Chiral coordinated nitrogen

It was argued that the relatively low ee in this case could be attributed to the large separation between the source of chirality and the reactive center, and so the reaction in Scheme 8.10 was investigated [19]. The chirality in the leaving group in compound 9 is closer to the reaction center than in the first studied substrate 8, since the stereocenter in 9 is in the position a to the coordinating nitrogen. 

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

The principle of asymmetric lithiation involves both the fact that formation of diastereomeric intermediates should involve different energies of activation, and the idea that, in directed lithiations, a lithium atom is coordinated with nitrogen or some other heteroatom (see above). When the coordinating nitrogen atom resides in a chiral environment, one of the two possible diastereomeric lithio intermediates is energetically favored for steric or other reasons. Thus one of the two possible intermediates should be formed preferentially, with the resulting condensation products reflecting the stereoselectivity of the lithiation. 

Asymmetric allylation of carbon nucleophiles has been carried out extensively using Pd catalysts coordinated by various chiral phosphine ligands and even with nitrogen ligands, and ee > 90% has been achieved in several cases. However, in most cases, a high ee has been achieved only with the l,3-diaryl-substitiitcd allylic compounds 217, and the synthetic usefulness of the reaction is limited. Therefore, only references are cited[24,133]. 

As described previously, lipophilic monoimidazole ligands form 2 1 complexes with the Zn2 + ion (n = 2 in Scheme 2) as active catalysts except for some sterically hindered ligands (Table 3, 5, 7), and bisimidazole ligands form 1 1 complexes (n = 1 in Scheme 2, Table 5). In this chiral system, the latter 1 1 complex accords with kinetic analyses for both L-47 and L,L-49 ligands as shown in Fig. 12 and Table 11. These conclusions seem to be reasonable since monoimidazole derivatives have only one imidazole nitrogen, while the other bisimidazole and chiral ligands have more than two nitrogen atoms which can effectively coordinate to the Zn2 + ion. 

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