We demonstrate the use of accelerated reactions with desorption electrospray ionization mass spectrometry (DESI-MS) as a tool for predicting the outcome of microfluidic reactions. DESI-MS was employed as a high throughput experimentation tool to provide qualitative predictions of reaction outcomes, so that vast regions of chemical reactivity space may be more rapidly explored and areas of optimal efficiency identified. This work is part of a larger effort to accelerate reaction optimization to enable the rapid development of continuous-flow syntheses of small molecules in high yield. In order to build confidence in this approach, however, it is necessary to establish a robust predictive connection between reactions performed under analogous DESI-MS, batch, and microfluidic reaction conditions. In the present work, we explore the potential of high throughput DESI-MS experiments to identify trends in reactivity based on chemical structure, solvent, temperature, and stoichiometry that are consistent across these platforms. N-alkylation reactions were used as the test case due to their ease of reactant and product detection by electrospray ionization mass spectrometry (ESI-MS) and their great importance in API synthesis. While DESI-MS narrowed the scope of possibilities for reaction selection among some parameters such as solvent, others like stoichiometry and temperature still required further optimization under continuous synthesis conditions. DESI-MS high throughput experimentation (HTE) reaction evaluation significantly reduced the search space for flow chemistry optimization, thus representing a significant savings in time and materials to achieve a desired transformation with high efficiency.

Mass spectrometry (MS) is used to monitor chemical reactions in droplets. In almost all cases, such reactions are accelerated relative to the corresponding reactions in bulk, even after correction for concentration effects, and they serve to predict the likely success of scaled‐up reactions performed in microfluidic systems. The particular chemical targets used in these test studies are diazepam, atropine and diphenhydramine. In addition to a yes/no prediction of whether scaled‐up reaction is possible, in some cases valuable information was obtained that helped in optimization of reaction conditions, minimization of by‐products, and choice of catalyst. In a variant on the spray‐based charged droplet experiment, the Leidenfrost effect was used to generate larger, uncharged droplets and the same reactions were studied in this medium. These reactions were also accelerated but to smaller extents than in microdroplets, and they gave results that correspond even more closely to microfluidics data. The fact that MS was also used for online reaction monitoring in the microfluidic systems further enhances the potential role of MS in exploratory organic synthesis.

Electrospray and Leidenfrost droplet accelerated reactions were used as a predictive tool for estimating the outcome of microfluidic synthesis as demonstrated by Wleklinski et al. Rapid analysis by electrospray-mass spectrometry (ESI-MS) also provided immediate feedback on reaction outcomes in flow reactions. Significant reaction acceleration was observed in electrospray relative to the corresponding bulk reaction. This rapid reaction screening and analysis method has allowed for the detection of previously unreported outcomes in the reaction between 5-chloro-2-(methylamino)benzophenone and haloacetyl chloride (halo = Cl or Br) in the continuous synthesis of diazepam. In our current study, a more detailed extension of the previous work, we report acceleration factors that are solvent dependent; additional byproducts that were observed on the microfluidic scale that were absent in the droplet reactions. Gaining insight from this combined droplet and microfluidic screening/rapid ESI-MS analysis approach, we have helped guide the synthesis of diazepam and showcased the potential of this method as a reaction optimization and discovery tool. Informed by these new insights, diazepam was synthesized in a high-yield two-step continuous flow process.