Indenyl ligand

Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

In the case of catalyst 3, the thiophene substitution in the 6,7 position controls the gap aperture between fluorenyl and indenyl ligands by repulsing steric interactions at the complex backside [10, 11]. This leads to increased stereoselectivities [9] and is responsible for a C2-symmetric-like polymerization mechanism, characterized by increasing isotacticities when the polymerization temperature is reduced [5, 11] (Fig. 7). 

Figure 1.8 Approximated transition states for primary propene insertion for model complexes with (a) isopropyl-bis(l-indenyl) ligand (isospecific) for the (R, R) coordination and (b) iso-propyl(cyclopentadienyl-9-fluorenyl) ligand (syndiospecific) for the R chirality at metal atom. Corresponding preinsertion intermediates, labeled a in Figures 1.7a,b, are sketched in Figures 1.4 and 1.6a, respectively.

According to analogous molecular mechanics analyses,38 this stereoselectivity mechanism would also operate for catalytic systems with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers,39,40 like those based on two unbridged 2-phenyl-indenyl ligands.40... 

It is worth noting that the calculations on models with 3-f-butyl-l-indenyl ligand (16) predict a substantial stereoselectivity but in favor of the opposite monomer enantioface [si and re for (R, R)- and (S,. S)-coordinated K-ligand, respectively],... 

Figure 1.10 Preinsertion intermediates for secondary propene insertion into primary polypropylene chain for (a) isospecific model complex based on (R, R)-coordinatedisopropyl-bis(l-indenyl) ligand and (b) syndiospecific model complex based on isopropyl(cyclopentadienyl-9-fluorenyl) ligand for R chirality at metal atom. Stereoselectivity of isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions (cf. Figures 1.4 and 1.10a). Stereoselectivity of syndiospecific model site is in favor of same monomer prochiral face for primary and secondary insertions (cf. Figures 1.6a and 1.1 Ob).

As an example, C2-symmetric isospecific models for homogeneous catalytic systems based on the (R, / -coordinated isopropyl-bis(l-indenyl) ligand and for heterogeneous catalytic systems based on TiCLt supported on MgCl2 are compared in Figure 1.19. These models correspond to minimum-energy preinsertion intermediates calculated to be suitable for primary propene insertion... 

Z-alkenes are reduced much more slowly than are E-isomers. For instance, the (E)-(l,2)-diphenylpropene is reduced in 9 h at 65 °C under 5.3 bar, while the Z-isomer reaches only 3% conversion after 48 h at 70°C under 133 bar of H2. These data are fully consistent with the model proposed to predict the stereochemical outcome of the reaction (see Scheme 6.7), which shows that, for the Z-isomers, the RL substituent points towards the indenyl ligands in the favored pathway, thus decreasing the reaction rate. 

Scheme 6.8) the substituent (R) on the nitrogen should point as far away as possible from the indenyl ligand. This model predicts that hydrogenation of syn- and cwti-imines give rise to enantiomers of opposite stereochemistry as observed experimentally. The model also predicts that if one takes into consideration the influence of Rs and RL substituents, the energy difference of the two possible pathways should be lower for the syn imines, giving rise to lower enan-tioselectivities. 

Enantioselective conjugate addition [40] has become truly useful with the aid of dialkylzinc, cationic copper catalyst, and a chiral ligand (Eq. 1, see also Chapt. 7) [41]. Magnesium-based reagents have found use in quantitative fivefold arylation of Cgo (Eq. 10.2) [42] and threefold arylation of C70 [43], paving ways to new classes of cyclopentadienyl and indenyl ligands with unusual chemical properties. 

Short branches, specifically ethyl branches up to about 2 mol%, are formed in the polymerization of ethylene by meso-ansa zirconocenes containing unsubstituted cyclo-pentadienyl and indenyl ligands [Melillo et al., 2002]. Ethyl branches form by an isomerization process in which the usual P-hydride transfer to monomer is immediately followed by reinsertion of the vinyl-terminated polymer into the formed ethyl-zirconium bond. 

In 2003, Gimeno, Bassetti and coworkers reported an unusual diastereoselective [2 + 2] cycloaddition of two C=C bonds under mild thermal conditions (Scheme 4.18) [128]. Heating the vinylidene complexes Rul leads to the bicyclic alkylidene complexes Ru2. In 2004, Sordo and coworkers investigated the mechanism of this [2 + 2] cycloaddition theoretically [25]. With model complexes in which the indenyl ligand was modeled with a Cp ligand, two different pathways (paths a and b) were studied, shown in Scheme 4.19. Path a considers a concerted process. In the stepwise pathway (path b), the vinylidene-to-alkyne tautomerization of R1 followed by... 

When dicarbonyl( 75-indenyl)(undecamethylcyclohexasilanyl)iron is treated with (z -Pr)2NLi and subsequently with Mel, migration of the cyclosilanyl group to the 2-position of the indenyl ligand takes place110. 

A further example of a zirconacyclopentane with an indenyl ligand system is the compound rac-(ebi)Zr(C4H8) 21 (ebi = ethylenebisindenyl) prepared by the treatment of a tetrahydrofuran (THF) solution of (ebi)ZrCl2 at -78 °C with magnesium... 

As noted above the hafnacyclopentanes 23b and 23c have been obtained via the intramolecular coupling of pendant alkenyl groups tethered to indenyl ligands, with 23b being the only structurally characterised hafnacyclopentane (Scheme 9).29... 

Ferrocenyl-iminophosphines, chiral, synthesis, 6, 204 l-(Ferrocenyl)indenyl ligands, in Ir complexes, 7, 386 Ferrocenylketimines, cyclopalladated, synthesis, 8, 286... 

Indenyl-fluorenyl systems, propylene polymerization, 4, 1068 Indenylidenes, in ROMP initiation, 11, 633 7]5-Indenyl ligand, in molybdenocene dihalides, 5, 573 Indenyl ligands, in cobalt(III) complexes, 7, 20 7]5-Indenyl ligands, in rhodium alkene complexes, 7, 197 2-(Indenyl)—phenoxo complexes, with mono-Cp Ti(IV),... 

Generally, ansa-metallocenes with bridges between the ligands incorporate a-olefin comonomers better than unbridged ones. It has also been shown that benzannelation of bridged indenyl ligands increases reactivity toward a-olefins. 

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