Reaction Behavior of Cumulene: Diels–Alder, Friedel–Crafts, and Pd-Catalyzed Domino Reactions

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Diels―Alder, Friedel―Crafts, and domino reactions mediated by Pd-catalyst of [3]cumulene 1 were investigated. Although the Diels―Alder reaction with cyclopentadiene occurred selectively on the C2, C3 double bond to give tetrasubstituted allenyl products, the Friedel―Crafts type addition of electron-rich heteroaromatics and subsequent protonation cleanly occurred on the C3, C4 double bond to afford tetrasubstituted conjugated dienes. In the Heck reaction with aryl iodide, the arylpalladium species was selectively inserted into the C3, C4 double bond and a subsequent C―H activation reaction with a neighboring phenyl group gave indene derivatives with a tetrasubstituted olefin moiety. Furthermore, Pd-catalyzed domino cyclization―allylation reaction of in situ generated [3]cumulene alkoxide afforded tetrasubstituted furan.