I'm conducting an organic synthesis involving Stille coupling. Stille coupling uses $\ce{Pd2(dba)3}$ and $\ce{P(o-tol)3}$ as catalysts. Both catalysts dissolves in toluene which is the solvent of my experiment. I have considered several ways to filter these catalysts (e.g. Celite filtration), and I just noticed there are syringe filters in my lab. Is it possible to filter Stille catalysts using syringe filter? I have not used the syringe filter, so I searched for its usage. It says that syringe filter can be used for removing particulates from solvent (also for HPLC preparation) according to its material and pore size. Syringe filter which I have is Whatman™ puradisc 25 TF 0.45 μm PTFE membrane with polypropylene housing. Diameter is 25 mm.
1 Answer
The short answer is no, as such this task is not possible.
You mention $\ce{Pd2(dba)3}$ and $\ce{P(o-tol)3}$ dissolve in the solvent of reaction; this describes an example of homogeneous catalysis, where the reagents share a phase in common. A simple filtration to remove the Pd catalyst will not work. It is like passing water of the ocean through a paper filter; the filtrate still contains the dissolved salt. The separation by Celite (or similar, by silica) added works because the catalyst adsorbs to the surface of grains of Celite in first place. And if these grains are larger in diameter than the pores of the filter in second place, the filtrate will contain less of the catalyst.
Thus passing the raw reaction mixture across a short plug of Celite, preferentially with pressure from above of the short column, rather than by gentle suction from below the glass filter / Buchner filter is your likely next unit operation ahead. Equally, it is better to start a filtration with a coarse filter in first place (which the pad of Celite may be suitable, too); large particles may rapidly clog the fine pores of a syringe filter. Because said adsorption of the catalyst to Celite competes with any other constituent of the reagent solution, you may loose some of your starting material / reaction product by this operation.
If you are interested to reuse the catalytic system (not a priori excluded from the present question), you may consider to change for heterogeneous catalysis instead. Then, catalyst and reagent do not share the same phase and the reaction takes place at the interface of the two. Either
a) perform the reaction under conditions of phase-transfer catalysis with water soluble catalysts. This may be achieved e.g., by addition of $\ce{-SO3H}$ groups to the ligand (e.g., TPPTS), or using otherwise charged catalysts (an example). Ideally, after completion of the reaction, similar to a liquid-liquid extraction, you simply let the two layers separate from each other.
.or.
b) You chemically bind your catalyst to a solid support, e.g., a polymer resin. Depending on the reaction conditions, some of the Pd will bleach out into the reaction mixture, yet the larger fraction of the catalyst will be removed from the reaction mixture by filtering-off the beads (an example) across a coarse filter.
The transition between homogeneous and heterogeneous conditions may require additional work to identify optimal reaction conditions. It depends on the scale of reagents to process and other parameters (e.g., acceptable concentration of remaining Pd in the product) if such an investment is justified.
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1$\begingroup$ Glass and Buchner filter retain material because of its granularity, column chromtography by physio-/chemisorption. Celite is more coarse than the silica you purchase to pack your (low pressure) column / flash chromatography; the consists of fine grains (around 40 to 60 µm in diameter) which frequently are fine enough to pass a filter, then trapped in the product; it remains undetected by 1H and 13 C NMR and may cause (especially on small scale [50 mg reactions]) yields greater 100%. Glass wool / sand pad below the silica + caution with the pressure applied aim to prevent this breakthrough. $\endgroup$ Commented Jun 11, 2021 at 12:11
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1$\begingroup$ If you work on real small scale, you may pack a column in a Pasteur pipette (an example). The Baran lab showcases a step-by-step procedure if you choose test tubes as column (entry), just in case you already have more material at disposition. The later takes some training for the later, but once you are used to it, it is quick and good enough. $\endgroup$ Commented Jun 11, 2021 at 12:12
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1$\begingroup$ If both Celite and silica for chromatography are in the shelf and for volumes of reaction greater than say 25 mL, I usually choose Celite (don't forget to wet the pad with the solvent of reaction prior, and to rinse a little after the filtration; both increase the volume of the filtrate compared to the initial volume) and then carefully concentrate the filtrate (reduced pressure) close to dryness. The examples with the Pasteur pipette column originally are about chromatography / separation of multiple fractions (at small scale); if you elute well, they may be used as a small scale filter, too. $\endgroup$ Commented Jun 12, 2021 at 11:57
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1$\begingroup$ If you lack experience for certain manipulations (perhaps especially at smaller scales), train yourself multiple times with a cheap mix in first place. Once you acquired the dexterity (when you may proceed without need to go back to the lab notes to read the next line, perform the work, and return to the notes to read the next line, etc.) you apply this on your «real stuff», expensive because you spent x hours of reaction + x amount of money. Here, why not mix toluene, vanilin (e.g., from the TLC stain) and active carbon / fine charcoal and test filtering and determine yield of recovery. $\endgroup$ Commented Jun 13, 2021 at 9:30
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1$\begingroup$ And see if your hosting group / scientific library has a primer / an introduction about typical lab techniques. As an example, I recall Zubrick's book as a gentle introduction in fairly accessible English, but your librarian / colleagues may indicate to similar references. On occasion, you find good tutorials on the web, too example. $\endgroup$ Commented Jun 13, 2021 at 9:37