Inherently chiral fullerenes

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). 

Of substantial interest were the chiroptical properties of these enantiomeric 60 derivatives with chiral functionalization patterns the mirror-image CD spectra of ( C)-2 and ( A)-2 displayed large Cotton effects between 250 and 750 nm with Ae values approaching 150 cm mmoL [25, 59]. These intensities of the CD bands are of similar magnitude to those measured for inherently chiral fullerenes (such as D2-C76 [78] and their derivatives [79]) but are much larger than those measured for non-racemic fullerene derivatives in which the chirality only results from the addends [25]. 

Fig. 8. CD spectra of a 52a with chiral side chains, hut an achiral fullerene addition pattern, and b the diastereoisomeric 48a and 49a having enantiomeric, inherently chiral fullerene functionalization patterns in addition to the (S)-configured side chains...

Fig. 16. Resolution of inherently chiral fullerenes. Top Kinetic resolution based on the differential reactivity of an optically pure osmium complex towards the enantiomers of 1 2" C76. Hawkins et al. utilized this method also for the resolution of and D2 C 4, Bottom Separation of the enantiomers of H2-C76 by application of the retro-Bingel reaction to each of two optically pure, diastereoisomeric Cyg derivatives having enantiomeric carbon cores...

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. 

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

Derivatives of achiral parent fullerenes in which the functionalization creates a chiral addition pattern on the spheroid, regardless of the addends being identical or different, have an inherently chiral functionalization pattern. The derivatives of chiral parent fullerenes automatically have an inherently chiral functionalization pattern. 

Figure 1.3. Schlegel diagrams of the enantiomers of 80,81-dimethoxy-2,3-(methano[l,2]ben-zenomethano)[76]fuIlerene.50 In all cases of inherently chiral carbon cages, the same numbering scheme is used for a given enantiomer of the parent fullerene and its derivatives.

Derivatives of Achiral Parent Fullerenes with an Inherently Chiral Functionalization Pattern... 

As the parent fullerene is achiral, its C-atom numbering can in principle be achieved with either one of the two mirror-symmetric numbering schemes. For a particular enantiomer of its derivatives with an inherently chiral functionalization pattern, however, a single numbering scheme only leads to the lowest set of locants for the addends (cf. Sections IV.A.l and IV.A.2) (Figure 1.4). 

Figure 1.6. Schlegel diagrams of two stereoisomers of tetrakis[( )-l-phenylbutyl] l,2 56,57-bis (methano)[70]fullerene-71,71J2,72-tetracarboxylate.35 54 The two types of stereogenic elements (inherently chiral addition pattern and stereogenic centers in the ester groups) are specified independently of each other. As can be seen from the descriptors (S,S,S,S, C) and (S fA), the depicted molecules are diastereoisomers.

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.

A. Fullerene Derivatives with an Inherently Chiral Addition Pattern... 

R,R,R,R, A)-20 (Figure 1.14), each as a diastereoisomeric pair in which the (R,R)-configured side groups are combined with inherently chiral addition patterns of opposite configuration (fC or fA) ,( After the separation of all isomers, they found that the formation of the bis-adducts 13-16 was only weakly diastereoselective (d.e. <10%). Furthermore the CD spectra of the diastereoisomeric pairs 13/14,15/16, and 17/18 are nearly mirror-image shaped which clearly proves the opposite configuration of the addition patterns in each pair and also the predominant chiroptical contribution of the distorted residual fullerene n-chromophorcs in comparison to that of the addends. 

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