Reversible Template-Directed Activation of Equatorial Double Bonds of the Fullerene Framework: Regioselective Direct Synthesis, Crystal Structure, and Aromatic Properties of T_h-C_(66)(COOEt)_(12)

摘要

Multiple additions to the f'ullerene core are remarkabh re-gioselective. By successive cyclopropanations of equatorial (e) 6-6 double bonds we were able to synthesize the T_h symmetric hexaadduct C_(66)(COOEt)_(12) (1). The preferred attack of 6-6 double bonds in c positions (Fig. 1) relative to addendsalready bound to the fullerene core can be explained by a consideration of both the thermody-namic stability of the different re-gioisomers and the enhanced orbital coefficients of frontier or-bitals in e positions of the corresponding precursor molecules. Meanwhile, other hexaadducts with an analogous addition pattern were synthesized by Diederich et al., who used spacer-controlled functionaliza-tion. and by Krautler et al. through six [4 + 2] cycloaddi-tions. The facile access to high yields of stableand highly sym-- metric oligoadducts of C_(60). which can be further funciionaiized, plays a very important role for example in the field of material science and biological applications of f'ullerene derivatives and is still one of the major synthetic challenges in fullerene chemistry. In this work we report for the first time on a new and very simple concept of a template-directed activation of the C_(60) framework, which allows the direct synthesis of 1 with very high regioselectivity and yields of up to 50%, as well as on its crystal structure and enhanced aromatic character relative to C_(60). Furthermore, we show that the stable compound 1 can be transformed quantitatively into the corresponding hexamalonic acid 2, which is of great importance with regard to modifications of the side chain.