Effect of substituents on TiO2 photocatalytic oxidation of trans-stilbenes

Article abstract of DOI:10.1246/bcsj.20180223

Photocatalytic reaction of trans-stilbene on TiO₂ particles produces benzaldehyde with high selectivity in acetonitrile-water mixed solvent. Introduction of electron-donating substituents to the benzene rings accelerated the reaction. On the other hand, the rate was decelerated by electron-withdrawing substituents. These results suggest that the reaction proceeds by hydroperoxo or hydroperoxy, which were formed on TiO₂ surface by UV irradiation, rather than free OH radicals.

Full text of DOI:10.1246/bcsj.20180223

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Effect of Substituents on TiO2 Photocatalytic Oxidation of trans-Stilbenes
Teruyuki Miyake, Yuichiro Hashimoto, Seihou Jinnai, Ryusei Oketani, and Suguru Higashida*
Advance Publication on the web October 24, 2018
doi:10.1246/bcsj.20180223
© 2018 The Chemical Society of Japan
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Effect of Substituents on TiO2 Photocatalytic Oxidation of trans-Stilbenes
Teruyuki Miyake, Yuichiro Hashimoto, Seihou Jinnai, Ryusei Oketani, and Suguru Higashida*
Osaka Prefecture University College of Technology, 26-12 Saiwai, Neyagawa, Osaka 572-8572
E-mail: <higashida@osaka-pct.ac.jp>
Suguru Higashida was born in Hyogo, Japan. He received B.Sc. from Kwansei Gakuin Univ. in 1986 and M.Ed. from
Osaka Kyoiku Univ. in 1988, and D.Eng in 1999 from Osaka University. In 1988, he became Research Associate at
Osaka Prefectural College of Technology, Japan, and to Associate Professor in 1998, and to Professor in 2010. His
research interests include photocatalytic oxidation of organic compounds and TiO2 photocatalyst.
Abstract
Photocatalytic reaction of trans-stilbene on TiO2 particles
This result suggested that the conversion of phenanthrene to a
lactone compound proceeds via a diacid compound, which is
produced photocatalytically by the cleavage of the double bond
of phenanthrene. This idea urged us to study the photocatalytic
reactivity of the double bonds of stilbene and related
compounds and to compare the reactivities with that of
phenanthrene. Although stilbenes, especially cis-stilbenes, have
structural similarity to phenanthrene, their reactivities are very
different because the double bond connecting the two aromatic
rings of stilbene has little contribution to their aromaticity.
Oxidative cleavage of the alkene double bond of trans-stilbene
leading to benzaldehyde has been one of the well-known
reactions of TiO2 photocatalytic oxidation of aromatic
compounds.9 In the case of non-aromatic alkenes, however, we
found that 1-decene was selectively converted to
1,2-epoxydecane using a TiO2 photocatalyst under oxygen
atmosphere.17 This result suggests that epoxide is formed as an
intermediate in oxidation of trans-stilbene. Confirmation of the
process and the mechanism will be helpful to get more
understanding of the reaction.
produces
acetonitrile-water
benzaldehyde
mixed
with
solvent.
high
selectivity
Introduction
in
of
electron-donating substituents to the benzene rings accelerated
the reaction. On the other hand, the rate was decelerated by
electron-withdrawing substituents. These results suggest that
the reaction proceeds by hydroperoxo or hydroperoxy, which
were formed on TiO2 surface by UV irradiation, rather than
free OH radicals.
Keywords: TiO2, Photocatalyst, Epoxidation
1. Introduction
Utilization of light energy to drive chemical processes has
been one of the most active research areas in chemistry. The
photocatalytic process using a TiO2 photocatalyst has widely
been studied and utilized to decompose pollutants in a mild
condition.1 Most organic compounds included in air or water
can be completely mineralized to CO2 and H2O.2 The reactions
on TiO2 photocatalyst are considered to be caused by reactive
oxygen species such as free hydroxy radical (•OH) and super
oxide radical anion (O2•-), which are generated by
photogenerated electron and hole pairs generated under
ultraviolet light irradiation.3-6 Although the photochemical
reaction on TiO2 photocatalyst is generally believed to proceed
by these oxidative species, detailed mechanisms still remain
unsettled and they seem to be strongly dependent on
experimental conditions.7 Many efforts are now directed
toward clarification of the mechanisms of TiO2 photocatalytic
reactions under different conditions.7-8
Examination of the effect of electron density at the
reactive position on the reactivity is a useful method to clarify
the reaction mechanisms. Stradiotto and co-workers recently
studied the TiO2 photocatalytic oxidation of aromatic rings
having different substituents, and found that electron-donating
groups such as methoxy group accelerate the reaction, while
electron-withdrawing groups such as trifluoro methyl group
decelerate the reaction.18
We thought that it should be
interesting to study the effect of the substituents on the reaction
of stilbenes by using stilbene derivatives having different
electron-donating and electron-withdrawing substituents, as
shown in Scheme 1.
Of photocatalytic reactions, those of aromatic compounds
have been extensively studied using TiO2 photocatalyst.9-12
During the course of their reactions, intermediates having
hydrophilic functional groups such as hydroxy (-OH) and
carboxy (-COOH) groups were produced before the complete
oxidation of the aromatic compounds into CO2 and H2O.11-12
Production of these variety of oxidation products have
motivated us to develop new organic synthetic methods using
TiO2 photocatalyst.13-14 We have reported that phenanthrene is
converted to a lactone derivative by the TiO2 photocatalytic
reaction albeit with a moderate yield.15 We recently found that
an aromatic diacid compound is photocatalytically converted to
lactone as a result of decarboxylation on UV-irradiated TiO2.16
2. Experimental
Scheme 1. Photocatalytic reactions studied; trans-stilbene (1),
mono-substituted trans-stilbenes (2, 3), and di-substituted
trans-stilbenes (4-6) were used as the substrates.

4-methoxybenzyltriphenylphosphoniumchloride was obtained
(3.79 g, 50%). 4-Methoxybenzyltriphenylphosphoniumchloride
(0.92 g, 2.2 mmol) thus obtained was added, together with
p-anisaldehyde (0.41 g, 3.0 mmol) and 3 mL of 1 M aqueous
sodium hydroxide solution, to 6 mL of methanol in a 50 mL
eggplant flask, and the mixture was stirred for 6 h at room
temperature to obtain 4,4’-dimethoxy-trans-stilbene 4. The
resulting product was collected by filtration and washed with
cooled methanol. The NMR data was as follows.
Scheme 2. Synthesis scheme of mono- or di-substituted
trans-stilbenes
In the experiments, reagent-grade stilbene
1 was
4, 4’-dimethoxy-trans-stilbene 4 (isolated yield 33%)
1H-NMR (400 MHz, CDCl3) : δ 3.82 (6H, s), 6.89 (4H, d, J =
8.8 Hz), 6.93 (2H, s), 7.43 (4H, d, J = 8.8 Hz)
purchased from Tokyo Chemical Industry Co. Ltd. and used
without further purification. The substituted stilbenes 2-6, as
shown in Scheme 1, were synthesized by Wittig reaction from
the corresponding substituted aromatic aldehydes and
substituted or non-substituted benzyl bromides (Scheme 2).19
Nuclear Magnetic Resonance (NMR) spectra were obtained on
a Jeol Delta V 5.0 JNM-ECS spectrometer. The chemical shift
values were referenced against for tetramethylsilane for
1H-NMR (0.0 ppm) and against CDCl3 (77.0 ppm) for
13C-NMR.
13C-NMR (100 MHz, CDCl3) : δ 55.31, 114.05, 126.11, 127.38,
130.41, 158.94
Preparation of 4, 4’-dibromo-trans-stilbene 5
4-Bromobenzylbromide (4.50 g, 18 mmol) and
triphenylphosphine (4.72 g, 18 mmol) were added to 10 mL of
toluene in a 100 mL eggplant flask, and the mixture was heated
o
at 100 C for 3 h. The resulting precipitate was collected by
filtration and washed with hexane, and 4-bromobenzyl
triphenylphosphoniumbromide was obtained (8.18 g, 88%).
4-Bromobenzyltriphenylphosphoniumbromide (0.51 g, 1.0
mmol) thus obtained was added, together with
4-bromobenzaldehyde (0.37 g, 2.0 mmol) and potassium
tert-butoxide (0.11 g, 1.0 mmol), to 9 mL of chloroform in a
100 mL eggplant flask, and the mixture was stirred for 44 h at
room temperature to obtain 4,4’-dibromo-trans-stilbene 5. The
resulting precipitate was collected by filtration and washed
with cooled methanol. The NMR data was as follows.
4, 4’-dibromo-trans-stilbene 5 (isolated yield 38%)
1H-NMR (400 MHz, CDCl3) : δ 7.02 (2H, s), 7.37 (4H, d, J =
8.8 Hz), 7.48 (4H, d, J = 8.8 Hz)
Preparation of mono-substituted trans-stilbenes (2 and 3)
Benzylbromide (12.3 g, 72 mmol) and triphenylphosphine
(18.8 g, 72 mmol) were added to 50 mL of toluene in a 200 mL
eggplant flask, and the mixture was heated at 70 oC for 3 h. The
resulting precipitate was collected by filtration and washed
with hexane, and benzyltriphenylphosphoniumbromide was
obtained
(30.1
g,
isolated
yield
97%).
Benzyl
triphenylphosphoniumbromide (0.95 g, 2.2 mmol) thus
obtained was added, together with p-anisaldehyde (3.0 mmol)
and 3 mL of 1 M aqueous sodium hydroxide solution, to 6 mL
of methanol in a 50 mL eggplant flask and the mixture was
stirred for
6
h
at room temperature to obtain
4-methoxy-trans-stilbene 2. Similarly, 4-nitro-trans-stilbene 3
was obtained using p-nitrobenzaldehyde as the aldehyde. The
resulting precipitate were collected by filtration and washed
with cooled methanol. For their purification, the products were
dissolved with ethyl acetate and neutralized with 10 mL of 1%
aqueous hydrochloric acid solution. The organic layer was
sequentially washed with saturated sodium hydrogen carbonate
solution and saturated sodium chloride solution. Then, the
organic layer was dried over anhydrous sodium sulfate and the
solvent was removed in vacuo with a rotary evaporator. The
products (2 and 3) were further purified by silica-gel column
chromatography. Their NMR data were as follows.
13C-NMR (100 MHz, CDCl3) : δ 121.62, 127.99, 128.07,
131.84, 135.84
Preparation of 4, 4’-dicyano-trans-stilbene 6
4-Cyanobenzylbromide (1.76 g, 9.0 mmol) and
triphenylphosphine (2.36 g, 9.0 mmol) were added to 10 mL of
toluene in a 100 mL eggplant flask, and the mixture was heated
o
at 110 C for 7 h. The resulting precipitate was collected by
filtration and washed with hexane, and 4-cyanobenzyltriphenyl
phosphoniumbromide was obtained (3.90 g, 94%).
4-Cyanobenzyltriphenylphosphoniumbromide (0.50 g, 1.1
mmol) thus obtained was added, together with
4-cyanobenzaldehyde (0.14 g, 1.0 mmol) and potassium
4-methoxy-trans-stilbene 2 (isolated yield 35%)
tert-butoxide (0.12 g, 1.1 mmol), to 10 mL of chloroform in
a
1H-NMR (400 MHz, CDCl3) : δ 3.84 (3H, s), 6.89 (2H, d, J =
8.6 Hz), 6.96 (1H, d, J = 16.0 Hz), 7.04 (1H, d, J = 16.0 Hz),
7.20 (1H, td, J = 7.6 Hz and 1.0 Hz), 7.36 (2H, td, J = 7.6 Hz
and 1.0 Hz), 7.44 (2H, d, J = 8.6 Hz), 7.49 (2H, dd, J = 7.6 Hz
and 1.0 Hz)
100 mL eggplant flask, and the mixture was stirred for 17 h at
room temperature to obtain 4,4’-dicyano-trans-stilbene 6. The
resulting precipitate was collected by filtration and washed
with cooled methanol. The NMR data was as follows.
4, 4’-dicyano-trans-stilbene 6 (isolated yield 64%)
1H-NMR (400 MHz, CDCl3) : δ 7.58 (2H, s), 7.85 (4H, d, J =
8.0 Hz), 7.87 (4H, d, J = 8.0 Hz)
13C-NMR (100 MHz, CDCl3) : δ 55.43, 114.10, 126.25, 126.70,
127.26, 128.83, 128.75, 128.91, 130.21, 137.74, 159.38
4-nitro-trans-stilbene 3 (isolated yield 51%)
13C-NMR (100 MHz, CDCl3) : δ 106.26, 114.94, 123.66,
126.50, 128.74, 137.15
1H-NMR (400 MHz, CDCl3) : δ 7.14 (1H, d, J = 16.0 Hz), 7.28
(1H, d, J = 16.0 Hz), 7.33 (1H, td, J = 7.2 Hz and 1.0 Hz), 7.40
(2H, td, J = 7.2 Hz and 1.0 Hz), 7.56 (2H, dd, J = 7.2 Hz and
1.0 Hz), 7.65 (2H, d, J = 8.4 Hz), 8.23 (2H, d, J = 8.4 Hz)
13C-NMR (100 MHz, CDCl3) : δ 124.10, 126.25, 126.84,
127.01, 128.83, 128.88, 133.28, 136.14, 143.37, 146.26
Preparation of di-substituted trans-stilbenes (4, 5 and 6)
Preparation of 4, 4’-dimethoxy-trans-stilbene 4
Photocatalytic Reactions
The photocatalytic reactions of 1-3 were carried out in a
Pyrex-glass test tube (φ = 18 mm), which contained each of 1-3
(0.5 mmol), acetonitrile-4% (w/w) water mixture (10 mL), and
TiO2 powder (40 mg, 0.5 mmol, 1 eq.); the concentration of 1-3
was 50 mmol/L. For the photocatalytic reactions of 4-6, since
their solubility of the symmetric stilbenes was so low, their
reactions were carried out in highly diluted solution (3.57
mmol/L) using 0.1 mmol of the compound; the volume of the
solution was 28 mL. P25 (Nippon Aerosil Co., Ltd.) was used
as the TiO2 photocatalyst for all the reactions, because we had
4-Methoxybenzylchloride (2.82 g, 18 mmol) and
triphenylphosphine (4.72 g, 18 mmol) were added to 10 mL of
toluene in a 100 mL eggplant flask, and the mixture was heated
o
at 100 C for 3 h. The resulting precipitate was collected by
filtration
and
washed
with
hexane,
and

Figure 1. Schematic of set-up for TiO2 photocatalytic
reactions.
found that this powder showed the highest activity for the
oxidation of trans-stilbene. The high photocatalytic activity
of this power for the oxidation of various organic compounds
has been attributed to the synergy effect between rutile and
anatase particles included in it.20
Figure 3. Time courses of photocatalytic reaction of
trans-stilbene and its derivatives; trans-stilbene 1 (black line),
methoxy-substituted trans-stilbene
2
(red line) and
During the reaction, the solution was magnetically stirred
to suspend the TiO2 particles and bubbled with oxygen gas at a
rate of 2.0 mL/min. In the absence of oxygen, the
nitro-substituted trans-stilbene 3 (blue line). Black broken
line stands for benzaldehyde produced from 1. The yield
reached 85% after UV light irradiation for 30 h.
photocatalytic reaction hardly proceeded.
A
500
W
oxygen source. Probably introduction of oxygen atoms to the
product proceeds via surface OH groups, which are supplied
from water molecules
super-high-pressure mercury lamp was used as the light source,
and deep UV component (λ < 350 nm) was filtered out with a
UV35 glass filter (Hoya Corporation) to prevent
photoexcitation of the substrates (Figure 1).
If the same type of reactions takes place for the reactions
of mono-substituted stilbenes, the expected products by the
oxidative cleavage of the double bond are benzaldehyde and
the corresponding mono-substituted benzaldehydes. They are
indeed observed in the products by the HPLC analysis (Figures
2b and 2c). However, we also observed some other products.
In the case of 2, which has a methoxy-substituent, an epoxide
and the cis-trans isomerized products were identified (Figure
2b). From 3, which has a nitro-substituent, more different
kinds of products were produced, judging from the appearance
of many small unassignable peaks in the HPLC chart (Figure
2c).
Concerning the conversion rates of the stilbenes, 2
showed the increased rate and 3 showed the decreased rate
compared with the non-substituted stilbene 1, as shown in
Figure 3. The results show the tendency of the conversion
rate to increase by the introduction of an electron-donating
group and to decrease by the introduction of an
electron-withdrawing group. However, concerning the effect of
the electron donating group, the increase in the rate should
partly be attributed to the production of the epoxide and
cis-trans isomerization.
All of the reactions were carried out at room temperature.
As the reactions proceeded, a small portion of the solution was
periodically sampled and analyzed by high performance liquid
chromatography (HPLC) with an instrument (HITACHI
L-6000 series) equipped with an octadecyl-silica column.
Mixture of acetonitrile and water (7:3) was used as an eluent.
3. Results and Discussion
Photocatalytic reaction of non-substituted trans-stilbene 1,
which was carried out as a control experiment, produced
benzaldehyde, as shown in Figure 2a. The yield reached 85%
after UV light irradiation for 30 h (Figure 3). However,
benzaldehyde was not obtained when the reaction was carried
out in acetonitrile containing no water. These results indicate
that 1 is oxidized with high selectivity using water as the
In order to further confirm the effect of the substituents,
introduction of the same substituents to both benzene rings of
stilbene at para-position is helpful. Using these substrates, the
analysis of the products becomes much easier because the same
substituted aldehydes are produced from a stilbene molecule by
the cleavage of the double bond. When di-substituted stilbenes
(4-6) are oxidatively cleaved at the double bond, the expected
products are 4-methoxy benzaldehyde (from 4, dimethoxy
group), 4-bromo benzaldehyde (from 5, dibromo group), and
4-cyanobenzaldehyde (from 6, dicyano group). In addition,
the effect of the substituents is expected to be enhanced
because of the doubled substituents.
In these experiments, the initial concentration was fixed
at 3.57 mmol/L for all the di-substituted and non-substituted
stilbenes and their reactivities were compared. Interestingly,
production of the cis forms was scarce from these symmetric
stilbenes, and the selectivity of the formation of the aldehydes
Figure 2. HPLC charts for the solutions of trans-stilbene
and mono-substituted trans-stilbenes before (starting
material) and after the TiO2 photocatalytic reaction; (a)
trans-stilbene 1, (b) methoxy-substituted trans-stilbene 2,
and (c) nitro-substituted trans-stilbene 3. Since the
reactivity of 3 was very low, photoirradiation was
continued for 20 h.

Scheme 3. Proposed mechanism of photocatalytic
Production of aldehydes from trans-stilbenes.
Figure 4. Time courses of photocatalytic reaction of
di-substituted and non-substituted trans-stilbenes; (a) decay
of substrates and (b) rise of products. Substrates used are
dimethoxy trans-stilbene 4 (red line), dibromo trans-stilbene
5 (blue line), non-substituted trans-stilbene 1 (black line), and
dicyano trans-stilbene 6 (magenta line). The products are
4-methoxy benzaldehyde (from 4, red line), 4-bromo
benzaldehyde (from 5, blue line), benzaldehyde (from 1,
black line), and 4-cyanobenzaldehyde (from 6, magenta line).
1,2-dioxetane by singlet molecular oxygen (1O2), which is
photochemically produced from molecular oxygen (3O2). Then,
the 1,2-dioxetane is automatically cleaved to aldehydes. In
this process, the formation of 1O2 is the key, which is produced
either by direct photoexcitation of 3O2 in the ground state or by
energy transfer from photoexcited substrates in their triplet
3
states to O2. Both of these processes proceed only under deep
UV light. In some earlier studies on TiO2 photocatalytic
reactions of stilbenes, these processes may have been
involved.9 However, in the present study, they do not
contribute to the actual reactions because we carefully removed
the deep UV component using the UV cut-off filter. Another
possibility of production of aldehydes from alkenes is the path
via epoxide. Once epoxide is formed, aldehydes are produced
by hydrolysis of the epoxide bond. In chemical epoxidation
of alkenes, 3-chloroperbenzoic acid (mCPBA) is commonly
used as the oxidant, and the reaction is known by the name of
Prilezhaev epoxidation.22 This reaction is driven by the
electrophilic oxygen O (δ+) in the OOH group of mCPBA and
is accelerated by introduction of electron-donating groups to
the alkenes.23 This tendency is in agreement with the results of
the photocatalytic reactions that stilbenes having electron
donating substituents show higher reactivity and also higher
yield of aldehydes, as mentioned above. In addition, we did
observe the photocatalytic production of epoxide from
methoxy-substituted stilbene and it was compared with the
standard sample by HPLC analysis, as shown in Figure 2b.
Taking into accounts these facts, we propose the
mechanism of photocatalytic production of aldehydes from
stilbenes, as shown in Scheme 3. First, titanium-based
was generally high except 6, which has electron-withdrawing
substituents. For 4 and 5, which have electron-donating
substituents, the rate of the reaction was higher than that of
non-substituted stilbene 1, as shown in Figure 4. The decay
rates for these stilbenes seem to be faster than the rise rates of
the products, which seems to be due to the formation of
epoxides as intermediates, as discussed later. The yields of
aldehydes at the stage of 10 h-irradiation reached 80% for 4,
and about 65% for 1 and 5.
These results clearly show the tendency of the rate of the
double-bond cleavage to increase by the introduction of
electron-donating substituents and to decelerate by the
introduction of electron-withdrawing substituents, which is the
same tendency observed using mono-substituted stilbenes.
It should be noted that benzoic acids, which are
commonly produced by conventional chemical oxidation of
stilbenes, were scarcely included in the products of the TiO2
photocatalytic reactions of 1-6 as far as the reactions were
continued for less than 10 h. We consider that, if the reactions
had been continued for longer time, benzoic acids would have
been produced from the aldehydes.
In photochemistry, production of aldehyde from stilbene
has been well known.21 In this case, stilbenes are converted to

peroxide species are formed on the surface of TiO2 by the holes
photogenerated in TiO2.24 Second, epoxides are produced from
stilbenes by surface hydroperoxo or hydroperoxy, rather than
free OH radicals, in a manner like Prilezhaev epoxidation.
Lastly, epoxide 7 is converted to aldehydes. Alternatively,
this final process may proceed via 1,2-diol compound 8,
feasibility of which was confirmed using 8 (R = H) as the
starting material ; the yield of benzaldehyde was obtained 92%
for 9 h by HPLC analysis. However, we couldn’t determine
two passes, because 1,2-diol compound 8 was not isolated due
to adsorption to TiO2 surface. As a whole, we consider that the
oxygen insertion to the double bond is the critical step, and that
the electron-donating groups introduced into stilbene enhance
this process. So far, many authors have suggested that free
hydroxy radicals (•OH) and super oxide radical anions (O2•-),
especially the former, are the active species in photocatalytic
oxidation of organic compounds. However, it is difficult to
explain our findings about the reaction of stilbenes on the basis
on the chemistry of these species. Although further intensive
studies are necessary to confirm the mechanisms, our proposed
mechanism, which give importance to the TiO2 surface in
photocatalytic reactions, may be useful to deepen the
understanding the mechanisms of many other photocatalytic
oxidation processes of organic compounds.
8.
9.
T. Ohno, K. Sarukawa, M. Matsumura, New J. Chem.
2002, 26, 1167.
M. A. Fox, M. Chanon, Photoinduced electron transfer
reactions: inorganic substrate and application, Elsevier
1988, 1.
10. T. D. Bui, A. Kimura, S. Ikeda, M. Matsumura, J. Am.
Chem. Soc. 2010, 132, 8453.
11. H. Yoshida, H. Yuzawa, M. Aoki, K. Otake, H. Itoh, T.
Hattori, Chem. Commun. 2008, 4634.
12. X. Pang, C. Chen, H. Ji, Y. Che, W. Ma, J. Zhao,
Molecules 2014, 19, 16291.
13. D. Friedmann, A. Hakki, H. Kim, W. Choi, D.
Bahnemann, Green Chem. 2016, 18, 5391.
14. Y. Shiraishi, T. Hirai, J. Photochem. Photobiol. C
Photochem. Rev. 2008, 9, 157.
15. S. Higashida, A. Harada, R. Kawakatsu, N. Fujiwara, M.
Matsumura, Chem. Commun. 2006, 2804.
16. R. Oketani, T. Miyake, S. Jinnai, T. Fukui, H. Tsujimoto,
M. Matsumura, S. Higashida. Chem. Lett. 2016, 45, 801.
17. T. Ohno, K. Nakabeya, M. Matsumura, J. Catal, 1998, 1,
76.
18. M. Bettoni, T. Del Giacco, F. Elisei, C. Rol, G. V.
Sebastiani, M. Stradiotto, J. Photochem. Photobiol. A
Chem. 2016, 324, 159.
19. S. Ceyhan, Y. Cetinkaya, A. Akdag, M. Balci,
Tetrahedron 2016, 72, 6815.
4. Conclusion
In literature photocatalytic reactions have mostly been
discussed in terms of mineralization of harmful or undesirable
materials. In these applications, photogeneration of strong
oxidation species is imperable and such species as free OH
radicals have been assumed to be involved in the efficient
photocatalytic reactions. However, it is also known that
different kinds of intermediates are produced, sometimes with
high selectivity, before their complete mineralization. Our
results showed that electron-donating and electron-withdrawing
substituents introduced to trans-stilbenes gave specific effects
on their TiO2-photocatalyzed reactions. The results suggested
that the reactions proceeded by hydroperoxo or hydroperoxy,
which were formed on TiO2 surface by UV irradiation, rather
20. T. Ohno, K. Tokieda, S. Higashida, M. Matsumura,
Applied Catalysis A: General 2003, 244. 383.
21. J. Eriksen, C. S. Foote, T. L. Parker, J. Am. Chem. Soc.
1977, 99, 6455.
22. P. Nikolaus. Ber. Dtsch. Chem. Ges. 1909, 42, 4811.
23. D. Swern, Org. React. 1953, 7, 378.
24. T. Ohno, T. Mitsui, M. Matsumura, J. Photochem.
Photobiol. A Chem. 2003, 160, 3.
than OH radicals.
We consider that discussion of
photocatalytic reactions on the basis of organic chemistry will
be important to deepen their understanding, which may also
contribute to open new methods in organic synthetic chemistry.
Acknowledgements
The authors are thankful to Dr. Michio Matsumura,
Professor Emeritus of Osaka University, for his suggestions
about the mechanisms of photocatalytic reactions.
This work was partly supported by Japan Society for the
Promotion of Science (JSPS) KAKENHI Grant Number
JP26107006 in Scientific Research on Innovative Areas
"Photosynergetics", and Kansai Research Foundation for
technology promotion (KRF).
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