Cobalt Metallacycles. 11. On the Transformation of Bis(acetylene)cobalt to Cobaltacyclopentadiene

Article abstract of DOI:10.1021/ja00345a039

Formation of η⁵-cyclopentadienyl(triphenylphosphine)cobaltacyclopentadienes (4) by the reaction of acetylenes with (η⁵-cyclopentadienyl)(triphenylphosphine)(acetylene)cobalt (1) has been investigated in detail.Kinetic studies indicate the intermediacy of (η⁵-cyclopentadienyl)bis(acetylene)cobalt (2), which cyclizes to coordinatively unsaturated η⁵-cyclopentadienylcobaltacyclopentadiene (3) by a spontaneous oxidative coupling reaction.Regioselectivity of the cyclization process is controlled by the steric factor of substituents rather than their electronic factor.The structures and bonding of intermediates 2 and 3 are studied with ab initio molecular orbital calculations.The transition state of the cyclization reaction is postulated to be a low-symmetry C_s conformation derived from an ''upright'' bis(acetylene)cobalt.This accounts for the observed regioselectivity that the acetylenic carbon bearing a bulky substituent becomes the α-carbon of the metallacycle.