Pseudo-asymmetric centres

Chiral centres and stereogenic centres Pseudo-asymmetric centres Unhelpful drawings Bond rotation Stereospecificity revisited... 

In addition to the four asymmetric centres carried into the macrocyclic backbone by the initial cyclohexane Ugsons, two pseudo-asymmetric centres located on the tertiary carbon atoms carrying the pendant nitro groups are present in the structure ofHjLlBb. 

The file benz-01. res on the accompanying CD-ROM contains a complete anisotropic model of a diabetes drug, which crystallizes in the monoclinic space group C2 with two molecules in the asymmetric unit. The model contains hydrogen atoms but not yet the solvent. The two independent molecules are related by a pseudo inversion centre, only violated by the chiral carbon atom. This one atom per molecule... 

The metal centre. In organic chemistry, chiral centres are usually associated with an asymmetric carbon atom, but this notion is of limited use for metal ions. Most tetrahedral metal ions are extremely labile, although pseudo-tetrahedral complexes such as (C5H5)MLL L" may be resolved into enantiomers. Octahedral centres with... 

The large negative values of AS in the reactions of the iridium(I) complexes have been rationalised in terms of increased solvation of the transition state, attributable to its increased dipole. Such a dipole results not only from the interaction of the alkyl halides with the metal centre, but also from deformation of the iridium complex from planar to pseudo-octahedral geometry as would occur in a three-centre cw-addition (Harrod and Smith, 1970). In some cases the values of AS and the influence of solvent on AH and AS have been explained in terms of a polar, asymmetric, three-centre transition state, in which the interaction of the metal is predominantly with the carbon centre (Ugo et al., 1972). 

The bicycUc polyethers give rise to complex structures. With lanthanum chloride and Bicy(4,2B,2B), the adduct corresponds approximately to a 1 1 stoichiometry, but a crystalline product is obtained only in the presence of water. The crystal structure analysis shows that partial hydrolysis takes place. In this, the behaviour is reminiscent of the 2 1 TEA complexes with 15C5. Indeed, the unit cell contains a pseudo-centrosymmetric dimer [LaCl(Cl,OH)L]2 with an OH Cl hydrogen bond across the centre of symmetry one site is occupied by Cl in one half and by OH in the other half. The ligand in this structure has an approximate twofold axis of symmetry. The O atoms form an end-capped trigonal prism around La(III) and the metal ion lies in the mean plane formed by the O atoms of the aliphatic unsubstituted chain. The other bicyclic polyether, Bicy(4B,2B,2B), forms 3 2 complexes with lanthanum nitrate. The asymmetric unit contains two dinitrato cations [La(N03)2L] and one pentanitrato anion [La(N03)5(Me0H)] . The La(III) ion is 11-coordinate in the anion and one of the cations, in which there is one bidentate and one monodentate nitrate ion. It is 12-coordinate in the other cation. 

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