Mechanism of unlike stereoselectivity in 1-alkene primary insertions: Syndiospecific propene polymerization by Brookhart-type nickel(II) catalysts

A theoretical QM/MM study of propene polymerization by a Brookhart-type Ni( II) catalyst suggests a mechanism of unlike chain end controlled stereosele ctivity for 1-alkene primary insertion. The chirality (si or re) of the ter tiary carbon of the growing chain nearest to the metal determines, in the a lpha-agostic-assisted transition state for monomer insertion, the preferent ial chain orientation around the metal-carbon bond (theta(1) approximate to -60 degrees or theta(1) approximate to 60 degrees, respectively). This chi ral orientation of the chain, in turn, discriminates by nonbonded repulsive interactions between propene enantiofaces (re or si, respectively). Analog ies and differences with respect to the well-established mechanism of chira l site controlled stereoselectivity for olefin insertion polymerizations ar e discussed.