Self-sensitized photooxygenation of a C60-cycloheptatriene dyad to form norcaradiene-derived endoperoxides

Article abstract of DOI:10.1002/ejoc.200901453

The [60]fullerene-cycloheptatriene dyad 1 readily underwent [4+2] cycloaddition of molecular oxygen at the cycloheptatriene moiety to give endoperoxides 5-e.xo and 5-endo. The involvement of singlet oxygen was verified by the complete inhibition of oxygenation on addition of -diazabicyclo[2.2.2] octane (DABCO), which is an efficient singlet-oxygen quencher. The observed facile oxygenation, was attributed to both the strong photosensitizing ability of the fuller-ene cage and to the valence isomerization of 1 to the norcaradiene form, which was facilitated by the bulky ierf-butyl groups. Competitive oxygenation of 1 and a related, cycloheptatriene lacking the fullerenyl group (2) resulted in the formation of endoperoxides of the two molecules in comparable yields. This result demonstrates that the cycloaddition is not an in-cage reaction but occurs after the singlet oxygen has diffused from the solvent cage.

Full text of DOI:10.1002/ejoc.200901453

FULL PAPER
DOI: 10.1002/ejoc.200901453
Self-Sensitized Photooxygenation of a C60–Cycloheptatriene Dyad to Form
Norcaradiene-Derived Endoperoxides
Masaaki Hanamura,[a] Jun Kamada,[b] Akiyo Amano,[b] Ken’ichi Takeuchi,[b]
Takao Okazaki,[a] Katsuyuki Hirai,[c] and Toshikazu Kitagawa*[a]
Keywords: Photooxidation / Fullerenes / Peroxides / Sensitizers / Valence isomerization
The [60]fullerene–cycloheptatriene dyad 1 readily under-
went [4+2] cycloaddition of molecular oxygen at the cyclo-
heptatriene moiety to give endoperoxides 5-exo and 5-endo.
The involvement of singlet oxygen was verified by the com-
plete inhibition of oxygenation on addition of 1,4-diazabicy-
clo[2.2.2]octane (DABCO), which is an efficient singlet-oxy-
gen quencher. The observed facile oxygenation was attrib-
uted to both the strong photosensitizing ability of the fuller-
ene cage and to the valence isomerization of 1 to the norcara-
diene form, which was facilitated by the bulky tert-butyl
groups. Competitive oxygenation of 1 and a related cyclo-
heptatriene lacking the fullerenyl group (2) resulted in the
formation of endoperoxides of the two molecules in compar-
able yields. This result demonstrates that the cycloaddition
is not an in-cage reaction but occurs after the singlet oxygen
has diffused from the solvent cage.
In this study we investigated the self-sensitized photooxy-
genation of a C60–cycloheptatriene dyad 1 (Scheme 1),
which afforded a mixture of stable endo- and exo-endo-
peroxides derived from the corresponding norcaradiene iso-
mers.
Introduction
It is well established that fullerenes C60, C70, and their
derivatives are photosensitizers that produce singlet oxygen,
based on studies of their photophysical characteristics[1]
and reactions.[2–7] The high quantum yield of singlet oxygen
production and its chemoselectivity in reactions with or-
ganic molecules make the process synthetically useful. In-
tramolecularly sensitized oxygenation of C60 and C70 deriv-
atives, which occurs spontaneously by irradiation in the
presence of molecular oxygen, allows the modification of
attached structures that accept oxygen. Reported examples
of such syntheses include [2+2][3] and [4+2][4] cycloadditions
to give cyclic peroxides (dioxetanes and endoperoxides,
respectively), ene reactions to give hydroperoxides,[5] and N-
and S-oxygenation of amines[6] and sulfides.[7] When a cy-
clic peroxide is the product, it often undergoes additional
transformations by cleavage of the O–O bond. However, the
initially formed peroxide can be isolated if its stability is
adequate, for example, self-sensitized photooxygenation of
a C60 derivative bearing a 9-anthryl group allowed the isola-
tion of a stable 9,10-endoperoxide.[4a]
Scheme 1. C60–cycloheptatriene dyad 1.
Adam and Balci[8] previously reported that unsubstituted
cycloheptatriene (CHT), which is in equilibrium with the
energetically higher norcaradiene (NCD) tautomer,[9] is
oxygenated to form a mixture of endoperoxides from both
tautomers (Scheme 2).[10]
[a] Department of Chemistry for Materials, Graduate School of
Engineering, Mie University,
Tsu, Mie 514-8507, Japan
Fax: +81-59-231-9416
E-mail: kitagawa@chem.mie-u.ac.jp
[b] Department of Energy and Hydrocarbon Chemistry, Graduate
School of Engineering,
Kyoto University, Kyoto 606-8501, Japan
[c] Instrumental Analysis Facilities, Life Science Research Center,
Mie University,
Tsu, Mie 514-8507, Japan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901453.
Scheme 2. Photooxygenation of the cycloheptatriene–norcaradiene
equilibrium mixture.
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Although the CHT–NCD equilibrium mixture contains with the NCD isomer (see the next section). The 13C NMR
multiple reaction sites, its regioselectivity can be controlled spectrum showed 30 sharp signals in the region between δ
with appropriate substituent(s). For instance, cycloheptatri- = 134.7 and 151.9 ppm, which were attributed to the sp2
enes with a π-electron-withdrawing substituent (CN, CHO carbon atoms of the C60 core, indicating that the product
or COCH3) on the methylene carbon are oxygenated with was a 1,2 adduct of C60 with Cs symmetry. The sp3 carbon
singlet oxygen via the NCD form.[11] We previously re- atoms of the C60 core were observed at δ = 54.6 and
ported that introduction of bulky alkyl substituents shifts 62.2 ppm. The cycloheptatrienyl ring exhibited four signals
the equilibrium towards the NCD form.[12,13] Based on this at 29.9 (C-1), 86.2 (C-2,7), 125.9 (C-4,5), and 143.8 (C-
result, we hypothesized that dyad 1 would selectively give 3,6) ppm, the last three signals also being extremely broad-
endoperoxides from the NCD isomer.
ened. The ethynylene linker and the tert-butyl groups ap-
peared at 83.4, 75.9, 34.4, and 29.4 ppm. The UV/Vis spec-
trum showed a narrow absorption band at 431 nm, which
is characteristic of 1,2-adducts of C60.[14] Each of these re-
sults is consistent with structure 1 in an equilibrium mixture
Results and Discussion
Synthesis of the [60]Fullerene–Cycloheptatriene Dyad
Dyad 1 was synthesized by nucleophilic addition of a with the NCD isomer.
cycloheptatrienylethynyllithium, prepared by lithiation of 2,
When the acidic work-up was performed with excess
to C60 and subsequent quenching with an equimolar TFA, the desired product 1 underwent an additional reac-
amount of trifluoroacetic acid (TFA; Scheme 3). The 1H tion to form allene 3 (Scheme 4). The structure of this prod-
NMR spectrum of the product showed the disappearance uct was determined on the basis of the two NMR doublet
of the ethynyl proton and the appearance of a proton at- signals of the 1,3-disubstituted allene at δ = 7.29 and
tached to the C60 cage. All the protons, except the tert-butyl 7.60 ppm (4JH,H = 6.0 Hz) and the aromatic proton signals
protons, were excessively broadened at room temperature corresponding to a 1,2,4-trisubstituted benzene. This result
(Figure 1, top spectrum) due to rapid valence tautomerism is consistent with our previous finding[15] that 7-ethynylcy-
Scheme 3. Synthesis of dyad 1.
Figure 1. Variable-temperature 1H NMR spectra (500 MHz) of 1 in CS2/CD2Cl2 (4:1, v/v). The numbering of the carbon atoms is
indicated in Scheme 1. * Impurity peaks.
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Self-Sensitized Photooxygenation of a C60–Cycloheptatriene Dyad
cloheptatrienes are isomerized to phenylallenes by an initial ethynyl (2) and phenylethynyl (4) derivatives. The lowest en-
protonation of the ethynyl carbon under acidic conditions. ergy isomer of 1 was eq-CHT, which has a greater stability
Allene 3 was also formed by treatment of isolated 1 with than 1-exo-NCD by 1.35 kcalmol–1. The relative energies
TFA. Therefore the amount of acid used to quench the re- of the other two isomers, 1-ax-CHT and 1-endo-NCD, were
action must not exceed the amount of butyllithium for this Ͼ3 kcalmol–1 higher than that of 1-eq-CHT, which is con-
synthesis.
sistent with the experimentally observed isomer distribu-
tion.
Table 1. Experimentally observed isomer populations, free energies,
and DFT-calculated energies of 7-(R-ethynyl)-2,5-di-tert-butylcy-
clohepta-1,3,5-trienes.
[b,c]
Tautomer pop-
ulation[a] [%]
∆G°25
Energy by DFT
calculations[b,d]
[kcalmol–1]
[kcalmol–1]
1 (R = HC60)
Scheme 4. Acid-catalyzed isomerization of 1.
eq-CHT
85.6[e]
14.4[e]
Ͻ1[e]
0.00[e]
0.61[e]
Ͼ1[e]
0.00
1.35
3.10
3.09
exo-NCD
endo-NCD
ax-CHT
CHT–NCD Isomerism
Ͻ1[e]
Ͼ1[e]
Dyad 1 demonstrated dynamic behavior in variable-tem-
1
2 (R = H)
perature H NMR measurements (Figure 1) that is typical
of rapid CHT–NCD exchange.[9] Although 1 can exist as
four structures, 1-eq-CHT, 1-ax-CHT, 1-exo-NCD, and 1-
endo-NCD, which are interchangeable by valence isomerism
and ring inversion (Scheme 5), the broad signals that were
eq-CHT
79.5[f]
16.9[f]
3.6[f]
0.00[f]
0.47[f]
0.94[f]
Ͼ1[f]
0.00[g]
1.17[g]
3.44[g]
3.47[g]
exo-NCD
endo-NCD
ax-CHT
Ͻ1[f]
1
4 (R = Ph)
observed in the H NMR spectrum at 15 °C split into only
eq-CHT
70.6[f]
26.4[f]
3.0[f]
0.00[f]
0.30[f]
0.96[f]
Ͼ1[f]
two signals at low temperatures. These signals were assigned
to 1-eq-CHT and 1-exo-NCD in a molar ratio of 85.6:14.4
at –100 °C, whereas the concentrations of the ax-CHT and
endo-NCD isomers were too low (Ͻ1%) to be detected.
exo-NCD
endo-NCD
ax-CHT
Ͻ1[f]
[a] Obtained from variable-temperature NMR measurements. [b]
Value determined relative to that of the eq-CHT isomer. [c] Experi-
mental Gibbs free energy. [d] Electronic energy obtained at the
B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level of theory. [e] In CS2/
CD2Cl2 (4:1, v/v) at –100 °C. [f] In CS2/CD2Cl2 (3:1, v/v) at
–120 °C; ref.[13a] [g] Ref.[15a]
1
Based on the H NMR spectroscopic data obtained at 15
and –100 °C, the values of ∆H° and ∆S° for the transforma-
tion of 1-eq-CHT into 1-exo-NCD were determined to be
0.10Ϯ0.16 kcalmol–1 and –3.0Ϯ1.0 calmol–1 K–1, respec-
tively.[16]
The proportion of the NCD form at equilibrium is negli-
gible for unsubstituted cycloheptatriene.[9] The increased
proportion (15–26%) of the exo-NCD form for 1, 2, and 4
is attributable to the presence of tert-butyl groups on C-3
and C-6. It is known that introduction of π-accepting sub-
stituent(s) on C-1 also lowers the energy of the NCD
form.[9,13b,17] Although the π system of the C60 sphere is
appreciably electronegative,[18] no increase in the NCD pop-
ulation was observed for fullerene derivative 1 relative to 2.
This result is mostly ascribed to the presence of an sp3 car-
bon atom between the ethynyl group and the fullerene π-
system, which inhibits the electron-withdrawing effect of
the C60 π-system.
Self-Sensitized Photooxygenation
Although 1 is stable in the dark, it was rapidly oxidized
under exposure to light and air (Scheme 6). When a CS2
solution of 1 (1.2ϫ10–3 ) was irradiated for 30 min with
a 500 W xenon lamp under oxygen, 1 was quantitatively
converted into endoperoxide 5. The MALDI-TOF mass
spectrum of the product exhibited a molecular ion peak
Scheme 5. Valence isomerism and ring inversion of ethynylcy-
cloheptatriene derivatives.
The DFT-calculated relative energies of the four possible ([M]+) at m/z = 980, which indicates the addition of two
1
isomers of 1 are listed in Table 1 along with those of the oxygen atoms to 1. The H and 13C NMR spectra showed
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two sets of oxygenated norcaradiene signals due to the for-
mation of exo and endo isomers. The 1H NMR signals were
assigned on the basis of the coupling constants between 2,4-
H and 3-H [3.5 Hz (exo) and 7.4 Hz (endo)]. The exo/endo
molar ratio was determined to be 60:40 based on signal
integration. Attempts to separate the isomers by
chromatography were not successful. The 13C NMR spec-
trum of the mixture showed 30 large peaks between δ = 134
and 151 ppm that were attributed to the fullerene cage of
5-exo and additional 29 peaks with lower intensities due to
5-endo, which indicates that both isomers have Cs sym-
metry. The signals of the sp3 carbon atoms of the fullerene
cage appeared at δ = 54.3 and 61.9 ppm for 5-exo and at δ
= 54.7 and 61.2 ppm for 5-endo. The carbon atoms of the
dioxatricyclic moiety were observed at δ = 9.26 (5-exo C-3),
17.2 (5-exo C-2,4), 128.5 (5-exo C-8,9), 8.41 (5-endo C-3),
17.6 (5-endo C-2,4), and 131.1 ppm (5-endo C-8,9), and the
ethynyl carbon atoms were detected at δ = 80.0 and
82.4 ppm (5-exo) and δ = 82.6 and 87.2 ppm (5-endo). The
UV/Vis spectrum showed a narrow absorption band at
431 nm, which further confirmed the pattern of addition on
the C60 sphere. This result is explained by the [4+2]-type
Scheme 7. Elementary processes in the photooxygenation of 1.
The steady-state treatment of the above process with re-
1
spect to O2 leads to a Stern–Volmer relationship [Equa-
tion (1)] that predicts a linear relationship between the over-
all rate of formation of 5 (kobs) and the quencher concentra-
tion.[22] For rate measurement, 1 was photolyzed in benzene
under exposure to air by using a 20 W tungsten lamp lo-
cated 50 cm away. The yields of 5-exo and 5-endo were de-
termined during the initial stage of the reaction. The total
yield of the two isomers decreased as the concentration of
DABCO added increased, giving the expected linear plot
shown in Figure 2. Addition of 100 equiv. of DABCO re-
sulted in the complete inhibition of the reaction even with
irradiation using a 100 W tungsten lamp for 3 h.
1
addition of singlet oxygen O2 to the NCD moiety.[19] Car-
bon disulfide is the solvent of choice because singlet oxygen
has a prolonged lifetime (200–250 µs) in this solvent.[20]
(1)
Scheme 6. Photooxygenation of 1.
Unlike the reported photooxygenation of the parent cy-
cloheptatriene (Scheme 2),[8] no CHT-derived [4+2] and
[6+2] cycloadducts were observed in the reaction of 1.
Adam and co-workers[11a] reported that 7-cyanocyclohepta-
triene, which contains an undetectable amount of the NCD
isomer at equilibrium,[9] gave an endoperoxide only from
the NCD tautomer. The exclusive formation of the NCD
Figure 2. Stern–Volmer plot for DABCO quenching of the photo-
adduct 5 observed in this study is consistent with the greater oxygenation of 1. Y0 is the yield of 5 without quencher; Y[DABCO]
is the yield of 5 with DABCO. Photolysis was carried out in CS2
proportion of the NCD tautomer of 1.
by irradiating the solution for 9 min with a 20 W tungsten lamp
from a distance of 50 cm. Values of Y0 were 22.5 and 24.2% at 25
and 0 °C, respectively.
The photooxygenation of 1 was effectively suppressed by
the addition of 1,4-diazabicyclo[2.2.2]octane (DABCO).
DABCO is a well-known singlet-oxygen quencher, which is
frequently used to test the involvement of singlet oxygen in
reaction sequences.[21] The most likely kinetic pathway for in air and light. However, 2 was also converted into the
the self-sensitization reaction of 1 is shown in Scheme 7. endoperoxides 6-exo and 6-endo (80:20) when irradiated in
Cycloheptatriene 2, which lacks the C60 moiety, is stable
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Self-Sensitized Photooxygenation of a C60–Cycloheptatriene Dyad
the presence of 0.1 equiv. of pristine C60 (Scheme 8).[23] This
demonstrates that C60-sensitized oxygenation also occurs
intermolecularly with an effectiveness similar to that of self-
sensitized oxygenation, even at low C60 concentrations (ca.
10–4 ).
Experimental Section
1
General: H and 13C NMR spectra were obtained by using JEOL
JNM-Lambda500 (1H, 500 MHz; 13C, 125 MHz) and JNM-AL300
(1H, 300 MHz; 13C, 75 MHz) spectrometers. Peak assignments were
based on DEPT, COSY, and/or H–C COSY measurements. UV/Vis
spectra were recorded by using a JASCO V-670 spectrophotometer.
High-resolution mass spectrometry was performed by using Ap-
plied Biosystems Voyager-DE PRO (MALDI-TOF) and JEOL
JMS-600H (EI) spectrometers. Gel permeation chromatography
(GPC) was carried out by using a Shodex H-2001 column. HPLC
was performed by using Nacalai tesque Buckyprep and 5PBB col-
umns with toluene and chloroform, respectively, as the eluents. 1,4-
Di-tert-butyltropylium perchlorate was synthesized as previously
reported.[26] THF was freshly distilled from sodium benzophenone
ketyl prior to use. Compound 2 was synthesized according to the
method described in our earlier reports[13a,15a] or by following the
modified procedure described below.
2,5-Di-tert-butyl-7-(trimethylsilylethynyl)cyclohepta-1,3,5-triene:
Butyllithium (2.6  in hexane, 2.13 mL, 5.54 mmol) was added
dropwise to a THF solution (50 mL) of trimethylsilylacetylene
(1.01 mL, 7.15 mmol) at –66 °C over 5 min. 1,4-Di-tert-butyltropyl-
ium perchlorate (1.40 g, 4.63 mmol) was added and the mixture
was stirred at room temperature for 30 min. Aqueous work-up af-
forded a pale-yellow oil (1.30 g), which was purified by flash col-
umn chromatography (SiO2, hexane) to afford 2,5-di-tert-butyl-7-
(trimethylsilylethynyl)cyclohepta-1,3,5-triene (223 mg, 16%) as col-
Scheme 8. C60-sensitized photooxygenation of 2.
Thus, it was of interest to determine whether sensitized
oxygenation is enhanced by the connection of CHT and C60
through an ethynylene linker, as in 1. This might allow an
in-cage process in which the generated singlet oxygen could
immediately attack the NCD moiety before escaping the
cage. To examine this possibility, a competitive oxygenation
reaction was carried out by irradiating a CS2 solution con-
taining equal concentrations of 1 and 2 (5.7ϫ10–4 ) in the
presence of air.[24] The yields of the oxygenated products 5
1
orless crystals; m.p. 91.6–92.7 °C. H NMR (300 MHz, CDCl3): δ
= 0.16 [s, 9 H, Si(CH3)3], 1.10 (s, 18 H, tBu), 1.89 (br. t, J = 5.5 Hz,
1 H, 7-H), 4.25 (br. s, 2 H, 1,6-H), 6.36 (br. s, 2 H, 3,4-H) ppm.
13C NMR (75 MHz, CDCl3): δ = 0.28 [Si(CH3)3], 29.40 [C(CH3)3],
30.86 (C-7), 34.74 [C(CH3)3], 65.82 (CϵC), 84.08 (CϵC), 109.03
(br., C-1,6), 124.92 (br., C-3,4), 143.92 (br., C-2,5) ppm. IR (KBr):
ν = 2176 (CϵC) cm–1. HRMS (EI, 70 eV): calcd. for C20H32Si
˜
and 6 measured midway through the reaction were compar- [M]+ 300.2273; found 300.2280. Further elution gave the 1,5-di-
able (48 and 56%, respectively).[25] No preferred oxygena-
tion of 1 was observed, which clearly indicates that the ad-
tert-butyl isomer (525 mg, 38%) as a pale-yellow oil and the 1,4-
di-tert-butyl isomer (309 mg, 22%) as colorless crystals.
dition of singlet oxygen is not an in-cage reaction but oc-
curs after its diffusion into the bulk solution. Consequently,
in the present system, the close proximity of C60 is not re-
sponsible for the observed efficient oxygenation.
2,5-Di-tert-butyl-7-ethynylcyclohepta-1,3,5-triene (2): A 0.88  solu-
tion of KOH in ethanol (18 mL, 16 mmol) was added to a solution
of 2,5-di-tert-butyl-7-(trimethylsilylethynyl)cyclohepta-1,3,5-triene
(162 mg, 0.538 mmol) in ethanol (50 mL). The mixture was heated
at reflux for 10 min and stirred for an additional 30 min at room
temperature. After aqueous work-up, the solvent was evaporated
to give a pale-yellow oil (116 mg). Purification by GPC (CHCl3)
afforded 2 (81.6 mg, 66%), which showed a 1H NMR spectrum
identical to that reported in ref.[15a,27]
.
Conclusions
1-[(3,6-Di-tert-butylcyclohepta-2,4,6-trien-1-yl)ethynyl]-1,2-dihydro-
[60]fullerene (1): The reaction and purification were carried out in
the dark. Butyllithium (2.64  in hexane, 0.097 mL, 0.26 mmol)
was added dropwise to a solution of 2 (58.4 mg, 0.256 mmol) in
THF (1 mL) at –77 °C over 5 min and the mixture was stirred for
We have synthesized a new fullerene derivative 1 by nu-
cleophilic addition of an organolithium to C60. The connec-
tion of an oxygen-accepting cycloheptatrienyl group to the
fullerene skeleton, which is an efficient photosensitizer, per-
mitted self-sensitized photooxygenation leading to the 25 min at room temperature. The solution was added dropwise to
a suspension of C60 (62 mg, 0.085 mmol) in THF (60 mL) over
10 min, during which the solution turned dark bluish-brown due
to the formation of the conjugate base of 1, R–CϵC–C60–. After
stirring the mixture for 50 min, a 0.05  solution of TFA (5.1 mL)
was added. The solvent was removed under vacuum and the residue
was extracted with CS2. The solvent was evaporated and the re-
maining solid was purified by GPC (toluene) to afford a dark-
brown solid of 1 (33.5 mg, 41% based on C60) as a rapidly exchang-
ing mixture of eq-CHT and endo-NCD isomers (84.2:15.8 at 15 °C,
85.6:14.4 at –100 °C). The 1H and 13C NMR spectra showed time-
averaged signals at room temperature. 1H NMR (500 MHz, CS2/
stable endoperoxide 5 by irradiation in the presence of air.
The photosensitivity of 1 was so high that even exposure to
low light from a distant, low-power bulb was sufficient for
complete oxygenation. The reaction was highly chemoselec-
tive such that the endoperoxide formed only from the nor-
caradiene tautomer, which was favored by the bulky tert-
butyl groups. Competitive oxygenation of 1 and 2 demon-
strated that the singlet oxygen, which was generated by pho-
tosensitization by the fullerene moiety of 1, diffused into
the bulk solution prior to cycloaddition.
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CD2Cl2, 4:1, v/v, 15 °C): δ = 1.25 (s, 18 H, tBu), 2.45 (br. s, 1 H, [5-endo, C(CH3)3], 54.33, 61.87 (5-exo, sp3, fullerene), 54.69, 61.20
1-H), 5.05 (br. s, 2 H, 2,7-H), 6.63 (br. s, 2 H, 4,5-H), 7.04 (s, 1 H,
(5-endo, sp3, fullerene), 80.02, 82.39, 82.55, 82.65, 83.37, 87.23 (5-
exo/5-endo, C-1,5 and -CϵC-), 128.47 (5-exo, C-8,9), 131.06 (5-
C60-H) ppm. 13C NMR (125 MHz, CS2/CD2Cl2, 4:1, v/v, 24 °C):
δ = 29.38 (CH3), 29.87 (C-1), 34.41 [C(CH3)3], 54.55, 62.17 (sp3, endo, C-8,9), 134.71, 135.83, 140.04, 140.06, 141.35, 141.41, 141.57,
fullerene), 75.91, 83.38 (-CϵC-), 86.19 (br., C-2,7), 125.85 (br., C-
4,5), 143.75 (br., C-3,6), 134.69, 135.94, 140.09, 140.15, 141.40,
141.48, 141.75, 141.80, 141.84, 141.93, 142.36, 142.38, 143.00,
144.34, 144.48, 145.13, 145.18, 145.27, 145.39, 145.47, 145.60,
145.98, 145.99, 146.15, 146.16, 146.50, 147.10, 147.37, 151.53,
141.72, 141.77, 141.80, 142.31, 142.34, 142.95, 144.23, 144.41,
145.07, 145.13, 145.21, 145.28, 145.36, 145.48, 145.92, 145.94,
146.10, 146.11, 146.31, 147.03, 147.31, 151.19, 151.22 (5-exo, sp2,
fullerene), 134.33, 135.70, 140.09, 140.26, 141.35, 141.43, 141.679,
141.684, 141.78, 141.84, 142.34, 142.36, 142.98, 144.26, 144.43,
144.91, 145.10, 145.15, 145.25, 145.27, 145.45, 145.96 (2 C), 146.10,
151.90 (sp2, fullerene) ppm. IR (neat): ν = 2233 (CϵC) cm–1. UV/
˜
Vis: λmax (ε [–1 cm–1]) = 211 (127000), 255 (102000), 306 (32600), 146.12, 146.33, 147.08, 147.34, 151.06, 151.36 (5-endo, sp2, fuller-
325 (32000), 431 (3670), 701 (734) nm. HRMS (MALDI-TOF):
ene) ppm. IR (CHCl ): ν = 2233 (CϵC) cm–1. UV/Vis: λ
(ε
˜
3
max
calcd. for C77H23 [M – H]+ 947.1794; found 947.1784.
[–1 cm–1]) = 212 (119000), 255 (105000), 307 (34800), 324 (34600),
431 (4000), 701 (558) nm. HRMS (MALDI-TOF): calcd. for
C77H24O2 [M]+ 980.1771; found 980.1731.
At –100 °C, the exchange was frozen. 1-eq-CHT: 1H NMR: δ =
1.14 (s, 18 H, tBu), 2.70 (t, J = 5.5 Hz, 1 H, 1-H), 5.55 (d, J =
5.5 Hz, 2 H, 2,7-H), 6.86 (s, 2 H, 4,5-H), 7.02 (s, 1 H, C60-H) ppm.
Photooxygenation of 2 to Form the Endoperoxide 6: A solution of
2 (27.9 mg, 0.122 mmol) and C60 (8.8 mg, 0.0122 mmol) in CS2
(90 mL) in a Pyrex vessel was irradiated with a 500 W xenon lamp
under bubbling oxygen for 30 min. The solvent was evaporated and
the residue was extracted with diethyl ether. Removal of the ether
gave a mixture of 6-exo and 6-endo (80:20) as colorless crystals
(25.2 mg). Flash column chromatography (SiO2, hexane) gave a
fraction consisting of pure 6-exo (7.9 mg) followed by mixtures of
6-exo and 6-endo [10.8 mg (91:9) and 4.9 mg (46:54)]. Total yield:
1
1-exo-NCD: H NMR: δ = 0.70 (t, J = 4.3 Hz, 1 H, 1-H), 1.26 (s,
18 H, tBu), 2.55 (d, J = 4.3 Hz, 2 H, 2,7-H), 5.61 (s, 2 H, 4,5-H),
6.93 (s, 1 H, C60-H) ppm.
Acid-Catalyzed Isomerization of 1 to 3: TFA (5 µL, 0.067 mmol)
was added to a solution of 1 (15.9 mg, 0.0168 mmol) in CDCl3
(0.6 mL) in an NMR tube. After 30 min, the solvent was evapo-
rated and the residue was purified by flash column chromatography
(SiO2, CS2) to afford 5.3 mg (33%) of 1-[3-(2,5-di-tert-butylphen-
yl)propa-1,2-dien-1-yl]-1,2-dihydro[60]fullerene (3) as a dark-brown
1
23.6 mg (74%). 6-exo: Colorless crystals; m.p. 104.9–105.9 °C. H
NMR (300 MHz, CDCl3): δ = 0.72 (m, 1 H, 3-H), 1.09 (s, 18 H,
tBu), 1.80 (d, J = 2.0 Hz, 1 H, -CϵCH), 1.876 (d, J = 3.3 Hz, 2
H, 2,4-H), 6.20 (s, 2 H, 8,9-H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 8.76 (C-3), 16.89 (C-2,4), 24.99 (CH3), 34.72 [C(CH3)3], 65.07
1
solid. H NMR (300 MHz, CS2/CDCl3, 4:1, v/v): δ = 1.29 (s, 9 H,
tBu), 1.54 (s, 9 H, tBu), 6.89 (s, 1 H, C60-H), 7.18 (dd, J = 8.3,
2.3 Hz, 1 H, 4-H), 7.29 (d, J = 5.9 Hz, 1 H, C60-C=C=C-H), 7.31
(d, J = 8.1 Hz, 1 H, 3-H), 7.60 (d, J = 6.1 Hz, 1 H, C60-C-H), 7.81
(d, J = 2.4 Hz, 1 H, 6-H) ppm. 13C NMR (125 MHz, CS2/CDCl3,
4:1, v/v): δ = 31.08 (CH3), 31.44 (CH3), 33.66 [C(CH3)3], 34.63
[C(CH3)3], 59.09, 63.75 (sp3, fullerene), 102.27, 103.70 (-CH=),
124.87, 126.08, 127.27, 130.28, 133.71, 135.00, 135.18, 135.88,
135.92, 140.027, 140.032, 140.16, 140.18, 141.35, 141.36, 141.39,
141.41, 141.61, 141.70, 141.71, 141.75, 141.76, 141.77, 141.78,
141.93, 141.94, 142.28, 142.29, 142.31, 142.96, 143.80, 144.277,
144.283, 144.41, 144.42, 145.08, 145.10, 145.11, 145.21, 145.23,
145.28, 145.30, 145.50, 145.51, 145.63, 145.900, 145.908, 145.917,
145.923, 146.07, 146.08, 146.11, 146.61, 146.62, 147.01, 147.21,
148.34, 152.73, 152.77, 153.95, 154.10 (sp2 carbons), 204.40
(-CϵC-), 83.09 (-CϵC-), 128.76 (C-8,9) ppm. IR (CHCl ): ν =
˜
3
3307 (H–CϵC), 2122 (CϵC) cm–1. HRMS (EI, 20 eV): calcd. for
C17H24 [M – O2]+ 228.1878; found 228.1879. 6-endo: 1H NMR
(300 MHz, CDCl3): δ = 1.12 (s, 18 H, tBu), 1.43 (td, J = 7.5,
2.6 Hz, 1 H, 3-H), 1.879 (d, J = 7.3 Hz, 2 H, 2,4-H), 2.01 (d, J =
2.6 Hz, 1 H, -CϵCH), 6.32 (s, 2 H, 8,9-H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 7.85 (C-3), 16.97 (C-2,4), 25.03 (CH3), 34.80
[C(CH3)3], 73.05 (-CϵC-), 83.23 (-CϵC-), 131.35 (C-8,9) ppm.
Photooxygenation of 1 in the Presence of DABCO: Compound 1
was dissolved in CS2 (5.7ϫ10–4 ) containing a known concentra-
tion of DABCO in a Pyrex tube. The solution was irradiated with
a 20 W tungsten lamp from a distance of 50 cm for 9 min at 25 or
0 °C under air. The solvent was removed under vacuum and the
remaining mixture was analyzed by 1H NMR to determine the
yield of 5. The results are given in Figure 2.
(=C=) ppm. IR (CHCl ): ν = 1942 (C=C=C) cm–1. UV/Vis: λ
˜
3
max
(ε [–1 cm–1]) = 211 (117000), 256 (88300), 309 (32700), 432 (3100),
704 (535) nm. HRMS (MALDI-TOF): calcd. for C77H24 [M]+
948.1873; found 948.1879.
Photooxygenation of 1 to Form the Endoperoxide 5: A solution of
1 (17.6 mg, 0.0185 mmol) in CS2 (15 mL) in a Pyrex vessel was
irradiated with a 500 W xenon lamp under bubbling oxygen for
30 min. The solvent was evaporated to give a dark-brown solid
(14.3 mg), which was shown to be an essentially pure mixture of 5-
exo and 5-endo (60:40) by NMR spectroscopy. Although this solid
was further purified by flash column chromatography (SiO2, Et2O/
Competitive Photooxygenation of 1 and 2: A solution containing
both 1 and 2 in CS2 (5.7ϫ10–4  each) in a Pyrex tube was irradi-
ated with a 20 W tungsten lamp from a distance of 50 cm for 1 h
at 0 °C under air. The solvent was removed under vacuum and the
remaining mixture was analyzed by 1H NMR to determine the
yield of 5 (48%) and 6 (56%).
CS2, 1:1) to give dark-brown crystals (8.8 mg, 50%), the isomers Calculations: Density functional theory (DFT) calculations[28] were
could not be separated even by recycle chromatography by using a
Buckyprep (Nacalai tesque) or GPC H-2001 (Shodex) column. H
performed by using the Gaussian 03 program.[29] The molecular
structures of the isomers of 1 were optimized at the B3LYP/6-
31G(d) level of theory starting from the B3LYP/3-21G-optimized
geometries, which were verified by frequency calculations to have
no imaginary frequencies. The B3LYP/6-31G(d)-optimized struc-
tures were found to be similar to those obtained at the B3LYP/
3-21G level of theory. The energies were computed by B3LYP/6-
311+G(2d,p) single-point calculations using the B3LYP/6-31G(d)
geometries and are summarized in Table S1 of the Supporting In-
1
NMR (300 MHz, CS2/CDCl3, 4:1, v/v): 5-exo: δ = 1.08 (t, J =
3.5 Hz, 1 H, 3-H), 1.18 (s, 18 H, tBu), 2.11 (d, J = 3.5 Hz, 2 H,
2,4-H), 6.26 (s, 2 H, 8,9-H), 6.89 (s, 1 H, C60-H) ppm; 5-endo: δ =
1.20 (s, 18 H, tBu), 1.77 (t, J = 7.4 Hz, 1 H, 3-H), 2.04 (d, J =
7.4 Hz, 2 H, 2,4-H), 6.49 (s, 2 H, 8,9-H), 6.81 (s, 1 H, C60-H) ppm.
13C NMR (125 MHz, CS2/CDCl3, 4:1, v/v): δ = 8.41 (5-endo, C-3),
9.26 (5-exo, C-3), 17.19 (5-exo, C-2,4), 17.63 (5-endo, C-2,4), 24.87
(5-exo, CH3), 24.94 (5-endo, CH3), 34.23 [5-exo, C(CH3)3], 34.29 formation.
3262
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Eur. J. Org. Chem. 2010, 3257–3264

Self-Sensitized Photooxygenation of a C60–Cycloheptatriene Dyad
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Acknowledgments
This work was supported by a grant from the Okasan-Kato Foun-
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[23] The higher yield of endo-5 compared with endo-6 is not consis-
tent with the equilibrium populations of the respective precur-
sors 1-endo-NCD and 2-endo-NCD (Ͻ1 and 3.6%, respectively,
Table 1), nor with the HOMO level of endo-NCD relative to
exo-NCD (1: –0.08 eV, 2: 0.00 eV, respectively, by DFT). It may
rather be due to other factors such as the rate of the exchange
reactions shown in Scheme 5.
[24] Because 1 and 2 contain similar proportions of the NCD iso-
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the relative probabilities of encountering singlet oxygen.
[25] By comparison, no endoperoxide was formed when 2 was irra-
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Received: December 13, 2009
Published Online: April 30, 2010
3264
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Eur. J. Org. Chem. 2010, 3257–3264