High chiral activators

The use of chiral amide ligands has been restricted to rhodium, where the catalyst precursor is [Rh(BH4)(amide)py2Cl2]. The work has been reviewed (10, 35) cinnamate derivatives were reduced to up to 57% ee, and hydrogenation of a carbon- nitrogen double bond in folic acid leads to tetrahydrofolic acid with high biological activity (308). 

Among optically active polymers, polysaccharide derivatives are particularly valuable. Polysaccharides such as cellulose and amylose are the most readily available optically active polymers and have stereoregular sequences. Although the chiral recognition abilities of native polysaccharides are not remarkable, they can be readily converted to the esters and carbamates with high chiral recognition abilities. The chiral recognition mechanism of these derivatives has been clarified to some extent. 

Using a chiral diphosphine Rh(I) complex as the catalyst, high catalytic activity and a good branched/linear ratio can be achieved, but in most cases the ee values remain moderate.122 The enantioselectivity of Rh(I)-catalyzed reactions depends on the amount of added chiral ligand because of the much higher catalytic activity of ligand-free rhodium species. Usually 4 6 equivalents... 

In contrast to chiral poisoning, the concept of chiral activation has also emerged recently. An additional activator selectively or preferentially activates one enantiomer of the racemic catalyst, resulting in much faster reaction, and gives products with high ee. 

The idea of enantioselective activation was first reported by Mikami and Matsukawa111 for carbonyl-ene reactions. Using an additional catalytic amount of (R)-BINOL or (/ )-5.5 -dichloro-4,4, 6,fi -tctramcthyl biphenyl as the chiral activator, (R)-ene products were obtained in high ee when a catalyst system consisting of rac-BINOL and Ti(OPri)4 was employed for the enantioselective carbonyl ene reaction of glyoxylate (Scheme 8-54). Amazingly, racemic BINOL can also be used in this system as an activator for the (R)-BINOL-Ti catalyst, affording an enhanced level of enantioselectivity (96% ee). 

Reactions of aldehydes with complexes 13—17 provide optically active homoallylic alcohols. The enantioselectivities proved to be modest for 13—16 (20—45% ee). In contrast, they are very high (> 94% ee) for the (ansa-bis(indenyl))(r]3-allyl)titanium complex 17 [32], irrespective of the aldehyde structure, but only for the major anti diastereomers, the syn diastereomers exhibiting a lower level of ee (13—46% ee). Complex 17 also gives high chiral induction (> 94% ee) in the reaction with C02 [32], in contrast to complex 12 (R = Me 11 % ee R = H 19% ee) [15]. Although the aforementioned studies of enan-... 

Song et al. [62] reported poly-salen Co(III) complexes 18, 19 as catalyst for HKR (Figure 5) of terminal alkene epoxides. The polymeric catalysts provided product epoxides with excellent conversion (>49%) and high chiral purity (ee s, 98%) and the catalytic system could be recycled once with retention of activity and enantioselectivity. 

In contrast to asymmetric deactivation, Mikami has reported a conceptually opposite strategy, asymmetric activation. A highly activated chiral catalyst can be produced by addition of a chiral activator (Scheme 8.11). This strategy has the advantage that the activated catalyst can afford products with a higher enantiomeric... 

HayasM et al. achieved high catalytic activity by using axially chiral iV-oxide catalyst 27. As compared to other organic catalysts, the reaction proceeded much faster, and high enantioselectivities were obtained with 0.01-0.1 mol% catalyst loading [53-55]. In 2005, Hoveyda and Snapper used a novel proline-based ahphatic A-oxide 28 for an asymmetric allylation (Scheme 19) [56],... 

A polymerization of a bulky methacrylate ester (e.g. trityl methacrylate) using an optically active anionic initiator can give an isotactic polymer, poly 1-methyl-1-[(trityloxy)carbonyl]ethylene of high optical activity owing to the formation of helical polymer molecules with units of predominantly one chirality sense. 

Similar results were obtained by Suzuki et al. (55) in their investigation of enantioselective addition of Et2Zn to various N-diphenylphosphinylimines with PAMAM dendrimers functionalized with 4 and 8 chiral amino alcohol groups. They observed 92% ee for the monomeric ligand, 43% ee for GO, and 30—39% ee for G1 when using N-diphenylphosphinylbenzaldimine as the substrate. Again, a high local concentration of chiral active sites leads to a decrease in enantioselectivity. 

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