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- Title
Achiral and Chiral Higher Adducts of C<sub>70</sub> by Bingel Cyclopropanation.
- Authors
Herrmann, Andreas; Rüttimann, Markus W.; Gibtner, Thomas; Thilgen, Carlo; Diederich, François; Mordasini, Tiziana; Thiel, Walter
- Abstract
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).
- Publication
Helvetica Chimica Acta, 1999, Vol 82, Issue 2, p261
- ISSN
0018-019X
- Publication type
Article
- DOI
10.1002/(SICI)1522-2675(19990210)82:2<261::AID-HLCA261>3.0.CO;2-X