Isomeric fullerenes

Chapter 32 - Computational chemistry of isomeric fullerenes and endofullerenes. 

Slanina, Z., Zhao, X., Uhlik, E, Lee, S. L., Adamow-icz, L. (2004b). Computing enthalpy-entropy interplay for isomeric fullerenes. International Journal of Quantum Chemistry, 99(5), 640-653. 

The NaOH-catalyzed aerobic direct dimerization of functionalized hydrofullerenes (152) was reported by Jin and Yamamoto (Scheme 8.74). The reaction is assumed to proceed through deprotonation and one-electron oxidation of 152, giving the isomeric fullerene dimers 153 in high yields. 

Diederich F, Whetten R L, Thilgen C, EttI R, Chao I and Alvarez M M 1991 Fullerene isomerism—isolation of C2y-... 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Following the discovery of a bulk fullerene preparation process in 1990, the covalent chemistry of these carbon allotropes has developed at a phenomenal pace. Frontier orbital (LUMO) and tether-directed functionalization concepts have been successfully applied to the regio- and stereoselective preparation of multiple covalent adducts of C60. These have found increasing applications in the construction of functional supramol-ecules. More recently, the sequence of Bingel reaction - retro-Bingel reaction has provided an elegant access to isomerically pure higher fullerenes and, in particular, to pure carbon enantiomers. 

Again it has to be noted that the frontier orbitals participating in such a valence isomerization are delocalized over the whole molecule [22]. This has consequences for the orbital symmetry and, thereby, a prior analogy with comparable processes involving 6 t-electrons only is not given. However, compared with smaller Jt-systems the selection rules for orbital symmetry controlled processes in fullerenes seem to be less restrictive, since a large number of tt-orbitals with small energy separation are available. Calculations at the AM 1 and PM3 level show that the photocycKzation... 

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],... 

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). 

The removal of methano addends from fuUerenes using CPE is an efficient and versatile procedure, which has heen successfully employed in the separation of enantiomers and constitutional isomers of the higher fullerenes, for the isomerization of his- and tris-adducts of Ceo, and more recently, for the selective removal of one addend in the presence of another. 

The electrochemistry of C86, C90, and C92 has been reported for isomeric mixtures of individual cages.8 As the cage size of the fullerene increases, the yield of fullerenes decreases significantly. In addition to this, the number of possible isomers also increases, which in turn makes separation of isomers difficult. Thus, no reports appear in the literature indicating the electrochemical behavior of a single isomer of empty... 

A first striking observation of this study was the dramatic difference in electron-accepting abilities of isomeric endohedral fullerenes. This difference reaches 0.22 V for the isomers of Yb C76 and even 0.3 V between two isomers of Yb C82. The cage size also influences the reduction potentials. The [CJ and [C2v] isomers of Yb C82 are the best electron acceptors of the family and the second isomer of Yb C76 the weakest electron acceptor. In general, however, these metallofullerenes are better electron acceptors than the available empty cages. The metallofullerenes with bigger cages, Yb C82 and Yb C84, also accept more easily a fifth electron than the others. 

Bellavia-Lund and Wudl (43) investigated the 1,3-dipolar cycloaddition of 2-(methoxyethoxy)methyl azide with [70]fullerene (199) (Scheme 9.43). Three isomeric triazolines 200-202 were obtained. Thermolysis of these triazolines gave the corresponding azafulleroids and fulleroaziridines, as a mixture, respectively. 

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