Polysiloxane-supported fullerene derivative as a new heterogeneous sensitiser for the selective photooxidation of sulfides to sulfoxides by 1O2

Article abstract of DOI:10.1039/b201141g

A heterogeneous photosensitiser for the generation of singlet oxygen was synthesized by covalent attachment of a methanofullerene derivative to polysiloxane-based Deloxan DAP beads and its performance in the photooxidation of sulfides to sulfoxides was investigated. As opposed to C₆₀ and many of its derivatives, the heterogeneous sensitiser can be readily used in protic solvents such as methanol which makes the photooxygenation highly selective, affording good to excellent yields of sulfoxides with aliphatic and benzylic sulfides. Moreover, the sensitiser is easily recovered at the end of the reaction by simple filtration.

Full text of DOI:10.1039/b201141g

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J O U R N A L O F
Polysiloxane-supported fullerene derivative as a new heterogeneous
sensitiser for the selective photooxidation of sulfides to sulfoxides
1
by O2
a
Daniel Latassa, Olivier Enger, Carlo Thilgen, Tilo Habicher, Heribert Offermanns
a
a
a
b
a
C H E M I S T R Y
and Fran c¸ ois Diederich
a
Laboratorium f u¨ r Organische Chemie, ETH-H o¨ nggerberg, CH-8093 Z u¨ rich, Switzerland.
E-mail: diederich@org.chem.ethz.ch; Fax: (141) 1-632-1109; Tel: (141) 1-632-2992
Gr u¨ naustrasse 2, D-63457 Hanau, Germany
b
Received 31st January 2002, Accepted 18th February 2002
First published as an Advance Article on the web 28th February 2002
A heterogeneous photosensitiser for the generation of
singlet oxygen was synthesized by covalent attachment of
a
methanofullerene derivative to polysiloxane-based
1
Deloxan DAP beads and its performance in the photo-
oxidation of sulfides to sulfoxides was investigated. As
opposed to C60 and many of its derivatives, the hetero-
geneous sensitiser can be readily used in protic solvents
such as methanol which makes the photooxygenation
highly selective, affording good to excellent yields of
sulfoxides with aliphatic and benzylic sulfides. More-
over, the sensitiser is easily recovered at the end of the
reaction by simple filtration.
1
Fig. 1 Structural and schematic representation of Deloxan DAP
beads.
organic solvents as well as in water, and shows very good
mechanical and chemical stability.
Concerning the sensitising moiety, fullerene-derived car-
1
Recent improvements in the functionalisation of fullerenes
have significantly accelerated and broadened research on C -
16
boxylic acid 1 was synthesised as previously reported.
It was attached to the primary amino groups of Deloxan
1
6
0
2
based advanced materials. The carbon cages have been known
for nearly a decade to be among the most efficient sensitisers
DAP beads by DCC–HOBt-mediated coupling, thus affording
the heterogeneous sensitiser 2 (Scheme 1). The well defined
and homogeneous photoactive units consist of a mono-
cyclopropanated derivative of C60 with a p-system that is
only slightly perturbed, an important aspect with regard to
3
for the generation of singlet oxygen. In terms of chemical
4
applications, a few literature reports are found on ene and
,5
5
,6
1
[
2
4 1 2]cycloaddition reactions of olefins with O generated
in the presence of fullerenes or fullerene derivatives as photo-
sensitisers. An interesting reaction which has not received any
particular attention in this context is the photooxygenation of
17
the quantum yield of singlet oxygen production.
1
Attachment of the fullerene to the Deloxan DAP beads was
confirmed by IR and MALDI-TOF spectroscopies. According
18
to the method of Berendsen and De Galan, the fullerene
7
sulfides which, depending on the conditions, affords sulfoxides
8
,9
or sulfones. Our interest in this chemical transformation was
raised by the observation that a mono-adduct of C60 having
thioether functionality was cleanly converted to the corre-
loading of the beads 2 was determined from the micro-
analytically measured percentage of carbon to be 3.7 6
24
21
mol g , which corresponds to 22% of functionalised
1
amino groups.
0
2 2
sponding sulfone upon standing in CH Cl solution under
1
0
1
ambient conditions. Chemical uses of O2 produced by
fullerenes are often hampered, however, by the limited
The performance of the heterogeneous sensitiser 2 in the
photooxidation of sulfides to sulfoxides (Scheme 2) was tested
on a variety of thioethers with alkyl, benzyl, or phenyl sub-
stituents. Methanol was used as the solvent of choice according
1
1
solubility of the carbon cages in most solvents or the difficult
recovery of soluble fullerene derivatives at the end of the
reaction. In contrast, polymer-based solid catalysts are easy to
isolate and to recycle after use and they are currently a hot
1
2
topic in both academic research and industrial applications.
We became interested in applying this concept to a fullerene-
1
based heterogeneous photosensitiser in a systematic study on
3
1
the selective photooxidation of sulfides to sulfoxides by O .
This reaction is best performed in polar solvents such as
2
1
4
methanol which is, however, a very poor solvent for
fullerenes. Therefore, heterogeneous sensitisation by poly-
mer-supported derivatives of C60 in methanol seemed quite
advantageous.
1
15
Our choice of solid support was Deloxan DAP,
2
a
macroporous (2.0–2.8 ml g ) polysiloxane bead (0.2–0.5 mm)
1
2
1
with a high loading (1.7 mmol g ) of functionalisable primary
amino groups developed by Degussa AG (Fig. 1). It has a high
1
Scheme 1 Synthesis of the Deloxan
DAP-based heterogeneous
photosensitiser 2. Reagents and conditions: i) DCC, HOBt, THF,
0 uC A 25 uC, 3 d.
2
21
specific surface area (350–500 m
g
), can be handled in
DOI: 10.1039/b201141g
J. Mater. Chem., 2002, 12, 1993–1995
This journal is # The Royal Society of Chemistry 2002
1993

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obtained with sensitiser 2 (Table 1, Entries 2 and 1). This result
can be explained by the very small amount of solubilised
fullerene or the limited accessible surface of the C60 crystallites.
In contrast, the total surface of the macroporous sensitiser 2,
carrying the photoactive C60 units, is very much larger and, in
addition, the appended fullerenes are presumably not aggre-
gating, which makes them easily accessible to the reagents.
When toluene, which is a much better solvent for fullerenes,
was used in combination with pure C60 as sensitiser, small
amounts of sulfoxide and sulfone were produced (Table 1,
Entry 3). Finally, photooxygenation of di(n-butyl) sulfide in
Scheme 2 Photosensitised oxidation of sulfides to sulfoxides.
to the observation of Foote and Peters that protic solvents,
minimising the physical quenching of singlet oxygen, afforded
8
better yields of sulfoxides than aprotic ones. Irradiation
was carried out with a 150 W medium pressure Hg lamp
1
and the progress of the reaction monitored by H-NMR
spectroscopy.
1
the presence of unmodified Deloxan DAP beads—i.e. not
After filtration of the sensitiser beads 2 at the end of
the reaction, the oxidation product was easily isolated from the
starting material by flash chromatography. In each case, the
sulfoxide was identified as the exclusive reaction product by
carrying any fullerene residues—afforded only traces of
sulfoxide. This clearly shows that the beads themselves, or
any impurity that might be included, did not have a
photosensitising effect.
In summary, we have prepared a polysiloxane-supported
fullerene derivative (2) as a new heterogeneous photosensitiser
for the generation of singlet oxygen and proven its effectiveness
in the photooxygenation of sulfides. Preliminary studies have
shown that these substrates are converted to sulfoxides with
high selectivity and in excellent to fair yields by singlet oxygen
generated in methanol in the presence of 2. The protic solvent
methanol plays a decisive role in terms of product selectivity.
1
comparison of its H-NMR spectrum to that of an authentic
sample.
Photooxygenation of di(n-octyl), dibenzyl, and benzyl
methyl sulfides, sensitised by 2, afforded the corresponding
sulfoxides in good to very good yields (Table 1, Entries 1, 6,
and 8). It is remarkable that no side or cleavage reactions
were observed with the benzylic substrates, in particular the
Pummerer rearrangement which is often described in this
1
9
context. Noticeably lower yields of sulfoxide were obtained
from the five-membered heterocycle tetrahydrothiophene
This emphasises the utility of a heterogeneous fullerene-derived
1
sensitiser combining the high quantum yields for O genera-
2
(
Table 1, Entry 4).
52%, Table 1, Entry 5) and from diisopropyl sulfide (28%,
tion of mono-functionalised C60 with its dispersion at the
molecular level. In addition, the heterogeneous sensitiser 2 can
easily be recycled, but the gradual decrease in activity observed
in subsequent reactions needs to be addressed in further studies.
Typical experimental procedure exemplified by the photooxid-
ation of di(n-octyl) sulfide to di(n-octyl) sulfoxide. Molecular
oxygen was bubbled through a suspension of the heterogeneous
photosensitiser 2 (13 mg beads w 4.9 mmol functionalised
fullerene w 1 mol% in relation to sulfide) in MeOH (25 ml)
at 25 uC. Di(n-octyl) sulfide (0.49 mmol) was added and the
mixture stirred and irradiated with a continuously water-
cooled Heraeus TQ 150 W medium pressure mercury lamp
located at 5 cm of the pyrex glass reactor. After 2 h, 2 was
filtered off, washed with MeOH (2 6 25 ml), CH Cl (25 ml),
2
0
In accordance with many literature reports almost com-
plete lack of reactivity was observed for diphenyl sulfide which
afforded only traces of sulfoxide (Table 1, Entry 7). The
markedly reduced nucleophilicity of sulfur in this case,
combined with an increase in steric hindrance as compared
to di(n-alkyl) sulfides may account for this result. Replacement
of one aryl group by an alkyl group activates the substrate
to some extent as observed for methyl phenyl sulfide (27%,
Table 1, Entry 9).
At the end of the reaction, the heterogeneous sensitiser 2
was easily recovered by filtration and recycled by washing
MeOH, CH Cl , Et O) and drying. Reuse in subsequent runs
2 2 2
(
2
2
27
still afforded clean sulfoxides but in noticeably lower yields,
showing that the effectiveness of the sensitiser gradually
decreases. As an example, di(n-octyl) sulfide provided 98%
of the corresponding sulfoxide in the first run, and after
recycling of 2, 60% and 54% in a second and third run,
respectively. This may be related to progressive oxidation of
25
and Et
5 uC, 12 h). The filtrate was evaporated in vacuo and the
sulfoxide was separated from the starting material by flash
chromatography (SiO , hexane–AcOEt 3 : 2). White solid
132 mg, 98%). Mp 71–72 uC (lit. 71–72 uC). IR (KBr):
2
O (25 ml) and dried prior to reuse (10 –10 mbar,
2
2
23
(
1
2
1
2911m, 2844w, 1466s, 1088m, 1005s, 933w, 722m. H-NMR
(CDCl , 300 MHz): 0.87 (t, J ~ 6.9, 6 H, CH ), 1.27–1.29 (m,
6 H, CH ), 1.34–1.45 (m, 4 H, CH ), 1.72–1.79 (m, 4 H, CH ),
.59–2.70 (m, 4 H, CH ). C-NMR (CDCl , 75MHz): 14.07,
2.65 (62), 28.96, 29.07, 29.22, 31.79, 52.58. FAB-MS: 275.2
the active chromophore or to cleavage of fullerene units
2
from the beads.
2
3
3
1
2
2
2
2
2
A few control reactions were run to ascertain the relevance of
the chosen combination of the heterogeneous photosensitiser 2
and the protic solvent methanol. When the photooxygena-
tion of di(n-octyl) sulfide was carried out in the presence of
crystalline C powder which is virtually insoluble in metha-
13
2
3
1
1
(
89, MH ), 297.2 (100, [M 1 Na] ).
6
0
1
1
nol, the yield of sulfoxide was only 7% as compared to 98%
Acknowledgement
1
We are grateful to Degussa AG and the Swiss National Science
Foundation for financial support and to Degussa AG for
Table 1 Oxidation of sulfides to sulfoxides by
presence of 2 or C60 as photosensitisers.
O
2
generated in the
a
1
providing the Deloxan DAP beads. We also thank Dr.
Stephan Wieland for fruitful discussions.
b
Entry Substrate
Sensitiser
Solvent
Time/h Yield (%)
1
2
3
4
5
6
7
8
9
a
(n-C
(n-C
8
8
8
H
H
H
17
17
17
)
)
)
2
2
2
S
S
S
2
MeOH
60 (cryst.) MeOH
2
6
3
4
6
8
4
4
4
98
7
Mixture
28
52
C
C
2
2
2
2
2
2
Notes and references
d
(n-C
60
Toluene
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
b
i
Pr
THT
2
Sc
1
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K. M. Kadish and R. S. Ruoff, Wiley Interscience, New York,
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Bn
Ph
2
S
S
98
2
Traces
90
27
BnMeS
MePhS
2
3
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c d
THT ~ Tetrahydrothiophene. Minor amounts of sulfoxide and
sulfone.
1
994
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