First enantiomerically pure C₇₀-adducts with a non-inherently chiral addition pattern

Abstract

By addition of an enantiomerically pure nitrile oxide to C 70 , four chiral isoxazolo [70]fullerene isomers were prepared and isolated in pure state: two constitutional isomers resulting from addition to one of the bonds radiating from the polar pentagons, and a constitutionally different pair of diastereoisomers resulting from addition to a hexagon-hexagon fusion site parallel to the equator of the fullerene. After elimination of the stereogenic center from the side chain, the former two adducts become achiral, while the latter two are separately transformed into enantiomeric fullerene derivatives with a non-inherently chiral addition pattern as sole stereogenic element. In the present work, chiral [70]fullerene derivatives with a non-inherently chiral functionalisation pattern were isolated in enantiomerically pure form for the first time. Their absolute configuration was assigned based on the good agreement between experimental circular dichroism (CD) data and CD spectra calculated using the ZINDO method.

Chiral Molecular Science: How were the absolute configurations of chiral molecules determined? “Experimental results and theories”

“…There are also other semiempirical MO methods, e.g., CNDO/S (Complete Neglect of Differential Overlap / Spectra), and ZINDO (Zerner Intermediate Neglect of Differential Overlap), which have been used for the calculation of CD and UV spectra of various chiral compounds: for some application examples, see Refs. .…”

Reductive Activation of C70 Equatorial Carbons and Structurally Characterized C70 δ-Adduct with Closed [5,6]-Ring Fusion

“…The chemistry of fullerenes is governed by the release of the strain energy caused by the deviation from the planarity of sp 2 -hybridized carbon atoms. , The principle is well observed for the reactions of C 70 , where the more strained carbons in the polar region (types a – c in Figure ) are more reactive with preferential formation of derivatives at the most and second most curved bonds (α- and β-bonds, Figure ) for ortho-addition reactions. − In contrast, the less strained carbon atoms in the flat equatorial region of C 70 are inert with very limited reports so far on the addition at the δ-bond. ,, Notably, the δ-adduct is the fifth most stable isomer predicted theoretically using C 70 H 2 as a model, next only to the α- and β-adducts and two para isomers with additions at d , d -carbons across the equator and a , c -carbons, which have all been obtained in significant amounts, − ,,, suggesting that the unavailability of the δ-isomer is likely due to the kinetic rather than thermodynamic factor.…”