Chirality lanthanide complexes with achiral

Unlike transition metal compounds such as Ru(phen)3, which are composed of achiral ligands but can be resolved into chiral enantiomers, no such examples of the resolution of chiral lanthanide compounds with achiral ligands have appeared in the literature. This is because lanthanide complexes are generally more labile than transition metal complexes. Just like transition metal complexes, however, it is possible in some cases to perturb the ground-state equilibrium concentration through the addition of a chiral environment compound. This is sometimes referred to as the Pfeiffer effect. An example of such an experiment is presented in Figure 4. In this... 

In the presence of a catalytic amount of chiral lanthanide triflate 63, the reaction of 3-acyl-l,3-oxazolidin-2-ones with cyclopentadiene produces Diels-Alder adducts in high yields and high ee. The chiral lanthanide triflate 63 can be prepared from ytterbium triflate, (R)-( I )-binaphthol, and a tertiary amine. Both enantiomers of the cycloaddition product can be prepared via this chiral lanthanide (III) complex-catalyzed reaction using the same chiral source [(R)-(+)-binaphthol] and an appropriately selected achiral ligand. This achiral ligand serves as an additive to stabilize the catalyst in the sense of preventing the catalyst from aging. Asymmetric catalytic aza Diels-Alder reactions can also be carried out successfully under these conditions (Scheme 5-21).19... 

Thus, we have developed catalytic asymmetric aza Diels-Alder reactions of imines with alkenes using a chiral lanthanide Lewis acid, to afford 8-hydroxyquinoline derivatives in high yields with high diastereo- and enan-tioselectivities. The characteristic points of this reaction are as follows, (i) Asymmetric aza Diels-Alder reactions between achiral azadienes and dienophiles have been achieved using a catalytic amount of a chiral source, (ii) The unique reaction pathway, in which the chiral Lewis acid activates not dienophiles but dienes, is revealed. In most asymmetric Diels-Alder reactions reported using chiral Lewis acids, the Lewis acids activate dienophiles [64, 65]. However, inverse electron-demand asymmetric Diels-Alder reactions of 2-pyrone derivatives have been reported [72]. (iii) A unique lanthanide complex including an azadiene and an additive, which is quite different from the conventional chiral Lewis acids, has been developed. 

The first is observed when the chiral solvating agent has a high association with the achiral lanthanide complex. Bonding of the CSA to the lanthanide (Ln) effectively creates a chiral lanthanide shift reagent (Ln-CSA) (Eqn [4]). This species then interacts... 

Inanaga and coworkers developed another type of lanthanide catalysts for asymmetric hetero Diels-Alder reaction (Scheme 13.23) [59, 60]. Benzaldehyde reacted with l-methoxy-3-(trimethylsiloxy)-l,3-butadiene in the presence of chiral Yb(III) phosphate complex, Yb[(R)-BNP]3, to afford the corresponding adduct with 77% yield and 70% ee. 2,6-Lutidine worked best as achiral additive in the system to improve reactivity as well as enantioselectivity (89% ee) [59b]. In modification studies of the chiral ligand to improve solubility and chiral environment of catalyst, partially reduced (R)-Hg-BNP was found more suitable than original (R)-BNP ligand in this reaction. Yb[(R)-H8-BNP]3 complex promoted the reaction in 98% ee (R=Ph) without any additives (Scheme 13.23a) [59c]. Yb[(P)-BNP]3 complex was... 

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