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 stereoselectivity for 1-alkene primary insertion. The chirality (si or re) of the tertiary carbon of the growing chain nearest to the metal determines, in the α-agostic-assisted transition state for monomer insertion, the preferential chain orientation around the metal-carbon bond (θ₁ ≈ 60° or θ₁ ≈ +60°, respectively). This chiral orientation of the chain, in turn, discriminates by nonbonded repulsive interactions between propene enantiofaces (re or si, respectively). Analogies and differences with respect to the well-established mechanism of chiral site controlled stereoselectivity for olefin insertion polymerizations are discussed.