Deracemisations under kinetic and thermodynamic control
Deracemisation reactions occur when a racemic mixture is converted into a nonracemic mixture by increasing the quantity of one enantiomer at the expense of the other. This process can take place under thermodynamic control, but when combined with crystallisation processes kinetic factors also play a role. This review summarises the different approaches that have been taken to achieve efficient deracemisations. Starting from examples in which spontaneous symmetry breaking was found to occur, attrition enhanced deracemisation will be discussed in which solid-solution equilibria drive the deracemisation process. The combination of detailed experimental studies and mathematical models resulted in a profound understanding of this complex process, which is applicable to all congomerate forming compounds with a racemisable stereocenter. Then, we focus on deracemisations that occur under full thermodynamic control. Especially the combination of supramolecular interactions with a racemisation process gives interesting results, albeit that they are less predictable. The review will end with the possibilities supramolecular helical structures that show dynamic helicity can offer in conjunction with asymmetric catalysis. Herein, the helical preference induced by a minute amount of chiral compound is relayed to high enantiomeric excesses in a variety of reactions.