Carbon-Carbon Bond Formation of α-Alkynylzirconacyclopentenes via Cyclization or a Cu/Pd-Mediated Cyclization/Cross-Coupling Sequence with Aryl Iodides

The cyclization of alpha-alkynylzirconacyclopentenes in the presence of CuCl was achieved cleanly to afford stereodefined 3-ylidenecyclopentenes. Coupling of the copper intermediates with a variety of aryl iodides catalyzed by Pd(PPh3)(4) afforded bis-arylated ylidenecyclopentenes; however, in the absence of copper salts, the coupling reaction of the same zirconacycles with aryl iodides was efficiently promoted by Pd(OAc)(2)/PPh3 in toluene to afford the monoarylated products. Activation of the central double bond of the cumulenic intermediate by a Pd(II) complex followed by nucleophilic attack of the Zr-sp(3) carbon bond to form an alkenylpalladium intermediate accounting for this novel cyclization is suggested.