Chelate rings, conformations

A number of substituted propane-l,3-diamine (64) compounds of the type [PtL2] and [PtX2L2] (X = Cl, Br, or NH3) have been described. Several of the latter species exhibited polymorphism which was attributed to either different H-bonded linkages or different chelate ring conformations in the two forms.In solution their n.m.r. spectra are consistent with a preference for a chair-type conformation with the Cl or OH axial.Similar n.m.r. spectral studies have also been reported for some related N-methyl substituted... 

Certain chiral phosphinite or aminophosphine ligands are also useful for amino acid synthesis. Hydrogenation with a CYCPHOS-Rh complex occurs rapidly to produce high optical yields. The efficacy of this reaction is ascribed to the flexibility of the ligand, which speeds the reaction and gives a fixed chelate ring conformation (26). 

There are practically no quantitative measurements concerning the equilibrium la lb. Dwyer and coworkers studied the correlation between chelate ring conformation and complex configuration in a series of ethylenediaminepropylenediamin-cobalt(III) complexes. From the results, they conclude that the methyl group of propylenediamine is always in an equatorial position25,26). 

For complexes with bidentate 1,2-diamines only one type of chelate ring conformation could be ascertained. The six-membered chelate rings of 1,3-diamines may exist in a chair 6a, skew boat 6b or boat 6c conformation. The least stable of the three types is 6c and was not taken into account so far. When the chelate ring carries... 

Most CD spectral studies of cobalt(III) complexes have been undertaken to investigate various sources of optical activity such as distribution of chelate rings, conformation of chelate rings, vicinal effect due to asymmetric carbon in an optically active ligand, and vicinal effect due to an asymmetric donor atom. Extensive reviews on these subjects have been written by Fujita and Shimura (1), Hawkins (2), and Mason (3). 

Chelate ring conformations in chelated diaminoethane (ethane-1,2-diamine, en), designated as 8 and X. Views looking into the N—M—N plane (centre H atoms bound to C atoms disposed roughly in the plane, Heq, and perpendicular to the plane, Hax, also included) and along the C—C bond (sides H atoms removed for clarity) are shown. Ready interconversion between the two conformations (which are mirror images) is possible, as only a small energy barrier exists between them. 

Nickel(ii) complexes of L-amino-acids have been studied by spectral methods and the results interpreted in terms of the chelate ring conformations. The reactions of LH- Cl" with NiCl and LH + CIO wjth NiCOj yield [NiLClJ... 

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. 

Assuming chelate ring conformations, which are analogous to those predicted to be the most strain free for trans-Co(cyclam)X2 " there are 8 meso and 32 racemic diastereoisomers for the Ni(CTH) system. These arise from the asymmetry of the four nitrogen donor atoms and from the asymmetry of two carbon atoms present in the macrocyclic ligand. The asymmetric carbons occur at positions 7 and 14 in 1,7-CTH (Structure II), and at positions 5 and 14 in 1,4-CTH (Structure I). Consideration of absolute and relative configurations is useful in the identification of various... 

The following summarizes the conclusions that have been made on the basis of the methyl proton resonance assignments (Table 3) and the observations made with respect to methyl group orientations and chelate ring conformations as a function of the symmetry (Table 1 and 2) for each of the... 

The CD curves of [Co(-)-pn(NH3)4]Cl3, S9 trans-[Co -)-pn2(NH3)2]Cl3,s and the vicinal or conformational effects of ( —)pn Ss in tris(diamine) complexes are strikingly similar. Such similarities through the series do not lead one to expect significant changes in the frequency intervals for tris-(diamine) complexes as chelate ring conformations change. 

SS (RR, RS, or SR) refers to the absolute configurations of the nitrogens of the secondary amines these are fixed by the chelate ring conformations. The absolute configurations of amino acids are designated S (for L) or R (for D). 

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