Carbon-carbon bond-forming reactions of cobaltacyclopentene/cobaltaoxanorbornadiene complexes with isocyanides, carbon monoxide, and alkynes

Article abstract of DOI:10.1021/om00095a010

Reaction of (η⁵-C₅H₅)Co[C(R)C(CO₂CH ₃)CH(CO₂CH₃)CH(CO₂CH ₃)][P(C₆H₅)₃] (R = C₆H₅, 2a; R = CO₂CH₃, 2b) with isocyanides, CO, and diphenylacetylene (DPA) afforded 1-amino-1,3-cyclopentadienes, cyclopent-2-enones, and (η⁵-C₅H₅)Co(η ⁴-1,3-cyclohexadiene) complexes, respectively. The related [2.2.1]-metallabicyclic cobaltaoxanorbornadiene complexes 1a and 1b reacted with the same reagents to afford identical products. Reactions with isocyanides and CO proceeded in two distinct steps: (1) co-ordination of the reactant to cobalt to form a new ligand-substituted cobaltacyclopentene complex and (2) insertion of the reactant into the metallacycle, followed by reductive elimination. Intermediates were not detected in the reactions with DPA. However, the isolation of isomeric η⁴-cis-5,6-dicarbomethoxy-1,3-cyclohexadiene complexes with CpCo coordinated on different faces of the diene suggested the intermediacy of a metallacycloheptadiene complex. Kinetic measurements established that the reaction of 1a with DPA was first order with respect to both reagents. A rate constant for the reaction of 2a with DPA was calculated from our data and the known equilibrium constant between 2a, 1a, and P(C₆H₅)₃. The agreement of the calculated rate with a published rate for the reaction confirmed that 1a is the kinetically significant intermediate in the carbon-carbon bond-forming reactions of 2a.