Amide-based nonheme cobalt(III) olefin epoxidation catalyst: Partition of multiple active oxidants CoV=O, CoIV=O, and Co III-OO(O)CR

Article abstract of DOI:10.1002/chem.201103916

A mononuclear nonheme cobalt(III) complex of a tetradentate ligand containing two deprotonated amide moieties, [Co(bpc)Cl₂][Et ₄N] (1; H₂bpc=4,5-dichloro-1,2-bis(2-pyridine-2- carboxamido)benzene), was prepared and then characterized by elemental analysis, IR, UV/Vis, and EPR spectroscopy, and X-ray crystallography. This nonheme Co^III complex catalyzes olefin epoxidation upon treatment with meta-chloroperbenzoic acid. It is proposed that complex 1 shows partitioning between the heterolytic and homolytic cleavage of an O-O bond to afford Co ^V=O (3) and Co^IV=O (4) intermediates, proposed to be responsible for the stereospecific olefin epoxidation and radical-type oxidations, respectively. Moreover, under extreme conditions, in which the concentration of an active substrate is very high, the Co-OOC(O)R (2) species is a possible reactive species for epoxidation. Furthermore, partitioning between heterolysis and homolysis of the O-O bond of the intermediate 2 might be very sensitive to the nature of the solvent, and the O-O bond of the Co-OOC(O)R species might proceed predominantly by heterolytic cleavage, even in the presence of small amounts of protic solvent, to produce a discrete Co ^V=O intermediate as the dominant reactive species. Evidence for these multiple active oxidants was derived from product analysis, the use of peroxyphenylacetic acid as the peracid, and EPR measurements. The results suggest that a less accessible Co^V=O moiety can form in a system in which the supporting chelate ligand comprises a mixture of neutral and anionic nitrogen donors. Copyright

Full text of DOI:10.1002/chem.201103916

DOI: 10.1002/chem.201103916
Amide-Based Nonheme Cobalt ACHTGNUTRNEUNG( III) Olefin Epoxidation Catalyst: Partition of
V IV III
À
Multiple Active Oxidants Co =O, Co =O, and Co OO(O)CR
[
a]
[a]
[b]
[a]
[a]
Young Joo Song, Min Young Hyun, Jun Ho Lee, Hong Gyu Lee, Jin Hoon Kim,
[
a]
[a]
[c]
[c]
Seung Pyo Jang, Jin Young Noh, Youngmee Kim, Sung-Jin Kim,
[
b]
[a]
Suk Joong Lee,* and Cheal Kim*
Abstract:
cobalt(III) complex of a tetradentate
ligand containing two deprotonated
amide moieties, [Co(bpc)Cl ][Et N] (1;
H bpc=4,5-dichloro-1,2-bis(2-pyridine-
A
mononuclear nonheme
termediates, proposed to be responsi-
ble for the stereospecific olefin epoxi-
dation and radical-type oxidations, re-
spectively. Moreover, under extreme
conditions, in which the concentration
of an active substrate is very high, the
CoÀOOC(O)R (2) species is a possible
tive to the nature of the solvent, and
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
the OÀO bond of the CoÀOOC(O)R
species might proceed predominantly
by heterolytic cleavage, even in the
presence of small amounts of protic
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
2
4
2
V
2-carboxamido)benzene), was prepared
solvent, to produce a discrete Co =O
and then characterized by elemental
analysis, IR, UV/Vis, and EPR spec-
troscopy, and X-ray crystallography.
intermediate as the dominant reactive
species. Evidence for these multiple
active oxidants was derived from prod-
uct analysis, the use of peroxyphenyl-
acetic acid as the peracid, and EPR
measurements. The results suggest that
reactive species for epoxidation. Fur-
thermore, partitioning between heterol-
ysis and homolysis of the OÀO bond of
III
This nonheme Co complex catalyzes
olefin epoxidation upon treatment with
meta-chloroperbenzoic acid. It is pro-
posed that complex 1 shows partition-
ing between the heterolytic and homo-
lytic cleavage of an OÀO bond to
the intermediate 2 might be very sensi-
V
a less accessible Co =O moiety can
Keywords: catalysts · cleavage re-
actions · cobalt · epoxidation · high-
valent species
form in a system in which the support-
ing chelate ligand comprises a mixture
of neutral and anionic nitrogen donors.
V
IV
afford Co =O (3) and Co =O (4) in-
Introduction
plexes of other planar tetradentate ligands, such as amido li-
gands, have been much less explored.
[3]
Because metalloenzymes efficiently catalyze biologically im-
portant oxygenation reactions under mild conditions, several
transition-metal-based catalysts have been designed and
used to mimic the efficient enzymatic OÀO bond-activation
Considerable interest is currently being paid to the
IV
chemistry of nonheme high-valent metal-oxo species (M =
V
O or M =O: M=Fe or Mn) as they have been identified
and/or implicated in the catalytic cycles of a number of non-
heme metal-containing enzymes that activate or evolve di-
[1]
and subsequent oxygenations . Iron and manganese por-
phyrins and related Schiff base complexes appear among
the most successful oxidation catalysts for the oxidation of
[1,2,4]
oxygen.
Although their structures and reactivities have,
to some extent, been well characterized and studied, the
[1,2]
V
IV
a wide range of organic substrates,
whereas cobalt com-
high-valent cobalt-oxo species (Co =O and Co =O) have
been much less explored.
[5,6]
This is probably because, to the
right of iron in the periodic table, the oxidation states with
four or fewer d electrons are known to be less accessible
and because the reactions of cobalt complexes with hydro-
peroxides often proceed through free-radical types of oxida-
tion reaction. In the literature, some cobalt(IV)-oxo spe-
cies and only a few cobalt(V)-oxo species have been pro-
posed to be the active intermediates for oxygen transfer in
reactions mediated by compounds of cobalt with oxidants
such as peracids and PhIO, and much is not known about
their oxidative reactivity.
[
a] Y. J. Song, M. Y. Hyun, H. G. Lee, J. H. Kim, S. P. Jang, J. Y. Noh,
Prof. Dr. C. Kim
Department of Fine Chemistry
Seoul National University of Science & Technology
Seoul 139-743 (Korea)
Fax : (+82)2-973-9149
[5]
[6]
E-mail: chealkim@snut.ac.kr
[
b] J. H. Lee, Prof. Dr. S. J. Lee
Department of Chemistry
Korea University, Seoul 136-701 (Korea)
Fax : (+82)2-3290-3145
E-mail: slee1@korea.ac.kr
During our research on modeling mononuclear nonheme
metal-containing enzymes, we have studied the chemistry of
nonheme metal (Fe and Mn) complexes of biscarboxamide
[
c] Dr. Y. Kim, Prof. Dr. S.-J. Kim
Department of Chemistry and Division of Nano Sciences
Ewha Womans University, Seoul 120-750 (Korea)
[7]
donor ligands because these types of ligands are highly de-
sirable due to their simple and inexpensive preparation. In
addition, the chemical reactivity of these systems can easily
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103916.
6094
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Chem. Eur. J. 2012, 18, 6094 – 6101

FULL PAPER
be tuned through modification
of substituent groups at the pyr-
idine ring or amide bridge.
Moreover, as these ligands pos-
sess an overall dianionic charge
(
by deprotonation of the two
[8]
amide moieties), they might
be expected to form high-oxida-
tion-state metal complexes. Scheme 1. Synthesis of cobalt
Therefore, we recently reported
ACHTUNGTRENNU(GN III) complex 1.
on monomeric amide-based
iron and manganese complexes that are capable of rapidly
catalyzing the epoxidation of olefins by the use of meta-
chloroperbenzoic acid (MCPBA), with high yields. More-
over, after using a mechanistic probe, the formation of the
absence of the v˜
A
H
U
G
R
N
N
(NÀH) stretching bands at approximately
À1
3400–3150 cm and the presence of the intense band at
À1
1653 cm in the IR absorption spectrum of the cobalt com-
plex indicated that the coordinated amide ligand is depro-
V
V
III
IV
[10]
1
Fe -oxo, Mn -oxo, Mn -OO(O)CR, and Mn -oxo moieties
was proposed from the reactions of iron and manganese
tonated.
In the H NMR spectrum, the appearance of
proton peaks of the benzene and pyridyl rings at 7.90–
9.76 ppm suggested that the complex 1 may have a low-spin
Co oxidation state. Addition of diethyl ether to a reaction
solution of 1 afforded dark brown crystals suitable for crys-
tallographic analysis. The crystal structure of 1 revealed a co-
ordination mode around the distorted octahedral metal
center (Figure 1), similar to the structure of the bpb ana-
The resultant cobalt complex is highly stable to
air and moisture; it can be stored for several months in air
without loss of catalytic activity.
[7]
complexes with MCPBA as oxidant. Accordingly, we
expect that cobalt complexes with the same amide-based
anionic chelating ligand could also produce the high-valent
cobalt-oxo species.
III
To extend the observations to nonheme cobalt complexes
in an effort to develop efficient, selective, and readily avail-
able catalysts, to observe the less accessible high-valent
[9,11]
logue.
V
IV
cobalt-oxo species (Co =O or Co =O), and to study the
III
OÀO bond cleavage mode of Co ÀOO(O)CR species and
the catalytic reactivity, we synthesized and then character-
ized an amide-based nonheme Co complex and applied it to
olefin oxidations with peracids as oxidants. We found that
the Co complex catalyzed olefin epoxidation by MCPBA
through multiple active oxidants.
Herein, we describe the synthesis and characterization of
a new and easily accessible cobalt complex, and its reactivity
toward oxidative functionalization of olefins by using
MCPBA under mild conditions. It is important to note that
mechanistic studies of the olefin epoxidation reactions pro-
moted by this catalyst provide evidence that the cobalt com-
plex shows partitioning between heterolytic and homolytic
V
IV
cleavage of the OÀO bond to afford Co =O (3) and Co =O
(
4) intermediates, proposed to be responsible for the stereo-
specific olefin epoxidation and radical-type oxidations, re-
spectively. Moreover, under extreme conditions, in which
the concentration of an active substrate is very high, the Co-
OOC(O)R (2) species might be a possible reactive species
for epoxidation. In protic solvents, the Co-OOC(O)R (2)
species might undergo predominantly heterolytic cleavage
V
to produce a Co =O species as the dominant epoxidizing in-
termediate.
Results and Discussion
The cobalt complex [Co ACHTUNGTRNENUG( bpc)Cl ] ACHTUNGTERNNNU[G Et N] (1; H bpc=4,5-di-
2 4 2
chloro-1,2-bis(2-pyridine-2-carboxamido)benzene) was syn-
thesized by reacting equimolar amounts of CoCl ·6H O and
2
2
the ligand H bpc in the presence of Et N/Et NCl in CH CN
2
3
4
3
[9]
(
Scheme 1; see the Experimental Section for details). The
Figure 1. Crystal structure of complex 1.
Chem. Eur. J. 2012, 18, 6094 – 6101
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www.chemeurj.org
6095

C. Kim, S. J. Lee et al.
Catalytic epoxidation of various olefins by using MCPBA
as a terminal oxidant with 1 as a catalyst was performed in
a CH Cl /CH CN (1:1) solvent mixture at room temperature
2-octene oxide was determined to be 1.3 (Table 1, entry 10).
This value is similar to those observed with the MCPBA/
À
[7a,9]
[(Fe
MCPBA/[(Mn AHCTUNTGRENNUN(G Me bpb)Cl ACHTUNGTERNNNU(G H O)] system (k /k =1.8).
2 2 cis trans
A
H
U
G
R
N
N
(Me bpb)Cl ]
system (k /k =1.7)
and the
2
2
3
2
2
cis trans
[12]
[7c,9]
(
see the Experimental Section for details). We confirmed
that direct substrate oxidation by MCPBA was negligi-
Aromatic olefins, such as styrene, cis-stilbene, and trans-
stilbene, afforded significant amounts of aldehyde products:
17.3% (benzaldehyde and phenylacetaldehyde) for styrene,
24.3% for cis-stilbene, and 24.1% for trans-stilbene
(Table 1, entries 11–13). This result suggests that significant
amounts of cobalt(IV)–oxo species might be produced by
the homolytic OÀO bond cleavage of CoÀOOC(O)R (2)
[
7,13]
ble.
Addition of a further portion of MCPBA regenerat-
ed the catalytic cycle, with the same yield as the first cycle.
Cyclic olefins, such as cyclopentene, cycloheptene, and cy-
clooctene, were oxidized to the corresponding epoxides (48–
64%; Table 1, entries 1–3), whereas the terminal olefin 1-
species formed from the reaction of catalyst
1 and
Table 1. Olefin epoxidations by MCPBA with cobalt catalyst
CH Cl /CH CN (1:1) at room temperature.
2 2 3
1 in
[5,16]
[
a]
MCPBA.
It has been proposed that some cobalt(IV)–oxo (4) spe-
Substrate
Product
1
[
5]
[6]
cies and only a few cobalt(V)–oxo (3) species are the
active intermediates for oxygen transfer in reactions mediat-
ed by compounds of cobalt with oxidants such as MCPBA
1
2
3
4
5
cyclopentene
cycloheptene
cyclooctene
1-octene
epoxide
epoxide
epoxide
epoxide
epoxide
48.4Æ1.4
55.4Æ1.2
63.9Æ1.7
14.5Æ0.2
43.6Æ2.7
6.2Æ0.5
7.3Æ0.3
40.1Æ4.5
11.1Æ0.3
36.5Æ1.2
46.3Æ9.6
9.5Æ1.1
39.7Æ1.3
1.3:1
42.2Æ1.4
9.0Æ0.1
8.3Æ0.1
21.1Æ0.4
21.5Æ0.5
24.3Æ0.0
42.5Æ1.4
24.1Æ0.5
[5]
and PhIO, whereas the reactions of cobalt complexes with
hydroperoxides often proceed through free-radical types of
oxidation reaction. On the basis of the results presented
cyclohexene
2
2
-cyclohexene-1-ol
-cyclohexenone
[16]
[3,5,6,16]
herein and previously,
we propose that the cobalt
6
cis-2-hexene
cis-oxide
trans-oxide
trans-oxide
cis-oxide
trans-oxide
trans-oxide
cis-/trans-oxide
epoxide
benzaldehyde
phenylacetaldehyde
cis-oxide
trans-oxide
benzaldehyde
trans-oxide
benzaldehyde
complex promotes partitioning between heterolytic and ho-
V
7
8
trans-2-hexene
cis-2-octene
molytic cleavage of the OÀO bond to afford Co =O and
IV
Co =O intermediates, proposed to be responsible for the
stereospecific olefin epoxidation and radical-type oxidations,
9
0
1
trans-2-octene
cis-/trans-2-octene
styrene
[3,5,6]
respectively.
1
1
To gain insight into the proposal that complex 1 promotes
partitioning between the heterolytic and homolytic OÀO
bond cleavage of CoÀOOC(O)R (2) species, we used perox-
1
2
3
cis-stilbene
yphenylacetic acid (PPAA) as a mechanistic probe, because
the cleavage mode could be easily determined by quantita-
1
trans-stilbene
tive determination of the degradation products derived from
[7,17,18]
PPAA.
Heterolytic cleavage of the OÀO bond affords
[
a] See the Experimental Section for details.
phenylacetic acid (PAA, 5), and homolytic cleavage affords
benzaldehyde (6), benzyl alcohol (7), and toluene (8)
through the benzyl radical. Furthermore, the direct reaction
of the acylperoxo intermediate and substrate affords PAA,
and apparently affects the OÀO bond cleavage mode. In the
octene was less efficiently oxidized to 1-octene oxide
Table 1, entry 4). Note that the reactions of cyclohexene af-
forded significant amounts of 2-cyclohexenone (7.3%) and
-cyclohexene-1-ol (6.2%) as the allylic hydrogen abstrac-
(
cyclohexene oxidation reaction catalyzed by 1 and PPAA,
PAA was the dominant degradation product of PPAA
(78.1% based on PPAA; Table 2, entry 1), with some
amounts of benzaldehyde (11.3%) and benzyl alcohol
(3.4%) generated from the homolytic OÀO bond cleavage,
thereby demonstrating that the CoÀOOC(O)R species gen-
2
tion products with cyclohexene oxide (43.6%) as a major
product (Table 1, entry 5). This indicated that the radical-
type oxidation reactions were, to a significant extent, in-
[14]
volved in the olefin epoxidation reactions. With cyclohex-
ene under anaerobic conditions, almost identical yields of
the products were obtained, thus indicating that oxygen
molecules were not involved in the olefin epoxidation reac-
tions. cis-2-Hexene and cis-2-octene were epoxidized to cis-
erated from the reaction of 1 and PPAA underwent parti-
tioning of 84% heterolysis and 16% homolysis (Table 2,
entry 1). To our knowledge, this is the first strong example
V
of the formation of a Co =O species that can be generated
2
-hexene oxide (40.1%) and cis-2-octene oxide (46.3%)
in nonheme cobalt complexes when using a peracid as the
oxidant.
On the other hand, as we and others have proposed,
[6,18]
along with some amounts of trans-2-hexene oxide (11.1%)
and trans-2-octene oxide (9.5%), which indicated moderate
stereochemical retention (78 and 83%, respectively;
namely that the MÀacylperoxo intermediate (M=Fe or Mn)
[15]
Table 1, entries 6 and 8).
trans-2-Hexene and trans-2-
might directly oxidize an olefin to an epoxide (when the
substrate is active or the concentration of the substrate is
high, the species MÀOOC(O)R (2’) might be gradually in-
octene were oxidized exclusively to trans-2-hexene oxide
and trans-2-octene oxide in low yields (36.5 and 39.7%, re-
spectively; Table 1, entries 7 and 9). In the competitive ep-
oxidation of cis- and trans-2-octene, the ratio of cis- to trans-
[7b,c,19]
volved in the epoxidation reaction),
we examined the
possibility of the direct olefin epoxidation by CoÀOOR spe-
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Chem. Eur. J. 2012, 18, 6094 – 6101

Amide-Based Nonheme Cobalt ACHUTNGRENNUG( III) Olefin Epoxidation Catalyst
FULL PAPER
[a]
Table 2. Yield of products derived from PPAA mediated by cobalt catalyst 1 in the presence of various substrates.
[
b]
[b]
[c]
Substrate
Heterolysis
5
Homolysis
6
Hetero/homo
[5/(6+7+8)]
Oxidation products
7
8
ACHTUNGTRENNUNG
1
2
3
cyclohexene
cis-2-octene
1-octene
78.1Æ2.0
75.3Æ1.3
68.8Æ3.1
11.3Æ0.4
12.8Æ0.3
14.7Æ0.1
3.4Æ0.2
3.0Æ0.2
2.4Æ0.1
–
–
–
84:16
83:17
80:20
oxide (39.7Æ1.7)
ol (8.6Æ1.0)
one (7.3Æ1.2)
cis-oxide (34.5Æ1.4)
oxide (12.3Æ0.2)
trans-oxide (7.0Æ0.2)
À3
[
a] Reaction conditions: substrate (0.5 mmol), catalyst (1.0ꢁ10 mmol), PPAA (0.02 mmol), solvent (1 mL, CH
–8 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. [c] Based on PPAA; ol, one, and oxide indicate cyclohexenol, cy-
clohexenone, and the corresponding epoxide to each olefin, respectively.
3 2 2
CN/CH Cl =1:1). [b] Based on PPAA;
5
Table 3. Yield of products derived from PPAA mediated by the catalyst 1 in the pres-
ence of 1-octene.
cies. We used two further substrates (an “easy one”,
cis-2-octene, and a “difficult one”, 1-octene), and
cyclohexene, to investigate the cleavage mode of
PPAA with catalyst 1. If the CoÀOOR species was
[a]
[b]
[b]
1
-Octene Heterolysis
Homolysis
Hetero/homo Oxidation
[c]
products
[mm]
5
6
7
8
[5/
A
H
U
G
R
N
U
G
1-octene oxide
involved in the epoxidation reaction, then the ratio
of heterolysis to homolysis would vary according to
1
2
3
4
5
0
50
100
200
500
59.1Æ4.2
58.9Æ7.8
71.6Æ5.1
71.7Æ11.6
68.8Æ11.3
12.1Æ0.4 3.1Æ0.8
15.4Æ0.2 2.7Æ0.0
16.7Æ0.3 2.7Æ0.1
15.6Æ1.1 2.6Æ0.2
14.7Æ0.4 2.4Æ0.1
–
–
–
–
–
80:20 (3.89)
76:24 (3.22)
79:21 (3.69)
80:20 (3.94)
80:20 (4.02)
0
8.6Æ0.2
9.6Æ0.2
10.3Æ0.4
12.3Æ0.2
[17a]
the type of substrate employed.
The percentage
of heterolysis to homolysis with catalyst 1 varied
little within possible experimental error—from
À3
[
a] Reaction conditions: substrate (0–0.5 mmol), catalyst (1.0ꢁ10 mmol), PPAA
0.02 mmol), solvent (1 mL, CH CN/CH Cl =1:1). [b] Based on PPAA; 5–8 indicate
phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. [c] Based
8
8
3
4:16 for cyclohexene to 83:17 for cis-2-octene to
0:20 for 1-octene—as shown in Table 2 (entries 1–
(
3
2
2
). This selectivity appeared to be nearly independ- on PPAA.
ent of the substrate employed.
The fact that the ratio of heter-
olysis to homolysis varied little
with the substrate implies that
there might be almost no inter-
action between the OÀO bond
[
a]
Table 4. Yield of products derived from PPAA mediated by the catalyst 1 in the presence of cyclohexene.
[
b]
[b]
[c]
Cyclohexene Heterolysis
Homolysis
Hetero/homo Oxidation products
[
mm]
5
6
7
8
[5/
A
H
U
G
R
N
N
(6+7+8)]
oxide
ol
one
0
1
2
3
4
5
0
50
59.1Æ4.2
61.0Æ8.2
65.1Æ5.6
71.8Æ5.4
78.1Æ5.0
12.1Æ0.4 3.1Æ0.8
18.1Æ0.3 2.5Æ0.3
16.3Æ2.1 3.4Æ0.6
14.8Æ0.1 3.3Æ0.3
11.3Æ0.6 3.4Æ0.2
–
–
–
–
–
80:20 (3.89)
75:25 (2.96)
77:23 (3.30)
80:20 (3.97)
84:16 (5.30)
À3
0
0
13.1Æ0.0 5.2Æ0.5 2.5Æ0.5
17.6Æ0.6 4.8Æ1.7 2.9Æ0.2
25.1Æ3.0 6.5Æ0.5 2.4Æ2.4
39.7Æ0.9 8.6Æ0.5 7.3Æ0.1
cleavage and substrate oxida-
tion.
100
200
500
To further examine the inter-
play between the OÀO bond [a] Reaction conditions: substrate (0.5 mmol), catalyst (1.0ꢁ10 mmol), PPAA (0.02 mmol), solvent (1 mL,
3 2 2
CH CN/CH Cl =1:1). [b] Based on PPAA; 5–8 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and
toluene, respectively. [c] Based on PPAA; ol, one, and oxide indicate cyclohexenol, cyclohexenone, and the
corresponding epoxide to each olefin, respectively.
cleavage and substrate oxida-
tion, we investigated the con-
centration effect of substrate, as
previously shown by us and
[
7,17a]
other groups.
If the CoÀ
OOR species was involved in the epoxidation reaction, then
the ratio of heterolysis to homolysis would vary according to
Similar results were also observed for cis-2-octene (Table S2
in the Supporting Information). These results suggest that
CoÀOOC(O)R (2) might be a possible reactive species for
[17a]
the concentration of substrate employed.
First of all, we
used two substrates that are difficult to oxidize, 1-octene
and trans-2-octene. We increased the concentration of sub-
strate 1-octene from 0 to 500 mm in the presence of catalyst
epoxidation under extreme conditions, in which the concen-
tration of an active substrate is very high.
Additionally, we studied the solvent effects because it has
1
. The ratio of heterolysis to homolysis varied little within
possible experimental error—from 80:20 (3.89) for 0 mm to
6:24 (3.22) for 50 mm to 79:21 (3.69) for 100 mm to 80:20
3.94) for 200 mm to 80:20 (4.02) for 500 mm, as shown in
been demonstrated previously that protic solvents facilitate
[21]
OÀO bond heterolysis to produce metal(V)–oxo species.
7
(
With complex 1, the reaction conditions with 0.050 mmol cy-
clohexene were chosen to examine the product distribution
of the mechanistic probe PPAA. As shown in Table 5 (en-
Table 3 (entries 1–5). Similar product ratios were also ob-
served for trans-2-octene (Table S1 in the Supporting Infor-
mation). Next, we changed the substrates to more easily oxi-
dized ones (cyclohexene and cis-2-octene). With cyclohex-
ene, the ratios of heterolysis to homolysis also varied only
a little except for at 500 mm and were, within experimental
error, from 80:20 (3.89) for 0 mm to 75:25 (2.96) for 50 mm
to 77:23 (3.30) for 100 mm to 80:20 (3.97) for 200 mm to
tries 1–6), when the amount of the protic solvent CH OH
3
was gradually increased from 0 to 20%, the heterolysis was
dramatically increased from 58 to 95% to produce a possible
[20]
V
reactive Co =O species. These results suggest that partition-
ing between the heterolytic and homolytic OÀO bond cleav-
age of PPAA is very sensitive to the nature of the solvent,
and that the cleavage of the OÀO bond of PPAA proceeds
8
4:16 (5.30) for 500 mm, as shown in Table 4 (entries 1–5).
predominantly by heterolytic cleavage, even in the presence
Chem. Eur. J. 2012, 18, 6094 – 6101
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6097

C. Kim, S. J. Lee et al.
Table 5. Yield of products derived from PPAA mediated by the catalyst 1 in the presence of cyclohexene in
protic solvent.
Next, we carried out a UV/
Vis study at low temperature
[
a]
[
b]
[b]
Solvent
Heterolysis
Homolysis
Hetero/homo Oxidation
(
À708C) to observe the pro-
[
b,c]
products
oxide
posed reactive intermediates:
(
MeCN/MeOH)
5
6
7
8
[5/
A
H
U
G
R
N
U
G
ol
one
III
V
Co ÀOOC(O)R, Co =O, and
1
2
3
4
5
6
10.0:0
9.8:0.2
9.5:0.5
9.3:0.7
9.0:1.0
8.0:2.0
40.8Æ2.6
58.3Æ2.6
70.6Æ2.1
73.8Æ4.0
75.5Æ2.2
75.9Æ0.5
26.0Æ1.4 3.2Æ0.2
16.9Æ0.6 2.8Æ0.1
6.0Æ0.1 3.2Æ1.0
3.8Æ0.1 2.0Æ0.1
2.8Æ0.3 1.8Æ0.1
2.7Æ0.3 1.6Æ0.0
–
–
–
–
–
–
58:42 (1.40)
75:25 (2.96)
88:12 (7.64)
93:7 (12.7)
94:6 (16.4)
95:5 (17.8)
À3
8.8Æ0.0 2.9Æ0.1 2.3Æ0.1
7.3Æ0.0 3.1Æ0.5 2.3Æ0.0
5.1Æ0.2 2.3Æ0.0 1.9Æ0.0
IV
Co =O. As soon as 1.2 equiva-
lents of MCPBA were added to
6.8Æ0.1 2.0Æ0.0 1.5Æ0.0 a solution of 1 either in the ab-
8.2Æ0.7 1.5Æ0.0 1.2Æ0.0 sence of or in the presence of
7.5Æ0.2 1.1Æ0.1 1.1Æ0.5
a substrate, cyclohexene, UV/
[
a] Reaction conditions: cyclohexene (0.05 mmol), catalyst (1.0ꢁ10 mmol), PPAA (0.02 mmol), solvent Vis spectra were recorded
(
1 mL). [b] Based on PPAA. [c] Oxide, ol, and one indicate cyclohexene oxide, cyclohexenol, and cyclohexe-
within 3 s and then every 30 s
over a period of 20 min (see
none, respectively.
Figure S1 in the Supporting In-
of small amounts of protic solvent, to produce a discrete
Co =O intermediate as the dominant reactive species.
formation). There was no spec-
V
tral change at À708C, which suggests that the reactive inter-
mediates might be spectroscopically not observable or pres-
ent in concentrations too low to be detected.
Based on our results, the most plausible mechanism for
the formation of the reactive species responsible for olefin
epoxidation could be as shown in Scheme 2. Peracid reacts
with a cobalt complex to form an initial cobalt–acylperoxo
In further efforts to detect the possible reactive species,
we used the electron paramagnetic resonance (EPR) spec-
troscopy method at low temperature (À408C). As expected,
the EPR spectrum of frozen solutions of complex 1 was
silent. However, an intermediate state frozen as soon as
complex 1 was mixed (for 5 s) with MCPBA at À408C in
a mixture of MeCN and CH Cl (1:1) showed clear EPR sig-
III
intermediate (Co ÀOOC(O)R (2)), which then undergoes
either a heterolytic (pathway a) or homolytic (pathway b)
V
IV
OÀO bond cleavage to afford Co =O (3) or Co =O (4) spe-
cies in aprotic solvent (CH Cl /CH CN=1:1). The resulting
2
2
3
V
Co =O intermediate formed from pathway a might be re-
sponsible for the olefin epoxidation that shows no formation
of allylic oxidation products and a high stereochemical re-
2
2
nals. There was a broad peak at a value of 5.28 g and a peak
with eight lines at a value of 2.003 g (Figure 2), which might
IV
be because of the oxidation of a bpc ligand, [Co CHTUTGNERNNUN(G bpc )]
IIIA
ox 2+
tention, whereas a Co =O complex generated with path-
way b might be ascribed to a radical-type oxidation that
species. A similar spectrum was reported by Wieghardt
[11a,b,22]
IIIA
ox 2+
shows the formation of allylic oxidation products and a loss
et al.
The spin state of the [Co
C
T
N
T
E
N
N
(bpc )]
species
III
3
of stereospecificity. In protic solvents, Co ÀOOC(O)R (2)
could be either a high-spin state (S= = ) or a low-spin state
2
1
6
undergoes predominantly the heterolytic cleavage (path-
(S= = ) because cobalt
A
H
U
G
R
N
U
G
2
V
way a) to produce a discrete Co =O intermediate as the
but a g value of 5.28 clearly indicates that the spin state of
this species should be a high-spin state. A possible explana-
dominant reactive species. Additionally, under extreme con-
ditions, in which the concentration of an active substrate is
very high, 2 might be a possible reactive species for epoxida-
tion (Scheme 2, pathway c). Therefore, under extreme con-
IV
tion for these results is that (bpc)Co =O species generated
from the homolysis of the (bpc)CoÀOOC(O)R intermediate
III
might be in equilibrium with a (bpc)Co -coordinated ligand
III
III
ox 2+
ditions, even in our amide-based nonheme Co -catalyzed
radical species, [Co AHCTUNGTERNNUN(G bpc )] . Results obtained by using
V
IV
epoxidation reaction, the multiple oxidants Co =O, Co =O,
the mechanistic probe with PPAA, as described earlier, re-
III
and Co ÀOOC(O)R might act simultaneously as key active
vealed that complex 1 showed partitioning between heterol-
V
intermediates, as recently shown with the amide-based non-
ysis (80%) and homolysis (20%) to afford EPR-silent Co =
III
[7b,c]
III
ox 2+
heme Mn complexes.
O species and EPR-active [Co AHCTNUGTERNNUN(G bpc )] species, respec-
tively. Therefore, we presume
III
that the amounts of [Co -
ox 2+
A
H
U
G
R
N
U
G
species generated
from the reaction of 1 and
MCPBA might be much less
than 20%, although the exact
III
ox 2+
amount of [Co ACHUNTGRNENUG( bpc )] spe-
cies was not determined, be-
cause we did not observe any
prominent UV/Vis change in
the region 500–800 nm at
À708C, which is the characteris-
tic absorption region of the
cation radical of the bpb ligand
Scheme 2. Plausible mechanism for the formation of the reactive species from the reaction of peracids with the
cobalt complex.
[11a,b,22]
series.
6098
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 6094 – 6101

Amide-Based Nonheme Cobalt ACHUTNGRENNUG( III) Olefin Epoxidation Catalyst
FULL PAPER
Experimental Section
General: Olefins, epoxides, cyclohexenol, cyclohexenone, methylene
chloride, acetonitrile, diethyl ether, triethylamine, tetraethylammonium
1
8
18
2 2 2
chloride hydrate, CoCl ·6H O, MCPBA (65%), and H O (95% O en-
richment) were purchased from Aldrich Chemical Co. and were used
without further purification. PPAA was synthesized according to the liter-
[
7a]
ature method. Product analyses for olefin epoxidation, partition reac-
1
8
tion of PPAA, and O incorporation reactions of cyclohexene oxide
were performed on either a Hewlett–Packard 5890 II Plus gas chromato-
graph interfaced with a Hewlett–Packard Model 5989B mass spectrome-
ter or a Donam Systems 6200 gas chromatograph equipped with a flame
ionization detector and a 30 m capillary column (Hewlett–Packard, DB-5
1
or HP-FFAP). H NMR spectra were recorded on a Bruker 250 instru-
3
ment in CDCl with TMS as the internal standard. Elemental analyses
for C, H, and N were performed on a Perkin–Elmer 240C instrument. IR
spectra were measured on a BIO RAD FTS 135 spectrometer as KBr
pellets. Low-temperature UV/Vis spectra were recorded on a Hewlett–
Packard 8453 spectrophotometer equipped with an Optostat variable-
temperature liquid-nitrogen cryostat (Oxford Instruments). EPR spectra
were recorded on a Jeol JES-TE300 ESR spectrometer with 100 kHz
field modulation.
III
ox 2+
Figure 2. X-band EPR spectrum of [Co ACTHUNGTRENNNUG
III
À
the reaction of [Co
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpc)Cl
2
]
3
2
2
at À408C. Inset: EPR spectrum in the range of 3200–3700 G. The spec-
trum with eight lines in the inset clearly indicates the coupling of the
ox
7
2
bpc radical species with the cobalt center (I= = ). Conditions: micro-
wave frequency=9.6465 GHz; microwave power=1 mW; modulation fre-
quency=100 kHz; modulation amplitude=10 G.
Synthesis of cobalt complex (1): Ligand H
2
bpc was synthesized as previ-
N] (1) was prepared by the reaction
O (0.12 g, 0.5 mmol) with H bpc (0.19 g, 0.5 mmol) in the
[7a,23]
ously reported.
of CoCl ·6H
2 4
[Co AHCTUNRTGENNUN(G bpc)Cl ] ACHTUNTGRENNNU[G Et
2
2
2
presence of triethylamine (140 mL, 1 mmol) and tetraethylammonium
chloride hydrate (0.17 g, 1 mmol) in MeCN (20 mL) at room temperatur-
[
11c,d]
Conclusion
e.
tained by layering diethyl ether on a solution of 1 in acetonitrile for
1
For 1, dark brown crystals suitable for X-ray analysis were ob-
1
week at room temperature (yield: 0.21 g, 65.0%); H NMR
[D ]DMSO, 400 MHz): d=9.76 (d, 2H; pyridyl-H ), 8.93 (s, 2H; benzyl-
3,5), 8.24 (t, 2H; pyridyl-H ), 8.03 (d, 2H; pyridyl-H ), 7.90 ppm (t, 2H;
); IR (KBr): n˜ =1653 cm (C=O); elemental analysis calcd
(%) for C26 CoN [Co(bpc)Cl [Et N] (2; 645.28): C 48.39, H
We have synthesized and characterized the mononuclear
nonheme Co complex 1 supported by a chelate ligand
having two deprotonated amide moieties. This complex cata-
(
6
6
III
H
5
3
À1
pyridyl-H
4
lyzes a wide range of olefin epoxidations by MCPBA,
H
30Cl
4
5
O
2
A
T
N
T
E
N
G
2
]
A
H
N
R
N
U
G
4
V
IV
III
4.69, N 10.85; found: C 48.45, H 4.50, N 11.03.
through multiple active oxidants Co =O, Co =O, and Co À
X-ray analysis: The diffraction data for compound 1 were collected on
a Bruker SMART AXS diffractometer equipped with a monochromator
in the MoKa (l=0.71073 ꢂ) incident beam. The crystal was mounted on
a glass fiber. The CCD data were integrated and scaled by using the
Bruker–SAINT software package, and the structure was solved and re-
fined with SHEXTL V6.12. Hydrogen atoms were located in the calcu-
lated positions. The crystallographic data and selected bond lengths for
compound 1 are listed in Tables S3 and S4 in the Supporting Information,
respectively. CCDC-794826 (1) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
OOC(O)R.
It is proposed that 1 shows partitioning between hetero-
lytic and homolytic cleavage of the OÀO bond to afford
V
IV
III
both less accessible Co =O and Co =O (or a Co -coordi-
nated ligand radical) intermediates responsible for the ste-
reospecific olefin epoxidation and radical-type oxidations,
respectively. Moreover, under extreme conditions, in which
the concentration of an active substrate is very high, the
CoÀOOC(O)R (2) is a possible reactive species for epoxida-
[
24]
tion. Furthermore, partitioning between the heterolytic and
homolytic OÀO bond cleavage of peracids is very sensitive
to the polarity of the solvent, such that cleavage of the OÀO
Catalytic olefin epoxidations by cobalt catalyst with MCPBA: MCPBA
0.02 mmol) was added to a mixture of substrate (0.5 mmol), cobalt cata-
lyst (0.001 mmol), and solvent (CH CN/CH Cl =1:1, 1 mL). The mixture
was stirred for 10 min at room temperature, even though the reaction
was complete within a few seconds even at À408C. The reaction was
monitored by GC/mass analysis of 20 mL 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
MCPBA. In the competitive reactions of cis- and trans-2-hexene, and cis-
and trans-2-octene, the concentration of the substrate was 0.25 mmol.
(
3
2
2
bond of peracids proceeds predominantly by heterolytic
cleavage in the presence of small amounts of protic solvent
V
to produce a discrete Co =O intermediate as the dominant
reactive species. This proposal was explained after a reactivi-
ty study in which PPAA was used as a mechanistic probe.
These results and others recently reported by our group
reveal the important role that supporting chelate ligands
having dianionic charge, through deprotonation of the two
amide moieties, play in influencing the formation of high-
valent metal–oxo species.
[
7]
Analysis of the OÀO bond cleavage products by cobalt catalyst with
PPAA: PPAA (0.02 mmol) was added to a mixture of substrate (0–
.5 mmol), cobalt catalyst (0.001 mmol), and solvent (CH CN/CH Cl =
3 2 2
1:1, 1 mL). The mixture was stirred for 10 min at room temperature. The
reaction was monitored by GC/mass analysis of 20 mL aliquots withdrawn
periodically from the reaction mixture. All reactions were run at least
three times and the average product yields are presented. Product yields
0
were based on PPAA. In a mixture of protic solvents (CH
the same reactions were performed by gradually varying the amount of
CH OH from 0 to 20%.
3 3
CN/CH OH),
3
Chem. Eur. J. 2012, 18, 6094 – 6101
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6099

C. Kim, S. J. Lee et al.
UV/Vis spectroscopic studies: Solutions of 1 (0.03 mm) in CH
2
Cl
1:1, 3 mL) were first cooled to À708C in a 1 cm UV cell. MCPBA
1.2 equiv, diluted in 30 mL CH Cl ) was then injected into the UV cell in
2
/CH
3
CN
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2 2
O , tBuOOH, and PhIO (iodosylben-
2
O
2
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Chem. Eur. J. 2012, 18, 6094 – 6101

Amide-Based Nonheme Cobalt ACHUTNGRENNUG( III) Olefin Epoxidation Catalyst
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[
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chloro(tetraphenylporphyrinato)cobalt
acids, initial transfer of an oxygen atom to cobalt
balt(V) species which, by rapid internal electron transfer, results in
has
been
suggested
that,
(III) with peroxycarboxylic
(III) yields a co-
in
the
reaction
of
A
H
U
G
R
N
U
G
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A
H
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Published online: March 29, 2012
Chem. Eur. J. 2012, 18, 6094 – 6101
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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