Controllable selectivity of photosensitized oxidation of olefins included in vesicles

Article abstract of DOI:10.1016/S0040-4020(00)00656-6

9,10-Dicyanoanthracene (DCA) sensitized photooxidation of α-pinene, trans-stilbene and trans,trans-1,4-diphenyl-1,3-butadiene in mixed surfactant vesicles was investigated. While the oxidation in homogeneous solution yields the products derived from both the energy transfer and the electron transfer pathways, that within Vesicles selectively yields either the singlet oxygen mediated or the superoxide radical anion mediated products depending on the status and location of the substrate and sensitizer molecules in the reaction medium. Upon incorporating the alkene in the bilayer membranes of one set of vesicles and DCA in another set of vesicles, the isolation of the substrate from the sensitizer prevents them from undergoing electron transfer. The singlet oxygen produced in the DCA-containing vesicles diffuses into the alkene-containing vesicles and reacts with the alkene. Thus, only the singlet oxygen oxidation products are obtained, and no product derived from the superoxide radical anion is detected. In contrast, incorporating both the sensitizer and the substrate within the same set of vesicles results in the two dissimilar molecules close to one another in the restricted space of the bilayer membranes of the vesicles. Thus, electron transfer from the alkenes to ¹DCA* is enhanced, and the efficiency of the intersystem crossing from ¹DCA* to ³DCA* is reduced. The photosensitized oxidation in this case only gives the products derived from the electron transfer pathway. Moreover, the reaction of singlet oxygen with DPB in vesicles results exclusively in the 1,2-cycloaddition products rather than the 1,4-cycloaddition product. This result indicates that the organized semirigid environment in vesicles prevents the olefin molecules from conformational change. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00656-6

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 7437–7442
Controllable Selectivity of Photosensitized Oxidation of Olefins
Included in Vesicles
Hong-Ru Li,a Li-Zhu Wub and Chen-Ho Tunga,
*
aContribution from the Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China
bThe Center of Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Received 27 April 2000; revised 3 July 2000; accepted 20 July 2000
Abstract—9,10-Dicyanoanthracene (DCA) sensitized photooxidation of a-pinene, trans-stilbene and trans,trans-1,4-diphenyl-1,3-buta-
diene in mixed surfactant vesicles was investigated. While the oxidation in homogeneous solution yields the products derived from both the
energy transfer and the electron transfer pathways, that within vesicles selectively yields either the singlet oxygen mediated or the superoxide
radical anion mediated products depending on the status and location of the substrate and sensitizer molecules in the reaction medium. Upon
incorporating the alkene in the bilayer membranes of one set of vesicles and DCA in another set of vesicles, the isolation of the substrate from
the sensitizer prevents them from undergoing electron transfer. The singlet oxygen produced in the DCA-containing vesicles diffuses into the
alkene-containing vesicles and reacts with the alkene. Thus, only the singlet oxygen oxidation products are obtained, and no product derived
from the superoxide radical anion is detected. In contrast, incorporating both the sensitizer and the substrate within the same set of vesicles
results in the two dissimilar molecules close to one another in the restricted space of the bilayer membranes of the vesicles. Thus, electron
1
1
3
transfer from the alkenes to DCAء is enhanced, and the efficiency of the intersystem crossing from DCAء to DCAء is reduced. The
photosensitized oxidation in this case only gives the products derived from the electron transfer pathway. Moreover, the reaction of singlet
oxygen with DPB in vesicles results exclusively in the 1,2-cycloaddition products rather than the 1,4-cycloaddition product. This result
indicates that the organized semirigid environment in vesicles prevents the olefin molecules from conformational change. ᭧ 2000 Elsevier
Science Ltd. All rights reserved.
Introduction
in this reaction, various efforts have been made in the past
decades, and remarkable control of the reaction pathway has
been obtained by the use of organized and constrained
media.6 In our previous works, we reported the photosensi-
tized oxidation of alkenes included within ZSM-5 zeolite7
or in water-swollen Nafion membranes.8 We trapped the
alkenes in the channels of ZSM-5 zeolites or within Nafion
clusters and isolated the photosensitizers in the surrounding
solution. The isolation of the substrate within the zeolite or
the water-swollen Nafion membranes from the sensitizer in
the external solution inhibits the electron transfer. On the
other hand, singlet oxygen can still be generated in the
solution, and is able to diffuse into the ZSM-5 channel or
the Nafion cluster to react with the alkene. Moreover, we
also trapped both the sensitizer and the alkene into the
Nafion cluster. The close contact between the sensitizer
and alkene molecules in the confined Nafion cluster leads
to efficient quenching of the singlet excited state of the
sensitizer by the alkene via an electron-transfer process.
As a result, the intersystem crossing from the singlet excited
state to the triplet state of the sensitizer cannot compete with
the quenching process by the alkene. Therefore, the subse-
quent triplet energy transfer to O2 cannot occur, and no
singlet oxygen mediated product was produced. Thus, we
could successfully direct the oxidation selectively toward
either the singlet oxygen mediated or the superoxide radical
Selective oxidation of alkenes by molecular oxygen is one
of the current challenges in the manufacture of organic
building blocks and industrial intermediates,1 and photo-
sensitized oxidation holds special promise for yielding
product specificity.2 There are two well-established types
of dye-sensitized photooxidation: the energy-transfer path-
way and the electron-transfer pathway.3 The energy-transfer
pathway involves energy transfer from the triplet sensitizer
to the ground-state oxygen to generate singlet oxygen (1O2),
then the generated singlet oxygen reacts with the substrate.4
In the electron-transfer photosensitized oxidation, electron
deficient sensitizers are generally used.5 Electron transfer
from the alkene to the sensitizer in its excited states results
in the alkene radical cation and the sensitizer radical anion,
which subsequently reduces O2 to give a superoxide radical
anion (OϪ2 ⅐). The generated superoxide radical anion reacts
with the alkene radical cation to yield the oxidation
products. Unfortunately, in many cases the two types of
photooxidation occur simultaneously, and the selectivity
of the oxidation reactions is very low. To gain selectivity
Keywords: selective oxidation; olefins; vesicles.
* Corresponding author. Tel.: ϩ86-10-64888046; fax: ϩ86-10-64879375;
e-mail: chtung@ipc.ac.cn
0040–4020/00/$ - see front matter ᭧ 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00656-6

7438
H.-R. Li et al. / Tetrahedron 56 (2000) 7437–7442
Scheme 1.
anion mediated products by controlling the status and
location of the substrate and sensitizer molecules in the
reaction media, which has not been observed in the solution
photooxygenation. In the present work, we extend the study
to a mixed surfactant vesicle medium to establish the scope
of this approach.
was measured to be ϳ4–5 nm. These vesicles were poly-
disperse with radii ranging from 80 to 150 nm, and the
average vesicle radius was ϳ100 nm. The vesicles were
stable at room temperature, and the dispersion solution
was optically clear. The substrates PE, TS and DPB and
the sensitizer DCA can all be easily incorporated into the
bilayer membranes of the vesicles by sonication, since they
are hydrophobic.12
Vesicles are widely used as simplified models of cell
membranes.9 Generally, vesicles are prepared from
double-tailed surfactants. Simple single-tailed surfactants
cannot form vesicles due to their relatively large hydrophilic
head effect. However, it was recently established that stable
vesicles could be simply produced by mixing commercially
available single-tailed cationic and anionic surfactants.10
This phenomenon arises from the strong electrostatic inter-
action between the oppositely charged head-groups of the
components.11 As a result, the mean effective head-group
area decreases considerably, while the hydrophobic volume
of the tails remains the same. Thus, this dynamic ion pairing
yields a pseudo-double-tailed zwitterionic surfactant, which
has the preferred geometry of a vesicle-forming surfactant.
In the present work we use such easily prepared and inex-
pensive vesicles as the reaction medium to conduct the
oxidation of a-pinene (PE), trans-stilbene (TS) and trans,
trans-1,4-diphenyl-1,3-butadiene (DPB) photosensitized by
9,10-dicyanoanthracene (DCA). With the isolation of the
substrate within one set of vesicles from the sensitizer in
another set of vesicles, we only obtained the products
derived from the singlet oxygen. On the other hand, by
incorporation of both the substrate and sensitizer molecules
in the same set of vesicles, we only observed the products
derived from superoxide radical anion.
The photosensitized oxidation was performed in two modes.
In the first mode (Mode 1), we incorporated the sensitizer
DCA in the bilayer membranes of one set of vesicles, and
solubilized the substrate in another set of vesicles. Equal
volumes of the two sets of vesicle dispersions were then
mixed to prepare the samples for irradiation. Although
sonication was carried out during preparation of the com-
ponent solutions, the final mixture was not sonicated. In this
way intermixing of solubilizates was prevented. We have
carried out a control experiment: the mixed solution
prepared from DCA-containing vesicles and substrate-
containing vesicles was stored in the dark at room tempera-
ture for one day, and then was irradiated. The products and
the efficiency of the product formation for the photosensi-
tized oxidation were found to be identical within experi-
mental error limit to those of the sample that was
immediately irradiated after the preparation. This observa-
tion suggests that the intervesicular exchange both of the
substrate and the sensitizer indeed did not occur, and
the photosensitized oxidation process in this mode involved
the generation of 1O2 in one vesicle and reaction with alkene
molecules in other vesicles. In the second mode (Mode 2)
both the sensitizer and the substrate were incorporated in the
bilayer of the same set of vesicles. Generally, the concen-
tration of the olefins was ϳ1.0×10Ϫ3 M corresponding to
thousands of substrate molecules in each vesicle (see
below), while the concentration of the sensitizer was
generally ϳ1.0×10Ϫ4 M. The samples were irradiated
under bubbling oxygen by using a 450 W Hanovia high
pressure Hg lamp as the light source, and a glass filter to
cut off the light with wavelength lϽ400 nm, ensuring the
absence of direct excitation of the alkene substrate. After
irradiation, the products were extracted with CH2Cl2 or ether
and analyzed by gas chromatography. Material balance for
all the reactions was generally greater than 95%, suggesting
that any unidentified products must be minor.
Results and Discussion
The vesicles used as the reaction medium in the present
study were prepared by sonicating an equimolar mixture
of a cationic surfactant (octyltrimethylammonium bromide,
8.2×10Ϫ2 M) and an anionic surfactant (sodium laurate,
8.2×10Ϫ2 M) in buffered solution (pHˆ9.2) for 30 min at
50ЊC. Vesicle formation was demonstrated by transmission
electron microscopy with negative technique (stained with
uranyl acetate). The unilamellar layer of the vesicle was
clearly shown in the electron microscope and its thickness

H.-R. Li et al. / Tetrahedron 56 (2000) 7437–7442
7439
Table 1. Product distribution in the DCA-sensitized photooxidation of PE
in solutions and in vesicles (error range is ϳ2%)
turn gives the inter-vesicular distance on average to be
1
ϳ134 nm.15 On the other hand, the species O2 is small
and uncharged and has a relatively long lifetime and proper-
ties that allow it to diffuse a long distance in non-viscosity
Media
1
2
3
4
1
CH3CN
CH2Cl2
Vesicles, mode 1
Vesicles, mode 2
52
85
100
0
32
10
0
13
5
0
3
0
0
media. The average diffusion length of O2 molecule in
aqueous solution is estimated to be ϳ780 nm.16 This diffu-
sion length is much larger than the intervesicular distance
55
4
41
1
estimated above. Thus, the O2 generated in DCA-contain-
ing vesicles is indeed capable of diffusing into the PE-
containing vesicles to react with the alkene molecules to
produce the ene product 1. In a previous work,14 we demon-
strated that in the experimental condition identical to that
described above ϳ20% of the generated singlet oxygen in
the sensitizer-containing vesicles could diffuse to other
vesicles to react with the substrate.
Oxidation of PE
It has been established that DCA can act both as an energy-
transfer sensitizer and an electron-transfer sensitizer.5 The
photooxidation of PE sensitized by DCA in homogeneous
solution followed by reduction of the reaction mixture with
sodium sulfite solution gave the ene product pinocarveol 1
and the non-ene products myrtenal 2, epoxide 3 and
aldehyde 4, as shown in Scheme 1. The ene product and
the non-ene products have been proposed to be derived from
the energy-transfer and electron-transfer pathways respec-
tively.13 The product distributions in acetonitrile and
dichloromethane are given in Table 1, which show signifi-
cant dependence on the solvents.
In contrast, the photooxidation in Mode 2 followed by
mixing the sample with a sodium sulfite aqueous solution
to reduce the reaction mixture exclusively gave the non-ene
products 2–4 (Scheme 1). No product derived from the
energy-transfer pathway was detected. The loading levels
used in the present study were hundreds of DCA and
thousands of PE molecules per vesicle. Thus, each DCA
molecule is surrounded by a number of PE molecules. The
high ‘local concentration’ of PE and the close contact
between DCA and PE molecules in the confined bilayer of
vesicles lead to efficient quenching of the singlet excited
state of DCA by PE via an electron-transfer process,
generating DCA radical anion and PE radical cation. As a
result, the intersystem crossing from the singlet excited state
to the triplet state of DCA can not compete with the quench-
ing process by PE. Therefore, the subsequent triplet energy
transfer to O2 can not occur, and no singlet oxygen mediated
product is produced. On the other hand, the DCA radical
anion generated above will efficiently undergo electron
transfer to oxygen to produce superoxide radical anion,
which subsequently reacts with PE radical cation located
in the same vesicle to yield the non-ene products 2–4.
The product distribution of the photosensitized oxidation of
PE in vesicles is dramatically altered compared with those
in the above homogeneous solutions, and remarkably
dependent on the experimental mode. The photosensitized
oxidation in Mode 1 followed by mixing the sample with a
sodium sulfite aqueous solution to reduce the reaction
mixture exclusively produced the ene product 1. No trace
of the non-ene products 2–4 was detected. Evidently, the
isolation of PE in the bilayer membranes of one set of
vesicles from DCA in another set of vesicles prevents
them from undergoing electron transfer. Thus, no photo-
oxidation products derived from the electron-transfer path-
way were detected. On the other hand, singlet oxygen can be
generated in the DCA-containing vesicles by the energy-
transfer from the triplet excited state of DCA to the ground
state of the oxygen. The vesicles used in this study have an
aggregation number (number of surfactant molecules per
vesicle) in the region of 105–106, and the average aggre-
gation number is ϳ7.2×105, as estimated from the vesicle
size and the volume of the surfactant molecule.14 Thus, at
surfactant concentration of 8.2×10Ϫ2 M the vesicle popula-
tion is equivalent to a molarity of ϳ1.0×10Ϫ7 M, which in
Oxidation of TS
As observed in the case of PE, the product distribution of the
DCA-photosensitized oxidation of TS incorporated in
vesicles significantly differs from that in homogeneous
solution, and is dependent on the experimental mode. The
DCA-photosensitized oxidation of TS in homogeneous
Scheme 2.

7440
H.-R. Li et al. / Tetrahedron 56 (2000) 7437–7442
Table 2. Product distribution in the DCA-sensitized photooxidation of TS
in solutions and in vesicles (error range is ϳ2%)
Table 3. Product distribution in the DCA-sensitized photooxidation of DPB
in solutions and in vesicles (error range is ϳ2%)
Media
5
6
7
8
Media
5
9
10
11
12
13
CH3CN
CH2Cl2
Vesicles, mode 1
Vesicles, mode 2
53
40
100
21
20
32
0
23
18
0
4
10
0
CH3CN
CH2Cl2
Vesicles, mode 1
Vesicles, mode 2
80
5
100
53
80
5
100
53
2
12
0
13
3
0
0
5
0
5
75
0
73
6
0
23
0
24
0
solution produced benzaldehyde 5, trans-2,3-diphenyl-
oxirane 6, benzil 7 and cis-stilbene 8 (Scheme 2).5c Table
2 shows the product distributions in acetonitrile and
dichloromethane. It has been well established that products
6, 7 and 8 are produced via the electron-transfer pathway,
while product 5 can be generated either via the energy-
transfer or the electron-transfer pathway.
Oxidation of DPB
The photo sensitized oxidation of DPB in homogeneous
solution has been extensively investigated.17 Irradiation of
oxygen-saturated DPB solution in CH2Cl2 containing DCA
with visible light gave benzaldehyde 5, cinnamaldehyde 9,
epoxide 10, ozonide 11, and endoperoxide 13 (Scheme 3).
In addition, a small amount of 1-phenylnaphthalene 12 was
detected. The product distributions in CH3CN and CH2Cl2
are shown in Table 3. Compound 13 is a product of
1,4-cycloaddition of singlet oxygen (1O2) to DPB. Products
10–12 are derived from the electron-transfer pathway.
Products 5 and 9 could be produced either via the energy-
transfer or the electron-transfer pathway.7 The photosensi-
tized oxidation of DPB in vesicles in Mode 1 gave 5 and 9 as
the unique products (Table 3 and Scheme 3). We believe
that these products are derived from the singlet oxygen
pathway. In contrast, the photosensitized oxidation of
DPB in vesicles in Mode 2 only produced the electron-
transfer mediated products 5, 9, 10 and 12 (Table 3). No
singlet oxygen products were detected. These observations
demonstrate once again that one can control the selectivity
in photosensitized oxidation of alkenes by incorporation of
the sensitizer and substrate either in different or the same
sets of vesicles.
The DCA-photosensitized oxidation of TS in vesicles in
Mode 1 yielded benzaldehyde 5 as the unique product.
The mass balance of this reaction was greater than 95%.
Evidently, this product was produced via the energy-transfer
pathway. As in the case of PE, the isolation of TS in the
bilayer membranes of one set of vesicles from DCA in
another set of vesicles inhibits the electron-transfer between
the substrate and the sensitizer. Hence, no superoxide anion
product was expected to be produced. The singlet oxygen
generated in the DCA-containing vesicles via the energy
transfer from the triplet state of DCA to the ground state
oxygen diffuses into the TS-containing vesicles and reacts
with TS to form 5. In contrast, the photosensitized oxidation
of TS in vesicles in Mode 2 only produced the electron-
transfer mediated products 5, 6 and 7 (Scheme 2). The
product distribution is shown in Table 2. Again, confining
the sensitizer and substrate molecules within the restricted
space of the bilayer membranes of vesicles enhances the
electron-transfer process and reduces the efficiency of
the intersystem crossing from the excited singlet state of
the sensitizer to its triplet state. Thus, only the products
derived from the electron-transfer pathway can be produced.
It is worth noting that the products in the reaction of DPB
with singlet oxygen in vesicles are remarkably different
from those in homogeneous solutions. The reaction in
homogeneous solution yielded endoperoxide 13,
a
Scheme 3.

H.-R. Li et al. / Tetrahedron 56 (2000) 7437–7442
7441
1
1,4-cycloaddition product of the diene to O2 as the unique
Experimental
product (Scheme 3). In sharp contrast, the photosensitized
oxidation in vesicles in Mode 1 produced benzaldehyde 5
and cinnamaldehyde 9 in quantitative yield as described
above. Evidently, these products were derived from an
intermediate dioxetane, a 1,2-cycloaddition product. The
preferential formation of the product of 1,2-cycloaddition
over that of 1,4-cycloaddition in vesicles is probably best
explained in terms of the greater difficulty of achieving the
necessary geometry for 1,4-cycloaddition in this organized
medium. It has been established that DPB in solution exists
in two conformational isomers: cisoid and transoid.18 At
equilibrium the main conformer is the transoid (ϳ99%),
and the cisoid is presented only in ϳ1%. The 1,4-cyclo-
addition of singlet oxygen to 1,3-diene to form endo-
peroxide is concerted and analogous to the Diels–Alder
reaction. This reaction requires a six-membered ring tran-
sition state. Only the cisoid conformer can satisfy such a
requirement, and in order to undergo 1,4-cycloaddition with
singlet oxygen the transoid conformer first has to be isomer-
ized to the cisoid one. Due to the kinetic equilibrium
between the two conformers in solution, the cycloaddition
can proceed until all the diene is converted to the
products. Obviously, in vesicles the organized semirigid
environment prevents DPB molecules from the conforma-
tional change. Thus, only the 1,2-cycloaddition products
were obtained.
General
Surfactants octyltrimethylammonium bromide and sodium
laurate were Aldrich products, and were recrystallized twice
from ethanol–ether before use. Doubly distilled water was
used throughout this work. TS, DPB and DCA were
purchased from Fluka and used as received. PE was Aldrich
product, and was distilled under reduced pressure under
nitrogen and stored in the dark before use. Gas chroma-
tography was performed on a Shimadzu GC-7A with a 3%
OV-17 column. Mass spectra were run on a VGZAB GC-
MS spectrometer. Electron microscopy was examined on a
Hitachi H-600 electron microscope.
Preparation of samples
To the equimolar mixture of octyltrimethylammonium
bromide and sodium laurate was added H2O buffered
solution (pHˆ9.2, Na2B2O7) to form a suspension solution
with the concentration of ϳ8.2×10Ϫ2 M for each surfactant.
A known amount of substrate was added to the above
suspension. The concentration of the substrate in the suspen-
sion was ϳ1.0×10Ϫ3 M. The suspension was sonicated for
30 min at 50ЊC. The solution became clear, and the
substrate-containing vesicles were produced. By using the
same procedure the DCA-containing vesicle dispersions
were prepared. The concentration of the sensitizer was
ϳ1.0×10Ϫ4 M. Mixing of equal volumes of the above
substrate- and sensitizer-containing vesicle dispersions
gave the Mode 1 irradiation samples.
Conclusion
The product distributions of DCA-photosensitized oxidation
of PE, TS and DPB in mixed surfactant vesicles are signifi-
cantly different from those in homogeneous solutions, and
remarkably dependent on reaction mode. By incorporating
the alkenes within the bilayer membranes of one set of
vesicles and DCA in another set of vesicles, the isolation
of the substrate from the sensitizer prevents them from
undergoing electron transfer. The singlet oxygen generated
in the sensitizer-containing vesicles can diffuse into the
substrate-containing vesicles to react with the substrate.
Thus, only the products derived from the energy-transfer
pathway are produced. In contrast, by incorporating both
the sensitizer and the substrate in the same set of vesicles,
the close contact between the substrate and sensitizer
molecules in the restricted space of the vesicle bilayer
membranes enhances the electron transfer from the
substrate to the singlet excited state of DCA, thus reduces
the efficiency of the intersystem crossing from the singlet
excited state to the triplet state of the sensitizer and the
subsequent formation of singlet oxygen. Therefore, the
photosensitized oxidation of the three alkenes exclusively
yields the products derived from the electron-transfer path-
way, and no singlet oxygen mediated product is detected.
Moreover, in the photosensitized oxidation of DPB, the
organized semirigid environment in vesicles prevents the
diene from the conformational change to adapt the neces-
sary geometry for 1,4-cycloaddition with singlet oxygen.
Thus, only the 1,2-cycloaddition products were obtained,
which was contrast with that in homogeneous solution
where the 1,4-cycloaddition product was the unique
product.
To prepare the Mode 2 samples both the substrate
(1.0×10Ϫ3 M) and the sensitizer (1.0×10Ϫ4 M) were added
to the surfactant suspension. Subsequently, the suspension
was sonicated as described above.
Negative staining electron microscopy
The vesicle samples were negatively stained with a 2%
(w/w) uranyl acetate solution and examined on a Hitachi
H-600 electron microscope.
Photooxidation of olefins and product analysis
The sample in a Pyrex reactor was bubbled with oxygen
during irradiation. A 450-W medium pressure Hanovia Hg
lamp was employed as the light source, and a glass filter was
used to cut off light with a wavelength below 400 nm. The
filter thus ensured the absence of direct excitation of the
olefin substrates. After irradiation, the products were
extracted with CH2Cl2 or ether, analyzed by GC, and
identified by their spectral properties and by comparing
with authentic samples.8,13e In the photosensitized oxidation
of PE, after irradiation the sample was mixed with a sodium
sulfite aqueous solution to reduce the reaction mixture, and
then the products were worked up as described above.
Acknowledgements
We thank the National Science Foundation of China, and the

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H.-R. Li et al. / Tetrahedron 56 (2000) 7437–7442
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for financial support.
8. Tung, C. H.; Guan, J. Q. J. Am. Chem. Soc. 1998, 120, 11874.
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