Synthesis, Characterization, and Efficient Catalytic Activities of a Nickel(II) Porphyrin: Remarkable Solvent and Substrate Effects on Participation of Multiple Active Oxidants

Article abstract of DOI:10.1002/chem.201702750

A new nickel(II) porphyrin complex, [Ni^II(porp)] (1), has been synthesized and characterized by ¹H NMR, ¹³C NMR and mass spectrometry analysis. This Ni^II porphyrin complex 1 quantitatively catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Reactivity and Hammett studies, H₂¹⁸O-exchange experiments, and the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe suggested that participation of multiple active oxidants Ni^II?OOC(O)R 2, Ni^IV-Oxo 3, and Ni^III-Oxo 4 within olefin epoxidation reactions by the nickel porphyrin complex is markedly affected by solvent polarity, concentration, and type of substrate. In aprotic solvent systems, such as toluene, CH₂Cl₂, and CH₃CN, multiple oxidants, Ni^II?(O)R 2, Ni^IV-Oxo 3, and Ni^III-Oxo 4, operate simultaneously as the key active intermediates responsible for epoxidation reactions of easy-to-oxidize substrate cyclohexene, whereas Ni^IV-Oxo 3 and Ni^III-Oxo 4 species become the common reactive oxidant for the difficult-to-oxidize substrate 1-octene. In a protic solvent system, a mixture of CH₃CN and H₂O (95:5), the Ni^II?OOC(O)R 2 undergoes heterolytic or homolytic O?O bond cleavage to afford Ni^IV-Oxo 3 and Ni^III-Oxo 4 species by general acid catalysis prior to direct interaction between 2 and olefin, regardless of the type of substrate. In this case, only Ni^IV-Oxo 3 and Ni^III-Oxo 4 species were the common reactive oxidant responsible for olefin epoxidation reactions.

Full text of DOI:10.1002/chem.201702750

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Accepted Article
Title: Synthesis, characterization, and efficient catalytic activities of a
nickel(II) porphyrin: remarkable solvent and substrate effects on
participation of multiple active oxidants
Authors: Cheal Kim, Hye Ahn, Jeong Bae, Min Kim, Kwon Bok, Ha
Jeong, and Suk Lee
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Synthesis, characterization, and efficient catalytic activities of a
nickel(II) porphyrin: remarkable solvent and substrate effects on
participation of multiple active oxidants
Hye Mi Ahn,[a] Jeong Mi Bae,[a] Min Jeong Kim,[b] Kwon Hee Bok,[a] Ha Young Jeong,[a] Suk Joong
Lee,*[b] Cheal Kim*[a]
Abstract: A new nickel(II) porphyrin complex, [NiII(porp)] (1), has
been synthesized and characterized by 1H NMR, 13C NMR and mass
spectrometry analysis. This NiII porphyrin complex 1 catalyzed
quantitatively the epoxidation reaction of a wide range of olefins with
meta-chloroperoxybenzoic acid (MCPBA) under mild conditions.
Reactivity and Hammett studies, H218O-exchange experiments, and
the use of PPAA (peroxyphenylacetic acid) as a mechanistic probe
suggested that participation of multiple active oxidants NiII-OOC(O)R
2, NiIV-Oxo 3, and NiIII-Oxo 4 in olefin epoxidation reactions by nickel
porphyrin complex is markedly affected by solvent polarity and
concentration and type of substrate. In aprotic solvent systems, such
as toluene, CH2Cl2, and CH3CN, multiple oxidants, NiII-OOC(O)R 2,
NiIV-Oxo 3, and NiIII-Oxo 4, operate simultaneously as the key active
intermediates responsible for epoxidation reactions of easy-to-oxidize
substrate cyclohexene, whereas NiIV-Oxo 3 and NiIII-Oxo 4 species
become the common reactive oxidant for difficult-to-oxidize substrate
perform efficient and selective hydrocarbon oxidations and for this
reason, a large number of corresponding high-valent iron- and
manganese-oxo intermediates have been investigated as
biomimetic chemical models of cytochrome P450[2–4] and as
catalysts in alkane hydroxylation, alkene epoxidation, and
sulfoxidation reactions in various oxygenation reactions.[5-7]
In recent years, nickel has been found in the active sites of
enzymes involved in oxidation processes. They include quercetin
dioxygenases,[8]
acireductone
dioxygenases,
CO
dehydrogenase,[9] acetyl-coenzyme A(CoA) synthase,[10] and
nickel superoxide dismutase.[11] Moreover, high-valent nickel-oxo
species have been postulated to be key reaction intermediates in
the catalytic cycle of several oxidation reactions.[12,13] However, a
very few such species have been isolated and well-characterized
to-date. For example, McDonald, Company, and Costas groups
reported the characterization and reactivity of NiIII-oxyl (or NiIV-
1-octene. In protic solvent system, a mixture of CH3CN and H2O (95/5), Oxo), ClONiIII-ligand radical, and NiIII-OR species, which could
the NiII-OOC(O)R 2 undergoes heterolytic or homolytic O-O bond
cleavage to afford NiIV-Oxo 3 and NiIII-Oxo 4 species by general-acid
catalysis prior to direct interaction between 2 and olefin regardless of
the type of substrate. In such case, only NiIV-Oxo 3 and NiIII-Oxo 4
species were the common reactive oxidant responsible for olefin
epoxidation reactions.
perform alkane hydroxylation, olefin epoxidation, and oxygen-
atom transfer to triphenyl phosphine.[14] Furthermore, Kakazawa
et al. obtained the crystal structure of a nickel(II)-acylperoxo
complex and showed its reactivities toward alkene, alkane, and
thioanisol.[15] Our and other groups also proposed that NiIII-Oxo
and NiIV-Oxo species are involved in alkene or alkane oxidation
reactions by studying reactivity via spectroscopic methods and
using mechanistic probe.[16] However, it is still not completely
clear which reactive species are responsible for oxygen atom
transfer in the catalytic oxygenation reactions and what factors
influence the nature of the reactive intermediates in synthetic
nickel complex systems.
Quite recently, we presented that multiple active oxidants, viz.
MnV=O, MnIV=O, and MnIII-OO(O)CR, operate simultaneously in
olefin epoxidation via manganese porphyrin and Mn(nonheme)
complexes, depending upon reaction conditions such as type of
reaction solution and concentration of the substrate.[17] In addition,
it has been proposed that the nature of solvent might significantly
affect partitioning between heterolysis and homolysis of the O-O
bond of a Mn-acylperoxo intermediate (Mn-OOC(O)R), and that
O-O bond cleavage of the Mn-OOC(O)R complex might proceed
predominantly by heterolytic cleavage in protic solvents.
Introduction
Selective oxidation of organic substrates using metal-based
catalysts is an exciting and challenging scientific goal and has
received great attention in the field of bio-inorganic chemistry
because it is selective, inexpensive, and closely related to the
biomimetic oxidation reactions of metalloenzymes.[1] Iron and
manganese are the major metals found in oxygenases that
[a]
[b]
H. M. Ahn, J. M. Bae, K. H. Bok, H. Y. Jeong, Prof. C. Kim
Department of Fine Chemistry, Seoul National University of Science
and Technology
Seoul 139-743, Korea
E-mail: chealkim@seoultech.ac.kr
M. J. Kim, Prof. S. J. Lee
In the current study, we are concerned with the use of
nickel(II) porphyrin complex for the catalytic epoxidation of olefins
for three principal reasons: 1) catalytic olefin epoxidation with Ni
porphyrin complex has not been reported to date. 2) Ni porphyrin
complex might have a good solubility in various solvents, because
most metal porphyrin complexes are usually soluble in various
solvents such as non-polar, polar, and protic solvents. Therefore,
Department of Chemistry, Korea University
Seoul 136-701, Korea
E-mail: slee1@korea.ac.kr
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/chem.201702750
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it is possible to study solvent effects on the participation of
multiple active oxidants generated from the reaction of a Ni(II)
porphyrin complex with oxidant according to solvent variation. 3)
Ni porphyrin complex with electron-donating groups might be
susceptible to Ni+4/Ni3+/Ni2+ redox reactions rendering the
oxidation reaction thermodynamically favorable, as Sadler et al.
showed with nickel-peptide complexes.[18]
Table 1. Epoxidation of cyclohexene by Ni(II) porphyrin complex 1 with
MCPBA in various solvent systems at room temperature.[a]
Conversion
[%][b]
Yield
[%][b]
Entry
1
Solvent
toluene
Product
cyclohexene oxide
2-cyclohexenol
78.0±2.7
100.0
70.0±4.8
-
Herein, we report a new nickel(II) porphyrin complex,
[NiII(porp)]
complex
1
(porp=tetrakis(2,6-di(n-
2-cyclohexenone
cyclohexene oxide
2-cyclohexenol
-
butoxy)phenyl)porphyrinato), which catalyzed the epoxidation
reaction of a wide range of olefins quantitatively in presence of
meta-chloroperoxybenzoic acid (MCPBA) under mild conditions.
Olefin epoxidation using this catalytic system is proposed to
involve multiple active oxidants, viz. NiII-OOC(O)R, NiIV-Oxo, and
NiIII-Oxo species, based on reactivity and Hammett studies,
H218O-exchange experiments, and the use of PPAA
2
3
4
CH2Cl2
CH3CN
98.7±1.3
-
2-cyclohexenone
cyclohexene oxide
2-cyclohexenol
trace
66.4±11.7
58.5±2.1
34.2±2.9
(peroxyphenylacetic acid) as
a
mechanistic probe. The
-
participation of multiple active oxidants was found to be
remarkably influenced by solvent polarity, concentration and type
of substrate.
2-cyclohexenone
cyclohexene oxide
2-cyclohexenol
-
CH3CN/H2O
(95/5)
15.5±1.4
-
-
Results and Discussion
2-cyclohexenone
Synthesis and characterization of Ni (II) porphyrin 1
[a] Reaction conditions: cyclohexene (0.035 mmol), catalyst (0.8x10-3
mmol), and MCPBA (0.07 mmol). [b] Based on cyclohexene.
Ni(II) porphyrin complex was prepared by condensation between
2,6-dibutoxybenzaldehyde (L1) and pyrrole followed by
metalation with NiCl2 (Scheme 1).[19] The final compound was
successfully characterized by 1H and 13C NMR and Matrix
Assisted Laser Desorption Ionization-Time of Flight (MALDI-TOF)
mass spectroscopy.
Therefore, we carried out the catalytic epoxidation reactions of
various olefins by catalyst 1 in CH2Cl2. The results of the
epoxidation reactions have been summarized in Table 2. Under
mild reaction conditions, cyclic olefins such as cyclopentene,
cycloheptene, and cyclooctene were oxidized to the
corresponding epoxides in excellent yields (99.2-100%, entries 1-
3), with conversions of 100%. Surprisingly, such quantitative
conversion is very rare in olefin epoxidation reactions catalyzed
by metal porphyrin complexes.[20] For cyclohexene (entry 4), it
afforded cyclohexene oxide (98.7%) as the major product along
with trace amount of 2-cyclohexenone. This suggested that free
radical oxidation process scarcely occurs in the epoxidation of
cyclohexene.[21] On the other hand, terminal olefins such as 1-
hexene and 1-octene, which are well-known as notably
challenging substrates to epoxidize,[22,23] were less efficiently
oxidized to the corresponding 1-hexene oxide and 1-octene oxide,
respectively (17.5% and 25.5%, entries 5 and 6).
Scheme 1. Preparation of 1.
cis-2-Hexene,
utilized
to
probe
the
epoxidation
stereochemistry, was predominantly converted to cis-2-hexene
oxide (96.0%; entry 9) and a small amount of trans-2-hexene
oxide (4.0%), indicating the high stereochemical retention (96%).
On the other hand, trans-2-hexene generated exclusively trans-2-
hexene oxide (85.7%; entry 8). The preference test for a mixture
of cis- and trans-2-hexene showed a ratio of 1.2, indicating a
similar preference to cis- and trans-olefin. The epoxidation
reactions using cis-2-octene and trans-2-octene showed
analogous results to cis- and trans-2-hexene (entries 9 and 10).
For styrene having an aromatic ring, styrene epoxide was
produced in a low yield (17.5%; entry 11), along with some
phenylacetaldehyde (39.2%) and trace amount of benzaldehyde
Epoxidation of various olefins catalyzed by a Ni (II) porphyrin
complex 1
To study the reactivity of Ni (II) porphyrin 1 toward olefins, we
performed catalytic epoxidation of cyclohexene with MCPBA as a
oxidant in various solvent systems, such as toluene, CH2Cl2,
CH3CN and a mixture of CH3CN and H2O (95/5) (Table 1). All
epoxidation reactions were complete within 2 min at room
temperature, and control experiments confirmed that direct
oxidation of the substrate by MCPBA was negligible. The best
result (100% conversion and 99% yield) was obtained in CH2Cl2.
2
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(0.5%). cis-Stilbene afforded both cis-stilbene oxide (16.8%; entry
12) and trans-stilbene oxide (21.0%) along with minor amounts of
benzaldehyde (1.0%) and 2-phenylacetophenone (7.5%),
whereas trans-stilbene formed trans-stilbene oxide (43.1%; entry
13) as the only major product in addition to small amounts of
benzaldehyde (3.2%) and 2-phenylacetophenone (5.6%). For
aromatic olefin epoxidation reaction, the formation of aldehyde
and ketone by-products indicated that either peroxo radical or
nickel(III)-Oxo species as the epoxidizing agent might be
responsible for the epoxidation of aromatic olefins. To the best of
our knowledge, 1 is the first nickel(II) porphyrin catalyst capable
of epoxidizing olefins and the most efficient among metal-
porphyrin complexes that showed quantitative conversions and
high epoxide yields.
[a] Reaction conditions: olefins (0.035 mmol), Ni(II) porphyrin complex
(0.8x10-3 mmol), and MCPBA (0.07 mmol). [b] Based on substrate.
Competition experiments of styrene and para-substituted
styrenes for Hammett study
To understand the nature of reactive intermediates responsible
for the olefin epoxidation, competitive reactions of four para-
substituted styrenes were carried out (Fig. 1). Results showed
that the rates of the epoxidation reactions with electron-rich
olefins were faster than those with electron-deficient ones. The
Hammett analysis gave a good linear relationship with ρ value of
-0.45, which indicated electrophilic character of the reactive
oxidant. This value is somewhat lower than those reported for the
epoxidation of styrene with [Ni(cyclam)](NO3)2 ( = -0.82),[24]
Table 2. Olefin epoxidations by Ni(II) porphyrin complex 1 with MCPBA in
CH2Cl2 at room temperature.[a]
[Ni(bisamidate)] (
=
-0.86),[14a] [Ni(II)(dpaq)Cl] (dpaq=2-
(bis(pyridin-2-ylmethyl)amino)-N-(quinolin-8-yl)acetamide) ( = -
0.72)[16a] and [Ni(TpCF3Me)(κ2-mCPBA)] (TpCF3Me = hydrotris(3-
trifluoromethyl-5-methylpyrazolyl)borate) ( = -1.35).[15a]
Conversion
[%][b]
Yield
[%][b]
Entry Substrate
Product
1
2
3
4
cyclopentene
epoxide
~100
~100
~100
~100
~100
cycloheptene
cyclooctene
cyclohexene
epoxide
~100
epoxide
99.2±1.5
98.7±1.3
-
0.8
epoxide
ρ = -0.45
-OCH3
2-cyclohexenol
2-cyclohexenone
epoxide
0.3
-CH3
trace
-H
-Cl
-CN
5
6
7
1-hexene
23.2±2.2
24.8±1.3
~100
17.5±0.5
25.5±0.8
96.0±1.5
4.0±0.1
85.7±0.5
95.2±0.1
3.9±0.2
93.3±0.9
17.5±0.3
0.5±0.1
39.2±0.3
16.8±0.0
21.0±0.0
1.0±0.4
7.5±0.6
43.1±1.1
3.2±1.2
5.6±0.5
-0.2
1-octene
epoxide
-0.7
-1.2
cis-2-hexene
cis-oxide
trans-oxide
8
9
trans-2-hexene
cis-2-octene
trans-oxide
96.8±0.2
99.6±0.2
-0.8
-0.4
0
σ+
0.4
0.8
cis-oxide
trans-oxide
Figure 1. Hammett plot for selective reactivities of styrene to para-substituted
styrenes by 1 with MCPBA.
10
11
trans-2-octene
trans-oxide
95.9±0.4
67.6±1.2
styrene
epoxide
Product distribution of the O−O bond cleavage of PPAA with
catalyst 1
benzaldehyde
phenylacetaldehyde
cis-stilbene oxide
trans-stilbene oxide
benzaldehyde
2-phenylacetophenone
trans-stilbene oxide
benzaldehyde
2-phenylacetophenone
We used phenylperacetic acid (PPAA)[17,21a,25] as a mechanistic
probe to gain insight into the reactive intermediates involved in
olefin epoxidation, because a quantitative determination of
degradation products derived from PPAA is very informative for
distinguishing heterolytic from homolytic cleavage of the peracid
O-O bond.[17,21a,26] When the O-O bond of the Ni(II)-acylperoxo
species (NiII-OOC(O)R 2), formed from the reaction of Ni(II)
porphyrin with peracid, was cleaved heterolytically, phenylacetic
acid (PAA (5), pathway (a) of Scheme 2) was generated. The
direct epoxidation of olefin by 2 also produced PAA (pathway (b))
and apparently varied the O-O bond cleavage mode. By contrast,
a homolytic cleavage of the O-O bond of 2 afforded benzyl alcohol
12
cis-stilbene
75.3±0.8
13
trans-stilbene
81.5±3.9
3
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(6), benzaldehyde (7) and toluene (8) through a rapid β-scission
of the acyloxy radical (pathway (c)).
[a] See Experimental section for details. [b] Hetero/Homo means the ratio of
heterolysis and homolysis.
First, a degradation experiment of PPAA with Ni(II) porphyrin
complex 1 was carried out in the absence of substrate in nonpolar
solvent toluene (entry 1 in Table 3 and entry 1 in Table S1). The
heterolytic cleavage product PAA was the dominant degradation
product of PPAA (71.6% based on PPAA), along with some
amounts of benzaldehyde (18.2%) and benzyl alcohol (4.2%)
formed via a homolytic O-O bond cleavage. These results
indicated that NiII-OOC(O)R species 2 generated from the
reaction of 1 and PPAA underwent 76.2 % heterolysis and 23.8 %
homolysis to produce high-valent NiIV-Oxo 3 and NiIII-Oxo 4
species. Importantly, these results indicated, for the first time, that
NiIV-Oxo species can be generated from the reaction of a nickel
porphyrin complex and peracids as an oxidant, whereas in only a
few cases in the past, NiIV-Oxo species were characterized or
proposed in non-heme nickel (II) complexes.[12,14a,c,16]
To further investigate the direct relation between the O-O
bond cleavage and olefin epoxidation, concentration effect of an
easy-to-oxidize substrate, cyclohexene, was studied.[17,21a,26a-c] If
NiII-OOR species 2 was involved in the epoxidation reaction, the
ratio of heterolysis to homolysis would increase with increase in
the concentration of cyclohexene.[26] This is because PAA as
heterolysis product is produced from the direct olefin epoxidation
by NiII-OOR species 2. When the concentration of cyclohexene
was increased from 0 to 160 mM, the ratio of the heterolysis to
homolysis also increased gradually from 3.2 (76.2/23.8) to 8.0
(88.9/11.1) (entries 1-5 in the fourth column of Table 3 and entries
1-5 in Table S1). This increase signified that NiII-OOC(O)R
species
2 contributed to the epoxidation with increasing
concentration of cyclohexene in toluene. To examine the type
effect of substrate on the cleavage mode of peracid, we further
changed the substrate from cyclohexene to 1-octene, more
difficult-to-oxidize substrate. The ratio did not vary within
experimental error (entries 1-5 in the fifth column of Table 3 and
entries 1-5 in Table S2). These results suggested that NiII-OOR
species 2 might not be a competent oxidant capable of oxidizing
the difficult-to-oxidize olefin.
We also studied the solvent effects on participation of multiple
active oxidants, because it has been demonstrated previously in
Mn(porphyrin) complexes that multiple oxidants, MnIII-OOC(O)R,
MnV=O, and MnIV=O, operate simultaneously as the key active
intermediates in aprotic solvents and that MnV=O species
becomes the common reactive intermediate in protic solvents.[17c]
A variety of solvents with different polarities for a detailed study
was used, ranging from toluene (dielectric constant: 2.4) to
CH2Cl2 (dielectric constant: 3.1) to CH3CN (dielectric constant:
5.8) to a mixture of H2O (dielectric constant: 9.0) and CH3CN
(5/95).
When a slightly more polar solvent (CH2Cl2) than toluene was
used, the partitioning of NiII-OOC(O)R species 2 in the absence
of cyclohexene was 81.5 vs. 18.5 (heterolysis vs. homolysis)
(entry 6 in the fourth column of Table 3 and entry 1 in Table S3).
An increase in concentration of cyclohexene showed a similar
pattern for the ratio of heterolysis to homolysis as shown in
toluene (entries 6-10 in the fourth column of Table 3 and entries
1-5 in Table S3). When the substrate was changed from
cyclohexene to 1-octene, the ratio did not vary within an
experimental error as shown in toluene (entries 6-10 in the fifth
column of Table 3 and entries 1-5 in Table S4). In the more polar
solvent CH3CN, the ratio (73.0/27.0) of heterolysis to homolyis in
the absence of cyclohexene was somewhat lower than those in
toluene and CH2Cl2 (entry 11 in the fourth column of Table 3 and
entry 1 in Table S5).[17c] Similarly, increasing the concentrations
of cyclohexene increased the ratios of heterolysis to homolysis
(entries 11-15 in the fourth column of Table 3 and entries 1-5 in
Table S5). With 1-octene, the ratio did not vary within the
experimental error (entries 11-15 in the fifth column of Table 3 and
entries 1-5 in Table S6). These results suggested that NiII-OOR
species 2 might not be a competent oxidant capable of oxidizing
Table 3. Olefin epoxidation by Ni(II) porphyrin complex 1 with PPAA as oxidant
in various solvent systems at room temperature.[a]
Solvent
(dielectric
constant)
Concentration Hetero/Homo
of olefin
Hetero/Homo
in 1-octene [b]
Entry
in cyclohexene [b]
1
2
toluene
(2.4)
0 mM
20 mM
40 mM
80 mM
160 mM
0 mM
3.2 (76.2/23.8)
3.9 (79.6/20.4)
4.6 (82.1/17.9)
5.4 (84.4/15.6)
8.0 (88.9/11.1)
4.4 (81.5/18.5)
4.7 (82.5/17.5)
7.7 (88.5/11.5)
13.1 (92.9/7.1)
22.7 (95.8/4.2)
2.7 (73.0/27.0)
2.8 (73.7/26.3)
3.4 (77.3/22.7)
3.6 (78.3/21.7)
4.1 (80.4/19.6)
3.8 (79.2/20.8)
4.4 (81.5/18.5)
4.2 (80.8/19.2)
5.2 (83.9/16.1)
4.7 (82.5/17.5)
3.2 (76.2/23.8)
3.8 (79.2/20.8)
3.9 (79.6/20.4)
3.8 (79.2/20.8)
3.9 (79.6/20.4)
4.4 (81.5/18.5)
3.5 (77.8/22.2)
3.7 (78.7/21.3)
3.8 (79.2/20.8)
4.0 (80.0/20.0)
2.7 (73.0/27.0)
2.6 (72.2/27.7)
2.5 (71.4/28.6)
2.0 (66.7/73.3)
2.4 (70.6/29.4)
3.8 (79.2/20.8)
3.5 (77.8/22.2)
4.1 (80.4/19/6)
3.8 (79.2/20.8)
3.9 (79.6/20.4)
3
4
5
6
CH2Cl2
(3.1)
7
20 mM
40 mM
80 mM
160 mM
0 mM
8
9
10
11
12
13
14
15
16
17
18
19
20
CH3CN
(5.8)
20 mM
40 mM
80 mM
160 mM
0 mM
CH3CN/H2O
(95/5)
(CH3CN : 5.8,
H2O : 9.0)
20 mM
40 mM
80 mM
160 mM
4
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the difficult-to-oxidize olefin in CH2Cl2 and CH3CN as shown in
toluene.
3 and NiIII-Oxo 4 species become the common reactive oxidant to
the difficult-to-oxidize olefin due to incompetent oxidizing power
of 2 (pathways A and C). In protic solvent system, the NiII-
OOC(O)R undergoes heterolytic and homolytic O-O bond
cleavage simultaneously to afford NiIV-Oxo 3 and NiIII-Oxo 4
species by general-acid catalysis prior to direct interaction
between 2 and olefin regardless of the type of substrate
(pathways D and E). In such case, only NiIV-Oxo 3 and NiIII-Oxo 4
species are the common reactive oxidant responsible for olefin
epoxidation reactions.
In order to observe the proposed reactive intermediates Ni-Oxo,
isotope labeling experiments were carried out to understand the
source of oxygen atoms found in epoxide products.[28] Epoxidation
of cyclohexene (10 equiv) by nickel complex 1 (0.8 mmol) and
MCPBA (20 equiv) was conducted in presence of excess H218O
(1111 equiv) in a mixture of H2O/CH3CN (5/95). The cyclohexene
oxide product contained a small amount (ca. 3%) of 18O derived
from H218O, demonstrating that high-valent Ni-Oxo species are
involved in the reactions of the catalysts with MCPBA, but oxygen
exchange between Ni-Oxo species and H218O might be much
slower than oxygen-atom transfer of Ni-Oxo species to the
substrate.
In the most polar and protic solvent system, e.g. a mixture of
H2O/CH3CN (5/95), the partitioning of NiII-OOC(O)R 2 in absence
of cyclohexene gave 79.2% heterolysis and 20.8% homolysis
(entry 16 in the fourth column of Table 3 and entry 1 in Table S7).
An increase in the concentration of cyclohexene showed,
surprisingly, no increase in the ratio of heterolysis to homolysis
unlike in aprotic solvents, toluene, CH2Cl2, and CH3CN (entries
16-20 in the fourth column of Table 3 and entries 1-5 in Table S7).
This result indicated that a protic solvent might induce the NiII-
OOC(O)R intermediate 2 to undergo O-O bond cleavage by
general-acid catalysis prior to direct interaction between 2 and
olefin.[27] With 1-octene, a nearly identical O-O bond cleavage
pattern was also observed as shown with cyclohexene (entries
16-20 in the fifth column of Table 3 and entries 1-5 in Table S8).
These results demonstrated again that the NiII-OOC(O)R
intermediate undergoes O-O bond cleavage by general-acid
catalysis prior to direct interaction between 2 and olefin regardless
of the type of substrate in the protic solvent system, and that NiIV-
Oxo 3 and NiIII-Oxo 4 species produced from the O-O bond
cleavage of NiII-OOC(O)R intermediate might be the common
reactive intermediate.
Scheme 3. Plausible mechanism for the formation of the reactive oxidants
responsible for the olefin epoxidation from the reaction of nickel porphyrin
complex 1 with peracid.
Scheme 2. Possible reactive intermediates formed from the reaction of PPAA
with nickel(II) complex.
Conclusions
We have synthesized and characterized the nickel porphyrin
complex 1, which efficiently and quantitatively catalyzed a wide
range of olefin epoxidation under mild conditions. To the best of
our knowledge, 1 is the first nickel(II) porphyrin catalyst capable
of epoxidizing olefins and the most efficient among all the metal-
porphyrin complexes reported to-date that showed quantitative
conversions and epoxide yields. Therefore, the present results
provide important information for designing of new oxidation
catalysts consisting of nickel porphyrin. On the other hand,
reactivity and Hammett studies, H218O exchange experiments,
and the use of PPAA as a mechanistic probe suggested that
participation of multiple active oxidants NiIV-Oxo 3, NiIII-Oxo 4, and
NiII-OOC(O)R 2 in olefin epoxidation reactions by nickel porphyrin
complex was markedly affected by several factors, such as
solvent polarity and concentration and type of substrate. In aprotic
solvent systems, such as toluene, CH2Cl2, and CH3CN, multiple
Mechanism
Based on our mechanistic studies, Scheme 3 shows the most
plausible mechanism for the formation of the reactive species
responsible for olefin epoxidation reaction catalyzed by Ni(II)
porphyrin complex. An initial nickel-acylperoxo intermediate (NiII-
OOC(O)R; 2), derived from reaction of a Ni(II) porphyrin complex
and peracid, undergoes either a heterolytic or homolytic O-O bond
cleavage to afford NiIV-Oxo 3 or NiIII-Oxo 4 species, or transfers
its oxygen atom directly to the substrate under the following two
conditions. In aprotic solvent systems such as toluene, CH2Cl2,
and CH3CN, the multiple oxidants, NiII-OOC(O)R 2, NiIV-Oxo 3,
and NiIII-Oxo 4, operate simultaneously as the key active
intermediates responsible for the epoxidation reactions of the
easy-to-oxidize olefin (pathways A, B, and C), whereas NiIV-Oxo
5
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10.1002/chem.201702750
Chemistry - A European Journal
FULL PAPER
oxidants, NiII-OOC(O)R, NiIII-Oxo, and NiIV-Oxo, operate
simultaneously as the key active intermediates responsible for the
epoxidation reactions of the easy-to-oxidize olefin, whereas NiIV-
Oxo and NiIII-Oxo species become the common reactive oxidant
to the difficult-to-oxidize olefin due to incompetent oxidizing power
of 2. In protic solvent system, the NiII-OOC(O)R undergoes
heterolytic and homolytic O-O bond cleavage simultaneously to
afford NiIV-Oxo and NiIII-Oxo species by general-acid catalysis
prior to direct interaction between 2 and olefin regardless of the
type of substrate. In such case, only NiIV-Oxo and NiIII-Oxo
species are the common reactive oxidant responsible for olefin
epoxidation reactions.
5,10,15,20-Tetrakis[2,6-di(n-butoxy)phenyl]porphyrin
(L2):
2,6-
Dibutoxybenzaldehyde (749 mg, 2.93 mmol) and pyrrole (428 mg, 2.93
mmol) were dissolved in dichloromethane (516 mL) in a 1-L Schlenk flask
equipped with a magnetic stirrer and degassed for 10 min. TFA (142.2 µL)
was added drop-wise, the flask was shield from light, and the resulting
mixture was stirred for 3 h at room temperature. After DDQ (800 mg) was
added, the solution was stirred for an additional 1 h. TEA (3.04 mL) was
then added and the reaction mixture was evaporated to dryness using a
rotary evaporator to yield a dark residue, which was purified by silica-gel
column chromatography (CH2Cl2) to afford pure product as a purple solid
(284 mg, 28.35 % yield). 1H NMR (CDCl3): δ 8.66 (s, 8H), 7.64 (t, 3JH-H
=
8.2 Hz, 4H), 6.95 (d, 3JH-H = 8.2 Hz, 8H), 3.77 (t, 3JH-H = 6.6 Hz, 16H), 1.00-
0.91 (m, 16H), 0.64-0.53 (m, 16H), 0.33-0.28 (m, 24H). 13C NMR (100 MHz,
CDCl3): δ (ppm) 144.4, 132.6, 129.2, 121.7, 102.3, 97.6, 91.3, 68.4, 30.8,
18.6, 13.4. MS (MALDI-TOF): m/z= 1191.79 for M+; Calcd. 1191.58.
More detailed mechanistic studies to elucidate the factors
such as general-acid and -base catalysis that influence the
partitioning of heterolysis versus homolysis of the NiII-OOC(O)R
intermediate are currently underway in this laboratory.
[5,10,15,20-Tetrakis(2,6-di(n-butoxy)phenyl)porphyrinato]nickel(II)
(1): To a solution of L2 (100 mg, 83.8 mmol) in DMF (25.0 mL) was added
NiCl2 (419 mmol) and the resulting solution was refluxed for 24 h. After
cooling, the reaction mixture was diluted with CHCl3 and washed with water.
The organic layer was collected and dried over anhydrous Na2SO4, and
the volatile was removed under reduced pressure. The resulting residue
was purified by silica-gel column chromatography (DCM) to afford pure 1
Experimental Section
General: Olefins, epoxides, 2-cyclohexen-1-ol, 2-cyclohexen-1-one,
benzaldehyde, acetonitrile, dichloromethane, dodecane and MCPBA
(65%) were purchased from Aldrich Chemical Co. and were used without
further purification. PPAA[17,21a,26a,b] and 2,6-dibutoxybenzaldehyde[29]
were synthesized according to the literature method. All other chemicals
were obtained from commercial sources and used without further
purification. All the reactions and manipulations of the porphyrin building
blocks were carried out under N2 with the use of standard inert-atmosphere
and Schlenk techniques unless otherwise noted. Solvents used in inert-
atmosphere reactions were dried and degassed using standard
procedures. Flash column chromatography was performed with 230-400
mesh silica gel using wet-packing method.
1
as a reddish purple solid (80 mg, 76.5 % yield). H NMR (CDCl3): δ 8.57
3
3
(s, 8H), 7.58 (t, JH-H = 8.2 Hz, 4H), 6.90 (d, JH-H = 8.2 Hz, 8H), 3.75 (t,
3JH-H = 6.6 Hz, 16H), 1.05-0.95 (m, 16H), 0.56-0.48 (m, 16H), 0.24-0.19 (m,
24H). 13C NMR (100 MHz, CDCl3): δ (ppm) 145.7, 131.8, 128.6, 121.7,
101.7, 96.7, 91.3, 68.4, 30.5, 18.3, 13.2. MS (MALDI-TOF): m/z= 1249.40
for M+; Calcd. 1248.26
Catalytic olefin epoxidations by MCPBA in the presence of Ni
porphyrin complex: MCPBA (0.07 mmol) was added to a mixture of
substrate (0.035 mmol), Ni porphyrin complex (0.8x10-3 mmol), and
solvent (CH2Cl2; 1 mL). The mixture was stirred for 10 min at room
temperature. Each reaction was monitored by GC/mass analysis of 20 μL
aliquots withdrawn periodically from the reaction mixture. Dodecane as an
internal standard was used to quantify the yields of products and
conversions of substrates. All reactions were run at least in triplicate, and
the average conversions and product yields are presented. Conversions
and product yields were based on substrate.
Instruments: 1H NMR spectra were recorded on a Varian AS400 (399.937
MHz for 1H and 100.573 MHz for 13C) spectrometer. 1H chemical shifts are
referenced to the proton resonance resulting from protic residue in
deuterated solvent and 13C chemical shift recorded downfield in ppm
relative to the carbon resonance of the deuterated solvents. Matrix
Assisted Laser Desorption Ionization–Time of Flight (MALDI-TOF) mass
spectra were obtained on a Bruker Daltonics LPF20 MALDI TOF Mass
Spectrometer Industry-Academic Cooperation Foundation, Yonsei
University. Product analysis for olefin epoxidation and PPAA experiment
were conducted by using a YL6500 gas chromatograph equipped with a
FID detector and a 30 m capillary column (Hewlett-Packard, DB-5 or HP-
FFAP). IR spectra were measured on a BIO RAD FTS 135 spectrometer
as KBr pellets. UV/Vis spectra were carried out with a Perkin Elmer model
Lambda 2S UV/Vis spectrometer.
Competitive reactions of styrene and para-substituted styrenes for
Hammett plot in the presence of Ni porphyrin complex: MCPBA (0.03
mmol) was added to a mixture of styrene (0.02 mmol) and para(X)-
substituted styrene (0.02 mmol, X = -OCH3, -CH3, -Cl, and -CN), Ni
porphyrin complex (0.8x10-3 mmol), and solvent (CH2Cl2; 1 mL). The
mixture was stirred for 10 min at room temperature. The amounts of
styrenes before and after reactions were measured 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 concentration of
para-substituted styrenes and Yi and Yf are the initial and final
concentration of styrene.[26b,28c]
2,6-Dibutoxybenzaldehyde (L1): To a solution TMEDA (7.1 mL, 71.71
mmol) and 1,3-dibutoxybenzene (13.37 g, 60.16 mmol) in diethyl ether was
added n-BuLi (2.5M in hexane, 26.14 mL, 65.33 mmol) at 0 oC over a 30
min period. The reaction mixture was stirred for 3 h under N2. After
warming up to room temperature, DMF (8.88 mL) was added drop wise
and the reaction was stirred for an additional 2 h. The mixture was
quenched with water and the product was extracted with ethyl acetate 4
times. The solution was evaporated to dryness using a rotary evaporator
to yield a yellow residue, which was recrystallized with hexane to afford
Analysis of the O−O bond cleavage products from the oxidation
reactions of substrates by PPAA in the presence of Ni porphyrin
complex: PPAA (0.04 mmol) was added to a mixture of substrate (0-0.16
mmol), Ni porphyrin complex (0.8x10-3 mmol), and various solvents
(toluene, CH2Cl2, distilled CH3CN, distilled CH3CN/H2O(95/5, 1 mL),
respectively. The mixture was stirred for 10 min at room temperature. Each
reaction was monitored by GC/mass analysis of 20 μL aliquots withdrawn
periodically from the reaction mixture. Dodecane as an internal standard
was used to quantify the yields of products and conversions of substrates.
All reactions were run at least in triplicate, and the average conversions
1
the product as pure white solid (8.6 g, 73.6 % yield). H NMR (CDCl3): δ
10.46 (s, 1H), 7.41 (t, 3JH-H = 8.2 Hz, 1H), 6.64(d, 3JH-H = 8.2 Hz, 2H), 4.08
(q, 3JH-H = 7.1 Hz, 4H), 1.78 (m, 4H), 1.41 (m, 4H), 1.01 (t, 3JH-H = 7.1 Hz,
6H).
6
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10.1002/chem.201702750
Chemistry - A European Journal
FULL PAPER
and product yields are presented. Conversions and product yields were
based on substrate or PPAA.
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Supplementary data
Supplementary data related to this article can be found at http: //
Keywords: Ni(II) porphyrin complex • olefin epoxidation • multiple
active oxidants • Ni-Oxo species • Ni(II)-OOC(O)R species
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10.1002/chem.201702750
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Hye Mi Ahn, Jeong Mi Bae, Min Jeong
Kim, Kwon Hee Bok, Ha Young Jeong,
Suk Joong Lee*, Cheal Kim*
Page No. – Page No.
Synthesis, characterization, and
efficient catalytic activities of a
nickel(II) porphyrin: remarkable
solvent and substrate effects on
participation of multiple active
oxidants
Multiple active oxidants: In the oxidation reactions catalyzed by a new nickel(II)
porphyrin complex, participation of the active oxidants is markedly influenced by
solvent polarity, and concentration and type of substrate. Three active oxidants,
NiIV-Oxo, NiIII-Oxo, and NiII-OOC(O)R, operate simultaneously in aprotic solvents,
whereas two active oxidants, NiIV-Oxo and NiIII-Oxo, operate in protic solvents.
9
This article is protected by copyright. All rights reserved.