Syndiotactic- and isotactic specific bridged cyclopentadienyl-fluorenyl based metallocenes; Structural features, catalytic behavior

The stereochemistry of propylene insertion/ propagation reactions with a variety of C-s and C-1 symmetric, bridged cyclopentadienyl-fluorenyl ligand containing metallocene catalysts are investigated. It is shown that active species with a local C-s 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 C-s 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 C-1 symmetric catalysts. The findings also reveal that C-2 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, C-1 symmetric enantiotopic catalysts.