Vesicle controlled selectivity in photosensitized oxidation of olefins

Article abstract of DOI:10.1039/b002597f

The photooxidation of α-pinene (α-PE) and trans,trans-1,4-diphenyl- 1,3-butadiene (DPB) sensitized by 9,10-dicyanoanthracene (DCA) in mixed surfactant vesicles was selectively directed toward either the singlet oxygen mediated or the superoxide radical an

Full text of DOI:10.1039/b002597f

Vesicle controlled selectivity in photosensitized oxidation of olefins
a
b
a
Hong Ru Li, Li Zhu Wu and Chen Ho Tung*
a
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing, 100101, China.
E-mail:chtung@ipc.ac.cn
Center of Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
b
Received (in Cambridge, UK) 3rd April 2000, Accepted 17th May 2000
The photooxidation of a-pinene (a-PE) and trans,trans-
,4-diphenyl-1,3-butadiene (DPB) sensitized by 9,10-di-
cyanoanthracene (DCA) in mixed surfactant vesicles was
selectively directed toward either the singlet oxygen medi-
ated or the superoxide radical anion mediated products by
controlling the status and location of the substrate and
sensitizer molecules in the reaction media.
chromatography. The material balance for all the reactions was
generally > 95%.
1
Photooxidation of a-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 and the non-ene products have
been proposed to be derived from the energy- and electron-
5
Dye-sensitized photooxidation of alkenes has been extensively
investigated.1 There are two well established types of dye-
sensitized photooxidation: an energy-transfer pathway and
electron-transfer pathway.2 The energy-transfer pathway in-
transfer pathways, respectively. The product distributions in
acetonitrile and dichloromethane are given in Table 1. The
product distribution of the photosensitized oxidation of a-PE in
vesicles is dramatically altered compared with that in homoge-
neous solutions, and is remarkably dependent on the experi-
mental mode. Photooxidation in mode 1 exclusively produced
the ene product 1 (Table 1). In contrast, photooxidation in mode
2 only gave the non-ene products 2–4 (Table 1). These
observations can be easily understood by consideration of the
status and location of the substrate and sensitizer molecules in
the reaction media. It has been established that DCA can act
both as an energy-transfer sensitizer and an electron-transfer
sensitizer.6 In mode 1, the isolation of a-PE in one set of
vesicles from DCA in another set of vesicles prevents them
volves energy transfer from the triplet sensitizer to ground-state
1
oxygen to generate singlet oxygen ( O
2
) which then reacts with
the substrate. In the electron-transfer photosensitized oxidation,
electron-deficient sensitizers are generally used. Electron
transfer from the alkene to the sensitizer in its excited states
results in formation of the alkene radical cation and the
sensitizer radical anion, which subsequently reduces O
the superoxide radical anion (O
2
to give
2
· ). The generated superoxide
2
radical anion then reacts with the alkene radical cation to yield
the oxidation products. Unfortunately, in many cases the two
types of photooxidation occur simultaneously, and the selectiv-
ity of the oxidation reactions is poor. Of the various approaches
to increase selectivity, use of organized and constrained media
from undergoing electron transfer. Thus, no non-ene products
1
were detected. On the other hand, O
2
can be generated in the
DCA-incorporating vesicles by energy transfer from the triplet
3
has shown considerable promise. Here, we report the photo-
excited state of DCA to the ground state of oxygen. The species
1
oxidation of a-pinene (a-PE) and trans,trans-1,4-diphenyl-
2
O is small and uncharged and has a relatively long lifetime
1
,3-butadiene (DPB) sensitized by 9,10-dicyanoanthracene
and properties which allow it to diffuse freely from one set of
vesicles to another set of vesicles where reaction with the
(DCA) in mixed surfactant vesicles. We found that the
oxidation could be directed selectively toward the products
derived either from the energy transfer pathway or the electron
transfer pathway by controlling the status and location of the
substrate and sensitizer molecules in the reaction media.
The vesicles used in the present study were prepared by
sonicating an equimolar mixture of a cationic surfactant
2
2
(
octyltrimethylammonium bromide, 8.2 3 10 M) and an
2
2
anionic surfactant (sodium laurate, 8.2 3 10 M) in buffered
4
solution (pH = 9.2 ) for 30 min at 50 °C. These vesicles were
found to be stable at room temperature and the dispersion
solution was optically clear. The photosensitized oxidation was
performed in two modes. In the first (mode 1), the sensitizer
DCA was incorporated in the bilayer membrane of one set of
vesicles and the substrate solubilized in another set of vesicles.
Equal volumes of the two sets of vesicle dispersions were then
mixed to prepare the samples for photolysis. Although sonica-
tion was performed during preparation of the component
solutions, the final mixture was not sonicated. In this way
intermixing of the solubilizates was prevented. In the second
mode (mode 2) both the sensitizer and the substrate were
incorporated in the bilayers of the same set of vesicles.
Scheme 1
Table 1 Product distribution in the DCA-sensitized photooxidation of a-PE
and DPB in solutions and in vesicles
PE
DPB
2
3
Generally, the concentration of the olefins was ca. 1.0 3 10
M corresponding to thousands of substrate molecules in each
vesicle, while the concentration of the sensitizer was generally
Media
MeCH
1
2
3
4
5
6
7
8
9
10
2
4
52 32 13
3
0
0
80 80
75 75 12
100 100
53 53 23
2
13
3
0
5
0
5
5
0
0
ca. 1.0 3 10 M. The samples were irradiated under oxygen by
using light with wavelength l > 400 nm, ensuring the absence
of direct excitation of the alkene substrate. After irradiation, the
CH
Vesicles (mode 1) 100
Vesicles (mode 2)
2
Cl
2
85 10
5
0
4
0
0
0
0
0
55
41
24
2 2
products were extracted with CH Cl and analysed by gas
DOI: 10.1039/b002597f
Chem. Commun., 2000, 1085–1086
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1085

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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
–7 and 9 (Table 1) and no singlet oxygen products were
detected. These observations demonstrate once again that the
selectivity in photosensitized oxidation of alkenes can be
directed by incorporation of the sensitizer and substrate in
different or the same set of vesicles.
It is of note 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 10, a 1,4-cycloaddition product
1
of the diene to O
2
, as the unique product (Scheme 2). In sharp
contrast, the photosensitized oxidation in vesicles produced
benzaldehyde 5 and cinnamaldehyde 6 in quantitative yield.
Evidently, these products were derived from an intermediate
dioxetane, a 1,2-cycloaddition product. It has been established
Scheme 2
7
that DPB in solution exists mainly in its transoid conformer,
and the cisoid form amounts to ca. 1%. The 1,4-cycloaddition of
singlet oxygen to the 1,3-diene to form the endoperoxide is
concerted and requires a six-membered ring transition state;
only the cisoid conformer can satisfy such a requirement.
Obviously, the organized semirigid environment in vesicles
prevents DPB from undergoing transoid to cisoid conforma-
tional change and thus only the 1,2-cycloaddition products were
obtained.
In conclusion, vesicles can be used to direct the photo-
sensitized oxidation of olefins either toward the singlet oxygen
mediated or the superoxide radical anion mediated products by
controlling the status and location of the olefin and sensitizer
molecules in the reaction media.
olefins produces the ene product 1. In mode 2, the DCA
molecule is surrounded by a number of a-PE molecules. The
high “local concentration” of a-PE and the close contact
between DCA and a-PE molecules in the confined bilayer of
vesicles leads to efficient quenching of the singlet excited state
of DCA by a-PE via an electron-transfer process, generating
DCA radical anions and a-PE radical cations. As a result,
intersystem crossing from the singlet excited state to the triplet
state of DCA can not compete with the quenching process by a-
2
PE. Thus, subsequent triplet energy transfer to O can not occur,
and no singlet oxygen mediated product is produced. On the
other hand, the generated DCA radical anions will efficiently
undergo electron transfer to oxygen to produce superoxide
radical anions, which subsequently react with a-PE radical
cations located in the same vesicle to yield the non-ene products
We thank the National Science Foundation of China, and the
Bureau for Basic Research, Chinese Academy of Sciences for
financial support.
2
–4.
Photosensitized oxidation of DPB in homogeneous solution
Notes and reference
has been extensively investigated.5 Irradiation of an oxygen-
,7
saturated DPB solution in CH Cl containing DCA with visible
light gave benzaldehyde 5, cinnamaldehyde 6, epoxide 7,
ozonide 8, 1-phenylnaphthalene 9 and endoperoxide 10
2
2
1 M. Prein and W. Adam, Angew. Chem., Int. Ed. Engl., 1996, 35, 477.
2 C. S. Foote, Photochem. Photobiol., 1991, 54, 659.
3
V. Ramamurthy and N. J. Turro, J. Inclusion Phenom. Mol. Recognit.
Chem., 1995, 21, 239.
(
Scheme 2) and the product distribution is shown in Table 1. 10
4
E. W. Kaler, A. K. Murthy, B. E. Rodriguez and J. A. Zasadzinski,
Science, 1989, 245, 1371.
1
2
is a product of 1,4-cycloaddition of singlet oxygen ( O ) to
DPB. Products 7–9 are derived from the electron-transfer
pathway. Products 5 and 6 could be produced either via an
energy- or electron-transfer pathway.6 The photosensitized
oxidation of DPB in vesicles in mode 1 gave 5 and 6 as the
unique products (Table 1 and Scheme 2). We believe that these
5
6
C. H. Tung and J. Q. Guan, J. Am. Chem. Soc., 1998, 120, 11874.
R. C. Kanner and C. S. Foote, J. Am. Chem. Soc., 1992, 114, 678; R. C.
Kanner and C. S. Foote, J. Am. Chem. Soc., 1992, 114, 682.
7 C. H. Tung, H. W. Wang and Y. M. Ying, J. Am. Chem. Soc., 1998, 120,
5179.
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