Efficient allylic oxidation of cyclohexene with oxygen catalyzed by chloromethylated polystyrene supported tridentate Schiff-base complexes

Article abstract of DOI:10.1016/j.molcata.2010.01.003

Four novel chloromethylated polystyrene supported tridentate Schiff-base metal complexes (PS-DA-M, M = Cu²⁺, Co²⁺, Ni²⁺ and Mn²⁺, respectively) were prepared by chloromethylated polystyrene and tridentate Schiff-base ligands, which were synthesized by 2,4-dihydroxyacetophotone with 2-aminopyridine. The complexes were characterized by the methods of ICP, FT-IR, XPS and TG/DTA. The catalytic activity of the complexes in the oxidation of cyclohexene with molecular oxygen was determined. The results showed that oxidation reaction was occurred in the allylic carbon atoms. The products are cyclohexene oxide, 2-cyclohexene-1-ol, 2-cyclohexene-1-one and 2-cyclohexene-1-hydroperoxide. Although all of four complexes showed good catalytic activity, PS-DA-Cu exhibited the best. The conversion of cyclohexene can be reached to 51.9% under the temperature of 343 K when using 2 mg of PS-DA-Cu to catalytic oxidation of 2 ml of cyclohexene. The influences of reaction temperature, the amount of catalyst, the reaction time were investigated, and the mechanism of cyclohexene oxidation was also discussed.

Full text of DOI:10.1016/j.molcata.2010.01.003

Journal of Molecular Catalysis A: Chemical 320 (2010) 56–61
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Efficient allylic oxidation of cyclohexene with oxygen catalyzed by
chloromethylated polystyrene supported tridentate Schiff-base complexes
a,b,∗
a
a
a
a,b
Yue Chang
, Yurong Lv , Feng Lu , Fei Zha , Ziqiang Lei
a
College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, Gansu, China
Key Laboratory of Eco-environment-related Polymer Materials Ministry of Education, Lanzhou 730070, Gansu, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel chloromethylated polystyrene supported tridentate Schiff-base metal complexes (PS–DA–M,
Received 13 October 2009
Received in revised form 1 January 2010
Accepted 6 January 2010
2+
2+
2+
2+
M = Cu , Co , Ni and Mn , respectively) were prepared by chloromethylated polystyrene and triden-
tate Schiff-base ligands, which were synthesized by 2,4-dihydroxyacetophotone with 2-aminopyridine.
The complexes were characterized by the methods of ICP, FT-IR, XPS and TG/DTA. The catalytic activity
of the complexes in the oxidation of cyclohexene with molecular oxygen was determined. The results
showed that oxidation reaction was occurred in the allylic carbon atoms. The products are cyclohexene
oxide, 2-cyclohexene-1-ol, 2-cyclohexene-1-one and 2-cyclohexene-1-hydroperoxide. Although all of
four complexes showed good catalytic activity, PS–DA–Cu exhibited the best. The conversion of cyclohex-
ene can be reached to 51.9% under the temperature of 343 K when using 2 mg of PS–DA–Cu to catalytic
oxidation of 2 ml of cyclohexene. The influences of reaction temperature, the amount of catalyst, the
reaction time were investigated, and the mechanism of cyclohexene oxidation was also discussed.
Available online 13 January 2010
Keywords:
Polymer-supported tridentate Schiff-base
Catalytic oxidation
Allylic oxidation
Cyclohexene
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
[5,7,10,19–24]. Meanwhile, it was observed that the oxidation effi-
ciency of molecular oxygen is poor. So the great demand for these
oxidation products and the high-energy intensity of the present
process warrant a replacement with a more effective catalytic pro-
cess [25–28].
Transition metal complexes as catalysts for the oxidation
of cyclohexene have attracted a deal of attention in recent
years, mainly for the oxidation products of cyclohexene and the
derivatives present the highly reactive carbonyl groups in the
cycloaddition reactions [1–3]. Of the variety of ligands employed
so far, the noteworthy examples include porphyrin derivatives,
macrocyclic, phthalocyanine, and Schiff-base [1,2,4–12]. Among
these substrates, Schiff-base transition metal complexes are the
attractive oxidation catalysts. It can be prepared simply and cheaply
for industrial applications. However, homogenous catalyst is diffi-
cult to be separated from reaction system, and thus influence the
recycle. To overcome these disadvantagements, Schiff-base com-
plexes were immobilized with zeolite [11], modified silica [13],
chitosan [14], and chloromethylated polystyrene [15–18].
In addition, during the process of catalytic reaction, these cata-
lysts need a mono-oxygen source such as H, PhIO, NaClO, to carry
out oxygen transferring to the olefin. However, these oxidants were
not environmental friendly. With much attention has been directed
toward the development of environmental, molecular oxygen as a
cheap, environmentally clean and convenient oxidant is suitable
In this work, chloromethylated polystyrene supported tri-
2
+
2+
dentate Schiff-base metal complexes (PS–DA–M, M = Cu , Co
,
2+
2+
Ni and Mn , respectively) were prepared by chloromethylated
polystyrene and tridentate Schiff-base ligand, which was synthe-
sized by 2,4-dihydroxyacetophotone with 2-aminopyridine. It was
found that cyclohexene could be effectively catalyzed by these
complexes under mild conditions using oxygen without reductant.
The allylic hydroperoxide was obtained as an important product,
which was suggested as an efficient allylic oxidation pathway.
2. Experimental
2.1. Materials and instruments
Chloromethylated polystyrene (PS) contains 4.5 mmol of Cl/g
and of 10% cross-linked di-vinybenzene. Cyclohexene was of chem-
ical purity. Other chemicals used were of analytical purity. ICP
measurements were carried out using a Perkin-Elmer ICP/6500.
FT-IR spectra were recorded on a Digilab Merlin FTS 3000 FT-
IR spectrophotometer in KBr pellets. The reaction products of
oxidation were determined by a HP 6890/5973 GC/MS instru-
ment and analyzed by a Shimadzu GC-16A gas chromatograph
∗ Corresponding author at: College of Chemistry & Chemical Engineering,
Northwest Normal University, Lanzhou 730070, Gansu, China.
Tel.: +86 931 797 1533; fax: +86 931 797 1261.
E-mail addresses: cy1126@nwnu.edu.cn, cy70@sina.com (Y. Chang).
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.01.003

Y. Chang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 56–61
57
Fig. 1. IR spectra of complexes (a) PS–DA, (b) PS–DA–Cu, (c) PS–DA–Mn, (d)
PS–DA–Ni, and (e) PS–DA–Co.
was discharged out of the glass reactor with the gas outlet tube.
After the gas outlet tube was closed, the reactor was put into
a paraffin oil path and heated to certain temperature. The con-
sumption of oxygen was measured by the gauge glass. After the
reaction, the products were analyzed by gas chromatograph and
GC/MS.
Scheme 1. The synthesis routine of PS–DA–M.
(
8
GL-16A gas chromatograph with a 5 m × 3 mm OV-17 column,
◦ ◦ ◦ ◦
0–200 C (10 C/min), Inj. 220 C, Det. 220 C). The thermal data
were recorded on a Perkin-Elmer TG-DTA 6300 instrument at a
◦
heating rate of 15 C/min.
2.2. Synthesis of chloromethylated polystyrene supported
3. Results and discussion
tridentate Schiff-base metal complexes (PS–DA–M)
3.1. Characterization
The method of preparation of 2,4-dihydroxyacetophotone was
the sameas mentionedin Ref. [29]. The yield of 2,4-dihydroxyaceto-
photone is 65% and m.p. is 417.4 K.
Chloromethylated polystyrene (5.0 g, equivalent to 22.5 mmol
of Cl) was added into 50 ml of acetone. Then acetone solution
The IR spectra of chloromethylated polystyrene supported tri-
−
1
dentate Schiff-base ligand show sharp bands around 3428 cm
−
1
due to the ꢀO–H vibrations. It gets a shift of 11–12 cm to the
lower wave number after chelated with metal ions, showing coor-
dination is through phenol oxygen atom in the complexation
(
30 ml) of 2,4-dihydroxyacetophotone (3.42 g, 22.5 mmol) was
−
1
added to the above suspension followed by anhydrous K CO3
(Fig. 1). Meanwhile, the band 1642 cm
due to ꢀC N in the free
2
−
1
(
2.0 g), KI (0.2 g) and PEG (0.05 g), respectively. The reaction mixture
ligand gets shifted to the lower wave number (ıꢀ = 11–20 cm ) in
was stirred and refluxed for 48 h under a nitrogen atmosphere. After
cooling, an orange polymer precipitated out, which was filtered and
washed with water and anhydrous ethanol, respectively. The solid
was dried in vacuum at 323 K for 24 h to obtain chloromethylated
polystyrene supported 2,4-dihydroxyacetophotone.
Chloromethylated polystyrene supported 2,4-dihydroxyaceto-
photone (2 g, 9.0 mmol) was added to 2-aminopyridine (0.84 g, 9.0
mmol) in 50 ml of anhydrous ethanol. The mixture was heated and
refluxed for 24 h and then cooled to room temperature. The prod-
ucts obtained were filtered off and washed with ethanol and dried
at 323 K in vacuum for 24 h to give polymer-supported tridentate
Schiff-base (PS–DA).
the metal complexes, indicating coordination is through pyridine
−
1
nitrogen. Several new bands in the complexes at 489–520 cm
,
−
1
411–419 cm are due to ꢀO–M and ꢀN–M, respectively, which are
absent in the spectrum of the ligand, further supporting the partic-
ipation of the oxygen atom and the nitrogen atom in complexation.
In order to prove the coordination of polymer-supported triden-
tate Schiff-base ligand with metal ion, corresponding small area
X-ray photoelectron spectroscopy of copper acetate, ligand and its
complex have been studied (Fig. 2). Compared to copper acetate, the
binding energy of Cu2p3/2 of complex increases 3.0 eV. The change
of binding energy of the Cu2p3/2 means a decrease of its election
density. On the other hand, binding energy of N1s1/2 of polymer
complex decreases 5.1 eV than that of the corresponding ligand of
PS–DA. The electronic state of the nitrogen atom in the polymer
complex is of higher electron density and, therefore, the electrons
in the copper atom may flow into the nitrogen atom to form an
N–Cu coordination bond. The O1s1/2 binding energy of the polymer
complex is raised of 0.6 eV than that of the corresponding support,
which indicates that oxygen atom in hydroxy group is bound with
metal ion. The structure of complex can be ensured as in Scheme 1.
The thermogravimetric analyses (TG) of the PS–DA and
PS–DA–Cu were measured under nitrogen atmosphere in the tem-
perature range from 293 K to 1023 K in order to investigate the
thermal stability. It was showed that degradation of the polymer
ligand (PS–DA) was from 484 K to 1023 K (Fig. 3). And the polymer-
supported copper complex (PS–DA–Cu) was gradually from 462 K
to 1023 K. The corresponding weight loss for PS–DA, PS–DA–Cu in
above temperature range was 86%, 67%, respectively.
PS–DA was added to Cu2+ solution dissolved in 50 ml of abso-
lute ethanol, and then the mixtures were refluxed for 24 h. After
the reaction, the solids were collected by filtration, washed with
ethanol and dried at 353 K in vacuum to give chloromethylated
polystyrene supported tridentate Schiff-base copper (II) complex
(
PS–DA–Cu). The synthesis routine of PS–DA–M was shown in
Scheme 1. PS–DA–Co, PS–DA–Ni and PS–DA–Mn were prepared as
the same way, respectively.
2.3. Oxidation reaction
Typical oxidation of substrate was performed according to
the procedure described in the literature [30]. The substrate and
the catalyst (chloromethylated polystyrene supported tridentate
Schiff-base metal complexes) were added to the special glass reac-
tor. The oxygen was filled from the gauge glass and the atmosphere

5
8
Y. Chang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 56–61
Fig. 2. XPS spectrum of Cu(OAc)2, PS–DA and PS–DA–Cu (Ref. C1s = 284.80 eV).
3.2. Oxidation of cyclohexene
plexes were listed in Table 1. As can be seen, all of the four
polymer-supported tridentate Schiff-base metal complexes have
catalytic activity for the oxidation of cyclohexene and high turnover
numbers have been obtained. Oxidation reactions occurred in
The results of cyclohexene oxidation catalyzed by chloromethy-
lated polystyrene supported tridentate Schiff-base metal com-
Table 1
Effect of various metal ions on oxidation.
Complexes
ICP (%)
Conversiona (%)
TONb (×104)
Selectivity of products (%)
EXO
–OH
C
O
–OOH
PS–DA–Mn
PS–DA–Cu
PS–DA–Co
PS–DA–Ni
0.68
7.23
0.78
1.64
38.1
51.9
22.2
32.4
3.1
8.7
1.5
12.5
10.6
17.2
33
22.7
26
25.3
40.9
10.6
14.7
48.7
24
54.1
48.6
0.47
2.29
1.17
Temperature 343 K; catalyst, 2 mg; cyclohexene, 2 ml; 1 atm O2 and 10 h.
a
Conversion and selectivity were determined by GC.
Moles of substrate converted per mole of metal in the catalyst.
b

Y. Chang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 56–61
59
Fig. 4. Relationship between consumption of O2 and the reaction time at the differ-
ent temperature (catalyst: 2 mg; cyclohexene: 2 ml).
Table 2
Effect of reaction temperature on oxidation.
Temperature (K)
Conversion (%)
Selectivity of products (%)
EXO
–OH
C
O
–OOH
3
3
3
3
3
13
23
33
43
53
12
19
32
51.9
39.7
11.1
9.3
7.8
1.5
2.5
26.1
18.8
27.6
33
13.8
14.5
24.6
40.9
48.6
48.9
57.2
37.4
24
Fig. 3. TG/DTA of PS–DA (a) and PS–DA–Cu (b).
44.3
4.5
Substract 2 ml; catalyst 2 mg; 10 h.
allyl carbon atoms and the products are cyclohexene oxide (EXO),
2
-cyclohexene-1-ol (–OH), 2-cyclohexene-1-one (–C O) and 2-
cyclohexene-1-hydroperoxide (–OOH) (Scheme 2). The conversion
of cyclohexene catalyzed by PS–DA–Cu is higher than that of
others, but the highest turnover numbers were obtained in the
presence of PS–DA–Mn. In the same conditions, the selectivity of 2-
cyclohexene-1-hydroperoxide in the presence of PS–DA–Co is the
highest.
In order to investigate the catalytic activity of PS–DA–M, a series
of experiments were carried out by choosing PS–DA–Cu as cata-
lyst to investigate the influence of reaction temperature, amount
of catalyst and reaction time.
(–OOH) decreases. It is clear that oxidation of cyclohexene is a free
radical reaction mechanism.
3.2.2. Effect of catalyst amount
Effect of catalyst amount on oxidation at temperature of 343 K
was shown in Table 3. Although the conversion of cyclohexene and
the selectivity for EXO are nearly unchangeable with the increas-
ing amount of catalyst, it is beneficial for the transformation of
2-cyclohexene-1-hydroperoxide to 2-cyclohexene-1-one and 2-
cyclohexene-1-ol.
3
.2.1. Effect of reaction temperature
The consumption of oxygen with 2.0 mg of PS–DA–Cu as a func-
3
.2.3. Effect of reaction time
The effect of reaction time on the oxidation was also investigated
tion of the temperature was shown in Fig. 4. It is clear that the low
temperature has the longer induce period and lower rate of oxy-
gen consumption. With the temperature rising, the consumption of
oxygen has increased and reaches the highest at the temperature of
in this system. As the reaction time is prolonged, the cyclohexene
conversion and the selectivity of 2-cyclohexene-1-one increased,
while the selectivity of cyclohexene oxide and 2-cyclohexene-1-ol
decreased (Fig. 5). For 2-cyclohexene-1-hydroperoxide, the selec-
tivity increases at first and then decreases. This phenomenon is
similar to the literature [14,31]. Additionally, the conversion of
cyclohexene maintained constant after 10 h reaction. It can be
explained that substantial decrease in the concentration of the
reaction mixture.
3
43 K. After that, the consumption of oxygen decreases. At the low
temperature, the energy was not sufficient for the activation of oxy-
gen molecules or the catalytic circulation. So raising temperature is
beneficial to the oxidation reaction. The conversion and selectivity
at the different temperatures were collected in Table 2. The conver-
sion of cyclohexene at 343 K is 4.3 times than at 313 K. Although the
conversion of cyclohexene increases quickly with the increase of
the temperature, the selectivity of 2-cyclohexene-1-hydroperoxide
Table 3
Effect of the amount of catalyst on oxidation.
Cat. (mg)
Conversion (%)
Selectivity of products (%)
EXO
–OH
C
O
–OOH
1
2
3
4
5
28
9
26.2
33
38
37.7
35.6
24
40
24
13.7
11.7
22.4
51.9
31.5
32.9
33.8
1.5
5.6
5.2
4.8
40.9
42.5
48.2
37
Scheme 2. The products of the reaction.
Temperature 343 K, substract 2 ml, 10 h.

6
0
Y. Chang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 56–61
Table 4
Reuse of the catalyst.
Recycling times
ICP (%)
Conversion (%)
Selectivity of products (%)
EXO
–OH
C
O
–OOH
1
2
3
4
5
7.23
6.0
–
–
5.2
51.9
47.2
46.4
45.5
41.6
1.5
7.9
4.4
2.4
4.6
33
40.9
33.7
32.8
31.6
32.5
24
35.7
23.5
34.3
31.8
22.6
39.1
31.5
30.9
Temperature 343 K; catalyst 2 mg; substract 2 ml; 10 h.
molecular oxygen forms 2-cyclohexene-1-hydroperoxide. Then,
2
1
-cyclohexene-1-hydroperoxide is decomposed to 2-cyclohexene-
-ol and 2-cyclohexene-1-one in the presence of catalyst.
Meanwhile, 2-cyclohexene-1-hydroperoxide can form cyclohex-
ene oxide and 2-cyclohexene-1-ol by epoxidation of cyclohexene.
As the rate of the formation of 2-cyclohexene-1-ol is faster
than the formation of 2-cyclohexene-1-one, the selectivity of 2-
cyclohexene-1-ol is higher than that of 2-cyclohexene-1-one at
reaction initial stage or at low temperature (Table 2 and Fig. 5).
At high temperature, 2-cyclohexene-1-ol can be continually oxi-
dized to 2-cyclohexene-1-one in the presence of catalyst. So
the selectivity of 2-cyclohexene-1-one is higher than that of 2-
cyclohexene-1-ol in final stage.
Fig. 5. The effect of reaction time on oxidation of cyclohexene (catalyst: 2 mg;
cyclohexene: 2 ml; 343 K).
3.4. Reuse of the catalyst
To investigate the reusability of PS–DA–Cu, the catalyst was sep-
arated by filtration after the first run, it was dried at 353 K under
vacuum and then used for the next run under the same conditions.
The data obtained were listed in Table 4. The conversion of cyclo-
hexene dropped from 51.9% to 47.2% after first use and then kept
relatively constant after next three runs, which could be mainly
attributed to the loss of the content of Cu on PS–DA–Cu during the
first use (from 7.23% to 6.0%). The selectivity of the products had no
great changes after four runs.
Scheme 3. The mechanism for the oxidation of cyclohexene.
3
.3. The proposed mechanism for the oxidation process
Based on the experimental results and other reports [15,31],
4. Conclusions
the mechanism for the oxidation of cyclohexene by the PS–DA–Cu
was proposed as shown in Scheme and the formation
of the products according to the proposed mechanism was
shown in Scheme 4. Firstly, the oxidation of cyclohexene with
3
Chloromethylated polystyrene supported tridentate Schiff-base
metal complexes (PS–DA–M, M = Cu2+, Co , Ni
2+
2+
and Mn ,
2+
respectively) were prepared and characterized. These complexes
were proved to be active and reusable catalysts for the oxi-
dation of cyclohexene with oxygen in the absence of reducing
agents. The products are ensured to be cyclohexene oxide (EXO),
2
-cyclohexene-1-ol (–OH), 2-cyclohexene-1-one (–C O) and 2-
cyclohexene-1-hydroperoxide (–OOH). The catalytic activity of
PS–DA–Cu was highest among the four metal complexes. The
conversion of cyclohexene can be reached to 51.9% under the
temperature of 343 K when using 2 mg of PS–DA–Cu to catalytic
oxidation of 2 ml of cyclohexene. The oxidation mechanism dif-
fers from the literature. And the catalyst could be reused for at
least four reaction cycles without considerable loss of reactivity
which endow chloromethylated polystyrene supported tridentate
Schiff-base metal complexes with a bright future in industrial appli-
cations.
Acknowledgement
The authors express their gratitude to Key Laboratory of Eco-
environment-related Polymer Materials Ministry of Education (ZD-
0
4-17).
Scheme 4. The formation of the products according to the proposed mechanism.

Y. Chang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 56–61
61
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