Versatile photosensitization system for 1O2-mediated oxidation of alkenes based on nafion-supported platinum(II) terpyridyl acetylide complex

Article abstract of DOI:10.1021/ol035012e

(Matrix presented) Platinum(II) terpyridyl acetylide complex (1) was incorporated into Nafion membranes as a photosensitizer, and the Nafion was immersed in an aqueous or organic solution of 7-dehydrocholesterol, α-pinene, or cyclopentadiene. This photosensitizer system can generate singlet oxygen (¹O₂) in high quantum yield to oxidize the alkenes in the solution outside the Nafion and can be easily removed from the reaction vessel at the end of the photooxidation.

Full text of DOI:10.1021/ol035012e

ORGANIC
LETTERS
2
003
Vol. 5, No. 18
221-3224
Versatile Photosensitization System for
1
3
O -Mediated Oxidation of Alkenes
2
Based on Nafion-Supported Platinum(II)
Terpyridyl Acetylide Complex
Dong Zhang, Li-Zhu Wu,* Qing-Zheng Yang, Xiao-Hong Li, Li-Ping Zhang, and
Chen-Ho Tung*
Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences,
Beijing 100101, China
chtung@ipc.ac.cn; lzwu@ipc.ac.cn
Received June 6, 2003
ABSTRACT
Platinum(II) terpyridyl acetylide complex (1) was incorporated into Nafion membranes as a photosensitizer, and the Nafion was immersed in
an aqueous or organic solution of 7-dehydrocholesterol, r-pinene, or cyclopentadiene. This photosensitizer system can generate singlet
1
oxygen ( O
2
) in high quantum yield to oxidize the alkenes in the solution outside the Nafion and can be easily removed from the reaction
vessel at the end of the photooxidation.
The oxo-functionalization of small abundant hydrocarbons
is the most important type of reaction in organic chemical
productions.1 For example, essentially almost all building
blocks for manufacture of plastics and synthetic fibers are
produced by oxidation of hydrocarbons. Among the various
2
oxidizing reagents, oxidation by molecular oxygen (O ) is
special promise.5,6 The most synthetically useful type of
hydrocarbon photooxidation is the dye-sensitized reaction
,2
1
involving transient singlet oxygen ( O
2
). For example, Diels-
Alder reaction of conjugated dienes, “ene” reaction of olefins
with allylic hydrogen, and dioxetane reaction of alkenes that
do not feature an allylic hydrogen belong to this type of
1
7
,8
photooxidation. However, examples of production of
chemicals by this type of photooxidation in industrial scale
are still limited. One of the reasons for this is that the
separation of the sensitizer from the products and unreacted
starting material at the end of the reaction is tedious; thus,
of particular interest in realizing economically advantageous
and environmentally benign processes.2
-4
However, the
reactivity of O
2
toward most organic molecules is inhibited
by its spin restriction, and here photoassisted processes hold
(
1) Haber, J. In PerspectiVes in Catalysis; Thomas, J. M., Zamaraev, K.
I., Eds.; IUAC/Blackwell Scientific Publications: London, 1992.
2) Sheldon, R. A.; Kochi, J. K. Metal-catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1981.
3) Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New
York, 1991.
(5) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. ReV. 2002, 102, 3811
and references therein.
(6) Tung, C.-H.; Wu, L.-Z.; Zhang, L.-P.; Chen, B. Acc. Chem. Res.
2003, 36, 39.
(7) Wasserman, H. H., Murray, R. W., Eds. Singlet Oxygen; Academic
Press: New York, 1979.
(8) Foote, C. S. Photochem. Photobiol. 1991, 54, 659.
(
(
(
4) Foote, C. S. ActiVe Oxygen in Chemistry; Chapman and Hall: New
York, 1995.
1
0.1021/ol035012e CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/06/2003

the sensitizer cannot be reused. Here we report a versatile
adsorbed into Nafion by immersing the polymer into a well-
stirred aqueous or methanol solution of the complex and is
not leached from the polymer during the photosensitized
reaction. Considering the hydrophobicity and the positive
charge of the complex, it is likely that the molecules of 1
are located in the fluorocarbon/water interface of the
membrane. The Nafion membranes used in the present study
and robust Nafion membrane-supported photosensitizer
1
system that is capable of generating O
2
with high quantum
yield and may be easily removed from the reaction vessel
1
and reused for photooxidation without loss of O
2
-generation
capacity.
Our photosensitizer system is based on a Nafion membrane-
supported transition metal complex. Nafion is a family of
polymers that consists of a perfluoronated backbone and short
+
were their sodium form (Nafion-Na ) and had the size of 2
× 0.4 × 0.0175 cm. The loading of the sensitizer was ca. 5
µmol/g Nafion, which allowed the optical density of a single
piece of membrane at the λmax of the MLCT transition over
1.0.
9
pendant chains terminated by sulfonic groups. When swollen
in water or methanol, the structure of Nafion is believed to
-
resemble that of an inverse micelle. The hydrated SO
3
headgroups are clustered together in a water-containing
pocket of ca. 40 Å in diameter, and the pockets are
interconnected with each other by short channels within the
perfluorocarbon matrix. This water-swollen Nafion can
incorporate high concentrations of aromatic hydrocarbons
and organic dyes.10,11 Furthermore, the concentration of
oxygen in water-swollen Nafion is more than 10 times greater
than in organic solvents.1 Thus, these optically transparent
membrane systems are particularly suitable for photooxida-
tion purposes.
The photosensitized oxidations were carried out in oxygen-
saturated solution of the substrate in which the sensitizer-
incorporated Nafion membranes were immersed. We have
demonstrated by EPR spectroscopy that singlet oxygen could
be generated in Nafion and could diffuse into the solution
15
to oxidize the substrate. It has been established that 2,2,6,6-
1
tetramethyl-piperidine (TMP) reacts with O
2
to give the
2,13
stable free radical nitroxide (TMPO), which can be readily
detected by EPR spectroscopy. Thus, we dissolved TMP in
oxygen-saturated methanol and immersed the complex-
incorporated Nafion membranes in the solution. Figure 2
Figure 1. Structure of the sensitizer.
Platinum(II) terpyridyl acetylide complex (1, Figure 1)14
is chosen as the sensitizer, because this complex absorbs light
1
in the visible region and may photochemically generate O
2
with high quantum yield (see below). Furthermore, this
complex is positively charged and with aromatic ligands. Due
to hydrophobic and electrostatic interactions, 1 can be easily
Figure 2. EPR spectrum of nitroxide radical generated by
irradiation of the oxygen-saturated TMP methanol solution, where
1
-incorporated Nafion was immersed (a) in the dark and (b) after
(9) Lee, P. C.; Meisel, D. J. Am. Chem. Soc. 1980, 102, 5477.
(10) Tung C.-H.; Guan, J.-Q. J. Am. Chem. Soc. 1998, 120, 11874.
(11) Lee, P. C.; Meisel, D. Photochem. Photobiol. 1985, 41, 21.
(12) Maldotti, A.; Molinari, A.; Andreotti, L.; Fogagnolo, M.; Amadelli,
the sample was irradiated for 100 s.
R. Chem. Commun. 1998, 507.
13) Ogumi, Z.; Kuroe, T.; Takehara, Z. J. Electrochem. Soc. 1985, 132,
601.
shows the EPR spectrum obtained after 100 s of irradiation
of the solution and clearly demonstrates the formation of
the nitroxide free radical. This observation can be interpreted
in terms of the fact that methanol can swell the complex-
incorporated Nafion membrane, thus enabling oxygen to
diffuse into the membrane from the solvent. Interaction
between oxygen and the triplet excited state of the complex
results in energy transfer to generate singlet oxygen, which
diffuses back to the solution to react with TMP resulting in
the nitroxide radical.
(
2
4
(
14) Complex 1 was prepared by the reaction of [Pt(trpy)Cl]Cl (trpy )
′-(4-methoxy-phenyl)-2,2′:6′,2′′-terpyridine) with 2 equivalent amounts of
HCCC6H4CCC6H5-4, which was synthesized by the literature method
Khatyr, A.; Ziessel, R. J. Org. Chem. 2000, 65, 3126) in DMF in the
(
presence of catalyst CuI and trimethylamine under nitrogen at room
temperature. Recrystallization of the crude product by diffusion of diethyl
ether vapor into an acetonitrile solution gave 1 as orange crystals. FAB-
+
1
MS: m/z 835(M ). H NMR (600 MHz, DMSO-d6) δ: 3.92 (s, 3H), 7.22
(
2
8
d, 2H, J ) 8.85 Hz), 7.46 (m, 3H), 7.52 (m, 4H), 7.59 (m, 2H), 7.91 (t,
H, J ) 6.64 Hz), 8.19 (d, 2H, J ) 8.81 Hz), 8.53 (t, 2H, J ) 8.13 Hz),
.85 (d, 2H, J ) 8.02 Hz), 8.97 (s, 2H), 9.12 (d, 2H, J ) 5.50 Hz). Anal.
Cacld for C38H26ClN3O5Pt‚0.5H2O: C, 54.07; H, 3.22; N, 4.98; O, 10.42.
-
1
Found: C, 53.84; H, 2.94; N, 4.81; 0, 10.38. IR: ν ) 2118 cm . UV: λab
3
-1
-1
(
6
CH3OH) ) 460 (ꢀ )7560 dm mol cm ). Emission: λem (CH3OH) )
(15) Zang, L. Y.; Frederik, J. G. M. K.; Bibhu R. M.; Hara, P. M.
Biochem. Mol. Biol. Int. 1995, 37, 283 and references therein.
20 nm.
3222
Org. Lett., Vol. 5, No. 18, 2003

Table 1. Oxidation Products of 7-Dehydrocholesterol (2) and R-Pinene (5) Photosensitized by 1-Incorporated in Nafion at Room
Temperature
substrate
photosensitizer
solvent
irradiation time (min)
products
conversion (%)
isolated yield (%)
TPP (1 × 10 5)a
-
CH3OH
CH3OH
CD3OD
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
60
60
60
180
180
120
120
3, 4
3, 4
3, 4
3
3
7
90
20
90
10
95
10
95
75:25
95, minor
95, minor
80
80
95
95
2
2
2
2
2
5
5
CH3OH-swollen 1/Nafion
CD3OD-swollen 1/Nafion
H2O-swollen 1/Nafion
D2O-swollen 1/Nafion
H2O-swollen 1/Nafion
D2O-swollen 1/Nafion
7
a
Photooxidation sensitized by TPP performed in homogeneous solution.
The first substrate we studied is 7-dehydrocholesterol
cholesta-5,7-dien-3â-ol, 2, Scheme 1). This substrate is an
60 min resulted in ca. 90% conversion (Table 1). Under
identical conditions, irradiation of the sample with 1-incor-
porated Nafion as the sensitizer for 60 min only led to ca.
(
2
0% conversion. The low quantum yield in the latter case is
1
probably due to the fact that O
membrane, and a fraction of the generated O
by the solvent during its diffusion into the outside solution.
2
was generated within Nafion
Scheme 1
1
2
was quenched
1
To increase the quantum yield of O
2
, which could diffuse
into the solution outside Nafion, we used deuterium methanol
1
CD
CD
3
OD instead of CH
OD is much longer than in CH
3
OH, because the lifetime of O
2
in
19
3
3
OH. Indeed, irradiation
of 1-incorporated Nafion membranes immersed in oxygen-
saturated solution of 2 in CD OD for 60 min gave ca. 90%
conversion and the mass balance was greater than 95%. We
endogenous compound of the body, involved in a variety of
biological processes, and some of its oxidation products
3
1
6
possess potent cytotoxic and antitumor activity. It was
reported that irradiation of oxygen-saturated solution of 2
in ethanol in the presence of eosin or rose bengal resulted
in cholesta-5R,8R-epidioxy-6-ene-3â-ol (3) and cholesta-5,8-
1
2
determined the quantum yield of the O that had diffused
into the solution outside Nafion by 9,10-diphenylanthrancene
20
bleaching method and found that this quantum yield was
3 3
ca. 0.56 in CD OD and ca. 0.21 in CH OH. The former value
dien-7â-hydroperoxide (4)1 as shown in Scheme 1. Both
7
1
is comparable with the quantum yield of O
TPP sensitizer in benzene (Φ ) 0.56).
2
formation for
1
products were derived from O
2
oxidation and the molar ratio
21
of 3 to 4 was ca. 2.88. We performed the photooxidation by
irradiation of 1-incorporated Nafion membranes that were
To save the expensive deuterium solvent, we performed
the photooxidation by water-swollen Nafion. We used a small
amount of D O to swell the complex-incorporated Nafion
2
membranes and immersed them in oxygen-saturated dichlo-
romethane where the substrate was dissolved. Since dichlo-
romethane cannot swell Nafion and is insoluble in water,
the water in Nafion cannot be extracted into the solution.
On the other hand, the substrate cannot diffuse into the
Nafion because it is insoluble in water. However, oxygen in
the outside solution can diffuse into the Nafion and undergo
-
3
immersed in oxygen-saturated solution of 2 (2.6 × 10 M)
in methanol. A glass filter was used to filter out the light
with λ < 450 nm; thus, 1 was selectively excited. After
irradiation, the products were isolated by column chroma-
tography on silica eluting with petroleum ether/ethyl acetate
1
18
and identified by H NMR and MS spectroscopy, and the
1
data of the HNMR and MS spectroscopy were in agreement
with those reported in the literature. The main product was
16
3
3
, and only a trace of 4 was obtained. The isolated yield of
was greater than 95% on the basis of the consumption of
energy transfer with the triplet state of the sensitizer. The
1
generated O
2
diffuses back to the outside solution to oxidize
the starting material. However, the quantum yield of the
product formation was low. For example, irradiation of
the substrate. Irradiation of such a system for 3 h resulted in
22
ca. 95% conversion (Table 1). This photosensitizer system
seems to be universal for various substrates because we can
change the outside solvent according to the substrate solubil-
ity.
oxygen-saturated solution of 2 in methanol in the presence
1
of tetraphenylporphrin (TPP), a typical O
2
sensitizer, for
(
16) Albro, P. W.; Bilski, P.; Corbett, J. T.; Schroeder, J. L.; Chignell,
C. F. Photochem. Photobiol. 1997, 66, 316.
17) Albro, P. W.; Corbett, J. T.; Schroeder, J. L. Photochem. Photobiol.
994, 60, 310.
18) Compound 3. MS: m/z 416 (M ). H NMR (400 MHz, CD3OD)
(
1
(19) Clennan, E. Tetrahedron 2000, 56, 9151.
(20) Diwu, Z.; Lown, J. W. J. Photochem. Photobiol. A: Chem. 1992,
64, 273.
(21) Rossbroich, G.; Garcia, N. A.; Braslavsky, S. E. J. Photochem. 1985,
31, 37.
(22) In contrast, under the same conditions, irradiation of the H2O-swollen
sample for 3 h only gave ca. 10% conversion.
+
1
(
δ: 0.79 (s, 3H, 13-CH3), 0.89 (s, 3H, 10-CH3), 3.80 (1H, 3R-H, m), 6.26
(
4
3
Hz).
d, J ) 8.5 Hz, 1H, 6-H) and 6.54 (d, J ) 8.5 Hz, 1H, 7-H). 4: MS: m/z
+
1
16 (M ). H NMR (400 MHz, CD3OD) δ: 0.66 (s, 3H, 18-CH3), 1.31 (s,
H, 19-CH3), 3.50 (1H, 3R-H, m), 4.57 (Η-7, bs), 5.73 (H-6, d, J ) 1.44
Org. Lett., Vol. 5, No. 18, 2003
3223

The second substrate we studied is (1S,5S)-(-)-R-pinene
5, Scheme 2]. It was established that the photosensitized
reported that 10 is an important chemical for synthesis of
prostaglandins. We carried out the photooxidation in a glass
tube that contained the mixture of dichloromethane and water
26
[
(
(
1:1 v/v) as shown in the Abstract graphic. The substrate
0.01 M) and thiourea were dissolved in the bottom dichlo-
Scheme 2
romethane layer and the top aqueous layer, respectively. A
piece of 1-incorporated Nafion membrane was swollen by
D O and immersed in the dichloromethane layer. During the
2
irradiation, oxygen was bubbled into the dichloromethane.
The generated 9 in dichloromethane reacted with thiourea
at the interface between water and dichloromethane to yield
the diol 10. Because 10 is very polar, it moved into the
aqueous layer. Thus, at the end of the reaction the unreacted
starting material remains in the dichloromethane layer, the
product is dissolved in the water layer and the sensitizer is
incorporated in the Nafion membrane. These components
in the reaction mixture were readily separated. Generally,
after 180 min of irradiation, the conversion of the starting
material was grater than 95%. Product 10 was obtained with
high purity by separation of the aqueous layer followed by
evaporation of the water and extraction with chloroform.
The isolated yield of 10 was close to 100% on the basis of
the consumption of the starting material.
It is of particular interest that in the above outlined
photooxidations, the sensitizer system is reusable. For
oxidation of 5 to form ene product 6 exclusively is a typical
example of singlet oxygen oxidation reactions.23 We con-
ducted this oxidation in the same manner as that for 2. After
reduction of the reaction mixture with sodium sulfite solution,
the product was isolated by column chromatography on silica
to give (1S,3S,5R)-(+)-trans-3-hydroxypin-2(10)-ene [(+)-
1
trans-pinocarveol, (+)-7]. The data of the H NMR spec-
troscopy of 7 were in agreement with those reported in the
2
3,24
27
literature,
and the isolated yield of this product is shown
O-swollen 1-incorporated
in Table 1. Evidently, by using D
2
Nafion as the sensitizer and dichloromethane as the solvent,
the oxidation proceeded with relatively high quantum yield
and the isolated yield of 7 was greater than 95%.
The third substrate we studied is cyclopentadiene (8,
Scheme 3). This diene undergoes [4 + 2] cycloaddition with
example, in the oxidation of 8, the D
2
O-swollen 1-incorpo-
rated Nafion membranes were reused 20 times without
1
2
significant loss of O -generation efficiency. We will continue
to extend the scope of the application of this sensitizer
system.
Scheme 3
Acknowledgment. Financial support from the Ministry
of Science and Technology of China (Grant Nos. G2000-
078104 and G2000077502) and the National Science Foun-
dation of China is gratefully acknowledged.
OL035012E
(
24) Foote, C. S.; Wexler, S. S.; Ando, W. Tetrahedron Lett. 1965, 4111.
1O
to yield cis-2-cyclopentene-1,4-diol (10). It has been
to yield epidioxide (9), which can be reduced by thiourea
(25) Kaneko, C.; Sugimoto, A.; Tanaka, S. Synthesis 1974, 876.
(26) Johnson, C. R.; Penning, T. D. J. Am. Chem. Soc. 1988, 110, 4726.
2
25
+
+ 1
(
27) Compound 10. MS: m/z 100 (M ), 82 (M - 18) . H NMR (400
MHz, CDCl3) δ: 1.54 (d.t, J ) 15 and 4 Hz, 1H), 2.70 (d.t, J ) 15 and 7
Hz, 1H), 4.65 (d, d, J ) 7 and 4 Hz, 2H), 6.03 (s, 2 H).
(23) Gollnick, K.; Schenck, G. O. Pure Appl. Chem. 1964, 9, 507
3224
Org. Lett., Vol. 5, No. 18, 2003