Catalytic syntheses of polycyclic compounds based on norbornadiene-2,5 in the presence of nickel complexes: II. cyclic dimerization of norbornadiene-2,5. Kinetics and mechanism of the process

Kinetics of the cyclodimerization of norbornadiene-2,5 (NBD) is studied in the presence of a catalytic system based on bis(η³-allyl)nickel. The forms of the rate laws characterized by different reaction orders with respect to NBD are determined. The influence of temperature and solvent nature on the process is studied. The thermodynamic parameters are determined. The structure of the products is shown to be determined by the structure of intermediates. The mechanism of the process, consisting of the following main steps, is proposed: (1) the formation of Ni(NBD)₂, which is the true catalyst; (2) the reversible addition of NBD to the indicated complex, resulting in the formation of Ni(NBD)₃ and Ni(NBD)₄ η-complexes and accompanied by a change in ligand coordination; (3) the oxidative addition of coordinated NBD molecules to a nickel atom that gives five and six-membered metallacyclic intermediates; and (4) the reductive elimination of nickel from them to form cyclic dimers. The conditions for the selective formation of individual isomers are proposed.