Buckyballs - Polyarenes - page 16

Fig. 5 A mixture of bis-
adducts ( left ) in the presence
of C 60 as dipolarophile
produces monoadduct ( right )
HPLC analysis of the reaction confirmed the quantitative formation of the
corresponding monoadduct (Fig. 5 )[ 59 ]. This interesting result opened new
avenues for improving the yields of mono-adduct formation from the usually
undesired bis-cycloadducts obtained as byproducts in 1,3-dipolar cycloaddition
reactions of azomethine ylides to C 60 . This observation has recently been used as
an indirect proof of the covalent attachment of pyrrolidine fragments to single-
walled carbon nanotubes (SWCNT). The authors applied the same conditions
described by Mart´n et al. [ 59 ] by heating a sample of pyrrolidino-SWCNT in the
presence of C 60 to act as dipolarophile and the corresponding pyrrolidinofullerene
compound was detected, thus confirming the efficient trapping of the thermally
generated azomethine ylide [ 60 ].
Guryanov et al. [ 61 ] described an alternative protocol to achieve the retro-
cycloaddition of pyrrolidinofullerenes. The authors reported the quantitative
retro-cycloaddition of pyrrolidinofullerenes under microwave irradiation in an
ionic liquid (1-methyl-3- n -octyl-imidazolium tetrafluorborate) without further
additives. The combination of microwave irradiation in an ionic liquid offers the
unique opportunity for very efficient flash-thermal activation in conjunction with a
strong stabilization of ionic intermediates or reactants. In this case, the ionic liquid
served as an ideal medium to solvate the incipient 1,3-dipole whose release in
solution was likely assisted by electrostatic interactions with the complementary
ions of the solvent. In agreement with the mechanism proposed by Filippone et al.
[ 62 ], cycloreversion occurred through the formation of a reactive 1,3-dipolar
intermediate which was expected to be stabilized by the ionic liquid environment.
Lukoyanova et al.[ 63 ] had reported an alternative way to induce the retro-
cycloaddition of pyrrolidinofullerenes using electrochemical techniques. The
authors induce the retro-process by controlled potential electrolysis (CPE) at an
applied potential determined from cyclic voltammetry experiments.
In order to determine whether the experimental conditions previously used for
the retro-cycloaddition of fulleropyrrolidines and fulleroisoxazolines [ 64 ] are suit-
able for 2-pyrazolinofullerenes, Delgado et al. [ 65 ] followed the above-mentioned
protocol: excess of dipolarophile, as well as copper triflate to facilitate the retro-
cycloaddition process. According to the experimental findings, C -aryl- N -aryl-2-
pyrazolino[60]fullerenes do not undergo an efficient retro-cycloaddition process
under a variety of experimental conditions, which reveals that these compounds are
thermally stable fullerene derivatives. In contrast, the presence of an alkyl chain in
the carbon atom of the pyrazole ring results in an easier cleavage of the 1,3-dipole,
leading to pristine C 60 in good yields (72%). These results show the importance of