Exploring the photoinduced electron transfer reactivity of aza[60]fullerene iminium cation

Article abstract of DOI:10.1016/j.tet.2010.10.006

Photolysis of (C₅₉N)₂ solutions in the presence of neutral π-donors, such as arenes and electron-rich alkenes leads to a series of novel aza[60]fullerene monoadducts. The key step of the reaction involves a photoinduced electron transfer from the donor molecule to the iminium cation of aza[60]fullerene, followed by radical coupling of the resulting aza[60]fullerenyl radical with an intermediate stabilized radical derived from the substrate. This type of reactivity has been proven efficient with arenes having oxidation potential higher than about 1.5 V. Simple olefins, such as tri- and tetra-methylethylene, as well as cyclohexene, can also participate in this kind of photoinduced electron transfer-initiated reaction with C ₅₉N⁺, affording the corresponding aza[60]fullerene derivatives. In the case of 2-methoxyprop-1-ene, 2,4-hexadiene, and β,β-dimethylstyrene, [2+2] cycloaddition reactions with the aza[60]fullerene carbon shell dominate, leading to a mixture of unidentified multiadducts.

Full text of DOI:10.1016/j.tet.2010.10.006

Tetrahedron 66 (2010) 9363e9369
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Exploring the photoinduced electron transfer reactivity of aza[60]fullerene
iminium cation
Manolis M. Roubelakis a, Georgios C. Vougioukalakis b, Leanne C. Nye c, Thomas Drewello c,
,
Michael Orfanopoulos a
*
a Department of Chemistry, University of Crete, Campus Voutes, Heraklion 71003, Greece
b Institute of Physical Chemistry, NCSR ‘Demokritos’, Agia Paraskevi Attikis, Athens 15310, Greece
c Department of Chemistry and Pharmacy, University of Erlangen-Nurnberg, Erlangen D-91058, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2010
Received in revised form 6 September 2010
Accepted 4 October 2010
Available online 13 October 2010
Photolysis of (C59N)2 solutions in the presence of neutral
p-donors, such as arenes and electron-rich
alkenes leads to a series of novel aza[60]fullerene monoadducts. The key step of the reaction involves
a photoinduced electron transfer from the donor molecule to the iminium cation of aza[60]fullerene,
followed by radical coupling of the resulting aza[60]fullerenyl radical with an intermediate stabilized
radical derived from the substrate. This type of reactivity has been proven efficient with arenes having
oxidation potential higher than about 1.5 V. Simple olefins, such as tri- and tetra-methylethylene, as well
as cyclohexene, can also participate in this kind of photoinduced electron transfer-initiated reaction with
C59Nþ, affording the corresponding aza[60]fullerene derivatives. In the case of 2-methoxyprop-1-ene,
Keywords:
Aza[60]fullerene
Photoinduced electron transfer
Iminium cation
Benzyltrimethylsilanes
Electron-rich olefins
2,4-hexadiene, and
shell dominate, leading to a mixture of unidentified multiadducts.
Ó 2010 Elsevier Ltd. All rights reserved.
b,
b-dimethylstyrene, [2þ2] cycloaddition reactions with the aza[60]fullerene carbon
1. Introduction
isomers that are impossible to isolate and characterize. For exam-
ple, one single addition on a [6,6] double bond in 2 can lead to 16
distinct isomers. As a result, only a few methods for the synthesis of
well-defined aza[60]fullerene adducts are known at present.3
The trapping of the azafullerenyl radical 1 was initially utilized to
afford adducts 3 and 4 (Fig. 2); radical 1 was produced from dimer 2
either thermally or photochemically in the presence of a hydrogen
atom donor, such as tributyl-tinhydride4 or diphenylmethane,5
respectively. In the same way, 9-alkyl-substituted fluorenes, 9,10-
dihydroanthracene, and xanthene gave the corresponding mono-
adducts 5, 6, and 7 (Fig. 2).6 A free radical chain mechanism has been
proposed for the production of these aza[60]fullerene derivatives.5ꢀ7
However, the most efficient way to prepare aza[60]fullerene
monoadducts involves the thermal treatment of the dimeric
(C59N)2 in the presence of air and excess toluene-p-sulfonic acid (p-
TsOH), a procedure that oxidizes the produced azafullerenyl radical
1 to aza[60]fullerene iminium cation C59Nþ (8, Scheme 1), which is
isoelectronic to C60. This entity can be easily trapped by nucleo-
philes, such as electron-rich aromatics,8,9 enolizable carbonyl
compounds,10 as well as alcohols and olefins,11 furnishing the cor-
responding azafullerene derivatives (Scheme 1). The presence of
both the oxygen and the acid is crucial for the above reactions to
take place. It has been proposed that oxygen acts as the oxidizing
agent, whereas toluene-p-sulfonic acid probably traps the reduced
oxygen species and adjusts the solution’s pH.8
Despite the very rich chemistry of C60 and the great number of
functionalization techniques that have been developed over the
years,1 similar progress regarding its nitrogen substituted coun-
terpart, aza[60]fullerene (1, Fig. 1), has been hampered thus far by
the low symmetry of the heterofullerene sphere and by the fact that
aza[60]fullerene is isolated as a dimer (2, Fig. 1).2 In other words,
the typical fullerene reactions can take place on any of the [6,6]
double bonds of the two balls, affording complicated mixtures of
N
N
N
2
1
(Cs)
Fig. 1. Azafullerenyl radical C59Nꢁ (1) and azafullerene dimer (C59N)2 (2).
* Corresponding author. Tel.: þ30 2810 545030; fax: þ30 2810 545001; e-mail
address: orfanop@chemistry.uoc.gr (M. Orfanopoulos).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.10.006

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M.M. Roubelakis et al. / Tetrahedron 66 (2010) 9363e9369
iminium cations and aza[60]fullerene iminium cation 8 (Fig. 3)
prompted us to investigate the (C59N)2 reactivity toward -electron
donors under photochemical conditions. Herein, we present the
photochemical addition of benzyltrimethylsilanes and electron-rich
alkenes to C59Nþ through a PET-initiated chain mechanism.
p
R
H
N
N
N
2. Results and discussion
3
4
5
2.1. Photoinduced electron transfer reactions between
R = H, Me, Et, Ph
iminium cation C59ND and substituted benzyltrimethylsilanes
O
In a preliminary communication, we have reported that photo-
chemically generated aza[60]fullerene iminium cation 8 reacts with
benzyltrimethylsilane (10b) to give monoadduct 11b (Scheme 2).16
(C59N)2, together with a large excess of benzyltrimethylsilane, was
irradiated in an o-dichlorobenzene (ODCB) solution in the presence
of air and p-TsOH. After semi-preparative HPLC purification of the
crude product, adduct 11b was isolated in 28% yield. The proposed
mechanism for the formation of 11b is illustrated in Scheme 2.
Iminium cation 8 derives from the oxidation of azafullerenyl radical
1, which is generated by the photochemical homolysis of (C59N)2.
Next, the key step of this reaction sequence, involving a photoin-
duced electron transfer from benzyltrimethylsilane 10b to iminium
cation 8, affords radical cation pair 12. Subsequent loss of the SiMe3þ
group leads to the neutral radical pair 13.14,15 Adduct 11b is even-
tually formed after radical coupling in 13 and self-sensitized pho-
tooxygenation17 of the resulting intermediate 14.
N
N
6
7
Fig. 2. Hydroaza[60]fullerene C59HN (3) and aza[60]fullerene monoadducts
C59(CHPh2)N (4), 5, 6, and 7.
N
p-TsOH / O2
N
N
ODCB, Δ
2
8
SiMe3
R
N
O
R1
R2
OH
R3
hv
H
N
+
N
R1
R3
R2
p-TsOH / O2
2
R
ODCB
8
10b
O
R2
R1
N
R3
PET
N
SiMe3
+
-SiMe3
N
N
Scheme 1. Aza[60]fullerene iminium cation reactivity.
It is also worth mentioning that aza[60]fullerene multiadducts12
have been successfully prepared and characterized starting from
open-cage fullerene derivatives.13
12
13
Arene donors14 as well as electron-rich olefins15 are well-known
for their ability to photochemically add to iminium cations of the
general type 9 (Fig. 3). The mechanism that has been established for
these reactions involves a photoinduced electron transfer (PET) from
O
N
N
hv
the
p-system of the arene/olefin to the single excited state of the
iminium cation in the first step, followed by the coupling of the two
O2
resulting radicals. The structural similarity between conventional
14
11b
Scheme 2. The photoinduced electron transfer reaction between C59Nþ (8) and ben-
zyltrimethylsilane (10b) affording 11b.
N
N
In the same context, Yoshida and co-workers have more recently
reported the reaction of an electrochemically generated N-acyli-
minium ion pool with a series of benzylsilanes.18 In agreement with
the discussion above, they proposed that the reaction proceeds
through a chain mechanism initiated by a single electron transfer
9
8
Fig. 3. Structural resemblance of aza[60]fullerene carbocation 8 with iminium cations
in general (9).

M.M. Roubelakis et al. / Tetrahedron 66 (2010) 9363e9369
9365
(SET) from benzylsilane to N-acyliminium cation, followed by
desilylation of the formed benzylsilane radical cation.
O
O
With the aim of further studying the PET reactivity of iminium
cation 8 toward benzyltrimethylsilanes of various oxidation poten-
tials, wesynthesized the series of benzyltrimethylsilanes depicted in
Table 1 (10a,cee), which are substituted with electron-withdrawing
or electron-donating moieties. In Table 1 we also provide the oxi-
dation potentials for these benzylsilanes. The lower the oxidation
potential of a substrate, the higher its reactivitywith formal iminium
cations.18 Benzyltributylstannane (15, Table 1) was also included in
our studies. Due to its lower oxidation potential, in comparison with
benzylsilanes 10aee, this compound was expected to be the most
reactive toward aza[60]fullerene iminium cation 8.
N
N
11c
11d
Fig. 4. Aza[60]fullerene monoadducts 11c and 11d.
Table 1
Oxidation potentials of benzyltrimethylsilanes 10 and benzyltributylstannane 15
Substrate
Oxidation potential (V)a
SiMe3
1.68
F
10a
SiMe3
1.68
1.62
10b
SiMe3
10c
SiMe3
1.55
1.37
1.24
10d
SiMe3
Fig. 5. 1H NMR (500 MHz, acetone-d6/CS2) spectrum of 11d.
MeO
10e
(Table 1), their reactions with C59Nþ were anticipated to proceed
smoothly furnishing the corresponding azafullerene monoadducts.
Nevertheless, as was clearly shown by HPLC analysis, azafullerene
dimer 2 was gradually consumed upon irradiation without leading
to the desired products. In this respect, irradiation of C60 solutions in
the presence of benzyltrialkylstannanes, such as 15, is known to lead
to single electron transfer from the stannanes to the fullerene
SnBu3
15
a
Taken from Ref. 18.
skeleton, affording the radical anion of C60 (C60 ꢀ).19 We speculate
ꢁ
All reactions were performed according to the experimental con-
ditions utilized in the preparation of adduct 11b.16 We initially chose
to study the photoinduced reaction of (4-fluorobenzyl)trimethylsi-
lane (10a) with 8. As shown by HPLC analysis, azafullerene dimer is
slowly consumed during this reaction, though without affording the
corresponding monoadduct, at least in a detectable amount. Given
that 10a has the same oxidation potential as 10b (vide supra), this is
a rather surprising observation. The lack of aza[60]fullerene mono-
adduct formationin this case can only beattributed to the existence of
the fluorine substituent, i.e., the electronics of 10a, which, neverthe-
less, are not translated into a higher oxidation potential.
that similar electron transfer reactions occur in the case of C59Nþ;
however, these reactions are obviously not taking place selectively
on the iminium cation segment of the cage, in order to afford the
corresponding a-amino radical/radical cation pair (12 in Scheme 2),
but, instead, on the carbon shell of C59Nþ leading to a complex
mixture of multiadducts, difficult to identify and isolate. It therefore
seems that the enhanced efficiency with which 10e and 15 can lose
one electron turns out to be a disadvantage in the case of their PET
reactions with (C59N)2.
Next, we examined the reactivity of trimethyl(2-methylbenzyl)
silane (10c) and trimethyl(4-methylbenzyl)silane (10d). Adducts
11c and 11d (Fig. 4) were isolated, respectively, in about 10% yield.
Unlike most known fullerene derivatives, the solubility of these
adducts in CS2 was rather poor. A representative 1H NMR spectrum
of derivative 11d is presented in Fig. 5.
(4-Methoxybenzyl)trimethylsilane (10e) and benzyltributyl-
stannane (15) were also subjected to PET reaction conditions in the
presence of in situ generated aza[60]fullerene iminium cation.
Taking into account the low oxidation potential of these compounds
2.2. Photoinduced electron transfer reactions between
iminium cation C59ND and electron-rich olefins.
Photochemical attachment of aliphatic chains on the
azafullerene core
As already mentioned in the introduction, simple alkenes are
known to photochemically react with iminium cations.15 In an ear-
lier work, Mariano and co-workers have reported that irradiation of
methanolic solutions containing iminium cation salts together with
unsaturated compounds such as isobutylene, cyclohexene, methyl

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M.M. Roubelakis et al. / Tetrahedron 66 (2010) 9363e9369
b
,
b
-dimethylacrylate, or 1,3-butadiene, leads to the corresponding
with H1 appearing as doublets at 5.40 (Jcis¼10.5 Hz) and 5.58 ppm
(Jtrans¼17.5 Hz), respectively. Furthermore, the singlet at 5.20 ppm
corresponds to one of the two geminal protons of 25b (H5 or H6),
whereas the peak of the other germinal proton overlaps with H2 at
about 5.40 ppm. The allylic proton of 25b (H4) is observed as
a quartet at 5.44 ppm (J¼6.5 Hz), due tothe splitting from the methyl
group CH33. Note that H4 resonates at lower fields in comparison to
typical allylic protons because besides being allylic, it is also next to
the azafullerene sphere, and, therefore, suffers further deshielding.
Both CH13 methyl groups appear as a singlet at 1.86 ppm, the CH32
group resonates also as a singlet at 2.12 ppm, while the CH33 methyl
group is split by H4 into a doublet (J¼6.5 Hz) observed at 1.72 ppm.
According to what we have already discussed above, concerning
the photoinduced electron transfer reactions of iminium cations,
the formation of the two isomers (25a and 25b, Scheme 3) can be
explained on the basis of the mechanism depicted in Scheme 4:
light-promoted electron transfer from the double bond of 16 to
C59Nþ leads to the formation of the radical cation pair 26. Next,
proton Ha or either proton Hb or Hc can be abstracted, affording the
neutral radical pair 27a or 27b, respectively. Finally, coupling of the
allylic radicals in 27a and 27b with azafullerenyl radical C59Nꢁ and
self-photooxygenation17 of the resulting azacompounds affords fi-
nal products 25a and 25b, respectively.
addition products. The established reaction mechanism involves
a photoinduced electron transfer from the p-system of the alkene to
the single excited state of the iminium cation.15a
The applicability of this functionalization methodology in the
field of azafullerene chemistry was studied by replacing formal
iminium cations with aza[60]fullerene iminium cation 8 and ap-
plying PET reaction conditions in the presence of olefins 16e24
(Fig. 6). The experimental procedure that was followed was the
same as in the case of benzylsilanes discussed earlier: C59Nþ was
photochemically produced in situ from (C59N)2, in an ODCB solu-
tion and in the presence of p-TsOH, oxygen, and the olefin.
H3C
H3C
H3C
H3C
CH3
CH3
H3C
H3C
CH3
CH3
16
17
18
HO
CH3HC CHCO2CH3
H3C
21
19
20
The mechanism proposed in Scheme 4 reveals an interesting
feature of the addition reactions between olefins and aza[60]ful-
lerene iminium cation (8). In particular, radical pairs 27a and 27b
lead to the final products after the attachment of C59Nꢁ to the most
substituted end of the intermediate allylic radical. If radical cou-
pling at the least substituted allylic carbon had taken place simul-
taneously, derivatives 25a0 and 25b0 (Fig. 8) would have been
formed as well. However, these derivatives were not detected in the
1H NMR spectra of the reaction products. On the basis of solely
stereochemical hindrance criteria, this result is rather surprising
and unanticipated. Indeed, it is well-established that similar radical
CH3
CH3
OCH3
CH3
22
23
24
Fig. 6. Olefin substrates 16e24.
The reaction of aza[60]fullerene dimer (2) with trimethyl-
ethylene (16) gave essentially one new peak in the corresponding
HPLC chromatograms, attributed to a new aza[60]fullerene adduct.
This adduct was isolated from the reaction mixture by column
chromatography (SiO2, toluene) and further purified by utilizing
a semi-preparative HPLC column. The same isolation and purifica-
tion procedure was followed for all azafullerene adducts we report
in this section.
couplings between a-amino radicals and allylic radicals, produced
from allylsilanes R2C]CHCH2SiR3, take place exclusively at the
least substituted terminus.20
The exclusive formation of adducts 25a and 25b, instead of their
isomers 25a0 and 25b0, can be explained in terms of the greater
stability of a tertiary or a secondary radical versus a primary one
(Scheme 5). Also note that a wide variety of free radicals can
multiply add to fullerene molecules leading to free radical mono-
and multiadducts.21 That being said, the primary radicals in radical
pairs 27a and 27b (Scheme 5) are very unstable and, therefore,
highly reactive toward the aza[60]fullerene carbon cage. Conse-
quently, no regioselectivity is expected during their addition re-
actions, and multiple addition products rather than monoadducts
25a0 and 25b0 will be formed. The stabilized secondary and tertiary
radicals (Scheme 5) on the other hand, are less reactive and can add
The new derivative was characterized by means of 1H, 1He1H
COSY, and HMQC NMR experiments, and found to consist of the two
inseparable isomers 25a and 25b (Scheme 3) in a w2.2/1 molar ratio.
A representative 1H NMR spectrum (500 MHz, CDCl3/CS2) of the 25a/
25b mixture is displayed in Fig. 7. The resonances between 5.1 and
6.6 ppm correspond to the vinylic protons of both isomers. Hence,
the H1 proton of 25a resonates at 6.47 ppm as a doublet of doublets,
because of its coupling with the two geminal protons H2
(Jcis¼10.5 Hz) and H3 (Jtrans¼17.5 Hz); H2 and H3 protons are coupled
H3C
(C59N)2
+
regioselectively to the a-amino radical center of azafullerenyl rad-
H3C
CH3
16
ical 1, affording compounds 25a and 25b that are eventually iso-
lated. In any case, and beyond the above rationalization, the
influence of stereoelectronic effects on the observed regiose-
lectivity cannot be ruled out.
2
hv
p-TsOH/O2
Moreover, the formation of isomer 25a at a greater percentage
than 25b (25a/25b¼w2.2/1) is observed due to the enhanced sta-
bility of the tertiary radical (in comparison with the secondary one).
This means that either the formation of radical pair 27a, bearing the
tertiary radical, is favored over the formation of 27b (i.e., Hþa ab-
straction is more favorable than Hþb or Hcþ abstraction), or that the
H2
H6
N
H3
2
CH3
H1
H5
O
1
3
CH3
O
CH3
1
CH3
H4
N
tertiary radical adds to the
a-amino radical center of 1 with an
+
enhanced regioselectivity in comparison with the secondary one.
Of course, both scenarios may occur simultaneously.
The photochemical reactions of aza[60]fullerene iminium cation
(8) with substrates 17 and 18 (Fig. 6) have also furnished the cor-
responding new adducts 28, 29a, and 29b illustrated in Fig. 9. We
25a
25b
Scheme 3. Photochemical addition of olefin 16 to azafullerene dimer 2.

M.M. Roubelakis et al. / Tetrahedron 66 (2010) 9363e9369
9367
Fig. 7. 1H NMR spectrum (500 MHz, CDCl3/CS2) of 25a/25b mixture.
Hb
PET
O
O
N
N
+
N
N
16
Ha
Hc
26
8
+
-Ha
+
-Hb+ or -Hc
25a'
25b'
Fig. 8. Hypothetical monoadducts 25a0 and 25b0 (not observed).
C59N
C59N
27a
27b
3o
1o
2o
1o
radical coupling
Scheme 5. Resonance structures of the allylic radicals in radical pairs 27a and 27b.
thermal addition of the structurally similar cyclooctene to C59Nþ has
been reported earlier (Scheme 7).11 This reaction has been proposed
to proceed via the nucleophilic addition of cyclooctene to C59Nþ (8),
followed by proton elimination, and gives four isomeric C59N/
cyclooctene adducts (Scheme 7). These isomers supposedly derive
after acid-catalyzed isomerization of the double bond on the
cyclooctenyl ring of the aza-adduct. Under the PETconditions of our
experiment, the formation of only one isomer has been achieved
from the reaction of cyclohexene with C59Nþ; double bond isomer-
ization of the cyclohexenyl ring in 30 does not take place in this case.
PET functionalization of C59Nþ with compounds 20 and 21
(Fig. 6) was also attempted but proved to be unsuccessful. Instead,
these reactions provided compounds 31 and 32, respectively
(Fig. 10). Apparently, acidic hydrolysis of the ester group in 20
produces a methanol molecule that attacks the azafullerene imi-
nium cation to give adduct 31. Product 32 is also the result of
a nucleophilic attack of the hydroxyl group of 21 to cation 8. Thus,
nucleophilic attack to iminium cation 8 is a much more efficient
process than photoinduced electron transfer, at least concerning
the double bonds of alkenes 20 and 21.
N
N
hv, O2
hv, O2
25a
25b
Scheme 4. Proposed mechanism for the photochemical addition of 16 to iminium
cation C59Nþ
.
speculate that these products are also formed via the mechanistic
pathway shown in Scheme 4. Tetramethylethylene (17), due to its
high symmetry, led to a single product (28), whereas 18, similarly to
16, gave a mixture of two isomers (29a and 29b).
The photochemical addition of cyclohexene (19, Fig. 6) to C59Nþ
afforded aza[60]fullerene derivative 30 (Scheme 6), as identified by
1H, 1He1H COSY, and HMQC NMR experiments. Note that the

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M.M. Roubelakis et al. / Tetrahedron 66 (2010) 9363e9369
suitable for this reaction type. With benzyltributylstannane and
benzyltrimethylsilanes having oxidation potentials lower than
about 1.5 V, photoinduced electron transfer toward C59Nþ occurs
readily, though not solely on the iminium cation moiety of C59Nþ,
leading to the formation of multiaddition products that are difficult
to isolate and identify. In the second part of our study it was shown
that simple olefins, such as tri- and tetra-methylethylene, as well as
cyclohexene, also participate in photoinduced electron transfer-
initiated reactions with C59Nþ, giving new aza[60]fullerene de-
rivatives that bear small aliphatic chains. These adducts are isolated
in relatively low yields (about 10%, see Experimental section)24
because the unstable free radical species that are formed during
the reaction are expected to attack the azafullerene carbon shell
non-regiospecifically. Finally, relatively electron-rich alkenes, such
O
O
O
N
N
N
28
29a
29b
Fig. 9. Novel aza[60]fullerene derivatives 28, 29a, and 29b.
O
as 2-methoxyprop-1-ene, 2,4-hexadiene, and b,b-dimethylstyrene,
react with azafullerene iminium cation C59Nþ, albeit in a [2þ2]
cycloaddition mode, providing unidentified azafullerene
multiadducts.
N
hv
(C59N)2
+
p-TsOH/O2
19
4. Experimental
30
4.1. General remarks
Scheme 6. Photochemical addition of cyclohexene to C59Nþ
.
All photochemical reactions were carried out using a 300 W
Xenon lamp as the light source and a Pyrex filter (>290 nm
wavelength). Nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker AMX-500 MHz spectrometer in the appro-
priate solvent. Chemical shifts are reported in parts per million
R
N
150 o
C
(C59N)2
+
p-TsOH/O2
downfield from Me4Si (
d¼0 ppm), by using the residual solvent
peak as an internal standard. Coupling constants (J) are in hertz. The
purchased reagents and solvents were used as-received without
further purification. High performance liquid chromatography
(HPLC) analyses were carried out on a Cosmosil 5C18-MS-II (4.6
IDꢂ250 mm) reverse phase column with detection at 326 nm. A
mixture of toluene/acetonitrile (60/40) was used as eluent at 1 mL/
min flow rate. Chromatography refers to flash column chroma-
tography and was carried out on SiO2 (silica gel 60, SDS,
230e400 mesh ASTM). Azafullerene adducts were further purified
by HPLC using a semi-preparative Cosmosil 5PBB (10 IDꢂ250 mm)
normal phase column. A mixture of toluene/hexane (70/30) was
used as eluent at 4 mL/min flow rate (detection at 326 nm).
Aza[60]fullerene (C59N)2,6b benzyltrimethylsilanes 10,18 and
benzyltributylstannane 1518 were prepared according to published
experimental procedures and were identified by comparing their
spectra with those reported in the literature.18,25,26
R :
Scheme 7. C59N/cyclooctene isomers.
O
O
OMe
O
N
N
31
32
4.2. General procedure for the photochemical reactions of aza
[60]fullerene iminium cation C59ND
Fig. 10. Molecular structures for compounds 31 and 32.
Finally, we also tested whether 22e24 (Fig. 6) exhibit PET re-
activity against C59Nþ. The double bonds of these olefins are fairly
electron-rich; however, their irradiation in the presence of (C59N)2
resulted in the formation of many different, unidentified multi-
adducts emanating from [2þ2] cycloadditions on the aza[60]ful-
Aza[60]fullerene (C59N)2 2 (20 mg, 13.8ꢂ10ꢀ3 mmol) together
with a 35-fold excess of p-TsOH (90 mg, 0.48 mmol) were dissolved
in 10 mL ODCB (HPLC grade). Next, 500 equiv of the p-donor (arene
or olefin) was added and the solution was irradiated with the Xe-
non lamp while bubbling with a gentle stream of air. The temper-
ature of the reaction was maintained at 0 ꢃC using an ice bath.
Reaction progress was followed by HPLC. All irradiations typically
lasted from 2 to 3 h (dimer 2 was not completely consumed). After
that, the reaction mixture was poured on a column of silica/toluene
in order to neutralize p-TsOH and get rid of the azafullerene mul-
tiadducts that are insoluble in ODCB. Elution with toluene gave
a brown-colored fraction containing the aza[60]fullerene adduct.
The solvent was then removed in vacuo at 60 ꢃC, and the remaining
solid was washed and centrifuged four times with acetonitrile HPLC
grade. Further purification of the reaction products was achieved by
semi-preparative HPLC. All aza[60]fullerene derivatives were col-
lected in about 10% isolated yield.
lerene carbon core. This kind of reactivity is well-established for
22,23
these substrates in their reactions with fullerene C60
.
3. Conclusion
The results presented herein comprise the first detailed study on
the photoinduced electron transfer reactivity of azafullerene imi-
nium cation C59Nþ, the least developed azafullerene functionali-
zation methodology. A series of arenes and electron-rich olefins
were utilized as substrates. Arenes with oxidation potentials higher
than about 1.5
V (benzyltrimethylsilane, trimethyl(2-methyl-
benzyl)silane and trimethyl(4-methylbenzyl)silane) are generally

M.M. Roubelakis et al. / Tetrahedron 66 (2010) 9363e9369
9369
4.2.1. Aza[60]fullerene adduct 11c. This adduct was isolated in about
10% yield (2.4 mg). Compound 11c: mp>360 ꢃC; 1H NMR (500 MHz,
acetone-d6/CS2): 7.3e7.0 (m, 4H), 5.81 (s, 2H), 2.69 (s, 3H) ppm.
References and notes
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d
4.2.2. Aza[60]fullerene adduct 11d. This adduct was isolated in
about 10% yield (2.2 mg). Compound 11d: mp>360 ꢃC; 1H NMR
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7.58 (d, J¼7 Hz, 2H), 7.25 (d, J¼7 Hz,
ꢀ
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2H), 5.74 (s, 2H), 2.42 (s, 3H) ppm.
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4.2.3. Aza[60]fullerene adducts 25a and 25b. This product mixture
was isolated in 15% yield (3.3 mg). Compounds 25a, 25b:
mp>360 ꢃC; 1H NMR (500 MHz, CDCl3/CS2):
d
6.47 (dd, Jcis¼10.5 Hz,
Jtrans¼17.5 Hz, 1H of 25a), 5.58 (d, Jtrans¼17.5 Hz, 1H of 25a), 5.44 (q,
J¼6.5 Hz, 1H of 25b), 5.40 (d, Jcis¼10.5 Hz, 1H of 25a), 5.40 (s, 1H of
25b), 5.20 (s, 1H of 25b), 2.12 (s, 3H of 25b), 1.86 (s, 6H of 25a), 1.72
(d, J¼6.5 Hz, 3H of 25b) ppm.
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4.2.4. Aza[60]fullerene adduct 28. This adduct was isolated in 11%
yield (2.5 mg). Compound 28: mp>360 ꢃC; 1H NMR (500 MHz,
CDCl3/CS2): d 5.36 (s, 1H), 5.18 (s, 1H), 2.21 (s, 3H), 1.90 (s, 6H) ppm.
4.2.5. Aza[60]fullerene adducts 29a and 29b. This product mixture
was isolated in 10% yield (2.2 mg). Compounds 29a, 29b: mp>360 ꢃC;
1H NMR (500 MHz, CDCl3/CS2):
d 6.05 (m, 1H of 29a), 5.37 (s, 1H of
29b), 5.37(m,1Hof29a), 5.26(s,1Hof29b), 5.21(t, J¼7Hz,1Hof29b),
2.10 (s, 3H of 29b),1.91 (m, 2H of 29b),1.89 (d, J¼5 Hz, 3H of 29a),1.84
(s, 6H of 29a), 1.21 (t, J¼7.5 Hz, 3H of 29b) ppm.
4.2.6. Aza[60]fullerene adduct 30. Derivative 30 was proven very
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HPLC column. However, its crude 1H NMR spectra, following silica
gel chromatography, were quite clean and fortunately allowed its
structure identification. Compound 30: 1H NMR (500 MHz, CDCl3/
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CS2):
d 6.24 (m, 1H), 6.17 (m, 1H), 5.43 (m, 1H), 2.62 (m, 1H),
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2.37e2.00 (m, 4H), 1.88 (m, 1H) ppm.
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Acknowledgements
24. Unfortunately, several attempts to observe the molecular ion of these adducts
in MALDI experiments were not successful. Instead, only the molecular ion
peak of the C59NO (m/z 738) fragment was observed for all samples.
25. Klein, J.; Medlik, A.; Meyer, A. Y. Tetrahedron 1976, 32, 51e56.
26. Hirao, T.; Morimoto, C.; Takada, T.; Sakurai, H. Tetrahedron 2001, 57, 5073e5079.
This work was co-funded by the Greek Ministry of Education
and the European Union through research and education action
programs PYTHAGORAS II 2005 and IRAKLITOS 2002.