Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: AlphaScent on August 26, 2019, 10:27:09 PM
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Hey all!
Does any one have a trick to removing Pd(PPh3)4 from a cross-coupling reaction? In my case I did a Negishi coupling.
VinylMgCl was cooled to 0C and THF soln of ZnBr2 is added and stirred for 15 min. My vinyl iodide was dissolved in THF and 0.03 eq Pd(PPh3)4 was added. The homogeneous soln was added to the organozinc reagent. Let stir at RT 2.5 hr.
Quench with 3 aliquots of sat. ammonium chloride and then add 1/2 aliquot of hexanes. Extract aqueous. Wash aqueous with ammonium chloride, water, brine, dry. NMR looks great except the Pd(PPh3)4.
The problem is the complex is soluable in hexane/THF so celite is a no go. Any help would be great. I know removing these types of catalysts can be a pain. I am trying to do a reaction on scale and a column is just not practical.
Thank you in advance!
Cheers!
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sorry, wash organics...im tired and didnt proof read...
Prost!
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maybe try to precipitate it by addition of a bad solvent for Pd(PPh3)4 to your hexane solution, make sure your compound is soluble obviously.
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I am trying to optimize the procedure. It seems that a scavenger is maybe the best way to go. Specifically TMT. There is a polymer bound Pd scavenger made by biotage.
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The pph3 part can be removed by letting it convert to the oxide then using zinc chloride. It will make an adduct that can be filtered out.
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@wildfyr
When you say let it convert to the oxide do you mean to atmospheric oxygen?
I have bought some TMT and it is on its way. I am interested to see how well it works. It seems it is used in industry as a Pd scavenger a lot. I have also found that simple activated charcoal works to soak up some Pd, and then a simple Si plug works to purify the material to about 95% purity.
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Are you sure its the Pd(PPh3)4 complex? In theory at least two PPh3 have to dissociate from the Pd in order for the reaction to proceed and I think you get other PdX2/4 species on workup, so you might just have the triphenylphosphine problem... which is in some ways worse. I've heard PPh3 referred to as the herpes of reagents, and not without cause.
If it is tetrakis, you could probably oxidize the PPh3 and force the Pd to complex a less soluble ligand like wildfyr suggested.
Take it you don't want to run a column?
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Yes, though I guess some weak inorganic (i.e. easily aqueous seperable) oxidizing agent could do it too.
My suggestion was to oxidize the PPh3 and complex that to the zinc to make an insoluble salt. I'm not sure what exactly to do with the Pd. Maybe if you can make it into some insoluble Pd salt like palladium oxide it can just be removed with celite filtration.
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Why not chromatography?
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I am trying to do a reaction on scale and a column is just not practical.
Thank you in advance!
Cheers!
@rolnor
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Sorry, missed that!
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I think you might be able to oxidize PPh3 to OPPh3 and precipitate it with ZnCl2 and then maybe chelate the palladium with EDTA and wash it away? I recall talking to a industry guy at a conference that mentioned having problems removing residual palladium after hydrogenation so they had to change the pathway completely but AFAIK he mentioned its possible to remove with edta.