Photophysical properties of three methanofullerene derivatives

Article abstract of DOI:10.1002/(SICI)1521-3765(19980210)4:2<270::AID-CHEM270>3.0.CO;2-5

The [6,6]-ring bridged methanofullerenes 61,61-bis[4-tert-butylbenzoate]-1,2-dihydro-1,2-methanofullerene[60] (1), 61,61-bis[4-(tert-butyldiphenylsilyloxymethyl)phenyl]-1,2-dihydro-1,2-methano fullerene[60] (2), and 61-[(ethoxycarbonyl)methylcarboxylate]-

Full text of DOI:10.1002/(SICI)1521-3765(19980210)4:2<270::AID-CHEM270>3.0.CO;2-5

FULL PAPER
Photophysical Properties of Three Methanofullerene Derivatives
²
Rene V. Bensasson,* Elisabeth Bienvenüe, Claude Fabre, Jean-Marc Janot, Edward J.
Land, Sydney Leach, Virginie Leboulaire, Andre Rassat,* SteÂphane Roux, and Patrick Seta
Abstract: The [6,6]-ring bridged meth-
anofullerenes 61,61-bis[4-tert-butylben-
zoate]-1,2-dihydro-1,2-methanofuller-
ene[60] (1), 61,61-bis[4-(tert-butyldiphe-
nylsilyloxymethyl)phenyl]-1,2-dihydro-
1,2-methanofullerene[60] (2), and 61-
[(ethoxycarbonyl)methylcarboxylate]-
1,2-dihydro-1,2-methanofullerene[60]
(3) were synthesized by the Diederich
method (3) or the Wudl method via
tosylhydrazone salts (1 and 2). Ground-
state absorption spectra of the metha-
nofullerenes in toluene and cyclohexane
solutions are presented and possible
assignments discussed. The sharp,
430 nm transition is associated with the
existence of less than 60 p electrons,
whereas a broad band peaking at 495 nm
as well as weak features in the 700 nm
region could be related to forbidden
transitions of C60. The allowed transi-
tions of C60 in the UV region are
modified relatively little in the metha-
nofullerenes. Laser flash photolysis and
pulse radiolysis techniques were used to
obtain triplet ± triplet absorption spectra
between 400 and 1100 nm and deter-
mine photophysical properties. The
three methanofullerenes have very sim-
ilar T± T spectra, with a strong peak at
720 nm whose molar absorption coeffi-
for 1O2(1Dg) production by energy trans-
fer from the triplet state of the three
methanofullerenes, implying that the
quantum yield of triplet production of
each is ꢀ 1. The results indicate that it is
possible for the spectroscopic and pho-
tophysical properties of methanofuller-
enes to vary little with the nature of the
methano adduct and to undergo only
slight modifications with respect to the
corresponding properties of C60. Up to
now, this has been found valid only when
the functionalization does not involve an
electron donor. The biological photo-
sensitization efficiencies of these meth-
anofullerenes are therefore expected to
be similar to those of C60 but with their
hydrophobicity and intracellular site
delivery modulated by the nature of
the methano-adduct.
1
cient is of the order of 14000m 1 cm ,
1
somewhat lower than the 20200m 1 cm
measured for the corresponding 750 nm
band of C60. Values near unity were
determined for FD, the quantum yield
Keywords: fullerenes
´
photo-
chemistry
triplet-state
´
synthetic methods
properties
´
´
UV/Vis spectroscopy
Introduction
The first synthesis of a methanofullerene[1] has been followed
by other related functionalizations.[2±5] Certain such modified
fullerenes have been shown to exhibit photosensitizing
properties towards living cells, enzymes, viruses, and DNA,
including photoinduced DNA cleavage[6] by singlet oxygen
production.
To help quantify the efficiencies of such photoprocesses for
modified fullerenes, we have synthesized three methanoful-
lerenes, using Diederich[7,8] and Wudl methods,[9] and deter-
mined their spectroscopic and photophysical properties.
These three methanofullerenes are: 61,61-bis[4-tert-butylben-
zoate]-1,2-dihydro-1,2-methanofullerene[60] (1), 61,61-bis[4-
(tert-butyldiphenylsilyloxymethyl)phenyl]-1,2-dihydro-1,2-
methanofullerene[60] (2), and 61-[(ethoxycarbonyl)methyl-
carboxylate]-1,2-dihydro-1,2-methanofullerene[60] (3), the
[6,6]-closed structures of which are shown here. For such
molecules, bridging can occur at either [6,6]- or [6,5]-ring
junctions. In [6,6] structures, an originally double bond of C60
[*] Prof. R. V. Bensasson
Laboratoires de Biophysique et de Photobiologie
Â
Museum National dꢀHistoire Naturelle, CNRS UA481
INSERM U201, 43 rue Cuvier, F-75231 Paris Cedex 05 (France)
Fax: Int. code ‡ (33)140793705
e-mail: rvb@mnhn.fr
E. Bienvenüe, J.-M. Janot, P. Seta
Â
 Â
Laboratoire des Materiaux et Procedes Membranaires
UMR 5635 du CNRS
1919 Route de Mende, F-34293 Montpellier Cedex 5 (France)
C. Fabre,[²] V. Leboulaire, A. Rassat, S. Roux
Laboratoire de Chimie, Ecole Normale Superieure
24 rue Lhomond, F-75231 Paris Cedex 05 (France)
Â
E. J. Land
CRC Department of Biophysical Chemistry
Paterson Institute for Cancer Research
Christie Hospital NHS Trust, Manchester M209BX (UK)
S. Leach
DAMAP, CNRS URA 812, Observatoire de Paris-Meudon
F-92195 Meudon (France)
[²] Deceased 25 March 1997.
270
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Chem. Eur. J. 1998, 4, No. 2

270±278
synthesis are converted exclusively to the [6,6]-closed, most
stable, isomer.[3,7,10]
CO2tBu
CH2OSitBuPh2
We studied the ground-state and triplet excited-state
absorption spectra of the three methanofullerenes 1, 2, and
3. The samples contained exclusively the [6,6]-closed isomer.
Using C60 as reference, we determined the quantum yield for
the sensitized formation of singlet oxygen, FD, the triplet
quantum yield of formation, FT, and the triplet molar
absorption coefficient, eT, for the three C61 derivatives.
CO2tBu
CH2OSitBuPh2
1
2
H
CO2CH2CO2Et
Results and Discussion
3
Methanofullerene syntheses: The methanofullerenes 1 and 2
have been prepared by the reaction of tosylhydrazone salts,[9]
formed in situ, with C60. The different steps in methanofuller-
ene 1 synthesis are described in Scheme 1. Starting from p-
dimethylbenzophenone, compound 4 was prepared with a
60% crude yield.[11] The white powder thus obtained is
contaminated with terephthalic acid as by-product of the
is bridged, whereas an originally single bond is bridged in [6,5]
structures. The two most stable isomers are the [6,6]-closed
and [6,5]-open.[2] Upon heating, the mixture of [6,6]-closed
and [6,5]-open isomers formed together in the course of the
O
O
Â
Â
Â
Abstract in French: Nous avons synthetise par la methode de
KMnO4
H2O
KOH
Â
Á
Diederich ou de Wudl trois methanofullerenes de type [6,6]
Â
Â
ferme: le 61,61-bis[4-tert-butylbenzoate]-1,2-dihydro-1,2-me-
thanofullerene[60] (1), le 61,61-bis[4-(tert-butyldiphenylsilyl-
oxymethyl)phenyl]-1,2-dihydro-1,2-methanofullerene[60] (2)
∆
HO2C
CO2H
Á
Â
4
Â
Â
Â
Á
Â
Â
et le 61-[(ethoxycarbonyl)methylcarboxylate] - 1,2-dihydro-
PCl5
Toluene∆
Â
Á
Á Â
1,2-methanofullerene[60] (3). Les spectres dꢀabsorption a lꢀetat
Â
Á
fondamental de ces methanofullerenes en solution dans le
O
O
Á
Â
Â
Â
toluene et le cyclohexane sont presentes et les attributions des
Pyridine
∆
Â
Á
 Á
bandes sont discutees. La bande etroite a 430 nm est associee a
ClOC
COCl
Â
Â
un defaut dꢀelectrons p par rapport au C60, tandis que la bande
t-BuO2C
CO2t-Bu
t-BuOH
Â
Á
large centree a 495 nm ainsi que les faibles bandes dans la
6
5
Â
Ã
Á
region de 700 nm pourraient etre dues a des transitions qui sont
TsNHNH2
∆
EtOH
interdites dans le cas du C60. Les transitions permises du C60
Â
dans lꢀultra-violet sont relativement peu modifiees dans les
H
Â
Á
Â
methanofullerenes. Les techniques de photolyse par eclair et de
radiolyse pulsee ont permis dꢀobtenir les spectres dꢀabsorption
N
S
N
Â
O2
Â
Â
Â
de lꢀetat triplet entre 400 et 1100 nm, caracterises pour les trois
Â
Á
methanofullerenes par un coefficient molaire dꢀabsorption de
t-BuO2C
CO2t-Bu
CO2t-Bu
1
lꢀordre de 14000m 1 cm au maximum dꢀabsorption a 720 nm,
Á
7
NaH, ODCB
inferieur a celui du C60 (20200m 1 cm au maximum dꢀab-
1
Â
Á
Á
Â
Â
Â
sorption a 750 nm). Pour les trois molecules etudiees, le
C60
Á
rendement quantique, FD, de production dꢀoxygene singulet,
∆
1
1
Â
O2( Dg), est proche de lꢀunite, ce qui implique un rendement
CO2t-Bu
Â
quantique de formation de lꢀetat triplet FT voisin de 1. Ces
Â
 Â
Á
resultats indiquent que les proprietes spectroscopiques et
1
Â
photophysiques de nos methanofullerenes varient peu avec la
Â
Â
nature des substituants du pont methano et ne presentent pas de
modifications importantes par rapport au C60. Ceci ne reste
vrai que si la fonctionalisation nꢀintroduit pas de donneur
Scheme 1. The synthesis of methanofullerene 1.
Â
Â
Á
Â
Â
dꢀelectron. Les methanofullerenes, du type etudie dans le
Â
Â
present travail, devraient conserver une efficacite de photo-
reaction (about 50%). Compound 4 was converted into 5 by
phosphorus pentachloride, and pure 5 was obtained by
fractional crystallization (corrected yield 72%).[12] Compound
5 was esterified by tert-butanol in the presence of pyridine,
giving 6 (77.8% yield), which was converted to 7 by
sensibilisateur proches de celle du C60. Des modifications
appropriees des substituants du pont methano devraient
modifier leur lipophilie et de ce fait moduler leur repartition
entre differents sites intracellulaires.
Â
Â
Â
Â
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271

R. V. Bensasson, A. Rassat et al.
FULL PAPER
tosylhydrazide in boiling ethanol (83% yield). Molecule 7
reacts with C60, in 1,2-dichlorobenzene (ODCB) and in the
presence of NaH, to give methanofullerene 1 (47.6% yield,
single isomer after equilibration and satisfactory structure
determination).[13]
The steps of the synthesis of methanofullerene 2 are
depicted in Scheme 2. Compound 8, prepared from p-
dimethylbenzophenone,[14] was hydrolyzed to 9 with the
Absorption spectra of the ground singlet state of the C61
derivatives
General characteristics: Room-temperature absorption spec-
tra of the three methanofullerenes 1, 2 and 3 in toluene and
cyclohexane are shown in Figures 1a and 1b, respectively.
In the visible region, the absorption spectra of the C61
methanofullerenes exhibit three distinct features whose
principal peaks and molar absorption coefficients in toluene
(Figure 1a) are:
1
for 1: a sharp band at 432 nm (2410m 1 cm ), a broad band
1
peaking at 492 nm (1520m 1 cm ), and a group of weak
O
O
Hg(ClO4)2
structures in the 650 ± 700 nm region whose strongest feature
BrH2C
HOH2C
1
is at 694 nm (219m 1 cm );
DME/H2O
30-40°C
2
1
for 2: similar features at 433 nm (2660m 1 cm ), 490 nm
2
8
9
1
1
(1910m 1 cm ) and 698 nm (215m 1 cm );
for 3: similar features at 429 nm (2620m 1 cm ), 492 nm
1
1
1
(1510m 1 cm ) and 691 nm (195m 1 cm ).
t-BuPh2SiCl
THF
DMAP
NEt3
O2
S
HN
Reflux
N
O
Toluene
Ph2t-BuSiOH2C
O
TsNHNH2
Ph2t-BuSi
2
2
11
10
C60
OSit-BuPh2
NaH
ODCB
∆
OSit-BuPh2
2
Scheme 2. The synthesis of methanofullerene 2.
assistance of mercuric ions[15] in dimethoxyethane/water
(72% yield). Derivative 9 was protected by tert-butyldiphen-
ylsilylchloride[16] (71% yield), then converted to 11 (87%
yield) with tosylhydrazide in boiling toluene. Tosyl hydrazone
11 reacts with C60, in 1,2-dichlorobenzene and in the presence
of NaH, to give methanofullerene 2 in 64% yield (corrected;
25% of starting C60 recovered), as a single isomer.
Methanofullerene 3 was synthesized according to Isaacs
and Diederich[8] and converted into the most stable isomer,[3,4]
the [6,6]-closed form, by heating. The characterization results
were consistent with those previously obtained by these
authors.
Figure 1. Ground-state absorption spectra, recorded at room temperature,
of methanofullerenes 1, 2, and 3: a) in toluene, 10 4 m, 400 ± 750 nm (inset:
detail of 380 ± 450 nm region); b) in cyclohexane, 10 4 m, 400 ± 750 nm
(inset: 10 5 m, detail of 200 ± 450 nm region).
In the aliphatic solvent cyclohexane, the corresponding
band features are blue-shifted by 2 ± 3 nm, and their absorp-
tion coefficients are slightly modified. The values for 3, for
1
1
example, are 426 nm (2900m 1 cm ), 491 nm (1530m 1 cm )
1
and 689 nm (252m 1 cm ).
The purity of the [6,6]-ring bridged methanofullerenes 1, 2,
and 3 was verified by chromatography, mass, infrared, 1H and
13C NMR spectroscopies. Complete details of the syntheses
and characterization of the [6,6]-ring bridged methanofuller-
enes 1, 2, and 3 and of intermediate compounds are described
in the Experimental Section.
These three new bands, not present in C60, are very similar
to those reported for several other methanofullerenes.[7,8] The
sharp band at ꢀ 430 nm has been considered as characteristic
of [6,6]-closed ring bridged methanofullerenes,[7] and is also
present in dihydrofullerene[17] and cyclohexyl-fused pseudo-
dihydrofullerenes.[17±19] This 430 nm peak is absent in [6,5]
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Methanofullerene
270±278
fulleroid structures.[7,20,21] The broad band peaking at around
500 nm is observed in [6,6]-closed ring fullerene structures
where the adduct is linked to the fullerene by a D bridge (e.g.,
C60O[22] and the closed-ring isomer of C61H2[23]) and is not
exhibited in cyclohexyl-fused pseudo-dihydrofullerenes.[17±19]
The longest wavelength absorption, around 700 nm for all
methanofullerenes, is also present in cyclohexyl-fused pseu-
do-dihydrofullerenes.[17±19] Complex functional group effects
on the spectra have been reported by Cardullo et al.[24] for
multigrafted C60 with cyclopropyl- and cyclohexyl-fused
functionalization.
It is striking that the sharp bands in the 400 ± 410 nm region,
which are characteristic structural features of C60, are
broadened and attenuated to become mere shoulders in the
spectra of 1, 2, and 3 (Figure 1a, insert). Furthermore, in the
latter the two sets of broad bands with maxima at ꢀ 550 and
ꢀ 600 nm that are prominent in C60 spectra do not appear.
Thus, these methanofullerene spectra are distinctively differ-
ent from those of [6,5] ring-open methanofulleroids[3,20] for
which spectral features similar to those of C60 are clearly
observed in the visible spectral region.
In C60, the allowed T1u ± 1Ag transitions, which occur in the
UV below 400 nm, arise by excitation from the HOMO
(symmetry hu) to the second excited configuration, LUMO‡1
(t1g), as well as from HOMO 1 (hg) to LUMO (t1u)
excitations.[26] The visible-region forbidden transitions, in-
duced by Herzberg ± Teller vibronic interactions, occur by the
lower-energy excitation from HOMO (hu) to LUMO.[26]
Let us consider how the orbitals, electronic states, and
transitions of C60 are transformed in going to methanofuller-
enes and methanofulleroids. In C60, the t1g LUMO is triply
degenerate. Arias et al.[21] have deduced from electrochemical
studies that this triple degeneracy appears to be approxi-
mately valid in methanofulleroid LUMOs, whereas in 58 p-
electron methanofullerenes, the LUMO is approximately
doubly degenerate. Indeed, methanofulleroids, which retain
60 p-type electrons, are considered to be isoelectronic with
C60.[10]
Further relevant information is provided by density-func-
tional calculations carried out by Curioni et al.[27] on the
structure and electronic properties of the two isomers
(methanofullerene and fulleroid) of C61H2 using the local-
density approximation. Their results show that although
symmetry is lowered in C61H2 relative to (Ih) C60, transitions
to the LUMO-derived states remain dipole-forbidden be-
cause the character of the corresponding orbitals is hardly
changed. This is consistent with the general weakness of the
transitions observed in the visible region of methanofuller-
enes in our work. A particularly interesting result of the
density functional calculations is that, in C61H2, the admixture
of new states localized on the methylene group occurs at
energies above that of states issuing from the LUMO‡1
configuration. In the methanofullerene isomer, but not in the
methanofulleroid variant, a new state occurs which has a large
amplitude on the methylene group. This state could be
responsible for the sharp band observed at ꢀ 430 nm in C61H2
and analogously in other methanofullerenes which contain a
bridge apex carbon. This assignment requires confirmation, in
particular by many-body calculations of the excited molecular
states.
We further note that a band at about 430 nm is not limited
to methano-bridged methanofullerene compounds, since a
similar band is also observed in di- and tetrahydrofullerenes
and in pseudo-dihydrofullerene derivatives resulting from
cycloadditions.[17,18,28] If there is a common electronic struc-
tural origin to this band in the different derivatives, which is
not certain, it would probably be related to the existence of
less than 60 p-type electrons on the carbon cages in each of
these cases. In this connection, we note that the 430 nm band
is not seen in methanofulleroids, which retain the 60 p-type
electrons in an expanded fullerene system where the adduct
hardly modifies the conjugated system of the C60 moiety.
Further theoretical work is required in order to clarify the
430 nm band assignment.
Having examined the possible origin of the sharp band at
ꢀ 430 nm, we now consider interpretation of the broad band
at ꢀ 495 nm. As discussed above, the adducts to C60 mainly
perturb the orbitals giving rise to Herzberg ± Teller-induced
forbidden transitions in the visible. Calculations show that the
HOMO ± LUMO gap in the prototype methanofullerene
On the other hand, the UV spectra of 1, 2, and 3 in
cyclohexane (Figure 1b, insert) exhibit strong bands which are
very similar to the strong features observed for C60 in this
spectral region in n-hexane and at the same wavelengths and
with similar absorption coefficients in cyclohexane.[25] The
values for the three C61 derivatives are given in Table 1, where
Table 1. Ground-state absorption of C60 and the methanofullerenes 1, 2,
and 3 in cyclohexane: peak wavelengths l (nm) and absorption coefficients
e (m 1 cm 1).
C60
1
2
3
l
e
l
e
l
e
l
e
257 158000
329 49500
256 130000
327 38000
258 110000
326 33600
258 122000
326 37000
they are compared with the corresponding bands of C60. Each
of these compounds also has a strong band in the 210 nm
region. Its absorption coefficient cannot be measured as
accurately as for the other bands because of increased solvent
absorption close to 200 nm. Note that the UV peak absorption
coefficients of 1, 2, and 3 in cyclohexane are smaller than
those of C60 in n-hexane or cyclohexane. The diminished value
is consistent with loss of two p-type electrons in going from
C60 (60 p-type electrons) to the [6,6] closed-ring methanoful-
lerenes (58 p-type electrons).
These results suggest that the adduct strongly perturbs the
numerous forbidden transitions of C60 that occur in the visible,
and possibly gives rise to some new states (see later), whereas
the allowed transitions in the ultraviolet are relatively less
affected, in energy and in transition strength, by interaction of
the adduct with the quasi-p system of electrons of the
fullerene cage.
Towards specific assignments: After these general consider-
ations, we now examine in more detail the perturbations
induced by the adducts, in order to interpret the features
observed in the visible-region spectra.
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R. V. Bensasson, A. Rassat et al.
FULL PAPER
C61H2 is not modified by more than 0.2 eV
with respect to C60.[27] We therefore suggest
that the 495 nm band in the methanofuller-
enes is derived from one or more of the
transitions corresponding to the g and d
bands in the 500 ± 620 nm region for C60, that
is, the 11T1g ± 11Ag, 11T2g ± 11Ag, 11T2u
±
[26]
[29±31]
11Ag
and possibly the 11Gg ± 11Ag
transitions of the Ih molecule.
The group of weak features close to
700 nm could also be derived from one of
these orbitally forbidden transitions in C60.
An alternative possibility is that the 700 nm
features arise from a spin- (and possibly
orbitally) forbidden transition to the lowest
triplet state. A similar assignment has been
proposed for the very much weaker bands of
C60 in this region,[26] as well as, on the basis of
theoretical calculations, for the similar but
stronger bands observed in C60 ± anthracene
[32]
Diels ± Alder adducts of C60
and in
C60H2.[17] It is our intention to eventually
test this possible assignment experimentally.
Finally, it is of interest that oxo- and
methylene-bridged C60 dimers have an es-
sentially featureless absorption spectrum in
the 400 ± 700 nm visible region.[33] In these
dimers, carbon ± carbon bonds link two C60
molecules, and the oxygen atom or the
Figure 2. Triplet-minus-singlet difference absorption spectra of: a) 1 and 2 in toluene obtained by
laser flash photolysis; and b) 1, 2, and 3 in benzene obtained by pulse radiolysis.
carbon of the methylene group forms the apex of a bridge
between these two cages. Although it is clear that this
structure severely perturbs the LUMO and the states issuing
from it, the dimer absorption spectra in the 200 ± 400 nm
region[33] show that perturbation of the configurations giving
rise to the allowed transitions mainly affects their relative
dipole oscillator strengths, but hardly effects their energies at
all.
The energies of the observed triplet ± triplet absorption
bands of C60 in the 300 ± 800 nm region fit reasonably well with
calculated values of allowed 3Hu 13T2g and 3Gu 13T2g
transitions.[40] In particular, the strong 750 nm band can be
assigned to the 23Gu 13T2g transition. It appears likely that
the 720 and 820 nm triplet ± triplet absorption features of the
three methanofullerenes are related to the 23Gu 13T2g and
23Hu 13T2g transitions of C60, which are predicted to give
rise to bands at 740 and 820 nm,[40] respectively, in this parent
fullerene.
Triplet absorption spectra: Figure 2a shows the triplet-minus-
singlet difference absorption spectra of 1 and 2 obtained by
laser flash photolysis of simple ꢀ 10 4 m solutions of the
methanofullerenes in toluene; Figure 2b shows the same
spectra, including that of 3, obtained after pulse radiolysis of
ꢀ 10 4 m solutions of the methanofullerenes in benzene in the
added presence of 10 1m biphenyl as triplet sensitizer.
It can be seen that the absorption profiles of all three
methanofullerenes are identical within experimental error,
with a maximum located at ꢀ 720 nm. The shape of this
720 nm absorption band is similar to that of the 750 nm
absorption of C60, except that for the methanofullerenes there
is an additional shoulder around 820 nm. This blue shift by
30 nm of the triplet absorption maximum as compared to the
750 nm lmax of C60 triplet absorption is also observed for other
methanofullerenes, including C61(COOEt)2.[34] The blue shift
is slightly larger for double and triple cyclopropanation of C60,
giving lmax at 690 nm and 650 nm, respectively.[34,35] For a
number of pseudo-dihydrofullerenes, including cyclohexyl-
fused and fulleroproline derivatives, the triplet absorption
maximum is located at ꢀ 700 nm.[17,18,36±39]
Determination of the quantum yield of singlet oxygen
production: Methanofullerene-sensitized production of sin-
glet oxygen, assessed by the amplitude of 1O2(1Dg) phosphor-
escence at 1270 nm, was compared with that obtained for C60.
The data for 1 given in Figure 3 allowed us to determine the
1
ratio of the slope a1D of O2 production sensitized by 1 as a
1
function of pulse energy, to that determined for O2 produc-
tion sensitized by C60, chosen as standard, asDt. The quantum
yield F1Dof singlet oxygen production by the methanofuller-
ene was then calculated from the slope ratio after correction
for the corresponding absorbances A532 of the solutions at
532 nm, the laser excitation wavelength [Eq. (1)].
st
FD1 ˆ (a1D/aDst)  (A /A5132)  FDst
(1)
532
The quantum yield of singlet oxygen production for C60, FsDt,
being near unity,[41,42] Equation (1) enabled us to determine a
FD value for the three C61 fullerene derivatives of near unity
(see Table 2), with a maximum error of 20%. Moreover, as FD
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Chem. Eur. J. 1998, 4, No. 2

Methanofullerene
270±278
Figure 3. Photosensitized singlet oxygen production in oxygen-saturated
perdeuterated toluene measured at its maximum concentration after the
1
st
&
Figure 4. Maximum transient absorbances, DAT, ( ), 720 nm and DAT,
*
(
), 750 nm, plotted as a function of laser pulse energy. (A532 ˆ 0.17 and 0.11
&
*
laser pulse and sensitized by the triplet of a) 1 ( ), b) C60
( ) plotted as a
for 1 and C60, respectively, in a 1-cm cell).
function of laser pulse energy. (A532 ˆ 0.20 and 0.24 for 1 and C60
,
respectively, in a 1-cm cell).
shown[44±46] that Equation (2) holds, where eG is the molar
Table 2. Quantum yield of methanofullerene-photosensitized production
of 1O2(FD) and molar absorption coefficients (eT in m 1 cm 1) at the
methanofullerene triplet maxima.
(eT eG)1 ˆ (FTst/F1T)  (aT1 /aTst)  esTt  (Ast/A1)
(2)
absorption coefficient of the molecule in its ground state.
Methanofullerene
FD
eT at 720 nm
laser flash photolysis pulse radiolysis
From Equation (2) and the data shown in Figure 4, a triplet
1
molar absorption coefficient of 14600 Æ 3000m 1 cm
at
1
2
3
1.04
0.96
0.95
14600
13800
±
14000
12000
15100
720 nm was obtained for methanofullerene 1, the ground-
state absorption at this wavelength being negligible (see
Figure 1). In an analogous manner, an eT of 13000 Æ
1
2600m 1 cm at 720 nm was obtained for 2.
can only be smaller or equal to the quantum yield of triplet
formation FT, the FT of the three C61 derivatives must also be
near unity.
By pulse radiolysis: Independent estimates of the molar
absorption coefficients of 1, 2, and 3 were obtained using the
energy transfer method[46,47] and pulse radiolytic excitation.
The lowest triplet of each methanofullerene in benzene was
produced by energy transfer from the triplet excited state of
another solute, biphenyl, with a well-characterized eT of
27100m 1 cm 1 at 365 nm,[47] present in greater concentration
(10 1m versus (1 ± 2) Â 10 4 m). Three reactions must be taken
into account in this system:
These high FT and FD values are similar to those observed
for bis(carboethoxy)[61]fullerene, 1,2-epoxy[60]fullerene,[43]
and cyclohexyl-fused [60]fullerenes.[17,36] It should be empha-
sized that functionalized fullerenes containing an electron-
donating moiety can undergo a photoinduced intramolecular
electron transfer process in polar solvents and exhibit a large
decrease of singlet !triplet intersystem crossing.[38,39]
1) Decay of the donor triplet 3B to its ground state BG:
k1
3B!BG;
kq
3
2) the energy-transfer reaction B‡C61!BG‡3C61;
Determination of the triplet molar absorption coefficient
By laser flash photolysis: Using the comparative method,[44±46]
it is possible to estimate the molar extinction coefficients of
the C61 fullerene derivative triplets. The concentration of
triplet molecules formed in the methanofullerene solution,
with unknown triplet molar absorbance coefficient eT but
known FT ( ꢀ 1), is compared with the concentration of triplet
formed in a solution of C60 chosen as standard with known eTst
3) decay of the acceptor triplet 3C61 to its ground state while it
k3
3
is being formed: C61!C61.
Typical values of the rate constants obtained for methano-
fullerene 1 were k1 ˆ 4.3  104 s , k2 ˆ k1‡kq[C61] ˆ 1.12 Â
1
1
1
106 s , and k3 ˆ 3.34  104 s .
The maximum absorbance of 3C61 at 720 nm reached at the
end of the energy-transfer reaction, corrected for decay of the
donor triplet 3B by routes other than energy transfer, is given
(20200m 1 cm ) at its 750 nm peak[40] and known FsTt of near
1
unity.[41,42] The maximum transient absorbance DA1T of the
triplet of unknown molar absorption coefficient, and of the
standard, DAsTt, are plotted as a function of laser pulse energy
(see Figure 4 for 1). The slopes of these plots, a1T and aTst, are
satisfactory straight lines. Furthermore, in the range of
laser pulse energies used, the depletion of the ground state
spans 1% to 10%; thus, we have favorable conditions for
using the comparative method.[44±46] For solutions such
that the ground state absorbance at 532 nm is A1 for the
unknown, and Ast for the standard, it can be easily
3
by Equation (3), and the maximum absorbance of C61
corrected for decay of C61 during its formation is given by
Equation (4). The correction for decay of the 3B state by
3
A720,cor ˆ A720,obs  [k2/(k2 k1)]
(3)
(4)
A720,cor ˆ A720,obs  exp[(lnk2/k3)/(k2/k3 1)]
routes other than energy transfer was typically ꢀ 4%, and
3
that for decay of C61 during its formation was typically
Chem. Eur. J. 1998, 4, No. 2
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0947-6539/98/0402-0275 $ 17.50+.25/0
275

R. V. Bensasson, A. Rassat et al.
FULL PAPER
ꢀ 11%. The resulting estimate for methanofullerene 1 is given
Experimental Section
in Equation (5). This value is in good agreement with
Materials: Reagents used were the best available commercially. C60 was
purchased from Technocarbo, Les Cyclades, Chemin de Camperousse, F-
06130 Plan de Grasse (France). Tetrahydrofuran and toluene were distilled
over sodium/benzophenone, 1,2-dichlorobenzene over calcium hydride,
pyridine over potassium hydroxide, and tert-butanol over sodium. All other
solvents were used as supplied (minimum purity 99%).
eT(3C61) at 720 nm ˆ eT (3B) at 365 nm  A720(3C61)/A365(3B)
1
ˆ 14000 Æ 1400m 1 cm
(5)
the corresponding molar absorption coefficient obtained in
the above laser flash photolysis determination. The corre-
sponding eT values obtained by pulse radiolysis for 2 and 3 are
collected in Table 2.
This decrease of the triplet molar absorption coefficient
compared with that of C60 has been observed for a number of
C60 derivatives such as hydrofullerenes[17] and cyclohexyl-
fused pseudo-dihydrofullerenes.[17,36,39]
Characterization: IR spectra were obtained on a Nicolet ± Impact 400D
FTIR spectrometer. 1H and 13C NMR spectra were measured on a Bruker
AC200 (250 MHz) instrument for the intermediate compounds and a
Bruker Avance DRX400 (400 MHz) instrument for the methanofullerenes.
Mass spectra were recorded on a Nermag R10-10S spectrometer [electron
impact (EI) or chemical ionization by CH4 or NH3]. Chromatographic
analysis was performed on a C18 Interchrom (150 Â 4.6)-type ODS2
column and a Waters 600E system controller equipped with a Waters 991
photodiode array detector (190 ± 800 nm). Data were collected on a NEC
Powermate SX/16 computer. Melting points were recorded on a Büchi 535
instrument.
A Kontron model Uvikon 940 spectrophotometer was
employed for spectral measurements of the ground-state C61 derivatives
(bandwidth 0.3 nm, data interval 0.1 nm). The sources of solvents were:
Prolabo RP, France (toluene); SDS, France (perdeuterated toluene); and
Fluka, Switzerland (cyclohexane, solvent for luminescence spectroscopy).
Conclusion
Time-resolved spectroscopic methods: For the determination of the C61
fullerene triplet properties, we used the complementary time-resolved
techniques of laser flash photolysis and pulse radiolysis. In the photolysis
experiments we used frequency-doubled, 532 nm, 6 ns pulses emitted by a
Nd:YAG laser (Quantel YG58510G, France), at a frequency of 10 Hz. The
detector for the triplet ± triplet absorption was an OMA3 built by
Princeton Applied Research with detector interface model 1461, gated
diode array model 1455BHQ and pulse amplifier model 1304, interfaced
with a Hewlett ± Packard 9000 computer. The laser pulse energy was
Functionalization of C60 by different methanofullerene syn-
theses leads to molecules which might have very different
chemical, mechanical, or solvatochromic properties.[3] How-
ever, as the electrochemical studies of Boudon et al.
showed,[48] the sp3 C atoms in the methano bridges of
methanofullerenes can act as insulators and greatly reduce
the effects of the nature of substituents. Our results on three
very different functionalizations enable us to confirm that the
photophysical properties of methanofullerenes could be
rather similar in certain cases, at least when the functional-
ization does not involve an electron donor.[37,19] The sharp
band at ꢀ 430 nm in the Sn S0 absorption spectrum could be
linked to the existence of less than 60 p-type electrons on the
carbon cages, since a similar band is observed in di- and
tetrahydrofullerenes and in pseudo-dihydrofullerene deriva-
tives resulting from cycloadditions,[17,18] but not in methano-
fulleroids; the latter retain the 60 p-type electrons in an
expanded fullerene system. We further observe that the
triplets of the three methanofullerenes we have studied have
common general characteristics, namely: a) a triplet inter-
system crossing efficiency FT and a corresponding quantum
yield of singlet oxygen production, FD, near unity; b) a 30 nm
hypsochromic shift of the main visible triplet absorption band
measured in mJ by
a
laser monitor (model 900) built by Laser
1
Instrumentation. The detector for the O2 phosphorescence emission with
lmax at 1.27 mm was a 3-mm diameter Judson J16 germanium photodiode,
the resulting voltage being applied to a Judson amplifier and fed to a
Nicolet model 206 digital oscilloscope. Each transient spectrum resulted
from an accumulation of spectra produced by 100 laser shots. The solvent
used for the 1O2 measurements was [D8]toluene, since the detection of 1O2
in this solvent is facilitated by its relatively long intrinsic lifetime ( ꢀ
320 ms). In the radiolysis measurements we used single 20 ± 100 ns pulses
of 9 ± 12 MeV electrons delivered by a Vickers linear accelerator.[50,51]
Analyzing light from
a xenon lamp was passed through a Kratos
monochromator onto an EMI 9558Q photomultiplier for wavelengths
between 350 and 780 nm, and a UDT PIN10 photodiode between 780 and
1080 nm. Changes in absorbance with time were recorded with a Tektronix
7612AD digitizer fitted with a DAN 486DX33 computer. Solutions of the
methanofullerenes in benzene (Romil, super purity solvent) were con-
tained in spectrosil quartz cells of 2.5 cm optical path length.
Bis(4-tert-butylbenzoate)methanone (6): Compound 5 (4.32 g, 14.1 mmol),
anhydrous tert-butanol (6.6 mL), and anhydrous pyridine (5.4 mL) were
refluxed under argon for 25 hours. After cooling, pyridinium hydrochloride
was filtered off, and the filtrate was extracted with dichloromethane
(100 mL) and washed with aqueous hydrochloric acid (0.1m, 50 mL) and
then water (50 mL). The organic extract was dried over sodium sulfate, and
the solvents were evaporated off. Subsequent recrystallization from hexane
afforded 6 as pure white crystals (4.175 g, 77.7%, m.p. 1228C). 1H NMR
(CDCl3): d ˆ 7.91 (d, 4H, J ˆ 8.25 Hz), 7.62 (d, 4H, J ˆ 8.25 Hz), 1.43 (s,
18H); 13C NMR (CDCl3): d ˆ 195.46 (s), 164.71 (s), 140.15 (s), 135.46 (s),
129.64 (CH), 129.34 (CH), 81.76 (O ± C), 28.07 (tBu); IR (KBr): nÄ ˆ 2999.1,
2977.9, 2931.6, 2869.9, 1710.8, 1658.7, 1502.5, 1477.4, 1460.0, 1406.0, 1392.5,
1367.4, 1313.4, 1298.0, 1253.7, 1163.0, 1120.6, 1016.4, 929.6, 873.7, 848.6,
3
as compared to the 750 nm main band of C60 in toluene or
benzene, lower than the 50 nm shift of a number of pseudo-
dihydrofullerenes, including cyclohexyl-fused and fulleropro-
line derivatives;[17,18,36±39] and c) a lower (by ꢀ 25%) triplet
3
molar absorption coefficient than that of C60.
The fact that our methanofullerenes have similar quantum
yields for singlet oxygen formation as does C60 itself suggests
that such derivatization, avoiding a coupling with an electron
donor, will not lower their biological photosensitization
efficiency were they to be used, for example, in photodynamic
therapy.[49] For such an application, appropriate adducts could
be chromophores absorbing in the red spectral region of
highest tissue transparency. Moreover, the possibility of
varying the adduct provides the opportunity to modify the
methanofullerene lipophilicity and consequently the site of
intracellular delivery.
‡
‡
721.3 cm 1; MS (NH3): m/z (%) ˆ 400 (100) [M‡NH4] , 383 (85) [M‡H] ,
‡
327 [M tBu‡2] , 309 (40); Rf ˆ 0.66 (hexane/ethyl acetate:4/1).
Bis(4-tert-butylbenzoate)methanotosylhydrazone (7): Diester 6 (2.31 g,
6.05 mmol, 1 equiv) in ethanol (80 mL) was refluxed under argon with
tosylhydrazide (2.25 g, 12.1 mmol, 2 equiv) for 3 hours, then stirred at room
temperature overnight. Ethanol was evaporated off, and crude 7 was
purified by flash chromatography on silica gel with CH2Cl2 as eluant. After
evaporation, pure 7 was obtained as white crystals (2.73 g, 82%). 1H NMR
276
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0947-6539/98/0402-0276 $ 17.50+.50/0
Chem. Eur. J. 1998, 4, No. 2

Methanofullerene
270±278
‡
(CDCl3): d ˆ 7.99 peaks (2p, 2H), 7.72 (m, 4H), 7.48 (s, 1H), 7.31 (2p, 2H),
7.2 (2p, 2H), 7.05 (2p, 2H), 2.29 (s, 3H), 1.45 (2p, 18H); 13C NMR (CDCl3):
d ˆ 165.06, 164.51, 152.18, 144.39, 139.45, 135.21, 134.56, 133.79, 133.04,
130.86, 129.72, 129.3, 128.23, 127.87, 127.11, 81.85, 81.32, 28.09, 21.6; IR
(KBr): nÄ ˆ 3158.1, 2977.7, 2931.4, 1710.7, 1596.9, 1477.3, 1458.0, 1405.9,
1368.3, 1351.0, 1311.4, 1298.9, 1256.5, 1185.1, 1168.7, 1121.5, 1054.9, 1018.3,
974.9, 882.3, 862.1, 846.6, 814.8, 782.0, 705.9, 681.8, 557.4, 549.6 cm 1; MS
744.6, 702.1, 609.5 cm 1; MS (NH3): m/z (%) ˆ 736 (20) [M‡18] , 720 (65)
‡
‡
[M‡2] , 719 (100) [M‡1] , 274 (80); Rf ˆ 0.59 (SiO2/toluene).
Bis[4-(tert-butyldiphenylsilyloxymethyl)phenyl]methanotosylhydrazone
(11): Compound 10 (2.824 g, 3.9 mmol, 1 equiv), p-toluenesulfonylhydra-
zide (0.904 g, 4.8 mmol, 1.2 equiv) and toluene (100 ml) were refluxed for
2 days under argon. Toluene was evaporated off under reduced pressure
down to 5 mL, and crude 11 was purified by flash chromatography on silica
gel (toluene) to yield pure 11 as a colorless, very viscous oil (2.2 g, 87%),
which solidified on standing. 1H NMR (CDCl3): d ˆ 8.04 (2p, 2H), 7.87 (m,
8H), 7.56 (m, 20H), 7.24 (2p, 2H), 5.01 (s, 2H), 4.92 (s, 2H), 1.31 (s, 9H),
1.25 (s, 9H); 13C NMR (CDCl3): d ˆ 154.35, 144.07, 143.29, 143.04, 135.58,
135.51, 135.20, 133.32, 133.14, 129.87, 129.74, 129.66, 129.57, 128.19, 127.91,
127.82, 127.74, 127.57, 127.08, 125.64, 65.12, 64.97, 26.66, 26.80, 21.61, 19.30;
IR (KBr): nÄ ˆ 3436, 3217, 3070, 2957, 2930, 2889, 2857, 1653, 1616, 1595,
1472, 1432, 1381, 1349, 1169, 1122, 1091, 825, 741, 715, 664, 610, 554,
‡
(NH3): m/z (%) ˆ 551 (100) [M‡1] ; Rf ˆ 0.37 (hexane/ethyl acetate:4/1).
61,61-Bis[4-tert-butylbenzoate]-1,2-dihydro-1,2-methanofullerene[60] (1):
C60 (0.885 g, 1.23 mmol, 1 equiv) was dissolved in anhydrous degassed 1,2-
dichlorobenzene (250 mL), after which sodium hydride (60% in oil,
0.249 g) and 7 (0.686 g, 1.25 mmol, 1 equiv) were introduced. The mixture
was heated stepwise (1 h at 508C, 2 h at 808C, 1 h at 1008C, 1 h at 1208C,
1 h at 1408C, and 1 h at 1608C) under argon. After cooling, the reaction
mixture was quenched with water (150 mL), stirred for 5 min, and filtered
on Celiteꢂ. The organic layer was decanted, washed with brine, and dried
over sodium sulfate, and the solvents were evaporated off. Crude 1 was
purified by flash chromatography on silica gel and eluted by hexane/
toluene (4/6). After evaporation, pure 7 was obtained as a brown powder.
The isomeric mixture was dissolved in chlorobenzene (100 mL) and
refluxed for 2 days under argon. Chlorobenzene was evaporated, and 1
(single isomer) was obtained after chromatography as a brown powder
(0.635 g, 47.6%). 1H NMR (CDCl3): d ˆ 8.17 (d, 4H, J ˆ 8.5 Hz), 8.10(d,
4H, J ˆ 8.5 Hz), 1.59 (s, 18H); 13C NMR (CDCl3): d ˆ 165.10, 147.42,
145.24, 145.21, 145.14, 144.73, 144.66, 144.54, 144.35, 143.81, 143.02, 142.96,
142.64, 142.14, 142.10, 140.98, 138.12, 132.02, 130.81, 130.06, 81.36, 78.22,
57.05, 28.19; IR (KBr): nÄ ˆ 2971.4, 1713.7, 1611.9, 1291.1, 1168.9, 1118.0,
512 cm 1; MS (NH3): m/z (%) ˆ 887 (75) [M‡1] , 274 (25), 206 (25), 204
‡
(30), 190 (50), 189 (45), 174 (100), 156 (10); Rf ˆ 0.29 (hexane/ethyl acetate
9:1) or 0.25 (toluene).
61,61-Bis[4-(tert-butyldiphenylsilyloxymethyl)phenyl]-1,2-dihydro-1,2-me-
thanofullerene[60] (2): C60 (0.2 g, 0.277 mmol, 1 equiv) was dissolved under
argon in anhydrous, degassed 1,2-dichlorobenzene (50 mL), after which 11
(0.26 g, 0.293 mmol, 1.3 equiv) and sodium hydride (60% in oil, 0.041 g,
1.03 mmol) were added. The mixture was heated stepwise (1.5 h at 508C,
2 h at 808C, 1 h at 1008C, 1 h at 1208C, 1 h at 1408C and 1 h at 1608C)
under argon. After cooling, the reaction mixture was quenched with water
(150 mL), stirred for 5 min and filtered on Celiteꢂ. The organic layer was
decanted, washed with brine, and dried over sodium sulfate, and the
solvents were evaporated off. Crude 2 was purified by flash chromatog-
raphy on silica gel and eluted by hexane/toluene (4/1) to obtain pure 2
(single isomer shown by HPLC, confirmed by NMR) as a brown powder
(0.395 g, 25% of starting C60 recovered, 64% yield). 1H NMR (CDCl3): d ˆ
‡
720.8, 527.3 cm 1; MS (NH3): m/z (%) ˆ 975 (1) [M] , 887 (10) [M
‡
‡
2(CO2tBu)] , 720 (25) [C60] , 224 (100); HPLC: tr ˆ 3 min (toluene/
acetonitrile: 45/55), tr ˆ 14.8 min (toluene/acetonitrile: 35/65); UV/Vis
(CH2Cl2): lmax(e) ˆ 223 (128482), 259 (136168), lmax(e) ˆ 223 (128000),
259 (136000), 329 nm (38000m 1 cm 1).
8.125 (2p, 4H), 7.75 (m, 8H), 7.45 (m, 16H), 4.89 (s, 4H), 1.18 (s, 18H); 13
C
Bis[4-(hydroxymethyl)phenyl]methanone (9): Mercuric oxide[15] (6.55 g,
0.0302 mol, 2.09 equiv) was introduced into dimethoxyethane (79 mL), and
then perchloric acid (70%) (5.6 mL, 9.352 g, 0.093 mol, 6.46 equiv) and
water (2 mL) were added. The mixture was stirred for 20 min at a
temperature between 40 and 608C. Distilled water (10 mL) and 8 (5.60 g,
0.0144 mol, 1 equiv) were then introduced. The mixture was stirred at 40 ±
608C for 1 h. Water (100 mL), potassium chloride (10 g), and ethyl acetate
(100 mL) were then introduced, and the mixture was stirred for a few
minutes. Potassium perchlorate was filtered off and rinsed with ethyl
acetate (20 mL). The mixture was then further extracted with ethyl acetate
(2 Â 100 mL). Organic extracts were pooled and washed with saturated
aqueous potassium carbonate solution (50 mL), and the mercuric carbo-
nate was filtered off. The organic layer was washed with brine (50 mL) and
dried over sodium sulfate. The solution was then evaporated, and the
resultant crude 9 was purified by flash chromatography on silica gel
(hexane/ethyl acetate (1/1), then ethyl acetate). Solvents were evaporated,
and pure 9 was obtained as a white solid (2.51 g, 72%). 1H NMR
([D6]DMSO): d ˆ 7.75 (d, 2H, J ˆ 8.1 Hz), 7.54 (d, 2H, J ˆ 8.1 Hz), 4.76 (d,
2H, J ˆ 5.75 Hz), 4.44 (t, 1H, J ˆ 7.75 Hz), 2.84 (s, 3H); 13C NMR
([D6]DMSO): d ˆ 195.6, 147.9, 136.0, 129.92, 126.94, 62.78; IR (KBr): nÄ ˆ
NMR (CDCl3): d ˆ 148.42, 145.41, 145.17, 145.08, 144.69, 144.64, 144.61,
144.24, 143.84, 143.00 (C60), 142.91, 142.27, 142.11, 141.00, 140.84, 138.20,
137.64, 135.59, 133.29, 130.75, 129.75, 129.03, 128.22, 127.74, 126.16, 79.18,
65.18, 58.03, 26.91, 19.34; IR (KBr): nÄ ˆ 3067, 3046, 3024, 2955, 2928.34,
2886.15, 2854.58, 1510, 1462, 1426, 1373, 1208, 1184, 1110.1, 1085.6, 1018,
823, 739, 701.3, 606, 575, 555, 526, 505 cm 1; MS (NH3): m/z (%) ˆ 1441 (75)
‡
‡
‡
[M‡NH4] , 1424 (20) [M‡1] , 1383 (10) [M 40] , 720 (30), 272 (50), 216
(100); HPLC: tr ˆ 7.17 min (toluene/acetonitrile: 45/55).
61-[(Ethoxycarbonyl)methylcarboxylate]-1,2-dihydro-1,2-methanofuller-
ene [60] (3): This compound was synthesized according to the method of
Isaacs and Diederich.[8] The characterization results were consistent with
those previously obtained by these authors.
Ã
Acknowledgments: A.R. and S.R. acknowledge the financial aid of Igol Ile
de France. We also appreciate support from the European Community
HCM program contracts CHRXCT 930178 (R.V.B.) and 94-0614 (R.V.B.,
S.L.), CNRS program GDR C60, (A.R., S.L., R.V.B.) and of the Cancer
Research Campaign (E.J.L.). We are grateful to D. Allan and B. W.
Hodgson for the maintenance of the pulse radiolysis equipment at the
Paterson Institute for Cancer Research Free Radical Research Facility,
Manchester (UK), operated with the support of the European Commission
through the Access to Large-Scale Facilities activity of the TMR Program.
3333.2, 3244.5, 2912.7, 2858.7, 1647.3, 1608.7, 1412.0, 1311.7, 1288.5, 1145.5,
1
1114.9, 1087.9, 1045.5, 1014.6, 929.7, 856.4, 821.7, 748.4, 686.7, 628.8 cm
;
‡
‡
MS (NH3): m/z (%) ˆ 260 (10) [M‡18] , 243 (100) [M‡1] , 135 (35).
Bis[4-(tert-butyldiphenylsilyloxymethyl)phenyl]methanone (10): Diol
9
Received: June 13, 1997 [F726]
(1.78 g, 7.36 mmol, 1 equiv) was dissolved in anhydrous tetrahydrofuran
(25 mL), and then triethylamine (1.637 g, 16.18 mmol, 2.2 equiv) and 4-
dimethylaminopyridine (0.090 g, 0.736 mmol, 0.1 equiv) were introduced.
The mixture was stirred for a few seconds at 08C. tert-Butyldiphenylsilyl-
chloride[16] (4.448 g, 16.18 mmol, 2.2 equiv) was introduced, and the
temperature was maintained at 08C for 2 h. The mixture was then stirred
overnight at RT. Triethylammonium chloride was filtered off and washed
with diethyl ether. Solvents were evaporated, and the resultant crude 10
was purified by flash chromatography on silica gel (hexane/ethyl acetate:
95/5, or pure toluene). Pure 10 was thus obtained as a colorless, very viscous
[1] T. Suzuki, Q. Li, C. Khemani, F. Wudl, Ö. Almarson, Science 1991, 254,
1186 ± 1188.
[2] F. Diederich, Y. Rubin, Angew. Chem. 1992, 104, 1123 ± 1146; Angew.
Chem. Int. Ed. Engl. 1992, 31, 1101 ± 1123.
[3] F. Diederich, L. Isaacs, D. Philp, Chem. Soc. Rev. 1994, 243 ± 255.
[4] F. Diederich, C. Thilgen, Science 1996, 271, 317 ± 323.
[5] F. Djojo, A. Herzog, I. Lamparth, F. Hampel, A. Hirsch, Chem. Eur. J.
1996, 2, 1537 ± 1547.
1
oil (3.59 g, 71%). H NMR (CDCl3): d ˆ 7.68 (m, 4H), 7.59 (m, 8H), 7.11
[6] A. S. Boutorine, H. Tokuyama, M. Takasugi, H. Isobe, E. Nakamura,
(m, 16H), 4.74 (s, 4H), 1.01 (s, 18H); 13C NMR (CDCl3): d ˆ 196.27, 145.77,
136.42, 135.53, 133.20, 130.15, 129.80, 127.79, 125.54, 65.14, 26.81, 19.3; IR
(NaCl) nÄ ˆ 3070.5, 3049.3, 2958.6, 2931.6, 2891.1, 2858.3, 1656.8, 1608.75,
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