Ansa catalyst

A very interesting FLP catalyst design was recently disclosed by Repo, Rieger and co-workers [38], Coined the ansa-aminoborane [39] FLP catalyst, it is based on the use of amines instead of phosphines as the Lewis basic component (Scheme 6.19). This catalyst is also active in 10 mol% quantities for the reduction of imines and enamines. The X-ray structure of the ansa-catalyst is illustrated below (Fig. 6.6). 

Scheme 6.19 Metal-free hydrogenation of imines and enamines using the ansa-catalyst 108...

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

Ansa-metallocenes with a bridged tethered donor functionality, R(Me)Si(C5Me4)2Ln CH(SiMe3)2 (Ln = Y, Sm, R = (CH2)nOMe), have been synthesized [139]. Use of these new catalyst precursors results in a significantly enhanced activity (2- to 5-fold) while the diastereoselectivity (e.g., for the IH of 2-amino-5-hex-... 

PP synthesized using TiCl4/Et3Al is mostly isotactic, but two minor fractions are also produced. One is a soluble, atactic PP, whilst the other fraction is a partially crystalline, elastomeric stereoblock of iso- and a-tactic PP sequences.98 Elastomeric PP may also be prepared using the ansa-titanoccnc complex, (26), (although this catalyst does undergo rapid deactivation).99 Stereoblock formation was attributed to an equilibrium mixture of slowly interconverting isospecific and aspecific catalyst sites. Other stereoblock PP materials have been prepared via chain transfer between two catalysts of different stereoselectivities.101,102... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

The asymmetric hydrogenation of acyclic imines with the ansa-titanocene catalyst 102 gives the chiral amines in up to 92% ee.684,685 This same system applied to cyclic imines produces the chiral amines with >97% ee values.684,685 The mechanism of these reductions has been studied 686... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

Chien JCW, Babu GN, Newmark RA, Cheng HH, Llinas GH (1992) Microstructure of elastomeric polypropylenes obtained with nonsymmetric ansa-titanocene catalysts. Macromolecules 25 7400-7402... 

The hydrogenation of imines is typically carried out with 1 mol% of the lantha-nocene catalyst under an H2 pressure of 13 bar at 90°C [127]. The best catalysts are based on Sm having Cp ligands, the ansa systems being unreactive. The rate and the total conversion are improved by the addition of PhSiH3, probably because it stabilizes the system (Table 6.16), and both are very sensitive to the... 

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. 

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. 

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),... 

The asymmetric version of the reaction utilizes Brintzinger s ethylene-bis(tetrahydroin-denyl) (EBTHI) ansa-metallocene approach [85]. Whereas complex B (Scheme 8.46) is still a catalyst for the Diels—Alder reaction, only low inductions are observed at room temperature. On cooling to —78 °C and using more reactive starting materials, a maximal induction of 52.4% ee was attained [86]. 

ANOVA (analysis of variance), commercial experimental design software compared, 8 398t Anoxic conditions, defined, 3 757t ansa-metallocenes, 16 90, 94 ansa-zirconocene catalysts,... 

Zinifex Ltd., 26 564 Zinin reduction, 2 490-491 Zircaloy tubes, 17 573 ansa-Zirconocene catalysts, C2-symmetrical, 16 114 ansa-Zirconocene complexes, 16 107 Zircon, 26 621-622... 

Constrained-geometry catalysts for C2H4 polymerization 88 that are counterparts of well-known ansa-metallocene systems have been prepared and shown to be active, in combination with MAO, toward polymerization of ethylene the product is almost entirely polyethylene, with ca. 1% of 1-octene obtained. The titanium complex was found to be four times as active as the zirconium species.1... 

In a related fashion, the resonance at 8 79.4 from the Zr(ansa-Fluorenyl) polymerization catalyst precursor 64, is consistent with substantial sp character at this carbon [2]. 

Finally, Brintzinger s synthesis of stereo-rigid ansa-metallocenes [18,19] provided the basis for modern high-activity metallocene catalysts and greatly increased their economic potential. However, although MAO activation and stereo-rigid structures of the new metallocene systems were extremely successful, the complexity of these catalytic systems was such that precise information about the structure of the active species remained elusive. 

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... 

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