No abstract

The electrochemical properties of covalent derivatives of the higher fullerenes C70, C76, C 2 v -C78, and D 3-C78, bearing bis(alkoxycarbonyl)methano addends, were investigated by cyclic voltammetry (CV) and/or steady-state voltammetry (SSV). Included in this study were a series of previously prepared mono- to tetrakisadducts of C70, monoadducts of C76, and bis- and trisadducts of C 2 v -C78 and D 3-C78. For all compounds, several (up to five) reduction steps and one or two oxidation steps were observed. The reduction steps usually required more energy than the corresponding processes in the parent fullerenes whereas the oxidation steps became facilitated with increasing degree of addition. A notable exception to this trend was the observation that two C76 monoadducts were more readily reduced than the parent fullerene and one was more difficult to oxidize. The electrochemical properties of C70 derivatives were found to closely resemble those of C60 adducts. Constitutional isomers displayed nearly identical redox characteristics. Only the first reduction process is reversible by CV or SSV in the entire series of C70 adducts whereas the electrochemically generated dianions of some of the derivatives underwent a rapid subsequent chemical reaction. The electrochemistry of covalent adducts of C76 and C78 differs substantially from that of the derivatives of the smaller carbon spheres C60 and C70. Among all compounds studied, monoadducts of C76 are the easiest to be reduced or oxidized and their electrochemically generated multiple anions are much more stable under the conditions of the SSV and CV experiments than those of the adducts of C70 and the C78 isomers. Several of the electrochemically generated trisanions of the adducts of C78 isomers underwent rapid chemical reactions. Indeed, a new bisadduct of C 2 v -C78, not detected in the regular synthetic mixture, was prepared via electrolysis of a trisadduct. This C 2-symmetrical bisadduct seems to be present as an intermediate during every electrolysis of other bis- and trisadducts of C 2 v -C78.

The regioselectivity of multiple cyclopropanations of C," with 2-bromopropanedioates in the presence of 1,8-diazahicyclo[5.4.0]undec-7-ene (DBU) as base (Bingel reaction) was investigated in a systematic study. Bisadduct formation occurred preferentially at the 6-6 bonds formed by the most pyramidalized sp2-C-atoms at the two opposite poles of the fullerene and, in the reaction with achiral bis[(ethoxycarhonyl)methyl] 2-bromopropanedioate (13a), yielded three constitutionally isomeric his(methano)fullerenes (Scheme 2). Two of them, C,-symmetrical ( i ) -l and (i)-Z, are chiral; a fact which had not been considered in previous investigations. Formation of the third, C2,-symmetrical isomer 3 was observed for the first time. Configurational descriptions for fullerene derivatives which possess a chiral chromophore as a result of specific functionalization patterns are proposed. Cyclopropanations of C,, with optically active bis[(S)-1-phenylbutyl] 2-bromopropanedioate (13b) yielded five optically active, C2-symmetrical his-adducts 7-1 1 which could be separated by preparative HPLC and fully characterized (Scheme 3, Fig. 4 ) . Compounds 7/8 and 9/10 represent two constitutionally isomeric pairs of diastereoisomers, and their circular dichroism (CD) spectra show pronounced Cotton effects mainly due to strong chiroptical contrihutions from the chirally functionalired fullerene chromophores (Fig. 7). Since the addition patterns on the fullerene surface in each pair of diastereoisomers have an enantiomeric relationship, their CD spectra closely resemble those expected for two enantiomers. In the third constitutional isomer 11, the addition pattern on the fullerene surface is C2,-symmetrical, and optical activity only results from the chiral addends. Its CD spectrum shows weak Cotton effects mainly from induced circular dichroism originating from the perturbation of the achiral fullerene chromophore by the attached chiral addends. Addition of diethyl2-brornopropanedioate (2 equiv.) to the C2-symmetrical racemic bis-adduct (+)-2 yielded a mixture of tris-adducts and one major, C,-symmetrical tetrakis-adduct (&)-4 which was isolated in pure form (Scheme 4 ) . Starting from the achiral C,,.-symmetrical bis-adduct 3, one single C,-symmetrical tris-(5) and one C,,-symmetrical tetrakis-adduct (6) were obtained as major products which were isolated and fully characterized (Scheme 5). The regioselectivity for introduction of a second addend in the same hemisphere of C,o is high and resembles the preferred pattern of bis-addition seen in the functionalization of C60.

Zufällig entdeckt wurde bei elektrochemischen Untersuchungen zur Stabilität von Anionen von 1 eine präparativ nützliche Methode: die Retro‐Bingel‐Reaktion, eine elektrochemische Methode zur Entfernung der Bis(alkoxycarbonyl)methano‐Bücken von Methanofullerenen. Sie wurde dazu verwendet, um die ersten Proben von enantiomerenreinem D2‐C76 mit gesicherter optischer Reinheit herzustellen.