Polymer coated magnetically separable organocatalyst for C[sbnd]N bond formation via aza-Michael addition

Article abstract of DOI:10.1016/j.tetlet.2016.09.093

A polyacrylamide coated magnetite (PAM@MNP) catalyst was synthesized by following a two step approach involving the reaction of magnetite (Fe₃O₄) particles with coupling agent 3-(trimethoxysilyl)propyl methacrylate followed by grafting of acrylamide and subsequent polymerization via surface initiated radical polymerization technique. The synthesized organocatalyst was used for a one-pot aza-Michael addition reaction of amines with electron deficient alkenes to give β-amino carbonyls. The magnetic properties of the synthesized organocatalyst provide it a facile recovery by external magnet which eliminates the problems arising during catalyst separation by conventional filtration.

Full text of DOI:10.1016/j.tetlet.2016.09.093

Accepted Manuscript
Polymer coated magnetically separable organocatalyst for C-N bond formation
via aza-Michael addition
Vineeta Panwar, Siddharth S. Ray, Suman L. Jain
PII:
S0040-4039(16)31286-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.09.093
TETL 48163
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 August 2016
23 September 2016
28 September 2016
Please cite this article as: Panwar, V., Ray, S.S., Jain, S.L., Polymer coated magnetically separable organocatalyst
for C-N bond formation via aza-Michael addition, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.
2016.09.093
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Graphical Abstract
Polymer coated magnetically separable organocatalyst for
C-N bond formation via aza-Michael addition
Leave this area blank for abstract info.
Vineeta Panwar,a Siddharth S. Raya and Suman L. Jaina*

1
Tetrahedron Letters
Polymer coated magnetically separable organocatalyst for C-N bond
formation via aza-Michael addition
Vineeta Panwar,a Siddharth S. Raya and Suman L. Jaina
aChemical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun-248005 (India)
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A polyacrylamide coated magnetite (PAM@MNP) catalyst was synthesized by following a two
step approach involving the reaction of magnetite (Fe3O4) particles with coupling agent 3-
(trimethoxysilyl)propyl methacrylate followed by grafting of acrylamide and subsequent
polymerization via surface initiated radical polymerization technique. The synthesized
organocatalyst was used for one-pot aza-Michael addition reaction of amines with electron
deficient alkenes to give β-amino carbonyls. The magnetic properties of the synthesized
organocatalyst provide it a facile recovery by external magnet which eliminates the problems
arising during catalyst separation by conventional filtration.
Available online
Keywords:
magnetic separation
organocatalyst
surface initiated radical polymerization
aza-Michael addition
heterogeneous catalysis
2009 Elsevier Ltd. All rights reserved.
Development of organocatalysts for organic
Alder20,21, Baylis–Hilman 22,23, Mannich24,25-27, Michael28,29
,
transformations is an area of current research interest as they
do not contain toxic metallic components and are considered to
be green, readily accessible and highly efficient1-5.
Organocatalysts generally possess higher stability thereby
simplifying handling and storage and the reactions can be
carried out under non-inert reaction conditions such as in air6-
12.Organocatalysts are simple organic compounds which show
higher solubility in most of the organic solvents; thus, the
separation of the organocatalyst from reaction mixtures and its
recycling is a challenging task. One of the most promising
approaches to overcome such limitations is the immobilization
or heterogenization of homogeneous organocatalyst to a
suitable support matrix, which makes the catalyst easily
recoverable and recyclable. Recently, magnetic nanoparticles
have emerged to be promising, robust, easily available support
materials for preparing the heterogeneous catalysts13-16 owing
to their magnetic properties and large surface area. The prime
advantage of using magnetic nanoparticles as support is the
facile recovery of catalyst using an external magnet which
solves the problem associated with catalyst loss during the
filtration at the end of the reaction17-18. Furthermore, magnetic
nanoparticles can be easily functionalized with various
coupling agents, which can provide a strong chemical
interaction between the organocatalyst and the metallic support
to prevent the leaching of the catalyst during the reaction.
Thus, magnetically separable organocatalysts have emerged to
be efficient, robust, and feasible substitutes to conventional
metal based catalysts for organic transformations. In addition,
the nano-size of MNPs provides efficient interaction between
catalyst and reactant molecules, which provides a way to
bridge the gap between homogeneous and heterogeneous
catalysts. So far, magnetically separable organocatalysts have
been used for various organic transformations such as
enantioselective C–C, C–N, C–O bond formation19, Diels–
Friedel–Crafts alkylation30, oxidation31,32 and carbohydrate
synthesis33.
Among the great variety of organocatalytic transformations,
aza-Michael addition occupy a very important position and has
received widespread attention for accessing a variety of
synthetically useful building blocks for the synthesis of
bioactive natural compounds34-37. This reaction involves the
interaction between α,β-unsaturated carbonyl compounds as
Michael acceptors and amines as Michael donors through the
hydrogen bonding between the carbonyl oxygen atom and the
amine NH proton. Thus, the amine donor should be electron
rich in nature. Owing to this reason, in general the reaction
rates are faster with aliphatic amines compared to those of
aromatic amines and very electron deficient amines such as 4-
nitroaniline did not react due to the decrease in nucleophilicity
of the nitrogen atom of 4-nitroaniline38. During the past decade,
a number of efficient organocatalysts both homogeneous as
well as immobilized have been developed for facile recovery
and recycling for aza-Michael addition reactions. Furthermore,
advanced strategies, for example using light39, microwave40-42
and ultrasonic irradiation43 have also been explored in the
literature. Although extensive research has been carried out
with organocatalysts for C-N bond formation through aza-
Michael addition, the use of magnetic nanoparticles
immobilized organocatalyst is rarely known. To the best of our
knowledge there is only one report is known on the use of
magnetic nano-organocatalyst for aza-Michael addition40. In
this report, magnetic nano-particles have been coated with
glutathione via post grafting method using ultrasonication.
Furthermore, the developed nano-organocatalyst has been used
o
for aza-Michael reaction at higher temperature (140 C) under
microwave irradiation using water as reaction media.

2
Tetrahedron
Herein, we report, a simple, cost effective and efficient
shows the characteristic bands of bending vibration of N–H
synthesis of magnetite functionalized polyacrylamide
(PAM@Fe3O4), a magnetically recoverable organocatalyst for
aza-Michael reaction under solvent less conditions (Scheme 1).
bond (1630 cm−1) and stretching vibration of N–H bond (Vas
N–H
,3400 cm−1; VsN–H , 3170 cm−1), respectively46. Comparison of
all three FTIR spectra clearly indicated the successful
functionalization of MNPs with polyacrylamide.
H
PAM@MNPs
R2
R1 NH2
R1: Ph
+
N
R1
R2
Solvent Free,
65 oC, 4h
CN
,
H3CO
O
R2:
Scheme 1: PAM@MNP-catalyzed aza-Michael addition
Synthesis and characterization of the catalyst
Preparation of polyacrylamide coated magnetic
nanoparticles, PAM@MNPs
The process for synthesis of polyacrylamide coated
magnetic nanoparticles is shown in Scheme 2, Fe3O4 magnetic
nanoparticles (MNPs) were synthesized by a chemical co-
precipitation of Fe2+ and Fe3+ ions under alkaline condition.
After that the MNPs were functionalized with 3-
trimethoxysilylpropyl methacrylate (silane-A) to afford vinyl
groups which were subsequently subjected to surface initiated
polymerization with acrylamide to get the desired
polyacrylamide coated MNPs (PAM@MNPs). The un-grafted
homopolymers/ oligomers were separated via repeated
washings with ethanol.
Fig. 1:FT-IR spectra of: a) neat MNPs; b) silane-A@MNPs; c) PAM@MNPs
The surface morphology of pure Fe3O4 (MNP), silane-A
functionalized Fe3O4 and polyacrylamide coated Fe3O4
nanoparticles were determined by FE-SEM as shown in figure
2. The FE-SEM image of Fe3O4 confirmed that the particles are
nearly spherical in shape with size ranging from 50–100 nm
(Fig. 2a). However, in the case of the silane-A functionalized
Fe3O4 NPs, the morphology was found to be slightly changed
from spherical to agglomerated cluster type structure (Fig. 2b).
However, after coating with PAM, clearly spherical shaped
nanoparticles were observed, without having any
agglomeration (Fig. 2c). Furthermore, the EDX pattern of all
three materials indicated the presence of desired elements
which further confirmed the successful formation of PAM
coated magnetic NPs (Fig. 2d-f).
O
OCH3
Fe3O4
MNPs
+
H3CO
Toluene
Reflux
Fe3O4
Si
O
OCH3
Silane-A
Silane-A@MNPs
Acrylamide
70oC
AIBN
O
OCH3
Si
H3CO
O
OCH3
n
O
m
O
Fe3O4
O
NH2
PAM@MNPs
Fe3O4
Scheme 2: Synthesis of polyacrylamide coated magnetic nanoparticles
(PAM@MNPs)
Figure 1 shows the FT-IR spectra of pure Fe3O4, silane-A
functionalized Fe3O4 and PAM@ MNPs, respectively. FT-IR
spectrum of Fe3O4 (Fig. 1a) showed characteristic peaks at 574
and 1384 cm-1 respectively which are attributed to the bending
vibration mode of Fe–O and stretching mode of Fe–O. The
peak at 3415 cm-1 is due to the –OH groups presented on the
surface of Fe3O4 nanoparticles44-45. The IR spectrum of silane-
A functionalized Fe3O4 (Fig. 1b) exhibited absorption peaks at
1710 and 1644 cm–1 related to the stretching vibration of C=O
and C=C bonds, respectively. The absorption peaks appeared at
1411 and 2910 cm–1 attributed to Si-O bond and asymmetric
stretching vibration of CH2 groups, respectively. Other peaks
appearing at ~1,100 attributed to the vibrational modes
involving the bridging oxygen atoms in Si-O bonds. All these
peaks revealed the successful functionalization and existence
of vinyl groups onto the surface of magnetite. In Figure 1c,
Fig. 2: SEM images of: a) Fe3O4 nanoparticles; b) silane-A@MNPs; c)
PAM@MNPs and EDX of: d) Fe3O4 nanoparticles; e) silane-A@MNPs; f)
PAM@MNPs

3
The XRD diffraction pattern of pure Fe3O4 is in well
agreement with the reported standard pattern of crystalline
magnetite (Figure 3a)47-48. The characteristic diffraction peaks
at (220), (311), (400), (422), and (511) revealed the inverse
cubic spinel structure of Fe3O4 and well matched with JCPDS
card No. 65–3107. Similarly these characteristic peaks also
appeared in silane-A functionalized Fe3O4 and polyacrylamide
functionalised Fe3O4, confirmed the presence of the magnetic
core (Figure 3b, c). However, the peak broadening in case of
PAM@Fe3O4, may be assumed to be due to the presence of
amorphous polymer coating.
Fig. 4: TGA thermogram of: a) Fe3O4 NPs; b) silane-A@MNPs; c) PAM@MNPs
Catalytic activity
After the successful synthesis and characterization of the
polyacrylamide coated magnetic nanoparticles (PAM@Fe3O4),
we decided to explore its catalytic potential for aza-Michael
reaction (Scheme 1). At first, reaction conditions were
optimized by choosing aniline and methyl acrylate as the
representative substrates for aza-Michael reaction. The results
are summarized in Table 1. The reaction was first conducted in
various solvents in order to choose an optimum reaction
o
medium for the reaction. In toluene at 65 C, the reaction was
found to be slow and afforded poor conversion (Table1, entry
1). Similarly, poor conversion was observed in polar aprotic
solvents such as acetonitrile and dichloromethane (Table1,
entry 2, 3). Polar protic solvents such as water and ethanol
provided better conversion (Table1, entry 4, 5); however
solvent less condition was found to be best, which further
makes the developed protocol benign from environmental
viewpoints. The reaction was found to be slow at room
temperature; however the conversion increased with
Fig. 3: XRD diffractogram of: a) Fe3O4 nanoparticles; b) silane-A@MNPs; c)
PAM@MNPs
Thermal stabilities of the synthesized materials were
determined by thermogravimetric analysis (TGA)49 as shown
in Fig. 4. In neat Fe3O4, a small amount of weight loss between
100-200 oC is assumed due to the evaporation of the adsorbed
water molecules. In Figure 4b, the major weight loss between
o
temperature and at 65 C it was obtained maximum. Further
increase in temperature did not influence the reaction to any
significant extent (Table1, entry 6). Similarly, 100 mg catalyst
was found to be optimum for this reaction and further increase
in catalyst amount did not provide any significant improvement
(Table1, entry 7). Based on these optimization experiments, we
o
250-300 C is attributed to the loss of silane-A functionalities
presented on the support of MNPs. In Figure 4c, sharp decrease
in weight loss between 200 and 650 ◦C was observed as
compared to neat Fe3O4 NPs and silane-A@MNPs. The initial
o
have chosen 65 C as optimum reaction temperature, 100 mg
o
weight loss below 140 C can be attributed to the evaporation
catalyst amount under solvent less condition. Next, we
compared the catalytic activity of the PAM@MNPs with neat
MNPs under identical conditions. The reaction with neat MNPs
was found to be very slow and provided only 30% yield of the
desired product (Table 1, entry 7). This is due to the
agglomeration of the bare MNPs which is in well agreement
with the suggested literature reports40,50. Furthermore, during
the recycling experiment with recovered neat MNPs, the
catalytic activity was found to be nil which again confirmed
the deactivation of the MNPs due to their agglomeration
tendency (Table 1, entry 7).
of water molecules in the polymer matrix. The major and sharp
weight loss between 200-650 oC was attributed to the
decomposition of the grafted polymer and other organic
components, which were presented on the surface of MNPs.

4
Tetrahedron
Table 1: Results of the optimization experimentsa
Table 2: PAM@MNP catalyzed aza-Michael addition of under solvent-free
conditiona
Entry
Solvent
Toluene
Yield (%)b
Ent
ry
Michael
acceptor
T/
(h)
Yield
(%)b
Amine
Product
1
2
3
4
5
6
50
Dichloromethane
Acetonitrile
Water
60
4
85
H
N
NH2
OCH3
O
OCH3
45
1.
2.
O
75
70
Ethanol
H
N
OCH3
NH2
OCH3
O
Without solvent
85a, 20c, 50d,92e
O
5
80
H3CO
-f, 65g, 90h,
H3CO
7
Without solvent
30i, -j
NH2
OCH3
O
aReaction conditions: aniline (1 mmol) , methy1 acrylate (1.5 mmol), and
catalyst (100 mg), solvent (2 ml), 65oC for 4h; bIsolated yield; cat room
temperature, at d50 oC, at e75 oC; fwithout catalyst, gusing 50 mg of
catalyst, h150 mg catalyst was used, iusing neat MNPs as catalyst,
jrecycling experiment using recovered neat MNPs as catalyst.
H
N
OCH3
3.
4.
4.5
7
72
76
Cl
O
Cl
H
N
NH2
CN
CN
CN
CN
H
N
NH2
CN
Deploying the above optimized reaction conditions, the
scope of this catalyst was then investigated for aza-Michael
reaction using a variety of substrates and the results are
summarized in Table 2. In general aromatic amines were found
to be less reactive (Table 2, entry 1-8) as compared to aliphatic
ones (Table 2, entry 10-11). However, aromatic amines
carrying either electron-donating or electron withdrawing
substituents were successfully reacted with unsaturated olefins
to produce their corresponding Michael adducts in moderate to
higher yields. However, very electron deficient amines such as
4-nitroaniline did not react even after prolonged reaction time,
which is due to the decrease in nucleophilicity of the nitrogen
atom (Table 2, entry 7,8). Alicyclic amine such as
cyclohexylamine also reacted smoothly under the described
conditions and afforded moderate product yield (Table 2, entry
9). Furthermore, the reaction rate with aliphatic amines such as
1-butylamine and allylamine was found be faster as compared
to aromatic amines and afforded higher product yield in
comparatively lower reaction time (Table 2, entry 10,11).
5.
6.
7.
3
6
75
82
-
H3CO
H3CO
NH2
H
N
CN
CN
Cl
Cl
H
NH2
N
10
CN
O2N
O2N
O2N
H
OCH3
N
OCH3
NH2
O
8.
12
-
O
O
O2N
H
N
OCH3
NH2
OCH3
OCH3
OCH3
9.
4.5
3.5
3.5
84
90
87
O
H
N
OCH3
H2N
10.
11.
O
In the previously known report40, magnetic nano-particles
have been coated with glutathione using post grafting method
using ultrasonication. In this case the covalent attachment of
organocatalyst on MNPs is not very much confirmed which
may hamper the long term stability of organocatalyst due to
leaching However, in the present study, the MNPs were
initially functionalized by silane-A linker to have covalent
immobilization of acrylamide followed by its polymerization to
get the leach proof stable catalyst. Also, in the previous report,
the magnetic organocatalyst has been used at higher
O
O
H
N
OCH3
CH2=CHCH2NH2
O
aReaction conditions: amine (1 mmol), alkene (1.5 mmol), and
PAM@MNPs (100 mg) at 65 oC; bIsolated yield; cusing bare MNPs as
catalyst; drecycling experiment using recovered MNPs as catalyst
Further, we evaluated the recycling of the recovered
PAM@MNPs organocatalyst for aza-Michael addition of
aniline with methyl acrylate as representative substrates. After
completion of the reaction the catalyst could easily be
separated with the help of an external magnet as shown in
Figure 5.
o
temperature (140 C) under microwave irradiation using water
as reaction media. In contrast, in the present study, the reaction
o
has been carried out at lower temperature i.e. 65 C under
solvent less conditions. These benefits of the present study
establish its superiority over the existing report.
Fig. 5: Separation of catalyst using external magnet

5
In a typical synthesis, 2 g of Fe3O4 nanoparticles were dispersed in 30
mL toluene by sonication for about h, and then 4mL of 3-
1
The recovered catalyst was washed with methanol, dried
and reused for subsequent runs under described optimized
reaction conditions. The recyclability of the catalyst was
checked for five runs. In all experiments the reaction time and
yield of desired product remained almost similar as shown in
Figure 6, establishing the efficient recycling of the catalyst.
(trimethoxysilyl) propylmethacrylate (silane-A) is added under the inert
atmosphere. The reaction mixture was stirred for 24 h under refluxing
condition. Afterwards, the silane A-functionalized Fe3O4 nanoparticles
obtained were separated by magnet, washed with ethanol, water and then
dried in a vacuum oven at 80 °C overnight.
Synthesis of polyacrylamide coated Fe3O4 nanoparticles
(PAM@MNPs)
Surface-initiated radical polymerization was performed to functionalize
silane A-modified Fe3O4 nanopartocles with polyacrylamide. In a typical
procedure, 1 g of silane A-modified Fe3O4 nanoparticles, 0.02 g of AIBN,
30 mL of ethanol and 10 mL of distilled water were all put together in a
three necked round bottom flask and sonicate it for 30 min to get uniform
dispersion and then heated at 70°C, with constant stirring under inert
atmosphere. Then 1g acrylamide dispersed in 20 mL ethanol and 10 mL
distilled water was added drop wise to the above reaction mixture and
again stirred for 4h in inert atmosphere. Finally the resultant product was
collected by magnetic separation, washed with ethanol several times, and
dried at 80°C in a vacuum oven.
General Procedure for the Synthesis of aza-michael product
Aniline (1 mmol), methyl acrylate (1.5 mmol) and catalyst (100 mg) were
o
placed into a round bottom flask and stirred at 60 C for 4 h. The progress
of the reaction was monitored with thin-layer chromatography (TLC).
After completion of the reaction, the reaction mixture was cooled to room
temperature and the catalyst was recovered by external magnet. The
reaction mixture was concentrated under reduced pressure and the crude
product was purified by column chromatography. Conversion of the
substrates and selectivity of the products was determined by GCMS. The
recovered catalyst was dried at 50 °C for 2 h and can be reused for
recycling experiments.
Fig. 6: Results of recycling experiment
Conclusion
A
novel magnetically separable organocatalyst i.e.
polyacrylamide coated magnetic nanoparticles was synthesized
by following simple two step strategy involving
a
1.
2.
3.
4.
5.
6.
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We acknowledge Director, CSIR-Indian Institute of
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Analytical science division is highly acknowledged for
providing characterisation results of the material. VP thanks
Council of Scientific and Industrial Research (CSIR) for
providing fellowship in the form of Senior Research
Fellowship (SRF).
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7
Highlights of the Research
 Polymer coated magnetically separable organocatalyst for C-N bond formation
 Prior Functionalization of MNPs for covalent immobilization of polyacrylamide.
 Synthesized organocatalyst was used for C-N bond formation via aza-Michael addition
 Magnetic organocatalyst was easily recovered by influence of the external magnet.
 Magnetic organocatalyst provided consistent activity for several runs.