Toward Continuous Deracemization via Racemic Crystal Transformation Monitored by in Situ Raman Spectroscopy

In this work, we demonstrate a semibatch solid-state deracemization process for N-(2-chlorobenzylidene)henylglycine amide (NCPA), a complex chiral polymorphic system that involves three types of crystalline racemates (racemic compound and conglomerate forms I and II). In this process, gradually fed metastable racemic compound crystals are converted in situ to crystals of the preferred (seeded) enantiomer under grinding conditions through a series of solvent-mediated transformations in a racemizing solution. The phase diagram for this system shows that while conglomerate form II is stable under the conditions examined (acetonitrile at 21 °C), form I crystals of a single enantiomer (used as seeds) are unstable at (nearly) racemic compositions and convert to the racemic compound upon addition of the racemization catalyst. Thus, care needs to be exercised in order to fully convert form I to form II before addition of the racemization catalyst in order to prevent the undesired crystallization of the racemic compound. This can be achieved by adding a small amount of water, which is found to enhance the nucleation and growth kinetics of the most stable conglomerate form II, eventually leading to complete deracemization. Importantly, we show that this special deracemization process can be easily monitored online by Raman spectroscopy, which gives access to the evolution of the solid-phase composition. For the studied system, this information can in turn be used to directly estimate the solid-phase enantiomeric excess online throughout the process, as long as conglomerate crystals of the counter enantiomer do not form.