[yesway]

Hello,

sorry for the bad joke in the title, could not resist.

I am preparing/purifying an arylboronic acid and I have heard about these things being in equilibrium with the trimeric anhydrides (boroxines).

So, naturally a few questions pop up for me in general handling for NMR sample preparation. The known procedure calls for acetone-d6 as the nmr solvent, but it also nicely dissolves in chloroform-d3.  Is it due to the higher water content in opened bottles of acetone/dmso-d6 that the free boronic acid is favoured and CDCl3 gives a mix of free boronic acid + oligomers? Or do polar solvents break up these anhydrides by themselves? Some people have said to add D2O to favour free acid formation or MeOD to make the B-ester. One book chapter wrote to keep these cmpds from decomposing in general is to store them slightly moist under an inert atmosphere, which would mean the free acid is also more stable in the long run! I am boron-boozled! 

I would be grateful if somebody with experience with analysis of this compound class could outline his favourite way of nmr sample preparation.

Many thanks in advance
G

[Dan]

I don't have any direct experience with these compounds, but it would make sense that using a coordinating (or wet) solvent like DMSO or acetone would break up oligomers.

Also - moronic acid is a real compound. I thought this was going to be a thread about terpene natural products...

[willug]

Yeah I've seen this before too. If you happen to run your boronic acids in dry DMSO you are quite likely to see these anhydrides. The addition of a drop of two of D2O is enough to hydrolyse them.

As it happens I've actually got before and after NMR's with p-nitrobenzeneboronic acid, but I'm not sure how to attach them...

[yesway]

Thank you all for your answers. Addition of a few drops of D2O into the CDCl3 solution of the cmpd gave nice nmr spectra! Still not sure how to store these compounds though.