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