Styrene, coordination

The coordination of styrene is expected to be strongly influenced by substituents that are neglected in the minimal QM model A. Thus, for sake of clarity, we do not present the styrene coordination results using model A. Depicted in Figure 8 are the three most stable styrene coordinated isomers, 8a-c. The coordination energies, which are also shown in Figure 8 in kcal/mol, reveal that the initial formation of the tt-complex is slow and reversible. In fact, only for isomer 8a is the styrene coordination exothermic and here it is only exothermic by 0.5 kcal/mol. Isomers 8a-c all have the olefinic bond of the styrene lying parallel to the plane defined by the P-Pd-Si atoms. No other sterically accessible isomers could be located where this bond lies parallel to this plane. Due to steric reasons, complexes with the olefinic bond perpendicular to this plane were found to be at least 8 kcal/mol less stable. 

Figure 8. Most stable styrene coordinated complexes. The styrene coordination energies are reported in kcal/mol. The coordination energies are reported as the energy of the tt-complex relative to the energy of the flee styrene molecule and the respective hydrido-silyl complexes.

The fact that isomers 8a, 8b and 8c are the lowest energy styrene coordinated complexes have potentially important ramifications that concern the modified-Chalk-Harrod mechanism and the regioselectivity observed in the hydrosilylation. With the modified-Chalk-Harrod mechanism olefin insertion into the M-Si bond follows styrene coordination. However, in all three isomers depicted in Figure 8, the coordinated hydride lies between the... 

If we were to assume that the reaction followed the Chalk-Harrod mechanism, then insertion of the olefin into the Pd-hydride bond in all three isomers 8a-c would lead to the correct regioisomer product. Thus, to some degree the regioselectivity of the hydrosilylation in this catalyst system is determined in the styrene coordination step. We will discuss the origin of the regioselectivity in more detail in Section 4. 

Systems modified by hybrid P-N ligands (68-75) were also found to be active for the copolymerization of styrene. In contrast to the N-N hgand [8, 9], their productivity is increased by increasing the pressure of carbon monoxide. In the case of the phosphine-dihydrooxazole ligands (68-72) the geometry of the ligand is very important for the steric control. The presence of only one substituent on position 4 of the dihydrooxazole ring (e.g., 69) is essential to achieve isotactic copolymerization [98]. Chirality associated with the presence of two different substituents in that position (Scheme 8.14, 72) is not sufficient to cause efficient i-enantio-face discrimination [99]. On the basis of these results, the model for styrene coordination (76) reported in Scheme 8.15 was assumed. 

Investigations on Mechanisms Initiated by Styrene Coordination and/or Chloride Dissociation... 

Styrene unit and the incoming styrene molecule. In Scheme 14.5, the incorporation of a coordinated styrene molecule is fulfilled via two simultaneous steps cis-migratory insertion, and a nucleophilic substitution of the phenyl moiety of the last unit of the growing polymer chain end by the newly incorporated unit. Sequentially after incorporating this unit, a new styrene coordinates enantioselect-ively to the metal center, leading to a mirror related active species that implies the syndioselective propagation of the polymer chain. ... 

New mechanistic insights were obtained in the rhodium(I)/aminophosphane-catalyzed styrene hydroformylation by using D2O as deuterium-labeling agent. H/D exchange of the rhodium-hydride complex and the reversibility of the styrene coordination were established. Based on the product compositions, protonolysis of the rhodium-acyl intermediate and a bimolecular reaction involving the rhodium-acyl and rhodium-hydride intermediates as the aldehyde-forming step were excluded [65]. 

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