Abstract
Oxabenzonorbornadienes 1 and 2 and azabenzonorbornadiene 3 undergo [2+2] cycloaddition with alkynes (PhC triple bond Ph, PhC triple bond CMe, PhC triple bond CCO2Et, PhC triple bond CCH(OEt)2, and HC triple bond C(CH2)4Me) in the presence of [Ni(PPh3)2Cl2], PPh3, and Zn powder in toluene to afford the corresponding exo-cyclobutene derivatives 4a-e, 5a-e, and 6 in fair to excellent yields. Under similar conditions. EtCO2C triple bond 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 4a, 4d, 4f, 6, and 14 at 500 degrees C affords dienes 13a-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 13a is readily deoxygenated by TiCl4 and Zn in THF to give a cyclooctatetraene derivative 16 in 89% yield.
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