[60]Fullerene supported on silica and γ-alumina sensitized photooxidation of olefins: Chemical evidence for singlet oxygen and electron transfer mechanism

Article abstract of DOI:10.1055/s-2004-820041

Fullerene C₆₀ supported on silica and γ-alumina (2% w/w C₆₀/SiO₂ and C₆₀/Al₂O₃) sensitizes the photooxidation of alkenes via singlet oxygen and/or electron transfer mechanism, depending on

Full text of DOI:10.1055/s-2004-820041

LETTER
971
[60]Fullerene Supported on Silica and g-Alumina Sensitized Photooxidation
of Olefins: Chemical Evidence for Singlet Oxygen and Electron Transfer
Mechanism
Photooxidation of
eO
lefins orgios C. Vougioukalakis,a Yiannis Angelis,a John Vakros,b George Panagiotou,b Christos Kordulis,b
Alexis Lycourghiotis,b Michael Orfanopoulos*a
a
Department of Chemistry, University of Crete, 71409 Iraklion, Crete, Greece
Fax +30(2810)393601; E-mail: orfanop@chemistry.uoc.gr
b
Department of Chemistry, University of Patras and Institute of Chemical Engineering and High-Temperature Chemical Processes
(ICE/HT), 26500 Patras, Greece
Received 31 January 2004
Herein we report the generation and trapping of 1O2 in a
Abstract: Fullerene C60 supported on silica and g-alumina (2%
‘two phase’ system, by using Silica and g-Alumina-bound
fullerene C60 as sensitizer and a well characterized singlet
w/w C60/SiO2 and C60/Al2O3) sensitizes the photooxidation of
alkenes via singlet oxygen and/or electron transfer mechanism, de-
oxygen olefin (2-methyl-2-heptene) acceptor. We also re-
port the photooxygenation of suitable aryl substituted ole-
fins by both singlet oxygen and/or electron transfer
mechanism.8
pending on the solvent and the substrate.
Key words: heterogeneous photooxidations, fullerene C60, alkenes,
singlet oxygen, electron transfer
Supported photosensitizers were prepared by depositing
C60 on g-Al2O3 [Akzo, specific surface area: 265 m2g–1,
size: 100–150 mesh powder, pore volume: 0.76 mLg–1]
and SiO2 [Alfa Aesar, specific surface area: 175–200
m2g–1, size: ca. 325 mesh powder, pore volume: 1.6
mLg–1] surfaces.9 Incipient wetness impregnation was
used for the above deposition. The impregnating solution
was prepared by dissolving the necessary amount of C60 in
1,2-dichloro-benzene in order a loading of 2% w/w C60 to
be achieved in the final catalysts. The impregnated
samples were dried at 180 °C for 4 hours in air. Impreg-
nated and/or final (dried) samples were characterized us-
ing UV/Vis diffuse reflectance spectroscopy, nitrogen
physioroption for specific surface area measurements and
carbon analysis.10
Fullerene C60 is an electron-deficient compound with rich
photophysical and photochemical properties.1 The triplet-
excited state of C60 is formed almost quantitatively with a
lifetime, in solution, between 40 and 280 ms.2 In the pres-
ence of molecular oxygen, energy transfer from the trip-
let-excited state of C60 to the ground state oxygen
produces singlet oxygen with quantum yield near unity
(Equation 1).1
3O2
hν
ISC
1C60
3C60
C60
1O2
Equation 1
This useful photochemical property characterizes C60 as
an efficient sensitizer for the mild homogeneous photoox-
ygenation of unsaturated substrates such as alkenes and
dienes.3 Additionally, fullerenes are easily reduced and
quenching of their triplet states by electron donors occurs
efficiently in polar solvents. An electron transfer mecha-
nism has been proposed in order to rationalize these re-
sults.4
Photosensitized oxygenations were carried out in a 4 mL
Pyrex cell containing a 0.03 M solution of 2-methyl-2-
heptene (1) in various solvents in the presence of 3.6 mg
of the insoluble catalyst (2% C60 on Al2O3 or 2% C60 on
SiO2, 10–4 M in C60) in 1 mL of solvent. The solution mix-
ture was irradiated for a period of 30 min, using a 300W
xenon lamp as the light source, while bubbling dry oxygen
at 0 °C. The photooxygenation reaction was monitored by
GC-MS (after reduction of the allylic hydroperoxides to
the corresponding alcohols with triphenyl phosphine) and
Furthermore, fullerene C60 derivatives that bear an oxidiz-
able group are easily oxidized in the presence of oxygen
and light. A self-sensitized photooxygenation mechanism
rationalizes the formation of the oxygenated adducts.5
However, the very small solubility of C60 in medium polar
to polar solvents limits its utility as a photosensitizer.6 In
previous studies, fullerene-coated beads have been pre-
pared and used as photocatalyst for the production of
singlet oxygen. However, in all cases, fullerene was de-
rivatized and bound to the surface by a chemical reaction.7
1
the products were identified by GC-MS and H NMR
spectroscopy. Allylic hydroperoxides 1a and 1b were the
1
only detected products by H NMR. The results are
summarized in Table 1.
As seen in Table 1, the ratio of the allylic hydroperoxides
1a:1b in non polar solvents is similar, within experimental
error, to those observed with conventional singlet oxygen
sensitizers such as tetraphenyl-porphyrine (TPP) or rose
bengal (RB).
SYNLETT 2004, No. 6, pp 0971–0974
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6.
0
5.
2
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0
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Advanced online publication: 25.03.2004
DOI: 10.1055/s-2004-820041; Art ID: G03404ST
© Georg Thieme Verlag Stuttgart · New York

972
G. C. Vougioukalakis et al.
LETTER
Table 1 Sensitized Photooxidation of 2-Methyl-2-heptene
nitrile. In hexane the allylic hydroperoxides 2a and 2b
were formed exclusively, in 9:1 ratio, whereas in aceto-
nitrile, along with allylic hydroperoxides 2a and 2b, small
amounts of acetophenone (2c), and 1-phenyl-1,2-dimeth-
yloxirane (2d) were formed. For comparison reasons, sen-
sitized photooxygenations of 2 were also performed in the
presence of three conventional sensitizers, rose Bengal
(RB), methylene blue (MB) and 9,10-dicyanoanthracene
(DCA). The first two, RB and MB, are well known singlet
oxygen photosensitizers, while DCA, an electron defi-
cient sensitizer, catalyzes the photooxidation reaction by
both electron transfer and/or singlet oxygen mechanism
depending on solvent polarity and substrate.13
+
HOO
OOH
1b
1
1a
Sensitizer
Solvent
Conversion
(%)a,b
1a:1bd
C60/SiO2
C60/Al2O3
C60/SiO2
C60/Al2O3
C60/SiO2
C60/Al2O3
C60/SiO2
C60/Al2O3
Hexanes
Hexanes
MeCN
MeCN
MeOH
CH3OH
DMSO
DMSO
H2O
46
24
46:54
46:54
52:48
52:48
47:53
47:53
54:46
54:46
43:57
43:57
43:57
43:57
50:50
52:48
16
Irradiation of 2 in the presence of MB or RB as the sensi-
tizers gave exclusively a mixture of the two allylic hydro-
peroxides 2a and 2b in a ratio 9:1. Hydroperoxides 2a and
2b are typical singlet oxygen products. The ratio of the al-
lylic hydroperoxides 2a and 2b has been shown to be in-
dependent on solvent polarity or the para-substitution of
the phenyl ring of arylalkene 2.14 These results are indic-
38
7
30
15
21
1
ative of O2 mechanism. Addition of DABCO, which
c
C60/SiO2
65
quenches the triplet state of C60 as well as singlet oxy-
gen,15 retarded the photocatalytic activity of both C60/SiO2
and C60/Al2O3.
c
c
C60/Al2O3
H2O
76
c
C60/SiO2
D2O
94
Furthermore, the DCA-sensitized photooxygenation of 2
in acetonitrile gave, apart of the allylic hydroperoxides 2a
and 2b, appreciable amounts of acetophenone 2c,
(Table 2).
C60/Al2O3
RB
D2O
100
100
100
MeCN
Hexanes
TPP
Acetophenone (2c) is most likely produced from 1,2-di-
methyl-2-phenyl-1,2-dioxetane (Equation 2) which, un-
der the reaction conditions, is expected to decompose to
cleavage products 2c and acetaldehyde. Epoxide 2d could
be formed by a mechanism similar to that suggested by
Bartlett and Landis (Scheme 1).16 Analogous mechanistic
rationalization has been reported previously on the DCA
sensitized photooxygenation of diphenylethylene.13 Simi-
larly, the C60/SiO2 and C60/Al2O3 sensitized photooxygen-
ation of 2 in acetonitrile gave, in addition to allylic
hydroperoxides 2a and 2b, small amounts of epoxide 2d.
These results are consistent with both singlet oxygen and
electron transfer mechanism.
a All measurements were taken after 30 min of irradiation.
b Determined by GC-MS after reduction to allylic alcohols with Ph3P:
error 1%.
c A small amount of MeCN (H2O or D2O/MeCN: 15/1) was used as a
co-solvent in order to dissolve the alkene in H2O and D2O.
d Determined by 1H NMR: error 3%.
Unlike the regioselectivity observed in intrazeolite11 pho-
tooxidations, at the present system, there is no notable
1
change in regioselectivity with that of the O2 homoge-
neous solution behavior. A reasonable explanation of this
result is that the mean pore diameter of silica and alumina
surfaces, used in this work, is in the order of 70–80 Å,
much larger than the corresponding zeolites of ca. 8 Å.
This fact, as well as the absence of cations in the present
system that could develop cation-p interactions, lead to
the absence of any induced special conformational effects
in the corresponding substrates.
O
C
O
C
O
Ph
+
CH3CHO
2c
Equation 2
In deuterated water the reaction conversion is higher than
that in non-deuterated water. It is well established that the
lifetime of O2 is significantly longer in deuterated than
protiated solvents. This solvent isotope effect has been
used in the past as strong evidence for the involvement of
1O2 in the reaction mechanism.12
To probe the C60/SiO2 and C60/Al2O3 sensitized photoox-
ygenation mechanism further and obtain additional infor-
mation on the electron transfer efficiency of these
surfaces, we photooxidized 1-methoxy-4-[1-(4-methox-
yphenyl)vinyl]benzene (3).
1
The results from the irradiation of 2-(E)-2-phenyl-butene
(2) with a variety of sensitizers and solvents are shown in
Table 2. C60 on SiO2 and C60 on Al2O3 effectively sensi-
tize the photooxidation of 2 both in hexane and aceto-
Substrate 3 is appropriate for an electron transfer test
since it is an electron rich olefin, not reactive under singlet
oxygen conditions. As presented in Equation 3, the C60/
SiO2 and C60/Al2O3 sensitized photooxygenation of 3 in
Synlett 2004, No. 6, 971–974 © Thieme Stuttgart · New York

LETTER
Photooxidation of Olefins
973
Table 2 Sensitized Photooxidation of 2-(E)-2-Phenyl-butene (2)a
O
O
HOO
OOH
2
2a
2b
2c
2d
Sensitizerb
Solvent
Irradiation time (min) Conversion (%)c
Relative yields (%)
2a + 2bc,d
2cc
2dc
–
Methylene blue
Rose bengal
DCA
CHCl3
MeCN
MeCN
Hexanes
MeCN
MeCN
Hexanes
MeCN
MeCN
30
30
30
20
60
80
60
60
80
97
93
100
100
80
–
–
–
99
20
–
–
e
C60/SiO2
100
100
0
100
89
–
C60/SiO2
7
4
C60/SiO2 (DABCO)
–
–
–
e
C60/Al2O3
100
100
0
100
94
–
–
C60/Al2O3
4
2
C60/Al2O3(DABCO)
–
–
–
a Solutions of 0.03 M.
b Solutions of 10–4 M.
c The reaction was monitored by GC-MS on a Supelco capillary column (SPB-5, 30m) in a Shimatzu GCMS-QP5050 (CI mass detector).
The error was 1%.
d The product ratio 1a:1b is, constant at 9:1, within experimental error.
e Photocatalysts were prepared by literature methods.9,10
acetonitrile gave exclusively and quantitatively the cyclic
MeO
OMe
MeO
endoperoxide 3a. In a previous work Gollnick and co-
workers showed that DCA photosensitizes the exclusive
formation of endo-peroxide 3a under similar conditions to
these reported here and shown in Equation 3.17 In a con-
trol experiment, free fullerene C60 under identical condi-
tions to those reported in Equation 3, gave no oxygenated
dimer 3a. Additionally, the production of 3a was
quenched when a small amount (<10% of the correspond-
ing alkene) of 1,2,4-trimethoxybenzene was added to the
reaction. This result is consistent with an electron transfer
C60 on SiO2 or Al2O3
CH3CN/hv/O2
O
O
MeO
MeO
OMe
3
3a
Equation 3
3O2
subs
mechanism, involving a donor 3 radical cation and the
sensitizer radical anion, since it is well established that do-
nor molecules with oxidation potentials lower than that of
the oxidizing alkene, quench the electron transfer path
from the alkene to DCA.13 It is not clear, at the moment,
why the irradiation of C60 supported silica and alumina
surfaces in acetonitrile promote electron transfer mecha-
nism to arylalkene substrates whereas free C60, under sim-
ilar conditions, does not. It is reasonable to assume that
special interactions may develop between C60 and alumi-
na or silica surfaces. These interactions may play a crucial
role on the electronic properties of the absorbed C60
molecule.
hv
–•
[sens + subs+• + O2
]
sens*
[sens–• + subs+•
]
sens
–•
O2
O
O
subs+•
O
O
O
O
O
O
–O2
2
2
2d
Scheme 1
Synlett 2004, No. 6, 971–974 © Thieme Stuttgart · New York

974
G. C. Vougioukalakis et al.
LETTER
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by a simple filtration, c) the heterogeneous nature of the
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Acknowledgment
We thank the Greek Secretariat of Research and Technology
(PENED 2001) for financial support. The Greek National Scholar-
ships Foundation (I. K. Y.) is also acknowledged for providing a
fellowship to G. C. Vougioukalakis.
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Synlett 2004, No. 6, 971–974 © Thieme Stuttgart · New York