Polydentate ligands chirality

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

The introduction of a sulfonate group by reaction with oleum is not limited to arylphosphanes. Tris(co-pbenylalkyl)phosphanes, P[(CH2) (C6H5)]3 (n = 1, 2, 3, and 6), can be sulfonated in the para position and to a lesser extent in the meta position (18). The technique of sulfonating water-insoluble ligand precursors can be applied to bidentate, polydentate, and chiral phosphanes (20-24) the compounds 1-3 are presented in Scheme 2 as examples. 

If we consider the geometry of the [M(en)3]"+ complex ion, we have further possibilities to consider. Whereas an octahedral complex with six identical ligands can only exist in one form, one with three didentate chelating ligands is chiral and can exist as two (non-superimposable) enantiomers (Fig. 2-7). The incorporation of polydentate ligands into a co-ordination compound may well lead to a rather considerable increase in the complexity of the system, with regard both to the stereochemical properties and any related chemical reactivity. 

Among the NHC ligands with N-substituents containing centers of chirality, polydentate ligands that combine the NHC unit with an anionic functional group have been developed recently. They thus combine two complementary ligating anchor units, which avoids the rotation of the chiral substituents around the C - N axis. Arnold and coworkers reported the synthesis of the... 

Another major contribution of polydentate ligands is the creation of a chiral pocket around the catalytic center providing, an appropriate chiral environment. The chiral pocket concept has been introduced by Trost for catalytic enantioselective allylic alkylation with the tetradentate aminophosphine ligand 33 [133]. The nucleophile fits into the chiral environment created by the chiral ligand and the allyl Pd intermediate. As a result, the enantiocontrol of the newly formed chiral center is very effective. In addition, the chiral control is likely to be efficient even at positions remote from the chiral ligand. That auxiliary has been widely... 

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. 

Chirality is often met with polydentate ligands at a number of different centres in the complex, and separation of all optical isomers is either impractical or, in effect, impossible. In many cases, working with a racemate has no significant influence on the chemistry, and optical resolution of complexes is attempted on only very limited occasions, such as where researchers wish to record the chiroptical properties, or where a chiral complex is required to assist in achieving a chiral reaction, such as use of a chiral complex as a catalyst in synthesis of organic molecules where a particular optical isomer is sought (exemplified in Chapter 9). 

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]. 

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... 

Closely related to the football ligands are the so-called sepulchrate ligands. One can be formed by the condensation of formaldehyde and ammonia onto the nitrogen atoms of tris(ethylenediamine)cobalt(llI). This results in tris(methylene)amino caps on opposite faces of the coordination octahedron. If the synthesis utilizes one of the (A, A)-enantiomers, the chirality of the complex is retained. Furthermore, the complex may be reduced to the corresponding cobalt(II) cation and reoxidized to co-balt(III) without loss of chirality. This is particularly unusual in that, as we shall see in the following chapter, cobalt(ll) complexes are quite labile in contrast to the stability of cobalt(III) complexes. Once again the extra stability of polydentate complexes is demonstrated. 

The use and significance of stereochemical reaction cycles in the reactions of chiral phosphines and phosphonium salts have been surveyed, and a major review of the chemistry of polycyclic C—P heterocycles, much of which is concerned with tertiary phosphines and phosphonium salts, has appeared. Procedures for the synthesis of a range of unidentate and polydentate phosphine ligands have been collected together in a single volume. Aspects of the chemistry of methylphosphines have been included in a review of recent developments in the chemistry of simple P-C compounds. ... 

Although unclear, these effects impose certain requirements on the substrate and the chiral ligands to be used. Thus, the well-known positive effects of having additional basic and coordinating functional groups present in the substrate are believed to stem from bi- or polydentate coordination of the substrate to the metal center. This renders the intermediate complexes more rigid and stereochemically uniform, and consequently the number of possible complex geometries is reduced. 

In 1969, Seebach et al. employed mixtures of -pentane and the tartrate derivative l,4-bis(dimethylamino)-2,3-dimethoxybutane (2) as a chiral co-solvent in -BuLi additions to PhCHO [25]. They obtained 1-phenyl-1 -pentanol in up to 33 % optical purity at -130°C and achieved higher enantioselectivities than with hexane/sparteine or hexane/2,3-dimethoxybutane mixtures under similar conditions. After further systematic studies [26-28], Seebach et al. showed that two equivalents of the polydentate amino ether ligand 3 yield an optical purity of 52% (-78°C,pentane) while a 1 1 ratio gives only 38% ee [29]. 

Modular Assembly of Chiral Catalysts with Polydentate Stereodlrectlng Ligands... 

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