Coordination chemistry chiral metal complexes

The demonstration of the optical activity of octahedral complexes was important in confirming Alfred Werner s intuitive ideas about coordination chemistry. Early work involved the resolution of complexes characterized by optical rotations. Modem instmments for optical rotatory dispersion were developed first, but circular dichroism (CD) spectra proved to be more useful. CD has been a powerful tool for detailed studies of the stereochemistry of octahedral complexes. Contributions to rotational strength of chelate ring conformational, configurational, and vicinal contributions are additive. Chiral metal complexes are now used in enantioselective synthesis of chiral pharmaceuticals. 

Metal-assisted enantioselective catalytic reactions are one of the most important areas in organic chemistry [1-3]. They require the appropriate design and the preparation of chiral transition metal complexes, a field also of major importance in modern synthetic chemistry. These complexes are selected on both their ability to catalyze a given reaction and their potential as asymmetric inducers. To fulfill the first function, it is absolutely required that the catalysts display accessible metal coordination sites where reactants can bind since activation would result from a direct interaction between the metal ion... 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

The above mechanism is novel in that it does not require the interaction of a carbonyl moiety with the metal center. Neither a ketone/Ru complex nor a Ru alkoxide is involved in the mechanism, and the alcohol forms directly from the ketone. This non-classical mechanism also explains the high functional selectivity for the C=0 group. When the chiral molecular surface of the Ru hydride recognizes the difference of ketone enantiofaces, asymmetric hydrogenation is achieved. This is different from the earlier BINAP Ru chemistry where the enantio-face differentiation is made within the chiral metal template with the assistance of heteroatom/metal coordination. Similar heterolyses of H2 ligands have been shown by Morris and others (92) to be the critical step in the mechanism of reaction processes related to the Noyori systems. 

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. 

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

Coordination chemistry and catalytic applications of metal complexes with chiral tertiary phosphines... 

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