Terminal and internal olefin epoxidation with cobalt(II) as the catalyst: Evidence for an active oxidant CoII-acylperoxo species

Article abstract of DOI:10.1021/jo3009963

A simple catalytic system that uses commercially available cobalt(II) perchlorate as the catalyst and 3-chloroperoxybenzoic acid as the oxidant was found to be very effective in the epoxidation of a variety of olefins with high product selectivity under mild experimental conditions. More challenging targets such as terminal aliphatic olefins were also efficiently and selectively oxidized to the corresponding epoxides. This catalytic system features a nearly nonradical-type and highly stereospecific epoxidation of aliphatic olefin, fast conversion, and high yields. Olefin epoxidation by this catalytic system is proposed to involve a new reactive Co^II-OOC(O)R species, based on evidence from H₂¹⁸O-exchange experiments, the use of peroxyphenylacetic acid as a mechanistic probe, reactivity and Hammett studies, EPR, and ESI-mass spectrometric investigation. However, the O-O bond of a Co^II-acylperoxo intermediate (Co^II-OOC(O)R) was found to be cleaved both heterolytically and homolytically if there is no substrate.

Full text of DOI:10.1021/jo3009963

Article
pubs.acs.org/joc
Terminal and Internal Olefin Epoxidation with Cobalt(II) as the
Catalyst: Evidence for an Active Oxidant CoII−Acylperoxo Species
Min Young Hyun, Soo Hyun Kim, Young Joo Song, Hong Gyu Lee, Young Dan Jo, Jin Hoon Kim,
In Hong Hwang, Jin Young Noh, Juhye Kang, and Cheal Kim*
Department of Fine Chemistry, Seoul National University of Science & Technology, Seoul 139-743, Korea
S
* Supporting Information
ABSTRACT: A simple catalytic system that uses commercially available
cobalt(II) perchlorate as the catalyst and 3-chloroperoxybenzoic acid as the
oxidant was found to be very effective in the epoxidation of a variety of olefins
with high product selectivity under mild experimental conditions. More
challenging targets such as terminal aliphatic olefins were also efficiently and
selectively oxidized to the corresponding epoxides. This catalytic system features a nearly nonradical-type and highly
stereospecific epoxidation of aliphatic olefin, fast conversion, and high yields. Olefin epoxidation by this catalytic system is
proposed to involve a new reactive CoII−OOC(O)R species, based on evidence from H218O-exchange experiments, the use of
peroxyphenylacetic acid as a mechanistic probe, reactivity and Hammett studies, EPR, and ESI-mass spectrometric investigation.
However, the O−O bond of a CoII−acylperoxo intermediate (CoII−OOC(O)R) was found to be cleaved both heterolytically
and homolytically if there is no substrate.
conditions, where the concentration of an active substrate is
very high, CoIII−OOC(O)R might be a possible reactive
species for epoxidation.6
INTRODUCTION
■
Strong interest has been paid recently to catalytic oxidation of
hydrocarbons by transition metal salts (or complexes) to
produce commercially important products related to pharma-
ceuticals, flavors, fragrances, etc.1 In particular, the epoxidation
of terminal alkenes remains a challenge in organic synthesis,
although the resulting 1,2-epoxides are versatile starting
materials for the synthesis of more complicated molecules.2
The epoxidation of terminal alkenes requires prolonged
reaction times and shows a low conversion, because the low
electron density of terminal olefins limits their reactivity for
electrophilic oxygen transfer.3 Therefore, the discovery of
efficient and practical epoxidation methods under mild
conditions and with inexpensive catalysts is an important goal
in both academia and industry.
Iron and manganese porphyrins and related Schiff base
complexes appear among the most successful oxidation
catalysts for the epoxidation of a wide range of olefins and
for the study of the reactive intermediates.7 However, cost
effectiveness and robustness are the major obstacles associated
with the application of these metal complexes in industrial
processes. Thus, there remains a need to develop an effective
method based on inexpensive and nontoxic catalysts for the
epoxidation of aliphatic internal and terminal alkenes under
mild conditions.2,3 In the search for efficient, less toxic, and
low-cost oxidation catalysts, we have developed a simple
catalytic system that shows efficient olefin epoxidation by 3-
chloroperoxybenzoic acid (MCPBA).
Much attention is currently being paid to the nature of
reactive intermediates (MIVO, MVO, or M−OOR; M =
Fe, Mn, Co) because they have been identified and/or
implicated in the catalytic cycles of a number of heme and
nonheme metal-containing enzymes that activate or evolve
dioxygen.4 Although their structures and reactivities have, to
some extent, been well characterized and studied, the M−OOR
species have been much less explored.5 Recently, our and other
groups suggested that, while the MIVO and MVO species
are major reactive intermediates in the catalytic cycle, the
species Mn−OOC(O)R is gradually involved in the oxidation
reaction if the substrate is active or the concentration of the
substrate is very high.5 Moreover, we have, quite recently,
suggested that (bpc)CoIII−OOC(O)R shows partitioning
between the heterolytic and homolytic cleavage of an O−O
bond to afford CoVO and CoIVO intermediates, proposed
to be responsible for stereospecific olefin epoxidation and
radical-type oxidations, respectively. In contrast, under extreme
Herein, we report a simple and highly efficient catalytic
system that uses commercially available cobalt(II) perchlorate
(Co(ClO4)2) (1) as the catalyst and MCPBA as the oxidant for
the epoxidation of a wide range of olefins, especially including
terminal olefins, with high yield and selectivity and fast
conversion at ambient temperature. It is important to note
that mechanistic studies of the olefin epoxidation reactions
promoted by this catalyst provide evidence that a CoII−
acylperoxo intermediate (CoII−OOC(O)R) shows partitioning
between heterolytic and homolytic cleavage of the O−O bond
to afford CoIIIO and CoIVO intermediates in the absence
of substrate, whereas, in the presence of substrate, an adduct
CoII−OOC(O)R might be a key intermediate for olefin
epoxidation, proposed to be responsible for a nearly non-
Received: May 19, 2012
© XXXX American Chemical Society
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a
Table 1. Olefin Epoxidations by MCPBA with Cobalt(II) Perchlorate (1) in CH3CN at Room Temperature
b
b
entry
substrate
cyclopentene
product
conversion (%)
yield (%)
1
2
3
4
epoxide
epoxide
epoxide
epoxide
100
81.3 1.4
c
cycloheptene
cyclooctene
cyclohexene
100
∼100 (93)
c
98.7 1.1
95.3 0.6
97.9 1.7 (92)
93.6 2.7
1.1 0.1
1.4 0.1
85.6 2.7
91.5 0.2
96.8 1.6
2.9 0.1
∼100
2-cyclohexen-1-ol
2-cyclohexenone
epoxide
5
6
7
1-hexene
1-octene
100
epoxide
93.6 0.1
100
cis-2-octene
cis-oxide
trans-oxide
8
trans-2-octene
trans-oxide
100
9
cis-/trans-2-octene
trans-1-phenyl-1-propene
styrene
cis-/trans-oxide
1-phenylpropene oxide
epoxide
1.0
10
11
100
99.8 1.5
74.7 1.4
5.2 0.4
2.7 0.3
49.0 3.4
40.5 2.5
4.5 0.7
86.5 2.2
98.2 0.2
benzaldehyde
phenylacetaldehyde
cis-oxide
12
13
cis-stilbene
100
trans-oxide
benzaldehyde
trans-oxide
trans-stilbene
100
benzaldehyde
5.6 0.5
a
b
Reaction conditions: substrate (0.035 mmol), catalyst (1.0 × 10−3 mmol), MCPBA (0.05 mmol), solvent (1 mL, CH3CN). Based on substrate.
c
Isolated yields (see Experimental section for details).
radical-type and highly stereospecific epoxidation of aliphatic
olefin. Evidence in support of this interpretation is based on
reactivity and Hammett studies, EPR and ESI-mass spectro-
metric investigation, H218O-exchange experiments, and the use
of peroxyphenylacetic acid (PPAA) as a mechanistic probe.
cis-2-Octene was predominantly epoxidized to cis-2-octene
oxide (96.8%), along with very small amounts of trans-2-octene
oxide (2.9%), indicating that the catalytic epoxidation reaction
occurs with high stereochemical retention (97%; entry 7).11
trans-2-Octene was oxidized exclusively to trans-2-octene oxide
and in high yield (∼100%). Competitive epoxidation of cis- and
trans-2-octene gave a value of 1.0 for the ratio of cis- to trans-2-
octene oxide (entry 9), indicating no preference for cis-olefin
epoxidation to trans-olefin epoxidation by the intermediate
generated in the reaction of cobalt(II) perchlorate (1) and
MCPBA. This selectivity for aliphatic trans-olefin is unusual,
because most of the metal-catalyzed systems described so far
have shown a major reactivity for cis-olefins, and there are only
two precedents to our knowledge for the reversal or the same
of cis/trans selectivity upon epoxidizing aliphatic or aromatic
olefins.2b,5f This value is nearly the same as for MCPBA
epoxidation.2b,8
With styrene, the dominant reaction involved the formation
of styrene oxide as the major product (74.7%) along with small
amounts of phenylacetaldehyde (2.7%) and benzaldehyde
(5.2%; entry 11). cis-Stilbene produced both cis-stilbene oxide
(49.0%) and trans-stilbene oxide (40.5%) with minor amounts
of benzaldehyde (4.5%; entry 12). trans-Stilbene was oxidized
to trans-stilbene oxide (86.5%) and small amounts of
benzaldehyde (5.6%; entry 13). It is of interest to note that
only small amounts of benzaldehyde were produced in the
aromatic olefin epoxidation reactions. The near-nonradical type
of product distribution of these aromatic olefins implies to us
that the peroxyl radical is minimally involved as the epoxidizing
agent because these species would be expected to oxidize
aromatic olefins to radical-induced rearranged products, based
on previous reports.5m,n,10 To the best of our knowledge, 1 is
the most effective cobalt catalyst that affords high conversions
and epoxide yields, minimal radical-type oxidation products,
and a high stereospecificity of aliphatic olefin in the epoxidation
of olefins by MCPBA.
RESULTS AND DISCUSSION
■
Catalytic epoxidation of various olefins using MCPBA as a
terminal oxidant with cobalt(II) perchlorate as catalyst was
performed in CH3CN at room temperature (see the
Experimental Section for details). CH3CN was found to be
the best solvent under the variety of conditions tested to
establish the optimal reaction protocol. This mixture was stirred
for 10 min at room temperature, although the epoxidation
reactions were completed even within 1 min at room
temperature (see the Supporting Information: Figure S1).
Control experiments showed that the epoxide is stable and does
not produce a diol under the conditions that were used in olefin
epoxidation, and that direct substrate oxidation by MCPBA was
negligible (Figure S1).5m,n,8
The epoxidation results are summarized in Table 1. Under
the reaction conditions, the cyclic olefins cyclopentene,
cycloheptene, and cyclooctene were oxidized to the corre-
sponding epoxides in excellent yields (81−100%; entries 1−3),
with conversions in the range of 99−100%. Interestingly, in
contrast to the results obtained with alkyl hydroperoxides as the
terminal oxidant in the presence of a variety of cobalt
complexes,9 the epoxidation reaction of cyclohexene produced
predominantly the epoxidation product (93.6%; entry 4), along
with trace amounts of cyclohexenone (1.4%) and cyclohexenol
(1.1%), suggesting a nearly nonradical oxidation process.10
Moreover, terminal alkenes, usually known as the least reactive
olefins in metal-catalyzed epoxidations,2,3 were readily oxidized
to the corresponding epoxides with high yields (86% and 92%;
entries 5 and 6).
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Next, we have carried out spectroscopic measurements by
EPR to observe the possible reactive intermediates, CoII−
OOC(O)R and CoO, and to determine the oxidation state
of the cobalt ion during the catalytic reaction. As expected, the
EPR spectrum of the frozen solution of Co(ClO4)2 indicated a
typical high-spin cobalt(II) (S = 3/2). The reaction solution that
was frozen 5 s and 1 min, respectively, after complex 1 was
mixed with MCPBA at −40 °C or room temperature in MeCN
was the same as the starting cobalt perchlorate (Figure S2).
This suggests that the cobalt ion retained its +2 charge during
the catalytic reaction and, moreover, that it is difficult to detect
the possible reactive intermediates with this spectroscopic
technique.
A further effort to detect the possible reactive species was
carried out by electrospray ionization mass spectrometry
experiments (ESI-MS) at low temperature (−40 °C). After 3
equiv of MCPBA were added to a solution of 1 (1 mM) in the
absence of and in the presence of substrate cyclohexene, we
tried to detect possible intermediates. However, we could not
observe any of them, suggesting that the reactive intermediates
might have very short lifetimes or be present in too low a
concentration to be detected.
used in studies of reaction mechanism to address the origin of
oxygen atoms found in oxidation products.15 Epoxidation of
cyclohexene by cobalt(II) perchlorate and MCPBA was
conducted in the presence of a large excess of H218O in
CH3CN (∼25−222 equiv; see Table S1 of Supporting
Information and the Experimental Section for details). The
product analysis by GC-MS showed no substantial 18O-
incorporation into cyclohexene oxide from H218O, whereas
cyclohexane hydroxylation showed 11% 18O-incorporation of
the cyclohexanol product in the previous study.1e These results
could be explained by the relative rate difference of oxygen
atom transfer and oxygen exchange, as previously reported,15 or
the direct epoxidation by CoII−OOC(O)R species.
To distinguish homolytic vs heterolytic cleavage of the
peracid O−O bond and to further examine the interplay
between the O−O bond cleavage and substrate oxidation,
PPAA as a mechanistic probe was used.4r,5m,n,16 When the O−
O bond of the coordinated anion (2) of PPAA undergoes
heterolytic O−O bond cleavage (Scheme 1; pathway a) or
Scheme 1. Plausible Mechanism for the Formation of the
Reactive Species from the Reaction of Peracids with Cobalt
Complex
Therefore, a nearly minimal amount of free radical oxidation
reaction, a high degree of stereospecificity of aliphatic olefin,
and no change of the oxidation state of cobalt(II) ion during
the catalytic reaction observed in our catalytic systems imply
that the possible reactive species produced in these catalytic
reactions might be two-electron oxidants CoIVO and CoII−
OOC(O)R.
Because an understanding of the nature and the mechanism
of formation of the reactive intermediates is crucial in designing
better catalysts, another investigation of the possible active
species responsible for this epoxidation reaction was under-
taken by examining the influence of substituent electronic
effects on the rate of epoxidation using styrene and para-
substituted styrenes. A Hammett plot analysis gave ρ = −1.20
which indicates that the active oxidant is electrophilic (Figure
1). The value is somewhat higher than those reported for the
epoxidation of styrenes using MCPBA itself (ρ = −0.76; see
Figure S3 of Supporting Information), MnIII tetraphenylpor-
phyrin (ρ = −0.41),12 Mn(salen) (ρ = −0.3),13 and
Mn(H,MePyTACN)(CF3SO3)2 (ρ = −0.67).14
directly oxidizes substrate (pathway c), phenylacetic acid (PAA,
5) is formed. In contrast, the homolytic cleavage of the O−O
bond of 2 gives benzaldehyde (6), benzyl alcohol (7), and
toluene (8) by a rapid β-scission of an acyloxyl radical
generated from its homolytic cleavage (pathway b). First, a
control experiment with PPAA as an oxidant under the same
reaction conditions as MCPBA was carried out in the absence
of substrate (Table 2; entry 1).
The heterolytic cleavage product, PAA (58% based on
PPAA), and the homolytic cleavage products, benzaldehyde
(15%) and benzyl alcohol (2.2%), were formed in a ratio of
77:23. These results suggest that heterolytic (77%) and
homolytic (23%) O−O bond cleavage of 2 occur simulta-
neously to produce CoIVO (3) and CoIIIO (4) species.
Next, we investigated the concentration effect of substrate, as
previously shown by us and other groups.5m,n,16 If the Co−
OOR species was involved in the epoxidation reaction, then the
ratio of 5 to (6 + 7 + 8) would vary according to the
concentration of substrate employed.16 We increased the
concentration of substrate cyclohexene from 0 mM to 140
mM in the presence of cobalt(II) perchlorate. Importantly, the
ratio of 5 to (6 + 7 + 8) varied from 3.4 (77:23) for 0 mM to
5.5 (85:15) for 20 mM to 7.5 (88:12) for 35 mM to 8.9 (90:10)
for 70 mM to 11.4 (92:8) for 140 mM, as shown in Table 2
(entries 1−5). The fact that the ratio of 5 to (6 + 7 + 8) varies
with substrate implies that there is an interaction between O−
O bond cleavage and substrate oxidation, that is, the initially
formed CoII−acylperoxo intermediate contributes to the
oxidation reaction, at least to some extent, as shown in Scheme
1 (pathway c).
Further mechanistic information arises from isotope labeling
experiments, because 18O-labeling experiments are commonly
Figure 1. Hammett plot for relative reactivities of styrene to para-
substituted styrenes with cobalt(II) perchlorate.
C
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Table 2. Yield (%) of Products Derived from Peroxyphenylacetic Acid (PPAA) Mediated by the Catalyst 1 in the Presence of
a
Cyclohexene
b
c
homolysis
oxidation products
b
entry cyclohexene (mM) heterolysis,
5
6
7
8
hetero (5)/homo (6 + 7 + 8)
oxide
ol
one
1
2
3
4
5
0
20
35
70
140
58
66
69
74
79
1
8
2
2
2
15
1
2.2 0.2
2.6 0.2
2.2 0.1
2.4 0.2
2.1 0.6
−
−
−
−
−
77/23 (3.4)
85/15 (5.5)
88/12 (7.5)
90/10 (8.9)
92/8 (11.4)
−
−
−
d
d
d
b
b
d
9.5 1.0
7.0 0.1
5.9 0.1
4.8 0.1
88
78
55
61
3
6
1
3
1.2 1.2
0
d
d
b
b
1.8 0.4
2.3 0.2
1.6 0.1
2.0 0.1
b
1.3 0.1
2.0 0.1
b
b
a
Reaction conditions: substrate (0−0.14 mmol), catalyst (1.0 × 10−3 mmol), PPAA (0.05 mmol), solvent (1 mL, CH3CN). Based on PPAA. 5, 6, 7,
and 8 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. ol, one, and oxide indicate cyclohexenol, cyclohexenone,
and the cyclohexene oxide, respectively. Based on cyclohexene.
c
d
Table 3. Yield (%) of Products Derived from Peroxyphenylacetic Acid (PPAA) Mediated by Cobalt(II) Perchlorate (1) in the
a
Presence of 1-Hexene
b
homolysis
b
b
entry
1-hexene (mM)
heterolysis,
5
6
7
8
hetero (5)/homo (6+7+8)
oxidation product: 1-hexene oxide
1
2
3
4
5
0
20
35
70
140
58
69
74
74
78
1
8
2
1
2
15
12
1
1
2.2 0.2
2.0 0.2
1.6 0.1
1.4 0.2
1.1 0.1
−
−
−
−
−
77/23 (3.4)
83/17 (4.9)
87/13 (6.7)
88/12 (7.6)
90/10 (9.3)
0
c
96
76
60
69
1
4
1
1
c
9.5 0.2
8.3 0.5
7.3 0.1
b
b
a
b
Reaction conditions: substrate (0−0.14 mmol), catalyst (1.0 × 10−3 mmol), PPAA (0.05 mmol), solvent (1 mL, CH3CN). Based on PPAA. 5, 6, 7,
and 8 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. Based on 1-hexene.
c
a
Table 4. Terminal Olefin Epoxidations by MCPBA with Cobalt(II) Perchlorate (1) in CH3CN at Room Temperature
with 1
without 1
b
b
b
b
entry
substrate
1-nonene
product
conversion (%)
yield (%)
conversion (%)
yield (%)
1
2
3
4
5
6
7
8
1,2-epoxynonane
73.7 2.1
88.6 1.2
90.7 1.5
88.6 2.2
88.7 1.1
85.9 3.6
90.6 1.1
91.4 4.0
72.6 2.4
78.7 1.1
81.9 1.7
77.5 0.9
77.9 1.7
73.6 2.7
84.1 1.0
80.5 3.1
5.6 1.1
10.0 0.7
14.9 1.5
17.3 2.0
27.1 0.6
31.0 1.7
31.1 0.5
7.8 1.3
5.7 1.0
4.9 0.2
10.1 0.1
12.4 0.3
19.1 1.3
17.1 2.1
30.2 1.0
7.7 1.2
1-decene
1,2-epoxydecane
1-undecene
1-dodecene
1-tridecene
1,2-epoxyundecane
1,2-epoxydodecane
1,2-epoxytridecane
1,2-epoxytetradecane
1,2-epoxypentadecane
vinylcyclohexane oxide
1-tetradecene
1-pentadecene
vinylcyclohexane
a
b
Reaction conditions: substrate (0.035 mmol), catalyst (1.0 × 10−3 mmol), MCPBA (0.05 mmol), solvent (1 mL, CH3CN). Based on substrate.
These results suggest that the CoII−OOC(O)R (2) might be
Scheme 1 shows the most plausible mechanism for the
formation of the reactive species responsible for olefin
epoxidation that may well explain our present epoxidation
reactions. Peracid reacts with a cobalt complex to form an initial
cobalt(II)−acylperoxo intermediate (CoII−OOC(O)R (2)),
which then undergoes either a heterolytic (pathway a) or
homolytic (pathway b) O−O bond cleavage to afford CoIVO
(3), or CoIIIO (4) species if there is no substrate. However,
when the substrate is present, the species Co−OOC(O)R (2)
might be a major oxidant responsible for the epoxidation
reaction (pathway c). More detailed mechanistic studies on the
factors that influence heterolysis vs homolysis and the lifetime
of CoII−OOC(O)R (2) are in progress in our laboratory.
Terminal olefins are a particularly challenging class of
substrate to epoxidize because of their relatively electron-
deficient nature,2,3 although the resulting 1,2-epoxides are
versatile starting materials for the synthesis of more
complicated molecules. Therefore, few simple catalytic systems
for terminal olefin epoxidation exist that provide rapid
conversion with high selectivity.2,3 Because our catalytic system
was proved to be very effective for terminal olefins (entries 5
and 6; Table 1), the present catalytic oxidation was applied to a
a potent reactive species for epoxidation, as previously
proposed for Mn(salen)-, Fe(porph)-, Re4 cluster-supported
Mn(saloph)-, Co(bpc)-, and Mn(bpc)-catalyzed epoxidation
with MCPBA as oxygen donor.5 In addition, this provides the
first indirect evidence for the existence of the active CoII−
OOC(O)R (2) species, analogous to the active species
([(TMG3tren)CoII−OIPh]) proposed in the reaction of the
cobalt(II) complex [CoII(TMG3tren)(OTf)](OTf) with
PhIO.17−19
Next, we changed the substrate to one that is more difficult
to oxidize (1-hexene), because it is proposed that, only when
the substrate is active or the concentration of the substrate is
high, the species M−OOC(O)R (2′) might be gradually
involved in the epoxidation reaction.5m,n,16 Surprisingly, a
similar concentration dependence was also observed for 1-
hexene (Table 3). That the difficult-to-oxidize substrate showed
the same concentration dependence is the first case of such an
observation, to the best of our knowledge, and again strongly
suggests that CoII−OOC(O)R (2) might be a major reactive
species for epoxidation and have potential enough to oxidize
the difficult-to-oxidize substrate.
D
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Isolated Yields. Cyclooctene oxide: Cyclooctene (244 μL, 1.75
mmol) and cobalt catalyst (0.05 mmol) were dissolved in 50 mL of
CH3CN. MCPBA (0.56 g, 2.5 mmol) wea added to the reaction
solution. After 10 min, the reaction mixture was filtered through silica
gel. Cyclooctene oxide was isolated from the filtrate through
evaporation, yielding 0.20 g of product (92%). Cycloheptene oxide:
The same method was used for the isolation of cycloheptene oxide.
Cycloheptene oxide was isolated from the filtrate through evaporation,
yielding 0.16 g of product (93%).
variety of terminal olefins. Representative results are shown in
Table 4. They were rapidly epoxidized in good to excellent
conversions (74−91%) and yields (73−84%), confirming that
our catalytic system is also very effective for a variety of
terminal olefins. For a comparison, in the absence of cobalt(II)
perchlorate, the yields of epoxide (5−30%) were very low
compared with those of epoxide in its presence within the same
time interval (sixth and seventh columns in Table 4).
Moreover, this catalytic system is a very rare case that shows
very efficient terminal olefin epoxidations among a variety of
cobalt catalysts.
18O-Labeled H218O experiments. To a mixture of cyclohexene
(0.01−0.02 mmol), cobalt catalyst (0.001 mmol), and H218O (10−40
μL, 0.556−2.22 mmol; 95% 18O enriched, Aldrich Chemical Co.) in a
dried solvent CH3CN (1 mL) was added MCPBA (0.01 mmol). The
reaction mixture was stirred for 3 min at room temperature and then
directly analyzed by GC/mass spectrometry. The 16O and 18O
composition in cyclohexene oxide were determined by the relative
abundance of mass peaks at m/z = 99 for 16O and m/z = 101 for 18O.
All reactions were run at least three times, and the average values are
presented.
Analysis of the O−O Bond Cleavage Products by Cobalt
Catalyst with PPAA. To a mixture of substrate (0−0.14 mmol),
cobalt catalyst (0.001 mmol), and solvent (CH3CN, 1 mL) was added
PPAA (0.05 mmol). The mixture was stirred for 10 min at room
temperature. Reaction was monitored by GC/mass analysis of 20 μL
aliquots withdrawn periodically from the reaction mixture. All
reactions were run at least three times, and the average product
yields are presented. Product yields were based on PPAA.
Competitive Reactions of Styrene and Para-Substituted
Styrenes for Hammett plot. To a mixture of styrene (0.03 mmol)
and para(X)-substituted styrene (0.03 mmol, X = OCH3, CH3, Cl, and
CN), cobalt catalyst (0.001 mmol), and solvent (CH3CN, 1 mL) was
added MCPBA (0.05 mmol). The mixture was stirred for 10 min at
room temperature. The amounts of styrenes before and after reactions
were determined by GC. The relative reactivities were determined
using the following equation: kx/ky = log(Xf/Xi)/log(Yf/Yi) where Xi
and Xf are the initial and final concentrations of substituted styrenes
and Yi and Yf are the initial and final concentrations of styrene.15b
CONCLUSION
■
In conclusion, the present work described a new method for
the fast epoxidation of a wide range of olefins using MCPBA as
terminal oxidant and using a commercially available Co(ClO4)2
as a catalyst. The selectivity for the formation of epoxides was
generally excellent enough to afford epoxides in high yields
almost irrespective of electronic and/or steric variation of the
substrates. Moreover, this catalytic system features a minimally
allylic oxidation reaction. In particular, terminal olefins, which
are normally least reactive to electrophilic oxidants and typically
require long reaction times, were found to undergo fast
conversion and high yields. These results suggest that the
system Co(ClO4)2/MCPBA in CH3CN is particularly suitable
to achieve high yields in alkene epoxidations under simple and
mild conditions and that high activity and stability can be
achieved without resorting to more extreme strategies such as
very electron-deficient or complicated ligands.3e
Reactivity and Hammett studies, EPR and ESI-mass
spectrometric investigation, H218O exchange experiments, and
the use of PPAA as a mechanistic probe suggest that CoII−
OOC(O)R might be a key active intermediate in the
epoxidation reaction. In contrast, the cobalt(II)−acylperoxo
intermediate (CoII−OOC(O)R) undergoes both a heterolytic
and homolytic O−O bond cleavage to afford CoIVO and
CoIIIO species, respectively, if there is no substrate. Future
studies will focus on attempts to understand the exact nature of
the reactive intermediate.
ASSOCIATED CONTENT
* Supporting Information
■
S
18O-Labeled H218O experiments, control experiments with and
without cobalt catalyst, X-band EPR spectra of Co(ClO4)2 and
the reaction solution frozen after Co(ClO4)2 was mixed with
MCPBA, and Hammett plot for relative reactivities of styrene
to para-substituted styrenes by MCPBA. This material is
available free of charge via the Internet at http://pubs.acs.org.
EXPERIMENTAL SECTION
■
General. Olefins, epoxides, cyclohexenol, cyclohexenone, acetoni-
trile, Co(ClO4)2·6H2O, MCPBA (65%), and H218O (95% 18O
enrichment) were purchased and used without further purification.
Peroxyphenylacetic acid (PPAA) was synthesized according to the
literature method.4r Product analyses for olefin epoxidation, partition
reaction of PPAA, and 18O-incorporation reactions of cyclohexene
oxide were performed on a mass spectrometer or a gas chromatograph
equipped with a FID detector using a 30-m capillary column (DB-5 or
HP-FFAP). EPR spectra were recorded on a spectrometer using 100-
kHz field modulation. Electrospray ionization mass spectra (ESI-MS)
were collected by infusing samples directly into the source using a
manual method. The spray voltage was set at 4.2 kV and the capillary
temperature at 80 °C.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chealkim@seoultech.ac.kr; fax: +82-2-973-9149; tel:
+82-2-970-6693.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from Converging Research Center Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2012001725, 2011K000675, and 2012008875) is gratefully
acknowledged. We thank Profs. Yong-Min Lee and Jaeheung
Cho (Ewha Womans University) for performing EPR and ESI-
mass spectrometry and for helpful comments.
Catalytic Olefin Epoxidations by Cobalt Catalyst with
MCPBA. To a mixture of substrate (0.035 mmol), cobalt catalyst
(0.001 mmol), and solvent (CH3CN, 1 mL) was added MCPBA (0.05
mmol). The mixture was stirred for 10 min at room temperature, even
though the reaction was complete within 1 min at room temperature
(Figure S1). Reaction was monitored by GC/mass analysis of 20 μL
aliquots withdrawn periodically from the reaction mixture. All
reactions were run at least three times, and the average product
yields are presented. Product yields were based on substrate. In a
competitive reaction of cis-2-octene and trans-2-octene, the concen-
tration of the substrate was 0.20 mmol, respectively.
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