Cobalt-Catalyzed Bicycloaddition of Alkynes with Conjugated Trienes Yielding Bicyclo[3.2.1]octadienes

Cobalt-catalyzed bicycloaddition of internal alkynes with conjugated trienes leads to the formation of bicyclo[3.2.1]octadienes. As a typical example, treatment of hex-3-yne (1a) with ((1E,3E)-hexa-1,3,5-trien-1-yl)benzene (2a) at room temperature in CH2Cl2 catalyzed by [CoBr2(dppp)] (10 mol %)/Zn (40 mol %)/ZnI2 (40 mol %) produces a bicyclic product, 5,6-diethyl-4-phenylbicyclo[3.2.1]octa-2,6-diene (3aa), as a single diastereomer in 58% yield. According to the controlled experiments, this reaction is considered to involve endo-6-styrylbicyclo[3.1.0]hex-2-ene (endo-6aa) as an intermediate, followed by the spontaneous vinylcyclopropane rearrangement to give 3aa.