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Author: Subject: Aldol condensation using Boric Acid
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[*] posted on 22-1-2008 at 12:23
Aldol condensation using Boric Acid

I've been wanting to try a procedure for acid-catalysed crossed-aldol condensation, using a Dean Stark apparatus:

Aldehyde and Ketone Condensation Reactions Catalyzed by Boric Acid
R. D. OFFENHAUERAN S. F. NELSEN J.Org.Chem., 33(2), (1968) 775-777

I plan on using various benzaldehydes with MEK, and acetone. Basic catalysis if often used with acetone, as the two conditions would produce the same product. On the other hand, acidic catalysis when using MEK lead to a product formed by condensation on the methylene carbon, on the the ethyll side chain, while basic catalysis favor condensation on the other chain (CO-CH3). This ables the synthesis of numerous isomers, compared to the condensations with acetone.

In this article, they use acetophenone as a methyl ketone. The problem with low boiling ketones such as MEK and acetone is that using them with a Dean Stark is very unpractical, as there azeotropes with water boil at low temperature, and their relatively high miscibility with water prevent efficient seperation of water in the trap, sending water back in the still and preventing the following of the recation by the amount of water formed.
So I though of using low boiling alkanes, mainly the cheap mixture of pentanes and hexanes, which have azeotropes between 45 and 60°c, roughly 5-10% water. PLus, the addition of hydrocarbons to mixtures of MEK and water greatly help the seperation of two phases (see US 6,121,497). Hopefull this will work well enough.
The catalysis seems to involve the formation of an enol-borate ester from the boric acid and the ketone, and apparently boron oxides are obtained at the end of the recation.

SO, I have a few questions:

-Has anyone ever used a Dean Stark with low boiling ketones such as MEK or Acetone? Any comments, ideas, advice?

-How easily does boric acid deshydrate to metaboric acid (HBO2)? I know this is usually done at temp around 170°C, but I think azeotropic removal of water would facilitate the reaction. Patent EP0327473 do this a 135°C, but patents are patents, and I don't really feel like using xylene to form the meatoboric acid, and thne distilling the solvant out to conduct the condensation... Would it be reasonable to think that a few hours reflux with an dean stark, using low boiling alkanes, could yield HBO2? I haven't found any conclusive answers on the web. And would preforming HBO2 before introducing the carbonyl compounds really be an advantage?
In the article they simply combine the boric acid with the reactants and reflux the hell out of it for 16-20H. But I would like to be able to moniter the progress of the recation by the amount of water formed (TLC willl also be sued in any case). So I'm not sure if I must count the amount of water produced by the deshydration of the boric acid. It is not specified if the solids they filter during workup up are boron oxide (B2O3, unlikely), metaboric acid (HBO2), or simply boric acid with various amounts of deshydrated products.

-If using preformed HBO2, would the end product be the free unsaturated ketone, or the corresponding enol-borate? When they use preformed "boron oxide" (B2O3 I presume), they add water after reflux to hydrolyse the borate ester and precipate the product. Would this be needed too with HBO2, or would it rather stay as is?

[Edited on 22-1-2008 by Klute]

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[*] posted on 22-1-2008 at 12:57


How easily does boric acid dehydrate to metaboric acid (HBO2)?

From my experience with hot boric acid as a flux for metalwork,
it dehydrates quite readily all the way to the oxide. Therefore, I
would think there is no need to go through the trouble with the
azeotrope or refluxing --- just bake it, taking care not to overbake it.

I am not so sure about this 170C temperature you mention ---
according to the Merck index, it dehydrates around 100C and, by
170C, is already molten and dehydrates further to pyroboric acid.

[Edited on 22-1-2008 by microcosmicus]

[Edited on 22-1-2008 by microcosmicus]
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