A simple and effective catalytic system for epoxidation of aliphatic terminal alkenes with manganese(II) as the catalyst

Article abstract of DOI:10.1002/chem.200800759

A simple catalytic system that uses commercially available manganese(II) Perchlorate as the catalyst and peracetic acid as the oxidant is found to be very effective in the epoxidation of aliphatic terminal alkenes with high product selectivity at ambient temperature. Many terminal alkenes are epoxidised efficiently on a gram scale in less than an hour to give excellent yields of isolated product (>90%) of epoxides in high purity. Kinetic studies with some C⁹-alkenes show that the catalytic system is more efficient in epoxidising terminal alkenes than internal alkenes, which is contrary to most commonly known epoxidation systems. The reaction rate for epoxidation decreases in the order: 1-nonene>cis-3-nonene> trans-3-nonene. ESI-MS and EPR spectroscopic studies suggest that the active form of the catalyst is a high-valent oligonuclear manganese species, which probably functions as the oxygen atomtransfer agent in the epoxidation reaction.

Full text of DOI:10.1002/chem.200800759

DOI: 10.1002/chem.200800759
A Simple and Effective Catalytic System for Epoxidation of Aliphatic
Terminal Alkenes with Manganese(II) as the Catalyst
Kam-Piu Ho, Wing-Leung Wong, Kin-Ming Lam, Cheuk-Piu Lai, Tak Hang Chan, and
Kwok-Yin Wong*[a]
Abstract: A simple catalytic system
that uses commercially available man-
ganese(II) perchlorate as the catalyst
and peracetic acid as the oxidant is
found to be very effective in the epoxi-
dation of aliphatic terminal alkenes
with high product selectivity at ambient
temperature. Many terminal alkenes
are epoxidised efficiently on a gram
scale in less than an hour to give excel-
lent yields of isolated product (>90%)
of epoxides in high purity. Kinetic stud-
ies with some C9-alkenes show that the
catalytic system is more efficient in ep-
oxidising terminal alkenes than internal
alkenes, which is contrary to most com-
monly known epoxidation systems. The
reaction rate for epoxidation decreases
in the order: 1-nonene>cis-3-nonene>
trans-3-nonene. ESI-MS and EPR spec-
troscopic studies suggest that the active
form of the catalyst is a high-valent oli-
gonuclear manganese species, which
probably functions as the oxygen atom-
transfer agent in the epoxidation reac-
tion.
Keywords: alkenes · epoxidation ·
homogeneous catalysis
· manga-
nese · oxidation
Introduction
salen ligand in Jacobsenꢀs catalyst for asymmetric epoxida-
tion of alkenes). However, cost effectiveness and robustness
are the major obstacles associated with the application of
these metal complexes in industrial processes. Although
manganese(II) sulfate in combination with bicarbonate and
hydrogen peroxide is known to be an effective catalytic
system for epoxidation of certain alkenes,[4c,8] this system
can only be applied to electron-rich and substituted alkenes,
and is inactive towards electron-deficient and aliphatic ter-
minal alkenes. There remains a need to develop an effective
method based on inexpensive and non-toxic catalysts for the
epoxidation of aliphatic terminal alkenes under mild condi-
tions. Herein, we report a simple and highly efficient catalyt-
ic system, which uses commercially available manganese(II)
Epoxidation of alkenes is an industrially important reaction
because epoxides are valuable chemicals widely used as in-
termediates in organic syntheses and as raw materials in the
manufacture of commodity products such as epoxy resins,
paints and surfactants.[1,2] Thus, there is a continuous motiva-
tion to develop new catalytic systems for the production of
epoxides with high yield and selectivity in a cost-effective
manner.[3] Transition-metal-catalysed epoxidation is one of
the most effective approaches for this transformation.[4]
Among the various transition-metal catalysts for epoxida-
tion, manganese stands out as the most efficient, economical
and environmentally benign. Various manganese complexes
ligated with salens,[5] porphyrins[6] and aromatic N-donor li-
gands[7] are known to be efficient catalysts for the epoxida-
tion of a wide range of alkenes. The ligands can often pro-
vide selectivity in the epoxidation process (e.g. the chiral
perchlorate (Mn(ClO4)2) as the catalyst and peracetic acid
A
(CH3CO3H) as the oxidant for the epoxidation of aliphatic
terminal alkenes with high selectivity at ambient tempera-
ture (Scheme 1).
[a] Dr. K.-P. Ho, Dr. W.-L. Wong, K.-M. Lam, C.-P. Lai,
Prof. T. H. Chan, Prof. K.-Y. Wong
Results and Discussion
Department of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University
Hung Hom Kowloon (Hong Kong SAR)
Fax : (+852)236-499-32
Screening of oxidantsand metal salts : The reactivity and se-
lectivity of catalytic epoxidation reactions usually show a
strong dependence on the reaction conditions and are par-
ticularly sensitive to the nature of the metal catalysts and
oxidants. In the trial experiments for screening possible re-
E-mail: bckywong@polyu.edu.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800759.
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FULL PAPER
Scheme 1. Catalytic epoxidation of terminal alkenes.
agents, a number of inexpensive oxidants and transition-
metal salts were studied systematically with 1-decene as the
substrate. Amongst the five commercial oxidants, only per-
acetic acid shows excellent reactivity (>99% conversion)
and epoxide yield (93%) with Mn(ClO4)2 as the catalyst in
G
acetonitrile/water (Figure 1). No epoxidation was observed
with other oxidants such as hydrogen peroxide, tert-butyl hy-
droperoxide and cumene hydroperoxide. In general, good
yields of epoxides and high selectivity were obtained by
using 2.4 equivalents of peracetic acid with respect to alkene
substrates (Figure 1, insert).
Figure 2. Effects of transition metals on the epoxidation of 1-decene with
2.4 equivalents of CH3CO3H as the oxidant in acetonitrile/water at room
temperature. Epoxide yields were determined by GC after 30 min.
salts are the only candidates suitable for the epoxidation of
aliphatic terminal alkenes under such simple oxidation con-
ditions with peracetic acid as the oxidant. After optimisa-
tion, only 0.4 mol% of Mn(ClO4)2 with respect to alkene
U
substrates was found to be sufficient to obtain good epoxide
yields (see Figure S1 in Supporting Information).
Optimisation of epoxidation conditions with Mn(ClO4)2 as
G
the catalyst: The excellent reactivity and selectivity results
of the Mn-catalysed epoxidation in the preliminary studies
prompted us to investigate the system in detail by focusing
on the effects of buffer additives, solvent and water content
in the reaction mixture. With respect to the reported stud-
Figure 1. Effects of oxidants on the epoxidation of 1-decene with
A
0.4 mol% of Mn(ClO4)2 salt in acetonitrile/water at room temperature;
A
ly influenced by pH and buffer additives because the epoxi-
dation reaction involves the formation of pH-sensitive per-
oxymonocarbonate as the key intermediate. In our system,
the additives are interestingly found not to participate in the
catalytic process; they intrinsically only act as pH buffers to
maintain slightly alkaline conditions in the reaction media.
As shown in Figure 3, several commercial buffer additives
were examined in the epoxidation of 1-decene. Acidic and
neutral buffer additives such as acetic acid, dihydrogen
phosphate and hexametaphosphate all gave almost no epox-
ide in comparison with the background level of epoxidation.
On the other hand, dramatically enhanced catalytic activities
in the epoxide production (85–93% epoxide yields) were
observed when alkaline buffers such as acetates, tartrates,
formates, carbonates, bicarbonates and hydroxides were
used in the epoxidation reaction. Among them, the best ad-
ditive was ammonium bicarbonate, which gave the highest
yield of 1,2-epoxydecane (93%). This is probably related to
its better solubility in the acetonitrile/water.
epoxide yields determined by GC after 30 min. The insert shows the ep-
oxide yields as a function of peracetic acid loading with 1-decene as the
substrate; epoxide yields were determined by GC after reaction of
15 min.
Control experiments indicate that the presence of the
MnII catalyst is crucial in the epoxidation because there is
no background reaction observed. To compare the catalytic
abilities of other metal salts in the epoxidation of the termi-
nal alkenes, some metal salts were screened under similar
reaction conditions (Figure 2). Along the first row of transi-
tion-metal salts (i.e. ScIII, T IVi, VIV, CrIII, MnII, MnIII, FeIII,
CoII, NiII, CuII and ZnII), all metal salts except those of Mn
were inactive towards 1-decene as they all gave epoxide
yields of less than 10%. The MnII salt, which gave excellent
selectivity and a 93% yield of the epoxide in 30 min at
room temperature, was unique in that it catalysed this reac-
tion effectively. In addition, a similar epoxidation reactivity
with a comparable yield (91%) was observed by using a
The optimal amounts of basic ammonium bicarbonate salt
used for the Mn-catalysed epoxidation were determined by
manganese(III) salt. These results indicate that manganese
A
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K.-Y. Wong et al.
Figure 4. Effects of solvents on the Mn-catalysed epoxidation of 1-
decene. Epoxide yields were determined by GC after reaction of 30 min.
Figure 3. Effects of buffer additives on the Mn-catalysed epoxidation of
1-decene. Epoxide yields were determined by GC after 30 min.
with an epoxide yield of 93%. Other solvents, regardless of
their protic or aprotic characters, afforded poor yields of
1,2-epoxydecane because they are susceptible to oxidation
under such conditions. For example, stable 2,2,2-trifluoro-
kinetic studies (see Figure S2 in Supporting Information).
Experimentally, small amounts of the bicarbonate buffer
(0.13 equivalents with respect to peracetic acid) were
enough to attain maximum epoxide production with high se-
lectivity. The addition of excess buffer showed no detrimen-
tal effect on the catalytic reaction as the epoxide yields were
still maintained. By calculation, the minimum amount of the
bicarbonate salt that must be added to the reaction aliquot
is just sufficient to neutralise the H2SO4 (ꢀ4.2 mol%),
which acts as the stabiliser in the commercial peracetic acid
stock solution.
Analytically, the H2SO4 content was simply extrapolated
from the titration of the peracetic acid with 0.5m ammonium
bicarbonate standard solution (see Figure S3 in Supporting
Information). Upon titration, three inflection points are re-
markably observed at pH 2.6, 6.2 and 10.6 which correspond
to the titration endpoints of H2SO4, CH3CO2H and
CH3CO3H, respectively. As predicted from the titration
curve, when all the H2SO4 content in the peracetic acid was
neutralised, the pH of the Mn-catalysed epoxidation system
shifted to less acidic. Through destabilising the peracetic
acid, it is believed that the peroxo oxygen atom could be ac-
tively transferred to the MnII catalyst to form high-valent
Mn–oxo species for catalysis.[9]
A
tively much higher than the other non-fluorinated alcohols.
On the other hand, strong chelating solvents (e.g. DMF and
DMSO (DMSO=dimethyl sulfoxide)) also retarded the cat-
alytic activity as their MnII-solvent complexes are intrinsical-
ly inactive in the catalysis.
Besides, the compatibility between solvents, substrates,
the Mn catalyst and the bicarbonate buffer seems to deter-
mine the success of the epoxidation reaction. A comparison
of acetonitrile with its analogues (i.e. propionitrile, butyroni-
trile and valeronitrile) indicates that acetonitrile was the
best choice as the reaction medium and the yields of 1,2-ep-
oxydecane dropped remarkably (from 93% to 19%) with
the chain length of alkyl nitrile under similar reaction condi-
tions (Figure 4). This can be attributed to the reduced solu-
bilities of the Mn catalyst and the bicarbonate buffer with
increasing hydrophobicity of the solvent. It is also consistent
with the experimental results, which show that the catalytic
activity was promoted significantly by addition of water
(10% (v/v)) to the acetonitrile mixture as the epoxide yields
were found to increase from 3% to 77% in the first 15 min
of the reaction (see Figure S4 in Supporting Information).
The solvent resistance to oxidation, the ability of the sol-
vent to form a complex with the MnII catalyst and the versa-
tility of in situ generated MnII–solvent complexes all control
the effectiveness of the catalytic reaction. Hence, the nature
of the solvent is another critical factor in this Mn-catalysed
epoxidation system. As shown in Figure 4, a number of
common solvents were screened as possible epoxidation
media. Among them, acetonitrile gave the best reactivity
Epoxidation of different aliphatic terminal alkeneswith the
Mn(ClO4)2/CH3CO3H/NH4HCO3 catalytic system in acetoni-
A
trile/water: The fruitful results of this simple Mn-catalysed
epoxidation system with 1-decene prompted us to investi-
gate its ability with other alkenes.[10] In Table 1, most ter-
minal aliphatic alkenes were epoxidised efficiently
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Chem. Eur. J. 2008, 14, 7988 – 7996

MnII-Catalysed Epoxidation of Terminal Alkenes
FULL PAPER
Table 1. Epoxidation of terminal aliphatic alkenes with the Mn catalyst
and peracetic acid in acetonitrile/water at room temperature.[a]
Kinetic and mechanistic studies of the Mn(ClO4)2/
N
CH3CO3H/NH4HCO3 catalytic system in epoxidation of ter-
minal alkenes: To have a better understanding of the reac-
tivity and regioselectivity of the Mn-catalysed system to-
wards terminal and internal alkenes, a number of kinetic ex-
periments were carried out with 4-vinyl-1-cyclohexene as
the model substrate. 4-Vinyl-1-cyclohexene was used be-
cause it is a non-conjugated alkadiene that bears one termi-
nal and one internal C=C double bond in the chemical
structure. As shown in Scheme 2, there are two possible
Entry Substrate
Conversion,
[%]
GC
Yield
Yield[b],[%] [%]
1
2
3
4
5
6
7
8
1-octene
1-nonene
1-decene
1-undecene
1-dodecene
1-tridecene
1-tetradecene
1-pentadecene
1,7-octadiene
1,8-nonadiene
1,9-decadiene
vinylcyclohexane
allycyclohexane
1H,1H,2H-perfluoro-1-
octene
>99
>99
>99
>99
>99
>99
90
86
91
93
94
92
93
85
63
78
91
92
91
94
36
83
87
90
87
91
90
–
75
–
9[c]
10[c]
11[c]
12
13
14[d]
>99
>99
>99
>99
>99
43
62
88
90
85
88
–
15[d]
1,1,2-trifluoro-1-octene
0
0
–
[a] Reaction conditions: alkenes (7 mmol), Mn(ClO4)2 salt (0.4 mol%),
A
NH4HCO3(aq) (0.5m, 4.2 mL), CH3CO3H (2.4 equiv, 3.4 mL), acetonitrile
(34 mL), 40 min reaction time. [b] GC yields were determined by GC
with tetradecane as the internal standard. [c] Small-scale reaction: alkene
Scheme 2. The regioselectivity of the Mn-catalysed epoxidation of 4-
vinyl-1-cyclohexene.
(0.5 mmol), Mn(ClO4)2 salt (0.64 mol%), CH3CO3H (4.8 equiv),
G
NH4HCO3(aq) (0.5m, 0.6 mL), acetonitrile (4.8 mL); diepoxides were
formed as the products. [d] Small-scale reaction: alkene (0.5 mmol), Mn-
A
0.3 mL), acetonitrile (2.4 mL).
stepwise-reaction pathways (Paths A and B) for the epoxi-
dation of 4-vinyl-1-cyclohexene (1). These include the for-
mation of diepoxides 4-epoxyethyl-1-cyclohexene 1,2-epox-
ide (4) via intermediate 4-vinyl-1-cyclohexene 1,2-epoxide
(2) and via intermediate 4-epoxyethyl-1-cyclohexene (3).
With respect to the kinetic diagram (Figure 5), the initial
rate of the monoepoxide 2 formation (k1–2 =10.1”10À5 msÀ1)
is much faster than that of the monoepoxide 3 (k1–3 =0 msÀ1)
in the Mn-catalysed reaction. Over 60% of the monoepox-
ide 2 was produced but no monoepoxide 3 was observed in
(>99% conversion) with the MnII catalyst on a gram scale
within 40 min. No reaction was observed even if 10 equiva-
lents of peracetic acid were used under the MnII-free condi-
tions. The Mn-catalysed system generally demonstrates ex-
cellent epoxide selectivity between alkenes with different
alkyl chain lengths. For the C8–C13 terminal alkenes
(Table 1, entries 1–6), both GC yields and yields of isolated
epoxide were found on average to be above 90%. As the
chain length got longer (Table 1, entries 7, 8), the conversion
and epoxide yields obviously declined. This is attributed to
poor solubility of the alkenes with longer alkyl chains in the
acetonitrile/water medium.
The terminal dienes, illustrated in entries 9–11 (Table 1),
can also be transformed into the corresponding diepoxides
effectively in the same manner. In particular, the conver-
sions of all the dienes were found to be over 99%. Good
yields of epoxides were obtained for both 1,9-decadiene
(90% yield) and 1,8-nonadiene (88% yield); however, the
epoxidation reaction only gave a fair yield of 1,2,7,8-diepox-
yoctane (62% yield). Meanwhile, the attachment of bulky
functional groups to the terminal C=C double bond, such as
vinyl cyclohexane and allyl cyclohexane (Table 1, entries 12
and 13), has no influence on the effectiveness of this MnII
catalyst. Both the substrates can achieve high conversions
(>99%) with excellent epoxide selectivities (>90%). At-
tempts were made to epoxidise the electron-deficient fluori-
nated terminal alkenes (entries 14 and 15). However, only
1H,1H,2H-perfluoro-1-octene showed very limited reactivi-
ty in the catalytic process with a 36% yield of epoxide by
GC.
Figure 5. Kinetic study of 4-vinyl-1-cyclohexene epoxidation with the Mn
&
catalyst in acetonitrile/water at 258C. 4-Vinyl-1-cyclohexene (1: ); 4-
!
vinyl-1-cylclohexene 1,2-epoxide (2; ); 4-epoxyethyl-1-cyclohexene (3;
*
N); 4-epoxyethyl-1-cyclohexane 1,2-epoxide (4; ).
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K.-Y. Wong et al.
the first 20 min. These results indicate that the Mn catalyst
preferentially epoxidises the electron-rich internal C=C
double bond even if it faces more steric hindrance in the
catalytic process. When the diene 1 content in the reaction
mixture dropped to 15% after 20 min, subsequent epoxida-
tion of the monoepoxide 2 got faster (k2–4 =4.4”10À5 msÀ1)
and eventually it was converted to diepoxide 4 as the final
product. A control experiment without the MnII catalyst
only showed the background epoxidation to form the mono-
competition reaction. The reactivity and regioselectivity are
unambiguously different from the general findings with in-
ternal electron-rich alkenes, which gives exclusive privileges
in catalytic epoxidation processes.[11]
A similar observation was also found in the epoxidation
of aliphatic 1-nonene and its geometric isomers by using the
MnII catalyst. As shown in entries 4–6 (Table 2), the initial
rate of terminal 1,2-epoxynonane formation was found to be
15.8”10À5 msÀ1 without any background epoxidation
(entry 4). The catalytic rate is considerably faster than the
oxidation of cis-3-nonene (entry 5) as the peracetic acid
itself reacts with the cis-isomer readily under MnII-free con-
ditions (k=4.9”10À5 msÀ1). This is in good agreement with
the results of the cyclohexene and vinylcyclohexane oxida-
tions (entries 2 and 3) reported above. Generally, there is no
significant catalytic effect observed for the Mn catalyst on
the trans isomer (entry 6) probably due to the steric hin-
drance.
In the 1-nonene/cis-3-nonene intermolecular competition
experiment, the rate of the Mn-catalysed cis-3-nonene epox-
idation (k=10.3”10À5 msÀ1) was maintained at a compara-
ble level in the reaction mixture; however, a remarkable
drop (ꢀ8 times) of 1-nonene epoxidation rate (k=1.6”
10À5 msÀ1) was observed (entry 7). As the active Mn catalyst
shows a higher preference to attack cis-type alkenes with
electron-rich C=C double bonds, the epoxidation rate for 1-
nonene always lags behind cis-3-nonene for the first 20 min
of the competition reaction, according to the kinetic dia-
gram (Figure 6). A remarkable turning point appeared after
20 min, at which the rate of 1,2-epoxynonane formation
drastically increased five times to k=7.9”10À5 msÀ1 (com-
pared with its initial rate k=1.6”10À5 msÀ1), when almost
all the cis-3-nonene was consumed. This unpredictable ki-
netic behaviour in the competition experiment is consistent
epoxide
2 at a very slow reaction rate (kcontrol =1.3”
10À5 msÀ1) (see Figure S5 in the Supporting Information).
A comparison of the initial rates of formation of the
mono
epoxide 2 (k1–2 =10.1”10À5 msÀ1) and monoepoxide 3
G
(k1–3 =0 msÀ1) proved that the Mn-catalysed epoxidation re-
action highly preferred to go through Path A rather than
Path B to produce diepoxide 4. This is easily understood by
the involvement of active high-valent Mn–oxo species,
which are typically electrophilic and attack effectively the
electron-rich internal C=C double bond in the catalytic pro-
cess. However, when the same studies were carried out inde-
pendently with different analogues of 4-vinyl-1-cyclohexene
(i.e. cyclohexene and vinylcyclohexane), the reactivity was
an amusing contrast. As listed in Table 2, the rate of forma-
tion of 1,2-epoxycyclohexane (entry 2) was found to be 9.8”
10À5 msÀ1 which is comparable to that of monoepoxide 2
(entry 1). Interestingly, the initial rate to produce the termi-
nal epoxide (k=20.8”10À5 msÀ1) from vinylcyclohexane
(entry 3) is two times faster than the reaction to form 1,2-
epoxycyclohexane (entry 2). By considering these kinetic re-
sults (Table 2, entries 1–3), the active Mn–oxo species gener-
ated in situ in the reaction mixture show better catalytic ac-
tivity in the epoxidation of terminal C=C double bonds;
nevertheless, they attack the sterically hindered dialkyl-sub-
stituted position in the diene 1 readily in the intramolecular
Table 2. The kinetic studies of the Mn-catalysed epoxidation of alkenes
with peracetic acid as the oxidant in acetonitrile/water at 258C.[a]
Rate constant k,”10À5 msÀ1
Entry Substrate
with Mn catalyst No cata-
lyst
1[b]
2
3
4
5
4-vinyl-1-cyclohexene
cyclohexene
vinylcyclohexane
1-nonene
cis-3-nonene
10.1 (0.5[c], 4.4[d]
)
1.3
2.7
0
0
4.9
2.3
9.8
20.8
15.8
14.2
2.8
6
trans-3-nonene
7[e]
8[e]
9[e]
1-nonene versus cis-3-nonene
1-nonene versus trans-3-nonene
cis-3-nonene versus trans-3-
nonene
1.6 (7.9[f]), 10.3 0, 4.1
1.9 (5.4[g]), 2.6
13.5, 2.4
0, 1.7
2.0, 1.5
[a] Reaction conditions for the kinetic studies: alkene substrates
(1 mmol) in of acetonitrile (4.8 mL), Mn(ClO4)2 salt (0.4 mol%),
AHCTREUNG
NH4HCO3(aq) (600 mL, 0.5m), and peracetic acid (500 mL) at 258C; the in-
itial rates (k) were determined from the reaction profiles at low conver-
sion of substrates (<20%). [b] The initial rates of monoepoxide 2 forma-
tion were determined by using 0.5 mmol of substrate. [c] The rate of
diepoxide 4 formation before all diene 1 was completely converted.
[d] The rate of diepoxide 4 formation after all diene 1 was completely
converted. [e] Intermolecular competition experiments. [f] Reaction rate
after 20 min. [g] Reaction rate after 140 min.
Figure 6. The competition study of 1-nonene versus cis-3-nonene in the
Mn-catalysed epoxidation reactions in acetonitrile/water at 258C. 1-
!
&
Nonene ( ), 1-nonene oxide ( ), cis-3-nonene (N), and cis-3-nonene
*
oxide ( ).
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Chem. Eur. J. 2008, 14, 7988 – 7996

MnII-Catalysed Epoxidation of Terminal Alkenes
FULL PAPER
with that observed in the intramolecular competition with 4-
vinyl-1-cyclohexene as the substrate (Figure 5). In both
cases, the epoxidation rates of terminal alkenes were found
to be much faster in the independent kinetic studies (en-
tries 2–5) even though the internal C=C double bonds
behave more competitively with the Mn catalyst (entries 1
and 7). Based on these kinetic results, the internal alkenes
seem not only to be competitors, but also to block the active
sites of the Mn catalyst for the attachment to the C=C
double bond of the terminal alkenes.
To account for this unusual reactivity of the Mn-catalysed
system, we postulate that the active species for the epoxida-
tion are high-valent Mn–oxo complexes with oxidation
states of +5 or even higher because most MnIV–oxo com-
plexes reported in the literature are generally not active in
the epoxidation of aliphatic terminal alkenes.[12] During the
competition reaction, the Mn–oxo oxygen atom transfers to
the alkenes through electrophilic attack of the C=C double
bond in which the effectiveness depends on the density of
the p electrons. As the dialkyl-substituted internal alkenes
(e.g. cis-3-nonene) donate more electrons to the C=C
double bond compared with the terminal alkenes (e.g. 1-
nonene), they are more susceptible to attack with the active
Mn–oxo species (Table 2, entries 1 and 7). However, the re-
activity showed an opposite trend in that the C=C double
bonds in the terminal alkenes were epoxidised faster in the
independent experiments (Table 2, entries 2–5). These find-
ings possibly indicate that the reaction rate is dependent on
the oxygen atom-transfer process.
In the kinetic studies of 1-nonene/cis-3-nonene intermo-
lecular competition (Figure 6), the catalytic epoxidation of
cis-3-nonene was dominant in the first 20 min with a net re-
action rate of 6.2”10À5 msÀ1 due to its structural superiority
in competing for the Mn–oxo active sites. When over 85%
of the cis-3-nonene was consumed after 20 min, the epoxida-
tion rate of 1-nonene was getting faster remarkably with k=
7.9”10À5 msÀ1 as the unusual competition from cis-3-nonene
decreased. In a rough estimation, the epoxidation rate of 1-
nonene was five times faster than its initial rate as well as
that of cis-3-nonene even if the peracetic acid concentration
was supposed to be much lower. When the reaction profiles
of 1-nonene epoxidation were followed in this competition
reaction, it was found, interestingly that the terminal al-
kenes were intrinsically less effective than the internal al-
kenes at coordination with the Mn–oxo species. However,
once the active sites were available, an enhanced epoxida-
tion rate was obtained. These results confirm that the
oxygen atom-transfer process is the rate-determining step
rather than the preference in binding to the Mn–oxo spe-
cies.
Figure 7. The competition study of 1-nonene versus trans-3-nonene in the
Mn-catalysed epoxidation reactions in acetonitrile/water at 258C. 1-
!
&
Nonene ( ), 1-nonene oxide
( ), trans-3-nonene (N), and trans-3-
*
nonene oxide ( ).
nonene took almost 3 h for complete conversion, which is
almost four times longer than the reaction time used in the
1-nonene/cis-3-nonene system. Furthermore, it is noteworthy
that the turning point for the dramatic increment in the ep-
oxidation rate observed in the 1-nonene/cis-3-nonene com-
petition reaction at 20 min did not occur. This is attributed
to the suppression of trans-3-nonone by blocking the active
Mn–oxo from the coordination to 1-nonene because over
50% of the trans-alkene still remained in the reaction mix-
ture. It is also in good agreement with the results obtained
in all of the kinetic studies. After 140 min, the turning point
of the reaction clearly appeared when over 95% of trans-3-
nonene was consumed by background oxidation and mean-
while, the Mn–oxo active sites became available.
Referring to the kinetic results listed in Table 2, the Mn-
catalysed epoxidation reaction is found to be controlled by
two key steps; that is, binding of the substrate onto the
active Mn–oxo and transfer of the Mn–oxo oxygen atom to
the C=C double bond. The binding affinity is mainly deter-
mined by the nucleophilicity of alkene C=C double bonds
towards the active Mn–oxo species. Hence, the internal al-
kenes (e.g. cis-2-nonene and cyclohexene) showed a high
preference to react faster in the competition reaction. The
rate-determining step in this Mn-catalysed reaction was
proven to be the process of Mn–oxo oxygen atom transfer
and therefore, the terminal alkenes (e.g. 1-nonene and vinyl-
cyclohexane) gave higher epoxidation rates in the indepen-
dent experiments. In terms of the structural geometry
(Table 2, entries 5, 6 and 9), it also indicates that the Mn cat-
alyst is highly sensitive to steric hindrance because only the
terminal alkenes and cis-isomers are readily epoxidised in
the catalytic manner (for the kinetic diagrams of these ex-
periments, see Figures S6 and S7 in the Supporting Informa-
tion). To compare, these experimental results apparently
contrast with the reported Mn-based systems[4–8] and give a
clue that the active species, generated in situ in the reaction
To get further justification, the 1-nonene/trans-3-nonene
intermolecular competition reaction was carried out under
similar conditions. trans-3-Nonene was used as a competitor
because it is a geometric isomer of 1-nonene with an inter-
nal C=C double bond and this trans-type double bond is
inert in the Mn-catalysed epoxidation system (Table 2,
entry 6). In Figure 7, the kinetic diagram indicates that 1-
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7993

K.-Y. Wong et al.
media, are not simply just the ligated or solvated Mn–oxo
complexes because these complexes generally show no such
regioselective properties. By combining these interesting re-
sults, it is suggested that the active Mn–oxo species in the
catalytic epoxidation could be some complicated polymetal-
lic Mn–oxo clusters rather than simple solvent–Mn–oxo
complexes.
To obtain more concise information to confirm the exis-
tence of the polymetallic Mn–oxo clusters during the cataly-
sis, ES-IMS measurements were conducted on the reaction
mixture to look for the possible species. As expected, a
number of high molecular mass species with manganese bi-
or oligonuclear structures were detected by using positive-
ion mode. In the control experiments without addition of
peracetic acid or by using acetic acid instead under similar
conditions, no such molecular ions were observed in the ES-
IMS analysis and hence, these could be the fragments or
molecules of the active species.[13]
and trinuclear Mn fragment ions as listed in Table S1 in the
Supporting Information. However, the exact structures and
conformations of the active Mn–oxo species are not yet
completely resolved.
The oxidation states of the Mn components were also
monitored by EPR spectroscopy during the epoxidation re-
action. As shown in Figure 9, the initial solution containing
peracetic acid and 1-decene in acetonitrile (without MnII
species) gave no visible signal (line a). After the MnII salt
The detectable fragment ions are summarised and as-
signed based on their major isotopic peaks (see Table S1 in
Supporting Information for detailed assignments). The as-
signment was attributed by comparing the isotopic ratios of
the chlorine atoms in the spectrum with the calculated
values because the isotopic contribution made by carbon,
hydrogen, oxygen and manganese atoms is insignificant in
the fragment ions. Undoubtedly, the mass series in the spec-
trum are dominated by the chlorine isotopes with a 2 amu
difference in the same mass range. In all cases, the assign-
ment of different fragment/molecule ions was found to be in
excellent agreement with the simulation on the isotopic
ratios. One of the fragment/molecule ions is selected and
shown in Figure 8 as an example (see the Supporting Infor-
mation for more detectable Mn–oxo cluster species).
Figure 9. EPR spectra of the Mn-catalysed reaction mixture: a) Control
experiment without Mn
surement at t=2 s after addition of NH4HCO3(aq); d) measurement at t=
5 min after addition of NH4HCO3(aq) Experimental conditions: Mn-
(ClO4)2 (1.3 mm), 1-decene (42.7 mm), NH4HCO3(aq) (50 mm) and perace-
A
G
.
AHCTREUNG
tic acid (9.6 equiv, 32 wt.%) in acetonitrile/water (9:1 v/v). EPR spectra:
9.4 GHz with 2 mW microwave power at 77 K.
was added, a remarkable signal appeared with a six-line pat-
tern centred at around g=2 (line b), which is consistent
with high-spin MnII (S=5/2) species. When NH4HCO3(aq)
was injected all at once in the reaction mixture, this signal
rapidly diminished in amplitude in the first 2 s (line c) and
no new EPR signal was observed. On the basis of these pre-
liminary results, we are speculating that the active species
for the catalytic reaction could be in low spin state because
the species are EPR silent in the measurements.[16]
Manganese clusters are the well-known catalysts in photo-
system II and behave as the water-oxidising complexes in
plants. However, to the best of our knowledge, there are no
literature reports of their applications in the epoxidation of
terminal alkenes. The structure of the Mn catalyst in its
active state during the catalysis is not clear yet. Neverthe-
less, we are speculating that it might be the polymetallic
manganese clusters (Mn3, Mn4 or higher nuclearity) with
high-valent MnV-oxo for oxygen atom transfer in the epoxi-
dation reaction.[17] The oxygen-transfer process in the cataly-
sis presumably follows the concerted mechanism because
the Mn catalyst gave epoxides with high stereospecificity.
This is also consistent with the observations that both mono-
alkyl- and cis-alkenes are epoxidised much more efficiently
than trans-alkenes in the kinetic studies.
Figure 8. The mass spectrum of the [Mn3
ion observed in the ESI-MS measurements. The insert is the simulated
isotopic distribution based on theoretical calculation.
N
A
Based on the ESI-MS information, the active species were
depicted as high-valent polymetallic Mn–oxo complexes or
clusters with rigid and giant structures formed in situ under
the reaction conditions.[14] Through simple calculations, the
possible oxidation states of the Mn in the spectrum are esti-
mated to be +4, +5 and +6. In addition, mixed valence sys-
tems[15] with +4/+5 and +5/+6 are also found in the bi-
7994
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Chem. Eur. J. 2008, 14, 7988 – 7996

MnII-Catalysed Epoxidation of Terminal Alkenes
FULL PAPER
acetic acid is added too quickly. The epoxide product was extracted into
pentane (3”40 mL), washed with 1m NaHCO3(aq) and dried over sodium
sulfate. The organic solvent was removed under vacuum by rotary evapo-
rator. The crude product was purified by a silica gel column with di-
chloromethane as the eluent to give the desired epoxides as a colourless
liquid. The epoxides were characterised by GCMS and 1H NMR spec-
trum, and compared with the known compounds.
Safety note: Manganese perchlorate salt is potentially explo-
sive and should always be handled with appropriate care al-
though we have not experienced any problems in these ex-
periments.
Epoxidation of 1,7-octadiene: Following the general procedure with 1,7-
Conclusion
octadiene (0.5 mmol), Mn(ClO4)2 salt (0.64 mol%, 100 mL, 0.02m in ace-
A
tonitrile), peracetic acid (500 mL, 4.8 equiv), NH4HCO3(aq) (600 mL, 0.5m
A simple catalytic system for the epoxidation of aliphatic
terminal alkenes has been successfully developed by using
commercially available manganese(II) perchlorate salt as
the catalyst and peracetic acid as the oxidant. Based on the
ESI-MS and EPR spectroscopy results, the active intermedi-
ate formed in situ is proposed to be the high-valent oligonu-
clear manganese species, which were first applied in the ep-
oxidation of terminal alkenes as effective catalysts. A
number of terminal alkenes were found to be epoxidised ef-
ficiently (<40 min) on a gram-scale with excellent yields of
the isolated products (>90%). The kinetic studies revealed
that the Mn-catalysed system shows higher catalytic activity
towards the terminal alkenes. The catalytic rate of epoxida-
tion was found to decrease in the order of 1-nonene>cis-3-
nonene>trans-3-nonene, which is a consequence of the
giant structure of the Mn–oxo species. To have a better un-
derstanding of the nature of the active catalyst in this
system, work is ongoing to isolate and obtain the structures
of the Mn–oxo clusters, which may lead to a new class of
catalysts for alkene epoxidation.
in water) in acetonitrile (4.8 mL) gave 1,2,7,8-diepoxyoctane (62%
1
yield). H NMR (400 MHz, CDCl3): d=2.84 (m, 2H), 2.68 (dd, 2H), 2.39
(dd, 2H), 1.52–1.37 ppm (m, 8H); GCMS: m/z: 142.
Epoxidation of 1,8-nonadiene: Following the general procedure with 1,8-
nonadiene (0.5 mmol), Mn(ClO4)2 salt (0.64 mol%, 100 mL of 0.02m ace-
A
tonitrile), peracetic acid (500 mL, 4.8 equiv), NH4HCO3(aq) (600 mL, 0.5m)
in acetonitrile (4.8 mL) gave 1,2,8,9-diepoxynonane (88% yield).
1H NMR (400 MHz, CDCl3): d=2.85 (m, 2H), 2.69 (dd, 2H), 2.41 (dd,
2H), 1.52–1.32 ppm (m, 10H); GCMS: m/z: 156.
Epoxidation of 1,9-decadiene: Following the general procedure with 1,9-
decadiene (0.5 mmol), Mn
CH3CN), peracetic acid (500 mL, 4.8 equiv) and 0.5m NH4HCO3(aq)
(600 mL) in acetonitrile (4.8 mL) gave 1,2,9,10-diepoxydecane (90%
A
AHCTREUNG
1
yield). H NMR (400 MHz, CDCl3): d=2.86 (m, 2H), 2.71 (dd, 2H), 2.41
(dd, 2H), 1.52–1.32 ppm (m, 8H); GCMS: m/z: 170.
Epoxidation of allylcyclohexane: Following the general procedure gave
1-cyclohexyl-2,3-epoxypropane (88% yield). 1H NMR (400 MHz,
CDCl3): d=2.92 (m, 1H), 2.73 (dd, 1H), 2.41 (dd, 1H), 1.75–1.66 (m,
6H), 1.42–1.22 (m, 5H), 1.12–0.83 ppm (m, 2H); GCMS: m/z: 140.
Epoxidation of vinylcyclohexane: Following the general procedure gave
epoxyethylcyclohexane (85% yield). 1H NMR (400 MHz, CDCl3): d=
2.69 (m, 2H), 2.49 (dd, 1H), 1.86 (d, 1H), 1.73–1.63 (m, 4H), 1.22–
1.07 ppm (m, 6H); GCMS: m/z: 126.
Conditionsfor kinetic and competition experiments : The activity and se-
lectivity of the catalytic system were determined by performing kinetic
experiments in a thermostatted bath at 258C. The experiments were typi-
cally conducted with the MnII salt (0.4 mol%) in a 10 mL round-bot-
tomed flask equipped with stirrer bar. Alkene substrates (1 mmol) in ace-
Experimental Section
Materials: All the solvents were of analytical reagent grade and were
used without further purification. Alkenes and epoxides were obtained
from Aldrich or Acros Organic and were used as received unless other-
wise noted. The standard compounds 1,2,7,8-diepoxyoctane, 1,2,8,9-di-
tonitrile (4.8 mL), Mn(ClO4)2 salt (0.32 mol%), NH4HCO3(aq) (600 mL,
G
0.5m) and peracetic acid (500 mL, 32 wt%) were added to the flask. The
reaction mixture was stirred vigorously. At regular time intervals, 50 mL
of the reaction mixture was withdrawn and diluted with pentane to 5 mL
in a volumetric flask. The sample was then analysed by GCMS with tetra-
decane as the internal standard.
epoxynonane, 1,2,9,10-diepoxydecane, 1-cyclohexyl-2,3-epoxypropane
and epoxyethylcyclohexane were synthesised and characterised following
the literature methods.[18] Transition-metal salts, peracetic acid (32 wt%
in dilute acetic acid), H2O2 (35 wt% solution in water), tert-butyl hydro-
peroxide (70 wt% in water), cumene hydroperoxide (88% in cumene)
and meta-chloroperoxybenzoic acid (purity ꢀ77%) were purchased from
Aldrich. The double-deionised water was purified by filtration through
an ion-exchange resin purification train (Millipore).
Conditions for ESI-MS analysis of the Mn–oxo cluster: The sample for
ESI-MS analysis was prepared in an ice bath at 08C. In a 5 mL round-
bottomed flask, Mn(ClO4)2 (0.02m) acetonitrile (100 mL) was added into
U
acetonitrile (2.4 mL) containing 0.5m NH4HCO3(aq) (300 mL) and perace-
tic acid (250 mL). The resulting mixture was stirred for 1 min and then
kept in a freezer at À258C for another 12 h. The resulting solution was
directly examined by ESI-MS without further dilution.
Instrumentation: A Hewlett–Packard 8900 GCMS equipped with EC-1
or EC-WAX columns (Alltech Associates, Inc.) was used for yield deter-
1
mination and identification. H and 13C NMR spectra were recorded on a
Bruker DPX-400 MHz NMR spectrometer. Tetradecane was used as an
internal standard in the quantitative GCMS measurements. EPR spectra
(77 K) were recorded at the X-band (9.4 GHz microwave frequency),
2 mW power and 25G modulation amplitude on a Bruker EMX EPR
spectrometer. Samples were placed in a 5 mm quartz tube and frozen in
liquid N2 before spectral measurement. ESI-MS analysis was carried out
by using a VG Micromass 7070F Mass Spectrometer.
Acknowledgement
We acknowledge the support from the Hong Kong Polytechnic Universi-
ty and the Research Grants Council Central Allocation Fund (CityU 2/
06C).
General proceduresfor gram-csale catalytic epoxidation of terminal al-
kenes: Alkene (7 mmol), acetonitrile (34 mL), NH4HCO3(aq) (4.2 mL,
0.5m in water) and Mn
A
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Received: April 22, 2008
Published online: July 10, 2008
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