I am prepairing some ligands in 3-4 steps which contain a catechol and ethylene glycol type of moiety and aldehyde group. The last step of that involves a schiff base reaction followed by reduction of the imine group using sodium borohydride. Up to the imine reduction it is ok but after adding borohydride, followed by the extraction with DCM-Water and washing 2-3 times with brine, whatever I am getting is not pure. ESI-MS reveals a [M+12] peak. NMR is also a little doubtful. After reading some literature I came to the conclusion that catechol (forming catecholaborane kind of things) and ethylene glycol type moieties are easily bound with boron.
My query is, did you face this kind of problem during synthesis? Please suggest how to purify my compounds? I am attaching some related literature here.
you can remove easily borane BH3 by DABCO in toluene at 50°C for 4h. DABCO-BH3 is removed by filtering on silicagel chromatography.
We used to cleave BF2-complexes of tetramic acids (Melophlins) simply by heating in methanol. Maybe it also works with catechol complexes.
There is a possibility of nitrogen-BH3 adduct formation, your mass [M+12] is a good evidence for that, and NMR can proof that as well.
In trying to form a stable pyrillium for ring-hetero-atom-substituion/exchange I saw several boron chelates. I have another article or two specifically listing medicinal properties of gamma-pyrones, "re-searching" for it at the moment.
To add - everything that has ended in a boron-chelate has been small amounts of"crap" so I haven't tried reclaiming starting material. Once I tried neutralizing what I thought was a product (boron-chelate) to restore organic solubility and I ended up with only starting material. I would presume adjusting the pH in solution would alleviate this problem. DMSO does some dastardly things with pyrillium salts as well, largely regenerating the starting material.
you can remove easily borane BH3 by DABCO in toluene at 50°C for 4h. DABCO-BH3 is removed by filtering on silicagel chromatography.
Yes, it could be helpful by using a strong base for the removal of borane. Or you can use HCl in THF and reflux it for some minutes.
Thanks for your suggestion.What pH I should maintain?.Otherwise there may be a chance for protonation of the secondary amine and it may go to the water part during extraction.I will definately try.
Know very about synthesis but because you wash with water there are 2 options you try to free the catechol from boron:
1) acidify the solution down to a pH of 4 to 5. B(OH)3 will be formed
2) add a diol that forms stronger complexes with borate. Examples are polyvinylalcohol (Mw is not relevant), guar gum but simple molecules such as D-mannose also does the job. D-mannose has a cis-diol functionality and in general molecules which have cis-diol functionality form strong complexes with borate.
If it is attached borane adduct or complex then you will need to detach with nitrogen containg bases and then remove with hexane...if it is free from ur product. i think also it will be soluble in hexane or any aromatic solvent such as toluene...
Hello, I think , that the easiest way is to acidify reaction mixture after the reduction is completed, because it removes (BH4)- ion like borane. Moreover you will break borane komplex with diol moiety which arises under basic conditions.
You should hydrolyze the B-O bond which can do it by using strong bases/acids or use from DABCO in a non-polar solvent such as xylene and toluene.
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