Achiral fullerenes

It should be mentioned that the addition of chiral residues to an achiral fullerene does not necessarily lead to a chiral addition pattern (cf. Section IV.C and Figure 1.1). In such a case the configuration of the chiral addend(s) only... 

If the addition of chiral or achiral addends creates a chiral addition pattern on an achiral fullerene core, irrespective of the addends being identical or different, the functionalization pattern is defined as inherently chiral. 

Chiral adducts can finally be obtained by functionalizing achiral fullerenes in such a way that chirotopic elements are exclusively located in the addends (no chiral core functionalization pattern). 

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...

Figure 6.5 (a) Schematic representation of the macromolecular helicity induction on poly(1 -co-5 ) upon complexation with L-alanine. The achiral fullerene and crown ether pendants represented by yellow and blue rings for clarity arrange in a helical array along the one-handed helical polymer backbone induced by noncovalent chiral interactions with L-alanine. (b) CD spectra of poly(1o.i-co-5o,) with L- and o-Ala HClO ([Ala-HCl04]/[poly(1o.,-co-5o,)] = 2) in dichloromethane-acetonitrile (8/2, v/v) at 25... 

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... 

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],... 

Fullerene, Cgo, undergoes photochemical 2 + 2-cycloaddition with /V,A-diethyl-4-methylpent-3-en-l-yn-l-amine to produce the stable Cgo-fused cyclobutenamine that is photo-oxidized to the dihydrofullerenone amide in high yield.15 The photochemical 2 + 2-cycloaddition of arylalkenes with Cgo has been shown to occur by a two-step mechanism involving the formation of a dipolar or diradical intermediate in the rate-determining step.16 The 2 + 2-photo-cycloaddition of cis- and irons-1 -(/j-methoxyphen-yl)-l-propene to C6o produces only trans-2 + 2-adduct. This is consistent with a two-step mechanism.17 The 2 + 2-photo-cycloaddition of cyclic 1,3-diones to Cgo results in the formation of two furanylfullerenes, one chiral and the other achiral. None of the expected De Mayo cyclooctane-1,3-dione addition products were formed.18... 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

Derivatives of achiral parent fullerenes in which the chirality of the functionalization pattern is due only to nonidentities among addends, have a noninherently chiral functionalization pattern. The analogy of this situation to that of a center of chirality should be noted. 

Derivatives of achiral parent fullerenes in which the addition of chiral residues does not create a chiral addition pattern on the fullerene surface. Their chiral elements are located exclusively in the addends. 

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). 

D3-C78), respectively (D2-C84 + D2d-C84), because the achiral isomers are CD inactive. The differential reactivity of the chiral reagent toward the enantiomers of D2-Cs4 is particularly remarkable in view of the roundness of this fullerene when compared to the helically twisted C76 (vide infra). 

Scheme 1.10. Achiral dimeric aza[70]fullerenes and their chiral cage-opened ketolactam precursors. The third ketolactam (right) is expected to afford a mixture of meso- and d, Z-6,6 -bi-5-aza[70]fullerene.

In the case of achiral, C2v-symmetric malonate addends, three constitutionally isomeric bis(methano)fullerenes are formed,35,54,107 an achiral one (C2v-symmetric 102), and two chiral representatives (C2-symmetric ( )-103 and ( )-104) which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 1.26).35,54... 

Figure 1.26. Double Bingel addition to C70 leads to an achiral top) and two inherently chiral (center and bottom) addition patterns. Combination of each of the latter with chiral ester moieties affords two diastereoisomeric pairs of enantiomers. The enantiomers of each pair were prepared separately by addition of either (R,R) or (S, -configured malonates to C70, and all stereoisomers were isolated in pure state. The black dots mark intersections of the C2-symmetry axis with the [70]fullerene spheroid. Next to the three-dimensional representations, constitution and configuration of the addition patterns are shown schematically in a Newman type projection along the Cs-axis of C70. Of the two concentric five-membered rings, the inner one corresponds to the polar pentagon closest to the viewer, and the attached vertical line represents the bond C(l)-C(2) where the first addition occurred. The functionalized bonds at the distal pole depart radially from the outer pentagon.

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