BIS-ADDUCTS THROUGH TETRAKIS-ADDUCTS OF C-60 BY REVERSIBLE TETHER-DIRECTED REMOTE FUNCTIONALIZATION AND SYSTEMATIC INVESTIGATION OF THE CHANGES IN FULLERENE PROPERTIES AS A FUNCTION OF DEGREE, PATTERN, AND NATURE OF FUNCTIONALIZATION

By the tether-directed remote functionalization method, a series of bi s- to hexakis-adducts of C-60, i.e., 1-7 (Fig. I), had previously been prepared with high regioselectivity. An efficient method for the remo val of the tether-reactive-group conjugate was now developed and its u tility demonstrated in the regioselective synthesis of bis- to tetraki s(methano)fullerenes (= di- to tetracyclopropafullerenes-C-60-I-h) 9-1 1 starting from 4, 5, and 7, respectively (Schemes 2, 4, and 5). This versatile protocol consists of a O-1(2) ene reaction with the two cycl ohexene rings in the starting materials, reduction of the formed mixtu re of isomeric allylic hydroperoxides to the corresponding alcohols, a cid-promoted elimination of H2O to cyclohexa-1,3-dienes, Diels-Alder, addition of dimethyl acetylenedicarboxylate, retro-Diels-Alder, additi on, and, ultimately, transesterification. In the series 9-11, ail meth ano moieties are attached along an equatorial belt of the fullerene. S tarting from C-2 upsilon-symmetrical tetrakis-adduct 15, transesterifi cation with dodecan-1-ol or octan-1-ol yielded the octaesters 16 and 1 7, respectively, as noncrystalline fullerene derivatives (Scheme 3). T he X-ray crystal structure of a CHCl3 solvate of 11 (Fig. 3) showed th at the residual conjugated pi-chromophore of the C-sphere is reduced t o two tetrabenzopyracylene substructures connected by four biphenyl-ty pe bonds (Fig. 5). In the eight six-membered rings surrounding the two pyracylene(= cyclopent[fg]acenaphthylene) moieties, 6-6 and 6-5 bond- length alteration (0.05 Angstrom) was reduced by ctr. 0.01 Angstrom as compared to the free C-60 skeleton (0.06 Angstrom) (Fig. 4). The crys tal packing (Fig. 6) revealed short contacts between Cl-atoms of the s olvent molecule and sp(2)- and sp(3)-C-atoms of the C-sphere, as well as short contacts between Cl-atoms and O-atoms of the EtOOC groups att ached to the methane moieties of 11. The physical properties and chemi cal reactivity of compounds 1-11 were comprehensively investigated as a function of degree and pattern of addition and the nature of the add ends. Methods applied to this study were UV/VIS (Figs. 7-11), IR, and NMR spectroscopy (Table 2), cyclic (CV) and steady-state (SSV) voltamm etry (Table 1), calculations of the energies of the lowest unoccupied molecular orbitals (LUMOs) and electron affinities (Figs. 12 and 13), and evaluation of chemical reactivity in competition experiments. It w as found that the properties of the fullerene derivatives were not onl y affected by the degree and pattern of addition but also, in a remark able way, by the nature of the addends (methane vs. but-2-ene-1,4-diyl ) anellated to the C-sphere. Attachment of multiple methane moieties a long an equatorial belt as in the series 8-11 induces only a small per turbation of the original fullerene pi-chromophore. In general, with i ncreasing attenuation of the conjugated fullerene pi-chromophore, the optical (HOMO-LUMO) gap in the UV/VIS spectrum is shifted to higher en ergy, the number of reversible one-electron reductions decreases, and the first reduction potential becomes increasingly negative, the compu ted LUMO energy increases and the electron affinity decreases, and the reactivity of the fullerene towards nucleophiles and carbenes and as dienophile in cycloadditions decreases.