Hi all,
Now a days boronate chemistry is done routinely by most of the chemists. I am synthesizing boronates in my lab. We generally do not purify the boronates (only filter the reaction mixture and use it as such for next step).
My questions are:
1. How chemists purify the boronates (boronic acids or boronic esters)?
2. What type of columns are proffered for purification (silica ot alumina)?
3. Is there any chemical modification techniques available for purification of boronates?
Please explain as this will help others also.
Best regards
Dear Tanwar,
Thanks for your concerns for others. The boronic acid can be recrystallized, derivatized to purify and column chromatographed. Recrystallization in benzene, dichloroethane, and EtOAc can be good to moderate yielding. Derivatization follows treating the impure product with the base, separating the resultant salt (obtained from base) by solvent extraction, and treating the salt with acid to afford the pure boronic acids. The chromatography (usually silica gel CC) is good for low and mild polar acids where acetone can be one of the eluents, the methanol as eluent have been tried for polar products. Silica Column filtration is also recommended.
My compounds are almost all oils, so recrystallization does not seem to be an option. I tried to distill my compounds, but they seem to be too heavy, or the vacuum too bad, or the Kugelrohr-apparatus not sufficiently airtight. Depending on your point of view. But still, distillation is still my best bet, if I can get the pressure down sufficiently. The Compounds are quite pure (>90%) but for coming coupling experiments they are supposed to be as pure as possible.
Up to a few days ago I thought that a flash column is out of the question, since with standard-eluent-mixtures I was not able to produce proper, TLCs that do not smear all the way. Then I found a Paper[1] I which they try to saturate the silica with Boric acid to improve the separation. According to the authors this does not necessarily work for Pyridin-Boronic acids. Of course I tried it anyways, and was disappointed. Does anyone know solvent systems where chromatography might still be possible? I thought about reverse phase, but that is probably too expensive.
Is anybody experienced on the topic and would like to share that knowledge? I find it odd, that the whole world is Suzuki-coupling, but there apparently are no standard purification Methods, when distillation and recrystallization are not an option. Or am I just not finding them?
Well, thank you for reading this post, I would be really happy for any kind of answer.
Best regards
Dear Habib, Thanks for your post though as an extension to Tanwar's query. For all to see it independently and fully exposed post it as part of your query. Anyway, thanks for the efforts.
Thanks for your post though as an extension to Tanwar's query. For all to see it independently and fully exposed, post it as part of your query. Anyway, thanks for the efforts.
I don't see any reason that you can not crystallize the product in the oil mixture which is, according to you ~90% pure, unless the major product itself is oil. Yes, you can not recrystallize unless you have crystallized it. Try its crystallization in a solo solvent as trial and hit or in a mixture of solvent. If your product is completely soluble in a certain solvent 'a'- dissolve it completely in a minimum volume of the solvent 'a' and add another solvent, solvent 'b' which produces a hazy/cloudy environment in the product's solution in 'a'. Heat the product mix in 'a' (or in 'a' and 'b') up and add some of the solvent 'b' (if added earlier, no need to add 'b') and cool immediately- the major product should come out as solid if that's solid by nature. The other method would be trituration with a solvent. You can also try this- put the product and solvent together and mix with an immiscible solvent, preferably, a combination of less and more polar solvents [still immiscible (Hexane-MeOH etc.)] and scratch the flask wall for some time, ~ 20-30 minutes! You can try slow freezing the product and decant out the unfrozen material. The traditional crystallization is way too much guess and the silica-based CC purification is tedious, tricky and many times does not work for all derivatives with a problem of elution and silica-stuck nature of the materials in the mixture and hard to get out. In my opinion, the purification through salt derivatization is still a good way for you unless there is some structural limitation. The boronic acid behaves very similarly to carboxylic acids. Anyway, give a try to some of the methods discussed and let us know. Hope they work!
Dear Riaz,
Many thanks for your valuable suggestions. I will apply them for my problem
regards
Dear Riaz,
As per your suggestions and based on my compounds polarity, I was able to purify my material (boronate ester) by CC using neutral alumina. The product got eluted in hexane itself.
Best Regards
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