istic absorption at 6.58-6.65 ppm due to the deshielding
effect of C60. In the 13C NMR spectra, the benzylic carbon
atom appears at δ 52.55-53.19 ppm, while the C60-H sp3
carbon resonates at 59.03-59.59 ppm. The corresponding
1H and 13C NMR signals of the compounds 6a-10a were
also identified either upfield or downfield shifted compared
to the corresponding NMR signals of 1a-5a (see the
Supporting Information). More importantly, the 13C NMR
spectra of compounds 1a-10a exhibit all of the patterns that
correspond to a C60 derivative with Cs symmetry induced
by the addition of two different addends (achiral) to the 6,6-
junction bond of C60. Thus, there are 30 13C NMR signals
(one or two of which may be overlapped) for the sp2 carbons
of the C60 core and two signals for the sp3-hybridized
fullerenyl carbons.
catalyzed reaction of 11 gave the two adducts 11a and 11b
in equimolar amounts (Scheme 2). Consequently, it is
Scheme 2. Product Distribution in the TBADT- and
DCA-Mediated Reactions of 4-Methylanisole 11 with C60
reasonable to assume that two different mechanisms operate
in the DCA- and TBADT-mediated reactions. As we already
mentioned, since previous results support an ET mechanism
on the DCA photocatalyzed reactions with aromatic sub-
strates,13 it is plausible that in this case the DCA photocata-
lyzed functionalization of C60 proceeds also through the same
mechanism. It is likely therefore that a HAT mechanism from
both -CH3 and -OCH3 groups of 11 predominates in the
corresponding TBADT-catalyzed reactions.14
Assuming that a HAT mechanism is operating in these
reactions, then the observed reactivity of compounds 1-11
should be mirrored in the C-H bond dissociation energies
(BDEs). For instance, it has been postulated that electron-
withdrawing substituents strengthen the benzylic C-H bond
and electron-donating substituents weaken it.15 It is thus not
unreasonable to expect that, when the para substituent is the
nitro group, the BDE of the benzyl C-H bond in p-
nitrotoluene 5 would be greater than that in toluene 1. On
this basis, 5 should be less reactive than toluene in hydrogen
abstractions, as found. Moreover, the longer reaction time
required in the reaction of p-dimethoxybenzene 7 compared
to 6 and 8 should be rationalized on the basis of the low
oxidation potential of 7, which probably causes a large
quenching of the reactive state of TBADT.16
To probe this mechanism further and obtain information
on the extent of bond breaking, we measured the intramo-
lecular primary isotope effect (PIE) of this reaction. To this
end, we prepared the R,R,R-trideutero-p-xylene 2-d3. The
reaction of 2-d3 was performed similarly with those described
previously in this work (Scheme 3), using a 400-fold excess
of the 2-d3 substrate with respect to C60. The ratio of these
products 2a-d3 and 2b-d3, which is the result of an intramo-
lecular isotopic competition between the -CH3 and -CD3
substituents of 2-d3, is proportional to the primary isotope
In the UV-vis absorption spectra, all of the compounds
1a-10a showed a weak absorption around 432 nm, which
is diagnostic for 1,2-adducts of fullerene (while 1,4-adducts
exhibit a broad absorption band at ca. 445 nm).3a The high
energy absorptions at ca. 330 and 258 nm are attributed to
the fullerene moiety.11
To explore the mechanism of this reaction, a series of
experiments were performed. Initially, the source of the
hydrogen atom in RC60H was investigated. The photocata-
lyzed reaction of 1 and 6 with C60 was performed in separate
experiments, first in a mixture of dry C6H5Cl/CD3CN (85:
15) and then in a mixture of C6H5Cl/CH3CN containing 0.5%
D2O. No measurable deuteration resulted in the first case,
while in the second set of experiments, the products 1a and
6a were highly deuterated (deuterium incorporation more
than 80% at the C60 sp3 carbon), as determined by 1H NMR
integration (see the Supporting Information). This result
indicates the formation of the fullerene anion as a reactive
intermediate in the TBADT-mediated reactions of C60.
Furthermore, compounds 1a and 6a do not exchange their
proton in a measurable amount (1H NMR) since no deuterium
incorporation could be detected after a sample of 1a or 6a
in a 85:15 mixture of C6H5Cl/CH3CN had been agitated with
D2O for several hours or even upon irradiation. Finally, no
H-D exchange observed (1H NMR) when a deaerated
solution of 1a (3 mg, 3.7 µmol) was irradiated in the presence
of TBADT and 0.5% D2O, under the experimental conditions
mentioned above.
It should be also mentioned that TBADT is known to react
with organic substrates either through an electron transfer
(ET) or a hydrogen atom transfer (HAT) mechanism.5b,12
Both mechanisms give rise to the same substrate-derived
organic radical R•, and hence, the distinction between these
two mechanistic paths is not straightforward. In order to
clarify this issue, we used 9,10-dicyanoanthracene (DCA)
as the photosensitizer in the reaction of 4-methylanisole 11
with C60. DCA is a well-established ET photosensitizer in a
polar solvent such as acetonitrile.13 The only product obtained
from this reaction was 11a, while the corresponding TBADT-
(14) The DCA catalyst is herein utilized for qualitative and not quantitive
purposes (e.g., for clarification of the mechanism of the corresponding
TBADT-mediated reactions). In this work, DCA was found to be unreactive
or much less reactive than TBADT, toward substrates 1-10. Therefore,
DCA should not be considered as an alternative catalyst to TBADT.
(15) (a) Zavitsas, A. A.; Pinto, J. A. J. Am. Chem. Soc. 1972, 94, 7390-
7396. (b) Zavitsas, A. A.; Rogers, D. W.; Matsunaga, N. J. Org. Chem.
2007, 72, 7091-7101.
(11) Hare, J. P.; Kroto, H. W.; Taylor, R. Chem. Phys. Lett. 1991, 177,
394-398.
(16) Easily oxidizable substrates exhibit larger quenching rate constants
of the reactive state of TBADT. At very low oxidation potentials back-
4-
5-
(12) (a) Duncan, D. C.; Fox, M. A. J. Phys. Chem. A 1998, 102, 4559-
4567. (b) Texier, I.; Delaire, J. A.; Giannotti, C. Phys. Chem. Chem. Phys.
2000, 2, 1205-1212.
electron transfer to W10O32 becomes favorable and no W10O32 is
observed; for example, in the case of N,N-dimethylaniline, no reduced
TBADT is observed; see: Tanielian, C.; Schweitzer, C.; Seghrouchni, R.;
Esch, M.; Mechin, R. Photochem. Photobiol. Sci. 2003, 2, 297-305. See
also reference 12a.
(13) (a) Eriksen, J.; Foote, C. S. J. Am. Chem. Soc. 1980, 102, 6083-
6088. (b) Eriksen, J.; Foote, C. S. J. Phys. Chem. 1978, 82, 2659-2662.
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