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Density Functional Theory Investigation into the Stereocontrol of the Syndiospecific Polymerization of Propylene Catalyzed by C s -Symmetric Zirconocenes

2007; American Chemical Society; Volume: 26; Issue: 8 Linguagem: Inglês

10.1021/om060786v

1520-6041

Simone Tomasi, Abbas Razavi, Tom Ziegler,

Organoboron and organosilicon chemistry

The mechanisms of the two most likely routes for the generation of stereodefects in the syndiospecific polymerization of propylene, mediated by the Cs-symmetric ansa-zirconocenes Me2CCpFluZrR+X- (1), Me2CCp(3,6-di-Me-Flu)ZrR+X- (2a), Me2SiCp(3,6-di-Me-Flu)ZrR+X- (2b), and Me2CCp(3,6-di-tBu-Flu)ZrR+X- (3) (X- = MeB(C6F5)3-, FAl(Bipf)3-, MAOMe-) are explored using density functional theory. Site epimerization events (also known as back-skip or missed insertion) generate defective single m dyads, while enantiomeric misinsertions are the main source of double mm triads. The most important factor affecting the frequency of enantiomeric misinsertion is the structure of the ligand, while the counterion and solvation have little effect. Site epimerization is also influenced by the nature of the ligand system, but the bulk and the charge distribution of the anion are more prominent factors. The inclusion of solvent effects in the modeling of the site epimerization is critical for explaining reactivity and to achieving at least qualitatively correct results. Application of the two-parameter probabilistic scheme developed by Farina and co-workers allows for the prediction of pentad distributions from the computed selectivities. The good agreement found between experiment and computations for the basic unsubstituted system invented by Ewen and Razavi validates the proposed model. Thus, the computed pentad distributions may be used for assessing the effectiveness of modifications to the ligands on syndioselectivity. Increasing the bulk of 3,6-substituents on the fluorenyl ligand leads to increased syndiotacticity, without significantly affecting the reactivity. By introducing a stereogenic center on the alkyl chain, it is possible to assess the influence of chain end control on stereoselectivity. The main effect of chain end control is to increase the stereoselectivity after a stereoerror.