Methanofullerenes, close

The 1,3-dipolar addition reaction of fullerene with diazo compounds is one of the most studied reactions in the early stage of the research on fullerene chemistry [7]. The reaction in solution first affords rather labile fulleropyrazoline derivative 26, which is readily converted to the so-called 5,6-open (a bond originally shared by five- and six-membered rings cleaved) fulleroid (27) or 6,6-closed (a bond shared by two six-membered rings remaining closed) methanofullerene derivatives (28) by heat (e.g., refluxing in toluene) or photoirradiation (UV light), respectively (Scheme 11) [7a]. 

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). 

Cycloaddition of diazoalkanes to 6-6 bonds was among the first reactions tested on the carbon cages41 and affords isolable fullerene-fused pyrazolines as primary adducts. Photochemically induced extrusion of N2 from the latter yields 6-6-closed methanofullerenes having the functionalized bond embedded in a cyclopropane substructure, whereas the thermal process affords 6-5 open homofullerenes in which a methylene group is bridging the open junction between a six- and a five-membered ring.15-17... 

Nogami s group (Tsuda et al., 1993) showed that dichlorocarbene, generated by pyrolysis of sodium trichloroacetate in a mixture of benzene and 1,2-dimethoxy-ethane, reacts with C o to give the [6,6]-closed methanofullerene, as shown by negative ion fast atom bombardment mass spectrometry (FAB MS) and by NMR spectroscopy. 

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. 

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-... 

In general, by [2 -1- 1]-photocycloaddition reactions methanofullerenes, fullerene epoxide, and azirid-inofullerenes as the corresponding oxygen and nitrogen analogues with a [6,6]-closed structure are obtained as shown in Figure 28.5. 

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