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‐l, 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‐bromopropane‐dioates) 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 thedesilylated 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‐l, 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 methane 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‐hydroxy‐ethoxy)ethyl] 1, 2‐methano[70]fullerene‐71, 71‐dicarboxylate ( 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.