzer).
GC-MS was carried out on a gas chromatograph (Perkin Elmer, Claurus 600) equipped with
thermal conductivity detector (TCD), and Elite wax column (30 mm length and 0.25 mm
diameter). UV-Visible spectroscopic analysis was done in a SHIMADZU UV-2550
spectrophotometer.
2
.2. Preparation of the catalyst
1
g of OMMT was dispersed in 50 mL DMF and then sonicated for 20 min. To achieve
theoretical copper loading in the clay/copper weight ratio of 1:1, 1:0.5 and 1:0.25 (coded as N1,
N0.5 and N0.25, respectively); 3.16 g, 1.58 g and 0.792 g of Cu(OAc) were dissolved in 30 mL,
2
1
5 mL and 8 mL DMF respectively. The Cu(OAc) solutions were then added to the clay
2
suspension and stirred for 24 h under ambient condition, followed by sonication for 10 min. The
Cu precursor was then reduced in the presence of montmorilonite by adding PhNHNH in the
2
optimized mole ratio of 1:3 (Cu(OAc) /PhNHNH ). The solution was stirred at room temperature
2
2
for 48 h followed by sonication for 15 min. The suspension so obtained consisting of Cu NPs
embedded in montmorilonite was centrifuged and washed repeatedly with acetone. The resulting
solid phase was dried at 45-50 ºC under vacuum (Scheme 2).
3

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<Scheme 2: Preparation of Cu decorated OMMT >
2
.3. Typical Procedure for the N-arylation of heterocycles
Cu-clay nanohybrid (N1, 5 wt%) was added to a mixture of bromobenzene (1.2 mmol),
0
imidazole (1 mmol) and K CO (2 mmol) in DMSO and stirred at 130 C. The reaction was
2
3
monitored by thin layer chromatography. After the reaction, the catalyst was separated out from
the reaction mixture by centrifugation and washed with distilled ethyl acetate. The recovered
catalyst was used for further reaction. The product was extracted with ethyl acetate and purified
using column chromatography on silica gel (ethyl acetate/hexane: 30/70). The purified products
were characterized by NMR and IR spectroscopy and ESI-MS technique.
3
. Characterization of the catalyst
3
.1. UV-visible spectroscopy
The formation of Cu NPs in clay system was first observed by UV–visible absorption studies
Fig. 1). The spectrum matches well with the literature value [29-32]. The broad surface-plasmon
(
band (approx. 588 - 612 nm) is due to the formation of Cu NPs that may shift depending upon
the particle size and shape. The broadness of the absorption band probably arises from the wide
size distribution of NPs.
<Fig. 1: UV-visible spectra of a) N1, N0.5 and N0.25 nanohybrids and b) OMMT +
Cu(OAc) without adding reducing agent >
2
4

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3
.2. XRD analysis
The X-ray diffractograms (Fig. 2a, b, and c) of the powder samples are well agreed with the
literature values [33, 34]. The peaks at 2 values 43.61º, 50.59º and 74.47º represent the [111],
[
200] and [220] Bragg’s reflections respectively of cubic structure of Cu. Besides these, the
peaks appearing at 35.62° and 61.81° correspond to CuO, though UV-visible spectrum is silent
about this. The peaks for the layered silicate were also observed, however the peak at 4.15º
corresponding to basal reflection [110] shifts to 3.64° indicating an increase in the gallery
spacing with increase in the Cu loading, and other peak observed was at 20.1º. The appearance of
low-angle peak suggests the possible coexistence of staked nanostructures made up of Cu NPs in
layered nanoclay. The intensity of the peak corresponding to Cu NPs increases with the increase
in the loading of the Cu into the layered silicate which might be due to the increase in particle
size of the Cu NPs. Sharp and more intense band suggests that the degree of crystallinity
increases with Cu concentrations. The oxidation of the Cu NPs might be responsible for the
presence of reflection peak for CuO NPs.
<Fig. 2: XRD of the catalysts (a) N0.25, (b) N0.5 and (c) N1>
3
.3. Morphology of the catalyst
The SEM & TEM micrographs of N1 (Fig. 3) revealed that the treated clay contained
heterogeneous particles.
<Fig. 3: (a) SEM of N1, (b) SEM of OMMT, c) SEM-EDX of the N1 catalyst, d) TEM of N1
and e) Histogram showing the particle size distribution>
5

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The white NPs in case of N1 were clearly seen (Fig. 3a) to anchor onto the surface of the clay
and well dispersed, exhibited no agglomeration. The EDX data (Fig. 3c) further support the
presence of the Cu in the clay. TEM image shows uniform distribution of the NPs over the clay.
The size of the NPs in N1 is found to be in the range of 3-13 nm (Fig. 3d). The size of the NPs in
N0.5 ranges from 2-20 nm (Fig. S5, ESI).
3
.4. The FT-IR and the TG analysis
From the FT-IR and the TG analyses of the catalyst we can predict that the basic structure of the
clay remained intact while the thermal stability was enhanced due to the modification of the
catalyst [35]. The FT-IR and TG of the OMMT and the catalyst are shown in the Fig. S6 and S7
(see the ESI).
4
. Results and Discussions
We found that Cu hybrid nanoclay was an effective catalyst for N-arylation of indole. The
reaction proceeds smoothly giving high yield of the products (Table 1). Among N1, N0.5 and
N0.25 catalysts, N1 had the highest activity for coupling of indole with bromobenzene providing
high yield of N-arylindole (Fig. 4).
<Fig. 4: Activity of catalysts N1, N0.5 and N0.25>
The reaction was optimized using various parameters (Table 1) such as reaction time,
temperature, solvent, base etc. Initially, the reaction of indole with bromobenzene was studied. In
the presence of 5 wt% of the catalyst and 2 equiv. K CO in DMSO at 130 ˚C, to our delight, the
2
3
6

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reaction afforded the desired product (94% yield). Of the different solvents tested DMSO was
clearly the best choice (Table 1). The choice of the base is also important for the effective and
the selective synthesis of the desired product. Temperature higher or lower than 130 °C makes
the reaction slower (Table 1). This might be due to the oxidation of the nano Cu to CuO. The
scope of the present catalytic system was extended to various indoles, other heterocycles like
imidazole, benzimidazole and other haloarenes viz., iodobenzene and chlorobenzene (Table 2).
The catalyst can effectively catalyze the reaction giving very high yield of product in very short
period. Except chlorobenzene, iodobenzene effectively couples with the heterocycles under the
mentioned condition to give the corresponding N-aryl heterocycles. The reaction proceeds
smoothly for the various substituents in indole moiety and aryl halide (Table 1). A 5 wt%
amount of the catalyst is found to be the optimum amount for the reaction.
<Table 1: Optimisation for the N-arylation of Indole with bromobenzene catalyzed by N1
catalyst >
<Table 2: N-arylation of various heterocycles with arylhalide catalyzed by N1 catalyst>
A plausible mechanism has been suggested based on the previous reports [10, 27-29]. The
reaction proceeds with the initial oxidative addition of the aryl halide over the Cu NPs. Further
the N-H heterocycles get coordinated with the Cu atom followed by reductive elimination to give
the desired N-arylated heterocycles. The base abstracts the N-H proton from the heterocycle
thereby generates a nucleophile.
7

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<Scheme 3: Plausible mechanism of the reaction>
The reusability of a catalyst is an important attribute to the industrial application. Therefore, the
recovery and reusability of our catalyst was investigated in the N-arylation of indole. After the
reaction, the catalyst was easily separated from the reaction mixture by centrifugation or by
simple filtration. The recovered catalyst was washed with dry acetone and then dried in a
th
desiccator over anhydrous CaCl . The catalyst has been reused up to 6 times with almost
2
th
consistent activity (Fig. 5). Even in the 6 run, the yield of N-arylated product was 88%, which
indicates excellent reusability of the catalyst.
<Fig. 5: Reusability of the catalyst N1for the N-arylation of Indole>
The recovered catalyst retains its stoichiometry as reflected from the powder XRD pattern (Table
and Fig. 6). However, the intensity of the CuO peak increases which indicates the oxidation of
3
Cu to CuO. The oxidation of the metallic Cu to CuO takes place very rapidly at higher
temperature and hence increasing the temperature of the reaction above 130 °C decreases the
yield of the product (Table 1). Little amount of leaching of Copper was observed in the
successive runs that might also contribute to the lowered yield.
<Table 3: XRD patterns of fresh and the recovered catalysts>
<Figure 6: XRD patterns of fresh and the recovered catalysts>
8

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5
. Conclusion
In conclusion, we have developed a new clay supported Cu nanohybrid catalyst system for the
effective N-arylation of heterocyles. The procedure is simple, general and affords good to
excellent yields. In comparison to the previous reports for the N-arylation of heterocycles with
Cu catalysts several features of this modified procedure can be established: 1) ligand free, 2)
high activity of the catalyst system, 3) considerably good stability of the catalyst, 4) high yield of
products in very short time, 5) sacrificial reducing agent (i.e. PhNHNH ) was used for
2
preparation of Cu NPs, 6) high recoverability and the reusability of the catalyst, 7) the support
i.e. OMMT helps the catalyst to disperse in the reaction mixture which adds the advantage of
homogeneous catalyst in the heterogeneous catalyst.
Acknowledgement:
DT thanks CSIR for the financial support to him.
Supplementary Information:
UV-visible spectra of N1, N0.5 and N0.25, XRD of the catalyst a) N0.25 b) N0.5 and C) N1,
EDX of N1, SEM of N1, TEM of N0.5, TG of OMMT and N1, FT-IR spectra of OMMT and N1
are available in supplementary information.
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2
1
1

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Legends
Schemes:
Scheme 1: N-arylation of indoles catalyzed by copper clay nanohybrid
Scheme 2: Preparation of Copper decorated OMMT
Scheme 3: Plausible mechanism of the reaction
Figures:
Fig. 1: UV-visible spectra of a) N1, N0.5 and N0.25 nanohybrids and b) OMMT +
Cu(OAc) without adding reducing agent
2
Fig. 2: XRD of the catalysts (a) N0.25, (b) N0.5 and (c) N1
Fig. 3: (a) SEM of N1, (b) SEM of OMMT without copper, c) SEM-EDX of N1, d) TEM
of N1 and e) Histogram showing the particle size distribution.
Fig. 4: Activity of catalysts N1, N0.5 and N0.25
Fig. 5: Reusability of the catalyst N1 for the N-arylation of Indole
th
Fig. 6: Powder XRD pattern of the recovered catalyst after 6 run
Tables:
Table 1: Optimisation for the N-arylation of Indole with bromobenzene catalyzed by N1
catalyst
Table 2: N-arylation of various heterocycles with arylhalide catalyzed by N1 catalyst
Table 3: XRD patterns of fresh and the recovered catalysts
1
2

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Scheme 1: N-arylation of indoles catalyzed by copper clay nanohybrid
X
N
Hybrid nanoclay (5wt%)
+
o
N
K2CO3, DMSO, 130 C
H
1
2
3a
X = Br, I
1
3

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Scheme 2: Preparation of Copper decorated OMMT
1
5 min
Cu(OAc)2
10 min sonication Phenylhydrazine
48 h stirring
Dispersed OMMT
in DMF
sonication
Copper Decorated OMMT
Washed with Actone And Dried at 45-50 C
4
8 h Stirring
0
1
4

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Scheme 3: Plausible mechanism of the reaction
ArNu
e
n
O
v
o
i
x
i
t
t
i
d
c
a
a
t
u
n
i
m
i
d
v
e
A
e
R
e
r
A
l
-
E
X
d
d
i
t
i
o
n
Ar
Nu
Ar
X
N
u
H
B
a
s
e
=
copper
NuH= Heterocycle
X= Br, I
nano particles containing NH
1
5

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Fig. 1: UV-visible spectra of a) N1, N0.5 and N0.25 nanohybrids and b) OMMT + Cu(OAc)2
without adding reducing agent
2
00
250
300
350
400
Wavelength (nm)
(a)
(b)
1
6

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Fig. 2: XRD of the catalysts (a) N0.25, (b) N0.5 and (c) N1
1
7

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Fig. 3: (a) SEM of N1, (b) SEM of OMMT without copper, c) SEM-EDX of N1, d) TEM of
N1 and e) Histogram showing the particle size distribution.
(a)
(b)
(c)
(d)
20
15
10
5
0
3
4
5
6
7
8
9
10 11 12 13
Particle Size (nm)
(e)
1
8

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Fig. 4: Activity of catalysts N1, N0.5 and N0.25
9
9
9
9
8
8
8
8
8
7
7
7
7
6
4
2
0
8
6
4
2
0
8
6
4
2
Yield
8
6
4
Time
N1
N0.5
N0.25
Catalyst
Reaction condition: indole (1.2 mmol), bromobenzene (1 mmol), catalyst (5 wt%),
K CO (2 equiv.), DMSO, stirring, 130 ˚C.
2
3
1
9

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Fig. 5: Reusability of the catalyst N1 for the N-arylation of Indole
100
8
6
4
2
0
0
0
0
0
1st
2nd
3rd
4th
5th
6th
Run
Reaction conditions: indole (1.2 mmol), bromobenzene (1 mmol), N1 catalyst (5 wt%),
K CO (2 equiv.), DMSO, stirring, 130 ˚C, time 5h
2
3
th
Fig. 6: Powder XRD pattern of the recovered catalyst after 6 run
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2
(In degree)
2
0

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Table 1: Optimisation for the N-arylation of Indole with bromobenzene catalyzed by N1 catalyst
Br
Copper Decorated OMMT
(
5 wt%)
N
+
o
DMSO, K CO , 130 C
2
3
N
H
Entry Solvent
Temperature Base
Catalyst
Base
Time
Yield
a
(
°C)
(wt%)
(mmol)
(h)
6
(%)
1
2
3
4
5
5
6
7
8
9
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
80
K CO3
2
3
5
5
5
5
5
5
5
5
5
5
2
2
2
2
2
2
2
2
2
2
2
30
52
75
78
94
77
72
85
80
40
33
80
K CO3
7
2
100
120
K CO3
2
6
K CO3
2
5
130
K CO3
2
6
130
KOH
6
130
NaOEt
6
150
K CO3
2
6
150
K CO3
2
6
Toluene
THF
Reflux
Reflux
K CO3
2
6
1
0
K CO3
2
6
a
isolated yields
2
1

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Table 2: N-arylation of various heterocycles with arylhalide catalyzed by N1 catalyst
Copper Decorated OMMT
(
5wt%)
Het-NH + ArBr
Het-N-Ar
o
DMSO, K CO , 130 C
2
3
a
Entry
1
Het-NH
ArX
Product
Yield (%)
Time (h)
5
Br
95
N
H
N
I
2
3
97
4.5
N
H
N
Br
78
73
7
N
H
N
OH
Br
OH
4
9
N
H
N
NH2
NH2
2
2

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Br
5
6
96
3
6
N
N
H
NO2
NO2
CH3
Br
CH3
92
N
N
H
Br
Br
7
8
85
90
8
7
CH3
CH3
N
N
H
CN
CN
N
H
N
Br
9
1
92
94
4
N
N
N
NH
Br
0
1
4.5
NH
N
Br
1
70
10
NH
N
N
N
2
3

ACCEPTED MANUSCRIPT
Br
N
N
1
1
2
3
86
7
9
N
N
H
Br
67
N
N
H
a
isolated yields
2
4

ACCEPTED MANUSCRIPT
Table 3: XRD patterns of fresh and the recovered catalysts
a
Catalyst
Fresh
Characteristic planes
111, 200, 220
Crystal system
Cubic
Space group
Fm¯3m
b
Recovered
111, 200, 220
Cubic
Fm¯3m
a
According to Miller indices
b
th
After the 6 run
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5

ACCEPTED MANUSCRIPT
Copper
nanoparticles
decorated
Organically
Modified
Montmorrilonite (OMMT) : An efficient catalyst for the N-arylation
of indoles and similar heterocycles
Dhrubajyoti Talukdar, Gautam Das, Suman Thakur, Niranjan Karak, Ashim Jyoti Thakur
Copper Decorated OMMT
(
5 wt%)
Het-NH+ ArBr
Het-N-Ar
o
DMSO, K CO , 130 C
2
3
Graphical abstract
2
6

ACCEPTED MANUSCRIPT
Highlights
1
2
3
4
. Cu nanoparticles decorated OMMT catalyzes the N-arylation of the NH-heterocycles.
. The procedure is simple, general, ligand free, and efficient.
. The nano hybrid system shows considerably good stability.
. High recoverability and the reusability of the catalyst.
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7