Kinetics, stereochemistry, and mechanism of the interconversions of cobaltaoxanorbornadiene and cobaltacyclopentene complexes

Article abstract of DOI:10.1021/om00153a031

The metallaoxanorbornadiene complexes 1-(cyclopentadienyl)-1-cobalta-5,7-dicarbomethoxy-3-methoxy-2-oxa-6-R- norbornadiene (1a, R = phenyl; 1b, R = carbomethoxy) reacted with P(C₆H₅)₃, P(OCH₃)₃, (CH₃)₃CNC, P(n-C₄H₉)₃, and n-C₃H₇NH₂ to afford the ligand-substituted cobaltacyclopentene complexes 2-6, respectively. No evidence was seen for activation of the coordinated ester group in 1. The equilibrium positions for the interconversion of cobaltacyclopentene/cobaltaoxanorbornadiene complexes were determined as a function of temperature. Thermodynamic data established that the formation of 1 was driven entirely by entropy. Kinetic measurements showed the reaction of 1 and ligand to be first order in both 1 and the ligand. Activation parameters for the reaction of 1 with P(C₆H₅)₃ and (CH₃)₃CNC were consistent with an associative mechanism in which the entering ligand displaced the coordinated ester group. Ligands attacked from the face of the metallacycle opposite the coordinated ester group. Isomeric complexes with ligand on the same face as the coordinated ester group were obtained in low yield as kinetic products of preparative scale reactions when the concentration of ligand was high and the steric bulk of the ligand was low. These complexes were observed only when intermolecular trapping of a coordinatively unsaturated cobaltacyclopentene intermediate became competitive with intramolecular trapping by the ester group. The results establish that cobaltaoxanorbornadiene complexes are the kinetically important intermediates in the reactions of cobaltacyclopentenes.