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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: jayana on December 14, 2012, 02:43:20 PM

Title: Pd black in Suzuki coupling reactions-how does it form & what tricks to avoid
Post by: jayana on December 14, 2012, 02:43:20 PM

Hello All,

I have been performing a Suzuki reaction between an electron-rich aryl bromide and an aryl-BF3K with EWG, and have noticed the reaction mixture going from orange to black within 30 to 60 minutes of reflux (solvent used was PhH/EtOH, base 2M Na2CO3 and catalyst-Pd(PPh3)4). The reaction either gets halted or half of the SM gets converted to the corresponding protodehalogenated material.

Could someone shed light on the mechanism for the formation of Pd-black (references/articles) & what tricks are there for avoiding it?  Do other Pd catalysts help?  What about additives/solvents?  Also, should you use a large excess of ArBF3K to avoid the reductive side reaction you are seeing?  (How is this usually avoided?).

Thank you.
Jayana

Title: Re: Pd black in Suzuki coupling reactions-how does it form & what tricks to avoid
Post by: discodermolide on December 14, 2012, 02:50:42 PM

Try using a different base.
I though this was not such a problem with the BF3 salts. According to Molander they seem to be better.

Title: Re: Pd black in Suzuki coupling reactions-how does it form & what tricks to avoid
Post by: OC pro on December 14, 2012, 04:56:19 PM

Suzukis are usually very fast. If the setup is good you will have a nice conversion. Almost no dehalogenation is observed.
Did you degas well? After dissolving everything, you should thoroughly evac and backfill with N2 or argon several times. Then add Tetrakis Pd.
How old is your Tetrakis? Still yellow-orangish crystals?