Methanofullerenes, structure

Isaacs, L., and Diederich, F. (1993) Structures and chemistry of methanofullerenes a versatile route into N-[(methanofullerene)carbonyl]-substituted amino acids. Helv. Chim. Acta 76, 2454-2464. 

Gubskaya VP, Berezhnaya LS, Gubaidullin AT, Faingold II, Kotelnikova RA, Konovalova NP, Morozov VI, Litvinov IA, Nuretdinov IA (2007) Synthesis, structure and biological activity of nitroxide malonate methanofullerenes. Org. Biomol. Chem. 5 976-981. 

F. Diederich, L. Isaacs, D. Philp, Syntheses, structures, and properties of methanofullerenes, Chemical Society Reviews, vol. 23, pp. 243-255,1994. 

Diazo compounds also undergo cycloaddition with fullerenes [for reviews, see (104),(105)]. These reactions are HOMO(dipole)-LUMO(fullerene) controlled. The initial A -pyrazoline 42 can only be isolated from the reaction of diazomethane with [60]fullerene (106) (Scheme 8.12) or higher substituted derivatives of Ceo (107). Loss of N2 from the thermally labile 42 resulted in the formation of the 6,5-open 1,2-methanofullerene (43) (106). On the other hand, photolysis produced a 4 3 mixture of 43 and the 6,6-closed methanofullerene (44) (108). The three isomeric pyrazolines obtained from the reaction of [70]fullerene and diazomethane behaved analogously (109). With all other diazo compounds so far explored, no pyrazoline ring was isolated and instead the methanofullerenes were obtained directly. As a typical example, the reaction of Cgo with ethyl diazoacetate yielded a mixture of two 6,5-open diastereoisomers 45 and 46 as well as the 6,6-closed adduct 47 (110). In contrast to the parent compound 43, the ester-substituted structures 45 and 46, which are formed under kinetic control, could be thermally isomerized into 47. The fomation of multiple CPh2 adducts from the reaction of Ceo and diazodiphenylmethane was also observed (111). The mechanistic pathway that involves the extrusion of N2 from pyrazolino-fused [60]fullerenes has been investigated using theoretical methods (112). 

Fulleropyrrolidines constitute an important family of Ceo derivatives that have the advantage over the methanofullerene systems in that they lead to stable reduced species, allowing the development of fullerene-based redox molecular switches. In order to promote mesomorphism in such a mono-adduct structure, Deschenairx et al. decided to modify the Ceo derivative by the use... 

Before we discuss experimental evidence for the structures of isomeric methanofullerenes, we return briefly to Wudl s and Diederich s observations that isomer mixtures are formed with diazoacetates under kinetic control and that they equilibrate to 8.28 as most stable isomer. It is surprising, however, that, for the parent (unsubstituted) methanofullerenes C61H2, it was not possible to thermally rearrange isomers into the compound corresponding to 8.28. When Smith et al. (1993) synthesized C61H2 by photolysis of the corresponding dihydropyrazoles, they obtained a mixture (4 3) of two isomers and observed neither thermal nor photolytic interconversion. 

The electronic structure of Cgo is best described as a fusion of [5]radialene and cyclohexa-l,3,5-triene substructures (Taylor, 1992 Hirsch, 1994). It is known that the double-bond character in the five-memberend rings in is low. Methano bridging can take place, however, as we have already seen (8.28-8.30), at the [6,5] or the [6,6] ring junctions and, in addition, valence isomerization is possible in both cases. This results in four isomeric methanofullerenes and eight, if the two substituents at the methano C-atom are different. Structure and naming of the four isomers are shown in Figure 8-4. 

The most convincing evidence for structure 8.36, i. e., the [6,6]-closed isomer was provided recently by an X-ray investigation carried out jointly by Diederich s group at ETH Zurich and by Gross and coworkers at the Universite Louis Pasteur in Strasbourg. They found that the 61,61-bis[4-(trimethylsilyl)buta-l,3-diynyl]-l,2-di-hydro-l,2-methanofullerene[60] (8.37) has bond lengths that are only compatible with a [6,6]-closed structure (Anderson et al., 1994b). 

At the same time, Osterodt et al. (1994) published the structure of another substituted methanofullerene[60], namely that of (3,4-dimethoxyphenyl)phenyl-methanofullerene[60] (8.36, R and R = C6H5 and 3,4-(CH30)2C6H3, respectively). A [6,6]-closed structure was also found for this compound. 

It was possible to thermally rearrange the [6,5]-open isomer of the two malonate derivatives and the diphenyl substituted compound to the [6,6]-closed isomers, but not the parent compound. These results also demonstrate that the preservation of the bonding pattern within 50 is the dominant cause for the structure of methanofullerenes. 

A comprehensive review on syntheses, structures, and chemical properties of methanofullerenes has been published by Diederich et al. (1994 b). 

Irradiation of the pyrazoline mixture 12, 13, and ( )-14 yielded methanofullerenes 15 and 16 (7 1) with a closed transannular 6 - 6 bond besides minor amounts of a third constitutional isomer (17), whereas thermolysis of the same mixture gave 17 and inherently chiral ( )-18 (4 l),both constituting core-modified fullerenes [76] (Scheme l).With open transannular 6-5 bonds , the functionalized regions of 17 and ( )-18 resemble methanoannulene sub-structures. [3 + 2]Cycloaddition of alkyl azides to C70 affords triazolines which are isolable when the temperature does not exceed ca. 50 °C. Similar to the reaction with diazomethane, three constitutionally isomeric adducts were observed [77,113], and the regioisomer of type 19 [113] with the substituent of the heterocycle located above the apex of C70, predominated over its regioisomer of type 20 in two in-... 

FIGURE 8.1 Chemical structure of (a) polystyrene (PS), (b) poly(methyl methacrylate) (PMMA), (c) 8-hydroxyqui-noline (8HQ), (d) 1-dodecanethiol (DT), (e) 2-naphthalenethiol (2NT), (f) polyaniline (PANI), (g) tetrathiafiilvalene (TTF), and (h) methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). 

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