Polystyrene-supported CuI-imidazole complex catalyst for aza-Michael reaction of imidazoles with α,β-unsaturated compounds

Article abstract of DOI:10.1016/j.molcata.2011.11.023

The polystyrene-supported CuI-imidazole complex catalyst was prepared and characterized by IR spectroscopy, elemental analysis, SEM/TEM and TG/DSC. The complex catalyst was globular with the size of 400-500 nm and showed a good thermal stability at high temperature. The immobilized Cu metal in the complex catalyst was about 0.85 mmol/g. By using the catalyst, aza-Michael reaction of imidazoles to α,β-unsaturated compounds was performed with good yields in 4-8 h. The catalyst showed an excellent recycling efficiency over five cycles without distinct leaching of metal from the polymer support.

Full text of DOI:10.1016/j.molcata.2011.11.023

Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
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Polystyrene-supported CuI–imidazole complex catalyst for aza-Michael reaction
of imidazoles with ␣,␤-unsaturated compounds
Lixia Lia,b, Zuliang Liua, Qilong Linga, Xiaodong Xinga,∗
a School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
b School of Environment, Jiangsu University, Zhenjiang 212013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The polystyrene-supported CuI–imidazole complex catalyst was prepared and characterized by IR spec-
troscopy, elemental analysis, SEM/TEM and TG/DSC. The complex catalyst was globular with the size of
400–500 nm and showed a good thermal stability at high temperature. The immobilized Cu metal in the
complex catalyst was about 0.85 mmol/g. By using the catalyst, aza-Michael reaction of imidazoles to
␣,␤-unsaturated compounds was performed with good yields in 4–8 h. The catalyst showed an excellent
recycling efficiency over five cycles without distinct leaching of metal from the polymer support.
© 2011 Elsevier B.V. All rights reserved.
Received 5 September 2011
Received in revised form
23 November 2011
Accepted 24 November 2011
Available online 3 December 2011
Keywords:
Polystyrene-supported CuI–imidazole
complex catalyst
Aza-Michael reaction
Imidazoles
␣,␤-unsaturated compounds
1. Introduction
chemicals, or produce products in moderate yield. Furthermore,
most of the catalysts could not be recycled.
Aza-Michael reaction (AMR, i.e. conjugated addition of nitro-
gen nucleophiles to ␣,␤-unsaturated compounds) is an important
C–N bond forming reaction and usually used for the synthesis
of ␤-amino carbonyl functionality which serves as an essential
intermediate in the synthesis of ␤-amino ketones, ␤-amino acids,
␤-lactam antibiotics, and chiral auxiliaries [1–3]. These compounds
were previously synthesized via the Mannich reaction and featured
by rigorous reaction conditions, low reaction yield and tedious
reaction pretreatment [4], which promoted to develop other more
effective and green synthesis methods. As noticed in the early
1960s, AMR is a less by-products and atomic economical reac-
tion. The AMR of imidazoles to ␣,␤-unsaturated compounds for the
synthesis of N-substituted imidazoles has been a current area of
investigation, in view of their pharmacological importance. To get
a simple, green and efficient synthesis procedure, many alternative
strategies have been developed in recent twenty years. In particu-
lar, various catalysts have been investigated such as: K2CO3/DMF
[5], Bi(NO)3 [6], guanidine [7], [Ti4H11(PO4)9]·nH2O [8], alkaline
protease and hydrolase [9–11]. Unfortunately, most of these proce-
dures require long reaction times (several days), highly dangerous
Recently, some alternative strategies with good yield have been
reported. Martín-Aranda reported [12] microwave-assisted syn-
thesis of N-substituted imidazoles using basic clay catalysts. Later
a number of studies about microwave-promoted Michael reac-
tions were reported [13–17]. Many studies showed that microwave
and sonochemical [18] could effectively promote AMR reactions
to get aim-products in very short time with good yields. There
are, however, limitations associated with such synthesis methods,
in particular poor reproducibility of reactions and hard control-
ling the reaction precisely [19], which are disadvantageous to
industrialize. Xia reported [20] a highly efficient KF/Al2O3 cata-
lyst for hetero-Michael addition of ␣,␤-ethylenic compounds in
room temperature, but these reactions required long reaction
time and the efficient of catalyst decreased rapidly after several
recyles. More recently, a series of ionic liquid catalyst system
[23] has been reported for the reaction of N-heterocycles with
␣,␤-unsaturated compounds at room temperature (rt), which usu-
ally was carried out at short time, however, the preparation and
treatment process of the catalyst was expensive and inconve-
nient. Recent years, N-arylation of imidazoles reactions catalyzed
by PS[DMVBIM][Pro]-CuI [24], Cu(acac)2 in [bmim][BF4] [25], PANI-
CuI [26] were reported. The results of these studies showed that the
copper catalysts were very efficient and reusable for such reactions.
In addition, Mirkhani had successfully bonded [Mn(salophen)Cl]
to imidazole modified polystyrene to use as biomimetic alkene
∗
Corresponding author.
E-mail address: xingxiaodong01@mails.tsinghua.edu.cn (X. Xing).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.11.023

L. Li et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
179
Scheme 1. Preparation of the PS–imCuI catalyst.
epoxidation and alkane hydroxylation catalyst with sodium peri-
odate [27]. Here, polystyrene-bound imidazole was not only
a support for immobilization of Mn(III) salophen but also a
heterogeneous axial base. Udayakumar reported [28] polystyrene-
supported palladium–imidazole complex catalyst for hydrogena-
tion of substituted benzylideneanilines. These results prompted
us to investigate the catalytic activity of polystyrene-supported
CuI–imidazole (PS–imCuI) complex in N-arylation reactions.
Polystyrene (PS) is one of the most widely used support
polymers in various catalyzed organic synthesis due to its easy
preparative protocol from inexpensive starting material, high envi-
ronmental stability and easy functionalised properties. In this
study, highly chloromethylated polystyrene beads (VBC-DVB) were
functionalised with imidazole, then CuI was anchored on the func-
tionalised beads to get the heterogeneous catalyst. FT-IR, SEM and
TG/DSC were employed to characterize the structure and property
of the catalyst. The catalytic performance of the synthesized cat-
alyst was investigated for ARM of imidazoles to ␣,␤-unsaturated
compounds.
chromatography (TLC) was performed on Merck precoated silica
gel 60-F254 plates.
2.2. Preparation of PS–CH2Cl
The PS–CH2Cl was synthesized via surfactant-free emulsion
polymerization according to the previous reports [29,30] and the
synthetic process was shown in Scheme 1. First, the glass reactor,
a 500 mL round-bottom (RB) flask, charged with 250 mL distilled
water was heated to 75 ◦C, the reaction temperature. Then stir-
ring at 450–550 rpm 13 mL comonomer mixture (volume ratio:
VBC:DVB = 98:2) was added. Following the mixture of V-50 (0.21 g,
2.95 × 10−3 mol/L) dissolved in deionized water (10 mL) was added.
The polymerization was continued under N2 for 6 h. The cooled
emulsion was dried by rotary evaporation followed by freeze-
drying to give a free-flowing fine white powder.
2.3. Preparation of PS–CH2–imidazole
Functionalisation of PS–CH2Cl with imidazole referenced to the
previous reports [31–34] and the synthetic process was shown in
Scheme 1(2) and (3). Imidazole (6.8 g, 0.1 mol) and KI (0.1 g) were
dissolved in 5 mL toluene, and the mixture was heated to 40 ◦C in
the presence of N2. Then sodium methylate (5.4 g, 0.104 mol) in
methanol (10 mL) was added dropwise with constantly stirring for
about 0.5 h to form imidazole sodium salt. The PS–CH2Cl particles
(1.8 g, 0.01 mol −CH2Cl) in acetonitrile (15 mL) were added to the
formed imidazole sodium salt solution. The reaction was continued
at 65 ◦C for 48 h. The functionalised beads were washed by ethanol
and dried under vacuum at 40 ◦C.
2. Experimental
2.1. Chemical reagents, method and equipments
Vinylbenzyl chloride (VBC, 95%, mixture of isomers, Meryer
Chemical Technology Shanghai Company), and divinylbenzene
(DVB, mixture of isomers, J&K chem. Co. Ltd., Shanghai, China)
were freed from inhibitors by reduced pressure distillation. 2,2ꢀ-
Azobis(2-amidinopropane) dihydrochloride (V-50), imidazoles,
sodium methoxide, dichloromethane, acrylonitrile, methyl acrylate
and other corresponding chemicals were commercially available
and used without further purification.
2.4. Preparation of PS–imCuI complex catalyst
IR spectra were recorded using a Nicolet IS10 FTIR spectrome-
ter. C, H and N contents were determined by dry combustion in an
autoanalyzer and analyses using Vario MICRO EL (Elementar Anal-
ysensyteme, Hanau, Germany). Cu content was determined using
Jarrel-Ash J-A1100 plasma-speetrometer. TG studies were carried
out using a METTLER TOLEDO SDTA851e instrument under nitro-
gen atmosphere. DSC studies were carried out using a METTLER
TOLEDO DSC823e instrument under nitrogen atmosphere. HPLC
studies were recorded using a Waters ODS-BP column and UV
detector, and determination wave-length was 254 nm. The 1H NMR
spectra were recorded with a FT-NMR 300 (300 MHz, bruker) spec-
trometer, and CCl3D-d6 was used as solvent. SEM was recorded on
a Leica S440i. TEM was recorded on a JEM-2100. ACME silica gel
(100 200 mesh) was used for column chromatography. Thin-layer
The synthetic process of PS–imCuI complex was shown in
Scheme 1(4) [34]. The mixture of PS–CH2–imidazole (2 g), cuprous
iodide (500 mg) and acetonitrile (30 mL) was stirred at room
temperature under nitrogen atmosphere for 48 h. The resultant
complex catalyst was filtered off and washed with acetonitrile fol-
lowed by acetone for several times. The residue was dried for 24 h
in air to afford the aquamarine complex catalyst.
2.5. General procedure for aza-Michael reaction of imidazoles
with ˛,ˇ-unsaturated compounds
In an oven dried 10 mL RB flask, imidazoles (1 mmol), ␣,␤-
unsaturated compound (1.1 mmol), PS–imCuI and solvent (2 mL)

180
L. Li et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
were taken and stirred at appropriate temperature for appropri-
ate time. After the completion of the reaction as monitored by TLC
(dichloromethane: petroleum ether = 1:1, V/V), the catalyst was fil-
tered off and washed with water, ethanol and acetone successively,
and then dried under room temperature several hours. The filtrate
was diluted with 15 mL ethyl acetate and washed with 5 mL sat-
urated aqueous NaCl solution. The organic layer was dried over
anhydrous Na2SO4 and concentrated to get the crude product.
The crude product was column chromatographed using mixture
of dichloromethane and petroleum ether as eluent. The products
were identified by NMR and compared with those reported in the
literature [9–11,22].
3. Results and discussion
3.1. Characterization of the catalyst
3.1.1. Infrared spectral studies
Fig. 1. IR-spectra of PS–CH2Cl (a) and PS–CH2–imidazole (b).
he IR spectra of PS–CH2Cl and PS–CH2–imidazole were recorded
and a comparative study was shown in Fig. 1. For PS–CH2Cl (Fig. 1a),
the characteristic peaks of polystyrene exhibited the C−H stretch-
ing vibration of aromatic ring at 3016 cm−1, and C−H asymmetric
and symmetric stretching vibrations of methylene at 2920 cm−1
and 2855 cm−1 [35], respectively. The peaks at 1609 cm−1 and
1507 cm−1 were due to the C−C skeleton vibration of aromatic
ring of polystyrene. Moreover, a typical peak at 1265 cm−1 was
attributed to stretching vibrations of the functional group −CH2Cl,
and another peak at 672 cm−1 was due to stretching vibrations of
C−Cl. The two typical peaks were practically omitted or were seen
as a weak band after introduction of imidazole on the polystyrene
and a peak assigned to the C N stretching vibration of imida-
zole ring at 1563 cm−1 [36,37] appeared at the same time(Fig. 1b),
which was indicated the successful attachment of imidazole on
the polystyrene support by covalent bond. In addition, no obvi-
ous band changes were observed when CuI was anchored on to
PS–CH2–imidazole beads.
3.1.2. Elemental analysis
Elemental analysis for PS–CH2Cl, PS–CH2–imidazole and
PS–imCuI complex catalyst were carried out and the data were
tabulated in Table 1. It can be estimated that the Cl content in
PS–CH2Cl was about 21.47% which meant that the polystyrene
beads could be highly functionalised with imidazole. The content
of nitrogen in PS–CH2–imidazole was 11.17%, which meant some
chloromethyl groups inside the polystyrene were not displaced
Fig. 2. SEM of PS–CH2Cl (A) and PS–imCuI catalyst (B), TEM of PS–CH2Cl (C) and PS–imCuI catalyst (D).

L. Li et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
181
Table 1
Elemental analysis of polymer support and polymer bound catalyst.
%C
%H
%N
%Cu
Scheme 2. Model reaction of N-substituted imidazoles using PS–imCuI complex
catalyst.
PS–CH2Cl
PS–CH2–imidazole
PS–imCuI
72.32
73.76
–
6.21
6.90
–
–
–
–
11.17
–
5.41%, 4.8%a, 4.76%b
a
surface could improve the thermal stability of PS–CH2Cl, and the
PS–imCuI catalyst was thermo-stable at high temperature.
After second cycle.
After fourth cycle.
b
3.2. Aza-Michael reactions of imidazoles with ˛,ˇ-unsaturated
compounds
by imidazole. Since the content of nitrogen was still very high,
the PS–CH2–imidazole could immobilize a lot of CuI. The amount
of metal was determined by stripping the bound metal from the
support and analyzed using inductively coupled plasma atomic
emission spectrometry. The content of Cu metal in PS–imCuI com-
plex catalyst were 5.41% (about 0.85 mmol/g), 4.8% and 4.76% at
beginning, after second and fourth cycle, respectively.
3.2.1. Catalyst activity of PS–imCuI in model reaction and
optimized reaction conditions
To evaluate the catalyst activity of PS–imCuI in AMR of imi-
dazoles with ␣,␤-unsaturated compounds, the reaction between
imidazole and acrylonitrile was chosen as a model reaction
(Scheme 2). The influences of solvent, temperature, catalyst dosage
and reaction time on the AMR process were investigated in detail
and the results were summarized in Table 2. Six kinds of solvent on
the reaction were studied at constant reaction temperature, cat-
alyst dosage and duration (Table 2, entries 1–6). Best yield was
obtained by using DMF as solvent (Table 2, entry 4). In addition,
it can be seen that reaction temperature had a great effect on
the reaction (Table 2, entries 4, 7–9). A higher reaction temper-
ature resulted in a better yields, and the tendency became slow
when the temperature over 70 ◦C. The influence of catalyst dosage
(molar ratio, relative to imidazole) on the reaction was carried out
at constant substrate concentration at a temperature of 75 ◦C using
DMF as a solvent. It is noteworthy that no product formation was
detected without PS–imCuI complex catalyst (Table 2, entry 10),
which meant that the reaction hardly carried out without cata-
lyst in DMF. When homogeneous CuI (8.5 mol%) was used instead
of PS–imCuI catalyst, very low amount of product formation was
observed (Table 2, entry 12b). Here most probably imidazole group
in the complex catalyst is playing a macro ligand which facilitates
the reaction as reported by using other nitrogen containing ligands
[38–40]. With the increase of the catalyst dosage, the yields increase
quickly. But the increase slow down when the catalyst usage over
0.1 g (Cu, 8.5 mol%). At constant substrate concentration, reaction
temperature of 75 ◦C, PS–imCuI (Cu, 8.5 mol%), DMF as solvent, the
influence of reaction duration time on the model reaction was stud-
ied (Table 2, entries 12, 14–16) at last. The reaction carried out 92%
3.1.3. SEM/TEM and TG/DSC studies
SEM of PS–CH2Cl (A) and PS–imCuI catalyst (B), and TEM of
PS–CH2Cl (C) and PS–imCuI catalyst (D) were shown in Fig. 2. It
was seen both PS–CH2Cl and PS–imCuI catalyst showed uniform
globular morphologies with the size of 400–500 nm. After the intro-
duction of CuI–imidazole complex, the size and the shape had no
obvious change.
TG/DSC analysis was carried out to investigate the thermal
stability of PS–CH2Cl and PS–imCuI catalyst in N2 atmosphere
with heating rate of 10 ◦C/min up to 500 ◦C. As shown in Fig. 3,
weight loss of PS–imCuI catalyst and PS–CH2Cl began at about
110 ◦C (Fig. 3aꢀ and a). In the range of 110–300 ◦C, no obvious
weight loss was observed from PS–imCuI catalyst. Comparatively,
obvious weight loss was observed from PS–CH2Cl. Correspond-
ingly for the DSC curve of PS–CH2Cl (Fig. 3b), a small exothermic
peak appeared around 142 ◦C, however, it did not appear in the
DSC curve of PS–imCuI catalyst (Fig. 3bꢀ). The weight loss and
the exothermic peak in the DSC curve of PS–CH2Cl were possibly
ascribed to the water inside the porous PS–CH2Cl particles. When
the CuI–imidazole complex was immobilized onto the PS–CH2Cl
surface in organic solvent, the water droved and the holes were
jammed. When the temperature further increased up to higher
than 300 ◦C, weight loss obviously displayed in TG curves of not
only PS–CH2Cl but PS–imCuI catalyst. A possible reason is that
their structures were destroyed or the chloromethyl groups and
the CuI–imidazole separated from the surface of PS. Finally, it
was observed that the residue weight of PS–imCuI and PS–CH2Cl
was about 65% and 25% at 475 ◦C, respectively. These results indi-
cated that the immobilization of CuI–imidazole onto the PS–CH2Cl
Table 2
Optimization of catalytic conditions for the AMR using imidazole and acrylonitrile.a
Entry
PS–imCuI (g)
Solvent
Temperature (◦C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0
0.05
0.1
0.15
0.1
0.1
0.1
MeOH
CH3CN
DMSO
DMF
Acetone
THF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
60
60
60
60
60
60
50
70
85
75
75
75
75
75
75
75
8
8
8
8
8
8
8
8
8
4
4
4
4
6
8
10
58
63
52
71
47
58
60
88
91
0
66
92, 5c
93
93
93
94
a
Reaction conditions: imidazole (1 mmol), acrylonitrile (1.1 mmol), solvent
(2 mL).
b
Determined by HPLC based on imidazole, corresponding pure product as internal
standard substance.
c
Fig. 3. TG-DSC patterns of PS–CH2Cl (a, b) and PS–imCuI catalyst (aꢀ, bꢀ).
CuI (8.5 mol%) used as catalyst.

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L. Li et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
The nitro-imidazole derivatives are pharmacological significance
as immunosuppressants, aldehyde dehydrogenase inhibitors and
radiotherapy synergists. It can be seen that the reaction of 4-nitro-
imidazole and 2-methyl-5-nitro-imidazole with acrylonitrile or
methyl acrylate could afford good or medium yield (Table 4, entries
9, 10 and 11) with this PS-supported CuI–imidazole complex cata-
lyst, which had not been reported by other supported CuI catalysts
[24,26]. In addition, extending the duration benzimidazole could
react with acrylonitrile and afford good yield (Table 4, entry 12).
Spectroscopic data of the products: 3-(1ꢀ-imidazole)-
propionitrile (Table 4, entry 1) 1H NMR (CDCl3) ı: 7.57 (s,
1H), 7.11 (d, 1H), 7.03 (d, 1H), 4.27 (t, 2H), 2.83 (t, 2H). Methyl
3-(1ꢀ-imidazole)-propionate (Table 4, entry 2) 1H NMR (CDCl3)
ı: 7.57 (s, 1H), 6.95 (s, 1H), 6.87 (s, 1H), 4.20 (t, 2H), 3.62 (s, 3H),
2.72 (t, 2H). Ethyl 3-(1ꢀ-imidazole)-propionate (Table 4, entry 3)
1H NMR (CDCl3) ı: 7.55 (s, 1H), 7.10 (d, 1H), 6.96 (d, 1H), 4.27 (t,
2H), 4.15(m, 2H), 2.77 (t, 2H), 1.24 (t, 3H). Methyl 2-methyl-3-
(1ꢀ-imidazole)-propionate (Table 4, entry 4) 1H NMR (CDCl3) ı:
7.56 (s, 1H), 7.12 (s, 1H), 6.89 (s, 1H), 4.00-4.25 (m, 2H), 3.68 (s,
1H), 2.47 (m, 1H), 1.20 (d, 3H). n-Butyl 2-methyl-3-(1ꢀ-imidazole)-
propionate (Table 4, entry 5) 1H NMR (CDCl3) ı: 7.56 (s, 1H), 7.09
(d, 1H), 6.98 (d, 1H), 6.09 (m, 1H), 5.54 (t, 1H), 4.14 (t, 2H), 1.94
(s, 3H), 1.65 (m, 2H), 1.40 (m, 2H), 0.94 (t, 3H). 3-[1ꢀ-(2-methyl
imidazole)]-propionitrile (Table 4, entry 6) 1H NMR (CDCl3) ı: 7.01
(s, 1H), 6.96 (s, 1H), 4.19 (t, 2H), 2.78 (t, 2H), 2.44 (s, 3H). Methyl
2-methyl-3-[1ꢀ-(2-methyl imidazole)]-propionate (Table 4, entry
7) 1H NMR (CDCl3) ı: 6.83 (s, 1H), 6.78 (s, 1H), 3.88–4.2 (m, 2H),
3.69 (s, 3H), 2.88 (m, 1H), 1.22 (d, 3H). Ethyl 3-[1ꢀ-(2-methyl
imidazole)]-propionate (Table 4, entry 8) 1H NMR (CDCl3) ı: 6.87
(s, 1H), 6.83 (s, 1H), 4.13 (m, 4H), 2.7 (t, 2H), 2.37 (s, 3H), 1.22 (t, 3H).
3-[1ꢀ-(4-nitro imidazole)]-propionitrile (Table 4, entry 9) 1H NMR
(CDCl3) ı: 7.96 (d, 1H), 7.61 (d, 1H), 4.40 (t, 2H), 2.98 (t, 2H). Methyl
3-[1ꢀ-(4-nitro imidazole)]-propionate (Table 4, entry 10) 1H NMR
(CDCl3) ı: 7.78 (s, 1H), 7.51 (s, 1H), 4.33 (t, 2H), 3.68 (s, 3H), 2.89
(t, 2H). 3-[1ꢀ-(2-methyl-5-nitro imidazole)]-propionitrile (Table 4,
entry 11) IR (neat): 1729, 1H NMR (CDCl3) ı: 7.79 (s, 1H), 4.29 (t,
2H), 2.89 (t, 2H), 2.55 (s, 3H). 3-(1ꢀ-benzimidazole)-propionitrile
(Table 4, entry 12) 1H NMR (CDCl3) ı: 8.01 (s, 1H), 7.83 (d, 2H),
7.35 (m, 2H), 4.50 (t, 2H), 2.90 (t, 2H).
Scheme 3. Production of N-substituted imidazoles using PS–imCuI complex cata-
lyst.
after 4 h (monitored by TLC), and as prolonging the duration to 10 h
the yield increased to 94%, which suggested that completion of the
reaction monitored by TLC was acceptable, and more reaction time
was not necessary.
From above studies, It was noted that PS–imCuI is efficient to
catalyze the AMR of imidazole and acrylonitrile, and the better reac-
tion conditions is DMF as solvent, 8.5 mol% (Cu) PS–imCuI catalyst
and reaction temperature 75 ◦C, which was also chosen to study the
latter reactions. The activity of the PS–imCuI catalyst in the AMR of
imidazole and acrylonitrile with other reported catalysts system
was further compared, as shown in Table 3. From the table, it can
be seen that the activity of PS–imCuI catalyst is more or less simi-
lar to the other reported systems, and the reaction is conducted at
shorter reaction time (4 h) in the present system.
3.2.2. Catalytic activity of PS–imCuI catalyst in different substrate
reaction
In order to further investigate the application of PS–imCuI cat-
alyst for AMR of different substrate, various activated alkenes such
as acrylonitrile, methyl acrylate, methyl methacrylate, ethyl acry-
late, and butyl methacrylate were reacted with imidazole, 2-methyl
imidazole, 4-nitro imidazole, and benzimidazole (Scheme 3). The
results were displayed in Table 4.
The results showed that PS–imCuI catalyst is efficient for these
AMR of imidazoles with ␣,␤-unsaturated compounds. It can be seen
that the yields were different for the reactions of imidazole with
different activated alkenes at same reaction conditions (Table 4,
entries 1–5). The yield of reaction of imidazole with acrylonitrile
was best, followed by with ethyl acrylate and butyl methacrylate,
and then with methyl acrylate, and the yield of reaction of imi-
dazole with methyl methacrylate was worst. The possible reasons
[41,42] are that the electronegativity of cyano group is bigger than
that of ester group in DMF so that the reaction of imidazole with
acrylonitrile is easier than with acrylate. The inductive effect of
long-chain alkoxy is bigger than that of short-chain alkoxy so that
the reaction of imidazole with ethyl acrylate is faster than with
methyl acrylate, and with butyl methacrylate is faster than with
methyl methacrylate [43]. Steric hindrance afford the reaction of
imidazole with methyl methacrylate slightly less yield than that
with methyl acrylate. Similar experiment phenomena were also
appeared in the reaction of various alkenes with 2-methyl imida-
zole and 4-nitro imidazole (Table 4, entries 6–8 and 9–10). It was
seemed that the imidazole with electron donating methyl was also
less active than simple imidazole (Table 4, entries 6 and 1), which
could be due to the stieric hindrance of the 2-methyl imidazole.
3.3. Recycling experiments
For any supported catalyst, it is important to know its ease of
separation and possible reuse. PS–imCuI catalyst can be easily sep-
arated by common filtration. The reusability of PS–imCuI catalyst
was evaluated in the AMR of imidazole with acrylonitrile at a con-
stant catalyst dosage of 8.5 mol%, substrate of 1 mmol imidazole
and 1.1 mmol acrylonitrile in 2 mL DMF at 75 ◦C with constant stir-
ring. As shown in Table 4 (entry 1), the reused PS–imCuI catalyst
still showed high activity for the yield of 3-(1ꢀ-imidazole) propioni-
trile after reusing for 5 times. It was noticed that there was a little
metal leaching from the catalyst in the first runs, and the color of the
catalyst became shallow after the first run. No metal leaching was
found at the later cycles. The possible reason was that there were
some CuI that was not immobilized well on the PS, which had not
Table 3
Comparison of activity of different catalyst system for the aza-Michael addition reaction of imidazole and acrylonitrile.
Entry
Catalyst
Reaction conditions
Yield (%)
Reference
1
2
3
4
5
6
7
[Ti4H11(PO4)9]·nH2O
KF/Al2O3
[bmim]OH
[Bmim]Im
Cu(acac)2 in [bmim][BF4]
PANI-CuI
[Ti4H11(PO4)9]·nH2O: 2 mol%, 50 ◦C, solvent-free, 5 h
KF/Al2O3: 10 mol%, rt, CH3CN, 20 h
[bmim]OH: 30 mol%, rt, 6 h
83
96
90
89
95
95
92
[8]
[20]
[21]
[23]
[25]
[26]
This study
[Bmim]Im: 2 mol%, rt, 1 h
Cu: 2 mol%, 60◦C, 8 h
Cu: 2.5 mol%, 60 ◦C, MeOH, 8 h
Cu: 8.5 mol%, 75 ◦C, DMF, 4 h
PS–imCuI

L. Li et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
183
Table 4
Aza-Michael reaction of imidazoles with ␣,␤-unsaturated compounds using PS–imCuI catalyst.a
a
Reaction conditions: imidazoles (1 mmol), alkenes (1.1 mmol), DMF (2 mL), 75 ◦C.
Determined by HPLC based on imidazole, pure product as internal standard substance.
Second cycle.
b
c
d
Fifth cycle.
e
Reaction temperature 85 ◦C.
f
Reaction temperature 100 ◦C.
been washed off when the catalyst was prepared. In addition, some
insoluble solid particles were found after fifth runs, which might
be CuI–imidazole complex that peeled off from PS–imCuI catalyst.
Metal estimation was carried out at the end of second cycle and at
the end of fourth cycle of the reaction (Table, entry 3). Percentage
of metal in the catalyst was no obvious change in both the cases.
From this it is evident that there is no distinct leaching of metal
from the catalyst. All these results indicated that PS–imCuI catalyst
was efficient and reusable.
4. Conclusion
In conclusion, the PS–imCuI catalyst was prepared and applied
in the C−N formation reaction. The catalyst showed a better

184
L. Li et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 178–184
thermal stability than that of PS–CH2Cl resin. Using the synthe-
sized catalyst, aza-Michael reactions of imidazoles with a variety
of ␣,␤-unsaturated compounds were developed with good yields
in 4–8 h. The PS–imCuI catalyst could be recovered by simple fil-
tration, and the yield had no significant decrease after reusing for 5
times. These preliminary results indicate that the PS–imCuI catalyst
was an excellent recyclable catalyst for the AMR of imidazoles with
␣,␤-unsaturated compounds and showed potential application in
industry. Further studies and additional applications are currently
underway.
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