Reaction monitoring is regarded as an important aspect in an array of chemistry environments — ranging from drug discovery and chemical synthesis through to understanding protein synthesis and natural products.
For a majority of synthetic organic and medicinal chemists, it is important to understand the optimal time needed to quench a reaction mixture for maximum yield and also to achieve real-time reaction monitoring. Advion’s expression Compact Mass Spectrometer (CMS) was specifically developed for chemists to improve their workflow directly at the bench.
The single quadrupole detector is user-friendly, can be easily maintained, and can be integrated with the industry’s widest range of novel sampling methods from ultra-high performance liquid chromatography to direct probe analysis. Users can quickly shift between the various sampling methods needed all through the chemist’s workflow.
Application: Real-Time Monitoring of a Suzuki Reaction Via LC/CMS and FIA
Every year, medicinal chemists are routinely tasked to personally synthesize more than a hundred new chemical entities (NCEs) for testing as potential pharmaceutical drug candidates. The aim is to prepare a high-yield and comparatively pure product through an improved synthetic method. Although LC/MS and TLC are normally used for guiding the outcomes of the reactions, these techniques are usually available only through shared open access of systems or through a central core facility. For immediate answers and real-time monitoring, the CMS can be directly fitted into a fume hood for hood-based applications in the analysis of chemical reactions using either flow injection analysis (FIA) or LC/CMS.
Figure 1. The CMS with reaction equipment directly inside of a hume hood.
The outcomes shown for the LC/CMS analysis and FIA analysis (Figures 2 and 3) of the Suzuki reactions to prepare p-Aminobiphenyl show the proof of principle for monitoring reactions in real time. Both outcomes demonstrate the increase of product/reactant ratio over time. Using a selective detector for real-time monitoring of the reaction mixture improves the chemist’s workflow, resulting in a maximum yield of the preferred product within a short duration of time.
Figure 2. LC/CMS analysis of Suzuki reaction.
Figure 3. FIA analysis of Suzuki reaction.
Application: Reaction Monitoring via TLC
Thin layer chromatography, or TLC in short, is a simple and low-cost method that offers vital information regarding synthetic reactions and is typically used for monitoring reactions in organic synthetic and medicinal laboratories. The Plate Express® TLC Plate Reader from Advion easily integrates with the CMS and offers compound structural data directly from TLC plates without requiring any extra preparation.
Figure 4. Experimental set up of the CMS and Plate Express.
The outcomes of a Suzuki reaction for the synthesis of 4-aminobiphenyl demonstrate an extracted ion chromatogram (XIC) of the decrease of the reactant ion and the increase of the product ion over time, thus indicating real-time monitoring. At 180 minutes, the reactant ion was no longer detected, signifying that the reaction was complete. The CMS combined with the Plate Express enables users to achieve real-time reaction monitoring by assessing the mass spectra for structural data directly from the TLC plate.
Figure 5. XIC of the synthesis of 4-aminobiphenyl.
Application: Monitoring Synthetic Reactions via ASAP®
The Atmospheric Solids Analysis Probe (ASAP®) from Advion enables direct and immediate mass analysis. The ASAP® can be simply dipped into a liquid or swiped over a solid, and then the probe can be directly inserted into the CMS’ ASAP-enabled APCI ion source for analysis (see Figure 6). The ASAP® does not need any sample preparation or chromatography and offers sensitive analysis of a variety of compounds within 30 seconds.
Figure 6. ASAP with sample inserted into the ASAP-enabled APCI ion source of the CMS.
Two experiments were demonstrated using the ASAP®. In the first experiment, the synthesis of 4-iodoisoquinoline applying the techniques of Artis and Buchwald could not be tracked by TLC. Yet, with the help of the ASAP®, the reaction product — after reacting at 110 °C for 22 hours at m/z 256.02 — was readily observed (see Figure 7).
Figure 7. Mass spectra of the synthesis of 4-iodoisoquinoline.
The outcomes of the second experiment to establish the optimum stop time for the reaction based on the work of Yaetko et al demonstrate an increased formation of 6-iodotryptophan as well as the acetamide-protected version of 6-iodotryptophan product ions as the presence of 6-iodoindole reactant ions decreases (see Table 1). In addition, the timed study demonstrates that the reaction was reaching a plateau at about 60 minutes, indicating that the reaction may have been ceased at 60 minutes.
Table 1. Presence of reactant and product ions over time
||% 6-IODOTRYPTOPHAN PROTECTED
Application: Monitoring Air-Sensitive Reactions via iASAP
A modification of this method is the inert ASAP (iASAP®), which enables effortless sampling of air-sensitive compounds from reactions directly from a Schlenk line or glove box (see Figure 8). Metal-based compounds have been found to be useful in a number of fields like energy, clinical, environmental, food safety, and so on. The final step in a synthetic process is to produce the metal complex, when ligands are adhered to the center of the metal. After these metal complexes are made, they can be used anywhere from anti-cancer drugs to stereospecific synthesis. Reaction conditions should be able to provide the required product, and side products should be reduced as much as possible to increase yield.
Figure 8. The iASAP, an inert modification developed for the original ASAP, for CMS analysis of air-sensitive samples.
During the synthesis of a molybdenum complex, the product crystals tend to precipitate from the solvent, making it difficult to directly monitor the products; however, the reaction products, thus formed, continue to stay in solution and therefore can be tracked. When the reaction was subjected to air, there was a reduction in the formation of lower mass Mo monomers (see Figure 10) compared to when the reaction was safeguarded from the air (see Figure 9). The reaction’s byproduct indicates the progress of the reaction, even when the formation of products is not directly monitored.
Figure 9. Mass spectra of reaction under nitrogen protection from air.
Figure 10. Mass spectra of reaction with air introduced.
This information has been sourced, reviewed and adapted from materials provided by Advion.
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