Multiple Cyclopropanations of C70. Synthesis and characterization of bis‐, tris‐, and tetrakis‐adducts and chiroptical properties of bis‐adducts with chiral addends, including a recommendation for the configurational description of fullerene derivatives with a chiral addition pattern

Abstract

AbstractThe regioselectivity of multiple cyclopropanations of C70 with 2‐bromopropanedioates in the presence of 1,8‐diazabicyclo[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[(ethoxycarbonyl)methyl]2‐bromopropanedioate (13a), yielded three constitutionally isomeric bis(methano)fullerenes (Scheme 2). Two of them, C2‐symmetrical (±)‐1 and (±)‐2, are chiral; a fact which had not been considered in previous investigations. Formation of the third, C2v‐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 C70 with optically active bis[(S)‐1‐phenylbutyl] 2‐bromopropanedioate (13b) yielded five optically active, C2‐symmetrical bis‐adducts 7–11 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 contributions from the chirally functionalized 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 C2v‐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 diethyl 2‐bromopropanedioate (2 equiv.) to the C2‐symmetrical racemic bis‐adduct (±)‐2 yielded a mixture of tris‐adducts and one major, C2‐symmetrical tetrakis‐adduct (±)‐4 which was isolated in pure form (Scheme 4). Starting from the achiral C2v‐symmetrical bis‐adduct 3, one single Cs‐symmetrical tris‐(5) and one C2v‐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 C70 is high and resembles the preferred pattern of bis‐addition seen in the functionalization of C60.