Buckyballs - Polyarenes - page 41

Disilirane additions were used to control the motion of the encapsulated Ce
atoms inside the C 80 cage of Ce 2 @C 80 . In this compound, the free random motion
of two Ce atoms is regulated under a hexagonal ring on the equator by the electron
donation from the silyl groups, exohedrically introduced, to the C 80 cage [ 211 ]. The
motion of La atoms encapsulated inside fullerenes has also been controlled
exohedrically, by the addition positions, in pyrrolidine adducts of La 2 @C 80 . Two
different compounds are obtained: in the [5,6]-adduct the two La atoms rotate rather
freely, whereas in the [6,6]-adduct the metallic atoms are in fixed positions inside
the cage [ 212 ].
The regioselectivity of 1,3-dipolar cycloaddition reactions of in situ generated
azomethine ylides can also be controlled by the trimetallic nitride cluster. In this
sense, Echegoyen et al. demonstrated that when the fullerene inner metal cluster
was Sc 3 N, the only product detected was the adduct at a [5,6]-junction (product of
thermodynamic control). On the other hand, the 1,3-dipolar cycloaddition reaction
occurred initially at a [6,6]-junction for the Y 3 N@C 80 , and underwent rearrange-
ment to the thermodynamically more stable Y 3 N@C 80 [5,6]-monoadducts upon
heating [ 213 ]. These experimental results, as well as the computational studies by
Poblet and Echegoyen, seem to indicate that, after thermalization of the kinetically
favored product, a pirouette-kind of mechanism gives rise to the [5,6]-monoadduct
that is thermodynamically preferred [ 214 ]. The rate of this rearrangement depends
on the internal cluster and on the pyrrolidine addend.
Pyrrolidine adducts have been used as the organic addend to connect electron-
acceptor Sc 3 N@C 80 ,Y 3 N@C 80 ,orLa 2 @C 80 units to powerful donors such as
ferrocene [ 215 ]or -extended tetrathiafulvalene derivatives (exTTFs) [ 216 ] in the

preparation of electron-donor-acceptor (D-A) systems that, upon photoexcitation,
yield radical ion pair states with remarkable lifetimes.
[2+2]Cycloadditions. The reaction of benzyne, generated from isoamyl nitrite
and anthranilic acid, with Sc 3 N@ I h -C 80 was successfully carried out recently to
afford both [5,6]- and [6,6]-monoadducts (Fig. 27 )[ 217 ]. Interestingly, when the
reaction was carried out with 2-amino-4,5-diisopropoxybenzoic acid instead
of anthranilic acid and under an aerobic atmosphere, in addition to the expected
[2+2] benzyne adducts, oxygenation of the [5,6] regioisomer produces an intriguing
third product, which is an open-cage metallofullerene. Under an inert atmosphere,
the reaction gave only the expected [2+2] adducts [ 218 ].
[2+1]Cycloadditions. The investigation of the Bingel reaction on endohedral
metallofullerenes demonstrates the remarkable regioselective control exerted by the
encapsulated species. The [2+1] cycloaddition of bromodiethylmalonate (the
Bingel-Hirsch reaction) in the presence of the non-nucleophilic base
1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) produced extremely stable derivatives
with Y 3 N@C 80 and Er 3 N@C 80 [ 219 ], while Sc 3 N@C 80 did not react under the same
experimental conditions. Under these conditions, the cyclopropanation of Sc 3 N@C 78
with diethyl bromomalonate produced only one monoadduct and one dominant sym-
metric bisadduct. The high regioselectivity in the second addition is supported by the
highest LUMO surface electron density value for the reactive bond, which corresponds
to the kinetically preferred site for a second nucleophilic attack [ 220 ].

ˀ