Metal centers, chirality

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

Hori, A., Kataoka, H., Akasaka, A., Okano, T. and Fujita, M. (2003) Reversible catenation of coordination rings with Pd(II) and/or Pt(II) metal centers chirality induction through catenation, J. 

Von Zelewsky has published many examples of the stereoselective synthesis of metal complexes using what he refers to as chiragen ligands. These are enantiopure natural compounds synthesized from the natural product (—)-a-pinene, which is combined with species such as bipyridine units to provide impressive control of metal-centered chirality.159-161 In this section, we will focus on the determination of absolute configurations in terms of stereospecific formation from different points of view in connection with absolute conformations in the ligands. 

Both A A and cis trans equilibria of siderophore complexes can exist in solution. The chirality of the ligand can impose a preferred metal-center chirality. In addition, the degree of this preference depends on the stereochemical rigidity of the ligand. In principle, the magnitude of the molar circular dichroism can be used as a measure for diastereoisomeric equilibria based on a comparison of the solid-state and solution ellipticity. Nevertheless, predictions of metal-center chiralities require theoretical calculations. For example, empirical-force-field calculations of iron(III) enterobactin show that the A orientation at the metal center is more stable than the A by 0.5 kcalmoH, which is consistent with the CD spectra. ... 

It is in the stereospecific polymerization of propylene that metallocene complexes display their astonishing versatility. Commercial Ziegler-Natta catalysts for isotactic polypropylene - based on combinations of TiCU, MgCl2, Lewis bases and aluminum alkyls - depend on a metal-centered chirality which exists at specific edge and defect sites on the crystal lattice to direct the incoming monomer in a particular orientation. These catalysts produce small amounts of undesirable atactic material due to the presence of achiral active sites. 

It is presumed that hemiporphycene will exhibit a rich metal coordination chemistry. Indeed, preliminary reports have indicated that complexes of divalent magnesium, zinc, nickel, and copper, trivalent iron, cobalt and rhodium, and tetra-valent tin may readily be prepared. Of particular interest in such metalation studies is the fact that metal complexes of hemiporphycene containing an axial substituent (e.g., 3.153 Figure 3.3.3) bear metal-centered chirality because of the dissymmetric nature of the ligand. Unfortunately, further details with regard to this point and other aspects of hemiporphycene coordination chemistry are still lacking at this time. 

Chiral complexes 376—379 were treated with some alkyl halides to check the diastereoselectivity of the formation of the metal-centered chirality created by an oxidative addition (Scheme 67). The reaction leading to 381—388 is highly diastereoselective in some cases complex 384 was characterized by X-ray crystallography. Diastereoselectivities were determined by H and P NMR. ... 

The stereochemistry of the reaction, observable when the starting complex has a metal-centered chirality, can vary. Brunner has shown that, if the recombination step is faster than the rearrangement of the 16-electron transition state or intermediate, the reaction occurs with retention of configuration at the metal center. In the opposite case, racemization is observed with the following mechanism (the rate being inversely proportional to [PPli3], the dissociation of this ligand must be involved) ... 

A number of metal-centered chiral catalysts have both Lewis acid and Lewis base functionalities (Figure 2.10) [58]. The same is true for chiral Bronsted acids, which bear both a Bronsted acidic site and a Lewis basic site. In this review, chiral catalysts where the Bronsted acidic site plays a chief role will be discussed and the catalytic system, where the covalent bond is formed between the catalyst and the substrate in the transition state, will not be included. 

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