Fullerenes chiral

Hawkins J M, Nambu M and Meyer A 1994 Resolution and configurational stability of the chiral fullerenes C-g, C g, and Cg. A limit for the activation energy of the Stone-Wales transformation J. Am. Chem. Soc. 116 7642-5... 

Concept With the discovery of the fullerenes, it has become evident that elemental carbon can exist in almost an infinite number of stable allotropes that are either molecular or polymeric in nature. Whereas achiral and chiral fullerenes can now be prepared in bulk quantities and methods for their regio- and stereoselective multiple functionalization are being developed in increasing numbers, the... 

This system allows also for the indication of the absolute configuration of inherently chiral fullerenes by introducing the stereodescriptors f C) and f A) ( P = fullerene s = systematic numbering C = clockwise A = anti-clockwise). 

The chiral fullerene C75 was also asymmetrically osmylated using the chiral ligands and (Scheme 8.8) [63]. In this way an optically active allotrope of a pure element was prepared. C75 contains 15 different types of [6,6] bonds. The pronounced regioselectivity of C7Q towards osmylation [58] suggests that specific bonds in C75 may be favored for an attack by OSO4. An analysis of the ab initio calculated curvature of 75 shows that two of the five pyracylene-type bonds are particularly distorted, which could enhance their reactivity [64]. Indeed HPLC analysis of C75[Os04L ] shows that two regioisomers are predominantly formed upon osmylation of C75 [63]. 

Various studies focused on applying the AD for kinetic resolutions of racemates. One of the most impressive examples was described by Hawkins, who discovered a kinetic resolution of a chiral fullerene [10]. The isolation of both enantiomers of this carbon allotrope became possible by a kinetic resolution using pseudo-enantiomeric ligands 10 and 11. Thus, performing the AD in the presence of 10 yielded (-)-C76 and the 0s04 adduct of (+)-C76. Reductive removal of the... 

While functionalized chiral nanotubes and chiral fullerenes may provide mo-lecularly well-defined materials in the future, their specialized chemistry is not covered in this overview. 

II. CONFIGURATIONAL DESCRIPTION OF CHIRAL FULLERENES AND FULLERENE DERIVATIVES WITH A CHIRAL FUNCTIONALIZATION PATTERN... 

A. Classification of Chiral Fullerenes and Chiral Fullerene Derivatives... 

Depending on the origin of their chirality, three classes of chiral fullerene derivatives can be distinguished (Figure l.l) 22 25... 

C. Examples for the Configurational Description of the Different Classes of Chiral Fullerene Spheroids... 

C. Resolution of Chiral Fullerenes and Assignment of Absolute Configurations to the Enantiomers of D2-C76... 

Following our protocol to separate fullerene isomers by the intermediacy of defined covalent adducts, the fraction of fullerene soot was subjected to Bingel cyclopropanation with bis (.S )-l -phenyl butyl malonate (cf. Section III.C).60 Among the products two chiral mono-adducts and four chiral bis-adducts were isolated in pure state and characterized. An analysis taking into consideration the symmetries of the compounds as deduced from H- or 13 C NMR spectroscopy, the magnitudes of their Cotton effects (these are relatively large, between ca. 10 and 200 M 1 cm-1 in the case of chiral fullerene... 

Further explorations should be done in the stereoselective synthesis of chiral fullerene derivatives, a field initiated by the kinetic resolution of D2-C through diastereoselective osmylation with 0s04 in the presence of alkaloid-derived ligands. Besides, a few diastereoselective macrocyclizations by bridging of C6o with enantiomerically pure, bifunctional reagents have been achieved with high d.e. s. The use of optically pure fullerene derivatives as bulky chiral auxiliaries... 

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