Selective photooxidation of styrene in organic-water biphasic media

Article abstract of DOI:10.1039/b411031e

The photooxidation of styrene by molecular oxygen was investigated in organic-water biphasic media at room temperature under UV light irradiation. The photoreaction gave a high selectivity (>90%) of the product, benzaldehyde, with a formative yield of 29%. This new method was also operative for the oxidation of several styrene derivatives, such as 3-methylstyrene, 4-methoxylstyrene, β-methylstyrene, and 1,1-diphenylethylene, into the corresponding aldehydes or ketones. It was demonstrated that both molecular oxygen and water were essential to these reactions. The mechanistic study revealed that the primary reaction involved mainly electron transfer from the excited styrene to triplet ground state molecular oxygen, forming the superoxide radical anion and styrene radical cation, followed by cleavage of the C=C bond via an adduct intermediate in the presence of water.

Full text of DOI:10.1039/b411031e

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P A P E R
Selective photooxidation of styrene in organic–water
biphasic media
Yanjun Ren, Yanke Che, Wanhong Ma, Xinzhi Zhang, Tao Shen and Jincai Zhao*
Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry,
The Chinese Academy of Sciences, Beijing, 100080 China. E-mail: jczhao@iccas.ac.cn;
Fax: +86-10-82616495
Received (in Montpellier, France) 19th July 2004, Accepted 20th August 2004
First published as an Advance Article on the web 9th November 2004
The photooxidation of styrene by molecular oxygen was investigated in organic–water biphasic media at
room temperature under UV light irradiation. The photoreaction gave a high selectivity (490%) of the
product, benzaldehyde, with a formative yield of 29%. This new method was also operative for the oxidation
of several styrene derivatives, such as 3-methylstyrene, 4-methoxylstyrene, b-methylstyrene, and
1,1-diphenylethylene, into the corresponding aldehydes or ketones. It was demonstrated that both molecular
oxygen and water were essential to these reactions. The mechanistic study revealed that the primary reaction
involved mainly electron transfer from the excited styrene to triplet ground state molecular oxygen, forming
the superoxide radical anion and styrene radical cation, followed by cleavage of the C C bond via an
adduct intermediate in the presence of water.
Q
years.34–35 Sheldon et al. reported catalytic oxidation of alcohol
in an environmentally friendly aqueous medium.36
1. Introduction
Selective oxidation of hydrocarbons is one of the most im-
portant reactions in organic synthesis.1–2 Stoichiometric oxida-
tion processes, such as the epoxidation of alkenes by peracids,
alkylperoxides and iodosobenzene,3–10 have been widely used.
However, those processes are expensive and produce large
amounts of toxic by-products. Therefore, the exploration of
a new oxidation process that has a minimal environment
impact and low cost is necessary and remains a great challenge.
A ‘green’ oxidation process, using hydrogen peroxide or,
ideally, molecular oxygen as the oxidant and water as the
solvent, is the most attractive oxidation technology for selec-
tive organic synthesis and the treatment of organic pollu-
tants.11–21 The key issue in the use of molecular oxygen as an
oxidant is product selectivity. Homogeneous catalytic oxida-
tion by O2 under thermal conditions in general exhibits very low
selectivities due to side reactions that are hard to control.1–2
In this regard, photoassisted oxidation processes can hold
special promise. For example, photoinduced reactions of O2
in many cases give some products that cannot be obtained by
dark reactions. More importantly, photochemical reactions
can be conducted at ambient temperature, which may minimize
the probability of secondary thermal reactions occurring and
thus improve the product selectivity.
Recently, Frei et al. have reported the selective photooxida-
tion of hydrocarbons by O2 in the gas phase over alkali- and
alkaline-earth-exchanged zeolites under visible light irradia-
tion.22–26 The reactions involve photoexcitation of a hydro-
carbon–O2 charge-transfer state. Tung et al. investigated the
microreactor-controlled product selectivity in O2-mediated
photooxidation of alkenes.27–31 Remarkable product selectivity
has been achieved in such a microreactor-controlled photo-
reaction. Ramamurthy et al. studied singlet-oxygen-mediated
photooxidation of olefins within dye-exchanged X and Y
zeolites.32–33 By the utilization of zeolite supercages as ‘‘an
active reaction cavity’’, the active oxygen has been directed
toward a particular face of the olefins and thus a high
selectivity was obtained. Organic reactions in water, on the
other hand, have attracted extensive attention in recent
In this work, we report a new approach for the selective
photooxygenation of styrene and its derivatives by O2 in
organic–water media under ambient conditions (Scheme 1).
The photoreaction gave a high selectivity of the product,
benzaldehyde. The reaction mechanism was investigated by
the aid of several characterization experiments including NMR
and ESR. The mechanistic study demonstrated that the pri-
mary reaction involves mainly electron transfer from the
excited styrene to triplet ground state molecular oxygen.
2. Experimental
2.1. Materials
Styrene, 4-methoxylstyrene, 3-methylstyrene, 1,1-diphenylethy-
lene, cis-stilbene, trans-stilbene, trans-b-methylstyrene, cyclo-
hexene, 1-octene, benzaldehyde, styrene oxide, benzoic acid,
1-phenyl-1,2-ethanediol, p-anisoldehyde, 3-methylbenzaldehyde,
benzophenone, cyclohexene oxide, 2-cyclohexen-1-one, and
2-cyclohexen-1-ol were purchased from Aldrich Co., Ltd., and
were used without further purification. Acetonitrile, acetone,
methanol, ethanol, chloroform and hydrogen peroxide were of
analytical reagent grade. 5,5-Dimethyl-1-pyrroline-N-oxide
(DMPO, Sigma) and 2,2,6,6-tetramethylpiperidine (TEMP,
Acros) were used as the ESR spin-trapping reagents. Deionized
and doubly distilled water was used throughout this study.
2.2. Photoreaction conditions
A 100 W medium pressure Hg lamp (Toshiba Lighting and
Technology of Japan) was used as the UV light source. Unless
otherwise noted, all the irradiation experiments were carried
out in a Pyrex vessel (l ¼ 330–400 nm). In a typical reaction,
Scheme 1
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
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the substrate (styrene: 100 ml, 0.9 mmol) and the solvent (5 ml)
were added to the vessel (20 ml) which was then sealed with a
septum. The reaction mixture was stirred and irradiated by
ultraviolet light under an oxygen atmosphere at ambient
pressure and temperature for the desired time. After the
reaction, the oxygenated products were extracted by diethyl
ether and analyzed by GC and GC/MS using toluene as an
internal standard.
2.3. Instrumentation and product analysis
Gas chromatographic analysis for the reaction kinetics was
performed on a Hitachi G-3900 GC spectrometer using a
hydrogen flame ionization detector, equipped with an integra-
tor processor. Nitrogen was used as the carrier gas. The inner
column (length 30 m; internal diameter 0.25 mm, film thickness
0.25 mm) was packed with a SGE BP-5 5% phenyl–methyl
siloxane film. The reaction products were identified by com-
parison of their retention times with the corresponding stan-
dard samples, using a Finnigan TRACE DSQ GC/MS
spectrometer with electron ionization. For GC analysis of the
styrene oxidation reaction, the following GC conditions were
used: the oven temperature was set at 100 1C, injection
temperature at 200 1C, and detection temperature at 250 1C.
Formaldehyde was analyzed on an HP SP-502 GC spectro-
meter.
The reaction yields were calculated on the basis of all
oxidation products. The selectivity was the fraction of benzal-
dehyde or the aldehydes among the oxidation products formed
from the photooxidation of styrene or the styrene derivatives.
Electron paramagnetic resonance (EPR) experiments were
conducted on a Brucker Model ESP 300E spectrometer at
room temperature. The irradiation source was a Quanta-Ray
Nd:YAG pulsed laser system (l = 355 nm; 10 Hz). The
reaction radicals were trapped by DMPO and TEMP. The
ESR center field was set at 3486.70 G, sweep width at 100 G,
microwave frequency at 9.82 GHz, modulation frequency at
100 KHz, and power at 5.05 mW. To minimize measurement
errors, the same quartz capillary tube was used throughout the
EPR measurements. In order to detect the active oxygen
species effectively, a small amount of dimethyl sulfoxide
(DMSO) was added to the styrene/H2O system since styrene
is insoluble in water.
Fig. 1 The product yield of benzaldehyde obtained after 11 h of
reaction under different conditions, bubbling with dioxygen during
reaction: (A) in aqueous medium under UV irradiation; (B) in aqueous
medium in the dark; (C) in ethanol under UV irradiation; (D) in
O2-free aqueous medium under UV irradiation.
media. In ethanol (i.e., absence of water; C), no benzaldehyde
was formed under the same conditions. In the absence of
dioxygen (D), the photooxidation of styrene did not occur in
either aqueous media or organic solvents such as chloroform,
acetone, acetonitrile, methanol, and ethanol. Therefore, both
water and dioxygen are essential to initiate the styrene photo-
oxygenation. In addition, in aqueous media the yield of
benzaldehyde reached 29% when dioxygen was continuously
added during reaction, whereas the yield was 15% if dioxygen
was added only before the UV irradiation. On the other hand,
the solution pH values and ionic strength had little influence on
the photooxygenation of styrene by O2.
3.2. Photooxygenation of other olefins with dioxygen
The photooxygenation of other terminal and internal alkenes
with dioxygen was also examined, for which 3-methylstyrene,
4-methoxylstyrene, 1,1-diphenylethylene, trans-stilbene, cis-
stilbene, trans-b-methylstyrene, cyclohexene and 1-octene were
selected as model substrates (Table 1). The experimental results
indicate that aromatic terminal alkenes such as 3-methylstyr-
ene, 4-methoxystyrene, 1,1-diphenylethylene can be selectively
oxidized to the corresponding carbonyl compounds with high
selectivity. The oxygenated products are 3-methylbenzalde-
hyde, 4-methoxybenzaldehyde and benzophenone, respec-
tively. Similar to styrene oxidation, the oxygenated product
yields and selectivity in aqueous media are all much higher
than those in organic solvents such as acetonitrile, chloroform
and acetone, under the same conditions, while the photo-
oxygenations did not occur in alcohol media. Interestingly,
the aromatic internal olefin, trans-b-methylstyrene, can also be
selectively oxygenated into benzaldehyde. The yield and selec-
tivity of benzaldehyde in aqueous media was also higher than
in organic solvents, such as acetonitrile. Almost 100% selec-
tivity could be achieved in aqueous media. This suggests that
Nuclear magnetic resonance (NMR) experiments were ex-
amined on a Bruker Avance DRX500 spectrometer using
tetramethylsilane (TMS) as an internal standard. Similarly, a
small amount of acetone was added to the mixture of styrene
and water in the 1H-NMR experiments so as to investigate the
interaction between styrene and water. Since the water reso-
nance obscures a large part of the spectrum, spin-spin relaxa-
tion methods were used to eliminate the water resonance.40,41
By making the spin-spin relaxation period long enough, the
water resonance can be completely eliminated and sample
resonances at the frequency of the water resonance can be
observed.
the C C double bond of the aromatic internal olefins can be
Q
oxidatively cleaved by O2 in aqueous media under UV irradia-
3. Results and discussion
tion. However, oxidative cleavage of the C C double bond of
Q
3.1. Effect of light, dioxygen and water on the oxygenation
of styrene
trans-stilbene and cis-stilbene could not be achieved in either
aqueous media or organic solvents under identical conditions.
Instead, cis-stilbene was photoisomerized to trans-stilbene,
possibly due to the stability of the conjugated phenyl substi-
tuents in the stilbene molecule. The oxidative cleavage of the
The effects of light, molecular oxygen and water on the
oxygenation of styrene were investigated under different reac-
tion conditions. Fig. 1 shows the product yield of benzalde-
hyde, obtained after 11 h. In the dark (B), styrene was hardly
oxygenated by dioxygen. Only a trace amount of benzaldehyde
(less than 1% of the initial styrene) was found in the reaction
mixture. Under ultraviolet light irradiation (A), selective oxy-
genation of styrene to benzaldehyde with a yield of 29% and a
high selectivity of 91% was achieved in the aerated aqueous
C C double bond of cyclohexene by O did not occur under
Q
2
UV irradiation in both aqueous and organic media. Only a
small amount of the autooxidation products, 2-cyclohexen-
1-one, 2-cyclohexen-1-ol and cyclohexene oxide, were found
in the reaction mixture. 1-Octene could not be oxidized
into carbonyl compound under identical conditions. This is
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Table 1 Photooxygenation of other olefins with dioxygena
Substrate
PhCH CH
Reaction media
Reaction time/h
Main product
PhCH
% Yield
% Selectivity
H2O
CH3CN
H2O
7
7
O
Q
15
3
91
83
80
73
98
66
88
Q
2
PhCH
O
Q
(Ph) C CH
Q
6
(Ph) C O37
8
Q
2
2
2
CH3CN
H2O
6
(Ph) C
2
O
1
Q
12
12
12
PhCH
O
18
6
Q
CH3CN
H2O
PhCH
O
Q
b
38
10
CH3CN
H2O
12
12
12
12
12
1
—
—
—
—
65
—
—
—
—
No reaction
No reaction
Isomerization
Isomerization
CH3CN
H2O
CH3CN
39
H2O
12
5
92
CH3CN
H2O
12
12
12
2
Traces
Traces
83
—
—
CH3CN
1-Octene
H2O
CH3CN
12
12
No reaction
No reaction
b
—
—
—
—
a
Reaction conditions: olefin 100 ml, solvent 5 ml, bubbling with dioxygen before irradiation. 4-Methoxystyrene 50 ml.
possibly due to the fact that both cyclohexene and 1-octene do
not absorb UV light (l ¼ 330–400 nm).
organic solvents, such as acetonitrile, chloroform and acetone,
the selectivity towards styrene oxide is much higher than that in
aqueous media (Table 2). In aqueous media, further photo-
oxygenation of benzaldehyde with dioxygen did not occur so
that only a small amount of benzoic acid (1%) was found in the
reaction products. Formation of 1-phenyl-1,2-ethanediol is
possibly due to the attack of water on the intermediate in the
reaction process. On the other hand, photooligomerization of
styrene was observed in aqueous media.
The influence of the mixed solvent chloroform–water on
styrene photooxidation was also studied (Table 3). It was
found that the photooxygenation of styrene was noticeably
enhanced upon addition of H2O into chloroform under UV
irradiation. For example, the yield of benzaldehyde was in-
creased by almost twofold (from 6 to 11%), when 10% H2O
(v/v) was added. Correspondingly, the selectivity of benzalde-
hyde also increased with increasing water content, from 78%
(in 10% H2O) to 85% (in 90% H2O). Thus, both the yield and
the selectivity of benzaldehyde are noticeably increased upon
addition of H2O into chloroform solutions.
In alcohol media, photooxygenation of styrene with dioxy-
gen under UV irradiation seems to be prevented. Less than 1%
benzaldehyde was observed, even after 11 h irradiation with
ultraviolet light. Upon addition of water to methanol solutions
(Fig. 3), the yield of benzaldehyde was noticeably increased.
When 80% water was added into methanol, the yield of
benzaldehyde increased to 8%. In aqueous media, the yield
of benzaldehyde was 15% under otherwise identical condi-
tions. These data suggest that addition of water can change the
photooxygenation pathway of styrene in alcohol solutions,
significantly promoting the photooxygenation of styrene with
dioxygen.
3.3. Effect of different solvents on the photooxygenation
of styrene
We also investigated the photooxygenation of styrene in
different solvents by O2 (Fig. 2). The distribution of oxyge-
nated products in the different media are illustrated in Table 2.
The photooxygenation yield of styrene with dioxygen is the
highest in aqueous media, but the lowest in methanol or
ethanol media. This shows that the nature of the solvent has
a dramatic effect on styrene photooxygenation. The yield of
benzaldehyde obtained in the aqueous, acetonitrile, chloro-
form, acetone, methanol and ethanol media was 29%, 6%,
6%, 5%, 1% and 1%, respectively. In alcohol media, photo-
oxygenation of styrene with dioxygen was scarcely observed.
ESR experiments indicated that no active oxygen species were
detected in alcohol reaction systems (see section 3.5 below),
probably since alcohols quench the styrene radical cations or
active oxygen species. The high yield (29%) and the high
selectivity (91%) of benzaldehyde may be related to the fact
that water noticeably accelerates the photooxygenation of
styrene by O with cleavage of the C C bond, forming mainly
Q
benzaldehyde with only traces of other oxides. By contrast, in
2
3.4. Photooxygenation of styrene with hydrogen peroxide
In aqueous media, when hydrogen peroxide was employed as
the oxidant for styrene photooxygenation under deaerated
conditions, only small amounts of benzaldehyde were observed
(less than 1% yield). However, in some organic solvents, such
as chloroform, acetonitrile and acetone, the yield of benzalde-
hyde became somewhat higher (Fig. 4). These results suggest
Fig. 2 The yield of benzaldehyde obtained from styrene photooxida-
tion in different solvents under UV irradiation. Reaction conditions:
styrene 0.9 mmol, solvent 5 ml, reaction time: 7 h, bubbling with
dioxygen during reaction.
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Table 2 Product distribution from the oxygenation of styrene with dioxygen under UV irradiationa
% Selectivity
Reaction media
% Yield (all products)
PhCH
O
PhCOOH
PhCH(OH)CH2OH
Q
H2O
17
3
91
83
76
84
—
—
1
13
1
3
7
1
CH3CN
CHCl3
Acetone
CH3OH
Ethanol
a
9
22
16
1
1
2
Trace
Trace
Trace
Trace
Trace
Trace
1
Trace
Trace
1
Reaction conditions: styrene 0.9 mmol, solvent 5 ml, charging with dioxygen before irradiation, reaction time: 7 h.
Table 3 Effect of mixed CHCl3–H2O solvent on the photooxygenation of styrene under UV irradiationa
% Selectivity
Solvent (by volume)
% Yield PhCH O
Q
PhCH
O
PhCOOH
PhCH(OH)CH2OH
Q
100% CHCl3
10% H2Ob
40% H2O
50% H2O
80% H2O
90% H2O
100% H2O
a
6
11
10
12
14
16
29
76
78
82
82
85
85
91
22
2
1
7
8
5
4
5
1
1
13
6
4
7
6
3
8
5
5
1
7
b
Reaction conditions: styrene 0.9 mmol, solvent 5 ml, reaction time: 12 h, bubbling with dioxygen during reaction. i.e., the ratio H2O : CHCl3 =
1 : 9 (v/v).
that the pathway of styrene photooxygenation by H2O2 is
different from that of O2 oxidation in aqueous media. When
both dioxygen and hydrogen peroxide were used simulta-
neously as the oxidant, the photooxygenation of styrene
proceeded effectively in aqueous media, but the yield of
benzaldehyde was lower than when using O2 alone. This
implies that the presence of dioxygen can promote the photo-
oxygenation of styrene in aqueous media, while the presence of
hydrogen peroxide may quench the photooxidation of styrene
in aqueous media. Interestingly, in alcohol media, the photo-
oxygenation of styrene with hydrogen peroxide resembles
somewhat that using dioxygen as oxidant. In alcohol media,
the photooxygenation of styrene by H2O2 could not proceed
effectively, the same as when using O2.
to eliminate the water resonance.40,41 NMR experiments in-
dicated that the presence of water had a great influence on the
chemical shifts of styrene. The chemical shifts of the hydrogens
of both the aromatic ring and the C C double bond were
Q
shifted to lower field. For instance, the chemical shift of
methylene ( CH ) moved from 5.133, 5.657 ppm to 5.226,
Q
5.792 ppm, the methenyl group (–CH ) signal shifted from
2
Q
6.633 ppm to 6.745 ppm and the aromatic ring signal from
7.274 ppm to 7.431 ppm. The hydrogen bond and the polar-
ization interaction between styrene and water molecules alter
the electron density of the styrene to a lower value, thus its
chemical shifts become bigger. The presence of water molecules
activates the C C double bond.
Q
The formation of hydrogen peroxide was observed during
styrene photooxygenation by O2 in aqueous media under
ultraviolet light irradiation, but its concentration was very
low. In order to verify the reaction mechanism, we compared
the oxidation reactions of styrene using Rose Bengal as a
sensitizer to generate singlet oxygen and by the photo-Fenton
3.5. Photooxidation reaction mechanism
1H-NMR techniques were used to investigate the interaction of
styrene and water. Since the water resonance obscures a large
part of the spectrum, spin-spin relaxation methods were used
Fig. 3 Changes of benzaldehyde yield with water content in the
photooxidation of styrene under UV irradiation in mixed alcohol–
water solution. Reaction conditions: styrene 0.9 mmol, solvent 5 ml,
purged with dioxygen before irradiation, reaction time: 8 h.
Fig. 4 Benzaldehyde yield using H2O2 as the oxidant for styrene
oxidation under UV irradiation in different solvents. Reaction condi-
tions: styrene 0.9 mmol, solvent 5 ml, H2O2 1 mmol, deaerated before
irradiation, reaction time: 8 h.
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Fig. 5 EPR signals of the DMPO–dOOH adducts as a function of irradiation time (left: styrene–H2O–DMSO system; right: styrene–CH3OH
system) in the dark and under UV irradiation. Styrene: 0.2 M, DMPO: 0.04 M.
reaction (Fe2+/H2O2) to produce hydroxyl radicals. The sen-
sitized photooxygenation of styrene was carried out in an
oxygen-saturated aqueous media of Rose Bengal under visible
light irradiation (l 4 420 nm). The photo-Fenton oxidation of
styrene was conducted under UV irradiation (l ¼ 330–400 nm)
in deaerated aqueous media. However, the yields of benzalde-
hyde from both the photosensitized oxidation and the photo-
Fenton reaction, 1% and 2%, respectively, after 12 h of
reaction, were much lower than the 29% obtained from styrene
photooxygenation under UV irradiation in oxygen-saturated
aqueous media. This demonstrates that the reactive intermedi-
ates for the styrene photooxygenation are neither singlet oxy-
gen, generated by photosensitization of Rose Bengal, nor
hydroxyl radical, generated by the photo-Fenton reaction.
The photooxygenation of styrene in water under UV irra-
diation may involve an electron transfer mechanism. In agree-
transfer occurs in the charge-transfer complex43 of styrene and
dioxygen, generating the superoxide radical anion and styrene
radical cation. Due to the polarization interaction between
styrene and a water molecule, the electron density of styrene is
altered so that the intermediate adducts are cleaved into
benzaldehyde and formaldehyde in the presence of water.
The formation of formaldehyde was confirmed by GC analysis
during the photooxygenation of styrene with dioxygen in
aqueous media. However, the amount of formaldehyde de-
tected in the reaction mixture was relatively low due to easy
photo-oligomerization of formaꢀldehyde. In comparison runs,
the active oxygen species (O2 d, OHd, or 1O2) were not
detected in the photoreaction performed in alcoholic solution
(Fig. 5). Probably the alcohol molecules quench the styrene
radical cation or the active oxygen species, leading to the
lower yield of the photooxygenation product of styrene with
dioxygen.
ꢀ
d
ment with this idea, the reactive intermediates, O2 and then
dOOH, were detected experimentally by EPR techniques
(Fig. 5). No EPR signals were observed when the reaction was
performed in the dark in aerated aqueous media. Under UV
irradiation, the characteristic sextet peaks of DMPO–dOOH/
4. Conclusions
A new method for benzaldehyde synthesis via the photooxida-
tion of styrene in organic–water biphasic media using molecu-
lar oxygen as the oxidant under ambient conditions was
reported. The study shows that both dioxygen and water are
essential to achieve the photooxygenation of styrene into
benzaldehyde with high yield and selectivity. The primary step
of this reaction involves mainly electron transfer from the
excited styrene to molecular oxygen, which gives the super-
oxide radical anion and styrene radical cation as intermediates;
ꢀ
d
O2 appeared gradually in the aerated aqueous media, and
their intensity increased with irradiation time. No EPR signals
ꢀ
d
of DMPO–dOOH/O2
adducts were observed in aerated
methanol media when the reaction was performed either in
the dark or under UV irradiation. Furthermore, upon addition
of SOD, an efficient quencher of O2 ꢀ, the photooxygenation
d
reaction of styrene was observed to be prevented. On the other
hand, with the addition of NaN3, an 1O2 quencher, the
photooxygenation reactions of styrene decreased only slightly.
It is well-known that 1,1-diphenylethylene cannot react with
singlet oxygen,42 but it could be easily be oxidized into
benzophenone with high selectivity under the present condi-
tions (Table 1). Based on the above results, we propose that the
reactive intermediate involved in the styrene photooxidation
process is mainly the superoxide radical anion. A possible
photoreaction mechanism is proposed in Scheme 2. Upon
irradiation with UV light, the excited styrene molecule trans-
fers its electron to triplet ground state dioxygen, or an electron
successive reactions involve cleavage of the C C bond in the
Q
adduct intermediate in the presence of water. By this method,
several other aromatic terminal alkenes and internal alkenes
can be also photooxygenated to the corresponding carbonyl
compounds.
Acknowledgements
For this work, the generous financial support by the Ministry
of Science and Technology of China (No. 2003CB415006), by
the National Science Foundation of China (No. 50221001,
No. 20133010, No. 20277038, and No. 20371048) and by the
Chinese Academy of Sciences is gratefully acknowledged.
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