Portal de Periódicos da CAPES

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

2004; Wiley; Volume: 205; Issue: 3 Linguagem: Inglês

10.1002/macp.200300153

1521-3935

Abbas Razavi, Vincenzo Bellia, Yves De Brauwer, Kai Hortmann, Liliane Peters, Sabine Sirole, Stephan Van Belle, Ulf Thewalt,

Synthetic Organic Chemistry Methods

Abstract Summary: 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. 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.