Interconversion of enantiomers

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

Fig. 9 Interconversion of enantiomers of A, A -dimethyl-2-(aminomethyl)phenyl iodo-telluronium iodide (18) and A, A -dimethyl-2-(aminomethyl)phenyl bromotelluronium bromide (19).

The interconversion of enantiomers can be viewed in general as requiring inversion at a particular atom or twisting about an axis of the molecule. Provided these processes are inhibited to some degree, the chirality of a molecule will be detectable so that any chiral species may be said to contain centres and/or axes and/or planes of chirality.115,116,117,118 The precise meanings and utility of these concepts are, however, a matter of some debate,115,116 129 and they have not been extensively applied to coordination compounds. 

Many chiral coordination compounds are very stable, and as such can be isolated, and studied by the various spectroscopic techniques described in Section 5.3. It is a very common situation in solution, however, that chiral coordination compounds may undergo internal rearrangements or ligand exchange with other excess ligand molecules in solution resulting in an interconversion of enantiomers. This process is... 

Tris-chelate complexes exist in enantiomeric configuration A and D about the metal atom, and when the chelating ligand is unsymmetrical, there are also geometrical isomers, cis and trans. Each geometrical isomer exists in enantiomeric forms thus there are four different molecules. In the case of tris complexes with symmetrical ligands, the process of inversion (interconversion of enantiomers) is important. When the metal ions are of the inert type, it is often possible to resolve the complex then the process of racemisation can be followed by measurement of optical rotation as a function of time. Possible pathways for racemisation fall into two broad classes those without bond rupture and those with bond rupture. 

We call the interconversion of enantiomers enantiomerization, the disappearance of optical activity by enantiomerization racemization, and the induction of optical activity by enantiomerization dera-cemization... 

When the reaction was run with optically active starting material, it was found that interconversion of enantiomers of the reactant occurred without any measurable isotope... 

It had previously been found that the rate of interconversion of enantiomers ( rac) is several times greater than the rate of isomerization of the geometric isomers (ifcjgom). In the cis system (7) little exchange is... 

Rapid interconversion of enantiomers of (4) occurs at room temperature, as was shown by the NMR data. Each of the two protons of each CH2 group is equivalent to the other. The H NMR spectrum is unchanged when the temperature is decreased to — 95°C (Scheme 23) <85JA590i>. 

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