On-line sampling and analysis of heterogenous reaction mixtures is a critical application of modern process analytical technology (PAT). This application note describes the monitoring of a heterogenous Suzuki coupling reaction using the compact Axcend InFocus™ sampling system coupled to the Axcend Focus LC®, all contained within a standard chemical fume hood. Over the course of the 3-h reaction, samples were taken every ~18 min for the first 1.5 h, and every ~30 min for the second 1.5 h. This process consumed a total of 333 μL of reaction mixture, representing <1% of the 50-mL reaction volume. The solid catalyst was captured using an in-line filter during sampling, which was subsequently flushed back into the reaction vessel. If desired, the system could be operated to collect the catalyst in a separate vessel.
On-line reaction monitoring is a critical PAT that enables automated periodic sampling and analysis of a reaction vessel to provide insights into reaction progression, product/impurity formation, and experimental optimization. For bench-scale reactions in which reaction volumes are small, sampling volumes must be kept to a minimum to avoid impacting the reaction conditions and consuming excess product. Additionally, the analytical instrumentation must be located near the reaction vessel to limit dilution due to Taylor dispersion and changing reaction conditions that occur during transfer in the sampling lines. Compact capillary-scale reaction monitoring LC systems have previously been used for monitoring homogenous reactions over hours or days requiring only microliters of sample.1
Frequently, reactions are conducted in heterogenous mixtures containing solid catalysts, precipitates, or solid salts. For these reactions, on-line monitoring requires filtration before analysis. Using the Axcend InFocus system, samples can be withdrawn in microliter sized aliquots, filtered, and delivered online to the LC for analysis. After sampling, the filter can be rinsed to reclaim trapped catalyst and to prevent clogging. To demonstrate the capabilities of the combined Axcend sampling-LC system, a heterogenous Suzuki coupling reaction was selected as relevant to both pharmaceutical and material science industries. The reaction was sampled every ~18 min for 1.5 h and then every ~30 min for an additional 1.5 h, consuming a total of 333 μL of sample. The use of compact, capillary-scale instrumentation for heterogenous reaction monitoring allows for automated analysis of bench-scale reactions while considerably limiting the sample and solvent consumed during the process.
A Suzuki coupling reaction between iodobenzene and phenylboronic acid pinacol ester was performed using a diphenylphosphine palladium (II) catalyst (SiliCat DPP-Pd) bound to ~100 μm particles as shown below:
The reaction was sampled using the InFocus sampling system fitted with a 47 cm long 250 μm inner diameter (i.d.). sample line (~23 μL volume). Samples were taken every ~18 min for the first 1.5 h, and every ~30 min for the subsequent 1.5 h. Samples (60-μL aliquots) were withdrawn from the reaction mixture through a 0.5-μm filter before filling the LC injection loop. After injection, the filter was flushed with 100 μL of methanol (MeOH) back into the reaction mixture, returning the catalyst and allowing heterogenous reaction sampling without rapidly clogging the filter.
| Column | Waters 150 mm x 0.3 mm ID, 1.8 μm particles, NanoEase HSS T3 |
| Mobile phase | A) Water with 0.1% trifluoroacetic acid (TFA) B) Acetonitrile with 0.1% TFA |
| Gradient | 70% B to 80% B over 10 min |
| Flow rate | 4 μL/min |
| Injection volume | 20 nL |
| Temperature | Ambient |
| UV-absorption detection wavelength | 255 nm |
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