[2 + 2 + 2]Cycloadditions of Alkynes to Furans and Thiophenes: A Cobalt-Mediated \"Enol Ether Walk\".
h5-Cyclopentadienylcobalt mediates the [2+2+2]cycloaddn. of two alkyne units to the 2,3-double bond in furans and thiophenes to give the corresponding complexed dihydrobenzoheterocycles, in turn capable of undergoing migration of the enol ether moiety along the periphery of the cyclohexadiene ligand. In some instances the cobalt-mediated alkyne addn. occurs by apparent C-H activation of the heterocyclopentadiene, resulting in the generation of butadienylated products.
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