Achiral and chiral higher adducts of C-70 by Bingel cyclopropanation

Five optically active isomeric C-70 bis-adducts with (R)-configured chiral malonate addends were prepared by Bingel cyclopropanation (Scheme 1) and th eir circular dichroism (CD) spectra investigated in comparison to those of the corresponding five bis-adducts with (S)-configured addends (Fig. 2). Pa irs of diastereoisomers, in which the inherently chiral addition patterns o n the fullerene surface have an enantiomeric relationship, display mirror-i mage shaped CD spectra that are nearly identical to those of the correspond ing pairs of enantiomers (Fig. 3, b and c). This result demonstrates that t he Cotton effects arising from the chiral malonate addends are negligible a s compared to the chiroptical contribution of the chirally functionalized f ullerene chromophore. A series of four stereoisomeric tetrakis-adducts (Fig . 4) was prepared by Bingel cyclopropanation starting from four stereoisome ric 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 inc reasing degree of functionalization (Fig. 5). Bingel cyclopropanations of C -70 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 diet hyl 2-bromomalonate and DBU generate 1, via the intermediacy of defined tet rakis-adducts (+/-)-16 and 17, respectively, a series of higher adducts inc luding hexakis-, heptakis-, and octakis-adducts (Table 1). A high regiosele ctivity was observed up to the stage of the hexakis-adducts, whereas this s electivity became much reduced at higher stages of addition. The regioselec tivity of the nucleophilic cyclopropanations of C-70 correlates with the co efficients of the LUMO (lowest unoccupied molecular orbital) and LUMO + 1 a t the positions of preferential attack calculated by restricted Hartree-Foc k self-consistent field (RHF-SCF) methods (Figs. 9-11). Based on prediction s from molecular-orbital calculations (Fig. II) and the analysis of experim ental C-13-NMR data (Fig. 7,a), the structure of a unique hexakis-adduct (( +/-)-22, Fig. 12),prepared from (+/-)-13, was assigned. The C-2-symmetrical compound contains four 6-6-closed methanofullerene sub-structures in its p olar 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 equa tor (at C(22)- C(23) and C(26)- C(27)). The 6-5-open sub-structures are for med by malonate additions to near-equatorial 6-5 bonds with enhanced LUMO c oefficients: followed by valence isomerization (Fig. 12).