Buckyballs - Polyarenes - page 7

and [5]radialenes, C 60 behaves as a highly strained electron-poor alkene. The chemical
reactivity is mainly driven by strain relief and, therefore, addition reactions have been
widely used [ 18 ]. Interestingly, although similar reactivity patterns have also been
observed for higher fullerenes, chemical reactivity tends to decrease significantly with
their size [ 19 - 21 ].
A variety of chemical reactions, namely nucleophilic additions, cycloaddition
reactions, free radical additions, halogenations, hydroxylation, and metal transition
complexations, have been reported for C 60 . However, addition reactions, electron
transfer reactions, and reactions involving the opening of the fullerene cage
(molecular surgery) have been studied in more detail. It is worth mentioning the
ease with which fullerenes are reduced by means of electron-rich chemical reagents
as well as electrochemically. Their oxidation, however, is considerably more
difficult to achieve. These experimental findings are in agreement with former
theoretical calculations which predicted that C 60 has a low energy LUMO which
is triply degenerated and, therefore, accepts up to six electrons in solution to form
up to the hexaanion [ 22 ]. The theoretical predictions were later confirmed by
electrochemical measurements recording from the monoanion to the hexaanion
using a toluene/acetonitrile 5:1 by volume solvent mixture at 10 C[ 23 ].

For a wider and more detailed study of the basic reactivity of fullerenes, the
reader is referred to the aforementioned monographs that comprehensively cover
the properties and chemical reactivity of fullerenes [ 19 - 21 ].

2 New Covalent Chemistry of Fullerenes

Significant effort is still being devoted to the chemical modification of fullerenes.
Even though most of them are based on the chemistry of electron-poor olefins,
fullerene curved double bonds have given rise to a quite peculiar fullerene chemis-
try. Remarkable examples of this reactivity have been provided by the use of fuller-
1,6-enynes, fullerene analogues of 1,6-enynes involving a highly reactive fullerene
double bond as the “ene” moiety. Thus, fulleropyrrolidines 1 bearing a propargyl
group on the C-2 of the pyrrolidine ring undergo an unusual thermal [2+2] cyclo-
addition reaction affording regioselectively a cyclobutene-fullerene derivative
2 (Scheme 1 )[ 24 ]. A different change in chemoselectivity is observed when an
internal alkyne is used in the fullerenynes. In that case, allenofullerene derivatives
(3) are obtained as a result of a formal “ene” reaction where the alkyne moiety with
the

CH group acts as an “ene” component, despite the unfavorable geometry
(Scheme 1 )[ 25 ].
Another example of the intriguing behavior of fullerene double bonds has
recently been reported by Bazan et al. in which fullerenes behave as a neutral
carbon based Lewis acid [ 26 ]. Thus, when C 60 reacts with the N-heterocyclic
carbene 4, that acts as a Lewis base, a thermally stable zwitterionic Lewis acid-base
adduct 5 is formed. The bulk of the substituents of carbene species, along with the
delocalization of its positive charge, prevent the expected cyclopropanation reac-
tion and a C-C single bond, with a length of 1.506 ˚

α

is formed instead (Scheme 2 ).