Cyclophane-Type Fullerene-dibenzo(18) crown-6 Conjugates with trans-1, trans-2, and trans-3 Addition Patterns: Regioselective Templated Synthesis, X-Ray Crystal Structure, Ionophoric Properties, and Cation-Complexation-Dependent Redox Behavior

摘要

The fullerene-crown ether conjugates (+-)-1 to (+-)-3 with trans-1 ((+-)-1), trans-2 ((+-)-2), and trans-3 ((+-)-3) addition patterns on the C-sphere were prepared by Bingel macrocyclization. The trans-1 derivative (+-)-1 was obtained in 30% yield, together with a small amount of (+-)-2 by cyclization of the dibenzol[18] crown-6 (DB18C6)-iethered bis-malonate 4 with C_(60) (Scheme 1). When the crown-ether tether was further rigidified by K~1-ion complexation, the yield and selectivity were greatly enhanced, and (+-)-1 was obtained as the only regioisomer in 50% yield. The macrocyclization, starting from a mixture of tethered bis-malonates with anti (4)and syn (10) bisfunctionalized DB18C6 moieties, afforded the trans-1 ((+-)-1, 15%), trans-2 ((+-)-2, 1.5%), and trans-3 ((+-)-3, 20%) isomers (Scheme 2). Variable-temperature ~1H-NMR (VT-NMR) studies showed that the DB18C6 moiety in C_2-symmetrical (+-)-1 cannot rotate around the two arms fixing it to the C-sphere, even at 393 K. The planar chirality of (+-)-1 was confirmed in ~1H-NMR experiments using the potassium salts of (S)-1, 1'-binaphthalene-2, 2'-diyl phosphate ((+-)-(S)-19) or (+-)-(1S)-camphor-10-sulfonic acid ((+-)-20) as chiral shift reagents (Fig. 1). The DB18C6 tether in (+-)-1 is a true covalent template: it is readily removed by hydrolysis or transesterification, which opens up new perspectives for molecular scaffolding using trans-1 fullerene derivatives. Characterization of the products 11 (Scheme 3) and 18 (Scheme 4) obtained by tether removal unambiguously confirmed the trans-1 addition pattern and the out-out geometry of (+-)-1. VT-NMR Studies established that (+-)-2 is a C_2-symmetrical out-out trans-2 and (+-)-3 a C_1-symmetrical in-out trans-3 isomer. Upon changing from (+-)-1 to (+-)-3, the distance between the DB18C6 moiety and the fullerene surface increases and, correspondingly, rotation of the ionophore becomes increasingly facile. The ionophoric properties of (+-)-1 were investigated with an ion-selective electrode membrane (Fig. 2 and Table 2), and K~1 was found to form the most stable complex among the alkali-metal ions. The complex between (+-)-1 and KPF_6 was characterized by X-ray crystal-structe analysis (Figs. 3 and 4), which confirmed the close langential orientation of the ionophore atop the fullerene surface. Addition of KPF_6 to a solution of (+-)-1 resulted in a large anodic shift (90 mV) of the first fullerene-centered reduction process, which is attributed to the electrostatic effect of the K~1 ion bound in close proximity to the C-sphere (Fig. 5). smaller anodic shifts were measured for the KPF_6 complexes of (+-)-2 (50 mV) and (+-)-3 (40 mV), in which the distance between ionophore and fullerene surface is increased (Table 3). The effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of (+-)-1 were investigated (Table 4). These are the first-ever observed effects of cation complexation on the redox properties of the C-sphere in fullerene-crown ether conjugates.