Ligands, chirally selective

In Table 44.1 selected rate constants for the hydrogenation of the dienes COD and NBD for various ligands (chiral and achiral) are summarized. As expected, for all systems investigated, the hydrogenation of NBD was faster than the hydrogenation of COD [13 a, c, 17]. 

Chirality Ligand chirality does not, of course, influence the selection of spheres ( ), it may, however, play a role in transport systems. 

In summary, the configuration of the desired product is controlled by the planar-chiral imine and ketimine ligand backbone. The selectivity of the reaction depends on both the chiral center and the communication of the side-chain with the ligand backbone. We tuned the side-chain to increase the enantioselectivity up to 90% ee. In the case of the amino alcohol ligands, chiral cooperativity is also observed. However, the influence of the planar chirality is much lower, whereas central chirality is dominant in this instance. In most cases the enantioselectivity is lower than for the ketimines. 

Different mechanisms have been devised to achieve chiral resolution by electromigration, but often we have to deal with mixed-mode separations rather than pure processes. In any case, chiral resolution results from stereospecific interactions of a chiral selector, with the enantiomers of the compound giving rise to a difference in migration velocity between the two entities. Chirally selective ligands, such as Cu(II)-L-histidine and Cu(II)-aspartame, have been used for derivatized amino acid mixtures. 

The trialkyl complexes see Alkyl Complexes) of (N-N)Pt are typically prepared by oxidative addition see Oxidative Addition), as shown in Scheme 58. The complex (N-N)Pt Mc2 with an imino-cyclohexane ligand, as shown in Scheme 11, undertakes protonation from different acids to give rise to A,A,C-tridentate and A,A,C,A-tetradentate chiral aminoalkyl species, respectively (Scheme 59). The chiral selectivity at the aminoalkyl carbon is opposite in... 

For PhsP-substituted ferrio-silanedioles of the type Cp(OC)(R 3P)Fe-Si(OH)2R the iron represents the center of chirality. Selective reaction of one of the two diastereotopical (OH)-ligands at the silicon should create a new chiral center, indicated by the formation of diastereomers. 

Table 1. Selected Phosphine Ligands Chiral at the Phosphorus Atom...

Table 2. Selected Monophosphine Ligands Chiral at Carbon(s)...

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

Although the target function of neutral CDs is to exert chiral selection in the EKC separation of optical isomers, they have often been used as auxiliary complex ligands as a means of improving resolution of closely eluting achiral positional and structural related compounds or to reduce significantly apparent retention factors. The separation of seven positional and structural naphthalenesulfonate isomers (pH 3.0 phosphate buffer/p-CD) " and five 2,4-dinitrophenylhydrazine (DNPH)-aldehyde derivatives in vehicular emission (pH 9.0 borate buffer/SDS/p-CD) are examples of neutral CD-mediated separations. 

Chmura et al. also prepared air and moisture resistant chiral imino phenoxide complexes of zirconium and titanium, 14 [16]. They envisioned to study the effect of supporting ligand chirality on the stereoselectivity of LA ROP reaction. But at the end, they did not gain acceptable evidence enable to support any relationship. They showed that all isolated polymers had similar and moderate heterotactic microstructure which implied simple chain end control mechanism and resulted to the selective racemic enchainment during the propagation process. First, they investigate polymerization in toluene at 80°C and ambient temperature in which titanium complexes were absolutely inactive and zirconium coxmterparts showed moderate activity after 2 and 24 hours, respectively. Then they checked out solvent free conditions at 130°C and received almost complete conversion after 30 minutes for both titanium and zirconium alkoxide complexes (Table 7.2, entry 33-36). In this condition, titanium coxmterpart, in contrast to zirconium, resulted to full atactic polymer. Their investigation also showed that zirconium complex retained its activity in moisture or with lactic acid impurity in crude monomer which is deleterious for most metal alkoxide catalysts. 

Figs. 7 and 8. Notably, the collection includes chiral ligands, a catalytically active ligand, luminescent ligands, and ligands with selective metal-ion sequestering... 

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