Chiral chelating ligands

As Stated earlier, the combination of an organolithium reagent with sparteine has been widely used to generate chiral carbanions. We will focus here on recent examples where the efficiency of sparteine was compared with other chiral chelating ligands. 

Chiral chelating ligands that contain only one phosphine and another donor atom have been successful in recent years. A readily synthesized example is the phosphino-oxazoline ligand 87 shown in Scheme 31. These types of ligand can be prepared in two steps from an amino alcohol, a substituted benzonitrile and either a electrophillic or nucleophilic phosphorus reagent. They have been used in a number of asymmetric reactions. 

The asymmetric hydroformylation of alkenes is catalyzed by a number of Rh and Pt complexes with chiral chelating ligands.69 A number of binaphthyl-based chiral phosphites10 and mixed phosphine-phosphite ligand like (22-XXIV) [ // ,S)-BINAPHOS ] have been found particularly effective, e.g., in the asymmetric hydroformylation of styrene in >90% optical yield.71... 

Similarly, ra 5-cyclopropanes were obtained from alkenes, such as styrene and 2,5-dimethyl-hexa-2,4-diene, with relative yields > 90% when a diazoacetate bearing a bulky ester group was decomposed by a copper catalyst with bulky salicylaldimato ligands. Several metal complexes with bulky Cj-symmetrlc chiral chelating ligands are also suitable for this purpose, e.g. (metal/ligand type) copper/bis(4,5-dihydro-l,3-oxazol-2-yl)methane copper/ethyl-enediamine ruthenium(II)/l,6-bis(4,5-dihydro-l, 3-oxazol-2-yl)pyridine cobalt(III)/ salen. The same catalysts are also suited for enantioselective reactions vide infra). For the anti selectivity obtained with an osmium-porphyrin complex, see Section 1.2.1.2.4.2.6.3.1. 

Half-sandwich Complexes with Chiral Chelating Ligands... 

On the other hand, the reaction of [(/7-cymene)Ru(/i-Cl)2]2 with the chiral chelating ligands 3.17 and 3.18 remarkably gave the respective cationic complexes 3.19 and 3.20 as single disatereomers (Figure As stated in the beginning of this section chiral... 

However, the most employed half-sandwich complexes with chiral chelating ligands were reported by Noyori et who designed a new type of active and robust Ru(II) catalyst precursors based on complexes of monotosylated diamines. The structure of one of these catalyst precursors 3.21 was determined by X-ray analysis and showed that the metal has (R) configuration (see Figure 3.6). 

One particularly fascinating class of pentacoordinate phosphorus compounds is the class of chiral spirophosphoranes with amino acid residues as chiral chelate ligands since they can serve as important structural features relevant to chiral phosphoryl transfer pathways. Therefore, their synthesis and stereochemistry were recently widely explored." ... 

To investigate the stereochemistry of pentacoordinated spirophosphoranes with chiral chelate ligands such as amino acids, the diastereoi-somers could be differentiated by reverse-phase high-performance liquid chromatography (HPLC), solution or solid-state circular dichroism (CD),... 

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