Fullerenes, absolute configuration

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 relative configuration within the precursor trisadducts is retained in the inherently chiral C -symmetrical hexakisadducts, while the absolute configuration has changed. The Cotton effects of such hexakisadducts are much less pronounced than those of their precursor tris-adducts, since the local symmetry of the fullerene core is very close to Tjj. 

Determination of the absolute configurations of these optically active fullerene derivatives was possible by comparison of their experimental and calculated circular dichroism (CD) spectra [96]. Tether controlled bis-cyclopropanation reactions have been extensively used to synthesize extended fimctional architectures such as dyads. 

Figure 30 Preparation of the pure enantiomers of D2-C84 by a retro-Bingel reaction of the fourth and fifth fractions of the bis-adducts, and CD spectra of the diastereo-isomeric bis-adduct (top right) as well as of the fullerene enantiomers (bottom right) in CH2C12. The absolute configurations of the fullerene spheroids reflect the structural drawings, but have not been assigned experimentally. Reprinted from ref. [71] with permission from Wiley-VCH.

Figure 1.7. A selection of inherently chiral higher fullerenes, for which certain or confident structural assignments have been made. In the case of D2-C76, the absolute configuration as well as a characteristic CD (circular dichroism) maximum of the shown enantiomer are indicated.

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

Figure 1.44. The different stereoisomers obtained by [2 + 2] photocycloaddition of 3-methyl-cyclohex-2-enone to Cgo (left, the newly formed stereogenic centers are marked by an asterisk), and illustration of a sector mle devised for the correlation of the absolute configuration of a 6-6 closed 1,2-adduct of Cgo to the CD band around 430 nm (right). According to the sign ((+) or (-)) of the sector in which an addend moiety (R1, R2,R3, or R4) is located, it is postulated to add a positive or negative contribution to the Cotton effect associated with the ca. 430 nm UV/Vis absorption of the fullerene chromophore.

As to the assignment of absolute configurations [46-49] to isolated fullerene enantiomers [50-52] or fullerene derivatives with a chiral functionalization pattern [53-611 (cf. also the particular case of a chiral core-modified Cgo derivative [621), much work remains to be done. Following the initial kinetic resolution of small amounts of the allotropes Dj-Cj yD -Cj and D2-Q4 by Hawkins et al [50, 511, Diederich and coworkers recently succeeded in obtaining optically pure C76 isomers by functionalization of ( )-Cy6 with an enantiopure chiral addend, separation of the resulting diastereoisomers, and subsequent removal of the functionality [521. Comparison of the obtained circular dichroism (CD) curves with calculated spectra allowed the assignment of their absolute configuration [46,491 (cf. Sect. 7). 

A major breakthrough in this respect has been the introduction of asymmetric metal catalysis for the preparation pyrrolidino[3,4 l,2][60]fullerenes with a complete control on the absolute configuration [53],... 

Figure 34.3 Absolute configuration assigned for both cw-2-carboximethyl-5-(p-methoxyphenyl)pyrrolidino[3,4 l,2][60]fullerene enantiomers using the sector rule. See insert for color representation of the figure.)...

Figure 1 compares TDDFT and experiment for the electronic Circular Dichroism (CD) spectrum of the chiral fullerene Cyg. A total of 240 optically allowed transitions were required to simulate the spectrum. The accuracy is clearly good enough to assign the absolute configuration of Cyg. TDDFT calculations of this size typically take less than a day on low-end personal computers. 

Absolute Configuration of Chiral Fullerenes and Covalent Derivatives from Their Calculated Qrcular Dichroism Spectra. 

FIGURE 55.6. Absolute configuration of chiral Cgo-fullerene cis-3 bisadduct 9 by X-ray internal reference method. 

Kessinger R, Thilgen C, Mordasini T, Diederich F. Optically active macrocycUc cis-3 bis-adducts of Cgo - regio and stereoselective synthesis, exciton chirality coupling, and determination of the absolute configuration, and first observation of exciton coupling between fullerene chromophores in a chiral environment. He/v. Chim. Acta 2000 83 3069-3096. 

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