Microwave-assisted nanocatalysis: A CuO NPs/rGO composite as an efficient and recyclable catalyst for the Petasis-borono-Mannich reaction

Article abstract of DOI:10.1039/c8ra05203d

A CuO NP decorated reduced graphene oxide (CuO NPs/rGO) composite was synthesized and characterized using various analytical techniques viz. XRD, TEM, SEM, UV-Vis, FT-IR, EDX, XPS and CV. The activity of the catalyst was probed for the Petasis-Borono-Mann

Full text of DOI:10.1039/c8ra05203d

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Microwave-assisted nanocatalysis: A CuO NPs/rGO
composite as an efficient and recyclable catalyst
for the Petasis-borono–Mannich reaction†
a
Cite this: RSC Adv., 2018, 8, 30280
Anshu Dandia, Sarika Bansal,a Ruchi Sharma,a Kuldeep S. Rathoreb
*
a
*
and Vijay Parewa
A CuO NP decorated reduced graphene oxide (CuO NPs/rGO) composite was synthesized and
characterized using various analytical techniques viz. XRD, TEM, SEM, UV-Vis, FT-IR, EDX, XPS and CV.
The activity of the catalyst was probed for the Petasis-Borono–Mannich (PBM) reaction of boronic acids,
salicylaldehydes, and amines under microwave irradiation (MW). The CuO NPs/rGO composite works as
a catalyst as well as a susceptor and augments the overall ability of the reaction mixture to absorb MW.
The synergistic effect of MW and CuO NPs/rGO resulted in an excellent outcome of the reaction as
indicated by the high TOF value (3.64 Â 10À3 mol gÀ1 minÀ1). The catalytic activity of the CuO NPs/rGO
composite was about 12-fold higher under MW compared to the conventional method. The catalyst was
recovered by simple filtration and recycled 8 times without significant loss in activity. This atom-
economical protocol includes a much milder procedure, and a catalyst benign in nature, does not
involve any tedious work-up for purification, and avoids hazardous reagents/byproducts and the target
molecules were obtained in good to excellent yields.
Received 17th June 2018
Accepted 15th August 2018
DOI: 10.1039/c8ra05203d
rsc.li/rsc-advances
compatibility make GO nanosheets a vital material in the
chemical sciences.5 GO shows great potential as a versatile
1. Introduction
catalyst for green and sustainable organic synthesis due to its
high surface area and easy recyclability.6
The chemical industry depends on synthetic organic processes
for the manufacture of value-added compounds for increasing
productivity, discovering new leads and generating novel ther-
apeutic agents against the vast numbers of potential drug
targets.1 In order to implement sustainable industrial develop-
ment, the use of catalysts is considered as one of the most
important objectives in process design nowadays.2 It is certain
that the development of catalysts that are low cost, have higher
product selectivity, and higher reaction yields and are easily
reusable is still in demand in current research scenarios.
Carbon-based nanostructures have made a profound impact in
many areas of science and technology due to their remarkable
properties.3 In this direction, graphene oxide (GO) nanosheets
have attracted considerable attention because of their prom-
ising applications in various elds.4 Ready functionalization,
good water-dispersion, reusability, non-toxicity, easy availability
in bulk quantities, a great deal of oxygen functionality in edge
and defect sites, such as hydroxyl (–OH), carboxylic (–COOH),
carbonyl (C]O), and epoxide groups (C–O–C) and high cell
Furthermore, multicomponent reactions (MCRs) have
emerged as powerful routes for the synthesis of various complex
molecules.7 MCRs in all areas of applied chemistry are very
popular because they offer a wealth of products, while requiring
only a minimum amount of effort. In this regard, the PBM
reaction, a three component coupling reaction involving
boronic acids or boronate esters, carbonyl compounds, and
amines, has received a great deal of interest in organic
synthesis.8 The PBM reaction with variation in all three
components of the reaction provides a useful tool in the
synthesis of structurally diverse and synthetically useful classes
of compounds9–15 such as a-amino acids, iminocyclitols, 2H-
chromenes, 2,5-dihydrofurans, 2-hydroxy morpholines etc. A
large number of compounds synthesized by the PBM reaction
have entered preclinical and clinical trial stages over the last few
years.16,17 Thus, due to the prociency of the PBM reaction,
various synthetic protocols have been developed. Several cata-
lysts and solvents have been employed to affect this
transformation.18–25
aCentre of Advanced Studies, Department of Chemistry, University of Rajasthan,
Jaipur, India. E-mail: dranshudandia@yahoo.co.in; parewavijay.parewa@gmail.com
bDepartment of Physics, Arya College of Engineering and IT, Jaipur, India. E-mail:
kuldeep_ssr@yahoo.com
In spite of subtle improvements of the reaction conditions,
somewhere down the line these reactions still lack versatility.
Most of these methods have limitations in terms of the use of
expensive and hazardous chemicals, longer reaction times (12–
24 h), product-diversity, tedious work-up and purication
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and spectral data of compounds. See DOI: 10.1039/c8ra05203d
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procedures, harsh reaction conditions, and unsatisfactory crystallographic planes of the CuO NP phase and rGO are
yields. Furthermore, the employed catalytic systems are more observed which indicates the successful modication of CuO
oen than non-recoverable, thus causing the turn over number NPs on the rGO sheets32 (Fig. 1a). These results indicate that
(TON) or turn over frequency (TOF) to plummet, which is CuO NPs were attached on the rGO surfaces and the regular
signicant from an industrial point of view. Therefore, further interlayer structures of GO were destroyed. The average particle
development of these catalytic systems will require advanced size of the CuO NPs was calculated to be 18 nm on the basis of
materials that can selectively catalyze chemical reactions with the Scherrer formula. It was conrmed by TEM imaging
high reactivity and can be recycled through simple separation (Fig. 1b) that the rGO sheet was modied with plenty of the CuO
and regeneration processes.
NPs (about 23 nm). Considering that the rGO sheet was trans-
Hence, taking into account the rationale behind the devel- parent, the overlap of CuO NPs demonstrates that the particles
opment of versatile and sustainable methodology and our could adsorb on both sides of the rGO. In the case of CuO NPs
interest in the exploration of expeditious synthesis of nano- alone (prepared by the same method in the absence of any
materials and heterocyclic compounds,26–30 herein we report the support), aggregation of CuO NPs led to a signicantly
synthesis, characterization and catalytic application of a CuO decreased surface area (Fig. 1c).
NPs–rGO composite for the PBM reaction of boronic acids,
It is clearly indicated by the SEM image that the rGO nano-
salicylaldehydes, and amines under microwave irradiation sheets have an irregular overlap of sheets in a layered structure
(Scheme 1).
due to further exfoliation during the reduction of GO to rGO.
Aer the one-pot reduction of the copper salt and GO, CuO NPs
were completely distributed on rGO sheets (Fig. 1d) and no
particles were scattered out of the supports, indicating a strong
interaction between the rGO support and the CuO NPs.
According to EDX analysis (Fig. 1e), the CuO NPs/rGO
composites contained the elements C, O, and Cu. The signals
of the C and O elements originated from the support material
(rGO sheets) and the signals of the O and Cu elements resulted
from the decorated CuO NPs.
UV-Vis absorption spectra (Fig. 2a) of GO, and the CuO NPs/
rGO composite show that the GO dispersion exhibits a charac-
teristic peak at 256 nm and a shoulder at 302 nm (low intensity)
corresponding to p–n transitions of the aromatic C–C bonds
and n–p transitions of the C–O bonds respectively (curve a).
Furthermore for the CuO NPs/rGO composite (curve b), the peak
at 302 nm almost disappeared and the peak at 262 nm shied to
274 nm, which shows that GO was reduced effectively.33
Furthermore, a new absorption band appeared at 362 nm that
could be assigned to the characteristic band of CuO,33b indi-
cating the presence of CuO (Fig. 2b) on the rGO sheets.
2. Results and discussion
2.1 Synthesis and characterization of CuO NPs/rGO
The CuO NPs/rGO composite was synthesized by a simple,
efficient and fast one-pot chemical route31 employing graphene
oxide (GO) as a precursor of rGO, Cu(OAc)2 monohydrate as
a precursor of CuO nanoparticles, and hydrazine hydrate as
a reducing agent. This synthetic protocol involves the protec-
tion, reduction and functionalization of graphene oxide in one
step (Scheme 2).
The CuO NPs/rGO composite was characterized by SEM,
TEM, XRD, UV-Vis, FT-IR, EDX, XPS and CV. In the XRD pattern
of the CuO NPs/rGO composite, diffraction peaks for
From the FT-IR spectrum of GO (Fig. 3a), the peaks at
3378 cmÀ1 and 1720 cmÀ1 could be assigned to –OH stretching
and C–O stretching vibrations respectively. The broad peak at
around 1200 cmÀ1 corresponds to a C–O–C vibration. Further-
more, four absorption peaks ranging from 1400 to 1550 cmÀ1
were observed due to aromatic C]C stretching of the GO sheet.
In the FT-IR spectrum of the CuO NPs/rGO composite (Fig. 3b),
the peaks are relatively weak compared to those of GO. These
results demonstrate the relative reduction of GO and the exis-
tence of strong interactions between the CuO NPs and the
surface functionalities of the rGO sheets.34
The Raman spectrum of GO shows a G band at around
1614 cmÀ1 and a D band at around 1356 cmÀ1 (Fig. 4; curve a).
The D line arises due to the breathing mode of phonons of A1g
symmetry near the K zone boundary; the G line originates from
the in-plane vibration of sp2 carbon atoms and a doubly
degenerate phonon mode (E2g symmetry) at the Brillouin zone
center. Both the Raman intensities of the D and G bands slightly
increased upon the adsorption of CuO NPs due to surface-
enhanced Raman scattering activity (curve b).35 Raman bands
Scheme 1 A CuO NPs/rGO composite catalyzed Petasis-Borono–
Mannich (PBM) reaction.
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Scheme 2 Schematic diagram for the synthesis of the CuO NPs/rGO composite.
of the CuO NPs also begin to show at 283, 323 and 647 cmÀ1 in (XPS) measurements (Fig. 5). The deconvoluted C 1s spectrum
the spectrum of the nanocomposite (curve b).33b
shows four peaks at 283.8, 284.6, 285.7 and 286.5 eV which
The successful preparation of the CuO NPs/rGO composite correspond to the C]C/C–C, C–O, C (epoxy)/C]O and
was also conrmed by X-ray photoelectron spectroscopic O–C]O functionalities respectively (Fig. 5a).36 The O 1s XPS
Fig. 1 (a) XRD pattern of the CuO NPs/rGO composite, (b) TEM image of the CuO NPs/rGO composite, (c) TEM image of CuO NPs, (d) SEM image
of the CuO NPs/rGO composite and (e) EDAX spectrum of the CuO NPs/rGO composite.
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Fig. 2 UV-Vis absorption spectra of (a) GO and the CuO NPs/rGO composite and (b) the peak for CuO in the CuO NPs/rGO composite.
also conrm the composition of CuO (2%) and rGO (98%) in
the catalyst.
The electrocatalytic responses of GO and the CuO NPs/rGO
composite were obtained at pH 12.0. It was observed from the
CV results that the response of GO was weak while the CuO NPs/
rGO composite showed a remarkable current peak of about 5.87
mA intensity at À0.43 V which conrmed the presence of CuO in
the CuO NPs/rGO composite (see ESI, Fig. S1†).
2.2 Catalytic activity of the CuO NPs/rGO composite
To examine the catalytic activity of the CuO NPs/rGO composite,
morpholine 1 (1 mmol), phenylboronic acid 2 (1 mmol), and
salicylaldehyde 3 (1 mmol) were employed as reactants for
Fig. 3 FT-IR spectra of (a) GO and (b) the CuO NPs/rGO composite.
a model reaction in dichloromethane (DCM) under microwave
irradiation. Initially, the mixture was treated with microwave
irradiation (400 W) at 70 ꢀC for 40 min without using any
catalyst. Under these conditions the reaction did not proceed
smoothly. These results encouraged us to optimize the reaction
conditions. In the subsequent study, different catalysts were
examined and the results are summarized in Table 1.
In contrast, Cu(OAc)2 showed lower catalytic activity, while
moderate yields were observed in the presence of various other
copper salts. When CuO NPs were used as a catalyst, the reac-
tion proceeded smoothly to afford the corresponding product in
reasonable amounts (Table 1, entry 7). Further experiments
were also performed to check the activity of GO and rGO sheets
for this catalyzed reduction reaction and they gave good yields
of the product 4a (Table 1, entries 8 and 9). In order to improve
the catalyst recovery and the prevention of aggregation of NPs in
the reaction mixture, CuO NPs were immobilized on a rGO
Fig. 4 Raman spectra of GO (spectrum a) and the CuO NPs/rGO support. It was found that the best results could be achieved by
composite (spectrum b).
using the CuO NPs/rGO composite as indicated by the high TOF
(3.64 Â 10À3 mol gÀ1 minÀ1). This result is in agreement with
our working hypothesis that the most surfaces of these attached
spectrum of the rGO–CuO nanocomposite shows peaks at
528.46, 531.45, and 532.8 eV from Cu–O, O–O and OH,
respectively (Fig. 5b).36 Furthermore, the appearance of two
signals at 944.3 eV and 965.1 eV due to Cu 2p1/2 and Cu 2p3/2
respectively in the Cu 2p XPS core level survey spectrum of the
CuO NPs/rGO composite suggests the formation of metallic
CuO NPs on the rGO nanosheets (Fig. 5c).36 The XPS results
CuO nanoparticles and rGO sheets are exposed to the reaction
environment. Hence, higher catalytic activity was observed with
the CuO NPs/rGO composite. These results show that this
method is superior to the other methods in terms of yield and
reaction time. The quantity of the catalyst used plays a vital role
for the formation of the desired product. The results summa-
rized in Table 1 clearly reveal that 10 wt% catalyst loading was
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Fig. 5 (a) C 1s, (b) O ls and (c) Cu 2p XPS survey spectra of the CuO NPs/rGO composite.
Table 1 Comparison of the CuO NPs/rGO composite with other
catalytic systems for the PBM reaction to afford 4aa
loading. Moreover, the composition of the CuO was character-
ized using an XPS technique.
In order to develop a viable approach, the model reaction
was investigated under different unconventional and conven-
tional conditions. Under MW, the catalytic activity of CuO NPs/
rGO was found to be 7-fold higher than with the conventional
method (Table 2).
TOF (Â10À3 mol
S. no. Catalyst
Time
Yieldb (%) gÀ1 minÀ1
)
1
—
40 min 26
25 min 35
25 min 50
25 min 40
25 min 41
25 min 43
15 min 71
40 min 57
40 min 58
—
2
3
4
5
6
7
8
CuNO3 (10 wt%)
Cu(OAc)2 (10 wt%)
CuSO4 (10 wt%)
CuCl2 (10 wt%)
CuI (10 wt%)
CuO NPs (10 wt%)
GO (10 wt%)
0.383
0.548
0.438
0.449
0.471
1.30
0.390
0.397
3.64
The enhancement of the catalytic activity under MW might
be due to the fact that the nanocatalyst acts as a susceptor37 and
absorbs microwave irradiation, thus it can serve as an internal
heat source for the reaction which enhances the overall capacity
of the reaction mixture to absorb MW and prevents the deacti-
vation of the nanocatalyst during the reaction. The literature
supports our working hypothesis that nanomaterials are selec-
tively heated more under MW compared to conventional heat-
ing due to differences in dielectric properties of the materials
and volumetric dielectric heating under MW.38 During the
optimization of the reaction conditions, the model reaction was
also studied by varying the microwave power (300, 400, and 500
W) and the temperature. It was concluded that a 400 W power
9
rGO (10 wt%)
10
11
12
CuO NPs/rGO (10 wt%) 7 min 93
CuO NPs/rGO (15 wt%) 7 min 93
CuO NPs/rGO (5 wt%) 7 min 35
2.28
2.88
a
Reactions were performed with salicylaldehyde (1 mmol), piperidine
(1 mmol) and phenyl boronic acid (1 mmol) in DCM under microwave
b
irradiation. Isolated yield. n.c.: not calculated.
ꢀ
output at 70 C was required to accomplish maximum conver-
adequate to catalyse the reaction, and excessive amount of
catalyst did not increase the yield remarkably. In addition, it
was also revealed that no other additive combinations such as
protic or Lewis acids were at all advantageous in this method.
Furthermore, we have also analyzed the results in terms of the
amount of CuO loading (1%, 2%, and 3%) in the CuO NPs/rGO
composite. The best results were observed when we used
10 wt% of the CuO NPs/rGO composite containing 2% CuO
sion to the product. The reaction was also investigated in
various solvents such as CH3CN, MeOH, 1,4-dioxane, THF, DMF
and DCM with all other parameters kept constant and the
progress of the reaction was checked through the use of TLC.
The yields were found to be 46, 63, 69, 32, 42 and 92%,
respectively. Hence, DCM was found to be the best solvent of
choice.
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Table 2 Dependency of the catalytic activity of CuO NPs/rGO on different unconventional and conventional conditionsa
TOF (Â10À3 mol
Entry
Condition
Catalyst
Temp. (ꢀC)
Time (min)
Yieldb (%)
gÀ1 minÀ1
)
1
2
3
4
5
6
Conventional
Ultrasound
Microwave
Microwave
Microwave
Microwave
CuO NPs/rGO (10 wt%)
CuO NPs/rGO (10 wt%)
CuO NPs/rGO (10 wt%)
CuO NPs/rGO (10 wt%)
CuO NPs/rGO (10 wt%)
CuO NPs/rGO (10 wt%)
70
70
50
60
70
80
60
30
7
7
7
65
71
68
78
93
93
0.297
0.651
2.66
3.05
3.63
3.63
7
a
Reactions were performed with salicylaldehyde (1 mmol), piperidine (1 mmol) and phenyl boronic acid (1 mmol) in DCM under microwave
b
irradiation. Isolated yield.
These excellent preliminary results encouraged us to further was occurring only due to the solid CuO NPs/rGO composite and
explore the applicability of the CuO NPs/rGO composite for the also showed that Cu was not detached from the catalyst during the
PBM reaction (Table 3). To study the scope and limitations of reaction. The ltrate was further analyzed by ICP-AES and there
this protocol, we employed a wide range of boronic acids, sali- was no metallic leaching in the ltrate. This whole experiment
cylaldehydes, and amines. In general, aryl boronic acids with conrms the heterogeneous nature of the presented catalytic
electron donating groups reacted more smoothly than those system and the presence of strong interactions between the CuO
possessing electron-withdrawing groups. Literature reports also NPs and the surface functional groups of the rGO sheets.
reveal that boronic acids bearing electron-decient substituents
Recycling experiments were performed by choosing the
do not allow for good migration from the boron to the iminium model reaction in DCM under microwave irradiation using the
carbon.39 Further extension of the scope to heteroatom con- CuO NPs/rGO composite as a solid catalyst. When the reaction
taining boronic acid substrates was also investigated. With was completed, the reaction mixture was ltered and a solid
reaction times of up to 40 min, heterocyclic boronic acids precipitate was dried along with the catalyst. Then, the solid
afforded good yields. Moreover, the electronic environments of precipitate was dissolved in acetone and the catalyst was
o-hydroxy aldehydes do not affect the reaction and the corre- recovered by ltration. The recovered catalyst was washed with
sponding products were generated in good to excellent yields. In water and ethanol and reused in 8 successive reaction cycles
addition, different secondary amines like piperidine and mor- without any signicant loss in its catalytic activity (Fig. 6).
pholine were also examined, were found to be effective
substrates and afforded the corresponding alkylaminophenols that the characteristics obtained from the TEM imaging of the
in high yields. fresh and used catalysts were similar, which suggests retention
The reason that the recycling experiments were successful is
A proposed reaction mechanism for this three-component of the structure and morphology of the CuO NPs/rGO composite
PBM reaction is outlined in Scheme 3. The CuO NPs/rGO aer repeated use as a catalyst. According to the ICP-AES
composite can serve as a Lewis acid catalyst for the reaction results, there was no metallic leaching in the nal product. In
of salicylaldehydes and amines to give the corresponding imine the case of liquid products, the reaction mixtures were sub-
(A). Aer that, the CuO NPs/rGO composite also facilitates the jected to centrifugation in order to recover the solid catalyst.
coordination between the oxygen anion of the imine (A) and the
boron atom of the boronic acid leading to the formation of
a tetracoordinate borate intermediate (B). Consequently, the
3. Experimental section
General information including the instruments used is
provided in the ESI.†
migration of the aryl moiety of the boronic acid from the boron
to the iminium carbon forms the stable intermediate (C) which
upon hydrolysis gives the desired product by the loss of a H3BO3
molecule. The CuO NPs/rGO composite may enhance the rates
of several of these transformations.
3.1 Preparation of graphene oxide (GO)
Graphene oxide (GO) was prepared using a modied Hummer’s
method (see ESI†).31
2.3 The heterogeneous nature and recyclability of the CuO
NPs/rGO composite
3.2 Synthesis of graphene oxide sheets decorated with CuO
nanoparticles
To conrm the heterogeneous nature of the CuO NPs/rGO
composite in a reaction, the model reaction was carried out The CuO NPs/rGO composite was synthesized by a one-pot
again under similar reaction conditions with catalyst procured chemical route. Firstly, 200 mg of the as-prepared GO was
from a previous cycle. Aer 4 min, the catalyst was separated from dispersed in 250 mL deionized water and ultrasonicated for
the reaction mixture. The reaction was continued with the ltrate 10 min using an ultrasonic probe. The obtained dispersion was
for another 30 min and the reaction conversion was monitored centrifuged at 10 000 rpm for 15 min to remove any unexfoliated
every 4 min. It was observed that further conversion was not GO. Then, Cu(OAc)2 monohydrate (1%, 2%, 3%) was dissolved in
observed even aer 40 min. These results revealed that the reaction this dispersion and the whole material was stirred at room
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Table 3 CuO NPs/rGO composite catalyzed synthesis of alkylaminophenolsa
a
Reaction of o-hydroxy aldehydes (1 mmol), secondary amines (1 mmol) and various boronic acids (1 mmol) in DCM under microwave irradiation.
temperature for 30 min. Aer that, a 20 mL hydrazine hydrate composite in DCM (15 mL) were introduced into a 50 mL
(5 mol LÀ1) solution was added slowly and the mixture was round-bottom ask. The ask was placed in a microwave
reuxed at 90 ꢀC under continuous stirring for 8 h. The obtained cavity and the reaction mixture was irradiated at 70 ꢀC for an
precipitate was separated by centrifugation (10 000 rpm for 15 appropriate time at 400 W. When the reaction was
min), washed with deionized water, and then dried under vacuum. completed, the resulting solid precipitate was ltered and
dried along with the catalyst. Then, the solid precipitate was
dissolved in acetone and the catalyst was recovered by
ltration. This solution was concentrated to generate the
crude product. The crude product was puried by crystalli-
zation from ethanol.
3.3 General procedure for the synthesis of
alkylaminophenols 4 by the PBM reaction
o-Hydroxy aldehydes (1 mmol), secondary amines (1 mmol),
boronic acids (1 mmol) and 10 wt% of the CuO NPs/rGO
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Scheme 3 Plausible mechanism for the CuO NPs/rGO composite catalyzed PBM reaction.
results obtained from XRD, SEM, TEM, XPS, EDX, FT-IR, UV-
Vis, Raman spectroscopy and cyclic voltammetry proved the
feasibility and reliability of the proposed method as a facile
route for the reduction of GO with in situ preparation and
modication with CuO NPs on its surface by a one pot
chemical route. Under microwave radiation, the catalytic
activity of the CuO NPs/rGO composite was 12 fold higher
than with the conventional method. The microwave proce-
dure coupled with the use of the CuO NPs/rGO composite
offers several advantages including cleaner reaction proles,
and highly economical, environmentally benign method-
ology involving shorter time durations, high yields, and
simple experimental and work up procedures.
Fig. 6 Recyclability of the CuO NPs/rGO composite.
Conflicts of interest
There are no conicts to declare.
4. Conclusions
Acknowledgements
We have reported a very simple, convenient and green
method for the selective synthesis of alkylaminophenols
derivatives via the PBM reaction of boronic acids, salicy-
laldehydes, and amines catalyzed by
composite under microwave irradiation. The excellent
Financial assistance from the DST-SERB, New Delhi is gratefully
acknowledged. We are thankful to the Malaviya National Insti-
tute of Technology, Jaipur and the Central Drug Research
Institute, Lucknow for the spectral analyses.
a CuO NPs/rGO
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