Copper-cationic salphen catalysts for the oxidation of cyclohexene by oxygen

Article abstract of DOI:10.1016/j.catcom.2014.10.002

The modified copper-cationic salphen catalysts were synthesized and used in the allylic oxidation of cyclohexene to 2-cyclohexen-1-ol and 2-cyclohexen-1-one with oxygen under mild conditions. Compared with their unmodified counterpart, the catalytic activ

Full text of DOI:10.1016/j.catcom.2014.10.002

Catalysis Communications 59 (2015) 97–100
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Copper-cationic salphen catalysts for the oxidation of cyclohexene
by oxygen
a
Amir Abdolmaleki a,b,c, , Soheila Rezaei Adariani
⁎
a
Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran
Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran
Center of Excellence in Sensors and Green Chemistry, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 August 2014
Received in revised form 30 September 2014
Accepted 2 October 2014
Available online 13 October 2014
The modified copper-cationic salphen catalysts were synthesized and used in the allylic oxidation of cyclohexene
to 2-cyclohexen-1-ol and 2-cyclohexen-1-one with oxygen under mild conditions. Compared with their unmod-
ified counterpart, the catalytic activities of modified catalysts are improved. The type of counteranion could affect
the reactivity of catalyst, which offers an opportunity to improve the catalysts via changing counteranions to
optimize the selectivity. The cation–anion interaction can be adjusted by different solvents, which in turn influ-
ences the catalyst reactivity. Furthermore, these novel catalysts can be reused without sacrificing the activity.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Copper-cationic salphen catalyst
Allylic oxidation
Counteranion
1. Introduction
catalyst reactivity could be improved by altering the type of counteran-
ion in ionic salts or changing the strength of the interaction between
Metal-salen complexes are an important class of coordination
compounds, and it has long been recognized that the catalysts can be
modified with various substituent groups on ligands [1–7]. However,
they suffer some problems in homogeneous medium, including difficul-
ty in separation from the reaction mixture and recycling. Immobiliza-
tion of the salen catalysts on some supports to create heterogeneous
catalysts was used to overcome these difficulties, however this method
suffers from different disadvantages, such as lower accessibility of
substrates, poor activity and also leaching of active species [8–18].
To solve these challenges, ionic compound was developed as an
efficient strategy owing to their intriguing physical and chemical
properties [19–24]. Modified ionic catalysts have obtained common
advantages of facilitating product isolation, reusing of the catalysts
and in some cases accelerating the reactions [25,26]. However, there
are a few reports on the effect of the counteranion on the reactivity of
these supported catalysts [17,26].
cation and anion.
2. Experimental
2.1. Materials and methods
All the chemicals were purchased from MERCK Chemical Co. O-
phenylene diamine was sublimated for further purification. 2,3-
Diaminophenazine was prepared and purified according to the
previously reported method [27,28]. Other solvents and chemicals
were purified by common procedures.
1H NMR (500 MHz) spectra were obtained in deuterated solvents on
a Bruker Avance 500 instrument (Bruker, Rheinstetten, Germany), UV–
Vis absorption spectra were obtained in DMSO (ca. 910–5 M) on a
JASCO-570 UV–Visible spectrometer and FT-IR spectra were obtained
as KBr pellets with a 680 plus-JASCO. Elemental analyses were per-
formed with a CHNS-932, Leco. The progress of reaction was monitored
and controlled by Shimadzu gas chromatograph (model 14A) equipped
with a wide bone OV-17 (60 m) capillary column and an FID detector.
The structure of products and by-products was further identified using
HP6890 GC/MS spectrometer by comparing the retention times and
the fragmentation patterns with authentic samples.
Herein, we designed and synthesized copper-cationic salphen,
whose catalytic activity could be improved using various counteranions,
Cu-[cationic salphen][X−]2 (Scheme 1). The most appealing features of
the catalysts were that the anion interacted with the cation, thus the
cation–anion interaction could easily deliver to the metal center. The
2.2. Preparation of the Cu-[cationic salphen][I−
]
2
⁎
Corresponding author at: Organic Polymer Chemistry Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,
Islamic Republic of Iran. Tel.: +98 31 3391 3249; fax: +98 31 3391 2350.
E-mail addresses: abdolmaleki@cc.iut.ac.ir, amirabdolmaleki@yahoo.com
(A. Abdolmaleki).
The cationic Cu-[cationic salphen][I−
according to the procedure already published in the literature [28];
]2 was prepared and purified
http://dx.doi.org/10.1016/j.catcom.2014.10.002
1566-7367/© 2014 Elsevier B.V. All rights reserved.

98
A. Abdolmaleki, S.R. Adariani / Catalysis Communications 59 (2015) 97–100
2.3. Synthesis of Cu-[cationic salphen][X−]2
X
X
Cu-[cationic salphen][X−]2: AgX (X = NO3, Br, CF3COO) or NH4PF6
(2 mmol) was added to the Cu-[cationic salphen][I−]2 (1 mmol) in ace-
tonitrile (50 ml), and then the mixture was stirred away from light at
room temperature for 3 h. The resulted mixture was filtered and the fil-
trate was evaporated under reduced pressure at 60 °C, and the obtained
H3C N
N CH3
N
O
N
solid was dried in vacuum. The Cu-[cationic salphen][X−
prepared.
]
was
2
Cu
But
O
tBu
Cu-[cationic salphen][NO−3 ]2: Yield: 95%. Atomic Absorption Spec-
troscopy: Cu-[cationic salphen][NO−3 ]2: Found: Cu, 8.38%. FT-IR (KBr):
tBu
But
3423, 2925, 2857, 1725, 1627, 1461, 1383, 1274, 1122, 408 cm−1
.
Cu-[cationic salphen][Br−]2: Yield: 91%. Atomic Absorption Spec-
troscopy: Cu-[cationic salphen][Br−]2: Found: Cu, 6.85%. FT-IR(KBr):
3430, 2923, 28621624, 1595, 1463, 1380, 1308, 1267, 1126, 1089,
Scheme 1. Structure of Cu-[cationic salphen][X−]2.
2,3-Diaminophenazine (0.21 g, 1 mmol), 3,5-di t-butyl-2-hydroxy
benzaldehyde (0.468 g, 2 mmol), and Cu(OAc)2·3H2O (0.23 g, 1 mmol)
in absolute ethanol were stirred under reflux condition for 12 h. Then,
the excess amount of methyl iodide was added and refluxed for 7 days.
A brown solid was obtained by removal of the unreacted methyl iodide
under reduced pressure. Yield (%) = 96; FT-IR (KBr): m = 3415, 2919,
2856, 1629, 1462, 1438, 1230, 1113, 743 cm−1; Anal. Calcd. (%) for
C44H54CuI2N4O2: C, 53.47; H, 5.51; N, 5.67; Found: C, 52.87; H, 5.36; N,
5.39. Atomic Absorption Spectroscopy: Cu-[cationic salphen][I−]2:
Found: Cu, 6.12%.
1033,781, 698, 535, 457 cm−1
.
Cu-[cationic salphen][PF−6 ]2: Yield: 88%. Atomic Absorption Spec-
troscopy: Cu-[cationic salphen][PF−6 ]2: Found: Cu, 6.03%. FT-IR(KBr):
3392, 2933, 2871, 1662, 1434, 1390, 1171, 1066, 945, 856, 728, 554,
418 cm−1
.
Cu-[cationic salphen][CF3COO−]2: Yield: 79%. FT-IR (KBr): 3442,
2929, 2868, 1687, 1629, 1476, 1430, 1288, 1201, 1127, 841, 795, 727,
455 cm−1
.
2.4. Cyclohexene oxidation catalyzed by Cu-[cationic salphen][X−]2
The Cu-[cationic salphen][X−]2 was used as catalysts for the allylic
oxidation of cyclohexene.
OH
O
A 25 ml three-necked round-bottomed flask was charged with
10 mmol cyclohexene, 10 ml CH3CN, and 0.1 mmol Cu-[cationic
salphen][X−]2. The resulting mixture was performed at 78 °C in a
water bath with fast stirring. The oxygen was flowing into the reactor,
after completion of the reaction, the solvent was removed by evapora-
tion under reduced pressure at 60 °C and the products were collected
by evaporation under reduced pressure at 100 °C. Subsequently, the
O2
+
0.1 mmol Catalyst
CH3CN
78 oC
Scheme 2. The oxidation of cyclohexene.
N
N
OH
N
NH2
NH2
t-Bu
CuCl2
+
N
O
N
O
N
Cu
Ethanol
t-Bu
t-Bu
t-Bu
Diaminophenazine
t-Bu
t-Bu
Copper salphen
CH3I/ reflux
X
I-
I-
H3C
X
H3C
N
N CH3
N
N
CH3
AgX or NH4PF6
CH3CN, RT, 4h
N
O
N
O
N
O
N
O
Cu
Cu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Copper- cationic salphen
Scheme 3. Synthesis of Cu-[cationic salphen][X−]2.
t-Bu
Copper- cationic salphen

A. Abdolmaleki, S.R. Adariani / Catalysis Communications 59 (2015) 97–100
99
Table 1
Table 3
The results of the oxidation of cyclohexene with Cu-[cationic salphen][X−
salphen.
]2 and copper
The recycling studies of Cu-[cationic salphen][PF6−]2 in the oxidation of cyclohexene.
Entry
Run times
Conversion (%)
Selectivity (%)
-ol
Entry
X
Time (h)
Conversion (%)
Selectivity (%)
-one
-ol
-one
1
2
3
4
5
Fresh
2nd
3rd
4th
5th
100
100
98
97
89
39.4
41.2
41.9
42.8
44.6
48.9
45.1
44.2
42.7
37.3
1
2
3
4
5
6
Copper salphen
24
16
16
16
16
16
23
100
100
100
100
100
65.3
23.9
26.5
28.7
39.4
24.1
32.6
64.1
59.2
56.6
48.7
63.9
Br−
NO−3
CF3COO−
PF−6
The oxidation of cyclohexene was performed with cyclohexene (6 mmol), catalyst (1%
molar percentage) in 5 ml CH3CN at 78 °C, 16 h.
I−
The oxidation of cyclohexene was performed with cyclohexene (6 mmol), catalyst (1%
molar percentage) in 5 ml CH3CN at 78 °C.
electric structure of the active center of the catalysts, which in turn ad-
justs the properties of the catalysts via altering the type of counteranion.
Activities of catalysts with different counteranions followed the order in
a trend expected of their nucleophilicity in CH3CN.
Different solvents were employed on the oxidation of cyclohexene
(Table 2) and due to the better solubility of the catalyst, the polar
solvent was more suitable for this reaction. The polarity of solvent had
influence on the cation and anion separation and the direct interaction
between anion and cation made the reactivity of the catalyst different
from each other. Different counteranions in ionic compounds can
change the electric structure of the active center of the catalysts,
which in turn adjusts the properties of the catalysts via altering the
type of counteranion.
catalyst was used without further purification. The progress of the
reaction was followed by GC (with benzyl ethanoate as the internal
standard). The products were analyzed by GC–MS. Besides, a series of
comparative experiments with different catalysts and different condi-
tions were carried out.
3. Results and discussion
The procedure for catalyst preparation is depicted in Scheme 2. The
Cu-[cationic salphen][X−
] was prepared by the reaction of Copper
2
salphen in the presence of excess methyl iodide as a reagent and a sol-
vent under reflux. The two CH3–N+ groups were formed in the salphen
ligand and neutralized by two iodide anions. To study the effect of
counteranions on the reactivity of these catalysts, the iodide anions
were exchange with different counteranions (Scheme 3). Altering the
counteranion only needs an ion change process which occurred in a
high yield (79–95%).
Finally, recyclability of the catalyst in the oxidation reaction was also
examined under optimized conditions. As shown in Table 3, the recov-
ery was accomplished in five sequential reaction runs with only a slight
loss of catalytic activity (less than 3% for 4 run) and selectivity.
4. Conclusions
3.1. The effect of the counteranion on the reactivity of the catalysts
Some new types of Cu-[cationic salphen][X−]2 complexes with two
cationic sites, as an excellent active catalyst on the allylic oxidation of
cyclohexene were synthesized. These easily reconstructed complexes
show better conversion compared to non-cationic complex. The types
of counteranion could affect the catalytic activity of the catalysts, and
also the reactivity and selectivity of catalysts change by changing the
solvent polarity due to the change of the cation–anion interaction.
Moreover, the catalyst could be used at least four times without signif-
icant loss of catalytic activity and selectivity.
The Cu-[cationic salphen][X−
] was found to be an effective and
2
selective catalyst for oxidation of cyclohexene to the corresponding 2-
cyclohexen-1-ol and 2-cyclohexen-1-one with oxygen as the oxidant
under mild conditions (Table 1). The conversion was measured by GC
method. Employing this system, cyclohexene was converted to 2-
cyclohexen-1-ol and 2-cyclohexen-1-one (depending on the reaction
condition, time and temperature) in high yields (up to 100% conver-
sion) and selectivity.
The effect of the counteranion in ionic compound shows the influ-
ence on the selectivity and the conversion of reaction, although non-
cationic salphen complex is noticeably less active in comparison to
Cu-[cationic salphen][X−]2. It was interesting to find that the type of
counteranion could affect the catalytic activity (Entry 2–6). Different
Acknowledgement
We gratefully acknowledge the partial financial support from the
Research Affairs Division, Isfahan University of Technology (IUT),
Isfahan. Further partial financial support from Iran Nanotechnology
Initiative Council (INIC), National Elite Foundation (NEF) and Center of
Excellence in Sensors and Green Chemistry (IUT) is also gratefully
acknowledged.
counteranions in ionic Cu-[cationic salphen][X−
] can change the
2
Table 2
The results of the oxidation of cyclohexene in different solvents over the Cu-[cationic
salphen][Br− 2 and Cu-[cationic salphen][PF6−]2.
]
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