Nickel-catalyzed [2+2] cycloaddition of alkynes with activated cyclic alkenes: Synthesis and novel ring expansion studies of cyclobutene products

Oxabenzonorbornadienes 1 and 2 and azabenzonorbornadiene 3 undergo [2 + 2] cycloaddition with alkynes (PhC drop CPh, PhC drop CMe, PhC drop CCO2Et, Ph C drop CCH(OEt)(2), and HC dropC(CH2)(4)Me) in the presence of [Ni(PPh3)(2) Cl-2], PPh3, and Zn powder in toluene to afford the corresponding exo-cyclo butene derivatives 4a-e, 5a-e, and 6 in fair to excellent yields. Under sim ilar conditions, EtCO2C drop CCO2Et does not react with 1 in toluene to giv e the [2 + 2] cycloaddition product, but in acetonitrile, the catalytic [2 + 2] cycloaddition proceeds and cycloadduct 4f is isolated in 83% yield. At high temperature, these cyclobutene derivatives readily undergo ring expan sion to yield the corresponding 8-membered carbocyclic dienes, Thus, flash vacuum pyrolysis of 4a, 4d, 4f, 6, and 14 at 500 degreesC affords dienes 13 a-d and 15 in 70-96% yields. This interesting ring expansion may be viewed as the insertion of an alkyne moiety into the carbon-carbon double bond of a cyclic olefin resulting in the enlargement of the ring by two carbons. Co mpound 13a is readily deoxygenated by TiCl4 and Zn in THF to give a cyclooc tatetraene derivative 16 in 89% yield.