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traces of C60On in varying ratios. The proportions of C60 and
We thank the EU FP7/2007–2013/(REA grant no. 290023) for
C60O obtained did not show any simple correlation with reaction funding and Dr S. van Berkel for tetrazole 12.
conditions (solvent, trapping alkene, degassed or anhydrous solvent,
Notes and references
‡ Handheld lamp at l = 254 nm gave 22% conversion after 3.5 hours.
irradiation wavelength).† Irradiation of pristine C60 gave oxidation to
C60O as previously reported.18 Oxidation of C60 occurred only at a
6,6-bond, an observation which is in accordance with previous
reports.19 Surprisingly, the use of non-degassed solvent shortened
the reaction time to around one quarter, but only marginally
increased the ratio of C60O/C60. These results together suggested
that C60O resulted from the in situ generation of singlet oxygen and
subsequent reaction with the photo-released C60 and not as a direct
result of photo-cleavage.
To test the necessity and role of dipolarophile traps, various
other alkenes were investigated as additives in the photocyclo-
reversion of fulleropyrazoline 7 (Table S2, ESI†). The rate of the
photocleavage could be improved in the presence of 3,3-dimethyl-
allylbromide or para-chlorostyrene. Pleasingly the cycloreversion
also proceeded efficiently without alkene (Table S2, entry 8, ESI†),
thus confirming the utility of this process even in the absence of a
trapping partner.
§ This cleavage process was induced by exposure to light from a
medium pressure mercury lamp and did not occur when 7, 10, 11,
and 13 were subjected to heat. Cycloreversion of fulleropyrazolines was
previously reported to occur only in the presence of Cu(OTf)2.15
´
1 (a) J. L. Delgado, N. Martın, P. de la Cruz and F. Langa, Chem. Soc.
Rev., 2011, 40, 5232; (b) F. Langa and F. Oswald, Targets Heterocycl.
Syst., 2004, 8, 120.
2 (a) D. M. Guldi and M. Prato, Acc. Chem. Res., 2000, 33, 695;
(b) F. Langa, P. de la Cruz, E. Espildora, A. de la Hoz,
J. L. Bourdelande, L. Sanchez and N. Martin, J. Org. Chem., 2001,
66, 5033; (c) A. Gouloumis, F. Oswald, M. E. El-Khouly, F. Langa,
Y. Araki and O. Ito, Eur. J. Org. Chem., 2006, 2344; (d) J. J. Oviedo,
´
´
M. E. El-Khouly, P. de la Cruz, L. Perez, J. Garın, J. Orduna, Y. Araki,
F. Langa and O. Ito, New J. Chem., 2006, 30, 93.
3 F. Langa and F. Oswald, C. R. Chim., 2006, 9, 1058.
4 (a) Y. Matsubara, H. Tada, S. Nagase and Z.-i. Yoshida, J. Org. Chem.,
´
1995, 60, 5372; (b) F. Langa, P. de la Cruz, J. L. Delgado, E. Espıldora,
´
M. J. Gomez-Escalonilla and A. de la Hoz, J. Mater. Chem., 2002,
´
12, 2130; (c) J. L. Delgado, P. de la Cruz, V. Lopez-Arza and F. Langa,
The reaction rate for photocleavage showed clear solvent
effects; it was enhanced in more polar solvents such as chloro-
form and benzonitrile and further enhanced in mixed aqueous–
organic solvent systems. This supports suggested formation of a
charge-separated diradical species (Scheme 2). Notably in the
presence of base (pyridine as solvent, DABCO as additive) the
photocleavage does not occur, presumably because the putative
pyrazoline radical cation is quenched under these conditions.20 In
this mechanism, under photoirradiation fulleropyrazolines form
a charge separated diradical species.2c,3,5b,6,21 The positive charge
that is formed can be stabilised particularly effectively by the N-1
methoxy group, which promotes bond cleavage to release free C60.
The breakdown products are discussed further in the ESI.†
Finally to test utility in future applications, the reaction was
tested in various mixed aqueous solvent systems (see ESI†);
Tetrahedron Lett., 2004, 45, 1651; (d) B. Lu, J. Zhang, J. Li, J. Yao,
M. Wang, Y. Zou and S. Zhu, Tetrahedron, 2012, 68, 8924.
´
5 (a) J. L. Delgado, P. de la Cruz, V. Lopez-Arza, F. Langa, D. B. Kimball,
M. M. Haley, Y. Araki and O. Ito, J. Org. Chem., 2004, 69, 2661;
´
(b) J. L. Delgado, P. de la Cruz, V. Lopez-Arza, F. Langa, Z. Gan,
Y. Araki and O. Ito, Bull. Chem. Soc. Jpn., 2005, 78, 1500.
¨
6 F. Hormann, W. Donaubauer, F. Hampel and A. Hirsch,
Chem. – Eur. J., 2012, 18, 3329.
7 (a) X. Zhang, A. Fan and C. S. Foote, J. Org. Chem., 1996, 61, 5456;
(b) G. Vassilikogiannakis and M. Orfanopoulos, J. Am. Chem. Soc.,
1997, 119, 7394; (c) G. Vassilikogiannakis and M. Orfanopoulos,
Tetrahedron Lett., 1997, 38, 4323; (d) G. Vassilikogiannakis and
M. Orfanopoulos, J. Org. Chem., 1999, 64, 3392.
8 (a) T. Da Ros and M. Prato, Chem. Commun., 1999, 663; (b) H.-C. Wu,
X. Chang, L. Liu, F. Zhao and Y. Zhao, J. Mater. Chem., 2010,
20, 1036.
9 E. M. Sletten and C. R. Bertozzi, Angew. Chem., Int. Ed., 2009,
48, 6974.
10 S. Aoki, N. Matsuo, K. Hanaya, Y. Yamada and Y. Kageyama, Bioorg.
Med. Chem., 2009, 17, 3405.
pleasingly, it proceeded efficiently in all and with rates enhanced 11 R. Huisgen, J. Org. Chem., 1976, 41, 403.
12 P. de la Cruz, A. Diaz-Ortiz, J. J. Garcia, M. J. Gomez-Escalonilla,
by the presence of water. This could vitally enable the use of
fulleropyrazolines under conditions suitable for biologically-
relevant photo-controlled substrates when e.g. used in putative
drug-delivery systems or for ‘1,3-dipole delivery’ to olefin-
containing biomolecules. The proof-of-principle for such a system
was demonstrated by the construction of the amino-acid-carrying
fulleropyrazoline 15 and subsequent successful ‘photo-release’
under aqueous conditions (see ESI†).
A. de la Hoz and F. Langa, Tetrahedron Lett., 1999, 40, 1587.
13 A. R. Hajipour, I. Mohammadpoor-Baltork and M. Bigdeli, J. Chem.
Res., 1999, 570.
14 C. Rodriguez-Emmenegger, C. M. Preuss, B. Yameen, O. Pop-
Georgievski, M. Bachmann, J. O. Mueller, M. Bruns,
A. S. Goldmann, M. Bastmeyer and C. Barner-Kowollik, Adv. Mater.,
2013, 25, 6123.
15 J. L. Delgado, F. Oswald, F. Cardinali, F. Langa and N. Martin, J. Org.
Chem., 2008, 73, 3184.
16 M. Kasha, J. Opt. Soc. Am., 1948, 38, 929.
In conclusion we have shown that the reactivity of fullero- 17 L. P. Hammett, J. Am. Chem. Soc., 1937, 59, 96.
18 K. M. Creegan, J. L. Robbins, W. K. Robbins, J. M. Millar,
pyrazolines may be tuned to enable highly efficient photo-
cleavage. The reaction proceeds in various solvent systems
and without the need for additional trapping agents to remove
1,3-dipolar species generated in situ. An electron donating
substituent on N-1 of the pyrazoline is sufficient to enable the
reaction to proceed; various substituents are tolerated at C-3.
This synthetic flexibility and the aqueous compatibility could
enable the use of C60 fullerenes as carriers for prodrugs or other
photocontrolled elements.
R. D. Sherwood, P. J. Tindall, D. M. Cox, A. B. Smith III,
J. P. McCauley, D. R. Jones and R. T. Gallagher, J. Am. Chem. Soc.,
1992, 114, 1103.
19 R. G. Bulgakov, Z. S. Kinzyabaeva, L. M. Khalilov and V. M. Yanybin,
Russ. J. Org. Chem., 2011, 46, 1776.
20 (a) Y.-P. Sun, B. Ma and G. E. Lawson, Chem. Phys. Lett., 1995,
233, 57; (b) G. E. Lawson, A. Kitaygorodskiy and Y.-P. Sun, J. Org.
Chem., 1999, 64, 5913.
21 N. Armaroli, G. Accorsi, J.-P. Gisselbrecht, M. Gross, V. Krasnikov,
´
D. Tsamouras, G. Hadziioannou, M. J. Gomez-Escalonilla, F. Langa,
J.-F. Eckert and J.-F. Nierengarten, J. Mater. Chem., 2002, 12, 2077.
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