[nox]

I'm making some aryl pinacol esters through the Miyaura borylation reaction. Following a prep by Buchwald (ACIE, 2007, 46, 5359) which uses 3 equivalents of B₂pin₂. The reaction works really well but for this one substrate the B₂pin₂ co-elutes with my product. Is there anything I can do to remove the excess  B₂pin₂ short of derivatizing my pinacol ester?

[discodermolide]

I'm making some aryl pinacol esters through the Miyaura borylation reaction. Following a prep by Buchwald (ACIE, 2007, 46, 5359) which uses 3 equivalents of B₂pin₂. The reaction works really well but for this one substrate the B₂pin₂ co-elutes with my product. Is there anything I can do to remove the excess  B₂pin₂ short of derivatizing my pinacol ester?

Try a work-up with ethanol amine.

[Bunsen Honeydew]

reduce B2Pin2 to PinB-H

[nox]

@disco: Wouldn't that sequester my aryl pinacol ester as well?

@Bunsen: Unfortunately my product contains an aldehyde, and I think under the conditions for making Bpin-H it would get destroyed (yes I could always protect and deprotect, but I really don't want to add 2 extra steps to what already is a pretty long sequence)

[g-bones]

I had a similar problem and this an issue for some of my other labmates as well.  You said you have an aldehyde, do you have anything that would be sensitive to oxidation?  The reason I ask is because I found passing my crude mixture through a short plug of silica (to get rid of black junk) then diluting the crude reaction mixture with ethyl acetate and refluxing with IBX (about two or three equivs) works WONDERS! column your material after that and I can almost guarantee it will fix your problem.  (see Chem. Sci., 2012, 3, 1338)
I also found that kugelrohr distillation works well at about 120 degrees celsius at about 0.1 torr for about an hour.  This is also in the SI of the paper cited above.  
Hope this helps!

[nox]

Hey g-bones thanks a lot for that It's pretty much just the aldehyde and that's it. The reason I'm being extra careful is because the aldehyde is deuterated at the formyl position, so it's a really precious intermediate and I absolutely don't want to commit any of it unless I have all the conditions optimized, which is why I'm running test reactions using a non-deuterated substrate.

I'm going to try running Buchwald's prep again but using less B₂pin₂, I have a feeling he only used 3 equivs so that the reaction could finish in an hour (to his credit the reaction really did finish in an hour, and the crude NMR was really clean except for the annoying B₂pin₂ and some pinacol presumably due to come B₂pin₂ that crapped out during the reaction).

Maybe using something like 1.1 equivs would improve the situation, since it seems like some B₂pin₂ always craps out in these reactions, so a tiny excess wouldn't hurt. Also better to have something like 90% conversion and fully converted/crapped out B₂pin₂ because at least I can separate the starting material from the product (although it'll still be a hard separation, at least they don't co-elute!)

The safer (but much less time and yield efficient) option is to protect the aldehyde as the dioxolane, do the borylation, then deprotect, although I'll probably only go down this route as a last resort if all else fails.

[discodermolide]

@disco: Wouldn't that sequester my aryl pinacol ester as well?

No I don't think so, it has a great affinity for boron.

[nox]

Sorry I should've clarified, when I say aryl pinacol ester, I mean the pinacol ester of boron on the aryl group.