High-throughput screening of organic reactions in microdroplets using desorption electrospray ionization mass spectrometry (DESI-MS): hardware and software implementation.

This study describes an automated system used for high throughput screening of reaction conditions based on accelerated reactions occurring in small volumes of reagents. Reaction mixtures are prepared in array format using a fluid handling robot and spotted on a flat polytetrafluoroethylene plate at densities up to 6144 per plate. The reaction and analysis steps are performed simultaneously using desorption electrospray ionization (DESI) to release microdroplets containing the reaction mixture from the plate for reaction prior to arrival at a mass spectrometer. Analysis rates are up to 1 reaction mixture per second and data are recorded in real time using an ion trap mass spectrometer. Beacon compounds are used to triangulate position on the plate and this allows tandem mass spectrometry (MS/MS) to be performed on confirm products of interest. Custom software allows the user to control the system. It is also used to receive data from the DESI mass spectrometer to screen the spectra for compounds of interest, to perform MS/MS and to save data. This custom software also communicates with the software controlling the fluid handling robot (Biomek i7) as well as the Beckman software used to prepare reaction mixtures and also the software that controls the solvent used as the DESI spray. Data were recorded for N-alkylation, N-acylation and N-sulfonylation reactions in three 8 hour experiments on successive days to establish the ruggedness and repeatability of the system. Repeatability is high (94-97%) over this period with false negative 6% (depending on noise threshold chosen). Plates containing 384 reaction mixtures are analyzed in 7 min by moving the DESI sprayer in steps under the sprayer instead of continuously.

High-Throughput Experimentation and Continuous Flow Evaluation of Nucleophilic Aromatic Substitution Reactions.

Nucleophilic aromatic substitution (SNAr) reactions were optimized using high-throughput experimentation techniques for execution under flow conditions. A total of 3072 unique reactions were evaluated with an analysis time of ∼3.5 s per reaction using a system that combines a liquid handling robot for reaction mixture preparation with desorption electrospray ionization (DESI) mass spectrometry (MS) for analysis. The reactions were performed in bulk microtiter arrays with and without incubation. In-house developed software was used to process the data and generate heat maps of the results. This information was then used to select the most promising conditions for continuous synthesis under microfluidic reactor conditions. Our results show that this HTE approach provides robust guidance for narrowing the range of conditions needed for optimization of SNAr reactions.

High Throughput Experimentation and Continuous Flow Validation of Suzuki-Miyaura Cross-Coupling Reactions.

Traditional methods to discover optimal reaction conditions for small molecule synthesis is a time-consuming effort that requires large quantities of material and a significant expenditure of labor. High-throughput techniques are a potentially transformative approach for reaction condition screening, however, rapid validation of the reaction hotspots under continuous flow conditions remains necessary to build confidence in high throughput screening hits. Continuous flow technology offers the opportunity to upscale the screening hotspots and optimize their output of the target compounds due to the exceptional heat and mass transfer ability of flow reactions that are conducted in a smaller and safer reaction volume. We report a robotic high throughput technique to execute reactions in multi-well plates that were coupled with fast mass spectrometric analysis using an autosampler to accelerate the reaction screening process. Palladium-catalyzed Suzuki-Miyaura cross-coupling reactions were screened in this system and a heat map was generated to identify the best reaction conditions for downstream scale-up under continuous flow. Here, high throughput experimentation reactions in 96-well plates were performed for 1 h at 50 °C, 100 °C, 150 °C, and 200 °C before diluting them into 384-well plates for mass analysis. With the aid of high throughput tools, 648 unique experiments were conducted in duplicate. The cross-coupling reactions were evaluated as a function of stoichiometry, temperature, concentration, order of addition, and substrate type. The hotspots from high throughput experimentation were examined using a microfluidic Chemtrix system that confirmed the positive reaction leads as true positives. Negative outcomes identified by these experiments were found to be true negatives by microfluidic reaction evaluation. Quantitation of product yields was performed using high-performance liquid chromatography-mass spectrometry (HPLC/MS-MS).

High throughput reaction screening using desorption electrospray ionization mass spectrometry.

We report the high throughput analysis of reaction mixture arrays using methods and data handling routines that were originally developed for biological tissue imaging. Desorption electrospray ionization (DESI) mass spectrometry (MS) is applied in a continuous on-line process at rates that approach 104 reactions per h at area densities of up to 1 spot per mm2 (6144 spots per standard microtiter plate) with the sprayer moving at ca. 104 microns per s. Data are analyzed automatically by MS using in-house software to create ion images of selected reagents and products as intensity plots in standard array format. Amine alkylation reactions were used to optimize the system performance on PTFE membrane substrates using methanol as the DESI spray/analysis solvent. Reaction times can be <100 µs when reaction acceleration occurs in microdroplets, enabling the rapid screening of processes like N-alkylation and Suzuki coupling reactions as reported herein. Products and by-products were confirmed by on-line MS/MS upon rescanning of the array.

Reaction screening and optimization of continuous-flow atropine synthesis by preparative electrospray mass spectrometry.

Preparative electrospray (ES) exploits the acceleration of reactions in charged microdroplets to perform a small scale chemical synthesis. In combination with on-line mass spectrometric (MS) analysis, it constitutes a rapid screening tool to select reagents to generate specific products. A successful reaction in preparative ES triggers a refined microfluidic reaction screening procedure which includes the optimization for stoichiometry, temperature and residence time. We apply this combined approach for refining a flow synthesis of atropine. A successful preparative ES pathway for the synthesis of the phenylacetyl ester intermediate, using tropine/HCl/phenylacetyl chloride, was optimized for solvent in both the preparative ES and microfluidics flow systems and a base screening was conducted by both methods to increase atropine yield, increase percentage conversion and reduce byproducts. In preparative ES, the first step yielded 55% conversion (judged using MS) to intermediate and the second step yielded 47% conversion to atropine. When combined in two discrete steps in continuous-flow microfluidics, a 44% conversion of the starting material and a 30% actual yield of atropine were achieved. When the reactions were continuously telescoped in a new form of preparative reactive extractive electrospray (EES), atropine was synthesized with a 24% conversion. The corresponding continuous-flow microfluidics experiment gave a 55% conversion with an average of 34% yield in 8 min residence time. This is the first in depth study to utilize telescoped preparative ES and the first use of dual ESI emitters for multistep synthesis.