Thermal Stability of [60]Fullerene Cycloadducts
additions,8 other fullerene-fused pentagonal heterocyclic rings,
like pyrazolino[60]fullerenes, also display interesting chemical,
electrochemical, and photophysical properties.9 Pyrazolino[60]-
fullerenes have been obtained by addition of nitrile imines to
[60]fullerene in good yields.10 Nitrile imines are efficiently
prepared by dehydrohalogenation of halohydrazones, obtained
in turn by chlorination or bromination of the corresponding
hydrazones. Despite the huge amount of work devoted to the
preparation of fullerene cycloadducts, much less is known
however on their chemical and thermal stability. The thermal
stability of chemically modified fullerenes is a fundamental
aspect in order to determine the best candidates to prepare
functional C60-based materials able to withstand the ambient
conditions to maintain their efficiency, thus preventing any
further chemical or thermal degradation. This issue is particu-
larly important in those photoelectronic devices, such us
photovoltaic cells, where oxygen, water, and extreme temper-
atures represent the main agents affecting the stability and
performance of organic sollar cells.11 Some of us have recently
reported the thermally induced transition metal-catalyzed quan-
titative retro-cycloaddition reaction of pyrrolidino[3,4:1,2]-
fullerenes12 as well as isoxazolino[3,4:1,2]fullerenes13 and
proved its utility as a new and useful protection-deprotection
protocol.12 In a very recent paper, this methodology has been
successfully applied to induce the retro-cycloaddition process
in pyrrolidino-SWCNTs (SWCNT: Single-wall Carbon Nano-
tube) derivatives, thus demonstrating the suitability of this
protocol to afford highly pure SWCNTs.14 Here, we report the
study carried out on the thermal stability of a series of 2-pyrazo-
lino[60]fullerenes bearing different substituents, (C-aryl-N-aryl
or C-alkyl-N-aryl) under the same experimental conditions used
previously for fulleropyrrolidines, which afforded pristine C60
quantitatively. The results indicates that C-substitution on the
pyrazole ring plays an important role in the course of this
reaction, being that l,3-diaryl-2-pyrazolino[60]fullerenes are
thermally stable compounds which are less susceptible to
undergo the retro-cycloaddition reaction under these conditions
and forming C60 in very low yields. In contrast, 1-aryl-3-alkyl-
2-pyrazolino derivatives are more vulnerable to these experi-
mental conditions, undergoing an efficient thermally induced
FIGURE 1. 2-Pyrazolino[60]fullerenes (1) and the isomer 1-pyrazolino-
[60]fullerenes.
transition metal-catalyzed retro-cycloaddition reaction. Related
1-pyrazolino-fused fullerenes are known to decompose easily
in refluxing toluene, affording a mixture of methanofullerenes.15
Recently, 2-pyrazolino[60]fullerenes (1) were obtained via
isomerization of 1-pyrazolines which, in turn, were synthesized
by 1,3-dipolar cycloadditions from alkyl diazoacetates (Figure
1).16 Thermal stability studies of these nitrogen-unsubstituted
2-pyrazolino[60]fullerenes showed decomposition in refluxing
o-dichlorobenzene, affording a mixture of methanofullerenes
through the loss of N2. However, no evidence of retro-cyclo-
addition reaction was reported under these conditions.
Herein we describe the unprecedented retro-cycloaddition
reaction of N-substitued 2-pyrazolino[60]fullerenes under ther-
mal or microwave assisted conditions to yield the parent [60]-
fullerene.
Results and Discussion
In order to determine if the experimental conditions previously
used for the retro-cycloaddition reaction of fulleropyrrolidines,12
and fulleroisoxazolines13 were suitable for 2-pyrazolino[60]-
fullerenes, we have followed the same protocol: excess of
dipolarophile (maleic anhydride) as well as the use of copper
triflate (CuTf2) to coordinate the metal to the nitrogen atoms
and, therefore, facilitate the retro-cycloaddition process. For
this purpose, we have synthesized a series of 2-pyrazolino[60]-
fullerenes bearing different substituents (Figure 2). Compounds
2a,17 2b,10c 2c,10d and 2e10a have previously been described by
some of us and were prepared according to the procedure
described in their respective literature.
(8) (a) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519. (b)
Tagmatarchis, N.; Prato, M. Synlett 2003, 768.
(9) Langa, F.; Oswald, F. C. R. Chimie 2006, 9, 1058.
(10) (a) Delgado, J. L.; de la Cruz, P.; Lo´pez-Arza, V.; Langa, F.;
Kimball, D. B.; Haley, M. M.; Araki Y.; Ito, O. J. Org. Chem. 2004, 69,
2661. (b) Langa, F.; de la Cruz, P.; Delgado, J. L.; Go´mez-Escalonilla, M.
J.; Gonza´lez-Corte´s, A.; de la Hoz, A.; Lo´pez-Arza, V. New J. Chem. 2002,
26, 76-80. (c) Esp´ıldora, E.; Delgado, J. L.; de la Cruz, P.; de la Hoz, A.;
Lo´pez-Arza, V.; Langa, F. Tetrahedron 2002, 58, 5821. (d) Delgado, J. L.;
de la Cruz, P.; Lopez-Arza, V.; Langa, F.; Gan, Z.; Araki, Y.; Ito, O. Bull.
Chem. Soc. Jpn. 2005, 78, 1500. (e) Matsubara, Y.; Tada, H.; Nagase, S.;
Yoshida, Z. J. Org. Chem. 1995, 60, 5372.
(11) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante,
M.; Heeger, A. J. Science 2007, 317, 222.
(12) (a) Mart´ın, N.; Altable, M.; Filippone, S.; Mart´ın-Dome´nech, A.;
Echegoyen, L.; Cardona, C. M. Angew. Chem., Int. Ed. 2006, 45, 110. (b)
Lukoyanova, O.; Cardona, C. M.; Echegoyen, L.; Altable, M.; Filippone,
S.; Mart´ın Dome´nech, AÄ .; Mart´ın, N. Angew. Chem., Int. Ed. 2006, 45,
7430.
(13) (a) Mart´ın, N.; Altable, M.; Filippone, S.; Mart´ın-Dome´nech, A.;
Mart´ınez-Alvarez, R.; Suarez, M.; Plonska-Brzezinska, M. E.; Lukoyanova,
O.; Echegoyen, L. J. Org. Chem. 2007, 72, 3840. (b) Da Ros, T.; Prato,
M.; Novello, F.; Maggini, M.; De Amici, M.; De Micheli, C. Chem.
Commun. 1997, 59. (c) Da Ros, T.; Prato, M.; Lucchini, V. J. Org. Chem.
2000, 65, 4289.
(14) Bruneti, F. G.; Herrero, M. A.; Mun˜oz, J. M.; Giordani, S.; Diaz-
Ortiz, A.; Filippone, S.; Ruaro, G.; Meneghetti, M.; Prato, M.; Va´zquez,
E. J. Am. Chem. Soc. 2007, 129, 14580.
2-Pyrazolino[60]fullerene derivatives 2d and 2f had not been
previously reported and were synthesized by 1,3-dipolar cyclo-
addition of the corresponding nitrile imines, which were
generated in situ from the corresponding hydrazones (see
experimental section).
The compounds to study were first heated at reflux in o-DCB
for 48 h, and small amounts (0.5 mL) of each reaction were
collected every 24 h, diluted, and submitted to HPLC analysis
(Table 1, o-DCB-24h and o-DCB-48 h). The results of these
experiments showed that pristine C60 was obtained in different
amounts, although the efficiency of the process in C-Aryl-N-
Aryl-2-pyrazolino[60]fullerenes was not very high (1-23%).
The same experiments perfomerd on C-Alkyl-N-Aryl-2-
(15) Suzuki, T.; Li, Q.; Khemani, K. C.; Wudl, F. J. Am. Chem. Soc.
1992, 114, 7301.
(16) Wang, G. W.; Li, Y. J., Peng, R. F.; Liang, Z. H.; Liu, Y. C.
Tetrahedron 2004, 60, 3921.
(17) Delgado, J. L.; de la Cruz, P.; Lo´pez-Arza, V.; Langa, F.
Tetrahedron Lett. 2004, 45, 1651.
J. Org. Chem, Vol. 73, No. 8, 2008 3185