Buckyballs - Polyarenes - page 15

Scheme 9 Different approaches to induce the retro-cyclopropanation reaction

benzocyclobutene rings fused to the [6,6]-bond were not affected by CPE [ 56 ]. This
singularity offered a new and versatile protecting/deprotecting group strategy. The
chemical retro-cyclopropanation of C 60 and C 70 mono-adducts was reported in
2000 by Diederich, Echegoyen and co-workers [ 57 ]. In this case, the methano-
addend was removed from C 60 and C 70 after heating at reflux with amalgamated
magnesium powder (10% HgBr 2 ) for 3 days, yielding 73% of C 60 and 63% of C 70 ,
respectively. In the case of bis-adducts of C 60 , the yield of recovered C 60 varied
between 13% and 48% and no isomerization reactions were detected. These
chemical conditions can selectively remove only the methano addends in the
presence of other functional groups such as pyrrolidines, offering a versatile
protecting/deprotecting group strategy (Scheme 9 )[ 57 ].

2.3.3 Retro-1,3-Dipolar Cycloaddition Reactions

Among the 1,3-dipolar cycloaddition reactions, the addition of azomethine ylides is
considered as one of the simplest and most efficient procedures for the functiona-
lization of fullerenes [ 58 ]. Azomethine ylides are reactive intermediates that can be
generated in several ways, although the decarboxylation of iminium salts, derived
from condensation of α

-amino acids with aldehydes or ketones, is the easiest and
most general procedure commonly followed. Pyrrolidinofullerenes are stable
compounds, although in the last few years a number of methodologies have been
described to promote the retro-cycloaddition reaction of these derivatives to afford
pristine C 60 fullerene.
Mart´netal.[ 59 ] have recently described the thermally induced retro-cycloaddition
of pyrrolidino[3,4:1,2]fullerenes. The authors studied the retro-cycloaddition process
on a series of pyrrolidinofullerenes under a variety of experimental conditions.
The best results were obtained heating the corresponding fulleropyrrolidine in the
presence of a dipolarophile such as maleic anhydride and copper triflate. Under
these conditions, the reaction led to the quantitative formation of the parent
unsubstituted C 60 in all cases. This methodology was also effective in inducing
the retrocycloaddition for

the mono-adduct mixture of three isomers of [70]

fulleropyrrolidine which afforded pristine C 70 in 95% yield.
Another important finding was the use of C 60 as a dipolarophile. A mixture of a
fullerene bis-adduct was heated to reflux in o -DCB in the presence of C 60 and