Complexes stereochemical properties

Low-symmetry molecular species can appear as a result of restricted reorientation and the interplay between stere-ogenic units. A tris-triarylmethane system containing chlorine atoms in all aromatic positions (3) has only three protons, one in each of the arylmethane units. However, the NMR spectrum of this species in solution (Figure 2) displays 16 proton signals. The unequal intensities of the peaks point out that they belong to different conformations in slow exchange. Analysis of the rather complex stereochemical properties of this system confirms the existence of six diastereomeric forms, five of which have Cl symmetry and therefore three proton signals, and one has C3 symmetry and contributes only one additional proton. 

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

Another hypothesis was provided by Mikio Shimitso (1982) on the basis of studies of steric effects in molecular models. It had been noted years previously that the fourth nucleotide at the 3 end of the tRNA molecules (referred to as the discrimination base) might have a recognition function. In the case of certain amino acids (i.e., their tRNA-amino acid complexes) this base pair, in combination with the anticodon of the tRNA molecule, can select the amino acid corresponding to the tRNA species in question this is done on the basis of the stereochemical properties of the molecule. Since the anticodon of a tRNA molecule and the fourth nucleotide of the acceptor stem are far apart in space, two tRNA molecules must complex in a head-to-tail manner. The pocket thus formed can then fit specifically to the corresponding amino acid. 

Synthetic models of myoglobin and hemoglobin are complex molecules that mimic the stereochemical properties of the protein active center [24] and have oxygen affinities similar to those measured for the protein [25-27]. The first heme model that reversibly binds oxygen (i.e. the picket-fence-oxygen complex Fe(TpivPP)(l,2-Melm)(02), shown in Fig. 3.3) was obtained in the early nine-teen-seventies by Collman and coworkers (TpivPP = tetrapivalami-nophenyl porphyrin 2-meIm = 2-methylimidazole) [18]. Research on synthetic models of the protein has led to a deeper understand-... 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

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. 

A second consequence of this sequential mechanism is even more surprising. The geometry about the metal centre in a sepulchrate is close to octahedral, and this is emphasised in Fig. 7-17. We discussed some of the stereochemical properties of complexes with three didentate chelating ligands in Chapter 2, and in the same way that the [Co(en)3]3+ cation may exist in the two enantiomeric A and A forms, so may the sepulchrate complex. These two enantiomers are shown in Fig. 7-17. The reaction of [Co(en)3]3+ with formaldehyde... 

Heterofunctionalized phosphines, with different denticities and/or donor sets, were successfully applied, at macroscopic and noncarrier added level, to the stabilization of the [M=0]3+, trans-[M02]+, [M=N]2+ and/ac-[M(CO)3]+ cores. The unique electronic and/or stereochemical properties of the tridentate PNS phosphine allowed the synthesis of 3+1 mixed-ligand complexes with an unprecedented in vivo robustness. Phosphines as bifunctional chelate ligands have been successfully applied for labeling different biologically active substrates, namely peptides and CNS-receptor avid molecules. 

Scheme 22. Stereochemical properties of the metal center M in octahedral [M(S<4)] complexes.

These cobalt(III) complexes have the same charge ( + 3) and essentially the same electronic state to each other, but they differ with respective to size, shape, and stereochemical properties for instance, [Co(en)3]3+ has only one molecular asymmetry about the metal center, but [Co(en)2(R,R-chxn)]3 +, [Co(en)(R,R-chxn)2]3 +, and [Co(R,R-chxn)3]3+ have chiral centers on the ligand besides the chirality about the metal center. In the excited state of [Eu(dpa)3]3" and [Eu(cda)3]6, A form - A form interconversion occurs with the first order rate constants of 15.8 and 29.6 s respectively. These values are much smaller... 

Figure 3.23 The structure of monomeric complex [Er(H20)3(EDTA)] [Er, black (large ball) O, grey N, black (small balls) C, white H, omitted)]. (Redrawn from the CIF file of N. Sakagami et al., Crystal structures and stereochemical properties of lanthanide(III) complexes with ethylenediamine-N, N, N, N -tetraacetate, Inorganica ChimicaActa, 288 (1), 7-16, 1999 [104].)...

The recent and continuing surge of interest in transition-metal complexes of o ,/3-unsaturated /3-keto amines (/3-amino ketones), especially with respect to their stereochemical properties, has resulted in increased efforts to secure general synthetic procedures for these complexes. Methods of preparation of these complexes have been reviewed recently. Although most of... 

A plausible intermediate for the enantio- and diastereoselective addition of diethylzinc to a-thioaldehyde catalyzed by 20o is shown in Fig. 3-4. When 20o ((—)-DFPE) is used in the ethylation of the racemic aldehyde 41, the (li)-enantiomer reacts faster than the (5)-enantiomer and the newly produced stereogenic center from (P)-41 has the S-configuration. This stereochemical property can be reasonably explained by considering the 7/6-fused bicyclic intermediate depicted. Compound (P)-41 has lower steric hindrance than (S)-41 in the reaction complex and ethylzinc preferentially attacks from the Si face of (7 )-41 to afford the S-configuration. Therefore, (i )-41 reacts faster than (S)-41 to afford (3S,4R)-42. 

The metabolism of the secreted steroid hormones is seemingly complex but can readily be understood in terms of a relatively small set of common biochemical transformations (B28). These major reactions are summarized in Fig. 4. Although the stereochemical properties and large number of positions open to metabolic attack give rise to a tremendous number and variety of metabolites through these reactions, they are in fact typical detoxication reactions common to a large number of nonsteroidal substances. 

Stereochemical Properties of N-Chelated Alkali Metal Complexes... 

Wilkinson catalyst, RhCl(PPh3)38 are prototypes of the OA reaction critically important to hydrogenation and homogeneous catalysis in general. It was now possible to directly observe the electronic and stereochemical properties of binding of a gaseous diatomic molecule on a metal complex and understand the factors that determine its activation toward cleavage. 

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