Polymeric nanoassembly of imine functionalized magnetite for loading copper salts to catalyze Henry and A3-coupling reactions

Article abstract of DOI:10.1016/j.reactfunctpolym.2021.104868

Poly(ethylenimine) was grafted on the magnetite (Fe₃O₄) nanoparticles, and subsequently reacted with different aldehydes to form the imine functionalities (Fe₃O₄-Imine NPs). Thus formed Fe₃O₄

Full text of DOI:10.1016/j.reactfunctpolym.2021.104868

Reactive and Functional Polymers 161 (2021) 104868
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Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Perspective Article
Polymeric nanoassembly of imine functionalized magnetite for loading
copper salts to catalyze Henry and A3-coupling reactions
,
b
,
,e,*
Prakash B. Rathoda c, K.S. Ajish Kumard, Anjali A. Athawalea, Ashok K. Pandeyb
a Department of Chemistry, Savitribai Phule Pune University, Pune 411007, India
b Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
c Department of Chemistry, Shree Sadguru Saibaba Science college, Ashti 442707, Maharashtra, India
d Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
e Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Poly(ethylenimine) was grafted on the magnetite (Fe3O4) nanoparticles, and subsequently reacted with different
aldehydes to form the imine functionalities (Fe3O4-Imine NPs). Thus formed Fe3O4-Imine NPs could host copper
salts generally used as the homogeneous catalyst in organic reactions with the stability prerequisite for the
recycling. The expected chemical structures of Fe3O4-Imine NPs were ascertained by the FTIR and CHNS ana-
lyses. The physical structure of Fe3O4-Imine NPs was found to be corn-like assemblies from the images obtained
by FESEM and HRTEM. The Fe3O4-Imine NPs assemblies possessed the superparamagnetic properties which
made possible to withdraw these particles from the reaction solution efficiently. The copper salt loading capacity
of the Fe3O4-Imine was lower than that of the Fe3O4-PEI, but the water content in the Fe3O4-Imine was higher
(10 wt%) than that of the Fe3O4-PEI (2.5 wt%). Among different aldehydes and copper salts used, the Fe3O4-
Imine formed by benzaldehyde and loaded with copper acetate salt exhibited better catalytic activity in the
Henry reactions using water as the solvent. Also, the Fe3O4-Imine showed better catalytic activity than that
obtained using the Fe3O4-PEI in both Henry reactions and A3-coupling of aldehyde, pyrrolidine, and phenyl
acetylene reactants. The several representative examples of these coupling reactions were studied to understand
the different parameters affecting the catalytic efficacy of copper acetate loaded Fe3O4-Imine.
Magnetite
Copper(II)-imine complex
Catalysis
Henry reaction
A3-couplings
1. Introduction
homogeneous catalysts without involving complicated synthetic chem-
istry [10–12]. It has been reported that the bare magnetite MNPs could
Green chemical processes require a design of the new multifunc-
tional nanocatalysts for the tandem reactions in one pot [1]. The
extensive studies are being carried out to explore the functional nano-
material as the catalysts due to their high surface area which makes
them intermediate between homogeneous and heterogeneous catalysts
[2]. The functionalized polymers could be used not only for anchoring of
the desired catalysts on the host matrix but also forming the functional
groups that can bind with the known homogeneous catalysts [3–6]. The
magnetic nanoparticles (MNPs) have been attracting special attention as
the heterogeneous catalyst supports because of their higher dispersion in
the reaction solution and also the superparamagnetic behavior which
enable their easy withdrawal from the reaction media under the mag-
netic field [7–9]. Among different MNPs, the iron oxide based magnetic
nanoparticles (MNPs) offers several advantages including anchoring
be used directly for catalyzing the selected organic reactions [12–14].
However, the functionalization of MNPs is a key factor to make a stable
and selective catalyst. Therefore, the iron based MNPs have been studied
extensively for the functionalization to generate the desired catalytic
sites [15–23]. The recent uses of the polymer grafted nanoparticles show
great potential in a variety of applications, including advanced polymer
nanocomposite fabrication, drug delivery, imaging, catalysis and lubri-
cation [24–26].
The copper is one of the prime elements in catalyzing the organic
transformations. This is because of its ability to form numbers of com-
plexes with the reagents and also due to their higher abundance which
makes it more cost effective and more sustainable than the precious
transition metal/noble metal based catalysts [27]. The general examples
of copper catalyzed reactions are the Click reactions, asymmetric
* Corresponding author at: Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.
E-mail address: ashokk@barc.gov.in (A.K. Pandey).
https://doi.org/10.1016/j.reactfunctpolym.2021.104868
Received 15 November 2020; Received in revised form 19 February 2021; Accepted 22 February 2021
Available online 26 February 2021
1381-5148/© 2021 Elsevier B.V. All rights reserved.

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
acetylide additions to carbonyl groups, radical alkylations, asymmetric
conjugate additions etc. [28–32]. Henry reactions, also known as the
nitroaldol reactions, are important in synthetic organic chemistry as
these reactions are used for the coupling of a nucleophilic nitroalkane
with an electrophilic aldehyde or ketone to produce a synthetically
useful β-nitro alcohol [33–37]. The examples of variations of the nitro-
aldol reactions are nitronate condensations, Retro-Henry reaction,
intramolecular Henry reaction etc. [38]. The base-catalyzed Henry re-
actions have the problems of various competitive side reactions such as
Nef-type reactions. Also, the base-catalyzed elimination of water can
lead to the formation of readily polymerizable nitro-olefin. To address
these problems, several homogeneous and heterogeneous catalysts,
including magnetic nanoparticles supported catalysts, have been
developed for the selective Henry reactions [39–47]. Biradar et al. have
reported a fixed bed reactor packed with the primary or secondary
amine-functionalized mesoporous silica (MCM-41) for performing
continuous Henry reaction [48]. It is reported in the literature that the
copper complexes are efficient in catalyzing this class of reactions
[42–47]. The copper has been immobilized on the amine rich magnetic
glycocyamine-modified chitosan which was found to have good cata-
lytic activity in the A3-coupling reactions of aldehydes, secondary
amines and phenyl acetylene for the propargylamines preparation [49].
The poly(ethylenimine) is known to have a higher affinity towards Cu
(II) ions [50]. However, the catalytic efficiency of the Cu(II)-loaded poly
(ethylenimine) coated magnetite has not been found to be higher for
Henry reaction in water [50]. The primary amine groups on poly(eth-
ylenimine) could be converted to imines with aldehydes, which may be
more hydrophilic and form a stable complex with copper salts [51].
Thus, the hydrophilic copper salts loaded imine polymeric assemble may
exhibit better catalytic activity in the organic reactions in water due to
their better compatibility.
(vinylbenzyl chloride) (PVBCl) and kept for a reaction for 12 h. Thus
formed PVBCl reacted particles (Fe3O4-PVBCl) were removed using an
external magnet and washed several times with DMF/ethanol. In the
final step of preparation, the Fe3O4-PVBCl particles were reacted with an
excess of PEI dissolved in ethanol at room temperature for 12 h with
continuous stirring, and washed thoroughly to obtain cleaned and dried
Fe3O4-PEI particles.
In a subsequent chemical modification, the amine functionality of
Fe3O4-PEI MNPs converted to imine by reacting with a different alde-
hyde such as benzaldehyde, sugar aldehyde, (+) camphor, camphor
sulphonic acid, 1-tert-butyl-4-methylenecyclohexane, and flavanone in
◦
DMF at 60 C under nitrogen atmosphere. The Fe3O4-Imine MNPs are
not very stable. To avoid the backward reaction, the Fe3O4-Imine MNPs
were washed with dried acetone and dichloromethane and then sus-
pended instantaneously in the solution containing copper salts in dry
acetone. Thus formed catalyst Fe3O4-Imine MNPs were characterized by
CHNS analysis, VSM, FESEM, HRTEM, and FTIR.
The NMR spectra were recorded by using Varian, 500 MHz, Inc., USA
and the sample was prepared by dissolving 5 mg of product sample in
CDCl3 and transferred into NMR tube. The LC-MS spectrometry was
carried out by using model number 410 Prostar Binary LC with 500 MS
IT PDA Detectors supplied by Varian, Inc. USA. The nitrogen analyses
were done at Sophisticated Analytical Instrument Facility (SAIF), IIT-
Bombay, Mumbai, India. Scanning Electron Microscopic imaging of
Fe3O4 particles was also carried out at SAIF using field emission gun
scanning electron microscopy (FESEM) (model JSM-7600F). Thermo-
gravimetric analysis (TGA) was carried out using STARe system
METLER TOLEDO instrument. For this analysis, the known weight of the
sample was taken in an alumina holder and thermo-grams was obtained
with heating rate of 10 ◦C minꢀ 1 from 30 ◦C to 900 ◦C, under dynamic
condition and in air atmosphere (50 mL minꢀ 1).
In the present work, the different imine-functionalized nano-
assemblies of the magnetite particles have been developed to host Cu(II)
salts generally employed for catalyzing the several organic reactions.
The objective of the present work is to study the catalytic efficiency of a
copper salt complexed by an imine functional group anchored on a
polymer coating of magnetite NP for the Henry and A3 coupling re-
actions. The magnetite NPs (Fe3O4) have been subjected to a series of
steps to anchor poly(ethylenimine) (PEI) (Fe3O4-PEI), which has been
subsequently converted to the imine functionality (Fe3O4-Imine) by
reacting with different aldehydes. Thus formed Fe3O4-Imine NPs have
been loaded with different copper salts such as copper acetate, copper
chloride and copper bromide. The Fe3O4-Imine polymeric nano-
structures have been characterized by the CHNS elemental analysis,
field emission scanning electron microscopy (FESEM), high resolution
transmission electron microscopy (HRTEM), Fourier transform infrared
spectroscopy (FTIR), thermo-gravimetric analysis (TGA), and vibrating
sample magnetometry (VSM). The catalytic activities of Fe3O4-Imine-Cu
(II) polymeric nanoassemblies have been studied for the selected ex-
amples of the Henry and A3-coupling reactions.
The catalytic activity of the Fe3O4-Imine-Cu(II) was studied in the
Henry and A3 multicomponent couplings reactions. The completion of
the reaction was monitored by using TLC. After completion of a reaction,
the solution was decanted and the Fe3O4-Imine-Cu(II) catalyst was
recovered by using an external magnet and wash with methanol or
ethanol. The collected catalyst dried at 50 ◦C in an oven under vacuum
for further use. The decanted solution was concentrated by using rota-
vapor, treated with silica, and purified by using the column chroma-
tography. The products were analyzed by NMR, IR and HRMS as given in
Supplementary Material (Appendix A).
3. Results and discussion
3.1. Grafting of PEI on Fe3O4 and conversion to imine
The commercially obtained Fe3O4 NPs (12 ± 3 nm size) were coated
with (3-aminopropyl)triethoxy silane (APTES) in ethanol and was then
reacted with poly(vinylbenzylchloride) (PVBCl) in DMF to form the
covalently linked polymer shell on Fe3O4 NPs as illustrated in Scheme 1,
which is similar to synthetic procedure described in our earlier work
[50]. The PVBCl coated Fe3O4 NPs were again reacted with low mo-
lecular weight poly(ethylenimine) (PEI) (Mw = 1300) to form covalent
links with the residual benzyl chloride units of PVBCl by replacing
covalently bonded chlorine atom as shown in Scheme 1.
2. Experimental
The details of reagents obtained commercially/prepared in the lab-
oratory are given in Supplementary Material (Appendix A). The for-
mation Fe3O4-PEI was carried out using a similar synthetic protocol
described in our earlier publication [50], and discussed here briefly. The
Fe3O4-Imine-Cu(II) catalyst was formed by suspending 1 g of Fe3O4
MNPs in 200 mL ethanol using sonicator, and added 100 mL of water +
3 mL ammonia solution. This solution mixture was again homogenized
with a sonicator and 2 mL of (3-aminopropyl)triethoxy silane (APTES)
was added. Finally, this solution was kept for overnight stirring. After
overnight stirring, the APTES was coated on Fe3O4 which were taken out
using an external magnet, washed thoroughly with ethanol and water,
and dried in the air after washing. For anchoring PEI, 1 g of APTES
coated Fe3O4 MNPs were dispersed in 200 mL of DMF having 2 g of poly
Thus formed Fe3O4-PEI NPs possessed higher concentration of the
amine sites. The amine groups on Fe3O4-PEI NPs then converted to imine
by reacting with different aldehydes in dry DMF solvent at 60 ◦C under
nitrogen atmosphere. The prepared Fe3O4-Imine NPs were loaded with
the copper salts in dry solvent acetone. It was observed that the Cu(II)
loadings made the anchored imine functionalities on Fe3O4-Imine NPs
stable and prevented the imine degeneration due to hydrolysis. The
different aldehyde functionality such as benzaldehyde (1a), sugar
aldehyde (2a), (+) camphor (3a), camphor sulphonic acid (4a), 1-tert-
butyl-4-methylenecyclohexane (5a), and flavanone (6a) were used for
the imine formation with Fe3O4-PEI as shown in Scheme 2. The primary
2

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
(aromatic,1250–1370 cmꢀ 1), N H (3280–3300 cmꢀ 1), and
–
–
CH2
(1470 cmꢀ 1) confirming the final chemical structure of Fe3O4-Imine NPs
depicted in Scheme 1.
The Cu(II) loadings on the Fe3O4-Imine NPs formed by using
different aldehydes were estimated by energy dispersive X-ray fluores-
cence analyses by EDS attached to FESEM. It is seen from data given in
Table S2 (Appendix A) that the Cu(II) loading was an order of ≈1.2 wt%
in all the samples of Fe3O4-Imine NPs irrespective of their imine func-
tional groups. However, the Cu(II) loading capacity of Fe3O4-PEI re-
ported in our earlier work was higher (≈ 2.6 wt%) under similar
conditions [50]. A similar trend was reported elsewhere wherein Cu
loading was found to be reduced after the conversion of a primary amine
in PEI to the imine functional groups [51]. Thermogravimetric analysis
showed 4-steps thermal degradation of the Fe3O4-Imine-Cu(II) (Struc-
ture 1a) NPs with total weight loss of ~18 wt%, see Fig. S2 (Appendix
A). The weight loss of 10% in first step of the thermogram of Fe3O4-
Imine-Cu(II) could be attributed to the loss of free water as bound water
◦
would be expected to released at higher temperature (up to 200 C).
Under similar conditions, 2.5% weight loss from the Fe3O4-PEI-Cu(II)
was observed [50]. This suggested that the water content in Fe3O4-
Imine-Cu(II) was higher than that in Fe3O4-PEI-Cu(II). The remaining 8
wt% weight loss from the Fe3O4-Imine-Cu(II) corresponded to the
amount of materials anchored on Fe3O4 NPs during coatings and sub-
sequent chemical modifications to form the imine functionalities. This is
a significant amount of loading of the material on Fe3O4, and was ex-
pected to affect the superparamagnetic properties of Fe3O4 NPs
adversely. The degradation of superparamagnetic properties of Fe3O4-
Imine nanoassembly with respect to pristine Fe3O4 NPs was studied by
vibrating sample magnetometer (VSM). It was observed that the satu-
ration magnetization of Fe3O4-Imine was drastically decreased with
respect to Fe3O4. However, the superparamagnetic properties were
retained i.e. no remanence and coercivity. It is seen from Fig. S3 (Ap-
pendix A) that the saturation magnetization was decreased systemati-
cally in each step of the formation from Fe3O4 to Fe3O4-Imine-Cu(II).
The Fe3O4-Imine-Cu(II) NPs having 4.5 emu gꢀ 1 saturation magnetiza-
tion could be retrieved easily using the external magnetic field.
3.3. Morphology of Fe3O4-Imine polymeric nanoassemblies
The morphology of the functionalized NPs was studied by FESEM.
The representative FESEM images shown in Fig. 1a suggested that the
Fe3O4-PEI NPs had spherical shapes with overlapping coatings. HRTEM
image given in Fig. 1b also showed that the coatings on Fe3O4-PEI NPs
were polymeric gel-like shell that encapsulated Fe3O4 NPs.
Scheme 1. Illustration of the chemical steps involved in the formation of
Fe3O4@Imine-Cu(II) catalyst using benzaldehyde (1a).
It was observed from the representative FESEM images given in
Fig. 2 that the Fe3O4-PEI NPs aggregated to form the corn-like polymer
assembly with varying length during reaction with an aldehyde. The
assembling of NPs in a long corn-like structure was related to reaction
conditions (DMF at 60 ◦C under nitrogen atmosphere) used for the for-
mation of imine which led to the fractal like growth. The representative
HRTEM images Fe3O4-Imine NPs are shown in Fig. 3. It is seen from
HRTEM images that it was the coated shell that formed the corn-like
nanoassemblies having Fe3O4 NPs embedded in the interior matrixes.
The illustration of morphological changes during different steps
involved in the formation of corn shaped Fe3O4-Imine assembly are
depicted in Scheme 3. The stability of these corn assembles was attrib-
uted to the interparticle crosslinking between free amine and residual
benzyl chloride units of PVBCl as well as overlapping of the polymer
shells of two adjacent particles.
objective of using different aldehyde was to provide rigidity to a copper
complex formed on the Fe3O4-Imine and also create the different local
environment around Cu(II).
3.2. Characterizations of Fe3O4-Imine catalysts
The formations of functional groups in different chemical steps
shown in Scheme 1 were monitored by the elemental analyses and FTIR
spectroscopy. As can be seen from Table S1 (Appendix A), the nitrogen
(0.39 wt%) and carbon (1.65 wt%) contents confirmed the coating of
APTES on Fe3O4 NPs. As expected, the carbon content increased to 3.92
wt% and nitrogen content decreased slightly (0.36 wt%) on reacting
APTES coated Fe3O4 NPs with PVBCl. Finally, the nitrogen content
increased to 0.46 wt% on reacting PVBCl coated NPs with PEI indicating
the formation of Fe3O4-PEI. The presence of expected functional groups
in each step shown in Scheme 1 was also confirmed by taking FTIR
3.4. Henry reaction using Fe3O4-Imine-Cu(II) catalyst
The Fe3O4-Imine-Cu(II) NPs were studied for their catalytic effi-
ciencies in the Henry type nitroaldol formation reactions. To optimize
the reaction conditions, a model reaction of 3-nitrobenzaldehyde with
nitromethane shown in Scheme 4 was conducted under different
spectra as shown in Fig. S1 (Appendix A). The FTIR spectrum showed the
ꢀ 1
vibration bands of Fe O (550–570 cm ), Si O (1030–1080 cmꢀ 1),
–
–
–
C
–
–
C C
–
– –
O
Si
Si (940–980 cmꢀ 1),
N
(1610–1660 cmꢀ 1),
3

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
Scheme 2. Different imine functional groups formed by reacting Fe3O4-PEI with: (a) benzaldehyde (1a), (b) sugar acrylamide (2a), (c) (+) camphor (3a), (d) (+)
camphor sulphonic acid (4a), (e)1-tert-butyl-4-methylenecyclohexane (5a), and (f) flavanone (6a).
Fig. 1. The representative FESEM (a) and HRTEM images of the gel-like Fe3O4-PEI (b).
reaction conditions, and results are summarized in Table 1.
temperature. As can be seen from Table 1, the Fe3O4-Imine-Cu(II)
catalyst having imine functional group 1a yielded 94% of product when
the reaction was conducted in water for 24 h (entry 1, Table 1). To
understand the solvent effect, the reaction was also performed in protic
solvents such as MeOH, EtOH, but only~40% of product was formed
The reaction shown in Scheme 4 did not proceed and did not yield
any product without any catalyst for an observation period of 24 h. In
the next step, this reaction was performed in the presence of various
Fe3O4-Imine catalysts shown in Scheme 1 in different solvents at room
4

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
Fig. 2. The representative FESEM images of Fe3O4-Imine showing the formation of the corn-like polymer nanoassemblies having varying lengths.
(entries 2 & 3, Table 1). This model reaction in the mixture of solvents
(MeOH-H2O (1:1)) afforded 60% of product formation (entry 4,
Table 1). Polar aprotic solvents such as DMF did not improve the yield to
a significant extent (entry 5, Table 1). The catalytic activities of Fe3O4-
Imine-Cu(II) catalysts loaded with various copper salts such as Cu
(OAc)2, CuBr2, CuCl2 were also studied. Among these, the Fe3O4-Imine-
Cu(II) catalyst loaded with Cu(OAc)2 showed better results than those
loaded with other copper salts. HPLC analysis of the nitroaldol adduct
revealed that it was a racemic mixture having 49:51 proportion when
Fe3O4-Imine-Cu(OAc)2 catalyst having functional group 1a was used.
To make an enantiomerically pure product, the Fe3O4-Imine-Cu(II)
catalysts (2a), (3a), (4a), (5a) and (6a) formed by using various chiral
aldehyde were studied. The HPLC analyses of the resultant product did
not show any enantiomeric excess with any of these catalysts used and
yields of the product also remained the same. This seemed to suggest
that the imine functional groups formed by using different chiral
aldehyde did not possess enough rigidity required for forming the
enantiomerically pure product. This may be due to the motion of the
hydrated polymer backbone which may offer random catalytic sites of
the Cu(II)-amine catalyst to the reactants. Since catalyst (1a) showed
better result in terms of yield, this catalyst under the optimized reaction
conditions with water as the solvent was subjected to further studies.
The reusability of the catalyst Fe3O4-Imine-Cu(II) was checked for six
successive cycling after washing, and no significant decrease in the
catalytic activity in the model reaction shown in Scheme 4 was
observed, see Fig. 4.
After optimizing the reaction conditions, the catalytic activity of
Fe3O4-Imine-Cu(OAc)2 was studied for a variety of aldehyde derivatives
reactants for the Henry reactions and results are shown in Table 2.
Among the aromatic unsubstituted aldehydes, benzaldehyde gave the
best yield (entry 3, Table 2). Similarly, among halogen derivatives (entry
4, and 6, Table 2), the bromo-derivative gave better yield. 2-
5

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
Fig. 3. The representative HRTEM images of the Fe3O4-Imine corn-like nanoassemblies.
Scheme 3. Representation of morphological changes during different steps involved in the formation of corn-shaped Fe3O4-Imine polymer nanoassemblies having
varying lengths.
Nitrobenzaldehyde gave better yield (96% yield) than with meta-
derivative probably due to the activation of aldehyde through second-
ary interaction of NO2 group with CHO group (entry 1–2, Table 2).
Reaction with aldehyde having preinstalled electron releasing substit-
uent (entry 5, Table 2) furnished the corresponding product in a good
yield. Reaction was found to be successful with even aliphatic aldehyde
(entry 8, Table 2) to afford corresponding adduct 60% yield. Reaction of
pyridine 3-carbaldehyde, 4-cyano benzaldehyde and aldehyde with
multiple substituents afforded nitroaldol product in a reasonably good
(entry 7 and 9, Table 2).
6

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
were optimized for the catalyst Fe3O4-Imine-Cu(OAc)2 (1a). As shown in
Table S3 (Appendix A), this model multicoupling reaction did not pro-
ceed in the absence of catalyst, or in the presence of intermediates
materials shown in Scheme 1, or in the presence Fe3O4-Imine-Cu(OAc)2
with water/methanol. But in the absence of the solvent, this reaction
proceeded well with ≈60% yield in the presence of Fe3O4-Imine-Cu
(OAc)2 irrespective of its functional groups formed by using different
aldehydes shown in Scheme 2. Thus optimized reaction conditions were
extended to the A3-coupling reactions involving aldehyde, pyrrolidine
and phenyl acetylene having different substituents. As can be seen from
Table 3, the A3-coupling reactions afforded a reasonable good yield in 6
h except for entry no. 4 in Table 3 which proceeded with a slightly lower
rate. The TON and TOF numbers for the model reaction showed shown
Scheme 4. Model Henry reaction studied for the optimization of the reaction
conditions at room temperature.
Table 1
Optimization and standardization of the reaction conditions for a model reaction
shown in Scheme 4 using different Cu(II) loaded Fe3O4-imine catalysts shown in
Schemes 1 & 2.
in Scheme 5 were found to be reasonably good as 287 and 47.8 hꢀ 1
respectively.
,
Entry
no.
Catalyst (structure)
Solvent
Time
(h)
Yield
(%)
1
2
3
4
5
Fe3O4-Imine-Cu(OAc)2
(1a)
water
24
36
36
48
48
94
40
39
60
20
3.6. Comparison with other catalysts
Fe3O4-Imine-Cu(OAc)2
(1a)
Methanol
Ethanol
The catalytic efficiency of Fe3O4-Imine-Cu(OAc)2 was compared
with its precursor Fe3O4-PEI-Cu(OAc)2 and other catalysts reported in
the literature for the model Henry reaction and multicomponent A3-
coupling reactions shown in Schemes 4 and 5, respectively. The multi-
component couplings reaction was first time studied and, therefore, no
literature was available. It is evident from the data given in Table 4 that
the catalyst with imine functionalization improved the yield of model
Henry reaction with respect to its precursor PEI. This could be attributed
to higher water content in Fe3O4-Imine-Cu(OAc)2 with respect to Fe3O4-
PEI-Cu(OAc)2 which would make Fe3O4-Imine-Cu(OAc)2 more
compatible with the aqueous medium containing reactants. This resul-
ted in the increase of yield to 94% with Fe3O4-Imine-Cu(OAc)2 as
compared to 65% in the presence of Fe3O4-PEI-Cu(OAc)2. However, this
trend was reversed when ethanol was used to conduct the Henry reac-
tion i.e. Fe3O4-PEI-Cu(OAc)2 gave better yield (80%) with respect to
Fe3O4-Imine-Cu(OAc)2 (39%) under similar conditions. This suggested
that the Fe3O4-PEI-Cu(OAc)2 was more compatible with an ethanol so-
lution containing reactants. It is seen from Table 4 that the Fe3O4-Imine-
Cu(OAc)2 exhibited comparable catalytic efficiency in the Henry reac-
tion with that reported in the literature using a homogeneous catalyst
such as DNA in water at 12 ◦C [36]. However, the TOF and TON numbers
were not reported in the literature, and hence quantitative comparison
could not be done. Similar to Henry reaction, the catalytic efficiency of
Fe3O4-Imine-Cu(OAc)2 in the A3-coupling reactions without any solvent
was considerably better as compared to that obtained by using Fe3O4-
PEI-Cu(OAc)2 under similar conditions as shown in Table 4.
Fe3O4-Imine-Cu(OAc)2
(1a)
Fe3O4-Imine-Cu(OAc)2
(1a)
Water:
Methanol
DMF
Fe3O4-Imine-Cu(OAc)2
(1a)
6
7
8
Fe3O4-Imine-CuBr2 (1a)
Fe3O4-Imine-CuCl2 (1a)
Fe3O4-Imine-Cu(OAc)2
(2a)
water
water
water
24
24
24
81
83
70
9
Fe3O4-Imine-Cu(OAc)2
(3a)
water
water
water
water
24
24
24
24
79
72
76
68
10
11
12
Fe3O4-Imine-Cu(OAc)2
(4a)
Fe3O4-Imine-Cu(OAc)2
(5a)
Fe3O4-Imine-Cu(OAc)2
(6a)
4. Conclusions
The imine-functionalized corn-like polymer nanoassemblies were
formed for loading the copper salts, which are generally used as ho-
mogeneous catalysts in the organic reactions. The corn-like polymer
nanoassemblies were formed due to stacking of the coated magnetite
particles held together by the crosslinking of overlapping polymer shells.
The formation of desired chemical structures on the magnetite NPs were
confirmed by the FTIR studies and CHNS analyses. It was observed that
≈1.2 wt% of Cu(II) could be loaded on the Fe3O4-Imine nanoassemblies
irrespective of their chemical structures. Among Cu(OAc)2, CuBr2, CuCl2
salts, the Cu(OAc)2 loaded Fe3O4-Imine nanoassemblies exhibited
maximum catalytic activity in a model Henry reaction involving reac-
tion of 3-nitrobenzaldehyde with nitromethane in the water at room
temperature. The catalyst developed in the present work was also found
to be successful in catalyzing the selected examples of multicomponent
couplings A3-coupling reactions of aldehyde, pyrrolidine, and phenyl
Fig. 4. The reusability Fe3O4-Imine-Cu(II) in model Henry reactions given in
Scheme 4. The reaction was carried out with 10 mg of the catalyst (Cu loading:
2.35 × 10ꢀ 4 mol/ g) and 5 mL water at RT.
3.5. A3-coupling reactions
◦
acetylene at 110 C temperature without the use of any solvent. The
The catalyst Fe3O4-Imine-Cu(OAc)2 (1a) was studied for catalyzing
the A3-coupling reaction of the type shown in Scheme 5.
TON and TOF numbers for the model A3-coupling reactions were found
to be reasonably good as 287 and 47.8 hꢀ 1, respectively. The catalyst
with imine functionalization (Fe3O4-Imine-Cu(OAc)2) improved the
The chemical conditions for the model reaction shown in Scheme 5
7

P.B. Rathod et al.
Table 2
Reactive and Functional Polymers 161 (2021) 104868
The variations of yields in the Henry reactions were studied in the presence of 10 mg Fe3O4-Imine-Cu(OAc)2 (1a) catalyst and 1 mmol of
aldehyde, 10 mmol of nitromethane in 1 mL of water at room temperature.
8

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
Scheme 5. Model A3-coupling reaction studied for the optimization of reaction conditions.
Table 3
The product yields obtained in the A3-coupling reactions studied in the presence of 10 mg Fe3O4-Imine-Cu(OAc)2 catalyst (1a) and 1 mmol of aldehyde, 1 mmol of
pyrrolidine, and 1 mmol of phenyl acetylene at 110 ◦C temp.
Table 4
Comparison of catalytic activity of Fe3O4-Imine-Cu(OAc)2 with other catalysts for the Henry reaction and Multicomponent couplings reaction given in Schemes 4 and
5, respectively.
Reaction
Catalyst
Solvent
Temp.
Time (h)
Yield (%)
Ref.
Henry reaction (Scheme 4)
Fe3O4-Imine-Cu(OAc)2
Fe3O4-Imine-Cu(OAc)2
Fe3O4-PEI-Cu(OAc)2
Fe3O4-PEI-Cu(OAc)2
DNA
Water
R.T.
R.T.
R.T.
R.T.
24
36
28
24
8
94
39
65
80
90
80
67
55
Present work
Present work
[50]
Ethanol
Water
Ethanol
Water
[50]
12 ◦
50 ◦
C
C
[36]
D-aminoacylase
DMSO
0.5
6
[52]
Multicomponent reaction (Scheme 5)
Fe3O4-Imine-Cu(OAc)2
Fe3O4-PEI-Cu(OAc)2
No solvent
No solvent
110 ◦
110 ◦
C
C
Present work
Present work
28
9

P.B. Rathod et al.
Reactive and Functional Polymers 161 (2021) 104868
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Declaration of Competing Interest
None.
Acknowledgment
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Authors are thankful to SAIF, IIT Bombay, Mumbai for FESEM and
CHN analyses, and Physics Department, University of Savitribai Phule
Pune University, Pune for VSM analyses. PBR thanks the Board of
Research in Nuclear Science (BRNS), DAE, India for the financial assis-
tance under BARC-University of Pune MoU for the Doctoral studies.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
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11