Nanocrystalline magnesium oxide-stabilized palladium(0): An efficient and reusable catalyst for synthesis of N-(2-pyridyl)indoles

Article abstract of DOI:10.1039/c5nj00074b

A selective and efficient catalytic process has been developed for the oxidative coupling between N-aryl-2-aminopyridines and alkynes using a nanocrystalline magnesium oxide (NAP-MgO)-supported palladium nanoparticle [NAP-Mg-Pd(0)] catalyst and CuCl₂ as an oxidant. The process involves the ortho C-H activation of N-aryl-2-aminopyridines to give N-pyridyl indoles in excellent yields and the true heterogeneity of the catalyst is verified by studying the recoverability and reusability for four cycles without significant loss of catalytic activity.

Full text of DOI:10.1039/c5nj00074b

NJC
PAPER
Nanocrystalline magnesium oxide-stabilized
palladium(0): an efficient and reusable catalyst
for synthesis of N-(2-pyridyl)indoles†
Cite this: New J. Chem., 2015,
39, 3399
Police Vishnuvardhan Reddy, Manne Annapurna, Pottabathula Srinivas,
Pravin R. Likhar and Mannepalli Lakshmi Kantam*
A selective and efficient catalytic process has been developed for the oxidative coupling between
N-aryl-2-aminopyridines and alkynes using a nanocrystalline magnesium oxide (NAP–MgO)-supported
palladium nanoparticle [NAP–Mg–Pd(0)] catalyst and CuCl2 as an oxidant. The process involves the
ortho C–H activation of N-aryl-2-aminopyridines to give N-pyridyl indoles in excellent yields and the
true heterogeneity of the catalyst is verified by studying the recoverability and reusability for four cycles
without significant loss of catalytic activity.
Received (in Porto Alegre, Brazil)
10th January 2015,
Accepted 30th January 2015
DOI: 10.1039/c5nj00074b
www.rsc.org/njc
O2À, lattice bound and isolated hydroxyl groups and anionic and
cationic vacancies. Nanocrystalline magnesium oxide, with its
Introduction
Carbon–carbon and carbon–heteroatom bond formation via three-dimensional structure and well-defined shape, acts as an
transition metal-catalyzed C–H activation of aromatics has excellent support for different metals.10 Our previous work showed
received great attention because it represents an atom- that a nanocrystalline magnesium oxide (NAP–MgO)-supported
economic strategy to construct complex structures.1 Recently, palladium nanoparticle [NAP–Mg–Pd(0)] is an active and efficient
various methods with C–H bond activation of arenes and the heterogeneous catalyst for different C–C coupling reactions,11
oxidative coupling with unsaturated molecules have been reduction of nitro compounds12 and oxidation of alcohols.13 Very
reported with high efficiency.2 These methods revealed that recently, our group has reported the NAP–Mg–Pd(0) catalyzed Heck
the C–H bond activation of arenes requires the directing group reaction of heteroaryl halides in the absence of ligand and base free
in order to direct the C–H functionalization to the ortho- conditions.14 In this direction, herein we wish to communicate an
position.3 Very recently, Li and co-workers reported Rh(III) and efficient and reusable heterogeneous NAP–Mg–Pd(0) catalyst for
Pd(II)-catalyzed C–H activation of N-aryl-2-aminopyridines, with oxidative ortho C–H activation of N-aryl-2-aminopyridines with
the pyridine moiety as a directing group,4,5 and the oxidative alkynes using CuCl2 as an oxidant.
coupling with alkynes to give substituted indole derivatives
which are widely present in natural products and pharma-
ceuticals.6 In spite of success, the development of environmentally
Results and discussion
benign and economical synthetic methods is still a challenging
task.7 Therefore, it is essential to develop a recoverable and XRD patterns of the fresh and used Pd supported on NAP–MgO
reusable catalytic system for oxidative C–H functionalization of catalysts are reported in Fig. 1. The fresh and used catalysts
arenes from industrial and environmental points of view.
displayed diffraction lines due to the metallic Pd phase (PCPDF
In recent years, nanocrystalline metal oxides have received # 88-2335) appearing at 2y = 40.0 and 46.51 and their corre-
much attention because of their usefulness as materials8 and sponding ‘d’ values are 0.225 and 0.195 nm. Both fresh and
more importantly as catalysts for organic transformations.9 The used catalysts exhibited the metallic Pd phase only. The
presence of edge-corner and other defect sites allows the absence of peaks due to Pd-halides may be either due to lower
nanostructured MgO materials to possess a high concentration amounts or below the X-ray detection limit.
of reactive surface ions, such as Lewis acid Mg2+ and Lewis base
The X-ray photoelectron spectra analysis of NAP–Mg–Pd(0)
fresh and used catalysts is presented in Fig. 2. The binding
energy (BE) values of Pd 3d5/2 and Pd 3d3/2 in the NAP–Mg–Pd(0)
fresh and used catalysts are 335.3, 340.4 and 335.5, 340.3 eV
respectively. These values clearly indicate the existence of Pd in
metallic form.15 However, the reduction of NAP–Mg–PdCl4 was
I & PC Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad – 500 607, India. E-mail: mlakshmi@iict.res.in; Tel: +91-40-27160387
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5nj00074b
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diphenylacetylene using NAP–Mg–Pd(0) as a catalyst in the
presence of an oxidant. In a typical experiment, 0.5 mmol
of N-phenyl-2-aminopyridine, 0.7 mmol of diphenylacetylene,
8.9 wt% (0.030 g) of NAP–Mg–Pd(0) and CuCl2 (2 equiv.) were
taken in 3 mL of DMA and the reaction mixture was stirred at
100 1C for 10 h. Interestingly, product 2a was obtained in 36%
yield (Table 1, entry 1). Encouraged by this result, various
solvents, catalysts and oxidants were used for further optimiza-
tion and the results are summarized in Table 1. Among the
oxidants studied (entries 1–6), the best result was obtained with
CuCl2 (Table 1, entry 1). On the other hand, no coupled product
was observed with other oxidants such as Ag2O, Ag2CO3, O2 and
TBHP (Table 1, entries 3–6). With the above results, it was
observed that the oxidant has a pronounced effect on the yields
of the reaction. Next, we examined the different solvents such
as DMF, DMSO, NMP and toluene (Table 1, entries 7–10),
wherein DMF afforded product 2a in an excellent yield of
86% (entry 7) and other solvents afforded 0% yield of the
product (entries 8–10). Similarly, the catalyst screening was
performed with other different heterogeneous palladium cata-
lysts such as Pd/C, LDH–Pd(0) and Mg–La–Pd(0) (Table 1,
entries 11–13). No coupled product was observed in the case
of Pd/C and LDH–Pd(0) (entries 11 and 12) and a significant
decrease in the product yield was observed when the reaction
was carried out with Mg–La–Pd(0) (entry 13) clearly showing
that the nano metal oxide supported palladium contributes
significantly for the reaction to proceed with ease. A decrease in
yields of the product was observed when the reaction was
performed at 75 1C and 60 1C (Table 1, entry 14 and 15). The
reaction was also carried out under the same optimum condi-
tions with low catalyst loading (0.015 g), however, the corres-
ponding product was obtained in 20% yield (Table 1, entry 16).
Fig. 1 XRD patterns of the fresh (a) and used (b) NAP–Mg–Pd(0) catalysts.
Table 1 Optimization of reaction conditiona
Entry
Catalyst
Oxidant
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
Pd–C
CuCl2
Cu(OAc)2
Ag2CO3
Ag2O
DMA
DMA
DMA
DMA
DMA
DMA
DMF
DMSO
NMP
Toluene
DMF
DMF
DMF
DMF
DMF
DMF
36
15
0
0
0
0
86
0
0
0
0
0
25
60c
50d
20e
O2 atm
TBHP
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
Fig. 2 X-ray photoelectron spectra of fresh and used NAP–Mg–Pd(0)
catalysts.
9
10
11
12
13
14
15
16
LDH–Pd(0)
Mg–La–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
NAP–Mg–Pd(0)
not complete during the in situ reduction as there was a trace
amount of Pd in the form of a halide at BE = 337.6 eV as observed
in both fresh and used catalysts.16 From these observations, it
indicates that Pd(0) species are intact after the reaction.
For optimization of the reaction conditions, we initiated our
a
Reaction conditions: 1a (0.5 mmol), 2a (0.7 mmol), catalyst (0.030 mg),
b
oxidant (2 equiv.), and 3 mL of solvent at 100 1C for 10 h. Isolated yields.
c
d
e
Reaction at 70 1C. Reaction at 60 1C. Reaction with (0.015 mg) of
study in the coupling of N-phenyl-2-aminopyridine (1a) with NAP–Mg–Pd(0).
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Table
2
NAP–Mg–Pd(0) catalyzed oxidative coupling of N-aryl-2-
with 1a to afford 2e relatively in lower yield but as a single
regioisomer, in which the phenyl group of the alkyne unit was
disposed adjacent to the nitrogen atom (Table 2, entry 5). The
electronic properties of the N-aryl group appear to have a minor
effect on the yield of the coupled products. The N-aryl-2-
aminopyridines having electron donating substituents at the
aminopyridines with alkynesa
t
Entry 1
Alkyne
Product Yieldb (%)
para-position of the N-aryl ring such as Me, OMe, Et and Bu
groups were coupled with diphenylacetylene to give the sub-
stituted indoles in good yields (Table 2, entries 6–9). Whereas,
the N-aryl-2-aminopyridines having electron withdrawing sub-
stituents, such as Cl and F atoms, at the para-position of the
N-aryl ring gave the corresponding products in excellent yields
(Table 2, entries 10 and 11). Slightly high yield of the coupled
product was obtained with 1h bearing OMe at the 3-position when
compared to 1c bearing OMe at the 4-position, but the C–H
activation occurs exclusively at the less hindered position (Table 2,
entry 12). The catalytic efficiency of NAP–Mg–Pd(0) was also studied
in the oxidative coupling reactions using other amino-heterocycles
such as N-aryl-2-aminoquinoline and N-aryl-2-aminopyrimidine.
From the above results, it is indicated that the yields and
selectivity of products in NAP–Mg–Pd(0) catalyzed oxidative coupling
reactions depend on various factors such as steric and electronic
factors and also the position of substituents (directing groups).
The spent catalyst was recovered from the reaction mixture
by simple centrifugation after the completion of the reaction.
The catalyst was washed with water and followed by diethyl
ether to remove any organic materials. It was dried at room
temperature and used for reusability study. The spent catalyst
was examined for the oxidative coupling reaction of N-phenyl-2-
aminopyridine with PhCRCPh under the same reaction con-
ditions. The catalyst showed consistent activity up to four cycles
(Fig. 3). No trace of leached Pd was detected in the spent
catalyst as determined by atomic absorption spectrometry
(AAS) studies of both fresh and spent catalysts.
1
R1 = R2 = C6H5
2a
86
2
3
4
5
1a
1a
1a
1a
R1 = R2 = 4-(OMe)-C6H4 2b
R1 = R2 = 4-(Me)-C6H4 2c
R1 = R2 = 4-(F)-C6H4
R1 = C6H5, R2 = Me
83
81
85
69
2d
2e
6
7
8
R1 = R2 = C6H5
R1 = R2 = C6H5
R1 = R2 = C6H5
2f
81
52
79
2g
2h
9
R1 = R2 = C6H5
2i
75
10
11
R1 = R2 = C6H5
R1 = R2 = C6H5
2j
85
87
2k
12
13
R1 = R2 = C6H5
2l
76
R1 = R2 = C6H5
R1 = R2 = C6H5
2m
2n
24
77
The TEM images of the fresh and used NAP–Mg–Pd(0)
catalysts are shown in Fig. 4. It appears that there is not much
change in the shape and size of the Pd particles in both fresh
and used catalysts. TEM analysis indicated that the nano Pd
particles have a mean diameter of 8 and 10 nm for fresh and
used catalysts. The morphology of the catalyst remains the
same even after 4 recycles.
14
a
Reaction conditions: 1 (0.5 mmol), 2 (0.7 mmol), NAP–Mg–Pd(0)
(0.030 mg), CuCl2 (2 equiv.) and 3 mL of solvent at 100 1C for
b
10 h. Isolated yields.
From the above study, we eventually concluded the optimized
reaction conditions: NAP–Mg–Pd(0) (0.030 g) as a catalyst in the
presence of CuCl2 (2 equiv.) as an oxidant in 3 mL of DMF as
solvent at 100 1C for 10 h.
Next, we turned our attention to explore the scope and
limitations of the NAP–Mg–Pd(0) catalyst in the oxidative
coupling reaction of various aminopyridines with alkynes
under optimized conditions and the results are summarized
in Table 2. Initially, we studied the coupling reaction of
N-phenyl-2-aminopyridine (1a) with various symmetrical and
unsymmetrical internal alkynes. As it can be seen from Table 2,
this protocol is rather general in nature for the reactions
of electron-rich and electron-deficient symmetrical alkynes
(Table 2, entries 1–4). In contrast, the unsymmetrically sub-
stituted alkyne such as methyl phenyl acetylene was coupled Fig. 3 Recyclability test.
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as received. All solvents used for experiments were dried using
standard procedures and distilled prior to use. Pd–C was pur-
chased from commercial sources and has 10% Pd on carbon.
Characterization
The X-ray diffraction (XRD) patterns of the fresh and used
samples were obtained on a Rigaku Miniflex X-ray diffracto-
meter using Ni filtered Cu Ka radiation (l = 0.15406 nm), at a
scan rate of 21 minÀ1, with a beam voltage and beam current of
30 kV and 15 mA, respectively. The X-ray photoelectron spectro-
scopic (XPS) analysis of the fresh and used NAP–Mg–Pd(0)
sample was performed using a Kratos Axis Ultra Imaging
X-ray photoelectron spectrometer equipped with a Mg anode
and a multichannel detector. Charge referencing was done
against adventitious carbon (C 1s, 284.8 eV). Shirley-type back-
ground was subtracted from the signals. The recorded spectra
were always fitted using Gauss–Lorentz curves to determine
the binding energies of the different elements. For the trans-
mission electron microscope (TEM) analysis, samples were
dispersed in methanol solution and dropped on a 200-mesh
Cu grid, and images were taken using a JEOL JEM 2100F high-
resolution transmission electron microscope at an acceleration
voltage of 200 kV. The 1H and 13C spectra were recorded on
1
Inova 500 and Avance 300 (300 MHz H and 13C) spectrometer
in CDCl3 using TMS as internal standard. The ACME silica gel
(60–120 mesh) was used for column chromatography purposes
and thin layer chromatography was performed on Merck pre
coated silica gel 60-F254 plates.
Preparation of catalysts
NAP–MgO was purchased from NanoScale Materials, Inc.
(Manhattan, USA). NAP–Mg–Pd(0) was synthesized following
the procedure reported earlier by our group.12 NAP–Mg–PdCl4:
NAP–MgO was calcined in air at 450 1C for 4 h (1.0 g; BET-surface
area = 147 m2 À1), treated with Na2PdCl4 (0.294 g, 1 mmol),
g
Fig. 4 Transmission electron micrographs of fresh and used (after 4th
cycle) NAP–Mg–Pd(0) catalysts.
dissolved in 100 mL decarbonated water with vigorous stirring for
12 h at room temperature under a nitrogen atmosphere to afford
brown coloured NAP–Mg–PdCl4. Then, the catalyst was filtered
and washed with deionized water and acetone and dried under
vacuum. NAP–Mg–Pd(0):NAP–Mg–PdCl4 (1.0 g) was reduced with
sodium borohydride (1.5 g, 39.6 mmol) in 20 mL of dry ethanol
with vigorous stirring for 3 h under a nitrogen atmosphere at room
temperature. Then, the reduced catalyst was filtered through a G-3
sintered glass funnel and washed with deionized water and
acetone and then dried under vacuum to get the black-coloured,
air stable NAP–Mg–Pd(0) (0.9 mmol of Pd per g, BET-surface area =
Conclusions
In summary, we have developed a first heterogeneous catalytic
protocol for the selective oxidative coupling between N-aryl-2-
aminopyridines and alkynes to give N-pyridyl indoles in excellent
yields using a nanocrystalline magnesium oxide (NAP–MgO)-
supported palladium nanoparticle [NAP–Mg–Pd(0)] catalyst and
CuCl2 as an oxidant. The catalyst works equally well in the ortho
C–H activation of N-aryl-2-aminopyridines as reported previously
by homogeneous catalysts. The recovery and reusability study of
the catalyst showed consistent catalytic activity up to four cycles.
116 m2 À1). The catalyst was used for the oxidative coupling
g
reaction between N-aryl-2-aminopyridines and alkynes.
A general procedure for the synthesis of N-(2-pyridyl)indoles
A reaction vessel was equipped with a magnetic bar and
charged with compound 1a (0.5 mmol), diphenylacetylene
(0.7 mmol), anhydrous CuCl2 (2 equiv.) and NAP–Mg–Pd(0)
Experimental section
Materials
NAP–MgO (commercial name: Nano Activet Magnesium Oxide (0.030 g), and 3 mL of DMF was added via a syringe at room
Plus) was purchased from Nano Scale Materials, Inc. (Manhattan, temperature. The reaction mixture was heated at 100 1C for 10 h
USA). All chemicals were purchased commercially and were used and then cooled to room temperature. The catalyst was
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Acknowledgements
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P.V.R. thanks CSIR, New Delhi, for the Senior Research Fellow-
ship and M. Annapurna is grateful for the UGC fellowship.
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