Epoxidation of cyclohexene with molecular oxygen by electrolysis combined with chemical catalysis

Article abstract of DOI:10.1007/s13738-014-0445-3

This paper describes an electrochemical coupling epoxidation of cyclohexene by molecular oxygen (O₂) under mild reaction conditions. Herein, the electroreduction of O₂ to hydrogen peroxide (H₂O₂) efficiently proceeds in a relatively environmentally friendly acetone/water medium containing electrolytes at 25-30 °C on a self-assembled H type of electrolysis cell with tree electrodes system, providing ca. 44.3 mM concentration of H₂O₂ under the optimal electrolysis conditions. The epoxidation of cyclohexene with in situ generated H₂O₂ simultaneously occurs upon catalysis by metal complexes, giving ca. 19.8 % of cyclohexene conversion with 78 % of epoxidative selectivity over the best catalyst 5-Cl-7-I-8-quinolinolato manganese(III) complex (Q₃Mn^III (e)). The present electrochemical coupling epoxidation result is nearly equivalent to the epoxidation of cyclohexene with adscititious H₂O₂ catalyzed by the Q₃Mn^III (e).

Full text of DOI:10.1007/s13738-014-0445-3

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0445-3
ORIGINAL PAPER
Epoxidation of cyclohexene with molecular oxygen by electrolysis
combined with chemical catalysis
•
Yanlong Wang Jie Deng Chao Zhang
•
•
•
•
•
Wenfeng Wu Qingji Xie Yachun Liu
Zaihui Fu
Received: 23 December 2013 / Accepted: 22 February 2014
Ó Iranian Chemical Society 2014
Abstract This paper describes an electrochemical cou-
pling epoxidation of cyclohexene by molecular oxygen
(O2) under mild reaction conditions. Herein, the electro-
reduction of O2 to hydrogen peroxide (H2O2) efficiently
proceeds in a relatively environmentally friendly acetone/
water medium containing electrolytes at 25–30 °C on a
self-assembled H type of electrolysis cell with tree elec-
trodes system, providing ca. 44.3 mM concentration of
H2O2 under the optimal electrolysis conditions. The
epoxidation of cyclohexene with in situ generated H2O2
simultaneously occurs upon catalysis by metal complexes,
giving ca. 19.8 % of cyclohexene conversion with 78 % of
epoxidative selectivity over the best catalyst 5-Cl-7-I-8-
quinolinolato manganese(III) complex (Q3MnIII (e)). The
present electrochemical coupling epoxidation result is
nearly equivalent to the epoxidation of cyclohexene with
adscititious H2O2 catalyzed by the Q3MnIII (e).
Keywords Electrolysis Á Chemical catalysis Á Molecular
oxygen Á Coupling reaction Á Cyclohexene epoxidation
Introduction
Epoxidation of olefins has outstanding importance in
organic synthesis [1], since the epoxy compounds can be
easily converted to a large variety of compounds [2, 3].
Therefore, there has been considerable interest in the
development of highly efficient epoxidation catalysts [3–
9]. For the epoxidation of olefins, iodosylarenes [10], t-
butyl hydroperoxide [11, 12], hypochlorite [13, 14] and
H2O2 [6, 15–18] are usually used as the oxidants
because they provide a good yield of epoxides under
relatively mild conditions. From an economic and
environmental viewpoint, molecular oxygen (O2) is more
attractive than the above oxidants and thus it is con-
sidered to be the ultimate oxidant. Some effective pro-
cesses upon catalysis by various transition metal b-
diketonate complexes [19–23], Co(II)-Salen [24, 25] or
Ru(II) complexes [26–28] have been developed for the
epoxidation of alkenes with O2. However, they generally
need to employ isobutyraldehyde as the reductant,
because the triplet nature of O2 hampers the reaction
with alkenes in its singlet state under mild reaction
conditions. The use of electrochemical control technique
can efficiently reduce O2 to generate hydrogen peroxide
(H2O2) [29, 30], so it has been successfully applied to
the epoxidation of alkenes with O2 combined with
transition metal Ru(II) [30], Fe(III) [31, 32] or Mn(III)
[33–35] complexes. Recently, we have reported that
8-quinolinolato manganese(III) complexes (Q3MnIII)
possessed a very excellent catalytic activity for the
epoxidation of alkenes with H2O2 in environmentally
Y. Wang Á J. Deng Á C. Zhang Á W. Wu Á Q. Xie Á
Y. Liu (&) Á Z. Fu (&)
National and Local United Engineering Laboratory for New
Petrochemical Materials and Fine Utilization of Resources, Key
Laboratory of Resource Fine-Processing and Advanced
Materials of Hunan Province and Key Laboratory of Chemical
Biology and Traditional Chinese Medicine Research (Ministry of
Education of China), College of Chemistry and Chemical
Engineering, Hunan Normal University, Changsha 410081,
China
e-mail: yacl315@163.com
Z. Fu
e-mail: fzhhnnu@126.com
123

J IRAN CHEM SOC
friendly acetone–water media [9, 36]. Here, we introduce
these relatively cheap Q3MnIII complexes into the elec-
trochemical epoxidation of cyclohexene with O2.
Controlled-potential electrolyses (CPE) experiments
were conducted by using a CorrTest CS Electrochemical
Workstation Model potentiostat and a self-assembled
electrochemical H-type reaction device (see Fig. 2). Glassy
carbon (20 mm 9 20 mm 9 3 mm), graphite (25 mm 9
25 mm 9 3 mm) and SCE electrodes were used as a
working electrode, a counter electrode, and a reference
electrode, respectively. All potentials here were reported
versus the SCE. Anode and cathode electrolysis solutions
were the same, consisting of acetone/water (30 mL, 3/1 V/
V), TEAP (3 mmol), NH4OAc (6 mmol) and HOAc
(3 mmol). The anode and cathode chambers was separated
by a salt bridge. Cyclohexene (1 mmol) and catalyst Q3Mn
(0.4 mol% relative to cyclohexene) were added to the
cathode chamber and the solution of the two chambers was
stirred at 30 °C for 8 h under controlled potential
(-0.75 V). An atmosphere of purified O2 saturated with
acetone and water was maintained over the reaction mixture.
After electrolysis reaction, the catalyst was separated from
the reaction mixture by filtration. The filtrate was analyzed
chromatographically with an Agilent 6890N gas chro-
matograph (GC) with an SE-54 quartz capillary column
(30 m 9 0.32 mm 9 0.25 lm) and a flame ionization
detector (FID) using cyclohexanone as an internal standard.
The column temperature was 100 °C. Both injector and
detector temperatures were set at 250 °C. In addition, the
electrolysis experiments for reducing O2 to generate H2O2
were carried out using the above-described conditions in the
absence of the substrate and catalyst, and the concentration
of H2O2 generated in the cathode chamber was also
monitored by KMnO4 chemical titration.
Experimental
Materials and reagents
8-Hydroxyquinoline (8-HQ (a)), 5-chloro-8-HQ (b), 5,7-
dichloro-8-HQ (c), 5,7-dibromo-8-HQ (d) and 5-chloro-7-
iodo-8-HQ (e) were purchased from Alfa Aesar. The other
commercially available chemicals, such as cyclohexene,
acetone, tetraethylammonium perchlorate ((Et)4NClO4,
TEAP), manganese acetate (Mn(OAc)2), ammonium ace-
tate (NH4OAc), acetic acid (HOAc), 30 wt% aqueous H2O2
and other solvents were of analytical grade. Distilled water
was used throughout this experiment.
Synthesis of metal complexes
A series of hexadentate binding 8-quinolinolato manganese
complexes (Q3MnIII (a–e), see Fig. 1) were prepared as
following the reported processes [9, 36]. For the sake of
comparison, we also prepared hexadentate binding
5-chloro-7-iodo-8-quinolinolato FeIII (Q3FeIII (e)) and ox-
obis (8-quinolinolato) VIV (Q2OVIV (a)), as well as the
tetradentate binding 8-quinolinolato CuII (Q2CuII (a)) and
salen–MnIIICl complexes based on our previously pub-
lished literature [9, 37, 38].
Cyclic voltammetric measurements and controlled-
potential electrolyses
Cyclic voltammetric (CV) measurement was conducted on
a three-electrode configuration and an Autolab electro-
chemical workstation (Eco Chemie, Holland). The working
electrode (WE), the counter electrode and the reference
electrode were a glassy carbon electrode (GCE, 3-mm-
diameter disk), a sheet of platinum foil, and a KCl-satu-
rated calomel electrode (SCE), respectively. All the CV
experiments were carried out at room temperature (20 °C).
RE
WE
CE
salt bridge
stirrer
Fig. 1 Structure of hexadentate binding 8-quinolinolato manganese
complexes (Q3MnIII (a–e)
Fig. 2 Self-assembled electro-chemistry reaction device
123

J IRAN CHEM SOC
100
(V/V, 3/1) under O2, a pair quasi-reversible redox wave
near (0.302 ? 0.594)/2 = 0.448 V was observed, assigned
to a redox process of the ligand’s hydroxyls in the dis-
torted Q3MnIII (e) [9]. Notably, the reducing potential of
O2 (-0.63 V) showed an obviously positive shift in the
presence of Q3MnIII (e), indicating that the Q3MnIII
(e) can promote the electrochemical reduction of O2. On
the other hand, the existence of Q3MnIII (e) could lead to a
significant decrease in the reducing current of O2, which
was likely due to a gradual deposition of Q3MnIII (e) on
the electrode surface.
1, N2 atmosphere, without Q3MnIII(e)
2, air atmosphere, without Q3MnIII(e)
3, O2 atmosphere, without Q3MnIII(e)
4, O2 atmosphere, with Q3MnIII(e)
50
0
1
2
-50
4
3
-100
-150
Electroreduction of molecular oxygen
-1.2
-0.8
-0.4
0.0
0.4
0.8
Potential / V
Figure 4a, b illustrates the effect of temperature and time
on the concentration of H2O2 generated by the electro-
chemical reduction of O2 in the absence of cyclohexene
and catalyst, in which the curve of H2O2 concentration
climbed continuously with the temperature or the time until
it achieved a maximum at 25 °C and 12 h, respectively.
After that, it fell rapidly due to an accelerating decompo-
sition of H2O2 at a relatively high temperature and after a
long time. The highest concentration of H2O2 (44.3 mM)
was obtained when the electrolysis temperature and time
were controlled at 25 °C and 12 h, respectively.
Fig. 3 Cyclic voltammogram curves of molecular oxygen in acetone/
water (3/1) medium containing 0.1 mol/L TEPA
Results and discussion
Cyclic voltammetric measurement
Figure 3 shows the cyclic voltammetric (CV) curves of an
acetone–water (V/V, 3/1) medium containing TEAP in
different atmospheres. In the range of scanning potential,
there were no redox current peaks observed under a N2
atmosphere (curve 1). When this CV measurement was
carried out under an air atmosphere, an obvious reducing
peak at pc = -0.65 V was observed in its CV curve 2.
Notably, this reducing peak in pure O2 atmosphere became
very strong and shifted to pc = -0.75 V, with the con-
comitant appearance of an oxidative peak at pa = 0.42 V
(curve 3). Undoubtedly, the reducing peak should be due
to the reduction process of molecular oxygen, as reported
by some literature [39–41]. Curve 4 illustrates that when
15 lM of Q3MnIII (e) was added to acetone/water medium
Epoxidation of cyclohexene with molecular oxygen
The electrochemical epoxidation of cyclohexene with O2
upon catalysis by various metal complexes was investi-
gated and the obtained data are summarized in Table 1.
This epoxidation did not occur under N2 atmosphere or in
the absence of the catalyst or without the cathode potential
(see entries 1, 2 and 3), but could proceed efficiently in the
case of electrochemical control technique combined with
the catalyst Q3MnIII (e) (entry 4), providing ca. 15.9 % of
A
B
-0.75V, 30oC
45
50
40
30
20
10
0
-0.75V, 12h
40
35
30
25
20
15
10
5
0
2
4
6
8
10
12
14
16
15
20
25
30
35
40
Temperature / oC
Time / h
Fig. 4 The temperature and time dependence of H2O2 concentration generated via electrochemical reduction of O2 in acetone/water media (V/V,
3/1) containing TEAP (0.1 mol/L), NH4OAc (0.2 mol/L) and HOAc (0.1 mol/L)
123

J IRAN CHEM SOC
Table 1 Electrochemical epoxidation of cyclohexene with dioxygen over various metal complexes
OH
O
Catalyst -0.75V
Acetone / Water
O
+
+
O2
A
C
B
Entry
Catalyst
Cathode potential
(V)
Cyclohexene conversion
(mol%)
Selectivity for
products (%)
A
B
C
1a
2
Q3Mn (e)
No
-0.75
-0.75
No
Trace
Trace
Trace
15.9
16.2
7.3
–
–
–
–
–
–
3
Q3Mn (e)
Q3Mn (e)
Q3Mn (e)
Q3Mn (a)
Q3Mn (b)
Q3Mn (c)
Q3Mn (d)
Q3FeIII (e)
Q2CuII (a)
Q2OVIV (a)
–
–
–
4
5b
-0.75
No
75
80
75
72
77
80
–
10
4
15
16
20
23
18
15
–
6
-0.75
-0.75
-0.75
-0.75
-0.75
-0.75
-0.75
-0.75
5
7
13.1
14.6
15.0
Trace
2.4
5
8
5
9
5
10
11
12
13
–
70
67
64
15
0
15
33
22
2.9
Salen–Mn–Cl
1.6
14
Reaction conditions: cathode, 30 mL of acetone/water containing 0.1 mol/L TEAP ? 0.2 mol/L NH4OAc ? 0.1 mol/L HOAc; cyclohexene,
1 mmol, catalyst: 0.4 %relative to mmol amount of substrate, oxygen atmosphere, 1 atm, reaction temperature and time, 30 °C and 5 h
A epoxycyclohexane, B cyclohexenol, C cyclohexanone
a
N2 atmosphere; other reaction conditions same as given above
b
Adding 0.3 mmol 30 wt% H2O2; temperature, 30 °C; time, 5 h
cyclohexene conversion with 75 % selectivity for epoxy-
cyclohexane. Moreover, the epoxidation result in the entry
4 is nearly equivalent to that of the Q3MnIII (e)-catalyzed
cyclohexene with adscititious H2O2 (Entry 5).
substituent. Compared to the unsubstituted Q3MnIII (a), the
5-chlorinated Q3MnIII (b) and especially 5,7-dihalogenated
Q3MnIII complexes (c–e) showed a higher epoxidative
efficiency and their activity followed an increasing
sequence of Q3MnIII (e) [ Q3MnIII (d) [ Q3MnIII
(c) [ Q3MnIII (b), most likely due to the increase of elec-
tron-donating effect of halogen substituents from Cl, Br to I
on the ligand’s aryl ring and the high resistance of halo-
genated 8-HQ ligands against oxidative degradation of the
Q3MnIII, as reported for chlorinated porphyrin–MnIII [42].
In the following experiments, the effects of various
reaction parameters such as the amount of substrate and
catalyst, cathode potential, reaction temperature and time,
as well as acetone/water volume ratio on the coupling
epoxidation were further checked using the best catalyst
Q3MnIII (e) as an example and the results are shown in
Table 2 and Figs. 5, 6 and 7. In the case of fixing substrate
amount as 5 mmol, the conversion of cyclohexene and the
utilization efficiency of the H2O2 generated via electrore-
duction of O2 rapidly increased with increasing catalyst
amount from 0.2 to 0.4 %; when the amount was higher
Entries 4 and 6–13 illustrate that the coupling epoxida-
tion was influenced by the type of catalysts. Among the
catalysts examined, a series of Q3MnIII (a–e) complexes
with a hexadentate binding structure exhibited a good
activity for this reaction (Entries 4, and 6–9, the conversion,
7.3–15.9 %; and the epoxidative selectivity, 72–80 %).
However, other metal complexes, including both the
hexadentate Q3FeIII (e) and Q2OVIV (a), as well as both the
tetradentate Q2CuII (a) and salen–MnIIICl, all provided very
poor epoxidative efficiency under the same reaction con-
ditions (entries 10–13). These facts suggest that the effi-
ciency of these catalysts is influenced drastically by their
metal ions, ligands and chelate modes, as previously
reported by us for the Q3MnIII complexes that catalyzed the
epoxidation of alkenes with H2O2 [9]. Entries 4 and 6–9
further show that the catalytic performances of Q3MnIII
complexes were largely influenced by their ligand’s
123

J IRAN CHEM SOC
Table 2 Effect of the amount of catalyst and substrate on the electrochemical epoxidation of cyclohexene with dioxygen over Q3Mn (e)
Entry
Catalyst amounta
(mol%)
Subtrate amount
(mmol)
Cyclohexene
conversion (%)
Used efficiency
of H2Ob2 (%)
Selectivity for
products (%)
A
B
C
1
0.2
0.4
0.6
0.8
0.4
0.4
0.4
0.4
0.4
0.4
5.0
5.0
5.0
5.0
1
4.6
10.6
6.8
43.3
99.8
63.5
37.3
28.2
58.4
100.0
100.8
81.6
76.4
74
70
72
68
75
75
70
69
81
80
4
22
22
21
21
15
17
20
21
15
17
2
8
7
3
4
4
11
10
8
5
15.9
12.5
7.2
6
2.5
7.5
10
5
7
10
10
4
8
5.3
9c
10d
21.3
32.1
5
3
Reaction conditions: cathode potential, -0.75 V, 30 mL of acetone/water (3/1) containing 0.1 mol/L TEAP ? 0.2 mol/L NH4OAc ? 0.1 mol/L
HOAc; oxygen atmosphere, 1 atm; reaction temperature and time, 30 °C and 5 h
A epoxycyclohexane, B cyclohexenol, C cyclohexanone
a
Relative to mmol amount of substrate
b
The estimated efficiency of H2O2 (%) = [(the amount (mmol) of epoxycyclohexane ? cyclohexenol ? 2 cyclohexanone/the amount (mmol)
of H2O2 generated by an electro-reduction of O2 in the absence of Q3Mn (e) (0.648 mmol)] 9 100 %
c
Adding 1.5 mmol 30 wt% H2O2, 30 mL of acetone/water (3/1) containing 0.2 mol/L NH4OAc ? 0.1 mol/L; temperature, 10 °C; time, 5 h
Adding 2.5 mmol 30 wt% H2O2, 30 mL of acetone/water (3/1) containing 0.2 mol/L NH4OAc ? 0.1 mol/L; temperature, 10 °C; time, 5 h
d
A
B
14
12
10
8
80
70
60
50
40
30
20
10
0
substrate, 1 mmol, potential, -0.75V, time, 5 h
Epoxycyclohexane
20
18
16
14
12
10
8
substrate, 1mmol, acetone/water, 3/1, time, 8 h
80
70
60
50
40
30
20
10
0
Cyclohexene conversion
Epoxycyclohexane
Cyclohexenone
Cyclohexene conversion
Cyclohexenone
Cyclohexenol
6
4
Cyclohexenol
-0.9
5/1
4/1
7/2
3/1
5/2
2/1
-1.0
-0.8
-0.7
-0.6
-0.5
Cathod potential / V
Volume ratio of acetone to water
Fig. 5 The effect of the volume ratio of acetone to water and cathode potential on the electrochemical epoxidation of cyclohexene with O2
(Q3MnIII (e), 0.4 %; temperature, 30 °C)
than 0.4 %, both the values greatly dropped (see entries 1–4
in Table 2). A large decrease in epoxidative efficiency at
high catalyst amount is likely due to the serious deposition
of catalyst on the electrode surface. In the case of fixing
catalyst amount at 0.4 %, when the amount of cyclohexene
was increased from 1 to 10 mmol, the conversion obviously
decreased from 15.9 to 5.3 %, while the utilization effi-
ciency greatly increased from 28.2 to 100.8 %, with a
concomitant slight decrease in epoxycyclohexane selectiv-
ity (see entries 5–8 in Table 2). This is likely due to a
decrease in substrate solubility. On the whole, the selec-
tivity for epoxycyclohexane slightly decreased with
increasing amount of substrate or catalyst, along with an
increase in cyclohexanone or cyclohexenol selectivity.
Entries 9 and 10 illustrate that the epoxidation of cyclo-
hexene with adscititious H2O2 under its optimal reaction
conditions could achieve ca. 76.4–81.6 % of the utilization
efficiency of H2O2, and this efficiency was lower than that
obtained from the electrocatalytic epoxidation of cyclo-
hexene with O2 under the optimized substrate and catalyst
123

J IRAN CHEM SOC
30
25
20
15
10
5
80
70
60
50
40
30
20
10
A
B
30oC
28
70
60
50
40
30
20
10
Epoxycyclohexane
Time, 5h
Epoxycyclohexane
24
20
16
12
8
Cyclohexene
Cyclohexenone
Cyclohexenol
Cyclohexenone
Cyclohexenol
Cyclohexene
4
2
4
6
8
10 12 14 16 18 20
Time / h
16
20
24
28
32
36
40
Temperature / oC
Fig. 6 The effect of reaction temperature and time on the electrochemical epoxidation of cyclohexene (5 mmol, Q3MnIII (e), 0.4 %, acetone/
water, 3/1, 30 mL; cathode potential, -0.75 V)
Fig. 7 Proposed scheme for the
Electrode surface
Ac
O
electrocatalytic epoxidation of
cyclohexene by molecular
oxygen
O
H
N
O
Heterolytic
cleavage
O
N
(O2)aq
2H2O
Mn+
O
N
Mn
N
N
O
O
-OAc-
N
HO
O
2e-
OAc-
N
O
O
O
H2O2
N
O
HOAc
Mn
N
N
Mn
N
O
HO
N
O
amounts. Figure 5a illustrates that the coupling epoxidation
was also influenced by the volume ratio of acetone to water;
when the ratio was between 2:1 and 3.5:1, the conversion of
cyclohexene gradually increased with increasing acetone
amount, likely due to the improvement of substrate solu-
bility. After that, the conversion slightly decreased. The
selectivity of products was not nearly influenced by the
volume ratio of acetone to water. Figure 5b illustrates that
the coupling epoxidation was influenced significantly by the
cathode potential; when the potential was controlled at
lower or higher than -0.75 V, epoxycyclohexane yield
drastically dropped. This is likely because the electro-
chemical reduction of O2 is not easy to proceed in the case
of cathode potential deviation from -0.75 V. Figure 6a and
b illustrates that the conversion of cyclohexene increased
continuously with temperature and time until it achieved a
maximum at 30 °C and 12 h; afterward, it decreased to a
great extent due to an accelerated decomposition of the
in situ generated H2O2 at a relatively high temperature and
after a long time. The effect of temperature and time on the
selectivity for products had a very similar, i.e., the selec-
tivity for epoxycyclohexane slowly increased and then
decreased with both the parameters. Under the optimal
conditions (cyclohexane, 5 mmol, catalyst, 0.4 %, acetone/
water volume ratio, 3.5/1, temperature, 30 °C, time, 12 h,
cathode potential, -0.75 V), the conversion of cyclohex-
ene, the selectivity for the epoxide and the utilization effi-
ciency for the in situ generated H2O2 were ca. 19.0, 71.0 and
85.1 %, respectively.
Reaction mechanism
Based on the results presented here, as well as those in
previous studies [29–35, 39–41], it is apparent that the
above data are better explained according to the following
reaction pathway (see Fig. 7). In this mechanism, molec-
ular oxygen is reduced to form H2O2 on the cathode’s
surface under the controlled cathode potential electrolyses.
Then the H2O2 in situ generated can selectively oxidize
cyclohexene to epoxycyclohexane upon catalysis by Q3-
MnIII via undergoing a pathway of forming a key inter-
mediate Q3MnV = O [9].
Conclusion
In summary, an electrochemical control technique com-
bined with Q3MnIII complexes has been developed as an
123

J IRAN CHEM SOC
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Acknowledgments We acknowledge the financial support for this
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