Ansa-metallocenes group

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... 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

Some of the drawbacks of the metallocene catalysts are their limited temperature stability and the production of lower-molecular-weight materials under commercial application conditions. It follows that they have a limited possibility for comonomer incorporation due to termination and chain-transfer reactions prohibiting the synthesis of block copolymers by sequential addition of monomers. This led to the development of half-sandwich or constrained geometry complexes, such as ansa-monocyclopentadienylamido Group IV complexes (67) 575,576... 

The butadiene ligand in the Group 4 metallocene complexes is very reactive towards olefins attached at the Cp rings. A typical example is the system 9 that instantaneously reacts further to yield 10 when generated from 8 (see Scheme 4). In contrast, the unsaturated (butadiene)ansa-metallocene complexes s-cis-13 and s-trans-13 are stable when prepared from 12 under similar conditions.15 Complex 12 was obtained by a ring closing olefin metathesis reaction from 11. 

The (s-cA-butadiene) Group 4 metallocenes adopt a a2,7i-type structure. The actual strength of the n-bonding component and, hence, the metallacyclopentene character of the complexes depends very much on the substitution pattern of the diene ligand6 and it is also strongly influenced by the nature of the bent metallocene unit. These various influences were recently analyzed for some ansa-metallocene/ 1,3-diene combinations by means of computational chemistry,83,84 and the results were compared with the dynamic features (AGfw of the ring-flip inversion process, solid... 

These bridged bis(cyclopentadienyl) samarium complexes exhibit various structures with regard to the bridging group and the position of substituents on the Cp rings.622 ansa-Metallocenes of the formula (77S 77S-C24H16)Ln(THF)2... 

Group IV Zwitterion ANSA Metallocene (ZAM) Catalysis for a Olefin Polymerization. US Patent 5,939,503, 1999. 

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]. 

Boron-bridged group-4 ansa-metallocene complexes 01EJI321. 

Ansa-metallocenes tethered to a silica surface through the bridging group have been prepared by reacting ClMeSi(Cp)2MCl2 (M = Zr, HI) with partially dehydroxylated silica, either by itself or modified by agents such as (MeO)3Si(CH2)3NH2. The catalyst activity in ethylene-1-hexene copolymerization is... 

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