Catalytic Carbon−Carbon Bond Activation and Functionalization by Nickel Complexes

The nickel alkyne complexes (dippe)Ni(PhC⋮CPh), 1, (dippe)Ni(MeC⋮CMe), 2, (dippe)Ni(MeO₂CC⋮CCO₂Me), 3, (dippe)Ni(CH₃OCH₂C⋮CCH₂OCH₃), 4, and (dippe)Ni(CF₃C⋮CCF₃), 5, were synthesized (dippe = bis(diisopropylphosphino)ethane) and characterized by ¹H, ³¹P, and ¹³C{¹H} NMR spectroscopy. Complexes 1, 2, and 3 were characterized by X-ray crystallography. The thermolysis of complex 1 or 2 (120 °C) in the presence of excess biphenylene and excess alkyne results in very slow catalytic formation of the corresponding 9,10-disubstituted phenanthrene. However, addition of ∼6 mol % O₂ (based on the metal complex) to the reaction mixture results in an acceleration in catalysis at lower temperatures (∼70−80 °C). The thermolysis of complexes 3 or 4 with excess biphenylene and excess alkyne leads to the alkyne cyclotrimerization product as the major organic species formed in the reaction. Fluorenone was catalytically produced by heating (dippe)Ni(CO)₂, biphenylene, and CO. Catalytic insertion of 2,6-xylylisocyanide into the strained C−C bond of biphenylene was also achieved by heating (dippe)Ni(2,6-xylylisocyanide)₂, excess biphenylene, and 2,6-xylylisocyanide. Mechanistic schemes are proposed for these reactions.