Evidence for the participation of two distinct reactive intermediates in iron(III) porphyrin complex-catalyzed-epoxidation reactions

Article abstract of DOI:10.1021/ja000289k

We have studied the competitive epoxidations of olefins with cis- and trans-stilbenes and with cyclooctene and trans-stilbene in iron porphyrin complex-catalyzed epoxidation reactions by H₂O₂, tert-butyl hydroperoxide (t-BuOOH), and

Full text of DOI:10.1021/ja000289k

J. Am. Chem. Soc. 2000, 122, 6641-6647
6641
Evidence for the Participation of Two Distinct Reactive Intermediates
in Iron(III) Porphyrin Complex-Catalyzed Epoxidation Reactions
Wonwoo Nam,*,† Mi Hee Lim,† Ha Jin Lee,† and Cheal Kim‡
Contribution from the Department of Chemistry and DiVision of Molecular Life Sciences,
Ewha Womans UniVersity, Seoul 120-750, Korea, and Department of Fine Chemistry,
Seoul National Polytechnic UniVersity, Seoul 139-743, Korea
ReceiVed January 26, 2000
Abstract: We have studied the competitive epoxidations of olefins with cis- and trans-stilbenes and with
cyclooctene and trans-stilbene in iron porphyrin complex-catalyzed epoxidation reactions by H2O2, tert-butyl
hydroperoxide (t-BuOOH), and m-chloroperoxybenzoic acid (m-CPBA) in protic solvent (i.e., a solvent mixture
of CH3OH and CH2Cl2) and aprotic solvent (i.e., a solvent mixture of CH3CN and CH2Cl2) at room temperature
under catalytic reaction conditions. The competitive epoxidations were also carried out with in situ generated
high-valent iron(IV) oxo porphyrin cation radical complexes in aprotic solvent under stoichiometric reaction
conditions. By determining the ratios of epoxide products formed in the competitive epoxidations, we were
able to conclude unambiguously that the reactive species generated in protic solvent are high-valent iron(IV)
oxo porphyrin cation radical complexes 3 and the intermediates formed in aprotic solvent are oxidant-iron
porphyrin intermediates 2. A protic solvent such as methanol is proposed to function as a general-acid catalyst,
thereby increasing the rate of O-O bond cleavage of 2 to form 3. In the absence of general-acid catalysis such
as in aprotic solvent, the rate of O-O bond cleavage of 2 is relatively slow and 2 transfers its oxygen to
olefins prior to the formation of 3. To further examine the effect of the general-acid catalysis on the nature of
epoxidizing intermediates, we carried out competitive epoxidations in the solvent mixtures of alcohol/CH2Cl2
using alcohols of varying pKa values and in the presence of an acid (i.e., HClO4) in aprotic solvent. The
product ratios were found to vary depending on the strength of the solvent acidity, demonstrating that the
reaction of 2 with olefin competes with the O-O bond cleavage of 2 that leads to the formation of 3. We also
reported for the first time that a high-valent iron(IV) oxo porphyrin cation radical intermediate containing
electron-deficient porphyrin ligand shows an unexpected preference for trans-stilbene over cis-stilbene in the
competitive epoxidations of cis- and trans-stilbenes.
Introduction
phyrin models, it has been generally believed that high-valent
iron(IV) oxo porphyrin cation radical intermediates 3 are the
only reactive species responsible for the oxygenation of
hydrocarbons (Scheme 1).1 A number of high-valent iron oxo
porphyrin complexes have been prepared at low temperature,
characterized with a variety of spectroscopic methods, and
directly used in the reactivity studies of oxygen atom transfer
reactions such as olefin epoxidation, alkane hydroxylation, and
N-demethylation.3 Also, there is direct and indirect evidence
that strongly supports the involvement of 3 as the reactive
species in the catalytic oxygenation of hydrocarbons by heme-
containing enzymes and iron porphyrin complexes.4,5
Elucidation of the structures of reactive intermediates re-
sponsible for oxygen atom transfer in catalytic oxygenation
reactions by heme and nonheme iron monooxygenase enzymes
and their model compounds has been the major goal of
biological and bioinorganic chemistry for the past three de-
cades.1,2 In heme-containing enzymes and their iron(III) por-
† Ewha Womans University.
‡ Seoul National Polytechnic University.
(1) Reviews for heme-containing iron enzymes and iron porphyrin
models: (a) McLain, J.; Lee, J.; Groves, J. T. In Biomimetic Oxidations
Catalyzed by Transition Metal Complexes; Meunier, B., Ed.; Imperial
College Press: London, 2000; pp 91-169. (b) Shimada, H.; Sligar, S. G.;
Yeom, H.; Ishimura, Y. In Oxygenases and Model Systems; Funabiki, T.,
Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997; pp
195-221. (c) Watanabe, Y. In Oxygenases and Model Systems; Funabiki,
T., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997;
pp 223-282. (d) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H.
Chem. ReV. 1996, 96, 2841-2887. (e) Ortiz de Montellano, P. R.
Cytochrome P450: Structure, Mechanism, and Biochemistry, 2nd ed.;
Plenum Press: New York, 1995. (f) Traylor, T. G.; Traylor, P. S. In ActiVe
Oxygen in Biochemistry; Valentine, J. S., Foote, C. S., Greenberg, A.,
Liebman, J. F., Eds.; Blackie Academic and Professional, Chapman and
Hall: London, 1995; pp 84-187. (g) Erman, J. E.; Hager, L. P.; Sligar, S.
G. AdV. Inorg. Biochem. 1994, 10, 71-118. (h) Montanari, F.; Casella, L.
Metalloporphyrins Catalyzed Oxidations; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 1994. (i) Meunier, B. Chem. ReV. 1992, 92,
1411-1456.
(2) Selected examples for nonheme iron enzymes and non-porphyrin iron
complexes: (a) Choi, S.-Y.; Eaton, P. E.; Kopp, D. A.; Lippard, S. J.;
Newcomb, M.; Shen, R. J. Am. Chem. Soc. 1999, 121, 12198-12199. (b)
Chen, K.; Que, L., Jr. Angew. Chem., Int. Ed. Engl. 1999, 38, 2227-2229.
(c) Valentine, A. M.; Stahl, S. S.; Lippard, S. J. J. Am. Chem. Soc. 1999,
121, 3876-3887. (d) Chen, K.; Que, L., Jr. Chem. Commun. 1999, 1375-
1376. (e) Lange, S. J.; Miyake, H.; Que, L., Jr. J. Am. Chem. Soc. 1999,
121, 6330-6331. (f) Shteinman, A. A. J. Biol. Inorg. Chem. 1998, 3, 325-
330. (g) Lipscomb, J. D.; Que, L., Jr. J. Biol. Inorg. Chem. 1998, 3, 331-
336. (h) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607-2624. (i)
Sam, J. W.; Tang, X.-J.; Peisach, J. J. Am. Chem. Soc. 1994, 116, 5250-
5256. (j) Valentine, J. S.; Nam, W.; Ho, R. Y. N. In The ActiVation of
Dioxygen and Homogeneous Catalytic Oxidation; Barton, D. H. R., Martell,
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7054.
10.1021/ja000289k CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/30/2000

6642 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Nam et al.
Scheme 1. Reactive Intermediates Capable of Oxygenating
Organic Substrates in Heme-Containing Enzymes and Iron
Porphyrin Models
enzymes and iron porphyrin models is an iron(III) peroxo
porphyrin complex 1 (Scheme 1).9-12 Valentine and co-workers
reported elegant results recently that in situ generated iron(III)
peroxo porphyrin complexes are powerful nucleophiles capable
of epoxidizing electron-deficient olefins.9 In cytochrome P-450
2B4 and aromatase enzymes, there is growing evidence that 1
is the reactive species responsible for the aldehyde deformylation
and aromatization reactions.11,12
Although it has been shown in a number of reports that
intermediates such as 2 and 3 are involved in iron(III) porphyrin
complex-catalyzed oxygenations of hydrocarbons, it is not
completely clear which reactive species (e.g., 2, 3, or both 2
and 3 at the same time) are responsible for oxygen atom transfer
in the catalytic oxygenation reactions performed at room
temperature and what factors are important to determine the
nature of the reactive intermediates in iron porphyrin model
systems. In this paper, we report the results of the competitive
epoxidations studied with cis- and trans-stilbenes and with
cyclooctene and trans-stilbene in iron(III) porphyrin-catalyzed
epoxidation of olefins by oxidants such as H2O2, tert-butyl
hydroperoxide (t-BuOOH), and m-chloroperoxybenzoic acid
(m-CPBA) in protic and aprotic solvents at room temperature.
We found from the studies that both the intermediates (i.e., 2
and 3) indeed function as reactive species in the epoxidation of
olefins by iron porphyrins at room temperature and that the
participation of 2 and 3 as reactive epoxidizing intermediates
is found to be controlled by the factors such as the solvent
system (i.e., protic and aprotic solvents), the presence of a proton
source in aprotic solvent, and the acidity of alcohol solvents.
Furthermore, we report for the first time that a high-valent
iron(IV) oxo porphyrin cation radical intermediate containing
nonbulky ortho-fluoro substituents at the phenyl groups of
electron-deficient porphyrin ligand shows an unexpected prefer-
ence for trans-stilbene over cis-stilbene in the competitive
epoxidations of cis- and trans-stilbenes.
In addition to the intermediacy of 3, recent studies provided
strong evidence that oxidant-iron(III) porphyrin intermediates
2 are capable of transferring their oxygen to hydrocarbons prior
to the formation of 3 (Scheme 1).6-8 Notably, Vaz et al. reported
elegant results that iron(III)-hydroperoxide (FeIII-OOH) and
iron(III)-hydrogen peroxide (FeIII-H2O2) intermediates function
as electrophilic oxidants in olefin epoxidation and alkane
hydroxylation reactions by cytochrome P-450 enzymes and their
mutants.6 In the mutants lacking threonine in the active site of
cytochromes P-450, the lifetime of iron(III)-hydroperoxide
species is increased, and this intermediate is capable of
oxygenating hydrocarbons prior to the formation of 3. The
involvement of 2 as reactive intermediates has also been
suggested in iron porphyrin complex-catalyzed oxygenations of
hydrocarbons by oxidants such as hydrogen peroxide and
peracids,4c,7,8 especially in the epoxidation of olefins by peracids
at low temperature.8 When the rate of O-O bond cleavage of
2 becomes slow such as in the cases where the reactions are
performed with electron-deficient iron porphyrin complexes,7a,8a
in nonpolar solvents (e.g., toluene),8a or at low temperature,4c
2 is able to transfer its oxygen to easily oxygenated organic
substrates such as olefins prior to the O-O bond cleavage of
2. Another reactive intermediate that has been proposed to effect
the oxidations of organic substrates in cytochrome P-450
(3) (a) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.;
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1998, 2167-2173. (c) Lee, K. A.; Nam, W. J. Am. Chem. Soc. 1997, 119,
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(5) (a) Matsui, T.; Ozaki, S.; Watanabe, Y. J. Am. Chem. Soc. 1999,
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Watanabe, Y. J. Biol. Chem. 1999, 274, 2838-2844. (c) Matsui, T.; Ozaki,
S.; Watanabe, Y. J. Biol. Chem. 1997, 272, 32735-32738.
(6) (a) Newcomb, M.; Shen, R.; Choi, S.-Y.; Toy, P. H.; Hollenberg, P.
F.; Vaz, A. D. N.; Coon, M. J. J. Am. Chem. Soc. 2000, 122, 2677-2686.
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Results and Discussion
We have studied competitive epoxidations with iron(III)
porphyrin complexes such as (meso-tetrakis(pentafluorophenyl)-
porphinato)iron(III) chloride [Fe(TPFPP)Cl], (meso-tetrakis(2,6-
difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and
(meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride
[Fe(TDCPP)Cl] (see Figure 1 for the structures of iron(III)
porphyrin complexes) and with oxidants such as m-CPBA, H2O2,
and t-BuOOH in protic solvent (i.e., a solvent mixture of CH3-
OH and CH2Cl2) and aprotic solvent (i.e., a solvent mixture of
CH3CN and CH2Cl2) at room temperature under catalytic
reaction conditions. Two sets of competitive epoxidations were
carried out with cis- and trans-stilbenes (eq 1) and with
cyclooctene and trans-stilbene (eq 2), and the yields of the
epoxide products and the product ratios are listed in Tables 1
and 2. We also carried out control reactions with cis-stilbene
(9) (a) Wertz, D. L.; Sisemore, M. F.; Selke, M.; Driscoll, J.; Valentine,
J. S. J. Am. Chem. Soc. 1998, 120, 5331-5332. (b) Selke, M.; Valentine,
J. S. J. Am. Chem. Soc. 1998, 120, 2652-2653. (c) Sisemore, M. F.; Selke,
M.; Burstyn, J. N.; Valentine, J. S. Inorg. Chem. 1997, 36, 979-984. (d)
Selke, M.; Sisemore, M. F.; Valentine, J. S. J. Am. Chem. Soc. 1996, 118,
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Chem., Int. Ed. Engl. 1996, 35, 206-208.
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Fe(III) Porphyrin Complex-Catalyzed Epoxidation Reactions
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6643
temperature. As the results of the competitive epoxidations of
cis- and trans-stilbenes are shown in Table 1 (see the column
of protic solvent, catalytic reaction), the ratios of cis- to trans-
stilbene oxides formed in the reactions of m-CPBA, H2O2, and
t-BuOOH were identical in each iron porphyrin complex within
experimental error (entries 1-3 for Fe(TPFPP)Cl, entries 4-6
for Fe(TDFPP)Cl, and entries 7-9 for Fe(TDCPP)Cl). Also,
the ratios of cyclooctene oxide to trans-stilbene oxide formed
in the competitive epoxidations of cyclooctene and trans-stilbene
by m-CPBA, H2O2, and t-BuOOH were identical in each iron
porphyrin complex (Table 2; see the column of protic solvent,
catalytic reaction). These results clearly demonstrate that the
reactions of the iron porphyrin complexes with the oxidants
generate 3 as reactive epoxidizing intermediates in a protic
solvent system (eq 3).4c,f If the reactive interemdiates generated
in the reactions of m-CPBA, H2O2, and t-BuOOH were different,
then the ratios of the oxide products formed in the competitive
epoxidations would be different (vide infra).
Figure 1. Structures of iron(III) porphyrin complexes used in this
study.
and trans-stilbene individually, to ensure that no isomerized
products were formed in the epoxidations of cis- and trans-
stilbenes under the reaction conditions employed, since the
Interestingly, the ratios of cis- to trans-stilbene oxides formed
in the competitive epoxidations of cis- and trans-stilbenes were
found to depend significantly on the nature of the iron porphyrin
complexes (Table 1, ratios of ∼0.5, ∼2, and >15 in the reactions
of Fe(TPFPP)Cl, Fe(TDFPP)Cl, and Fe(TDCPP)Cl, respec-
tively), and these results are interpreted in terms of the steric
and electronic effects of the iron porphyrins on the cis-trans
selectivity. The high ratio of cis- to trans-stilbene oxides
observed in the Fe(TDCPP)Cl reaction is ascribed to the steric
effect of the bulky ortho-chloro substituents at the phenyl groups
of the porphyrin ligand, since the approach of trans-stilbene to
the iron oxo moiety is difficult due to the steric hindrance
between the phenyl groups of trans-stilbene and the phenyl
groups of the porphyrin ligand.15 The relatively low ratios
of cis- to trans-stilbene oxides observed in the reactions of
Fe(TPFPP)Cl and Fe(TDFPP)Cl may be due to the fact that
the size of the ortho-fluoro substituents at the phenyl groups of
the porphyrin ligands is smaller than that of the ortho-chloro
substituents at the Fe(TDCPP)Cl complex. In addition, the cis-
to trans-epoxide ratios obtained in the reactions of Fe(TPFPP)-
Cl (i.e., ratio of ∼0.5) and Fe(TDFPP)Cl (i.e., ratio of ∼2) were
different as well. Since these two iron porphyrins contain the
same ortho-fluoro substituents at the phenyl groups of the
porphyrin ligands, we suggest that the different cis- to trans-
epoxide ratios are caused by the different electronic properties
of the iron porphyrin complexes. The results also indicate that
the intermediate of the more electron-deficient iron porphyrin
complex (i.e., (TPFPP)+•FeIVdO) shows an unexpected prefer-
ence for trans-stilbene over cis-stilbene (i.e., cis- to trans-
epoxide ratio of 0.5), and, to the best of our knowledge, this is
the first time to observe the preference of trans-stilbene over
cis-stilbene in iron porphyrin-catalyzed competitive epoxidations
of cis- and trans-stilbenes (vide infra).15
formation of isomerized products (e.g., the formation of trans-
stilbene oxide in the epoxidation of cis-stilbene) would result
in giving false product ratios in the competitive epoxidation
reactions. The control reactions showed at most only trace
amounts of isomerized trans-stilbene oxide formation in the
cis-stilbene epoxidations and no formation of cis-stilbene oxide
in the trans-stilbene epoxidations (Supporting Information,
Table S1). In addition, an intriguing observation that we
made in the control reactions was that the yields of trans-stilbene
oxide formed in the epoxidations of trans-stilbene by Fe-
(TPFPP)Cl and Fe(TDFPP)Cl in protic solVent were unusually
high and comparable to the yields of cis-stilbene oxide formed
in the epoxidation of cis-stilbene by the iron porphyrin
complexes (Table S1; compare the yields of cis-oxide in
cis-stilbene epoxidation with the yields of trans-oxide in
trans-stilbene epoxidation in the first column).13 In the absence
of the iron porphyrin catalysts, no epoxide formation was
observed in the reactions of H2O2 and t-BuOOH. The epoxi-
dations of olefins by m-CPBA carried out in the presence of
iron porphyrin complexes are iron porphyrin complex-catalyzed
reactions.14
Competitive Epoxidations in Protic Solvent. The competi-
tive epoxidations of cis- and trans-stilbenes and of cyclooctene
and trans-stilbene were first carried out in a protic solvent
system (i.e., a solvent mixture of CH3OH and CH2Cl2) at room
Competitive Epoxidations in Aprotic Solvent. When the
identical competitive epoxidations were carried out in an aprotic
solvent system (i.e., a solvent mixture of CH3CN and CH2Cl2),
(13) It has been shown previously that only small amounts of trans-
stilbene oxide are yielded in the epoxidation of trans-stilbene by iron(III)
porphyrin complexes and iodosylbenzene: Groves, J. T.; Nemo, T. E.;
Myers, R. S. J. Am. Chem. Soc. 1979, 101, 1032-1033.
(14) (a) Traylor, T. G.; Kim, C.; Richards, J. L.; Xu, F.; Perrin, C. L. J.
Am. Chem. Soc. 1995, 117, 3468-3474. (b) Traylor, T. G.; Lee, W. A.;
Stynes, D. V. J. Am. Chem. Soc. 1984, 106, 755-764.
(15) (a) Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 5786-
5791. (b) Traylor, T. G.; Xu, F. J. Am. Chem. Soc. 1988, 110, 1953-1958.
(c) Ostovic, D.; Bruice, T. C. J. Am. Chem. Soc. 1988, 110, 6906-6908.
(d) Traylor, T. G.; Miksztal, A. R. J. Am. Chem. Soc. 1989, 111, 7443-
7448. (e) He, G.-X.; Mei, H.-Y.; Bruice, T. C. J. Am. Chem. Soc. 1991,
113, 5644-5650. (f) Nappa, M. J.; Tolman, C. A. Inorg. Chem. 1985, 24,
4711-4719.

6644 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Nam et al.
Table 1. Competitive Epoxidations of cis- and trans-Stilbenes by Iron(III) Porphyrin Complexes with Various Oxidants in Protic and Aprotic
Solvents under Catalytic Reaction Conditions and by in Situ Generated (Porp)+•FeIVdO Complexes in Aprotic Solvent under Stoichiometric
Reaction Conditionsa
protic solvent, catalytic reaction
aprotic solvent, catalytic reaction
(Porp)+•FeIVdO in aprotic solvent
product yields (%)b
product yields (%)b
product yields (%)b
ratio of cis-
ratio of cis-
ratio of cis-
iron
porphyrin
cis-
trans-
to trans-
oxide
cis-
trans-
to trans-
oxide
cis-
trans-
to trans-
oxide
entry
oxidant
oxidec
oxidec
oxidec
oxidec
oxidec
oxidec
1
2
3
4
5
6
7
8
9
Fe(TPFPP)Cl m-CPBA
28
18
18
62
46
39
76
44
30
59
45
48
31
20
18
0.5 ( 0.1
0.4 ( 0.1
0.4 ( 0.1
2.0 ( 0.2
2.3 ( 0.4
2.2 ( 0.4
>15
88
11
8 ( 1
0.6 ( 0.2d
nde
12 ( 2
4 ( 1d
nde
18
34
48
41
0.5 ( 0.1
2.4 ( 0.4
>15
H2O2
t-BuOOH
Fe(TDFPP)Cl m-CPBA
H2O2
t-BuOOH
Fe(TDCPP)Cl m-CPBA
H2O2
17d
nde
86
29d
nde
7
14
48d
nde
80
12d
nde
tracef
traced,f
nde
tracef
tracef
tracef
>15
tracef
>15
36d
nde
>15d
nde
t-BuOOH
>15
a
See Experimental Section for detailed experimental procedures. All reactions were run at least in triplicate, and the data reported represent the
average of these reactions. b Based on the amounts of oxidants used. c cis-Oxide and trans-oxide stand for cis-stilbene oxide and trans-stilbene
oxide, respectively. d Reactions were run in the presence of 5-chloro-1-methylimidazole (0.1 mmol), since the epoxidation of olefins by H2O2 in
aprotic solvent did not yield the oxide products in the absence of the imidazole.23 e Not determined due to either low product yields or formation
of isomerized trans-stilbene oxide product in the epoxidation of cis-stilbene. f Yields were less than 3% based on oxidants used.
Table 2. Competitive Epoxidations of Cyclooctene and trans-Stilbene by Iron(III) Porphyrin Complexes with Various Oxidants in Protic and
Aprotic Solvents under Catalytic Reaction Conditions and by in Situ Generated (Porp)+•FeIVdO Complexes in Aprotic Solvent under
Stoichiometric Reaction Conditionsa
protic solvent, catalytic reaction
aprotic solvent, catalytic reaction
(Porp)+•FeIV)O in aprotic solvent
product yields (%)b
product yields (%)b
product yields (%)b
ratio of co-
ratio of co-
ratio of co-
iron
porphyrin
co-
trans-
to trans-
oxide
co-
trans-
to trans-
oxide
co-
trans-
to trans-
oxide
entry
oxidant
oxidec
oxidec
oxidec
oxidec
oxidec
oxidec
1
2
3
4
5
6
7
8
9
Fe(TPFPP)Cl
m-CPBA
H2O2
t-BuOOH
10
8
7
31
21
17
67
36
24
87
71
60
53
36
32
0.1 ( 0.05
0.1 ( 0.05
0.1 ( 0.05
0.58 ( 0.15
0.58 ( 0.20
0.53 ( 0.20
>15
87
8
11 ( 3
0.3 ( 0.1d
nde
23 ( 5
1.6 ( 0.4d
nde
3
22
59
31
0.1 ( 0.05
0.8 ( 0.2
>15
11d
nde
92
36d
nde
Fe(TDFPP)Cl m-CPBA
4
27
H2O2
t-BuOOH
40d
nde
93
25d
nde
Fe(TDCPP)Cl m-CPBA
H2O2
tracef
tracef
tracef
tracef
traced,f
nde
>15
tracef
>15
66d
nde
>15d
t-BuOOH
>15
nde
a
See Experimental Section for detailed experimental procedures. All reactions were run at least in triplicate, and the data reported represent the
average of these reactions. b Based on the amounts of oxidants used. c Co-oxide and trans-oxide stand for cyclooctene oxide and trans-stilbene
oxide, respectively. d Reactions were run in the presence of 5-chloro-1-methylimidazole (0.1 mmol).23 e Not determined due to the low yields of
epoxide products. f Yields were less than 3% based on oxidants used.
the product ratios were found to be changed dramatically,
especially in the reactions of Fe(TPFPP)Cl and Fe(TDFPP)Cl
with m-CPBA (Tables 1 and 2; compare the product ratios in
the columns of protic solvent and aprotic solvent). The ratios
of cis- to trans-stilbene oxides obtained in the competitive
epoxidations of cis- and trans-stilbenes in aprotic solvent were
much higher than those obtained in protic solvent (Table 1, see
the column of aprotic solvent, catalytic reaction). The ratio of
∼0.5 in protic solvent was changed to 8 in aprotic solvent for
the reactions of Fe(TPFPP)Cl with m-CPBA and the ratio of
∼2.0 in protic solvent was changed to 12 in aprotic solvent for
the reactions of Fe(TDFPP)Cl with m-CPBA (Table 1; compare
entries 1 and 4 in the columns of protic solvent and aprotic
solvent). The same trend was observed in the competitive
epoxidations of cyclooctene and trans-stilbene, in which the
ratios of cyclooctene oxide to trans-stilbene oxide were much
higher in aprotic solvent than in protic solvent (Table 2). For
examples, the ratios of 0.1 and ∼0.6 in protic solvent were
changed to 11 and 23 in aprotic solvent for the reactions of
Fe(TPFPP)Cl and Fe(TDFPP)Cl with m-CPBA, respectively
(Table 2; compare entries 1 and 4 in the columns of protic
solvent and aprotic solvent). The finding of the dependence of
the product ratios on the solvent systems (i.e., protic versus
aprotic solvents) leads us to suggest that the reactive intermedi-
ates formed in aprotic solvent are different from the high-valent
iron(IV) oxo porphyrin cation radical intermediates generated
in protic solvent. If the reactive epoxidizing intermediates
generated in the protic and aprotic solvents were the same
species (e.g., 3), we would expect to observe that the product
ratios would be similar in both solvent systems (vide infra).
Another intriguing observation that we made in aprotic
solvent was the dependence of the epoxide product ratios on
the kinds of oxidants used. For example, the ratios of cis- to
trans-stilbene oxides formed in the competitive epoxidations
of cis- and trans-stilbenes by Fe(TPFPP)Cl were 8 and 0.6 for
the reactions of m-CPBA and H2O2, respectively (Table 1,
entries 1 and 2 in the column of aprotic solvent), and the ratios
obtained with Fe(TDFPP)Cl were 12 and 4 for the reactions of
m-CPBA and H2O2, respectively (Table 1, entries 4 and 5 in
the column of aprotic solvent). The dependence of the product
ratios on the structures of the oxidants was also observed in
the competitive epoxidations of cyclooctene and trans-stilbene,
in which the product ratios obtained in the reactions of m-CPBA
and H2O2 were greatly different (Table 2; compare the product
ratios in entries 1 and 2 for the Fe(TPFPP)Cl reactions and
entries 4 and 5 for the Fe(TDFPP)Cl reactions in the column
of aprotic solvent). These results further support that a common
intermediate such as 3 is not responsible for the olefin
epoxidations in aprotic solvent, since the generation of the
common intermediate in the reactions of m-CPBA and H2O2

Fe(III) Porphyrin Complex-Catalyzed Epoxidation Reactions
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6645
should give the same ratios of epoxide products, as we have
observed in the protic solvent system.
Scheme 2. Proposed Mechanism for Olefin Epoxidation in
Protic and Aprotic Solvents
We have suggested above that the different product ratios
observed in protic and aprotic solvents are due to the presence
of two different reactive intermediates in the protic and aprotic
solvents. However, since we could not rule out a possibility
that the different product ratios in the protic and aprotic solvents
are caused by the difference of solvents, we therefore prepared
high-valent iron(IV) oxo porphyrin cation radical complexes4a,16
and directly used the in situ generated iron oxo intermediates
in the competitive epoxidations in aprotic solVent.17 As shown
in Tables 1 and 2 (see the columns of (Porp)+•FeIVdO in
aprotic solvent), the ratios of cis- to trans-stilbene oxides and
cyclooctene oxide to trans-stilbene oxide obtained with
(TPFPP)+•FeIVdO and (TDFPP)+•FeIVdO were different from
those obtained in aprotic solvent but the same as those obtained
in protic solvent. These results lead us to conclude that the
reactive species generated in the protic solvent are 3 as we have
suggested above and that the intermediates generated in the
aprotic solvent are not 3 but, possibly, 2 even at room
temperature (eq 4).8
hindrance between the phenyl groups of trans-stilbene and the
phenyl groups of the porphyrin ligand of high-valent iron oxo
intermediate might not be correct. Instead, we propose that the
selectivity of cis-stilbene over trans-stilbene in the PhIO
reactions may be due to the generation of iodosylbenzene-iron
porphyrin adducts as reactive epoxidizing intermediate in aprotic
solvents such as CH2Cl22i,8a,18,19 and that the selectivity of cis-
stilbene by the iodosylbenzene-iron porphyrin intermediates
is caused by the steric hindrance between the phenyl groups of
trans-stilbene and the bulky PhIO bound to iron.
Effect of General-Acid Catalysis on the Rate of O-O
Bond Cleavage of 2. We have demonstrated above that 3 is
the common reactive intermediate in protic solvent, whereas 2
is a potent reactive species in aprotic solvent. Then, how are
two different reactive species responsible for olefin epoxidations
in protic and aprotic solvents? A plausible explanation is that
protic solvent functions as general-acid catalysis.20 In the
presence of general-acid catalysis, the rate of O-O bond
cleavage of 2 is accelerated, resulted in forming 3 as the reactive
species in a fast rate (Scheme 2, pathway A). In the absence of
general-acid catalysis such as in aprotic solvent, the rate of O-O
bond cleavage of 2 is relatively slow and 2 transfers its oxygen
to olefins prior to the formation of 3 even at room temperature
(Scheme 2, pathway B). To further examine the effect of
general-acid catalysis on the formation of two different reactive
intermediates such as 2 and 3, we first carried out the
competitive epoxidations of cis- and trans-stilbenes and of
cyclooctene and trans-stilbene in the presence of an acid (i.e.,
HClO4) in aprotic solvent and found that the product ratios
changed dramatically and became similar to those obtained with
(Porp)+•FeIVdO complexes (Table 3; see data in 1-A and 2-A).
These results clearly indicate that the presence of proton in
aprotic solvent accelerates the rate of the O-O bond cleavage
of 2 to form 3 as epoxidizing oxidant (Scheme 1, pathway A).21
We also studied the competitive epoxidations in the solvent
mixtures of alcohol/CH2Cl2 using alcohols of varying pKa
values, since it has been shown by Traylor and co-workers that
the rate of hydroperoxide O-O bond cleavage is significantly
affected by the alcohol acidity.20 As shown in Table 3 (see data
in 1-C and 2-C), the ratios of cis- to trans-oxides and
cyclooctene oxide to trans-stilbene oxide gradually increased
as the acidity of the alcohols decreased, demonstrating that the
rate of the O-O bond cleavage of 2 is affected by the strength
On the basis of the results that 2 may be the reactive
epoxidizing intermediate in aprotic solvent, the observation of
the high ratios of cis-olefin oxides to trans-stilbene oxide
observed in the reactions of m-CPBA with Fe(TPFPP)Cl and
Fe(TDFPP)Cl in aprotic solvent (Tables 1 and 2; see entries 1
and 4 in the column of aprotic solvent) is attributed to the steric
effect of the bulky m-CPBA bound to the iron porphyrins, since
the approach of trans-stilbene to 2 is highly restricted by the
steric hindrance between the phenyl groups of trans-stilbene
and the bulky m-CPBA bound to iron.8a Also, the relatively
low ratios of cis-olefin oxides to trans-stilbene oxide observed
in the H2O2 reactions, compared to the product ratios obtained
in the m-CPBA reactions, are due to the fact that the size of the
hydrogen atom in iron(III) hydroperoxide porphyrin intermediate
(e.g., R ) H in (Porp)FeIII-OOR, 2) is much smaller than that
of the acyl group in acylperoxo-iron(III) porphyrin complex
(e.g., R ) C(O)(3-Cl-Ph) in (Porp)FeIII-OOR, 2). In addition,
it is worth noting that both the postulated (Porp)FeIII-OOH
intermediate and the high-valent iron(IV) oxo porphyrin cation
radical intermediate containing nonbulky ortho-fluoro substit-
uents at the phenyl groups of highly electron-deficient porphyrin
ligand (i.e., (TPFPP)FeIII-OOH and (TPFPP)+•FeIVdO) show
a selectivity of trans-stilbene over cis-olefins in the competitive
epoxidations of cis- and trans-olefins. As we have noted in the
previous section, this is the first time to observe that trans-
stilbene is more reactive than cis-stilbene in iron porphyrin-
catalyzed competitive epoxidations of cis- and trans-stilbenes.15
These results further suggest that the previous interpretation that
the preference of cis-stilbene over trans-stilbene in the competi-
tive epoxidations of cis- and trans-stilbenes by the reactions of
iron(III) porphyrin complexes with iodosylbenzene (e.g., the
ratio of 15 for the selectivity of cis-stilbene over trans-stilbene
in the reactions of Fe(TPP)Cl and PhIO)15a was due to the steric
(18) (a) Yang, Y.; Diederich, F.; Valentine, J. S. J. Am. Chem. Soc. 1991,
113, 7195-7205. (b) Yang, Y.; Diederich, F.; Valentine, J. S. J. Am. Chem.
Soc. 1990, 112, 7826-7828. (c) Nam, W.; Valentine, J. S. J. Am. Chem.
Soc. 1990, 112, 4977-4979.
(19) The ratios of cis- to trans-stilbene oxides formed in the competitive
epoxidations of cis- and trans-stilbenes by PhIO catalyzed by Fe(TPFPP)-
Cl and Fe(TDFPP)Cl in aprotic solvent were 4 and 7, respectively, indicating
that the reactive species generated in the PhIO reactions are not high-valent
(16) Fujii, H. Chem. Lett. 1994, 1491-1494.
(17) Although we tried to prepare acylperoxo-iron(III) porphyrin com-
plexes to use the intermediates directly in the competitive epoxidation
reactions, we failed to prepare the acylperoxo-iron(III) porphyrin complexes
by reacting Fe(TPFPP)OH and Fe(TDFPP)OH with m-CPBA at -45 °C in
aprotic solvent.8a Instead, we observed the formation of ferryl-oxo porphyrin
complexes, (Porp)FeIVdO, in the reactions.
iron(IV) oxo porphyrin cation radical complexes such as (TPFPP)+•FeIV
d
O and (TDFPP)+•FeIVdO: Nam, W.; Han, H. J.; Lee, H. J.; Kim, C.
Unpublished results.
(20) Traylor, T. G.; Xu, F. J. Am. Chem. Soc. 1990, 112, 178-186.
(21) Groves, J. T.; Watanabe, Y. J. Am. Chem. Soc. 1988, 110, 8443-
8452.

6646 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Nam et al.
Table 3. Effects of Proton and Alcohol Acidity on the Ratios of Epoxide Products Formed in the Competitive Epoxidations of cis- and
trans-Stilbenes and of Cyclooctene and trans-Stilbene Studied with Fe(TPFPP)Cl and Fe(TDFPP)Cl as Catalyst and m-CPBA as an Oxidant
under Catalytic Reaction Conditionsa
cis-stilbene versus trans-stilbene
cyclooctene versus trans-stilbene
yields (%) of productsb
yields (%) of productsb
ratio of
ratio of
cis-
trans-
cis- to
trans-oxide
cyclooctene
oxide
trans-
cyclooctene oxide
to trans-oxide
oxidec
oxidec
oxidec
1. Fe(TPFPP)Cl as the Catalyst
A. proton effect in aprotic solventd
presence of HClO4
no
88
30
18
11
41
41
8 ( 1
0.7 ( 0.1
0.5 ( 0.1
87
9
3
8
40
31
11 ( 3
0.2 ( 0.1
0.1 ( 0.05
yes
B. (TPFPP)+•FeIVdO as an oxidante
C. effect of alcohol acidity in protic solventf,g
CH3OH/CH2Cl2
30
40
62
86
50
40
23
13
0.6 ( 0.1
1.0 ( 0.1
2.7 ( 0.5
6.6 ( 0.8
18
28
53
78
59
48
27
12
0.3 ( 0.1
0.6 ( 0.1
2.0 ( 0.4
6.3 ( 0.8
CH3CH2OH/CH2Cl2
(CH3)2CHOH/CH2Cl2
(CH3)3COH/CH2Cl2
2. Fe(TDFPP)Cl as the Catalyst
A. proton effect in aprotic solventd
presence of HClO4
no
86
43
34
7
16
14
12 ( 2
2.7 ( 0.3
2.4 ( 0.4
92
36
22
4
18
27
23 ( 5
2.3 ( 0.7
0.8 ( 0.2
yes
B. (TPFPP)+•FeIVdO as an oxidante
C. effect of alcohol acidity in protic solventf,g
CH3OH/CH2Cl2
54
53
57
85
27
26
27
11
2.0 ( 0.2
2.0 ( 0.3
2.1 ( 0.3
7.7 ( 0.5
35
40
35
86
42
45
40
11
0.8 ( 0.2
0.9 ( 0.2
0.9 ( 0.2
8 ( 2
CH3CH2OH/CH2Cl2
(CH3)2CHOH/CH2Cl2
(CH3)3COH/CH2Cl2
a General reaction conditions were the same as described in Tables 1 and 2 unless otherwise indicated. b Based on the amounts of oxidants used.
c cis-Oxide and trans-oxide stand for cis-stilbene oxide and trans-stilbene oxide, respectively. d Reactions were run in the presence of HClO4 (5 ×
10-3 mmol). e Data were obtained from Tables 1 and 2. f Reactions were run in a solvent mixture (total volume of 2.5 mL) of alcohol and CH2Cl2
(3:1). Since alcohols used as solvent may function as an axial ligand and it has been reported that there is a significant axial ligand effect on the
reactivity of 3,3h the studies of the alcohol acidity effect were performed by blocking the axial position of iron porphyrin complex with an imidazole
(5-chloro-1-methylimidazole, 0.1 mmol) to eliminate a possibility of the axial ligand effect. g See ref 20 for the pKa values of the alcohols.
of the alcohol acidity and that the rate of the O-O bond
cleavage of 2 slows down as the acidity of the alcohols
decreases. Furthermore, the results of the effect of alcohol
acidity on the product ratios imply that the reaction of 2 with
olefin (Scheme 1, pathway B) is competing with the O-O bond
cleavage of 2 that leads to the formation of 3 (Scheme 1,
pathway A) and that, in some cases, both the intermediates, 2
and 3, effect the olefin epoxidation at the same time, depending
on the acidity of the alcohol solvents functioning as general-
acid catalysis.
In conclusion, we have demonstrated unambiguously (1) that
two intermediates, 2 and 3, are involved as reactive species in
iron porphyrin complex-catalyzed epoxidation reactions at room
temperature and (2) that an important factor to determine the
nature of reactive intermediates by controlling the rate of the
O-O bond cleavage of 2 is general-acid catalysis. The present
results are relevant to those recently reported by Vaz et al. that
several reactive intermediates can effect the functionalization
of organic substrates by cytochrome P-450 enzymes and that 2
becomes the reactive epoxidizing intermediate by disrupting the
presumed proton-transfer pathway in site-directed mutants.6,11
Also, there is growing evidence that both iron(III)-hydroperoxide
and high-valent iron oxo intermediates are capable of oxygenat-
ing hydrocarbons in methane monooxygenases and non-por-
phyrin iron complexes, depending on the reactivities of hydro-
carbons such as olefins and alkanes.2 Furthermore, the significant
effect of the general-acid catalysis on the rate of the O-O bond
cleavage of 2 has been well documented in peroxidase enzymes
that the distal histidine residue functions as general-acid catalysis
that increases the rate of formation of compound I.22 Finally,
our present results suggest that the previous results of the kinetic
and mechanistic studies of oxygen atom transfer reactions that
had been performed with an assumption that 3 was the reactive
intermediate in iron porphyrin-catalyzed oxygenation reactions
should be reevaluated because the reactive species responsible
for oxygen atom transfer in the catalytic reactions in aprotic
solvent might not be 3 but 2 in some cases.
Experimental Section
Materials. Methanol (anhydrous), dichloromethane (anhydrous), and
acetonitrile (anhydrous) were obtained from Aldrich Chemical Co. and
purified by distillation over CaH2 prior to use. All reagents purchased
from Aldrich Chemical Co. were the best available purity and used
without further purification unless otherwise indicated. m-CPBA
purchased from Aldrich Chemical Co. was purified by washing with
phosphate buffer (pH 7.4) followed by water and then dried under
reduced pressure. H2O2 (30% aqueous) and tert-butyl hydroperoxide
(t-BuOOH, 70% aqueous) were purchased from Fluka and Sigma,
respectively. Fe(TPFPP)Cl was obtained from Aldrich. Fe(TDFPP)Cl
and Fe(TDCPP)Cl were obtained from Mid-Century Chemicals. Fe-
(TPFPP)(CF3SO3), Fe(TDFPP)(CF3SO3), and Fe(TDCPP)(CF3SO3)
were prepared by stirring equimolar amounts of the chloride iron(III)
porphyrins with Ag(CF3SO3) followed by filtering through a 0.45 µM
filter. The resulting solution was used immediately for further studies.
Instrumentation. Product analyses for the epoxidations of cis- and
trans-stilbenes were performed on an Orom Vintage 2000 HPLC
equipped with a variable wavelength UV-200 detector. Detection was
made at 215 nm. Products were separated on a Waters Symmetry C18
reverse phase column (4.6 × 250 mm), eluted first with 50% methanol
in water for 15 min and then with 85% methanol in water for 10 min
at a flow rate of 1 mL/min. The yields of cyclooctene oxide were
determined by a Hewlett-Packard 5890 II Plus gas chromatograph
equipped with a FID detector using a 30-m capillary column (Hewlett-
(22) (a) Erman, J. E.; Vitello, L. B.; Miller, M. A.; Shaw, A.; Brown,
K. A.; Kraut, J. Biochemistry 1993, 32, 9798-9806. (b) Newmyer, S. L.;
Ortiz de Montellano, P. R. J. Biol. Chem. 1995, 270, 19430-19438. (c)
Rodriguez-Lopez, J. N.; Smith, A. T.; Thorneley, R. N. F. J. Biol. Inorg.
Chem. 1996, 1, 136-142.

Fe(III) Porphyrin Complex-Catalyzed Epoxidation Reactions
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6647
Packard, HP-5). UV-vis spectra were recorded on a Hewlett-Packard
8453 spectrophotometer equipped with an OptostatDN variable-temper-
ature liquid-nitrogen cryostat (Oxford Instruments).
Catalytic Competitive Epoxidations in Protic and Aprotic
Solvents. Reactions were performed at ambient temperature under argon
atmosphere unless otherwise indicated. All reactions were run in at
least triplicate, and the data reported represent the average of these
reactions.
The competitive epoxidations of cis- and trans-stilbenes and of
cyclooctene and trans-stilbene in protic solvent were carried out as
follows: oxidants (0.05 mmol, diluted in 0.3 mL of CH3OH/CH2Cl2
(3:1)) were slowly added over a period of 20 min to a stirred solution
containing an iron porphyrin complex (1 × 10-3 mmol) and equal
amounts of competing olefins (0.15 mmol each) in a solvent mixture
(2.2 mL) of CH3OH and CH2Cl2 (3:1). The reaction mixture was further
stirred for 10 min and directly analyzed by HPLC and/or GC. Product
yields were determined by comparison against standard curves.
The reaction conditions in aprotic solvent were as follows: oxidants
(0.05 mmol, diluted in 0.3 mL of CH3CN/CH2Cl2 (1:1)) were slowly
added over a period of 20 min to a stirred solution containing an iron
porphyrin complex (1 × 10-3 mmol) and equal amounts of competing
olefins (1.0 mmol each) in a solvent mixture (2.2 mL) of CH3CN and
CH2Cl2 (1:1). The competitive epoxidations studied with H2O2 were
carried out in the presence of 5-chloro-1-methylimidazole (0.1 mmol),
since the epoxidation of olefins by iron porphyrin complexes and H2O2
in aprotic solvent did not yield epoxide products in the absence of the
imidazole.23 The reaction mixture was further stirred for 10 min and
directly analyzed by HPLC and/or GC with known authentic samples.
Competitive Epoxidations Studied with in Situ Generated High-
Valent Iron(IV) Oxo Porphyrin Cation Radical Complexes. The
reactions of triflate iron(III) porphyrin complexes (1.0 × 10-3 mmol)
with 2 equiv of m-CPBA in a solvent mixture (0.5 mL) of CH3CN and
CH2Cl2 (1:1) at -45 °C gave the formation of green intermediates.
The formation and stability of the intermediates were confirmed by
taking low-temperature UV-vis spectra of the green solutions. The
spectra showed broad absorption bands around 550-750 nm, charac-
teristic of porphyrin cation radical complexes (Supporting Information,
Figure S1).4a,16 After substrates (equal amounts of competing olefins,
0.03 mmol each, diluted in 0.2 mL of CH2Cl2) were added to a reaction
solution containing in situ generated (Porp)+•FeIVdO (1.0 × 10-3 mmol)
at -45 °C, the reaction mixture was stirred for 10 min at -45 °C and
then directly analyzed by HPLC and/or GC.
Acknowledgment. Financial support for this research from
the Korea Science and Engineering Foundation (96-0501-01-
01-3, 1999-2-122-002-4), Brain Korea 21 Project, and the
Ministry of Education of Korea (BSRI-98-3412) is gratefully
acknowledged.
Supporting Information Available: Figure S1 displays
UV-vis spectra of high-valent iron(IV) oxo porphyrin cation
radical complexes and Table S1 contains data of control
reactions (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(23) Nam, W.; Lee, H. J.; Oh, S.-Y.; Kim, C.; Jang, H. G. J. Inorg.
Biochem. Accepted for publication.
JA000289K