1
$\begingroup$

I am conducting an organic synthesis involving Stille coupling, using $\ce{Pd2(dba)3}$ and $\ce{P(o-tol)3}$ as catalysts. Reaction solvent is toluene in which the catalysts also dissolve. Many texts said that celite pad can filtrate these catalysts, but I failed. I tried vacuum filtration using buchner filter, and poured $\pu{0.0087 g}$ $\ce{Pd2(dba)3}$ in $\approx \pu{25 mL}$ toluene. $\ce{Pd2(dba)3}$ in toluene has dark red color, but filtrate also has same color. TLC spot was also in same position. Is there any other way than column chromatography?

$\endgroup$
3
  • 2
    $\begingroup$ Filtration is not going to remove the phosphine, or the phosphine oxide which is also likely to be present as they are both toluene soluble. You are going to have to run chromatography. $\endgroup$ – Waylander Jun 21 at 6:46
  • $\begingroup$ Thank you for advice. Does filtration work for Pd catalyst? I saw many papers using Pd catalyst employ Celite filtration. (although I failed) $\endgroup$ – Krang Lee Jun 21 at 6:59
  • 2
    $\begingroup$ Celite filtration only removes fine solids, so may take out degraded catalyst. You still have to run chromatography to get rid of tin residues. $\endgroup$ – Waylander Jun 21 at 7:21
6
$\begingroup$

Your question is actually two: how do I remove residual catalyst, and how do I remove residual ligand. The situation is complicated yet further by the fact that both catalyst and ligand are capable of being oxidised, for example the phosphine ligand becoming the phosphine oxide during the course of the reaction due to adventitious oxygen.

As pointed out, Celite only really removes particles - it can often be good enough to carry on, but won’t get rid of everything. You can also try filtering through scavengers - these are resins designed to specifically ligate metals.

On a small scale, the easiest option is to concentrate the reaction mixture and purify by silica chromatography. Depending on the polarity of your compound, it’s often possible to elute the molecule of interest whilst retaining the catalyst/ligand.

On larger scales, chromatography becomes challenging and time consuming. Carefully designed work ups, crystallisations, and precipitations can give pure material with incredibly low ppm values of residual metal.

The key question is always whether the material is clean enough for the next step. Often, the easiest and most efficient option is to filter through Celite, evaporate, and carry on as it only really matters whether your final step is clean (for example if you do a Buchwald and a subsequent Boc deprotection, it can be significantly easier to clean the material up after the second step wheee you can make use of a strongly basic centre).

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.