Base configurations chelates

This result corresponds to the conformational analysis which predicts 24 as the most stable one. As already mentioned, it is possible to fix the absolute configuration of a given isomer by a suitable choice of substituents. A selection between different geometrical isomers — e.g. on the base of chelate ring size — seems to be less easy. It... 

TS, which is usually based on the chair (Zimmerman-Traxler) model. This pattern is particularly prevalent for the allylic borane reagents, where the Lewis acidity of boron promotes a tight cyclic TS, but at the same time limits the possibility of additional chelation. The dominant factors in these cases are the E- or Z-configuration of the allylic reagent and the conformational preferences of the reacting aldehyde (e.g., a Felkin-type preference.)... 

The synthesis of the C(l)-C(6) subunit was based on addition of an enol boronate to 3-benzyloxypropanal through TS-1. Immediate reduction of the chelate is also stereoselective and provides the intermediate 13. These steps establish the configuration at C(2)-C(5). 

No information concerning the distribution of the unpaired electron on the chelate ligands of four-coordinated low-spin Co(II) Schiff base complexes is available from single crystal EPR spectra. In particular, no nitrogen hf interaction, which is sensitive to the ground state configurations, is observed. 

Inversion at the N center is coupled to conformational changes in a chelate ring. The kinetics of inversion at asymmetric N centers in complexes of tetraaza linear or macrocyclic ligands have received scant attention. There are five configurational isomers of the planar complex Ni([14]aneN4) +, Sec. 3.1.1. The interconversions between such structures are base catalyzed with second-order rate constants covering a small range from 1.2 x 10 to 2.4 x 10 M- s- Refs. 108-110. 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

The interpretation and prediction of the relationship between the configuration of the newly formed chiral center and the configuration of the amine are usually based on steric differentiation of the two faces of the imine anion. Most imine anions that show high stereoselectivity incorporate a substituent which can hold the metal cation in a compact transition state by chelation. In the case of entry 2 in Table 1.3, for example, the observed enantioselectivity is rationalized on the basis of transition state L. 

For diorganotin dicarboxylates, the symmetrically chelated structure has been proposed from infrared and far-infrared spectra (9). A nonsymmetrically chelated configuration and a partial bridging of acetoxy groups have also been proposed based on infrared data (10, 11). 

The complex bis(ethylxanthato)(py) Zn contains zinc in a trigonal plane composed of the pyridine and a pair of sulfur atoms from the xanthate ligands (Zn—N = 2.03 A Zn— S = 2.29 A).873 In contrast, the xanthate ligands in bis(ethylxanthato)(l,10-phen)Cd are bidentate, thus conferring a pseudooctahedral configuration on the metal (Cd—N = 2.38 A Cd—S = 2.64, 2.72 A). Zinc chelates of the stoichiometry ZnL have been isolated (HL = several Schiff bases derived from S-methyldithiocarbazate).875... 

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