Efficient and convenient epoxidation of alkenes to epoxides with H2O2 catalyzed by Co(OAc)2 in ionic liquid [C12py][PF6]

Article abstract of DOI:10.1007/s13738-015-0695-8

A simple, efficient, and eco-friendly procedure for the epoxidation of alkenes to epoxides with H₂O₂ catalyzed by Co(OAc)₂ in ionic liquid [C₁₂py][PF₆] has been developed. The reactions were carried out with alkene, Co(OAc)₂ (0.1 mmol), [C₁₂py][PF₆] (10 mL), and H₂O₂ (30 %, 11 mmol) at room temperature for 2-6 h. This atom-economical protocol affords the target products in good to high yields (88-98 %). The products can be separated by a simple extraction with organic solvent, and the catalytic system can be recycled and reused without loss of catalytic activity. Graphical abstract: A simple, efficient, and eco-friendly procedure for the epoxidation of alkenes to epoxides with H₂O₂ catalyzed by Co(OAc)₂ in ionic liquid [C₁₂py][PF₆] has been developed.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-015-0695-8

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0695-8
ORIGINAL PAPER
Efficient and convenient epoxidation of alkenes to epoxides
with H O catalyzed by Co(OAc) in ionic liquid [C py][PF ]
2
2
2
12
6
1
1
1
Yu‑Lin Hu · Yi‑Wen Liu · De‑Jiang Li
Received: 4 April 2015 / Accepted: 30 June 2015
©
Iranian Chemical Society 2015
Abstract A simple, efficient, and eco-friendly procedure
Introduction
for the epoxidation of alkenes to epoxides with H O cata-
2
2
lyzed by Co(OAc) in ionic liquid [C py][PF ] has been
Epoxides are important classes of chemicals used exten-
sively for the preparation of a variety of fine or special
chemicals such as natural products, drugs, polymer mate-
rials, etc., [1, 2]. Therefore, there has been tremendous
interest in developing efficient methods for the synthesis of
these molecules, and a well-known method constitutes the
epoxidation of alkenes. Traditional methods for perform-
ing such a transformation generally involve the use of stoi-
chiometric amount of the strongest oxidizing reagents (i.e.,
H PO , peroxy acids, H WO /FAp, etc.) [3–5], and suffer
2
12
6
developed. The reactions were carried out with alkene,
Co(OAc) (0.1 mmol), [C py][PF ] (10 mL), and H O
2
2
12
6
2
(
30 %, 11 mmol) at room temperature for 2–6 h. This atom-
economical protocol affords the target products in good to
high yields (88–98 %). The products can be separated by
a simple extraction with organic solvent, and the catalytic
system can be recycled and reused without loss of catalytic
activity.
Graphical abstract A simple, efficient, and eco-friendly
procedure for the epoxidation of alkenes to epoxides with
H O catalyzed by Co(OAc) in ionic liquid [C py][PF ]
3
5
2
4
from considerable drawbacks such as low yield, harsh or
delicate reaction condition, and a large amount of waste by-
products. Oxidation with NaClO [6, 7], PAA [8], TBHP [9,
2
2
2
12
6
has been developed.
1
1
0], PhIO [11, 12], NaIO [13, 14], CHP [15], oxone [16–
8], peracetic acid [19, 20], TADOOH [21], PPO [22], and
4
30% H2O2
H2O
other reagents [23–28] have also been developed for this
conversion. However, these protocols are generally asso-
ciated with one or more disadvantages, such as complex
handling procedures, problematic side reactions, the use of
expensive, toxic and moisture-sensitive reagents, long reac-
tion times, low yields, and environmental concerns associ-
ated with the use of poorly manageable catalysts. Hydrogen
peroxide (H O ) is an economical, environmentally friendly
R2
cobalt acetate
R2
R1
R1
O
ionic liquid [C12py][PF6]
recycled catalytic system
2
2
and atom-efficient oxidant which leads to only water after
the reaction [29]. This reagent has, however, high activation
energy, making catalysis necessary [30], and a number of
catalytic oxidation processes based upon the combination
of transition metals or organocatalysts and H O have been
Keywords Epoxidation · Alkene · H O · Cobalt acetate ·
Ionic liquid
2
2
2
2
*
Yu-Lin Hu
huyulin1982@163.com
developed for such a conversion [31–37]. However, most of
the procedures still suffered from the use of expensive rea-
gents, difficulties in work-up, long reaction times, environ-
mental hazards, waste control, etc. Consequently, search for
new and environmentally benign synthetic methodologies
1
College of Materials and Chemical Engineering,
China Three Gorges University, Yichang 443002,
People’s Republic of China
1
3

J IRAN CHEM SOC
General procedure for epoxidation reactions
For a typical experiment, alkene (10 mmol), Co(OAc)2
(
1 mmol), and an aqueous solution of H O (30 %,
2 2
1
1 mmol) were sequentially added to 10 mL of [C py]
12
[
PF ]. The obtained mixture was stirred vigorously at room
6
temperature for the appropriate time (Table 2), the reac-
tion progress was monitored by GC. Upon completion of
the reaction, cyclohexane was used to extract the organic
compounds (3 × 10 mL). The cyclohexane solution was
washed with water (3 × 10 mL), and then dried over anhy-
drous Na SO . The solvent was removed and the residue
Scheme 1 Catalytic epoxidation of alkenes to epoxides
2
4
was distilled under vacuum to give the desired pure prod-
uct. The rest of the ionic liquid and the catalyst were recov-
ered by decantation of water produced in the reaction and
concentration under vacuum. Fresh substrates were then
recharged to the recovered catalytic system and then recy-
cled under identical reaction conditions. The target sub-
strates were characterized by NMR and Elemental analysis
or compared with their authentic samples. Spectroscopic
data for selected products are as follows.
for epoxidation of alkenes that address these drawbacks
remains to be of value and interest.
Ionic liquids (ILs) have attracted considerable atten-
tion during the past few years due, in some cases, to
their favorable properties such as very low vapor pres-
sure, wide liquid temperature range, good ionic conduc-
tivity, excellent electrochemical properties, and strong
ability to dissolve many chemicals [38, 39]. Therefore,
ILs have recently gained recognition of scholars from
various fields such as chemical synthesis [40], analytical
and separation science [41], electrochemistry [42], bio-
catalytic transformations [43], and catalysis [44, 45]. In
continuation of our interest in exploring green oxidation
reactions in ionic liquids, we report herein a new, efficient
and environmentally friendly protocol for the epoxida-
tion of alkenes to epoxides with H O catalyzed by cobalt
2‑Phenyloxirane (Table 2, entry 1)
Yield: 92 %; colorless oil; bp 193–195 °C/760 mm (Ref.
[46] 192–194 °C/760 mm).
2‑Ethyl‑3‑phenyloxirane (Table 2, entry 2)
2
2
1
acetate (Co(OAc) ) in ionic liquid N-dodecylpyridinium
Yield: 95 %; colorless oil; H NMR (500 MHz, DMSO-d ):
2
6
hexafluorophosphate ([C py][PF ]) under mild condi-
δ = 0.97 (t, 3H, CH ), 1.54 (m, 2H, CH ), 2.85 (m, 1H,
1
2
6
3
2
1
3
tions (Scheme 1). Furthermore, we demonstrate that the
catalytic system can be recycled and reused without any
significant loss of catalytic activity.
CH), 3.78 (d, 1H, CH), 7.39–7.46 (m, 5H, Ar–H) ppm; C
NMR (500 MHz, DMSO-d ): δ = 12.4, 23.1, 56.2, 62.7,
6
126.3, 127.8, 128.9, 137.6 ppm; anal. calcd for C H O:
1
0
12
C, 81.01; H, 8.14; O, 10.77. Found: C, 81.04; H, 8.16; O,
1
0.80.
Experimental
2
‑(O‑Tolyl)oxirane (Table 2, entry 3)
Apparatus and reagents
1
Yield: 94 %; colorless oil; H NMR (500 MHz, DMSO-d ):
6
All the chemicals were from commercial sources without
any pretreatment. All reagents were of analytical grade.
The ionic liquids were synthesized according to the litera-
ture procedure [39]. H NMR spectra and C NMR spectra
were recorded on a Bruker 500-MHz spectrometer using
δ = 2.47 (s, 3H, CH ), 2.83 (m, 2H, CH ), 3.79 (m, 1H,
3
2
1
3
CH), 7.23–7.41 (m, 4H, Ar–H) ppm; C NMR (500 MHz,
DMSO-d ): δ = 18.5, 51.7, 54.3, 123.4, 124.9, 125.8,
6
1
13
128.4, 134.1, 138.7 ppm; anal. calcd for C H O: C, 80.53;
9
10
H, 7.48; O, 11.89. Found: C, 80.56; H, 7.51; O, 11.92.
DMSO-d as the solvent with tetramethylsilane (TMS) as
6
an internal standard. GC analyses were performed on a Shi-
madzu-14B gas chromatography equipped with HP-1 capil-
lary column (30 m, 0.25 mm, 0.25 μm). Elemental analy-
sis was performed on a Vario EL III instrument (Elmentar
Anlalysensy Teme GmbH, Germany).
2‑(2,4‑Dimethylphenyl)oxirane (Table 2, entry 4)
1
Yield: 97 %; colorless oil; H NMR (500 MHz, DMSO-
d ): δ = 2.31 (s, 3H, CH ), 2.43 (s, 3H, CH ), 2.86 (m, 2H,
6
3
3
CH ), 3.81 (m, 1H, CH), 7.03–7.14 (m, 3H, Ar–H) ppm;
2
1
3

J IRAN CHEM SOC
1
3
C NMR (500 MHz, DMSO-d ): δ = 18.7, 22.6, 51.9,
(3‑Propyloxiran‑2‑yl)methanol (Table 2, entry 10)
6
5
4.1, 124.3, 125.5, 131.2, 132.8, 134.6, 139.5 ppm; anal.
1
calcd for C H O: C, 81.02; H, 8.14; O, 10.76. Found: C,
Yield: 97 %; colorless oil; H NMR (500 MHz, DMSO-
1
0
12
8
1.04; H, 8.16; O, 10.80.
d ): δ = 0.94 (t, 3H, CH ), 1.35-1.46 (m, 4H, CH CH ),
6
3
2
2
2
.54 (m, 1H, CH), 2.67 (m, 1H, CH), 3.62 (m, 2H, CH ),
2
1
3
2
‑(4‑Methoxyphenyl)oxirane (Table 2, entry 5)
3.85 (brs, 1H, OH) ppm; C NMR (500 MHz, DMSO-d6):
δ = 14.5, 21.4, 34.6, 54.9, 61.5, 65.2 ppm; anal. calcd for
C H O : C, 62.02; H, 10.36; O, 27.53. Found: C, 62.04;
1
Yield: 98 %; colorless oil; H NMR (500 MHz, DMSO-
6
12
2
d ): δ = 2.79 (m, 2H, CH ), 3.82 (m, 1H, CH), 3.89 (s, 3H,
H, 10.41; O, 27.55.
6
2
1
3
CH ), 7.05 (m, 2H, Ar–H), 7.21 (m, 2H, Ar–H) ppm;
C
3
NMR (500 MHz, DMSO-d ): δ = 51.3, 54.5, 56.4, 117.4,
7‑Oxabicyclo[4.1.0]heptane (Table 2, entry 11)
6
1
25.9, 131.6, 158.7 ppm; anal. calcd for C H O : C, 71.95;
9 10 2
H, 6.69; O, 21.28. Found: C, 71.98; H, 6.71; O, 21.31.
Yield: 98 %; colorless oil; bp 129–131 °C/760 mm (Ref.
[46] 128–130 °C/760 mm).
1
,4‑Di(oxiran‑2‑yl)benzene (Table 2, entry 6)
2
‑Isobutyloxirane (Table 2, entry 12)
1
Yield: 96 %; colorless oil; H NMR (500 MHz, DMSO-
1
d ): δ = 2.76 (m, 2H, CH ), 3.85 (m, 1H, CH), 7.24 (s, 4H,
Yield: 96 %; colorless oil; H NMR (500 MHz, DMSO-
6
2
1
3
Ar–H) ppm; C NMR (500 MHz, DMSO-d ): δ = 50.7,
d ): δ = 0.93 (dd, 3H, CH ), 1.37 (t, 2H, CH ), 1.69 (m,
6
6
3
2
1
3
5
4.1, 124.5, 137.2 ppm; anal. calcd for C H O : C, 74.02;
1H, CH), 2.47 (m, 2H, CH ), 2.62 (m, 1H, CH) ppm;
C
1
0
10
2
2
H, 6.18; O, 19.71. Found: C, 74.06; H, 6.21; O, 19.73.
NMR (500 MHz, DMSO-d ): δ = 21.7, 25.6, 43.2, 49.5,
6
5
4.8 ppm; anal. calcd for C H O: C, 71.92; H, 12.06; O,
6 12
2
‑(4‑Nitrophenyl)oxirane (Table 2, entry 7)
15.95. Found: C, 71.95; H, 12.08; O, 15.97.
1
Yield: 90; light yellow solid; H NMR (500 MHz, DMSO-
d ): δ = 2.91 (m, 2H, CH ), 3.95 (m, 1H, CH), 7.68 (m,
Results and discussion
6
2
1
3
2
H, Ar–H), 8.13 (m, 2H, Ar–H) ppm; C NMR (500 MHz,
DMSO-d ): δ = 51.7, 55.3, 56.4, 125.2, 126.8, 143.4,
The initial study was carried out using phenylethylene as
the substrate to optimize the reaction conditions, and the
results are summarized in Table 1. Initial reaction screen-
ing led to disappointing results in the absence of catalyst
and ionic liquid, the reaction proceeded very slowly, and
the yield was only 12 % after 24 h (Table 1, entry 1). The
results mean that the oxidant H O alone does not work
6
1
49.2 ppm; anal. calcd for C H NO : C, 58.13; H, 4.25;
8 7 3
N, 8.46; O, 29.04. Found: C, 58.18; H, 4.27; N, 8.48; O,
2
9.06.
1
‑(2‑(Oxiran‑2‑yl)phenyl)ethanone (Table 2, entry 8)
2
2
1
Yield: 88 %; colorless oil; H NMR (500 MHz, DMSO-d ):
effectively in the reaction. The effects of different ionic
liquids, such as [C py][PF ], [C py][PF ], [C py][PF ],
6
δ = 2.56 (s, 3H, CH ), 2.87 (m, 2H, CH ), 3.89 (m, 1H,
3
2
4
6
6
6
8
6
CH), 7.43-7.58 (m, 3H, Ar–H), 7.93 (m, 1H, Ar–H) ppm;
[C py][PF ], [C py][PF ], [C py][BF ], [C py][OTf],
10 6 12 6 12 4 12
and [C py]Br, were tested with Co(OAc) as catalyst in the
12 2
1
3
C NMR (500 MHz, DMSO-d ): δ = 30.5, 52.1, 54.5,
6
1
23.7, 126.8, 129.3, 131.6, 134.2, 139.5, 200.7 ppm; anal.
reaction (Table 1, entries 2–9), it was observed that [C py]
12
calcd for C H O : C, 74.05; H, 6.18; O, 19.71. Found: C,
[PF ] demonstrated the best performance (Table 1, entry 6).
1
0
10
2
6
7
4.06; H, 6.21; O, 19.73.
The different effects of ILs in the reaction may be attributed
to their different characteristics of melting points. A review
of the literature [47] shows that the kind of anion and the
symmetry of cation are the two important factors giving the
melting points, which would influence the different abilities
of stabilizing and dissolving the oxidant H O and the cata-
lyst. Under the same conditions, the IL which stabilizes and
dissolves them more easily will lead to a larger increase in
the effective reactant concentration, which increases the
encounter probabilities between the substrate and reactive
2
‑Benzyloxirane (Table 2, entry 9)
1
Yield: 95 %; colorless oil; H NMR (500 MHz, DMSO-d ):
6
δ = 2.51 (m, 2H, CH ), 2.63 (m, 2H, CH ), 3.05 (m, 1H,
2
2
2
2
1
3
CH), 7.25-7.43 (m, 5H, Ar–H) ppm; C NMR (500 MHz,
DMSO-d ): δ = 40.3, 47.2, 55.7, 125.4, 127.8, 128.6,
6
1
1
37.7 ppm; anal. calcd for C H O: C, 80.53; H, 7.48; O,
1.90. Found: C, 80.56; H, 7.51; O, 11.92.
9 10
1
3

J IRAN CHEM SOC
Table 1 Optimization of the conditions for epoxidation of phenyleth-
100
80
2
ylene with H O2
O
60
40
20
0
3
0% H O
2 2
reaction conditions
Entrya
Yield (%)b
Ionic liquid
Catalyst
Time (h)
1
2
3
4
5
6
Cycle
1
2
3
4
5
6
7
8
9
–
–
24
6
3
3
3
3
3
3
3
8
8
8
3
6
3
3
12
77
81
87
90
92
86
75
85
71
67
75
84
71
81
79
[C py][PF ]
Co(OAc)2
Co(OAc)2
Co(OAc)2
4
6
Fig. 1 Repeating reactions using recovered catalytic system. The
reactions were carried out with phenylethylene (10 mmol), recovered
catalytic system ([C py][PF ], Co(OAc) ), in the presence of H O
[C py][PF ]
6
6
[C py][PF ]
8
6
1
2
6
2
2
2
[C py][PF ]
Co(OAc)2
10
6
(30 %, 11 mmol) at room temperature for 3 h
[C py][PF ]
Co(OAc)2
12
6
[C py][BF ]
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
–
12
4
[C py][OTf]
epoxides in good to high yields (Table 2, entries 1–12).
Various functionalities such as alkyl, alkoxy, acetyl and
nitro groups can tolerate the reaction. Surprisingly, the
epoxidation of alkyl alkenes (Table 2, entries 10–12) to
the corresponding epoxides is faster and more efficient
than aryl alkenes (Table 2, entries 1–9). In addition, it was
also observed that the electronic nature of the substituents
on the aromatic ring has some impact on the reaction rate.
Substrates with electron-withdrawing substituents (Table 2,
entries 7 and 8) are less reactive than those with electron-
donating substituents (Table 2, entries 3–6), and a decrease
in the isolated yield or reaction rate.
The excellent results of the catalytic system suggest the
epoxidation reaction has a particular reaction mechanism.
According to the literatures [48, 49] and the observations
in our reactions, taking the epoxidation of phenylethylene
with H O as an example, a possible mechanism is pro-
12
[C py]Br
12
10
11
12
13
14
15
16
a
–
[C py][PF ]
12
6
[C py][PF ]
WO3
12
6
[C py][PF ]
Fe(OAc)3
Cu(OAc)2
Mn(OAc)2
FeBr3
12
6
[C py][PF ]
12
6
[C py][PF ]
12
6
[C py][PF ]
12
6
The reactions were carried out with phenylethylene (10 mmol),
catalyst (1 mmol), IL (10 mL), and H O (30 %, 11 mmol) at room
temperature
2
2
b
Isolated yield
species, and so the higher rate or yield of the reaction is
observed. For a blank test (Table 1, entry 10), a lower yield
of the product was obtained when the same reaction condi-
tion was used in the absence of ionic liquid. In addition, the
catalyst is crucial for this reaction, and its lack leads to a
much lower yield (Table 1, entry 11). Besides Co(OAc)2,
we also tried to use other types of catalysts in this model
reaction (Table 1, entries 12–16), and the results showed
2
2
posed (Scheme 2). In the reaction, the catalyst Co(OAc)2
provides a source of Co(II) (1), which reacts with the oxi-
dant H O to form Co(III) = O (2). Then, 2 reacts with
2
2
the substrate to form transition state 3. 3 then very rapidly
affords Co(II) to yield the desired product. The Co(II) is
then re-oxidized to Co(III) = O by H O to complete the
2
2
that Co(OAc) demonstrated the best performance in terms
of yield and reaction rate. Therefore, the combination of
H O , Co(OAc) and [C py][PF ] was chosen as the opti-
2
catalytic cycle. It looks like that the formation of 3 from 2
is the rate-determining step.
2
2
2
12
6
mal conditions for further exploration.
In addition, the catalytic system could be typically
recovered and reused for subsequent reactions with no
appreciable decrease in yields and reaction rates (Fig. 1).
The recycling process involved the removal of the prod-
uct from the catalytic system, by a simple extraction with
organic solvent. Fresh substrates were then recharged to
the aqueous layer of catalytic system and the mixture was
heated to react once again.
With these results in hand, we subjected other alkenes
to the epoxidation reactions, and the results are listed in
Table 2. It is clear that various types of aryl and alkyl alk-
enes can be successfully epoxidized to the corresponding
Conclusion
In conclusion, we have demonstrated an efficient and con-
venient epoxidation of alkenes to epoxides using H O /
2
2
Co(OAc) catalytic system in the ionic liquid [C py][PF ].
2
12
6
A wide range of aryl and alkyl alkenes were found to be
applicable to the catalytic system. Mild reaction condi-
tions, simplicity of operation, high yields, stability, easy
isolation of products, and excellent recyclability of the
catalytic system are the attractive features of this methodol-
ogy. The scope, definition of the mechanism and synthetic
1
3

J IRAN CHEM SOC
Table 2 Catalytic epoxidation of alkenes to epoxides
a
b
Entry
Substrate
Product
Time / h
3
Yield / %
O
1
92
O
2
3
4
5
3
3
3
3
95
94
97
98
O
O
O
O
O
O
O
6
7
3
6
96
O
O
c
90
O N
2
O N
2
O
O
O
c
8
9
6
88
O
3
2
95
97
O
1
0
1
OH
OH
O
1
2
2
98
96
O
1
2
a
Unless otherwise noted, the reactions were carried out with alkene (10 mmol), Co(OAc) (1 mmol), [C py][PF ] (10 mL), and H O (30 %,
2
12
6
2
2
1
1 mmol) at room temperature
b
Isolated yield
c
The reaction was carried out at 50 °C
1
3

J IRAN CHEM SOC
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(
application of this reaction are currently under study in our
laboratory.
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