Chiral complexes, determination

Platinum-thiourea complexes have been extensively studied because of their biological activity [54], but few have been used in catalysis. Neutral thioureas are able to coordinate to metal centres through their sulfur atom (Scheme 9) [55,56] monomeric (I) and oligomeric (II) species are known for Rh [57], and an X-ray structure has also been determined for the chiral complex III [58]. In many complexes hydrogen bonding has been observed... 

The X-ray structure of [cp Ir(CNR)Cl2], CNR = l,3,4,6-tetra-0-acetyl-2-deoxy-2-isocyano-Q,, 3-D-glucosc, has been determined.428 The synthesis of chiral complexes of Ir111 with Q-amino acid anions, L-L, of general formula [cp Ir(Cl)(L-L )] (252), and their NMR spectroscopic characterization, have been detailed. The X-ray structures of (252), L-L = L-proline and [cp Ir(Cl)(L-His-OH)]Cl, His = histidine, are described.429 C-allylglycinate binds in a terdentate manner in (253), which has been characterized by X-ray diffraction studies.430 C-vinylglycinate forms complex (254). 

Ma and co-workers extended use of chiral guanidine catalysts to the addition of glycine derivatives to acrylates [121], Addition products were achieved in high yield with modest enantioselectivity (Scheme 67). The ferf-butyl glycinate benzophenone imines generally provided better enantiomeric ratios than the ethyl glycinate benzophenone imines. Based on this observation, the authors hypothesized that an imine-catalyst complex determines the stereochemical outcome of the product. 

Finally, optical isomerism is even more rare. The first example of the determination of the absolute configuration of such a chiral complex is shown in Fig I2.15.26 This... 

M. Sawada et al., Chiral recognition in host-guest complexation determined by the enantiomer-labeled guest method using fast atom bombardment mass spectrometry. J. Am. Chem. Soc. 117, 7726-7736 (1995)... 

Notably, as was observed for neutral supraminols [51,60], the complex (R,R)-29 (R,R)-44 assumes a helical motif in which chirality is determined by the absolute configuration of the trans-1,2-diaminocyclohexane. 

In this paper, the chirodiastaltic energy between the inorganic host and the complex guest is calculated using the molecular mechanics. The results imply that there is chiral recognition effect between the inorganic chiral motifs and chiral complex templates. The absolute configuration of the chiral templates determines that of the chiral motifs in the networks. 

Determination of % ee via Pt NMR. The cis chiral complex has been used to determine enantiomeric purity of asymmetric allenes and allylic alcohols and ethers via complexation and Pt NMR spectroscopy. The complexes (3) and (4) are generated by displacement of ethylene from (1) by the alkene recovery of the alkene and (1) is effected by the reverse sequence employing excess ethylene. ... 

HPLC-CSPs are based on molecules of known stereochemical composition immobilized on liquid chromatographic supports. Single enantiomorphs, diastereomers, diastereomeric mixtures, and chiral polymers (such as proteins) have been used as the chiral selector. The chiral recognition mechanisms operating on these phases are the result of the formation of temporary diastereomeric complexes between the enantiomeric solute molecules and immobilized chiral selector. The difference in energy between the resulting diastereomeric solute/CSP complexes determines the magnitude of the observed stereoselectivity, whereas the sum total of the interactions between the solute and CSP chiral and achiral, determines the observed retention and efficiency. 

When the absolute structure has been determined, the result must be correlated with some physical property of the crystal, otherwise the result has no use to the chemist. The obvious correlation is with the direction of rotation of the plane of plane-polarized light, that is, whether the compound or crystal is dextrorotatory or levorotatory. Another correlation can be made with crystal appearance this was shown for zinc blende with its matte and shiny faces, and for silica and sodium ammonium tartrate crystals for the disposition of their hemihedral faces. If such data are not available, it may be necessary to list physical properties of diastereomers made with chiral complexing agents. Then, whenever this same compound is encountered by a chemist, its absolute structure is well known. 

In the ligand polarization mechanism for optical activity, the potential of the electric hexadecapole component, Hxy(x>-y>), produces a determinate correlation of the induced electric dipole moment in each ligand group which does not lie in an octahedral symmetry plane of the [Co Ng] chromophore (Fig. 8). The resultant first-order electric dipole transition moment has a non-vanishing component collinear with the zero-order magnetic moment of the dxy dxj yj transition in chiral complexes, and the scalar product of these two moments affords the z-component of the rotational strength, RJg, of the Aj -> Ti octahedral excitation. 

Another important aspect is the selection of the organometallic catalyst precursor responsible for generation of the active catalytic species. The suitable combination of metal complex and chiral ligand determines the efficiency of the process,... 

Consider, for example, Werner s ingenious determination of the structures of cis- and trans-[Pt(NH3)2Cl2] (2), and his demonstration of the octahedral structure of six-coordinate complexes through the optical resolution of [Co(en)2(NH3)X]2+ (X = Cl, Br) (3). We are now able, by modern techniques, not only to perform these demonstrations quickly, but, in the case of chiral complexes, to show the actual configurations of the isomers (A,5). ... 

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