Metallocenes, ansa metallocene-catalyzed

Llinas GH, Dong SH, Mallin DT, Rausch MD, Lin YG, Winter HH, Chien JCW (1992) Crystalline-amorphous block polypropylene and nonsymmetric ansa-metallocene catalyzed polymerization. Macromolecules 25 1242-1253... 

Borrelli, M. Busico, V. CipuUo, R. Ronca, S. Budzelaar, P. H. M. Selectivity of metallocene-catalyzed olefin polymerization A combined experimental and quantum mechanical study. The ansa-Me2Si(Ind)2Zr and anM-Me2C(Cp)(Flu)Zr systems. Macromolecules 2003, 36, 8171-8177. 

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). 

A series of novel, bridged Zr(IV)-boratabenzene complexes have been synthesized and structurally characterized (Scheme 28).48 The geometries of these adducts closely resemble those of classical ansa-metallocenes. In the presence of excess MAO, these boratabenzene complexes catalyze the polymerization of olefins. 

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... 

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. 

Finally, Brintzinger et al. have reported a different type of ansa-metallocene, which can be readily prepared enantiomerically pure, and which catalyzes the... 

The catalysts are Tim hydrides which are generated in situ by reduction of enantio-merically pure L TiCl2 with butyllithium and PhSiH3 they are also highly effective in the asymmetric hydrogenation of imines and enamines.20 The lutetium ansa-metallocene complex (22-XVII) catalyzes the deuteration of 1-pentene (63% e.e.)21 Related samarium compounds hydrogenate imines.22... 

Because ansa metallocenes had been found to be effective polymerization catalysts, exploration of chelating bis(arylimido) complexes has been undertaken, typical examples being (48-51). In general, these have distorted octahedral structures with mo-n in the range 1.73-1.75A and ZMo=N-R in the range 155-162°. Complex (51), which is chiral, catalyzed the kinetic resolution of styrene oxide with ane.e of 30%. 

The efficacy of ansa metallocenes as polymerization catalysts has stimulated research into chelating bis(arylimido) complexes, including those with chiral ligands. The first of these, (62), and derivatives (63) and (64) (n= 1, 2) were reported by Gibson et al. in 1996.121 The complexes display distorted octahedral structures with d(Mo=N), Z(Mo=N—C) and Z(N=Mo=N) in the ranges 1.725-1.754 A, 155-162° and 100-103°, respectively. Complexes featuring strained, seven-membered, unsymmetrical ansa bis(imido ligands), e.g., (65), have also been reported.113 The first chiral bis(imido)-MoVI complex, C2-symmetric (66), catalyzes the kinetic resolution of styrene oxide and enantioselective trimethylsilylcyanation of benzaldehyde with 30% and 20% e.e., respectively.151... 

In an effort to enhance the activity and syndiospecificity of 6 -symmetric ansa-Cp-Flu metallocene complexes in the MAO-co-catalyzed polymerization of propylene, a large number of fluorenyl-substituted ansa-Cp-Flu metallocene complexes have been prepared,879-881 including the di-/z r/-butyl-substituted derivative 1129.882 The hafnocene... 

Brintzinger and coworkers produced a titanium complex with a binaphthyl linked ansa metallocene ligand [56[. Upon activation with nBuLi, this complex catalyzed the reductions of B and 2 phenyl 1 pyrroline in 76 and 98% ee, respectively. Titanium catalysts with chiral amido cyclopentadienyl ligands have also been reported, but were considerably less stereoselective than the metallocene based ones [57]. 

The copolymerization of ethylene with 1-olefins catalyzed by homogeneous Cp2ZrCl2—MAO catalysts and catalysts supported on HY zeolite shows a loss in activity on heterogenization, an increase in molecular weight, and a decrease in comonomer incorporation (Table 7). The ansa-metallocenes Et-(Ind)2MCl2 (M = Zr, Hf) display no attenuation of comonomer incorporation. The authors hypothesize that the smaller metallocenes are in the pores of the zeolite, into which diffusion of the comonomer is difficult, and the ansa-metallocenes, not adsorbed inside the pores, are located on the exterior and are therefore more accessible to comonomer. 

Thereafter, many reports on lanthanocenes for stereoselective MMA polymerization have appeared (Fig. 15). Table 1 summarizes some examples of lanthanide-catalyzed stereoselective polymerizations of MMA. Marks et al. developed Ci-symmetric ansa-metallocenes of lanthanides that give isotactic poly(MMA) [ 125]. They introduced a (+)-neomenthyl group on a Cp ring and achieved high isotactidty (31, [mm]=0.94, Mn= 1.04x10, M /Mn>1.8). Similar... 

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