Bridged metallocenes

Recently the ylide 16 or the corresponding protonated ligands allowed, in presence of metallated bases of groups I and II (Li, K, Ba), the synthesis of the first phosphonium bridged metallocene 17 (dicyclopentadienylide) (Scheme 12). Chiral kalocene and barocene, observed only in racemic forms, have thus been obtained [57]. 

Isospecific Polymerization by Ansa-Type (.Bridged) Metallocenes... 

Chiral C2-symmetric ansa-metallocenes, also referred to as bridged metallocenes, find extensive use as catalysts that effect asymmetric C—C bond-forming transformations [4]. In general, bridged ethylene(bis(tetrahydroindenyl))zirconocene dichloride ((ebthi)ZrCl2) 1 or its derived binaphtholate ((ebthi)Zrbinol) 2 [5] and related derivatives thereof have been extensively utilized in the development of a variety of catalytic asymmetric alkene alkylations. 

Chiral, Ci-symmetric (asymmetric) bridged metallocenes, 16 108-109 Chiral, C2-symmetric bridged metallocenes, 16 104-108 Chiral, C2-symmetric catalysts, racemic mixture of, 16 106 Chiral, C2-symmetric unbridged metallocenes, 16 108 Chiral catalysts, 16 395 Chiral centers, in biochemical compounds, 17 402... 

In the case of co-polymers with cyclic olefins other than NB, Naga et al. performed the co-polymerizations of ethylene or propylene with cyclopentene by using a number of bridged metallocene catalysts, and then investigated the peak melting temperatures and crystalline structures of the resultant co-polymers, all containing cyclopentane units. In 2004, Waymouth and Lavoie reported on the catalytic properties of a series of... 

Direct reaction of fulvenes with metal atoms appears to afford a useful synthetic route to alkyl-bridged metallocenes (122), e.g.,... 

The special interest in silicon-bridged metallocene units stems from the observation that silicon atoms possess electronic transmission characteristics of unsaturated alkene or alkyne fragments158-161. 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

In catalysts obtained from achiral non-bridged metallocenes of class I with C2v molecular symmetry (double helical), such as Cp2MtX2, the positions of the coordinated monomer and of the alkyl ligand are not chirotopic and, therefore, the catalyst control is completely lacking (Table 3.1) [68],... 

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

It should be emphasised that all open-structure isopropylidene-bridged metallocenes have much lower activities towards ethylene polymerisation... 

Introduction of another ruthenium bipyridine moiety or bridging metallocene (ferrocene, cobaltocenium) results in sensors that specifically bind chloride anions (Figure 16.15). The structural modification of the amide receptor results in a decease in cavity dimensions and significant rigidity of the macrocycle. Therefore it cannot accommodate hydrogen phosphate anions, but only much smaller CF [45, 46]. 

This reaction can operate starting from a lot of fulvenes. Some allyl-bridged metallocenes can be synthesized by an unusual intramolecular HNMe2 elimination and a fulvene coupling (equation 5). ... 

For example, the cyclocondensation of cyclopentadiene with 2,5-hexanedione in the presence of sodium affords 4,7-dimethylindene in 65 % yield. Subsequent standard transformations yield the desired bridged-metallocene structures. 

Chiral an a-metallocene complexes have become useful catalysts in asymmetric polymerization reactions [73]. While enantio-resolution of a <3-metaIlocene race-mates cannot yield more than 50% of a particular enantiomer, the readily accessible racemate of a biphenyl-bridged metallocene complex (we abbreviate to bi-phecp -M M = Ti, Zr) has been quite recently reported to give enantio-pure ansa-titanocene and -zirconocene complexes through binol-induced asymmetric trans-... 

Figure 7-16. binolL-induced a.symmetric transformation of biphenyl-bridged metallocene complexes. 

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