[hans50316]

Now I'm working on borylation reaction. For my reactant, I have two bromo group on para position.
I used bis(pinacolato)diboron, Pd(dppf)₂Cl₂, potassium acetate, and oxygen free DMF, the reaction was carried at 80 degrees and under N₂ atmosphere. However, the main product was substituted by methyl group on both side.
(I'm not sure if i get any boronic ester substituted compound yet.)

Any thought about why the side reaction went so well?

[Dan]

Your description is confusing, can you draw a scheme please?

[TheUnassuming]

I've done this reaction with several different substrates and never seen that... where are you getting the methyl boron species that gets coupled?
Double check all your starting material purities, something seems off.

[hans50316]

Sorry for the misleading. I drew my reactant in a more specific way this time. Actually the borylation of both side can be done when using dioxane as solvent. However if I used DMF, i only got the compound as draw below.

And according to Miyaura's paper, higher polarity solvent tend to give good yield, so i'm confused about the result now.   

[TheUnassuming]

Hrmm... in general I avoid using DMF when I need to heat it up very much.  It tends to decompose and is probably the source of your random methyl groups that are getting attached.  Why don't you want to use dioxane?

[hans50316]

Dioxane works, but with low yield. I was just thinking  if I can make the yield higher with different solvent.  Thanks for your reply, I never considered  DMF might decompose my compound.

[TheUnassuming]

There are other solvents compatible with pd coupling, though dioxane is the only one I've used for BPin. 
With dioxane, are you getting decomposition/side reactions?

[hans50316]

I'm guessing I have dehalogenation as side reaction, not really sure about that.
Besides, the product seems to decompose on TLC plate, I guess it's not stable.

[TheUnassuming]

Dehalogenated products shouldn't decompose on silica, though if I was going to run a Pd mediated dehalogenation I would probably use DMF . 
So you get complete conversion of the starting material to mostly an unknown product that isn't acid stable + some desired product?  What other functional groups do you have on the molecule (I'm assuming you aren't using 1,4 dibromobezene without other functionalities)?

[hans50316]

We don't have oxygen free dioxane, but we dry dioxane and use balloon filled with N₂.
And after reaction, I still can observe starting material  on TLC. I'm not sure if it's because the solvent.
I have bromide at the end of R group and ether at the middle.

[k1mng]

Starting material sounds column stable to me. Is it possible that your low yield with dioxane is because you have oxygen in your solution which would oxidise your catalytically active Pd[0]? To make a solvent oxygen free, you can degas it by bubbling N₂ into it for a period of time. The dioxane needn't be dry, it just shouldn't have oxygen.

EDIT: Start with dry dioxane, I forgot to consider that the product could be activated by OH^- and couple with the starting material if water is present. Sorry!

[TheUnassuming]

Agreed.  If your solvent has O2 that will kill your turnover, giving side reactions a better relative rate.   Especially for any organometallic chemistry you should as a general rule degas your solvent before use.  The notvoodoo site has a good writeup on the common ways:
http://www2.chem.rochester.edu/~nvdtest/test/pages/how-to.php?page=degas_solvents

[hans50316]

When I said ''not oxygen free dioxane'', I mean we don't keep dioxane bottle in a glove box which has O₂ content lower than 10ppm.

Before reaction, we purge N₂ to the reactor, than evacuate by oil pump, repeat three times and start reaction.

Not sure if this is enough or we should have one bottle in glove box.

[k1mng]

Do you mean you purge/evac the vessel without the solvent? Doing this would dry + remove O₂ from the vessel but you will still introduce O₂ into the vessel via the dioxane that you add thereafter from the bottle unless you perform one of the degassing methods.

[hans50316]

Right, I should have added dioxane first than degas the vessel.
Thanks for all your advice, I'll try it again.