Methanofullerene molecular scaffolding: Towards C-60-substituted poly(triacetylenes) and expanded radialenes, preparation of a C-60-C-70 hybrid derivative, and a novel macrocyclization reaction

The synthesis of (E)-hex-3-ene-1,5-diynes and 3-methylidenepenta-1,4-diynes with pendant methano[60]-fullerene moieties as precursors to C-60-substituted poly(triacetylenes) (PTAs, Fig. 1) and expanded radialenes (Fig. 2) is described. The Bingel reaction of diethyl (E)-2,3-dialkynylbut-2-ene-1,4-diyl bis(2-bromopmpanedioates) 5 and 6 with two C-60 molecules (Scheme 2) afforded the monomeric, silyl-protected PTA precursors 9 and 10 which, however, could not be effectively desilylated (Scheme 4). Also formed during the synthesis of 9 and 10, as well as during the reaction of C-60 with the desilylated analogue 16 (Scheme 5), were the macrocyclic products 11, 12, and 17, respectively, resulting from double Bingel addition to one C-sphere. Rigorous analysis revealed that this novel macrocyclization reaction proceeds with complete regio- and diastereoselectivity. The second approach to a suitable PTA monomer attempted N,N'-dicyclohexylcarbodiimide(DCC)-mediated esterification of (E)-2,3-diethynylbut-2-ene-1,4-diol (18, Scheme 6) with mono-esterified methanofullerene-dicarboxylic acid 23; however, this synthesis yielded only the corresponding decarboxylated methanofullerene-carboxylic ester 27 (Scheme 7). To prevent decarboxylation, a spacer was inserted between the reacting carboxylic-acid moiety and the methano C-atom in carboxymethyl ethyl 1,2-methano[60]fullerene-61,61-dicarboxylate (28, Scheme 8), and DCC-mediated esterification with diol 18 afforded PTA monomer 32 in good yield. The formation of a suitable monomeric precursor 38 to C-60-substituted expanded radialenes was achieved in 5 steps starting from dihydroxyacetone (Schemes 9 and 10), with the final step consisting of the DCC-mediated esterification of 28 with 2-[1-ethynyl(prop-2-ynylidene)]propane-1,3-diol (33). The first mixed C-60-C-70 fullerene derivative 49, consisting of two methano[60]fullerenes attached to a methano[70]fullerene, was also prepared and fully characterized (Scheme 13). The C-s-symmetrical hybrid compound was obtained by DCC-mediated esterification of bis[2(2-hydroxyethoxy)ethyl] 1,2-methano[70]fullerene-71,71-dicarboxyl (46) with an excess of the C-60-carboxylic acid 28. The presence of two different fullerenes in the same molecule was reflected by its UV/VIS spectrum, which displayed the characteristic absorption bands of both the C-70 and C-60 mono-adducts, but at the same time indicated no electronic interaction between the different fullerene moieties. Cyclic voltammetry showed two reversible reduction steps for 49, and comparison with the corresponding C-70 and C-60 mono-adducts 46 and 30 indicated that the three fullerenes in the composite fullerene compound behave as independent redox centers.