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≡CPh, PhC≡CMe, PhC≡CCO2Et, PhC≡CCH(OEt)2, and HC≡C(CH2)4Me) in the presence of [Ni(PPh3)2Cl2], PPh3, and Zn powder in toluene to afford the corresponding exo-cyclobutene derivatives 4 a–e, 5 a–e, and 6 in fair to excellent yields. Under similar conditions, EtCO2C≡CCO2Et does not react with 1 in toluene to give the [2+2] cycloaddition product, but in acetonitrile, the catalytic [2+2] cycloaddition proceeds and cycloadduct 4 f is isolated in 83 % yield. At high temperature, these cyclobutene derivatives readily undergo ring expansion to yield the corresponding 8-membered carbocyclic dienes. Thus, flash vacuum pyrolysis of 4 a, 4 d, 4 f, 6, and 14 at 500 °C 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. Compound 13 a is readily deoxygenated by TiCl4 and Zn in THF to give a cyclooctatetraene derivative 16 in 89 % yield.