Ansa-ligand-synthesis

By contrast, the synthesis of syndiotactic PP, s-PP, is generally catalyzed by Cs-symmetry ansa- metallocenes. For example, (16)/MAO affords PP with a pentad (rrrr) content of 86% at 25 °C.77 The stereoselectivity is highly sensitive to ligand variation. For example, substitution at the 3-position of the Cp ring with a methyl group affords heterotactic PP,78 whilst the Bu analog favors i-PP production.50,75,79... 

The synthesis and characterization of enantiomerically pure ansa-cyclopentadienyl organolanthanides Me2Si(ButCp)[(+)- i o-Men-Cp]Ln(CH(SiMe3)2 and their use as precatalysts for asymmetric olefin hydrogenation have been reported. In a one-pot reaction starting from 6,6-dimethylfulvene, methyllithium, and dimethyldichloro-silane the desired product Me2Si(ButCp)Cl was obtained, which was alkylated with Na[(+)-row-Men-Cp] to afford the neutral ligand. Reaction with BunLi afforded the dilithium salt as a colorless crystalline solid (Scheme 161). 

Cj-symmetric structures having one of the two Cp ligands endowed with bilateral symmetry (such as fluorenyl) have several synthetic advantages, the main being the absence of a meso-isomer (Figure 28). We recall here that the latter is instead formed as the undesired byproduct in the synthesis of the C2-symmetric chiral ansa-metallocenes. 

Although the chirality of cydophanes was recognized more than 50 years ago by Liittringhaus and Gralheer in ansa compounds [14], chiral cydophanes have been applied as auxiliaries and ligands in asymmetric synthesis only recently. Some examples of this exciting use of cydophanes will be given. 

According to an early hypothesis21, stereoregular isotactic polymerization requires the presence of chirality within the catalyst. Thus, with achiral metallocenes mostly atactic polymers are obtained. Chiral ansa-metallocenes with substituted cyclopentadienyl ligands, or especially with indenyl and tetrahydroindenyl ligands, are effective in stereoselective polymer synthesis. 

WUd, F. R. W. P Zsolnai, L. Huttner, G Brintzinger, H. H. ansa-Metallocene derivatives. IV. Synthesis and molecular structures of chiral a/wa-titanocene derivatives with bridged tetrahydroindenyl ligands. J. Organomet. Chem. 1982, 232, 233-247. 

Jany, G Fawzi, R. Steimann, M. Rieger, B. Synthesis of enantiomerically pure ethylene-bridged ansa-zirconocene and -hafnocene complexes bearing fluorenyl, indenyl, octahydroflnorenyl, and tetrahydroindenyl ligands. Organometallics 1997,16,544-550. 

Examples of metallocenes with two dimethylsilyl interannular linkers are more numerous than those composed strictly of C—C and C—H bonds. Royo first reported the synthesis of ansa-metallocenes with a [ (CH3)2Si 2(C5H3)2l ligand array. Relevant metallocene diehloride and dimethyl complexes 4a-d are depicted in Figure 4.5. X-ray erystal struetures have been obtained for dichloride complexes 4a and 4b and dimethyl complex 4c (see Table4.1). In all three struetures, the cyclopentadienyl rings are eclipsed. Despite a wealth of synthetie and crystallographie data,... 

Planar chiral cyclopentadienyhnetal complexes (M=Zr, Ti, Co, Fe, Mo) are stereoselective catalysts, which have received attention in a number of as3munetric synthetic applications [42]. The conpling of l-(p-tolyl-snlfinyl)naphthalene-2-carboxylate ester 50 with indenyl lithium 51 and fluorenyl Uthinm 52 provides access to l-(2 -methyl-3 -indenyl)naphthalene (53) and 9-(l -naphthyl)fluorene (54), respectively, as single rotamers (Scheme 8.12) [43-45]. The atropisomer-ism arises fiom hindered rotation abont the bond between the cyclopentadienyl ring and the naphthalene moiety. Ligands 53 and 54 are building blocks for the enantiospecific synthesis of planar chiral bidentate ansa zirconocene complexes 55 and 56 [46,47]. 

Introduction of bulky substituents on Cp/indenyl ligands or bridging moieties can improve the racemo/meso ratios in the synthesis of ansa-metallocenes [42,63,64,65]. Exclusive synthesis of the racemic isomer was even achieved using tert-butyl and trimethylsilyl substituents. However, polymerization using the corresponding ansa-zirconocene was not described [66], although the ansa-yttrocene complex 8 produced isotactic polypropylene [51]. 

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