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G. C. Vougioukalakis, M. Orfanopoulos / Tetrahedron Letters 44 (2003) 8649–8652
is relatively weak (18 kcal/mol).5 When the thermal
Table 1. Reactions of the azafullerene radical 2 with sub-
stituted fluorenes 5a–d
homolysis of the azafullerene dimer is followed by
oxidation of the azafullerene radical C59N to the aza-
fulleronium cation C59N+, monomeric adducts with
electron rich aromatics,6 carbonyl compounds,7 alco-
hols and olefins,8 can be isolated. The proposed mecha-
nism of these functionalizations involves nucleophilic
trapping of the C59N+ cation by the corresponding
nucleophile.
The first two reported azafullerene derivatives, both
produced by trapping the azafullerene radical 2, are the
hydroazafullerene C59HN (3)9 and the diphenylmethane
adduct C59(CHPh2)N (4),10 shown in Scheme 1. To the
best of our knowledge, these are the only isolated and
characterized azafullerene adducts to date, whose for-
mation involves a radical mechanism.
Adduct
R
Yield (%)a
Reaction time (h)
6a
6b
6c
6d
H
Me
Et
41
30
35
20
9
8
6
4
Ph
a Based on the amount of isolated adduct.
In our effort to study further the mechanism of this
reaction by deuterium isotope effect measurements, we
observed that apart from diphenylmethane,10 none of
its alkylated derivatives reacted with the azafullerene
dimer. However, the corresponding fluorene alkyl
derivatives, which are structurally similar to diphenyl-
methane, reacted smoothly with this compound.
Scheme 2. Synthesis of substituted fluorenes 5b–d.
Here we report an efficient preparation and characteri-
zation of four new azafullerene adducts 6a–d (Table 1)
by the thermal reaction between the azafullerene radical
and: 9H-fluorene (5a), 9-methyl-9H-fluorene (5b), 9-
ethyl-9H-fluorene (5c) and 9-phenyl-9H-fluorene (5d).
Substituted fluorenes 5b–d were synthesized quantita-
tively by reduction of the corresponding alcohols with
the etherated boron trifluoride-triethylsilane system11
(Scheme 2). Substrate 5a is commercially available.
like all the adducts 6a–d, shows a similar absorption
pattern to those previously reported for other aza-
fullerene adducts.12
The solutions of adducts 6a–d are green and their
UV-VIS spectra are identical to that of (C59N)2. These
adducts, as well as all the azafullerene monoadducts
reported so far, have characteristic absorptions at 380,
450, 595, 730 and 817 nm. The FT-IR spectra of 6a–d
are also very similar with strong characteristic absorp-
tions at 524 and 1510 cm−1.
Substituted azafullerenes 6a–d were obtained by the
reaction of the azafullerene radical 2 with the corre-
sponding fluorenes 5a–d (Table 1). In a typical experi-
ment, 300 equiv. of the substituted fluorene were added
to a degassed (5 vacuum/argon cycles) solution of 1 (20
mg) in 40 ml HPLC grade 1,2-dichlorobenzene
(ODCB). The mixture was heated at 160°C for 4–9 h
under argon and the reaction was monitored by HPLC,
equipped with a Separon C18 reverse phase column, at
326 nm. After distillation of ODCB, the crude mixture
was washed 4–6 times with acetonitrile and/or acetone
(centrifugation) in order to remove the remaining
fluorene. Flash chromatographic purification on SiO2
using hexane as eluent (the crude reaction mixture was
loaded with CS2), afforded 6a–d as black solids (yields
20–41%, as shown in Table 1). The remaining aza-
fullerene dimer elutes first, followed by the more polar,
green colored azafullerene adducts 6a–d. All adducts
Scheme 3. Substituted diphenylmethanes 7a–c.
It is worth mentioning here that diphenylmethane
reacts slower than fluorene with the azafullerene radi-
cal. For example after 48 h heating at 180°C the
reaction with diphenylmethane gave the adduct in 42%
yield, whereas in the case of fluorene the same yield was
obtained only after 9 h heating at 160°C. However,
substituted diphenylmethanes 7a–c (Scheme 3), which
are structurally similar to the corresponding fluorenes
6a–c, gave no adducts with azafullerene under the same
experimental conditions. Instead, progressive decompo-
sition of the azafullerene dimer was observed and no
further investigation of its fate was attempted. Appar-
ently, the observed dramatic change in reactivity
1
were characterized by H NMR, 13C NMR, COSY and
HMQC experiments, as well as with UV-VIS, FT-IR
spectroscopy and mass spectrometry. A representative
1H NMR spectrum of the adduct between the aza-
fullerene radical 2 and 9-methyl-9H-fluorene is pre-
sented in Figure 1. The assignment of the aromatic
hydrogen absorptions of the azafullerene adducts was
based on the COSY and HMQC experiments. In Figure
2 the 13C NMR spectrum of 6c is also presented, which
Vougioukalakis, Georgios C.
