title: Cobalt-mediated cyclic and linear 2:1 cooligomerization of alkynes with alkenes: a DFT study.
abstract: The mechanism of the cobalt-mediated [2 + 2 + 2] cycloaddition of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) has been studied by means of DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct "collapse" pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co-C sigma-bond rather than inter- or intramolecular [4 + 2] cycloaddition. The resulting seven-membered metallacycle 7 is a key intermediate which leads to either CpCo-complexed cyclohexadiene 5 or hexatriene 13. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization observed occasionally in these reactions. With aromatic double bonds, a C-H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed investigations of high- and low-spin potential energy surfaces are presented. The reactivity of triplet cobalt species was found kinetically disfavored over that of their singlet counterparts. Moreover, it could not account for the formation of CpCo-complexed hexatrienes. However, triplet cobalt complexes cannot be ruled out since all unsaturated species appearing in this study were found to exhibit triplet ground states. Consequently, a reaction pathway that involves a mixing of both spin-state energy surfaces is also described (two-state reactivity). Support for such a pathway comes from the location of several low-lying minimum-energy crossing points (MECPs) of the two surfaces.
authors.affiliation: Université Pierre et Marie Curie-Paris 6, Laboratoire de Chimie Organique (UMR CNRS 7611), Institut de Chimie Moléculaire (FR 2769), case. 229, 4 place Jussieu, F-75252 Paris Cedex 05, France. gandon@ccr.jussieu.fr
authors.forename: Vincent
authors.initials: V
authors.lastname: Gandon
authors.forename: Nicolas
authors.initials: N
authors.lastname: Agenet
authors.forename: K Peter C
authors.initials: KP
authors.lastname: Vollhardt
authors.forename: Max
authors.initials: M
authors.lastname: Malacria
authors.forename: Corinne
authors.initials: C
authors.lastname: Aubert
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D000480:Alkynes / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D003198:Computer Simulation; D003500:Cyclization; D008956:Models, Chemical; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D000475:Alkenes; D000480:Alkynes; D009942:Organometallic Compounds; D003035:Cobalt