Quasi-racemates, crystallization

E. Vedejs, R. W. Chapman, S. Lin, M. Muller, D. R. Powell, Crystallization-induced asymmetric transformation vs quasi-racemate formation in tetrava-lent boron complexes, /o r a/ of the American Chemical Society 2000, 122, 3047. 

The stereochemical correlations between the enantiomorphic crystals and their chiral inhibitors, namely that the additive affects only the enantiomer of the same absolute configuration, provides us with a new method for the determination of absolute configuration on a relative scale. This is revealed independently by morphological changes and enantioselective occlusion of additives. The method is related to the quasi-racemate technique of Fredga [18], but has the advantage of a wider applicability. 

For example, the racemic N-acetyl-phenylglycine (AcPG) can be reacted with methyl (S phenylglycinate (MePG). The resolution can be treated as a recrystallization of a quasi enantiomeric mixture with eeo=50% from water when the less soluble diastereoisomeric salt (namely, the heterochiral quasi-racemic mixture) crystallized containing (R)-AcPG in good enantiomeric excess (ee 79%). 

When two couples of enantiomers have a high degree of similarity, the S enantiomers can fit in the same crystal lattice that is, a complete solid solution. This mechanism of substitution expands across the binary system (idem for the two R enantiomers). By contrast, the couples of antipodes (5 l-i 2 and S2-RI) lead to the formation of the so-called quasi-racemic compounds and , which can be considered as co-crystals (Figure 13.12). Prior to the emergence of the X-ray diffraction technique using the anomalous... 

Ordered binary crystals were obtained via cocrystallization of quasi-racemic cyclopalladated complexes in a borderline regime between inertness and lability.Braunstein ef al. devised routes for anchoring a cyclopalladated complex 72 inside a nanoporous alumina membrane. 

Pseudo-racemates are solid solutions (mixed crystals) of equimolecular proportion of the (+) and (—) enantiomer. Quasi-racemates are racemic solid solutions involving two very similar chiral substances. 

Quasi-racemate is a mixture of equimolar amounts of two analog chiral molecules that have the same constitution but opposite configuration. Although components of quasi-racemate are chemically different, they have similar geometry, charge distribution, can replace each other in the crystal lattice, and are stiU able to form a racemic crystalline phase. 

In order to pin-point the properties of order-disorder we concentrated on molecules whose racemates are centrosymmetric and the resolved enantiomorphs, therefore, quasi-centrosymmetric. These systems are advantageous since the enantiomeric crystal contains two independent molecules related to each other by a pseudo centre of inversion, and replaced in the racemate by a centrosymmetric pair. 

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