IRA-200 resin-supported platinum(II) complex for photooxidation of olefins

Article abstract of DOI:10.1016/j.tet.2007.03.148

Cyclometalated platinum(II) 4,6-diphenyl-2,2′-bipyridine complex is supported on a commercially available cationic ion-exchange resin (amberlite IRA-200). Photophysical and ESR determinations indicate that the combined photosensitized system is able to generate singlet oxygen with high quantum yield upon irradiation of light in the visible region. The platinum(II) complex loaded on IRA-200 resin is stable and the photooxidation occurs in an environmentally friendly way. Only a simple filtration is needed to recycle the expensive metal catalyst.

Full text of DOI:10.1016/j.tet.2007.03.148

Tetrahedron 63 (2007) 4907–4911
IRA-200 resin-supported platinum(II) complex for
photooxidation of olefins
*
Ke Feng, Li-Zhu Wu, Li-Ping Zhang and Chen-Ho Tung
Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry
and Graduate University, The Chinese Academy of Sciences, Beijing 100080, PR China
Received 19 January 2007; revised 22 March 2007; accepted 23 March 2007
Available online 27 March 2007
Abstract—Cyclometalated platinum(II) 4,6-diphenyl-2,20-bipyridine complex is supported on a commercially available cationic ion-
exchange resin (amberlite IRA-200). Photophysical and ESR determinations indicate that the combined photosensitized system is able to
generate singlet oxygen with high quantum yield upon irradiation of light in the visible region. The platinum(II) complex loaded on IRA-
200 resin is stable and the photooxidation occurs in an environmentally friendly way. Only a simple filtration is needed to recycle the expensive
metal catalyst.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
same functional sulfonic group as that in Nafion, and there-
fore may simplify the surface modification in mesoporous
silica systems.
Though singlet oxygen (1O2) is a versatile reagent for oxida-
tion reactions including cycloaddition reactions to produce
endoperoxides and ene reactions to provide allylic hydroper-
oxides,1–4 the reactivity of molecular oxygen toward most
organic molecules is inhibited by its spin restriction. Over
the past decades, chemists have made tremendous efforts
to realize an effective oxidation reaction by using molecular
oxygen as oxidized reagent, and photosensitized processes
have been demonstrated to be very promising.1–4
In the present work, we shall report that cyclometalated
platinum(II) 4,6-diphenyl-2,20-bipyridine complex can be
loaded on a commercial cationic ion-exchange resin—
amberlite IRA-200. The combined system with relatively
low cost is able to generate singlet oxygen upon irradiation
of light in the visible region, and photosensitized oxidation
occurs in an environmentally friendly way. The quantum
yield for the product formation in this system is slightly
lower than that in SBA-15, but much higher than that in
Nafion case. The platinum(II) complex loaded on IRA-200
resin is also more stable than that in homogenous solution;
only a simple filtration is needed to recycle the expensive
metal catalyst.
Square-planar platinum(II) polypyridyl complexes are ap-
pealing from photochemical perspective due to the unique
properties of open axial coordination site, high population
of triplet state and strong visible absorption.5–13 In the
course of exploring the photochemical reactivity, we re-
cently found that platinum(II) polypyridyl complexes can
operate the energy transfer process with molecular oxygen
to generate singlet oxygen under visible light.14–17 However,
the high costs and low stability in solution render their syn-
thetic application impractical. In this regard, we have incor-
porated platinum(II) polypyridyl complexes into Nafion14,15
and mesoporous silica SBA-1517 by the virtue of noncova-
lent interaction to accomplish the photosensitized oxidation.
As an extension of the previous works, we are interested in
resin-supported platinum(II) complex for photooxidation
of olefins because ion-exchange resin18,19 (1) is much
cheaper than Nafion and SBA-15; (2) IRA-200 resin has
2. Results and discussion
The loading of cyclometalated platinum(II) complex
Pt(L)Cl onto the commercial cationic ion-exchange resin
IRA-200 was achieved by introducing the resin into the so-
lution of Pt(L)Cl. As depicted in Scheme 1, an appropriate
amount of the IRA-200 resin (200 mg) was added to
a Pt(L)Cl acetonitrile solution at a suitable concentration
(0.20 mM, 10 mL). The mixture was stirred for 3 h until
the color of platinum(II) complex solution faded. After the
resin was filtered, rinsed, and dried, the loading level of
0.54 wt % resin powder was obtained. With a similar ap-
proach, a series loadings of platinum(II) complex (0.27,
* Corresponding author. Tel./fax: +86 10 82543580; e-mail: lzwu@mail.
ipc.ac.cn
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.148

4908
K. Feng et al. / Tetrahedron 63 (2007) 4907–4911
Scheme 1. The loading of platinum(II) complex on amberlite IRA-200 resin.
0.54, 1.07, 2.15, 3.22, 4.30 wt %) were prepared for further
experimental use.
Figure 1 shows the absorption profiles of platinum(II)
complex in acetonitrile solution and on IRA-200 resin.
Pt(L)Cl in acetonitrile exhibits intense vibronic-structured
absorption bands at wavelengths below 320 nm with extinc-
tion coefficients of the order of 104 dm3 molꢀ1 cmꢀ1, which
is assigned to the intraligand (IL) character of the cyclo-
metalated polypyridyl ligand, and a less intense band at
380–480 nm with extinction coefficients of the order of
103 dm3 molꢀ1 cmꢀ1, which is ascribed to dp(Pt)/p*
metal-to-ligand charge transfer (MLCT) state. The diffusion
reflectance UV–vis spectroscopy (UV-DRS) is quite similar
to those absorption bands of IL state and MLCT state of
Pt(L)Cl in solution, suggesting that the platinum(II) com-
plex was immobilized onto the IRA-200 resin.
Figure 2. The emission spectrum of platinum(II) complex-loaded IRA-200
powder with loading level of 2.15 wt %.
The emission spectrum of platinum(II) complex-loaded
IRA-200 resin shows the emissive band in the range of
500–700 nm with the maximum at 538 nm (Fig. 2). The life-
time of 2.15 wt % platinum(II) complex-loaded IRA-200
powder was determined to be 2.25 ms. With reference to
spectroscopic work on the Pt(L)Cl in Nafion and SBA-15,
the reaction of 2,2,6,6-tetramethylpiperidine (TMP) and sin-
glet oxygen can be easily detected by ESR spectroscopy.20
Therefore, we introduced TMP into acetonitrile solution,
in which platinum(II) complex-loaded resin was suspended.
Figure 3 shows the ESR spectrum of the sample. Obviously,
nitroxide radicals TMPO were generated when oxygen
3
the emissive state was assigned as a typical MLCT state
of platinum(II) complex.
To assess the 1O2 generation ability of platinum(II) complex-
loaded IRA-200 sample, electron spin resonance (ESR)
spectroscopy was used to examine the reactive oxygen
species. As we know, the stable free radical nitroxide
(2,2,6,6-tetramethylpiperidine oxide, TMPO) arising from
Figure 3. ESR spectra of nitroxide radical generated by irradiation of
platinum(II) complex-loaded IRA-200 powder suspended in an oxygen
saturated acetonitrile solution of TMP. (a) In the dark; (b) the sample was
continuously scanned for 10 cycles under irradiation; the inset shows the
intensity’s increasing trend of the third peak in a scale of ca. 400 s.
Figure 1. The absorption profile of platinum(II) complex in acetonitrile
solution and on IRA-200 resin. The loading level of 2.15 wt % was used.

K. Feng et al. / Tetrahedron 63 (2007) 4907–4911
4909
saturated TMP acetonitrile solution was irradiated under the
laser beam. The signal intensity was increased gradually as
the irradiation time was prolonged. Control experiments re-
veal that light, platinum(II) complex, oxygen, and TMP are
all essential for the production of the ESR signal. Appar-
thiourea to give corresponding cis-cyclodiols 7 and 10.21
The endoperoxy intermediate 922 could be directly observed
at room temperature if no thiourea was added into the sys-
1
tem. H NMR spectrum indicated that the conversion of 8
at high concentration (0.1 M) was 70% and the yield of 9
was about 100%. At the same time, (1S,5S)-(ꢀ)-a-pinene
(11) underwent ene reaction to exclusively afford 12.
Subsequent reduction with triphenylphosphine, (1S,3S,5R)-
(+)-trans-3-hydroxypin-2(10)-ene [(+)-trans-pinocarveol,
(+)-13]23 was obtained as an unique product with 25%
conversion monitored by GC detection. The photooxidation
of 7-dehydrocholesterol (14) afforded products 15 and
16,24,25 a mixture of [4+2] cycloaddition and ene reaction,
3
ently, the emissive MLCT excited state of platinum(II)
complex on IRA-200 operates the energy transfer process
1
with molecular oxygen to generate O2 efficiently and per-
sistently in this combined system.
Photosensitized oxidation of olefins was performed at room
temperature. Three types of 1O2 reactions, [2+2], [4+2] cy-
cloaddition, and ene reaction,4 were involved (Scheme 2).
A small amount (5 mg) of 0.54 wt % platinum(II) com-
plex-loaded IRA-200 powder was added into the reaction
vessel that contains 5 mL of substrates 1 or 3 solution
(1.0 mM). The mixed solution was irradiated with a 500 W
Hg lamp with oxygen bubbled in. A quartz jacket with water
circulation was used to cool the lamp. A light filter was
placed outside to cut off light below 450 nm and guarantee
irradiation with visible light. After irradiation, the mixture
was centrifuged under 3000 rpm for 15 min, and the upper
liquid was separated for further gas chromatograph (GC) de-
tection. These two rich electron stilbene derivatives 1 and 3
underwent [2+2] cycloaddition to afford the dioxetane inter-
mediates, which were quickly converted to their correspond-
ing aldehyde 2 and ester 4. The GC results indicated that the
activities of the reactions are very high. Both the conversion
and yields were close to 100%. The oxidation of olefins un-
der fixed ratio of resin to substrate [5 mg/5 mL (1.0 mM)]
was also undertaken, which exhibited a nearly linear rela-
tionship with the loading level of IRA-200. This observation
indicated that the oxidation rate of photooxidation depended
on the loading level of platinum(II) complex on IRA-200
resin. Despite of this, a certain extent of photosensitizer
leaching was detected in the reaction vessel as the loading
level was increased to 4.30 wt %. In this situation,
0.54 wt % platinum(II) complex-loaded IRA-200 was em-
ployed as the photosensitizer for all reactions.
1
the molar ratio of 3:1 could be directly validated from H
NMR spectroscopy. Product 15 could be separated by col-
umn chromatography on silica, while product 16 could not
endure such treatment. All the above results were summa-
rized in Table 1 and the mass balance was greater than 95%.
Substrate 3 was selected to study reaction kinetics and recy-
cle of the catalyst. Time course study revealed that the photo-
oxidation was completed in about 90 min (Fig. 4). Almost
no decrease in efficiency of the combined photosensitizer
Table 1. Conversion and yield for 1O2 photooxidation of olefins
Substratea Solvent
Irradiation Product Convt. Yield
time (h)
(%)
(%)
1
3
5
8
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CD3CNg
CH3CN
CH3CN
CD3CNg
2
1.5
3
3
3
2
3
4
2
2
4
7
10
9
9
13
15, 16
15, 16
w100b w100b
w100b w100b
—
—
>95c,d,e
>95c,d
>95c,d
w100f
—
70f
22c
—
11
14
w100c
95, —c,d
72, 28c
>95c
a
The loading level of catalyst was 0.54 wt %.
Concentration of the substrates was about 1.0 mM.
Concentration of the substrates was about 0.01 M.
Yields were calculated based on the consumption of the substrates.
Oligomers of cyclopentadiene were excluded.
b
c
d
e
f
Cyclodienes 5 and 8 underwent [4+2] cycloaddition with
1O2 to yield endoperoxides, which could be reduced by
Concentration of the substrates was about 0.1 M.
Tetramethylsilane was used as the internal standard.
g
OH
Pt(L)-IRA
O2, h ( >450nm)
Ph
H
H
O2, h ( >450nm), thiourea
PhCHO
2
7
Pt(L)-IRA
Ph
O
1
6
5
OH
OH
O
O2, h ( >450nm)
Ph
H3CO
OCH3
Ph
PhCOOCH3
4
Pt(L)-IRA
O2, h ( >450nm), thiourea
10
3
Pt(L)-IRA
O
,
2
h
(
8
OH
>
4
5
9
P
t
(
L
)
OOH
OH
0
n
O2, h ( >450nm)
PPh3
-
I
R
m
)
A
O
O
Pt(L)-IRA
11
12
13
O2, h ( >450nm)
+
O
O
Pt(L)-IRA
HO
HO
HO
OOH
14
15
16
Scheme 2. Photooxidation of olefins.

4910
K. Feng et al. / Tetrahedron 63 (2007) 4907–4911
Figure 4. Kinetic plot and repeating reaction of substrate 3.
was observed in the first three runs, though in the next three
runs the conversion decreased to 90% and in the final run the
conversion was 85%. The platinum(II) complex-loaded on
IRA-200 resin is much stable than that in solution. Only
a simple filtration is needed for the separation and recycle
of the expensive metal catalysts. Compared with the hetero-
genous systems in our previous work, the quantum yield26 for
the product formation (0.24) in IRA-200 resin system is
slightly lower than that of SBA-15 (0.33); while both sys-
tems are much more efficient than that in Nafion case.
Hitachi F-4500. UV-DRS spectra were measured on JASCO
UV-550. Solid state luminescent decay profiles of IRA-200
resin-supported platinum(II) complex were obtained by Ed-
inburgh LP 920 at 538 nm using the third harmonic (355 nm)
of a pulse Nd: YAG laser as excitation resource. Gas chro-
matograph was monitored on Shimadzu GC-14B. ESR spec-
troscopic experiments were carried out at room temperature
(298 K) with a Bruker ESP 300E spectrometer.
Acknowledgements
3. Conclusion
We are grateful for financial support from the National
Science Foundation of China (Nos. 20333080, 20332040,
20472091, 50473048, 20472092, and 20403025), the
Ministry of Science and Technology of China (Grant Nos.
2003CB716802, 2004CB719903, 2006CB806105, and
G2007CB808004), and the Bureau for Basic Research of
the Chinese Academy of Sciences.
Cyclometalated platinum(II) 4,6-diphenyl-2,20-bipyridine
complex was successfully supported on a commercial avail-
able cationic ion-exchange resin IRA-200. Photophysical
and ESR determinations demonstrate that the IRA-200
resin-supported platinum(II) complex is able to generate sin-
glet oxygen with high quantum yield upon irradiation of
light in the visible region. Cycloaddition and ene reactions
of singlet oxygen with a range of substrates could occur ef-
ficiently upon introduction of platinum(II) complex-loaded
IRA-200 resin into the reaction vessel. The IRA-200 resin-
supported platinum(II) complex is a mild and recyclable
alternative to solution-phase photooxidation sensitizer. Only
a simple filtration is needed for the separation and recycle
of the expensive metal catalyst.
References and notes
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K. Feng et al. / Tetrahedron 63 (2007) 4907–4911
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(dt, J¼15 and 7 Hz, 1H), 4.65 (dd, J¼7 and 4 Hz, 2H),
6.03 (s, 2H). Compound 10: MS: m/z 114 (M+); 1H NMR
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