Org Prep Daily

December 17, 2011

Fighting Pd colloids with salt

Filed under: procedures — milkshake @ 12:54 am

I have been running some debenzylations of a macromolecule with the Pearlman catalyst  in water. The hydrogenation often results in reaction mixtures with persistant dark colloids. I have seen this kind of problem before, with small molecule-hydrogenations on Pd/C though it was never quite as bad. I suppose this polymer loves to stabilize Pd nanoparticles in water.  Pre-activating the Pearlman catalyst with hydrogen prior the substrate addition does not help much.

I noticed that in this case 1) Celite and other brands of diatom-based filtration materials are ineffective for removing the dark colloids but filtration through a thick pad of charcoal actually works, to a degree, especially when combined with disposable plastic submicron Millipore filtration setup (pilfered from biologists);  filtration through charcoal tends to be slow and in some cases charcoal alone does not remove the colloids completely. 2) Saturating the hydrogenation reaction mix with salt before the filtration breaks the Pd colloids – they actually coalesce into a perfectly filterable precipitate and are removed  with the Pd-C catalyst.

I wonder if this high-salinity trick (in a polar solvent) could be employed in workup of other reactions suffering from product discoloration by colloids, i.e.  Ru-catalyzed periodate oxidations or olefin metathesis.

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