stereochemistry

CARLO THILGEN, ISABELLE GOSSE, AND FRANÇOIS DIEDERICH 15
The helicity of carbon nanotubes was among the most revealing discoveries
by Ijima and co-workers in the early 1990s. 89 It was later confirmed by STM
(scanning tunneling microscopy) images of single nanotube surfaces resolved
at the atomic level. 90 It appears that the helicity of nanotubes is of prime
importance for potential technological applications, as it is closely related to
their electronic structure and properties. 13
C. Resolution of Chiral Fullerenes and Assignment
of Absolute Configurations to the Enantiomers of D2-C76
Isolation of the first chiral carbon molecule, D2-C76, as a racemic mixture2 generated strong interest in the separation of its optical antipodes and the
investigation of their chiroptical properties. In 1993 Hawkins and Meyer effectuated
a small-scale kinetic resolution by asymmetric Sharpless osmylation of
racemic C76, usingOsO4complexes with an enantiomerically pure ligand
derived from a cinchona alkaloid4 (Figure 1.9). Partial reaction of (±)-D2-
C76 with the osmium reagent afforded enantiomerically enriched unreacted C76
and diastereoisomerically enriched osmates. Reduction of the latter with SnCl2
yielded [76]fullerene enriched in the other enantiomer. Use of a pseudoenantiomeric
alkaloid-derived ligand in the osmylation afforded an enrichment of
the opposite optical C76 antipodes in the starting material and the product,
respectively (Figure 1.9). Being able thus to obtain each C76 enantiomer in an
enriched form, Hawkins and Meyer recorded the first CD spectra of the chiral
carbon molecules. 4 Repetition of the experiment with C78 and C84 afforded
the respective data for the optical antipodes of D3-C78 and D2-C84. 5 In these
experiments it was possible to start from the isomeric mixtures (C2v-C78 +
L 1
H 3CO
H
O
N
N
(±)-C 76
(+)-C76(OsO 4L 1 )
(−)-C76
OsO4
OsO4
(±)-C 76
(+)-C 76
+ SnCl2 +
(−)-C 76(OsO 4L 2 )
N
N
O
H
L 2
OCH 3
Figure 1.9. Kinetic resolution of D2-C76 based on a differential reactivity of enantiomerically
pure osmium complexes toward the optical antipodes of the fullerene.