Pyridine-grafted graphene oxide: a reusable acid-base bifunctional catalyst for the one-pot synthesis of β-phosphonomalonates via a cascade Knoevenagel-phospha Michael addition reaction in water

Article abstract of DOI:10.1039/c5ra13083b

A new acid-base bifunctional catalyst was synthesized by postgrafting of pyridine onto GO. This new catalyst was characterized by different methods such as FT-IR, XRD, TGA, TEM, SEM, EDS, UV-Vis and elemental analysis and employed as a reusable acid-base bifunctional catalyst for the one-pot synthesis of β-phosphonomalonates via a cascade Knoevenagel-phospha Michael addition reaction of aldehydes, malononitrile and dialkyl phosphites in water.

Full text of DOI:10.1039/c5ra13083b

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PAPER
Pyridine-grafted graphene oxide: a reusable acid–
base bifunctional catalyst for the one-pot synthesis
of b-phosphonomalonates via a cascade
Knoevenagel–phospha Michael addition reaction
in water†
Cite this: RSC Adv., 2015, 5, 96532
*
Sara Sobhani and Farzaneh Zarifi
A new acid–base bifunctional catalyst was synthesized by postgrafting of pyridine onto GO. This new
catalyst was characterized by different methods such as FT-IR, XRD, TGA, TEM, SEM, EDS, UV-Vis and
elemental analysis and employed as a reusable acid–base bifunctional catalyst for the one-pot synthesis
of b-phosphonomalonates via a cascade Knoevenagel–phospha Michael addition reaction of aldehydes,
malononitrile and dialkyl phosphites in water.
Received 5th July 2015
Accepted 2nd November 2015
DOI: 10.1039/c5ra13083b
www.rsc.org/advances
reaction is impossible due to the neutralization that forms
inactive salts. In addition, recovery and reuse of both catalytic
species are oen problematic in homogeneous systems.8 To
Introduction
The “greening” of global chemical processes has become an
important task of modern synthetic chemistry. One major
drawback in modern chemical production processes is the use
of highly toxic and ammable organic solvents.1 Learning from
living systems, there has been a growing interest in the devel-
opment of water medium organic synthesis since water is safe,
renewable, nontoxic and nonammable.2 Moreover, the organic
synthesis process is usually comprised of iterative reactions.3
The iterative reaction is generally conducted individually and
step-by-step with the corresponding catalyst. Aer each step of
reaction, the product and catalyst are isolated and puried,
followed by adding a new catalyst and other reactants as well as
solvent media for the subsequent step of the cascade reaction.4
Obviously, this is the most time- and cost-demanding and
waste-producing manual operation.5 To reduce the number of
purication steps and improve overall efficiency, the use of one-
pot chemical reactions has been recognized as a promising
approach. However, the difficulties associated with the design
of well-controlled isolated catalytic centres which are able to
catalyze each reaction step efficiently, limit the wide range of
general applications for one-pot chemical processes.6 Learning
from enzymes, most studies have focused on homogeneous
catalysts with multiple kinds of active sites for synergistically
driving one-pot cascade reactions.7 Obviously, the use of both
strong acidic and basic species in a homogeneous one-pot
address this dilemma, heterogeneous catalysts recently attrac-
ted much attention for their remarkable advantages of easier
recycling and xing acidic and basic groups to avoid neutrali-
zation reaction.9 Solid matrixes such as silica, clay, and polymer
are the common solid materials which have been used for
hosting acidic and basic functions.10 There are several evident
barriers in the synthesis of these bifunctional catalysts such as
the tedious protection–deprotection procedures or the uncon-
trollable and inhomogeneous positioning of catalytically active
species.11 Therefore, the development of a simple and efficient
method for the fabrication of robust acid–base bifunctional
heterogeneous catalysts to synthesis complex molecules in one
step is urgently needed.
Graphene and related two-dimensional sp2-hybridized
materials have invoked incredible research interest owing to
their inherent optical, electrochemical and mechanical prop-
erties.12 Graphene oxide (GO), the oxygenated graphene sheets
covered with hydroxyl, epoxy and carboxyl groups offers excel-
lent aqueous dispersity, acidic properties and tremendous
opportunities for further chemical modications.13 Notably, GO
exhibits as solid acid owing to the existence of carboxylic acids
on the edges. Taking advantage of the presence of intrinsic
carboxylic acids on GO, acid–base bifunctional GO catalysts
have been synthesized by a simple postgraing of basic func-
tional groups onto GO.14 The acid–base bifunctional GO cata-
lysts have exhibited excellent activity in one-pot cascade
reactions owing to the unique two dimensional structure with
low mass transfer limitation and suitable surface acidity of GO.
Meanwhile, these heterogeneous catalysts could be easily recy-
cled and reused repetitively.15 However, despite the excellent
Department of Chemistry, College of Sciences, University of Birjand, Birjand, Iran.
E-mail: ssobhani@Birjand.ac.ir; sobhanisara@yahoo.com; Fax: +98 56 32202065;
Tel: +98 56 32202065
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra13083b
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catalytic activity and reusability of bifunctional GO catalysts, spectroscopy (EDS), ultraviolet-visible spectroscopy (UV-Vis)
rare studies reaches GO functionalization and catalytic perfor- and elemental analysis and employed as a reusable acid–base
mances in cooperative catalysis, even though this area has bifunctional catalyst for the one-pot synthesis of b-phospho-
immense potential.
nomalonates via cascade Knoevenagel–phospha Michael addi-
Within organophosphorus compounds, phosphonates are tion reaction in water.
an important class of molecules. They have broad applications
in catalysis,16 medicinal,17 and material chemistry.18 There are
Results and discussion
several methods for the synthesis of phosphonates. Among
various methods to generate P–C bonds, the conjugate addition
of phosphorus nucleophiles to activated alkenes (phospha-
Michael reaction) holds a fundamental position in the phos-
phonate synthesis.19 These protocols proceed through two-pot
procedure in which P–C and C–C bond formations occur in
separate steps. Actually, these methods are not multi-
component reactions and use of a Knoevenagel adduct as
starting material is required.20 Over the last few years, coupling
of P–C and C–C bond forming events during one-pot phos-
phonate synthesis has been attracted much attention in
organophosphorus synthesis. To date, Brønsted/Lewis acids,
bases, ionic liquids and micellar solution have been reported
for the one-pot synthesis of b-phosphonomalonates via cascade
Knoevenagel–phospha Michael addition reaction.21 Even
though, phospha-Michael addition could be proceed by these
methods, most of the existing protocols suffer from limitations
such as low yields, long reaction times, using hazardous organic
solvents or unrecyclable and/or expensive catalysts and tedious
work-up procedures leading to copious amount of toxic wastes.
Additionally, in most of these methods, poisonous and nasty
smell trialkyl phosphites is used as the phosphorus nucleophile
and few example employ dialkyl phosphites, which fulls the
criteria of atom economy.21g,h,22 More importantly, in spite of the
prominence of bifunctional catalysts for efficient promotion of
one-pot cascade reactions, the reports for one-pot synthesis of
b-phosphonomalonates catalyzed by bifunctional catalysts are
rare in the literature.20b,22 Therefore, a simple and green proce-
dure for one-pot synthesis of b-phosphonomalonates catalyzed
by an efficient, inexpensive and reusable bifunctional catalyst
employing diethyl phosphite as the phosphorous component
remains a major challenge in organophosphorus synthesis.
In this paper, in continues of our recent works on the
development of new heterogeneous catalysts23 and to benet
the efficiency of bifunctional catalysts in one-pot cascade reac-
tions, we report the synthesis of pyridine-graed GO as a new
acid–base bifunctional catalyst (Py–GO, Scheme 1) by post-
graing of pyridine onto GO. This new catalyst was character-
ized by different methods such as Fourier transform infrared
spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravi-
metric analysis (TGA), transmission electron microscopy (TEM),
scanning electron microscopy (SEM), energy dispersive
Synthesis and characterization of Py–GO
Pyridine-graed GO (Py–GO) was synthesized by the procedure
outlined in Scheme 2. As indicated in Scheme 2, GO was reacted
with an appropriate concentration of 3-mercaptopropyl trime-
thoxysilane to give mercapto-functionalized GO. The obtained
mercapto-functionalized GO was subjected to react with 3-
(chloromethyl)pyridine hydrochloride to obtain Py–GO. The
synthesized Py–GO was characterized by FT-IR, XRD, TGA, TEM,
SEM, EDS, UV-Vis and elemental analysis.
FT-IR spectra of GO and Py–GO are shown in Fig. 1a and b,
respectively. Vibration peaks with a strong intensity at 3400,
1710, 1610, 1200, and 1050 cmꢀ1 in the FT-IR spectrum of GO
were assigned to the stretching vibrations of O–H, C]O, C]C,
C–OH and C–O–C bonds, respectively. These peaks reveal the
presence of hydroxyl, carboxyl, and epoxy groups on the GO. In
the spectrum of Py–GO, typical bands at about 2900 and 2830
attributed to C–H stretching vibrations from the silane chain. A
peak at 1570 cmꢀ1 was observed, which was related to the
stretching vibrations of C]C bonds. Si–O–Si and Si–O–C
stretching modes of silane chain were indicated as a strong
broad doublet at about 1110 and 1030 cmꢀ1. An absorption
band at 1430 cmꢀ1 due to the ring stretching vibration of C]N
was also seen (Fig. 1b). These results indicated that pyridine was
successfully graed onto GO.
The XRD pattern of GO (Fig. 2a) shows a sharp peak at 2q ¼
11.9ꢁ with d-spacing of 0.73 nm, which can be ascribed to the
(0 0 1) of GO, suggesting that graphite powder was oxidized to
GO. As reported usually, the natural graphite reveals a basal
reection (0 0 2) peak at 2q ¼ 26.6ꢁ.24 In Py–GO, the diffraction
peak (0 0 1) became very weak and broad appeared at 2q ¼ 8.2ꢁ
Scheme 1 Pyridine-grafted GO (Py–GO).
Scheme 2 Synthesis of Py–GO.
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The TEM image of Py–GO is shown in Fig. 4a. It is evident
from Fig. 4a that the immobilized catalyst has a layered struc-
ture. The crumpled nanosheet can be seen in both TEM and
SEM images (Fig. 4b and c) due to an agglomerated phase. EDS
mapping demonstrates that S, O, and N elements (Fig. 4d–f) are
homogeneously distributed on the surface of Py–GO, which
indicates that sulfur and pyridine groups are homogeneously
anchored on GO. The crumpling characteristic of the Py–GO,
along with the homogeneous distribution of the active sites
should be benet for its catalytic performances, because reac-
tants and products can easily access or leave the active sites on
both sides of the two-dimensional with limited mass transfer
resistance.
Fig. 1 FT-IR spectra of (a) GO and (b) Py–GO.
The UV-Vis spectrum of GO (Fig. 5a) shows a strong
absorption peak at 230 nm, which is attributed to the p / p*
transition of graphite C–C bonds and a shoulder at ꢂ300 nm is
assigned to the n / p* transition of C]O. The loading of
pyridine leads to signicant changes in the absorption spec-
trum of GO (Fig. 5b). An absorption peak at 230 nm in GO was
shied to 280 nm in Py–GO revealing the graing of pyridine
onto GO.
One-pot synthesis of b-phosphonomalonates catalyzed by Py–
GO
Fig. 2 XRD patterns of (a) GO and (b) Py–GO.
In continuous of our recent research programs on the develop-
ment of novel methods for the synthesis of phosphonate deriv-
atives,21a–d,i–k,26 in this paper, we have studied the applicability of
pyridine graed GO (Py–GO) as a new acid–base bifunctional
catalyst for the one-pot synthesis of b-phosphonomalonates via
with d-spacing of 1.0 nm (Fig. 2b), which indicates that the
organic groups were successfully tethered onto the GO. Another
weak and broad diffraction peak centred at around 22.5ꢁ
emerged for Py–GO, indicating that a part of the GO was
changed into graphene.25
The thermal decomposition of Py–GO and GO was examined
by thermogravimetric analysis (Fig. 3a and b). As shown in
Fig. 3a, Py–GO has a 2.6% mass loss below 145 ꢁC, resulting
from the evaporation of adsorbed water and a 6.8% mass loss
ꢁ
from 145 to 280 C, which is due to the removal of the oxygen-
containing functional groups. The mass loss of GO is obviously
higher than that of Py–GO (Fig. 3b), especially the second mass
loss, which demonstrates an obvious decrease in the amount of
oxygen-containing functional groups and a profound reduction
of GO in Py–GO. The last mass loss of 13.4% is found around
400 ꢁC in the TGA of Py–GO that is caused by the decomposition
of organic parts located on its surface.
Elemental analysis showed that loading of pyridine on the
GO was 0.9 mmol gꢀ1 based on nitrogen content (N ¼ 1.27%).
Fig. 4 (a) TEM image, (b and c) SEM images of Py–GO and the cor-
Fig. 3 TGA curve of (a) Py–GO and (b) GO.
96534 | RSC Adv., 2015, 5, 96532–96538
responding quantitative EDS element mapping of (d) S, (e) O and (f) N.
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one-pot cascade reaction, clearly demonstrates a cooperative
behaviour of intrinsic carboxylic acids and pyridine active sites
on the same framework in Py–GO.
To conrm the generality of the present method, we next
examined the reaction of a variety of aldehydes with malono-
nitrile and diethyl phosphite under the optimized reaction
conditions. The results of this study are depicted in Table 2.
As indicated in Table 2, benzaldehyde and different
substituted benzaldehydes containing electron-donating and
electron-withdrawing groups underwent one-pot reaction and
gave the corresponding b-phosphonomalonates in good to high
yields (entries 1–10). The reaction of aliphatic aldehydes pro-
ceeded well and produced the desired products in good yields in
longer reaction time, probably due to their relatively weaker
electrophilicity (entries 11 and 12).
Fig. 5 UV-Vis spectra of (a) GO and (b) Py–GO.
cascade Knoevenagel–phospha Michael addition reaction of
aldehydes, malononitrile and diethyl phosphite.
Furthermore, we have evaluated the generality of the pre-
sented method for the one-pot synthesis of b-phosphonomalo-
nates from 4-chlorobenzaldehyde with different phosphite
esters and active methylene compounds (Table 3). As it is shown
in Table 3, both the yields and the reaction time of 4-chlor-
obenzaldehyde and malononitrile with different phosphite
esters are relatively dependent of the phosphorus compound
(entries 1–5). The reaction proceeded with higher yield in
shorter reaction time using dialkyl phosphites (entries 1 and 2)
compared with trialkyl phosphites (entries 3–5). The higher
activity of dialkyl phosphites is might be due to the role of the
basic sites of Py–GO catalyst to accelerate phospha-Michael
addition step by P–H bond deprotonation. In addition to
malononitrile, some other active methylene compounds were
also examined to carry out the reaction with 4-chlor-
obenzaldehyde and diethyl phosphite (entries 6–8). The results
showed that the reaction involving ethyl cyano acetate worked
well and the desired product was obtained in 70% yield.
At rst, one-pot reaction of benzaldehyde, malononitrile and
diethyl phosphite was chosen to optimize the reaction condi-
tions such as solvents and molar ratio of the catalyst at 100 C
ꢁ
(Table 1). We have found that in the presence of 3 mol% of Py–
GO, the corresponding b-phosphonomalonate was obtained in
ꢁ
the best yield at 100 C under aqueous conditions (entry 2). By
lowering the amount of the catalyst to 1 mol%, the desired
product was obtained in lower yield (entry 7). A similar reaction
at a lower temperature required longer reaction time and
produced the desired product in lower yield (Table 1, entry 8). In
order to show the role of Py–GO as an acid–base bifunctional
catalyst, similar reactions in the absence of the catalyst and in
the presence of G, GO, pyridine and a mixture of pyridine and
GO were also examined (Table 1, entries 9–13). It was found that
no reaction was occurred in the absence of any catalyst, and
lower yields (20–65%) of the desired product were achieved by
G, GO (with acidic sites), pyridine (as a base) and a mixture of
pyridine and GO. The high catalytic activity of Py–GO for this
Table 2 Synthesis of a variety of b-phosphonomalonates catalyzed by
Py–GO
Table 1 One-pot reaction of benzaldehyde, malononitrile and diethyl
phosphite under different conditions
Entry Catalyst
Solvent Time (h) Yielda (%)
Entry
Aldehyde
Product
Time (h)
Yielda (%)
1
2
3
4
5
6
7
Py–GO (3 mol%)
DMF
H2O
—
EtOH
CH3CN
Toluene
H2O
H2O
H2O
H2O
H2O
1
1
2
2
2
2
1
2
3
3
3
3
3
60
90
78
73
52
30
80
75
—
20
53
30
65
Py–GO (3 mol%)
Py–GO (3 mol%)
Py–GO (3 mol%)
Py–GO (3 mol%)
Py–GO (3 mol%)
Py–GO (1 mol%)
Py–GO (3 mol%)
—
G (3 mol%)
GO (0.03 g)
Pyridine (27 mL)
Pyridine (27 mL) + GO (0.03 g) H2O
1
2
3
4
5
6
7
8
Benzaldehyde
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
1
1
2
3
3
90
80
73
78
87
80
73
82
68
85
75
70
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
3-Methoxybenzaldehyde
4-Chlorobenzaldehyde
3-Chlorobenzaldehyde
2-Chlorobenzaldehyde
4-Bromobenzaldehyde
3-Bromobenzaldehyde
4-Hydroxybenzaldehyde
Butyraldehyde
b8
c9
10
11
12
13
9
9
10
11
12
10
11
12
H2O
Heptanaldehyde
a
a
Isolated yield. Reaction conditions: benzaldehyde (1 mmol),
Isolated yield. Reaction conditions: aldehyde (1 mmol), malononitrile
malononitrile (1 mmol), diethyl phosphite (2 mmol), catalyst, (1 mmol), diethyl phosphite (2 mmol), catalyst (3 mol%), H2O (3 mL),
b
c
temperature ¼ 100 ꢁC. Temperature ¼ 80 ꢁC. No catalyst.
100 ꢁC. All of the products are known.20b,e,21c,g,k
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Table One-pot synthesis of b-phosphonomalonates from 4-
Paper
3
chlorobenzaldehyde with different phosphite esters and active
methylene compounds catalyzed by Py–GO
Entry Phosphite ester
X
Y
Product Time (h) Yielda (%)
Fig. 7 FT-IR spectrum of Py–GO after six times reuse.
1
2
3
4
5
6
7
8
HP(O)(OEt)2
HP(O)(O-iso-Pr)2 CN
P(OMe)3
P(O-iso-Pr)3
P(OEt)3
HP(O)(OEt)2
HP(O)(OEt)2
HP(O)(OEt)2
CN
CN
CN
CN
CN
CN
5
1
1
1
3
1
2
24
24
87
80
68
50
65
70
13
14
13
5
CN
CN
CN
CN
reaction of benzaldehyde, malononitrile and diethyl phosphite
in water at 100 C, the mixture was centrifuged to remove the
ꢁ
solid catalyst. Then the mother solution was allowed to react for
another 3 h under the same reaction conditions. In the absence
of the catalyst, any increase in the conversion value was not
detected. It demonstrated that any pyridine leaching from Py–
GO to the solution was not occurred and the present catalysis
was heterogeneous in nature.
CO2Et 15
b
CO2Et CO2Et
NO2
—
—
—
0
H
a
Isolated yield. Reaction conditions: 4-chlorobenzaldehyde (1 mmol),
active methylene group (1 mmol), phosphite ester (2 mmol), catalyst
b
(3 mol%), H2O (3 mL), 100 ꢁC. All of the products are known.19a,20b,21g Trace.
A plausible mechanism for the one-pot reaction of aldehydes,
malononitrile and diethyl phosphite in the presence of Py–GO is
However, no product was produced when diethyl malonate or depicted in Scheme 3. The process represents a typical cascade
nitromethane were used in this one-pot reaction under the reaction by double-activation. In the initial step of the catalytic
same conditions.
Because the recycling of the catalyst are important issues
from both practical and economical viewpoints, we next inves-
cycle, the acid sites present in the GO activate the carbonyl group
in the aldehyde and the basic sites activate the malononitrile to
form the Knoevenagel product. In the second step, the cyanogroup
tigated the reusability of the Py–GO catalyst in the reaction of in the Knoevenagel product were activated by the acid sites in the
benzaldehyde, malononitrile with diethyl phosphite under the catalyst. Diethyl phosphite was deprotonated by basic sites of Py–
optimized reaction conditions (Fig. 6). Aer 1 h in each run, the GO. The activated a, b-unsaturated malonate underwent Michael
catalyst was easily recovered from the reaction mixture by addition with deprotonated phosphorus nucleophile to give the
centrifugation and reused aer washing with EtOAc and drying. corresponding b-phosphonomalonate.
As seen in Fig. 6, Py–GO showed excellent catalytic performance
upon at least six runs. FT-IR spectrum of the catalyst aer six
times reuse (Fig. 7) was compared with that of the fresh catalyst
In order to show the cooperative behaviour of intrinsic
carboxylic acids and pyridine active sites on the same framework
of Py–GO in these reactions, we have performed the one-pot
(Fig. 1b). Comparison of the FT-IR spectra of the fresh and reaction of benzaldehyde, malononitrile, and diethyl phosphite
reused catalyst persuasively validates that the structure of the in the presence of a catalytic amount of solid catalysts (3 mol%)
Py–GO remains unchanged aer the coupling reaction (Fig. 7).
Furthermore, elemental analysis of Py–GO was carried out aer
six times reuse and the results showed that the loading amount
of the catalyst remained intact.
To check whether any pyridine leached out from the solid
catalyst to the solution, aer proceeding of 60% of the coupling
Fig. 6 Reusability of the Py–GO in the coupling reaction of benzal-
dehyde, malononitrile and diethyl phosphite under the optimized Scheme 3 A plausible mechanism for the one-pot reaction of alde-
reaction conditions.
hydes, malononitrile and diethyl phosphite catalyzed by Py–GO.
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dispersed GO in toluene, slowly heated to 105 ꢁC and stirred at this
temperature for 20 h. The resulting mercapto-functionalized GO
was centrifuged, washed 3 times with methanol, ethanol and
CH2Cl2 and dried under vacuum.
Table 4 Comparison of the catalytic activity of Py–GO with various
reported acidic and basic catalysts for the synthesis of b-phospho-
nomalonate 1
Entry
Catalyst
Time (h)
Yielda (%)
TONb
TOFc
1
2
3
4
5
6
7
8
NPS-g-Fe2O3
g-Fe2O3–La(OTf)2
HClO4–SiO2
AP-SiO2
5
3
10
5
3
3
3
1
20
40
20
10
55
40
58
90
6
13
6
1
4
0.6
0.6
6
4
6
30
Synthesis of pyridine-graed GO (Py–GO)
3-(Chloromethyl)pyridine hydrochloride (0.15 g) and triethyl-
amine (0.13 mL) were added to mercapto-functionalized GO
(1.7 g) in toluene (15 mL), and the mixture was reuxed for 18 h.
The solid material was then centrifuged, washed with water,
methanol, CH2Cl2 and dried under vacuum.
3
Py–g-Fe2O3
18
13
19
30
Piperidine–MNPs
Piperazine–MNPs
Py–GOd
a
Isolated yield, reaction conditions: benzaldehyde (1 mmol), malononitrile
General procedure for the synthesis of b-
phosphonomalonates
(1 mmol), diethyl phosphite (2 mmol), catalyst (3 mol%), temperature ¼
of catalyst. c TOF ¼ turnover frequency ¼ TON per hour. d This work.
b
100 ꢁC. TON ¼ turnover number ¼ ratio of moles of product to moles
A mixture of aldehyde (1 mmol), malononitrile (1 mmol),
diethyl phosphite (2 mmol) and Py–GO (3 mol%) was reuxed at
100 ꢁC for an appropriate time (Table 2). The catalyst was
separated by centrifugation. The supernatant was extracted with
ethyl acetate (3 ꢃ 10 mL). The organic layer was dried over
anhydrous Na2SO4. The solvent was removed under reduced
pressure, and the resulting product (1–15) was puried by
chromatography on silica (n-hexane/EtOAc: 1/1).
including acidic [n-propylsulfonated-g-Fe2O3 (NPS-g-Fe2O3),21a
g-Fe2O3–La(OTf)2 (ref. 21b) and HClO4/SiO2 (ref. 20c)] and basic
catalysts [3-aminopropylated-SiO2 (AP-SiO2),21c Py–g-Fe2O3,23c
piperidine–g-Fe2O3 and piperazine–g-Fe2O3 (ref. 21d)] (Table 4).
As is evident from Table 4, Py–GO as an acid–base bifunctional
catalyst is the most effective catalyst in terms of TON and TOF
leading to the formation of b-phosphonomalonates.
Conclusion
Experimental
We have successfully synthesized a new inexpensive and reus-
able acid–base bifunctional catalyst by postgraing of pyridine
onto GO. This pyridine-graed GO was successfully used as an
acid–base cooperative catalyst for the one-pot synthesis of
b-phosphonomalonates via cascade Knoevenagel–phospha
Michael addition reaction of aldehydes, malononitrile and
dialkyl phosphites in water. Efficiency and reusability of the
catalyst and using aqueous conditions make this protocol
a green process for the synthesis of b-phosphonomalonates in
good to high yields. Employment of odourless dialkyl phos-
phite, which fulls the criteria of atom economy, instead of
nasty smell trialkyl phosphite is the other advantage of this
protocol.
Chemicals were purchased from Merck Chemical Company. NMR
spectra were recorded on a Bruker Advance DPX-400 and 250 in
CDCl3 as solvent and TMS as internal standard. The purity of the
products and the progress of the reactions were accomplished by
TLC on silica-gel polygram SILG/UV254 plates. FT-IR spectra were
recorded on a Shimadzu Fourier Transform Infrared Spectropho-
tometer (FT-IR-8300). TEM analysis was performed using TEM
microscope (Philips CM30). Scanning electron microscopy (SEM)
and energy dispersive spectroscopy (EDS) were determined by
using a SEM Phenom, model PROX. Thermogravimetric analysis
(TGA) was performed using a Shimadzu thermogravimetric
analyzer (TG-50). Elemental analysis was carried out on a Costech
4010 CHNS elemental analyzer. Power X-ray diffraction (XRD) was
performed on a X'Pert Pro MPD diffractometer with Cu Ka (l ¼
0.154 nm) radiation. The UV-Vis spectra were carried out by using
a Shimadzu UV-160 A spectrophotometer.
Acknowledgements
Financial support of this project by University of Birjand
Research Council is appreciated.
Synthesis of graphene oxide (GO)
Notes and references
A 9 : 1 mixture of concentrated H2SO4/H3PO4 (360 : 40 mL) was
added to a mixture of graphite akes (3.0 g) and KMnO4 (18.0 g).
The reaction was then heated to 50 ꢁC and stirred for 12 h. The
reaction was cooled to rt and poured onto ice (ꢂ400 mL) with
30% H2O2 (3 mL). Finally, the resulting oxidized product was
collected by centrifuging, washing abundantly with 5% HCl
solution and deionized water and dried under vacuum.
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