electron-withdrawing group on their methano-bridge carbon,
which efficiently stabilizes the cyclopropane moiety toward
a ring-opening reaction.4 Recently, we have reported the first
synthesis of methano[60]fullerenes bearing an electron-
donating group (amino group) on the methano-bridge carbon,
in which the rearrangement of an electron-withdrawing group
on the methano-bridge carbon was the key reaction.5 Con-
sidering the electron-donating characteristic of the amino
group, aminomethano[60]fullerenes would be suitable as the
starting materials to realize our concept. Here we report
unprecedented transformation of aminomethano[60]fullerenes
via the ring-opening reaction of the cyclopropane rings and
the easy hydrolysis of the resultant aldehyde/ketones.
Table 1. The Ring-Opening Reaction of
Aminomethano[60]fullerenes 1
yield
(%)a
entry
base (equiv)
DBU (2.1)
Et3N (2.1)
DMAP (2.1)
pyridine (2.1)
pyridine (2.1)/Et3N (cat.)
pyridine (2.1)/DMAP (cat.) 2c (Et)
pyridine (2.1)/DMAP (cat.) 2b (Me)
pyridine(2.1)/DMAP (cat.) 2d (Bn)
pyridine (2.1)/DMAP (cat.) 2e (Ph)
product (R)
2c (Et)
2c (Et)
2c (Et)
2c (Et)
2c (Et)
b
1
2
3
4
5
6
7
8
9
-
According to the procedure we have recently reported,
seven kinds of aminomethano[60]fullerenes were prepared
as trifluoromethanesulfonic acid (HOTf) salts (1a-g‚HOTf).5
As an initial study to investigate the properties/reactivities
of these unprecedented species, we attempted to generate
free aminomethano[60]fullerenes 1a-g by the dissociation
of the ion pairs. For example, when 1c‚HOTf was treated
with triethylamine, to our surprise the starting material
1c‚HOTf was immediately converted into the ketone 2c (60%
isolated yield) instead of the expected free amine 1c (Table
1, entry 2). The ketone 2c was unambiguously identified by
1H/13C NMR and MS spectra, which are in good agreement
with the reported ones.6 Worth noting is the fact that such a
dramatic change in the structure, involving a C-C bond
cleavage, took place readily and cleanly under mild condi-
tions.
60
64
77
64
87
86
90
90
88
10 pyridine (2.1)/DMAP (cat.) 2f (4-MeOC6H4)
11 pyridine (2.1)/DMAP (cat.) 2g (4-(4-iPrC6H4)C6H4) 76
12 pyridine (2.1)/DMAP (cat.) 2a (H)
70c
a Isolated yield. b A complex mixture. c A mixture of the target compound
2a and 1,2-dihydro[60]fullerene 3 was obtained (2a:3 ) 77:23), which could
not be separated because of the poor solubility of 2a to common organic
solvents.
With the optimized conditions in hand, we then applied
the ring-opening reaction to the transformation of various
aminomethano[60]fullerenes into the corresponding carbonyl
compounds. As a result, it was clearly proved that a wide
range of substituents on the methano-bridge carbon were
tolerant in our method, including alkyl, benzyl, and aryl
groups, as well as proton (Table 1, entries 6-12). Worth
noting is the unique structure of the resultant aldehyde/
ketones 2a-g, in which a carbonyl group is directly
connected to a [60]fullerene core and is expected to bring a
significant effect on the chemical/physical properties of the
[60]fullerene moiety.7 To the best of our knowledge, most
of the carbonyl compounds 2a-g have not been synthesized
elsewhere, except for 2c, which has been synthesized by a
photochemical reaction in unsatisfactory yield.6,8 Among
A further detailed study revealed that the choice of the
base was one of determinant factors to control this reaction.
The use of 1,8-diaza[5,4,0]bicycloundec-7-ene (DBU) in the
place of triethylamine caused complicated side reactions
(Table 1, entry 1). Contrary to this, relatively weak bases
could efficiently promote this reaction to give the ketone 2c
in moderate to good yields (entries 3 and 4). Interestingly,
the addition of a catalytic amount of 4-(dimethylamino)-
pyridine (DMAP) to pyridine led to the notable improvement
of the yield, which was superior to those achieved by using
either of the two bases alone (entry 6 vs entries 3 and 4).
Although the role of DMAP is unclear at present, such a
special effect was undoubtedly attributable to some charac-
teristic of DMAP, because the use of triethyamine in the
place of DMAP merely resulted in the moderate yield of 2c
(entry 5).
(6) Siedschlag, C.; Luftmann, H.; Wolff, C.; Mattay, J. Tetrahedron 1997,
53, 3587.
(3) (a) Win, W. W.; Kao, M.; Eiermann, M.; McNamara, J. J.; Wudl, F.
J. Org. Chem. 1994, 59, 5871. (b) Knight, B.; Martin, N.; Ohno, T.; Orti,
E.; Rovira, C.; Veciana, J.; Vidal-Gancedo, J.; Viruela, P.; Viruela, R.; Wudl,
F. J. Am. Chem. Soc. 1997, 119, 9871. (c) Yanilkin, V. V.; Toropchina, A.
V.; Morozov, V. I.; Nastapova, N. V.; Gubskaya, V. P.; Sibgatullina, F.
G.; Azancheev, N. M.; Efremov, Y. Y.; Nuretdinov, I. A. Electrochim. Acta
2004, 50, 1005.
(4) For selected examples of methano[60]fullerenes functionalized with
electron-withdrawing group(s), see: (a) Bingel, C. Chem. Ber. 1993, 126,
1957. (b) Bestmann, H. J.; Hadawi, D.; Ro¨der, T.; Moll, C. Tetrahedron
Lett. 1994, 35, 9017. (c) Benito, A. M.; Darwish, A. D.; Kroto, H. W.;
Meidine, M. F.; Taylor, R.; Walton, D. R. M. Tetrahedron Lett. 1996, 37,
1085. (d) Hino, T.; Kinbara, K.; Saigo, K. Tetrahedron Lett. 2001, 42, 5065.
(e) Hamada, M.; Hino, T.; Kinbara, K.; Saigo, K. Tetrahedron Lett. 2001,
42, 5069.
(7) For theoretical studies on the properties of 1-substituted 1,2-dihydro-
[60]fullerenes, see: (a) Van Lier, G.; Safi, B.; Geerlings, P. J. Chem. Soc.,
Perkin Trans. 2 1998, 349. (b) Amat, M. C.; Van Lier, G.; Sola, M.; Duran,
M.; Geerling, P. J. Org. Chem. 2004, 69, 2374.
(8) For selected examples of 1-substituted 1,2-dihydro[60]fullerenes,
see: (a) Hirsch, A.; Soi, A.; Karfunkel, H. R. Angew. Chem., Int. Ed. Engl.
1992, 31, 766. (b) Fagan, P. J.; Krusic, P. J.; Evans, D. H.; Lerke, S. A.;
Johnston, E. J. Am. Chem. Soc. 1992, 114, 9697. (c) Komatsu, K.; Murata,
Y.; Takimoto, N.; Mori, S.; Sugita, N.; Wan, T. S. M. J. Org. Chem. 1994,
59, 6101. (d) Anderson, H. L.; Faust, R.; Rubin, Y.; Diederich, F. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1366. (e) Timmerman, P.; Anderson, H.
L.; Faust, R.; Nierengarten, J.-F.; Habicher, T.; Seiler, P.; Diederich, F.
Tetrahedron 1996, 52, 4925. (f) Murata, Y.; Motoyama, K.; Komatsu, K.;
Wan, T. S. M. Tetrahedron 1996, 52, 5077. (g) Tanaka, T.; Komatsu, K.
Synth. Commun. 1999, 29, 4397. (h) Chronakis, N.; Vougioukalakis, G.
C.; Orfanopoulos, M. Org. Lett. 2002, 4, 945. (i) Chen, Z.-X.; Wang,
G.-W. J. Org. Chem. 2005, 70, 2380.
(5) (a) Tada, T.; Ishida, Y.; Saigo, K. Org. Lett. 2005, 7, 5897. (b) Tada,
T.; Ishida, Y.; Saigo, K. Synlett 2007, 235.
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