Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: reflux on December 04, 2009, 09:28:02 AM
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Is it better to store a TBAF/THF solution at room temp than in the fridge? We keep ours in the fridge but solids start to form (there is ~5% water in solution, maybe it is freezing). Ideally we would like to eliminate the water but that causes decomposition of the TBAF into Bu3N and F2. A organic solvent soluble and anhydrous fluoride source would be ideal for us but there are not too many options (NaF, KF, and CsF with 18-C-6 or TBAT (a fluorosilicate developed by Deshong) are some good alternatives but do not work well for our methodology).
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We keep our TBAF in the fridge as well, though sometimes I've questioned if this is really necessary. It takes a long time for those solids to redissolve as the solution warms up to room temp. I was under the impression that the solids were the TBAF crashing out at cold temps....thus you have to wait for it to fully dissolve again in order to have your solution of known molarity.
If we want dry fluoride sources, people have used TBAT with success.
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Not sure what your methodology is, but we've recently been battling a reaction where TBAF isn't the best source. We've had a lot of luck with CsF in NMP, which will run at r.t. I can't imagine why it should work any worse than TBAF. It can be run anhydrously as well. You can also use an inorganic fluoride source (NaF, KF, CsF) in the presence of a phase tranfer catalyst (TBAB, etc...) to get the same effect as TBAF but under anhydrous conditions.
You can buy solid TBAF, but as you may imagine, it doesn't stay dry for long and is really hydroscopic.
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We dry ours and then leave it at room temp under high vac
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We dry ours and then leave it at room temp under high vac
How? How much water does this leave? Storing "anhydrous TBAF" (0.2 to 0.3 equiv H2O under vacuum for 4 days is supposed to give you 80% Hoffman elimination. (http://pubs.acs.org/doi/abs/10.1021/jo00160a041)
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I've done a lot of reactions using TBAF (indolizations/desilylations) and in my experience the presence of some water was essential to the well-being of the reaction (or at least it didn't harm it, as we could prove by deliberately adding a small amount of water to the reaction mixture without decreasing the yield). We always kept the TBAF at room temperature.