Recent studies have shown that total deracemization of a racemic suspension of a conglomerate forming compound can be attained in the presence of a racemizing agent through either attrition enhanced deracemization or temperature cycles. We experimentally investigate the deracemization of N-(2-methylbenzylidene)-phenylglycine amide, in the presence of DBU as racemizing agent in a mixture of isopropanol and acetonitrile (95/5 w/w), at several different operating conditions. Based on several experiments, we determine how the operating parameters influence the temperature cycles, by varying the initial enantiomeric excess, the cooling rate, the operating temperature range, and the system volume. We examine how each parameter affects the phenomena characterizing the temperature cycles, e.g., total process time or total number of cycles to attain deracemization. Finally, we discuss in general how to improve the performance of the process.

A suspension of crystals of both enantiomers of a conglomerate forming chiral compound can be deracemized by applying temperature cycles in the presence of a racemizing catalyst that enables in solution the conversion of the undesired enantiomer into the desired one. We aim at showing through experiments that, seemingly paradoxically, accelerating the racemization reaction by increasing the catalyst concentration speeds up the deracemization process if all the other parameters are left unchanged. We prove this by deracemizing, via temperature cycles, crystals of N-(2-methyl-benzylidene)-phenylglycine amide (NMPA) suspended in either pure acetonitrile (ACN) or a mixture of isopropanol/ACN (95:5 w/w) in the presence of different concentrations of the base catalyst 1,8-diazabicyclo-[5.4.0]­undec-7-ene (DBU). The applied periodic temperature cycles follow a rather standard protocol. Prior to that, we have fully characterized the racemization reaction rate, as a function of enantiomers’ concentration, catalyst concentration, and temperature, thus proving that the reaction is second order in NMPA and DBU, and estimating the Arrhenius parameters for the two solvent systems.

Solid-state deracemization via temperature cycles is a technique that has been shown to be effective to isolate the pure enantiomer of a conglomerate-forming compound. This process has a large number of operating parameters that can be adjusted according to system-specific properties. On the one hand, this feature makes the process flexible and prone to optimization. On the other hand, the design space is so large that experimental optimization of the process can become long and cumbersome. In this work, we achieve two results. First, we show that deracemization via temperature cycles works very effectively for two new experimental systems, namely, the chiral compounds 2-(benzylideneamino)-2-(2-chlorophenyl)acetamide (CPG) and 3,3-dimethyl-2-((naphthalen-2-ylmethylene)amino)butanenitrile (tLEU). Second, we propose a new approach for the design of an effective deracemization process via temperature cycles for a new compound. Therefore, in this work, we investigate the effect of different operating conditions, namely, the initial enantiomeric excess, the cooling rate, the temperature range, and the catalyst concentration, on the performance of deracemization via temperature cycles for the new compounds CPG and tLEU and for N-(2methylbenzylidene)phenylglycine amide (NMPA), which was already studied in a previous paper. On the basis of these outcomes, we conclude by proposing a model-free screening strategy for the design of an effective deracemization process via temperature cycles for a new compound.

In this work, we examine the chromatographic purification of the chiral amide N-(2-methyl-benzylidene)-phenylglycine amide (NMPA). Specifically, we find that its adsorption behavior on an AY polysaccharidic chiral stationary phase (amylose tris(5-chloro-2-methylphenylcarbamate)), with acetonitrile as mobile phase, follows an uncommon type-1 mixed Langmuir isotherm. Moreover, we measure the racemization rate of NMPA when polymer-supported 1,8-diazabicycloundec-7-ene (DBU) is used as a catalyst in a fixed-bed reactor. We also determine the system's solid−liquid equilibria at different temperatures. This characterization is necessary for the design of an integrated process where simulated moving bed (SMB) chromatography is combined with racemization for the recycle of the undesired enantiomer. On the basis of the physicochemical properties above, three-column intermittent SMB (3C-ISMB) experiments were run at different concentrations and compositions to prove that an enantiopure product can be obtained when the separation is operated at high feed concentrations (nonlinear chromatographic conditions) in the case not only of a racemic feed but also of nonracemic feeds, which are relevant for the integrated process when racemization is slow, and thus incomplete.

We address the purification of the target enantiomer of a chiral compound from its racemic mixture, through simulated moving bed (SMB) chromatography either as a standalone or combined with the recycle and racemization of the undesired enantiomer. We carry out a comparative assessment of the two processes with focus on the role of the racemization kinetics, on the effect of the selectivity of the chiral stationary phase, and of the upstream symmetric synthesis of the racemate. The analysis is general thanks to the methods adopted for the optimal design of the integrated process (with racemization) and for the sizing of the SMB unit, as well as for the use of the four generalized Langmuir adsorption isotherms. In this way, we determine quantitative criteria that show that the standalone SMB process outperforms the integrated process when the performance of the racemization catalyst or the chiral stationary phase is poor and the racemic feed is inexpensive.

Solid-state deracemization via temperature cycles is a promising technique that combines crystallization and racemization in the same batch process to attain enantiomer purification. This method is particularly attractive because the target enantiomer can be isolated with a 100% yield, and a large number of operating parameters can be adjusted to do this effectively. However, this implies that several choices need to be made to design the process for a new compound. In this work, we provide a solution to this dilemma by suggesting a simplified model-free design approach based on a single dimensionless parameter, that is, the dissolution factor, that represents the cycle capacity. This quantity is obtained from a novel rescaling of the model equations proposed in previous work and acts as a handy design parameter because it only depends on the operating conditions, such as the suspension density, the enantiomeric excess, and the difference in solubility between high and low temperatures in the cycle. With extensive modeling studies, supported by experimental results, we demonstrate the primary and general effect of the dissolution factor on the deracemization process and thus its relevance for the process design. Through both experiments and simulations, we rationalize and evaluate the process performance when periodic and non-periodic temperature cycles are applied to the deracemization of virtual and real compounds with different properties, that is, growth rate and solubility. Based on the approach proposed here, we clarify how the combined effect of more operating conditions can be exploited to obtain quasi-optimal process performance, which results superior when deracemization via periodic temperature cycles is performed.

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Inspired by deracemization via temperature cycles, which enables the collection of crystals of the desired enantiomer from an initially racemic mixture, we focus in this work on an alternative batch process, namely crystallization-induced deracemization. This process starts with a suspension of enantiomerically pure crystals, which undergoes a simple cooling crystallization, coupled with liquid-phase racemization. The experimental and model-based analysis of such a process, carried out here, revealed that: (i) deracemization via temperature cycles is a safe choice to operate with high enantiomeric purity, although its throughput is limited by the suspension density; (ii) if the distomer is less prone to nucleation, crystallization-induced deracemization is a simple process; however, its performance is strongly limited by the solubility; (iii) the purity achieved with crystallization-induced deracemization can be increased by utilizing large seed mass and by optimizing the cooling profile or catalyst concentration. Alternatively, the purity increases via partial dissolution of the seeds, which resembles the heating part of the deracemization process via temperature cycles.