Optically active macrocyclic cis-3 bis-adducts of C-60: Regio- and stereoselective synthesis, exciton chirality coupling, and determination of the absolute configuration, and first observation of exciton coupling between fullerene chromophores in a chiral environment

A series of optically active cis-3 bis-adducts, such as (R,R,C-f)-16 (Schem e 6), was obtained regio- and diastereoselectively by Bingel macrocyclizati on of C-60 with bis-malonates, which contain optically active tethers deriv ed from 1,2-diols. The absolute configuration of the inherently chiral addi tion pattern in cis-3 bis-adducts had previously been determined by compari son of calculated and experimental circular dichroism (CD) spectra. Full co nfirmation of these earlier assignments was now obtained by an independent method based on semiempirical AM1 ('Austin Model 1') and OM2 ('Orthogonaliz ation Method 2') calculations combined with H-1-NMR spectroscopy. It was fo und computationally that bis-malonates [CHR(OCOCH2COOEt)](2), which contain (R,R)- or (S,S)-butane-2,3-diol derivatives as optically active tethers, p referentially form out-out cis-3 bis-adducts of C-60 as a single diastereoi somer in which the alkyl groups R adopt a gauche conformation, while the tw o glycolic H-atoms are in an antiperiplanar (ap) and the eater linkages to the fullerene in a gauche relationship (Figs. 2 and 5). In contrast, in the less favorable diastereoisomer, which should not form, the alkyl groups R adopt an np and the H-atoms a gauche conformation, while the eater bridges to the fullerene remain, for geometric reasons, locked in a gauche conforma tion. According to the OM2 calculations, the geometry of the fully staggere d tether in the free bis-malonates closely resembles the conformation of th e tether fragment in the bis-adducts formed. These computational prediction s were confirmed experimentally by the measurement of the coupling constant between the vicinal glycolic H-atoms in the H-1-NMR spectrum. For (R,R,C-f )-16, (3)J(H,H) was determined as 7.9 Hz, in agreement with the ap conforma tion, and, in combination with the calculations, this allowed assignment of the C-f-configuration to the inherently chiral addition pattern. This conf ormational analysis was further supported by the regio- and diastereoselect ive synthesis of cis-3 bis-adducts from bis-malonates, including tethers de rived from cyclic glycol units with a fixed gauche conformation of the alky l residues R at the glycolic C-atoms. Thus, a bis-malonate of (R,R)-cyclohe xane-1,2-diol provided exclusively cis-3 bis-adduct (R,R,C-f)-20 in 32% yie ld (Scheme 7). Incorporation of a tether derived from methyl 4,6-O,O- benzy lidene-alpha -D-glucopyranoside into the bis-malonate and Bingel macrocycli zation diastereoselectively produced the cis-3 stereoisomer (alpha ,D,(f)A) -22 (Scheme 8) as the only macrocyclic bis-adduct. If the geometry of the a lkyl groups R at the glycolic C-atoms of the tether component deviates from a gauche relationship, as in the case of tethers derived from exo cis- and trans-norbornane-2,3-diol or from trans-cyclopentane-1,2-diol, hardly any macrocyclic product is formed (Schemes 5 and 9). The absolute configuration s of the various optically active cis-3 bis-adducts were also assigned by c omparison of their CD spectra, which are dominated by the chiroptical contr ibutions of the inherently chiral fullerene chromophore (Figs. 1, 3, and 4) . A strong chiral exciton coupling was observed for optically active macroc yclic cis-3 bis-adducts of C-60 with two appended 4-(dimethylamino)benzoate ((S,S,C-f)-26; Fig. 6) or meso-tetraphenylporphyrin ((R,R,C-f)-28; Fig.7) chromophores. Chiral exciton coupling between two fullerene chromophores wa s observed for the first time in the CD spectrum of the threitol-bridged bi s-fullerene (R,R)-35 (Fig. 9).