Nitrogen enriched mesoporous organic polymer anchored copper(ii) material: An efficient and reusable catalyst for the synthesis of esters and amides from aromatic systems

Article abstract of DOI:10.1039/c4dt03838j

A new copper-grafted mesoporous poly-melamine-formaldehyde (Cu-mPMF) has been synthesized from melamine and paraformaldehyde in DMSO medium, followed by grafting of Cu(ii) at its surface. Cu-mPMF has been characterized by elemental analysis, powder XRD, HR TEM, FE-SEM, N₂ adsorption study, FT-IR, UV-vis DRS, TGA-DTA, EPR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The Cu-grafted mesoporous material showed very good catalytic activity in methyl esterification of benzylic alcohols and amidation of nitriles. Moreover, the catalyst is easily recoverable and can be reused seven times without appreciable loss of catalytic activity in the above reactions. The highly dispersed and strongly bound Cu(ii) sites in the Cu-grafted mesoporous polymer could be responsible for the observed high activities of the Cu-mPMF catalyst. Due to strong binding with the functional groups of the polymer, no evidence of leached copper from the catalyst during the course of reaction emerged, suggesting true heterogeneity in the catalytic process. This journal is

Full text of DOI:10.1039/c4dt03838j

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Nitrogen Enriched Mesoporous Organic Polymer Anchored Copper(II)
Material: an Efficient and Reusable Catalyst for the Synthesis of Esters
and Amides From aromatic systems
Rostam Ali Molla,a, Md. Asif Iqubal,b, ‡ Kajari Ghosh,a,‡
Kamaluddin,*b S. M. Islam*,a
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia 741235, W.B., India
bDepartment of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India
‡These authors contributed equally to this manuscript
Abstract
A new copperꢀgrafted mesoporous polyꢀmelamineꢀformaldehyde (CuꢀmPMF) has been
synthesized from melamine and paraformaldehyde in DMSO medium, followed by grafting of
Cu(II) at its surface. CuꢀmPMF has been characterized by elemental analysis, powder XRD, HR
TEM, FEꢀSEM, N2 adsorption study, FTꢀIR, UVꢀvis DRS, TGAꢀDTA, EPR spectroscopy,
Raman spectroscopy and Xꢀray photoelectron spectroscopy (XPS). The Cuꢀgrafted mesoporous
material showed very good catalytic activity in methyl esterification of benzylic alcohols and
amidation of nitriles. Moreover, the catalyst is easily recoverable and can be reused for seven
times without appreciable loss of catalytic activity in the above reactions. The highly dispersed
and strongly bound Cu(II) sites in the Cuꢀgrafted mesoporous polymer could be responsible for
the observed high activities of the CuꢀmPMF catalyst. Due to strong binding with the functional
groups of the polymer, no evidence of leached copper from the catalyst during the course of
reaction occurred, suggesting true heterogeneity in the catalytic process.
Key words: Copper, mesoporous, polyꢀmelamine, esterification, amidation, recoverable
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* Author to whom correspondence should be addressed.
Prof. Sk. Manirul Islam
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
Phone: + 91- 33-2582-8750, Fax: 91-33-2582-8282, E-mail: manir65@rediffmail.com
Prof. Kamaluddin
Department of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India. Phone: + 91-
01332285796, E-mail: kamalfcy@iitr.ac.in
Introduction:
Arylated carbonyl compounds are important motifs in many natural products,
pharmaceuticals and bioactive molecules, which have always drawn chemists attention in
organic synthesis.1 In particular, aryl esters and amides derivatives are found at the core of
numerous important intermediates that are widely applied in the synthesis of bulk chemicals,
drugs, synthetic materials and polymers.2,3
Traditionally, esters are prepared by the reaction of activated carboxylic acid derivatives
with alcohols.4 Recent developments in palladiumꢀcatalyzed threeꢀcomponent esterification
reactions of alcohols, aryl halides, and CO provide a more direct approach for ester synthesis.5,6
The oxidative esterification of aldehydes or alcohols is also reported for the synthesis of esters.7
As the required aldehydes are normally synthesized by selective oxidation of alcohols,8,9 the
direct conversion of alcohols into esters in the presence of catalysts is certainly a step forward
towards green, economic, and sustainable processes.10ꢀ13 These oneꢀpot procedures reported
usually require the use of highly expensive noble metals such as gold,10 palladium,11 iridium,12
or ruthenium13 as catalysts.
On the other hand, one of the most common methods for amide synthesis employs
activated derivatives of the carboxylic acid, such as esters14 together with carbodiimides,
2

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phosphonium, or uronium salts as additives.15 Other general procedures for amide synthesis
include the wellꢀestablished name reactions.16ꢀ20 Besides, catalytic procedures have been
developed including oxidative amidation of aldehydes21 or alcohols,22 aminocarbonylation of
aryl halides23or terminal alkynes,24 rearrangement of oximes and coupling with amines, cross
coupling of amides with aryl and alkenyl halides,25 and transamidation of amides with amines.26
Besides these protocols, a less frequently reported synthetic method is the coupling of nitrile
with a mine, which has been reported to be performed in the presence of ruthenium27 or
platinum28 catalysts. The use of earthꢀabundant firstꢀrow transition metals instead of catalysts
based on highly expensive noble metals is interesting and important from academic, industrial,
and sustainability aspects.
Copper is one of the most abundant metals on the earth, and it is one of the most
inexpensive and environmentally friendly. We describe our efforts on copperꢀcatalyzed methyl
esterification reactions with peroxides, which serve as both the oxidant and the source of the
methyl group. To the best of our knowledge, copperꢀcatalyzed direct esterification of benzylic
alcohols in the absence of methanol and copper catalyzed secondary amides synthesis are rare to
date.29, 30 Previously reported most of these protocols used homogeneous copper catalysts.
Homogeneous catalysts have some disadvantages, such as they may easily be destroyed during
the course of the reaction and they cannot be easily recovered after the reaction for reuse.31, 32 To
get rid from these serious issues related to homogeneous catalyst, it is thus desirable to develop
an efficient methodology to avoid all these difficulties.33
Various supports such as silica, carbon, and zeolites among have been used for the
immobilization of homogeneous copper salt.34 Among the different type of mesoporous
materials, the organic mesoporous polymers are a class of advanced materials, which possess the
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textural porosities of mesoporous materials (wellꢀordered pores, large surface area and tunable
structures) and simultaneously the advantages of organic polymers (high hydrophobicity,
containing aromatic sections and higher stability in acid or base media in comparison to
silicabased materials).35
In this context, mesoporous organic polymers carrying the reactive functional groups at the
surface of the mesopores36 can provide an ideal tethering agent for the active metals to bind at its
surface strongly. Due to extensive crossꢀlinking, polymeric materials are highly desirable
material for long term stabilization of the entrapped metal centres, which reduces the possibility
of leaching of the metal under reaction conditions.37
Herein, we report a novel copper doped mesoporous polyꢀmelamineꢀformaldehyde (Cuꢀ
mPMF) material that exhibits superb activities towards esterification of benzyl alcohol and
amidation of nitriles. The high activity of CuꢀmPMF is attributed to its condensed network
structure which contains large number of nitrogen atoms. Significant amounts of nitrogen have
been incorporated into comparable inorganic framework materials38 and this is found to be
beneficial for the stabilization of copper acetate.
Experimental Section
Chemicals
Melamine and paraformaldehyde were purchased from SigmaꢀAldrich. All other
reagents and chemicals were purchased from commercial sources and were used without
further purification. Solvents were dried and distilled through standard procedure.
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Physical measurements
The FTꢀIR spectra of the samples were recorded from 400 to 4000 cmꢀ1 on a Perkins
Elmer FTꢀIR 783 spectrophotometer using KBr pellets. UVꢀVis spectra were taken using a
Shimadzu UVꢀ 2401PC doubled beam spectrophotometer having an integrating sphere
attachment for solid samples. Thermogravimetric analysis (TGA) was carried out using a
Mettler Toledo TGA/DTA 851e. Surface morphology of the samples was measured using a
scanning electron microscope (SEM) (ZEISS EVO40, England) equipped with EDX
facility. Powder Xꢀray diffraction (XRD) patterns of different samples were analyzed with
a Bruker D8 Advance Xꢀray diffractometer using Ni–filtered Cu Kα (λ=0.15406 nm)
radiation. High resolution transmission electron microscopy (HRꢀTEM) images of the
mesoporous polymer were obtained using a FEI TECHNAI G20 transmission electron
microscope operating at 200 kV. Specific surface area of the sample was measured by
adsorption of nitrogen gas at 77 K and applying the BrunauerꢀEmmettꢀTeller (BET)
calculation. Xꢀray photoelectron spectroscopy (XPS) was performed on an Omicron
nanotech operated at 15 kV and 20 mA with a monochromatic Al Kα Xꢀray source. The
EPR (electron paramagnetic resonance) spectrum of the copper catalyst was recorded for
the solid sample at room temperature by a JESꢀFA200 ESR spectrometer (JEOL). NMR
spectra were recorded on a Varian Mercury plus NMR spectrometer (1H NMR at 300 and
500 MHz) in pure deuterated solvents.
Synthesis of mesoporous poly-melamine-formaldehyde polymer (mPMF): Melamine
(0.378 g, 3 mmol) and paraformaldehyde (1.8 eq, 0.162 g, 5.4 mmol) were mixed with 3.36
ml (overall concentration of 2.5 M) of dimethyl sulfoxide (DMSO) in a 15 ml Teflon
container secured in a steel reactor. The reaction mixture was heated to 120 0C in an oven for
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1 h. The reactor was then carefully removed from the oven for stirring on a magnetic stirrer
0
plate to obtain a homogeneous solution. It was then heated in the oven to 170 C for 72 h.
The reaction was allowed to cool to room temperature, and the obtained solid was crushed,
filtered, and washed with DMSO, acetone, tetrahydrofuran (THF) and CH2Cl2. The resulting
white solid was dried under vacuum at 80 °C for 24 h.
HN
N
N
HN
N
NH
N
HN
NH
N
N
N
N
NH
N
2
Paraformaldehyde
DMSO
HN
HN
N
N
N
H
N
H
H
HN
N
N
170 0C
N
N
N
N
H N
NH
2
2
HN
N
NH
N
HN
N
NH
N
HN
NH
HN
N
NH
N
N
N
N
N
N
N
N
N
N
N
N
H
N
H
N
N
N
H
N
H
H
H
H
H
Scheme1. Schematic diagram showing the formation of mesoporous polymer mPMF
Synthesis of Cu-mPMF catalyst: In typical synthesis, 1 gm mesoporous polymer in methanol
(10 mL) was stirred along with Cu(OAc)2 (300 mg). It was then heated to reflux for 24h and
green coloured solid was filtered and dried under vacuum at 80 °C for 24 h. Copper(II) loading
to the mPMF was determined to be 14.20% by AAS.
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MeOH
24h Stir
Cu(OAc)2
Cu(OAc)2
+
mPMF
Scheme 2. Schematic diagram showing the formation of copper(II) grafted mesoporous mPMF
General Procedure for Cu-mPMF Catalyzed Esterification of Benzylic Alcohols: Benzylic
alcohol (0.3 mmol), CuꢀmPMF (20 mg), K2CO3 (0.6 mmol), TEAB (0.15 mmol), TBHP (2.4
mmol, 70% aqueous solution) and DMSO (2.0 mL) were added to a 50 mL round bottom flask
0
and the reaction mixture was heated in an oil bath at 100 C for 16h under 1 atm O2 pressure.
The reaction was quenched with a saturated solution of Na2SO3 (for removal of excess TBHP),
and the mixture was extracted with ethyl acetate. The organic solvent was removed under
vacuum, and purification by chromatography on a silica gel column using a mixture of petroleum
ether and ethyl acetate afforded the desired product.
General Procedure for Cu-mPMF Catalyzed Esterification of Acids:
Acids (0.3 mmol), CuꢀmPMF (20 mg), TBHP (1.8 mmol, 70% aqueous solution), and DMSO
(2.0 mL) were added to a 50 mL round bottom flask in air, and the reaction mixture was heated
0
in an oil bath at 100 C for 16h. The reaction was quenched with a saturated solution of Na2SO3
(for removal of excess TBHP), and the mixture was extracted with ethyl acetate. The organic
solvent was removed under vacuum, and purification by chromatography on a silica gel column
using a mixture of petroleum ether and ethyl acetate afforded the desired product.
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General procedure for the secondary amide synthesis:
Nitriles (1 mmol), anilines (1.3 mmol) and CuꢀmPMF (20 mg) in water (5mL) were put into a 50
0
mL round bottom flask. The reaction mixture was stirred at 80 C for 14h and then cooled to
room temperature and extracted with ethyl acetate. The organic layer was dried over Na2SO4 and
the solvent was removed under reduced pressure. The residue was purified by silicaꢀgel column
chromatography to afford the corresponding product.
Result and discussion
Characterization of Cu-mPMF material
X-ray diffraction: In Figure 1 powder Xꢀray diffraction pattern of mPMF and CuꢀmPMF are
shown. The mesoporous polymer, mPMF shows one broad diffraction peak cantered at 2θ =21,
which correspond to characteristic peak of the mPMF. Nevertheless, like other porous polymers
based on crossꢀlinking,39 mPMF is mostly amorphous. In the XRD pattern of the CuꢀmPMF
shows the additional diffraction peaks at 2θ = 10.67, 12.85 and 25.74 degrees could be assigned
to presence of crystalline features originated due to the binding of Cu(OAc)2 at the pore
surface.39 Thus, overall intensity of the CuꢀmPMF increased than mPMF due to the crystalline
features of CuꢀmPMF.
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Figure 1 powder XRD pattern of mPMF and CuꢀmPMF materials.
HR-TEM analysis: Fig. 2 demonstrates HRꢀTEM images of copperꢀgrafted mesoporous polyꢀ
melamineꢀformaldehyde material. A foamꢀlike interconnected mesoporous network structure was
observed under HRꢀTEM images (2a and 2b). Figure 2c and 2d suggested the existence of high
electron density dark spots of dimension ca. 7 nm throughout the specimen, which are assigned
to copper(II)ꢀparticles in CuꢀmPMF.
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Figure 2 (aꢀd) HRꢀTEM images of CuꢀmPMF material.
Scanning electron micrographs (SEM) analysis: The scanning electron micrographs of the
CuꢀmPMF are shown in Fig. 3A and B. This FEꢀSEM images indicates uniform spherical
morphology of the polymer CuꢀmPMF material and spherical particles get aggregated among
themselves to form large assembly of particles. Energy dispersive spectroscopy analysis of Xꢀ
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rays (EDAX) data of the CuꢀmPMF (Fig.S1) also inform that the attachment of metal on the
surface of the polymer matrix. The elemental mapping (Fig. 3 cꢀf) indicates the presence of
carbon, nitrogen, oxygen and Cu ions in the composites.
Figure 3 (a) ꢀ (b) scanning electron micrographs and (c)ꢀ(f) elemental mapping of the CuꢀmPMF
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Specific surface area
The surface area of the CuꢀmPMF determined by N2ꢀadsorption study reveals that Cuꢀ
mPMF exhibits specific surface area of about 170.4 m²/g (Fig.4). The BET surface areas of
mPMF is 930.0 m2 gꢀ1 with an average pore size of 15.7 nm.23 Considerable decrease in the
surface area to 170.4 m²/g in CuꢀmPMF suggests that copper acetate particles are anchored at the
surface of the mPMF.
Figure 4 N2 adsorption/desorption isotherms of CuꢀmPMF
Raman spectroscopy
In addition to the optical spectroscopy, Raman spectroscopy also provides useful
information regarding the interaction of metal and mesoporous mPMF. Raman spectra of the
mPMFꢀCu shows two well documented D and G band at 1412 and 1551 cmꢀ1 respectively
(Fig.5). The D band at 1412 cmꢀ1 is attributed due to the defect or disorder in carbon atom and G
band at 1551 cmꢀ1 is attributed due to sp2 in plane vibration of carbon atom. Raman spectroscopy
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of mesoporous CuꢀmPMF showed different intensity peaks from each component. The peaks at
970 and 665cm−1 corresponded to T band and copper acetate respectively.40
Figure 5 Raman spectroscopy of CuꢀmPMF
Electron paramagnetic resonance study
Electron paramagnetic resonance (EPR) of copperꢀgrafted mesoporous organic polymer
CuꢀmPMF at room temperature is shown in Figure 6. In the mesoporous organic polymer Cu is
anchored at the surface of the polymer matrix through the donor nitrogen sites. The spectrum
shows four wellꢀdefined hyperfine lines which arises due to the coupling of the unpaired electron
with the nuclear spin of Cu(II).41 The g and A values of the fresh catalyst are 2.05 and 326.8 mT
respectively.
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Figure 6 The Xꢀband EPR spectrum of Cu(II) in (a) fresh and (b) used catalyst for CuꢀmPMF
X-ray photoelectron spectroscopy (XPS): The mesoporous polymelamine formaldehyde
grafted copper acetate sample contains carbon and nitrogen as the major elements. The XPS data
showed peaks centred at 956.1, 936.6, 404.1 and 287.8 eV (Fig. 7). These binding energies
correspond to Cu2p1/2, Cu2p3/2, N1s and C1s, respectively. The C peak at 287.8 eV is identified
as sp2ꢀbonded carbon (C=N) in good agreement with literature. N1s spectrum signals centered at
404.1 corresponds to nitrogen present in copper grafted mPMF. The binding energy of the
Cu2p3/2 peak at 936.3 eV and indicates that copper species are only present as Cu(II) state in
mesoporous CuꢀmPMF. The binding energy values 936.6 and 956.1 eV corresponding to the spin
orbit splitting components of Cu 2p3/2 and Cu 2p1/2 in the +2 oxidation state of copper agrees
well with the reported literature values. There is also a satellite peak present in the XPS
spectrum, which is a feature of 2p → 3d satellite peak of Cu in +2 oxidation state.42
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Figure 7 Xꢀray photoelectron spectroscopy (XPS) analysis of CuꢀmPMF.
Catalytic activities:
Since supported copper systems exhibit catalytic activity in a wide range of the
industrially important processes and have been extensively studied, we have decided to
investigate the catalytic activity of CuꢀmPMF in the field of esterification of benzylic alcohols
and synthesis of secondary amides from nitriles and anilines.
Cu-mPMF catalyzed estrification:
To test the catalytic activity of the present catalyst, first esterification of benzylic alcohols
0
was tested at 100 C for 16h and TBHP was used as the methyl source as well as oxidant. For
optimization of reaction conditions, we choose the benzyl alcohol as a substrate for the
esterification reaction (Scheme 3). In this case benzyl alcohol is selectively converted to methyl
benzoate in the presence of CuꢀmPMF, tertꢀbutyl hydroperoxide (TBHP) and TEAB in DMSO at
0
100 C under oxygen atmosphere (1 atm). But the conversion of benzyl alcohol into desired
product was increased when base was introduced.
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O
C
OCH3
Cu-mPMF / Solvent
OH
+
TBHP
TEAB /Base
100 0C / O2(1 atm)
R
R
Scheme 3 Cu-mPMF catalyzed esterification of benzyl alcohol
It was found that sodiumethoxide afforded a 33 % yield of desired ester. But in the presence of
potassium carbonate 85% desired ester was formed. From Table 1 it was cleared that DMSO was
the only effective solvent for this reaction. Only a low yield was obtained when DMSO was
replaced by DMF. No target product was observed in other solvents such as toluene and THF.
Table 1 effect of solvent and base on the esterification of benzyl alcohol
Entry
Solvent
DMSO
DMSO
DMSO
DMF
Base
NaOEt
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Yield (%)
1
2
33
67
85
6
3
4
5
THF
NR
NR
82
NR
78
6
Toluene
DMSOꢀD6
DMSO
DMSO
7a
8b
9c
16

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10
DMSO
ꢀ
15
Reaction conditions: Benzyl alcohol (0.3 mmol), CuꢀmPMF (20 mg), TBHP (2.4 mmol),
TEAB (0.15 mmol), base (0.6 mmol), Solvent (2 mL), 100 0C, O2 (1 atm), 16 h.
a TBHP in decane instead of its aqueous solution (70 wt % TBHP).
b DTBP or DCP oxidant used instead of TBHP.
c Reaction carried out at 90 0C.
The present esterification was very sensitive to the reaction temperature, when temperature was
reduced to 90 0C from 100 0C a lower yield (78%) was obtained (Table 1, entry 9). Interestingly,
no reaction occurred in the absence of TBHP or when TBHP was replaced with other oxidant
like diꢀtertꢀbutyl peroxide (DTBP) or dicumyl peroxide (DCP). Under the same conditions, the
control experiment showed that the yield would be reduced in the absence of oxygen. The
0
maximum yield was obtained in the presence of CuꢀmPMF, K2CO3, TEAB in DMSO at100 C
and oxygen (1atm) (Table 1, entry 3). In this reaction benzyl alcohol first oxidized to acid under
the reaction condition. Literature study suggests that in the presence of copper catalyst TBHP
decomposes to generate the methyl radical. Next, acyloxyl radical is generated from benzoic acid
by the reaction with tertꢀbutoxyl or tertꢀbutylperoxyl radical and then coupling of methyl radical
and acyloxyl radical gives the corresponding ester. When other oxidants instead of TBHP
(methyl source) were used no desired product was formed, only a mixture of aldehyde and acid
were obtained. To confirm whether the methyl came from TBHP or DMSO, we carried out the
control reaction in DMSOꢀd6 with TBHP. An 82% yield of ester was obtained with no deuterated
ester detected, thus demonstrating the methyl group came from TBHP.
To examine the scope of this esterification reaction, a variety of various benzylic alcohol
derivatives were examined under optimized conditions. The experimental results are summarized
in Table 2. As shown in Scheme 3, the protocol well tolerated diverse electronic and steric
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substituents of the benzylic alcohol, as well as heterocyclic substrates, all resulting in good to
excellent yields. The substitution of electron withdrawing and electron donating groups on the
aryl ring of alcohol has influence on the outcome of the reaction. Benzylic alcohols bearing
either electronꢀdonating or electronꢀwithdrawing substituents, afforded the corresponding esters
in good to excellent yields. Strong electronꢀdonating groups such as methoxy group containing
benzylic alcohol resulted in slightly excess yields than benzylic alcohol with electronꢀ
withdrawing groups, such as Cl, Br and NO2 (Table 2, entries 6ꢀ8). Even heteroaromatic proved
effective for this esterification reaction as depicted with the thiophene ring system leading to
products (Table 2, entry 12). Unfortunately, the present catalytic system is less effective for
aliphatic compound (Table 2, entry 15).
Table 2 CuꢀmPMF Catalyzed Methyl Esterification of Various Benzylic Alcohols
Entry
1
Substrate
Product
Yield (%)
85
O
OH
C
OMe
O
OH
2
3
88
95
C
OMe
O
OH
C
OMe
MeO
MeO
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OMe
OMe
O
4
5
75
83
C
OH
OH
OMe
OMe
MeO
MeO
MeO
MeO
O
C
OMe
OMe
O
OH
6
7
8
9
77
76
C
OMe
OMe
Br
Br
Cl
O
OH
C
Cl
OH
O
58
68
C
O2N
OMe
OMe
O2N
O
OH
C
NC
NC
Br
Br
O
10
C
71
OH
OMe
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OH
NO2
O
11
12
64
C
NO2
OMe
OMe
O
OH
C
72
78
S
S
O
OH
C
13
14
OMe
O
OH
C
89
49
OMe
O
C
OH
15
OMe
Reaction conditions: Alcohol (0.3 mmol), CuꢀmPMF (20 mg), TBHP (2.4 mmol),
TEAB (0.15 mmol), K2CO3 (0.6 mmol), DMSO (2 mL), 100 0C, O2 (1 atm), 16 h.
After the successful application of the oxidative methyl esterification from benzylic
alcohols, we tried to extend this methodology to carboxylic acids as substrates. It was found that
base and additive TEAB were not necessary in the methyl esterification of the acids and 1.8
mmol of TBHP was sufficient for the reaction. Subsequently, various acid derivatives were
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examined, and representative results are listed in Table 3. Similar to benzylic alcohols different
substituted acids were subjected to the esterification reactions, as shown in Table 3. The results
indicate that acids with electronꢀwithdrawing or electronꢀdonating groups were all wellꢀtolerated
and provided the corresponding products in good to excellent yields. As shown in Table 3,
benzoic acid with methoxyl groups at different positions on the phenyl ring all worked smoothly
under CuꢀmPMF/ TBHP catalytic system, giving 84–97% yields of the desired esters (Table 3,
entries 2ꢀ4). The electronꢀwithdrawing group substituted acids (Table 3, entries 5ꢀ9) could also
be efficiently converted to the expected esters. The hetero cyclic substrates were reacted
smoothly under the optimized conditions (Table 3, entries 13, 14). The use of acids instead of
alcohols provides almost similar yield of desired product. This observation also suggests that the
above reaction (Scheme 3) may follow the following reaction sequence, alcohol
acyloxyl radical ester.
acid
Table 3 CuꢀmPMF Catalyzed Methyl Esterification of Various Acids
Entry
Substrate
Product
Yield (%)
O
COOH
C
1
92
OMe
COOH
O
C
2
97
OMe
MeO
OMe
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COOH
COOH
OMe
O
C
3
4
84
MeO
OMe
OMe
OMe
OMe
O
Br
87
55
Br
C
OMe
OMe
COOH
O
C
5
OMe
O2N
NO2
COOH
O
C
NO2
6
7
64
66
OMe
NO2
COOH
O
C
OMe
NO2
NO2
COOH
O
C
8
67
OMe
NC
CN
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COOH
O
C
9
74
77
OMe
C
O
C
O
COOH
O
C
10
11
OMe
O
COOH
C
91
89
OMe
O
COOH
12
C
OMe
O
13
14
73
65
C
C
COOH
COOH
S
S
OMe
O
O
O
OMe
Reaction conditions: Alcohol (0.3 mmol), CuꢀmPMF (20 mg), TBHP (1.8 mmol),
DMSO (2 mL), 100 0C, 16 h.
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Cu-mPMF catalyzed amides synthesis
After successful synthesis of various esters from benzylic alcohols, we have extended the
applications of our newly synthesized CuꢀmPMF for the synthesis of secondary amides from
various nitriles and anilines.
NH2
H
N
CN
Cu-mPMF
+
H2O / 80 0C
O
Scheme 4 CuꢀmPMF catalyzed secondary amide synthesis
At the initial stage, a model reaction of phenylacetonitrile and aniline was carried out in aqueous
0
acetonitrile solvent at 80 C. A yield of 48 % N, 2ꢀdiphenylacetamide was obtained. Less polar
solvent like toluene was less effective. When this reaction was carried out in water medium 85 %
yield was obtained and thus water was chosen as the most effective solvent. This coupling
reaction was found to be highly sensitive to the reaction temperature. At lower temperatures (50ꢀ
0
0
65 C) only low to moderate yield was obtained. A reaction temperature of 80 C was found to
be optimal for the model reaction. Also the reaction was carried out for different time ranging
from 10 h to 16 h and it was found that at 14 h the conversion was 77% at given conditions.
Literature study showed that nitrile directly reacted with amine to form an unstable amidine
intermediate which can be easily hydrolyzed to give corresponding amide.43
To understand the substrate scope of this reaction, a wide range of phenylacetonitriles
and anilines were reacted under the optimized reaction conditions (Table 4). Good to excellent
yields of secondary amides were obtained in most cases. Electron donating or electronꢀ
withdrawing substituents on nitriles or anilines produce good to excellent yields of secondary
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amides, electron donating groups are more effective than electron withdrawing, indicating that
the reaction is slightly sensitive to electronic effects. Anilines or nitriles bearing substituent at
ortho position resulted in lower yields than substituents at meta or para position. For example, 4ꢀ
methoxyaniline gave the desired product in 92% and, while 4ꢀchloro and fluoroaniline resulted in
76% and 72% yields, respectively (Table 4, entries 11, 12).
Table 4 CuꢀmPMF Catalyzed secondary amides synthesis from nitriles and anilline
Entry
1
Nitriles
Anilines
Product
Yield (%)
77
NH2
H
N
CN
CN
O
NH2
NH2
H
N
2
3
80
83
O
O
H
N
CN
NH2
NH2
H
N
CN
85
92
4
5
O
H
N
CN
O
MeO
MeO
H
N
NH2
6
CN
61
O
CF3
CF3
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H
N
NH2
7
8
CN
68
O
Cl
Cl
H
NH2
N
CN
63
86
O
Br
Br
NH2
H
N
CN
9
O
NH2
NH2
CN
CN
H
N
84
76
10
11
O
O
H
N
Br
Cl
F
Br
NH2
H
N
12
13
CN
72
68
58
O
Cl
NH2
H
N
CN
O
F
NH2
H
N
C
O
14
H3C
CN
Reaction conditions: Nitriles (1 mmol), Anilines (1.3 mmol), CuꢀmPMF (0.1 mmol) in H2O (5
mL) at 80 0C for 14 h.
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Possible reaction mechanism:
Although the exact reaction sequence was not determined, based on our experimental
results and former mechanistic studies44ꢀ49, the reaction mechanisms of these reactions are
proposed in Scheme 5. In case of methyl esterification reaction various literatures suggested that
TBHP can produce methyl radical via tertꢀbutoxyl and tertꢀbutylperoxyl radicals. In the presence
of oxidants and additive benzyl alcohol is transformed into corresponding acid. Subsequently,
the acyloxyl radical is generated from benzoic acid by the reaction with tertꢀbutoxyl or tertꢀ
butylperoxyl radical. Finally, coupling of the acyloxyl and methyl radicals gives the desired
ester. But in case of amide synthesis reaction, exact reaction pathway is not clear to us. Various
literatures47ꢀ49 suggest that initially nitrile is hydrolyzed to give primary amide then it reacts with
amine to give secondary amide. On the other hand, some groups suggested that nitrile first reacts
with amine to give an unstable intermideate (amidine) which can be easily hydrolyzed to
secondary amide.
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TBHP
O
C
O
C
O
.
.
OH
.
[O]
OH
T-BuO or T-BuOO
O
C
O
.
CH3
NH
O
C
Ar-NH2
H2O
Cu-mPMF
Ar
Ar
C
NHAr
NHAr
Ar-CN
Scheme 5 Proposed reaction mechanism of CuꢀmPMF catalyzed esterification and amidation
Catalyst reusability
Recovery and catalyst reuse are important issues in the heterogeneous catalysis. We
studied the reusability of the present heterogeneous copper catalyst in the esters and amides
synthesis reactions (Fig. 8). After completion of the reaction, the catalyst was recovered by
simple filtration and washed with ethyl acetate followed by acetone then dried in reduced
0
pressure at 40 C. The recovered catalyst was employed in the next run with further addition of
substrates in appropriate amount under optimum reaction conditions. The catalyst show almost
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same activity up to 7 reaction cycles. No catalyst deterioration was observed, confirming the
high stability of the heterogeneous catalyst under the reaction conditions. Further, no evidence
for leaching of copper or decomposition of the catalyst was observed during the catalytic
reaction and no metal could be detected by atomic absorption spectroscopic measurement of the
filtrate after removal of catalyst. These studies clearly demonstrated that metal was intact to a
considerable extent with the heterogeneous support, and there is no significant amount of
leaching occurred during reaction.
Figure 8 Catalyst reusability test of the mPMF supported copper catalyst.
Conclusions
In conclusion, we have reported the preparation and characterization of mesoporous
polymer supported Cu(II) complex and its successful applications for the methyl esterification of
benzylic alcohols in the presence of TBHP, which use as the methyl source. Both electronꢀrich
and electronꢀdeficient benzylic alcohols could be applied and the nature of the base, temperature
and the solvent system proved crucial for the transformation. The present system is highly air
and moisture stable and the catalyst can be synthesized readily from inexpensive and
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