Magnetic nanoparticles coated by acidic functionalized poly(amidoamine) dendrimer: Effective acidic organocatalyst

Article abstract of DOI:10.1016/j.catcom.2012.08.018

A novel magnetic Br?nsted acid catalyst was synthesized based on growing poly(amidoamine) dendrimers on the surface of magnetic nanoparticles. After the dendronizing process, the MNP coated PAMAM was functionalized by chlorosulfuric acid to form an acid catalyst. Because of dendrimer coating of MNPs, catalyst shows good loading level of acidic groups on the surface. Also zwitterion nature of catalyst surface improves the catalytic activity. This new catalyst is proven to be highly effective in the synthesis of α-aminophosphonate compounds in a green way.

Full text of DOI:10.1016/j.catcom.2012.08.018

Catalysis Communications 28 (2012) 86–89
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Magnetic nanoparticles coated by acidic functionalized poly(amidoamine)
dendrimer: Effective acidic organocatalyst
⁎
Ali Pourjavadi , Seyed Hassan Hosseini, Seyedeh Talieh Hosseini, Seyyed Alireza Aghayeemeibody
Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2012
Received in revised form 12 August 2012
Accepted 13 August 2012
Available online 21 August 2012
A novel magnetic BrØnsted acid catalyst was synthesized based on growing poly(amidoamine) dendrimers on
the surface of magnetic nanoparticles. After the dendronizing process, the MNP coated PAMAM was func-
tionalized by chlorosulfuric acid to form an acid catalyst. Because of dendrimer coating of MNPs, catalyst shows
good loading level of acidic groups on the surface. Also zwitterion nature of catalyst surface improves the
catalytic activity. This new catalyst is proven to be highly effective in the synthesis of α-aminophosphonate
compounds in a green way.
Keywords:
© 2012 Elsevier B.V. All rights reserved.
Magnetic nanoparticles
Poly(amidoamine) dendrimer
BrØnsted acid catalyst
α-Aminophosphonate
1. Introduction
find new protocols for synthesis of novel heterogeneous catalysts to
overcome these problems. A rationalized choice for solving this prob-
Despite of the lower activity of heterogeneous catalysts than ho-
mogenous ones, in the last decade, a lot of efforts have been done to
develop novel methods for heterogenizing homogeneous catalysts in
order to combine the advantages of both homogeneous and het-
erogeneous catalysts [1–5]. In order to achieve this matter, homoge-
neous catalysts were immobilized on the surface of solid supported
materials such as mesoporous silica [6–8], polymers [9,10] and mag-
netic nanoparticles [11,12]. Among the solid support materials, mag-
netic nanoparticles (MNPs) were more interesting for immobilization
of homogeneous catalysts, because their magnetic response causes
simple separation of catalyst by using an external magnet. Although
MNPs always tend to agglomerate due to their dipole–dipole in-
teraction, it was found that coating of MNPs by polymers could help
to prevent their aggregation and improve their chemical stability
[13].
However these heterogenized catalysts can be easily separated
from the reaction mixtures, but their activity is significantly less than
their homogeneous counterparts. The activity of heterogeneous cat-
alysts strongly depended on the loading amount of their immobilized
homogenous part. Moreover the short lengths of soluble homogenous
part (spacer) on the solid catalyst surface make difficult interaction
between substrates and immobilized homogenous catalyst. The prob-
lem of low activity could be overcome by using a longer spacer group
with the multi catalytic site. For these reasons, there is still a need to
lem in the heterogeneous catalysts is immobilization of polymers,
especially dendrimers on the solid surface.
Dendrimers, a class of macromolecules with highly branched
structure and globular shape, possess unique properties such as high
density of active groups, good structural homogeneity, intense inter-
nal porosity and good biocompatibility [14]. Among the various types
of dendrimers, poly(amidoamine) (PAMAM) dendrimers are partic-
ularly interesting because of their globular structure, mimicking the
three-dimensional structure of biomacromolecules, and their good
biocompatibility [15]. PAMAM dendrimers in the homogenous forms
[16–18] or the immobilized heterogeneous forms [19–21] have been
used intensively as catalyst in the several catalytic organic transforma-
tions. Interestingly, in many cases, the dendritic catalysts were found to
be more efficient or selective than the traditional analog catalyst.
Herein, we describe synthesis of a novel magnetic organocatalyst
which magnetic Fe3O4 nanoparticles were coated by sulfamic acid
functionalized PAMAM dendrimer. The synthesized catalyst was used
in the synthesis of α-aminophosphonate in the mild reaction con-
dition and excellent yields.
α-Aminophosphonates are an important class of compounds in
modern pharmaceutical chemistry [22–24] and their synthesis has
received considerable attention because of their structural analogy to
α-amino acids. Therefore, a number of methods have been developed for
synthesis of α-aminophosphonate family. These methods include Lewis
metal acids, such as FeCl3 [25], LiClO4 [26], InCl3 [27], ZrCl4 [28], SbCl3/
Al2O3 [29], SiO2@TaCl5 [30] and BrØnsted acids such as Amberlite-IR 120
[31], CF3CO2H [32] and sulfamic acid [33]. However, many of these
methods have some drawbacks such as using expensive, toxic and
⁎
Corresponding author. Tel./fax: +98 2 166165311.
E-mail address: purjavad@sharif.edu (A. Pourjavadi).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.08.018

A. Pourjavadi et al. / Catalysis Communications 28 (2012) 86–89
87
moisture sensitive catalysts, long reaction times, low yield of pro-
ducts and tedious separation procedures. Therefore, the develop-
ment of an effective, convenient, and green protocol for synthesis of α-
aminophosphonates is still a challenge.
To growing dendrimer on the surface of MNPs, the surface of
nanoparticles should have amine groups for initiation. Amine func-
tionalization of MNP surface was performed by (3-aminopropyl)
triethoxysilane under reflux condition in ethanol/water mixture for
24 h.
Propagation of PAMAM dendrimer onto the surface of MNPs, up to
the third generation, was prepared using standard methods intro-
duced by Tomalia [35] and co-workers (Scheme 1). The amino groups
on the surface of MNPs react with methyl acrylate in a Michael-type
addition reaction to form the amino propionate ester. Subsequent
amidation of the ester groups with excess amount of ethylenediamine
completes the first generation. Repetition of these two reactions forms
the next generation of PAMAM dendrimer. Simple magnetic filtration
and solvent rinsing served to remove excess reagents.
After preparation of PAMAM (G3) coated MNPs (MNP@PAMAM),
an excess amount of chlorosulfuric acid was added to a dispersed
dichloromethane solution of MNP@PAMAM for two purposes: (1)
conversion of terminal amine groups of PAMAM to sulfamic acid and (2)
the neutralization of inner tertiary amine groups in PAMAM dendrimer.
The nucleophilic attack of terminal amine groups to ClSO3H and sub-
stitution with chloride will result in sulfamic acid functions. Tertiary inner
amine groups initially react with ClSO3H in an acid–base reaction type. In
this step, counterion of neutralized tertiary inner amine groups will be
ClSO3− or released Cl−. Although, the excess amount of chlorosulfuric acid
2. Result and discussion
2.1. Synthesis of MNP@DSO3H
The magnetic nanoparticles (MNPs) were synthesized by co-
precipitation method in the basic condition. A solution of iron (II)
and iron (III) was vigorously stirred (700 rpm) in deionized water under
N2 atmosphere. Then, ammonia solution was dropwise added to the
mixture at 70 °C, immediately followed by the addition of NH3 solution;
MNP black precipitate was formed. Silica coating is one of the most ideal
methods for protection of MNPs, due to its high chemical and thermal
stabilities, large surface areas and good compatibilities [34]. For silica
coating procedure, the resulted MNPs were ultrasonically suspended in
ethanol/water mixture and pH of solution was adjusted on 10 by adding
ammonium solution. Tetraethyl orthosilicate (TEOS) was dropwise added
to the solution at 50 °C in the presence of a constant nitrogen flux. The
mixture was stirred for 6 h to allow the silica shell to grow on the surface
of the nanoparticles.
Scheme 1. Synthetic route to catalyst.

88
A. Pourjavadi et al. / Catalysis Communications 28 (2012) 86–89
will cause chloride anion replacement with ClSO3−. After washing several
times with water, H2O molecules react with ClSO3− anions and convert
them to HSO4− anions. AgNO3 test shows that there is no chloride ion
(b2%) in the final catalyst structure.
Table 1
Control experiments in synthesis of α-aminophosphonate.a
Entry Catalyst
Amount of
Solvent Time
(min) (%)
Yieldb TOF
catalyst (mg)
(h−1
)
1
2
3
4
5
6
7
8
–
–
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
CH2Cl2 45
CH3CN 45
Et2O
THF
3 h
120
60
25
25
25
25
40
1b
18
78
95
94
95
91
79
90
86
57
71
42
31
63
87
–
2.2. Catalyst activity of MNP@DSO3H
Fe3O4@SiO2
20
–
MNP@(CH2)3NHSO3Hc 30
50.0
MNP@DSO3Hd
MNP@DSO3H
MNP@DSO3H
MNP@DSO3He
MNP@DSO3Hf
MNP@DSO3He
MNP@DSO3He
MNP@DSO3He
MNP@DSO3He
MNP@DSO3He
Amberlite-IR 120e
H2SO4
20
10
5
5
5
5
5
5
5
107.5
212.8
430.2
1236.2
1117.9
679.2
649.0
430.2
535.1
316.9
4.07
To investigate the catalyst activity of MNP@DSO3H, we choose the
synthesis of α-aminophosphonate by three-component reaction of
various aldehydes, amines and triethyl phosphite (Fig. 1).
Table 1 shows control experiment and optimization of reaction
conditions for synthesis of α-aminophosphonate. Reaction between
benzaldehyde, aniline and triethyl phosphite was chosen as model
reaction. As seen in Table 1, the reaction does not proceed without
any catalyst at room temperature. Using Fe3O4@SiO2 as catalyst at
room temperature gave only 18% yield in 2 h. For the activity com-
parison we used homogenous sulfamic acid and sulfamic acid func-
tionalized MNP and the results are shown in Table 1. When MNP@DSO3H
was used in the reaction medium, yield of reaction was increased to 95%
in 25 min at room temperature under solvent free condition. Comparison
between Entries 4 and 5 shows that the dendrimer coated catalyst is
more active than sulfamic acid functionalized MNP. It may correspond to
the zwitterion nature of MNP@DSO3H surface. To optimize the catalyst
loading in the reaction we reduced the amount of catalyst and it was
found that 5 mg of catalyst was enough for 3 mmol scale of reaction. For
optimization of reaction solvent, similar condition was performed in
various solvents, but solvent free condition gave better yields. As seen in
Table 1, in the optimum condition a good turn over frequency (TOF) is
obtained.
9
10
11
12
13
14
15
16
45
45
5
Hexane 45
100
10
10
Neat
Neat
Neat
45
45
120
8.23
4.21
NH2SO3H
a
Reaction condition: benzaldehyde (1 mmol), aniline (1 mmol) and triethyl phosphite
(1.3 mmol) at room temperature.
b
Isolated yield.
Loading amount of protons is 0.73.
Loading of protons in MNP@ DSO3H was 1.06 mmol/g.
3 mmol scale.
c
d
e
f
5 mmol scale.
3. Conclusion
In conclusion, we have developed green magnetic catalyst for the
synthesis of α-aminophosphonate in solvent free condition at room
temperature. Because of the ionic surface of MNP@DSO3H, dendrimer
coated catalyst shows high catalytic activity. Increasing the catalytic
functional groups on the surface of MNPs by generation of PAMAM
caused MNP@DSO3H and was used in the low weight percent com-
pared with substrates. Catalyst was recycled and reused for seven
times without considerable loss of catalytic activity. Also, easy magnetic
separation of the catalyst is an additional suitable aspect of our catalyst.
Subsequently, we investigated the diversity of MNP@DSO3H in the
synthesis of α-aminophosphonate. Reaction was performed under the
optimized condition with the various aromatic and aliphatic aldehydes
and amines and the corresponding α-aminophosphonate products were
obtained in good to excellent yields (Table 2). Hindered aldehydes and
highly deactivated aldehydes were also reacted under the same condition
and obtained good yields.
2.3. Catalyst recycling
Table 2
The recyclability of the MNP@DSO3H was investigated in the
syntheses of α-aminophosphonate by choosing a reaction between
benzaldehyde, aniline and triethyl phosphite as modeling reaction at
room temperature and solvent free condition. After completion of the
reaction, monitored by TLC, catalyst was magnetically separated (Fig. 2)
and reused in another reaction vessel under the same condition.
As seen in Fig. 3 even after seven cycles, products were obtained in
high yield.
Synthesis of α-aminophosphonate catalyzed by MNP@DSO3H.a
Entry
R1CHO
R2NH2
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
ph-
ph-
ph-
ph-
ph-
ph-
ph-
ph-
ph-
25
30
25
35
35
20
20
30
20
35
20
25
25
40
30
30
35
2 h
25
30
35
40
20
35
30
91
90
92
89
88
95
93
88
92
79
92
86
91
89
90
90
82
N.R.
90
85
90
86
94
92
90
4-(OH)ph-
4-(Cl)ph-
4-(Me)ph-
4-(OMe)ph-
4-(NO2)ph-
3-(NO2)ph-
2-(NO2)ph-
Furfuryl-
isobutyr-
Morpholine
ph-
ph-
ph-
ph-
ph-
ph-
ph-
ph-
9
ph-
ph-
ph-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4-(Br)ph-
4-(Me)ph-
4-(NO2)ph-
4-(Cl)ph-
3-(Cl)ph-
2-(Cl)ph-
4-(Cl),2-(NO2)ph-
Cyclohexyl-
4-(NO2)ph-
4-(Br)ph-
4-(NO2)ph-
4-(OMe)ph-
4-(Cl)ph-
4-(Cl)ph-
4-(Cl)ph-
4-(Cl)ph-
4-(NO2)ph-
4-(NO2)ph-
Furfuryl-
4-(OMe)ph-
a
Reaction condition: aldehyde (3 mmol), amine (3 mmol), triethyl phosphite (3.5 mmol)
and MNP@DSO3H (5 mg), room temperature, solvent free.
b
Fig. 1. Synthesis of α-aminophosphonate catalyzed by MNP@DSO3H.
Isolated yield.

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89
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.08.018.