Metallocene ligands

It is noteworthy that a clear enhancement of selectivity for the linear hydro-formylation product is observed only with cdpp (Table 5.2-1, entry e). With all other ligands, the n/iso ratios are in the 2 to 4 range. While this is in accordance with known results in the case of PPI13 (entry a) and dppe (entry c) (in comparison to monophasic hydroformylation [69]) and also with reported results in the case of Natppts (entry b in comparison to the biphasic hydroformylation of 1-pentene in [BMIM][PF(3] [46]), it is more remarkable for the bidentate metallocene ligand dppf... 

Ionic States Resulting From Lower Energy Ionizations of Open-Shell Metallocenes, Ligand Field Energies and Experimental Ionization Energies (eV)... 

These simple first-order relationships confirm that the heteroferricinium SOMO has little interaction with the metallocene ligands [49]. 

Brintzinger and coworkers produced a titanium complex with a binaphthyl linked ansa metallocene ligand [56[. Upon activation with nBuLi, this complex catalyzed the reductions of B and 2 phenyl 1 pyrroline in 76 and 98% ee, respectively. Titanium catalysts with chiral amido cyclopentadienyl ligands have also been reported, but were considerably less stereoselective than the metallocene based ones [57]. 

A very different type of catalyst was developed by Buchwald et al. [6] the chiral Ti complex with Brintzinger s ansa-metallocene ligand (ebthi) is extraordinarily effective for the enantioselective hydrogenation of cychc imines with high optical yields (>97% ee). Unfortunately, the activity and productivity of this Ti catalyst are relatively low compared to Rh- and Ir-diphosphine catalysts. The stereochemical outcome of the reaction can be predicted by straightforward steric arguments. Acyclic imines are reduced with lower enantioselectivity, probably due to isomerization problems and the presence of syn/anti isomers. Studies with multifunctional imines or in presence of other substrates revealed that the scope of the Ti-ebthi catalyst is rather Hmited. Partial or total catalyst inhibition is observed in presence of most functional groups, expect amines, alcohols, acetals, and halides [39]. 

Cyclopentadiene, whose deprotonation in the presence of a base gives the relatively stable cyclopentadienyl anion, has been found to be a suitable substrate for palladium-catalyzed arylation [74, 75]. By using excess aryl bromides, it is completely arylated to produce pentaarylcyclopentadienes (Eq. 29). Metallocenes, typically zirconocene dichloride, are perarylated similarly [74, 76]. Pentaarylcyclopentadienes have been applied as the components of electroluminescent devices [77] and bulky metallocene ligands [78].Di-tcrt-butylphos-phinoferrocene is also pentaarylated efficiently (Eq. 30) [79]. In this case, another mechanism other than Scheme 1 (B) seems to be involved possibly coordination-assisted palladation participates (see Sect. 3.2). 

Natppts (entry b in comparison to the biphasic hydroformylation of 1-pentene in [BMIM][PF6] [6]), it is more remarkable for the bidentate metallocene ligand dppf... 

A cationic metallocene ligand with sterically dissimilar cyclopentadienyl rings joined to a positively charged coordinating transition metal atom, and a stable noncoordinating counter anion for the metallocene cation... 

Effect of Metallocene Ligand Structure on Chain Transfer Reactions in Propylene Polymerization at 40-60 °C with MAO Cocatalyst... 

The same workers have also addressed the issue of stereoselectivity by computing competing pathways for MMA polymerization initiated by the isoselective cyclopentadienyl-ania-indenyl complex 34. °° In the first phase of this study, the geometry of the metallacyclic enolate and the direction of monomer approach were examined. The authors concluded that the incoming MMA monomer prefers to approach the metal center from the same side as the indenyl unit (i.e., approach modes 34.1 and 34.3 in Figure 23.11) since this forces the propagating metallacyclic enolate to move away from the more hindered side of the an a-metallocene ligand. In addition, in order for facile C-C bond formation to occur with the monomer, the enolate C=C bond must be proximal, that is, also on... 

A model assuming that Cp substituents distal to the bridge experience steric non-bonded contacts with the monomer methyl group, perhaps mediated by the chain end, accounts for the specificity of the chiral metallocenes that produce isotactic, atactic, syndiotactic, hemiisotactic, and random or block cotactic polypropylenes. The tacticities as well as the microstructures of these polymers are accomodated by these simple concepts, the geometry of the metallocene ligands, and by generally accepted fundamental aspects of the polymerization and stereochemical control mechanisms. 

---