Hello,
I am working on a C-C coupling of a strong electron accepting aryl-cl and Bu3Sn-Benzonitrile. I was able to synthesize my product but the yield is very low.
I always have a big residue/ side products. It consists of some insoluble black powder and also a soluble dark red, like strong red wine, compound. Also a sideproduct is the dimerisation of the organo-tin compound.
Does anybody have more experience about the catalyst degradation? How does it look like usually?
Leo Sagurna dis you follow a specific literature procedure? Unchanged Pd(PPh3)4 would be yellow. The observation of a black color certainly indicates the formation metallic palladium. Perhaps you can get some more information from the attached reference. Good luck!
Thanks Frank T. Edelmann for your answer. I saw through the paper but couldn't really find any observations covered my question.
I am not that experienced in crosscouplings of other molecoules.
I dissolved the aryl-Cl and 5 m% of the cat in warm dioxane and then slowly add the oranostannane. All under Ar and with about 10 min degassing by bubbling Ar through the solution.
The starting solution is yellow and becomes organge to brownish by adding the stannane. After some hours it is a black suspension. Also some deep redblackish unknown slightly soluble side product emerges which covers anything other.
My product is yellowbrownish, the educt is bright yellow, the dimer of the homocoupling is yellow.
Do I even have active catalyst left after some hours or can I assume that everything is decomposed? Does someone know if the catalyst can also change to that red colour?
I also tried other protocols. Exchange of solvent with THF gives similar results. Additives like CsF, CuI or exchange of L (PtBu3 or Xphos) or change of solvent to NMP, DMSO didn't give any yield. I am unfortunately stuck to the aryl-chloride since there is no alternative.
https://pubs.rsc.org/-/content/articlelanding/1999/p1/a808261h/unauth#!divAbstract
Leo Sagurna the question remains: Did you follow a literature procedure? I wonder what has been described on the reference.
There is no reported C-C coupling with my educt yet, probably because it doesn't work so well. But I tried anyways. So I was told take 5 m% of catalyst and I read that many solvents are tolerated in Stille coupling. I tried these "standard conditions". My educt is only soluble and stable in THF and dioxane thats how I came there. I don't have any sources to this procedure. This is how I got the best results.
But since my yield didn't satisfied me I was looking for improvements, especially protocols that used aryl-chlorides. So I came to following procedures which I tried with no success.
Mee, S. P. H., Lee, V., & Baldwin, J. E.: Stille coupling made easier - The synergic effect of copper(I) salts and the fluoride ion. Angewandte Chemie - International Edition 2004 43(9), 1132–1136
Littke, A. F., Schwarz, L., Fu, G. C.: Pd/P(t-Bu)3: A mild and general catalyst for stille reactions of aryl chlorides and aryl bromides. Journal of the American Chemical Society 2002 124(22) 6343–6348
Naber, J. R., & Buchwald, S. L.: Palladium-catalyzed Stille cross-coupling reaction of aryl chlorides using a pre-milled palladium acetate and XPhos catalyst system. Advanced Synthesis and Catalysis 2008 350(7–8), 957–961
I couldn't yield any product. Thats the experiments where I changed the solvent, the ligand or any additives.
I hope this answers your question.
Dear Leo Sagurna many thanks for the information. I must admit that I'm not an expert enough to tell you exactly what went wrong in your case...😢
Catalyst Pd(PPh3)4 has yellow colour before coupling where as black color due formation metallic palladium after C-C coupling .
Thank you for your answer. The yellow colour I was aware of since I started the reaction.
My question is more considered to the states in between. Is there a general colour for the state after oxidative additon or after transmetallation or is this always different? How do your reactions look during the reaction? Is there a trend that the active species usually looks like or are there big differences each time?
As I repeat my question maybe more understandable:
My reaction looks like this. Educt (aryl-chloride - which is known to act maybe more like an acid-chloride) is bright yellow, the catalyst is a greenish yellow. In the heat combined they form a orange yellow solution. After I put the organotin reagent it quickly turns darker, brownish. After half an hour I can't say because it is to dark to discover any colour. Probably but I can not say there is already a suspension formed.
The black colour due to the metallic palladium. How fast does it occur in your reactions? I assume the metallic-pd is not an active species of the catalyst.
So my hope is if maybe I learn more just about how Stille-coupling generally looks like I can maybe presume some direction for improvements.
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