Manganese acetate in pyrrolidinium ionic liquid as a robust and efficient catalytic system for epoxidation of aliphatic terminal alkenes

Article abstract of DOI:10.1002/asia.201000109

Green epoxides! A novel and simple ionic liquid/manganese acetate catalytic system has been developed for the rapid and selective oxidation of aliphatic terminal alkenes to epoxides. It provides an efficient, reusable, and scalable protocol for the green synthesis of epoxides from various aliphatic terminal alkenes.

Full text of DOI:10.1002/asia.201000109

COMMUNICATION
DOI: 10.1002/asia.201000109
Manganese Acetate in Pyrrolidinium Ionic Liquid as a Robust and Efficient
Catalytic System for Epoxidation of Aliphatic Terminal Alkenes
Kam-Piu Ho, Wing-Leung Wong, Lawrence Yoon Suk Lee, Kin-Ming Lam,
Tak Hang Chan, and Kwok-Yin Wong*[a]
In recent years, growing attention has been paid to the ap-
plications of room temperature ionic liquids (RTILs) as a
new class of reaction media in diverse chemical processes.
The characteristics and the most important applications of
ionic liquids (ILs) have been comprehensively reviewed in
the past decade.[1] The unique properties of ILs, for instance,
low melting point and negligible vapour pressure with essen-
tially no volatile organic emission, make them a good candi-
date to comply with contemporary environmental standards
and economical requirements.[2]
Intense interests on the use of ILs in catalytic reactions
stem from their potential benefits, particularly the ease of
product separation and therefore high recyclability of the
catalysts.[1,3] Numerous possible combinations of cations and
anions offer convenient ways to fine-tune the physicochemi-
cal properties of ILs, such as polarity, viscosity, and miscibil-
ity with other reagents. Most metal catalysts are soluble in
ILs without the modification of ligands, and usually the non-
coordinating anions of ILs leave the active sites of catalysts
still available for the substrates. These distinctive properties
are advantageous in improving biphasic catalytic systems,
which often suffer from problems associated with the incom-
patibility amongst the reaction components.[1a,4–6]
achieve excellent results comparable to those of the homo-
geneous catalysis in organic solvents,[9] the successful exam-
ples are usually limited to internal alkenes. The epoxidation
of terminal alkenes in IL, particularly of long-chained ali-
phatic alkenes, often face difficulties owing to poor miscibil-
ity among the reaction components, which leads to slow re-
action rate and low conversion yield. Effective catalytic ep-
oxidation of aliphatic terminal alkenes and unfunctionalized
olefins in ILs still remains as a challenge.
Our group has been interested in the development of ILs
as “green” media for metal-catalyzed oxygenations of organ-
ic molecules.[10] The epoxidation of alkenes catalyzed by
manganese (Mn) salts is particularly attractive, because they
are inexpensive, relatively non-toxic, and very effective in
catalyzing the epoxidations.[11] We have previously demon-
strated the catalytic oxidation of alkenes by MnII/
Me4NHCO3 in a RTIL (1-butyl-3-methylimidazolium tetra-
fluoroborate, [bmim]BF4) with H2O2 as the terminal oxi-
AHCTUNGTRENNUNG
dant.[10a,c] Unfortunately, the [bmim]BF4-MnII/Me4NHCO3
system is inactive towards aliphatic terminal alkenes. In gen-
eral, we experienced two major difficulties in using
[bmim]BF4 as the medium for oxidations: (i) the low misci-
bility of non-polar aliphatic terminal alkenes, such as 1-
octene, with the polar [bmim]BF4 medium; and (ii) the insta-
bility of [bmim]BF4 under very oxidizing conditions. Despite
the extended reaction time and excessive amount of oxidant
used, only moderate conversions with poor epoxide selectiv-
ity were observed. Herein, we report a novel and robust IL
system for the rapid epoxidation of a number of aliphatic
terminal alkenes under relatively mild conditions using MnII
One of the most extensively studied catalytic reactions in
which ILs are used as a reaction medium is the oxidation of
organics, such as epoxidation of alkenes. Since the first
report on manganeseACTHNUTRGNEUNG(III)-salen complex which successfully
catalyzed the epoxidation of alkenes in IL,[7] a series of imi-
dazolium-based ILs have been tested.[1a–c,3a–b,8] Although
these newly developed IL-based catalytic systems seem to
acetate (MnACHTNUGRTENUNG(OAc)2) as an effective ligand-free catalyst pre-
cursor (Scheme 1). This homogeneous catalytic system is de-
signed to be simple, recyclable, easy to separate the prod-
ucts, resistant to oxidative degradation, scalable, and effi-
cient for the oxidation of unfunctionalized terminal alkenes.
The surfactant-like IL, 1·C12H25SO4 (Scheme 1, 1=1-
methyl-1-propylpyrrolidinium, C12H25SO4 =dodecyl sulfate),
was used as a reaction medium for the epoxidation of al-
[a] Dr. K.-P. Ho, Dr. W.-L. Wong, Dr. L. Y. S. Lee, K.-M. Lam,
Prof. T. H. Chan, Prof. K.-Y. Wong
Department of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong SAR (China)
Fax : (+852)2364 9932
E-mail: bckywong@polyu.edu.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000109.
kenes with MnACHTNUTRGNEUNG(OAc)2 as the catalyst. From our previous
1970
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1970 – 1973

phatic terminal alkenes was investigated using 1-octene as
the model substrate and peracetic acids (32 wt.% in dilute
acetic acid) as the oxidant at 208C (Table 1). Control experi-
Table 1. The epoxidation of 1-octene catalyzed by MnACHTUNGTRENNUNG(OAc)2 in
1·C12H25SO4.[a]
Entry Mn
[mol%]
(OAc)2 1-Octene Conv. Yield[b] Selectivity TOF
ACHTUNGTRENNUNG
G
A
[%]
[%]
[%]
ACHTUNGTRENNUNG ]
[hÀ1
Scheme 1. Catalytic epoxidation of aliphatic terminal alkenes in a simple
ionic liquid–manganese(II) catalyst system.
1[c]
2
3
0.5
0.2
0.4
0.5
0.6
0.5
0.5
0.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0
65
90
100
100
23
67
100
0
–
–
63
87
94
93
23
67
82[g]
97
97
94
93
100
100
82
650
450
400
333
92
4
5
study in ligand-free Mn-catalyzed epoxidation of alkenes in
acetonitrile using peracetic acid as the oxidant, the active
species is likely to be a high-valent polymetallic mangane-
se(V) oxo species which is EPR silent.[10e] In addition, the
main key to successful conversion is the compatibility be-
tween reaction medium, inorganic metal catalyst, buffer so-
lution, and the substrate. 1·C12H25SO4 was designed to ac-
6[d]
7[e]
8[f]
268
240
20
[a] Reaction conditions: 1·C12H25SO4 (1.0 g), NaHCO3 (150 mg), H2O
(0.38 mL), CH3CO3H (3.4 equiv), at 208C, 30 min. [b] Yield estimated by
GC with tetradecane as an internal standard. [c] Control experiment
using water instead of 1·C12H25SO4. [d] Using 1.4 equiv of CH3CO3H.
[e] Using 2.4 equiv of CH3CO3H. [f] Reaction time: 50 min. [g] Yield of
isolated product.
commodate the inorganic salts (MnACHTUNGRTENUNG(OAc)2 and NaHCO3),
water, and non-polar organic substrates in order to form a
homogenous phase for catalysis. The dodecyl sulfate, a
common phase transfer agent, was selected as a counterion
to enhance the compatibility with aliphatic terminal alkenes.
In this study, the pyrrolidinium cation was chosen rather
than imidazolium or pyridinium because of its stability
ments performed in water instead of 1·C12H25SO4 or in the
absence of Mn
0.2 mol% of Mn(OAc)2 was used in 1·C12H25SO4, only 65%
of 1-octene was converted. However the reaction proceeded
smoothly with a good selectivity (97%) of 1,2-epoxyoctane
(Table 1, entry 2). As the catalyst loading was increased to
0.5 mol%, the epoxidation was greatly improved to achieve
complete conversion and excellent yield within 30 min
(94%, Table 1, entry 4). It was found that 0.5 mol% of Mn-
ACHTUNGERTN(NUNG OAc)2 showed poor reactivity. When
AHCTUNGTRENNUNG
À
À
against oxidative degradation. Structurally, the C C and C
N bonds in pyrrolidinium are less susceptible to oxidation
than the unsaturated C=C and C=N bonds in the other cat-
ions. The stability of different ILs against the oxidative deg-
radation was compared by reacting them with peracetic acid
for 30 min at 208C (Figure 1). The amount of peracetic acid
AHCTUNGRTEG(NNUN OAc)2 is optimal and further increase in catalyst loading
(0.6 mol%) has no effect on either the reaction time or the
epoxide selectivity. Under these optimized conditions, high
conversions (>99%) and excellent yields of epoxide
(>93%) were achieved.
In order to explore the potential of this system for the
large scale synthesis of aliphatic terminal epoxides, 20 mmol
of 1-octene was used under the same conditions (Table 1,
entry 8).[12] The reaction was completed in 50 min and the
product of 1,2-epoxyoctane was isolated in good yield
(82%). The crude product collected by simple extraction
and decantation from the IL medium showed excellent
1
purity in GC-MS, H and 13C NMR characterizations, even
without further purification by flash column chromatogra-
phy.
Figure 1. A comparison of the stability of different ionic liquids in per-
acetic acid (CH3CO3H, 32 wt.% in dilute acetic acid) at 208C for 30 min.
The catalytic epoxidation of other terminal alkenes were
also surveyed under similar conditions. In most cases, com-
plete conversion with high selectivity was achieved
(Table 2). Aliphatic terminal alkenes such as 1-heptene, 1-
nonene, and 1-decene were oxidized smoothly to give over
80% yields of isolated products (Table 2, entries 1–3). How-
ever, as the chain length of the substrate increases to C11,
that is, 1-undecene, the catalytic reactivity declined consider-
ably because of its poor miscibility with the reaction
medium. The conversion was not completed (74%) even
after the prolonged reaction time of 50 min, and 69% yields
of 1,2-epoxyundecane oxide was isolated.[13] For 1-dodecene
consumed in the reaction indicates the degree of IL degra-
dation, thus the stability of IL. The pyridinium and imidazo-
lium-based ILs used up significant amounts of peracetic acid
(8–26%) and exhibited obvious color changes. In the case of
1·C12H25SO4, only 2% of peracetic acid was used up without
any change in color. These results confirmed that
1·C12H25SO4 is more robust and suitable as a medium for
our study.
Once the IL medium was optimized for the stability and
compatibility with reaction reagents, the catalytic ability of
the 1·C12H25SO4–MnACTHNUTRGNEUNG(OAc)2 system for the oxidation of ali-
Chem. Asian J. 2010, 5, 1970 – 1973
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www.chemasianj.org
1971

K.-Y. Wong et al.
COMMUNICATION
Table 2. Epoxidation of various terminal alkenes in the 1·C12H25SO4–Mn-
cellent conversion (100%) and yield (94%) obtained in
1·C12H25SO4 (Table 3, entry 4) can be attributed to the sta-
bility of pyrrolidinium cation under strongly oxidizing reac-
tion conditions. As noted previously, oxidant was wasted in
the degradation of pyridinium or imidazolium-based ILs.
The stability and reusability of the 1·C12H25SO4 medium
was tested by successively repeating the epoxidation of 1-
octene. Upon the completion of each reaction, the product
was extracted with pentane and the remaining 1·C12H25SO4
medium was reused upon addition of fresh peracetic acid.
For the first three cycles, comparable yields of isolated prod-
uct were obtained. However, during the recycling process,
insoluble sodium acetate (from peracetic acid) and manga-
nese oxide were formed and accumulated in the IL medium.
It seems that these insoluble substances affect the catalytic
activity of the system significantly as they accumulated up
to a certain concentration (Figure 2). For example, at the
4th cycle the conversion dropped to 45% because of salting
out effect caused by high concentration of acetate leading to
ACHTUNGTRENNUNG
(OAc)2 catalytic system.[a]
Entry
Alkenes
Conversion [%]
Yield of isolated product [%]
1
2
3
4
100
100
100
74
81
87
87
69
5
6
100
100
89
87[b]
7
8
100
100
82[b]
88[b]
9
100
0[c]
[a] Reaction conditions: 1·C12H25SO4 (1.0 g), NaHCO3 (150 mg), H2O
(0.38 mL), alkene (2.5 mmol), Mn(OAc)2 (0.5 mol%), peracetic acid
AHCTUNGTRENNUNG
(3.4 equiv), at 208C for 30 min. [b] Only di-epoxides were produced and
yields were estimated by GC with tetradecane as an internal standard.
The di-epoxides were identified with the known compounds by GC-MS.
[c] No styrene oxide was obtained; styrene was over-oxidized to benzoic
acid.
poor solubility of Mn
were then filtered off and the IL was reused with fresh Mn-
(OAc)2 catalyst (0.5 mol%) added to the medium, and com-
ACHTUNGERTN(NUNG OAc)2 in the IL. The precipitates
AHCTUNGTRENNUNG
Table 3. A comparison of different ionic liquids used as the reaction medium for the catalytic epoxidation of
1-octene.[a]
(C12), the conversion dropped
to 38% under the same condi-
tions. In general, the reactivity
was found to decrease as the
alkyl chain of the terminal
alkene gets longer (>C10).
Entry
Ionic Liquids
Conversion [%]
Yield of epoxide[b] [%]
1
2
65
74
63
69
The 1·C12H25SO4–MnACTHNUTRGNENUG(OAc)2
3
4
84
77
94
system can epoxidize sterically
bulky terminal alkenes. Vinyl-
cyclohexane (Table 2, entry 5),
was epoxidized smoothly to
afford a good isolated yield of
89%, which is comparable to
those of straight-chained al-
kenes. Moreover, di-enes such
100
[a] Reaction conditions: 1·C12H25SO
(OAc)2 (0.5 mol%, 0.04m in water), peracetic acid (3.4 equiv), at 208C, 30 min. [b] GC yields were determined
with tetradecane as an internal standard.
4ACHTUNGTRENN(NUG 1.0 g), NaHCO3 (150 mg), H2O (0.38 mL), 1-octene (2.5 mmol), Mn-
AHCTUNGTRENNUNG
as 4-vinyl-1-cyclohexene, 1,8-nonadiene, and 1,9-decadiene
were successfully converted into the corresponding di-epox-
ides in good yields (yields of isolated products >82%)
under similar conditions (Table 2, entries 6–8). Competition
experiments were carried out using 4-vinyl-1-cyclohexene as
the model substrate. It was found that epoxidation occurs
through a stepwise-reaction. The internal cyclic C=C bond
was oxidized first and then followed by the terminal C=C
bond. The reaction eventually gave the di-epoxide as a sole
product.[14]
The effect of using different IL media in the catalytic ep-
oxidation of 1-octene was also investigated by comparing
the conversion rate and epoxide yield. For all the ILs stud-
À
ied, C12H25SO4 was used as a common counterion and only
Figure 2. Reuse of 1·C12H25SO4 medium with Mn
epoxidation of 1-octene. (At cycle 5 and 8, precipitates of manganese
oxide and sodium acetate were filtered off; and fresh Mn(OAc)2 catalyst
(0.5 mol%) was added to the ionic liquid medium for further experi-
ments).
ACHTUNGRTEN(NNUG OAc)2 as catalyst for
the cationic component was varied. Table 3 shows that when
the reaction was carried out in pyridinium or imidazolium-
based ILs, the conversions were incomplete. The epoxide
yields in those ILs were less than 77%. Presumably, the ex-
AHCTUNGTRENNUNG
1972
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Epoxidation of Aliphatic Terminal Alkenes
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plete conversion with good yields of isolated product were
achieved in subsequent catalytic cycles (from 5th to 7th cy-
cle, 100% conversion, ~80% yield of isolated product). The
catalytic system shows stable performance as the excess
sodium acetate and manganese oxide were removed regular-
ly and fresh MnACHTUNGTRENNUNG(OAc)2 was added as the catalyst (Figure 2,
8th and 9th cycles). Under such circumstances, the ionic
liquid (1·C12H25SO4) medium can be reused constantly.
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Typical procedures for the catalytic epoxidation of terminal alkenes in
1·C12H25SO4–MnACHTUNGTRENNUNG(OAc)2: 1·C12H25SO4 (1.0 g), H2O (0.38 mL), MnACHTUGNTREN(NUGN OAc)2
(100 mL ,0.075m in H2O), NaHCO3 (150 mg), and 1-octene (2.5 mmol)
were added to a 10 mL round-bottomed flask fitted with a water-cooling
jacket. The flask was then sealed with a rubber septum. Peracetic acid
(3.4 equiv, 1.8 mL, 32 wt.% in dilute acetic acid) was added in a dropwise
manner over 1 min. The mixture was stirred vigorously for 30 min at
208C. Crude products were extracted with pentane (4ꢁ6 mL), washed
with 1m NaHCO3 solution, and then dried over sodium sulfate. After the
removal of solvents, 1,2-epoxyoctane oxide was isolated as a colourless
liquid. The epoxide was characterized by GC-MS, 1H NMR, and
13C NMR.
Recycling procedures of the 1·C12H25SO4 medium with MnACHTUNTGRNENG(U OAc)2 as cat-
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alyst for the epoxidation of 1-octene: After each experiment, the residual
pentane in 1·C12H25SO4 was removed by a rotary evaporator. The recov-
ered brownish ionic liquid was then reused for the next run. After every
three cycles of reaction, the ionic liquid medium was diluted with di-
chloromethane (5 mL) and the insoluble manganese oxides and sodium
acetates were filtered off. The ionic liquid was reused for catalysis after
the removal of organic solvent under vacuum and with the addition of
new MnACHTUNGTRENNUNG(OAc)2 catalyst (0.5 mol%, 0.04m in water).
[12] Caution: Large scale reaction is highly exothermic and the peracetic
acid should be added slowly. The reaction should be proceeded with
care.
Acknowledgements
We gratefully acknowledge the support from the Hong Kong Polytechnic
University and the Research Grants Council Collaborative Research
Fund (CityU 2/06C).
[13] The modified reaction conditions for 1-undecene, such as increased
MnACTHNUTRGEN(NUG OAc)2 or peracetic acid loading and longer reaction time, result-
ed in poor epoxide selectivity owing to the over-oxidation.
[14] For detailed experimental results of the competition study, see the
Supporting Information.
Keywords: catalysts · epoxidation · green chemistry · ionic
liquid · manganese
Received: February 7, 2010
Revised: April 24, 2010
Published online: June 24, 2010
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1973