Reaction of Aryl and Vinyl Halides with Zerovalent Nickel-Preparative Aspects and the Synthesis of Alnusone

Zerovalent nickel complexes such as bis(1,5-cyclooctadiene)nickel and tetrakis(triphenylphosphine)nickel react rapidly with aryl and vinyl halides to produce the symmetrical coupling products, a low-temperature analogue of the Oilman reaction. The reaction proceeds through oxidative addition of the organic halide to Ni(0), and the reactivity of the Ni(II) intermediates has been examined. Arylnickel halide complexes decompose rapidly to biaryls in DMF. The coupling of simple vinyl halides proceeds with isomerization of the double bond but 3-haloacrylates give efficient coupling with retention of geometry. Cyclizations to form ortho-bridged biaryls are efficient in simple cases (6-, 7-, 8-, 9-, 10-, and 14-membered rings) but fail with an ortho-disubstituted case. The 13-membered meta-bridged cyclic biphenyl, alnusone, is prepared efficiently with the crucial aryl halide coupling to form the ring proceeding in 50% yield. A side reaction promoted by the presence of protons and with certain ortho-substituted aryl halides is reduction of the aryl halide to the arene. This process can be enhanced by deliberate addition of acid during reaction with Ni(0) and a series of aryl halides underwent successful reduction.