Highly efficient and selective epoxidation of alkenes by photochemical oxygenation sensitized by a ruthenium(II) porphyrin with water as both electron and oxygen donor

Article abstract of DOI:10.1021/ja0295218

Visible light irradiation of a reaction mixture of carbonyl-coordinated tetra(2,4,6-trimethyl)-phenylporphyrinatoruthenium(II) (Ru^IITMP(CO)) as a photosensitizer, hexachloroplatinate(IV) as an electron acceptor, and an alkene in alkaline aqueous acetonitrile induces selective epoxidation of the alkene with high quantum yield (Φ = 0.6, selectivity = 94.4% for cyclohexene and Φ = 0.4, selectivity = 99.7% for norbornene) under degassed conditions. The oxygen atom of the epoxide was confirmed to come from a water molecule by an experiment with H₂¹⁸O. cis-Stilbene was converted into its epoxide, cis-stilbeneoxide, without forming trans-stilbeneoxide, trans-Stilbene, however, did not exhibit any reactivity. Under neutral conditions, an efficient buildup of the cation radical of Ru^IITMP(CO) was observed at the early stage of the photoreaction, while an addition of hydroxide ion caused a rapid reaction with the cation radical to promote the reaction with reversion to the starting Ru^IITMP(CO). A possible involvement of a higher oxidized state of Ru such as Ru^IV, Ru^V, Ru^VI through a dismutation of the Ru^III species was excluded by an experiment with Ru^VITMP(O)₂. Decarbonylation of the Ru complex was also proven to be invalid. A reaction mechanism involving an electron transfer from the excited triplet state of Ru^IITMP(CO) to hexachloroplatinate(IV) and subsequent formation of OH^--coordinated Ru^III species, leading to an oxo-ruthenium complex as the key intermediate of the photochemical epoxidation, was postulated.

Full text of DOI:10.1021/ja0295218

Published on Web 04/19/2003
Highly Efficient and Selective Epoxidation of Alkenes by
Photochemical Oxygenation Sensitized by a Ruthenium(II)
Porphyrin with Water as Both Electron and Oxygen Donor
Shigeaki Funyu, Takehiko Isobe, Shinsuke Takagi, Donald A. Tryk, and
Haruo Inoue*
Contribution from the Department of Applied Chemistry, Graduate Course of Engineering,
Tokyo Metropolitan UniVersity, 1-1 Minami-ohsawa, Hachiohji, Tokyo 192-0397 Japan, and
CREST, Japan Science and Technology
Received November 29, 2002; E-mail: inoue-harou@c.metro-u.ac.jp
Abstract: Visible light irradiation of a reaction mixture of carbonyl-coordinated tetra(2,4,6-trimethyl)-
II
phenylporphyrinatoruthenium(II) (Ru TMP(CO)) as a photosensitizer, hexachloroplatinate(IV) as an electron
acceptor, and an alkene in alkaline aqueous acetonitrile induces selective epoxidation of the alkene with
high quantum yield (Φ ) 0.6, selectivity ) 94.4% for cyclohexene and Φ ) 0.4, selectivity ) 99.7% for
norbornene) under degassed conditions. The oxygen atom of the epoxide was confirmed to come from a
18
water molecule by an experiment with H
2
O. cis-Stilbene was converted into its epoxide, cis-stilbeneoxide,
without forming trans-stilbeneoxide. trans-Stilbene, however, did not exhibit any reactivity. Under neutral
II
conditions, an efficient buildup of the cation radical of Ru TMP(CO) was observed at the early stage of the
photoreaction, while an addition of hydroxide ion caused a rapid reaction with the cation radical to promote
II
the reaction with reversion to the starting Ru TMP(CO). A possible involvement of a higher oxidized state
IV
V
VI
III
of Ru such as Ru , Ru , Ru through a dismutation of the Ru species was excluded by an experiment
VI
with Ru TMP(O)
2
. Decarbonylation of the Ru complex was also proven to be invalid. A reaction mechanism
involving an electron transfer from the excited triplet state of Ru TMP(CO) to hexachloroplatinate(IV) and
subsequent formation of OH -coordinated Ru species, leading to an oxo-ruthenium complex as the key
intermediate of the photochemical epoxidation, was postulated.
II
-
III
1
. Introduction
of the most promising candidates for the reaction. Ultaraviolet
light irradiation of a TiO2 electrode connected with a Pt electrode
in an aqueous electrolyte induces oxygen evolution from the
For the past several decades, extensive efforts have been made
in the field of artificial photosynthesis, which involves photo-
chemical reactions to convert light energy into chemical energy
via endoergonic processes. Recent efforts on photoredox systems
that can induce actual chemical reactions have focused much
attention on the reduction terminal end of the photoredox
systems rather than on the oxidation terminal end. Efficient
hydrogen evolution and even carbon dioxide reduction have
been reported. Only a very few reactions, however, can actually
be coupled in the oxidative terminal end.3 Incorporation of a
water molecule into the oxidative terminal end, that is, oxidation
of water as an ideal electron donor, should be one of the central
10
TiO2 surface and hydrogen evolution from the Pt surface. This
phenomena, that is, the photochemical splitting of water, is often
referred to as the “Honda-Fujishima effect”. The finding of
the Honda-Fujishima effect on the photochemical splitting of
(
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3
-6
subjects of study. Among various approaches for the oxida-
tion of water,
7
-9
a semiconductor such as TiO2 has been one
(
(
1) (a) Kalyanasundaram, K.; Gratzel, M. Photosensitization and Photocatalysis
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5734
9
J. AM. CHEM. SOC. 2003, 125, 5734-5740
10.1021/ja0295218 CCC: $25.00 © 2003 American Chemical Society

Epoxidation of Alkenes by Photochemical Oxygenation
A R T I C L E S
water by TiO210 afforded a strong impact and has led to the
The highest quantum yields of the photochemical epoxidation
turned out to be 0.60 for the formation of cyclohexeneoxide
from cyclohexene, with a selectivity of 94.4%, and 0.40 for
norborneneoxide formation, with 99.7% selectivity. The reaction
serves as one of the typical photoreactions that can incorporate
a water molecule both as an oxygen atom donor and as a two-
electron donor.
recent progress in the field.1
1,12
Even though the light energy
utilized for the excitation of TiO2 is discouragingly limited to
that in the ultraviolet region, due to the large band gap (3.0 eV
for rutile and 3.2 eV for anatase) of TiO2,13 interesting reports
have recently appeared that visible light can drive similar
reactions on semiconductors such as TiO2-xNx and In1-xNix-
11,12
TaO4.
Dye-sensitized solar cells using nanocrystalline TiO2
The synthesis of epoxide compounds usually requires rather
strong oxidizing agents such as hydroperoxides.18 From the
viewpoints of synthetic methodology, this photochemical
epoxidation with water as an oxygen atom donor by visible light
would afford a novel synthetic method without using any strong
which can utilize the visible light energy have greatly improved
the energy conversion efficiency and have attracted much
1
4
attention. On the other hand, metal complexes have been
another potential candidate for the water splitting reaction.
Binuclear ruthenium complex has been reported to exert an
elegant four-electron oxidation of water,3h,i,15 and binuclear
manganese complexes have also been reported to exhibit a
1
8
oxidizing agent. When H2 O is used in this photochemical
18
epoxidation, it can serve as a synthetic tool of O labeling in
the corresponding epoxides.
7
,8,16
similar reaction.
The interesting reactions, however, that
2. Experimental Section
have thus far been reported are only driven by chemical or
electrochemical oxidation upon the metal complexes and have
not yet been connected with photoreactions. The difficulties in
designing a photochemical reaction system with metal com-
plexes for water oxidation would involve subjects to be solved
such as (1) how the water molecule could be incorporated in
the electron donor system of the photoreaction, (2) how the
highly stereochemical arrangement among the metal complexes
could be achieved for the four-electron oxidation processes, and
2
.1. Materials. The free base porphyrin, tetra(2,4,6-trimethyl)-
2
phenylporphyrin (H TMP), was synthesized according to the report of
Lindsey.19 The carbonyl-coordinated tetra(2,4,6-trimethyl)phenylpor-
II
phyrinatoruthenium(II) (Ru TMP(CO)) was further synthesized from
H
2
TMP and Ru
3
(CO)12 by procedures similar to those reported for the
20a
VI
tertraphenylporphyrin complex.
oxidation of Ru TMP(CO) by m-chloroperbenzoic acid in dichlo-
romethane according to the literature.2 Acetonitrile (HPLC grade) was
used as received from Nakalai Tesque. K Pt Cl and [(CH CH CH -
2 6 3 2 2
2 4 2 6
CH ) N] Pt Cl were used as received from Aldrich. Distilled water
2
Ru TMP(O) was prepared by
II
1a
IV
(3) how the self-decomposition ascribed to the high oxidation
IV
state of the metal complexes could be avoided. The oxidation
of water should involve one of the following: (1) a one-electron
process,17 (2) a two-electron one, or (3) a multielectron one.
The one-electron process, even the one-electron oxidation of
the hydroxide ion, needs a rather high oxidation potential (>2.0
V), and the resultant hydroxide radical is too reactive to be
handled. The multielectron one, such as the four-electron
process, requires a highly specific stereochemical arrangement,
as described above, although the thermodynamic requirement
for the process is milder. To avoid the difficulties of these two
processes, among the actual photoreactions, we have recently
focused our attention on the two-electron oxidation of water,
which also has a mild thermodynamic requirement, sensitized
was passed through an ion-exchange column (G-10, ORGANO Co.).
The specific resistance of the water was below 0.1 µS/cm . Cyclohexene
2
3
(Tokyo kasei) was distilled under nitrogen before use and was stored
under nitrogen. Styrene (Tokyo kasei) was purified by passing through
an alumina column (eluent: hexane) before use and was stored under
nitrogen. Cyclooctene, norbornene, and 1-hexene (all from Tokyo kasei)
were distilled under nitrogen and were passed through an alumina
column (eluent: hexane) before use. trans-Stilbene (Tokyo kasei) was
purified by vacuum distillation, and cis-stilbene was passed through
18
an alumina column before use (eluent: hexane). H
was used as received from EURISO-TOP.
2
O (96.5% content)
2.2. Measurements. UV-vis spectra were measured on a Shimadzu
UV-2400PC. Gas chromatographic analyses were performed on a
Shimadzu GC-17A equipped with a TC-17 column (GL Sciences Inc.
4-6
by several high-valent metalloporphyrins.
Here, we report
3
0m, 60-250C) and a mass spectrograph (Shimadzu QP-5000) as a
II
that a ruthenium porphyrin as a sensitizer induces a highly
selective epoxidation of alkenes with water both as an oxygen
donor and as a two-electron donor upon visible light irradiation.
detector. The practical detection limit of the GC-MS was ca. 10-7 M.
Quantitative analysis was carried out in the selected ion monitoring
(SIM) detection mode. Fast atom bombardment (FAB) mass spectra
were measured on a JEOL JMS-LX1000 with the use of m-nitrobenzyl
alcohol matrix.
2.3. Photochemical Oxygenation Reaction. All of the samples for
the photoreactions were degassed by seven repeated freeze-pump-
(
11) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625.
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(
2
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(
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1
J. AM. CHEM. SOC.
9
VOL. 125, NO. 19, 2003 5735

A R T I C L E S
Funyu et al.
The major product was cyclohexene oxide (97.4% selectivity),
with small amounts of 2-cyclohexenol (1.5%) and trans-2-
chlorocyclohexanol (1.1%). The selectivity is defined as the
percentage of each product among all of the reaction products.
The quantum yield for the formation of cyclohexene oxide was
0
.16. The turnover number of the reaction after 60 min
irradiation, based on the initial concentration of sensitizer, was
4. Clearly, the reaction proceeded catalytically. Both K2PtCl6
3
II
and Ru TMP(CO) were necessary for the photochemical
epoxidation of cyclohexene. The net chemistry of the reaction
can be expressed as follows:
hυ
2
-
C H + H O + Pt(IV)Cl
8
6
10
2
6
II
Ru TMP(CO)
2
-
C H O + Pt(II)Cl + 2HCl (1)
6
10
4
An experiment with H2 O revealed that an 18O atom was
18
quantitatively incorporated into the oxygenated products under
1
8
degassed conditions. When H2 O (32.2% content) was used in
1
8
the photoreaction, incorporation of O into the oxygenated
products was 32.2% in cyclohexane oxide and 31.8% in
2-cyclohexenol, respectively. This clearly indicates that a water
molecule serves as an oxygen donor in the photochemical
oxygenation reaction. Photoreactions with other alkenes under
alkaline conditions also afforded epoxide as almost an exclusive
product, as indicated in Table 1.
cis-Stilbene was converted into its epoxide, cis-stilbeneoxide,
without forming trans-stilbeneoxide. Interestingly, trans-stilbene,
however, did not exhibit any reactivity. Contrary to the results
obtained here, a dioxo-complex of ruthenium octaethylporphy-
Figure 1. (a) The spectral changes under basic condition (aqueous
VI
rin, Ru OEP(O)2, which has only a slight steric hindrance
II
-5
acetonitrile solution (10% H2O) containing Ru TMP(CO) (1.0 × 10 M),
around the metal center, has been reported to afford both the
cis-form and the trans-form of the epoxide from cis-stilbene
and even exhibit a reactivity for trans-stilbene, in a study of
P-450 model systems where alkene was converted to the
corresponding epoxide by a catalytic action of the ground-state
IV
-4
-3
K2Pt Cl6 (5 × 10 M), cyclohexene (1.0 × 10 M), and KOH (1.5 ×
-
3
1
0
M)). (b) The spectral changes under neutral condition (aqueous
II
-5
acetonitrile solution (10% H2O) containing Ru TMP(CO) (1.0 × 10 M),
IV
-4
-3
K2Pt Cl6 (5 × 10 M), cyclohexene (1.0 × 10 M), and KOH (0 M)).
thaw cycles under 10-5 Torr. The degassed samples in a 1 × 1 × 4.5
22
ruthenium porphyrins. In the present photochemical system
cm quartz cell (EIKO-SHA) were irradiated with monochromatic light
II
with Ru TMP, which has substantial steric hindrance in the
(420 nm) through an interference filter KL-42 and a sharp cutoff filter
vicinity of the metal center due to the methyl groups in the
ortho-positions on the meso-substituted phenyl ring, trans-
stilbene, with its extended molecular structure, might not be
able to sufficiently access the metal center of the ruthenium
L-39 from a 500W Xe arc lamp (USHIO 500-DKO). The photoreaction
was monitored with UV-visible spectroscopy. The reaction mixture
after the photoreaction was vacuum-distilled, and the products were
analyzed by GC-MS spectroscopy. All of the procedures were
performed at ambient temperature.
2
3,24
complex, as observed in P-450 model systems.
The lack of
2
.4. Labeling Experiment with the Use of 18O. The 18O content of
formation of the isomerized form, trans-stilbeneoxide, from cis-
stilbene strongly suggests that a concerted, stereoregulative
mechanism is operating in the photochemical epoxidation.
The overall view of the reactivities of alkenes examined here
does not necessarily show a direct correlation between the
reactivities and the oxidation potentials of the alkenes (Table
1
8
16
18
the water, H
2
O/(H
2
O + H
O. The photoreactions were carried out under exactly
the same conditions as were used for the experiment carried out with
2
O), was adjusted to 32.2% by mixing
1
6
18
2 2
H O and H
1
6
2
H O. The reaction mixtures after the photoreaction were vacuum-
18
distilled, and the content of O in the reaction product was determined
by GC-MS (Shimadzu QP-5000).
1), but other factors such as steric ones could also be controlling
the reactivity, which supports the idea that the reaction proceeds
in a concerted manner on the metal center of the complex.
3. Results and Discussion
3
.2. Photochemical Epoxidation under Neutral Conditions.
3
.1. Photochemical Epoxidation of Alkene Sensitized by
II
Under neutral conditions (without added KOH), however, the
photoreaction exhibited a somewhat different aspect. In the
initial period of the light irradiation (20 s), the original
Ru TMP(CO) under Alkaline Conditions. A degassed aque-
ous acetonitrile solution (10% H2O) containing Ru TMP(CO),
K2Pt Cl6, cyclohexene, and KOH was irradiated with visible
II
IV
light (420 nm). Upon visible light irradiation for 60 min, K2-
PtCl6 completely disappeared, while the absorption band of the
porphyrin (411.5 nm) gradually shifted to 412.0 nm, with
decreasing intensity, as shown in Figure 1a.
(
22) Leung, W. H.; Che, C. M. J. Am. Chem. Soc. 1989, 111, 8812.
(23) Liu, C. J.; Yu, W. Y.; Che, C. M.; Yeung, C. H. J. Org. Chem. 1999, 64,
7365.
(24) Groves, J. T.; Han, Y.; Engen, D. V. J. Chem. Soc., Chem. Commun. 1990,
436.
5736 J. AM. CHEM. SOC.
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VOL. 125, NO. 19, 2003

Epoxidation of Alkenes by Photochemical Oxygenation
Table 1. Photochemical Epoxidation of Alkenes Sensitized by Ru TMP(CO)
A R T I C L E S
II
a
Ru TMP(CO)] ) 1.0 × 10 M, [K2PtCl6] ) 5.0 × 10 M, [alkene] ) 1.0 × 10 M. Visible light (420 nm) was irradiated. b Reference 31. c 20 s.
II
-5
-4
-3
[
d
e
II
-5
-3
-3
5
40 min. [Ru TMP(CO)] ) 2.0 × 10 M, [[C4H9]4N2PtCl6] ) 2.5 × 10 M, [alkene] ) 1.0 × 10 M. Visible light (420 nm) was irradiated.
absorption band (411.5 nm) disappeared completely and shifted
to 391 nm, as shown in Figure 1b. On prolonged irradiation
1,2-diol was observed in the initial stages (shorter than 20 s),
but its appearance on prolonged light irradiation, as described
above, strongly suggests that the diol is not directly formed by
the photoreaction, but the initial photoproduct, the epoxide,
undergoes conversion into the diol upon acidification of the
system during the photoreaction. In fact, under alkaline condi-
tions, the diol was not detected at all (Table 1). Even though
trans-2-chlorocyclohexanol formed as the major product under
neutral conditions on prolonged irradiation, and did not form
under alkaline conditions, there still may be a possibility that
trans-2-chlorocyclohexanol is the initial reaction product and
the epoxide is derived from trans-2-chlorocyclohexanol by ring
closure under alkaline conditions. The initial product distribution
under neutral conditions, however, clearly indicates that the
primary photoproduct should be the epoxide and that other
products are derived from the epoxide upon acidification and
the appearance of the chloride ion, as shown in eq 1, in the
reaction system during the photoreaction.
(540 min), the absorption at 391 nm gradually shifted to 418
nm. Within the initial 20 s light irradiation, the products obtained
were cyclohexene oxide (87%), 2-cyclohexenol (4%), trans-2-
chlorocyclohexanol (5%), cyclohexanone (2%), and trans-1,2-
dichlorocyclohexane(2%) (Table 1). The apparent quantum yield
of the products was 0.07. The selectivity and the reactivity for
the epoxide were both lower than those obtained under alkaline
conditions. On prolonged light irradiation (540 min), the
selectivity dropped substantially for cyclohexene oxide (1%),
2
-cyclohexenol (4%), trans-2-chlorocyclohexanol (59%), trans-
cyclohexane-1,2-diol (32%), and trans-1,2-dichlorocyclohexane
4%). trans-2-Chlorocyclohexanol became the major product.
(
IV
Because the consumption of K2Pt Cl6 during the photoreaction
induces the formation of HCl, as indicated in eq 1, the chloride
ion can give rise to the chlorinated compounds, and acidification
of the reaction mixture affects the product distribution, as
6
observed in the case of antimony(V)porphyrin. Actually, the
3.3. Improvement of the Reactivity. This photochemical
epoxidation is quite an attractive reaction, because it obviously
involves the water oxidation process, as described later. The
reactivity, however, could further be improved. The reaction
mechanism, as described later, indicates that the photochemical
pH of the reaction mixture decreased down to 3.2 after 540
min light irradiation in the neutral sample. In the dark,
cyclohexeneoxide was confirmed to be efficiently converted into
trans-2-chlorocyclohexanol and cyclohexane-1,2-diol in a 65:
-
4
II
3
5 ratio under acidic conditions ([HCl] ) 7.5 × 10 M) in
formation of the cation radical of Ru TMP(CO) is one of the
aqueous acetonitrile (9/1 v/v%). No formation of cyclohexane-
key steps that governs the total reactivity, as described later.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 19, 2003 5737

A R T I C L E S
Funyu et al.
To improve the photochemical reactivity, a promising approach
should be to improve the efficiency of the cation radical
formation by increasing the concentration of electron acceptor.
We have found that potassium hexachloroplatinate (K2PtCl6)
is an efficient electron acceptor for the present photochemical
epoxidation, but its solubility in aqueous acetonitrile is small.
-
4
The present condition (5 × 10 M) is nearly the practical
solubility limit in the present solvent system. Other more soluble
electron acceptors are required to improve the reactivity. After
III
an extensive search for other electron acceptors, only Co (acac)3
III
and Fe (NO3)3 were found to exhibit reactivity, although small,
while organic acceptors such as methyl viologen and quinones
IV
exhibited very little reactivity. Only hexachloroplatinate turned
out to be an efficient electron acceptor. Among the hexachlo-
IV
roplatinate salts, tetrabutylammonium hexachloroplatinate was
found to be more soluble than the potassium salt. The solvent
system was also modified by adding benzene to aqueous
acetonitrile to improve the solubility of the hexachloroplatinate.
By choosing these experimental conditions, we were finally able
to successfully find an efficient system for the photochemical
Figure 2. Effect of the intensity of the incident light on the photochemical
epoxidation under the condition of aqueous acetonitrile solution (10% H2O)
II
-5
IV
-4
containing Ru TMP(CO) (1.0 × 10 M), K2Pt Cl6 (5 × 10 M),
-3
-3
cyclohexene (1.0 × 10 M), and KOH (1.5 × 10 M).
II
epoxidation. Under the following conditions, [Ru TMP] )
II
upon one-electron electrochemical oxidation of Ru TMP(CO).
If the dismutation is involved in the photoreaction, the rate of
the reaction should be proportional to the square of the light
intensity, but that was not the case. As shown in Figure 2, the
yield of the photochemical oxygenation was approximately
proportional to the light intensity.
-
5
-3
-3
2
[
× 10 M, [KOH] ) 1.5 × 10 M, [alkene] ) 1 × 10 M,
-3
[(C4H9)4N]2PtCl6] ) 2.5 × 10 M in acetonitrile/benzene/
water (8/1/1 v/v%), visible light irradiation (λ ) 420 nm)
induced an efficient epoxidation of cyclohexene, with a quantum
yield of 0.6 and a selectivity of 94.4%. Norbornene also showed
a high quantum yield (0.4) for the photochemical epoxidation,
with a selectivity of 99.7% (Table 1).
The absence of the dismutation may be due to the steric
hindrance of the bulky substituents in RuTMP. In fact, a
completely different product distribution, with less reactivity,
3
.4. Is a Higher Oxidation State of Ru Involved in the
Photochemical Epoxidation? Before discussing the reaction
mechanism of the photochemical epoxidation, we first examined
the possibility of whether the reaction involves a higher
VI
was observed, as described below, when Ru TMP(O)2 was
IV
employed as a sensitizer, where the Ru species could play a
key role. Groves et al. have reported an oxygenation reaction
IV
V
VI
oxidation state of Ru such as Ru , Ru , and Ru , which are
VI
of alkene catalyzed by Ru TMP(O)2 with the redox mechanism
known to be strong oxidizing agents.1
5,21
Some ruthenium
VI
IV
II 21b
involving the cycles among Ru -Ru -Ru . We examined
complexes are known to exhibit P-450 oxygenation in the
ground state, where dissolved molecular oxygen or another
appropriate oxidizing agent is required.21 Oxidation-reduction
VI
the high valent species, Ru TMP(O)2, for the reaction with
cyclohexene in the dark under exactly the same experimental
-3
conditions ([KOH] ) 1.5 × 10 M, in CH3CN/H2O (9/1 v/v
V
III
cycles among high oxidation states such as Ru -Ru and
%)) as were used for the present photoreaction. 2-Cyclohexene-
VI
IV
21b,i
Ru -Ru have been proposed.
The possibility involving
1
9
-ol was the exclusive reaction product with a selectivity of
high oxidation states of Ru, however, is rather questionable for
this photoreaction, as described below. The appearance of the
cation radical (391 nm species) and its trapping by hydroxide
ion are convincing evidence for the formation of a hydroxide-
coordinated Ru species in the photoreaction, which is presumed
9% in this case. We examined further a photoreaction sensitized
VI
by the high valent species, Ru TMP(O)2. A reaction mixture
VI
-5
-3
of Ru TMP(O)2 (1.0 × 10 M), cyclohexene (1.0 × 10 M),
-4
-3
K2PtCl6 (5.0 × 10 M), and KOH (1.5 × 10 M) in a
degassed aqueous acetonitrile solution (10% H2O) was irradiated
III
to be the Ru species. Supposing this process, however, we
VI
with 420 nm light. The starting Ru TMP(O)2 had already been
transformed into the Ru species before light irradiation, and
the Ru species was the light absorbing species. The photo-
III
found that a further oxidation of the Ru species (I) to higher
IV
oxidation states by K2PtCl6 (Ered ) 0.73V vs NHE) in the
reaction system is quite unfavorable. Absence of an induction
period for the photochemical formation of the epoxides strongly
IV
products were cyclohexeneoxide (27%), 2-cyclohexenol (56%),
trans-2-chlorocyclohexanol (8%), and cyclohexen-2-one (5%),
respectively. These results indicate that oxidation states higher
III
suggests that a further light irradiation step involving the Ru
species would not contribute substantially to the oxygenation.
III
than Ru are not involved in the present photochemical
II
In the case of Ru octaethylporphyrin, one-electron oxidation
epoxidation.
II
IV
of the Ru complex has been reported to afford the Ru species
A possible contribution of decarbonylation from the starting
via a dismutation process (two Ru species producing a RuII
III
26
complex to the photochemical oxygenation was also excluded
species and a Ru species). Furthermore, the RuIV species is
readily converted to its µ-oxo-dimer, which is well characterized
by the appearance of new absorption bands at 375, 510, and
IV
25
by FAB-MS and IR studies on the sample for the initial 20 s
II
irradiation. The starting Ru TMP(CO) remained unchanged.
5
70 nm. However, the characteristic bands of the µ-oxo-dimer
(26) (a) Hopf, F. R.; O’Brien, T. P.; Scheidt, W. R.; Whitten, D. G. J. Am.
Chem. Soc. 1975, 97, 277. (b) Vogler, E. M.; Kunkely, H. Ber. Bunsen-
Ges. Phys. Chem. 1976, 80, 425. (c) Farrell, N.; Dolphin, D. H.; James, B.
R. J. Am. Chem. Soc. 1978, 100, 324. (d) Hoshino, M.; Kashiwagi, Y. J.
Phys. Chem. 1990, 94, 673.
were not observed during the present photoreaction and even
(25) Mosseri, S.; Neta, P.; Hambright, P. J. Phys. Chem. 1989, 93, 2358.
5
738 J. AM. CHEM. SOC. VOL. 125, NO. 19, 2003
9

Epoxidation of Alkenes by Photochemical Oxygenation
A R T I C L E S
Scheme 1
3
.5. Reaction Mechanism of the Photochemical Epoxida-
the photoreaction and presumably exerts a key role in the
product-determining step in the reaction mechanism. A simple
electron transfer from the alkene to the cation radical of Ru -
II
tion. Because Ru TMP(CO) is almost nonfluorescent, with a
lifetime shorter than 35 ps,2 the excited state responsible for
the reaction should be the excited triplet state of the complex,
which has a lifetime sufficiently long for the reaction to take
place. The triplet lifetime was estimated to be 64 µs under basic
1b
II
TMP(CO) (Eox ) 1.03 V vs NHE) is most unfavorable, because
the oxidation potentials of the alkenes (Table 1) indicate that
the electron undergoes a considerable endoergonic process, in
-3
V
6
conditions ([KOH] ) 1.5 × 10 M) in aqueous acetonitrile (9/1
contrast to the case for Sb TPP. In fact, the cation radical is
stable enough to be observed under neutral conditions, even in
the presence of alkene, as described above (Figure 1b).
Even though the reaction mechanism is not yet fully
understood, one of the most probable reaction schemes for the
photochemical oxygenation is, thus, postulated in Scheme 1.
v/v%) by a laser flash photolysis experiment. Supposing the
II
electron transfer from the excited Ru Complex to be diffusion-
controlled as the maximum limit (1010
-1
-1
M
s ), we found
6
-1
that the maximum rate would become 5 × 10 s with [K2-
-
4
PtCl6] ) 5 × 10 M. This cannot compete with the singlet
decay rate (<35 ps), while the reaction can proceed within the
triplet lifetime. The triplet state is considered to be the porphyrin
II
Upon visible light irradiation of Ru TMP(CO), an electron
2-
transfer from the triplet excited porphyrin to PtCl would form
6
20
II
+•
triplet state rather than MLCT one. An electron transfer from
the triplet Ru TMP(CO) (Eox ) -0.67 V vs NHE, ET (excited
triplet state energy) ) 1.70 eV ) to K2PtCl6 (Ered ) 0.73 V vs
NHE) would be sufficiently exoergic. The electrochemical
oxidation of Ru TMP(CO) (1.0 × 10 M) at +1.30 V versus
NHE in acetonitrile in the presence of tetrabutylammonium
perchlorate (0.1 M) as a supporting electrolyte afforded a species
with absorption maxima λmax of 391, 589, 646 nm, which are
exactly the same for the species observed within the initial 20
s light irradiation under neutral conditions. A further addition
a porphyrin cation radical (Ru -P ), because the first oxidation
peak (1.03 V vs NHE) in the cyclic voltammogram is considered
to correspond to an oxidation of the porphyrin ligand, with the
II
27
2
8
II
III 20a,29
second peak (1.44 V vs NHE) corresponding to Ru /Ru .
Coordination of hydroxide ion or water to the one-electron
II
-4
III
oxidized species of the starting complex would form a Ru
III
species (I). Although the formation of the Ru species from
Ru -P is an endoergonic process, stabilization by coordina-
tion of OH or H O to the Ru center may render the
II
+•
-
III
2
conversion possible. Successive deprotonation would result in
-
3
of alkaline aqueous acetonitrile ([KOH] ) 1.5 × 10 M in
the formation of the metal-oxo complex (II). The metal-oxo
complex (II) induces a two-electron oxidation through a reaction
CH3CN/H2O ) 9/1 V/V) in the presence of cyclohexene
-
3
I
(
1.0 × 10 M) to the electrochemically oxidized species
with the alkene to form the corresponding epoxide and the Ru
induced a rapid reversion to one with λmax ) 411.5 nm.
species (III), leading to the formation of the porphyrin radical
II
-5
II
-•
II
2-
Treatment of Ru TMP(CO) (1.0 × 10 M) with Fe(ClO4)3
5 × 10 M) in acetonitrile also afforded a species (λmax )
92, 589, 646 nm) very similar to those obtained in the
electrolysis. The 391 nm species is well characterized as the
anion (Ru -P )(IV), which is reoxidized to Ru -P by PtCl
.
6
-
5
II
(
3
Because the first reduction peak (-0.63 V vs NHE) of Ru -
TMP(CO) in the cyclic voltammetry is considered to be the
2
0a,29
reduction of porphyrin ring,
the presumed reduced species
II
+• 29
I
II
-
cation radical of the complex (Ru -P ). The behavior
described above is very suggestive in elucidating the mechanism
of photoreaction. The appearance of the 391 nm species in the
photoreaction under neutral conditions (but not under alkaline
conditions) strongly suggests that the cation radical is formed
upon light irradiation but that it is trapped by the hydroxide
ion to result in a promotion of the oxygenation cycle. The
coordination of hydroxide ion or water to the metal center of
complex would be reasonable for forming a reactive species
such as a metal-oxo complex for oxygen transfer. The hydroxide
ion also prevents the acidification of the reaction mixture during
Ru would readily result in the formation of Ru -P . The
reoxidization to Ru -P by K PtCl (+0.73 V vs NHE) is
II
2
6
sufficiently exoergonic.
A metal-oxo complex is presumably involved in the present
photochemical oxygenation, without the presence of a powerful
oxidizing agent, wherein the key step is a two-electron oxidative
activation of water, which can be thought of as a four-electron
reduced form of molecular oxygen. This should be in sharp
contrast to the well-known P-450 oxygenation system and its
model reactions, which require either molecular oxygen and a
(
30) (a) McMurry, T. J.; Groves, J. T. Cytochrome P-450: Structure, Mechanism,
and Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum Press: New
York, 1986; pp 1-28. (b) Mansuy, D. Pure Appl. Chem. 1987, 59, 759
and references therein. (c) Meunier, B. Chem. ReV. 1992, 92, 1411 and
references therein.
II
(
(
(
27) The triplet excited energy (E
T
) of Ru TMP(CO) was estimated from the
phosphorescence (729 nm) in EPA.
28) Dean, J. D. Lange’s Handbook of Chemistry; McGraw-Hill Book Co.: New
York, 1985; pp 6-14.
29) (a) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin, D.; James, B. R. J.
Chem. Soc., Chem. Commun. 1981, 982. (b) Barley, M. H.; Dolphin, D.;
James, B. R. J. Chem. Soc., Chem. Commun. 1984, 1499.
(31) (a) Ostovic, D.; Bruice, T. C. Acc. Chem. Res. 1992, 25, 314. (b)
Arasasingham, R. D.; He, G. X.; Bruice, T. C. J. Am. Chem. Soc. 1993,
115, 7985. (c) Traylor, T. G.; Xu, F. J. Am. Chem. Soc. 1988, 110, 1953.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 19, 2003 5739

A R T I C L E S
Funyu et al.
two-electron reduction or powerful oxidizing agents.21,30 The
oxygenation reaction through a metal-oxo complex is especially
crucial from the viewpoint of artificial photosynthesis, because
the complex can directly transfer an oxygen atom derived from
a water molecule to organic compounds, and water acts as a
two-electron donor in the reaction cycle.
JA0295218
5740 J. AM. CHEM. SOC.
9
VOL. 125, NO. 19, 2003