Fullerenes methanofullerenes

The most important classes of functionalized [60]fullerene derivatives, e.g. methanofullerenes [341, pyrrolidinofullerenes [35], Diels-Alder adducts [34i] and aziridinofullerene [36], all give rise to a cancellation of the fivefold degeneration of their HOMO and tlireefold degeneration of their LUMO levels (figure Cl.2.5). This stems in a first order approximation from a perturbation of the fullerene s 7i-electron system in combination with a partial loss of the delocalization. 

A methanofullerene derivative possessing an ammonium subunit has been prepared and subsequently shown to form a supramolecular complex with a porphyrin-crown ether conjugate <06T1979>. The synthesis and study of these fullerene-containing supramolecular photoactive devices have also been reported <06CRC1022>. 

An alternative synthetic approach, first developed by Bingel225 allowed the efficient nucleophilic cyclopropanation of fullerenes via their reaction with bromomalonate derivatives in the presence of base. This approach, the most reliable method for the synthesis of functionalized methanofullerenes, combined the advantages of mild... 

Fig. 1.3 Fullerene bearing 7 [3-alanine, Baa, and nitroxide malonate methanofullerene... 

Novel nitroxide malonate methanofullerenes (Fig. 1.3), thanks to the presence of nitroxide radicals and fullerene moiety, are able to protect cells from toxic side effects of cyclophosphamide (Gubskaya et al., 2007). Experiments were carried out on mice, in which leukemia P-388 was transplanted. Cyclophosphamide or fullerene individually injected did not increase the average life span of the animals, while the combination of the anticancer drug and nitroxide fullerene derivative resulted in the survival of 70% animals, classifying these compounds as promising modifiers of biological reaction for tumor therapy. 

The first carbon nanostructure that was covalently modified to alter surface properties was the C60 fullerene. After its discovery in the mid-1980s, numerous functionalization methods were developed for obtaining two important classes of derivatives, methanofullerenes and fullerene Diels-Alder adducts. 

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]. 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

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). 

A few other interesting molecular architectures exhibiting uncharacteristic electrochemical behavior have been constructed on the basis of the methanofullerene building block. These include two methanofullerene-substituted bipyridine ligands complexed to rhenium and ruthenium ((35) and (36) in Fig. 17), which were prepared as possible candidates for photoinduced electron transfer processes [138]. In addition, three fullerene crown ether conjugates ((38), (39), and (40) in Fig. 18) have... 

For methanofullerenes, the triplet-triplet absorption appears arround 720 nm, for cyclohexylfused derivatives, pyrrolidino- and dihydro-fullerene, the absorption appears arround 700 nm [62,71,72]. 

UV-vis spectroscopy and NMR investigations clearly demonstrate that isomer 59b is the 6,6-adduct of C6o with C2v symmetry, namely the methano fullerene. There is crucial evidence supported by H and 13C NMR for the isomer 59a being the 5,6-adduct of C6o with Cs symmetry. No isomerization of the pure fulleroid to the methanofullerene or vice versa upon photolysis is observed as reported by Wudl et al. [238], Thus, the isomeric ratio of the two isomers reveals the formation ratio of carbene and diazo compound during the reaction. 

Other methanofullerenes (not of the Bingel-type) were also found to be unstable after several reduction processes (68-70, Figure 23), [9-11] and under CPE they led to the isolation of [60]fullerene. A more recent study was conducted in THF to avoid the well-known reactivity of CH2CI2 towards the polyanions of C60 [72] and to explore the mechanisms involved during adduct removal [9b,10], Surprisingly, an electrochemically induced intermolecular adduct transfer was observed for the spiromethanofullerenes studied, and the regio-isomer distribution found in THF differed significantly from that obtained when the compounds are prepared by a direct synthetic route [10], The proposed mechanism for the formation of... 

Figure 25 Cyclic voltammograms in CH2Cl2(- -0.1 M Bu4NPF6) at 100 mV s-1 for (a) (—) trans-l fullerene-dibenzo[18]crown-6 (( )-37) (b) ( ) after 2e CPE and reoxidation. Reprinted from Chemical Retro-cyclopropanation Reactions in Methanofullerenes Effect of the 18-crown-6 moiety, 1, M. A. Herranz, et al. 299-303, copyright 2003, with permission from Elsevier.

Besides the 1,3-dipolar cycloaddition of azomethine ylides to C60, the Bingel cycloprop anation reaction is widely used for regioselective functionalization of fullerenes. In principle, this versatile modification involves the generation of carbon nucleophiles from a-halo esters and their subsequent addition to C60 [19]. The addition takes place exclusively on double bonds between two six-membered rings of the fullerene skeleton, yielding methanofullerenes. As shown in Scheme 2, addition of diethylbromomalonate to C60, in the presence of an auxiliary base... 

The novelty of this type of organic functionalization is explained in terms of the wide number of different methanofullerene materials that can be reached due to the presence of esters moieties either properly functionalized carbon nucleophiles can be constructed that ultimately would add to C60, or additional chemical transformation to the addend group of an already synthesized methanofullerene could occur. Thus, theoretically, any chemical group carrying novel physical, electronic, magnetic, or mechanical properties that fulfills the appropriate requirements for the construction of novel organofullerene hybrid materials can be properly designed and, thus, attached on the fullerene skeleton. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

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... 

In the context of a completely different type of chemistry, an unexpected rearrangement occurred upon reaction of C60 with l-methoxy-l-[2-(trimethylsilyl)ethoxy]carbene and afforded a mixture of 1,4-adduct ( )-146 (Figure 1.35) and the corresponding 1,2-adduct instead of the anticipated methanofullerene.299 A crucial role in this reaction course was ascribed by Wudl, Warkentin, and co-workers to the nucleophilicity of the dialkoxycar-bene, the electrophilicity of the fullerene and the (3-stabilization of a positive charge by the trimethylsilyl group. 

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