The covalently templated bis-functionalization of C70, employing bis-malonate 5 tethered by an anti-disubstituted dibenzo[18]crown-6 (DB18C6) ether, proceeds with complete regiospecificity and provides two diastereoisomeric pairs of enantiomeric C70 crown ether conjugates, (±)-7a and (±)-7b, featuring a five o'clock bis-addition pattern that is disfavored in sequential transformations (Scheme 1). The identity of (±)-7a was revealed by X-ray crystal-structure analysis (Fig. 6). With bis-malonate 6 containing a syn-disubstituted DB18C6 tether, the regioselectivity of the macrocylization via double Bingel cyclopropanation changed completely, affording two constitutionally isomeric C70 crown ether conjugates in a ca. 1 : 1 ratio featuring the twelve (16) and two o'clock ((±)-15) addition patterns, respectively (Scheme 3). The X-ray crystal-structure analysis of the twelve o'clock bis-adduct 16 revealed that a H2O molecule was included in the crown ether cavity (Figs. 7 and 8). Two sequential Bingel macrocyclizations, first with anti-DB18C6-tethered (5) and subsequently with syn-DB18C6-tethered (6) bis-malonates, provided access to the first fullerene bis-crown ether conjugates. The two diastereoisomeric pairs of enantiomers (±)-28a and (±)-28b were formed in high yield and with complete regioselectivity (Scheme 9). The cation-binding properties of all C70 crown-ether conjugates were determined with the help of ion-selective electrodes (ISEs). Mono-crown ether conjugates form stable 1 : 1 complexes with alkali-metal ions, whereas the tetrakis-adducts of C70, featuring two covalently attached crown ethers, form stable 1 : 1 and 1 : 2 host-guest complexes (Table 2). Comparative studies showed that the conformation of the DB18C6 ionophore imposed by the macrocyclic bridging to the fullerene is not particularly favorable for strong association. Reference compound (±)-22 (Scheme 4), in which the DB18C6 moiety is attached to the C70 sphere by a single bridge only and, therefore, possesses higher conformational flexibility, binds K+ and Na+ ions better by factors of 2 and 20, respectively. Electrochemical studies demonstrate that cation complexation at the crown ether site causes significant anodic shifts of the first reduction potential of the appended fullerene (Table 3). In case of the C70 mono-crown ether conjugates featuring a five o'clock functionalization pattern, addition of 1 equiv. of KPF6 caused an anodic shift of the first reduction wave in the cyclic voltammogram (CV) by 70 to 80 mV, which is the result of the electrostatic effect of the K+ ion bound closely to the fullerene core (Fig. 14). Addition of 2 equiv. of K+ ions to C70 bis-crown ether conjugates resulted in the observation of only one redox couple, whose potential is anodically shifted by 170 mV with respect to the corresponding wave in the absence of the salt (Fig. 16). The synthesis and characterization of novel tris- and tetrakis-adducts of C70 are reported (Schemes 5 and 6). Attempts to prepare even more highly functionalized derivatives resulted in the formation of novel pentakis- and hexakis-adducts and a single heptakis-adduct (Scheme 7), which were characterized by 1H- and 13C-NMR spectroscopy (Fig. 10), as well as matrix-assisted laser-desorption-ionization mass spectrometry (MALDI-TOF-MS). Based on predictions from density-functional-theory (DFT) calculations (Figs. 12 and 13), structures are proposed for the tris-, tetrakis-, and pentakis-adducts.
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Jan 3, 2018
Daria V Berdnikova + 3
Open Access
