Photocatalytic aerobic epoxidation of alkenes under visible light irradiation by an iron(III) porphyrin with Mg-Al hydrotalcite anionic clay

Article abstract of DOI:10.1246/cl.2009.1098

In photocatalytic aerobic oxidation of cyclohexene catalyzed by 5, 10, 15, 20-tetrakis(pentafluorophenyl)porphyriniron(III) chloride [Fe(TPFPP)Cl] under visible light irradiation, the coexistence of solid bases was found to enhance the selectivity to cyclohexene oxide drastically. In the absence of solid bases, however, only the allylic oxidation of cyclohexene was catalyzed by the porphyrin complex acting as a sensitizer. Among the various metal oxides investigated, hydrotalcite exhibited the highest conversion of cyclohexene and selectivity to cyclohexene oxide. Copyright

Full text of DOI:10.1246/cl.2009.1098

1098
Chemistry Letters Vol.38, No.11 (2009)
Photocatalytic Aerobic Epoxidation of Alkenes under Visible Light Irradiation
by an Iron(III) Porphyrin with Mg–Al Hydrotalcite Anionic Clay
Kentaro Teramura,Ã1 Kentaro Ogura,2 Takashi Sugimoto,2 Hideo Tsuneoka,2 Tetsuya Shishido,2 and Tsunehiro TanakaÃ2
1Kyoto University Pioneering Research Unit for Next Generation, Kyoto University, Kyoto 615-8510
2Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510
(Received July 22, 2009; CL-090684; E-mail: kentaro.teramura@kupru.iae.kyoto-u.ac.jp)
In photocatalytic aerobic oxidation of cyclohexene cata-
present. Chloroform (solvent) was used as such without purifica-
tion. The photocatalytic reaction was carried out in a closed
batch system using Pyrex glass vessels; the reaction mixture
was illuminated via a glass window, and a rubber balloon was
used to maintain the system at atmospheric O2 pressure. The
synthesized Mg–Al hydrotalcite (100 mg) was dispersed in a
chloroform solution (4 mL) of Fe(TPFPP)Cl (5:0 Â 10À2 mM)
and cyclohexene (12.5 mM). The liquid suspension was first
bubbled with O2 for 5 min and then irradiated with a 200-W
Hg-Xe lamp equipped with fibre optic waveguide, a collective
lens, and a mirror. The organic products were analysed with
an FID gas chromatograph and a GC-mass spectrometer.
lyzed by 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-
iron(III) chloride [Fe(TPFPP)Cl] under visible light irradiation,
the coexistence of solid bases was found to enhance the selec-
tivity to cyclohexene oxide drastically. In the absence of solid
bases, however, only the allylic oxidation of cyclohexene was
catalyzed by the porphyrin complex acting as a sensitizer.
Among the various metal oxides investigated, hydrotalcite ex-
hibited the highest conversion of cyclohexene and selectivity
to cyclohexene oxide.
The one-step insertion of oxygen atoms into hydrocarbons
(alkanes and alkenes) to afford alcohols, aldehydes, ketones,
and epoxy compounds is a highly attractive reaction since par-
tially oxidized derivatives are raw materials for various products
such as polymers, aromatic compounds, agricultural chemicals,
drugs, and synthetic compounds. Molecular oxygen is the most
desirable oxidizing agent if it can be activated at ambient tem-
peratures and atmospheric pressure. Photocatalytic systems can
be used for selective and environmentally friendly oxidation
with molecular oxygen as the oxidant. Molecular oxygen is ac-
tivated by certain photocatalysts and in some photocatalytic re-
action systems, although enhancement of the photocatalytic ac-
tivity of these catalysts is presently difficult.1–3 Our research
group has investigated the environmentally friendly selective
oxidation of various hydrocarbons in gas4–7 and liquid8–11 phases
using orthovanadate [(V=O)O3]-like species dispersed on vari-
ous supports as photocatalysts. In addition, it is reported that
MnIII porphyrin complexes adsorbed on solid bases enhance
the catalytic activity in epoxidation reactions.12 Solid bases in
the reaction system may interact with the MnIII porphyrin com-
plex and change the electronic state of Mn. This implies that
novel photocatalytic aerobic oxidations can be carried out using
a combination of homogeneous and heterogeneous catalysts. In
this study, we investigate the aerobic epoxidation of cyclohex-
ene in a novel photocatalytic reaction system under visible light
irradiation using FeIII porphyrin as the homogeneous catalyst
and various metal oxides as heterogeneous catalysts.
Fe(TPFPP)Cl was purchased from Sigma-Aldrich Corp. and
used as such without purification. Mg–Al hydrotalcite anionic
clay was synthesized by the following coprecipitation method.
An aqueous solution containing magnesium nitrate and alumi-
nium nitrate was slowly added to an aqueous NaOH solution
at room temperature and pH 10.0, and the resulting solution
was aged for 12 h at 333 K. The solution was then filtered, and
the filtrate was washed with distilled water and dried at 378 K
for 12 h in an oven. Prior to use, cyclohexene (substrate) was dis-
tilled for removal of water and impurities and subsequently
passed through an alumina column for removal of any peroxides
In this study, we employed pure Fe(TPFPP)Cl and
H2TPFPP (free-base porphyrin) as the homogeneous catalyst
for the epoxidation of cyclohexene under visible light irradiation
at ambient O2 pressure and room temperature. Small amounts of
selective oxidation products were hardly obtained in the case of
Fe(TPFPP)Cl. When H2TPFPP is used for the photocatalytic
aerobic oxidation of cyclohexene, only 2-cyclohexen-1-ol and
2-cyclohexen-1-one (allylic oxidation products) are obtained,
suggesting that H2TPFPP acts as a sensitizer to generate singlet
oxygen species under visible light irradiation. We carried out the
photocatalytic aerobic oxidation of an alkene using a porphyrin
complex such as Fe(TPFPP)Cl and found that certain solid bases
dramatically enhance the conversion of alkene and the selectiv-
ity to the epoxy compound. Figure 1 shows the yields of the se-
lective oxidation products obtained in the photocatalytic aerobic
oxidation of cyclohexene in the presence of Fe(TPFPP)Cl and
12
cyclohexene oxide
10
2-cyclohexen-1-ol
2-cyclohexen-1-one
8
cyclohexanol
cyclohexanone
6
4
2
0
HT*
MgO CaO ZnO Al2O3 ZrO2 TiO2 SiO2 Nb2O5 Ta2O5 WO3
Figure 1. Yields of selective oxidation products for the photo-
oxidation of cyclohexene in the presence of O2 by various solid
metal oxides with Fe(TPFPP)Cl after 8 h of visible light irradia-
tion: cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexen-
1-one, cyclohexanol, and cyclohexanone from the left.
Fe(TPFPP)Cl: 5:0 Â 10À2 mM, cyclohexene: 12.5 mM, solid
material: 100 mg, solvent: chloroform, volume: 4 mL. ÃHT:
Hydrotalcite.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.11 (2009)
1099
Table 1. Yields of selective oxidation products for the photoox-
idation of various alkenes in the presence of O2 by Fe(TPFPP)Cl
with hydrotalcite after visible light irradiationa
various solid metal oxides after 8 h of visible light irradiation.
We found that a solid base catalyst is necessary to obtain epoxy
compounds in the proposed photocatalytic reaction system.
Solid bases such as MgO, CaO, and ZnO enhanced the conver-
sion of cyclohexene to cyclohexene oxide with high selectivity,
while amphoteric metal oxides such as Al2O3, ZrO2, and TiO2
showed a moderate additive effect. However, inert materials
such as SiO2 and solid acids such as Ta2O5, Nb2O5, and WO3
did not contribute to the enhancement of the photocatalytic ac-
tivity. The photocatalytic activity, therefore, was thought to de-
pend only on the acid–base property of the added solid metal ox-
ide. Using solid base catalysts with a layered hydrotalcite struc-
ture, maximum conversion of cyclohexene to cyclohexene oxide
could be achieved with high selectivity to cyclohexene oxide.
Oxidation of cyclohexene under visible light irradiation in the
presence of Fe(TPFPP)Cl and hydrotalcite afforded cyclohexene
oxide as the main product and 2-cyclohexen-1-ol, 2-cyclohexen-
1-one, cyclohexanol, and cyclohexanone as the by-products.
These products were hardly formed when the reaction was car-
ried out in the presence of Fe(TPFPP)Cl or hydrotalcite, indicat-
ing that the Fe–porphyrin complex photocatalytically works
in coordination with hydrotalcite. We concluded that both
Fe(TPFPP)Cl, a homogeneous catalyst, and a solid base such
as hydrotalcite, which acts as a heterogeneous catalyst, enhance
the yield of the product formed in the novel photocatalytic oxi-
dation reaction under visible light irradiation.
Reaction time Conversionb
Substrates
Products (Selectivityc/%)
/h
/%
OH
O
OH
O
20
52.4
O
(54.5)
(51.0)
(50.8)
(62.6)
(2.4)
(6.4)
(3.0)
(3.8)
O
20
20
48.0
40.8
O
21.5
n. d.
74.5
n. d.
O
(5.1)
O
aFe(TPFPP)Cl: 5:0 Â 10À2 mM, cyclohexene: 12.5 mM, solid
material: 100 mg, solvent: chloroform, volume: 4 mL.
bConversion: consumption of substrate normalized to initial
amount of substrate. cSelectivity: amount of product normalized
to consumption of substrate.
tive aerobic epoxidation of certain alkenes under visible light ir-
radiation.
We optimized the reaction conditions (concentrations of
cyclohexene, Fe(TPFPP)Cl, and added hydrotalcite) for the
photocatalytic aerobic epoxidation of cyclohexene in the
Fe(TPFPP)Cl/hydrotalcite system. The initial rate of formation
of cyclohexene oxide was 2.35 mmol hÀ1. The amounts of the se-
lective oxidation products increased with the photoirradiation
time, corresponding to the diminution of cyclohexene concentra-
tion. After 20 h of photoirradiation, the conversion of cyclohex-
ene and the selectivity to cyclohexene oxide were 52.4 and
54.5%, respectively. The turnover number (TON) of cyclohex-
ene upon the addition of Fe(TPFPP)Cl reached 131. Apparent
quantum efficiency of the photocatalytic aerobic epoxidation
of cyclohexene by Fe(TPFPP)Cl with hydrotalcite is 0.61% at
411 nm. The amounts of the selective oxidation products and di-
minution rate of cyclohexene were virtually constant after 20 h
because of the decomposition of Fe(TPFPP)Cl. No Fe(TPFPP)Cl
was detected in the solution after 24 h of photocatalytic reaction,
as was confirmed from the UV–vis absorption spectra. Even af-
ter the porphyrin complex was completely consumed, the selec-
tive oxidation products could be regenerated by adding fresh
Fe(TPFPP)Cl to the reaction mixture. Table 1 shows the yields
of the selective oxidation products obtained in the photocatalytic
aerobic oxidation of various alkenes using the Fe(TPFPP)Cl/hy-
drotalcite system under visible light irradiation. Cyclic alkenes
such as cyclooctene and norbornene and aromatic alkenes such
as styrene could be oxidized in chloroform under visible light
irradiation to selectively afford the corresponding epoxy deriva-
tives; however, 1-hexene was not oxidized under these condi-
tions. The above results showed that the Fe(TPFPP)Cl/hydrotal-
cite system functions as an effective photocatalyst for the selec-
This study was partially supported by the Program for Im-
provement of Research Environment for Young Researchers
from Special Coordination Funds for Promoting Science and
Technology (SCF) commissioned by the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
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Published on the web (Advance View) October 24, 2009; doi:10.1246/cl.2009.1098