Photooxygenation of aromatic alkenes in zeolite nanocavities

Article abstract of DOI:10.1039/b205672k

Aromatic alkenes such as styrene (1), 1,1-diphenylethene (2), and cis- and trans-stilbenes (3), but not triphenylethene (4), formed their contact charge-transfer (CCT) complexes with O₂ in solutions. In zeolite NaY, the CCT absorption band was observed only for 1 and 3. Irradiation of alkenes 1-4 included in the zeolite nanocavities under O₂ produced benzaldehyde and benzophenone as the major oxygenation products. In particular, for 3 and 4, the photooxygenation competed with a photoelectrocyclic reaction, which subsequently yielded phenanthrenes as the exclusive photoproducts under O₂ in solution. It is likely that the oxygenation products were produced through the alkene cation radicals and superoxide anion radical generated by excitation of the CCT complexes and/or photoinduced electron transfer from the excited alkenes to O₂. YAG laser (266 nm) excitation of 1 included in the zeolite cavities under vacuum produced its alkene cation radical and the trapped electron, Na₄³⁺, both of which were quenched by O₂. On the basis of the optimum structure for the guest molecules obtained by semi-empirical molecular orbital calculations (AM1), it is suggested that the photooxygenation reaction was regulated by the electrostatic interaction between the guest molecules and alkali-metal cations in the nanocavities as well as by the strong electrostatic field which stabilized the ion radical pairs generated.

Full text of DOI:10.1039/b205672k

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Photooxygenation of aromatic alkenes in zeolite nanocavities
Masanobu Kojima,*a Miyuki Nakajoh,a Chie Matsubarab and Shuichi Hashimotoc
a Department of Bioscience and Biotechnology, Shinshu University, Kami-ina,
Nagano 399-4598, Japan. E-mail: mkojima@gipmc.shinshu-u.ac.jp;
Fax: ϩ81-265-77-1629; Tel: ϩ81-265-77-1627
2
b The United Graduate School of Agricultural Science, Gifu University, Gifu 501-1193, Japan
c Chemistry Department and Advanced Engineering Courses, Gunma College of Technology,
Maebashi, Gunma 371-8530, Japan
Received (in Cambridge, UK) 11th June 2002, Accepted 12th August 2002
First published as an Advance Article on the web 26th September 2002
Aromatic alkenes such as styrene (1), 1,1-diphenylethene (2), and cis- and trans-stilbenes (3), but not triphenylethene
(4), formed their contact charge-transfer (CCT) complexes with O2 in solutions. In zeolite NaY, the CCT absorption
band was observed only for 1 and 3. Irradiation of alkenes 1–4 included in the zeolite nanocavities under O2
produced benzaldehyde and benzophenone as the major oxygenation products. In particular, for 3 and 4, the
photooxygenation competed with a photoelectrocyclic reaction, which subsequently yielded phenanthrenes as the
exclusive photoproducts under O2 in solution. It is likely that the oxygenation products were produced through the
alkene cation radicals and superoxide anion radical generated by excitation of the CCT complexes and/or
photoinduced electron transfer from the excited alkenes to O2. YAG laser (266 nm) excitation of 1 included in the
zeolite cavities under vacuum produced its alkene cation radical and the trapped electron, Na43ϩ, both of which were
quenched by O2. On the basis of the optimum structure for the guest molecules obtained by semi-empirical molecular
orbital calculations (AM1), it is suggested that the photooxygenation reaction was regulated by the electrostatic
interaction between the guest molecules and alkali-metal cations in the nanocavities as well as by the strong
electrostatic field which stabilized the ion radical pairs generated.
Introduction
Finding ways to regulate the photophysical and photochemical
behavior of organic molecules by means of organized and
constrained media has recently become an interesting and
important area of research.1–3 As one of the more promising
media, zeolite nanocavities have attracted much attention in the
past ten years with respect to the stabilization of the photo-
excited species4 and the enhancement of intersystem crossing
for organic guest molecules by metal ions,5 in addition to the
steric effect6–9 and the regioselectivity10 due to the restricted
spaces. We are particularly intrigued by the phenomenon,
reported by Frei et al.,11–15 in which an enhanced intermolecular
interaction takes place between aliphatic alkenes such as
2,3-dimethylbut-2-ene and an oxygen molecule (O2) in the
alkali-metal cation-exchanged Y zeolites. Surprisingly, the
contact charge-transfer (CCT) complexes formed by the inter-
action exhibited extremely strong absorption spectra up to the
visible wavelength region. Excitation of the CCT absorption
band using a visible laser light seemed, on examination of the
infrared spectra, to cause an electron transfer from the alkenes
Fig. 1 Aromatic alkenes used for photooxygenation in CH2Cl2 and in
NaY.
to O2, which generated the monomer alkene cation radicals and
superoxide anion radical (O2 Ϫ), and finally formed oxygenation
ؒ
products through the dioxetanes and hydroperoxides.11–15 We
have been studying the photoinduced electron transfer (PET)
reaction of the CCT complexes in solutions between aromatic
alkenes like styrenes and O2.16–21 This study aims to establish
whether zeolite nanocavities have potential as photochemical
reaction vessels for the CCT complexes of aromatic alkenes.
In a previous paper,22 we reported that stilbenes included in
alkali-metal cation-exchanged Y zeolites were photooxygenated
by excitation of the CCT complexes. In this paper, we first
report in detail on the differences in the spectroscopic and
photochemical behavior of aromatic alkenes [styrene (1),
1,1-diphenylethene (2), stilbenes (3), and triphenylethene (4);
Fig. 1] under O2 in zeolites and in solutions. Secondly, we
suggest that the zeolite nanocavities stabilize not only the CCT
complexes but also the ion radical intermediates generated by
excitation of the CCT complexes and/or PET reaction between
the guest molecules and O2. This stabilization is probably
due to the strong electrostatic field. Finally, it is suggested that
the electrostatic interaction of metal ions with the guest
molecules in the zeolite nanocavities is another important
factor in regulating the photooxygenation of organic guest
molecules, particularly for 1,2-diphenylethenes 3 and 4.
1894
J. Chem. Soc., Perkin Trans. 2, 2002, 1894–1901
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b205672k

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Measurements of CCT absorption spectra
Experimental
Change in absorption spectra caused by interaction between
1–4 and O2 in MeCN was measured using a Shimadzu UV-2100
UV-visible double-beam spectrophotometer, as reported in the
case of 1,1-diarylethenes.21 For the zeolite samples, the spectra
were measured using a Shimadzu ISR-260 integrating sphere
assembly attached to the spectrophotometer and a 30 × 30 × 4
mm quartz cell. In the solution the absorption band attributed
to the CCT complexes was observed using a 0.1 mm path-
length quartz cell for a solution of 2 × 10Ϫ2 M 1 and a 10 cm
path-length quartz cell for the solutions of 1 M 1, 0.05 M 2, 3a,
and c-3b, and 0.002 M t-3b; however, the CCT absorption band
for 4 could not be seen using a 0.1 M solution. On the other
hand, in the case of the zeolite samples, the new absorption
band attributed to the CCT complexes appeared only for 1 and
3 (excluding c-3a), but not for 2 or 4. It was confirmed that the
CCT absorption band was formed reversibly by removing and
introducing O2.
Materials
Alkenes 1 and 2 were commercial products from Wako Pure
Chem. Ind., Inc. cis- (c-3a) and trans-stilbene (t-3a) and 4 were
purchased from Aldrich, and trans-4,4Ј-dimethoxystilbene
(t-3b) was obtained from Acros Organics. cis-4,4Ј-Dimethoxy-
stilbene (c-3b) was prepared by irradiation of t-3b in benzene
under air using a high-pressure mercury lamp (Riko UVL-
400HA) and isolated by column chromatography. All the
aromatic alkenes were purified by distillation under reduced
pressure or recrystallized prior to use. Zeolite NaY (SiO2,
67.3%; Al2O3, 20.3%; Na2O, 12.4%; Lot No. 3001) and silica gel
60 (particle size, 0.040–0.063 mm; 230–400 mesh ASTM) were
obtained from Tosoh and Merck, respectively.
3,6-Dimethoxyphenanthrene (14b) was prepared as follows: a
100 mL solution of 2 × 10Ϫ3 M (1 M = 1 mol dmϪ3) t-3b in
MeCN was irradiated under nitrogen using a high-pressure
mercury lamp through a Pyrex filter at room temperature for
3 h. After the solvent was removed using a rotary evaporator,
Irradiation of alkene in solution
the products were analyzed by means of H and 13C NMR
1
The alkenes dissolved in CH2Cl2 (0.1 M for 1, 0.05 M for 2 and
4, 1 × 10Ϫ3 M for 3) were irradiated under O2 and under
degassed conditions in a Pyrex tube using a high-pressure
mercury lamp (>290 nm; effective excitation wavelength, 313
nm). The starting alkenes and products were analyzed using
GC and HPLC.
(JEOL Alpha-400 NMR spectrometer) and were identified
as the corresponding isomeric dihydrophenanthrenes (15b).
When these were oxidized by addition of a small amount of
iodine to the solution, 14b was yielded as the sole product; 14b:
MS(m/z) 238 (Mϩ, 100%), 223 (45%), 195 (50%), 152 (43%);
13C NMR(CDCl3) δ = 155.1, 131.0, 130.0, 127.1, 124.2, 116.5,
1
104.3, 55.6 (OMe); H NMR(CDCl3) δ = 7.96 (d, 2H), 7.79
(d, 2H), 7.56 (s, 2H), 7.24 (dd, 2H), 4.02 (s, 6H, OMe).
Irradiation of zeolite and silica gel samples
The zeolite and silica gel samples (200–500 mg) were irradiated
in a quartz cell (30 × 30 × 4 mm) under O2 and under vacuum
at room temperature using 313 nm light from a high-pressure
mercury lamp and 254 nm light from a low-pressure mercury
lamp (Riko UVL-160LA). After irradiation the photoproducts
for the zeolite samples were extracted with 50 mL of CH2Cl2
and 0.1–1 mL of water, filtered using a PTFE [poly(tetrafluoro-
ethylene)] membrane filter, and analyzed by GC and HPLC. In
the case of the silica gel sample, the products were extracted
with 50 mL of methanol and analyzed by GC.
Measurements of oxidation potentials
The oxidation potentials (Eox) of 1–4 were determined in
MeCN by means of cyclic voltammetry using platinum as work-
ing and counter electrodes, Ag/Agϩ as a reference electrode, and
0.1 M tetraethylammonium perchlorate as a supporting electro-
lyte.21 For irreversible oxidation, the peak potential was taken
as the oxidation potential: Eox/V = 1.66 (1), 1.50 (2), 1.16 (t-3a),
1.25 (c-3a), 0.70 (t-3b), and 0.70 (c-3b) and 1.20 (4).23
Loading of alkenes into zeolite cavities
Molecular orbital calculation
NaY (1 g) was activated at 500 ЊC under air for 20 h using an
EYELA electric furnace TMF-1000. The activated zeolite,
cooled in a desiccator, was added into a 50 mL anhydrous
pentane or cyclohexane solution containing 10–40 mg of 1–4.
The mixture was stirred for 1 h and then kept in the dark at
room temperature. After 24 h the zeolite sample was filtered,
washed with anhydrous pentane or cyclohexane, and dried
under vacuum. The filtrate for 1, 2, and 4 was analyzed, in order
to confirm that the alkenes had been completely adsorbed
in the zeolite cavities, using Shimadzu GC-8A and GC-14A gas
chromatographs with a flame ionization detector and, respect-
ively, either a G-100 or a G-450 capillary column (Chemicals
Evaluation and Research Institute) attached. The filtrate for 3
was analyzed using a Tosoh CCP8020 high-pressure liquid
chromatograph (HPLC) with a Tosoh silica-60 packed column
attached. The number of guest molecules over the number of
available supercages <S> was 0.15, 0.46, and 0.61 for 1, 0.11
and 0.43 for 2, 0.19 for 3, and 0.08 for 4.
Geometry optimization for 1–4 and their complexes which
interacted with sodium ion (Naϩ) was carried out using the
AM1 method in MOPAC 97 installed in CSC Chem3D Ver. 4.5
for Windows.
Diffuse reflectance laser photolysis
The experimental set-up for a nanosecond diffuse reflectance
laser photolysis, applicable to the detection of transient species
in optically inhomogeneous and light-scattering systems, was
similar to that described previously.24 We adopted the approach
reported by Wilkinson et al.25 in which percentage absorption is
employed to describe the transient optical absorption signal
following laser excitation of the guest-doped zeolite:
absorption (%)(λ, τ) = 100[1 Ϫ R(λ, τ)/R0(λ, τ)]
where R and R0 denote the intensities of the diffuse reflected
light with and without excitation, respectively. The transient
absorption spectra were corrected for luminescence by subtract-
ing a laser-only shot signal from the initial signal trace.
Adsorption of 4 on silica gel
Silica gel (1 g) was activated at 500 ЊC under air for 5 h using
an electric furnace. The activated silica gel, cooled to room
temperature in a desiccator, was added into a 20 mL anhydrous
pentane solution containing 10 mg of 4. The mixture was
stirred for 10 min and then kept in the dark at room tempera-
ture without a stopper in order to remove the solvent. After 10
h, the silica gel sample adsorbing 4 was dried under vacuum, as
in the preparation of the zeolite samples.
Results and discussion
Formation of the CCT complexes
As in the cases of 4-substituted styrenes, 1-arylcyclohexenes,
and 1,1-diarylethenes,16–21 the absorption band attributable to
J. Chem. Soc., Perkin Trans. 2, 2002, 1894–1901
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the CCT complexes comprising 1 and O2 in MeCN were
observed using a 10 cm path-length cell in a 310 to 450 nm
wavelength region, as can be seen in Fig. 2a. When the
Fig. 3 Diffuse reflectance absorption spectra for 1 (a: <S> = 0.15) and
t-3a (b: <S> = 0.19) in NaY observed under O2 (0.025–1 atm).
for 1 and 3, in contrast to the case of aliphatic alkenes reported
by Frei et al.11–15 In the case of 2 and c-3a, which formed the
CCT complexes with O2 in MeCN, no new absorption band
attributable to the CCT complexes was observed. This indicates
that zeolite nanocavities do not necessarily favor the formation
of the CCT complexes for aromatic alkenes. As discussed later,
this is probably due to the electrostatic interaction of the
alkenes with the metal cations in the cavities.
Fig. 2 Absorption spectra for CCT complexes comprising 1 and O2 in
MeCN: (a) 1 M and (b) 2 × 10Ϫ2 M.
absorption spectra for 1 were measured using a 2 × 10Ϫ2
M
solution and a 0.1 mm path-length cell under O2, the intensity
of the absorption band between 230–260 nm was ca. 0.15
greater than that measured under argon, whereas no significant
spectral change was observed in the 310 to 450 nm wavelength
region, as can be seen in Fig. 2b. The increase in intensity of the
absorption band was reversed by exchanging argon for O2;
therefore, this spectral change is due to the formation of the
CCT complex between 1 and O2. Because the absorption
spectra for the CCT complexes were previously measured at
higher concentrations, the CCT absorption band was observed
only in wavelength regions longer than those attributed to the
organic substrates.26–28 The present results obtained for 1 in a
lower concentration indicate that the CCT absorption band
may overlap with that of the substrate itself, as can be seen
in UV absorption spectra for 1 and aromatic hydrocarbons
measured in the cryogenic oxygen matrix.29 The CCT
absorption bands were also observed for 2 and 3 in MeCN in
the 310–360 and 375–500 nm regions, respectively.
In NaY the new absorption was observed under O2 only for
1 and 3 (though not for c-3a), as shown in Fig. 3. For 1 the
intensity of the absorption at around 250 nm increased with
increase in the partial pressure of O2, as can be seen in Fig. 3a;
in addition, it was found that this spectral change could be
reversed by changing the partial pressure. Thus, we attributed
the new absorption to the formation of the CCT complex. The
CCT band observed was similar to that of 1 in a lower concen-
tration in MeCN (Fig. 2b), although the weak absorption band
could also be seen in a longer wavelength region in the case of
t-3a and t-3b.22 For the zeolite samples, a CCT absorption
band overlapping with that of the guest molecules was clearly
observed by means of diffuse reflectance spectra, similar to the
case of 4-methoxystyrene and methoxybenzene adsorbed in
NaY.30 However, no significant enhancement in the formation
of the CCT complexes was seen in the visible wavelength region
On the basis of the oxidation potentials measured by cyclic
voltammetry, 4 (Eox/V = 1.20 vs. Ag/Agϩ) is more electron-
donating than 1 and 2 (1.67 and 1.50 V). However, the CCT
complex comprising 4 and O2 was not formed either in MeCN
or in NaY. Therefore for the formation of the CCT complex,
both the formation of structures with less steric hindrance as
well as the property of donating electrons are important. As in
1
the reaction of O2 with a carbon–carbon double bond, the
CCT complex would require a strict geometry of oxygen with
the π systems.31
Photochemical reaction of aromatic alkenes under O2 in solution
and in zeolite
(a) For phenylethene (1). Irradiation of 1 in CH2Cl2 under
argon using 313 nm light yielded [2 ϩ 2] and [2 ϩ 4] cyclodimers
9 as the dominant products (distribution of product, 95%) as
found by Brown (Entry 1 in Table 1).32 However, under O2,
production of benzaldehyde (5a, 44%) and styrene oxide
(7, 20%) competed with the dimer formation (30%), as shown in
eqn. (1) and Entry 2, Table 1. When a zeolite sample of 1 (<S>
= 0.46) adsorbed in NaY was irradiated under vacuum using
313 nm light, cyclodimers 9 were the dominant products (84%,
Entry 3) as in the photoreaction in CH2Cl2 under argon.
Formation of 14% 5a is probably due to a small amount of
air leaking in during irradiation. It is worth noting that form-
ation of 9 was entirely suppressed when the zeolite sample was
irradiated under O2 and that 5a was formed in a yield of 75%
(Entry 4). Phenylacetaldehyde (6) was produced only in NaY as
a photooxygenation product; however, it was confirmed that 7
was unstable in the cavities and readily gave 6 in the dark.
Although photodimerization under vacuum in NaY was seen
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J. Chem. Soc., Perkin Trans. 2, 2002, 1894–1901

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Table 1 Photooxygenation of 1 in CH2Cl2 and in NaY
Distribution of productsb (%)
Entry
Conditions
Conversion (%)
Total yielda.(%)
5a
6
7
8
9
c
1
2
3
4
5
6
Ar/CH2Cl2/>290 nm
O2/CH2Cl2/>290 nm
11
21
7
20
5
41
43
9
62
30
29
2
44
14
75
7
—
—
—
22
—
17
3
—
6
2
3
—
Trace
95
30
84
Trace
93
c
20
—
—
—
—
c d
Vacuum/NaY/>290 nm
c d
O2/NaY/>290 nm
d e
Vacuum/NaY/254 nm
d e
O2/NaY/254 nm
10
73
10
a Total yield of products based on the amount of 1 consumed. b Yield (%) based on the initial amount of 1 = conversion × (total yield/100) ×
(distribution of products/100). c Irradiated through a Pyrex filter using a high-pressure mercury lamp (effective excitation wavelength, 313 nm) for
2 h. d <S> = 0.46. e Irradiated through a quartz filter using a low-pressure mercury lamp for 2 h.
Table 2 Photooxygenation of 2 in CH2Cl2 and in NaY
Distribution of productsc (%)
Entry
Conditionsa
Conversion (%)
Total yieldb (%)
10
11
12
13
1
2
3
4
Degassed/CH2Cl2
O2/CH2Cl2
3
35
3
—
43
—
50
—
51
—
74
—
—
—
20
—
21
—
—
—
28
—
6
Vacuum/NaYd
O2/NaYd
53
a Irradiated through a Pyrex filter using a high-pressure mercury lamp (>290 nm; effective excitation wavelength, 313 nm) for 5 h. b Total yield of
products based on the amount of 2 consumed. c Yield (%) based on the initial amount of 2 = conversion × (total yield/100) × (distribution of
products/100). d <S> = 0.11.
to proceed exclusively, the efficiency was lower than that in
CH2Cl2 (see the conversion and total yield in Entries 1 and 3).
This may be due to the small number of <S> and also to the
fact that the diffusion rate of guest molecules between the
cavities was slower than that in solution.33 When the zeolite
sample was irradiated using 254 nm light (Entries 5 and 6), the
product distribution was similar to that obtained using 313 nm
light, although the CCT absorption maximum was observed
around 250 nm, as can be seen in Fig. 3a.
may occur through an autooxidation mechanism, as reported
for 2-phenylpropene and 2-methyl-1,1-diphenylpropene.34
Accordingly, NaY might be an unfavorable environment for
autooxidation, as in the case of 1, due both to the small number
of <S> and to the fact that the diffusion rate of guest molecules
between the cavities is slower than that in solution.
(2)
(1)
(c) For 1,2-diphenylethene (3). When c- and t-3a were
irradiated using 313 nm light under O2 in CH2Cl2, PhH, MeCN,
and cyclohexane, phenanthrene (14a) was yielded quantitatively
in all the solvents as the final photostable product [eqn. (3)].
As mentioned already, the CCT complexes for 3a are formed
in solutions; however, the result clearly shows that the photo-
chemical reaction through excitation of the complexes does not
occur at all in solutions. This is probably because a photo-
electrocyclic reaction in solutions occurs much faster than
photooxygenation.
(b) For 1,1-diphenylethene (2). We have already reported on
a study of the photochemical reactions of the CCT complexes
between 1,1-diarylethenes and O2, in which 3,3,6,6-tetraaryl-
1,2-dioxanes and benzophenones were the major products
through the PET reaction of the complexes.20,21 With respect to
2, benzophenone (10) was analyzed in the present study
together with other photooxygenation products in order to
re-examine the reaction in detail, as shown in Table 2 and eqn.
(2). Under degassed conditions in CH2Cl2 and under vacuum in
NaY (Entries 1 and 3), no significant photoreaction occurred
using 313 nm light. In contrast, diphenylmethane (11),
1,1-diphenylethanol (12), and diphenylacetaldehyde (13)
together with 10 were found to be the main oxygenation
products. Both in CH2Cl2 and in NaY, 10 was obtained as the
major product (51 and 74%, respectively); however, it should be
noted that, on the basis of the conversion and distribution of
the products (Entries 2 and 4), the reactivity of 2 and product
selectivity to give 10 was ca. 20% greater in NaY than in
CH2Cl2. Although 11 was yielded as a specific product in NaY,
suppressing the formation of 12 and 13, it was found that 11
was a secondary product from 13. Therefore, the photo-
decomposition of 13 to 11 seems to occur more readily in NaY
than in solution. Although an enhancement of the fragmen-
tation might be an interesting characteristic of the zeolite
cavities, we have not pursued this area because it was not a
photoreaction specific to the cavities. The production of 12
(3)
For c- and t-3b, on the other hand, irradiation of the
solutions under O2 gave a different product distribution;
namely, no dimethoxyphenanthrene 14b was produced, and
J. Chem. Soc., Perkin Trans. 2, 2002, 1894–1901
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Table 3 Product distribution yielded on irradiation of 3 in NaY under O2
Distribution of products (%)
Entry
3
Conditionsa b
Total yieldc (%)
c-3
t-3
14
5
d
1
2
3
4
5
6
c-3a
c-3a
t-3a
t-3a
c-3b
t-3b
NaY/C6H12
NaY
NaY/C6H12
NaY
NaY
NaY
82
24
86
43
47
46
83
74
24
56
82
46
17
11
76
30
12
38
Trace
5
Trace
7
—
—
—
10
—
7
6
16
d
a Irradiated through a Pyrex filter using a high-pressure mercury lamp (>290 nm; effective excitation wavelength, 313 nm) for 4 h. b <S> = 0.19.
c Total yield of starting 3 and products. d Irradiated while being stirred in cyclohexane.
Table 4 Photooxygenation of 4 in CH2Cl2 and in NaY and on silica gel
Distribution of
productsc (%)
Entry
Conditionsa
Conversion (%)
Total yieldb (%)
5a
10
17
1
2
3
4
5
6
Degassed/CH2Cl2
O2/CH2Cl2
0
38
6
49
21
83
—
90
9
24
6
—
5
—
33
—
3
—
12
—
29
—
5
—
83
100
38
100
92
Vacuum/NaYd
O2/NaYd
Vacuum/silica gel
O2/silica gel
60
a Irradiated through a Pyrex filter using a high-pressure mercury lamp (>290 nm; effective excitation wavelength, 313 nm) for 5 h. b Total yield of
products based on the amount of 4 consumed. c Yield (%) based on the initial amount of 4 = conversion × (total yield/100) × (distribution of
products/100). d <S> = 0.08.
instead, dimethoxydihydrophenanthrene isomers 15b were
formed together with 4-methoxybenzaldehyde (5b). It has been
reported that some dihydrophenanthrenes formed from the
corresponding cis-stilbenes are stable even under O2; therefore,
the addition of oxidants stronger than O2 is useful in the
preparation of phenanthrenes.35,36 In fact, when 2 × 10Ϫ3 M t-3b
was irradiated in MeCN under nitrogen using a high-pressure
mercury lamp for 3 h, isomeric 15b was observed using
NMR. Addition of a small amount of iodine into the isomers
produced 14b as the sole product. Therefore, it was found that
15b was particularly stable under O2 and oxidized very slowly to
14b.
When NaY including 3a was irradiated while being stirred
in cyclohexane under O2 using 313 nm light, cis–trans iso-
merization was the dominant reaction and formation of 14a
was almost entirely suppressed, as shown in Entries 1 and 3,
Table 3. This is probably because photooxygenation of the
solvent competed with the photooxygenation of 3a, as found by
Frei et al.14 In contrast, when the zeolite sample dried under
vacuum was irradiated without the solvent under similar
conditions, a significant amount of 5a (10% for c-3a and 7% for
t-3a, Entries 2 and 4) was produced, together with c- and t-3a
(ca. 85%) and 14a (ca. 6%). For 3b, irradiation of the dried
NaY sample with 313 nm light caused cis–trans isomerization
and the production of 5b as in the photoreaction in solutions
(Entries 5 and 6). However, no 14b or 15b was detected by
HPLC. These results suggest that zeolite cavities are an
extremely unfavorable environment for the photoelectrocyclic
reaction of stilbenes.
Entry 3). This might be either because air leaked in during
irradiation or because the dihydrophenanthrene produced from
4 was stabilized in the cavities and reacted with O2 to give 17
when the reaction cell was opened. Therefore, as indicated
above, it seems likely that the zeolite cavities have a suppression
effect on the electrocyclic reaction of stilbene derivatives. In
order to further confirm this effect, photooxygenation of 4
adsorbed on the silica gel surface was carried out, as shown in
Entries 5 and 6. Under degassed conditions, as in NaY, a small
amount of 17 (yield, 6%) was produced; in contrast, 17 (yield,
55%) was the dominant product under O2, formed on the
surface through a photoelectrocyclic reaction. Accordingly,
there can be no doubt that the photoelectrocyclic reaction
became remarkably slow in the zeolite nanocavities. We suggest
that this is due to the electrostatic interaction of the alkenes
with Naϩ in the cavities, as discussed below on the basis of the
semiempirical molecular orbital calculations.37–39
(4)
Transient absorption spectra observed on 266 nm laser excitation
of 1 in NaY
Although photooxygenation of organic substrates by excitation
of the CCT band has been widely studied in solution, few
transient absorption spectra for reactive intermediates
generated in the reaction have been reported. For example,
Tsubomura et al. studied flash photolysis of aniline derivatives
in solutions in oxygenated and deoxygenated conditions.40 The
transient spectra, caused by the cation radical of the anilines,
were observed in deoxygenated ethanol, but not in oxygenated
ethanol. In addition, Ogilby et al. observed singlet oxygen (1O2)
at 1270 nm in a time-resolved experiment subsequent to pulsed
(d) For triphenylethene (4). As can be seen in Table 4, under
degassed conditions, no photoreaction of 4 occurred in CH2Cl2
(Entry 1). Under O2, on the other hand, 4 underwent an
exclusively electrocyclic reaction to give 9-phenylphenanthrene
(17, 83%) in CH2Cl2, as in the case of 3a (Entry 2). It is
noteworthy that photooxygenation of 4 in NaY to yield 5a and
10 (33 and 29%, respectively) competed with the formation of
17 (38%, Entry 4), whereas a comparatively small amount of 17
was formed under vacuum (conversion, 6%; total yield 9%;
1898
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UV laser photolysis of the CCT absorption bands of organic
molecules with O2.41,42 In contrast with the studies of reactions
in solution, there has been no investigation of transient species
generated by photolysis of the CCT complexes between organic
guest molecules and O2 formed in zeolites. As shown in Fig. 4a,
Fig. 5 Optimum structure of 1/Naϩ complex (I) calculated using AM1
Method.
Fig. 5; see Fig. 1 for the carbon numbers). This interaction site is
similar to that for t-3a reported previously.48 For 2 there are
several interaction sites, as shown in Fig. 6 (complexes II,
III, and IV); however, on the basis of the heat of formation
(∆Hf): 358 kJ molϪ1 for II, 362 kJ molϪ1 for III, and 380 kJ
molϪ1 for IV, the most favorable interaction with Naϩ can be
seen to occur between the two benzene rings of 2. When c-3a
interacts electrostatically with Naϩ, the more stable interaction
occurs between the two benzene rings (complex V, ∆Hf = 348 kJ
molϪ1) rather than on the ethylene bond (complex VI, ∆Hf
= 351 kJ molϪ1), as shown in Fig. 7. However, given the small
difference in ∆Hf between V and VI, these complexes may
coexist in the cavities. Similar interaction seems to occur for 4,
as can be seen in Fig. 8 (complexes VII and VIII; ∆Hf = 462 kJ
molϪ1 and 460 kJ molϪ1, respectively). The complexes VII and
VIII are more stable than the other complex IX (∆Hf = 465 kJ
molϪ1), in which Naϩ interacts with the π-electrons mainly on
the ethylene bond. The differences in ∆Hf among the three
complexes are relatively small (2∼5 kJ molϪ1); therefore, these
complexes would coexist in the zeolite cavities, as in the case
of 2 and 3. As noted in the discussion of the distribution of
photooxygenation products for 3 and 4, the rate of the photo-
electrocyclic reaction giving 14 and 17 is slower in the zeolite
cavities than in solutions. This is probably because of the
interaction of Naϩ with the π-electrons of the two benzene
rings attached to the ethylene bond (see complexes V and
VII), as found in the case of naphthalene49 and cis-1,2-
diphenylcyclopropane,39 which suppresses the electrocyclic
reaction.
Fig. 4 Transient absorption spectra observed on 266 nm laser
excitation of 1 (<S> = 0.46 and 0.61) in NaY under vacuum (a) and at
0.2 atm O2 (b).
Photooxygenation in zeolite nanocavities
we measured the transient absorption spectra observed by
excitation of 1 included in NaY using a Nd:YAG-laser
photolysis system (266 nm; laser power, 2 mJ) under vacuum.
The transient absorption bands appearing at around 330 and
(a) Role of CCT complex. It is surprising to discover that
zeolite cavities have the effect of stabilizing the CCT complexes
comprising aliphatic alkenes and O2.11–15 In the case of the
aromatic alkenes employed in this study, however, the form-
ation of the CCT complexes is not necessarily accelerated in
NaY; rather, suppression of the formation was seen for 2 and
c-3a. Therefore, the photooxygenation of 1–4 observed is
understood to proceed through excitation of the CCT
complexes and/or PET reaction between excited 1–4 and O2 to
ϩ
ؒ
520 nm were attributable to a cation radical of 1 (1
)
43 and the
3ϩ 24,44,45
trapped electron (Na4
)
according to the literature. We
could not determine if this ionization process was mono-
photonic or biphotonic because, due to the limitation of our
system, the range through which the power of the laser could
be adjusted was too narrow. However, on the basis of the
ionization potential of 1 (8.50 eV)46 and the energy of 266 nm
light (4.66 eV), it is likely that this ionization occurred through
a biphotonic process even though the zeolite cavities lowered
the ionization potential to some extent through the effect of
the strong electrostatic field.47 At 0.2 atm O2 both absorption
Ϫ
ؒ
generate the alkene cation radicals and O2
.
It is expected that the strong electrostatic field in the zeolite
cavities stabilizes the CCT complexes, as suggested in previous
studies using aliphatic alkenes and alkyl substituted
benzenes.11–15,50,51 However, in the case of the aromatic alkenes,
the formation of the CCT complexes are not essential in
initiating photooxygenation. For 2 and 4, photooxygenation in
NaY occurred even though the CCT complexes were not
observed. Therefore, the acceleration of the oxygenation
observed particularly for 1 and 2 seems to be due to stabiliz-
ation, by the electrostatic field, of the alkene cation radicals and
intensity of 1 ϩ and Na43ϩ sharply decreased, as can be seen in
ؒ
3ϩ
Fig. 4b. This indicates that Na4 is rapidly oxidized by O2 to
Ϫ
ϩ
yield O2 , while 1 reacts with O2 or O2 Ϫ. This result suggests
ؒ
ؒ
ؒ
that the reaction of 1 with O2 or O2 Ϫ is an important process
in the production of the oxygenation products of 1 in the
cavities.
ϩ
ؒ
ؒ
O2 Ϫ, generated as the reactive intermediates.
ؒ
Electrostatic interaction of Na؉ with alkenes
(b) Mechanism. We propose Scheme 1 and 2 as the
mechanism for the photooxygenation of 1–4 in NaY. As shown
in Scheme 1, excitation of the CCT complexes and/or PET
reaction of alkenes 1 and 2 with O2 would generate the alkene
Using the AM1 Method, the optimum structure of 1–4 and
their complexes with Naϩ were calculated as in the case of
4-methoxystyrene and azobenzene.30,48 In the case of 1, Naϩ
interacts with π-electrons on C2 and C3 carbons (complex I in
cation radicals and O2 Ϫ, followed by cycloaddition between
ؒ
J. Chem. Soc., Perkin Trans. 2, 2002, 1894–1901
1899

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Fig. 6 Optimum structure of 2/Naϩ complexes (II, III, and IV) obtained by AM1 calculation.
corresponding epoxide is not understood at this stage. However,
the instability of the epoxide in the zeolite cavities is due to the
existence of Brønsted acid sites.52,53 For 1,2-diphenylethenes 3
and 4, the electrostatic interaction of Naϩ with the two benzene
rings, as shown in Scheme 2, makes an electrocyclic reaction
slower than in solutions, due to steric hindrance on the bond
formation. Therefore, it is probable that photooxygenation
through excitation of the CCT complex and/or PET reaction
between the alkenes and O2 competes with the electrocyclic
reaction. Finally, it is possible that 1O2 is generated by excitation
of the CCT complex and by sensitization of O2 by the excited
alkenes. However, it is known that aromatic alkenes are
1
less reactive with O2 than aliphatic alkenes54 and also that
Fig. 7 Optimum structure of c-3a/Naϩ complexes (V and VI).
1O2 reaction gives oxygenation products different from
those yielded through PET;55–57 accordingly we conclude
that 1O2 is not the reactive oxygen species in the present
reaction.
the ion radical pair to give dioxetanes 16, which readily
decomposes to 5 and 10, or by the reaction of the alkene
cation radicals with O2. The route for the production of the
Fig. 8 Optimum structure of 4/Naϩ complexes (VII, VIII, and IX).
Scheme 1 Mechanism for photooxygenation of 1 and 2 in NaY: R = H and Ph.
1900
J. Chem. Soc., Perkin Trans. 2, 2002, 1894–1901

View Article Online
Scheme 2 Mechanism for photooxygenation of 3a and 4 in NaY: R = H and Ph.
26 D. F. Evans, J. Chem. Soc., 1953, 345.
27 D. F. Evans, J. Chem. Soc., 1956, 1351.
Acknowledgements
28 H. Tsubomura and R. S. Mulliken, J. Am. Chem. Soc., 1960, 82,
The authors are grateful to the Ministry of Education,
Science, Sports and Culture of Japan for partial support of this
research by a Grant-in-Aid for Scientific Research awarded to
M. K. (No. 10640487) and also to Mr M. Hiratsuka for his
assistance in the measurement of laser photolysis.
5966.
29 S. Hashimoto and H. Akimoto, J. Phys. Chem., 1989, 93, 571.
30 C. Matsubara and M. Kojima, Res. Chem. Intermed., 2001, 27,
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31 N. J. Turro, Tetrahedron, 1985, 41, 2089.
32 W. B. Brown, J. Am. Chem. Soc., 1968, 90, 1916.
33 A. V. Barzkin and S. Hashimoto, J. Chem. Phys., 2000, 113, 2841.
34 T. Kanno, M. Hisaoka, H. Sakuragi and K. Tokumaru, Bull. Chem.
Soc. Jpn., 1981, 54, 2330.
35 J. Bendig, M. Beyermann and D. Kreysig, Tetrahedron Lett., 1977,
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1901