First Direct Evidence for Stereospecific Olefin Epoxidation and Alkane Hydroxylation by an Oxoiron(IV) Porphyrin Complex

Article abstract of DOI:10.1021/ja0368204

We report in this study that an oxoiron(IV) porphyrin complex bearing electron-deficient porphyrin ligand, (TPFPP)Fe^IV=O (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion), shows reactivities similar to those found in oxoiron(IV) porphyrin π-cation radicals. In the epoxidation of olefins by the (TPFPP)Fe^IV=O complex, epoxides were yielded as major products; cyclohexene oxide was the sole product formed in the epoxidation of cyclohexene, and stilbenes were stereospecifically oxidized to the corresponding epoxide products. More striking results were obtained in alkane hydroxylation reactions; the hydroxylation of adamantane afforded a high degree of selectivity for tertiary C?H bonds over secondary C?H bonds, and the hydroxylation of cis-1,2-dimethylcyclohexane yielded a tertiary alcohol product with >99% retention of stereochemistry. The latter result demonstrates that an oxoiron(IV) porphyrin complex hydroxylates alkanes with a high stereospecificity. Isotope labeling studies performed with H₂¹⁸O and ¹⁸O₂ in the olefin epoxidation and alkane hydroxylation reactions demonstrated that oxygen atoms in oxygenated products derived from the oxoiron(IV) porphyrin complex. Copyright

Full text of DOI:10.1021/ja0368204

Published on Web 11/05/2003
First Direct Evidence for Stereospecific Olefin Epoxidation and Alkane
Hydroxylation by an Oxoiron(IV) Porphyrin Complex
Wonwoo Nam,*,† Se-Eun Park,† In Kyung Lim,† Mi Hee Lim,† Jongki Hong,‡ and Jinheung Kim§
Department of Chemistry, DiVision of Nano Sciences, and Center for Biomimetic Systems, Ewha Womans
UniVersity, Seoul 120-750, Korea, Hazardous Substance Research Team, Korea Basic Science Institute,
Seoul 136-701, Korea, and Department of Chemical Technology, Changwon National UniVersity,
Kyungnam 641-773, Korea
Received June 21, 2003; E-mail: wwnam@ewha.ac.kr
Table 1. Epoxidation of Olefins and Hydroxylation of Alkanes by
High-valent oxoiron(IV) porphyrins have been frequently invoked
as the key oxidants in the catalytic cycle of heme-containing en-
zymes.1 Especially, oxoiron(IV) porphyrin π-cation radicals, re-
ferred to as compound I, are believed to carry out oxygen atom
transfer reactions in the catalytic oxidation of organic substrates
by cytochromes P450 and iron porphyrin models.2 Indeed, it has
been demonstrated that in situ-generated oxoiron(IV) porphyrin
π-cation radicals oxygenate olefins and alkanes to the corresponding
epoxides and alcohols efficiently and stereoselectively.3 In contrast,
oxoiron(IV) porphyrins, referred to as compound II, have been
considered to be such poor oxidants that they can only oxygenate
triphenylphosphine to triphenylphosphine oxide until Groves and
co-workers reported that an oxoiron(IV) porphyrin complex, (TMP)-
FeIVdO (TMP ) tetramesitylporphinato dianion), is able to oxidize
olefins.4,5 However, the (TMP)FeIVdO complex reacts with olefins
with a selectivity which is very different from that observed for
(TMP)+•FeIVdO, an oxoiron(IV) porphyrin π-cation radical com-
plex.5 Moreover, there is no report yet that oxoiron(IV) porphyrins
are able to activate C-H bonds of alkanes, thereby yielding alcohol
products. In this Communication, we report that an oxoiron(IV)
porphyrin complex bearing electron-deficient porphyrin ligand oxy-
genates olefins and alkanes with reactivities similar to those found
in oxoiron(IV) porphyrin π-cation radicals. To the best of our know-
ledge, this study provides the first example of an oxoiron(IV)
porphyrin complex that conducts two-electron oxidations of olefins
to epoxides and of alkanes to alcohols with a high stereoselectivity.
Treatment of an iron(III) porphyrin complex, Fe(TPFPP)Cl
(TPFPP ) meso-tetrakis(pentafluorophenyl)porphinato dianion) 1,
with m-chloroperbenzoic acid (m-CPBA) in the presence of a small
amount of H2O in a solvent mixture of CH3CN and CH2Cl2 at 25
°C resulted in the formation of an oxoiron(IV) porphyrin complex,
(TPFPP)FeIVdO 2 (Supporting Information, Figure S1 for UV-
vis spectra of 1 and 2).6 Titration experiments show that the
complete conversion of 1 to 2 required 4 equiv of m-CPBA
(Supporting Information, Figure S2 for UV-vis spectral changes
upon the addition of different amounts of m-CPBA). In addition,
the stability of 2 was found to depend significantly on the amounts
of H2O present in reaction solutions (Supporting Information, Figure
S3).7 Therefore, 2 was prepared by reacting 1 with 4 equiv of
m-CPBA in the presence of H2O and directly used in reactivity
studies. When olefins were added to a reaction solution containing
2, the intermediate 2 reverted back to an iron(III) porphyrin complex
with clear isosbestic points at 476, 524, and 566 nm (Supporting
Information, Figure S4).8 Product analysis of the reaction mixture
revealed that epoxides were yielded as major products (Table 1A),
demonstrating that 2 is capable of oxygenating olefins to the cor-
2a,b
substrate
products
yields (%)c
k
obs × 103 s-1 d
A. Epoxidation of Olefins
cyclooctene oxide
cyclohexene oxide
cis-stilbene oxide
trans-stilbene oxide
cyclooctene
cyclohexene
cis-stilbene
trans-stilbene
43 ( 5
35 ( 4
17 ( 3
21 ( 4
3.5 ( 0.3
12 ( 2
2.1 ( 0.2
6.9 ( 0.5
B. Hydroxylation of Alkanes
triphenylmethane
adamantane
triphenylmethanol
1-adamantanol
2-adamantanol
39 ( 4
33 ( 4
3 ( 1
6.4 ( 0.5
1.8 ( 0.2
2-adamantanone
(1R,2R or 1S,2S)-1,2-
dimethylcyclohexanol
1 ( 1
cis-1,2-dimethyl-
cyclohexane
29 ( 4
1.6 ( 0.2
a Reactions were run at least in triplicate under argon. b In general, 2 (2
mM) was prepared by adding 4 equiv of m-CPBA (8 mM, in 20 µL of
CH3CN) to a reaction solution containing 1 (2 mM) and H2O (15 µL) in a
solvent mixture (0.5 mL) of CH3CN and CH2Cl2 (3:1) in a 0.1-cm UV cell
at 25 °C. Substrate (0.2 M, in 20 µL of CH2Cl2) was then injected into the
UV cell, and spectral changes of 2 were directly monitored by a UV-vis
spectrophotometer. Product analyses were performed with GC/GC-MS or
HPLC, and product yields were determined by comparison against standard
curves prepared with authentic samples. c Yields were calculated on the
basis of the amount of 2. d Pseudo-first-order rate constants for the reduction
of 2 to [FeIII(TPFPP)]+ upon the addition of 100 equiv of substrate were
determined by monitoring the absorbance change at 547 nm.
responding epoxide products. In the case of cyclohexene, cyclo-
hexene oxide was yielded as a major product, and the formation of
allylic oxidation products such as cyclohexenol and cyclohexenone
was not observed. In the epoxidation of cis- and trans-stilbenes,
cis- and trans-stilbene oxides were yielded, respectively, and the
formation of isomerized epoxide products and benzaldehyde was
not detected, indicating that the epoxidation of olefins by 2 is highly
stereospecific. Table 1 also shows that the reaction rates of 2 toward
olefins were in the order of cyclohexene > trans-stilbene >
cyclooctene > cis-stilbene.9
Isotope labeling studies were then performed with H218O and
18O2, to understand the nature of oxidizing species and the origin
of oxygen atoms in epoxide products (see Supporting Information
for experimental details). When the cyclooctene epoxidation was
carried out with 2 in the presence of H218O, we found that ∼30%
of the oxygen in cyclooctene oxide derived from the labeled water
(Scheme 1A) and the degree of 18O incorporated into the epoxide
product increased linearly with the increase of the amounts of H218
O
in the reaction mixture (Supporting Information, Figure S6).10 Fur-
thermore, a significant increase of 18O-incorporation was also ob-
served when 2 was incubated in reaction solution containing H218O
prior to the addition of cyclooctene (Supporting Information, Figure
S7). The cyclooctene epoxidation by 2 was then carried out under
18O2 atmosphere, to ensure that O2 does not play a significant role
in the olefin epoxidation. In this reaction, only a trace amount of
18O was incorporated from 18O2 into the epoxide product (Scheme
† Ewha Womans University.
‡ Korea Basic Science Institute.
§ Changwon National University.
9
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J. AM. CHEM. SOC. 2003, 125, 14674-14675
10.1021/ja0368204 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Isotope Labeling Studies
oxoiron(IV) porphyrins with greater oxidative reactivities and to
elucidate mechanisms of oxygen atom transfer from the oxoiron-
(IV) porphyrin complex to olefins and alkanes are currently
underway in this laboratory.
Acknowledgment. This work was supported by a grant from
MOST through the Creative Research Initiative Program.
Supporting Information Available: Text containing experimental
details for isotope labeling studies and Figures S1-S7 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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(8) The formation of an iron(III) porphyrin complex may be the result of a
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The reactivity of 2 was then examined in alkane hydroxylation
reactions (see Table 1B). In the hydroxylation of triphenylmethane
by 2, triphenylmethanol was yielded as the only detected product.
When the triphenylmethane hydroxylation was carried out in the
presence of H218O, 50% of the oxygen atom in the triphenylmetha-
nol product derived from the labeled water (Scheme 1A). Further-
more, when the triphenylmethane hydroxylation was carried out
under 18O2 atmosphere, less than 2% 18O was incorporated into
the triphenylmethanol product (Scheme 1B). The results of the
isotope labeling studies demonstrate clearly that the oxygen in the
triphenylmethanol product derives from 2 and that O2 does not play
a significant role in the alkane hydroxylation. In the hydroxylation
of adamantane, a high degree of selectivity for tertiary C-H bonds
over secondary C-H bonds was observed (i.e., a 3°/2° ratio of
∼25, normalized on a per-hydrogen basis). Such a high 3°/2° ratio
has been observed in the catalytic hydroxylation of adamantane
by iron complexes of porphyrin and non-porphyrin ligands.12,13 Most
significantly, the alkane hydroxylation by 2 was found to be highly
stereospecific. In the hydroxylation of cis-1,2-dimethylcyclohex-
ane,3b,13,14 (1R,2R or 1S,2S)-1,2-dimethylcyclohexanol, the tertiary
alcohol with the methyl groups cis to each other, was the only de-
tected product, and the formation of (1R,2S or 1S,2R)-1,2-dimeth-
ylcyclohexanol, the epimer with the methyl groups trans to each
other, was not observed at all. This result demonstrates unambigu-
ously that an oxoiron(IV) porphyrin complex hydroxylates alkanes
stereospecifically. The same stereospecificity was also observed
in the hydroxylation of the tertiary C-H bond of cis-decalin (data
not shown).12a Finally, it should be noted that the oxidizing power
of 2 is not as strong as its oxoiron(IV) porphyrin π-cation radical
species, (TPFPP)+•FeIVdO,6a so that 2 cannot activate alkanes with
stronger C-H bonds such as cyclohexane even at 25 °C.15,16
In conclusion, we have demonstrated here that an oxoiron(IV)
porphyrin complex is able to conduct two-electron oxidations of
olefins to epoxides and of alkanes to alcohols with reactivity patterns
similar to those found in oxoiron(IV) porphyrin π-cation radicals.
Such a finding in iron porphyrin models suggests that oxoiron(IV)
porphyrins in cytochromes P450 and recently isolated nonheme
oxoiron(IV) complexes17 may be able to effect olefin epoxidation
and alkane hydroxylation. Furthermore, the present results lead us
to propose that this oxoiron(IV) porphyrin complex is a “third
electrophilic oxidant” that may be involved in oxygen atom transfer
reactions by cytochromes P450.18 Studies designed to synthesize
(9) The observations that the reaction rates of 2 toward olefins depend on
olefin substrates (Table 1) and the amounts of cyclohexene (Supporting
Information, Figure S5) demonstrate that 2 is indeed involved in the olefin
epoxidation reactions.
(10) Meunier, B.; Bernadou, J. Top. Catal. 2002, 21, 47-54.
(11) It has been shown that compound II of peroxidase exchanges its oxygen
with labeled water readily at pH 7: Hashimoto, S.; Nakajima, R.;
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(15) We have shown previously that 1 associated with m-CPBA hydroxylates
alkanes including cyclohexane at -40 °C under catalytic reaction
conditions and suggested an acylperoxo-iron porphyrin as a reactive species
(see ref 6a). In the reactions, the yields of alcohol products were high,
and only trace amounts of 18O were incorporated from H218O into the
products when the reactions were carried out in the presence of H218O. In
contrast, alkane hydroxylations by 2 did not occur at -40 °C, and the
18O-incorporation from H218O into oxygenated products was high at room
temperature. These different reactivity patterns indicate that the hydroxy-
lating intermediate generated in the reaction of 1 and m-CPBA under
catalytic conditions at -40 °C is different from 2.
(16) A (TPFPP)+•FeIVdO complex, prepared in the reaction of 1-CF3SO3 (2
mM) with 1.2 equiv of m-CPBA at -40 °C, hydroxylates cyclohexane
with kobs ) 9.6 × 10-2 s-1, yielding cyclohexexanol (37%) as a major
product. The low reactivity of 2, as compared to the (TPFPP)+•FeIVdO
complex, was also observed in the hydroxylation of cis-1,2-dimetyhlcy-
clohexane, in which the (TPFPP)+•FeIVdO complex disappeared im-
mediately upon the addition of the substrate at -40 °C and yielded 55%
of alcohol product, whereas 2 was stable in the presence of the substrate
at -40 °C and yielded no alcohol product under the identical conditions.
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