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 (<b>1a</b>) with ((1<i>E</i>,3<i>E</i>)-hexa-1,3,5-trien-1-yl)benzene (<b>2a</b>) 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 (<b>3aa</b>), as a single diastereomer in 58% yield. According to the controlled experiments, this reaction is considered to involve <i>endo</i>-6-styrylbicyclo[3.1.0]hex-2-ene (<i>endo</i>-<b>6aa</b>) as an intermediate, followed by the spontaneous vinylcyclopropane rearrangement to give <b>3aa</b>.