A surfactant-like ionic liquid with permanganate dissolved as a highly selective epoxidation system

Article abstract of DOI:10.1016/j.catcom.2015.05.020

A ligand-free catalytic epoxidation system using permanganate in a surfactant-like ionic liquid (IL) medium was developed. The results indicate that the IL takes crucial effects in the epoxide selectivity. The loading of permanganate is also found critical in preventing over-oxidation of epoxides. The system with 0.3 mol% permanganate and 3.5-equivalent CH₃CO₃H is able to achieve excellent yields and selectivity of epoxides. The study of epoxidation with KMnO₄ in IL medium reveals an unusual oxidation behavior of permanganate not found in traditional solvents.

Full text of DOI:10.1016/j.catcom.2015.05.020

Catalysis Communications 69 (2015) 25–28
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Catalysis Communications
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Short communication
A surfactant-like ionic liquid with permanganate dissolved as a highly
selective epoxidation system
b,
Yu-Jing Lu a, Wing-Leung Wong b, , Cheuk-Fai Chow
⁎
⁎
a
Institute of Natural Medicine and Green Chemistry, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China
Department of Science and Environmental Studies, Centre for Education in Environmental Sustainability, The Hong Kong Institute of Education, 10 Lo Ping Road, Tai Po, Hong Kong, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2015
Received in revised form 19 May 2015
Accepted 27 May 2015
Available online 30 May 2015
A ligand-free catalytic epoxidation system using permanganate in a surfactant-like ionic liquid (IL) medium
was developed. The results indicate that the IL takes crucial effects in the epoxide selectivity. The loading of
permanganate is also found critical in preventing over-oxidation of epoxides. The system with 0.3 mol% perman-
ganate and 3.5-equivalent CH3CO3H is able to achieve excellent yields and selectivity of epoxides. The study
of epoxidation with KMnO4 in IL medium reveals an unusual oxidation behavior of permanganate not found in
traditional solvents.
Keywords:
© 2015 Elsevier B.V. All rights reserved.
Epoxidation
Ionic liquid
Permanganate
1. Introduction
2. Results and discussion
Oxidation of alkenes to epoxides is an important industrial process
[1–3]. Chemists are searching for new technology to use green reagents,
solvents, and reaction media for epoxidation [4–6]. Potassium perman-
ganate (KMnO4) is an inexpensive and widely used oxidant for organic
oxidations [7]. The process is also recognized as environmental-friendly
because manganese dioxide can be separated and recycled [8]. None-
theless, some practical problems of over-oxidations, degradations, and
formation of a multitude of products are observed occasionally.
In recent years, a number of new oxidation systems have been re-
ported with KMnO4 in IL including oxime derivative oxidation [9], oxi-
dative deprotection of aromatic hydrazones [10], oxidation of arenes
[11] and alcohols [12,13]. Among these examples, ILs were addressed
to play crucial roles. Recent studies also demonstrate that reactions in
ILs show different thermodynamic and kinetic behaviors from conven-
tional solvents [14]. Some systems are known to improve reaction per-
formance to give better reaction rate, selectivity, and yields [15–18].
Epoxidation of alkenes with 1-methyl-3-butylimidazoliumdeca-
tungstate in [bmim][BF4] was reported [19], while permanganate in IL
has not been investigated although many epoxidation reactions
and inert alkane oxidations are investigated with Mn-catalysts
[20–23]. In this study, we reported a new system with permanga-
nate in a surfactant-like IL for transformation of terminal alkenes
to epoxides.
MPP+DS− is composed of 1-methyl-1-propylpyrrolidinium cation
and a long-chained alkyl sulfate anion and is able to accommodate
both inorganic salts and non-polar aliphatic alkenes as a homogenous
phase for catalysis because dodecyl sulfate is an excellent phase transfer
agent. MPP+DS− is thus prepared as a recyclable medium for catalysis.
MPP+ is able to form an ionic chelation with MnO−4 in-situ to enhance
the miscibility of the reaction mixture (Scheme 1).
MPP+DS− is not a room temperature IL (m.p. 64 °C). It is a hygro-
scopic salt absorbing moisture to form a viscous gel. Using small
amounts of de-ionized water (0.4 mL/g) is able to prepare a viscous
solution. A homogenous gel was obtained by mixing MPP+DS− with
KMnO4(aq). The gel was characterized with ESI-MS and UV–vis
(Fig. 1) and it was found that MnO−4 was trapped by MPP+ as the species
of [MPP+]2MnO−4 is identified with the corresponding mass (m/z) =
374.86 Da. The species corresponding to [MPP+]2DS− (m/z =
521.24 Da) is also observed. This indicates that the permanganate
is “dissolved” in MPP+DS− through ionic exchange. In addition, a set of
characteristic permanganate absorption peaks from 450 to 600 nm
(λmax = 457, 475, 488, 506, 526, 546, and 569 nm) is observed in
the UV–visible spectrum.
KMnO4 is a well-known oxidant for C–H bond oxidation but no re-
port was found for alkenes epoxidation [20,21]. KMnO4 in IL system
was investigated in the epoxidation reaction with 1-octene and com-
mercial CH3CO3H. Table 1 summarizes the experimental results. In gen-
eral, reactions were found very effective and rapid to produce epoxides
in excellent yields and selectivity. Control experiments, without using IL
as the medium or in the absence of KMnO4, showed poor reactivity with
CH3CO3H as the oxidant (Table 1, entries 1–2). However, when adding
⁎
Corresponding authors.
http://dx.doi.org/10.1016/j.catcom.2015.05.020
1566-7367/© 2015 Elsevier B.V. All rights reserved.

26
Y.-J. Lu et al. / Catalysis Communications 69 (2015) 25–28
however, the reaction was not completed. As increasing KMnO4 loading
to 0.3 mol%, the epoxidation was efficiently to achieve 100% conversion
within 30 min (Table 1, entry 4). Further increasing KMnO4 (0.6 mol%)
did not give extra improvements in both reactivity and reaction time for
complete conversion but slightly reducing the yield of epoxides
(Table 1, entry 6). The reactions were able to achieve high yields of ep-
oxide (N92%) under optimized conditions.
The comparison of using isolated MPP+MnO4− and KMnO4 in
MPP+DS− for epoxidation was studied. Under the same conditions,
99% conversion and 93% epoxide yield were obtained with MPP+MnO4−
as the catalyst. The result is very comparable with KMnO4 (Table 1,
entry 5). The findings imply that no significant difference from using
isolated MPP+MnO4− or KMnO4 in IL medium because in both cases
they are likely having the same active Mn-oxo species for epoxidation.
It is noteworthy that the present epoxidation system does not give ob-
servable alkyl peroxides, which are found as the primary products and
Scheme 1. Epoxidation of aliphatic terminal alkenes in MPP+DS− medium with KMnO4 or
isolated catalyst MPP+MnO4− using CH3CO3H as the oxidant.
0.15 mol% KMnO4 into MPP+DS− with CH3CO3H, the reaction was
proceeded effectively. As shown in Table 1 (entry 3), 1-octene was oxi-
dized to 1,2-epoxyoctane selectively without over-oxidation occurred;
(A)
(B)
O
S
N+
MnO4
2
N+
-
O-
11
O
O
2
(C)
Fig. 1. (A) ESI-MS analysis (positive mode) of IL medium, and (B) a MS spectrum of IL trapped with MnO−4 as the counter-ion after the addition of KMnO4 to MPP+DS−; (C) the UV–vis
spectrum of the mixture of KMnO4 in MPP+DS− and the isolated MPP+MnO−4
.

Y.-J. Lu et al. / Catalysis Communications 69 (2015) 25–28
27
Table 1
studied. The corresponding di-epoxides were produced exclusively in
good isolated yields (N80%).
KMnO4–MPP+DS− system for epoxidation of aliphatic terminal alkenes.a
The preliminary results demonstrate that KMnO4-IL is an effective
and selective epoxidation system for a number of terminal alkenes. It
is noteworthy that the epoxidation condition of the system is simple,
at room temperature, and without using organic solvents. The active
species responsible for oxygen transfer during the epoxidation is con-
sidered to be permanganate because control experiments prove that
CH3CO3H is almost inactive. The use of permanganate for oxidation of
organic substrates and its oxidation mechanisms has long been studied
[25–28] but it is rarely found selectivity towards epoxides in alkene
oxidation. Under acidic conditions, oxidative cleavage is the typical
reaction path (Fig. 2(A)) to give ketones, carboxylic acids, and carbon
dioxide as the end products. However, with catalytic amounts of per-
manganate as the catalyst and CH3CO3H as the oxidant in IL medium,
the reaction gives unexpected high selectivity for epoxides. We specu-
late that the IL medium may take crucial effects in preventing alkenes
from over-oxidation (path A). We know that catalytic amounts of
permanganate in MPP+DS− are solvated by MPP+ (Fig. 2(C)). The
pyrrolidinium permanganate species is also observed by ESI-MS
(Fig. 1A). With respect to our results, a possible oxygen transfer mech-
anism for alkene epoxidation is proposed in Fig. 2(B). The Mn-oxo of
the permanganate is “solvated” by surrounding MPP+ and may render
it requiring more activation energy for interaction with the double
bond of the substrate. Consequently, the C_C double bond interacts
with one of the MnVII_O oxo and followed by forming a three
membered-ring transition state via a concerted pathway to give ep-
oxide as the product. The reduced MnV-species is either regenerated
to permanganate by CH3CO3H or undergoes disproportionation to
form colloidal MnIVO2 and permanganate [29]. The catalytic system
can be re-used for three times in good yields (80–85%); however, at
the fourth cycle, only about 40% yields of 1,2-epoxyoctane were ob-
tained due to the formation of colloidal MnO2 which precipitated
out in the medium. The catalytic system can be resumed by adding
KMnO4 to the reaction medium.
Entry
Alkene substrate,
mmol
KMnO4, mol%
Conversion, %
Yield, %
1b
2
3
4
5
1-Octene
1-Octene
1-Octene
1-Octene
1-Octene
1-Octene
1-Octene
1-Heptene
1-Nonene
1-Decene
1-Undecene
1,8-Nonadiene
1,9-Decadiene
2.5
2.5
2.5
1.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.3
0
b2
0
74
100
99
100
99
N99
N99
98
75
N99
N99
b2
0
70
93
92
0.15
0.3
0.3
0.6
0.3
0.3
0.3
0.3
0.3
0.3
0.3
6
90
7c
8
93d
80d
83d
85d
70d
80d
82d
9
10
11e
12f
13f
Conditions: 1.0 g MPP+DS−, 0.15 g NaHCO3, 0.4 mL H2O, 3.5 equivalents of CH3CO3H;
a
20 °C, 30 min to complete conversion; yields estimated by GC with tetradecane as internal
standard.
b
Control experiment using de-ionized water as the reaction medium.
c
Isolated MPP+MnO−4 catalyst.
d
Isolated yields.
Reaction run for 50 min.
Only di-epoxides were obtained.
e
f
are completely transformed into the unsaturated ketones and alcohols in
the triosmium dodecacarbonyl catalyzed oxidation system [24].
Other terminal alkenes were also surveyed. In most cases, high con-
version and selectivity for epoxides were achieved. For aliphatic termi-
nal alkenes, 1-heptene, 1-nonene and 1-decene were oxidized smoothly
to give 80%–85% isolated yields (Table 1, entries 8–10). However, as the
aliphatic chain length of alkenes increases to C11, 1-undecene, the reac-
tivity was found declined remarkably due to its poor miscibility with the
medium. The conversion was not completed even extending the reac-
tion time to 50 min; only 70% yields of 1,2-epoxyundecane was isolated.
Moreover, di-enes including 1,8-nonadiene and 1,9-decadiene were
Fig. 2. (A) Typical cleavage oxidation of alkene with permanganate to give carboxylic acid and CO2; (B) proposal mechanism for epoxidation of alkenes with permanganate in MPP+DS−
(C) permanganate ion was solvated by MPP+ in IL medium.
;

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Y.-J. Lu et al. / Catalysis Communications 69 (2015) 25–28
Table 2
3. Conclusion
Control experiments with KMnO4 as the oxidant in different reaction media.a
In conclusion, a new KMnO4-IL catalytic system for selective oxidation
of aliphatic terminal alkenes to epoxides was developed. The system was
demonstrated to be able to produce high yields and selectivity under
simple conditions such as ligand-free and solvent-free with the non-
toxic, inexpensive, and largely available reagents (KMnO4, CH3CO3H).
The system also reveals an unusual oxidation behavior of permanganate
in IL medium.
Entry Medium
KMnO4 equivalent Conversionb, Products observedb
%
1c
2c
3d
Water
1
4
1
b1
2
Epoxide (0%)
Heptanoic acid (100%)
Epoxide (0%)
Heptanoic acid (100%)
Epoxide (4%)
Water
Acetonitrile
9
Heptanoic acid (74%)
2-Octanone (12%)
Other (10%)
4d
5e
6e
Acetonitrile
Ionic liquid
Ionic liquid
4
1
4
94
14
98
Epoxide (b1%)
Acknowledgment
Heptanoic acid (77%),
2-Octanone (8%),
Other (14%)
We acknowledge the funds received from the National Nature
Science Foundation of China (81473082 and 21102021). We also ac-
knowledge the supports received from the Department of Science and
Environmental Studies, the Centre for Education in Environmental Sus-
tainability, and the Hong Kong Institute of Education.
Epoxide (13%)
Heptanoic acid (69%)
2-Octanone (11%)
Other (7%)
Epoxide (2%)
Heptanoic acid (91%)
2-Octanone (2%)
Other (5%)
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