Formation of arenes and of tetracarbonyl(hexatrienediyl) dicobalt ("flyover") complexes from Co-2(CO)(8)

The cobalt carbonyl mediated cyclotrimerization of alkynes to yield both ar enes and "flyover" complexes (which contain a C6Co2 ring) has been reexamin ed with particular regard to the intermediacy of cobaltacyclopentadienes. S ince the latter can now be obtained in controlled fashion from (alkyne)Co-2 (CO)(6) and can react with a third alkyne under mild conditions by using tr imethylamine-N-oxide as promoter, the incorporation of three different alky nes into both arenes and flyovers has become possible. Cobaltacyclopentadie nes, like their precursors, catalyze alkyne cyclotrimerization, but flyover s do not do so under comparable conditions and hence are not on the main re action path to arenes. In some circumstances cleavage of cobaltacyclopentad ienes must occur to account for the observed arene products. Crystal struct ures of two flyover complexes are reported.