Polydentate ligands, asymmetric

Like BINOL, salicylaldehyde imines have become very important in asymmetric catalysis and a variety of polydentate ligands prepared from chiral monoamines and diamines are employed in oxidation reactions, carbenoid reactions and Lewis acid catalyzed reactions. As in the previous section, this section emphasizes the effect of the phenol moiety on the asymmetric catalysis. An imine derived from a chiral 1-phenethylamine and salicylaldehyde was employed in the copper catalyzed asymmetric cyclopropanation by Nozaki, Noyori and coworkers in 1966, which is the first example of the asymmetric catalysis in a homogeneous system . Salicylaldehyde imines with ethylenediamine (salen) have been studied extensively by Jacobsen and Katsuki and their coworkers since 1990 in asymmetric catalysis. Jacobsen and coworkers employed the ligands prepared from chiral 1,2-diamines and Katsuki and coworkers sophisticated ligands possess chirality not only at the diamine moiety but also at the 3,3 -positions. 

Replacement of three C-substituents in phosphines by oxy groups produces triesters of phosphorous acid, also called phosphites. Such phosphites play a crucial role as mono- and bidentate ligands, but also as polydentate ligands, in numerous catalytic applications [1]. In Rh-catalyzed hydroformylation, they are indispensable in regioselective transformations at each scale. Several chiral phosphites based on rather complicated alcohols have also been designed for use in asymmetric hydroformylations [2]. 

Phosphorus nucleophiles Asymmetric Michael addition of diaryl phosphine oxides to a,/ -unsaturated A-acylated oxazolidinones (299) has been reported to proceed with excellent enantioselectivities (<99% ee) in the presence of a catalyst generated from Et2Zn and the polydentate ligand (301). °... 

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

Polydentate polyphosphine metal complexes freqnently show greater catalytic activity than the monophosphine-type complexes so far discussed. In addition, asymmetric polydentate complexes (whether the asymmetry arises on the P atom or another atom) may catalyse asymmetric syntheses, that is, the production of a single optical isomer, sometimes of nearly 100% optical purity (Chapter 13.2). In the presently known field of asymmetric catalytic ligands, P componnds figure prominently and are of ever-growing importance. BINAP (12.372d) represents a family of such ligands, many of which have C2 symmetry (12.373). 

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