Catalytic role of Cu(I) species in Cu2O/CuI supported on MWCNTs in the oxidative amidation of aryl aldehydes with 2-aminopyridines

Article abstract of DOI:10.1016/S1872-2067(15)60964-0

Cu₂O and CuI were supported on multiwalled carbon nanotubes (MWCNTs) using a wet impregnation method, and the resulting materials were fully characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, transmission electron microscopy, and temperature-programmed desorption with ammonia analysis. The results of these experiments revealed that Cu₂O and CuI were deposited on the MWCNTs in the cubic and γ phases, respectively. These results also showed that the Cu-containing MWCNTs exhibited weak to strong electron-accepting (Lewis acidic) properties. The catalytic activities of these materials were studied for the synthesis of biologically significant N-(pyridin-2-yl)benzamides via the oxidative amidation of aryl aldehydes with 2-aminopyridines. The yields of the products were in the range 50%-95% with 100% selectivity. Notably, the CuI/MWCNT catalyst was much more effective than the Cu₂O/MWCNT catalyst with respect to the isolated yield of the product, although the latter of these two catalysts exhibited much better recyclability. A preferential interaction was observed between the polar nature of the acid-activated MWCNTs and the ionic Cu₂O compared with covalent CuI. The differences in these interactions had a significant impact on the rate of the nucleophilic attack of the amino group of 2-aminopyridine substrate on the carbonyl group of the aryl aldehyde.

Full text of DOI:10.1016/S1872-2067(15)60964-0

Chinese Journal of Catalysis 36 (2015) 1825–1836
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Catalytic role of Cu(I) species in Cu2O/CuI supported on MWCNTs in
the oxidative amidation of aryl aldehydes with 2‐aminopyridines
H. N. Hareesha, K. U. Minchithaa, N. Nagarajub, N. Kathyayini a,*
a Department of Nanobiosciences, Centre for Emerging Technologies, Jain Global Campus, Jain University, Jakkasandra Post, Kanakapura Taluk,
Ramanagara District–562112, Karnataka, India
b Catalysis Research Laboratory, Department of Chemistry, St. Joseph’s College and Research Centre, 46 Langford Road, Shanthinagar Bengaluru–560027,
Karnataka, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Cu2O and CuI were supported on multiwalled carbon nanotubes (MWCNTs) using a wet impregna‐
tion method, and the resulting materials were fully characterized by powder X‐ray diffraction, Fou‐
rier transform infrared spectroscopy, scanning electron microscopy with energy dispersive X‐ray
spectroscopy, transmission electron microscopy, and temperature‐programmed desorption with
ammonia analysis. The results of these experiments revealed that Cu2O and CuI were deposited on
the MWCNTs in the cubic and γ phases, respectively. These results also showed that the
Cu‐containing MWCNTs exhibited weak to strong electron‐accepting (Lewis acidic) properties. The
catalytic activities of these materials were studied for the synthesis of biologically significant
N‐(pyridin‐2‐yl)benzamides via the oxidative amidation of aryl aldehydes with 2‐aminopyridines.
The yields of the products were in the range 50%–95% with 100% selectivity. Notably, the
CuI/MWCNT catalyst was much more effective than the Cu2O/MWCNT catalyst with respect to the
isolated yield of the product, although the latter of these two catalysts exhibited much better recy‐
clability. A preferential interaction was observed between the polar nature of the acid‐activated
MWCNTs and the ionic Cu2O compared with covalent CuI. The differences in these interactions had
a significant impact on the rate of the nucleophilic attack of the amino group of 2‐aminopyridine
substrate on the carbonyl group of the aryl aldehyde.
Received 23 June 2015
Accepted 12 August 2015
Published 20 November 2015
Keywords:
Cuprous oxide
Cuprous iodide
Multiwalled carbon nanotube
2‐Aminopyridine
Oxidative amidation
© 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
or active material. In general, materials with a porous texture
are preferred as solid supports for heterogeneous catalysts
Research towards the development of solid, acid‐base and
redox catalysts for organic transformations under environ‐
mentally benign conditions is a particularly active area in het‐
erogeneous catalysis [1,2]. There are several important ad‐
vantages associated with heterogeneous catalysis, including
simple experimental procedures, mild reaction conditions, and
the generation of minimal chemical waste. Heterogeneous cat‐
alysts can be solid catalysts or catalysts supported on an inert
because their high surface area. In these cases, the catalytically
active species are uniformly dispersed on the surface of the
porous support. In several instances, supported materials have
been shown to exhibit improved catalytic performance com‐
pared with the free catalysts, as well as offering significant re‐
ductions in the effective cost of the active catalyst [3–6].
Multiwalled carbon nanotubes (MWCNTs) are currently
recognized as potential supports for catalytically active transi‐
* Corresponding author. Tel: +91‐98‐86506387; Fax: +91‐80‐27577211; E‐mail: nkathyayini45@gmail.com
DOI: 10.1016/S1872‐2067(15)60964‐0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 11, November 2015

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H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
tion metals/metal oxides, and the catalytic activities of these
systems have been explored in a variety of different organic
transformations for the synthesis of fine chemicals [7–10].
MWCNTs have high surface area because of their small size and
cage‐like mesoporous structures. Although pristine MWCNTs
are chemically inert, they can be oxidized by an acid treatment
process, which makes them chemically reactive through the
formation of surface defects and oxofunctionalities [11]. These
structural features of functionalized MWCNTs make them
suitable for the uniform distribution of catalytically active spe‐
cies on their surface. Furthermore, the mesoporous structure of
MWCNTs facilitates intraparticle mass transfer processes in the
reaction medium, which promote chemical interactions be‐
tween the reactant molecules [12,13]. Based on their unique
properties, MWCNTs are considered to be good materials for
the development of novel supported transition‐metal oxide
catalysts.
The amide bond is a key functionality in a wide range of
important chemicals, including peptides, polymers, and phar‐
maceutical agents. Considerable research efforts have been
directed towards the development of efficient and economical‐
ly viable methods for the synthesis of amide bonds using dif‐
ferent organic transformations [14,15]. Amides are traditional‐
ly synthesized by the reaction of activated carboxylic acid de‐
rivatives with an amine using an suitable coupling reagent, e.g.,
propylphosphonic anhydride (T3P), N,N,N′,N′‐tetramethyl‐O‐
(benzotriazol‐1‐yl)uronium tetrafluoroborate (TBTU), 1‐bis
(dimethylamino)methylene‐1H‐1,2,3‐triazolo[4,5‐b]pyridinium
3‐oxide hexafluorophosphate (HATU) and dicyclohexylcar‐
bodiimide (DCC). Surasani et al. [16] reported the synthesis of
amides via a C–N coupling reactions using a palladium catalyst
under homogeneous conditions. Kathiravan et al. [17] reported
the synthesis of a series of amides from esters and amines us‐
ing a Ru‐complex. In a separate study, Porco and co‐workers
[18] reported the use of Zr(O‐t‐Bu)4 in combination with an
activator (e.g., 1‐hydroy‐7‐azabenzotriazole (HOAt) or
1‐hydroxybenzotriozole (HOBt)) for the formation of amides
from esters and amines.
Cu‐based catalysts are used extensively in organic trans‐
formations [19]. In recent years, considerable progress has
been made towards the development of copper‐catalyzed oxi‐
dative amidation reactions to provide access to a wide range of
amide derivatives. For example, Li et al. [20] reported the oxi‐
dative amidation of aldehydes with amine hydrochlorides in
the presence of a CuI‐AgIO3 catalyst using aqueous tert‐butyl
hydroperoxide as an oxidant. In a later study, Kathiravan et al.
[17] described the Cu‐catalyzed reactions of amides derived
from 8‐aminoquinoline with a variety of aryl halides for the
synthesis of substituted amides. Primary, secondary, and ter‐
tiary amides can also be synthesized using copper sulfate or
copper(I) oxide as a catalyst with tert‐butyl hydroperoxide as
an oxidant [21]. For example, Yang et al. [22] described the
synthesis of N‐(pyridin‐2‐yl)benzamides via the oxidative ami‐
dation of aryl aldehydes with 2‐amino pyridines in the pres‐
ence of CuI, while Diwakar et al. [23] reported the synthesis of
the same derivatives from benzoyl chloride and 2‐amino
5‐bromopyridine in the presence of pyridine.
It is understood from the literature that the catalytic activity
of Cu‐based catalysts can be significantly influenced by the
physical and chemical properties of the Cu species. Further‐
more, consideration of the reported protocols reveals that the
reagents and catalysts commonly used in the processes are
generally expensive, toxic and non‐recyclable and usually re‐
quire tedious work up procedures for the separation of the
product. Therefore, there is an urgent need to develop im‐
proved catalytic processes to overcome these issues and pro‐
vide facile access to a wide range of N‐(pyridin‐2‐yl)ben‐
zamides and their derivatives that could be used to support
structure‐activity relationship studies.
In this study, we have developed a series of catalysts for the
oxidative amidation of benzaldehydes with 2‐aminopyridines.
These catalysts were prepared by depositing Cu2O and CuI on
MWCNTs using a wet impregnation method. This research was
conducted based on the following considerations: (1) the im‐
portance of developing new and efficient heterogeneous cata‐
lysts for the synthesis of fine chemicals; (2) the application of
copper‐based catalysts in organic transformations, especially
oxidative amidation reactions; (3) the biological application of
N‐(pyridin‐2‐yl)benzamides [23,24]; and (4) the significance of
understanding the synergetic effect of the mesoporous struc‐
ture of MWCNTs and the catalytically active transition‐metal
ion species deposited on their surfaces. The resulting binary
composites of MWCNTs with Cu2O and CuI were evaluated in
terms of their surface properties and their catalytic activities
towards the synthesis of N‐(pyridin‐2‐yl)benzamides from aryl
aldehydes and 2‐amino pyridines via an oxidative amidation
reaction.
2. Experimental
2.1. Materials
MWCNTs were purchased from Reinste Nano Ventures Pri‐
vate Ltd. and used as received. Concentrated H2SO4, concen‐
trated HNO3, Cu(OAc)2, CuI, Cu2O, and Na2SO3 were obtained
from Merck. All of the chemicals used in the current study were
purchased as the analytical grade and used without further
purification.
2.2. Preparation of the catalysts
2.2.1. Acid activation of the MWCNTs
A portion of the MWCNTs (0.5 g) was immersed in a 1:3
(v/v) mixture of concentrated HNO3 and concentrated H2SO4
(100 mL), and the resulting mixture was heated on a mantle at
80 °C for 6 h. The mixture was then cooled to room tempera‐
ture before being carefully diluted with 2 L of deionized water.
The resulting mixture was stirred overnight with a magnetic
stirrer before being filtered to give a solid, which was washed
with deionized water until the pH of the filtrate was neutral.
The solid was then dried in air at 80 °C overnight to give the
activated MWCNTs, which were stored in a desiccator prior to
being used. This material will be referred here after as
F/MWCNT (functionalized multiwalled carbon nanotubes).

H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
1827
2.2.2. Preparation of Cu2O/MWCNT and CuI/MWCNT catalysts
MWCNTs containing 1, 5, 10, 15, 20, and 25 wt% Cu2O or
CuI were prepared using a wet impregnation method. Cu2O was
deposited on the MWCNTs by reducing Cu(OAc)2 in the pres‐
ence of Na2SO3, while CuI was impregnated on the MWCNTs
the surfaces of the catalysts. For the NH3‐TPD measurements, a
small sample (0.1 g) of the catalyst was initially degassed at
100 °C for 45 min under a stream of He. The material was then
cooled to 50 °C and held at this temperature for 45 min under a
steady stream of NH3. During this time, the NH3 molecules were
adsorbed onto the acidic sites on the surface of the solid cata‐
lyst. Desorption studies were conducted by increasing the
temperature from 50 to 550 °C in the presence of He.
using CuI.
A typical procedure for the preparation of
Cu2O/MWCNT containing 20 wt% Cu2O is as follows. A mixture
of F/MWCNT (0.11 g), Cu(OAc)2 (0.084 g), and Na2SO3 (0.05 g)
in deionized water (20 mL) was mixed for 2 h in a mortar and
pestle to give a homogeneous mixture. The resulting catalyst
was initially dried at 100 °C overnight and then heated at 200
°C for 3 h in a hot air oven. The catalyst was washed with a
small volume (ca. 100 mL) of deionized water to remove any of
the unwanted water soluble products formed during the chem‐
ical reaction between Cu(OAc)2 and Na2SO3, which can be rep‐
resented as 2Cu(OOCCH3)2 + Na2SO3 +2H2O → Cu2O + Na2SO4 +
4CH3COOH.
The washed catalyst was then heated at 200 °C to remove
any physically adsorbed water molecules. CuI/MWCNT con‐
taining 20 wt% CuI was prepared in a similar manner using by
mixing F/MWCNT (0.16 g) and CuI (0.040 g) with deionized
water (20 mL) in a mortar and pestle until a homogeneous
mixture was obtained. The resulting paste was then subjected
to the sequential drying, washing, and drying steps described
above for the preparation of Cu2O/MWCNT to give the desired
CuI/MWCNT. MWCNTs containing different weight percent‐
ages of Cu2O and CuI were prepared using the wet impregna‐
tion method and have been represented as 1%–25%
Cu2O/MWCNT and 1%–25% CuI/MWCNT, respectively.
2.4. Catalytic activity studies
The catalytic activities of the F/MWCNT, 1%–25%
Cu2O/MWCNT, and 1%–25% CuI/MWCNT catalysts were
studied using the amidation reaction of benzaldehyde with
2‐aminopyridine as a model reaction. It was noteworthy that
only 20% Cu2O/MWCNT and 20% CuI/MWCNT exhibited a
good catalytic activity, hence only these two catalysts were
checked for their catalytic activity in the amidation of substi‐
tuted benzaldehydes with 2‐aminopyridines. For this reason,
these two catalysts were selected for further evaluation to as‐
sess the scope and limitation of the amidation reaction using a
series of substituted benzaldehydes and 2‐aminopyridines
(Scheme 1).
In a typical procedure, a mixture of substituted benzalde‐
hyde (5 mmol) (I), substituted 2‐aminopyridine (4.5 mmol)
(II), and catalyst (0.1 g) in THF (10 mL) was heated at 80 °C for
6 h in a round‐bottom flask fitted with a water cooled conden‐
ser in the presence of air. The progress of the reaction was
monitored by thin layer chromatography (TLC). Upon comple‐
tion of the reaction, the mixture was cooled to room tempera‐
ture and diluted with ethyl acetate (10 mL). The resulting sus‐
pension was then filtered to allow for the recovery of the solid
catalyst. The filtrate was evaporated to dryness under vacuum
to give a residue, which was purified by column chromatog‐
raphy over silica gel eluting with a 7:3 (v/v) mixture of
n‐hexane and ethyl acetate to give N‐(pyridin‐2‐yl)benzamide
(III).
2.3. Characterization of the catalysts
The F/MWCNT, 20% Cu2O/MWCNT, and 20% CuI/MWCNT
catalysts were characterized using a variety of different analyt‐
ical techniques, including powder X‐ray diffraction (PXRD),
Fourier transform infrared spectroscopy (FT‐IR), scanning
electron microscopy with energy dispersive X‐ray spectroscopy
(SEM‐EDS), transmission electron microscopy (TEM), and tem‐
perature‐programmed desorption with ammonia (NH3‐TPD).
These analyses were conducted to develop a better under‐
standing of the physico‐chemical properties of these catalysts
because they showed a high level of catalytic activity towards
the synthesis of N‐(pyridin‐2‐yl)benzamides via the oxidative
amidation of aryl aldehydes with 2‐aminopyridines. PXRD pat‐
terns were recorded on a Panalytical X’Pert pro X‐ray diffrac‐
tometer using monochromated Cu Kα radiation (λ = 0.154 nm)
in the 2θ range of 2°–70° with a scanning rate of 2°/min. The
morphological characteristics and elemental compositions of
the catalyst particles were investigated by SEM‐EDS on a Model
system. TEM images of the catalysts were recorded using a
Tecnai transmission electron microscope at 200 kV. Samples of
the catalyst were dispersed in ethanol under ultrasonic irradia‐
tion at a frequency of 43 kHz for 2 h at room temperature. A
drop of the resulting suspension was placed on a carbon‐coated
copper grid to record the TEM images. NH3‐TPD experiments
were measured using a Mayura Analytical instrument to de‐
termine the strength and concentration of the acidic sites on
All the products were fully characterized by melting point
(M.P.), FT‐IR, 1H NMR, and LC‐MS analysis. FT‐IR spectra were
recorded as KBr pellets on a Nicolet Model Impact 400D FT‐IR
spectrometer with 4 cm–1 resolution. 1H NMR spectra were
recorded at 400 MHz on a Bruker Advance NMR spectrometer
using CDCl3 as solvent. TMS (δ = 0) was used as an internal
1
reference for the H NMR analysis. Melting points were rec‐
orded in open capillary tubes and have been reported as the
uncorrected values. LC‐MS analysis was conducted on a Shi‐
madzu QP 5000 LC‐MS instrument.
R1 = H, 4-C2H5, 4-OC2H5, 4-CN, 4-C (CH3)2, 4-F, 4-Cl, 4-N (CH3)2
R2 = H, 4-CH3, 4,6-(CH3)2
Scheme 1. Synthesis of N‐(pyridin‐2‐yl)benzamides via the oxidative
amidation of aryl aldehydes with 2‐amino pyridines.

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H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
3. Results and discussion
3.1. Characterization of catalysts
3.1.1. PXRD results
1733 (C=O)
1632 (C=C)
(1)
PXRD patterns of the F/MWCNT, 20% Cu2O/MWCNT, and
20% CuI/MWCNT catalysts are shown in Fig. 1. The diffraction
peaks at 25.96° and 43.49° were attributed to the (002) and
(100) hkl planes of the graphite structure of the MWCNTs. The
diffraction peaks at 2θ = 28.41°, 36.28°, 42.06°, and 61.38° for
the 20% Cu2O/MWCNT were attributed to the (110), (111),
(200), and (220) crystal planes of the cubic phase of Cu2O
(JCPDS 78‐2076) [25–27]. The 20% CuI/MWCNT catalyst con‐
tained diffraction peaks at 2θ = 25.60°, 29.48°, 42.30°, 49.96°,
61.23°, and 67.43°, which were attributed to the (111), (200),
(220), (311), (400), and (331) planes of crystalline γ‐CuI, re‐
spectively (JCPDS 00‐083‐115) [28,29].
622 (Cu2O)
(2)
(3)
495 (CuI)
611 (CuI)
2000 1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm1)
Fig. 2. FT‐IR spectra of the F/MWCNT (1), 20% Cu2O/MWCNT (2), and
20% CuI/MWCNT (3) catalysts.
3.1.2. FT‐IR analysis
The FT‐IR spectra of the F/MWCNT, 20% Cu2O/MWCNT,
and 20% CuI/MWCNT catalysts are shown in Fig. 2. The ab‐
sorption bands at 3500 and 1628 cm–1 were assigned to hy‐
droxyl (–OH) and C=C bonds, respectively, while the absorption
band at 1733 cm–1 was assigned to the carbonyl group (C=O) of
the F/MWCNT. Functionalization is an important step for the
chemical activation of carbon nanotubes because the resulting
functional groups provide a platform for the formation of
strong binding interactions between the surface of the support
and the active metal species [30]. However, the absorption
bands observed for the 20% CuI/MWCNT and 20% Cu2O/
MWCNT catalysts were more intense than those observed for
the F/MWCNT. For example, the 20% Cu2O/MWCNT catalyst
exhibited IR bands around 622 cm–1, which is the characteris‐
tic of the lattice vibrations of the Cu2O [31] present on the sur‐
face of the carbon nanotubes. Furthermore, the FT‐IR spectrum
of the 20% CuI/MWCNT catalyst contained absorption bands at
611 and 495 cm–1, which are the two characteristic vibrations
of γ‐CuI [32]. The results of these FT‐IR studies therefore con‐
firm that the Cu species have been successfully deposited on
the MWCNTs.
3.1.3. SEM‐EDS results
The SEM images and EDS profiles of the F/MWCNT, 20%
Cu2O/MWCNT, and 20% CuI/MWCNT catalysts are shown in
Fig. 3. These SEM images revealed that there were significant
changes in the morphological characteristics of the F/MWCNT
following the deposition of copper oxide/iodide. In these cata‐
lytic materials, the surfaces of the tubular structures of the
F/MWCNT were associated with different sized particles,
which could be attributed to the copper oxide/iodide particles.
Taken together with their EDS profiles, the SEM images of these
materials allowed for the determination of the atomic percent‐
ages of carbon, oxygen/iodide, and copper.
Element
C
O
Wt%
91.08
8.02
500 nm
500 nm
500 nm
Element Wt%
C
O
Cu
-CuI
Cubic Cu2O
MWCNT
87.44
4.29
8.27
(3)
(2)
(1)
Element Wt%
C
O
I
68.08
4.39
6.55
Cu
18.10
0
10
20
30 40
2/(o )
50
60
70
Fig. 3. SEM‐EDS images of the F/MWCNT (a), 20% Cu2O/MWCNT (b),
and 20% CuI/MWCNT (c) catalysts.
Fig. 1. PXRD patterns of the F/MWCNT (1), 20% Cu2O/MWCNT (2), and
20% CuI/MWCNT (3) catalysts.

H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
1829
3.1.4. TEM analysis
TEM images of the 20% Cu2O/MWCNT and 20% CuI/
MWCNT catalysts are shown in Fig. 4. The TEM image of the
20% Cu2O/MWCNT catalyst in Fig. 4(a) clearly shows that Cu2O
nanoparticles were deposited on the inner and outer walls of
the MWCNTs. The high resolution TEM image of the 20%
Cu2O/MWCNT catalyst in Fig. 4(b) clearly indicates that the
Cu2O nanoparticles were also present within the tubular struc‐
ture of the MWCNTs. In the case of 20% CuI/MWCNT, agglom‐
erated CuI particles were observed together with the tubular
structure of the MWCNTs (Fig. 4(c)).
(1)
(2)
(3)
100
200
300
400
500
600
3.1.5. NH3‐TPD
Temperature (oC)
NH3 can interact with the surface of a material by phy‐
sisorption or chemisorption processes through the lone pair of
electrons on its nitrogen atom. The strength of this interaction
depends on the electron accepting properties (i.e., Lewis acidi‐
ty) of the interacting species with strongly electron accepting
species forming stronger interaction with ammonia molecules.
It has been reported that NH3 desorption peaks in the range of
50–250 °C correspond to weakly acidic sites on the surface of
the catalyst, while those in the range of 250–350 °C are indica‐
tive of moderately/strongly acidic sites. Furthermore, NH3 de‐
sorption peaks above 350 °C correspond to strongly acidic sites
on the surface of the catalyst [33]. The NH3 desorption peak
area can therefore be used to represent the concentration of
the acidic sites on the surface of a catalyst.
The NH3‐TPDprofiles of the F/MWCNT, 20% Cu2O/MWCNT,
and 20% CuI/MWCNT catalysts are shown in Fig. 5. The
F/MWCNT catalyst contained two desorption peaks in the
ranges of 100–300 and 300–550 °C, which indicated the pres‐
ence of weakly acidic sites, as well as a low concentration of
moderately/strongly acidic sites. In contrast, the 20% Cu2O/
MWCNT catalyst contained desorption peaks in the ranges of
150–300 and 300–450 °C, which indicated the presence of high
concentrations of both weakly and moderately strong acidic
sites. In the case of 20% CuI/MWCNT, the NH3‐TPD profile
contained a strong desorption peak in the range of 100–200 °C,
which indicated a high concentration of weakly acidic sites. The
20% CuI/MWCNT catalyst also contained a broad desorption
peak in the range of 350–500 °C, which indicated a low con‐
Fig. 5. TPD‐NH3 profiles of the F/MWCNT (1), 20% Cu2O/MWCNT (2),
and 20% CuI/MWCNT (3) catalysts.
centration of strongly acids sites. The differences in the
NH3‐TPDprofiles of the three different catalysts therefore indi‐
cated that there were significant differences in their electron
pair accepting properties.
Thus, the data provided above clearly demonstrate that the
Cu(I) species had been successfully deposited on the surface of
the carbon nanotube support as the cubic phases of Cu2O and
γ‐CuI. Notably, these results clearly show that these materials
exhibit different electron accepting properties.
3.2. Catalytic activity studies
The catalytic activities of the F/MWCNT, 20% Cu2O/
MWCNT, and 20% CuI/MWCNT catalysts were investigated
towards the reaction of benzaldehyde with 2‐aminopyridine in
tetrahydrofuran (THF) at 80 °C over 6 h. The activities of Cu2O
and CuI were also investigated for the same reaction under the
same conditions (Table 1). The results of these screening ex‐
periments revealed that F/MWCNT did not exhibit any catalytic
activity towards this reaction (Table 1, entry 5). In contrast, the
Cu2O/MWCNT and CuI/MWCNT materials both exhibited cata‐
lytic activity towards the reaction, with the isolated yield in‐
creasing as the weight percentage of Cu2O and CuI deposited on
the MWCNTs increased. The isolated yield of the desired prod‐
(a)
(b)
(c)
Cu2O
Cu2O
100 nm
Fig. 4. TEM images of 20% Cu2O/MWCNT (a, b) and 20% CuI/MWCNT (c) catalysts.

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H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
the oxidative amidation of aryl aldehydes with 2‐amino pyri‐
Table 1
dines. In this study, we have investigated the amidation of a
series of benzaldehydes with 2‐amino pyridines under hetero‐
geneous conditions using air as an oxidizing agent and obtained
the corresponding products in isolated yields of 85%–93%.
There are several important advantages to our newly devel‐
oped process, including the use of an environmentally benign
oxidizing agent, a clean protocol for the isolation of the product,
and the reusability of the catalyst.
It is well known that the catalytic yield in heterogeneous
catalytic process depends on the relative adsorption and de‐
sorption of the reactants and products from the catalyst, re‐
spectively. With this in mind, we have conducted several ex‐
periments and made an attempt to understand the electron
accepting properties of Cu2O and CuI supported on MWCNTs
on the outcome of this oxidative amidation reaction.
The higher activity of the 20% CuI/MWCNT catalyst com‐
pared with the 20% Cu2O/MWCNT catalyst can be explained as
follows. CuI is more covalent in character than Cu2O. This dif‐
ference in the covalent properties of these materials can be
attributed to the higher polarizability of the large iodide ion
(206 pm) compared with the oxide ion (126 pm). Furthermore,
Cu2O can interact more effectively with the polar groups pre‐
sent on the functionalized carbon nanotubes than CuI because
it can form polar‐polar interactions much more readily than
non‐polar‐polar interactions. The formation of strong interac‐
tions between Cu2O and the MWCNT support would lead to the
deposition of Cu2O not only on the surface of the support but
also throughout the bulk of the support. In contrast, most of the
CuI would be deposited on the surface of the support because
of its relatively weak interaction with the polar groups present
on the functionalized carbon nanotubes. Most of this material
would therefore be available for the reactants, leading to the
observed higher level of catalytic activity. Furthermore, the CuI
catalyst would leach much more readily from the support into
the reaction because of its weak interactions with the function‐
al groups on the surface of the support owing to its non‐polar
nature. This would explain the observed decrease in the cata‐
lytic activity of this material when it was recycled and reused in
subsequent runs.
Isolated percentage yield of N‐(pyridin‐2‐yl)benzamide with different
catalysts used.
Entry
1
2
3
4
Catalyst
Cu2O
CuI
Cu2O
CuI
Quantity of catalyst (g)
Yield (%)
20
0.02
0.02
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
43
45
85
5
6
7
8
F/MWCNT
NC
NC
8
25
40
1% Cu2O/MWCNT
5% Cu2O/MWCNT
10% Cu2O/MWCNT
15% Cu2O/MWCNT
20% Cu2O/MWCNT
25% Cu2O/MWCNT
1% CuI/MWCNT
5% CuI /MWCNT
10% CuI/MWCNT
15% CuI/MWCNT
20% CuI/MWCNT
25% CuI/MWCNT
9
10
11
12
13
14
15
16
17
85
85
10
28
42
65
93
93
Reaction conditions: benzaldehyde 5 mmol, 2‐aminopyridine 4.5 mmol,
THF 10 mL, 80 °C, 6 h.
NC: No conversion.
uct reached a plateau in both cases at 20% loading (Table1,
entries 10 and 16). Notably, the Cu2O and CuI catalysts exhib‐
ited 100% selectivity for the expected N‐(pyridin‐2‐yl)ben‐
zamide products, with low isolated yields (Table 1, entries
1–4). The 20% Cu2O/MWCNT and 20% CuI/MWCNT catalysts
afforded isolated yields that were significantly higher than
those achieved with pure Cu2O and CuI respectively (85% ver‐
sus 45% and 93% versus 85%, respectively). These observa‐
tions indicated that the support was playing an important role
in enhancing the catalytic activities of Cu2O and CuI. The recy‐
clability of these catalysts was also investigated and the results
revealed that the 20% Cu2O/MWCNT catalyst could be recycled
up to four times without any discernible decrease in its catalyt‐
ic activity. In contrast, the 20% CuI/MWCNT catalyst showed a
significant reduction in its catalytic activity after each cycle. For
example, the 20% CuI/MWCNT catalyst gave an isolated yield
of only 25% after its fourth cycle. From these experimental
analysis, it may be inferred that copper compounds exhibit
catalytic activity towards the oxidative amidation of benzalde‐
hydes with 2‐aminopyridines and that their catalytic activities
can be enhanced by depositing these materials on an MWCNT
support. A review of recent reports from the literature revealed
several studies pertaining to the use of copper compounds as
homogenous catalysts in oxidative amidation reactions
[20,22,34]. For example, CuI, CuBr, CuCl, and Cu(OAc)2 have
been used as catalysts in the presence of air as an oxidizing
agent under homogeneous conditions to obtain amidation
products with yields in the range of 5%–45% [22]. Further‐
more, the use of CuBr2 and Cu(OTf)2 in the presence of a sur‐
factant with I2 as an oxidizing agent has been used to synthe‐
size amides with yields in the range of 80%–93% [34]. Howev‐
er, to the best of our knowledge, there have been no reports in
the literature concerning the use of heterogeneous catalysts for
3.3. Generality of the catalytic activity of the Cu2O/MWCNT
In light of the reusability of the 20% Cu2O/MWCNT catalyst,
we conducted a series of additional experiments to assess the
scope of this catalyst using a variety of substituted benzalde‐
hydes and 2‐aminopyridines. The aromatic ring of the benzal‐
dehyde was decorated with a variety of different functional
groups, including 2‐C2H5, 4‐C3H7, 4‐OC2H5, 4‐F, 4‐Cl, 4‐CN, or
4‐N(CH3)2, while the 2‐aminopyridine was substituted with a
4‐CH3 or 4,6‐(CH3)2 group.
The nature of the substituent on the aromatic ring of the
benzaldehyde had a significant effect on the yield of the ami‐
dation reaction. For example, the presence of an electron‐do‐
nating substituent (i.e., 4‐C2H5, 4‐C3H7, or 4‐OC2H5) on the aro‐
matic ring of benzaldehyde gave much higher yields of the
product than the presence of an electron‐withdrawing substit‐

H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
1831
uent (i.e., 4‐F, 4‐Cl, and 4‐CN) (Table 2).
The mechanism proposed above is consistent with the elec‐
tron‐accepting properties of the Cu species and the observed
NH3‐TPD results. This mechanism is also consistent with the
observed variations in the reactivity of the benzaldehyde sub‐
strate depending on the electron‐withdrawing/donating prop‐
erties of the substituent on the aromatic nucleus. The precise
nature of the substituent would have a significant influence on
the rate of the nucleophilic attack of the 2‐aminopyridine on
the carbonyl carbon of the aldehyde. Similarly, given that the
copper in CuI is less electropositive than the Cu in Cu2O, it
would interact to a much lesser extent with the lone pair of
electrons on the N atom of the pyridine ring. The lone pair of
electrons on the N atom of the amino group (–NH2) would
therefore be more available for the nucleophilic attack of the
A plausible reaction mechanism for the oxidative amidation
of the aryl aldehydes with 2‐aminopyridines in the presence of
a copper‐based catalyst is shown in Scheme 2. According to this
mechanism, the lone pair of electrons on the nitrogen atom of
the 2‐amino pyridine substrate would coordinate to a Cu atom
on the MWCNTs to form intermediate A. The subsequent com‐
bination of an aryl aldehyde with A would lead to the formation
of the complex intermediate B via the nucleophilic attack of the
amino group of the 2‐aminopyridine on the carbonyl group of
the aryl aldehyde. This step would be followed by a proton
transfer to produce hemiaminal C, which would undergo an
oxidation reaction to give the desired N‐(pyridin‐2‐yl)ben‐
zamide product D with the regeneration of the catalyst.
Table 2
Catalytic activities of the 20% Cu2O/MWCNT and 20% CuI/MWCNT catalysts towards the oxidative amidation reactions of a series of different aryl
aldehydes with substituted 2‐amino pyridines.
Yield (%)
Cu2O CuI
Yield (%)
Entry
R1
R2
Product
Entry
R1
R2
Product
Cu2O
CuI
O
1
H
H
85
81
93
91
10
2‐(C2H5)
4‐CH3
85
80
95
N
H
N
O
2
2‐(C2H5)
H
11
4‐C(CH3)2
4‐CH3
86
N
H
N
3
4
4‐C(CH3)2
H
H
80
64
85
58
12
13
4‐OC2H5
4‐F
4‐CH3
4‐CH3
79
63
85
70
4‐F
5
6
4‐Cl
H
H
70
82
75
86
14
15
4‐Cl
4‐CH3
4‐CH3
65
50
72
63
O
4‐OC2H5
4‐CN
N
H
N
NC
7
4‐N(CH3)2
H
65
72
16
17
4‐N(CH3)2
4‐CH3
60
58
69
66
O
8
9
4‐CN
H
H
52
84
58
92
4‐N(CH3)2 4,6‐CH3
N
H
N
NC
4‐CH3

1832
H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
806; 1H NMR (400 MHz, DMSO‐d6): δ 8.90 (br s, 1H), 8.35 (d, J =
R2
Cu(I)/MWCNT
6.2 Hz, 1H), 8.33–8.31 (m, 2H), 8.21 (d, J = 5.4 Hz, 1H),
7.92–7.88 (m, 2H), 7.78–7.72 (m, 1H), 7.23–7.19 (m, 1H). MS
(EI) m/z: 233 [M+H]+.
Catalyst
Regeneration
N
NH2
..
R2
Reactant 1
Coordination
OH
R2
[o]
Oxidation
O
4‐Ethoxy‐N‐(pyridin‐2‐yl)benzamide* (Table 2, entry 6):
Yellow solid. M.P.: 130–132 °C; IR (cm–1): 3438, 3180, 2966,
2930, 2874, 1680, 1613, 1573, 1532, 1412, 1278, 1166, 1094,
997, 884, 816; 1H NMR (400 MHz, DMSO‐d6): δ 8.90 (br s, 1H),
8.33–8.21 (m, 2H), 8.30 (d, J = 4.4 Hz, 1H) 8.14–8.10 (m, 1H),
7.96–7.95 (m, 2H), 7.84 (d, J = 5.6 Hz, 1H), 7.69–7.64 (m, 1H),
2.92 (q, J = 7.2 Hz, 2H), 1.31 (t, J = 7.0 Hz, 3H); MS (EI) m/z: 243
[M+H]+.
N
N
H
R1
N
..
NH2 (A)
(C)
R2
Cu(I)/MWCNT
R2
+
N
N
H
R1
NH2
N
..
(D)
H
R1
N-(pyridin-2-yl)benzamides
O
O
H
R2
N
NH2
..
..
R1
4‐(Dimethylamino)‐N‐(pyridin‐2‐yl)benzamide** (Table 2,
entry 7): Yellow solid. M.P.: 128–130 °C; IR (cm–1): 3191, 2959,
1681, 1638, 1630, 1610, 1574, 1528, 1413, 1296, 1278, 1166,
842, 756; 1H NMR (400 MHz, DMSO‐d6): δ 9.68 (br s, 1H),
8.68–8.62 (m, 2H), 8.56 (d, J = 6.8 Hz, 1H), 8.48 (d, J = 5.2 Hz,
1H), 6.82–6.76 (m, 2H), 6.56 (d, J = 4.8 Hz, 1H), 6.46–6.42 (m,
1H), 3.98 (s, 3H), 3.86 (s, 3H); MS (EI) m/z: 242 [M+H]+.
4‐Cyano‐N‐(pyridin‐2‐yl)benzamide* (Table 2, entry 8):
Reddish brown solid. M.P.: 117–119 °C; IR (cm–1): 3438, 3205,
2997, 2230, 1680, 1606, 1571, 1506, 1303, 1222, 1162, 1099,
997, 853; 1H NMR (400 MHz, DMSO‐d6): δ 9.20 (br s, 1H),
8.98–8.95 (m, 2H), 8.88 (d, J = 7.2 Hz, 1H), 8.68 (d, J = 5.6 Hz,
1H), 8.52–8.46 (m, 2H), 7.89 (d, J = 6.2 Hz, 1H), 7.26–7.23 (m,
1H); MS (EI) m/z: 224 [M+H]+.
Reactant 2
Combination
O
H
R1
(B)
Scheme 2. Plausible reaction mechanism for synthesis of N‐(pyridin‐
2‐yl)benzamides in the presence of the Cu(I)/MWCNT catalyst.
carbonyl carbon atom of the aldehyde, which would explain the
higher product yield observed for the 20% CuI/MWCNT cata‐
lyst versus the 20% Cu2O/MWCNT catalyst.
3.4. Spectroscopic data of N‐(pyridin‐2‐yl)benzamides
N‐(4‐Methylpyridin‐2‐yl)benzamide* (Table 2, entry 9):
Yellow solid. M.P.: 109–111 °C; IR (cm–1): 3442, 2922, 2856,
1690, 1662, 1595, 1420, 1293, 1091, 1010, 891, 841, 759; H
NMR (400Hz, DMSO‐d6): δ 9.85 (br s, 1H), 8.24 (s, 1H),
8.02–8.02 (m, 2H), 7.98–7.93 (m, 2H), 7.49–7.34 (m, 3H), 2.37
(s, 3H); MS (EI) m/z: 213 [M+H]+.
N‐(Pyridin‐2‐yl)benzamide* (Table 2, entry 1): Yellow solid.
M.P.: 80–82 °C; IR (cm–1): 3640, 3346, 1655, 1599, 1534, 1440,
1322, 1260, 750, 714; 1H NMR (400 MHz, DMSO‐d6): δ 8.98 (br
s, 1H), 8.42 (d, J = 8.4 Hz, 1H), 8.24 (d, J = 3.2 Hz, 1H), 7.95 (d, J =
7.2 Hz, 2H), 7.79–7.75 (m, 1H), 7.58 (d, J = 1.7 Hz, 1H),
7.54–7.47 (m, 2H), 7.08–7.05 (m, 1H); MS (EI) m/z: 199
[M+H]+.
2‐Ethyl‐N‐(pyridin‐2‐yl)benzamide* (Table 2, entry 2): Pale
yellow solid. M.P.: 116–118 °C; IR (cm–1): 3438, 3188, 2762,
2715, 2490, 1603, 1542, 1465, 1394, 1370, 1285, 1139, 997,
806; 1H NMR (400 MHz, DMSO‐d6): δ 9.50 (br s, 1H), 8.16–8.14
(m, 1H), 7.91 (s, 1H), 7.51 (d, J = 1.64 Hz, 1H), 7.40–7.41 (m,
1H), 7.39 (d, J = 2.4 Hz, 2H), 7.35(m, 2H), 3.10 (q , J = 6.9 Hz,
2H), 1.35 (t, J = 6.98 Hz, 3H); MS (EI) m/z: 227.1 [M+H]+.
4‐Isopropyl‐N‐(pyridin‐2‐yl)benzamide** (Table 2, entry 3):
Yellow solid. M.P.: 124–126 °C; IR (cm–1): 3746, 3166, 1905,
1666, 1599, 1518, 1454, 1387, 1289, 1158, 1112, 859; 1H NMR
(400 MHz, DMSO‐d6): δ 9.11 (br s, 1H), 8.5 (d, J = 2.2 Hz, 1H),
8.35–8.29 (m, 2H), 8.21 (d, J = 3.2 Hz, 1H), 7.82 (d, J = 6.8 Hz,
1H), 7.77–7.72 (m, 2H), 7.68–7.63 (m, 1H), 3.12 (s, 1H), 1.53 (s,
6H); MS (EI) m/z: 241 [M+H]+.
4‐Fluoro‐N‐(pyridin‐2‐yl)benzamide* (Table 2, entry 4):
Yellow solid. M.P.: 106–109 °C; IR (cm–1): 3343, 3052, 2923,
1655, 1599, 1532, 1438, 1320, 1075, 1027, 750; 1H NMR (400
MHz, DMSO‐d6): δ 9.17 (br s, 1H), 8.29–8.25 (m, 2H), 8.16 (d, J =
8.2 Hz, 1H), 7.95 (d, J = 7.8 Hz, 1H), 7.89–7.85 (m, 2H),
7.63–7.58 (m, 1H), 7.68–7.63 (m, 1H); MS (EI) m/z: 217
[M+H]+.
4‐Chloro‐N‐(pyridin‐2‐yl)benzamide* (Table 2, entry 5):
Off‐white solid. M.P.: 115–118 °C; IR (cm–1): 3340, 3004, 2839,
1666, 1599, 1532, 1461, 1416, 1289, 1253, 1211, 1024, 849,
1
2‐Ethyl‐N‐(4‐methylpyridin‐2‐yl)benzamide* (Table 2, en‐
try 10): Yellow solid. M.P.: 123–125 °C; IR (cm–1): 3441, 3240,
2923, 2803, 1655, 1606, 1570, 1521, 1413, 1373, 1293, 1199,
943, 887, 689; 1H NMR (400 MHz, DMSO‐d6): δ 9.16 (br s, 1H),
8.33 (s, 1H), 7.77–7.71 (m, 1H), 7.51(d, J = 7.2 Hz, 1H), 7.43 (t, J
= 7.2 Hz, 1H), 7.32 (d, J = 7.8 Hz, 1H), 7.26–7.23 (m, 1H), 6.88 (s,
1H), 2.90 (q, J = 7.2 Hz, 2H), 2.38 (s, 3H), 1.28 (t, J = 7.2 Hz, 3H);
MS (EI) m/z: 241 [M+H]+.
4‐Isopropyl‐N‐(4‐methylpyridin‐2‐yl)benzamide** (Table 2,
entry 11): Yellow solid. M.P.: 122–124 °C; IR (cm–1): 3434,
3191, 2920, 1676, 1616, 1567, 1446, 1419, 1281, 1222, 1158,
845, 704; 1H NMR (400 MHz, DMSO‐d6): δ 8.90 (br s, 1H), 8.29
(s, 2H), 2.88 (s, 2H), 7.34 (d, J = 7.2 Hz, 2H), 6.90 (s, 1H), 2.98 (s,
1H), 2.41 (s, 3H), 1.28 (s, 6H); MS (EI) m/z: 255 [M+H]+.
4‐Ethoxy‐N‐(4‐methylpyridin‐2‐yl)benzamide* (Table 2,
entry 12): Yellow solid. M.P.: 130–133 °C; IR (cm–1): 3423,
3096, 2850, 1689, 1602, 1530, 1485, 1409, 1258, 1229, 1019,
1009, 889; 1H NMR (400 MHz, DMSO‐d6): δ 9.19 (br s, 1H), 8.32
(s, 1H), 8.13 (d, J = 4.8 Hz, 1H), 7.92 (d, J = 8.4 Hz, 2H), 7.33 (d, J
= 8.0 Hz, 2H), 6.93 (d, J = 4.8 Hz, 1H), 2.75 (q, J = 7.6 Hz, 2H), 2.4
(s, 3H); MS (EI) m/z: 257 [M+H]+.
4‐Fluoro‐N‐(4‐methylpyridin‐2‐yl)benzamide* (Table 2,
entry 13): Yellow solid. M.P.: 108–111 °C; IR (cm–1): 3456,
3200, 2977, 2878, 1673, 1624, 1578, 1510, 1438, 1399, 1226,
1015, 885, 758. 1H NMR (400 MHz, DMSO‐d6): δ 9.17 (br s, 1H),

H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
1833
8.29 (s, 1H), 8.10 (d, J = 4.2 Hz, 1H), 7.99 (d, J = 5.2 Hz, 2H),
7.19–7.15 (m, 2H), 6.94 (d, J = 4.0 Hz, 1H), 2.43 (s, 3H); MS (EI)
m/z: 231 [M+H]+.
4‐Chloro‐N‐(4‐methylpyridin‐2‐yl)benzamide* (Table 2,
entry 14): Off‐white solid. M.P.: 119–121 °C; IR (cm–1): 3389,
3118, 2988, 1647, 1628, 1550, 1498, 1428, 1258, 1226, 1001,
856, 740, 738; 1H NMR (400 MHz, DMSO‐d6): δ 9.10 (br s, 1H),
8.23 (s, 1H), 8.18 (d, J = 4.8 Hz, 1H), 7.97–7.94 (m, 2H),
7.50–7.47 (m, 2H), 6.86 (d, J = 5.2 Hz, 1H), 2.53 (s, 3H); MS (EI)
m/z: 247 [M+H]+.
4‐Cyano‐N‐(4‐methylpyridin‐2‐yl)benzamide* (Table 2, en‐
try 15): Brown solid. M.P.: 121–124 °C; IR (cm–1): 3197, 3110,
2950, 2248, 1670, 1610, 1465, 1423, 1340, 1258, 1219, 1020,
877, 745; 1H NMR (400 MHz, DMSO‐d6): δ 9.21 (br s, 1H), 8.21
(s, 1H), 8.18–8.10 (m, 2H), 7.96 (d, J = 5.6 Hz, 1H), 7.46–7.41 (m,
2H), 6.91 (d, J = 5.6 Hz, 1H), 2.41 (s, 3H); MS (EI) m/z: 238
[M+H]+.
4‐(Dimethylamino)‐N‐(4‐methylpyridin‐2‐yl)benzamide**
(Table 2, entry 16): Yellow solid. M.P.: 119–122 °C; IR (cm–1):
3451, 3078, 2950, 2899, 2850, 1638, 1620, 1558, 1298, 1226,
874, 789; 1H NMR (400 MHz, DMSO‐d6): δ 10.2 (br s, 1H), 8.29
(s, 1H), 8.23 (d, J = 8.8 Hz, 1H), 8.15 (d, J = 5.2 Hz, 1H), 6.85 (d, J
= 3.6 Hz, 1H), 6.64 (d, J = 8.8 Hz, 1H), 6.52 (m, 2H), 4.04 (s, 6H),
2.38 (s, 3H), 2.37 (s, 3H); MS (EI) m/z: 256 [M+H]+.
from the reaction mixture by filtration and washed sequentially
with ethyl acetate and acetone before being dried at 200 °C in a
hot air oven for 12 h and reused in a fresh reaction. The 20%
Cu2O/MWCNT catalyst showed good recyclability with no dis‐
cernible decrease in its catalytic activity after four cycles, as
indicated by a consistently high yield of the desired N‐(pyri‐
din‐2‐yl)benzamide product. In contrast, the activity of the
20% CuI/MWCNT catalyst decreased significantly with every
run, as exemplified by a significant decrease in the yield of the
desired N‐(pyridin‐2‐yl)benzamide product with each cycle
(Fig. 6).
Upon completion of the fourth cycle, the two catalysts were
separated from their reaction mixtures and dried at 200 °C. The
FT‐IR spectra and PXRD patterns of the catalysts were subse‐
quently recorded and the results are shown in Figs. 7 and 8,
respectively. It is noteworthy that the FT‐IR spectrum of the
20% Cu2O/MWCNT catalyst contained a lattice vibration for
Cu2O at 622 cm–1. Furthermore, the PXRD pattern of this cata‐
lyst contained diffraction peaks at 2θ = 28.41°, 36.28°, 42.06°,
and 61.38°, which correspond to the (110), (111), (200), and
(220) crystal planes of Cu2O, respectively. However, the FT‐IR
spectrum of 20% CuI/MWCNT contained two characteristic
absorption bands at 611 and 495 cm–1. Furthermore, the ab‐
sence of any diffraction peaks for CuI confirmed that this mate‐
rial was not present on the MWCNTs. Further experiments to
4‐(Dimethylamino)‐N‐(4,6‐dimethylpyridin‐2‐yl)benzamide
** (Table 2, entry 17): Yellow solid. M.P.: 140–142 °C; IR (cm–1):
3367, 3310, 2922, 2850, 1676, 1640, 1618, 1589, 1523, 1420,
1
1408, 1356, 1245, 996, 856; H NMR (400 MHz, DMSO‐d6): δ
(2)
8.93 (br s, 1H), 8.19 (s, 1H), 8.01 (d, J = 4.4 Hz, 1H), 7.80 (d, J =
8.8 Hz, 2H), 6.79 (d, J = 5.2 Hz, 1H), 6.63 (d, J = 9.2 Hz, 2H), 2.96
(s, 3H), 2.91 (s, 3H), 2.30 (s, 3H); MS (EI) m/z: 270 [M+H]+.
* Reported compounds; **New compounds.
(1)
3.5. Reusability of the catalysts
Cu2O
The recyclability properties of the 20% Cu2O/MWCNT and
20% CuI/MWCNT catalysts were evaluated against the oxida‐
tive amidation reaction of benzaldehyde with 2‐aminopyridine
for the synthesis of N‐(pyridin‐2‐yl)benzamide using THF as a
solvent. Upon completion of the reaction, catalyst was removed
3500 3000 2500 2000 1500 1000 500
Wavenumber (cm1)
Fig. 7. FT‐IR spectra of the 20% Cu2O/MWCNT (1) and 20% CuI/
MWCNT (2) catalysts after four reaction cycles.
100
20% Cu2O/MWCNT
20% CuI/MWCNT
CuI
Cu2O
80
60
40
20
0
(2)
(1)
1
2
3
4
Cycles
10
20
30
40
50
60
70
2/(o )
Fig. 6. Recyclability of the 20% Cu2O/MWCNT and 20% CuI/MWCNT
catalysts based on the yield of N‐(pyridin‐2‐yl)benzamide following the
reaction of benzaldehyde with 2‐aminopyridine at 80 °C for 6 h in THF.
Fig. 8. PXRD patterns of the 20% Cu2O/MWCNT (1) and 20% CuI/
MWCNT (2) catalysts after four reaction cycles.

1834
H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
compare the amounts of copper in the fresh and recycled cata‐
lysts using a colorimetric method indicated that there had been
no significant leaching of copper from the 20% Cu2O/MWCNT
catalyst. However, in the case of 20% CuI/MWCNT, these ex‐
periments revealed that 34% of the copper had leached from
the catalyst after four cycles. These experiments therefore re‐
vealed that 20% Cu2O/MWCNT is much better than 20%
CuI/MWCNT with respect to its reusability.
100
80
60
40
20
0
3.6. Effect of the catalyst amount, solvent, temperature, and
duration on the yield over 20% Cu2O/MWCNT
Further experiments were conducted to assess the catalytic
activity of the 20% Cu2O/MWCNT catalyst for the synthesis of
N‐(pyridin‐2‐yl)benzamide by varying the amount of catalyst,
as well as investigating different solvents, temperatures, and
reaction times. The results of these experiments are presented
in Figs. 9–11.
The catalytic activity of the 20% Cu2O/MWCNT catalyst was
measured by varying the amount of catalyst added to the reac‐
tion from 0.01 to 0.2 g (Fig. 9). The results showed that the
catalytic activity increased as the catalyst amount increased up
to 0.1 g of catalyst, where the yield of the desired product
reached a plateau. Based on these results, a catalyst charge of
0.1 g was selected as the optimum amount of catalyst for the
reaction.
The oxidative amidation reaction was conducted in the
presence of several different solvents, including THF, DMF,
EtOAc, chloroform, DCM, and ethanol, to determine the effects
of these different solvents on the yield of the product, and the
results are shown in Fig. 10. It is noteworthy that THF and DMF
provided good yields of N‐(pyridin‐2‐yl)benzamide, although
the use of DMF complicated the isolation of the desired prod‐
uct. The use of polar aprotic solvents (e.g., THF and DMF) gave
high yields of the desired product, while polar protic solvents
such as ethanol gave much lower yields. This difference in the
yield could be attributed to the presence or absence of hydro‐
gen bonding interactions between the amino groups of the
2‐aminopyridine substrate and the solvent, which could have a
significant impact on the availability of the amino group for the
Solvent
Fig. 10. Effect of different solvents on the synthesis of N‐(pyridin‐2‐
yl)benzamide in the presence of the 20% Cu2O/MWCNT catalyst at 80
°C for 6 h.
amidation reaction. Based on these results, THF was identified
as the most efficient solvent for the synthesis of N‐(pyridin‐2‐
yl)benzamides. It is noteworthy that these observations are
consistent with the proposed mechanism.
The effect of the temperature and reaction time on the yield
of the oxidative amidation of benzaldehyde with 2‐aminopyride
in the presence of the 20% Cu2O/MWCNT catalyst were also
studied. The temperature was varied in the range of 40–120 °C
at intervals of 20 °C, with the respective reactions being con‐
ducted over 2, 4, 6, 12, and 24 h (Fig. 11). It is noteworthy that
the reaction did not proceed at room temperature even after 24
h. The results revealed that the highest isolated yield of the
product was obtained at 80 °C after a reaction time of 6 h. In‐
creasing the reaction time to 8, 12, or 24 h at 80 °C did not lead
to an increase in the isolated yield of the product. Furthermore,
increasing the reaction temperature above 80 °C did not lead to
an increase in the yield of the product.
4. Conclusions
Cu2O and CuI supported on acid activated MWCNTs were
100
80
100
80
60
40
20
0
40 oC
80 oC
120 oC
60 oC
100 oC
60
40
20
0
0
5
10
15
20
25
0.00
0.05
0.10
0.15
0.20
Time (h)
Catalyst amount (g)
Fig. 9. Effect of different amounts of the 20% Cu2O/MWCNT catalyst on
the isolated yield of the product. Reaction conditions: benzaldehyde 5
mmol, 2‐aminopyridine 4.5 mmol, THF, 80 °C, 6 h.
Fig. 11. Effect of the reaction temperature and reaction time on the
activity of the 20% Cu2O/MWCNT catalyst towards the oxidative ami‐
dation of benzaldehyde with 2‐aminopyridine.

H. N. Hareesh et al. / Chinese Journal of Catalysis 36 (2015) 1825–1836
1835
found to be an efficient heterogeneous catalysts for the synthe‐
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leaching of the catalytically active component into the reaction
mixture. This catalyst also demonstrated a broad substrate
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the synthesis of N‐(pyridin‐2‐yl)benzamides.
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Acknowledgments
The authors thank DST‐SERB for the financial support for
this research work (Project number: SERB/F/3690/2013–14).
Authors thank Dr. Krishna Venkatesh, CEO, Centre for Emerg‐
ing Technologies and Dr. Chenraj Roychand, President Jain
University trust for their constant support in encouraging this
research work.
Graphical Abstract
Chin. J. Catal., 2015, 36: 1825–1836 doi: 10.1016/S1872‐2067(15)60964‐0
Catalytic role of Cu(I) species in Cu2O/CuI supported on MWCNTs in the oxidative amidation of aryl aldehydes with
2‐aminopyridines
H. N. Hareesh, K. U. Minchitha, N. Nagaraju, N. Kathyayini*
Jain University, India; St. Joseph’s College and Research Centre, India
Several composite Cu(I) species supported on MWCNTs have been synthesized using a wet impregnation method. Characterization re‐
vealed the formation of cubic phase Cu2O and γ‐phase CuI on the MWCNTs. The 20% Cu2O/MWCNT and 20% CuI/MWCNT catalysts were
efficient for the synthesis of N‐(pyridin‐2‐yl)benzamides under mild reaction conditions with 100% selectivity and good yields.

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