Chiral eutectic mixture

The topical homochirality problem is presently being investigated in several research laboratories across the world. One new object of study is systems with eutectic mixtures. The addition of chiral dicarboxylic acids that co-crystallise with chiral amino acids to aqueous mixtures of d- and L-amino acids allows tuning of the eutectic composition of the amino acids in several cases, these systems yield new eutectic compositions of 98% ee or higher. Thus, solid mixed crystals with a ratio... 

When a diastereomeric mixture is an eutectic mixture, the difference in solubility between the diastereomers can be roughly correlated to the difference in thermodynamic stability between them if the conditions for crystallization, such as temperature and solvent, are unchanged. Then it becomes meaningful to estimate the stabilities of a pair of diastereomeric crystals and to observe the correlation between the difference of the stability of the pair of diastereomeric crystals and the efficiency of the chiral discrimination. 

Wang, Y., R. LoBrutto, R.W. Wenslow, and 1. Santos (2005). Eutectic composition of a chiral mixture containing a racemic compound. Organic Process R D9, 670-676. 

Chiral (L-, or D-form) separation from an optical isomer is essential for its commercial use. Preferential crystallization is often used as a separation technique of a chiral from optic isomers, but it is invalid when racemic compounds are formed, as it is in the production of mandelic acid by chemical reaction. However, by using high pressure crystallization, it becomes possible to separate a chiral from Sie mixture in the region of racemic conq)ound formation under atmospheric pressure, when the eutectic conq)ositions shift toward the racemic composition under high pressure. 

The asymmetric polymerization in crystalline architectures provides an excellent environment to conduct the absolute asymmetric synthesis of polymers, and also provides an effident route for the ampHfication of chirality. Mirror-symmetry breaking might occur either through total asymmetric transformations, either in enantiomorphous crystals that have self-assembled from achiral molecules, or within racemic crystalline architectures which are delineated by chiral rims or surfaces when one of the chiral faces is blocked by an interface. The self-assembly of nonracemic mixtures into a mixture comprising eutectic compositions of a racemic compound and an enantiomorphous assembly, followed by asymmetric transformation, provides a series of thermodynamically controlled, alternative routes for the effident ampHfication of homochirality. 

Solubility data at the eutectic point for a conglomerate system (equivalent to the solubdity of the conglomerate) can be determined experimentally by measuring the total concentration in a saturated solution equilibrated with both enantiomers. The solubihty of each enantiomer should be half of this value. Solubihty data at the eutectic point for a racemic-compound-forming system, hence the eutectic ee, can be determined by measuring the concentrations of each enantiomer in a saturated solution, which is in equilibrium with a solid mixture of one enantiomer and the racemic compound. Because of the advancement of chiral HPLC in the past years, satisfactory results can be readily obtained and with a reasonable amount of material if an appropriate solvent is chosen. The same approach is applicable for obtaining the solubility of a racemic compound, where the solubility equals the total concentration of R and S in the supernatant saturated with racemic compound. If no material with ee of 0% is available, then the solubility of the racemic compound can be calculated from the following equation ... 

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