Chemistry Reference
In-Depth Information
Fig. 4 Site-, regio-, diastereo-, and enantio-selective cycloaddition onto C
70
The application of this methodology to higher fullerenes has also been achieved
with excellent selectivity. Indeed, higher fullerenes are characterized by a more
complex covalent chemistry due to the minor degree of symmetry. In C
70
, the
number of different 6,6 double bonds is increased compared to the single kind of 6,6
double bond in C
60
and, therefore, even higher control is necessary to face the
distinct levels of selectivity encountered.
The very mild conditions used for this transformation allowed direction of
cycloaddition of chiral N-metalated azomethine ylides toward the more reactive
α
double bond (C
25
-C
8
according IUPAC nomenclature) with an almost complete
site-selectivity. Despite the use of unsymmetrical dipoles, all the complexes used
afforded the pyrrolidines bearing methoxycarbonyl group in the polar region of C
70
as major product with good levels of regio-selectivity (Fig.
4
)[
47
].
Finally, for this regioisomer all four possible stereoisomer were obtained by the
use of the suitable metal chiral complex with high values of diastereo- and enantio-
selectivity [
43
-
45
].
Theoretical calculations (B3LYP/LANL2DZ) indicated a stepwise mechanism
for this cycloaddition where the first step is critical for the stereochemical outcome.
Furthermore, the high regioselectivity has been accounted for by the calculated
nucleophilic and electrophilic Fukui indexes.
Chiral functionalization of endofullerenes represents one step further in the
application of chiral metal catalysis. The first chiral endohedral metallofullerenes
were recently prepared by using such methodology on a racemic mixture of a non-
IPR metallofullerene derivative, La@C
72
(C
6
H
3
Cl
2
)[
48
]. This mono-functionalized
metallofullerene was chosen due to the calculated energy level of the LUMO orbital
being suitable to give rise to 1,3-dipolar cycloaddition. Despite this complexity,
eight optically pure bis-adducts of La@C
72
(four from the clockwise starting
material enantiomer and four from the anticlockwise enantiomer) were isolated
using non-chiral HPLC. These endohedral fulleropyrrolidines resulted from the
addition onto only 2 sites among 108 possible addition sites. For each site, two 2,5-
disubstituted pyrrolidine regioisomers are formed with fixed (2
S
,5
S
) and optical
purities as high as 98% (Scheme
7
). All the isomers feature a strong Cotton effect
that is dominated by the inherent chirality of the carbon core. The four isomers
