Chiral metallocene

Keywords stereoselective DIels-Alder reaction catalysts, DIels-Alder chiral metallocene catalyst review... 

More recently, a very efficient asymmetric carbolithiation of N,N-dimethyl-aminofulvene 30, leading to a chiral cyclopentadienide anion, was reported by Hayashi et al. [6] for the synthesis of chiral metallocenes (Scheme 6). By adding an aryl lithium such as 31 complexed with a chiral ligand on fulvene 30, a cyclopentadienide ion 32 bearing a stereogenic center at the a position was generated. This anion was reacted with [RhCl(nbd)]2 to yield... 

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

Subsequent to these studies by the author s group, Whitby and co-workers reported that enantioselective alkylations of the type illustrated in Scheme 6.6 can also be carried out with the non-bridged chiral zirconocene 31 [19]. Enantioselectivities are, however, notably lower when alkylations are carried out in the presence of 31. For example, this new chiral metallocene affords 29 and 30 (Scheme 6.5) with 82% and 78% ee, respectively. 

These recent results illustrate for the first time that metallocene-based chiral Lewis acids can serve effectively in providing [4+2] cycloaddition products with excellent levels of enantiofacial selectivity. Perhaps more importantly, the reported NMR studies and the observed dramatic solvent effect should pave the way for future endeavors in the rational design of better chiral metallocenes. 

Methyl substituents, in chiral metallocene catalysts, 16 106 v-Methyl substituents, in chiral metallocene catalysts, 16 106 Methyl tert-amyl ether (TAME), 12 404-405... 

Metal complexes of pinene-fused boratabenzene ligands, analogous to chiral metallocenes that have found application in catalysis and enantioselective synthesis, have been prepared.122-124 With late transition metals such as Mn and Fe, the complexes are obtained as mixtures of diastereomers (e.g., 97) with the sterically less congested exo form predominating, but the bis(ligand) Zr complex 98 was obtained as the pure exo,exo product.124 A lithium... 

Unsymmetrically disubstituted (and hence chiral) metallocenes (with point group C,) such as the ferrocene 122) provide a rather special problem they were first defined as planar chiral 19) and the specification of molecular chirality (descriptors Rp and Sp) applied accordingly19). Several authors still classify these structures as planar chiral21 23-24). 

In recent years we have developed a new type of planar-chiral ligand system which is based on a phosphaferrocene skeleton equipped with an additional donor function Y [8]. This structure is closely related to the well-known ferrocene-type ligands in that a CH unit of the latter has been replaced by a P atom. The phosphaferrocene moiety serves as both a chiral metallocene-type backbone and as a donor group via the phosphorus atom lone pair. These new ligands are unique in their topological architecture and show interesting ligand properties. 

Hinsichtlich ihrer Molekiilgeometrie sind chirale Metallocene am ehesten mit chiralen p-Cyclophanen oder Ansaverbindungen 12) und be-ziigUch ihrer optischen Stabilitat mit optisch aktiven Spiranen vergleich-bar. ... 

Abb. 3. Anwendung der Sequenzregel auf chirale Metallocene. Als Beispiel wurde ein Keton vom Tjrp (7) gewahlt. B — BUckrichtung. 

The experimental observation was that C2-symmetric metallocene complexes of zirconium (Fig. 6) produced isotactic polymers, while Cs-symmet-ric metallocene complexes (Fig. 6) produced syndiotactic polymers. Pure MM calculations with frozen core showed that the stereoselectivity is not related to direct interactions of the -ligands of the chiral metallocene with the entering monomer, but to interactions of the -ligands with the growing chain. It is therefore the chirally oriented growing chain which discriminates between the prochiral faces of the propene monomer. For C2-symmetric complexes, identical enantiofacial orientation in all insertion steps results in isotactic polymer formation for Cs-symmetric complexes the enantiofacial orientation alternates between insertion steps and leads to syndiotactic polymers. 

The choice of solvent has had little, if any, influence on the majority of Diels-Alder reactions.210,211 Although the addition of a Lewis acid might be expected to show more solvent dependence, generally there appears to be little effect on asymmetric induction.118129 However, a dramatic effect of solvent polarity has been observed for chiral metallocene triflate complexes.212 The use of polar solvents, such as nitromethane and nitropropane, leads to a significant improvement in the catalytic properties of a copper Lewis acid complex in the hetero Diels-Alder reaction of glyoxylate esters with dienes.213... 

Brintzinger, H. H., Fischer, D., Miilhaupt, R., Rieger, B. and Waymouth, R. M., Stereospecific Olefin Polymerisation with Chiral Metallocene Catalysts , Angew. Chem., Int. Ed. Engl., 34, 1143-1170 (1995). 

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