Ansa-Titanocene complexes

Asymmetric Catalysts. The first synthesis of a uniform elastic homopolymer of propylene was reported by Chien in 1990. " The polymer was made using an asymmetrically substituted ansa-titanocene complex 42 which was activated by MAO... 

The basic compound of Brintzinger s ansa-titanocene complexes is ethylenebis-(tetrahydroindenyl)titanium dichloride, (EBTHI)TiCl2. Further analogues ((EBTHI)TiH, (EBTHI)Ti(Me)2, and (EBTHI)Ti(CO)2) have been wddely used for asymmetric hydrogenation, hydrosilylation, and Pauson-Khand reaction (121). Novel optically active titanium complexes containing a linked amido-cyclopentadienyl ligand have been developed and used for asymmetric hydrogenation (122). 

The polymerization activities for several ansa-titanocene complexes have been reported by Miyashita, Mabika, and Suzuki [22]. The catalytic activity increases by decreasing the bite angle, the angle of the Cp centroid-Tl-Cp centroid (Table 2.5). The activities of ansa-titanocene complexes are lower than those of the monocyclopentadienyl complexes. 

Recently an ansa-titanocene dicarbonyl complex containing a siloxy bridge has been reported by Curtis et al. (78). The reaction sequence is shown below. 

Alkenes without functional groups are difficult to hydrogenate enantioselec-tively with noble-metal catalysts. Titanium complexes with a C2 symmetric chiral ligand framework, as in the ansa-titanocene (22-XVI), reduce aryl-substituted C=C bonds in very high isolated and optical yields 19... 

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-... 

The reaction of (EBI)H2 [EBI = ethylene-1,2-bis(indenyl)] and titanium amides did not give the expected ansa-titanocene products. Even with the azetidine complex, Ti(NC3H6)4 reacts with only a single amine elimination to give the mono(indenyl) derivative (C9H6CH2CH2C9H7)Ti(NC3H6)3. By contrast, -zirconocene and hafnocene are easily obtained by the amine elimination process. It is likely that increased steric crowding around the smaller Ti disfavors the second amine elimination.674... 

The first example of catalytic asymmetric hydrogenation of N,N dialkyl enamines was reported by Buchwald and Lee in 1994. By using 5 mol% chiral ansa titanocene catalyst [(S,S,S) (EBTHI)TiO binaphtho] (EBTHI = ethylenebis(tetrahydroindenyl)), they achieved excellent enantioselectivities (up to 98% ee) in the hydrogenation of (1 arylvinyl)amines [50]. In 2000, Boner used chiral rhodium diphosphine complexes for the hydrogenation of 2 N piperidinylethylbenzene and 2 alkyl 1,3,3 trimethyle neindoline and obtained the tertiary amines in moderate enantiomeric excesses [51]. 

Ansa-metallocene complexes have been studied for olefin polymerization, e.g., polymerizing i-PP with an ansa-titanocene. The bridged ligands change the catalysts and the polymer structures, also affecting MW and MWD... 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

With ansa(chiral) titanocenes, zirconocenes, and hafnocenes in combination with methylalumoxane (MAO) it is possible to obtain highly isotactic polypropene [366-374]. When changing the symmetry of the complex, different structures of the polypropene are yielded. The activity of these hydrocarbon soluble catalysts are extremely high. 

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