Synthesis, separation, and characterization of optically pure C-76 mono-adducts

Nucleophilic Bingel cyclopropanation of D-2-C-76 with bis[(S)-1-phenylbutyl] 2-bromomalonate in toluene in the presence of base yielded three constitutionally isomeric pairs of diastereoisomeric mono-adducts together with one other constitutional isomer. All seven mono-adducts were isolated in optically pure form by prep. HPLC on a (S,S)-Whelk-O1 chiral stationary phase. They represent the first optically pure adducts of an inherently chiral fullerene. Characterization by UV/VIS, CD, C-13- and H-1-NMR spectroscopy allowed identification of pairs of stereoisomers and symmetry assignments: the two pairs of diastereoisomers which were isolated as the major product possess C-1 symmetry, whereas the third pair of diastereoisomers, which is a minor product, is C-2-symmetrical. The circular dichroism spectra of the optically active C-76-adducts showed very pronounced Cotton effects resulting from strong chiroptical contributions of the chiral fullerene chromophore with the maximum observed de values being twice as high than those previously measured for optically active adducts of achiral fullerenes with a chiral addition pattern. Whereas the regioselectivity of mono-additions to C-70 correlates with the degree of local bond curvature and the regioselectivity of multiple Bingel cyclopropanations of C-60 with electronic parameters such as coefficients of the lowest unoccupied molecular orbital (LUMO), no such simple predictive correlations exist for the nucleophilic addition to C-76. Despite full spectral characterization, an unambiguous structural assignment of the isolated compounds was not possible, except for the two C-2-symmetrical isomers. Based on considerations of local bond curvature and the previous experiences with the chemistry of C-70, the structures of the C-2-symmetrical stereoisomers were assigned as (S,S,C-f)-3 and (S,S,(f)A)-3, resulting from addition to the polar alpha-type C(1)-C(6) bond.