Nickel-promoted first ene-diyne cycloaddition reaction on C60:: Synthesis and photochemistry of the fullerene derivatives

A novel method for the construction of a fused cyclohexadiene ring on C-60 based on a nickel-promoted [2+2+2] cycloaddition of 1,6-diynes is described. Treatment of C-60 With terminal 1,6-diynes(HC equivalent to CCH2)(2)X) in the presence of NiCl2(PPh3)(2), Zn, and PPh3 at 90 OC:in toluene afforded [2+2+2] bicyclic hexadiene derivatives (X = C(CO2Me)(2) (2a), C(CO2Et)(2) (2b), C(COMe)(2) (2c), CH2 (2d), O (2e), NSO2-p-C6H5CH3 (2f), C(SO2Ph)(2) (2g), and CC(O)CH2C(CH3)(2)CH2C(O) (2h) in good yields. Spectral data for products 2a-h indicated that the cycloaddition of diynes to C-60 occurs across a 6,6-ring junction on the fullerene. On the basis of the established chemistry of metal-mediated [2+2+2] cycloaddition, a mechanism is proposed to account for the present nickel-mediated reaction. All the hexadiene derivatives 2a-h in solution are readily oxidized by molecular oxygen in the presence of Light at ambient temperature. The oxidation process of compound 2a in chloroform-d was monitored by H-1 NMR spectroscopy, and the results showed that 2a first reacted with molecular oxygen to form peroxide 3 and was subsequently converted to dialdehyde 4 and C-60- containing polymeric material. Photochemical properties of some cyclohexadiene derivatives were then investigated. Upon irradiation (350 nm), compounds 2a-c, 2f, and 2h readily underwent [4+4] cycloaddition to give the corresponding bisfulleroids 5a-c, 5f, and 5h in excellent yields.