A novel, efficient, and recyclable biocatalyst for Michael addition reactions and its iron(iii) complex as promoter for alkyl oxidation reactions

Article abstract of DOI:10.1039/c5cy01034a

Chitosan-g-poly(2-cyano-1-(pyridin-3-yl)allyl acrylate) (Cs-g-PCPA), having a grafting percentage (G%) of 64%, was prepared and was then subjected to complexation with iron(iii) (Cs-g-PCPA/Fe(iii)). The results of the catalytic studies demonstrated that Cs-g-PCPA serves as an efficient, recyclable and eco-friendly, basic catalyst for Michael addition reactions, which produce adducts in high yields under mild conditions. Moreover, a Cs-g-PCPA-supported iron(iii) complex was prepared and was then characterized by using FESEM, XRD, TGA, and XPS. In addition, the EDS plot of the Fe(iii) chelate confirms the presence of a relatively high amount (13%) of Fe(iii). The Cs-g-PCPA (G% = 64)-supported iron(iii) complex (Cs-g-PCPA/Fe(iii)) in the presence of H₂O₂ can be used to oxidize methyl pyridazinyl carbonitriles to form the corresponding fused furan derivatives efficiently. These polymeric catalysts are stable such that they can be recycled and reused more than five times without losing their catalytic activity.

Full text of DOI:10.1039/c5cy01034a

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A novel, efficient, and recyclable biocatalyst for Michael additions
and its iron (III) complex as promoter for alkyl oxidation reactions
Khaled D. Khalil,a,b Enas I. Ibrahim,a Fakhreia A. Al-Sagheera,*
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract. Chitosan-g-poly(2-cyano-1-(pyridin-3-yl)allyl acrylate) (Cs-g-PCPA), having a grafting percentage G% of 64%, was
prepared and was then subjected to complexation with iron (III) (Cs-g-PCPA/Fe(III)). The results of catalytic studies
demonstrated that Cs-g-PCPA serves as an efficient, recyclable and eco-friendly, basic catalyst for Michael additions, which
produce adducts in high yields under mild conditions. Moreover, Cs-g-PCPA supported Iron (III) complex was prepared
and was then characterized by using FESEM, XRD, TGA, and XPS. In addition, EDS plot of the Fe(III)-chelate, confirms the
presence of a relatively high amount (13%) of Fe(III). Cs-g-PCPA (G% = 64) supported Iron (III) complex (Cs-g-PCPA/Fe(III))
in presence of H2O2 can be used to oxidize methyl pyridazinyl carbonitriles to form the corresponding fused furan
derivatives efficiently. These polymeric catalysts are stable so that they can be recycled and reused more than five times
without losing their catalytic activity.
DOI: 10.1039/x0xx00000x
www.rsc.org/
naturally occurring polysaccharide that is usually obtained by
alkaline hydrolysis of chitin. This polymer has many
1. Introduction.
Many investigations carried out recently have focused on the
development of new green synthetic methodologies, which
reduce and/or prevent the use and/or production of
environmental pollutants.1-3 "Twelve Principles of Green
Chemistry" have been suggested as guidelines to increase the
efficiencies of synthetic methods, use less toxic solvents,
reduce the steps of the synthetic routes and minimize
wastes.3-5 Catalysis of chemical reactions has many green
beneficial features including lower activation energies that
results in lower temperature processes, the use of catalytic
rather than stoichiometric amounts of materials, increased
chemo- and stereo-selectivities that cause less by-product
formation, and the decreased utility of processing and
separation agents.6 Heterogeneous catalysis, in particular,
conforms to the goals of green chemistry by providing simple
methods for product/catalyst separations.7,8 Chitosan, Cs, is a
advantages, including being renewable, biodegradable,
relatively economical, and having a low toxicity. Recently,
chitosan has been used as a “green” support for several
transitional metal catalysts and as a solid base catalyst for
many organic reactions.9-11 Much earlier, chitosan was
successfully used as an efficient biocatalyst for Michael
addition reactions of active methylene compounds to electron
poor alkenes.12,13 This process, which has attracted enormous
attention as one of the most efficient C–C bond forming
reaction in organic synthesis, is typically carried out in
presence of homogeneous or heterogeneous basic catalysts.14-
16 The major problems associated with the use of chitosan as a
catalyst for these processes are its lower catalytic activity and
difficulty of being recycled because it forms a gel. To overcome
these problems, a new catalyst, Cs-g-PCPA, was generated by
homogeneous grafting of the novel monomer, 2-cyano-1-
(pyridin-3-yl)allyl acrylate (CPA), onto chitosan using similar
reported methodology.17-19 The chitosan graft cross-linked
copolymer Cs-g-PCPA was found to have higher thermal
stability than the unmodified chitosan and, as a result, it can
be used effectively for higher temperature catalytic
reactions.20
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Chitosan has been reported to a have high metal ion 2.2 Preparation of Chitosan-g-iron (III) supported catalyst
adsorption affinity.21,22 Tomczak et al. has shown that the
The chitosan graft copolymer Cs-g-PCPA (64%G) was
chitosan Fe(III) chelate contains surface adsorbed Fe3+ ions
converted into a Fe(III) chelate (Cs-g-PCPA(64%G)/Fe(III)) by
through nitrogen and oxygen atoms on the surface of
stirring in a pH 1.8 aqueous solution containing Fe(NO3)3 for 5
chitosan.23,24 The Fe(III) ions adsorption capacity of chitosan
h.21-23 The degree of Fe(III) incorporation of the polymer was
grafted with pyridine containing monomers is expected to be
determined by calculating the difference between the Fe3+
higher than that of the non-grafted chitosan owing to the
concentration of the aqueous solution before and after
presence of pyridine moieties in graft side chains. Recently,
adsorption onto the polymer by using atomic absorption
Tang et al. prepared chitosan-supported iron (III) chelates and
spectrophotometry. Under these conditions, the maximum
used them as powerful catalysts for the oxidation of
adsorption of Cs-g-PCPA(64%G)/Fe(III) was 0.089 mmol Fe3+/g
cyclohexane with air. 25
chitosan. The formed Fe(III) supported catalyst was converted
Based on the above observation, chitosan-g-poly(2-cyano-1- into dark brown spherical beads by neutralization with sodium
(pyridin-3-yl)allyl acrylate) supported iron(III) complex, Cs-g- hydroxide, which were kept under distilled water before use.
PCPA/Fe(III), was prepared in the form of beads and shown to Various Fe (III) ion binding sites exist in the graft copolymer,
be an effective promoter of methyl group specific oxidation including s C-2 (NH2), C-3 (OH) of the chitosan backbone and in
reactions of methylpyridazinone in the presence of hydrogen the CN, C=O, N centres of the graft side chains. As a
peroxide.
consequence, the iron chelates are likely heterogeneous in
nature.
2. Results and discussion.
2.3 Characterization of Chitosan-g-(PCPA) copolymer and its
iron (III) complex
2.1 Preparation of graft copolymer Cs-g-PCPA
2-cyano-1-(pyridin-3-yl)allyl acrylate (CPA, 1) was prepared
The Cs-g-PCPA graft copolymer sample, with 64%G, was
prepared and confirmed by studying the FTIR spectra which
was in agreement with our reported data.20
and was then employed in homogeneous grafting to chitosan
using potassium persulfate and sodium sulfite as redox
promoters (Scheme 1).20 The grafting was demonstrated by
analysing FTIR spectral differences between the chitosan and
the copolymer.25
Surface morphologies of chitosan, Cs-g-PCPA graft copolymer
(64%G), and iron(III) chelated Cs-g-PCPA (64%G) (see below)
are displayed in Fig. 1 A, B , and C. By viewing these images, it
can be seen that the surface of chitosan is fibrous in shape
while that of the grafted sample is less fibrous. The FESEM of
Cs-g-PCPA (64%G)/Fe(III), displayed in Fig. 1C, shows that the
O
.
Cs
X
O
X = O, NH
CN
N
1
surface has undergone
a remarkable change owing to
CH
O
H2
C
O
coordination with Fe(III) ions. The unique structure of the
chitosan is completely destroyed and replaced by irregular
shapes with some large, well-developed crystals distributed on
the surface. The energy dispersion composition (EDS) plot of
the Fe(III)-chelate, given in Fig. 1D, confirms the presence of a
relatively high amount (13%) of Fe(III). Interestingly, the
amount of Fe(III) has not changed markedly (12%) after
carrying out the catalytic oxidation reaction of the product 15
for 3 times (Scheme 5).
HO
CN
NH2
HOH2
C
NH
O
O
HO
HOH2
2
O
O
O
H2
C
HO
NHCOCH3
O
O
N
C
H
C
O
C
H2
O
O
Graft Copolymer
Cs-g-PCPA
NC
N
Scheme 1. Homogeneous grafting of 2-cyano-1-(pyridin-3-
yl)allyl acrylate ( ) onto chitosan to yield the grafted polymer
Cs-g-PCPA ).
1
(2
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XPS spectroscopy was employed to obtain information about
the elemental composition and chemical states on the surface
of Cs-g-PCPA(64%G)/Fe(III) (Fig. 3 A and B) analysis of the
spectra shows that a peak exists that corresponds to a binding
energy between 700-720 eV, which is characteristic of Fe3+
cations. In addition, in a manner characteristic for Fe3+,28
satellite peaks are observed at ca. 720 and 733 eV on the high
binding energy sides of the main peaks in the Fe(2p) spectrum
(Fig. 3 B). These findings indicate that chelation of Fe(III) with
the copolymer has taken place in the bulk and at the surface of
the grafted polymer, a phenomenon that is in accord with the
results of FESEM measurements which show that large regular
crystals are present on the surface of the particles.
Fig. 1. FESEM images of A) chitosan; B) Cs-g-PCPA (64%G); C)
Cs-g-PCPA(64%G)/Fe(III); D) EDS of Cs-g-PCPA (64%G)/Fe(III).
In Fig. 2 A, B, and C are shown XRD patterns of pure chitosan
powder, chitosan graft 64% copolymer, and its Fe(III) chelate
respectively. The pattern of chitosan contains a characteristic
peak at 2
chitosan and 64%G copolymer, Fig. 2 A and B, show weakness
in the intensity peak of chitosan (2 = 19.8°) upon grafting and
that at the same time appearance of new peak at 2 = 17.1°.
The intensity increase of the (2 = 17.1°) peak is associated
θ
= 19.8°.26,27 Inspection of the XRD patterns of the
θ
θ
θ
with the production of long polymer chains in poly-2-cyano-1-
(pyridin-3-yl)allyl acrylate moieties introduced by grafting.
Fig. 3. (A) XPS spectra for Fe(III)-grafted chitosan (64%G), (B)
Moreover, in Fig
2 C, Fe(III) chelates showed a clear
Fe(2p) spectrum.
improvement in the crystallinity which is attributed to the
coordination of Fe with the binding sits of the copolymer.
AFM provides a high resolution, sub-nanometre scale mapping
of the topography of the surface of a material. The AFM of
chitosan (Fig. 4 A) yields a 3D image that shows the presence
of a smooth textured surface topography, indicating the
absence of detectable agglomerates. In Fig. 4 B is displayed the
AFM of the %G = 64 copolymer, which shows that it has an
irregular undulating surface, attributed to the randomly
distributed graft regions. In Fig. 4 C is displayed the 3D image
of the surface of Cs-g-PCPA(64%G)/Fe(III), which is comprised
of
a random distribution of broad and intense peaks
associated with the presence of Fe(III).
Fig. 2. XRD patterns of (A) chitosan25; B) Cs-g-PCPA (64%G)25;
C) Cs-g-PCPA (64%G )/ Fe(III).
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dimedone
4 (1.40 g, 10 mmol) was carried out in 25 mL
absolute ethanol using 1, 5, 10, 15, and 20% wt. of catalyst
under the same conditions (Fig. 5). The catalyst loading 10%wt
was found to be the optimal quantity. Moreover, the catalyst
was reused four times and the results showed that the graft
copolymer can be reused as such without significant loss in its
catalytic activity (Table 1).
Table 1. Recyclability of the chitosan graft copolymer as basic
catalyst.
Recycled Catalyst
Fresh
State of Catalyst
Catalyst
(1)
94
(2)
93
(3)
92
(4)
92
Product 5
(%Yield)
95
Fig. 4. AFM – 3D images of (A) chitosan Cs, (B) Cs-g-PCPA
(64%G), and (C) Cs-g-PCPA (64%G)/Fe(III).
2.4 Application of the graft copolymer as
heterogeneous Michael addition catalyst
a
basic
120
320
300
280
260
240
220
200
180
160
100
80
60
40
20
0
The effectiveness of the grafted chitosan polymer Cs-g-PCPA
as a basic catalyst in Michael addition reactions was explored.
Loading Catalyst %wt vs %Yield
Loading Catalyst %wt vs Time (min.)
Benzylidene-malononitrile
Michael addition reaction with dimedone
10%wt Cs-g-PCPA beads to yield the chromene-3-carbonitrile
derivative in an excellent yield (>95%) (Scheme 2). Moreover,
this compound has been prepared previously by employing
3
was observed to participate in a
4
promoted by
5
5
0
5
10
15
20
25
30
other homogeneous basic catalysts.12,13,15,16 One of the
difficulties associated with using the unmodified chitosan as a
catalyst for these reactions is that separation of the product
from the catalyst. This difficulty is caused by the fact that this
polysaccharide readily forms a gel. In contrast, the catalytically
more active grafted copolymer does not form a gel during
workup and, as a result, it is a superior catalyst for these
reactions. Accordingly, the grafted copolymer catalyst can be
readily separated by simple filtration and reused after it is
washed with hot methanol and dried in an oven at 100 °C for 2
h for more than five times without loss of catalytic activity.
Loading Catalyst %wt
Fig. 5. Optimization of the chitosan graft copolymer as basic
catalyst.
Reactions of benzylidene-malononitrile
thiobarbituric acids 6a and
have been described earlier.29-31
These processes produce the pyridopyrimidines when
ammonium acetate is employed as the catalyst. In contrast,
reactions of with 6a and occur more efficiently (80%) to
produce pyranopyrimidines 7a and when carried out in
3 with barbituric and
b
8
3
b
b
ethanolic solutions containing 10%wt of Cs-g-PCPA beads
(Scheme 2). In a study evaluating the possibility of carrying out
a one pot reaction, we observed that stirring independent
In order to estimate the appropriate catalyst loading, a model
reaction of benzylidene-malononitrile
3 (1.54 g, 10 mmol) and
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COOEt
Ar
mixtures of 6a and
b containing benzaldehyde, malononitrile
O
S
/ dioxane
NC
COOEt
CN
+
Ar
and Cs-g-PCPA beads leads to efficient formation of 7a and b,
NC
NH2
Cs-PCPA
S
10
9a,b
9a, Ar = phenyl,
11a,b
respectively. The results of additional investigations
9b, Ar = 2-(N-methylpyrrolyl)
demonstrated that Cs-g-PCPA serves as a catalyst for classical
COOEt
CN
COOEt
Gewald's reactions29,30 of 3-oxoalkanenitriles 9a and
b with
Ar
Ar
COOEt
CN
Ar
S
O
+
Cs-PCPA
Cs-PCPA
CN
ethyl cyanoacetate 10 in 1,4-dioxane solutions containing
NC
S-
CN
Cs_PCPAH+
CN
elemental sulfur. These processes generate the thiophene
B
A
9a,b
- Cs_PCPA
derivatives 11a and
b
(Scheme 3).
COOEt
NH
Ar
COOEt
NH2
Ar
Ph
CN
CN
O
H
NC
NC
S
S
3
O
O
O
11a,b
C
4
NH
NH
11a, Ar = phenyl,
11b, Ar = 2-(N-methylpyrrolyl)
Cs-PCPA
X
6a,b
O
Ph
O
6a, X = O,
6b, X = S
CN
Scheme 3. Cs-g-PCPA (64%G) catalysed Gewald's synthesis of
NH2
Cs-PCPA
thiophene.
NH4OAc
5
O
Ph
O
Ph
CN
CN
CN
Substituted enaminonitriles 12a and
substrates for the synthesis of new quinolinone derivatives
13a and , containing piperidenyl or morpholinyl substituents
at the C-2 position (Scheme 4). In fact, 12a and were found
b are promising
HN
CN
HN
+
6a,b
Ph-CHO
X
NH2
N
H
O
X
Cs-PCPA
NH2
N
H
N
H
b
7a,b
7a, X = O,
7b, X = S
8
b
Scheme 2. Cs-g-PCPA (64%G) catalysed Michael addition
to react with dimedone in ethanol in the presence of 10%wt of
Cs-g-PCPA as heterogeneous catalyst to afford quinolinone
derivatives 13a and b in high yields (ca. 90%). The structures of
these products were assigned based on the results of X ray
crystallographic analysis (Fig. 6A and B).
reactions.
A plausible mechanism for this process involves the formation
of ylidene intermediate , which is cyclization of intermediate
B occurs readily to form the aromatic thiophene products, 11a
and via elimination of the catalyst (Scheme 3). The findings
A
b
of a further exploration of this process showed that the yields
of the thiophenes are highly dependent on the nature of
solvent, with the use of 1,4-dioxane leading to much higher
yields than with ethanol.
A
B
Fig. 6. (A) X-ray crystal structure of 13a; (B) X-ray crystal
structure of 13b
.
A reasonable mechanism for these processes involves initial
Cs-g-PCPA catalysed Michael addition reaction between the
enaminonitriles and dimedone to produce intermediate I. This
intermediate then cyclizes to give the aminopyran
intermediate II that undergoes an allowed 1,3-nitrogen shift, in
agreement with that previously published by our group,13,32,33
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accompanied by ring opening to afford imine intermediate IV
.
relatively higher yields than previously reported34 (Scheme 5).
The reaction mixture of the catalyst in glacial acetic acid with
H2O2 afforded the maximum yield of the oxidation products
(Table 2). As shown in Table 2, it is clear that the solvent of
choice for this reaction is the glacial acetic acid where higher
yields were obtained.
Finally, IV intermediate readily cyclizes to give 13a and b.
O
O
NC
Cs_PCPA
EtOH
+
N
N
O
N
Y
12a,b
12a, Y = CH2,
12b, Y = O
Y
4
13a,b
Y = CH2, O
Y
N
O
O
O
NC
CH3
Cs_PCPA
EtOH
+
NH2
CN
O
N
O
CN
H2O2 / AcOH
Cs_PCPA-Fe
O
Y
4
12a,b
I
N
N
O
N
N
Ar
Ar
Y
N
O
15a,b
15a, Ar = phenyl,
15b, Ar = p-Chlorophenyl
14a,b
14a, Ar = phenyl,
14b, Ar = p-Chlorophenyl
Y
O
1,3-shift
N
O
NH2
O
NH2
III
Scheme 5. Oxidation of alkyl pyridazinones utilizing Cs-g-PCPA
II
(64%G)/Fe(III).
O
O
Cs_PCPA
Table 2. Solvent effects on the oxidation reaction of methyl
pyridazinones a
Y
N
N
O
Y
H2N
O
HN
V
IV
- H2O
Entry
Substrate
Solvent Product Yield
O
1
2
3
4
5
6
7
8
9
H2O
EtOH
AcOEt
CH3CN
AcOH
H2O
N.R. b
8
N
N
Y
13a,b
13a, Y = CH2,
13b, Y = O
14a
15a
44
Scheme 4. Cs-g-PCPA (64%G) catalysed synthesis of
quinolinone derivatives 13a and
66
b
.
72
1.6 Applying the chitosan graft–Fe(III) chelate as a co-catalyst
for oxidation of alkyl pyridazinones
N.R.b
14
EtOH
AcOEt
CH3CN
AcOH
One of the major challenges in organic synthesis is the
development of methods for the controlled oxidation of non-
activated alkyl C-H bonds.3 Earlier reports24 of the use of Fe(III)
supported chitosan chelates in oxidations of alkyl groups
encouraged us to carry out a brief exploration of oxidation
reactions of heterocyclic compounds promoted by Cs-g-PCPA
/Fe(III) chelates. In this effort, we observed that controlled
14b
15b
48
58
10
66
a
Reactions were conducted with 1 mmol substrate,
with 10% wt of the Cs-g-PCPA (G=64%)/Fe(III) chelate
and 5 mmol of 30% hydrogen peroxide.
oxidation at the methyl groups in the pyridazinones 14a and
b
b N.R. Reaction didn't occur.
can be performed utilizing the Cs-g-PCPA (64%G)/Fe(III)
chelate as an efficient Fe(III)-supported catalyst. Accordingly,
The proposed mechanism of oxidation of pyridazinones via
chitosan graft–Fe(III) chelate may be postulated as in Scheme
6. The reaction was started with oxidation of methyl
independent mixtures of 14a and
b containing Cs-g-PCPA
(64%G)/Fe(III) react in the presence of excess hydrogen
peroxide to form the fused furan derivatives 15a and
b in
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substituent utilizing H2O2/Fe3+ oxidizing mixture to afford the and after adsorption at wavelength 248 nm using an atomic
corresponding 1° alcohol. Because of the binding capability of absorption spectrophotometer (Perkin-Elmer 3100 Model).
Fe(III) catalyst with CN group, non-isolable intermediate was Single crystal X-ray crystallographic analysis was performed by
catalysed to underwent heterocyclization process to afford the using a RIGAKU RAPID II. The XRD measurements were
final product 15a and
b
via intermediates B and C.
performed at room temperature on chitosan, grafted chitosan
and Fe(III)/copolymer. Siemens diffractometer model D500
(Germany) operating in the reflection mode with Cu-Kα
radiation (35kV, 30mA) was used for the analysis. X-ray
photoelectron spectroscopy (XPS) was conducted using a
model VG Scientific 200 spectrometer (UK) equipped with
MgKα radiation (1253 eV) and operated at 23 kV and 13 mA.
All binding energy values (in eV) were determined with respect
to the C1s line (284.6 eV) originating from adventitious carbon,
the standard deviation of the peak position being within ±0.02
eV. Surface topography was analysed via AFM using the
Nanoscope IV Multimode Atomic Force Microscope.
H
H
O+
:
O
Fe(III)-Cs
CH3
H2C
N-
CN
N
Cs-Fe(III)
H2O2
N
N
N
Cs-Fe(III)
O
N
O
N
O
N
B
Ar
Ar
A
Ar
14
- Cs-Fe(III)
H
O
O
NH
NH2
N
N
O
O
N
N
Ar
Ar
C
15
Scheme 6. Proposed mechanism for oxidation of alkyl
pyridazinones utilizing Cs-g-PCPA (64%G)/Fe(III) as a catalyst.
3.3 Preparation of Basic Catalyst Chitosan-g-PCPA (Cs-g-
PCPA) beads
3. Experimental
3.1 Materials
3.3.1
Preparation of 2-Cyano-1-(pyridin-3-yl)allyl acrylate,
CPA, vinyl monomer
Medium molecular weight grade chitosan (deacetylated 90%,
M = ca. 35.000) was purchased from Aldrich. DABCO (1,4-
diazabicyclo[2.2.2]octane) was purchased from Aldrich.
Potassium persulfate K2S2O8 (KSS) and sodium sulfite Na2SO3
(SS) were purchased from Riedel-deHaen and Fluka,
respectively. Acrylonitrile was purchased from Aldrich and
purified by treatment with 10% NaOH for 30 min to remove
the inhibitor and then distilled at 77°C. Pyridine-3-carbonitrile
was purchased from Aldrich. All other chemicals of analytical
grade were used without further purification.
Recently, Fakhreia et al has reported a novel methodology to
modify chitosan through grafting with 2-Cyano-1-(pyridin-3-
yl)allyl acrylate, CPA 25
synthesized by
.
The latter vinyl monomer was
reacting 2-(Hydroxy(pyridin-3-
yl)methyl)acrylonitrile, Morita Baylis Hillman (MBH) adduct of
acrylonitrile and nicotinaldehyde in the presence of DABCO as
a catalyst under neat conditions, with acryloyl chloride.
3.3.2
Preparation of the graft copolymer Cs-g-PCPA, under
optimum conditions 25
3.2 Apparatus and Instrumentation
A 1.00 g chitosan powder was stirred in 50 mL distilled water
to which K2S2O7 (3.5 x 10-3 M) and NaHSO3 (1.75 x 10-3 M) with
a ratio of 1:0.5 were added respectively. CPA (2.14 g, 10 mmol)
was added and then mixture was stirred at 65oC for 4 h.
Filtration gave the graft copolymer, which was washed with
water and was then extensively extracted with hot ethanol
using the soxhlet extraction technique to remove the
unreacted monomer. Finally the graft copolymer Cs-g-PCPA
was dried.
Field emission scanning microscopy (FESEM) was carried out
using a model Leo (Zeiss) Remotely Operationable Variable
Pressure Field Emission SEM with EDX (Energy Dispersive X-Ray
Spectrometers) and Cryo attachment. Powder samples used in
the measurements were made by compression of the polymer
and then coating with gold. The degree of Fe3+ adsorption on
the chitosan grafted copolymer was calculated based on the
difference of Fe3+ concentration in aqueous solutions before
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ARTICLE
Journal Name
3.3.3
Preparation of Cs-g-PCPA
chelate: Fe / Cs-g-PCPA
–
supported Iron(III) heterogeneous catalysts for Michael addition reactions. The
results of this study show that Cs-g-PCPA (G = 64%) serves as a
superior catalyst for Michael additions as compared to
An aqueous solution containing Fe(NO3)3 was used to absorb
Fe(III) ions on the surface of Cs-g-PCPA grafted chitosan beads
of 64 %G.20,21 The degree of adsorption was calculated based
on the difference of Fe3+ concentration in aqueous solution
before and after adsorption at wavelength 248 nm. For Fe(III)
adsorption onto Cs-g-PCPA, a contact time of 5 h at pH 1.8 was
used for stirred mixture of Cs-g-PCPA beads (0.05 g) and 100
mL (10 mM) of Fe(NO3)3 solution in 250 mL beaker. The
amount adsorbed on the beads was calculated according to
the following equation:
chitosan itself and suggest that the new basic catalyst can
replace other more toxic old organic catalysts, like piperidine
and pyridine, in Michael additions. The chitosan graft cross-
linked copolymer Cs-g-PCPA was found to have higher thermal
stability than chitosan and, as a result, it can be used
effectively for higher temperature reactions. Finally, the iron
(III)-supported chitosan copolymer complex, Cs-g-PCPA (G =
64%)/Fe (III), was shown to be a promising catalyst for
selective methyl group oxidation reactions of methyl
substituted heterocycles.
( C0 - Ce ) V
Adsorption Capacity (qe)
=
W
Acknowledgements
Where C0 is the initial Fe(III) concentration (ppm), Ce is the
final concentration of Fe(III) after the adsorption time, V is the
volume of Fe(III) solution (mL) and W is the used weight of
graft copolymer Cs-g-PCPA beads (g). The yellowish-coloured
beads containing adsorbed Fe3+ were washed thoroughly with
distilled water and stored in the distilled water for further use.
Under the previous conditions, the maximum adsorption
capacity was obtained as 0.091 mmol Fe3+/g chitosan.
The authors wish to acknowledge financial support provided
by the Kuwait University. They also their appreciation for
technical support from the E.M unit and the general facilities
projects GS01/01, GS01/03, GS01/05, GS03/01, GS 03/08
under the GFS program. Also, the authors would like to thank
the Nanoscopy Science Center at Kuwait University.
Notes and references
1. Brundtland, Our common future, "Our common future:
The World Commission on Environment and
Development", Oxford: Oxford University Press, 1987.
3.3.4
Conversion of catalyst powder to the form of spherical
beads
2. A. S. Bommarius, B. R. Riebel, Biocatalysis:
Fundamentals and Applications,Wiley-VCH, Verlag
GmBH & KGaA: Weinheim (Germany), 624 pp, 2004.
Chitosan (or modified chitosan) was converted to spherical
bead shapes as previously reported35 by dissolving 2.00 g of
powder in 60 mL of 5% (v/v) acetic acid solution. The viscous
solution was sprayed into 500 mL of a stirred 0.50 M NaOH
solution as a precipitation bath. In the bath neutralization of
the acetic acid occurred within the chitosan gel and thereby
promoting coagulation to form spherical uniform chitosan gel
beads. Impurities trapped in the pores of chitosan beads were
removed by rinsing with distilled water. Filtration and air-
drying gave beads that could be stored under distilled water.
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Catalysis Science & Technology
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DOI: 10.1039/C5CY01034A
Graphical Abstract
A novel, efficient, and recyclable biocatalyst for Michael additions and its iron (III)
complex as promoter for alkyl oxidation reactions
Khaled D. Khalil, Enas I. Ibrahim, Fakhreia Al-Sagheer
O
HN
O
Ph
O
Ph
O
CN
Ph
CN
CN
O
NC
Ar
CN
NH
CN
NH
X
N
Ar
HN
H
Cs_PCPA
Cs_PCPA
X
N
H
O
O
2
2
O
CN
HN
O
HN
Cs_PCPA
N
+
N
O
N
H
N
O
N
H
Y
Y
CH
3
N
Cs_PCPA-Fe
H O /AcOH
N
N
O
N
2
2
Ar
Ar
CN
NH
2
O
O