Achiral and Chiral Higher Adducts of C₇₀ by Bingel Cyclopropanation

Five optically active isomeric C70 bis-adducts with (R)-configured chiral malonate addends were prepared by Bingel cyclopropanation (Scheme 1) and their circular dichroism (CD) spectra investigated in comparison to those of the corresponding five bis-adducts with (S)-configured addends (Fig. 2). Pairs of diastereoisomers, in which the inherently chiral addition patterns on the fullerene surface have an enantiomeric relationship, display mirror-image shaped CD spectra that are nearly identical to those of the corresponding pairs of enantiomers (Fig. 3, b and c). This result demonstrates that the Cotton effects arising from the chiral malonate addends are negligible as compared to the chiroptical contribution of the chirally functionalized fullerene chromophore. A series of four stereoisomeric tetrakis-adducts (Fig. 4) was prepared by Bingel cyclopropanation starting from four stereoisomeric bis-adducts. A comparison of the CD spectra of both series of compounds showed that the magnitude of the Cotton effects does not decrease with increasing degree of functionalization (Fig. 5). Bingel cyclopropanations of C70 in Me2SO are dramatically faster than in apolar solvents such as CCl4, and the reaction of bis-adducts (±)-13 and 15 with large excesses of diethyl 2-bromomalonate and DBU generated, via the intermediacy of defined tetrakis-adducts (±)-16 and 17, respectively, a series of higher adducts including hexakis-, heptakis-, and octakis-adducts (Table 1). A high regioselectivity was observed up to the stage of the hexakis-adducts, whereas this selectivity became much reduced at higher stages of addition. The regioselectivity of the nucleophilic cyclopropanations of C70 correlates with the coefficients of the LUMO (lowest unoccupied molecular orbital) and LUMO+1 at the positions of preferential attack calculated by restricted Hartree-Fock – self-consistent field (RHF-SCF) methods (Figs. 9 – 11). Based on predictions from molecular-orbital calculations (Fig. 11) and the analysis of experimental 13C-NMR data (Fig. 7, a), the structure of a unique hexakis-adduct ((±)-22, Fig. 12), prepared from (±)-13, was assigned. The C2-symmetrical compound contains four 6−6-closed methanofullerene sub-structures in its polar regions (at the bonds C(1)−C(2), C(31)−C(32), C(54)−C(55), and C(59)−C(60)), and two 6−5-open methanofullerene sub-structures parallel to the equator (at C(22)−C(23) and C(26)−C(27)). The 6−5-open sub-structures are formed by malonate additions to near-equatorial 6−5 bonds with enhanced LUMO coefficients, followed by valence isomerization (Fig. 12)