Syndiotactic‐ and Isotactic Specific Bridged Cyclopentadienyl‐Fluorenyl Based Metallocenes; Structural Features, Catalytic Behavior

Summary: The stereochemistry of propylene insertion/propagation reactions with a variety of Cs and C1 symmetric, bridged cyclopentadienyl‐fluorenyl ligand containing metallocene catalysts are investigated. It is shown that active species with a local Cs symmetry and enantiotopic coordination positions, in the general contest of the chain migratory insertion, behave syndioselective. However, the bilateral symmetry rule, “central dogma” of syndioselectivity, is not a catholic prerequisite and may be ignored by the Cs systems whenever symmetric “excess” in steric bulk is present. An asymmetric “excess” of steric forces, on the other hand, will perturb the migratory insertion process completely and induce a full tactic inversion in resulting C1 symmetric catalysts. The findings also reveal that C2 symmetry and helicity of homotopic catalysts, conditions thought to be essential for high isoselectivity, can be lifted in light of recent developments in high performance centrally chiral, C1 symmetric enantiotopic catalysts. Ligand outline, chain orientation, and monomer coordination mode with enantiomorphous active species. image Ligand outline, chain orientation, and monomer coordination mode with enantiomorphous active species.